Exploration of Spectrodynamic Behavior of a Naphthalene-Based Schiff Base (E)-2-((Naphthalen-1-ylimino)methyl)-4-nitrophenol: Synergistic Effects of ESIPT and Excimers.

Excited-state intramolecular proton transfer (ESIPT) and excimer formation are unimolecular and bimolecular reactions, respectively. It is not frequent to observe coupling between these two excited-state processes. In this work, we have shown the coupling between excimer formation and ESIPT processes in a custom-designed fluorophore (E)-2-((naphthalen-1-ylimino)methyl)-4-nitrophenol (NMNP) in different matrices using steady state and time-resolved fluorescence spectroscopic measurements followed by quantum chemical calculations. Extensive spectral analyses of NMNP in different media reveal the occurrence of pre-equilibrium excited-state intramolecular proton transfer (ESIPT) between the enol form or normal form (N*) and tautomeric form or keto form (T*), where excimer (E*) formation is facilitated by delayed population decay and well-ordered lattice packing, resulting in triple emissions of N* (400-450 nm), T* (520-550 nm), and E* (below 450 nm) with lifetime values 0.12-0.50 ns, 1.80-2.50 ns and >5.0 ns, respectively. Theoretical potential energy curves (PECs) along the proton transfer coordinates using density functional theory (DFT) and time-dependent density functional theory (TDDFT) suggest a low energy barrier for the excited state proton transfer process.