Charge Instability in Quadrupolar Chromophores:  Symmetry Breaking and Solvatochromism

Abstract

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.

Enhanced Two‐Photon Absorption of Organic Chromophores: Theoretical and Experimental Assessments

“…The reliability of a three-state model for quadrupolar chromophores with enhanced TPA response has been widely tested and accepted (e.g., [90,157,158] ). Similar to the two-state case, MIX [155] or ρ [158] parameter can be chosen to describe molecular properties in the framework of the three-state model.…”

“…[146][147][148][149] Since only a few electronic transitions often predominate in the nonlinear resonant spectra of organic molecules, effective few-state models have become very popular for rational molecular design of NLO-phores. [90,103,127,[150][151][152][153][154][155][156][157][158][159][160] For branched structures, the Frenkel exciton model has been shown to provide a valuable qualitative tool to connect the photophysical properties of branched chromophores to those of their corresponding monomeric counterpart. [113,120,[161][162][163] Theoretical limits for TPA activities have also been explored as well.…”

Observing Second‐Order Nonlinear Optical Properties by Symmetry Breaking in Centrosymmetric Furan‐Containing Oligoaryl Cyclophandienes

“…[2][3][4] Couplings between electron and molecular vibrations in cyanine dyes and related linear conjugated systems are known to lead to symmetry breaking. [4] The electron localization in the excited state of the phenylenevinylene-based conjugated polymers has been rationalized by a conformational kink mechanism.…”

“…[9a] Quadrupolar organic dyes have been shown to be solvatochromic and symmetry breaking may take place. [3] Hyper-Rayleigh scattering (HRS) at the doubled frequency, 2w, has been observed for centrosymmetric dichloro-and tetrachloroethenes in solution. [10] Although the intensities are relatively low, an involvement of cooperative scattering by neighboring molecules [10] including solvent molecules [11] has been suggested to account for these observations.…”

Sensitivity of the photophysical properties of organometallic complexes to small chemical changes

“…For example, for large molecular systems, one can also describe the nuclear dynamics quantum mechanically 32 and include the effect of the environment such as the solvent. 33,34 Such models can capture universal behavior that is not sensitive to microscopic details. For example, the single impurity Anderson model can describe the Kondo effect in a diverse range of systems including magnetic impurities in metals, quantum dots in semiconductor heterostructures, carbon nanotubes, and single molecule transistors.…”