Dendritic and linear macromolecular architectures for photovoltaics: a photoinduced charge transfer investigation.

Dendrimers have been previously shown to provide significant advantages in both excited-state energy transfer and charge transfer. However, this architecture causes one of the charges to be encapsulated and thus not available for charge separation over long distances. We conceived dendron-rod-coils as scaffolds that could have the architectural advantage of the dendrimers, while still providing a possible conduit for charge separation. In this study, we have designed and synthesized dendron-rod-coil-based donor-chromophore-acceptor triads and have compared these with dendron-rod and rod-coil diads. We have then evaluated the relative abilities of these molecules in photoinduced charge transfer. Our studies reveal that dendron-rod-coil could indeed be the ideal architecture for efficient photoinduced charge separation.

Probing the periphery of dendrimers by heterogeneous electron transfer.

The accessibility of the electroactive periphery was studied and compared for dendrimers and linear analogs by heterogeneous electron transfer using microelectrodes.

Energy and charge transfer dynamics in fully decorated benzyl ether dendrimers and their disubstituted analogues.

We examine the photophysics of a series of molecules consisting of a benzthiadiazole core surrounded by a network of benzyl ether arms terminated by aminopyrene chromophores, which function as both energy and electron donors. Three classes of molecules are studied: dendrimers whose peripheries are fully decorated with aminopyrene donors (F), disubstituted dendrimers whose peripheries contain only two donors (D), and linear analogues in which a pair of benzyl ether arms link two donors to the central core (L). The electronic energy transfer (EET) and charge transfer (CT) rates are determined by fluorescence lifetime measurements on the energy donors and electron acceptors, respectively. In all three types of molecules, the EET time scales as the square root of the generation number G, consistent with the flexible nature of the benzyl ether framework. Transient anisotropy measurements confirm that donor-donor energy hopping does not play a major role in determining the EET times. The CT dynamics occur on the nanosecond time scale and lead to stretched exponential decays, probably due to conformational disorder. Measurements at 100 degrees C confirm that conformational fluctuations play a role in the CT dynamics. The average CT time increases with G in the L and D molecules but decreases for the F dendrimers. This divergent behavior as G increases is attributed to the competing effects of larger donor-acceptor distances (which lengthen the CT time) versus a larger number of donors (which shorten the average CT time). This work illustrates two important points about light-harvesting and charge-separation dendrimers. First, the use of a flexible dendrimer framework can lead to a more favorable scaling of the EET time (and thus the light-harvesting efficiency) with dendrimer size, relative to what would be expected for a fully extended dendrimer. Second, fully decorated dendrimers can compensate for the distance-dependent slowdown in CT rate as G increases by providing additional pathways for the CT reaction to occur.

Dendrimer analogues of linear molecules to evaluate energy and charge-transfer properties.

[reaction: see text] We have designed and synthesized difunctionalized dendrimers containing two donors in the periphery and an acceptor at the core to serve as scaffolds for comparison with linear analogues to investigate the advantage of dendritic scaffolds for energy and charge transfer. Comparison of these dendrimers with the fully decorated dendrimers provides information on the advantage of chromophore density in energy/charge transfer from periphery to the core.

Light harvesting dendrimers.

Tree-like dendrimers with decreasing number of chromophores from periphery to core is an attractive candidate for light-harvesting applications. Numerous dendritic designs with different kinds of light-collecting chromophores at periphery and an energy-sink at the core have been demonstrated with high energy transfer efficiency. These building blocks are now being developed for several applications such as light-emitting diodes, frequency converters and other photonic devices. This review outlines the efforts that are based on both conjugated and non-conjugated dendrimers.