Theoretical design of conjugated diradicaloids as singlet fission sensitizers: quinones and methylene derivatives

The electronic structures of 206 carbonyls and methylene derivatives based on conjugated cyclic hydrocarbons are computationally studied in this work using theoretical methods of quantum chemistry. The singlet open-shell nature of the ground state and its influence on the low-lying excited states is analyzed for 90 carbonyl (quinone, Q), 90 methylene (quinone dimethide, QDM) and 26 carbonyl-methylene (quinone methide, QM) mixed derivatives in the pursuit of new promising candidates for singlet fission sensitizers. Non-negligible diradical character is observed for most of the studied molecules, which is mainly determined by the nature and the relative position of the substituting groups in the bare rings. In general, the methylene group enhances to a greater extent the diradical character and the following trend is observed: y0(QDM) > y0(QM) > y0(Q). This feature leads to a decrease in the energy of the S0 → S1 and, especially, the S0 → T1 transitions, facilitating the accomplishment of the singlet fission energetic conditions: 2T1–S1 ≤ 0 (C1) and 2T1–T2 < 0 (C2). For all the methylene derivatives, these transitions have π → π* character, while some carbonyl-containing molecules, in particular those with low diradical character, show transitions with n → π* character, due to the presence of the lone pairs of the oxygen atom. From the total set of 206 molecules analyzed, 10 molecules with intermediate diradical character may be considered as potential candidates to undergo singlet fission efficiently.