Singlet Fission: From Coherences to Kinetics.

Singlet fission, in which an initially excited singlet state spontaneously splits into a pair of triplet excitons, is a process that can potentially boost the efficiency of solar energy conversion. The separate electronic bands in organic semiconductors make them especially useful for dividing a high-energy singlet exciton into a pair of lower-energy triplet excitons. Recent experiments illustrate the role of spin coherence in fission, while kinetic models are used to describe how triplet and singlet states interact on longer time scales.

The dynamics of singlet fission in crystalline tetracene and covalent analogs.

Singlet fission (SF) is a spin-allowed process in which an excited singlet state spontaneously splits into a pair of triplet excitons. This relaxation pathway is of interest as a mechanism for increasing the efficiency of photovoltaic solar cells, since ionization of the triplets could produce two charge carriers per absorbed photon. In this Account, we summarize our recent work on trying to understand how SF occurs using both covalent and noncovalent assemblies of tetracene.

Quantum beats in crystalline tetracene delayed fluorescence due to triplet pair coherences produced by direct singlet fission.

A detailed analysis of the oscillations seen in the delayed fluorescence of crystalline tetracene is presented in order to study the mechanism of singlet fission. Three quantum beat frequencies of 1.06 ± 0.

The dependence of singlet exciton relaxation on excitation density and temperature in polycrystalline tetracene thin films: kinetic evidence for a dark intermediate state and implications for singlet fission.

The excited state dynamics of polycrystalline tetracene films are studied using femtosecond transient absorption in combination with picosecond fluorescence, continuing work reported in an earlier paper [J. J. Burdett, A.

Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission.

The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20-100 ns as opposed to 2 μs or longer in single crystals.

Electronic energy migration on different time scales: concentration dependence of the time-resolved anisotropy and fluorescence quenching of Lumogen Red in poly(methyl methacrylate).

Electronic energy transfer plays an important role in many types of organic electronic devices. Forster-type theories of exciton diffusion provide a way to calculate diffusion constants and lengths, but their applicability to amorphous polymer systems must be evaluated. In this paper, the perylenediimide dye Lumogen Red in a poly(methyl methacrylate) host matrix is used to test theories of exciton motion over Lumogen Red concentrations (C(LR)'s) ranging from 1 x 10(-4) to 5 x 10(-2) M.

Using two-photon excitation to control bending motions in molecular-crystal nanorods.

Molecular-crystal nanorods composed of 9-anthracenecarboxylic acid can undergo reversible bending due to molecular-level geometry changes associated with the photodimerization of the molecules in the crystal lattice. The use of highly focused near-IR femtosecond laser pulses results in two-photon excitation of micrometer-scale regions and is used to induce transient bends at various locations along the length of a single 200-nm-diameter nanorod. Bending can be observed in nanorods with diameters as small as 35 nm, and translational motion of a single nanorod could be induced by sequential bending of longer segments.