On the Effect of Donor Strength on the Photoluminescence Performance in Mono-substituted N-Donor Triarylmethyl Radicals

Abstract

The functionalization of light-emitting triarylmethyl radicals with electron donating moieties can significantly increase their photoluminescence quantum yield . As luminophores in light-emitting diodes, such open-shell radicals can be used to overcome the problem of spin-statistics inherent to conventional closed-shell emitters. However, so far the functionalization of triarylmethyl radicals with donors of varying strength has been limited by the restricted reactivity of the triarylmethyl radical, constraining optimization of performance to empirical trial and error approaches. Here, we make use of the reliable reactivity of N-heterocyclic donors in radical-mediated aromatic substitutions, allowing us to systematically investigate the effect of donor strength on the emission characteristics of triarylmethyl radicals. As a single descriptor proxy to the donor strength, we employ the ionization energy IE of the donor moiety determined by density functional theory calculations. A systematic bathochromic shift of the emission wavelength \lambda_max is observed for increasing donor strength, while maximum \phi values are obtained for medium-strength donors. We rationalize these effects with a simple model based on the Marcus theory supported by quantum chemical calculations and electron paramagnetic resonance. This allows us to understand the effect of the donor strength on both \lambda_max and \phi, enabling the design of improved light-emitting radicals in the future.