Solvent Dependence Distinction of Vibronic Coherences and Enhancing Twisted Donor-Acceptor Systems' Absorption via Controlled Intensity Borrowings

Distinguishing vibrational from vibronic coherences in pump-probe spectroscopy remains a fundamental bottleneck without multidimensional techniques. Further, twisted intramolecular charge-transfer (TICT) states exhibit weak absorption at orthogonal geometries, limiting their use in OLEDs/bioimaging. Moreover, thermally activated delayed fluorescence (TADF) triplet efficiency drops in polar media via larger ΔEST and non-radiative decay. We introduce an unprecedented strategy for donor-acceptor (D-A) type molecules based on solvent-polarity sorting via a 35-femtosecond pump-probe technique only: LE-CT vibronic coherences emerge under resonance in n-hexane; DCM stabilizes CT, enlarges the electronic gap, and quenches vibronic signals, but vibrational coherences persist. We also demonstrate how LE-CT mixing can boost TICT absorption: the effective transition dipole moment (TDM) scales with the angle θ between the TDMs of the two states. Aligned TDMs enhance absorption, while anti-aligned yield quasi-dark states. Finally, a Dynamic TADF (non-polar)/TTA (polar) switching strategy is presented: synthesized molecule A1 shows deep-blue TADF in hexane (222% emission gain), converts to TTA in DCM (179% gain), reflecting polarity robust superiority to TADF only emitters. A2 yields pure blue TTA (CIE 0.15, 0.06; QY 0.44/0.69 degassed). We anticipate this study to enable versatile, high-efficiency D-A materials and help distinguish vibrational and vibronic coherences.