Modulating Heavy-Atom-Free Triplet-State Dynamics in Benzimidazole-Based D−π–A Molecules: Distinct Roles of Solvent Polarity and Protic Interactions

Efficient triplet-state generation in heavy-atom-free organic molecules is highly desirable for photonic applications. Here, we investigate the triplet exciton dynamics of benzimidazole-based donor-π-acceptor (D-π-A) molecules with different acceptor groups (-OH, -NO2 and -C(CN)2). The transient absorption spectroscopy reveals that nitro-substituted BVI-NO2 in DMSO exhibits obvious intersystem crossing (ISC, 863.4 ps) and triplet-state population (1.53 μs), which is different from the other two molecules. More interestingly, in aprotic solvents, the ISC times are prolonged (1154 ps in DMF, 1887 ps in trichloroethylene) with the decreasing polarity, while the triplet decay shortens to 1.268 and 0.453 μs. In contrast, in protic solvents (methanol and ethanol), ISC lifetimes significantly accelerated to 34.29 and 92.07 ps, accompanied by shortened triplet lifetimes (0.3053 and 0.3591 μs). Theoretical calculation demonstrates that nitro substitution enables a favorable S1(π, π) → T4(n, π*) transition, triggering the absence of the ISC channel in hydroxyl and malononitrile molecules. And protic hydrogen-bonding interactions induce intermolecular orbital mixing and create additional S1-Tn transition channels, further enhancing ISC. These results establish nitro substitution as an effective heavy-atom-free strategy for triplet generation and highlight that proton-induced intermolecular interactions can modulate triplet yields in solvent-assisted doping systems.