Vanillin-based merocyanine dyes: Multiparametric stimuli-responsive dyes for vapor, solution, and solid-state environments

New merocyanine-type dyes were synthesized through a simple and efficient route using low-cost starting materials. This process involved the condensation of cationic heterocyclic compounds containing an activated methyl group with 3-methoxy-4-hydroxybenzaldehyde (vanillin) under basic conditions, yielding the dyes in good yields (54-66%). Photophysical studies revealed pronounced positive solvatochromism (Δλ ≈ 70 nm, λ abs = 484-560 nm) associated with a solvent-dependent modulation of the ground-state electronic structure, reflecting changes in charge distribution between quinoid and cyanine-like limiting forms. The cyanine-like description corresponds to a resonance hybrid characterized by delocalized electronic density, rather than a discrete valence structure. The dyes exhibit emission originating from an intramolecular charge transfer (ICT) excited state, as supported by Lippert-Mataga correlations. Qualitative, proof-of-concept sensing experiments were carried out with the model dye 5b and its conjugated acid 5bH to illustrate their potential as optical probes, rather than fully optimized sensors. Additionally, dye-doped silica matrices based on 5b were prepared, enabling rapid naked-eye identification of acidic and basic vapors and hydrogen peroxide under controlled laboratory conditions.