Photoisomerization and Ultrafast Dynamics of Phenylazothiazoles: Theoretical Perspective

Phenylazothiazole (PAT) is a novel heteroaryl azo photoswitch that undergoes trans (E) to cis (Z) photoisomerization under visible light, making it promising for biological applications. However, the quantum yield of the E-to-Z isomerization is significantly lower than that of the Z-to-E process, limiting its practical utility. In this study, we employ nonadiabatic dynamics simulations to investigate the ultrafast dynamics of the E-to-Z photoisomerization. Through a systematic analysis of electronic structure methods, we demonstrate that spin-flip time-dependent density functional theory (SF-TDDFT) provides a reliable description of the electronic excited states, particularly at conical intersections, yielding results consistent with multireference methods. Based on two-dimensional potential energy surfaces, we reveal that E-PAT initially relaxes along the torsional coordinate to reach the minimum of the S₂ state, followed by two distinct pathways returning to the ground state. One pathway involves a planar minimum in the S₁ state, predominantly leading back to the E isomer rather than the Z isomer, which explains the lower E-to-Z quantum yield. Additionally, we explore the substituent effects on the optical properties and thermal isomerization of PAT derivatives, showing that substituents not only induce a redshift in the absorption spectrum but also modulate the activation barrier of ground-state isomerization. These findings provide valuable theoretical insights into the photoisomerization mechanism of PATs and offer guidance for designing optoelectronic materials with tunable optical properties.