Elucidation of Excitation Energy Dependent Correlated Triplet Pair Formation Pathways in an Endothermic Singlet Fission System

Abstract

Singlet fission is the spin-allowed conversion of a photogenerated singlet exciton into two triplet excitons in organic semiconductors, which could enable single-junction photovoltaic cells to break the Shockley-Queisser limit. The conversion of singlets to free triplets is mediated by an intermediate correlated triplet pair (TT) state, but an understanding of how the formation and dissociation of these states depend on energetics and morphology is lacking. In this study, we probe the dynamics of TT states in a model endothermic fission system, TIPS-Tc nanoparticles, which show a mixture of crystalline and disordered regions. We observe the formation of different TT states, with varying yield and different rates of singlet decay, depending on the excitation energy. An emissive TT state is observed to grow in over 1 ns when excited at 480 nm, in contrast to excitation at lower energies where this emissive TT state is not observed. This suggests that the pathway for singlet fission in these nanoparticles is strongly influenced by the initial sub-100 fs relaxation of the photoexcited state away from the Franck-Condon point, with multiple possible TT states. On nanosecond time scales, the TT states are converted to free triplets, which suggests that TT states might diffuse into the disordered regions of the nanoparticles where their breakup to free triplets is favored. The free triplets then decay on μs time scales, despite the confined nature of the system. Our results provide important insights into the mechanism of endothermic singlet fission and the design of nanostructures to harness singlet fission.

New insights into the design of conjugated polymers for intramolecular singlet fission

“…This theory predicts that emission is strongest when the singlet state (1e–1h) is nearly in resonance with the TT state (2e–2h), similar to what is observed here. We note that a similar phenomenon has recently been reported in molecular crystals, where a distinct redshifted-delayed emission signal was observed and assigned to direct the TT emission 40 , 41 . While the prediction of direct TT emission in polymers is based on purely electronic considerations and does not involve phonon-mediated intensity borrowing arguments that have recently been invoked in molecular crystals 40 , both reports suggest that symmetry breaking is a necessary condition for the observation of direct TT emission.…”

New insights into the design of conjugated polymers for intramolecular singlet fission

“…This theory predicts that emission is strongest when the singlet state (1e-1h) is nearly in resonance with the TT state (2e-2h), similar to what is observed here. We note that a similar phenomenon has recently been reported in molecular crystals, where a distinct redshifteddelayed emission signal was observed and assigned to direct the TT emission 40,41 . While the prediction of direct TT emission in polymers is based on purely electronic considerations and does not involve phonon-mediated intensity borrowing arguments that have recently been invoked in molecular crystals 40 , both reports suggest that symmetry breaking is a necessary condition for the observation of direct TT emission.…”

Controlling the structures of organic semiconductor–quantum dot nanocomposites through ligand shell chemistry

“…29 Fitting was performed using the SasView software package (version 4.2.2). 30 Small-angle X-ray scattering SAXS measurements were carried out on a Xeuss 2.0 instrument equipped with an Excillum MetalJet liquid gallium X-ray source. Scattering data were collected for 900 s using collimating slits of 0.5 Â 0.6 mm (''high flux'' mode).…”