How Does the Trans–Cis Photoisomerization of Azobenzene Take Place in Organic Solvents?

Abstract

The trans-cis photoisomerization of azobenzene-containing materials is key to a number of photomechanical applications, but the actual conversion mechanism in condensed phases is still largely unknown. Herein, we study the n, pi* isomerization in a vacuum and in various solvents via a modified molecular dynamics simulation adopting an ab initio torsion-inversion force field in the ground and excited states, while allowing for electronic transitions and a stochastic decay to the fundamental state. We determine the trans-cis photoisomerization quantum yield and decay times in various solvents (n-hexane, anisole, toluene, ethanol, and ethylene glycol), and obtain results comparable with experimental ones where available. A profound difference between the isomerization mechanism in vacuum and in solution is found, with the often neglected mixed torsional-inversion pathway being the most important in solvents.

Athermal photofluidization of glasses

“…This slow thermal relaxation is indicative of an activated, barrier-limited decay process, as typically described by an Arrhenius law, t(U) ¼ t r exp(U/k B T), where 1/t r is the rate of barrier-crossing attempts on the molecular orientational fluctuation time scale, at most t r B20 ps 18 , U is a barrier height and t(U) is the characteristic time of exponential relaxation, as sketched in Fig. 4a.…”

“…Effectively complete bypass of the thermal process occurs at light intensity sufficiently low to produce no observable effects of average heating. The success of azobenzenes in converting photon energy to effective local fluidization can be attributed to their distinctive photo-induced change in molecular shape 16,17 , combined with, importantly, relatively slow (B10 ps) dynamics 18 of the resulting energy transfer to their local environment.…”

Absorption and Emission Spectra of a Flexible Dye in Solution: A Computational Time-Dependent Approach

“…Although a lot of work has been done to describe GS structures and reliable FFs are available in the literature, much less effort has been devoted to the development of "ad hoc" FFs for the EES conformations. 35,[47][48][49][50] Recently, the JOYCE 51 parameterization protocol, devised to obtain accurate and specific FFs from QM computed data, has been extended to the automated treatment of both GS and EES. 50 In the present work, the above mentioned "dynamic" approach is applied to compute the electronic spectra of a flexible dye, namely the 4-naphthoyloxy-1-methoxy-2,2,6,6-tetramethylpiperidine (NfO-TEMPO-Me) molecule (see Figure 2), in toluene solution, where solvent molecules are modeled explicitly.…”

Kinetic Monte Carlo simulations for birefringence relaxation of photo-switchable molecules on a surface

“…40,41 The situation is even more complicated when the isomerization occurs in a crowded environment because neighboring molecules then most likely interfere. 42 In our model the isomerization process is drastically simplified: it consists of a simple switch from the straight to the bent needle and vice-versa. Also, we assume that the conformational change happens instantaneously, a reasonable assumption given the relative time scales involved.…”

“…1), while the isomerization occurs on the picosecond scale even in relatively dense organic solvents. 42 In order to incorporate isomerization as described in Sec. II in the kinetic Monte Carlo simulation, we define the following set of rules:…”