Two Thin Film Polymorphs of the Singlet Fission Compound 1,3-Diphenylisobenzofuran

Polycrystalline thin films of 1,3-diphenylisobenzofuran (1) with a morphology referred to here as α exhibit highly efficient singlet fission, producing two triplet states for every absorbed photon at 77 K, and about 1.4 triplet states per absorbed photon at room temperature. However, the triplet yield depends strongly on the specific crystalline form of 1, and for the morphology referred to as β the triplet yields are roughly an order of magnitude smaller. In this study, α, β, and mixed α/β films of 1 are prepared by thermal evaporation and solution drop-casting, and the structural and photophysical differences that may account for the very different triplet quantum yields are explored. The crystallites of 1 in thin films have been identified with two bulk crystal polymorphs grown from solution and structurally characterized. Analysis of absorption spectra of the films reveals a 600 cm^-1 blue shift in the onset and a unique spectral profile for the form α crystallites as compared to form β. Intermolecular interactions between columns of slip-stacked molecules are different in the two polymorphs, and this likely gives rise to the much smaller triplet quantum yield for β-1.