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Porphyrin Nanostructures as Light Harvesting Antennas for Dye-Sensitized Solar Cells

The overall goal of the funded work is develop novel porphyrin-based materials that will be implemented as both light harvesting and electron conductive structures in next generation dye sensitized solar cells (DSSCs).  The need for these novel approaches stems from the growing energy demands throughout the developed and developing world, an increased focus on sustainable and renewable energy sources, inefficiency and high cost associated with traditional silicon-wafer photovoltaic cells.  The approach to this development has focused on the well characterized porphyrin (TPPS) and a series of peptidic scaffolds. 

TPPS is known to spontaneously self assemble into large, excitonically coupled nanostructures under very low pH conditions.  These structures, commonly referred to as J-aggregates due to the signature red-shifted absorption peak of the aggregate, have significant potential for application to DSSCs in that they both conduct electrons efficiently and can absorb electromagnetic radiation in the UV and visible ranges.  The three dimensional arrangement of individual TPPS monomers within the self assembled nanostructure is highly structured, requiring proper alignment between monomers to allow excitonic coupling and formation of the J-aggregate, not simply an aggregate.  The drawback to direct application of this material is primarily related to the extremely low pH conditions required for the aggregates to spontaneously self assemble ( < pH 1.5 ) which provides logistic hurdles to the widespread production and manufacturing of a TPPS based DSSC as well as potential restrictions on material lifetime.  In an attempt to circumvent this restriction, we have developed a series of peptide scaffolds to bind and orient the TPPS monomers with the goal of inducing a J-aggregate structure at higher pH conditions closer to neutral.  Previous results on our initial designs were successful in promoting scaffolded J-aggregate formation up to pH 3.6, a significant enhancement over spontaneous aggregation conditions.   Our work over the past year has focused on three distinct areas of peptide-TPPS aggregates: (1) binding order within the peptide-TPPS aggregate, (2) reversibility of J-aggregate formation, and (3) alternate peptide sequences to promote J-aggregate formation.

The majority of effort was focused on determining which sites on the originally designed peptide scaffold had the greatest affinity for the TPPS monomers.  This was a critical step in that the original peptide scaffold was shown to bind three TPPS molecules and each of the presumed binding sites were different in composition and local environment.  The N-terminal binding site was potentially influenced by the N-terminal amino charge, the middle binding site had an altered sequence topology, and the C-terminal site could be influenced by the carboxyl at the C-terminus of the peptide.  The experimental design relied on differential fluorescence quenching experiments using three peptide constructs with a naturally fluorescent Tryptophan residue incorporated near one of the binding sites.  Control experiments showed that the Trp inclusion did not affect the ability of the peptide to bind TPPS or to form J-aggregates under conditions similar to the parental scaffold.   Our results show a significant preference for the N-terminal binding site which can potentially be attributed to the molecular geometry of the N-terminal charge favorably interacting with the anionic sulfonates on the TPPS.

The second area of focus was to determine the stability and reversibility of J-aggregate formation.  Since the ultimate goal of the project is to develop chemical materials compatible with DSSCs, we want to ensure that our nanostructures are both robust and regenerable in the event of degradation.  Due to the current pH sensitivity of the aggregates, the most likely mode of aggregate breakdown is due to some environmentally stimulated change in pH.  As such our efforts focused on using pre-formed peptide-TPPS aggregates and cycling the environmental pH conditions while monitoring the spectroscopic signatures of the J-aggregate.  Our results indicate that the aggregate formation event is clearly reversible over several cycles of pH flux, however there appears to be a kinetic barrier upon aggregate-reformation that is significantly longer than upon initial formation.  We are currently investigating the cause of this kinetic barrier.  These investigations are looking at both TPPS ionization states as well as the kinetics of peptide folding/unfolding which we have previously shown to be linked to stable peptide induced aggregate formation.

Our final and most recent set of experiments is using a set of sequence-varied peptide scaffolds for J-aggregate formation.  Our initial sequences have modified the original parent to reduce helical propensity in the peptide since helix content increased concomitantly with reduction of J-aggregate.  One sequence replaced highly helical Leu residues with Alanine, adding flexibility and increasing solubility of the scaffold while the other construct incorporated the non-natural amino acid di-amino-propionic acid as the cationic moieties for TPPS binding.  Both scaffolds retained the ability to bind TPPS and form J-aggregates at low pH.  No enhancement of pH range has been observed at the current time.  Circular dichroism experiments indicate that these new constructs contain somewhat less helical content compared to the parental scaffold which will generate future generation peptides.  The most exciting result was found with a much shorter peptide sequence in which the cationic functionality was changed from an amine to a guanidine group.  This peptide was only 5 amino acids in length can only accommodate binding one TPPS molecule.  However, a distinct J-aggregate peak was induced by peptide binding, indicating that this peptide may be nucleating aggregate formation on a scale larger than itself.  This is a key goal for development of peptide scaffolded aggregates since the scaffolds will become unwieldy and cost prohibitive as sizes increase.