Photochromism of dye containing Schiff base-metal complex: A revisit through spectro-kinetic, thermodynamic and theoretical analyses for the design of a molecular logic gate

Abstract

Photochromic compounds attract tremendous necessity because of their dynamic skill for optical memory media, photo-printing and UV-sensor, ophthalmic and sunglass lenses, and most importantly molecular logic gates. Here, a novel photochromic material based on a dye containing Schiff base-metal complex has been designed and developed. The metal coordination of photochromic ligand gives a unique and potentially glorious photo-physical property. In dark, the material displays no absorption band (Turn-OFF) above 500 nm, but exhibits chelation-enhanced intense green fluorescence (Turn-ON) (λex = 365 nm, λemi = 501 nm). Upon UV irradiation, the material shows strong absorption band [λmax = 554 nm] (Turn-ON) but FRET leads to weak emission (Turn-OFF) in THF solution. The interesting ‘ON-OFF’ behavior encourages the development of UV light controlled molecular logic gate. The material displays moderate quantum yield (0.082), excellent fatigue resistance (>5 cycles) and solvent polarity dependent photochromism. The designed d10-metal (Zn) guided photochromism is understood in the light of spectro-kinetic, thermodynamic and theoretical analyses. The analyses establish that the keto-enol tautomerism through zinc metal guided excited-state intramolecular proton transfer (MG-ESIPT) is the key factor for the photochromism under study. The forward reaction (enol to keto i.e., greenish yellow to pink) is the temperature independent metal promoted UV light induced phenomenon, but the backward reaction is strongly temperature dependent one. The backward reaction follows 1st order kinetic with activation energy of 53.16 kJ/mol or 12.71 kcal/mol (spectro-kinetic data). The photochromism understudy is basically photo-thermal reversible process (PTRP) (thermodynamic study). The theoretical analyses [DFT, TS, PES, and FMO] provide a deep insight into the mechanistic understanding of the photochromic process.