Design of singlet fission chromophores with cyclic (alkyl)(amino) carbene building blocks

We use MRSF-TDDFT and NEVPT2 methods to design singlet fission chromophores with the building blocks of cyclic (alkyl)(amino)carbenes (CAACs). CAAC dimers with C-2, C-4, and p-phenylene spacers are considered. The substitutions with trifluoromethyls and fluorine atoms at the C position are investigated. The electronegative substituents enhance the accepting capability of the C while maintaining it as a quaternary C atom. The phenylene-connected dimers with the two substitutions are identified as promising candidates for singlet fission chromophores. The cylindrically symmetric C-2 and C-4 spacers allow for substantial structural reorganizations in the S-0-to-S-1 and S-0-to-T-1 excitations. Although the two substituted dimers with the C-4 spacer satisfy (or very close to satisfy) the primary thermodynamics criterion for singlet fission, the significant structural reorganizations result in high barriers so that the fission is kinetically unfavorable.