Technical: This project aims to address basic questions that arise in connection with the solid-state photophysics of conjugated polymers and oligomers, which are of broad interest in optoelectronic device applications. The questions are: Why is the photoluminescence (PL) quantum yield in conjugated polymer films so much lower than in solution even though the excited lifetimes are at least as long in the films? What fraction of paired charges with uncorrelated spins on adjacent conjugation segments will recombine to form singlet excited states and how does this depend on morphology? Do energy transfer and formation of interchromophore states following photoexcitation compete with charge transfer in donor/acceptor systems typical of organic photovoltaics? During the project, Kristi Kiick's group at the University of Delaware will synthesize rigid peptides as scaffolds where conjugated oligomers can be placed at specific places on the peptide through reactions with judiciously placed non-natural amino acids. Lewis Rothberg's group at the University of Rochester will perform steady-state and transient spectroscopy to measure how the altering the relative positions and orientations of the chromophores changes the photophysics. By changing the distance between two identical oligomers on a peptide scaffold, it is possible to simulate the photophysical behavior of conjugated polymers as they go from dilute solution to films. The project is designed to measure singlet-triplet branching in charge recombination that is a limiting factor in fluorescent organic light-emitting diode technology, to determine whether and when energy transfer can compete with charge transfer and what the consequences are for efficiency of photoinduced charge separation, and to find out whether photogeneration of interchromophore species can suppress charge generation in donor/acceptor systems characteristic of organic photovoltaics.
The project addresses basic research issues in a topical area of materials science with high technological relevance, and is expected to provide prescriptive information on how chromophores would be best organized for electroluminescent device applications. This project provides student training in a highly collaborative and interdisciplinary environment, including organic synthesis, materials characterization, and laser spectroscopy. Involving students in closely coordinated collaborative research is an important part of their training and especially valuable in materials science.
Organic light-emitting diodes based on small "conjugated" molecules for display technology have become commercially viable but remain expensive and have performance characteristics inadequate for lighting applications. Conjugation is a structural feature that makes the materials well suited to visible light absorption and emission as well as transporting charge, properties critical to light emitting applications and emerging applications for solar energy conversion using organic photovoltaic devices. One potential game changer in the economics is incorporating the conjugated units in polymeric materials that can be processed by screen printing and low cost thin film coating methods. Those efforts have been hampered because conjugated units in close proximity often exhibit "aggregation quenching", a phenomenon meaning that polymers exhibit very high luminescence efficiency in dilute solution will not do so in the solid (thin film) state where molecular chromophores interact. The purpose of our project is to understand the reason for the aggregation quenching phenomenon in detail and which packing geometries of chromophores lead to reduction of luminescence efficiency. This knowledge can help to design proper molecular strcutures and illustrate structures to be avoided in the design of electroluminescent polymers. The approach that we took was to take a common class of conjugated molecules of well defined structure and to "scaffold" them onto a rigid backbone made from a peptide of known and fixed geometry. We were able to place pairs or three conjugated fluorescent molecules in specific locations and orientations with respect to one another so that we could investigate how separation and relative orientation mediate aggregation quenching. What we found is that there are substantial decreases in luminescence when the chromophores are oriented so that their backbones can overlap. Moreover, we found one particular geometry that exhibits behavior in terms of fluorescent efficiency and excited state lifetime much like conjugated polymer films. We also studied the mechanism by which the luminescence is reduced and confirmed our hypothesis (based on studies of the fluorescent polymers) that there is a very fast relaxation process that competes with formation of the luminescent state. The nature of that process remains to be understood and will form a useful basis for theoretical work to explain why that unusual process happens in certain molecular packing geometries. The general scheme we developed for creating multichromophore systems of well defined geometry may also have far reaching implications for understanding complex conjugated systems with multiple chromophores such as photosynthetic centers and arrangements of charge donors and acceptors used in developing organic photovoltaics.
