Exciton Fission and Fusion in Bis(tetracene) Molecules with Different Covalent Linker Structures

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Abstract
Bichromophoric molecules can support two spatially separated excited states simultaneously and thus provide novel pathways for electronic state relaxation. Exciton fission, where absorption of a single photon leads to two triplet states, is a potentially useful example of such a pathway. In this paper, a detailed study of exciton fission in three novel phenylene-linked bis(tetracene) molecules is presented. Their spectroscopy is analyzed in terms of a three-state kinetic model in which the singlet excited state can fission into a triplet pair state, which in turn undergoes recombination on a time scale longer than the molecule's radiative lifetime. This model allows us to fit both the prompt and delayed fluorescence decay data quantitatively. The para-phenylene linked bis(tetracene) molecules 1,4-bis(tetracen-5-yl)benzene (1) and 4,4‘-bis(tetracen-5-yl)biphenylene (2) show intramolecular exciton fission with yields of ∼3%, whereas no delayed fluorescence is observed for tetracene or the meta-linked molecule 1,3-bis(tetracen-5-yl)benzene 3. Analysis of the temperature-dependent fluorescence dynamics yields activation energies for fission of (10.0 ± 0.6) kJ/mol for 1 and (4.1 ± 0.5) kJ/mol for 2, with Arrhenius prefactors of (1.48 ± 0.04) × 108 s-1 for 1 and (1.72 ± 0.02) × 107 s-1 for 2. The observed trends in activation energies are reproduced by ab initio calculations of the independently optimized singlet and triplet energies. The calculations indicate that electronic coupling between the two tetracene units is primarily through-bond, allowing differences in fission rates to be qualitatively explained in terms of the linker structure as well. Our results show that it is important to consider the effects of the linker structure on both energy relaxation and electronic coupling in bichromophoric molecules. This study provides insight into the structural and energetic factors that should be taken into account in the design of exciton fission molecules for possible solar cell applications.