Synthesis and Optical Property Studies of Blue-Light Organic Radicals Based on N-Heterocyclic Carbenes*

Because of the open-shell electronic structures, organic radicals have many special properties and can be applied to various fields, such as molecular magnets, spintronics, organic rechargeable batteries, electron paramagnetic resonance imaging and field-effect transistors. Specifically, organic radicals provide an alternative method to overcome the efficiency limitation of organic light-emitting diodes (OLEDs) based on conventional fluorescent organic molecules. They have doublet-spin properties arising from unpaired electronics and can be designed to have rapid emission on nanosecond timescales for exploitation in OLEDs with up to 100% internal quantum efficiency. However, luminescent radicals are still rare and often have narrow highest occupied molecular orbital (HOMO)-singly occupied molecular orbital (SOMO) gap, so the luminescent colors are mainly focused on the long wave range of visible light, such as orange and red. It is difficult to obtain luminescent radicals with blue emission. In this work, to further enrich the types of luminescent radicals and broaden the luminescence range of radicals, we design and synthesize novel radical precursors [2a]I and [2b]I by palladium-catalyzed coupling reaction of N-heterocyclic carbenes (NHCs) and 9-(4-iodophenyl)-9H-pyrido[2,3-b]indole, which was characterized by nuclear magnetic resonance spectroscopy, high-resolution electrospray ionization mass spectrometry, and single-crystal X-ray diffractometry analyses. Subsequently, two neutral luminescent radicals 3a and 3b were successfully prepared by single electron reduction using KC8 as a reducing agent. The experimental results show that radicals 3a and 3b have blue emission in tetrahydrofuran solution, and their maximum emission wavelengths are 450 and 428 nm, respectively. In addition, it is found that the fluorescence emission energy of radicals 3a and 3b is much higher than the maximum absorption energy given by the absorption spectrum, indicating an obvious Anti-Kasha emission phenomenon. Theoretical calculations further confirm that the fluorescence originates from the higher energy electronic excited state (D3) rather than the lowest energy-excited state (D1). This work shows that luminescent radicals with blue emission can be constructed by using NHCs as the skeleton unit, which provides a new research idea for the controlled synthesis of stable luminescent radicals.