Structure–behavior study of a family of “hybrid cyanine” dyes which exhibit inverted solvatochromism

Abstract

The inverted solvatochromism of twenty dyes containing an electron-donor phenolate conjugated with an electron-withdrawing nitro-substituted phenyl ring was analyzed in terms of the dye structure and substituents. Structural factors that increased the difference between the electrophilicities of the donor and acceptor moieties, or the donor-acceptor strength of the phenolate dyes, also increased the sensitivity of the dyes to solvent-polarity changes and red-shifted their solvatochromic absorption bands.

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

“…Multiparametric regression analysis allowed a comparison of the effect of specific solvent properties on their solvatochromic behavior. The trends observed were confirmed experimentally by studying the spectral response of some dyes, 6 b and 14 (Scheme ), to specific environmental changes (by using an organic solvent like dichloromethane), brought about by the addition of a co‐solvent or of a cationic species. Thus, besides their full characterization, these novel compounds provided additional experimental and theoretical information that helped us to understand and rationalize the solvatochromic inversion of this interesting family of “hybrid cyanine” dyes.…”

“…The synthesis of compounds 6 a – 13 a was performed as shown in Scheme , by using a methodology described by Stock et al., through the condensation of 5‐nitro‐2‐thiophenecarboxyaldehyde or 5‐nitro‐2‐furaldehyde with the corresponding aniline in the presence of acetic acid as the catalyst and ethanol as the solvent. The 2,6‐substituted aminophenols were prepared according to a methodology described by Johnson et al .…”

“…Compounds 6 b and 14 were chosen as model “hybrid cyanine” solvatochromic dyes. Dichloromethane was chosen as a reference solvent, capable of mixing in all ratios with a more polar solvent and of solubilizing an organic electrolyte.…”

“…We have termed these compounds “hybrid cyanine” dyes, as their molecular structure may be considered a hybrid of class I and class II dyes . As a consequence, solvatochromic plots employing a class I dye as a reference, like the commonly employed E T (30) scale based on pyridinium N ‐phenolate betaine 1 , or a class II dye, such as probe 2 , inevitably yield poor correlations with the solvatochromic shifts of these “hybrid” compounds.…”

“…We have therefore proposed the reference “hybrid” dye 5 (X=CH, R 1 =R 2 =H) to evaluate the effect of different structural features on the solvatochromic behavior of this family of compounds. E T values for all of these derivatives correlate linearly with the E T ref values of the unsubstituted reference stilbene, in around 30 solvents with widely different polarities . The simple two‐parameter formula [Eq. ]…”