Excited-State Decay Paths in Tetraphenylethene Derivatives

The photophysical properties of tetraphenylethene (TPE) compounds may differ widely depending on the substitution pattern, for example, with regard to the fluorescence quantum yield φ_f and the propensity to exhibit aggregation-induced emission (AIE). We report combined electronic structure calculations and nonadiabatic dynamics simulations to study the excited-state decay mechanisms of two TPE derivatives with four methyl substituents, either in the meta position (TPE-4mM, φ_f = 0.1%) or in the ortho position (TPE-4oM, φ_f = 64.3%). In both cases, two excited-state decay pathways may be relevant, namely, photoisomerization around the central ethylenic double bond and photocyclization involving two adjacent phenyl rings. In TPE-4mM, the barrierless S₁ cyclization is favored; it is responsible for the ultralow fluorescence quantum yield observed experimentally. In TPE-4oM, both the S₁ photocyclization and photoisomerization paths are blocked by non-negligible barriers, and fluorescence is thus feasible. Nonadiabatic dynamics simulations with more than 1000 surface hopping trajectories show ultrafast cyclization upon photoexcitation of TPE-4mM, whereas TPE-4oM remains unreactive during the 1 ps simulations. We discuss the chances for spectroscopic detection of the postulated cyclic photoproduct of TPE-4mM and the relevance of our findings for the AIE process.