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

Abstract

Eight compounds with phenols as electron-donating groups and 5-nitrothiophen-2-yl or 5-nitrofuran-2-yl acceptor moieties in their molecular structures were synthesized. The crystalline structures of six compounds were obtained. Their corresponding phenolate dyes were studied in 29 solvents and the data showed that in all cases a reverse solvatochromism occurred. The results are explained in terms of the ability of the medium to stabilize the electronic ground and excited states of the probes to different extents. The frontier molecular orbitals were analyzed for the protonated and deprotonated forms of the compounds. The calculated geometries are in agreement with the X-ray structures determined for the compounds and it was verified that after their deprotonation an increase in the electron delocalization occurs. Radial distribution functions were calculated for the dyes in water and n-hexane to analyze different solvation patterns resulting from the interaction of the solvents with the dyes. Data obtained by using the Catalán multiparameter equation revealed that the medium acidity is responsible for hypsochromic shifts, whereas the solvent basicity, polarizability, and dipolarity contributed to bathochromic shifts of the solvatochromic band of these dyes. Two model "hybrid cyanine" dyes were used in the design of simple experiments to demonstrate that the solvatochromic behavior of these dyes in solution can be tuned with careful consideration of the properties of the medium.

The Simulation of Solvent Polarizabilities and Dipolarities with Polarizable Continuum Model

“…This classification is ultimately arbitrary because the definition of solvent polarity can vary with the scale and probe employed. For instance, Reichardt’s E T (30) scale is normally employed, with the result that dyes that do not possess the E T (30) electron-donor and/or -accepting groups may yield plots with significant dispersion. , …”

“…For instance, Reichardt's E T (30) scale is normally employed, 21 with the result that dyes that do not possess the E T (30) electron-donor and/or -accepting groups may yield plots with significant dispersion. 30,31 Continuum models have been employed to mimic the behavior of different dyes with variable success. In a comparison of 33 dyes that exhibited negative (15), positive (5) and reverse (13) solvatochromism, Domi ́nguez et al 32 examined the average contributions of specific and nonspecific solvent effects to their solvatochromism, concluding that polarizability does not play any significant role for negative solvatochromic dyes; by contrast, polarizability and dipolarity are essential in the solvatochromism of positive and a majority of inverted dyes.…”

Influence of Solvent Properties on Two Individuals Presented in the Excited State – Reverse Fluorosolvatochromism of CNAacDMA**

“…[5,6] A third option is possible if a dye shows positive and negative solvatochromism in different range of solvent polarity; it is so called inverted or reverse solvatochromism. An example of compound exhibiting reverse solvatochromism are classical Brooker's merocyanines [7] or 4-(nitrostyryl)phenolate, [8] styrylpyridinium salts, [9] pyridinium phenolate betaine dyes. [10] However, the directional fluorochromism described by us, next to pyrrolo [3,2-b] pyrrole-diketopyrrolopyrrole [11] is, to the best of our knowledge, only the second such case.…”