Interchain delocalization of photoinduced neutral and charged states in nanoaggregates of lengthy oligothiophenes

Photoluminescence (PL) and photoinduced absorption (PIA) spectroscopy measu rements were performed on two lengthy oligothiophenes, G3-T11-G3 and G3-T17 -G3, doubly end-substituted with third generation poly-benzyl ether dendron s. These oligothiophenes form well-defined nanoaggregates in dichloromethan e solution upon cooling. The molecularly dissolved and aggregated phases in terconvert reversibly in a narrow temperature range. PL and PIA spectroscop y were used to investigate the optical signatures of photoexcited singlet, triplet, and charged states as a function of aggregation. The extent of agg regation could be controlled by varying the temperature. Both the fluoresce nce and the triplet absorption spectra of the aggregated phase were signifi cantly bathochromically shifted when compared to the spectra of the isolate d molecules in solution. These bathochromic shifts indicate that interchain delocalization of the singlet and triplet photoexcitations occurs within t he dendritic nanoassemblies. Charged states of G3-T11-G3 and G3-T17-G3 were selectively created by photoexcitation in the presence of an external elec tron acceptor (tetracyanoethylene). The principal absorption bands of the c harged states shift to lower energy upon aggregation. Surprisingly, new hig h-energy bands are observed in the PIA spectrum of the aggregated phase. Th ese transitions are clear signatures of two-dimensionally delocalized polar onic charge carriers within the nanoaggregates (i.e. intermolecular delocol ization over the constituent molecules within the aggregate).