Light-Harvesting in Carbonyl-Terminated Phenylacetylene Dendrimers:  The Role of Delocalized Excited States and the Scaling of Light-Harvesting Efficiency with Dendrimer Size

Abstract

The photophysics of a family of conjugated phenylacetylene (PA) light-harvesting dendrimers are studied using steady-state and time-resolved optical spectroscopy. The dendrimers consist of a substituted PA core surrounded by meta-branched PA arms. The total number of PA moieties ranges from 3 (first generation) to 63 (fifth generation). By using an alcohol/ketone substituent at the dendrimer core, we avoid through-space Forster transfer from the peripheral PA donors to the core acceptor (in this case, the carbonyl group), which simplifies the analysis of these molecules relative to the perylene-terminated molecules studied previously. The delocalized excited states previously identified in smaller dendrons are seen in these larger dendrimers as well, and their influence on the intersite electronic energy transfer (EET) is analyzed in terms of a pointdipole Forster model. We find that these new delocalized states can both enhance EET (by decreasing the spatial separation between donor and acceptor) and degrade it (by lowering the emission cross section and shifting the energy, resulting in poorer spectral overlap between donor and acceptor). The combination of these two effects leads to a calculated intersite transfer time of 6 ps, in reasonable agreement with the 5-17 ps range obtained from experiment. In addition to characterizing the electronic states and intersite energy transfer times, we also examine how the overall light-harvesting efficiency scales with dendrimer size. After taking the size dependence of other nonradiative processes, such as excimer formation, into account, the overall dendrimer quenching rate k Q is found to decrease exponentially with dendrimer size over the first four generations. This exponential decrease is predicted by simple theoretical considerations and by kinetic models, but the dependence on generation is steeper than expected based on those models, probably due to increased disorder in the larger dendrimers. We discuss the implications of these results for dendrimeric light-harvesting structures based on PA and other chemical motifs. IntroductionDendrimer molecules represent a fast-growing field of research in macromolecular organic chemistry. Their multigenerational branched architecture allows the synthetic chemist to associate an exponentially growing number of active units around a central chemical group located at the core of the dendrimer. If the active units are conjugated molecules that absorb light and the core is an energy acceptor or trap, then such dendrimers can be used as light-harvesters, 1 in analogy to natural photosynthetic systems. 2,3 Such systems have received considerable attention as light-harvesting elements for solar energy conversion. The goal is to build a molecule with a huge effective absorption cross section, where the initially absorbed energy is then transferred from the peripheral donors to the acceptor (trap) at the base of the dendrimer. Many variants of light-harvesting architectures have been constructed, and their electronic energy transfe...

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