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Exciton fission (EF) is a process where an initially excited singlet exciton spontaneously splits into a pair of triplet excitons.  This process, which is mainly observed in organic crystals, may provide a way to enhance the efficiency of organic photovoltaics by 30%.  There are two advantages of organic materials for this application.  First, EF is an intrinsic property of the solid state structure, and does not require quantum confinement.  Second, EF produces more robust product states, since triplet lifetimes on the order of microseconds and diffusion lengths on the order of microns can be obtained in molecular crystals.  The possible advantages of exciton fission have motivated our current study of tetracene, the best-studied molecule where this phenomenon has been observed.  Our research has two main areas of inquiry:

1)  What are the dynamics of exciton fission in solid tetracene samples, and how are they affected by sample morphology and the delocalization of the singlet exciton?

2)  Is it possible to efficiently harvest the triplets that are produced via fission as electron hole pairs?

During the course of our study, several other groups published conflicting papers as to whether EF occurs in thin films of pentacene, a molecule closely related to tetracene.  These papers all relied on a single type of measurement to justify their conclusions.  We undertook a multifaceted study of tetracene in order to provide multiple observables that could better constrain the conclusions.  We also compared results in dilute solution with those of the solid film in order to isolate effects due to aggregation, like electronic delocalization.

            Picosecond luminescence measurements have been used previously to both establish a timescale for the fission process and to measure the lifetime of the triplets in various types of solid samples.  In single crystals of tetracene, our measurements agree with those of previous workers that the initially excited singlet fluorescence decays within about 200 ps.  After this initial decay, both single crystals and polycrystalline evaporated films show a long-lived delayed fluorescence component due to recombination of two triplet excitons back into a singlet.  Thus the delayed fluorescence lifetime can be used to estimate the triplet lifetime as well.  For single crystals, this lifetime ranges between 5 and 20 microseconds.  For the polycrystalline films, the delayed fluorescence lifetime is much shorter, on the order of 100-200 ns.  The origin of this shortened triplet lifetime could be either nonradiative relaxation of the triplets back to the ground state, which would limit the amount of time they have to be converted into electron-hole pairs, or it could result from trapping or conversion into a charge-separated state.  These last two outcomes would be more desirable from the point of view of ultimately producing electron hole pairs.  To directly monitor the triplet and charge-separated states, we turn to transient absorption spectroscopy. 

            Our approach to the transient absorption spectroscopy is two-pronged.  First, we want to establish the excited state properties of molecular tetracene in solution.  Once we understand its properties at the level of individual molecules, we can then see how those properties are modified by aggregating the molecules together into a crystal.  We have performed transient absorption on tetracene in solution with ~100 fs time resolution.  Surprisingly, there is very little literature on the spectroscopy of the polyacene molecules in solution, despite their importance in organic electronics.  We found that the transient absorption spectrum of tetracene in toluene is remarkably complicated, with at least three induced absorption features centered at 450 nm, 650 nm, and 1200 nm.  All these features can be assigned to known electronic transitions.  These features all decay at the same rate as the fluorescence, indicating that they originate from the first singlet excited state, representing transitions to higher lying excited states. 

            Transient absorption signals from ultrathin films of tetracene are much stronger than what would be expected based on solution measurements.  This enhanced nonlinear optical response is consistent with the idea that the initially excited singlet states are delocalized, superradiant excitons, rather than localized excitations on a single molecule.  The largest signal component at 530 nm decays on a timescale of 10 ps or so, faster than the instrument response function of our earlier luminescence measurements.  Based on our analysis of the transient absorption and photoluminescence measurements, we have concluded that the photoluminescence signal is dominated by defect states, and that the intrinsic singlet state is highly delocalized, undergoing EF within 10 ps.  The triplet states produced in this way relax to the ground state on a 100 ns timescale, but do not produce other, long-lived charge separated states.  We are now working on rubrene, a molecule closely related to tetracene but which can be made in amorphous form, where complications like crystal defects and delocalized states should play a much smaller role.

            To summarize, we are now actively engaged in making samples and characterizing the time-resolved dynamics of EF candidate molecules in both molecular (solution) and solid state form.  Our initial results point to the complexity of the EF process and confirm that delocalization and defect states play a key role in the optical response of the crystalline state. 

            In addition to our study of tetracene photophysics, the post-doc partly supported by this grant has also worked on making organic polymer nanostructures with novel properties.  By imprinting a dye-doped polymer surface with nanorods, we showed that the photoluminescence output could be increased by a factor of two or more.  When these nanorods are doped with both a luminescent dye and magnetic nanoparticles, they can be detached from the surface and then rotated and translated using an external magnetic field.  This subarea of research on polymer-based materials may be eventually lead to multifunctional, remote-controlled nanostructures.