Substituent Effect on Conformational Preferences in Ground and Excited States of Selected Schiff Bases: An Insight from Theoretical Calculations

2-Hydroxy aromatic Schiff bases that exhibit ESIPT (excited state intramolecular proton transfer) or TICT (twisted intramolecular charge transfer) photodeactivation pathways emerge as promising candidates for fluorescent sensors. In this computational work the influence of various substituents, differing in the electronic properties, on conformational preferences in the ground and S-1 excited state for a series of 2-hydroxy-1-naphthylmethylene-hydrazine-based ligands is systematically studied using (TD)DFT calculations. In order to shed light on physical factors which might determine the obtained conformational preferences, extensive bonding analyses are performed. The results highlight the crucial role of a substituent's ability to form not only well-established intramolecular hydrogen bonds (e.g., O-H center dot center dot center dot N) but also unintuitive nonclassic weak interactions (e.g., C-H center dot center dot center dot O, C-H center dot center dot center dot N, and C-H center dot center dot center dot H-C) in the modulation of the equilibrium between naphthol-imine and keto-amine forms, and planar or twisted conformations, and, thus, in determination of photophysical properties of the considered bases.