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

Abstract

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.

Reverse Solvatochromism of Imine Dyes Comprised of 5‐Nitrofuran‐2‐yl or 5‐Nitrothiophen‐2‐yl as Electron Acceptor and Phenolate as Electron Donor

“…Some experiments were performed to verify this possibility (see Figures S43 and S44 in the Supporting Information). The results are similar to those obtained previously for a series of (nitrostyryl)phenolate solvatochromic dyes . Therefore, based on these experiments, the possibility of the formation of ion pairs in non‐polar media was discarded.…”

(E)-2,6-Dibromo-4-{2-[1-(1H,1H,2H,2H-perfluorooctyl)pyridinium-4-yl]ethenyl}phenolate methanol disolvate, a fluoroponytailed solvatochromic dye

“…The crystal structure of an acceptor-substituted conjugated 2,6-dibromophenol derivative (refcode SULSAV), displaying visible solvatochromism, has been reported (Stock et al, 2015).…”

A generalized reversal model for the solvatochromism of merocyanines

“…One of the most widely employed scales is based on the solvatochromism of the pyridinium‐N‐phenolate betaine B30 (Scheme ), the E T (30), or the E T N scale, in its normalized version 9a . It has been used to assess the polarity of a large number of solid and liquid systems in the literature, and among them, to solutions of the compounds analysed in this work . However, as will be discussed below, it correlates poorly with the solvatochromic response of Class I dyes, where neither the donor, nor the acceptor fragments are charged groups.…”