Nonadiabatic Ensemble Simulations of cis-Stilbene and cis- Azobenzene Photoisomerization Amanda J. Neukirch,* ,†,‡ Logan C. Shamberger, ‡ Enrique Abad, § Barry J. Haycock, ‡ Hong Wang, ‡ Jose ́ Ortega, ∥ Oleg V. Prezhdo, †,⊥ and James P. Lewis* ,‡ † Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, United States ‡ Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506-6315, United States § Computational Biochemistry Group, Institute of Theoretical Chemistry, University of Stuttgart, 70569 Stuttgart, Germany ∥ Departmento de Física Teó rica de la Materia Condensada and Conensed Matter Physics Center (IFIMAC), Universidad Autó noma de Madrid, Madrid 28049, Spain ⊥ Department of Chemistry, University of Rochester, Rochester, New York 14627, United States ABSTRACT: Structurally, stilbene and azobenzene molecules exist in closed and open cis and trans forms, which are able to transform into each other under the influence of light (photoisomerization). To accurately simulate the photoisomerization processes, one must go beyond ground-state (Born−Oppenheimer) calculations and include nonadiabatic coupling between the electronic and vibrational states. We have successfully implemented nonadiabatic couplings and a surface-hopping algorithm within a density functional theory approach that utilizes local orbitals. We demonstrate the effectiveness of our approach by performing molecular dynamics simulations of the cis−trans photoisomerization in both azobenzene and stilbene upon excitation into the S 1 state. By generating an ensemble of trajectories, we can gather characteristic transformation times and quantum yields that we will discuss and compare with ultrafast spectroscopic experiments. ■ INTRODUCTION Both stilbene and azobenzene can be interconverted between their cis- and trans-isomers by light of different wavelengths (Figure 1). Each isomer has distinct spectral and geometric properties that allow these molecules to serve as ideal model systems for molecular transducers in light-driven devices and optical switches. 1 The scientific community has invested considerable effort to harness the molecular motion of systems on the macroscopic scale. Applications range from optical storage devices, 2,3 regulating channels in the ligand-binding domain of proteins, 4 photo-orientation of liquid crystals, 5,6 control of peptide conformations, 7 modifying surface properties on oriented films, 8 and control of CO 2 adsorption in porous metal organic frameworks (MOF). 9 A detailed description of the photoisomerization process, especially in azobenzene, remains elusive, despite a plethora of novel applications and considerable theoretical and exper- imental studies. There are four possible pathways for isomerization in azobenzene: rotation, inversion, concerted inversion, and inversion-assisted rotation. 10−14 In the rotational pathway, the NN π-bond breaks, allowing for free rotation around the N−N bond, 13 and the C−NN−C dihedral angle changes, while the NN−C angles remain fixed at ∼120°. In the inversion mechanism, the C−NN−C angle remains fixed at 0°, but one of the NN−C angles increases to 180°. 14 A linear transition state is produced in the concerted inversion mechanism when both NN−C bond angles increase to 180°. Finally, in the inversion-assisted rotation mechanism, there are large changes in both the C−NN−C angle and the NN− C angles simultaneously. No barrier exists along the rotational pathway after excitation into the S 1 state. According to previous computational research, the conical intersection between the S 0 and S 1 states exists when the C−NN−C dihedral angle is ∼90° and the NN−C angle is ∼140°. These facts have Received: November 13, 2013 Published: December 4, 2013 Figure 1. Simplified excitation and reaction schemes for azobenzene. Stilbene can be represented in a similar manor. Article pubs.acs.org/JCTC © 2013 American Chemical Society 14 dx.doi.org/10.1021/ct4009816 | J. Chem. Theory Comput. 2014, 10, 14−23