Photoluminescence of donor-acceptor carbazole-based molecules in amorphousand powder forms

We present absorption and photoluminescence features of four samples of car bazole molecules substituted with various electron-acceptor groups. These m olecules named 1-(N-ethylcarbazolyl)-2-substituted-2-cyanovinylene contain in their structure the electron-donor carbazole nucleus and cyanovinylene b earing either another nitrile function, an ethylester, a phenyl, or a para- nitrophenyl groups. It is shown that depending on the strength of the donor -acceptor internal charge transfer, both the absorption and emission spectr a are more or less redshifted. It is found that the ethyl-ester derivative displays the best relative photoluminescence efficiency among all the sampl es and its peak is measured at 490 nm when taking amorphous thin film. The microcrystalline powder form of the same material exhibits spectral narrowi ng and shift of the peak emission. We obtain further narrowing of the emiss ion band and further redshifting of the emission when we illuminate, transv ersely, a glass capillary containing the crystalline sample by an ultraviol et light-emitting diode. (C) 2000 American Institute of Physics. [S0021-897 9(00)07610-6].