Multidimensional Potential Energy Surfaces Resolved at the RASPT2 Level for Accurate Photoinduced Isomerization Dynamics of Azobenzene

Flavia Aleotti,Alberto Arcioni,Marco Garavelli,Claudio Zannoni,Artur Nenov,Roberto Berardi,Lorenzo Soprani

doi:10.1021/acs.jctc.9b00561

Flavia Aleotti, Alberto Arcioni + Show 5 more

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https://doi.org/10.1021/acs.jctc.9b00561

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Abstract

We have used state-of-the-art ab initio restricted active RASPT2 computations using a 16 orbitals, 18 electrons active space to produce an extended three-dimensional map of the potential energy surfaces (PESs) of the ground and first nπ* excited states of azobenzene along CNNC torsion and the two CNN bending angles, which are the most relevant coordinates for the trans-cis photoisomerization process. Through comparison with fully unconstrained optimizations performed at the same level of theory, we show that the three selected coordinates suffice to correctly describe the photoisomerization mechanism and the S1-S0 crossing seam. We also provide a map of the nonadiabatic coupling between the two states in the region where they get closer in energy. Eventually, we show that treating the two CNN bending angles as independent coordinates is fundamental to break the symmetry and couple the two electronic states. The accuracy of the S0 and S1 PESs and couplings was validated with semiclassical dynamics simulations in the reduced space of the scanned coordinates, showing results in good agreement with published full-coordinate dynamics.

Highlights

Azobenzene and its derivatives are well known for their ability to undergo a significant and ultrafast structural change when irradiated with light
Through comparison with fully unconstrained optimizations performed at the same level of theory, we show that the three selected coordinates suffice to correctly describe the photoisomerization mechanism and the S1-S0 crossing seam
We provide a map of the non-adiabatic coupling between the two states in the region where they get closer in energy

Summary

Introduction

Azobenzene and its derivatives are well known for their ability to undergo a significant and ultrafast structural change when irradiated with light (photoisomerization). A pronounced dependence of the photoisomerization quantum yield on the excitation wavelength has been documented,[10,11] which is in violation of Vavilov’s extension of Kasha’s rule ( known as the Kasha-Vavilov’s rule). This dependence suggests that different mechanisms take place when the system is irradiated with visible light (excitation to S1, nπ∗ in nature) or with UV light (excitation to S2, ππ∗ in nature). According to the latest computational works,[11,12,14] on the ground state (thermal reaction) a huge barrier is observed along torsion, while the potential energy surface (PES)

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