Linkage dependence on charge transfer in organic dimers with application to intra-molecular singlet fission

Singlet fission (SF) photophysics is currently being investigated intensively because of the possibility that the process can be utilized to double the photoconductivity of organic solar cells. Interest in SF materials has shifted from intermolecular to intramolecular singlet fission (iSF) in recent years. The iSF compounds currently being investigated are mostly dimers of tetracene and pentacene linked by other entities. Experimental work has shown that the rate of singlet fission depends very strongly on the actual connectivity between the outer chromophores and the linker molecules. Thus, for example, with the phenyl group as the linker, and tetracene and pentacene as the chromophores, the rate is two orders of magnitude faster with para linkage than with meta linkage. Similar results are obtained with naphthalene as the linker molecule; different connectivities can give SF rates different by factor of about 20 or greater. We explain this enormous dependence of the SF rates on connectivity within fully many-body calculations. In particular we show that there is a one-to-one correlation between SF rates and the oscillator strength of a charge-transfer absorption from the ground state that occurs at wavelengths intermediate between the monomer absorptions.