Photophysical and photochemical studies on bis(Dendron) poly(aryl ether) dendrimers: Intramolecular triplet energy transfer in poly(aryl ether) dendrimers via a folded conformation

A series of Fréchet-type bis(dendron) poly(aryl ether) dendrimers ([BP-Gn]2-NBD, n = 1–3) with the benzophenone (BP) chromophores and the norbornadiene (NBD) group attached to the periphery and the core, respectively, were synthesized, and their photophysical and photochemical properties were investigated by using steady-state and time-resolved techniques. Intramolecular triplet energy transfer from the peripheral BP chromophores to the core NBD group in [BP-Gn]2-NBD occurs with the efficiency of ca. 84%, 56% and 48%, and with the rate constant of 1.9 × 107, 1.2 × 107, 1.1 × 107 s−1 at room temperature for generations 1–3, respectively. Selective excitation of the BP groups resulted in the isomerization of NBD to the quadricyclane (QC). The quantum yields of the intramolecular photosensitized isomerization for [BP-Gn]2-NBD are measured to be ca. 0.147, 0.098 and 0.087 for generations 1–3, respectively. The light-harvesting ability of these molecules increases with increasing the number of peripheral chromophores. The rate constant of the triplet–triplet energy transfer decreases inconspicuously with increasing generation, which validates that the intramolecular triplet energy transfer mainly takes place via a through space mechanism involving the closest donor and the acceptor groups by folding of the dendrimer structure, and the bis(dendron) system takes more congested conformation than the corresponding monodendron system.