Realignment of Local and Charge-Transfer Excited States in Promoting Room-Temperature Phosphorescence of Organic Aggregates

Aggregation significantly affects the organic room-temperature phosphorescence (RTP), especially in molecules with donor (D)–acceptor (A) architectures but is yet to be rationally understood. By constructing different D–A aggregates in solid films, substantial modulation of both fluorescence and phosphorescence from the locally excited (LE) and intramolecular charge-transfer (ICT) states were achieved. Systematic investigations reveal that the aggregation-sensitive dihedral angle (φ) between D and A units controls the electronic communications, leading to enhanced D–A coupling and reduced ICT at shrinking φ for the varied excited-state energies, but the LE energy is unaffected due to the absent D–D stacking. These different behaviors result in obvious energy realignment, especially at heavily aggregated structures, exerting dramatic effects on triplet exciton harvesting for varied luminescent behaviors. At heavy aggregation in films/crystals, the singlet ICT (1ICT) grows energetically closer to the singlet LE (1LE) and even higher than the triplet LE (3LE), which makes the internal conversion from 1LE to 1ICT and the intersystem crossing from 1ICT to 3LE more efficient, promoting RTP from the molecular aggregates. This work with in-depth photophysical insights into excited-state energy realignment caused by aggregation sheds important light on the understanding of RTP emission in solid states and the development of efficient RTP materials with D–A structures.