Tuning the role of charge-transfer states in intramolecular singlet exciton fission through side-group engineering

Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells. The challenge of controlling molecular packing and energy levels in the solid state precludes clear determination of the singlet fission pathway. Here, we circumvent this difficulty by utilizing covalent dimers of pentacene with two types of side groups. We report rapid and efficient intramolecular singlet fission in both molecules, in one case via a virtual charge-transfer state and in the other via a distinct charge-transfer intermediate. The singlet fission pathway is governed by the energy gap between singlet and charge-transfer states, which change dynamically with molecular geometry but are primarily set by the side group. These results clearly establish the role of charge-transfer states in singlet fission and highlight the importance of solubilizing groups to optimize excited-state photophysics.S.L. thanks AGS(O) Scholarship support from A*STAR Singapore. J.W. acknowledges financial support from MOE Tier 3 grant (MOE2014-T3-1-004). This work was supported by the Engineering and Physical Sciences Research Council, U.K. (Grant numbers EP/M005143/1 and EP/G060738/1). D.H.P.T. and N.D.M.H. acknowledge the Winton Programme for the Physics of Sustainability. K.C. and J.M.H. acknowledge support from a Rutherford Discovery Fellowship to J.M.H