Thermochromism of intramolecular charge transfer emission bands. Probing the temperature dependence of Franck—Condon factors

A Failure of Continuum Theory: Temperature Dependence of the Solvent Reorganization Energy of Electron Transfer in Highly Polar Solvents

“…Figure presents the dependence of , −Ts , λ s , and −Δ F s on the magnitude of the solvent dipole moment, with all other solvent parameters characteristic of acetonitrile. A maximum temperature dependence of λ s and −Δ F s is predicted in weakly polar solvents, with m ≃ 1.5 D. Two previous studies of CT state thermochromism detected maximum temperature dependence of the Stokes shift in solvents with m ≃ 1.2 D, although the solvents investigated differed by more than just the solvent dipole moments. 5b, For solvent dipoles m ≤ 1.5 D the entropies and free energies are approximately equal 47 Equation 57 can be used to approximate the entropic barrier of ET in weakly polar solvents.…”

A Thermodynamic Analysis of the π* andET(30) Polarity Scales

“…Although the classical continuum formulations of both issues (optical transition and ET) emerged almost simultaneously , and have long been developing largely independently, there is now a growing desire to get a rigorous description in terms of intermolecular forces shifting the research of ET reactions toward model systems amenable to spectroscopic methods. It is the combination of steady state and transient optical spectroscopy that becomes a powerful method of studying elementary mechanisms of ET and testing theoretical concepts. − The classical treatments of ET and optical transition have been facing serious problems when extended to nonpolar and weakly polar solvents, since the classical reorganization energies observed are much higher than surmised from electrostatic interactions in linear dielectric formalisms. ET reorganization energies (not to be confused with the solvation reorganization energy mentioned above) due to the classical nuclear modes, as extracted from band-shape analyses of absorption spectra in apolar liquids, were found to fall in the range 0.2−0.4 eV. − The further partitioning into internal vibrations and solvent degrees of freedom is however still uncertain.…”

“…It is the combination of steady state and transient optical spectroscopy that becomes a powerful method of studying elementary mechanisms of ET and testing theoretical concepts. − The classical treatments of ET and optical transition have been facing serious problems when extended to nonpolar and weakly polar solvents, since the classical reorganization energies observed are much higher than surmised from electrostatic interactions in linear dielectric formalisms. ET reorganization energies (not to be confused with the solvation reorganization energy mentioned above) due to the classical nuclear modes, as extracted from band-shape analyses of absorption spectra in apolar liquids, were found to fall in the range 0.2−0.4 eV. − The further partitioning into internal vibrations and solvent degrees of freedom is however still uncertain. Following continuum treatments, the nonpolar part of solvent reorganization energy is usually taken to be zero. 21b,d To the contrary, recent resonance Raman spectroscopy measurements, applied to quantify the intramolecular component, point to a rather high (0.2−0.3 V) contribution of the solvent component. , In view of the widespread interest in the ET problematics, we include in section 3C the calculations of solvent reorganization energies for betaine-30.…”