Fluorescence, room temperature phosphorescence and photodegradation of carbazole compounds in irradiated poly(methyl methacrylate) matrices

A blue fluorescence of N-ethylcarbazole (EtCz) and of its dimer N,N′-diethyl-3,3′-bicarbazyl ((EtCz)2) as dopants of poly(methyl methacrylate) (PMMA) matrices, under laser pulsed UV irradiation, was observed. The fluorescence lifetimes were found to be 16.1 and 11.6 ns for EtCz and (EtCz)2, respectively. Concomitantly, a green phosphorescence occurred at room temperature and peaking at 444 nm for EtCz and 486 nm for (EtCz)2. From both luminescence spectra, singlet–triplet energy transition was estimated to be 0.56 eV for EtCz and 0.45 eV for (EtCz)2. Moreover, under UV irradiation, photoluminescence spectra exhibited a strong degradation with exposure time, accompanied by the appearance of new bands in the 400–500 nm range. Species associated to these new signals exhibited shorter lifetimes than those of the main peaks. Photodegradation of the doped PMMA matrices were not observed under nitrogen atmosphere, whereas in the presence of oxygen gas, they readily appeared. In the same way, thin film of pure (EtCz)2 did not show any modification under vacuum, whereas in air the photochemical evolution clearly appears. Possible causes of the oxygen-sensitive photodegradation of both the EtCz and (EtCz)2-doped PMMA matrices, were discussed.