Interchain Delocalization of Photoinduced Neutral and Charged States in Nanoaggregates of Lengthy Oligothiophenes

Photoluminescence (PL) and photoinduced absorption (PIA) spectroscopy measurements were performed on two lengthy oligothiophenes, G3-T11-G3 and G3-T17-G3, doubly end-substituted with third generation poly-benzyl ether dendrons. These oligothiophenes form well-defined nanoaggregates in dichloromethane solution upon cooling. The molecularly dissolved and aggregated phases interconvert reversibly in a narrow temperature range. PL and PIA spectroscopy were used to investigate the optical signatures of photoexcited singlet, triplet, and charged states as a function of aggregation. The extent of aggregation could be controlled by varying the temperature. Both the fluorescence and the triplet absorption spectra of the aggregated phase were significantly bathochromically shifted when compared to the spectra of the isolated molecules in solution. These bathochromic shifts indicate that interchain delocalization of the singlet and triplet photoexcitations occurs within the dendritic nanoassemblies. Charged states of G3-T11-G3 and G3-T17-G3 were selectively created by photoexcitation in the presence of an external electron acceptor (tetracyanoethylene). The principal absorption bands of the charged states shift to lower energy upon aggregation. Surprisingly, new high-energy bands are observed in the PIA spectrum of the aggregated phase. These transitions are clear signatures of two-dimensionally delocalized polaronic charge carriers within the nanoaggregates (i.e. intermolecular delocalization over the constituent molecules within the aggregate).