High stability and luminescence efficiency in donor–acceptor neutral radicals not following the Aufbau principle

Abstract

With their unusual electronic structures, organic radical molecules display luminescence properties potentially relevant to lighting applications; yet, their luminescence quantum yield and stability lag behind those of other organic emitters. Here, we designed donor-acceptor neutral radicals based on an electron-poor perchlorotriphenylmethyl (PTM) or tris(2,4,6-trichlorophenyl) methyl (TTM) radical moiety combined with different electron-rich groups. Experimental and quantum-chemical studies demonstrate that the molecules do not follow the Aufbau principle: The singly occupied molecular orbital (SOMO) is found to lie below the highest (doubly) occupied molecular orbital (HOMO). These donor-acceptor radicals have a strong emission yield (up to 54%) and high photo-stability, with estimated half-lives reaching up to several months under pulsed UV laser irradiation. Organic light-emitting diodes (OLEDs) based on such a radical emitter show deep-red / near-infrared emission with maximal external quantum efficiency of 5.3 %. Our results provide a simple molecular-design strategy for stable, highly luminescent radicals with non-Aufbau electronic structures.

Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

“…In fact, DFT calculations clarified that the lowest photoexcited state of the PyBTM radical is a doublet state corresponding to the one‐electron excitation from β‐HOMO to β‐SOMO (SUMO). The lowest photoexcited state of the weak CT type luminescent π‐radicals was also a doublet state corresponding to the one‐electron intramolecular charge‐transfer excitation from β‐HOMO in the carbazole (NCz in Figure ) moiety to β‐SOMO (SUMO) in the PTM (perchlorotriphenylmethyl) moiety, according to the DFT calculations . It should be noted that α‐SOMO is occupied in the lowest photoexcited doublet state (D 1 ) of these typical luminescent radicals.…”

“…As described in the previous section, organic radicals are usually non‐luminescent owing to the EISC or other non‐radiative energy relaxation pathways shown in Figure b and c, even when a chromophore is attached to the radical. However, a stable luminescent radical was first reported in 2006, since then, the luminescent properties of stable organic π‐radicals have become a research topic of great interest . Typical luminescent organic π‐radicals are shown in Figure .…”

“…However,a stable luminescentr adical was first reported in 2006, [32] since then, the luminescentp roperties of stable organic p-radicals have become ar esearcht opic of great interest. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] Typical luminescent organic p-radicalsa re shown in Figure 12. In the first report of al uminescent radical, the maximum quantum yield (F F )w as 53 %f or af luorescence emission of l = 628 nm in ac yclohexanes olutiono fT TM-NCz.…”

“…Luminescent radicals have the potential to raise the upper limit of the internal quantum efficiency to 100 % owing to their unique electronic/spin structures . The tris(2,4,6‐trichlorophenyl)methyl (TTM) radical and its derivatives have been found to be promising candidates to produce novel materials for OLEDs . In this review, we focus on the excited‐state and dynamics of photostable non(weakly)‐luminescent as well as luminescent π‐radicals, which are located on counter sides to each other.…”

“…[19][20][21][22][23][24][25][26][27][28][29][30][31] Furthermore, luminescent organic p-radicalsh ave also been developed. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] One of the straightforwarda pplications of luminescence is organic light-emitting devices (OLEDs). [49] Luminescent radicals have the potentialt or aise the upper limit of the internal quantum efficiencyt o1 00 %o wing to their unique electronic/spin structures.…”