Recent progress in ferrocene- and azobenzene-based photoelectric responsive materials

Abstract

Ferrocene and its derivatives are commonly used electrochemically responsive materials. Ferrocene derivatives have attracted much attention due to their high chemical stability, characteristic electrochemical response and high liquid crystallinity. Azobenzene and its derivatives are important light responsive materials and possess outstanding photochemical properties due to the presence of an azo bond which can isomerize reversibly between trans- and cis-forms under different illumination conditions. These kinds of compounds have been designed for information storage devices, molecular switches, molecular devices and so on. By combining a ferrocene group with an azobenzene group, novel functional materials with outstanding properties of both ferrocene and azobenzene can be obtained. These novel functional materials can be used for wide range of applications like high density information storage, ion recognition, DNA detection, etc. This review summarizes recent progress in ferrocene- and azobenzene-based photoelectric responsive materials, including their synthesis, properties and applications.

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1. Introduction

Ferrocene is an organometallic compound comprising a ferrous ion sandwiched by two cyclopentadienyl rings¹ and it was discovered in 1951.² Ferrocene derivatives have attracted much attention due to their excellent properties like electrochemical activity, thermal and photochemical stability and liquid crystallinity. Ferrocene derivatives show unique and stable electrochemical properties due to the redox changes between Fe(II) and Fe(III), which can be detected by the cyclic voltammetry (CV).³ Ferrocene compounds show different properties like polarity, intermolecular force, solubility etc. in the different oxidation states (+2 and +3) of the iron atom. Taking advantage of these property changes, researchers have applied ferrocene derivatives in information storage devices,⁴ molecular switches,⁵ chemical detection⁶ and so on. Elena Marchante et al.⁷ constructed an electrically driven and readable monolayer switch based on a solid electrolyte with an electroactive ferrocene unit. Tae-Lim Choi et al.⁸ reported that ferrocene-based polythiophenes can be applied in information storage devices. F. Le Floch et al.⁹ reported their application in DNA detection.

Azobenzene is a diazene compound in which two hydrogen atoms are substituted with two phenyl rings. Azobenzene has two configurations: trans- and cis-forms. The trans-form is thermodynamically steady state and the cis-form is dynamically steady state. Typically, azobenzene molecule exists in thermodynamically stable state (trans-form) but on exposure to UV irradiation, it undergoes isomerization and changes into cis-form. As external stimulation is removed, cis-form of azobenzene slowly recovers to trans-form.^10,11 Azobenzene with different configurations has different physical and chemical properties such as UV-vis spectrum, dipole moment and solubility.^12,13 Based on different properties of cis- and trans-forms of azobenzene, azobenzene and its derivatives have been applied in various fields, such as information storage devices,^14,15 biosensors,^12,16 nano-carriers^17,18 and so on. Fernández et al.¹⁹ synthesized epoxy-based nanostructured thermosets linked with azobenzene groups and found that these materials can achieve numerous writing-erasing cycles without photodegradation. Blasco et al.²⁰ synthesized a novel photoresponsive azobenzene-containing miktoarm star polymer which can self-assemble in water to form stable vesicles. Upon UV irradiation, such polymer shows photo-induced morphological change due to isomerization of azobenzene, which increases the permeability of the membrane and can be used for controlled drug release.

Recently, scientists devoted their studies on smart materials which can response to external stimuli, such as temperature,^21–23 pH,^24,25 light,^26–28 electric (redox)^29,30 and so on. Among these external stimuli, light and electric (redox) stimuli are considered excellent, as these are fast, efficient and need no additives to the system. So, numerous efforts have been made for photoelectric responsive materials.^31–33 Masato Kurihara et al.³⁴ synthesized meta-ferrocenylazobenzene (m-FcAB or 3-FcAB) and found that ferrocenyl group affected the isomerization of azobenzene by electronic effect. So much attention has been paid to combine ferrocene group with azobenzene group. By integrating ferrocene with azobenzene, the novel functional materials containing outstanding properties of both ferrocene and azobenzene can be obtained and used for wide range of applications, like high density information storage,³⁵ ion recognition,^36,37 molecular devices,³⁸ etc. This review provides an overview on the synthesis, properties and applications of different photoelectric responsive materials containing ferrocene and azobenzene including compounds, self-assembled monolayers and polymers.

2. Ferrocene- and azobenzene-based photoelectric responsive materials

Ferrocene- and azobenzene-based photoelectric responsive materials usually include compounds, self-assembled monolayers and polymers.

2.1 Ferrocene- and azobenene-based compounds

Many researchers devoted their work on the synthesis of ferrocene- and azobenzene-based compounds to combine photochemical and electrochemical properties and to get photoelectric responsive materials.

2.1.1 Synthesis of ferrocene- and azobenzene-based compounds. Due to long time of research, the synthesis methods of ferrocene- and azobenzene-based compounds have been well studied. Several methods have been used for the synthesis of ferrocene- and azobenzene-based compounds. The most commonly used methods are substitution and coupling reactions.

In substitution reactions, ferrocene derivatives containing carboxyl groups and azobenzene derivatives containing hydroxyl or amino groups are used as reactants. Usually, ferrocenyl carboxylic acid is reacted with oxalyl chloride under anhydrous and anaerobic conditions to get high reactivity intermediate and then this intermediate is reacted with hydroxyl or amino group of azobenzene to get the targeted product. Chao Li et al.³⁶ synthesized ferrocenyl azobenzene derivatives by the substitution reactions of 4-aminoazobenzene with chlorocarbonyl ferrocene (Scheme 1).

Scheme 1 Synthetic route of ferrocenyl azobenzene derivatives.³⁶

Coupling reaction is a reaction referred to a process in which two organic moieties react to form one product. Benjamin J. Coe et al.³⁹ synthesized 4-ferrocenyl-2′-methyl-4′-nitroazobenzene by the coupling reaction of 2-nitroso-5-nitrotoluene with 4-aminophenylferrocene and studied its structure. Yan et al.⁴⁰ utilized Sonogashira coupling reaction to synthesize ferrocenylethynyl-terminated azobenzene derivatives by the reaction of ethynylferrocene with different iodo-substituted azobenzene derivatives (Scheme 2).

Scheme 2 Synthesis of ferrocenylethynyl-terminated azobenzene derivatives.⁴⁰

In addition to the above methods, click chemistry has also been applied to synthesize ferrocenyl azobenzene derivatives. Yang et al.⁴¹ synthesized a water-soluble ferrocenyl derivative (4-ferrocenyl-1,1′-azobenzene-3,4′-disulfonic acid) in aqueous medium. They found that cationic form of ferrocene had the ability to react with aromatic diazonium salts in the dilute acidic environment (Scheme 3). Synthesis of ferrocenyl azobenzene based derivatives by click chemistry is easy and environmentally friendly.

Scheme 3 Synthesis of water-soluble monosubstituted ferrocene derivatives.⁴¹

2.1.2 Properties of ferrocene- and azobenzene-based compounds. Ferrocene has electro-responsive properties and azobenzene has photo-responsive properties. The combination of ferrocene and azobenzene results in the dual responsive (photoelectric dual responsive) properties of the new compounds, but in actual, these new compounds show some extra properties due to the electronic influence between these two moieties.

The positions of substituents affect photoelectric responsive performance of ferrocene- and azobenzene-based molecules. Masato Kurihara et al. synthesized m-FcAB whose isomerization could transform from trans-form to cis-form under green light.³⁴ Employing m-FcAB, they successfully controlled the reversible isomerization of azobenzene with a single green light by adjusting the redox state of azobenzene, as shown in Scheme 4. This work was inspired by a discovery about azoferrocene that trans-to-cis isomerization could be triggered by a green light irradiation which excited a low-lying metal-to-ligand charge transfer (MLCT) band.⁴² MLCT is d–π* transition from Fe(II) d-orbital to π*-orbital of the bridging ligand, while in the case of para-ferrocenylazobenzene(p-FcAB), the same phenomenon was not observed.

Scheme 4 Redox-conjugated photoisomerization pathway.³⁴

It is found that attachment of the metal atom with azobenzene by a proper π-conjugated system enables electron communication between metal center and azobenzene group. Tatsuaki Sakano et al. synthesized a series of azobenzene-introduced azaferrocenophanes and found that photochemical behaviour of azobenzene could be influenced by the oxidation state of the molecule.^43–45 The compounds at the photostationary state underwent thermal isomerization of cis-form to trans-form. After one-electron oxidation, the isomerization of the compounds responded more rapidly than before oxidation. They concluded that intramolecular electron transfer was responsible for rapid isomerization and then proposed the possible mechanism (Scheme 5).

Scheme 5 The possible intramolecular electron transfer mechanism of azaferrocenophane.⁴⁵

Different numbers and positions of substituents affect the properties of ferrocenyl azobenzene derivatives. Jian-Feng Yan et al. prepared ferrocenylethynyl-terminated derivatives and investigated the effect of number and position of ferrocenylethynyl group on the properties of azobenzene.⁴⁰ It was found that monoferrocenylethynyl derivatives were more stable in air and moisture while diferrocenylethynyl derivatives were unstable and slowly decomposed in solution. The diferrocenylethynyl derivatives exhibited totally overlapped redox peaks which showed that the two ferrocenyl groups were electrochemically equivalent. Also it was found that m-ferrocenylethynyl derivative with high electron density on ferrocene showed stronger interactions between azo group and alkynyl group than p-ferrocenylethynyl derivative from CV studies. UV-vis studies showed the same results. Aiko Sakamoto et al.⁴⁶ synthesized a series of ferrocenyl azobenzenes with different electronic effect and different positions of the substituent groups and studied their photoelectric responsive performance (Scheme 6). It was found that introduction of electron-withdrawing groups on azophenyl group resulted in positive effect on isomerization of azobenzene with single green light.

Scheme 6 Ferrocenylazobenzenes with different substituent groups.⁴⁶

2.1.3 Applications of ferrocene- and azobenzene based compounds. Ferrocene- and azobenzene-based compounds show excellent properties which enable these molecules for various applications, such as ion detection, biosensors, molecular switches and so on.

Anions widely exist in nature and play extremely important roles in many areas, such as environment, biology, medicine and so on. The special natures of anions, such as big ionic radius, pH sensitivity, complex space configuration and easy solvent influence, make it more difficult for anion recognition in comparison with cation recognition. So anion recognition is a challenge for researchers. Among various chemosensors, colorimetric chemosensors are attractive, as their recognition processes can be easily observed from UV-vis spectroscopy. Usually conventional colorimetric chemosensors contain chromophore, such as azobenzene, which linked to an anion-binding part directly.⁴⁷ The interaction of anion with receptor affects optical signals of the chromophore even it leads to the colour change of the system. On the other hand, ferrocene is also a popular building block for anion recognition because anion binding induces negative shift of the redox potential.⁴⁸ The combination of azobenzene and ferrocene moieties in a same chemosensor results in better recognition effect. Antonia Sola et al. synthesized a chemosensor based on ferrocenyl-guanidine decorated with a chromogenic aryl azo moiety which exhibited good sensitivity and affinity for acetate ions in acetonitrile.⁴⁹ The recognition process could be monitored by CV and UV-vis spectroscopy and also by “naked-eye” because of a colour change from yellow to orange. Chao Li et al. successfully synthesized two kinds of colorimetric probe molecules, 4-amideferrocenylazobenzene and N,N′-bisamideazophenylferrocene for anion recognition.³⁶ They used amide linkage as anion binder through hydrogen bonding while ferrocene and azobenzene as signal transmission units for selective recognition of F⁻ and/or H₂PO₄⁻. The recognition process has a colour change from yellow to salmon pink. Xiaoting Zhai et al. continued this study to investigate the effect of different substituents of ferrocenyl azobenzene on anion recognition.³⁷ The results showed that electron-donating groups weakened hydrogen bonding and produced negative effect on the sensitivity of recognition and electron-withdrawing groups strengthened hydrogen bonding and produced positive effect on recognition sensitivity, as shown in Scheme 7.

Scheme 7 The possible mechanism of anion recognition of the synthesized ferrocenyl azobenzenes upon addition of F⁻ and H₂PO₄⁻.³⁷

Nucleic acids detection is one of the forefront researches in biochemistry and requires detection techniques of high sensitivity and high precision. The detection techniques mainly include mass spectroscopy, radioactivity, quartz microbalance, fluorescence and so on. All these detection techniques have some obvious drawbacks. The electrochemical and photochemical techniques are the most promising techniques with merits of high sensitivity, short response time, low costs, etc.⁵⁰ Peptide nucleic acid (PNA) oligomers are DNA mimics and have got much attention due to their prominent properties, such as sequence-selective binding for DNA or RNA, excellent stability in biological environment and no electrostatic repulsion with negatively charged DNA or RNA.⁵¹ So, it is quite a good method to choose PNA monomers modified with ferrocenyl azobenzene for DNA detection. Jindu Li et al. synthesized a PNA monomer of thymine (Fc-Azo-T) labelled by ferrocenyl-azobenzene with good electrochemical and photochemical activity.⁵² They found that detection limit of DNA in ethanol could reach up to 10⁻⁶ M by differential pulsed voltammogram (DPV) and could reach up to 10⁻⁷ M by UV-vis spectroscopy. Sheng Liu et al. designed three ferrocenyl azobenzene labelled PNA monomers of cytosine, adenine and guanine (Fc-Azo-C, Fc-Azo-A, Fc-Azo-G).⁵³ The lowest DNA detection limit of these three PNA monomers could down to 10⁻⁸ M in ethanol.

Cyclodextrin (CD) is a natural product which is obtained by the reaction of cyclodextrinase with starch and has many attractive functions. One of the most widely used functions is molecular recognition. β-CD is a kind of CD with seven glucose units. As both ferrocene group and azobenzene group can form stable complex through host–guest interactions with β-CD, a lot of researches have been done and found that these complexes have promising prospects in biological applications and functional materials. Several studies show that β-CD can form a complex with ferrocene and trans-azobenzene but separate from ferrocene ion and cis-azobenzene.⁵⁴ Liangliang Zhu et al. synthesized an asymmetric guest compound containing ferrocene and azobenzene moieties.⁵⁵ They found that the guest molecule can form a host–guest system with β-CD complex and the complexation sites and stoichiometries can be modulated by electrical redox and photoirradiation (Scheme 8). Avik Samanta et al. designed an azobenzene-ferrocene conjugated N,N′-bis(3-aminopropyl)ethylenediamine which could respond to metal ions, UV irradiation and oxidation stimuli.⁵⁶ Under normal conditions, it could form inclusion complex with β-CD and aggregated on the host β-CD vesicles. As any of these three stimuli was produced, the conformation of the guest molecule became changed and its affinity for the host vesicles dramatically reduced, which resulted in the destruction of the complexes (Scheme 9). It has been reported that binding affinity of ferrocene for β-CD is much higher than azobenzene.^57,58 Based on this information, Jing Zheng et al. designed a remote-controlled release system of DNA for living cells based on cooperatively mediated host–guest interactions through light and competitive unsheathing,⁵⁹ as shown in Scheme 10. They used mercapto-β-CD modified gold nanoparticles (AuNPs) serving as a carried agent, azobenzene labelled on a releasing oligonucleotide acting as a photo switch and ferrocene serving as “enhancers”. Since the binding affinity of ferrocene for β-CD is stronger, azobenzene could separate from β-CD more easily with the presence of ferrocene, thus this oligonucleotide releasing process needed less UV-light irradiation and did less harm to people's health.

Scheme 8 The interconversion network of the dual-driven (redox and light-driven) ensembles between the four states.⁵⁵

Scheme 9 Stimuli-responsive conformational changes in the molecular structure of azobenzene–ferrocene conjugated N,N′-bis(3-aminopropyl)ethylenediamine.⁵⁶

Scheme 10 Schematic representation of remote controlled-release system of DNA for living cells based on cooperatively mediated host–guest interactions through light and competitive unsheathing.⁵⁹

Recently interest in the use of molecular gels for designing functional materials has been increased rapidly. Researchers devoted their studies to design low molecular weight gelators which can regulate sol–gel transition. Rouzbeh Afrasiabi et al. recently designed a photoresponsive discrete metallogelator by the introduction of photochromic azobenzene group and redox-active ferrocene moiety into peptide (Scheme 11).⁶⁰ The designed molecule showed supramolecular self-assembly based on the dipeptide unit and showed gel–sol transition under the stimuli of ultrasound, light, heat and redox signals. This smart material showed potential in NOT logic gate operation and XOR operations.

Scheme 11 Molecular structure of the metallogelator.⁶⁰

Molecular machines and devices have attracted much attention due to their potential applications in biology, materials and various molecular machineries such as motors, shuttles, switches have been reported.^61,62 Takahiro Muraoka et al. designed a pair of light-driven molecular scissors using ferrocene as pivot, azobenzene as handle driven by visible and ultraviolet lights and two phenyl groups as blades.^63,64 The photoisomerization of azobenzene under different light caused the ferrocene pivotal rotation, and then interlocked to the blade parts of open–close motion. The follow-up work indicated that by changing the redox state of the ferrocene pivot, the reversible open-close motion of the blades could be realized under single UV light, as shown in Scheme 12. In another work, Takahiro Muraoka et al. used the molecule scissors (1) bearing zinc porphyrin units as a host molecule and nitrogenous base, compound 2 as a guest molecule.^65,66 These two compounds could form a stable complex. Photoisomerization of azobenzene in response to UV and visible light could induce a pedal-like motion of zinc porphyrin units and then caused the twist rotation of guest molecule 2 in clockwise and counterclockwise directions, repeatedly (Scheme 13). This work has important significance in biological supramolecular machines and molecular devices.

Scheme 12 Schematic representation of the sequential operations of molecular scissors on UV irradiation and redox of the ferrocene pivot.⁶⁴

Scheme 13 Design and concept of light-powered molecular pedal.⁶⁵

2.2 Ferrocene- and azobenzene-based self-assembled monolayers

The attachment of the functional self-assembled monolayers (SAMs) with semiconductive, metallic, carbon-based substrates, is a popular method to get functional surfaces or interfaces, which have numerous applications such as electronic devices, photovoltaic devices, catalysis, biological/chemical sensing, wetting control and corrosion inhibition. Ferrocene and azobenzene moieties show unique properties, so researchers have paid considerable attention on the synthesis, properties and applications of ferrocenylazobenzene-functionalized SAMs.

2.2.1 Synthesis of ferrocene- and azobenzene-based SAMs. Ferrocene- and azobenzene-based SAMs can be prepared in two steps. First step involves the synthesis of ferrocene- and azobenzene-based compounds and second step involves the SAM-modifications on the substrates. Usually researchers make some pretreatments for the substrates and immerse these substrates in a solution of ferrocene- and azobenzene-based compounds under proper conditions. Several different synthesis approaches have been used for ferrocene- and azobenzene-based SAMs according to different anchoring groups. Chongqing Li et al. designed a novel ferroceneylazobenzene SAM on an indium–tin oxide (ITO) electrode by the adsorption of 4-(4′-11-ferrocenyl-undecanoxyphenylazo)benzoic acid on preprocessed silanized ITO substrate surface covalently, as shown in Scheme 14.³³ Toshihiro Kondo et al. constructed a SAM containing ferrocene and azobenzene on gold electrode through the reaction of 4-nitro-(1-azobenzyl)-1′-(6-mercaptohexyl)ferrocene (NAzFcC6SH) with Au (Scheme 15).⁶⁷

Scheme 14 Synthesis of FcAzo/ITO.³³

Scheme 15 Synthesis of NAzFcC6SH SAM.⁶⁷

2.2.2 Properties of ferrocene- and azobenzene-based SAMs. It is well known that ferrocene and azobenzene show stable redox properties and reversible photo-induced isomerization, respectively. The self-assembly of ferrocene- and azobenzene-based SAMs on the substrates influence their photoelectrical properties.

Numerous studies demonstrate that the photoresponse isomerization of azobenzene moieties is severely affected by free volume of azobenzene in SAMs. Wang et al. found that in a SAM constructed by azobenzene-terminated alkanethiols, 5% or less concentration of azobenzene was necessary for free isomerization of azobenzene moieties.⁶⁸ Nakagawa et al. found that in consideration of azobenzene isomerization, the critical area needed for SAMs composed of alkyl-azobenzenes was about 45 Ų per molecule.^69–71 The critical free volume is the guarantee for isomerization of azobenzenes in SAMs. Toshihiro Kondo's work⁶⁷ indicated that 100% trans-azobenzene groups self-assembled on a gold electrode and formed a compact monolayer which showed no response to UV light while 20% cis-azobenzene groups formed a loose monolayer and showed free isomerization under UV-vis light. Chongqing Li et al. considered the surface coverage an important parameter for azobenzene isomerization based on the same reason.³³

Electrochemical properties of ferrocene moieties can be influenced by SAMs structure. Hua-Zhong Yu et al. found that conformation change and protonation of azobenzene groups could cause spatial inhibition for electron transfer of SAMs.⁷² Electron transfer rate through a SAM can be affected by the redox microenvironment, such as monolayer composition, intermolecular interaction and anionic species in the supporting electrolyte. Toshihiro Kondo et al. reported a work⁶⁷ that CV of a gold electrode modified with 20% cis- and 80% trans-forms of azobenzene SAM showed two pairs of peaks in the potential range between 0 and +750 mV, one belonging to azobenzene moieties and another belonging to ferrocene moieties. In sequent second scan, the wave of azobenzene disappeared, the redox potential of ferrocene became more negative and the peak separation of ferrocene redox wave was smaller. This process became reversible after one week UV irradiation. This phenomenon indicated that charge-transfer rate of the ferrocene group was faster in cis-azobenzene than in trans-azobenzene (Scheme 16). Chongqing Li et al. reported that charge transfer rate between ferrocene and ITO electrode slowed down on UV irradiation because the cis-azobenzene SAM possessed smaller porosity and more compact barrier between redox species and the ITO electrode (Scheme 17).³³ It was concluded that the electrochemical properties of ferrocene groups, including charge-transfer rate and redox potential could be manipulated by photo induced structural change of the azobenzene moieties in the SAMs.

Scheme 16 Schematic models at the interface: (a) between 100% trans-NAzFcC6SH SAM modified gold electrode and (b) between 20% cis-NAzFcC6SHSA modified gold electrode.⁶⁷

Scheme 17 Schematic illustration of FcAzo/ITO electrode and its reversible photoisomerization affect on the transportation of chloride ion in SAM.³³

2.2.3 Applications of ferrocene- and azobenzene-based SAMs. Ferrocene- and azobenzene-based SAMs show wide range of applications due to their excellent photoelectrical properties, for example, data storage devices, chemical sensors, photovoltaic devices, molecule devices and so on.

Kosuke Namiki et al.³⁵ designed a 3-ferrocenylazobenzene monolayer on an ITO electrode in which azobenzene moiety exhibited reversible isomerization under single green light by adjusting the redox state of ferrocene moiety just like compound 3-FcAB (Scheme 18). This photoisomerization system was more stable and could use fine-focused light as radiation source. So it is more suitable to apply in new density optical data storage devices.

Scheme 18 Schematic representation of the redox-coupled single-light isomerization of 3-FcAzo/ITO.³⁵

Dean J. Campbell et al.⁷³ designed a self-assembled monolayer of ferrocenyl-azobenzene butanethiols on Au electrode with densely packed azobenzene moieties. The resulting SAM exhibited almost no electrochemical accessibility for large charge-compensating cations upon their reduction, while small charge-compensating cations (H⁺ and Li⁺) had access for this SAM (Scheme 19). Thus, electron transfer processes from the electrode surface to the self-assembled film could be gated by choosing specific film structure or adjusting charge-compensating cation concentration or size. From another perspective, this SAM exhibited selectivity toward different size of ions, allowed small ions to pass through but inhibited the penetration of large ions, which means it could be used for selective ion recognition.

Scheme 19 Schematic depiction of the selective penetration of small cations and proton sources in SAM.⁷³

Daniel G. Walter et al. constructed a photoactive SAM consisting of 99% cis-azobenzene derivatives.⁷⁴ By adjusting ferrocene site, its structure was amplified by the photo driven signal via an electrochemical response, which could be used as a “photoswitchable diode” (Scheme 20).

Scheme 20 Schematic illustration of photoswitchable diode.⁷⁴

Nano scale sized molecular devices can be constructed on substrates based on the interaction with β-CD. Itamar Willner et al. designed a light-driven “molecular train” on Au-electrode surface.⁷⁵ The assembly system was consisted of a molecule linked with Au electrode surface containing an azobenzene group and long alkyl group, a ferrocene-modified β-CD (Fc-β-CD) assembling with alkyl group, and a bulky anthracene group at tail. Responding to the light driven azobenzene isomerization, Fc-β-CD could move on different sites of the molecule bonded on Au surface (Scheme 21). The assembly functional optoelectronic system could be used for optical information record and electronic signal transition. Marco Frasconi et al.⁷⁶ studied a reversible self-assembly of β-CD modified gold nanoparticles (β-CDAuNPs) on a mixed SAM, formed by co-adsorption of redox-active ferrocenylalkylthiols and n-alkanethiols on Au surfaces. This dual controlled self-assembly can be applied to logic gate operations (Scheme 22).

Scheme 21 Organization of MT monolayer assembly on an Au-electrode and its photoinduced translocation.⁷⁵

Scheme 22 Truth table for an “AND” logic gate system.⁷⁶

2.3 Ferrocene- and azobenzene-based polymers

In the last several decades, scientists focused on the production of functional materials that can response to external stimuli. Ferrocene-containing polymers have been hot research topics for a long time since with the discovery of their response to electrical stimulus. Similarly, azobenzene-containing polymers have always been popular topics in the study because of their potential applications in stimuli-responsive materials and photoswitches owing to their fast and reversible photoisomerization. The combination of ferrocene and azobenzene moieties in the same polymer results in materials with photoelectric responsive properties, which have broad application prospects.

2.3.1 Synthesis of ferrocene- and azobenzene-based polymers. Several different polymerization methods have been used for the synthesis of ferrocene- and azobenzene-based polymers.

For the synthesis of linear structure polymers containing ferrocene and azobenzene groups on backbones, polycondensation reaction is a good choice. Zareen Akhter et al. prepared some ferrocene containing aromatic azo polyesters by the polycondensation reaction of 1,1′-ferrocenedicarbonyl chloride and different aromatic diols containing azo groups synthesized by diazotization coupling reaction (Scheme 23).⁷⁷ These polymers showed unique physical and chemical properties. Alss S. Abd-El-Aziz et al. synthesized a monomer containing ferrocene and azobenzene moieties. Polymerization of this functional monomer was accomplished with sulfur based nucleophiles and oxygen (Scheme 24).⁷⁸ The obtained polymers displayed excellent thermal stability and great solubility in organic solvents.

Scheme 23 Synthesis of ferrocene-containing aromatic azo polyesters prepared by solution polycondensation.⁷⁷

Scheme 24 Synthesis of polymers with azobenzene and ferrocene in backbone.⁷⁸

Several methods have been used for the synthesis of polymers containing ferrocene in the backbone and azobenzene in the side chains. Xiao-Hua Liu prepared two novel metallomesogenic polymers containing poly(ferrocenylsilane) in the main chain and 4-pentoxy-4′-hydroxyhexanoxyazobenzene or 4-pentoxy-4′-hydroxyundecyloxyazobenzene acrylate in the side chains.⁷⁹ Poly(ferrocenylsilanes) with high molar mass were obtained via ring-opening polymerization of silicon-bridged [1]ferrocenophane precursors and then azobenzene-based mesogens were attached to the backbone by hydrosilylation reaction (Scheme 25). These metallomesogenic polymers showed good thermotropic liquid-crystalline properties. Wael A. Amer et al. once synthesized a series main-chain ferrocene-based polyesters containing azobenzene in side chains (MFPAS) by solution polycondensation reaction and post-polymerization azo-coupling reaction (Scheme 26).⁸⁰ They also synthesized a variety of main-chain ferrocene-based polymers with different aromatic units by an interfacial polycondensation reaction (Scheme 27).⁸¹

Scheme 25 Synthesis of poly(ferrocenylsilane)s containing azobenzene in the side chain.⁷⁹

Scheme 26 Synthesis of MFPAS.⁸⁰

Scheme 27 Synthesis of main-chain ferrocene-based polymers containing aromatic units.⁸¹

Side chain ferrocenylazobenzene-containing polymers can be obtained by post-modification of the precursor polymers. Kosuke Namiki et al. synthesized polymers containing 3-FcAB by the modification of poly(2-bromoethyl acrylate) precursors (Scheme 28).⁸²

Scheme 28 Synthesis of meta-ferrocenylazobenzene-containing acrylic polymers.⁸²

Among all kinds of branched macromolecules, dendrimers and star-shaped macromolecules get much attention due to their outstanding properties of high solubility and low viscosity. The preparation of star-shaped macromolecules can be done by two methods. First is the divergent method, which involves polymerization of linear chains with reactive groups with multifunctional core. Second is convergent method, which involves the use of multifunctional core to initiate polymerization of the monomers. Alaa S. Abd-El-Aziz et al. synthesized varieties of azobenzene and organoiron containing branches with multifunctional cores via metal-mediated nucleophilic aromatic substitution and ester condensation reactions to afford multi-substituted three- and four-arms star-shaped oligomers (Scheme 29).⁸³ This complex oligomer possessed a tri-ether core to which arene cyclopentadienyliron cations, aryl-azo dye and ferrocene moieties were attached.

Scheme 29 Reaction of oligomer with ferrocene carboxylic acid.⁸³

2.3.2 Properties of ferrocene- and azobenzene-based polymers. Ferrocene- and azobenzene-based polymers show electrochemical and optical properties which can be detected by CV and UV-vis spectroscopy. The thermal stability of polymers makes them more suitable as functional materials but some characteristics of these polymers affect their photoelectric properties. Polymers exhibit various structural diversity because they have numerous internal degrees.⁸⁴ Generally polymers in solution are in a state of random coil with some groups buried inside and some groups staying outside. In the solid state, these polymers form different aggregated morphologies. Both configurations and aggregated morphologies can affect properties of these polymers. Kosuke Namiki et al. synthesized 3-FcAB containing polymers by solution polymerization (polymer A) and emulsion polymerization (polymer B) and studied their electrical and light responsive properties in solution and also in solid state.⁸² Both polymers formed spherical particles in solid state and polymer A had bigger outside diameters than polymer B. Table 1 shows the electronic spectral changes and isomerization behaviour of 3-FcAB in polymers. When in solution of 1,4-dioxane, λ_max value of π–π* band for polymer A was not shifted, whereas for polymer B it was shifted to longer wavelength, indicating the azobenzene chromophores of polymer A were buried inside and experienced a more polarizable environment than dioxane. When in the solid state, the photoisomerization behaviours of polymers showed that π–π* band was shifted to a longer wavelength due to J-aggregates formation of 3-FcAB in polymers.

Table 1 Summary of the electronic spectral changes and isomerization behaviors of 3-FcAB in polymers⁸²

Sample	π–π*/nm	MLCT/nm	%MLCT	%cis molar ratio
				UV	Green
a Polymer was dissolved by ultrasonication.b A 1,4-dioxane solution was cast on a glass plate.
(Polymer A)
1,4-Dioxane	326	440	8	59	15
Solid^b	344	450	20	10	5
(Polymer B)
1,4-Dioxane^a	330	436	9	54	17
Chloroform^a	325	441	18	10	9
Solid^b	339	450	15	12	5

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It is well known that polymers containing ferrocene and azobenzene moieties show liquid crystalline property. Liquid crystals exhibit both liquid and crystalline characteristics like fluidity of liquid and nematic property of crystals which can be used in liquid crystalline materials, photovoltaic cells, light-emitting diodes and so on. Azobenzene and its derivatives, as photochromic liquid crystal molecules have been widely utilized in liquid crystalline materials and the configuration change of azobenzene moieties can change order degree of liquid crystalline materials.⁸⁵ Singh et al. reported that presence of ferrocene groups in the polymers contributed in liquid crystallinity and ferrocene groups also showed mesogenic property.^86,87 Ferrocene based polymers containing azobenzene groups are promising candidates as mesomorphic materials with redox property. Xiao-Hua Liu et al.⁷⁹ synthesized two novel polymers with poly(ferrocenylsilane) main chain and azobenzene side chains. Both polymers displayed nematic mesophase. Wael A. Amer et al.⁸⁰ synthesized three novel MFPASs and studied their properties. All these synthesized polymers showed excellent photoisomerization and thermotropic liquid-crystalline properties.

2.3.3 Applications of ferrocene- and azobenzene-based polymers. In the modern society of information explosion, people have to deal with and store the growing amount of information, so information storage devices are playing increasingly important roles in our daily life. Compared with conventional inorganic semiconductor memory devices, organic materials show many outstanding advantages in data storage application due to their structural diversity, good flexibility, low manufacturing cost and so on.^88–90 The memory devices based on small molecular compounds have poor stability, low device yield and short retention time, while polymeric materials exhibit promising application prospects in information storage devices.⁹¹ As ferrocene is a desired electrochemical storage group, ferrocene-based polymers play important roles in electrochemical storage devices. Jing Xiang et al. synthesized several new conjugated ferrocene-containing poly(fluorenylethynylene)s (PFcFEs) and constructed two terminal single layer devices (ITO/polymer/Al) which exhibited flash memory behaviour.⁹² On the other hand, azobenzene-based polymers, as irreplaceable optical data storage materials are still being widely studied in enormous researches.^93–95 So, polymers containing ferrocene and azobenzene moieties have much potential applications in high density information storage devices. Kosuke Namiki et al. synthesized 3-FcAB-containing polymers whose structures can be changed in four states under green light and redox stimuli (Scheme 30).⁸² These polymer particles are promising candidates for nano-sized high density memory devices.

Scheme 30 Photochromic behaviour of 3-FcAB-containing polymer.⁸²

Surface relief gratings (SRGs) have attracted much attention since Todorov et al. stated that chromophore could make large optical birefringence by photoisomerism on exposure to polarized light.⁹⁶ Azo polymers have been hot topics of the researchers and can be used as perfect thin films for SRGs.⁹⁷ When the polymer films are irradiated with low power laser radiations, photoinduced cyclic isomerizations of azo chromophores cause polymer chains migration, thus form the SRGs. Yaning He et al.⁹⁸ and Eun-Ju Ha et al.⁹⁹ synthesized different azo polymers and studied their photo-induced surface-relief-gratings behaviours. They found that the structure of backbone affected the inscription rate of azo polymers. Polyferrocenylsilane (PFS) material is an important kind of stimuli-responsive materials. The oxidation of Fe(II) induces configuration change in the polymer chains because of the electrostatic repulsion of Fe(III), and then results in structure change of the material.¹⁰⁰ Rumman Ahmed et al. synthesized PFS-based diblock copolymers containing azobenzene chromophores through ionic self-assembly route and obtained organometallic SRGs (Scheme 31).¹⁰¹ When PFS chains were oxidized, the modulation depth was increased because of the repulsion of charged Fe(III) electroactive centres. Furthermore, the azobenzene chromophores could be removed by oxygen plasma since it was found that azobenzene chromophores in SRGs were unfavorable for some applications.^102,103 Such gratings have potential applications in templates, non-reversible holographic data storage devices and simple read-only ID tags.

Scheme 31 (A) Chemical structure of the hierarchical PFEMS₁₁₂-b-PFAMS(EO)₁₁₂ complex; (B) AFM morphology profiles of the PFEMS₁₁₂-b-PFAMS(EO)₁₁₂ SRGs (a) before and (b) after 2 minutes of oxidation; (c) combined plots of the height profiles, indicating similar modulation depth of the oxidized and reduced samples; SEM micrographs of the SRGs (d) before and (e) after oxygen plasma etching (scale bars = 500 nm).¹⁰¹

3. Conclusion

In this review, the recent progress on the synthesis, properties and applications of ferrocene- and azobenzene-based photoelectric responsive materials including compounds, self-assembled monolayers and polymers were summarized. Ferrocene- and azobenzene-based materials possess unique photoelectric responsive properties and show wide range of applications, such as high-density information storage devices, chemical sensors, molecular devices and so on. From the above summary, the studies from synthesis to applications of ferrocene- and azobenzene-based compounds are very vast, while the exploration of ferrocene- and azobenzene-based polymers is very rare. So, in the future, further studies should be done on ferrocene- and azobenzene-based photoelectric responsive polymers and develop a wider range of applications.

Acknowledgements

Financial supports from the National Natural Science Foundation of China (21272210, 21472168 and 21372200), the International Science and Technology Cooperation Project of Ministry of Science and Technology of China (2009DFR40640), the Science and Technology Program of Zhejiang Province (2013C24001 and 2013C31146), the Science and technology innovation team of Ningbo (2011B82002) are gratefully acknowledged.

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