Negative Solvatochromism of the Intramolecular Charge Transfer Band in Two Structurally Related Pyridazinium—Ylids

Two charge transfer compounds based on pyridazinium ylids were studied by electronic absorption spectroscopy in binary and ternary solutions, with the purpose of evaluating their descriptors of the first singlet excited state and to estimate the strength of the intermolecular interactions in protic solvents. The molecular descriptors of the excited state were comparatively estimated using the variational method and the Abe model of diluted binary solutions. Analysis of electronic properties using density functional theory was performed for several key solvents, in order to understand the solvatochromic behavior. The DFT calculations revealed that, in the polar and strongly interacting solvents, the carbanion and the terminal group become a stronger electron acceptor. The bathochromic shift of the ICT band was confirmed using DFT calculus. The ability of the two ylids to recognize and discriminate the solvents was analyzed with principal component analysis and with cluster analysis. Although the study was performed in 24 solvents, the results showed that the ylids were most sensitive to alcohols, so they can be a useful tool to identify and classify different types of low-alcoholic solvents.