Mapping the Excited State Dynamics of Gas-Phase Spiropyran with Time-Resolved Photoelectron Spectroscopy.

We report a femtosecond time-resolved photoelectron spectroscopic study of the excited state dynamics of spiropyran (SP3mt) in the gas phase. Using 266 nm pump and 200 nm probe pulses with an overall instrument response function of 160 fs, the experiment directly resolves the complete early-time evolution following photoexcitation. Following rapid internal conversion (SP-S2 → SP-S1), the excited-state population bifurcates into two competing pathways with a branching ratio close to 70/30: one channel leads to merocyanine formation via a multi-configurational excited state, and the other channel involving intersystem crossing (ISC) to a long-lived triplet-state (T2) manifold. Calculations at the MS-CASPT2/6-31G* level support the spectral assignments and the identification of the key transient intermediates. Our results provide a comprehensive picture of isolated spiropyran photoisomerization dynamics and clarify the elementary branching dynamics that control the competition between productive ring opening and nonreactive relaxation.