Thermosolvatochromism of Betaine-30 in CH₃CN

Abstract

The solvatochromic dye betaine-30 is thermochromic as well, due to the temperature dependence of solvent polarity, which strongly influences the wavelength of the visible absorption band. We report an analysis of the temperature-dependent absorption spectrum of betaine-30 in CH 3 CN, incorporating the internal-mode displacements determined from the resonance Raman profiles at room temperature. The temperature-dependent solvent reorganization energy λ solv associated with the visible transition of betaine-30 influences the width and position of the absorption spectrum and is relevant to theories for the rate of return electron transfer. We have previously determined λ solv for betaine-30 in acetonitrile and deuterated acetonitrile from analysis of the room-temperature absorption and resonance Raman profiles using time-dependent spectroscopy theory. In this work, we present a revised set of normal-mode displacements, including the contribution from a torsional mode of betaine-30 at 133 cm -1 , obtained from an analysis of the room-temperature Raman profiles in CH 3 -CN and CD 3 CN. These displacements are then kept fixed, and the temperature-dependent absorption spectrum of betaine-30 in acetonitrile is modeled to obtain the solvent reorganization energy, 0-0 energy, and transition moment as a function of temperature. The solvent reorganization energy λ solv is found to decrease with increasing temperature, consistent with decreasing solvent polarity but opposite to the prediction of dielectric continuum theory. In contrast to our previous analysis, the nonlinear solvent response is included in the model, and the amplitude of the solvent response is found to be smaller in the excited than the ground electronic state, due to the decrease in solute dipole moment in the excited electronic state.

Solvent Reorganization Entropy of Electron Transfer in Polar Solvents

“…The main result of our simulations is the confirmation of the earlier theoretical 8,28,31 and experimental 21,26,31,34,[36][37][38][39][40]42 findings of the positive value of the reorganization entropy (negative slope of the reorganization energy with increasing temperature, Figure 1) both at constant-volume and constant-pressure conditions (Table 1). We have also performed simulations of λ s in water at constant temperature of T ) 300 K and varying pressure (NPT MD).…”

Activation entropy of electron transfer reactions

“…In strongly polar solvents, the temperature derivative of c 0 is mostly influenced by the high-frequency dielectric constant, and continuum S λ is nagative. In this case, the predictions of the continuum model significantly depart from both the microscopic calculations and many experimental measurements [12,43,44,45,46,47,50], showing positive reorganization entropies.…”

The thermochromism of the ET(30) betaine in a micro-heterogeneous medium: A spectral and dynamics simulation study

“…A different behavior had been observed in solvents like ethanol and acetonitrile [3,4], where a small, but perceptible shift of the solvatochromic band of 1 was recorded with the rising temperature. This situation was also observed in our systems, after the polymeric micelle had been formed.…”

“…In one case, in solutions of methanol [4], the observation of an isosbestic point was interpreted as arising from two differently solvated forms of the betaine in equilibrium. A second behavior was observed in solvents like ethanol [4] and acetonitrile [3], where a small, but perceptible shift of the solvatochromic band of 1 was recorded with the rising temperature.…”

“…The thermochromism of the E T (30) betaine has been investigated in pure solvents [2][3][4], and in binary solvent mixtures [5][6][7][8]. In pure solvents, two different situations have emerged from the spectral variations of the probe with the temperature.…”