A Motif for Generating Coherently Degenerate States in A-D-D-A Type Organic Small Molecules

Encouraged by the superior performance of the D-A-A-D dimer in generating Wannier excitons, we developed a new motif to construct A-D-D-A type organic small molecules. The donor moieties consist of 4,4’-dimethyl-4”-((4-amino)styryl)-triphenylamine, while the acceptor is an s-triazine derivative. DFT studies, using two detection techniques, confirmed that a pair of Wannier exictons is generated in one of the newly designed compounds studied here. Based on the analysis of molecular orbitals and the UV-Vis spectrum, A-D-D-A-2 has been identified as the Wannier molecule. Although A-D-D-A-1 is not confirmed as a Wannier molecule based on the molecular orbital inspection, it may still generate Wannier excitons; however further studies with new techniques are needed to confirm this. A brief discussion of the double excitation in A-D-D-A-2 is also provided. The motif used in the formation of Wannier molecules of A-D-D-A type dimers, i.e. A-D-D-A-2, is the same motif as in the other high performing organic small molecules. This suggests that these high-performance organic molecules may already generate Wannier excitons, which could be the primary reason for their exceptional performance as photosensitizers in solar cells.