Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes

Abstract

The doublet-spin nature of radical emitters is advantageous for applications in organic light-emitting diodes (OLEDs), as it avoids the formation of triplet excitons that limit the electroluminescence efficiency of non-radical emitters. However, radicals generally show low optical absorption and photoluminescence yields. Here we explain the poor optical properties of radicals based on alternant hydrocarbons, and establish design rules to increase absorption and luminescence yields for donor-acceptor-type radicals. We show that non-alternant systems are necessary to lift the degeneracy of the lowest energy orbital excitations; moreover, intensity borrowing from an intense high-lying transition by the low energy chargetransfer excitation enhances the oscillator strength of the emitter. We apply these rules to design tris[2,4,6trichlorophenyl] methyl (TTM)-pyridoindolyl derivatives with high photoluminescence quantum yield (>90%). OLEDs based on these molecules showed pure-red emission with over 12% external quantum efficiency. These insights may be beneficial for the rational design and discovery of highly luminescent doublet emitters.

Solid‐State Room‐Temperature Near‐Infrared Photoluminescence of a Stable Organic Radical

“…[9,10] Luminescent organic radicals have attracted considerable attention [11][12][13][14][15][16][17][18] because of their unusual photophysical properties such as long-wavelength emissions, absence of heavy-atom effects, [19,20] and high electroluminescence quantum efficiency. [21][22][23][24] These characteristics stem from the doublet spin state of isolated radical molecules with an unpaired electron. Radicals in the aggregated state also have unique luminescent properties associated with unique electronic states generated by intermolecular spin-spin interactions.…”

Sensitive and Repeatable Photoinduced Luminescent Radicals from A Simple Organic Crystal

“…Another important reason is that organic radicals suffered from aggregationcaused quenching of their luminescence in condensed state, by electron transfer or spin-exchange interaction. [6] As a result, there are few reports of photoinduced radicals with luminescence. [2a, 7] Recently, Tang et al reported the phosphorus-containing compound, tris(4-chlorophenyl)phosphine, which can generate stable solid-state emissive radicals under UV irradiation.…”

Highly Efficient Multifunctional Luminescent Radicals

“…Since there is no inhibition in the transition of doublet excitons, the upper limit of internal quantum efficiency (IQE) of organic light-emitting diodes (OLEDs) exploiting luminescent radicals as emitters has been theoretically increased to unit. [10][11][12][13][14][15][16] In addition to OLED applications, luminescent radicals have also shown other potential applications, such as cation-responsive turn-on fluorescence, 17 triplet sensitization, 18 magnetoluminescence, 19,20 circularly polarized luminescence, 21,22 and photodynamic therapy. 23 However, until now, there have been no reports regarding fluorescent sensing of luminescent radicals in an environment of mutual solubility with water.…”

“…Meanwhile, the borrowed intensity from the higher-energy transitions can augment the oscillator strength of the charge-transfer (CT)-type emission and was verified. 13 Since the excited states of radicals are hybridizations of local and CT states, the impact of solvent polarity on radical luminescence will be limited in terms of the intensity borrowing. Based on the above idea, herein, we obtained two luminescent radicals, 2αPyID-TTM and 2δPyID-TTM, as shown in Figure 1, in which the tris (2,4,6-trichlorophenyl)methyl (TTM) radical moiety connects with two α-carboline (αPyID) and two δ-carboline (δPyID) units, respectively.…”