Uncovering the excited-state intramolecular proton transfer and triplet formation pathways of 3-methyl-6-hydroxy-m-phthalic acid

Three dominant photorelaxation pathways—internal conversion, intramolecular proton transfer, and triplet formation—originating from the S1 state of 3-methyl-6-hydroxy-m-phthalic acid are investigated theoretically. Potential energy profiles and spin–orbit coupling parameters associated with the six low-lying excited electronic states (S1, S2, T1, T2, T3, and T4) are analyzed to elucidate the mechanistic details of these photorelaxation pathways. Full-dimensional quantum wavepacket simulations indicate that the wavepacket initiated on S1 would move through the S1–S2 conical intersections and accumulate in the S2 state. This nonadiabatic population transfer feature, combined with small barrier energies on S1 and S2, allows the molecule to display proton transfer from both states. The efficient S1–T3 intersystem crossing is the third event, where the rate of this event increases with the elongation of the proton transfer coordinate (O–H distance). Vibronic coupling in the triplet manifold enables an ultrafast T3 to T1 decay. In addition, the proton transfer and triplet formation features are studied in nonpolar and aprotic polar solvents to interpret the experimental findings.