Dual conformational emission and vibrational coherence in a sulfone-embedded narrowband emitter

Organic luminescent narrowband emitters have emerged as promising candidates for wide-color-gamut displays. However, the comprehensive photo-induced structural and vibrational dynamics responsible for the spectral broadening remain poorly understood. In this study, ultrafast spectroscopy is employed to elucidate the excited state dynamics of a sulfone-embedded narrowband emitter, 2tCPD. An intrinsic dual emission, which originates from a reversible conformational transition between boat and chair structures on a timescale of ∼100 ps, is revealed. In aromatic toluene solution, the metastable chair conformation is stabilized by forming a sandwich-like toluene-2tCPD-toluene complex through weak π-π interaction, thereby switching the dominant emission to the chair form. Furthermore, the direct observation of coherent oscillations enables the visualization of vibronic coupling in 2tCPD, where the key vibrational mode at 180 cm^-1 oscillates along the excited-state potential energy surface of the boat conformation. This mode, which involves the bending vibration of sulfone and out-of-plane bending of carbonyl and peripheral phenyl groups, mainly drives the structural reorganization during the electronic transition. These findings provide mechanistic insights into conformation-dependent emission in narrowband emitters and underscore the crucial vibrational information in guiding the molecular design toward high color-purity organic fluorophores.