Excited-State Processes in First-Generation Phenyl-Cored Thiophene Dendrimers

First generation dendrimers with three oligothiophene arms (meta-arranged, 3G1-nS) and four arms (ortho- and para-arranged, 4G1-nS) connected to a central phenyl core were investigated spectroscopically in solution. In all dendrimers, on an ultrafast time scale (<10 ps), two "cooling" processes convert the initially generated, "hot" exciton into the geometrically relaxed, "cold" exciton. A decrease in the triplet yield, particularly evident for the 4-arm dendrimers; intersystem crossing rate; and nonradiative triplet decay time with increasing number of bridging thiophene units n all meet with expectations from prior studies on linear oligothiophenes. A relatively fast internal conversion process (>0.6 ns^-1) is observed in both dendrimer series, possibly due to increased twisting about the phenyl core that reduces the triplet yields considerably with respect to oligothiophenes. An anomalous shifting of the triplet-triplet absorption spectra characterizes the 4G1-nS dendrimers as unique from the 3G1-nS series in terms of the hindrance of torsional motion and confinement of excited states enforced by the arrangement of dendrons.