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Electroabsorption spectra of multipolar chromophores

Electroabsorption (EA) spectroscopy is a useful tool to investigate electronic properties of solvated dyes. Following Liptay, EA spectra can be fitted from the linear absorption spectrum and its first and second derivatives, to extract the variation of the dipole moment and of the molecular polarizability upon photoexcitation. In the lack of specific models, the Liptay approach, developed for dipolar dyes, is currently applied to multipolar dyes. We discuss EA spectra of quadrupolar and octupolar dyes, based on essential state models. A perturbative treatment of the applied field allows to express the numerically exact EA spectra of multipolar dyes: the second-derivative contribution vanishes for quadrupolar dyes, and for both quadrupolar and octupolar dyes a contribution appears due to the electric field-induced absorption to a dark (two-photon allowed) state. A new fitting procedure is then proposed for EA spectra of quadrupolar and octupolar chromophores, leading to reliable estimates of the molecular parameters. The method is applied to a few dyes of interest for two-photon absorption applications.