Photoswitchable Materials Research Articles

Visible‐Light‐Triggered Molecular Photoswitch Based on Reversible E/Z Isomerization of a 1,2‐Dicyanoethene Derivative

AbstractA designed bis(dithienyl) dicyanoethene‐based, strictly E/Z photoswitch (4TCE) operates through state‐selective (E and Z isomer) photoactivation with visible light. The E and Z isomers of 4TCE exhibit remarkably different spectroscopic characteristics, including a large separation (70 nm) in their absorption maxima (λmax) and a 2.5‐fold increase in molar extinction coefficient from cis to trans. The energetically stable trans form can be completely converted to the cis form within minutes when exposed to white light, whereas the reverse isomerization occurs readily upon irradiation by blue light (λ<480 nm) or completely by thermal conversion at elevated temperatures. These features together with excellent thermal stability and photostability of both isomers make this new E/Z photoswitch a promising building block for photoswitchable materials that operate without the need for UV light.

Visible-light-triggered molecular photoswitch based on reversible E/Z isomerization of a 1,2-dicyanoethene derivative.

A designed bis(dithienyl) dicyanoethene-based, strictly E/Z photoswitch (4TCE) operates through state-selective (E and Z isomer) photoactivation with visible light. The E and Z isomers of 4TCE exhibit remarkably different spectroscopic characteristics, including a large separation (70 nm) in their absorption maxima (λ(max)) and a 2.5-fold increase in molar extinction coefficient from cis to trans. The energetically stable trans form can be completely converted to the cis form within minutes when exposed to white light, whereas the reverse isomerization occurs readily upon irradiation by blue light (λ<480 nm) or completely by thermal conversion at elevated temperatures. These features together with excellent thermal stability and photostability of both isomers make this new E/Z photoswitch a promising building block for photoswitchable materials that operate without the need for UV light.

A highly efficient silole-containing dithienylethene with excellent thermal stability and fatigue resistance: a promising candidate for optical memory storage materials.

Diarylethene compounds are potential candidates for applications in optical memory storage systems and photoswitchable molecular devices; however, they usually show low photocycloreversion quantum yields, which result in ineffective erasure processes. Here, we present the first highly efficient photochromic silole-containing dithienylethene with excellent thermal stability and fatigue resistance. The photochemical quantum yields for photocyclization and photocycloreversion of the compound are found to be high and comparable to each other; the latter of which is rarely found in diarylethene compounds. These would give rise to highly efficient photoswitchable material with effective writing and erasure processes. Incorporation of the silole moiety as a photochromic dithienylethene backbone also was demonstrated to enhance the thermal stability of the closed form, in which the thermal backward reaction to the open form was found to be negligible even at 100 °C, which leads to a promising candidate for use as photoswitchable materials and optical memory storage.

Synthesis of 3′-allylindoline spirobenzopyrans derived from 3-allyl-3H-indoles

In this study, 8 new spirobenzopyrans were synthesized. A novel, three-step, facile route for the synthesis of 3′-allylindoline spirobenzopyrans via 3-allyl-3H-indoles was developed. The newly synthesized spirobenzopyrans were evaluated for their photochromic properties. The presence of an allyl moiety at the 3′ position did not disturb the photochromic response. The steric effects of the diallyl groups at the 3′ position affected the interconversion between colored and colorless forms. Therefore, the allyl chain in 3′-allylindoline spirobenzopyrans can be utilized to attach these compounds to a molecular matrix. Consequently, this synthetic methodology could be readily applied to the creation of new photo-switchable materials.

New azobenzene-based chiral-photochromic substances with thermally stable Z-isomers and their use for the induction of a cholesteric mesophase with a phototunable helix pitch

Novel photosensitive azobenzene-containing substances with sorbitol- and lactate-based chiral fragments were synthesized to serve as chiral-photochromic dopants in cholesteric mixtures. Owing to the presence of chlorine or methyl lateral substituents in the azobenzene fragment of the dopants, the photoinduced Z-form of their molecules is highly stable. The photooptical properties of the prepared cholesteric mixtures were studied and it was shown that UV-irradiation of the planarly-oriented films results in a shift of the selective light reflection wavelength to a long-wavelength spectral region due to E–Z isomerization of the azobenzene moieties. This process is thermally and photooptically reversible and the rate of blue-light-induced helix twisting is comparable with the rate of the untwisting process, whereas the thermal relaxation completes only after ca. 40 days at room temperature (for a chlorine-substituted chiral-photochromic dopant). It was found that this value is almost two orders of magnitude less in comparison with the non-substituted chiral-photochromic dopant also studied in this work. The obtained results demonstrate the promising properties of the synthesized compounds for creation of novel photoswitchable cholesteric materials.

Femtosecond spectroscopy reveals huge differences in the photoisomerisation dynamics between azobenzenes linked to polymers and azobenzenes in solution.

Femtosecond fluorescence up-conversion spectroscopy of two azobenzenes covalently attached to the side chain or linked by covalent bonds at each end into the main chain of polybutylmethacrylate polymer colloids with different cross-linking ratios reveals dramatic differences in the excited-state dynamics compared to the monomer chromophores in solution due to strong mechanical forces in the complex micronetworks. For the azobenzene derivative DR1 in the polymer side chain, the measurements determined an increase of the mean excited-state lifetime after irradiation at λ = 475 nm to 〈τ〉 = 5.5 ps from 〈τ〉 = 0.5 ps for the monomer. For the cross-linked BAAB in the polymer main chain, an increase of 〈τ〉 was found of more than a factor-of-20. Moreover, with a lifetime of τ = 430 ps, ≈12% of the molecules in the tightly (1 : 10) cross-linked polymer were found to remain in the excited state about 100 times longer than observed for the monomer chromophore. These results are of high relevance for applications of photoswitchable polymer materials.

A Dual-Modality Photoswitchable Supramolecular Polymer

Covalent or noncovalent linked polymers with stimuli-responsive properties have been well researched as a kind of advanced functional materials. However, little effort has been devoted to establishing a bridge for switching between covalent polymers and noncovalent polymers. Actually, such unitive system is promising because it can combine their chemical advantages of two types of polymers in a single and tunable platform. Herein, by taking advantage of reversible photodimerization of coumarins and host-guest assemblies with γ-cyclodextrin (γ-CD), we demonstrate a simple and effective way to construct a dual-modality supramolecular polymer, which can be switched between a noncovalent polymer and its corresponding covalent polymer in response to light stimuli. Moreover, this unique switchable polymer can also be employed to construct a dual-stimuli responsive supramolecular hydrogel with the surfactant cetyl trimethylammonium bromide (CTAB). This methodology establishes a bridge between the two "polymer mansions" and is promising to open a new class of photoswitchable materials.

Fast and fully reversible photochromic performance of hybrid sol–gel films doped with a fused-naphthopyran

A new type of photochromic naphthopyran with a fused structure was embedded in an organic–inorganic amine–alcohol–silicate hybrid gel producing colourless, transparent and flexible photochromic films. Under UV-vis light these materials developed, in less than 30 s, intense yellow-orange colours that faded completely in the dark in 3 min. This behaviour contrasts with the usual performance of common naphthopyrans that always generate two coloured species which fade with different kinetics, leading to the formation of a residual colour that persists for several minutes/hours. The structural design of this new naphthopyran prevents the formation of the long-lived coloured species and therefore fully reversible photo-switchable materials can be easily obtained.

W-Band Time-Resolved Electron Paramagnetic Resonance Study of Light-Induced Spin Dynamics in Copper–Nitroxide-Based Switchable Molecular Magnets

Molecular magnets Cu(hfac)(2)L(R) represent a new type of photoswitchable materials based on exchange-coupled clusters of copper(II) with stable nitroxide radicals. It was found recently that the photoinduced spin state of these compounds is metastable on the time scale of hours at cryogenic temperatures, similar to the light-induced excited spin state trapping phenomenon well-known for many spin-crossover compounds. Our previous studies have shown that electron paramagnetic resonance (EPR) in continuous wave (CW) mode allows for studying the light-induced spin state conversion and relaxation in the Cu(hfac)(2)L(R) family. However, light-induced spin dynamics in these compounds has not been studied on the sub-second time scale so far. In this work we report the first time-resolved (TR) EPR study of light-induced spin state switching and relaxation in Cu(hfac)(2)L(R) with nanosecond temporal resolution. To enhance spectral resolution we used high-frequency TR EPR at W-band (94 GHz). We first discuss the peculiarities of applying TR EPR to the solid-phase compounds Cu(hfac)(2)L(R) at low (liquid helium) temperatures and approaches developed for photoswitching/relaxation studies. Then we analyze the kinetics of the excited spin state at T = 5-21 K. It has been found that the photoinduced spin state is formed at time delays shorter than 100 ns. It has also been found that the observed relaxation of the excited state is exponential on the nanosecond time scale, with the decay rate depending linearly on temperature. We propose and discuss possible mechanisms of these processes and correlate them with previously obtained CW EPR data.

Photochromic Alkynes as Versatile Building Blocks for Metal Alkynyl Systems: Design, Synthesis, and Photochromic Studies of Diarylethene-Containing Platinum(II) Phosphine Alkynyl Complexes

The synthesis of newly designed photochromic dithienylethene-containing ethynylthiophene and ethynylthieno[3,2-b]thiophene has been described, and their incorporation as versatile ligands into the platinum(II) phosphine system was demonstrated. All platinum(II) complexes have been successfully characterized by (1)H and (31)P NMR spectroscopies, positive fast atom bombardment (FAB) mass spectrometry, as well as elemental analysis. One of the complexes has been characterized by X-ray crystallography. Their photophysical, photochromic, and electrochemical properties have been studied. Upon photoexcitation, all the photochromic diarylethene-containing alkynes and platinum(II) complexes exhibited reversible photochromism. The thermal bleaching kinetic of complex 6 was studied in toluene at 298 and 313 K. Complexes 1, 3, and 4, which contained the labile chloro- ligand, represent a new class of versatile building blocks for photoswitchable functional materials.

The research front on molecular functional switches

The 4 International Workshop on Organic Photoswitchable Multifunctional Molecules and Materials was held between 25 and 27 October 2009 in East China University of Science & Technology (ECUST), Shanghai. It was a great event for academic circles, and at the same time provided an excellent chance for communication between international scholars from the field of organic photoswitchable functional materials. Along with a successful series of workshops on this topic, such as those held by Japan-France (Shonan, 2006), France-Russia (Saissac, 2007) and France-Japan (Arras, 2008), ECUST (China) and CNRS's GDRI (France) joined together to host this continued workshop on novel photoswitchable multifunctional molecules and materials. This workshop covered all aspects from molecular design to devices using photo-switchable materials, and was intended as a forum for further interdisciplinary discussion and international cooperation. The workshop was presided over by Prof. Yu Pei and Prof. He Tian. During the workshop, more than 50 experts and 50 Ph.D. candidates from Canada, Germany, Italy, Hong Kong, France, Japan, South Korea, Sweden, Singapore, the USA and China presented and discussed the latest research findings, interpretations and ideas relating to organic photoswitchable multifunctional materials. Many famous professors came to the gathering, with the aim of better promoting the development of advanced materials and also mutual understanding. Following the workshop, we have received 17 high quality papers from the scholars for a special issue of Frontiers of Chemistry in China. This issue provides a unique opportunity for scientists in this area to exchange ideas and share in the achievements in the field of organic functional materials, mainly focussing on organic chemosensors and photochromic molecules. As we know, the most convenient way to introduce a photoswitching function into a molecular system is to use photochromic units as photo-functional units. A particularly interesting photochromic unit is diarylethene, which reversibly changes its π-conjugation and structure upon irradiation. In this workshop, many lectures were given on photochromic diarylethenes. Therefore, taking into consideration the requirements of most international participants, we decided to launch another special issue on the topic of photochromic molecules, namelyDyes and Pigments (Guest Editors: Prof. Neil Branda and Prof. He Tian), which will include papers mostly from foreign scientists. Of course, this issue in Frontiers of Chemistry in China should reflect some of the progress in the photochromic field in China. Two research articles by Prof. Yi Chen et al. and Prof. Shou-Zhi Pu et al. report new photochromic diarylethene compounds for holographic optical recording and two-photon absorption, respectively. Owing to their direct toxic effects on human beings, animals and plants, chemical warfare agents (CWAs) and their mimics have become widespread in agriculture and chemical warfare. A short review by Prof. Chun-Hua Yan and his coworkers contains recent reports on the design of fluorescent molecular switches and their advantages in the detection of CWAs. It is believed that extensive interest in this field has accelerated the development of novel fluorescent molecular switches and detection techniques. Seven research articles (from the groups of Yun-Bao Jiang, Xiao-Jun Peng, JianZhang Zhao, Wei-Hong Zhu, Jian-Li Hua, Juan Xie and Yong-Shu Xie) show how to construct new organic fluorescent sensing molecules with highly selectivity and sensitivity and even with a new signal-transduction mechanism. It is interesting to note that a new switching molecule based on ferrocene can be controlled and detected by an aggregation-induced emission (AIE) phenomenon (research article by Dr. Dong Zhang). For organic compounds, switching at the molecular level is important in materials for sensing, separation, and drug delivery. In response to changes in temperature, concentration, media, light, time, and the presence or absence of other ions or molecules, organic compounds can aggregate or deaggregate, which triggers various chemical, physical, and biological phenomena. Prof. Shu Wang and his coworkers prepared a water-soluble conjugated polyfluorene with He TIAN Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, China E-mail: tianhe@ecust.edu.cn Front. Chem. China 2010, 5(2): 113–114 DOI 10.1007/s11458-010-0100-1

X‐ray Absorption Spectroscopy as a Probe of Photo‐ and Thermally Induced Valence Tautomeric Transition in a 1:1 Cobalt–Dioxolene Complex

The role of the cobalt ion in the entropy- and optically-driven valence tautomeric (VT) interconversion exhibited by the [Co(Me(2)tpa)(DTBdiox)](PF(6))C(6)H(5)CH(3) complex (Me(2)tpa = bis (6-methyl-(2-pyridylmethyl))(2-pyridylmethyl)amine, DBdiox = 3,5-ditertbutyl-dioxolene) is established by means of X-ray absorption spectroscopy (XAS). Analysis of the pre-edge features at 6 and 300 K in the Co K-edge XAS spectra using a ligand field multiplet approach allows us to obtain detailed information on the electronic structures of the metal ion in the two redox isomers. The temperature dependence of the spectra confirms the occurrence of a thermally induced VT transition and suggests that nucleation and distortion of the phase boundaries take place during the process. Moreover, optically induced metastable state formation is monitored at low temperatures--with a high degree of reproducibility--without changing the position of measurement on the sample during the experiment. This result paves the way for the use of such a highly sensitive technique for the investigation of photoswitchable materials in non-crystalline and nanostructured environments.

New photochromic xerogels composites based on nitrosyl complexes

New photoswitchable hybrid materials based on mononitrosyl complexes with excellent optical properties have been obtained by sol–gel process. Inclusion in silica matrix prepared from tetramethoxysilane precursor leads to new materials in which the ruthenium complex [RuCl(NO)py4](PF6)2·1/2H2O (py = pyridine) is stabilized as crystalline nanoparticles with diameters between 2 and 15 nm. Photochromic properties are maintained and have been evidenced by infrared spectroscopy under irradiation (λ = 473 nm) at low temperature (T = 100 K). The reversible transfer from the ground state (GS) to the metastable state (MS1) is about 40% in the composite, which is close to the value observed on the most studied sodium nitroprusside (50% on pure material).

Ordering phenomena and photoorientation processes in photochromic thin films of LC chiral azobenzene-containing polymer systems

The processes of mesogen ordering in nematic and cholesteric side-chain polymer systems were studied by polarized optical spectroscopy. For this purpose, nematic azobenzene-containing copolyacrylate was synthesized and its cholesteric mixture with chiral dopant was prepared. Thin films of copolymer and mixture were prepared by spin-coating using polyimide-coated and rubbed glass and quartz substrates. Film thickness was comparable or less than helix pitch value. It was found that azobenzene and phenylbenzoate side groups of nematic copolymer are oriented after annealing along a rubbing direction. Cholesteric mixture demonstrates a more complicated behaviour, the direction of orientation of the side-chain groups is rotated at certain angle depending on the film thickness. Dichroism in azobenzene groups’ absorbance is lower for cholesteric mixture and decreases with film thickness increasing. A possible mechanism of ordering of the side-chain groups in these thin films was proposed based on the helical order formation. Photooptical switching of the orientation and an influence of the nonpolarized and polarized light on the cholesteric thin films were studied demonstrating a possibility of using such thin films of photochromic cholesterics as promising photoswitchable materials.

Photoisomerisation in single molecules of nitroprusside embedded in mesopores of xerogels

Photoswitchable hybrid materials are successfully prepared by embedding guanidinium nitroprussides (GuNP, (CN3H6)2[Fe(CN)5NO]) into mesopores of transparent xerogel monoliths. The such prepared hybrid materials exhibit a higher photostability than the corresponding GuNP solutions, whereby the chemical stability of the [Fe(CN)5NO]2−-anion in titania gel is nearly infinite. By irradiation with light in the blue-green spectral range one nitrosyl isomer is formed by a 180° rotation of the NO ligand changing the Fe–NO into a Fe–ON coordination (SI), which is detected by the shift of the ν(NO) stretching vibration from 1945 cm−1 (Fe–NO) to 1820 cm−1 (Fe–ON). Consequently there is enough space around the NO-ligand for such movement in xerogel mesopores. The embedding in silica xerogels increases the achievable population of the isomeric nitrosyl configuration to about 15% with respect to a single crystalline powder where only 9% are reached.

Reversible Photoswitching of Ferromagnetic FePt Nanoparticles at Room Temperature

There has been a great interest in developing photoswitchable magnetic materials because of their possible applications for future high-density information storage media. In fact, however, the examples reported so far did not show ferromagnetic behavior at room temperature. From the viewpoint of their practical application to magnetic recording systems, the ability to fix their magnetic moments such that they still exhibit room-temperature ferromagnetism is an absolute requirement. Here, we have designed reversible photoswitchable ferromagnetic FePt nanoparticles whose surfaces were coated with azobenzene-derivatized ligands. On the surfaces of core particles, reversible photoisomerization of azobenzene in the solid state was realized by using spacer ligands that provide sufficient free volume. These photoisomerizations brought about changes in the electrostatic field around the core-FePt nanoparticles. As a result, we have succeeded in controlling the magnetic properties of these ferromagnetic composite nanoparticles by alternating the photoillumination in the solid state at room temperature.

Photocontrolled Magnetization of CdS-Modified Prussian Blue Nanoparticles

The first photocontrollable magnetic nanoparticles containing CdS and Prussian blue (PB) have been created using reverse micelles as nanoreactors. Photoinduced electron transfer from CdS to PB in the reverse micelle changed the magnetic properties of the composite nanoparticles from ferromagnetic to paramagnetic. The magnetization in the ferromagnetic region below 4 K was substantially decreased after UV light illumination and could be restored almost to its original level by thermal treatment at room temperature. This novel strategy of designing composite nanoparticles containing photoconductive semiconductors and magnetic materials to create photoswitchable magnetic materials may open many possibilities in the development of magneto-optical devices.

Mechanistic studies on the photochromism of [e]-annelated dimethyldihydropyrenes.

The photochemistry of several photochromic arene [e]-annelated dimethyldihydropyrenes (DHPs) was studied. These compounds have much larger photochemical ring opening quantum yields than the simple DHPs, making them potentially more useful as building blocks for photoswitchable materials.

Synthesis, photophysics and photochemistry of novel luminescent rhenium(I) photoswitchable materials

A series of new mono- and bi-nuclear rhenium(I) complexes, [Re(CO)3(N–N)L]ClO4 and [{Re(CO)3(N–N)}2L′][ClO4]2[where N–N = 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) and L = 4-phenylazopyridine (phazo), 4-styrylpyridine (stypy), 4-pyridyl-2-ethylbenzene (NB); L′= 4,4′-azopyridine (azo), 1,4-bis(4-pyridyl-2-ethyl)benzene (NBN)] are prepared and shown to exhibit rich photophysical and photochemical behaviour; the X-ray crystal structure of a binuclear [{Re(CO)3(bpy)}2(NBN)][PF6]2 complex is determined.