Jet-Cooled Spectroscopy of Amino-Substituted Cinnamates.

The photochemical and photophysical relaxation pathways of UV filters play a critical role in determining their stability and safety. Here, we present comprehensive gas-phase spectroscopic studies of cinnamate derivatives substituted on the phenyl group with an amino group in the para (M4AC), meta (M3AC), and ortho (M2AC) positions, while the influence of increasing the electron-donating character of the substituent has been investigated by studies on the para-dimethylamino derivative (MeM4AC). Using Resonance Enhanced Multiphoton Ionization (REMPI), UV-UV depletion, pump-probe ion yield measurements, and photoelectron velocity-map imaging (VMI), complemented by electronic structure calculations, we characterize their electronically excited-state structure and dynamics. Compared to previously studied hydroxy- and methoxy-substituted cinnamates, all four derivatives exhibit substantial changes in electronic structure and excited-state behavior. Amino substitution is found to lead to pronounced red shifts of the S1 absorption band and to modify the ordering of the low-lying excited states. Time-resolved measurements show nanosecond excited-state lifetimes that are dominated by radiative and nonradiative relaxation from the S1 state to the ground state, while population transfer to long-lived 1nπ* states -that was previously shown to promote intersystem crossing to the triplet manifold in hydroxy- and methoxy-substituted cinnamates- is strongly suppressed. These results demonstrate that amino substitution has a profound influence on the properties of cinnamate-based UV filters, providing a new molecular design approach to improve UV-absorbing molecular materials.