Solvent Polarity as a Diagnostic for Vibronic/Vibrational Coherences Distinction in an Efficient TADF-TTA Switchable Emitter.

Distinguishing vibrational from vibronic coherences in donor-acceptor (D-A) emitters remains a central challenge in excited-state spectroscopy, particularly for thermally activated delayed fluorescence (TADF) systems where temperature-dependent analysis is often compromised by conformational changes and altered charge-transfer (CT) populations. Here, we introduce a pump-probe-only, solvent-polarity-based strategy to resolve this problem using 1,8-naphthalimide D-A chromophores. In nonpolar n-hexane, resonant LE-CT coupling generates persistent coherent beating, whereas in polar dichloromethane, the CT state is stabilized, the LE-CT gap widens, and vibronic coherence is quenched while vibrational oscillations remain comparatively robust. This establishes a practical diagnostic for identifying LE-CT vibronic mixing without recourse to temperature variation or two-dimensional spectroscopy. Beyond coherence assignment, the same LE-CT coupling explains intensity borrowing and the appearance of CT absorption even in near-orthogonal geometries, providing a route to estimate the effective CT transition dipole and to rationalize enhanced optical absorption. Finally, we show unprecedented polarity-controlled switching between TADF and triplet-triplet annihilation (TTA) in a single deep-blue emitter, thereby linking coherence physics, LE-CT electronic structure, and triplet harvesting into a unified framework for designing polarity-robust optoelectronic materials.