Photoinduced Cis Research Articles

Photopyroelectric Investigation of the trans–cis Isomerization Effect on the Nematic- Isotropic Phase Transition of a Liquid Crystalline Azobenzene

Photopyroelectric calorimetry has been applied to the study of the effects of different concentration of photo-induced cis isomeric molecules on the characteristics of the nematic – isotropic phase transition in p,p’-diheptylazobenzene (7AB) samples. The investigations could be carried out by monitoring the behavior of the thermal diffusivity over the phase transition before and during the sample irradiation with UV light of varying intensity. Among other, it is shown that the monitoring of the time dependence of the thermal diffusivity at fixed sample temperature enabled the comparative study of the dynamics of the trans–cis and cis–trans isomeric transitions. It was shown that the increasing cis isomers concentration caused an increase in the fraction of material undergoing the phase transitions and increased the cis–trans conversion rate. Finally, polarization microscopy observations of the sample texture, which could be carried out together with the calorimetric evaluations proved useful for a more comprehensive analysis of the obtained results.

Photo-Rewritable Glaring Patterns Composed of Stripe Domains in Nematic Elastomers.

Dynamic ordered micropatterns in polymeric materials provide an effective approach for the on-demand tuning of optical properties toward a smart optical material. In this study, it is shown that glaring patterns exhibiting strong anisotropic light diffusion can be developed at specific locations in nematic liquid-crystal elastomers with light-sensitive azobenzene units. Glaring originates from the stripe domains of the nematic directors that self-organize in light-irradiated regions after a simple uniaxial stretching and releasing process without any complicated lithographic technique. The nematic order transiently reduced by the photo-induced cis azobenzene isomers unlocks entropic elasticity, which induces local uniaxial shrinkage that causes buckling of the directors forming stripe domains. The written pattern on the film is tangibly visible with the backlight owing to the difference in anisotropic light diffusion. Furthermore, this pattern can be erased by light irradiation or thermal annealing. These films can be applied to optical elements for achieving augmented luminaries, security labeling, and sign-sheeting applications.

Modelling ultrafast dynamics at a conical intersection with regularized diabatic states: An approach based on multiplicative neural networks

• Neural network (NN) potentials for quantum dynamics of cis–trans photoisomerization. • Multiplicative NNs adapted to regularized diabatic states at a conical intersection. • NN potentials match a sum-of-products form required by MCTDH wavefunction propagation. • Good agreement found for highly correlated dynamics with up to 13 degrees of freedom. Neural-network (NN) potentials are employed in conjunction with the Multiconfiguration Time-Dependent Hartree (MCTDH) method in order to simulate an ultrafast photoinduced cis–trans type isomerization process induced by a conical intersection. To this end, NN potentials are fitted to a diabatic potential of regularized diabatic states type [Köppel et al., J. Chem. Phys. 115, 2377 (2001)], which entirely relies on adiabatic potential information. Multiplicative NNs are employed which match the sum-of-products form of the multiconfigurational MCTDH wavefunction. Good agreement with the reference dynamics is obtained for simulations of the highly correlated dynamics with up to 13 degrees of freedom. This study contributes to developing NN methodologies suitable for photochemical dynamics at complex excited-state topologies.

Azopolymers with imide structures as light-switchable membranes in controlled gas separation

In the paper, we demonstrate a series of azobenzene polyimides with azomoieties located in the main chain for potential applications as membranes for controlled gas separation. Since azobenzene derivatives can exist in two forms, that is, thermodynamically stable trans and photoinduced cis, both isomers can be utilized for membrane preparation. The long term stability of the cis form is a precondition for such an application. Thus, the effect of polymer structure on the stability of cis-trans isomerization process in polymer films was investigated. The prepared azopolymers exhibited high stability of cis isomers and the return cis-trans reaction was not completed during 7 days after switching off the excitation light (405 nm). The stability of the cis-trans isomerization was discussed in the context of the fragility parameter. To the best of our knowledge, it is the first time when the main chain azopolymer was tested as a membrane for the gas separation. Two types of azomembranes, that is, bearing azobenzene derivatives as trans, and cis isomers and a reference membrane without azochromophore were prepared. Our investigations showed that the presence of trans-azomoieties increases the permeability of the gases (N2, O2, He and CO2) in comparison to the reference polyimide and that the permeability increase is not accompanied by any losses of the selectivity.

Configurational E/Z and photo-induced cis-trans isomerism in the Pd(II) complex of asymmetrical N,N-methyl-ethyl-N′-benzoylthiourea

The palladium complex prepared from an asymmetrically di-substituted N,N-methyl-ethyl-N′-benzoylthiourea ligand exists in chloroform as a pure mixture of the configurational isomers cis-ZZ-[Pd(L-κS,O)2], cis-EE-[Pd(L-κS,O)2] and cis-ZE-[Pd(L-κS,O)2]. These have been have been characterized by 1H, 13C, HMBC and 1D NOE difference NMR spectroscopy in organic solvents. The single-crystal X-ray diffraction structures of crystals of the cis-EE-[Pd(L-κS,O)2] isomer isolated from either acetonitrile or dichloromethane solutions in the dark, confirmed its molecular structure. In acetonitrile, the analytically pure mixture of the cis-ZZ/ZE/EE-[Pd(L-κS,O)2] complexes undergo reversible photo-induced cis → trans isomerization when irradiated with polychromatic light to form all the configurational isomers trans-ZZ/ZE/EE-[Pd(L-κS,O)2]. In the absence of light, the latter trans-complexes slowly revert back to trans-ZZ/ZE/EE-[Pd(L-κS,O)2] in the dark in both acetonitrile and chloroform-d solutions, as monitored by RP-HPLC and 1H NMR spectroscopy as a function of time at 25 °C. Estimates of the relative rate of the trans → cis reversion reaction in the dark, is 2.8 × 10−2 min−1 in acetonitrile, compared to that in chloroform-d of ca 0.38 × 10−2 min−1.

Temperature dependent characteristics of photo-induced birefringence in different types of azo materials

At different temperatures, a semiconductor laser with a wavelength of 650 nm is used as probe light, and an Nd:YAG continuous laser with a wavelength of 532 nm is selected as pump light. The azo samples are placed between a pair of orthogonal polarizers with the vertical direction clockwise and counterclockwise 45 degrees, respectively. The polarization direction of the pump light is set to be the vertical direction. In order to reduce the effect of the stray light, a chopper is placed in the optical path of the probe light. The signal of photo-induced birefringence is recorded by a phase-locked amplifier (NF-LI5640). The photo-induced birefringences of the doped azo material, the azo polymer and the azo liquid crystal polymer are measured respectively, and the dynamic processes of photo-induced birefringence are fitted by a double e-index model. The experimental results show that with the influence of the pump light, photo-induced birefringences of the three types of azo materials rise rapidly at first and then gradually tend to reach their own saturation state because of the photo-induced cis and trans isomerism and the photo-induced molecular orientation properties of azo molecules. The photo-induced birefringence shows a tendency to increase at first and then decrease with the temperature increasing, which can be understood as a competitive mechanism. The photo-induced birefringence depends on the photo-induced orientation and irregular thermal motions of azo groups. In the range below the glass transition temperature of the samples, the increase of the temperature of samples contributes to the rearrangement of the azo molecules due to the influence of the pump light. When the temperature of the samples is higher than the glass transition temperature, molecular chains begin to move. The irregular thermal motions of azo components and polymer molecules are aggravated. This destroys the orientations of the polymer molecules and results in the drop of the photo-induced birefringence. Comparing the doped azo material with the azo polymer sample, the azo liquid crystal polymer sample exhibits not only a larger photo-birefringence, but also the photo-induced birefringence that does not change obviously after the pump light has been turned off, which means that the azo liquid crystal polymer sample has long optical storage properties. This shows that the azo liquid crystal polymer material is an ideal polarization-sensitive optical recording medium, which is expected to be used in the fields of optical storage, polarization holography and optical information processing.

Photoinduced metal-free diboration of alkynes in the presence of organophosphine catalysts

In sharp contrast to the transition metal-catalyzed diboration of alkynes with diboron species such as B2pin2, which affords vicinal cis-diborated alkenes, the organophosphine-catalyzed addition of B2pin2 to alkynes proceeds under metal-free conditions upon photoirradiation to give the corresponding vicinal trans-diborated alkenes with good stereoselectivity. For example, the photoinduced diboration of 1-octyne with B2pin2 in the presence of tris(o-methylphenyl)phosphine afforded 1,2-diborated octene with excellent trans selectivity (trans/cis=95/5). The influence of various organophosphines on the diboration is described in detail. The diboration does not occur in the dark or in the absence of an organophosphine catalyst. Moreover, ESR (electron spin resonance) measurements of the reaction mixture revealed a radical species around g=2.003, supporting a radical pathway for the organophosphine-catalyzed diboration. In addition, the photoinduced cis to trans isomerization of vicinal cis-diborated alkenes in the presence/absence of organophosphines is discussed.

Reversible photoinduced bi-state polymer solar cells based on fullerene derivatives with azobenzene groups

[6,6]-Phenyl-C61-butyric acid-4′-hydroxyl-azobenzene ester (PCBAb) was synthesized and used as the acceptor in the fabrication of reversible UV–VIS response bi-state polymer solar cells (PSCs) based on the photoinduced cis–trans isomerization of PCBAb. The device can be switched between “active” and “sleep” by the irradiation of UV and visible light, respectively. The active device has a PCE of 2.0%. With UV irradiation, the device goes to “sleep” with a lowered PCE (0.4%), and simultaneously decreased Jsc, Voc and FF, while after visible light treatment, the device is made “active” again. The mechanism of the bi-state process involves the different electron mobilities of the isomers.

Synthesis of a double-stranded spiroborate helicate bearing stilbene units and its photoresponsive behaviour

The sodium ion-triggered extension and contraction motions along with photo-induced cis–trans isomerisation of a photoresponsive spiroborate-based double-stranded helicate were investigated.

Photophysical and Electrochemical Investigations of the Fluorescent Probe, 4,4′-Bis(2-benzoxazolyl)stilbene

In solution, 4,4'-bis(2-benzoxazolyl)stilbene (BBS) was found to exhibit consistently high absolute fluorescence quantum yields (Φ(fl) ≥ 0.88) and a monoexponential lifetime, both independent of BBS concentration. The BBS steady-state and time-resolved photophysics were investigated by different techniques to understand the various deactivation pathways. Nonradiative deactivation of BBS singlet excited state by intersystem crossing was found to be negligible. Other than fluorescence, the excited state of BBS was found to be deactivated by trans-cis photoisomerization. At low concentrations (≈5 μg/mL), UV spectroscopy and laser flash photolysis showed concordant results that the photoinduced cis isomer gradually replaced the original absorption spectrum of the pure trans isomer. However, at high concentrations (≈0.2 mg/mL), (1)H NMR and DOSY measurements confirmed that irradiating BBS at 350 nm induced a conversion from the trans-BBS into its cis isomer by photoisomerization. It was further found that the stilbene moiety of both isomers was photocleaved. The resulting photoproduct was an aldehyde that was oxidized under ambient conditions to its corresponding carboxylic acid, i.e., 4-(1,3-benzoxazol-2-yl)benzoic acid. The structure of the photoproduct was unequivocally confirmed by X-ray diffraction. Spectroscopic investigation of BBS showed a limited photoisomerization after irradiation at 350 nm of a trans solution. The BBS electrochemistry showed irreversible oxidation, resulting in an unstable and highly reactive radical cation. Similarly, the cathodic process was also found to be irreversible, giving rise to a radical anion and showing its n-doping character.

Colorimetric and fluorescent pH and Cu2+ probes induced by photoisomerization of a maleonitrile-based Salen ligand

Both photo-induced cis-trans-isomers of a maleonitrile-based Salen ligand can be used as pH probes covering a broad pH range through three different mechanisms but upon undergoing the formation of a stable complex and Cu(2+)-promoted hydrolysis, respectively, they exhibit totally different responses and mechanisms for sensing Cu(2+).

QM/MM Trajectory Surface Hopping Approach to Photoisomerization of Rhodopsin and Isorhodopsin: The Origin of Faster and More Efficient Isomerization for Rhodopsin

The photoinduced cis-trans isomerization dynamics of rhodopsin and isorhodopsin are studied using a newly developed hybrid QM/MM trajectory surface hopping MD scheme based on the Zhu-Nakamura theory for nonadiabatic transitions. Rhodopsin and isorhodopsin have 11-cis and 9-cis forms of retinal as chromophore and the two proteins are isomerized to bathorhodopsin enclosing the all-trans form. The simulation reproduced faster and more efficient isomerization in rhodopsin than in isorhodopsin. In the excited state, rhodopsin shows a straightforward dynamics, whereas isorhodopsin dynamics is rather complicated and in a back-and-forth manner. The latter complicated dynamics would be mainly due to a narrow space near the active dihedral angle ═C8-C9═C10-C11═ (ϕ9) created by Thr 118 and Tyr 268 in opsin. Rhodopsin gives bathorhodopsin only while isorhodopsin yields a byproduct. The rigorous selectivity in rhodopsin would be another reason why rhodopsin is selected biologically. Comparison with our previous opsin-free investigations reveals that opsin tends to confine the twist of the active dihedral to only one direction and funnels transitions into the vicinity of minimum energy conical intersections (MECI). The twist-confinement totally blocks simultaneous twisting of ϕ9 and ϕ11 (═C10-C11═C12-C13═) and enhances the quantum yields. The opposite rotation of ϕ9 and ϕ11 ("wring-a-wet-towel" motion) takes place upon photoexcitation, which also does without opsin. The wring-a-wet-towel motion is dynamically enhanced in comparison with the one expected from locations of the MECI. The present simulation reveals that the Weiss-Warshel model for cis-trans photoisomerization is not applicable for rhodopsin because the branching ratio after transition is crucial.

Photoisomerization of fluorinated 1,3-dimethyl-5-propenyl uracils

The photo-induced cis / trans isomerization of double bonds in fluorinated derivatives of 1,3-dimethyl-5-propenyl uracils was investigated. The more sterically congested isomers were the major products in the observed photostationary states. The results of DFT calculations suggest that photo-induced cis / trans isomerization of this compounds can run through the formation of radical species, which are the transition states produced during photoexcitation. Preference of the formation of more sterically hindered isomers in photostationary state one can rationalize on the basis of the difference in oscillator strength and the role of equilibration between conformers s-cis / s-trans . ► Photo-induced cis / trans isomerization of fluorinated 1,3-dimethyl-5-propenyl uracils. ► DFT calculations suggest the formation of radical species as the transition states. ► The more sterically congested isomers were the major products in the photostationary states. ► Stereoselectivity is a result from the difference in oscillator strength and the role of s- cis / s-trans equilibration. We investigated the photo-induced cis / trans isomerization of double bonds in fluorinated derivatives of 1,3-dimethyl-5-propenyl uracils. When 1,3-dimethyl-5-( E -1,2,3,3,3-pentafluoropropenyl)uracil was irradiated by light λ > 300 nm, we obtained the photostationary state involving equilibration with 1,3-dimethyl-5-( Z -1,2,3,3,3-pentafluoropropenyl)uracil where the more sterically congested isomer Z was the major product ( E : Z = 1:1.5). Irradiation of Z isomer as the starting material also led to the identical photostationary state. In the case of 1,3-dimethyl-5-(1,3,3,3-tetrafluoropropenyl)uracil the more sterically congested isomer E was even more abundant ( Z : E = 1:7.5). The results of DFT calculations suggest that photo-induced cis / trans isomerization of these compounds can run through the formation of allylmethylene biradical species, which are the transition states produced during photoexcitation. Preference of the formation of more sterically hindered isomers in photostationary state one can rationalize on the basis of the difference in oscillator strength and the role of equilibration between conformers s-cis / s-trans .

A Nonadiabatic Ab Initio Dynamics Study on Rhodopsin and Its Analog Isorhodopsin: Chemical Dynamics Reasons behind Selection of Rhodopsin by Life

Abstract The structural difference between rhodopsin and isorhodopsin is only in the cis-position of the chromophore, but the difference leads to a large discrepancy in photoisomerization period and quantum yield. The photoinduced cis–trans isomerization dynamics of the two chromophores are investigated using a Quantum Mechanics/Molecular Mechanics trajectory surface hopping scheme. Rhodopsin shows a straightforward and fast excited-state dynamics whereas the isorhodopsin dynamics in the excited state is complicated due to differences in retinal motions and space gaps formed by surrounding residues. Consequently, the isorhodopsin → bathorhodopsin reaction is slower and less efficient. Photoexcitation of rhodopsin gives bathorhodopsin only, whereas isorhodopsin yields an analog with 9,11-di-cis-retinal in addition to bathorhodopsin. These differences explain why life uses rhodopsin rather than isorhodopsin.

Evolution of the Environment of Guest Molecules in Dynamically Heterogeneous Matrices of Poly(ethyl methacrylate) and Poly(n-butyl methacrylate) Far below Tg

Photoinduced cis–trans isomerization has been used to measure the lifetime of structural heterogeneities in the matrices of poly(ethyl methacrylate) (PEMA) and poly(n-butyl methacrylate) (PnBMA) far below Tg. Nonequilibrium distribution of probe molecules over their environments was formed by a partial trans → cis isomerization. To monitor the equilibration process, the back-isomerization was started after different delay times. The change in quantum yield of back-isomerization with delay time is interpreted to result from the change in the environment of probe molecules. The times required for the environments to change were estimated over a temperature range of Tg – 113 K to Tg – 73 K for PEMA and Tg – 65 K to Tg – 25 K for PnBMA. It was found that the lifetimes of different environments differ from each other by up to 2 orders of magnitude and correlate with the isomerization probability: the environment of the molecules which isomerize faster changes more often. For both matrices, the value of 55 kJ/mol was estimated for the activation energy of the lifetime of the least mobile environments while the lifetime of the most mobile environments depends on temperature only weakly.

UV-enhanced Ordering in Azobenzene-Containing Polystyrene-block-Poly(n-Butyl Methacrylate) Copolymer Blends

Reversible photocontrol on the ordering of block copolymers through the disorder-to-order transition (DOT) is important to promote long-range laterally ordered nanostructures over a macroscopic scale by “photo-combing”, similar to the classic zone refining used to produce highly pure, large single crystals. In this report, we have synthesized two deuterated polystyrene-b-poly(n-butyl methacrylate-r-2-(4-(phenylazo)benzoate)ethyl methacrylate) (PS-d8-b-P(nBMA-r-AzoEMA)) block copolymers containing photoisomerizable azobenzene functionalities. These two block copolymers and their binary mixtures exhibit an entropy-driven lower DOT (LDOT). Upon UV irradiation, the photoisomerization of azobenzene led to a gain in entropy, originating from conformational changes and density variations. Such an increase in entropic contributions significantly enhanced the ordering, thereby driving the phase-mixed copolymer blends toward a fully ordered state, as evidenced by small-angle neutron scattering (SANS) and specular neutron reflectivity (NR). This process could also be reversed by exposure to visible light via photoinduced cis−trans isomerization of azobenzene.

Photoresponsive magnetization reversal in green fluorescent protein chromophore based diradicals

We present density functional theory based calculations on six diradicals in which imino nitroxide (IN) and IN or oxo- or phospha-verdazyl radical centres are linked by fragments of structures called cyan, blue and green fluorescent protein, respectively. In the latter two variants the indolyl fragment of cyan fluorescent protein (CFP) is replaced by phenyl and phenol moiety to obtain blue fluorescent protein Y66F GFP (BFPF) and green fluorescent protein (GFP). The photoinduced cis–trans isomerization of these diradical substituted chromophores is accompanied by changes from antiferro- to ferro-magnetic spin coupling on the radical centres. The calculations in this work provide predictions of the change of magnetic response to photo excitation. The estimated magnetic exchange coupling constants associated with cis–trans isomerization of the chromophores can be useful parameters in designing radical substituted isomeric fluorescent chromophores. Upon irradiation with light of appropriate wavelength, the dark trans diradicals turn into their fluorescent cis isomers. Therefore, photoinduced magnetic crossover from antiferromagnetic to ferromagnetic regime associated with the change in color would be noticed in all three cases. This is a novel observation in case of the systems with GFP chromophore and its variants. These diradicals are potentially very useful in different applications and biocompatibility of such systems makes them prospective in different in vivo treatments. Moreover, change in color associated with magnetic crossover for these diradicals increases their suitability as biological taggers.

Evidence for the phantom state in photoinduced cis–trans isomerization of stilbene

Abstract Transient absorption spectra of cis-stilbene in solution, recorded in the range 275–690 nm with 80 fs instrument response, reveal two successive evolving bands. One at 630 nm decays on a subpicosecond scale; it corresponds to vertically-excited nearly planar S1 state. The other, growing and decaying band at 340 nm, is shown to be consistent with theoretically predicted phantom state p∗ of perpendicular molecular geometry. The lifetime of p∗ is determined to be τ p ∗ = 0.4 ps in acetonitrile and 0.3 ps in hexane.

A vision for ultrafast photoisomerization

We propose a simple physical mechanism to explain the ultrafast first step of vision, a photoinduced cis to trans rotation of retinal. In the ground state, the torsional stability of π bonds is countered by Coulomb interactions acting between the π lobes; the torsional dependence for Coulomb interactions is absent in the often-used Ohno approximation, but restored with our formula. After photoexcitation, the bonding weakens causing the destabilizing terms to dominate. The twist in the ground state due to steric interactions surrounding the 11- cis bond increases the initial torque and thus the speed of the reaction.

Dynamical Heterogeneity in Glassy o-Terphenyl. 2. Measurement of Environment Structure Lifetime Using Reversible Reactions

A new method is proposed for measuring the lifetime of heterogeneities in a glassy matrix. UV-vis absorption spectroscopy has been used to monitor the kinetics of photoinduced cis-->trans isomerization of 1-naphthyl-azomethoxybenzene (NAMB) in o-terphenyl (OTP) below T(g). The dependence of isomerization rate on the duration of dark interval after generation of cis molecules was established; an increase in the dark interval causes a decrease in isomerization rate. This dependence is shown to be due to the change in the local environment of NAMB molecules. The time required for the environment to change was estimated over a temperature range of 234 (T(g) - 9 K) to 241.5 K (T(g) - 1.5 K). The change in the environment of the guest molecules has been interpreted in terms of the exchange processes. The values obtained for the exchange time coincide with the rotation times of the NAMB molecule. Also, it is shown that the time of structural rearrangement of the environment near the fast reacting molecules is of the same order of magnitude as the time of structural rearrangement of the environment near the slow reacting ones.