Singlet Fission in Diphenylhexatriene Amide Derivatives: Effects of Intermolecular Hydrogen Bonds in Crystals

Singlet fission (SF) is a photophysical process where one high-energy singlet exciton (S*) interacts with an adjacent ground-state molecule to produce two low-energy triplet excitons (T) via a correlated triplet pair [(TT)]. As an exciton-multiplication process, SF can potentially increase the photon-to-electricity conversion efficiency of solar cells and optoelectronic devices. In organic materials, SF efficiency depends strongly on the E(S*)/E(T) energetics and the extent of electronic coupling between the two interacting molecules. Strong intermolecular interactions such as hydrogen bonds are often effectively utilized to control molecular arrangement and electronic coupling in crystals; however, our understanding of the effects of hydrogen bonds on solid-state SF is still insufficient. Here, we investigate the SF kinetics in a series of newly synthesized 1,6-diphenyl-1,3,5-hexatriene (DPH) derivatives with amide [CONHR (R = Me, Et, Pr, Bu) and NHCOMe] ring substituents. Crystallographic and FT-IR spectral analyses confirm the presence of NH···OC hydrogen bonds and the resulting close proximity of molecules. Fluorescence decay measurements and kinetics analysis reveal that S* → (TT) rates are not very different for the amides and unsubstituted hydrocarbon DPH, while (TT) → T + T and (TT) → S* are clearly faster in the amides than in DPH. Considering similar energetics for all trienes, the acceleration can be attributed to enhanced electronic coupling in the amide crystals.