Rapid energy transfer in a dendrimer having π-conjugated light-harvesting antennas

We investigate rapid energy transfer (ET) and its temperature dependence in a star-shaped stilbenoid phthalocyanine (SSS1Pc) dendrimer having π-conjugated light-harvesting (LH) antennas, and develop an appropriate model. In SSS1Pc, an intense core photoluminescence (PL) band appears under the selective excitation of the absorption bands of the LH antenna due to highly efficient ET at room temperature (RT). The transient response of core-absorption bleaching and the temporal behaviours of the PL intensities of the core and antenna reveal that ET from the LH antenna occurs rapidly prior to achieving quasi-equilibrium in the photoexcited state of the LH antenna. In addition, it is also clarified that the ET quantum efficiency in SSS1Pc degrades at temperatures lower than ~100 K. To understand these results, we develop an ET model based on a π-conjugating network between the LH antenna and the core that accounts for steric hindrance between the LH antenna and the torsional vibration of the LH-antenna subunit. This model reveals that highly efficient ET occurs at RT through the π-conjugated network mediated by the thermally activated torsional vibration of the LH-antenna subunit.