Diradical Character Based Design for Singlet Fission of Condensed-Ring Systems with 4nπ Electrons

By applying the diradical character based molecular design guideline for singlet fission (SF), we investigate the feasibility of efficient SF in condensed-ring pi-conjugated molecules with 4n pi electrons (n = 4, 5,...), i.e., antiaromatic polycyclic hydrocarbons composed of five- and six-membered rings. The multiple diradical character (y(i)), which takes a value between 0 (closed shell) and 1 (pure open shell), is defined as the occupation number of the lowest unoccupied natural orbital (LUNO) + i (i = 0, 1,...) calculated using the approximately spin-projected spin-unrestricted Hartree-Fock method. The excitation energies are also evaluated using the tuned long-range corrected time-dependent density functional theory method with the Tamm-Dancoff approximation to examine the energy level matching conditions for SF: (i) 2E(T-1) - E(S-1) similar to 0 or <= 0 and (ii) 2E(T-1) E(T-2) < 0, where S-1, T-1, and T-2 represent singlet first, triplet first, and triplet second excited states, respectively. It turns out that the energy level matching conditions are satisfied in relatively small-size antiaromatic condensed-ring molecules with y(0) similar to 0.4 and y(1)/y(0) < 0.2, i.e., intermediate diradical character without significant tetraradical character. This finding also demonstrates the reliability of our design guidelines presented in our previous study.