The efficient triplet states formation of Se-modified PDI dimers and tetramers in solvents

The triplet excited states of molecules play an important role in photophysical processes, which has attracted great research interest. Perylene diimide (PDI) is a widely studied material closely associated with the generation of triplet states, and it is highly anticipated to become an electron acceptor material for improving photovoltaic conversion efficiency. In this work, we prepared dimers and tetramers composed of selenium-modified PDI-C5 (N,N'-bis(6-undecyl) perylene-3,4,9,10-bis(dicarboximide)) units. We investigated the photophysical processes of these dimers and tetramers in chloroform and toluene using UV-visible absorption spectroscopy, fluorescence spectroscopy, and femtosecond transient absorption spectroscopy. Both the dimers and tetramers undergo efficient triplet state formation processes in the solvents. Solvents with higher polarity facilitate charge transfer thereby promote the triplet states formation. The differences in the configurations of the dimer and tetramer molecules lead to variations in triplet states generation. The twisted angles in the tetramer restricted the intramolecular electronic coupling, posing certain hindrances to exciton coupling and lowering the intramolecular CT characteristics. The emission of excimer in tetramers also competes with the triplet states formation. The research demonstrates the influence of various factors on the generation of triplet states of PDI oligomers.