Developing intelligent control of photoresponsive materials: from switch-type to multi-mode

Abstract

Most photoresponsive materials are controllably switched between two functional states with contrary relationship, which is defined as switch-type control logic. However, the world is not black and white. When facing real-world applications, the photoresponsive materials are working under complex stimuli. Therefore, developing the control logic of photoresponsive materials from switch-type to multi-mode is essential, meaning the materials could switch among multiple (n > 2) functional states by varying light conditions. In this tutorial review, the light-switchable optical, electrical, chemical, mechanical and morphological properties of photoresponsive materials with switch-type control logic is introduced. The multi-mode control of photoresponsive materials is discussed from two aspects: (1) switching between multiple stationary states, termed as multi-stable control and (2) switching between multiple nonequilibrium states, termed as multi-stage control. The potential applications and future challenges are envisioned to encourage further and continuous developments for the multi-mode photoresponsive materials.

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Gaolu Zhu

Gaolu Zhu obtained his Master's degree from Sichuan Agricultural University in 2023 under the supervision of Prof. Shaobo Zhang. He is currently a PhD candidate at the University of Electronic Science and Technology of China, working under the guidance of Prof. Dongsheng Wang and Prof. Yonghao Zheng. His research focuses on photochromic molecules and materials.

Fanxi Sun

Fanxi Sun received his PhD in Optical Engineering from the University of Electronic Science and Technology of China in 2025, under the supervision of Prof. Dongsheng Wang and Prof. Yonghao Zheng. He earned his MSc from Nanyang Technological University, Singapore, and his B.Eng. from Sichuan University. His research interests include photoresponsive materials, solid-state photoswitching, and single-molecule electronics.

Xu Deng

Xu Deng received his PhD in 2013 from the Max-Planck-Institute for Polymer Research. After serving as a postdoctoral fellow at UC Berkeley and Lawrence Berkeley National Laboratory, he joined the University of Electronic Science and Technology of China as a professor in 2015. In 2017, he was pointed by the president of the Max-Planck-Institute as the head of the MPIP-UESTC partner group. He is interested in understanding wetting dynamics and physical chemistry at interfaces. In 2021, he was admitted as a Fellow of the Royal Society of Chemistry. In 2022, he was awarded the Friedrich Wilhelm Bessel Research Award of the Alexander von Humboldt Foundation, Germany.

Yonghao Zheng

Yonghao Zheng received MChem (2008) and PhD (2011) degrees from Durham University under the supervision of Prof. Martin R. Bryce. He spent two periods of time (2012–2014 and 2016) in Prof. Fred Wudl's group at the University of California Santa Barbara as a postdoctoral researcher. In 2015, he moved to Rice University as a postdoctoral researcher in Prof. James M. Tour's group. In 2016, he became a Professor at the University of Electronic Science and Technology of China. His current research is mainly focused on the development of stable radicals in single-molecule devices and photoresponsive materials.

Chen Wei

Chen Wei obtained her Bachelor and PhD degrees in Physics in 2009 and 2014 from Nankai University, China. She worked as a visiting scholar between 2012 and 2014 at the Wyant College of Optical Sciences, University of Arizona, collaborating with Professor Nasser Peyghambarian. Currently, she is a full Professor at the School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China. Her research interests encompass mid-infrared fiber lasers, nonlinear fiber optics, all-optical switching and ultrafast fiber lasers, aiming to precisely control functionalities of photoresponsive materials with well-constructed laser systems.

Dongsheng Wang

Dongsheng Wang received his PhD degree from Max-Planck-Institute for Polymer Research (MPIP) under the supervision of Prof. Dr Hans-Jürgen Butt in 2017, then he joined University of Electronic Science and Technology of China (UESTC) and worked as an associate professor. He was appointed by the president of the Max-Planck-Institute as the head of the MPIP-UESTC partner group in 2019. He is interested in photoresponsive molecules, materials and devices. In the year 2024 he proposed “self-adaptive photochromism”, where the color of materials switches to remain the same as the environment. He was awarded the “Emerging Investigators” title of Journal of Materials Chemistry and Chemical Communications in 2022 and 2023.

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Key learning points

(1) The importance and urgency of multi-mode control of photoresponsive materials.

(2) Multi-stable control based on switching between multiple photostationary states.

(3) Multi-stage control based on switching between multiple nonequilibrium states.

(4) Potential applications of multi-mode photoresponsive materials.

(5) The challenges for the further developments of multi-mode photoresponsive materials.

1. Introduction

Photoresponsive materials, which reversibly switch their properties and functionalities in response to light irradiation, have garnered significant attention due to their wide-ranging potential applications,¹ including chip manufacturing,^2,3 information storage,^4,5 optoelectronics,^6,7 surface engineering,^8,9 nanotechnology^10,11 and biomedicine.^12,13 Compared to other stimuli, light irradiation offers superior control over molecular structures, chemical reactions, and material functionalities due to its non-contact, rapid response, and spatiotemporal precision.^14,15 This control capability is not only applicable to organic photoresponsive materials, but also extends to emerging systems such as 2D carbon-based photoresponsive materials.^16,17 However, the switching of most photoresponsive materials is limited between two functional states with contrary relationship, which is in analogy with the on/off switching of a light source (Fig. 1). The switch-type control logic limits the application of photoresponsive materials in environments with complex stimuli. This limitation arises from the fact that photoresponsive molecules generally isomerize between two photostationary states. In addition, due to the differences in thermodynamics, photoresponsive molecules are typically classified into P-type and T-type: P-type photoresponsive molecules are bistable where the isomerization to both directions is triggered by light irradiation. By contrast, T-type photoresponsive molecules undergo photoisomerization but spontaneously relax back to their original state in the absence of light.

Fig. 1 Development of the control logic from traditional switch-type to multi-mode for photoresponsive materials.

Conventional switch-type photoresponsive materials have gained extensive use for studies in controlling simple functions, such as single-layer information encryption, monotherapy drug delivery and simple motion of photoactuators. As the smart materials develop, the switch-type control logic meets problems in performing complex functions, such as multi-layer information encryption, combination drug delivery and complex motion of photoactuators. The demand for advancing the control logic of photoresponsive materials from switch-type to multi-mode is in analogy with the human desire for controlling the light source in a more precise manner, rather than simply being limited to on and off (Fig. 1). The multi-mode control logic allows photoresponsive materials to switch between multiple (n > 2) functional states, and has become a critical and urgent scientific goal in this field. In recent years, extensive research has focused on designing molecular and material structures, and then adjusting light conditions such as wavelength,^18–21 intensity,^22–25 time^26–29 and focus location^21,23,30 to achieve multi-mode control of photoresponsive materials. The multi-mode control of photoresponsive materials could be categorized into two types: (1) switching between multiple stationary states, termed as multi-stable control; (2) switching between multiple nonequilibrium states, termed as multi-stage control (Fig. 1).

Unlike the pioneer reviews talking about targeted applications on specific photoresponsive molecules and materials,^15,31,32 this tutorial review focuses on the developments of multi-mode control logic for photoresponsive materials. We first introduced the photoresponsive materials with switch-type control of key properties, including optical, electrical, chemical, mechanical and morphological properties. Secondly, the multi-mode photoresponsive materials are discussed mainly from the aspects of multi-stable and multi-stage control logic. The potential applications and future challenges of the multi-mode control logic of photoresponsive materials were further envisioned. We expect this review will encourage researchers to realize the importance and urgency of developing the control logic of photoresponsive materials from switch-type to multi-mode, which makes the materials smarter.

2. Switch-type photoresponsive materials

Photoresponsive molecules, also known as photoswitches, can undergo reversible isomerization between two stationary states, such as trans–cis for azobenzene (Azo),^33,34spirocyclic–merocyanine for spiropyran (SP) and spirooxazine (SO),^35–37 open–closed for diarylethene (DAE),^38,39 and linear–cyclic for donor–acceptor Stenhouse adduct (DASA)^40,41 (Fig. 2). The conformational switching results in significant changes in molecular characteristics, including geometry (size, planarity), conjugated structure and polarity (dipole moment).^10,42,43 The switching behavior of photoresponsive materials strongly depends on their physical state. For example, DAE with minimal conformational change exhibits reversible photoisomerization in both solutions and solid state. In contrast, photoresponsive molecules with large structural changes during the isomerization process (e.g. Azo, SP, and DASA) switch efficiently in solutions but often face suppressed switching in the solid state due to the limited free space, unless incorporated into polymers, liquid crystals and materials with nano-framework structures. Therefore, the critical properties including optical, electrical, chemical, mechanical and morphological properties of photoresponsive materials could be switched under controlling of light, making them attractive for various applications (Table 1). However, limited by the isomerization of photoresponsive molecules, the properties of most photoresponsive materials can only switch between two functional states with contrary relationships, exhibiting switch-type control logic.

Fig. 2 Chemical structure and isomerization of common photoresponsive molecules.

Table 1 Summary of light-switched properties, molecular mechanisms, and applications of photoresponsive materials with switch-type control logic

Light-switched property	Molecular mechanism		Applications
Optical property (color, luminescence, transmittance, reflectance)	Conjugated structure, molecular orientation	Switching of conjugated structure leads to dramatical variation of absorption spectra; switched orientation of photoresponsive liquid crystal molecules induces variable refraction to light of materials	Information storage,^44,45 secrecy,^46–54 anti-counterfeiting,^51,52,55–59 displaying,^46,60–62 sensors,^52,63–69 smart windows^70–76
Electrical property (conductivity)	Conjugated structure, conformation	Switching of conjugated structure and conformation leads to changes in delocalization of π electron energy levels and frontier orbits	Single molecular junction,^77–88 organic field-effect transistors (OFETs)^89–101
Chemical property (wettability, solubility, chemical activity)	Molecular polarity, electron distribution, functional groups	Molecular polarity affects the affinity of materials to water and solvents, which are interrelated to the wettability and solubility; electron distribution and functional groups affects chemical activity	Controllable wetting on surfaces,^102–108 supramolecular self-assembly,^109–112 drug delivery,^113–120 microfluids,^121–125 nanoreactors,^126–131 selective chemical reaction^132–138
Mechanical property (rigidity, viscoelasticity)	Intermolecular interaction (van der Waals force, hydrogen bonding, π–π interaction)	Isomerization leads to variation of the intermolecular aggregation and condensed matter physics of the photoresponsive materials	Photolithography,^139–141 self-healing materials,^142–146 3D printing,^147–150 switchable adhesives,^142,151–155 solar thermal fuels,^156–166 gas separation,^167–171 tissue engineering^172–175
Morphological property (size, shape)	Conformation, intermolecular interaction	Isomerization leads to rearrangement of polymer chains and further deforms the bulk materials. The shape change could be precisely controlled by materials design	Photoactuators,^61,176–183 artificial muscle,^184–186 robots,^187,188 microfluids^189–192

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2.1 Optical properties

Photoresponsive materials exhibit switchable optical properties under controlling of light irradiation (Fig. 3), such as color, luminescence, transmittance, and reflectance. Typically, the phenomenon where the color of photoresponsive materials is switched by light irradiation is widely known as photochromism. This is because the switching of the conjugated structure induces dramatic variations in the absorption spectra of photoresponsive molecules. For example, typical SP/SO exhibits a thermally stable spirocyclic state, which is colorless due to negligible absorption in the visible region. UV light irradiation induces the formation of the merocyanine state, which has extended conjugation over the entire molecule. The sharply increased absorption enables colorless-to-colored switching of SP/SO.¹⁹⁴ Another example is the linear–cyclic isomerization of DASA, while the strong color comes from the push–pull triene π-bridge of linear DASA, visible light deforms the conjugated structure and generates the colorless cyclic isomer.¹⁹⁵

Fig. 3 Photoresponsive materials with switch-type control of optical properties. (a) Photochromic solid powders by embedding DASA into the nanocages of MOFs, visible light and heat induced reversible color change of the materials. Reproduced with permission.⁴⁶ Copyright 2022, Elsevier. (b) Photoresponsive fluorescent switches by modifying NDI with DASA, visible light and heat reversibly turn on and turn off the luminescence of DASA-NDI. Reproduced with permission.¹⁹³ Copyright 2018, Elsevier. (c) Smart windows by integrating Azo into liquid crystal molecules, with photographic images showing the transparent state in the dark and light-scattering state under sunlight. Reproduced with permission.⁷⁰ Copyright 2022, Royal Society of Chemistry.

Under the control of light irradiation and heat, photoresponsive materials exhibit reversible color-switching between two states, which is widely known as photochromism. Almost all the photoresponsive molecules have been reported on their photochromic applications, even the color-switching of typical Azo is subtle (yellow-orange), and this slight shift can still be observed with the naked eye. The photochromism includes positive and negative:¹⁹⁶ (1) the positive photochromism indicates photoresponsive molecules (i.e. SP/SO) exhibit a colorless pristine state and light-induced colorless-to-colored transition; (2) the negative photochromism indicates photoresponsive molecules (i.e. DASA) exhibit a colored pristine state and light-induced colored-to-colorless transition. Wang and coworkers reported solid powdery materials with negative photochromism controlled by visible light and heat by embedding DASA into the nanocages of metal–organic frameworks (MOFs).⁴⁶ The isomerization kinetics of DASA is closely interrelated with the physicochemical environment inside the nanocages. When using MIL-101(Cr) with randomly oriented benzene rings around its nanocages, photochromism is fast and efficient, comparable to that observed in solutions (Fig. 3a). The solid powders are further applied as smart additives for the fabrication of photochromic bulk materials.

When integrating with fluorescent molecules, the variations in absorption spectra allow photochromic molecules to control material luminescence via Förster resonance energy transfer (FRET). As a typical example, 1,8-naphthalimide (NDI) exhibits strong fluorescence around 600 nm. Qu and coworkers reported a photoresponsive fluorescent switch by modifying NDI with DASA, which exhibits characteristic absorption at ∼623 nm (Fig. 3b).¹⁹³ The luminescence of DASA-NDI is intercepted due to the spectral overlap between fluorescence and absorption. Visible light induces linear-to-cyclic isomerization of DASA and gradually enhancing fluorescence intensity, which can subsequently be reversed by heating.

Materials with light-switchable transmittance and reflectivity are important for designing and fabricating smart windows (Fig. 3c).⁷⁰ Ikeda and coworkers reported smart windows fabricated by integrating small amounts of Azo into liquid crystal molecules. The reversible trans–cis isomerization of Azo induces variations in the orientation of liquid crystal molecules within photopolymerized networks, thereby regulating the refractive index. When the refractive index of the liquid crystals matches that of the polymeric matrix, incident light is transmitted, otherwise, the incident light is scattered.

2.2 Electrical properties

Photoresponsive materials exhibit light-switchable electrical properties, such as conductivity (Fig. 4). Light irradiation reversibly switches the planarity and conjugated structure of the photoresponsive molecules, resulting in significant variations in their electronic energy levels and frontier orbitals.^197,198 Moreover, the switchable molecular geometry changes alter the delocalization of π electrons across the conjugated structures, closely influencing electronic conductivity.¹⁹⁹ Azo, a widely studied photoresponsive molecule, shows reversible isomerization between trans and cis under controlling of light and heat. The trans isomer has a planar molecular structure, whereas the cis form exhibits approximately a 60° twist between the benzene rings. The distortion of molecular structure during the trans–cis isomerization induces conductance switching along several orders of magnitude. Guo and coworkers reported a single-molecule junction by connecting Azo based photoresponsive molecules between graphene nanogap electrodes through the formation of robust covalent amide bonding (Fig. 4a). The single-molecule junction was subsequentially treated with sequential exposure to 254 nm UV light and visible light (>460 nm), while the corresponding current was monitored in real-time under constant source–drain and gate voltage biases. Exposure to UV light triggers the trans-to-cis isomerization of Azo, increasing the HOMO–LUMO gap, and causing a sharp decrease in the conductance from ∼0.7 μA to ∼0.5 μA, which is recovered after visible light irradiation.⁷⁷

Fig. 4 Photoresponsive materials with switch-type control of electrical properties. (a) Photoresponsive single-molecule junction by connecting Azo between the graphene nanogap electrodes, time trace of the drain current of the single-molecule junction indicates the switching of conductance under a sequence of UV and visible light irradiation. Reproduced with permission.⁷⁷ Copyright 2013, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Schematic illustration of a graphene–DAE–graphene junction controlled by UV and visible light irradiation. Reproduced with permission.⁷⁸ Copyright 2016, Science. (c) Top-gate organic thin-film transistors by assembling a monolayer of DAE on the surface of a gold electrode; the source-leakage current could be reversibly switched by controlling the UV and visible light irradiation. Reproduced with permission.⁸⁹ Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

DAE as a typical P-type photoresponsive molecule, reversibly isomerizes between open and closed under controlling of UV and visible light irradiation. The closed state shows delocalized π electrons spreading throughout the entire molecule; conversely, in the open form, the thiophene ring twists out of the cyclopentene plane, forming a non-conjugated bent structure that interrupts electron transmission. Therefore, the open and closed isomers exhibit dramatically different molecular conductivity, while the molecular length is only slightly changed during the isomerization. Guo and coworkers fabricated a photoresponsive single-molecule junction by anchoring a DAE derivative between the graphene nanogap electrodes (Fig. 4b). UV light irradiation with a wavelength of 365 nm induces the open-to-closed isomerization of DAE, which sharply increases the current from ∼0.08 nA to ∼0.29 nA, generating a high switch ratio of ∼110. On the other hand, visible light irradiation results in recovery of the original current.⁷⁸ The non-contact control, efficient and fast response make the photoresponsive single-molecule junction attractive in optoelectronic nanodevices, such as molecular transistors and logic units. However, due to the technical limitations, the photoresponsive single-molecular devices are still in their infancy.

Besides the single-molecular conductance, plenty of research have been focused on fabricating photoresponsive organic field-effect transistors (OFETs) with light-tunable conductivity. Samori and coworkers reported a series of OFETs fabricated by forming a self-assembled monolayer of photoresponsive molecules (e.g. Azo,^98,200 SP,^101,201 DAE^89,202,203) on electrode surfaces. For example, a top-gate organic thin-film transistor was fabricated by assembling a monolayer of DAE onto a gold electrode surface, enabling the modulation of charge injection through controlled isomerization of DAE (Fig. 4c). When the DAE is in closed form, the source-leakage current of the transistor is high, which is attributed to the conjugated structure facilitating charge transport across the monolayer. The source-leakage current could be reversibly switched under controlling of UV and visible light irradiation. After irradiation with 313 nm UV light, the DAE undergo open-to-closed isomerization, leading to an increase in the measured drain current. Visible light irradiation decreases the current to ∼35% of the closed state, resulting in a switch ratio of ∼2.86.⁵¹

2.3 Chemical properties

Photoresponsive materials exhibit controllable switching of chemical properties between two states, such as wettability, solubility and chemical activity (Fig. 5). This is because the polarity, electron distribution and functional groups on the photoresponsive molecules switch with the variation of molecular structure and geometry during the isomerization. For example, typical trans Azo consists of two coplanar phenyl groups connected by a nitrogen–nitrogen double bond, resulting in a planar geometry with a nonpolar character (dipole moment = ∼0.5 D) and stable chemical activity. By contrast, the polarity is increased for the cis isomer (dipole moment = ∼3 D),²⁰⁶ which increases the hydrophilicity and chemical activity. Moreover, the spirocyclic-to-merocyanine and linear-to-cyclic isomerization of SP and DASA generate zwitterionic isomers with high dipole moment (∼18 D for merocyanine SP,³⁵ ∼16 D for cyclic DASA²⁰⁷).

Fig. 5 Photoresponsive materials with switch-type control of chemical properties. (a) Reversibly switching the wettability on the surface with an arylazopyrazole self-assembled monolayer by UV and visible light irradiation. Reproduced with permission.²⁰⁴ Copyright 2020, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Controlling the assembly of Au nanoparticles by immobilization of thiol-functionalized Azo molecules on the surface. UV light irradiation induces aggregation of the nanoparticles to form a three-dimensional crystal, which is redispersed in the solution after visible light irradiation. Reproduced with permission.²⁰⁵ Copyright 2007, The National Academy of Sciences of the USA. (c) Photoresponsive ligands with light-switched binding affinity with metal-porphyrins based on the trans–cis isomerization of azopyridine. Reproduced with permission.¹³⁴ Copyright 2011, American Chemical Society.

Glorius and coworkers reported switching wettability on surfaces by controlling the trans–cis isomerization of an arylazopyrazole self-assembled monolayer (Fig. 5a).²⁰⁴ Arylazopyrazole exhibits similar molecular structure and chemical properties as Azo. Under dark or visible light irradiation, the arylazopyrazole monolayer in trans results in a water contact angle of ∼84.6°; UV light irradiation gradually induces a hydrophobic-to-hydrophilic transition, and the water contact angle slightly decreases to ∼80.7°. Importantly, light irradiation does not cause surface decomposition or contamination, and the wettability switching can be reversed through sequential UV and visible-light treatments. Introducing micro and nanostructure on the surface could further increase the contact area between the droplet and photoresponsive materials, which enhances the light-switching of wettability. Wang and coworkers reported the modification of silica microparticle surfaces with DASA molecules to enhance the roughness of photoresponsive materials. The water contact angles could be switched with a modulation range of ∼40° under controlling of visible light irradiation and heat. On the other hand, the water contact angles on a smooth surface modified with DASA are switched between ∼71° and ∼66°.²⁰⁸

The switched dipole moment of photoresponsive molecules induces variation of intermolecular aggregation in specific solutions, which have been used in controlling the self-assembly of polymers and micro/nanoparticles. Grzybowski and coworkers modified Au nanoparticles by immobilizing thiol-functionalized Azo on the surface in a low concentration (Fig. 5b). UV light irradiation increases the intermolecular interaction between Azo, which further induces the aggregation of Au nanoparticles to form three-dimensional crystals. The nanoparticles could be redispersed in the solution after visible light irradiation or heat.²⁰⁵

Specifically, the isomerization of photoresponsive molecules generates functional groups and binding sites, increasing chemical reactivity toward certain reactions. For example, Herges and coworkers reported photoresponsive ligands with controllable binding affinity with metal-porphyrins based on the trans–cis isomerization of azopyridine (Fig. 5c).¹³⁴ Specifically, the association constant between trans azopyridine and nickel porphyrin is as high as 10.25 L mol⁻¹, sharply decreasing to 0.3 L mol⁻¹ after UV light irradiation. Barner-kowollik and coworkers reported a Michael addition reaction selectively occurred between cyclic DASA and thiol groups. The linear-to-cyclic isomerization of DASA generates a 2-cyclopentenone structure due to the deformation of triene π- is vulnerable to be nucleophilically attacked during the (1,4) addition reaction.¹³² Additionally, spirocyclic and merocyanine SP exhibit significantly different affinities to protons.³⁶ The binding constant of merocyanine SP for protons (pK_a ≈ 2.25) is much higher than that of spirocyclic SP (pK_a ≈ 7.15³⁵). This enables SP to reversibly release and recapture protons under the control of UV and visible light, acting as a photoacid.²¹²

2.4 Mechanical property

Photoresponsive materials exhibit light-switchable mechanical properties, including rigidity and viscoelastic properties, which result from the switching of either intermolecular interaction (i.e. van der Waals force, hydrogen bonding, π–π interaction) or self-assembled structure (Fig. 6). These intermolecular interactions are modulated by changes in the geometrical structure and polarity of the photoresponsive molecules. For example, trans Azo exhibits strong intermolecular π–π interactions in the solid state due to its planar geometry, which are significantly weakened upon trans-to-cis photoisomerization. On the other hand, the switched self-assembled structure is originated from the variable chemical properties of photoresponsive materials.

Fig. 6 Photoresponsive materials with switch-type control of the mechanical property. (a) UV-light-induced crystal-to-liquid transition of EtO-Az-EG, where the switching is reversed by visible light irradiation. Reproduced with permission.²⁰⁹ Copyright 2012, Royal Society of Chemistry. (b) Azo contained polymers exhibit transition between solid and liquid under alternating UV and visible light irradiation, which dramatically switches the T_g of the polymers. Reproduced with permission.²¹⁰ Copyright 2016, Springer Nature. (c) The photoresponsive host–guest interaction between Azo and α-CD results in sol–gel transition of supramolecular hydrogels by controlling the UV and visible light irradiation. Reproduced with permission.²¹¹ Copyright 2010, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The switched intermolecular interaction in the solid-state makes Azo attractive for controlling the rigidity of photoresponsive materials, which show light-induced phase transition between the solid and liquid state. In the year 2012, Okui and coworkers for the first time reported a UV light induced solid-to-liquid transition by an amphiphilic molecule (EtO-Az-EG) consisting of a hydrophobic stiff Azo rod and a hydrophilic flexible tetra(ethylene glycol) chain (Fig. 6a).²⁰⁹ Crystalline transEtO-Az-EG has a melting temperature of 50 °C. Nevertheless, UV light irradiation transforms the crystalline solid to liquid at room temperature. The transition is reversible under visible light irradiation. This is because of the ortho-alkylation, which prevents the benzene rings of Azo from being fully coplanar, leading to the weakened intermolecular π–π stacking interactions. These further lower the energy required for the transition from crystalline phase to isotropic phase. Besides, by integrating Azo as the side functional group of polymers, the glass transition temperature (T_g) of polymeric materials could be reversibly switched under controlling of UV and visible light irradiation.²¹⁰ Wu and coworkers reported the first photoresponsive polymer with light induced transition between solid and liquid (Fig. 6b).²¹⁰ When the Azo side group is in the trans state, the photoresponsive polymer shows a T_g of 48–68 °C, making the polymeric material in a glassy state at room temperature. After treating with UV light irradiation, the T_g sharply decreases to −10 °C along with the trans-to-cis isomerization of the Azo side group. These induce a solid-to-liquid transition of the polymeric material. Subsequently, visible light irradiation restores the T_g to its initial value, inducing a liquid-to-solid transition.

Viscoelasticity as an important mechanical property of polymeric materials, indicates the variation tendency of intermolecular interaction between polymer chains under external stress. Polymeric materials with three-dimensional crosslinked structures (e.g. hydrogels, organogels, and elastomers) exhibit typical viscoelastic property, which could be controlled by light after integrating with photoresponsive molecules. In 2006, Harada and coworkers demonstrated that a mixture of host polymer containing cyclodextrin (CD) and guest polymer containing Azo exhibited reversible viscosity switching under UV and visible light irradiation.²¹³ This is attributed to the photoresponsive host–guest supramolecular interaction between CD and Azo. In aqueous environment, trans Azo tends to enter the hydrophobic cavity of CD to form a 1 : 1 host–guest complex, whereas cis Azo is more hydrophilic and spontaneously slides out of the cavity. In the year 2010, the same group reported a photoresponsive hydrogel with a controllable sol–gel transition (Fig. 6c).²¹¹ The hydrogel remains in the gel state due to the strong host–guest interaction between trans Azo and α-CD (association constant K_a = 1100 M⁻¹). UV light irradiation dramatically decreases the association constant (K_a = 4.1 M⁻¹), which induces a gel-to-sol transition of the hydrogel. From the year 2015 to 2018, Wu and coworkers reported a series of red-light-responsive host–guest interactions between methoxyl-/isopropyl-substituted Azo and β-/γ-CD.^214,215 As a result, the viscoelasticity of the hydrogels could be therefore switched by long-wavelength-light, which is attractive in controlling biomacromolecules (e.g. proteins, DNA) release in vivo and cell growth and migration in tissue engineering.

Besides, while the photoresponsive molecules form self-assemblies, the mechanical property of materials is closely interrelated with the assembled structure and morphology. For example, some studies indicate in the SP-functionalized polyelectrolyte solutions,^216–218 photoisomerization between nonpolar spirocyclic and zwitterionic merocyanine alters intra- and inter-chain conformation and electrostatic interactions, which further induces variation of the rheological behavior. Raghavan and coworkers developed photorheological fluids by doping SP into the reverse micelles self-assembled from lecithin and sodium deoxycholate,²¹⁹ where the viscosity could be reversibly switched under UV light irradiation. Viscoelastic fluids consisting of long-wormlike micelles are formed in the dark, which exhibit a viscosity of η* = 2.5 Pa s. Upon UV light exposure, the spirocyclic-to-merocyanine isomerization of SP switches the self-assembled structure into short-chain micelles, leading to a viscosity drop of up to 10-fold, which recovers once the light is removed. This behavior is attributed to the micelle length changes driven by the differing interactions of lecithin with nonpolar spirocyclic and zwitterionic merocyanine. Zou and coworkers investigated the reversible coordination between Ca²⁺ and SP-doped biocompatible lecithin under UV and visible light irradiation,²²⁰ and explored the light-controlled transition between sol and gel. They incorporated SP into a two-component organogel composed of lecithin and Ca²⁺, which is in a gel state because of the coordination between lecithin and Ca²⁺. UV light irradiation generates SP in the merocyanine form, which induces the formation of merocyanine-Ca²⁺ coordination, leading to a 12-fold decrease in viscosity and a gel-to-sol transition. The viscosity and sol–gel transition are reversible under controlling of UV and visible light irradiation.

2.5 Morphological property

Photoresponsive materials exhibit light-controllable morphological properties between two stationary states, allowing remote manipulation of shape and size without direct contact (Fig. 7). The light-switchable morphological property mainly comes from the movement of photoresponsive molecules during the isomerization process, which induces rearrangement of polymer chains and further deforms the bulk materials. Most T-type photoresponsive molecules (e.g. Azo, SP/SO and DASAs) exhibit significant molecular deformation under controlling of light and heat, including the dissociation/formation, rotation and inversion of chemical bonds.^{35,41,206,221,222} In contrast, the P-type photoresponsive molecules (e.g. DAE), whose isomerization to both directions is controlled by light, exhibit negligible molecular deformation during the isomerization.^206,223

Fig. 7 Photoresponsive materials with switch-type control of morphological properties. (a) Sliding of a silicone oil slug in the tubular microactuator fabricated by Azo-containing liquid crystal polymers; the sliding of the slug is driven by light-induced deformation of the tube wall. Reproduced with permission.¹⁸⁹ Copyright 2016, Springer Nature. (b) Schematic illustration of 635 nm NIR-light-induced bending of a liquid crystal polymer film through a low-power excited triplet–triplet annihilation-based upconversion luminescence technique. Reproduced with permission.¹⁸⁰ Copyright 2013, American Chemical Society. (c) Light-controlled switching of supramolecular hydrogels between an expanded state and a contracted state based on a [c2]daisy chain structure composed of α-CD and Azo. Reproduced with permission.¹⁸⁴ Copyright 2016, Springer Nature. (d) Light-controlled bending of artificial muscle based on the [c2]daisy chain structure composed of α-CD and stilbene derivatives, and the deformation process could be accomplished within 3 s. Reproduced with permission.¹⁸⁵ Copyright 2018, American Chemical Society.

The anisotropic configurations of photoresponsive materials amplify molecular-level deformation to the macroscopic scale. The anisotropic configurations can be formed by either internal or external forces. Internal forces refer to weak intermolecular interactions such as hydrogen bonding, π–π stacking and electrostatic forces, which guide the formation of anisotropic nanostructures (e.g. liquid crystals, supramolecular assemblies) in photoresponsive materials. External forces, such as tension and friction, guide the formation of oriented structures in photoresponsive films, gels and elastomers.

Liquid crystal elastomers consisting of Azo functional groups have been extensively studied for the design and fabrication of deformable materials under the control of light irradiation and heat. While the trans-to-cis isomerization of Azo chromophores reduces the order of liquid crystal molecules, resulting in macroscopic shrinking or bending of photoresponsive materials. Yu and coworkers have reported a series of liquid crystal elastomers based on Azo since the year 2003. Functional groups with an Azo structure have been introduced into the side chains of polymers to fabricate photoresponsive liquid crystal elastomers. For example, they fabricated a photoresponsive liquid crystal network thin film using Azo-containing polymers. The bending of the thin film could be precisely controlled by linearly polarized light. Under 366 nm UV light irradiation, the film bends toward the direction of polarized light, and fully recovers after 540 nm visible light irradiation.¹⁷⁹ Therefore, the bending direction of the film could be well-controlled by the irradiation direction. Based on the deformation of photoresponsive materials, the liquid crystal polymers are further applied in fabricating tubular microactuators to achieve light-controlled sliding of fluid slugs (Fig. 7a).¹⁸⁹ When exposed under asymmetric light irradiation with a gradient of intensity, the localized expansion and contraction of the tubular microactuators generate capillary force to propel the fluid slugs. The sliding direction and speed of the fluid slugs could be precisely controlled by varying the intensity of light irradiation. Consider that by using UV light and short-wavelength visible light irradiation with high energy it is possible to induce decomposition of photoresponsive molecules, which does not benefit the long-term durability of liquid crystal elastomers. Yu and coworkers reported soft actuators responsive to red and near-infrared (NIR) light by a low-power excited triplet–triplet annihilation-based upconversion luminescence technique (Fig. 7b).¹⁸⁰ A sensitizer is excited under 635 nm red light irradiation, transfers energy to an annihilator, and emits 470 nm blue light. The emitted blue light induces trans-to-cis isomerization of the azotolane group in the liquid crystal polymer, leading to the bending and contraction of photoresponsive material. Long-wavelength light provides better tissue penetration and biocompatibility, making it attractive in biomedical applications. On the other hand, because of the slight molecular deformation during the isomerization process, DAE has been reported to reversibly switch the surface morphology of diaromatic single crystals.²²⁴

With the photoresponsive crosslinkers, the morphological property of hydrogels and elastomers could be controlled by light irradiation, exhibiting light-induced motions such as shrinking and bending, which are widely applied in photoactuators, robots and artificial muscles. Harada and coworkers proposed a novel photoresponsive artificial muscle by constructing the supramolecular crosslinker with a [c2]daisy chain structure based on the supramolecular host–guest interaction between Azo and α-CD (Fig. 7c).¹⁸⁴ The artificial muscle is in an expanded state while the trans Azo enters the hydrophobic cavity of α-CD. Under 365 nm UV light irradiation, the trans-to-cis isomerization of Azo leads to dissociation of the host–guest intermolecular complex. Because cis Azo slides out of the hydrophobic cavity of α-CD, the length of the [c2]daisy chain decreases and induces shrinking of the artificial muscle. Visible light irradiation with a wavelength of 430 nm expands the artificial muscle to the initial state. The same group further reported improving the light-controlled rate for the artificial muscle by fabricating the [c2]daisy chain structure composed of α-CD and stilbene (Fig. 7d).¹⁸⁵ Similar to Azo, the stilbene in trans form forms a 1 : 1 host–guest complex with α-CD. UV light irradiation with a wavelength of 280 nm triggers the trans-to-cis isomerization of stilbene and induces bending of the artificial muscle within 3 s. Additionally, the dried artificial muscle also shows fast response to light irradiation.

3. Strategies to achieve multi-mode control of photoresponsive materials

In daily life switches are widely used in controlling the functions of household appliances between two states with contrary logic, such as on–off of light (Fig. 1). With the developments of human society, more intelligent and precise control of devices and equipment is required: (1) for intelligent control, the device and equipment could be selectively switched among multiple working modes, achieving complex functionality (e.g. mode dial of wash machine, camera); (2) for precise control, the working status of devices and equipment could be continuously tuned between two states (e.g. rotating speed of electric fan, luminance of light). Generally, the intelligent control for the electric device and equipment is realized based on the integrating and programming of multiple basic logic controllers; on the other hand, the precise control is achieved by varying the input current. The success of intelligent and precise control in household appliances has inspired efforts to develop control logic in photoresponsive materials, moving from switch-type to multi-mode systems.

In the last chapter, we discussed the mechanism, design and application of switch-type photoresponsive materials. The optical, electrical, chemical, mechanical and morphological properties of the photoresponsive materials could be controllably switched between two stationary states with a contrary relationship. In many cases, switch-type photoresponsive materials perform reliably in the fields of biomedicine (e.g. drug delivery, tissue engineering), electronic engineering (e.g. information storage), and chemical engineering (e.g. nanoreactors). However, it remains challenging to further improve the intelligent and precise control of multi-mode switching for photoresponsive materials.

In the following part, the multi-mode control of photoresponsive materials is discussed from two aspects: (1) switching among multiple (n > 2) photostationary states, also termed as multi-stable control; (2) switching among multiple nonequilibrium states, also termed as multi-stage control.

3.1 Multi-stable control: switching between photostationary states

The multi-stable control of photoresponsive materials is stated as “switching between multiple photostationary states” (Fig. 8). However, most photoresponsive molecules are isomerized between two photostationary states (also known as “photoswitches”), which becomes the intrinsic limitation for the multi-stable control. Two strategies have been proposed to achieve multi-stable control in photoresponsive materials: (1) from the aspect of chemistry, design and synthesis of molecular machines with multiple (n > 2) photostationary states; (2) from the aspect of material science, introduce multiple photoresponsive molecules with distinct spectral properties into a single material system.

Fig. 8 Schematic diagram of multi-stable control of photoresponsive materials through chemistry-based and material-based methods.

(1) Chemistry-based methods: photoresponsive molecules with multiple stationary states. For the most photoresponsive molecules, the isomerization process involves multiple intermediates between two stationary states. However, these intermediates often possess high molecular energy and are difficult to precisely control, which is the main reason for the limited isomerization between two photostationary states. With the help of integrating different types of stimuli, many photoresponsive molecules exhibit multiple (n > 2) stationary states, such as the protonation/deprotonation of heteroayl Azo derivatives and SP/SO in merocyanine state under control of acid and base.^35,225,226 However, the multi-mode control of photoresponsive materials relies on pure light-controlling. Therefore, it is important to explore ways of increasing the number of photostationary states. Feringa and coworkers have been working on the molecular machines with light-driven switching among multiple (n = 3 or 4) photostationary states.^227,228 A typical molecular motor is based on an overcrowded alkene with a stereo center, where two identical chiral components are connected by a central C=C bond (Fig. 9). A typical molecular motor is based on an overcrowded alkene with a stereo center, where two identical chiral components are connected by a central C=C bond (Fig. 9). The molecular motor is in a thermodynamically stable state ((P,P)-trans-1) in the dark, which is switched to the (M,M)-cis-1 after UV light irradiation with the wavelength longer than 280 nm. This step could be reversed by visible light irradiation. The (M,M)-cis-1 state is unstable, and thermal fluctuation induces helicity inversion and further generates a (P,P)-cis-1 state. UV light irradiation coverts the (P,P)-cis-1 into (M,M)-trans-1 with a higher molecular energy, which undergoes a thermal-induced spiral inversion, returning to its initial conformational state ((P,P)-trans-1).

Fig. 9 Schematic illustration of the isomerization process of the typical molecular motor between four stationary states.

Interestingly, under continuous UV irradiation above a critical temperature, the molecular motor exhibits continuous C=C bond rotation, behaving like a molecular machine. The isomerization process of the molecular motor involves both photochemical and thermally driven steps. Therefore, precise control of light and heat conditions is essential for effective operation of the molecular motor.

Because of the controllable isomerization of the molecular motor between four stationary states, the multi-stable control of photoresponsive materials have been investigated. Feringa and coworkers developed a photoresponsive supramolecular self-assembly capable of switching among three distinct aggregated states under the control of light irradiation and heat (Fig. 10a).²²⁸ Hydrophilic segments were introduced into the molecular motor to form an amphiphilic structure, where the chiral and geometric variation of the molecular motor is closely interrelated to the macroscopic assembled morphology. As mentioned before, the molecular motor shows two distinct thermodynamically stable states, (P,P)-trans-M1 and (P,P)-cis-M1, which switches to the metastable states (M,M)-trans-M1 and (M,M)-cis-M1 after 312 nm UV light irradiation. The molecular motor in (P,P)-cis-M1 self-assembles into a helical fiber structure with the maximum width of 7.8 ± 0.7 nm and the minimum is 3.8 ± 0.4 nm in aqueous solution (Fig. 10b). After sequential UV irradiation and heating, the supramolecular self-assembly switches to a micelle structure with the average diameter of 6.2 ± 0.8 nm, and the molecular motor is stabilized in the (P,P)-trans-M1 state (Fig. 10c and d). The micelle structure further transfers into a worm-like micelle structure after UV light irradiation, which is unstable and switches back to the initial helical fiber structure after heating. The switching of supramolecular self-assembly between three distinct aggregated states indicates the chemical property of the molecular motor could be controlled by light. Besides, the optical property of the self-assembly is accordingly switched due to the varied absorption spectra of the molecular motor during isomerization. The supramolecular self-assembly is further applied in controlling luminescence of aggregation-induced emission, where the color and brightness of the fluorescence could be switched between three stationary states.

Fig. 10 Application of molecular motors in multi-stable light-control of photoresponsive materials. (a) Schematic illustration of the isomerization and switched supramolecular self-assembly of the molecular motor (M1) by controlling the 312 nm UV light irradiation and heat. Cryo-TEM images of the self-assembled structure of (P,P)-cis-M1 (b), (P,P)-trans-M1 (c) and (M,M)-cis-M1 (d) in water. Reproduced with permission.²²⁸ Copyright 2022, American Chemical Society. (e) Schematic illustration of the unidirectional light-driven molecular motor used as a photoresponsive organocatalyst. Reproduced with permission.²²⁹ Copyright 2011, American Association for the Advancement of Science.

The unidirectionally rotated molecular motor possesses a unique dynamic stereochemical property. Feringa and coworkers reported a molecular motor that can selectively catalyze a Michael addition reaction by modifying with a 4-dimethylaminopyridine Brønsted base and a thiourea hydrogen-bond donor group (Fig. 10e).²²⁹ The spatial relationship between these groups could be switched between remote, proximity in M-helicity and proximity in P-helicity by controlling the UV light irradiation and heat, which is closely affecting the catalytic performance of the molecular motor. When the molecular motor is in the (P,P)-trans-1 or (M,M)-trans-1 state, the Brønsted base and hydrogen-bond donor group are positioned far apart, resulting in a low-yield (∼7%) nonselective catalyzation for the Michael addition reaction, where the product is in racemic mixture (R,S) with an enantiomeric ratio of ∼1 : 1. On the other hand, the molecular motor in the (M,M)-cis-1 state selectively induces the formation of (S) enantiomer with an obviously increased yield of ∼50%; the (P,P)-cis-1 selectively produces the (R) enantiomer with the yield of ∼83%. These indicate the potential in the light-controllable nanoreactor of the molecular motor. Moreover, the molecular motor exhibits different spin injection barriers when in different isomerization states, which leads to switching of the electron transport efficiency.²³⁰ These make the molecular motor attractive in controlling the electrical property of photoresponsive materials between multiple stationary states.

Although the molecular motor can isomerize among four distinct stationary states, it still meets challenges in fabricating multi-mode photoresponsive materials: (1) the isomerization of the molecular motor could be well-controlled in solutions, which however is hindered in the solid-state. The intermolecular aggregation increases the difficulty of C=C bond rotation, as well as changing the molecular energy of stationary states; (2) compared with the trans–cis isomerization, the chiral switching of the molecular motor does not generate a significant difference in the molecular property (e.g. polarity, geometrical size, conjugation), which may limit the observable switching of key properties of multi-mode photoresponsive materials. Therefore, it still meets challenges to fabricate multi-mode photoresponsive materials with switchable electrical, chemical, mechanical and morphological properties by molecular motors. For example, Feringa and coworkers introduced a hydrophobic perfluorobutyl group modified molecular motor onto an Au substrate surface. While the isomerization of the molecular motor could be controlled among four stationary states, the wettability on the surface is switched between only two states.²³¹ The materials’ property is closely interrelated with the chemical structure of molecules, and to generate significant variation in the photoresponsive materials’ property it is critical to achieve obvious changes in molecular structure under light-control.

(2) Material-based methods: introduce multiple photoresponsive molecules into one material system. Compared with seeking molecular machines with multiple photostationary isomers, the combining of well-studied photoresponsive molecules with distinct spectral properties into a single material system is a more direct yet effective approach to realize multi-mode control in photoresponsive materials. Also known as orthogonal light-control, incident light with various wavelength selectively triggers the isomerization of photoresponsive molecules and switches the materials among multiple functional states, and the optical, electrical, chemical, mechanical and morphological properties of the photoresponsive materials could be controlled.

As an important optical property, the color of photoresponsive materials has been switched among multiple stationary states with the orthogonal light-control strategy, which results in multi-stable photochromism. Read de Alaniz and coworkers reported a series of DASA molecules, where the absorption spectra could be tuned by varying the electron-donating and -withdrawing moieties. DASA molecules with absorption maxima at 560 nm and 623 nm, respectively, were used for the orthogonal light-control (Fig. 11a).²³² In both solutions and polymeric films, the mixture of DASA molecules switches among four colored states under control of 514 nm green light, 650 nm red light and heat. The photochromic materials are in a blue state under darkness, which is contributed by the linear DASA molecules. Under 514 nm green light or 650 nm red light irradiation, one of the DASA molecules undergoes linear-to-cyclic isomerization, inducing the color-switching to cyan and purple, respectively. White light or irradiating with green and red light simultaneously generates a colorless state for the photochromic materials, indicating both the DASA molecules are cyclic. Therefore, the linear–cyclic isomerization of DASA molecules could be selectively controlled by varying the wavelength of incident light.

Fig. 11 Photochromic materials with multiple stationary color states. (a) Schematic illustration of orthogonal light-control of DASA molecules in solutions and polymer films. Reproduced with permission.²³² Copyright 2016, American Chemical Society. (b) Schematic illustration of programmable encryption achieved by integration of DASA and SP molecules. Reproduced with permission.²³³ Copyright 2024, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) The color of SAP materials could be switched spontaneously to match the surrounding light environment. Reproduced with permission.²³⁴ Copyright 2024, American Association for the Advancement of Science.

Besides the orthogonal light-control strategy, integration of positively and negatively photochromic molecules achieves programmable color-switching. By varying the input sequence of light irradiation and heat, the output color state of photoresponsive materials could be controllably switched, which is closely interrelated to the input conditions. For example, Wang and coworkers reported programmable information encryption by integrating DASA (negative photochromism) and SP (positive photochromism) (Fig. 11b).²³³ The input stimuli are controlled by varying the sequence of 365 nm UV light irradiation, 520 nm green light irradiation and heat, termed as “012”, “120”, “210”, “021”, “201” and “102”. The information is recorded by the invisible inks of DASA and SP through handwriting and ink-jet printing, which could be switched among four distinct states according to the input stimuli.

The previous examples focus on the “active control” of photoresponsive materials switching among multiple photostationary states. Wang and coworkers recently reported photoresponsive materials which are adaptive to ambient light environments and switch color accordingly, which is termed self-adaptive photochromism (SAP) (Fig. 11c). The SAP is realized based on the complementary color theory. Specifically, a series of DASA molecules with the absorption spectra range of 500–700 nm are integrated to fabricate SAP materials. Because of the even and well-distributed absorption spectra, the SAP materials are in a black state in the dark. While the incident light with a specific wavelength induces the linear-to-cyclic isomerization of one DASA molecule, an absorption gap is generated on the spectrum of SAP materials, switching the color to the same of the incident light. The SAP materials are able to mimic the color and pattern of the surrounding environment without the participation of electronic devices, which have great potential in active camouflage.²³⁴

The multi-stable control for the chemical property of photoresponsive materials has been achieved and widely applied in controlling switching of nanodevices, nanoreactors and self-assemblies between multiple stationary states. Asanuma and coworkers reported orthogonal light-control of a DNA nanodevice by modifying the oligonucleotides (R bar and L bar) with two Azo molecules, where the trans-to-cis isomerization was selectively triggered by 340 nm and 390 nm UV light, respectively (Fig. 12a).²³⁵ Under irradiation with different wavelengths, the nanodevice undergoes a seesaw-like motion due to the reversible isomerization of Azo between trans and cis state. This is because only in the planar trans does Azo stabilize the duplex, and the trans-to-cis isomerization leads to dissociation of the DNA duplex. The DNA nanodevice could be switched among four distinct states under controlling of 340 nm, 370 nm, 390 nm and 450 nm light irradiation. Bruns and coworkers reported a photoresponsive nanoreactor controlled by 525 nm green light and 630 nm red light irradiation based on the reversible photoisomerization of DASA.²³⁶ These DASA molecules were incorporated into amphiphilic block copolymers via reversible addition–fragmentation chain-transfer radical polymerization, which further self-assemble into polymersomes with the average diameter of 30–200 nm (Fig. 12b). After being illuminated with the corresponding wavelength of light, the linear-to-cyclic isomerization of DASA increases the permeability of polymersome membranes, thereby activating the encapsulated enzymes (e.g. horseradish peroxidase, glucose oxidase) to catalyze biochemical reactions. By alternately irradiating with green and red light, the cascade reactions can be repeatedly started and stopped, achieving independent control over different enzymatic reactions. Klajn and coworkers reported orthogonal light-control of self-assembly and disassembly of gold nanoparticles with different sizes by controlling the trans/cis isomerization of two Azo molecules (Azo-1 and Azo-2) immobilized on the nanoparticles’ surface (Fig. 12c).²³⁷ Specifically, 420 nm blue light irradiation induces trans-to-cis isomerization of Azo-1, resulting in the aggregation of gold nanoparticles with the diameter of 2.5 nm, while the 365 nm UV light irradiation disassembles the aggregates. The switching process of Azo-2 and gold nanoparticles with the diameter of 5.5 nm is reversed. Therefore, the self-assembly of nanoparticles could be controllably switched between three independent states in the dark, UV light and blue light irradiation.

Fig. 12 Photoresponsive materials with multi-stable light-controlled chemical property. (a) Orthogonal light-control of a DNA nanodevice by modifying the oligonucleotides (R bar and L bar) with two Azo molecules. Reproduced with permission.²³⁵ Copyright 2011, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Photoresponsive nanoreactors controlled by green and red light irradiation based on the reversible photoisomerization of DASA. Reproduced with permission.²³⁶ Copyright 2018, American Chemical Society. (c) Orthogonal light-control of self-assembly and disassembly of gold nanoparticles by controlling the photoisomerization of Azo-1 and Azo-2 immobilized on the nanoparticles’ surface. Reproduced with permission.²³⁷ Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Besides, with the orthogonal light-control strategy, the morphological property of photoresponsive materials has been successfully switched among multiple stabilized states, which is attractive in fabricating smart soft actuators. Broer and co-workers developed photoresponsive microactuators using inkjet printing and light-induced polymerization of resins combining Azo molecules with the absorption peaks at 358 nm and 490 nm (A3MA and DR1A in Fig. 13a).²³⁸ UV light irradiation generates a cisA3MA and a transDR1A, which induces bending of the A3MA part; on the other hand, visible light generated a transA3MA and a cisDR1A, which induces bending of the DR1A part. Simultaneous UV and visible light irradiation lead to full-length bending of the microactuators. This selective actuation realizes a cilia-like motion, accelerated by weak cis absorption and thermal recovery. The same group reported a photoresponsive liquid crystal elastomer with programmable actuation by controlling 405 nm purple light and 530 nm green light irradiation. The protonation significantly varies the photoresponsiveness of Azo, switching the photoresponsive molecule from an azomerocyanine (1-AM) form into a hydroxyazopyridinium (1-HAP) form (Fig. 13b).²³⁹ For the 1-AM containing polymer film, 530 nm green light irradiation induces significant bending (∼50°), while 405 nm purple light irradiation generates a lesser extent (∼20°); on the other hand, the acid-treated film (now in the 1-HAP form) exhibits minor bending (∼10°) under green light irradiation and significant bending (∼50°) under purple light irradiation. Therefore, by controlling the protonated and light irradiated conditions, the actuator could be switched among three stationary states.

Fig. 13 Photoresponsive materials with a multi-stable light-controlled morphological property. (a) Photoresponsive microactuators switching between four independent stationary states based on the photoisomerization of A3MA and DR1A. Reproduced with permission.²³⁸ Copyright 2009, Springer Nature. (b) Liquid crystal elastomers with programmable actuation by controlling the light irradiation and protonation. Reproduced with permission.²³⁹ Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.2 Multi-stage control: switching between nonequilibrium states

The multi-stage control of photoresponsive materials is stated as “switching between multiple nonequilibrium states”. Between two photostationary states, the isomerization process of photoresponsive molecules follows the first-order kinetics, where the rate and conversion efficiency are closely interrelated with irradiated conditions. Therefore, the isomerization equilibrium is expected to be stabilized at any stage among the photostationary states, and the photoresponsive materials could be switched between multiple nonequilibrium states by well-controlling the irradiated conditions (i.e. light intensity and irradiation time). As a typical example, the brightness of a table lamp could be tuned among multiple states by controlling the input current (Fig. 14).

Fig. 14 Schematic diagram of multi-stage control of photoresponsive materials by P-type and T-type photoresponsive molecules.

After removal of light irradiation, photoresponsive molecules exhibit different thermal-relaxation performance from the high-molecular-energy photostationary state to the low-molecular-energy pristine state, which could be classified into P-type and T-type: (1) P-type photoresponsive molecules such as DAE exhibit a bistable characteristic, with the photoisomerization to both directions triggered by light irradiation. The photoresponsive molecules are able to maintain the nonequilibrium state after removing the light source, which is advantageous for achieving multi-stage control; (2) on the other hand, T-type photoresponsive molecules (e.g. Azo, SP/SO, DASA) exhibit a spontaneously occurring thermal relaxation from the higher-energy photostationary state to the lower-energy state. Therefore, it is challenging to achieve multi-stage control with the T-type photoresponsive molecules, because the nonequilibrium states are difficult to stabilize.

(1) Multi-stage control based on P-type photoresponsive molecules. DAE as a typical P-type photoresponsive molecule with picosecond response time, excellent fatigue resistance and high thermal stability, has been extensively applied in the fields of optical storage, optoelectronic devices and sensing/imaging.³⁸ In addition, the excellent properties of DAE can be expressed not only in solution, but also in the solid state.^51,240 The multi-stage control of the optical property for the photoresponsive materials has been attractive in information secrecy. Li and coworkers reported multi-stage control of optical data storage based on a NIR fluorescent bistable molecular switch (TDI-4(DTE-TPE)) constructed by covalently coupling aggregation-induced emissive tetraphenylethylene (TPE), DAE and NIR emissive terrylene diimide (TDI) into a single molecule (Fig. 15a).²⁴¹ The TDI-4(DTE-TPE) molecules were doped into wafer to fabricate an optical storage medium film. The erasing, writing, and reading process is dominated by 405 nm, 621 nm and 680 nm light irradiation, respectively. By well-controlling the exposure time under 621 nm light, a prototype of 32-level optical information storage is realized, while each storage unit can store 5 bits of data, significantly increasing the storage density.

Fig. 15 Materials with multi-stage light-controlled optical and electrical properties based on DAE molecules. (a) Photoisomerization process of TDI-4(DTE-TPE) and the mechanism of multi-stage optical storage by controlling the irradiation time of 621 nm red light, the erasing and reading process are proceeded by 405 nm and 680 nm light irradiation. Reproduced with permission.²⁴¹ Copyright 2023, Royal Society of Chemistry. (b) Schematic illustration of the photoisomerization of DAE-Me and the non-volatile optoelectronic devices with multi-stage light-controlled electrical properties. Reproduced with permission.²⁴² Copyright 2016, Springer Nature.

The open–closed photoisomerization of DAE switches the molecular conjugation and electron distribution, which is applicable in the field of optoelectronic devices. Samori and coworkers have developed a series of non-volatile optoelectronic devices based on DAE molecules capable of multi-stage light-controlled electrical behavior. For example, a multi-level non-volatile flexible photoelectric storage thin-film transistor is achieved based on an organic binary mixture of poly(3-hexylthiophene) (P3HT) and DAE derivatives (DAE-Me) as the active layer, which is switched on and off by irradiation with UV and green light (Fig. 15b).²⁴² The two photostationary states of DAE possess different electronic energy levels. By well-controlling the light irradiation conditions (i.e. intensity and time), the degree of isomerization of DAE-Me in the device could be precisely controlled, achieving multi-stage control of current levels. By controlling the number of pulses, these current levels could be gradually adjusted, achieving a multi-stage switching between 256 (8-bit storage) current levels. Importantly, the current levels show almost no change after being stored in the dark for 500 days, indicating the excellent non-volatility.

Unlike traditional P-type photoresponsive molecules, which exhibit a slight change in molecular form during the photoisomerization process (i.e. extending of chemical bonds, torsion of planar structure), hydrazone shows the trans–cis isomerization with dramatic variation of molecular geometry under controlling of UV light (λ ∼340 nm) and visible light irradiation (λ ∼450 nm) (Fig. 16).²⁴³ Besides, hydrazone exhibits a transition performance between P-type and T-type photoresponsive molecules with tunable and super-long thermo-relaxation half-life (τ_1/2) ranges from a few days to several years, depending on the substituted functional groups and chemical environment.^244,245

Fig. 16 Materials with multi-stage light-controlled optical, chemical and morphological properties based on hydrazone. (a) Liquid crystal elastomer film with multi-stage control of photoactuation by developing hydrazone containing polymers, complex photoactuation has the potential to be achieved by well-controlling the sequence, intensity and time of UV and blue light irradiation. Reproduced with permission.²⁴⁹ Copyright 2024, American Chemical Society. (b) Multi-stage control of DOX delivery from polymeric micelles self-assembled from an amphiphilic methoxy poly(ethylene glycol)-b-poly(ethylenediamine-L-glutamate) block copolymer incorporated with hydrazone. Reproduced with permission.²⁵¹ Copyright 2020, Royal Society of Chemistry. (c) Photoisomerization between the trans and cis forms of hydrazone containing chiral dopants – the liquid crystal film exhibits gradual transition from red to blue after irradiating with 442 nm blue light irradiation for different times. Reproduced with permission.²⁵² Copyright 2019, American Chemical Society.

Ivan and coworkers reported liquid crystal materials with multi-stage photochromism by forming a helical twisting structure with hydrazone containing chiral dopants. The reflectance of the liquid crystals is gradually altered by well-controlling the trans/cis isomer ratio of the hydrazone dopants, resulting in photochromism from red to blue under different times of 442 nm blue light irradiation. Particularly, as the irradiation time increased, the reflection wavelength gradually red-shifted from 450 nm to 1600 nm (Fig. 16a). This multi-stage control enables multiple color on the same liquid crystal film, demonstrating its potential applications in the fields of displaying,^243,246,247 smart windows,²⁴⁸ and optical sensors.^249,250

Chen and coworkers reported multi-stage drug delivery control using polymeric micelles, self-assembled from an amphiphilic methoxy poly(ethylene glycol)-b-poly(ethylenediamine-L-glutamate) block copolymer incorporated with hydrazone (Fig. 16b).²⁵¹ When the hydrazone is in the cis state, the amphiphilic di-block copolymers self-assemble into a core–shell nanostructure where the hydrophilic drug molecules doxorubicin hydrochloride (DOX) are encapsulated into the core of micelles. Blue light irradiation with the wavelength of 450 nm triggers the cis-to-trans isomerization of hydrazone, which gradually induces decomposition of the micelles and releases DOX. The micelles are irradiated by blue light for 5 min and then kept in the dark for 55 min, and a burst of DOX release is achieved right after blue light is released in the first three irradiation cycles, which is immediately stopped when the light source is removed, resulting in a quantitatively multi-stage control.

Katsonis and coworkers reported a liquid crystal elastomer film exhibiting multi-stage control of photoactuation by developing hydrazone containing polymers (Fig. 16c).²⁵² Blue light irradiation with the wavelength of 405 nm increases the intermolecular aggregation among hydrazone units, which induces shrinking of the polymers and results in a bending towards the direction of light irradiation. On the other hand, UV light irradiation generates a bending opposite to the direction of light irradiation. The bending angle of the liquid crystal elastomer film is closely interrelated with the irradiation time and could be maintained at any nonequilibrium states. By well-controlling the sequence, intensity and time of UV and blue light irradiation, complex photoactuation has potential to be achieved on the liquid crystal elastomer film.

(2) Multi-stage control based on T-type photoresponsive molecules. Compared with P-type photoresponsive molecules with a bistable characteristic, the multi-stage control of T-type photoresponsive molecules meets greater challenges, especially in solid-state photoresponsive materials: (1) thermal relaxation rapidly destabilizes the nonequilibrium states once the irradiation source is removed; (2) the isomerization process of T-type photoresponsive molecules often aligns with significant variation in molecular geometry, which is hindered in the solid-state materials. Therefore, to achieve multi-stage control of T-type photoresponsive molecules, it is important to build a reliable physicochemical environment to improve the isomerization in the solid-state.
(a) Multi-stage control of optical properties of photoresponsive materials. Wang and coworkers demonstrated that the photoisomerization of DASA in the solid-state could be significantly promoted by ester containing molecules.⁵⁴ In the follow-up study, the same group reported multi-stage light-control of information encryption by precisely controlling the isomerization dynamics of DASA molecules on a paper surface (Fig. 17a).²⁵³ The DASA molecules exhibit tunable isomerization dynamics by varying the concentration of ester molecules. The invisible inks containing ester molecules were printed onto the paper surface pretreated by DASA solution, under white light irradiation, and the DASA molecules in different regions exhibited distinct isomerization kinetics, resulting in a grayscale image with up to 8-bit resolution. In addition to conventional 1-bit information (e.g. text, symbols, binary images), this approach enables encryption of more realistic information, which improves the security level in secrecy and anti-counterfeiting. Cui and coworkers reported encapsulating SP molecules into the nanopores of cadmium-based MOFs (ZJU-128⊃SP), which dramatically improves the isomerization of SP in the solid-state (Fig. 17b).²⁵⁴ By precisely controlling the exposure time of UV light irradiation, SP in the nanopores gradually isomerizes from the spirocyclic state to merocyanine, which switches the blue fluorescence of ZJU-128⊃SP into red. By irradiating a quick response code (QR code) recorded by ZJU-128⊃SP with UV light and controlling the exposure time at different regions, a 3D color code for dynamic information encryption is realized.

Fig. 17 Materials with multi-stage light-controlled optical properties based on T-type photoresponsive molecules. (a) Multi-stage light-control of information encryption by printing ester containing invisible inks on the paper surface pretreated by DASA solution; grayscale image up to 8 bits could be recorded. Reproduced with permission.²⁵³ Copyright 2023, Royal Society of Chemistry. (b) Multi-stage light-control of fluorescence gradually switching from blue to red by encapsulating SP molecules into the nanopores of MOFs, and 3D color codes for dynamic information encryption is realized. Reproduced with permission.²⁵⁴ Copyright 2023, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

(b) Multi-stage control of electrical properties of photoresponsive materials. The dramatic variation of conjugated structure and electron distribution of T-type photoresponsive molecules during the isomerization process makes them attractive in fabricating optoelectronic devices with multi-stage control of electrical properties. Samori and coworkers reported photoresponsive hybrid van der Waals heterostructures by integrating Azo molecules with 2D semiconductors such as MoS₂ or WeS₂ (Fig. 18a).²⁵⁵ The isomerization dynamics of Azo on the 2D semiconductors surface is tuned by controlling the density of immobilization, which is closely affecting the intermolecular aggregation. Under 365 nm UV light irradiation, trans Azo on the device surface gradually switches to the cis state, and the transition yield could be maintained for over 15 h at any equilibrium state after removing the light source. Therefore, multi-stage control of device conductivity is achieved by precisely controlling the exposure time of UV light irradiation. The erase process is manipulated by visible light irradiation, which induces the cis-to-trans isomerization of Azo on the surface. The programming-erasing cycles are reversible by sequentially treating the devices with UV and visible light irradiation. Wang and coworkers reported multi-stage optical modulation of 2D logic-in-memory devices by assembling DASA molecules on the surface of graphene (Fig. 18b).²⁵⁶ By varying the carbon spacer lengths (n = 1, 5, 11, and 17) on the electron-donating and -withdrawing moieties of DASA, the assembled structure of DASA on the surface of graphene could be controlled. A short carbon spacer generates a self-assembled film of DASA with strong intermolecular interactions, which limits the visible-light-induced linear-to-cyclic isomerization. In contrast, an excessively long spacer (n = 17) induces crystallization of alkyl chains, which sharply increases the rigidity of the film and hinders the photoisomerization of DASA. By precisely controlling the 520 nm visible light intensity and irradiation time, the devices exhibit a stepwise increase in electric conductivity, indicating a logic-in-memory characteristic. The multi-stage switching between multiple nonequilibrium states sharply increases the storage capacity of the devices.

Fig. 18 Materials with multi-stage light-controlled electrical properties based on T-type photoresponsive molecules. (a) Multi-stage control of photoswitching between multiple nonequilibrium memory states by immobilizing Azo molecules onto the surface of 2D semiconductors such as MoS₂ or WeS₂, and a UV light pulse is used for programming and visible light is used for erasing. Reproduced with permission.²⁵⁵ Copyright 2021, American Chemical Society. (b) Multi-stage optical modulation of 2D logic-in-memory devices by well-controlling the isomerization of DASA on the surface, 520 nm visible light is used for writing and heat is used for erasing. Reproduced with permission.²⁵⁶ Copyright 2023, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

(c) Multi-stage control of chemical properties of photoresponsive materials. The multi-stage control of chemical properties of T-type photoresponsive molecules has been attractive in precisely controlling the reaction and delivery of nanoreactors and drug carriers. Specifically, precise control over the timing, location, and dosage of drug release ensures high therapeutic efficacy and reduced toxicity or side effects. Xu and coworkers reported multi-stage control of membrane permeability by integrating Azo derivatives (GlyAzoCns) into phospholipid-based self-assembled liposomes (Fig. 19a).²⁵⁷ UV light irradiation switches the Azo from a hydrophobic trans state to a hydrophilic cis state, which opens the nanovalve in the lipid layer and increases the permeability for water-soluble cargo molecules, such as doxorubicin hydrochloride (DOX). Precisely controlling the exposure time of UV light irradiation achieves multi-stage release of DOX, and the released dosage is closely interrelated with the intensity and time of light irradiation. The liposomes exhibited distinct stepwise release behavior under intermittent UV exposure; notably, after 13 irradiation cycles (totalling 9.75 min), the cumulative DOX release was comparable to that observed after continuous 10 min exposure. These indicate the potential of precise and quantitative multi-stage control of drug release by controlling the light irradiation. Boyd and coworkers reported on-demand release of hydrophilic drug molecules (5-deoxy-5-fluorouridine (5-dFu)) by precisely controlling the isomerization process between spirocyclic and merocyanine of SP in the self-assembled bilayer of amphiphilic poly(ethylene oxide)-b-PSPA (PEO-b-PSPA) diblock copolymers (Fig. 19b).²⁵⁸ Upon UV exposure (λ < 420 nm), the SP molecules are converted to the hydrophilic zwitterionic merocyanine form, enhancing bilayer polarity and permeability, which induces rapid release of encapsulated 5-dFu. Visible light (λ₂ > 450 nm) triggers the merocyanine-to-spirocyclic isomerization and results in a quick close of the drug release. On-demand release of 5-dFu is achieved by alternating UV (2 min) and visible light (15 min) irradiation, thus offering the possibility of precise and multi-stage control of drug release.

Fig. 19 Materials with multi-stage light-controlled chemical properties based on T-type photoresponsive molecules. (a) Multi-stage control of DOX release from the liposomes formed by phospholipid integrated with GlyAzoCns. The released dosage could be precisely controlled by the intensity and time of UV light irradiation. Reproduced with permission.²⁵⁷ Copyright 2016, American Chemical Society. (b) On-demand controlled 5-dFu release from the self-assembled vesicles of PEO-b-PSPA diblock copolymers, where the on-demand release of 5-dFu is achieved by alternating UV and visible light irradiation. Reproduced with permission.²⁵⁸ Copyright 2015, American Chemical Society.

(d) Multi-stage control of morphological properties of photoresponsive materials. Due to the obvious changes in molecular geometry and structure during the isomerization process, precise control of the isomerization dynamics of T-type photoresponsive molecules realizes multi-stage control of the morphological property of photoresponsive materials, which is attractive in fabricating microrobots with complex light dominated actuations. White and coworkers reported liquid crystal elastomer materials with precise multi-stage control of actuations by introducing o-fluorinated Azo (oF-Azo) into polymers via radical-mediated thiol-acrylate chain transfer reaction.²⁵⁹ Under 365 nm UV light irradiation, the trans-to-cis isomerization of oF-Azo induces bending of the liquid crystal elastomer, where the bending angle is closely interrelated to the exposure time. Notably, the liquid crystal elastomer exhibits persistent deformation of the morphology for over 72 hours after removal of the UV light irradiation, which is attributed to the long-term thermal stability of the cis isomer of oF-Azo (with a half-life of over 700 days). The liquid crystal elastomer film is selected to fabricate a flat pattern of a box (Fig. 20). UV light irradiation for different amounts of time gradually folds the film into a box in 5 min, which could be maintained for up to 24 hours in the dark. Importantly, the morphology of the box could be fixed at any nonequilibrium state between closed and open, indicating the multi-stage control of the morphologic properties. The closed box reopens upon 2 minutes of 405 nm purple light irradiation, returning to its original flat configuration.

Fig. 20 Materials with multi-stage light-controlled morphological properties based on T-type photoresponsive molecules. Schematic illustration of a liquid crystal elastomer film consisting of oF-Azo exhibiting multi-stage control of bending, and the film gradually transforms into a box under control of 365 nm UV light irradiation. Reproduced with permission.²⁵⁹ Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

4. Potential applications of multi-mode photoresponsive materials

Compared to photoresponsive materials based on the switch-type control logic, which are switching between two functional states with contrary relationship, with the multi-mode control logic, the optical, electrical, chemical, mechanical and morphological properties of photoresponsive materials could be precisely and controllably switched among multiple stationary (multi-stationary control) or nonequilibrium (multi-stage control) states. These make the photoresponsive materials sensitive in the environment with complex stimuli and exhibit the specific function accordingly. The potential applications of multi-mode photoresponsive materials are discussed as follows (Table 2 and Fig. 21).

Table 2 Summary of the potential applications of photoresponsive materials with multi-mode control logic

Light-switched property	Applications
	Multi-stable control	Multi-stage control
Optical property	Programmable information encryption,^233,260 optically tuned metasurface,^21 active camouflage,^234 multi-colored smart windows, displaying technology, self-adaptive photochromism^234	Encryption for grayscale information,^253 time-resolved secrecy and anticounterfeiting,^254,261 smart windows with precisely tunable transmittance, optical filters with dynamical transmittance
Electrical property	Native decimal arithmetic on chips, single-molecular transistors with complex functionalities, full-colored optical sensors	Logic-in-memory devices,^28,255,256 synapse-like devices, nanodevices with precisely tunable conductivity, optical sensors with tunable sensitivity
Chemical property	Nanoreactors with selective chemical reactivity,^236 programmable self-assembly into multiple nanostructures,^20,235,237 selective catalysis,^229 drug delivery for compound medicine	On-demand drug delivery,^257,258,262 membranes with controlled permeability, gradually switched self-assembly, controlling droplet sliding on surface
Mechanical property	Programmable tissue engineering, multi-step nanoimprinting technique	Artificial muscles with tunable tensile force, on-demand delivery of biomacromolecules
Morphological property	Complex motions in one photoactuator, light-controlled robot	Time-programmable photoactuator,^252,259 photoactuator with precisely controlled motions (e.g. bending and twisting angles)

---

Fig. 21 Potential applications of photoresponsive materials with multi-mode control logic of optical, electrical, chemical, mechanical and morphological properties.

4.1 Multi-mode control of optical properties

The photoresponsive materials with controllably switched optical properties (e.g. color, luminescence, transmittance, reflectance) among multiple states offer potential applications in the fields of secrecy and anticounterfeiting, photodetector, displaying, information storage and smart windows. Taking secrecy and anticounterfeiting as the example, as a traditional design, while the photochromic materials used as invisible inks switch between two colors, the information is controlled between a “fake” and a “truth”. On the other hand, increasing the number of switchable colors develops a more complex secrecy system, which brings more “fake” states, those switched by controlling light irradiation conditions and lowers the risk of information leakage. When applied in smart windows, the multi-mode switching of color and transmittance enables the photoresponsive materials adaptive to the surrounding environment of light irradiation. The performance of smart windows could be accordingly changed, resulting in a dynamically tunable light condition indoor.

4.2 Multi-mode control of electrical properties

The light-switchable conductivity between two distinct states (i.e. conducting and nonconducting) makes photoresponsive materials attractive in logical calculus under the control of light irradiation. As a traditional strategy, the conducting state is represented as “1” and the nonconducting state as “0”. The information could be written and read by light irradiation on microchips, which achieves a binary arithmetic. While the conductivity of photoresponsive materials is developed to multi-mode control logic, more complicated operation, such as native decimal arithmetic, is expected to be achieved on the microchips. Moreover, devices with unique functionalities, such as synaptic and logic-in-memory devices could be realized.

4.3 Multi-mode control of chemical properties

The chemistry of photoresponsive molecules is closely interrelated to their polarity, solubility and reactivity, which is attractive in fabricating photoresponsive materials and devices with switchable hydrophobicity/hydrophilicity, drug-delivery systems, photoresist and nanoreactors. Photoresponsive drug-delivery systems are important in biomedicine, for their advantage that the loaded drug molecules could be precisely and controllably released at the lesion location. More importantly, the release process is controlled by a clean and contactless stimulus. Encapsulating drug molecules into polymeric self-assemblies is a common strategy to fabricate drug-delivery systems, while the chemical property of polymers is switched by light, the changed self-assembled structure induces drug release. Limited by the switch-type control logic of photoresponsive materials, most drug-delivery systems exhibit controlled release for single drug molecules. By developing the multi-mode control logic of the chemical property of the photoresponsive materials, it is expected to achieve the controlled release of compound medicine.

4.4 Multi-mode control of mechanical properties

With the light-switchable mechanical properties (e.g. rigidity and viscoelasticity), photoresponsive materials are attractive in controlling cell growth and migration in tissue engineering, delivery of biomacromolecules from hydrogels and nanoimprinting on surfaces. For example, reversible nanoimprinting has been achieved by switching the rigidity of Azo containing photoresponsive polymers. Nanopatterns could be fabricated on the surface of polymer films by the nanoimprinting technique after switching the polymers into a soft state by UV light irradiation. The nanostructure is stabilized after visible light irradiation by increasing the rigidity of polymers. The nanopatterns could be erased by UV light irradiation and the polymer films are able to be reused for nanoimprinting. For the traditional strategy, while the rigidity of the photoresponsive polymers is switched between two stationary states, a simple nanostructure could be formed on the surface. Improving the multi-mode control of the mechanical property has potential to fabricate nanopatterns with complex structures by a multi-step nanoimprinting technique, and the details of the nanostructure could be well-controlled by varying the force of nanoimprinting.

4.5 Multi-mode control of morphological properties

Photoresponsive materials with light-switchable morphological properties have been attracting broad interest in the fabrication of photoactuators, artificial muscle and microrobots. Taking photoactuators as an example, limited by the switch-type control logic of traditional photoresponsive materials, the motions including extension, rotation, bending and twisting of the actuators could be switched between two stationary states. Aiming to achieve light-controlled complex actuation of robots, the development of multi-mode control of the morphologic properties of of photoresponsive materials is important. For the multi-stable control, different types of motions could be integrated into a single actuator, which could be selectively triggered by the wavelength of light. For the multi-stage control, the key parameters such as bending angle and twisting direction could be precisely controlled by the intensity and time of light irradiation.

5. Future challenges

5.1 Multi-stable and multi-stage control of solid-state photoresponsive materials

Photoresponsive molecules exhibit fast and efficient isomerization in dilute solutions because the long distance between molecules ensures enough free space necessary for the isomerization process. Therefore, multi-stable and multi-stage control of photoresponsive molecules is accessible in diluted solutions and soft materials. For real-world applications, solid-state materials are important because of their stable mechanical property. However, the intermolecular aggregation is dramatically increased in the solid-state materials, which hinders the proceeding of the isomerization process. Therefore, it faces great challenges to achieve multi-stable and multi-stage control of solid-state photoresponsive materials.

5.2 Precisely controlling the photoresponsive materials in complex environments

Different to the light irradiation conditions in the lab, the stimuli are complex in real-world applications, including light, heat, pH, water and chemicals. Because of the conjugated structure and push–pull nature, the photoresponsive molecules are easily disturbed by the other stimuli. For example, acidity, metal cations and nucleophilic anions have been reported to induce spirocyclic-to-merocyanine isomerization of SP, and water molecules coordination has been reported to induce linear-to-cyclic isomerization of DASA. Therefore, it remains a challenge to achieve precise multi-mode control of photoresponsive materials in the environment with complex stimuli.

5.3 Further improving the light-controlled rate and accuracy

For the multi-stage control of photoresponsive materials, the isomerization equilibrium is expected to be stabilized at any stage between the photostationary states. The isomerization process of the photoresponsive molecules follows the first-order kinetics, where the rate and conversion efficiency are closely interrelated to the irradiation conditions (i.e. wavelength and intensity). These indicate under the same irradiation conditions the first half of photoisomerization is fast, which is getting slower with the proceeding of isomerization. Therefore, achieving a balance between improving the light-controlled rate and accuracy for multi-stage control of photoresponsive materials remains a substantial challenge. While the photoisomerization is too fast, it is difficult to precisely stabilize the isomerization process at any stage; while the photoisomerization is too slow, good control of the isomerization process is possible, but the controlling efficiency decreases.

5.4 Improving stability and fatigue-resistance under long-time exposure

In addition to advancing precise switching and multi-mode responsiveness, addressing long-term stability under light exposure is essential for the practical applications of photoresponsive materials, especially for commercial concerns. On the one hand, it is necessary to maintain fast and efficient isomerization of photoresponsive molecules under light control; while on the other hand, the stability of the chemical structure should be maintained under light irradiation. This is a huge challenge. Besides, the fatigue-resistance and chemical robustness are important for application under continuously changing light environments. This challenge has the potential to be addressed by either improving the stability of photoresponsive molecules via chemical strategies, or endowing photoresponsive molecules with regenerative capability, where the chemical structure could be reformed after decomposition.

5.5 Developing a responsive type of “adapt the environment” for the photoresponsive materials

For most photoresponsive materials, including the ones with switch-type and multi-mode control logic, the responsive type is “active control”. This means the switching of the photoresponsive materials is based on the subjective wishes of human beings, which is realized by selectively controlling the light irradiation conditions. Based on combining the multi-stable and multi-stage control logic, the photoresponsive materials might be brought to life, indicating a responsive type of “adapt the environment”. Our group previously reported self-adaptive photochromism, which is when the color of photoresponsive materials is adaptively switched and always remains the same with the environment. Adaptivity of photoresponsive materials requires developments for both the multi-stable and multi-stage control logic. For example, multi-stable control logic enables the photoresponsive materials switched among multiple stationary-colored states; multi-stage control logic ensures the precise brightness and transition of the color. Besides the color, we envision the photoresponsive materials are able to switch to adapt to the light irradiation environment, which is smarter than the conventional photoresponsive materials based on “active control”. This tutorial review as well as the developments of self-adaptive photoresponsive materials raises an open scientific question on the future of smart materials.

Author contributions

Gaolu Zhu: writing – original draft, writing – reviewing & editing, investigation, visualization; Fanxi Sun: visualization, writing – reviewing & editing, investigation; Yinghao Ji: investigation; Hongtao Hu: investigation; Mengyao Yang: investigation; Yichen Zhang: investigation; Xu Deng: writing – reviewing & editing; Yonghao Zheng: writing – reviewing & editing, supervision; Chen Wei: writing – reviewing & editing, investigation. Dongsheng Wang: conceptualization, writing – reviewing and editing, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (52203134, 62375041 and 22375029) and the Foundation of Science & Technology Department of Sichuan Province (2023ZYD0037, 2024YFHZ0307 and 2024NSFSC0249).

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