Triplet state formation in photoexcited slip-stacked perylene-3,4:9,10- bis(dicarboximide) dimers on a xanthene scaffold

Two covalent perylene-3,4:9,10-bis(dicarboximide) (PDI) dimers in which the PDI molecules are attached to a xanthene (Xan) scaffold in which the long axes of the two π-π stacked PDI molecules are slipped by 4.3 and 7.9 Å were prepared. These dimers are designed to mimic J-aggregates and provide insights into the photophysics of triplet state formation in PDI aggregates that target organic electronics. Using ultrafast transient absorption and stimulated Raman spectroscopy, the mechanism of ³PDI formation was found to depend strongly on a competition between the rate of Xan^•+- PDI^•- formation involving the spacer group and the rate of excimer-like state formation. Which mechanism is favored depends on the degree of electronic coupling between the two PDI molecules and/or solvent polarity. Singlet exciton fission to produce ³PDI does not compete kinetically with these processes. The excimer-like state decays relatively slowly with τ = 28 ns to produce ³PDI, while charge recombination of Xan ^•+-PDI^•- yields ³PDI more than an order of magnitude faster. The perpendicular orientation between the π orbitals of PDI and the Xan bridge provides a large enough orbital angular momentum change to greatly increase the intersystem crossing rate via Xan ^•+-PDI^•- → ³PDI charge recombination. These results highlight the importance of understanding inter-chromophore electronic coupling in a wide range of geometries as well as the active role that molecular spacers can play in the photophysics of covalent models for self-assembled chromophore aggregates.