The
photophysical properties of tetraphenylethene (TPE) compounds
may differ widely depending on the substitution pattern, for example,
with regard to the fluorescence quantum yield ϕf 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, ϕf = 0.1%) or in the ortho position (TPE-4oM,
ϕf = 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.