Understanding High Fluorescence Quantum Yield and Simultaneous Large Stokes Shift in Push-Pull Molecular Dyads in Polar Solvent*

Donor-acceptor (D-A) systems are potential candidates for bioimaging, sensing, photonic and photovoltaic applications. Recently, large Stokes shift (SS) with remarkably high fluorescence quantum yield (Φ_F) in polar solvents were reported for a pyrene and styrene-based D-A chromophore (pentafluorostyryl-aminopyrene, StyPy) and also for diphenylphenanthroimidazole (PPI) and triazolopyridine (TP) based D-π-A dyads with varied phenyl π-bridge length (PPI-P_n-TP, n =1, 2). Solvent driven stabilization of twisted intramolecular charge-transfer (TICT) emissive-state for the StyPy and of ICT for PPI-P_n-TP dyads were claimed to be responsible for the observed large SS. However, TICT/ICT generally corresponds to decoupling of frontier molecular orbitals (FMOs) between the D and A units, which is expected to reduce Φ_F.

To microscopically understand these experimental observations, we performed quantum-chemical calculations, which revealed a partial CT assisted large excited-state relaxation (λ) at the low-lying singlet excited-state for StyPy and a substantial λ involving primarily rotation of the PPI phenyl-ring directly linked to the π-bridge for PPI-P_n-TP, leading to an almost planarized and structurally rigid emissive state. Importantly, while a greater FMOs overlap at the planar excited-state together with an increased structural rigidity yield high Φ_F, a significant λ is identified be the root-cause for the observed large SS in polar solvents for both D-A dyads.