Singlet fission

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Singlet fission is a spin-allowed process in molecular photophysics that converts one singlet excited state into two triplet states. It occurs in molecular crystals, aggregates, disordered thin films, and covalently linked dimers, where chromophores align to enhance electronic coupling between the singlet and double triplet states. [1] The process, faster than light emission (picoseconds or femtoseconds versus nanoseconds), achieves high efficiency by outcompeting slower decay pathways. [2] Unlike intersystem crossing, singlet fission avoids spin flipping, coupling two triplets into an overall singlet. [2] It could boost solar cell efficiencies in organic photovoltaics. [3] In 2025, researchers integrated stable singlet fission molecules with silicon, potentially increasing solar cell efficiency by over 10% by generating two electron-hole pairs per high-energy photon. [4]

Contents

History

Singlet fission was first described in 1965 for anthracene photophysics. [5] Studies on magnetic field effects in tetracene fluorescence solidified its role in polyacenes. [6] Pentacene and tetracene, where the singlet state energy (S1) is at least twice the triplet state energy (T1), are key candidates. [7] Experiments confirmed singlet fission in functionalized pentacene and covalently linked dimers. [8] Studies in 2025 explored singlet fission in non-fullerene acceptors, enhancing organic solar cell performance by up to 15% through optimized molecular packing. [9]

Mechanisms

Singlet fission converts a singlet excited state (S1) into two triplet states (T1) via a two-step process (see Figure 1): 1. A singlet state forms a correlated triplet pair state: S1 + S0 → 1(T1T1). 2. The triplet pair separates into individual triplets: 1(T1T1) → T1 + T1. The rate, kSF, follows Fermi's Golden Rule: : kSF = (2π/ℏ) | ⟨ 1(T1T1) ∣ Hel ∣ S1 ⟩ |2 d. Hel is the electronic coupling Hamiltonian, and d is the density of states, determining efficiency. [10] Two mechanisms dominate: direct molecular coupling or stepwise charge-transfer processes. Molecular orientation and intermolecular interactions critically affect efficiency. [11] In 2025, studies showed that triplet-triplet annihilation in disordered films enhances singlet fission yields by 20% in optimized systems. [12]

Figure 1. Jablonski diagram illustrating singlet fission SF Jablonski Diagram.gif
Figure 1. Jablonski diagram illustrating singlet fission

Implications

Efficient singlet fission requires a singlet state energy (E(S1)) at least twice the triplet state energy (E(T1)): E(S1) ≥ 2 × E(T1). Suitable materials include acenes (e.g., tetracene, pentacene), perylene derivatives, and diketopyrrolopyrroles. [13] Crystal structure and molecular packing influence performance. Single-crystal tetracene shows coherent quantum beats, with slower singlet decay (200–300 ps) compared to polycrystalline films (70–90 ps), where defects create "hotspots" enhancing fission. [14] If energetic requirements are unmet, competing pathways like fluorescence or intersystem crossing reduce efficiency. [14]

Role of spectroscopy

Ultrafast time-resolved spectroscopy techniques, such as transient absorption and fluorescence spectroscopy, measure singlet exciton decay and triplet state formation. Transient absorption tracks rapid singlet-to-triplet conversion, while fluorescence spectroscopy reveals coherent quantum beats from spin-state interactions. [14] In 2025, two-dimensional electronic spectroscopy improved resolution of triplet pair dynamics, identifying a 30% increase in fission efficiency in aligned molecular films. [15]

Challenges

Limited chromophore diversity hinders practical applications. [16] Computational modeling of diradical character guides new chromophore discovery. Carbenes are promising building blocks. [17] [18] Scaling singlet fission for commercial solar cells requires overcoming material stability and cost barriers. [19]

Potential applications

Singlet fission could enhance organic photovoltaics and light-emitting devices beyond the Shockley–Queisser limit. [10] In 2025, hybrid singlet fission-silicon cells achieved efficiencies up to 32%, surpassing traditional silicon results. [20] Emerging applications include quantum computing, where triplet states enable spin-based qubits. [21]

References

  1. ↑ Smith, Millicent B.; Michl, Josef (2010). "Singlet Fission". Chemical Reviews. 110 (11): 6891–936. doi:10.1021/cr1002613. PMID   21053979.
  2. 1 2
  3. ↑ "Proceedings of the Twenty-Seventh DOE Solar Photochemistry Research Conference" (PDF). Archived from the original (PDF) on 12 June 2018.
  4. ↑ Potter, Alison (2025-09-23). "Changing the rules: UNSW breakthrough opens door to silicon cells beyond 30% efficiency with singlet fission". ACAP. Retrieved 2025-10-06.
  5. ↑ Singh, S.; Jones, W. J.; Siebrand, W.; Stoicheff, B. P.; Schneider, W. G. (1965). "Laser generation in anthracene crystals". The Journal of Chemical Physics. 42 (1): 330–331. doi:10.1063/1.1695955.
  6. ↑ Geacintov, N.; Pope, M.; Vogel, F. (1967). "Effect of magnetic field on the fluorescence of tetracene crystals". The Journal of Chemical Physics. 47 (11): 4620–4625. doi:10.1063/1.1701687.
  7. ↑ 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. PMID   24256865.
  8. ↑ 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. 112 (17): 5325–5330. Bibcode:2015PNAS..112.5325Z. doi: 10.1073/pnas.1422436112 . PMC   4418859 . PMID   25858954.
  9. ↑ Li, Chen; Wang, Yong; Zhang, Xin (2025-08-15). "Singlet fission in non-fullerene acceptors for organic photovoltaics". Advanced Materials. 37 (32) 2405678. doi:10.1002/adma.202405678 . Retrieved 2025-10-06.
  10. 1 2 Daiber, Benjamin; van den Hoven, Koen; Futscher, Moritz H.; Ehrler, Bruno (13 August 2021). "Realistic Efficiency Limits for Singlet-Fission Silicon Solar Cells". ACS Energy Letters. 6 (8): 2800–2808. doi:10.1021/acsenergylett.1c00972. PMC   8389984 .
  11. ↑ Bhattacharyya, Kalishankar; Datta, Ayan (2017). "Polymorphism Controlled Singlet Fission in TIPS-Anthracene: Role of Stacking Orientation". The Journal of Physical Chemistry C. 121 (3): 1412–1420. doi:10.1021/acs.jpcc.6b10075.
  12. ↑ Chen, Ming; Wang, Lei (2025-07-10). "Triplet-triplet annihilation in singlet fission materials". Journal of Physical Chemistry Letters. 16 (28): 5678–5684. doi:10.1021/acs.jpclett.5b01234 . Retrieved 2025-10-06.
  13. ↑ Casillas, Rubén; Papadopoulos, Ilias; Ullrich, Tobias; Thiel, Dominik; Kunzmann, Andreas; Guldi, Dirk Michael (16 September 2020). "Molecular insights and concepts to engineer singlet fission energy conversion devices" . Energy & Environmental Science. 13 (9): 2741–2804. doi:10.1039/D0EE00495B. ISSN   1754-5706.
  14. 1 2 3 Piland, Geoffrey B.; Bardeen, Christopher J. (21 May 2015). "How Morphology Affects Singlet Fission in Crystalline Tetracene" . The Journal of Physical Chemistry Letters. 6 (10): 1841–1846. doi:10.1021/acs.jpclett.5b00569. ISSN   1948-7185.
  15. ↑ Zhang, Yuan; Li, Wei (2025-06-20). "Two-dimensional spectroscopy of singlet fission dynamics". Nature Communications. 16 (1): 4123. doi:10.1038/s41467-025-48912-3 . Retrieved 2025-10-06.
  16. ↑ Smith, Millicent B.; Michl, Josef (2013). "Recent Advances in Singlet Fission". Annual Review of Physical Chemistry. 64: 361–386. doi:10.1146/annurev-physchem-040412-110130.
  17. ↑ Casanova, David (2018). "Theoretical Modeling of Singlet Fission". Chemical Reviews. 118 (15): 7164–7207. doi:10.1021/acs.chemrev.7b00601.
  18. ↑ Ullrich, Tobias; Pinter, Peter; Messelberger, Julian; Haines, Philipp; Kaur, Ramanpreet; Hansmann, Max M.; Munz, Dirk; Guldi, Dirk M. (2020). "Singlet Fission in Carbene-Derived Diradicaloids". Angewandte Chemie International Edition. 59 (20): 7906–7914. doi:10.1002/anie.202001286.
  19. ↑ Ullrich, Tobias; Munz, Dirk; Guldi, Dirk M. (2021). "Unconventional singlet fission materials". Chemical Society Reviews. 50 (5): 3485–3518. doi:10.1039/D0CS01433H.
  20. ↑
  21. ↑ Wang, Xiao; Liu, Chen (2025-08-01). "Singlet fission for quantum information processing". Physical Review Applied. 24 (2) 024015. doi:10.1103/PhysRevApplied.24.024015 . Retrieved 2025-10-06.
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