基于氮杂环卡宾蓝光有机自由基的合成及其光学性质研究

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

发光自由基因同时具有发光、导电、顺磁、可逆的氧化还原等特性,使其在有机光电领域具有广阔的应用前景.然而,目前已报道的发光自由基通常具有窄的最高占据轨道(HOMO)-单电子占据轨道(SOMO)能隙且分子结构单一,因此发光颜色主要集中在橙色、红色等可见光的长波范围,难以获得具有蓝色发射的自由基.为进一步丰富发光自由基的种类、拓宽自由基的发光范围,本工作以氮杂环卡宾和9-(4-碘苯基)-9H-吡啶并[2,3-b]吲哚为前驱体通过钯催化的偶联反应得到了自由基前驱体[2a]I和[2b]I,进而通过单电子还原成功制备了两例发光自由基3a和3b.实验表明,自由基3a和3b在四氢呋喃溶液中具有蓝色的发射,其最大发射波长分别为450和428nm.此外,对比发现自由基的荧光发射能量高于对应吸收谱最大吸收波长处的能量,表现出明显的Anti-Kasha发射现象.本工作表明以氮杂环卡宾为骨架单元可有效构筑具有蓝色发射的自由基,为可控合成稳定的发光自由基提供了新的研究思路.

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 doubletspin 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.