Singlet fission

Such theoretical studies have yielded useful insight about the material properties that may be important for efficient singlet fission. It has not been possible, however, to distinguish between the competing... 

Intramolecular singlet fission systems hold particular appeal for device applications because such materials could potentially eliminate the dependence of triplet formation on intermolecular packing in the solid... 

However, the development of experimental observations of intramolecular singlet fission calls the 2Ag-mediated model into question (Fig. 2). In a pair of transient absorption studies on isolated chains of polydiacetylene (1), Lanzani et al. conclusively demonstrated intramolecular singlet fission, observing rapid (<200-fs) formation of triplet excitons accompanied by loss of the spectral signature of Due to... 

The underlying mechanism of this process was resolved in a detailed study of the excitation energy dependence in the photophysics of another polyene, poly(3-dodecylthienylenevinylene) 3 (Fig. 3). In this system as well, intramolecular singlet fission could be observed within well-isolated polymer chains in solution. Similarly to 2, under band-edge excitation 3... 

Fig. 2 Polyenes and the 2 state, (a) Schematic representation of the singlet 2Ag state in a polyene, showing its equivalent description either as a triplet-triplet or as soliton-antisoliton pair. Adapted from ref. 33. (b) Polyene-type structures discussed in the text. 1 Polydiacetylene [34,35], 2 Poly(diethyldipropargylmalonate) 136], 3 Poly(3-dodecylthienyl-enevinylene) [37], 4 Polyibenzodithiophene thiophene dioxide) [38]. R-groups denote solubilizing chains, (c) Models of singlet fission in polyenes, mediated by formation of 2Ag (left) or directly from IBu (right).

An additional property of intramolecular singlet fission observed in all of these polymeric systems is worth noting the triplets formed show an anomalously short lifetime. ... 

In contrast to intramolecular, solution-based fission, diffusional fission requires the collision of two molecules, one photoexcited and the other in its ground state, to produce a triplet exciton on each molecule (Fig. 5). Two important faetors distinguish collisional singlet fission systems molecules have the geometric freedom to find an optimal singlet fission geometry, and the singlet fission rate is limited to the rate of the diffusion of the molecules in the solvent. 

Fig. 5 Diffusional intermolecular singlet fission, (a) Energy diagram of singlet fission via a [T-T] intermediate. In TIPS-pentacene (13) formation of two triplet excitons is exothermic whilst for TIPS-tetracene (14) this process is endothermic, reflected in the respective triplet yields for the two molecules (b). (c) Schematic depicting the collision of TIPS-pentacene in solution to form the excimer intermediate. Adapted from ref, 28,...

Better resolution of an excimer intermediate in fission was achieved in a more recent study of TIPS-tetracene solutions (Fig. 7). In this work, the diffusion-limited dynamics, endothermic energetics and unusually sharp triplet exciton absorption features enabled identification of a spectroscopically distinct intermediate state in transient absorption and photoluminescence. In contrast to TIPS-pentacene, TIPS-tetracene represents a typical tetracene system where singlet fission is endothermic by 200 meV. This endothermicity is well known to have a drastic effect on the rate of fission in the solid state where triplet formation occurs three orders of magnitude slower in films of tetracene than in pentacene. Recent work suggests that fission in tetracene may not require thermal activation. Notably, the decay of singlet excitons and the rise of triplet exciton absorption have been shown to occur independent of temperature. A low-lying, dark intermediate state in tetracene was invoked to explain these observations, however it had been difficult to isolate such a state experimentally. ... 

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