Molecular Design Principle for Efficient Singlet Fission Based on Diradical Character and Exchange Integral: Multiple Heteroatom Substitution Effect on Anthracene

We theoretically investigate significant impacts of diradical characters and exchange integrals on the first singlet (S-1) and triplet (T-1) excitation energies related to singlet fission (SF), from which a series of C-2h-symmetrically diazadibora-substituted anthracenes [(BN)(2)-anthracenes] are newly proposed as potential efficient SF chromophores. From the analytical expressions of electronic structures for a model system with two electrons in two orbitals, a tuning of exchange integrals represented by localized natural orbitals is shown to be effective to vary E(S-1) and E(T-1), in addition to the control of diradical characters, both of which are expected to result in SF isothermal or exothermic [E(S-1) - 2E(T-1) approximate to 0 or > 0]. The applicability of this strategy to realistic molecules is verified for multiple heteroatom substitution to anthracene, namely, (BN)(2)-anthracenes, by the complete active space configuration interaction method and the time-dependent density functional theory method. Our calculations show that (BN)(2)-anthracenes exhibit a wide range of diradical characters and enhanced exchange integrals, enabling flexible tunings of E(S-1) and E(T-1). Accordingly, several (BN)(2)-anthracenes are found to be promising candidates for efficient SF. Such a behavior of diradical character can be understood from resonance structures associated with the captodative effect. Moreover, a concrete strategy of tuning exchange integrals is demonstrated in terms of frontier molecular orbital distributions. The present results open up novel molecular design guidelines for efficient SF based on the tuning of exchange integrals in addition to diradical characters.