Dynamic Excited-State Localization Induced by Jahn-Teller Distortion Observed by Coherent Vibrational Spectroscopy

Molecular symmetry is a central design element in functional materials, yet its dynamic modulation in the excited state and its consequences for optoelectronic properties remain largely unexplored, particularly in main-group p-block element complexes. We address this knowledge gap by investigating unique Al(III) dinuclear triple-helical complexes that combine high symmetry with twisted π-conjugated systems and achieve exceptional optical properties of unusually large Stokes shifts and high photoluminescence quantum yields. Using transient absorption spectroscopy with a 10 fs pump pulse, we detected coherent vibrational oscillations overlapped with transient absorption/stimulated emission signals. Analysis of the dephasing times of oscillatory signals revealed photoexcitation-triggered Jahn-Teller distortions in these high-symmetry p-block complexes, evidenced by a specifically short dephasing time constant of 410 fs associated with intraligand twisting vibrations. Our findings demonstrate that excited-state symmetry breaking, strongly coupled with intraligand twisting vibrations, is crucial in determining the remarkable photofunctional properties of large Stokes shifts and high photoluminescence quantum yields. This work elucidates the fundamental mechanisms underlying the performance of these Al(III) complexes and provides a conceptual framework for designing next-generation photofunctional materials by harnessing dynamic symmetry changes.