Symmetry Breaking in Octupolar Chromophores: Solvatochromism and Electroabsorption

In this contribution, we adopt an essential-state description for octupolar (AD3 or DA3) chromophores (where A is an electron-acceptor and D is an electron-donor) that also accounts for the coupling of electrons to molecular vibrations and for solvation effects. The first excited state of octupolar chromophores is always multistable and can therefore support symmetry breaking. In particular, symmetry is always broken in the relaxed excited state of octupolar dyes in polar solvents, with consequent localization of the excitation on one of the dipolar molecular branches. This rationalizes the common observation of strongly solvatochromic fluorescence spectra for octupolar chromophores. The model is validated through the comparison with experimental data. The essential-state model is also adopted to derive a perturbative expression for the electroabsorption spectrum: if compared with the formalism derived for dipolar molecules, a new term appears for octupolar chromophores, due to the field activation of an otherwise dark transition. The importance and implications of this term are discussed.