Synthesis of an advanced metal-guided photochromic system for molecular keypad lock: detailed experimental findings and theoretical understanding

Abstract

The dye-containing Schiff base metal complex is a new member of the photochromic family with advantages, such as long-wavelength absorption, high molar absorption coefficient, large Stokes shift, facile synthesis, good photoresponse, and excellent fatigue resistance. The system is highly promising for the fabrication of simple devices such as photoprinting, photopatterning, and UV light sensing. In this work, an excellent metal-guided photochromic (MG-PC) material was synthesized using Rhodamine B hydrazide, trihydroxybenzaldehyde and Zn²⁺via Schiff base chemistry with a view to construct a molecular keypad lock. All the issues related to photochromism (photophysical properties, kinetics, thermodynamics, photostationary state, quantum yield, fatigue resistance and effect of various controlling factors) have been understood and tuned accordingly to achieve the target of a quick responsive MG-PC system. The significantly low activation energy (E_a = 27.53 kJ mol⁻¹) for the thermal bleaching justifies the achievement of the target. The MG-PC material involves photo-tautomerism (PT), which consists of photo-coloration (conversion of a pale green-yellow enol to a pink color keto form exclusively under UV light) and thermal bleaching (return back to original enol form under dark condition below the critical temperature, 50 °C). The PT proceeds through the metal-guided excited state and ground state intramolecular proton transfer as evident from the large Stokes shift (λ_ex = 365 nm, λ_em = 581 nm). Theoretical analyses (DFT, TD-DFT, PES, FMO, and others) provide deep insight into the mechanistic aspects, and help in understanding the experimental evidence. The present work attempts to solve the long-standing problem associated with the mechanistic path (the passage of proton transfer, transition states, FMO, etc.) involved in photochromism. The photo-responsiveness is categorized firmly for the first time as a photo-thermal reversible process (T-type). The developed system demonstrates a large scope to design molecular logic gates and keypad lock.