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.