Cis Photoisomerization Research Articles

Photochemical liquefaction and softening in molecular materials, polymers, and related compounds

Recent developments in materials showing photo-induced liquefaction and softening, including molecular materials, gels, and polymers, are described. A macrocyclic azobenzene molecule that has long alkoxy chains was first reported to show a photo-induced solid–liquid phase transition at room temperature through trans–cis photoisomerization. The structure–property relationship for understanding the photo-induced phase transition has since been investigated by using model compounds with a simple molecular design. Such azobenzene-based materials also exhibited dynamic motion on glass and water surfaces; these motions are driven by continuous liquefaction/crystallization and dissolution of the liquefied material. Photo-induced liquefaction has been further developed in middle molecular and polymeric materials. The effects of the alkyl chain length, functional groups, and number of azobenzene units on the photo-induced behavior have been extensively investigated. In addition, by using photoresponsive gelators, the photo-induced quasi-solid–liquid (gel–sol) phase transition of gels was accomplished. Moreover, very recently, a photoresponsive plasticizer that can plasticize “photo-inactive” polymers by light irradiation has been proposed. This review focuses on the recent developments on the photo-induced liquefaction and softening of azobenzene-based materials. The trans–cis photoisomerization of azobenzene units incorporated in molecular materials, polymers, and gels induces a variety of state changes such as solid–liquid, gel–sol, and glass–rubber transitions of those materials isothermally. Several novel applications such as reusable and environmentally friendly photoresists, reworkable adhesives, and self-healing materials can be expected using the photochemical liquefaction and softening.

Photoluminescence emission enhancement and coordination-driven isomer-locking effects of Cr(III) on azobenzene-bearing tweezer anchored to binaphthdiaza-crown macrocycles

In this report, the synthesis of a new bi-macrocycle tweezer linked through an azobenzene moiety, L was undertaken. The characterization of the tweezer was accomplished by employing IR, 1H and 13C NMR, UV–vis, fluorescence spectroscopy and CHN microanalysis while the UV–visible spectroscopy illustrated a trans ⇆ cis photo-isomerization for L. According to our preliminary fluorescence observations, a metal-ion chemosensing behavior was established for trans-L isomer towards Cr(III) ion among the library metal ions. The quenching of the respective trans ⇆ cis isomerization in the presence of Cr(III) ion suggested the preference of a strong interaction between Cr(III) ion with L, leading to isomer-locking of the trans-L isomer. Since the Job plots for L in the presence of Cr(III) metal ion suggested a 1:2 ratios for the interaction of Cr(III) with L, an electronic effect of Cr(III) ion with the phenolic groups on L was suggested as the main factor for the observed isomer-locking process in trans-L. The binding constant and the detection limit for Cr(III) ion by L were also assessed as 1.55 × 108 M−1 and 10−8 M, respectively, suggesting L as a new chemosensor for Cr(III) metal ion.

Photochemical Properties of Supramolecular Dyad with Pyrenylethenylquinoline as a Photochrome

Spectral and photochemical properties of a bichromophoric dyad with two photochromes, 2-[(2-(pyren-1-yl)ethenyl]quinoline (PEQ) bridged by ten methylene groups have been investigated. Stationary spectroscopy does not show substantial interaction between the PEQ groups in the dyad, but the appearance of another, more long-living component in the f luorescence decay kinetics indicates possible excimer formation. The formation of the chemically active excimer is also confirmed by the intramolecular [2+2] photocycloaddition reaction to give tetrasubstituted cyclobutane with two vicinal pyrene substituents. This reaction competes with trans–cis photoisomerization of the dyad. It has been proposed that the π-stacking interaction of two PEQ groups with a long system of π-bonds plays an important role in the excimer formation followed by the photocycloaddition reaction.

Synthesis, Structural Characterization, and Luminescence Switching of Diarylethene-Conjugated Ru(II)-Terpyridine Complexes by trans-cis Photoisomerization: Experimental and DFT/TD-DFT Investigation.

We synthesized and thoroughly characterized a new family of diarylethene-conjugated mononuclear Ru(II)-terpyridine complexes and investigated in detail their photophysical, electrochemical, and spectroelectrochemical behaviors. Interestingly, the compounds show moderately strong room-temperature luminescence predominantly from their 3MLCT state with luminescence lifetime varying between 8.43 and 22.82 ns. Because of the presence of diarylethene unit, all the monometallic complexes underwent trans-to-cis photoisomerization upon interaction with UV light with substantial changes in their absorption and luminescence spectra. Reverting back from the cis to the trans form is also made possible upon treatment with visible light or by heat. Trans-to-cis isomerization leads to almost complete quenching of luminescence, while backward cis-to-trans isomerization gives rise to restoration of the original luminescence for all the complexes. Thus, "on-off" and "off-on" emission switching was made possible upon successive interaction of the complexes with UV and visible light. Computational investigation involving density functional theory (DFT) and time-dependent DFT methods was done for proper assignment of the experimental absorption and emission spectral bands in the complexes. Finally, experimentally observed trend on the absorption and emission spectral behaviors of the complexes upon photoisomerization was also compared with the calculated results.

Photoswitching Behavior of 5-Phenylazopyrimidines: In Situ Irradiation NMR and Optical Spectroscopy Combined with Theoretical Methods.

The photoswitching behavior of 5-phenylazopyrimidines was investigated by optical methods and NMR spectroscopy with in situ irradiation sustained by mathematical modeling and DFT calculations. Irradiation of various compounds with electron-donating groups on the pyrimidine ring and substituents with electron-withdrawing as well as electron-donating substituent in the para-position of the phenyl ring were examined. All compounds could be successfully converted to the cis isomer; this isomerization and the subsequent thermal fading were studied. Switching cycles can be repeated without signs of photodegradation for most of the compounds, which makes them adept to molecular photoswitches. Interestingly, the chloro and cyano derivatives can be switched without UV light, which makes them vis(π → π*)-vis(n → π*) photoswitches. Surprisingly equal trans-to- cis photoisomerization quantum yields for π → π* and n → π* excitation indicate the blocking of the inversion pathway following π → π* excitation. In contrast to that, DFT computations suggest the inversion mechanism for the reverse thermal cis-to- trans isomerization of 5-phenylazopyrimidines.

Reaction‐Induced Phase Separation: A Strategy to Synthesize Azobenzene‐Modified All‐Optical PDLC Devices

AbstractThis work presents the synthesis of all‐optical polymer dispersed liquid crystals (PDLC) devices, in which the reversible trans–cis photoisomerization of azobenzene groups is used to induce the transition in the liquid crystal (LC) phase, activating the film from an opaque to a translucent state. The appropriate morphology is obtained through a reaction‐induced phase separation strategy using an epoxy–amine matrix modified with azobenzene precursors and the addition of a thermoplastic polymer (polystyrene, PS), which promotes the phase separation and the preferential location of the LC in separated domains. Formulations with liquid crystal content of 50 wt%, 5 wt% of modifier (PS), and 10 wt% of azo‐precursor present a reversible on–off optical response when irradiating the sample with the activation beam at λ = 488 nm. It is demonstrated that the liquid crystal responds according to the expected cooperative movement of orientation with the azobenzene groups. Based on the study of the PDLC devices obtained, its potential technological application can be confirmed.

Trans–cis isomerization energies of azopyridines: a calorimetric and computational study

Azobenzenes undergo reversible trans–cis photo-isomerization and have been studied extensively as photo-responsive materials. Despite their similar photochemistry, azopyridines have received relatively little attention; for example, their isomerization energies are presently unknown. In comparison with azobenzenes, azopyridines offer additional opportunities for materials design through hydrogen bonding and coordination chemistry. Here we report the isomerization energies for all three symmetrical azopyridines (i.e., the 2,2′-, 3,3′-, and 4,4′-isomers) through a combined experimental and computational study. Heat of isomerization was measured in the liquid state, with o-terphenyl introduced to suppress crystallization. We obtain ∆Eiso = 25.2 ± 0.6, 42.6 ± 0.6, and 35.0 ± 1.8 kJ mol−1 for 2,2′, 3,3′, and 4,4′-azopyridine, respectively. For azobenzene, we obtain ∆Eiso = 47.0 ± 1.3 kJ mol−1, in agreement with the literature value and validating our method. Theoretical calculations yielded gas-phase ∆Eiso in reasonable agreement with experiment and explain the low isomerization energy of 2,2′-azopyridine on the basis of a low-energy cis conformer. Because of the smaller van der Waals volume of the pyridine N relative to the phenyl CH, the two aromatic rings in the cis isomer can approach closer to coplanarity, leading to greater π-conjugation and lower conformational energy.

Small biomolecule dopant retinals: Electron blocking layer in P3HT:PCBM type organic solar cells

We present a comparative study of the photophysics and electron/hole properties of all-trans retinal-benzimidazole type molecules decorated with different electronic moieties (such as –OCH3, –N(CH3)2, –F, –CF3) in organic photovoltaic (OPV) devices in solution and on solid thin films. Steady-state spectra of synthesized dyes give large Stokes shifts (6887–13152 cm−1) in studied solvents. Decay times of these dyes were found to be substituent dependent giving a bi-exponential decay for fluorine containing retinals. Trans to cis photoisomerization rate constants of synthesized dyes were found to be about 3.3–16.4 × 10−6 s−1. Using a cyclic voltammetry measurements, HOMO and LUMO energy levels of fluorine-substituted dyes shift to lower values as compared to that of unfluorinated derivatives. We compared unusual electron blocking behavior of methoxy- and N,N-dimethylamino-substituted derivatives (Ret-I and Ret-II, respectively) in bulk heterojunction solar cells (BHJ-SCs) incorporating an active layer of P3HT:PCBM doped with Ret derivatives at various weight ratios. Hole mobility values for fluorine containing retinals were found to be about 1.0 × 10−4 and 7.1 × 10−4 cm2/V s for Ret-III and Ret-IV dyes, respectively.

Shape-memory and self-healing functions of DNA-based carboxymethyl cellulose hydrogels driven by chemical or light triggers.

Photoresponsive nucleic acid-based carboxymethyl cellulose (CMC) hydrogels are synthesized, and their application as shape-memory and self-healing functional matrices are discussed. One system involves the preparation of a carboxymethyl cellulose hydrogel crosslinked by self-complementary nucleic acid duplexes and by photoresponsive trans-azobenzene/β-cyclodextrin (β-CD) supramolecular complexes. Photoisomerization of the trans-azobenzene to the cis-azobenzene results in a hydrogel exhibiting lower stiffness due to the separation of the azobenzene/β-CD bridging units. The hydrogel is switched between high and low stiffness states by the cyclic and reversible light-induced isomerization of the azobenzene units between the trans and cis states. The light-controlled stiffness properties of the hydrogel are used to develop a shape-memory hydrogel, where the duplex bridging units act as permanent memory in the quasi-liquid shapeless state of the hydrogel. A second system in the study is a carboxymethyl cellulose hydrogel crosslinked by the K+-stabilized G-quadruplex bridging units and by trans-azobenzene/β-CD complexes. The resulting hydrogel includes dual-trigger functionalities, where the trans-azobenzene/β-CD complexes can be reversibly formed and dissociated through the trans and cis photoisomerization of the azobenzene units, and the K+-stabilized G-quadruplexes can be reversibly dissociated and reformed in the presence of 18-crown-6-ether/K+-ions. The signal-responsive crosslinked hydrogel reveals controlled stiffness properties, where the hydrogel crosslinked by the trans-azobenzene/β-CD and K+-ion-stabilized G-quadruplex reveals high stiffness and the hydrogel crosslinked only by the K+-ion-stabilized G-quadruplexes or only by the trans-azobenzene/β-CD complexes reveals low stiffness properties. The controlled stiffness properties of the hydrogel are used to develop shape-memory hydrogels, where the trans-azobenzene/β-CD complexes or the K+-ion-stabilized G-quadruplexes act as permanent memories in the shapeless and quasi-liquid states of the hydrogels. In addition, the hydrogel that includes two types of stimuli-responsive crosslinking units is used as a self-healing matrix, where each of the triggers guides the self-healing processes.

Photoinduced Pedalo‐Type Motion in an Azodicarboxamide‐Based Molecular Switch

AbstractWell‐defined structural changes of molecular units that can be triggered by light are crucial for the development of photoactive functional materials. Herein, we report on a novel switch that has azodicarboxamide as its photo‐triggerable element. Time‐resolved UV‐pump/IR probe spectroscopy in combination with quantum‐chemical calculations shows that the azodicarboxamide functionality, in contrast to other azo‐based chromophores, does not undergo trans–cis photoisomerization. Instead, a photoinduced pedalo‐type motion occurs, which because of its volume‐conserving properties enables the design of functional molecular systems with controllable motion in a confined space.

Photoinduced Pedalo-Type Motion in an Azodicarboxamide-Based Molecular Switch.

Well‐defined structural changes of molecular units that can be triggered by light are crucial for the development of photoactive functional materials. Herein, we report on a novel switch that has azodicarboxamide as its photo‐triggerable element. Time‐resolved UV‐pump/IR probe spectroscopy in combination with quantum‐chemical calculations shows that the azodicarboxamide functionality, in contrast to other azo‐based chromophores, does not undergo trans–cis photoisomerization. Instead, a photoinduced pedalo‐type motion occurs, which because of its volume‐conserving properties enables the design of functional molecular systems with controllable motion in a confined space.

Solvent dependent trans → cis photoisomerization of N, N′-diacetylindigo studied by femtosecond time-resolved transient absorption spectroscopy

Solvent dependence of trans→cis photoisomerization of N,N′-diacetylindigo (DAI) was investigated by femtosecond transient absorption (TA) spectroscopy. A strong and sharp singlet excited state absorption band centered at 595nm was observed in the TA spectrum and the differential absorbance (ΔAbs) and the wavelength at the maximum of the band were periodically modulated by coherent vibrational wavepacket motions. However, even in the “reactive solvent”, ethyl acetate (EtAc), where the isomerization takes place, the formation of cis-DAI was unobservable in our experimental time window of 2ns which means that isomerization is occurring in a longer time scale. Preceding studies suggest that intermolecular hydrogen-bond (H-bond) with the solvent quenches the excited state and blocks the isomerization. Interestingly, we have observed that dimethylformamide (DMF) blocked the isomerization more efficiently than methanol. The excited state lifetime was shorter in DMF compared to EtAc solution suggesting that weak H-bonding with the formyl group of DMF is sufficient to prevent the slow trans→cis photoisomerization of DAI.

Photoswitchable Arylazopyrazole-Based Ruthenium(II) Arene Complexes

A new family of donor-functionalized photoswitchable arylazopyrazole-based ligands (3–5) was synthesized and characterized. The new ligands have been employed to prepare a series of novel photoswitchable half-sandwich ruthenium(II) cymene complexes of the type [(η6-p-cymene)Ru(L)Cl]+ (L = 1-(2-methylenepyridyl)-4-(phenyldiazenyl)-3,5-dimethyl-1H-pyrazole (6a), 1-(2-methylenepyridyl)-4-((4-bromophenyl)diazenyl)-3,5-dimethyl-1H-pyrazole (6b), 1-(2-benzothiazolyl)-3,5-dimethyl-4-arylazopyrazole (7)). All of the complexes have been fully characterized by 1H NMR, 13C NMR, and UV–vis spectroscopy and elemental analyses. In addition, the structure of complex 6a was determined by X-ray crystallography. The UV–vis spectroscopic studies show that both the ligands and metal complexes exhibit excellent trans to cis photoisomerization of the arylazopyrazole moiety upon irradiation with 365 nm UV light. The cis isomer of the compounds can be switched back nearly quantitatively to the more stable trans form with 530 nm ...

Isothermal Chirality Switching in Liquid‐Crystalline Azobenzene Compounds with Non‐Polarized Light

AbstractSpontaneous mirror‐symmetry breaking is a fundamental process for development of chirality in natural and in artificial self‐assembled systems. A series of triple chain azobenzene based rod‐like compounds is investigated that show mirror‐symmetry breaking in an isotropic liquid occurring adjacent to a lamellar LC phase. The transition between the lamellar phase and the symmetry‐broken liquid is affected bytrans–cisphotoisomerization, which allows a fast and reversible photoinduced switching between chiral and achiral states with non‐polarized light.

Spectral and photochemical properties of hybrid organic\u2013inorganic nanosystems based on CdS quantum dots and merocyanine ligands

The spectral and photochemical properties of hybrid organic–inorganic nanosystems (HNSs) were investigated. HNS consisted of CdS quantum dots (QDs) functionalized with ligands containing the isothiouronium anchor group linked by a polymethylene chain with photochromic merocyanine (MC). The HNS synthesis was carried out via a microwave-assisted one-pot technique. Energy transfer from the QDs to MC in the HNS was observed and resulted in QD fluorescence quenching and MC sensitization. Compared to the free MC, trans–cis photoisomerization of MC in the HNS was suppressed and its photodestruction was accelerated. In addition, upon HNS photolysis by visible light with energy higher than the threshold, the photosensitized destruction of the QDs (which did not absorb the applied light) occurred. The observed effects were proposed to be caused by MC adsorption on QDs surface, which leads to the restriction of the MC photoisomerization and population of the surface electron trap states of the QDs.

Photonics of 18-crown-6-containing styryl dye and its complexes with metal cations

Using absorption, luminescence, 1H NMR, and laser kinetic spectroscopies, the photophysical processes and photochemical reactions of 4-[(E)-2-(2,3,5,6,8,9,11,12,14,15-decahydro-1,4,7,10,13,16-benzohexaoxacyclooctadecin-18-yl)vinyl]-1-ethylpyridinium perchlorate and its complexes with lithium, sodium, potassium, calcium, barium, silver, and lead perchlorates in MeCN have been studied. The styryl dye and its complexes with metal cations are capable of emitting normal (prompt) and delayed fluorescence and enter into the trans–cis photoisomerization reaction. The dye molecules in the triplet state participate in the processes of degradation of electronic excitation energy. Triplet–triplet absorption is observed only in the presence of lead cations with a maximum at 470 nm and the deactivation rate constant of the triplet state k = 2.5 × 104 s–1 in a deoxygenated solution.

The novel photoresponsive oligomers containing azo derivatives of sulfamerazine for spontaneous surface relief grating inscription

In this work, a novel photoresponsive materials has been successfully developed. We have synthesized the series of azopolymers containing derivatives of sulfamerazine and studied their photochromic properties. The polymers were obtained from methacrylate azomonomers, butyl methacrylate and isobornyl methacrylate by radical polymerization. Spectral properties and photoisomerization kinetic constants were determined for thin transparent films, using UV–Vis spectroscopy. The maximum absorption of the films was observed at 432–440nm. The obtained materials showed ability to reversible trans–cis photoisomerization. Reversibility of the process was confirmed during ellipsometric measurements. The change of the real part of the complex refractive index induced by laser irradiation was between 0.010 and 0.053. Additionally, some properties of described polymers and azobenzene derivatives were calculated and were utilized in order to better understand the differences in the photochromic behaviour of the new materials. Moreover, we determined the suitability of the obtained azopolymers for spontaneous surface relief grating inscription. Spontaneous surface relief grating recording on the thin azopolymer films was carried out using set-up with one laser beam. The spontaneous SRG was successfully recorded in all investigated polymers.

Reversible Stereoselective Folding/Unfolding Fueled by the Interplay of Photoisomerism and Hydrogen Bonding.

A linear molecular architecture equipped with complementary three-fold hydrogen-bonding units embedded with a photoswitchable trans-tetrafluoroazobenzene moiety was synthesized. The transto cis photoisomerism of the azobenzene unit induced drastic changes in the molecular architecture as a result of intramolecular hydrogen bonding as evidenced by NMR spectroscopy and size exclusion chromatography. A minute stereogenic element in the linear trans state enabled stereoselective folding into the cis state, thus producing a globular architecture with enhanced chiroptical property.

Reversible Stereoselective Folding/Unfolding Fueled by the Interplay of Photoisomerism and Hydrogen Bonding

AbstractA linear molecular architecture equipped with complementary three‐fold hydrogen‐bonding units embedded with a photoswitchable trans‐tetrafluoroazobenzene moiety was synthesized. The transto cis photoisomerism of the azobenzene unit induced drastic changes in the molecular architecture as a result of intramolecular hydrogen bonding as evidenced by NMR spectroscopy and size exclusion chromatography. A minute stereogenic element in the linear trans state enabled stereoselective folding into the cis state, thus producing a globular architecture with enhanced chiroptical property.

Reversible trans⇌cis photoisomerizations of [Re(CO)3(ph2phen)(stpyCN)]+ towards molecular machines.

In our search for light powered molecular devices, a novel fac-[Re(CO)3(ph2phen)(trans-stpyCN)]+ complex was synthesized to show switchable trans-cis configurations of the coordinated stpyCN ligand through efficient and reversible photoassisted isomerizations. Controlled photolyses of acetonitrile solutions led to spectral changes ascribed to reversible trans⇌cis photoisomerization processes. A remarkable quantum yield for the back cis-to-trans isomerization was obtained, with the same magnitude of the trans-to-cis photoprocess, and curiously, both trans- and cis-isomers are emissive at room temperature. Photochemical and photophysical characterization studies for fac-[Re(CO)3(ph2phen)(stpyCN)]+ provided insights into the isomerization and emissive light-driven pathways that are resulted from the interactions between the close-lying intraligand and charge transfer states. The reversible cis-trans photoisomerization has potential to be exploited as light-powered molecular motors and geometry regulators in molecular machines.