Solvatochromic behavior of dyes with dimethylamino electron-donor and nitro electron-acceptor groups in their molecular structure

Abstract

Six dyes with N,N‐dimethylaminophenyl and 4‐nitrophenyl or 2,4‐dinitrophenyl groups in their molecular structures were prepared and characterized. These compounds have different conjugated bridges (CC, CN, and NN) connecting the electron‐donor and the electron‐acceptor groups. All compounds are solvatochromic, with reverse solvatochromism occurring. The solvatochromic band observed in each spectrum for the dyes is due to a π ➔ π* transition, of an intramolecular charge transfer nature, which occurs from the electron‐donor N,N‐dimethylaminophenyl group to the electron‐acceptor group in the molecules, which is reinforced by the structures of the compounds optimized by applying density functional theory, which exhibit high planarity. The reverse solvatochromism was explained considering two resonance structures. The benzenoid form is better stabilized in less polar solvents and characterizes the region displaying positive solvatochromism, while the dipolar form is better stabilized in more polar solvents, in the region of negative solvatochromism. The Catalán multiparametric approach was used to study the contribution of solvent acidity, basicity, dipolarity, and polarizability to the solvatochromism exhibited by the compounds. These compounds are good candidates for the investigation of the polarizability and, to a lesser extent, the dipolarity of the medium, with very little interference from specific interactions of the solvent through hydrogen bonding. Copyright © 2014 John Wiley & Sons, Ltd.

A generalized reversal model for the solvatochromism of merocyanines

“…The solvatochromic behaviour of Brooker's merocyanine 6 (Scheme ) has been a controversial subject for three decades …”

“…Plots of transition energies E T vs. E T N solvent polarity values for compounds 17 , 18 , 19 . Hyperbolic fitting curves (see Supporting Information) are shown with an extended or amplified solvent polarity axis.…”

“…Compounds 17 , 18 , 19 constitute another example of a reversal shift with structural changes (Scheme ).…”

“…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.…”

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

“…[4] This compound is knownf or itsv ery pronounced positive solvatochromism, with its visible solvatochromic band beingd isplaced to increasing l max values when the polarityo f the mediumi si ncreased. It has also been verified that some families of solvatochromic dyes, for instance, the classical Brooker's merocyanine (dye 3) [5] or compound 4, [6] interestingly present ar eversal in their solvatochromic behavior.F or these dyes,anegative solvatochromism is verified for the most polar solventst hrought os olvents exhibiting intermediate polarity, followed by ar eversal in the solvatochromism, from negative to positive, occurringw ith af urther reduction in the polarity of the solvent.…”