Intramolecular proton transfer of 2-hydroxy-1-naphthaldehyde semicarbazone and thiosemicarbazone in ground and lowest excited singlet states: A comparative experimental and computational study

Photophysics of 2-hydroxy-1-naphthaldehyde semicarbazone (2HNS) and the corresponding thiosemicarbazone (2HNT) are explored in n-heptane and methanol as solvents, focusing on the intramolecular proton transfer (IPT) in the ground (S0) and the first excited singlet (S1) states using absorption, steady state and time-resolved fluorometric techniques. The feasibility of the IPT process in the two molecular systems has also been compared. Experiments confirm that for both the compounds IPT takes place in the lowest excited singlet state (S1), but not in the ground state (S0). Ab-initio quantum chemical calculations provide support to the experimental findings. Simulated potential energy curves (PEC) in the two electronic states imply that the IPT process is endothermic in the S0 state but becomes exothermic in the S1 state for both the probes. PECs also reveal that compared to the ground state the activation barrier for the IPT process is reduced appreciably in the S1 state. The IPT process, thus, becomes feasible both thermodynamically and kinetically in the S1 state but not in the S0 state. The experiments and calculations, however, reveal that the excited state intramolecular proton transfer process is relatively more viable for 2HNS compared to 2HNT. Further, the work demonstrates that any of the suitably chosen structural parameters leading to the unique transition state and yielding the same values of the reaction parameters can be taken as the reaction coordinate to follow the progress of the intramolecular prototropic process.