The photoisomerization of azobenzene is a prototypical reaction of various light-activated processes in material and biomedical sciences. However, its reaction mechanism has been under debate for decades, partly due to the challenges in computational simulations to accurately describe the molecule's photodynamics. A recent study (J. Am. Chem. Soc. 2020, 142 (49), 20680-20690) addressed the challenges by combining the hole-hole Tamm-Dancoff Approximated (hh-TDA) density functional theory (DFT) method with the ab initio multiple spawning (AIMS) algorithm. The hh-TDA-DFT/AIMS method was applied to first-principles non-adiabatic dynamics simulation of azobenzene’s photodynamics in the vacuum. However, it remains necessary to benchmark this new method in realistic molecular environments against experimental data. In the current work, the hh-TDA-DFT/AIMS method was employed in a quantum mechanics/molecular mechanics setting to characterize the trans azobenzene’s photodynamics in explicit methanol and n-hexane solvents, following both the S1 (nπ*) and S2 (ππ*) excitations. The simulated absorption and fluorescence spectra following the S2 excitation quantitatively agree with the experiments. However, the hh-TDA-DFT method overestimates the torsional barrier on the S1 state, leading to an overestimation of the S1 state lifetime. The excited-state population decays to the ground state through two competing channels. The reactive channel partially yields the cis azobenzene photoproduct and the unreactive channel exclusively leads to the reactant. The S2 excitation increases the decay through the unreactive channel and thus decreases the isomerization quantum yield compared to the S1 excitation. The solvent slows down the azobenzene's torsional dynamics on the S1 state, but its polarity minimally affects the reaction kinetics and quantum yields. Interestingly, the dynamics of the central torsion and angles of azobenzene plays a critical role in determining the final isomer of the azobenzene. This benchmark study validates the hh-TDA-DFT/AIMS method’s accuracy for simulating the azobenzene’s photodynamics in realistic molecular environments.

First-principles non-adiabatic dynamics simulation of azobenzene photodynamics in solutions

ACS Fall 2021

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