Quintet-triplet mixing determines the fate of the multiexciton state produced by singlet fission in a terrylenediimide dimer at room temperature.
basic_science · Level V
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- Record sourced from PubMed, PMID 30948629.
- Also identified by DOI 10.1073/pnas.1820932116 and PMC identifier 6486782.
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Abstract
Singlet fission (SF) is a photophysical process in which one of two adjacent organic molecules absorbs a single photon, resulting in rapid formation of a correlated triplet pair (T<sub>1</sub>T<sub>1</sub>) state whose spin dynamics influence the successful generation of uncorrelated triplets (T<sub>1</sub>). Femtosecond transient visible and near-infrared absorption spectroscopy of a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI<sub>2</sub>), in which the two TDI molecules are directly linked at one of their imide positions, reveals ultrafast formation of the (T<sub>1</sub>T<sub>1</sub>) state. The spin dynamics of the (T<sub>1</sub>T<sub>1</sub>) state and the processes leading to uncoupled triplets (T<sub>1</sub>) were studied at room temperature for TDI<sub>2</sub> aligned in 4-cyano-4'-pentylbiphenyl (5CB), a nematic liquid crystal. Time-resolved electron paramagnetic resonance spectroscopy shows that the (T<sub>1</sub>T<sub>1</sub>) state has mixed <sup>5</sup>(T<sub>1</sub>T<sub>1</sub>) and <sup>3</sup>(T<sub>1</sub>T<sub>1</sub>) character at room temperature. This mixing is magnetic field dependent, resulting in a maximum triplet yield at ∼200 mT. The accessibility of the <sup>3</sup>(T<sub>1</sub>T<sub>1</sub>) state opens a pathway for triplet-triplet annihilation that produces a single uncorrelated T<sub>1</sub> state. The presence of the <sup>5</sup>(T<sub>1</sub>T<sub>1</sub>) state at room temperature and its relationship with the <sup>1</sup>(T<sub>1</sub>T<sub>1</sub>) and <sup>3</sup>(T<sub>1</sub>T<sub>1</sub>) states emphasize that understanding the relationship among different (T<sub>1</sub>T<sub>1</sub>) spin states is critical for ensuring high-yield T<sub>1</sub> formation from singlet fission.