Excited state intramolecular proton transfer of 2-hydroxy-1-naphthaldehyde semicarbazone: A combined fluorometric and quantum chemical study

Excited state intramolecular proton transfer (ESIPT) of 2-hydroxy-1-naphthaldehyde semicarbazone (2HNS) has been studied in n-heptane and methanol solvents exploiting absorption, steady state and time-resolved fluorometric techniques. Experimental results reveal that although the intramolecular proton transfer (IPT) reaction does not take place in the ground state (S0), the process is feasible in the lowest excited singlet state (S1). Ab initio quantum chemical calculations have been carried out to judge the feasibility of the IPT process in the different electronic states. Density functional theory (DFT) has been used for the ground state calculations while those relevant to the lowest excited singlet state have been performed exploiting the time dependent density functional theory (TDDFT) using the B3LYP functional at 6-31G** level. The distances of the transferable hydrogen from the oxygen atom (wherefrom it is dislocated during the IPT reaction) as well as the nitrogen atom (to which it gets attached) have been taken as the independent reaction coordinates. The calculations reveal that either of these two parameters may be used to monitor the progress of the prototropic process since both lead to the same prototropic species passing through the same transition state. Simulation of the potential energy curves (PECs) and potential energy surfaces (PESs) leads to visualize the advancement of the reaction. Calculations imply that formation of the tautomer in the S0 state leads to endothermicity, while the process is exothermic in the S1 state. The calculated activation energy (Eact) of the process also decreases in the excited state. Thus, although the reaction is not feasible in the ground state, both the thermodynamic (ΔH) and the kinetic (Eact) factors favor the prototropic process to occur in the S1 state. This article reveals that any of the suitably and logically chosen structural parameters giving the same value of the reaction parameters and resulting in the same transition state may serve as the reaction co-ordinate.