Single molecule (SM) spectroscopy has been conducted to study exciton-like intramolecular charge transfer (ICT) coupling dynamics in two model dendritic systems (as shown in Figure 1) containing the branched ICT interactions. Both S-2 and TOZ-2 bear a common electron-donating moiety (D) in the central part, connected in each branch to an electron-withdrawing group (A) through a conjugated linker. In contrast to TOZ-2, the branches in S-2 are additionally terminated by electron-donating groups. The strong-coupling and stepwise photobleaching of the ICT branches in the dendrimers, depending on torsional disorder have been demonstrated at the SM level (see Figure 2). The fluorescence from delocalized ICT excited state over two branches in push-pull molecules have been observed at SM level, which can not be distinguished by means of conventional experiments probing the averaged behavior of large ensembles of molecules. The study of these two different molecules using SM spectroscopy could help us to determine the nature of interbranch ICT coupling for the electronic delocalization and CT phenomena.

J. Am. Chem. Soc. 2009，131，5742－5743

Figure 1. Molecular structures and normalized absorption (solid lines) and fluorescence (dashed lines) spectra in CHCl3 of (a) S-2 and (b) TOZ-2.

Figure 2. (a) Representative SM fluorescence-intensity time traces, (b) intensity-ratio and (c) lifetime-ratio histograms for the molecules, showing two-step photobleaching, and (d) the corresponding LS/IS correlation plots for S-2 (blue) and TOZ-2 (red).