Reverse solvatochromism of iminophenolate dyes comprised of 5– nitrofuran–2–yl or 5–nitrothiophen–2–yl as electron acceptor

Compounds 1a-8a, with phenols as electron–donating groups and 5–nitrothiophen–2–yl or 5–nitrofuran–2–yl acceptor moieties in their molecular structures (Figure 1A), were synthesized and studied as probes of pure solvents polarities. Their corresponding phenolate dyes (1b-8b) were studied in 29 solvents with different polarities, which exhibit different colors (Figure 1B), confirming that the compounds are solvatochromic species. The solvatochromic bands in the vis region of the spectra are a result of –* electronic transitions, with an intramolecular CT from the electron–donor phenolate moiety towards the electron–acceptor 2– nitrothiophenyl or 2–nitrofuranyl moiety. Figure 1C shows a plot of the transition energies obtained as a function of the ET(30) solvent polarity values for dye 5b, these being similar to those obtained for the other dyes. In all cases, a reverse solvatochromism was verified.The crystalline structures of six compounds were obtained (Figure 2A). Nearly complete coplanarity between the phenolic ring and the nitro–substituted heterocyclic fragment was verified, with small dihedral angles between the two rings. The frontier molecular orbitals were analyzed for the protonated and deprotonated forms of the compounds (Figure 2B). The calculated geometries are in agreement with X–ray structures determined for the compounds and an increase in the electron delocalization occurs after their deprotonation. Data obtained using the Catalán multiparameter equation revealed that the medium acidity is responsible for hypsochromic shifts, while the solvent basicity, polarizability, and dipolarity contributed to bathochromic shifts of the solvatochromic band of these dyes