Photochromic properties of some azomaleimide derivatives and DFT quantum chemical study of thermal cis-trans isomerization pathways

New Insights on Solvent-Induced Changes in Refractivity and Specific Rotation of Poly(propylene oxide) Systems Extracted from Channeled Spectra

Investigation of chiroptical polymers in the solution phase is paramount for designing supramolecular architectures for photonic or biomedical devices. This work is devoted to the case study of poly(propylene oxide) (PPO) optical activity in several solvents: benzonitrile, carbon disulfide, chloroform, ethyl acetate, and p-dioxane. To attain information on the interactions in these systems, rheological testing was undertaken, showing distinct variations of the rheological parameters as a function of the solvent type. These aspects are also reflected in the refractive index dispersive behavior, from which linear and non-linear optical properties are extracted. To determine the circular birefringence and specific rotation of the PPO solutions, the alternative method of the channeled spectra was employed. The spectral data were correlated with the molecular modeling of the PPO structural unit in the selected solvents. Density functional theory (DFT) computational data indicated that the torsional potential energy-related to the O1-C2-C3-O4 dihedral angle from the polymer repeating unit-was hindered in solvation environments characterized by high polarity and the ability to interact via hydrogen bonding. This was in agreement with the optical characterization of the samples, which indicated a lower circular birefringence and specific rotation for the solutions of PPO in ethyl acetate and p-dioxane. Also, the shape of optical rotatory dispersion curves was slightly modified for PPO in these solvents compared with the other ones.

Synthesis and photochromism of catechol-containing symmetrical azobenzene compounds

Symmetrical azobenzene derivatives with two catechol groups, 1d-4d, were synthesized as kinds of novel compounds, and the structures were confirmed using mass spectrometry and nuclear magnetic resonance spectroscopy. These compounds could attain photostationary state rapidly in solution upon UV irradiation, and their photochromism had good reversibility. Substituents and their positions on azobenzene chromophore had obvious influence on the maximum absorption and photochromic performances of these as-synthesized compounds. Electron-donating group on ortho positions could contribute to the redshift π-π* band. The sulfonamide group that is bonded to dopamine molecules and azobenzene rings caused a negligible n-π* transition of cis isomer, resulting in photobleaching upon UV irradiation. Among the four compounds, 4d had the strongest electron-donating ortho-methoxy substituents and lower planarity; thus its absorption could decrease more significantly upon UV irradiation of the same intensity, and its cis-to-trans conversion could be up to 63%. Furthermore, owing to the presence of catechol groups, 4d showed an effective affinity and adhesion to substrate, and on the surface of substrate, a weak colour change could be observed upon UV irradiation, but the reversibility was poorer than that in solution.

A DFT study on the isomerization mechanism of azobenzene derivatives on silicon substrates

The cis–trans isomerization mechanism of azobenzenesulfonamide derivatives on silicon substrates was investigated using DFT. The most favorable cooperative mode of the N2 inversion of the L followed by the N1 inversion of the R was proposed.

Acid-catalysed liquid-to-solid transitioning of arylazoisoxazole photoswitches

Molecular photoswitches play a vital role in the development of responsive materials. These molecular building blocks are particularly attractive when multiple stimuli can be combined to bring about physical changes, sometimes leading to unexpected properties and functions. The arylazoisoxazole molecular switch was recently shown to be capable of efficient photoreversible solid-to-liquid phase transitions with application in photoswitchable surface adhesion. Here, we show that the arylazoisoxazole forms thermally stable and photoisomerisable protonated Z- and E-isomers in an apolar aprotic solvent when the pK a of the applied acid is sufficiently low. The tuning of isomerisation kinetics from days to seconds by the pK a of the acid not only opens up new reactivity in solution, but also the solid-state photoswitching of azoisoxazoles can be efficiently reversed with selected acid vapours, enabling acid-gated photoswitchable surface adhesion.

First thermal studies on visible-light-switchable negative T-type photochromes of a nitrile-rich series

Representatives of visible-light-switchable nitrile-rich negative T-type photochromes (NRP) were synthesized. The temperature effect on the thermal relaxation of the photoinduced form of NRP was studied for the first time. It was found that regardless the temperature the reverse thermal reaction in ethanol and 1,4-dioxane proceeded according to a first-order kinetic equation, while an unusual pseudo-zero order of the dark reaction was observed in acetonitrile. Using the obtained experimental data, the activation energy as well as the activation parameters, such as entropy and enthalpy of activation, were calculated for the first time for thermal relaxation of NRP photochromes. It was also shown that an initial form of NRP photochromes bearing a tricyanofuran acceptor was more temperature resistant than the initial form of photochromes with a tricyanopyrrole acceptor.

The temperature effect on the thermal stability of both the initial and photoinduced forms of nitrile-rich T-type photochromes was studied for the first time.

Experimental and theoretical study of bidirectional photoswitching behavior of 5,5′-diphenylhydantoin Schiff bases: synthesis, crystal structure and kinetic approaches

Herein, the synthesis and characterization of four novel 5,5′-diphenylhydantoin Schiff bases containing different aromatic species are presented.

Theoretical insights into a colorimetric azo-based probe to detect copper ions

In the present study, a colorimetric azobenzene-based probe (AZO 1) was reported that exhibits high selectivity toward Cu²⁺ and undergoes a red to yellow colour change upon its detection. Density functional theory (DFT) calculations were carried out to investigate the mechanism of the probe discoloration. The differences in the binding energies of complexes of 2 : 1 and 1 : 1 stoichiometry indicated that a two-step complexation process takes place as the Cu²⁺ content increases. However, the calculated absorption spectra suggested that a significant colour change would only be observed for the 1 : 1 AZO 1 : Cu²⁺ complex. A HOMO–LUMO electronic transition was a key factor for the blue shift of the absorption bands of the probe. Further studies indicated that solvent molecules participate in the complexation and that the presence of the o-methoxy group in AZO 1 led to formation of an octahedral complex because of the additional chelating site. A significant change in the conformation of AZO 1, namely the rotation of the N,N-di(carboxymethyl)amino group around the N–C_Ar bond by approximately 90°, resulted in a larger HOMO–LUMO energy gap, and the corresponding alteration of the intramolecular charge transfer (ICT) from the N,N-di(carboxymethyl)amino group to the phenyl ring led to the observed colour change.

DFT calculations indicated that the rotation of the N,N-di(carboxymethyl)amino group around the N–CAr bond by approximately 90°, resulted in a larger HOMO–LUMO energy gap, and led to the observed colour change.