Singlet fission

Jablonski-like diagram for singlet fission.

In the area of spectroscopy, singlet fission is a spin-allowed process whereby a singlet excited state converts to two triplet states. The phenomenon is observed in molecular crystals, aggregates, disordered thin films, and covalently-linked dimers, where the chromophores are oriented such that the electronic coupling between singlet and the double triplet states is large. The process often occurs on a picosecond or femtosecond timescales, thereby out-competing fluorescence. The process is distinct from intersystem crossing, in that singlet fission does not involve a spin flip, but results in two triplets coupled into an overall singlet. [1] It has been proposed that singlet fission in organic photovoltaic devices could improve the photoconversion efficiencies.[2]

History

The process of singlet fission was first introduced to describe the photophysics of anthracene in 1965.[3] Early studies on the effect of the magnetic field on the fluorescence of crystalline tetracene solidified understanding of singlet fission in polyacenes.

Acenes, Pentacene and Tetracene in particular, are a prominent candidates for singlet fission. The energy of the triplet states are smaller than or equal to half of the singlet (S1) state energy, thus satisfying the requirement of S1 ≥ 2T1. Singlet fission in functionalized pentacene compounds has been observed experimentally.[4] Intramolecular singlet fission in covalently linked pentacene and tetracene dimers has also been reported.[5]

The detailed mechanism of the process is unknown. Particularly, the role of charge transfer states in the singlet fission process is still debated.

References

  1. ↑ Smith, Millicent B.; Michl, Josef (2010). "Singlet Fission". Chemical Reviews. 110 (11): 6891–936. doi:10.1021/cr1002613. PMID 21053979.
  2. ↑ http://science.energy.gov/~/media/bes/csgb/pdf/docs/Solar_photochemistry_2004.pdf
  3. ↑ Singh, S.; Jones, W. J.; Siebrand, W.; Stoicheff, B. P.; Schneider, W. G. J. Chem. Phys. 1965, 42, 330.
  4. ↑ Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H. (17 November 2013). "Singlet exciton fission in solution". Nature Chemistry. 5 (12): 1019–1024. Bibcode:2013NatCh...5.1019W. doi:10.1038/nchem.1801.
  5. ↑ Zirzlmeier, Johannes; Lehnherr, Dan; Coto, Pedro B.; Chernick, Erin T.; Casillas, Rubén; Basel, Bettina S.; Thoss, Michael; Tykwinski, Rik R.; Guldi, Dirk M. (9 April 2015). "Singlet fission in pentacene dimers". Proceedings of the National Academy of Sciences: 5325–5330. doi:10.1073/pnas.1422436112.
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