Multiple ESIPT and triplet formation pathways in photoexcited 2-mercapto-5-methyl-1,3-benzenedicarboxaldehyde

Abstract

Simultaneous dynamical events happening upon photoexcitation of 2-mercapto-5-methyl-1,3-benzenedicarboxaldehyde (MMB) are studied using quantum wavepacket simulations. Several crossings among S1, S2, S3, T3, and T4 states are identified using the potential energy profile analyses in vibrational coordinate space. Simulations in the singlet manifold show nonadiabatic population transfer to higher states (S2 and S3) within a few tens of femtoseconds upon initiating the wavepacket on S1. Such population exchange opens up multiple excited-state intramolecular proton transfer (ESIPT) pathways involving all three singlet states, i.e., S1, S2 and S3. The singlet-triplet dynamics using spin-orbit coupling constants reveal that the triplet generation via S1 to T3/T4 is a minor deactivation pathway and can compete with the proton transfer process.