Excited-State Symmetry-Breaking Dynamics in a Centrosymmetric Quadrupolar Emitter: Cascaded Relaxation Pathways

Excited-state symmetry breaking (ES-SB) is a photophysical process wherein centrosymmetric quadrupolar molecules acquire a pronounced dipolar character after photoexcitation. This ultrafast excitation localization and the emergence of dipolar excited states stem from the interplay of electronic coupling, vibronic interactions, and environmental polarization, whose respective contributions are often difficult to disentangle in spectroscopic analysis. In this work, we investigate a linearly aligned centrosymmetric quadrupolar dye with a D-π-A-π-D motif to elucidate how solvent effects and electronic coupling between branches cooperatively govern the onset and degree of ES-SB. By combining femtosecond broadband transient absorption spectroscopy using Laporte-forbidden transitions as symmetry markers with an essential-state model incorporating vibronic coupling and solvation effects, we directly tracked ES-SB dynamics in real time and mapped the associated relaxation potential energy surfaces. Our results reveal that, in polar solvents, ES-SB proceeds via a cascaded relaxation pathway: an initial symmetry-preserving step along symmetric vibrational coordinates, followed by symmetry breaking along antisymmetric vibrational coordinates. Furthermore, we establish a quantitative criterion for ES-SB onset (Stokes-shift energy >3V, where V is electronic interbranch coupling). This work clearly elucidates the dynamics of ES-SB and establish a predictable and tunable framework for rationally controlling ES-SB behavior in quadrupolar chromophores.