Tuning spin gap in anthracene and tetracene for singlet fission: An exact PPP model study in the VB basis

We have studied donor-acceptor (D-A) substituted anthracene and tetracene to explore the strength and nature of substitutions that satisfy the energy criteria for singlet fission. We have carried out exact it electron calculation on symmetrically substituted anthracene and tetracene by employing the Valence Bond technique and exactly solving the PPP model. The singlet manifold for tetracene spans a space of 7:450 million VB diagrams while the triplet spans a space of approximate to 902 million. The calculations are made possible by using a new symmetry which is a direct product of the inversion symmetry and the electron-hole (alternancy) symmetry which exists for symmetric D-A substituted tetracene. We have studied the dependence of singlet and triplet gaps on D-A strength. We have also varied the number and position of donors and acceptors to find optimal substitution for possible singlet fission. We find that for anthracene, single pair D/D substitution at)beta - beta' satisfies the criterion for the singlet fission, with the heat of formation of singlet fission (HSF) increasing with the strength of substitution. Four site substitution in anthracene is SF favourable only for the case where two D/A and D/D pairs are at terminal carbons. For tetracene HSFs decrease with the strength of substitution for both D/A and D/D substitution. For substitution at the alpha- position, HSF is negative only for a relatively low site energy value of 2.0 eV. Four site substitution favours the singlet fission except for substitution at alpha- alpha' positions. (C) 2017 Elsevier B.V. All rights reserved