Energy Migration in Dendritic Oligothiophene-Perylene Bisimides

A series of novel oligothiophene-perylene bisimide hybrid (DOTPBI) dendrimers up to the second generation (G0, G1, and G2) were investigated. Optical measurements such as nonlinear optical and time-resolved spectroscopy, including two-photon absorption, fluorescence upconversion, and excited state transient absorption were carried out. Results of these measurements revealed the ability of these molecules to undergo intramolecular fluorescence resonance energy transfer (FRET) from the dendritic oligothiophenes (DOT) to the perylene bismide (PBI) moiety. The delocalization length and the photoinduced electron transfer (PET) rate were investigated as a function of dendrimer generation. A fast energy transfer process from the DOT dendron to the PBI core was observed. For the case of the G2 dendrimer, with relatively large thiophene dendrons attached to the bay area of the perylene bisimide, the PBI core is highly twisted and its ability to self-assemble into π–π stacked aggregates is destroyed. As a result, among the three generations studied, G1, which has the best two-photon cross section and the most efficient energy transfer, is the best light harvesting material.