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Abstract
The functionalization with donor moieties can significantly increase the photoluminescence quantum yield \phi of light-emitting triarylmethyl radicals. As luminophores in light emitting diodes, such open-shell radicals can overcome the problem of spin-statistics inherent to conventional closed-shell emitters. However, so far the donor-functionalization of triarylmethyl radicals has been limited by the capricious 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 substitution, allowing us to systematically investigate the effect of donor strengths on the emission characteristics of triarylmethyl radicals. As a single descriptor proxy to the donor strength, we employ the ionization energy IE determined by density functional theory (DFT) 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 using Marcus theory for intramolecular charge transfer, allowing us to derive design strategies and understand the effect of the donor strength on both \lambda_max and \phi.
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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