Energy transfer dynamics in light-harvesting small dendrimers studied by time-frequency two-dimensional imaging spectroscopy

Rapid energy transfer dynamics in light-harvesting small dendrimers (star-shaped stilbenoid phthalocyanine: SSS2Pc-m, m=1,2) having different numbers of oligo(p-phenylenevinylene) periphery antennas was studied by time-frequency two-dimensional (2D) real-time pump-probe imaging spectroscopy; the dendrimers having one light-harvesting periphery antenna for each aromatic ring (SSS2Pc-1) are expected to maintain a planer structure, leading to good π-conjugation, while those having two periphery antennas for each aromatic ring (SSS2Pc-2) have a large steric hindrance between the periphery antennas. Under the selective excitation of the periphery antennas (∼400 nm), the time-frequency 2D image of the absorbance changes in SSS2Pc-1 shows the transient absorption due to the core around 500–600 nm and the ground-state bleaching of the Q-band around 700 nm with a rise time of 0.5 ps. The transient absorption also shows clear oscillatory components of ∼0.6 Thz, which might come from the torsional motion of the peripheries. These results show that rapid and highly efficient energy transfer occurs from the periphery antennas to the core, and that the torsional vibration plays an important role for the energy transfer. The experimental results obtained in SSS2Pc-1 are compared with those in SSS2Pc-2 having a large steric hindrance.