How β-Ionone Ring Dynamics and Conformational Heterogeneity Control Carotenoid Photophysics

Carotenoid ultrafast photophysics has been studied for decades, but fully resolving their excited-state relaxation mechanisms remains a central, driving pursuit. Here, we investigate mini-beta-carotene (m5) as a tractable model system, coupling high time-resolution transient absorption spectroscopy with ab-initio high-level multireference quantum chemistry calculations. We reveal the key and often overlooked role of the beta-ionone ring-backbone torsional coordinate, which acts as a dynamic "conjugation length control knob" during excited-state relaxation. The torsion of these rings produces ground-state conformational heterogeneity that shapes excited-state spectroscopic signatures and governs the non-radiative relaxation pathway. Explicitly accounting for this coordinate allows us to decipher the ultrafast dynamics and provide a unified interpretation of the experimental transient absorption spectral features. We introduce a general framework based on ring-backbone coupling that bridges simple polyenes and complex carotenoids across varying chain lengths.