Luminescent Radicals Based on the Tris(trichlorophenyl)methyl acceptor: How to Choose the Donor Component

The donor–acceptor (D-A•) radicals containing the tris(2,4,6-trichlorophenyl)methyl (TTM•) acceptor have recently received much attention in view of their efficient luminescence. Since TTM• is structurally alternant, based on the orbital-pairing features described for alternant hydrocarbons, it was proposed that the D-A• molecule could exhibit emissive properties only when the donor component is nonalternant. This hypothesis seemed to be validated by the synthesis of the alternant TTM•-tetracene system, which was measured to be nonemissive. While some experimental findings have deviated from the alternant rule, the underlying mechanism remains unclear. Here, we investigate quantum mechanically the excited-state properties of a series of TTM•-acene radicals. The results of our high-level calculations highlight that alternant hydrocarbons should not be disregarded in the design of radical emitters, rationalize the absence of emission in TTM•-tetracene, and lead to a set of simple rules to obtain highly luminescent TTM-D emitters.