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This Type G ACS PRF Research Grant investigates composite systems with light harvesting and charge transport functionalities for insight relevant to solar energy conversion. Carbon nanotubes sensitized with porphyrin molecular aggregates are the targeted systems. In solar cells utilizing these materials, photoexcited porphyrin molecules transfer electrons to a single-wall carbon nanotube (SWCNT). The SWCNT then transports the electrons directly to an attached electrode. SWCNT are effective conduits for charge transport in photovoltaics due to their high conductivities and ability to function as scaffolds for light harvesting assemblies. While the physical properties of these materials are promising, the body of research in this area is fairly sparse. Careful study of photophysical mechanisms will improve insight and inform applications.

Initial work conducted in our lab is challenged by heterogeneity in SWCNT lengths and diameters found with commercially obtained samples. Concomitant with efforts involving the SWCNT, we have investigated the possibility that self-assembled cylindrical molecular aggregates (CMA) composed of porphyrin molecules and/or cyanine dyes can accomplish both the light harvesting and charge transport functionalities which are the emphasis of this research. To this end, CMAs composed of cyanine dyes and several porphyrin compounds have been prepared and characterized. We find that the cyanine dye CMAs have much greater photostability than those involving porphyrins. Therefore, most of the experiments performed to date have focused on the electronic relaxation mechanisms of these materials.

Several femtosecond laser spectroscopies have been applied to a particular class of self-assembled double-walled CMAs prepared with derivatives of 5,5’,6,6’-tetrachloro-benzimidacarbocyanine. The experiments show that energy transfer from the inner to outer cylinder wall takes place in less than 1 picosecond. Energy transfer occurring parallel to the long axes of the cylinders is presently under examination. Remarkably, the data exhibit clear signatures of correlated excited state (i.e., exciton) energy level fluctuations, which cause electronic coherences to persist for about 100 fs following photoexcitation. Furthermore, the aggregate’s spatially localized excited states facilitate a real-space interpretation of the dynamics, which provides insight into how the environment (e.g., solvent and molecular scaffold) controls these non-radiative dynamics. The finding of correlated exciton fluctuations is important because this particular CMA is one of only a handful of systems known to possess these many-body correlations, which are known to have a substantial influence on light harvesting efficiency in biological antennae.

The discovery of exciton correlations in the CMA has motivated further study of this phenomenon in two closely related light harvesting proteins found in the phycobilisome antenna of cyanobacteria, Allophycocyanin (APC) and C-Phycocyanin (CPC). Both of these proteins harvest light with pigment dimers. This well-defined dimer electron structure makes these systems ideal for quantitative investigation of exciton correlations and their effects on light harvesting efficiency. Experiments performed to date have revealed several interesting insights. (i) Despite similar structures, the nature of electronic states in APC and CPC is quite different. APC absorbs radiation with exciton states delocalized over two pigments, whereas the excited states localize to individual pigments in CPC. Our experiments suggest that environmental motion explains this difference; solvation is almost a factor of two faster in CPC than in APC. (ii) Another interesting result finds that APC achieves sub-100 fs internal conversion by way of a vibrational mode characterized by hydrogen out-of-plane (HOOP) wagging motion of an ethylene moiety. The HOOP accepting mode allows electronic relaxation to occur 3-5 times faster in APC than in CPC. Retinal proteins use a similar nuclear mode to trigger a cascade of events responsible for human vision.

To summarize, much progress has been made during the past two years addressing the fundamental issues emphasized in the proposed research. Technical obstacles involving nanotube samples have initially challenged careful physical study of these materials. In parallel with these efforts, we have examined several molecular aggregates that self-assemble into cylinders with lengths on the order of a micron. These materials are motivated by the idea that the light harvesting and charge transport functionalities, which are the emphasis of this research, may be achieved porphyrin and/or cyanine dye molecules self-assembled into cylindrical nanostructures. Future work will examine prospects for separating charge subsequent to CMA light absorption. Recent literature finds that platinum metal particles can be absorbed to the porphyrin CMA for charge separation and catalysis.

This ACS PRF grant has helped to initiate the PI’s research program. These funds were instrumental for setting up an experimental lab and identifying viable research directions. Undergraduate and graduate students have participated in this research. ACS PRF support is acknowledged in several publications.