The Negative Solvatochromism of Reichardt‘s Dye B30 – A Complementary Study

Abstract

The UV/Vis spectra of a hypothetical negative solvatochromic dye in a solvent are theoretically calculated assuming the classical damped harmonic oscillator model and the Lorentz‐Lorenz relation. For the simulations, the oscillator strength of the solvent was varied, while for the solute all oscillator parameters were kept constant. As a result, a simple change of the oscillator strength of the solute can explain the redshift and intensity increase of the UV/Vis band of the solute. Simulated results are compared with measured UV/Vis spectroscopic data of 2,6‐diphenyl‐4‐(2,4,6‐triphenylpyridinium‐1‐yl) phenolate B30 (Reichardt‘s dye) Significant correlations of the absorption energy (1/λmax) with the molar absorption coefficient ϵ as function of solvent polarity are demonstrated for several derivatives of B30. The approach presented is only applicable to negative solvatochromism. Therefore, while the approach is vital to fully understand solvatochromism, it needs to be complemented by other approaches, e. g., to describe the changes of the chemical interactions based on the nature of the solvent, to explain all its various aspects.

The Global Polarity of Alcoholic Solvents and Water – Importance of the Collectively Acting Factors Density, Refractive Index and Hydrogen Bonding Forces

“…[ 1 , 3 , 23 , 24 , 25 ] Thus, the decrease of as a function of N is more pronounced for tertiary alcohols than for primary or secondary ones indicating a higher oscillator strength and/or a stronger coupling of the solvent oscillators with those of the solute and consequently a stronger redshift in accordance with the previous study. [8a] The high significance of the correlations is impressive and indicates reliable relationships (Figure 2 ).…”

“…Recently, it could be demonstrated that electromagnetic coupling of electronic excitations of solutes B1 , B26 , and B30 , respectively, (Figure 1 ) with those of the solvent plays a major role. [8a] In the simplest case, this coupling is described by the frequency dependent Lorentz‐Lorenz relation, which predicts red shifts and also intensity changes with the same order of magnitudes like those that are experimentally observed. [ 8b , 8c , 8d ]…”

“…This result is a clear indication that the E T (30) parameter for primary alcohols is essentially determined by volume properties of the solvent and not by specific hydrogen bonds, which is consistent with the theory of electrostatic coupling between solvent and solute. [ 8a , 41 ]…”

“…Currently, it has been demonstrated that the molar absorption energy 1/ λ max [related to E T (30), Equation (1)] and the molar absorption coefficient of B1 , B26 and B30 , respectively, are related to each other. [8a] Analysing these relationships, the quotient of the molar absorption energy 1/λ max (in cm −1 ) and the molar absorption coefficient ϵ (in cm 2 /mol) [Eq. (8a) or (8b)] correspond to the N , because the ratio 1/(λ max ϵ) has the same physical unit.…”

“…This result is in full agreement with the results of our earlier study, according to which the electromagnetic coupling of the oscillators of solvent and solute determine the measured value. [8a] Therefore, in this work, we want to refine the so‐called D HBD quantity from Ref. [21b] which allows an extension of our concept in Ref.…”