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

ABSTRACT

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.