Unveiling the π-Chain Effect on Charge Transfer and Charge Recombination Among Donor−π–Acceptor Material Systems

Charge transfer plays an important role in photophysical and photochemical reactions. However, the factors affecting the excited charge-transfer state are unclear. Here, two donor−π–acceptor dyads with an excellent blue fluorescence quantum yield are designed by integrating 1,2-diphenylphenanthroimidazole (PPI) as an electron donor and 1,2,4-triazolopyridine (TP) as an electron acceptor through phenyl (P) bridges. In the solvents dichloromethane (DCM) and dimethyl formamide (DMF), the dynamics of intramolecular charge transfer (ICT) of the two dyads (TP-P-PPI and TP-P-P-PPI) is located at the Marcus normal region, while the dynamics of charge recombination (CR) is situated at the Marcus inverted region. Therefore, TP-P-P-PPI with a long π-chain exhibits a longer lifetime of ICT but a shorter lifetime of CR than TP-P-PPI does with a short π-chain. In contrast, when the two dyads are spin-coated into a film, the dynamics of ICT and CR processes of the two dyads are restored to be positively correlated with the π-chain length because of the inhibition of intramolecular torsion between PPI and TP in the excited state of the film. This work demonstrates a specific approach via the molecular torsion to tune the dynamics of the ICT and CR among donor−π–acceptor systems.