Charge instability in quadrupolar chromophores: symmetry breaking and solvatochromism.

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.