Stretchable organic room temperature phosphorescence systems

Abstract

Organic room temperature phosphorescent (RTP) materials, with their excellent properties such as high exciton utilization efficiency, long lifetime, and low toxicity, show promising application prospects in many key fields. Organic RTP systems can be divided into stretchable and inelastic materials. Stretchable organic RTP materials exhibit more advantages in dynamic information encryption, human–computer interface, and biomedical sensing applications owing to their stretchability, fatigue resistance, and self-healing ability. They could solve the problems of rigid materials, which struggle to match dynamic environments and meet practical requirements. Herein, we review the recent research progress on stretchable organic RTP systems according to their construction strategy, including physical doping of phosphorescent chromophores into stretchable polymers and polymerization of phosphorescent chromophores with polymer monomers. Finally, the prospects and scientific challenges in this promising field are critically analyzed to provide helpful guidance for further development of advanced stretchable RTP systems.

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Wanjuan Gao

Wanjuan Gao is currently a postgraduate student at Changchun University of Science and Technology, studying in Changchun, Jilin Province. Before this, she obtained her bachelor degree from Jilin Institute of Chemical Technology in 2023. Now she is mainly engaged in the preparation and characterization of flexible organic room temperature phosphorescence materials.

Dan Li

Dan Li is currently an associate professor of Changchun University of Science and Technology. She graduated from Northeast Normal University with the master degree in chemistry in the group of Prof. Zhongmin Su under the supervision of Prof. Dongxia Zhu in 2020. She received her PhD degree in chemistry from Tianjin University in 2023, under the supervision of Prof. Zhen Li and Prof. Ben-zhong Tang. Her research interests focus on multi-component organic room temperature phosphorescence materials.

Hua Feng

Hua Feng is currently a doctoral student (Class of 2022) at Changchun University of Science and Technology, studying in Changchun, Jilin Province. She received her master degree in 2022 from Liaoning University. Her research focuses on the preparation and characterization of organic room-temperature phosphorescent materials via host–guest doping strategy.

Zhongmin Su

Prof. Zhongmin Su received his BS (1983) and then his PhD in inorganic chemistry under the supervision of Prof. Rong-Shun Wang and Prof. Chi-Ming Che from Northeast Normal University (1997). He has been a full professor at Northeast Normal University since 1994. Later, he worked as a visiting scholar in the group of Prof. Chi-Ming Che and Prof. Guan-Hua Chen (The University of Hong Kong), in Prof. Koji Ohta's group (National Industrial Technology Research Institute, Japan), and in Prof. N. Roesch's group (Technical University of Munich, Germany). His research interests focus on functional material chemistry and quantum chemistry.

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1. Introduction

In recent years, organic room temperature phosphorescence (RTP, all the abbreviations in this review are listed in Table 1) materials have shown important potential in biological imaging, information encryption, flexible displays, sensors and other fields owing to their longer luminescence lifetimes, large Stokes shifts, and high signal-to-noise ratios.^1–10 Two primary approaches to realize highly efficient RTP performance have been proposed, namely, achieving an effective intersystem crossing (ISC) rate to generate triplet excitons and the use of rigid environments to suppress non-radiative transitions.^11–20 In terms of suppressing non-radiative transitions and molecular motion, traditional RTP materials have mostly relied on crystal engineering and rigid polymer matrices (such as polyvinyl alcohol and polymethyl methacrylate) to achieve efficient phosphorescence emission. However, the brittleness of crystalline materials and the rigidity of these amorphous materials limit their machinability and practical application scenarios, especially in fields requiring mechanical flexibility (such as wearable devices).^21–29

Table 1 List of abbreviations used in the paper

Abbreviation	Full form	Abbreviation	Full form
RTP	Room temperature phosphorescence	PRET	Phosphorescence resonance energy transfer
ISC	Intersystem crossing	FM	Figure-of-merit
Flou.	Fluorescence	BA	Boric acid
IC	Internal conversion	PDMS-OH	Hydroxyl silicone oil
Non-rad.	Non-radiative transition	TPB-3NH2	1,3,5-Tris-(4-aminophenyl)benzene
Phos.	Phosphorescence	RTPPs	Room temperature phosphorescent putties
Φ ISC	Yield of intersystem crossing	ΔEST	Singlet–triplet energy gaps
τ P	Phosphorescence lifetime	k ISC	Intersystem crossing rate
Φ ET	Energy transfer efficiency	PBS	Polyborosiloxane
QY	Quantum yield	QCl	Chloro-substituted amphoteric quinoline
PhQY	Phosphorescence quantum yield	AA	Acrylic acid
PVA	Polyvinyl alcohol	PMACa	Poly(calcium maleate)
PAA	Poly(acrylic acid)	NCCS	Nonconventional chromophores
PU	Polyurethane	NTLs	Non-traditional luminogens
PDMS	Polydimethylsiloxane	PR	Polyrotaxane
IL	Ionic liquid	CD	Cyclodextrins
CTE	Clustering-triggered emission	WPU	Waterborne polyurethane
PVB	Polyvinyl butyral resin	NPR	Pseudopolyrotaxane
TP	Triphenylene	NPEG	Naphthalene modified polyethylene glycol
TpB	Triphenylen-2-ylboronic acid	CNDs	Carbon dots
TpBe	Tetramethyl-2-(triphenylen-2-yl)-1,3,2-dioxaborolane	EY	Eosin Y
W-hydrogel	Wood hydrogel	THDI	Hexamethylene diisocyanate trimer
RhB	Rhodamine B	BTD-HEA	Benzothiadiazole-based dialkene
TS-FRET	Triplet to singlet Förster resonance energy transfer	DMSO	Dimethyl sulfoxide
LPL	Long-persistent luminescence	RDRP	Reversible deactivation radical polymerization
TAED	Tetraacetylethylenediamine	GCP	Gradient copolymer
P(n-BA)	Poly(n-butyl acrylate)	ATRP	Atom-transfer radical polymerization

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To break through the limitations of rigid materials, flexible RTP materials have gradually become a research hotspot, and some elastic organic RTP crystals have been reported.^30–33 Unfortunately, non-stretchable organic RTP crystals still cannot meet practical demands. Therefore, a range of stretchable organic RTP materials has been fully developed, which combine stretchability and RTP emission to exhibit unique advantages in flexible display, dynamic anti-counterfeiting and encryption, and wearable device applications due to their reversible deformation ability, resistance to mechanical stress and dynamic optical response.^34–45

This review systematically summarizes the recent progress in stretched organic RTP systems and discusses these systems by focusing on their design strategies. In particular, the materials are mainly classified according to their construction strategies, which include physically doping phosphorescent chromophores into stretchable polymers and the polymerization of phosphorescent chromophores with polymer monomers (Fig. 1(A)). In addition, future directions and opportunities in the field of stretchable organic RTP systems are discussed. It is hoped that this review will promote the development of organic stretched RTP systems in the future.

Fig. 1 Schematic diagram of the construction strategies for stretchable RTP systems.

2. Construction strategies for stretchable RTP systems

2.1 Physically doping phosphorescent chromophores with stretchable polymers

At present, large-area organic RTP materials with good flexibility can be prepared by doping phosphorescent chromophores into a polymer matrix. However, the above materials are prone to breakage or fracture under stress, leading to the RTP intensity and lifetime being significantly reduced or even quenched. This limits their application in the field of flexible optoelectronics. Therefore, it is very important to develop stretchable organic RTP materials with good mechanical properties and stable optical properties. In the preparation of stretchable organic RTP materials, phosphorescent chromophores are usually doped into the stretchable polymer matrix through molecular interactions such as hydrogen bonds, ionic bonds and host–guest inclusion. The stretchable polymer matrix not only limits non-radiative transitions but also provides stretchable properties. Polyvinyl alcohol (PVA), poly(acrylic acid) (PAA), polyurethane (PU) and polydimethylsiloxane (PMDS) are typically selected as stretchable polymer matrices. PVA and PAA, which are rich in hydroxyl groups and carboxyl groups, are generally prepared into RTP gels by forming hydrogen bond networks that demonstrate excellent stretchability. For PU, the degree of microphase separation between the soft segments and hard segments determines its stretchability. Regulation of the stretchability could be achieved by controlling factors such as the type of soft segments and the aggregation degree of hard segments. Moreover, in PDMS, the main chain of is composed of Si–O bonds and the side chains are methyl groups. This unique molecular structure endows it with excellent chemical stability and flexibility. This good elasticity and significant stretchability enable it to withstand a certain degree of deformation without breaking.

2.1.1 Hydrogen bonds. Doping phosphorescent chromophores into a stretchable polymer matrix through hydrogen bond interactions is an important strategy to construct high-performance stretchable RTP materials. As a dynamic and reversible non-covalent interaction, hydrogen bonds can effectively fix the phosphorescent molecules and inhibit non-radiative transition and molecular motion, thus significantly improving the quantum yields and lifetime of the phosphorescence.^46–52 Simultaneously, the flexibility of the hydrogen bond network can endow the material with excellent stretchable deformation ability and realize cooperative optimization of optical properties and mechanical flexibility.^53–56

Polyvinyl alcohol (PVA), as a flexible polymer matrix rich in hydroxyl groups, has become one of the most widely used matrices in RTP host–guest doping systems due to its excellent film formation, transparency and controllable hydrogen bond network. The large number of hydroxyl groups in the molecular chain can interact with the phosphorescent chromophores through hydrogen bonds to form a stable system, which could not only suppress molecular vibration to reduce non-radiative transitions but also extend the lifetime due to the rigid microenvironment of PVA.^57–62 In addition, the high transparency and solution processability of the PVA matrix make it easy to prepare large-area flexible films or fibers, which are suitable for information encryption, flexible displays and other fields.

Zhang and co-workers reported PVA-based ionogels (PVA/PAMNaIL) with high stretchability (≈400%), toughness (≈20 MJ m⁻³), electrical conductivity (8.4 ms cm⁻¹), and an ultra-long afterglow lifetime (112.4 ms) (Fig. 2(A)).⁶³ They enhanced the toughness and extended the RTP of the ionogels via salting-out-induced microphase separation, which resulted in the formation of an ionic liquid (IL)-rich phase (soft) for stretching and ionic conduction and a polymer-rich phase (stiff) for energy dissipation and clustering-triggered phosphorescence. The salted-out ions could fully interact with the hydroxyl groups of the polymer, driving the IL beyond its gel-phase boundary. This led to the aggregation of molecular chains and subsequently resulted in the formation of a microphase-separated morphology within the ionogel. In the IL-rich phase, the IL screens ionic bonds and forms a few hydrogen bonds, building highly stretchable networks and providing pathways for effective charge conduction. Conversely, the polymer-rich phase with multiple hydrogen bonds is no longer a gel but is stiff, which limits non-radiative deactivation and enhances RTP performance. Moreover, a polymer-rich phase with abundant unsaturated bonds may exhibit strong non-traditional phosphorescence in the aggregated state, i.e., clustering-triggered emission (CTE).

Fig. 2 (A) Schematics of stretchable RTP PVA-based ionogels.⁶³ Copyright 2024 Wiley-VCH GmbH. (B) PVB-based afterglow systems.⁶⁴ Copyright 2024 American Association for the Advancement of Science. (C) Schematic illustration of the preparation of room temperature phosphorescent W-hydrogel from natural wood.⁶⁵ Copyright 2024 Springer Nature. (D) Molecular structure of the polymeric energy donor in polyurethane.⁶⁶ Copyright 2025 ACS Publications. (E) Emission mechanism of room-temperature phosphorescent polymers.⁶⁷ Copyright 2024 Royal Society of Chemistry. (F) Chemical structure and luminescence schematic diagram of amorphous RTP copolymer.⁶⁸ Copyright 2023 Springer Nature.

The hydrophilicity of PVA may lead to the destruction of the hydrogen bond network in high-humidity environments, resulting in phosphorescence quenching. Recently, Yang and co-workers reported highly robust phosphorescent systems based on commercially available polyvinyl butyral resin (PVB), a random copolymer derived from PVA, which balanced the mutually exclusive features of a rigid hydrophilic hydrogen bond network and elastic hydrophobic constituent as the matrix, significantly improving its environmental stability (Fig. 2(B)).⁶⁴ Rigid triphenylene (Tp) derivatives (TpB and TpBe) were physically doped into PVB to obtain two doped systems (TpB@PVB and TpBe@PVB), which exhibited remarkably ultralong RTP emission with a lifetime near 6.0 s and an unusual afterglow duration of over 30 s. Impressively, the doped films could maintain a bright afterglow for over 25 seconds after being stored in refrigerators, soaked in natural water for a month, or even subjected to strong collisions and impacts. This indicates that the PVB matrix has a dual-network structure with strong hydrogen bonds and hydrophobicity. This not only effectively reduced the non-radiative transitions of triplet excitons but also effectively isolated the triplet excitons from quenching by moisture and oxygen in the air, thus significantly stabilizing and prolonging the luminescence of the triphenylene derivatives.

In addition to PVA-based polymer matrices, some other polymer matrices also have been developed. Chen and co-workers developed a wood hydrogel (W-hydrogel) via the in situ polymerization of acrylamide in the presence of delignified wood (Fig. 2(C)).⁶⁵ As a result of the molecular hydrogen bond interactions between the components of delignified wood and polyacrylamide, the W-hydrogel exhibited a tensile strength of 38.4 MPa and green RTP emission with a lifetime of 32.5 ms. Moreover, the tensile strength and RTP lifetime were increased to 153.8 MPa and 69.7 ms upon treating the W-hydrogel with ethanol. Additionally, the W-hydrogel was used as an energy donor together with Rhodamine B (RhB) to produce red afterglow emission at 600 nm via a triplet-to-singlet Förster resonance energy transfer (TS-FRET) process with an efficiency of up to 77.8%.

Subsequently, Ramakrishna and co-workers constructed a series of long-persistent luminescence (LPL) polyurethane fibers with high phosphorescence quantum yields (PhQY) and long lifetimes via wet spinning (Fig. 2(D)).⁶⁶ The unconventional phosphor tetraacetylethyle-nediamine (TAED) and fluorescent dyes were doped into polyurethane fibers by wet spinning. The obtained fibers, aided by a hydrogen bond network and efficient phosphorescence resonance energy transfer (PRET), presented a high PhQY of 70.51%, a long phosphorescence lifetime exceeding 1 s, and a maximum figure-of-merit (FM) of 751 ms at room temperature. Additionally, these LPL fibers exhibited excellent mechanical properties, including strong tensile strength and resistance to quenching effects from acid–base and heat environments. Even after 50 cycles of 500% stretching, the fibers retained their optical properties. Finally, the LPL fibers were successfully applied in flexible displays, advanced anticounterfeiting, and fashion design.

Wu and co-workers achieved ultralong RTP emission in a polymeric viscous flow state with free chain motion through a facile B–O click reaction between boric acid (BA), polyvinyl alcohol (PVA), and hydroxyl silicone oil (PDMS-OH) (Fig. 2(E)).⁶⁷ A commercial phosphor (1,3,5-tris-(4-aminophenyl)benzene, TPB-3NH₂) was doped into the matrix through hydrogen bonding between TPB-3NH₂, BA and PDMS-OH. The yielded room-temperature phosphorescent putties (RTPPs) exhibited long lifetimes under ambient conditions (up to 2.39 s). These unexpected results could be explained by the “three birds with one stone” role of BA: (i) dynamically cross-linking the PDMS chain to form PBS; (ii) narrowing singlet–triplet energy gaps (ΔE_ST) and accelerating k_ISC; and (iii) providing a bridging action that significantly alleviates the immiscibility between PVA and polyborosiloxane (PBS). Moreover, the multicolor afterglow could be adjusted from green to yellow via effective TS-FRET. Impressively, utilizing its viscous liquid features combined with RTP performance, RTPPs could be easily applied in complex models, handiwork, and anti-counterfeiting.

Zhao and co-workers presented a concise and facile strategy to fabricate persistent RTP elastomers with high stretchability and robust optical properties by blending ionic RTP polymers and PVA into a PDMS matrix (Fig. 2(F)).⁶⁸ Multicolor persistent RTP elastomers could be easily achieved by modifying the chemical structure of monomers to display different phosphorescence colors from blue to red. In addition, the prepared elastomers exhibited good mechanical properties compared to pure PDMS film, which was attributed to the addition of ionic RTP polymers and PVA, which provided extra hydrogen bonding, ion–dipole, and dipole–dipole interactions in the PDMS matrix. Importantly, their optical properties were well maintained even after multiple cycles of bending, twisting, and stretching. Moreover, the persistent RTP elastomers were used to print an anti-counterfeiting label on the PDMS substrate, which retained intense afterglow emission upon bending and stretching.

2.1.2 Ionic bonds. In addition to hydrogen bond interactions, ionic bonds also play an important role in constructing efficient and controllable stretchable RTP materials. By virtue of their strong binding force, high directivity and environmental responsiveness, ionic bonds can not only anchor phosphorescent chromophores tightly in a flexible matrix through electrostatic action to effectively inhibit molecular non-radiative transition but also endow materials with self-healing ability and stimulus-responsiveness through the dynamic dissociation–recombination of ion pairs. Tang and co-workers synthesized a series of ionogels using poly(acrylic acid) (PAA) as the polymer matrix, chloro-substituted amphoteric quinoline (QCl) as the chromophore, and imidazolium-based ionic liquid (IL) as the solvent, achieving stretchable and long-lived RTP ionogels with a tensile yield strength of 53 MPa, tensile strain of 497%, Young's modulus of 782 MPa, toughness of 111.2 MJ m⁻³, and lifetime of 113.05 ms (Fig. 3(A)).⁶⁹ The strong and dynamic interactions of the polymer, phosphor and ILs were essential to the stretchable and long-lived RTP ionogels. In the ionogels, the polymer chains were essentially held together by the covalent crosslinks and strong non-covalent interactions (H-bonds and electrostatic interactions). As the IL content was increased, the solvent facilitated the motion of the segments, and the distance between the polymer chains was long, resulting in few non-covalent crosslinks. Thus, the ionogels exhibited high stretchability. The experimental results demonstrated that the electrostatic interactions between the cations of QCl and anions of acrylic acid (AA) and ILs played a significant role in establishing long-lived RTP ionogels.

Fig. 3 (A) Design of a glassy ionogel with stretchability and long-lived phosphorescence.⁶⁹ Copyright 2024 Wiley-VCH GmbH. (B) Schematic illustration of the preparation process and physical interactions in the PVA, PVA/PMANa, PVA/PMACa, and PVA/PMACa-DS hydrogels.⁷⁰ Copyright 2024 Wiley-VCH GmbH. (C) Interactions and structural characterization of the Alg/PMA/Ca hydrogel.⁷¹ Copyright 2024 Wiley-VCH GmbH.

In 2024, Wang and co-workers reported a non-traditional poly(vinyl alcohol)/poly(calcium maleate) (PVA/PMACa) hydrogel based on ionic bond cross-linking (Fig. 3(B)).⁷⁰ The obtained poly(vinyl alcohol)/poly(calcium maleate)-DS (PVA/PMACa-DS) hydrogels exhibited excellent mechanical properties with tensile strengths of up to 15 MPa, due to the presence of strong hydrogen bonding and especially ionic bonding. The PVA/PMACa-DS hydrogels showed varied phosphorescence emission colors from blue to yellow-green with a maximum lifetime of 13.4 ms. Experiments and theoretical calculations demonstrated that ionic crosslinking between Ca²⁺ and the nonconventional chromophores prevented the contact of the nonconventional chromophores with water molecules and hence restricted nonradiative decay, leading to RTP emission. To solve the problem of the phosphorescence emission being easily quenched by water and dissolved oxygen, their group innovatively introduced two strategies, namely, the introduction of more nonconventional chromophores (NCCs) and the construction of strong ionic crosslinking (Fig. 3(C)).⁷¹ The novel nonaromatic RTP hydrogels were prepared using two types of non-traditional luminescent polymers, sodium alginate and a polymeric carboxylate, which were not RTP emissive or very weakly emissive in aqueous environments. The prepared hydrogel materials exhibited excellent anti-swelling properties and stable underwater RTP emission with a maximum phosphorescence lifetime of up to 451.1 ms even after being immersed in water for several months. The clustering of the NCCs due to ionic interactions effectively inhibited contact between the non-traditional luminogens (NTLs) and water, and hence, stable RTP emission from hydrogel was achieved. Furthermore, wet-spinning technology was employed to facilitate the rapid and large-scale preparation of RTP hydrogel fibers for long-term aqueous-environment applications.

2.1.3 Host–guest inclusion. Host–guest inclusion interaction is a typical supramolecular chemical force and plays a key role in the construction of stretchable RTP materials. By matching the size of a rigid host (such as cyclodextrin, cucurbituril or calixarene) and a phosphorescent guest, a stable inclusion complex is formed, which can not only effectively inhibit the non-radiative transition of the guest molecule but also impart the material with excellent elasticity and stimulus-responsiveness by taking advantage of the dynamic reversible interaction between the host and guest.

Polyrotaxanes (PR) formed by threading polymers through cyclodextrins (CD) have been extensively exploited for supramolecular soft materials such as adhesives, elastomers, and hydrogels because of their unique topological structure, which can effectively enhance the mechanical properties of the materials. Liu and co-workers reported a supramolecular elastomer based on waterborne polyurethane (WPU) and a pseudopolyrotaxane (NPR) formed by α-CD and naphthalene-modified polyethylene glycol (NPEG), which exhibited reversible mechanically responsive RTP enhancement behavior in the mechanical stretching process (Fig. 4(A)).⁷² The supramolecular elastomer exhibited stable RTP properties (τ = 762.34 ms) and afterglow emission even in water, high-temperature, and chemical environments (acid, alkali, and organic solvents). Impressively, reversible phosphorescence emission (the phosphorescence intensity increased three times under 200% strain) was observed, as the stretching further suppressed the non-radiative transition and vibration of NPR. The experimental results showed that the introduction of CDs with abundant hydroxyl groups not only realized long-lifetime phosphorescent emission through the macrocyclic confinement effect, but also enhanced the mechanical properties via forming hydrogen bonds with the WPU chains. Inspired by its exciting properties, the supramolecular RTP elastomer was successfully applied to information security and encryption.

Fig. 4 (A) Schematic illustration of mechanically stretched α-CD pseudopolyrotaxane elastomers with reversible phosphorescence behavior.⁷² Copyright 2024 Wiley-VCH GmbH. (B) Schematic illustration of the synergistic assembly of PU@β-CD-HP and the formation of multicolor delayed fluorescence.⁷³ Copyright 2024 Elsevier. (C) Digital images of the circular hydrogels.⁷⁴ Copyright 2023 Elsevier.

Similarly, making use of the elasticity of polyurethane (PU), they constructed a novel flexible supramolecular phosphorescent elastomer that exhibited long lifetime, high tensile strength, and reversible photo-thermal responsiveness (Fig. 4(B)).⁷³ Experimental results showed that the host–guest inclusion and hydrogen bonding interactions of β-CD-HP could effectively suppress non-radiative transitions and enhance luminescence performance. The supramolecular PU@β-CD-HP film showed blue-green phosphorescence, with an afterglow lasting up to 18 s and a lifetime of 1211 ms. In addition, it exhibited distinctive reversible photo-thermal stimuli-responsive characteristics, and the phosphorescence performance of this photo-thermal response was maintained even after numerous cycles. Furthermore, benefitting from the excellent mechanical properties of the polyurethane chain, the PU@β-CD-HP film exhibited high tensile strength and toughness, and could be stretched up to three times its original length. Leveraging the adaptable and reversible photo-thermal responsiveness of PU@β-CD-HP film, the resultant polymer exhibited versatile utility in the fabrication of materials for diverse applications such as information encryption and photoprinting. The host–guest inclusion interaction is one of the most important interactions in supramolecular polymers. It is possible to spontaneously repair cracks or fractures in supramolecular materials to increase their service life through highly directional non-covalent interactions. Liu and co-workers developed a multicolor supramolecular intelligent hydrogel with ultralong phosphorescence, high tensile strength and self-healing properties, which was constructed through the in situ thermally initiated polymerization of acrylic acid-modified β-cyclodextrin (β-CD-DA), acrylic acid-modified adamantane (Ad-DA) and acrylic acid (AA), and then carried out noncovalent binding with carbon dots (CNDs) (Fig. 4(C)).⁷⁴ The hydrogel showed green RTP emission with an afterglow of up to 10 s and a lifetime of 1261 ms. Significantly, the hydrogel network could be easily stretched to 18 times its original length and reversibly and quickly recovered. The experimental results showed that the hydrogel could heal itself rapidly after being cut into pieces due to the host–guest inclusion interaction between β-CD and adamantane. In addition, this phosphorescent hydrogel exhibited good phosphorescence energy transfer with the fluorescent dyes Rhodamine B (RhB) and Eosin Y (EY) through an efficient TS-FRET process, showing high energy transfer efficiencies (Φ_ET) of 99.9% and 99.3%, respectively.

2.2 Polymerization of phosphorescent chromophores with polymer monomers

Recently, the anchoring of phosphorescent chromophores in a stretchable polymer matrix via covalent bonds has become an important strategy to break through the limitations of physical doping and achieve high stability in stretchable RTP systems. Compared with noncovalent interactions dependent on physical doping (such as hydrogen bonding and host–guest inclusion), covalent bonding strategies permanently fix phosphorescent chromophores to polymer chains through chemical bonds, effectively avoiding phosphorescence quenching caused by the vibration and rotation of the molecules, and at the same time giving the materials higher environmental tolerance. Moreover, this close connection can also enhance the mechanical properties of the material.

Using phosphorescent chromophores directly as functional monomers to participate in the polymerization of polymer monomers is an innovative strategy to construct highly dispersible and stable RTP stretchable systems. In this strategy, phosphorescent chromophores and polymer monomers are polymerized to build a covalent network, which fundamentally avoids the phase separation problems caused by physical doping and endow the material with adjustable mechanical properties and luminescence characteristics. Based on the type of polymer monomers selected, there are two design methods: the polymerization of phosphorescent chromophores with stretchable polymer monomers, and the copolymerization of phosphorescent chromophores with soft segment and hard segment monomers to balance the luminescence and mechanical properties by regulating the microenvironment of the segments.

2.2.1 Phosphorescent chromophores polymerized with stretchable polymer monomers. Yang and co-workers proposed a novel ultra-low temperature visualization detection method based on a flexible cross-linked polymer system, which overcame the problems of brittleness at low-temperature and complex detection techniques (Fig. 5(A)).⁷⁵ Specifically, multifunctional aromatic amines were rationally selected as chain extenders and subjected to polycondensation reaction with the crosslinker and curing agent hexamethylene diisocyanate trimer (THDI). In this system, the abundant carbonyl groups and N atoms facilitated the generation of triplet excitons, and the involvement of the phosphors in the network structure restricted their molecular motion and guaranteed the stability of the triplet excitons. Additionally, the polymer films exhibited oxygen-consumption properties and showed excellent luminescence properties, including long phosphorescence lifetime and high phosphorescence quantum yield after a brief period of irradiation. Impressively, the obtained film exhibited ultra-sensitive temperature response performance under cryogenic conditions, with the phosphorescence color rapidly transforming from blue to green as the temperature increased. Based on this amazing phenomenon, a flexible temperature sensing technology for cryogenic conditions was developed to monitor the temperature visually and in real-time.

Fig. 5 (A) Chemical structures of the soft block and the hard block. (B) Chemical structures of phosphors and schematic representation for the preparation of persistent RTP polymer films.⁷⁵ Copyright 2024 Wiley-VCH GmbH. (C) Schematic diagram of the polymerization reaction initiated by the multifunctional phosphorescent compound to generate a gel.⁷⁶ Copyright 2024 Wiley-VCH GmbH. (D) Schematic diagram of reversible deactivation radical polymerization.⁷⁷ Copyright 2023 ACS Publication. (E) Schematic diagram of the rational design of stretchable RTP polymers.⁷⁸ Copyright 2024 Springer Nature. (F) Design of stretchable RTP block copolymers via ATRP.⁷⁹ Copyright 2024 ACS Publications.

He and co-workers developed benzothiadiazole-based dialkene (BTD-HEA), a multifunctional phosphorescent emitter with a remarkable intersystem crossing yield (Φ_ISC, 99.83%) (Fig. 5(B)).⁷⁶ It could achieve red phosphorescence emission at 640 nm in dimethyl sulfoxide (DMSO) solution at room temperature. Due to the fact that BTD-HEA molecules possess a high ability to generate triplet excitons and have a diene structure, they could also be effectively used as a photoinitiator and a crosslinking agent. They could efficiently initiate the polymerization reactions of various acrylamide/acrylate monomers within 120 s, thus enabling the preparation of a series of flexible RTP gels. Moreover, BTD-HEA served as an effective UV photoinitiator, aiding in the polymerization of diverse monomers and enabling visual monitoring of the gelation point via its phosphorescence colorimetric effect, providing a more sensitive alternative to traditional rheological methods.

2.2.2 Phosphorescent chromophores copolymerized with soft segment and hard segment monomers. Fabricating a hard–soft microphase-phased structure by designing a block copolymer is also a construction strategy for stretchable RTP materials. The phosphorescent chromophores are copolymerized with the soft and hard segments. The dispersed hard microphase determines the phosphorescence properties of the copolymers, and the mechanical performance is dependent on the continuous soft microphase in the nanometer dimension.

Huang and co-workers proposed controlling the growth of macromolecular chains through reversible deactivation radical polymerization (RDRP), achieving the regulation of micro–nano structures and functionalization of polymers at various scales (Fig. 5(C)).⁷⁷ RDRP could offer a gradient copolymer (GCP) architecture with controlled heterogeneities, which combined hard segments and flexible segments. In this work, the hard phase poly(acrylic acid) (PAA) suppressed nonradiative transitions, which gave rise to excellent RTP performance of the bulk GCP samples with a lifetime of up to 1180 ms. The soft phase poly(n-butyl acrylate) (P(n-BA)) provided excellent chain mobility, which afforded intrinsic flexibility, stretchability and healing ability. In contrast to the doping system, the intrinsically flexible RTP polymers exhibited excellent stability and uniformity of RTP emission under various strains of up to 200%. The utilization of RDRP enabled the synthesis of GCP nanoobjects with tunable compositions, solid contents, and viscosity, which were compatible with both inkjet printing and screen printing. This study on the integration of RDRP into RTP materials opened a new platform for developing more advanced light-emitting materials.

Huang and co-workers adopted atom-transfer radical polymerization (ATRP) to prepare block copolymers based on its precise control of molecular weight and composition, on-demand functionality, and wide utilization of diverse monomers (Fig. 5(E)).⁷⁸ Because its multiple carbonyl groups and nitrogen heteroatom could effectively facilitate the population of triplet states through an efficient ISC process and its rigid planar molecular skeleton could restrict molecular motions to suppress non-radiative decay, a naphthalimide derivative was selected as the ATRP initiator and phosphorescence chromophore. They chose polyacrylic acid (PAA) as the hard block, because its abundant carboxyl groups could construct strong hydrogen bonds to suppress the non-radiative transitions of the triplet excitons. Meanwhile, alkyl methacrylate showed high ATRP activity, and the hydrophobic alkyl chains helped to prevent phosphors from being quenched by surrounding moisture. Therefore, alkyl methacrylate monomers were introduced to prepare the soft blocks. By simultaneously incorporating stiffness and flexibility into the carefully designed block copolymers, they managed to obtain stretchable phosphorescent materials. The copolymers demonstrated an intrinsic stretchability of 712%, maintaining an ultralong phosphorescence lifetime of up to 981.11 ms. In addition, these copolymers enabled multistage volume data encryption and stretchable afterglow display.

In the same year, based on this strategy, Gu and co-workers achieved a maximum stretchability of 667% for the obtained copolymer, with a phosphorescence lifetime of 728 ms (Fig. 5(F)).⁷⁹ The formation of a hard–soft microphase separation structure played a crucial role in obtaining stretchable RTP materials. The dispersed hard microphase determined the phosphorescence properties of the copolymers, and the mechanical performance was dependent on the continued soft microphase in the nanometer dimension. Moreover, the stretchability of copolymers could be precisely regulated by modulating the molecular structure and the content of the second soft blocks. This study provided an effective strategy for obtaining stretchable RTP materials and offered guidelines for designing and synthesizing multifunctional RTP materials for advanced applications in flexible and organic electronics.

3. Conclusions and perspectives

Stretchable organic RTP materials successfully overcome the tradeoff between mechanical flexibility and environmental stability in traditional phosphorescent materials through innovative molecular design and structural regulation, providing a transformative solution for the flexible optoelectronics, smart sensing and biomedical fields. In this review, we have summarized recent developments in stretched organic RTP systems with a focus on the construction strategies of the materials, i.e., the physical doping of phosphorescent chromophores into a stretchable polymer and the polymerization of phosphorescent chromophores with polymer monomers. Determining how to balance the luminescent and mechanical properties is the key to developing stretchable RTP systems. On one hand, forming strong interactions among the phosphor, polymer chains and solvent in RTP gels can help to resolve the conflict between stretchability and persistent phosphorescence. Generally, polymer gels enhance stretchability, owing to the enhanced free volume of segment motion. Additionally, strong interactions, such as hydrogen bond, ionic bond and host–guest inclusion interactions, can suppress the non-radiative transitions of phosphors to achieve efficient RTP. On the other hand, fabricating a hard–soft microphase structure by designing a block copolymer is also a construction strategy for balancing the flexibility and rigidity of stretchable RTP materials. The phosphorescent chromophores are copolymerized with the soft and hard segments. The dispersed hard microphase determines the phosphorescence properties of the copolymers, and the mechanical performance is dependent on the continuous soft microphase in the nanometer dimension.

Although stretchable RTP systems have seen rapid development, they are still in the preliminary research stage. In order to further promote breakthroughs in this field, future research needs to deepen the exploration of construction methods, multi-model response and application (Fig. 6).

Fig. 6 Prospects and scientific challenges of stretchable room-temperature phosphorescent materials.

The first area for exploration is the diversification of stretchable matrices and new construction strategies. Currently, stretchable matrices are mostly limited to traditional polymers such as polyurethane and polydimethylsiloxane. Therefore, it is urgent to develop new dynamic stretchable matrices to expand the performance boundaries. It may be feasible to construct biomimetic dynamic networks. Based on the self-healing function of natural polymers (such as silk protein and elastin), materials with high tensile and anti-fatigue properties could be designed. In addition, the development of degradable RTP should be attempted. For example, taking advantage of a biodegradable matrix based on polylactic acid and polycaprolactone, environmentally friendly smart materials incorporating phosphorescence functionality could be developed.

The second is the design of stimulus-responsive stretchable RTP materials. Overcoming through the limitations of a single luminous function and developing systems with multi-mode responsiveness, such as mechanical force, temperature and humidity response, is the key to improving practicability. Through the introduction of dynamic covalent bonds (such as disulfide bonds and borate ester bonds), reversible changes in RTP emission color or lifetime occur during stretching and compression. For example, an elastomer containing spiropyran units could be designed to achieve red phosphorescence switching using a forced-open loop reaction for visual sensing of mechanical distribution. Alternatively, a combination of humidity/temperature sensitive polymer segments could be used to develop phosphorescence–swelling co-responsive materials.

The third is the deep application of flexible wearables and health monitoring. The purpose of developing all materials lies in achieving practical applications. The mechanical compatibility, breathability and biocompatibility of stretchable RTP materials with human skin provide unique advantages in the field of health monitoring. In the field of flexible wearables, due to their excellent flexibility and stretchability, they could conform to the complex curves of the human body and be made into products such as smart clothing, bracelets, and protective gear. Additionally, in the area of health monitoring, the elastomer can maintain close contact with the skin and monitor physiological indicators such as skin surface temperature, humidity, and sweat composition. Changes in the phosphorescence signals could provide early warnings of health problems such as skin diseases and metabolic abnormalities, offering convenient and efficient means for personal health management and medical diagnosis, and promoting the intelligent and personalized development of the medical and health industry.

Stretchable RTP systems are moving from “performance optimization” to “function creation”, and the deep integration with flexible electronics, biomedical and artificial intelligence will give birth to a new generation of smart devices. Through innovation in construction strategies, the expansion of responsiveness dimensions and the achievement of application scenarios, they are expected to achieve the leap from laboratory to industrialization in a few years, providing core material support for green energy and personalized medicine.

Author contributions

D. L. and W. G. conceived the project. Z. S. gave valuable suggestions. All figures were integrated by W. G. and H. F., W. G. and D. L. wrote the manuscript.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the National Natural Science Foundation of China (no. 22405025, 22271023), and the Education Department of Jilin Province (JJKH20250479KJ) are gratefully acknowledged.

Notes and references

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