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.