咔唑衍生物能带结构的电化学方法研究

Electrochemical Method for Determination of the Energy Band Structure of Carbazole Derivatives

有机电致发光材料能带结构的表征对于材料的优化选择和有机电致发光器件的研究非常重要。循环伏安法具有简单、操作方便的特点,被广泛地用于表征有机材料的能带结构。用循环伏安法研究了一系列咔唑衍生物的最高占有分子轨道(HOMO)能级。实验结果表明:给电子基团有利于提高咔唑衍生物的空穴传输能力;吸电子基团则会降低该类材料的空穴传输能力;而具有共轭作用的取代基的衍生物则可以作为空穴阻挡材料。这对咔唑衍生物的设计合成具有指导意义。

It is very important to obtain data of energy band structure for optimizing and choosing organic electroluminescent materials. There are mainly four methods can be used such as UV-Vis absorption spectroscopy, quantum chemistry calculation, the photoemission analysis and cyclic voltammetry to obtain the band gap, HOMO and LUMO energy level respectively. Because the energy level can be determined easily by the electrochemical method, the cyclic voltammetry is widely applied. In the present work, a series of carbazole deri-(vatives) have been studied. The experimental values of HOMO energy level have been gained through the onset potential in the cyclic voltammetric curves. The experimental results showed that the HOMO energy level of N-methylcarbazole is 5.99 eV which is lower than those of other N-alkylcarbazole derivatives for the σ-π hyperconjugation between methyl and carbazyl. The electron-withdrawing carbonyl on the compound ethyl N-carbazylacetate(6) makes it have lower level of the HOMO (energy) level than carbazole. The HOMO energy level of CBP is the lowest for the conjugation of aryl which reduces the density of electron cloud on the nitrogen atom dramatically. It is informed that the HOMO energy level is significantly influenced by the characteristics of the substituent. The electron-donating group can improve the ability of hole transfer of carbazole derivatives, but the electron-withdrawing group is just contrary. The carbazole derivatives with conjugated aryl have relative low HOMO energy level and can be used as hole barrier materials. The experimental conclusions are constructive to direct the design and synthesis of organic electroluminescent materials.