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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.
Supplementary materials
Title
Supporting Information
Description
Experimental Procedures, General Methods, Synthetic Procedures and Spectroscopic Data, NMR Spectra of new Closed-Shell Compounds, X-Band EPR Spectra of new Open-Shell Compounds, Absorption and Emission Spectra of new Open-Shell Compounds, High resolution Mass Spectra (MALDI (DCTB)) of new Compounds, Emission Properties of the Radicals and Ionization Energy of the corresponding Substituent, Natural Transition Orbitals for TTM-Bta, Computational Calculations, Ionization Energies of the Substituents, Geometries, Oscillator Strengths, Rate Constant of radiative Relaxation, Rate Constant of non-radiative Transition, Comparing Rate Constants,
Marcus Theory for intramolecular Charge Transfer applied to the 3-State Model, General Considerations, Steady State Approximation, Graphs for linear dependence of q_(CT,0) and ∆CTG° on ionization energy IE, Graphs for the Gibbs energy of activation ∆CTG^‡ for the charge transfer process as a function of q_(CT,0) and ionization energy IE of the donor moiety, Plot of k_(LE-CT) as a function of the ionization energy IE.
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