On the determination of molecular polarizability changes upon electronic excitation from the solvent shifts of absorption band maxima

Abstract

A linear relationship was established between the polarizability differences of the ground and excited state molecules (Δα) from electrochromism measurements and pMW, where p is the slope of the plot of absorption band maxima against the Lorenz-Lorentz function ϕ( n 2) = ( n 2 − 2)/( n 2 + 2), n is the refractive index of the solvent and MW is the relative molecular weight of the solute. The Δα values of porphyrins, aromatic hydrocarbons, polar and ionic π-electronic molecules were estimated by using this dependence as a calibration line. The average polarizability is always larger in the excited states (exception: the S 1 state of azulene with Δα=0±2 Å 3). For different transitions in a given compound Δα increases, as their energy and oscillator strength grow (exceptions: the S 3 band of azulene at 296 nm and the S 2 band of bacteriochlorophyll a at 575 nm). The magnitudes of Δα are relatively small in the case of S 1 bands in porphyrins. A linear correlation was found between Δα/MW and the average matrix-induced dipole moment change responsible for linear broadening of spectral holes in an external electric field.