Methyl substitution enhanced photoisomerization of trans, trans -1,4-diphenyl-1,3-butadiene: direct ab initio trajectory surface hopping dynamic simulations

Single methyl group substitution on the p-position of the phenyl ring (tt-DPB-me1) or the conjugated CC bond (tt-DPB-me2) has been found to enhance the photoisomerization efficiency for two trans,trans-1,4-diphenyl-1,3-butadiene (tt-DPB) derivatives by performing direct ab initio trajectory surface hopping dynamics simulations. With implementation of the Zhu-Nakamura global switching algorithm, on-the-fly trajectory surface hopping dynamics simulations based on the ground state and first excited state potential energies and their gradients calculated by the two state averaged complete active space self-consistent field method with basis set 6-31G were propagated up to 3000 femtoseconds. Four-hundred sampling trajectories have been performed for both tt-DPB-me1 and tt-DPB-me2, and five distinctive photoisomerization pathways were observed for both of them. Among which, One Bond Flipping (OBF) and Hula-Twist (HT) are the dominant photoisomerization mechanisms. The lifetime of the S₁ state is estimated to be 1423.0 fs (819.0 fs), and the photoisomerization quantum yields are 0.088 (0.378) in tt → ct, 0.070 (0.015) in tt → tc and 0.073 (0.065) in tt → cc for tt-DPB-me1 (tt-DPB-me2). By analyzing the dynamics simulation data, it can be concluded that closer methyl substitution with respect to the central CC double bond results in a higher percentage of the corresponding photoisomerization products. The present simulation results are in agreement with the ultrafast spectroscopy measurements, which unveil the photoisomerization mechanisms of tt-DPB derivatives and present useful physical insights on how to tune the photoisomerization of the substituted DPB.