Synthesis and solvatochromism of dipodal and tripodal dyes derived from Brooker’s merocyanine

Abstract

Brooker’s merocyanine (BM) is a classic solvatochromic probe, which exhibits a band in the visible region sensitive to changes in the polarity of the medium. BM shows a reversion in its solvatochromism, which has inspired much research on studies involving different families of dyes. We synthesized four novel dipodal and tripodal solvatochromic dyes (1b–4b) inspired by BM molecular structure. Characterization of the compounds in their protonated form used spectrometric and differential scanning calorimetry techniques. Dyes 1b–4b have an electron–donating phenolate moiety connected through a CHCH conjugated bridge to two or three N–methylpyridinium electron–acceptor groups. The probes exhibit negative solvatochromism in the solvents studied, showing very different colors with the change in the polarity of the solvent. Multiparameter strategies of Kamlet–Abboud–Taft (KAT), Catalán, and a modified KAT fitted the experimental data. The HBD acidity of the solvent represented the main contribution to the solvation of the probes. Performing electronic–structure calculations of the compounds at LC–BLYP/ma–def2–SVP/D4 level and employing water and DMSO as implicit solvents (SMD model) using software ORCA5.0 revealed that the longest–wavelength absorption bands in the dyes are due to a π → π* transition, from the phenolate (electron–donating) group to the N–methylpyridinium (electron–acceptor) groups. The calculations did not reproduce the experimentally observed negative solvatochromism of the dyes in the solvents studied. However, comparing the theoretical and experimental data of the present study with data from the literature provided important information regarding the results obtained.