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

Aggregation significantly affects the organic room-temperaturephosphorescence (RTP), especially in molecules with donor (D)-acceptor(A) architectures but is yet to be rationally understood. By constructingdifferent D-A aggregates in solid films, substantial modulationof both fluorescence and phosphorescence from the locally excited(LE) and intramolecular charge-transfer (ICT) states were achieved.Systematic investigations reveal that the aggregation-sensitive dihedralangle (& phi;) between D and A units controls the electronic communications,leading to enhanced D-A coupling and reduced ICT at shrinking & phi; for the varied excited-state energies, but the LE energy isunaffected due to the absent D-D stacking. These differentbehaviors result in obvious energy realignment, especially at heavilyaggregated structures, exerting dramatic effects on triplet excitonharvesting for varied luminescent behaviors. At heavy aggregationin films/crystals, the singlet ICT ((ICT)-I-1) grows energeticallycloser to the singlet LE ((LE)-L-1) and even higher than thetriplet LE ((LE)-L-3), which makes the internal conversion from (LE)-L-1 to (ICT)-I-1 and the intersystem crossing from (ICT)-I-1 to (LE)-L-3 more efficient, promoting RTP from themolecular aggregates. This work with in-depth photophysical insightsinto excited-state energy realignment caused by aggregation shedsimportant light on the understanding of RTP emission in solid statesand the development of efficient RTP materials with D-A structures.