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Modeling Singlet Fission in Rylene and Diketopyrrolopyrrole Derivatives: The Role of the Charge Transfer State in Superexchange and Excimer Formation

Miller, Claire E., Wasielewski, Michael R., Schatz, George C.
The Journal of Physical Chemistry C 2017 v.121 no.19 pp. 10345-10350
Gibbs free energy, energy, models, physical chemistry, solar energy
Singlet fission (SF) is being explored as a way to improve the efficiency of organic photovoltaics beyond the Shockley-Queisser limit; however, many aspects of the SF mechanism remain unresolved. The generally accepted mechanisms provide simplified models of SF that equivocate over whether a charge transfer (CT) state is involved in SF. A one-step superexchange model allows the CT state to act as a virtual state, reducing the effect of large Gibbs free energy values from SF rate calculations. Also, extending superexchange to an excimer-mediated process allows for further refinement of the triplet formation model. Application of the superexchange and excimer-mediated models to a variety of rylene and diketopyrrolopyrrole derivatives provides new insights into the role of the CT and excimer states, providing a semiquantitative description of SF that is dictated by the CT state energy.