The photo-orientation of azobenzene in viscous solutions, simulated by a stochastic model

Dual photoisomerization mechanism of azobenzene embedded in a lipid membrane

The photoisomerization of chromophores embedded in biological environments is of high importance for biomedical applications, but it is still challenging to define the photoisomerization mechanism both experimentally and computationally. We present here a computational study of the azobenzene molecule embedded in a DPPC lipid membrane, and assess the photoisomerization mechanism by means of the quantum mechanics/molecular mechanics surface hopping (QM/MM-SH) method. We observe that while the trans-to-cis isomerization is a slow process governed by a torsional mechanism due to the strong interaction with the environment, the cis-to-trans mechanism is completed in sub-ps time scale and is governed by a pedal-like mechanism in which both weaker interactions with the environment and a different geometry of the potential energy surface play a key role.

Isomerization and reorientation of Disperse Red 1 in poly(ethyl methacrylate)

Irradiation of azobenzene-containing polymer materials with light causes cis-trans isomerization and reorientation of azobenzene moieties and thereby changes in the optical properties of the materials. In this study, the film of poly(ethyl methacrylate) doped with the azobenzene derivative Disperse Red 1 (DR1) has been irradiated with the linearly polarized light of 546 nm. The time profiles of optical anisotropy and absorbance measured during irradiation have been analyzed using a kinetic model. Based on the analysis of the time profiles, we conclude that the light-induced reorientation of DR1 molecules occurs in confined environments where trans → cis isomerization is hindered, while in roomy environments, there is no reorientation. In the confined environment, reorientation occurs due to the environmental changes caused by the isomerization attempts of the DR1 molecule. The polymer environment affects thermal cis → trans and light-induced trans → cis isomerizations of the DR1 molecule differently, suggesting that the spatial requirements for these processes to proceed are different. The thermal isomerization does not result in the reorientation of DR1 molecules in roomy environments.

Towards New Chiroptical Transitions Based on Thought Experiments and Hypothesis

We studied supramolecular chirality induced by circularly polarized light. Photoresponsive azopolymers form a helical intermolecular network. Furthermore, studies on photochemical materials using optical vortex light will also attract attention in the future. In contrast to circularly polarized light carrying spin angular momentum, an optical vortex with a spiral wave front and carrying orbital angular momentum may impart torque upon irradiated materials. In this review, we summarize a few examples, and then theoretically and computationally deduce the differences in spin angular momentum and orbital angular momentum depending on molecular orientation not on, but in, polymer films. UV-vis absorption and circular dichroism (CD) spectra are consequences of electric dipole transition and magnetic dipole transition, respectively. However, the basic effect of vortex light is postulated to originate from quadrupole transition. Therefore, we explored the simulated CD spectra of azo dyes with the aid of conventional density functional theory (DFT) calculations and preliminary theoretical discussions of the transition of CD. Either linearly or circularly polarized UV light causes the trans–cis photoisomerization of azo dyes, leading to anisotropic and/or helically organized methyl orange, respectively, which may be detectable by CD spectroscopy after some technical treatments. Our preliminary theoretical results may be useful for future experiments on the irradiation of UV light under vortex.

All-optical switching in dye-doped DNA nanofibers

All-optical switches are introduced which are based on deoxyribonucleic acid (DNA) in the form of electrospun fibers, where DNA is semi-intercalated with a push-pull, luminescent nonlinear pyrazoline derivative. Optical birefringence is found in the organic nanofibers, with fully reversible switching controlled through continuous-wave laser irradiation. The photoinduced signal is remarkably large, with birefringence highlighted by optically-driven refractive index anisotropy approaching 0.001. Sub-millisecond characteristic switching times are found. Integrating dye-intercalated DNA complex systems in organic nanofibers, as a convenient and efficient approach to template molecular organization and control it by external stimuli, might open new routes for realizing optical logic gates, reconfigurable photonic networks and sensors through physically-transient biopolymer components.

Wavelength Dependence of the Reorientation Efficiency of Azo Dyes in Polymer Matrixes

Irradiation with linearly polarized light of azobenzene-containing polymeric matrixes causes reorientation of azobenzene molecules. In this study, the optical light-induced anisotropy of amorphous poly(alkyl methacrylates) doped with an azo compound was measured at different temperatures and at two irradiation wavelengths. To describe a decrease in the efficiency of anisotropy formation with temperature, a model of molecule reorientation is suggested which includes the probability of molecule reorientation per one isomerization as a basic parameter. The probability of molecule reorientation was found to depend on irradiation wavelength. Comparing the anisotropy time profiles at different irradiation wavelengths, we concluded that, upon each photon absorption, the molecule most likely makes an attempt to reorient even without isomerization, i.e., the reorientation occurs by a mechanism predicted by Persico and co-workers in their theoretical works. Also, we infer that the reorientation is facilitated by the photon energy absorbed by a molecule.