Abstract: The fluorescence spectra of carbazole and N-ethyl carbazole mixed with DCB and chloro substituted benzene were studied in this paper. The photophysical behavior of carbaaole was quite different from N-alkyl carbazole derivatives. Exciplex may not be formed between carbazole and various acceptors. Only quenching of carbazole fluorescence can be observed. We proposed that the reasonable explanation is due to the difference in electrical density in N-atom of carbazole and N-alkyl carbazole derivatives. Current theoretical and experimental results indicate N-alkylation of carbazole has a substantial effect on the nitrogen "lone electron pair". Electron density on nitrogen is increased Lowering of electron density on nitrogen will promote the formation of encounter complex which causes the quenching of fluorescence and restricts the conversion of encounter complex to equilibrium exciplex. Chlorosubstituted benzene were used to check this assumption. The experimental results support our proposal.

Key words: photophysics, exciplex, carbazole, dicyanobenzene, N-ethyl carbazole