Electroabsorption spectra of quadrupolar and octupolar dyes in solution: beyond the liptay formulation

We present theoretical models for electroabsorption (EA) spectra of polar and multipolar (quadrupolar and octupolar) organic chromophores in solutions. Based on essential-state models for the electronic structure, we obtain closed expressions for the EA spectra of these dyes. For polar dyes, we regain the well-known Liptay result, which relates the EA spectrum to the linear absorption spectrum and its first and second derivatives. For multipolar dyes, an additional contribution is found, which is due to the field-induced absorption toward dark states: when the dark states are close in energy to allowed states, this term leads to spurious second-derivative-like structures in the EA spectrum. This is particularly well-apparent for quadrupolar dyes where second-derivative contributions to the EA signal are symmetry-forbidden. The extension of essential-state models to account for slow degrees of freedom, including molecular vibrations and polar solvation, leads to a more-realistic description of optical spectra but hinders the analytical treatment of EA. However, numerically exact EA spectra can be obtained along the same lines that have been recently developed for linear and two-photon absorption spectra of (multi)polar organic chromophores in solution. Essential-state models offer the possibility for a joint analysis of linear and nonlinear spectra (including EA) of (multi)polar dyes: reliable information can then be extracted also on molecular properties for systems where overlapping signals from allowed and dark states would hinder the standard analysis of EA spectra.