Nonadiabatic dynamics simulation of photoisomerization mechanism of photoswitch azodicarboxamide: Hydrogen bonding effects

Photoinduced Forward and Backward Pedalo-Type Motion of a Molecular Switch

“…In order to efficiently perform extended molecular dynamics (MD) simulations that enable covering the full photodynamic process, we initially focused on benchmarking the DFT approach with respect to the multireference method. Geometry optimization of the molecule in its electronic ground state (S 0 ) showed, in line with previous calculations, , that the central azodicarboxamide moiety of the molecular switch has a nonplanar configuration. The optimized structural parameters at the CAM-B3LYP and the MP2 level are in very good agreement (Table and Figure a).…”

“…This conclusion appears to be at odds with the results reported by Liu et al where it was concluded that trans–cis isomerization is the main relaxation pathway . However, the discrepancy can be clarified by the fact that the previously employed CASSCF approach fails to accurately describe (a) the potential energy landscape (see Supporting Information Table S1), and consequently also the energetics of the CIs, and (b) the explicit solvent bulk, which could favor a pedalo-type volume-conserving mechanism with respect to a space-demanding cis–trans isomerization process.…”

“…This conclusion appears to be at odds with the results reported by Liu et al where it was concluded that trans−cis isomerization is the main relaxation pathway. 17 However, the discrepancy can be clarified by the fact that the previously employed CASSCF approach fails to accurately describe (a) the potential energy landscape (see Supporting Information Table S1), and consequently also the energetics of the CIs, and (b) the explicit solvent bulk, which could favor a pedalo-type volume-conserving mechanism with respect to a spacedemanding cis−trans isomerization process. Our CASPT2 and TDDFT protocol together with the QM/MM approach, which has been successfully employed to model a variety of photoinduced phenomena in organic chromophores, 28,29 indicates that dynamic correlation and an explicit representation of the solvent environment able to model solvent bulk effects, which cover both electrostatic and steric contributions from the solvent surrounding, need to be included for an accurate description of these systems (Table S1) and to deliver an experimentally consistent mechanistic scenario.…”

“…On the basis of vibrational fingerprints obtained by time-resolved infrared spectroscopy in combination with static density functional theory (DFT) calculations, we concluded that photoexcitation of the azodicarboxamide-molecular switch induced a pedalo-type motion. However, more recently such a motion has been called into question in theoretical studies of Liu and co-workers in which the excited-state dynamics of the bare chromophore in the gas phase and of the chromophore with two water molecules were simulated at the multiconfigurational CASSCF level. In contrast to the pedalo-type motion, they proposed that N=N trans–cis isomerization, known from other azobenzene-based systems, − takes place.…”

“…In this framework, the dynamics of the azodicarboxamide core involved in the electronic excitations is described at the QM level while the four azodicarboxamide phenyl substituents and the molecular embedding are represented at the MM level (Figure b). Notably, and in contrast to previous studies, , we incorporated solvation effects by including an explicit representation of the chloroform solvent molecules at the MM level (including electrostatic and steric influence, Figure c). The importance of the embedding on the excited-state dynamics has been proven previously for a wide range of different systems. , The QM/MM setup involves three different layers.…”