We present a joint theoretical and experimental work aimed to understand the spectroscopic behavior of multipolar dyes of interest for nonlinear optics (NLO) applications. In particular, we focus on the occurrence of broken-symmetry states in quadrupolar organic dyes and their spectroscopic consequences. To gain a unified description, we have developed a model based on a few-state description of the charge-transfer processes characterizing the low-energy physics of these systems. The model takes into account the coupling between electrons and slow degrees of freedom, namely, molecular vibrations and polar solvation coordinates. We predict the occurrence of symmetry breaking in either the ground or first excited state. In this respect, quadrupolar chromophores are classified in three different classes, with distinctively different spectroscopic behavior. Cases of true and false symmetry breaking are discriminated and discussed by making resort to nonadiabatic calculations. The theoretical model is applied to three representative quadrupolar chromophores:  their qualitatively different solvatochromic properties are connected to the presence or absence of broken-symmetry states and related to two-photon absorption (TPA) cross-sections. The proposed approach provides useful guidelines for the synthesis of dyes for TPA application and represents a general and unifying reference frame to understand energy-transfer processes in multipolar molecular systems, offering important clues to understand basic properties of materials of interest for NLO and energy-harvesting applications.

Absorption and steady-state emission spectra of two fluorescent dyes are measured in a series of aprotic solvents with similar refractive index and different polarity. The spectra are interpreted in terms of a two-state electronic model accounting for the coupling to internal vibrations and to an effective solvation coordinate. The proposed approach naturally accounts not only for solvatochromic shifts of absorption and emission bands but also for the evolution of band shapes with solvent polarity and for the observation of nonspecular absorption and fluorescence bands. The good agreement between experimental and calculated spectra confirms the validity of a two-state picture for the low-energy spectral properties of these donor−acceptor molecules, provided that the molecular polarizability is fully accounted for. The role of conformational degrees of freedom in flexible chromophores is also addressed.

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.