Synthesis and Solvatochromism of Substituted 4‑(Nitrostyryl)phenolate Dyes

4-(Nitrostyryl)­phenols 2a–9a were
synthesized, and by deprotonation in solution, the solvatochromic
phenolates 2b–9b were formed. Their
absorption bands in the vis region of the spectra are due to π–π*
electronic transitions, of an intramolecular charge-transfer nature,
from the electron-donor phenolate toward the electron-acceptor nitroarene
moiety. The frontier molecular orbitals and natural bond orbitals
were analyzed for the protonated and deprotonated forms. The calculated
geometries are in agreement with X-ray structures observed for 4a, 6a, and 8a. The HOMO–LUMO
energy gaps suggest that, after their deprotonation, an increase in
the electron delocalization is observed. In the protonated compounds,
the HOMO is primarily localized over the phenol ring and the CC
bridge. After deprotonation, it extends toward the entire molecule,
including the NO2 groups. The solvatochromism of each dye
was studied in 28 organic solvents, and it was found that all compounds
exhibit a reversal in solvatochromism, which is interpreted in terms
of the ability of the media to stabilize their electronic ground and
excited states to different extents. The Catalán multiparameter
equation is used in the interpretation of the solvatochromic data,
revealing that the most important contribution to the solute/solvent
interaction is the hydrogen-bond donor acidity of the solvent.