Spectroscopic and lasing characterization of electroluminescent organic materials

摘要

Some electroluminescent molecules are investigated by optical spectroscopy. Their lasing ability is studied in liquid solutions using a low�Q laser oscillator arrangement. In neat films wave-guided amplification of spontaneous emission is investigated. Some studies concerning the distributed feedback laser action of neat films on corrugated gratings are made.udThe investigated materials are the triphenylamine molecule TPA, the triphenylamine starbust molecule m-MTDAB, the triphenylamine dimer TPB, the napthyl-diphenylamine dimer beta-NPD, the oligomer dicarbazovinylene-MEH-benzene 2CzV-MEH-B, and the phenylene-bipyridine polymer PPBpy.udThe molecule triphenylamine (TPA) in tetrahydrofuran (THF) and the starburst triphenylamine oligomer 1,3,5-tris(3-methylphenylphenylamino)benzene (m-MTDAB) in THF and as neat film are characterized. The fluorescence decay is studied by picosecond laser excitation and time-resolved signal detection. Reverse saturable absorption is observed in picosecond laser nonlinear transmission measurements (laser duration 35 ps, laser wavelength 347.15 nm). The lasing ability of the compounds is tested by picosecond laser transverse pumped amplification or attenuation of spontaneous emission. The excited-state absorption cross-section spectra of the samples in the fluorescence spectral regions are extracted from the amplification/attenuation of spontaneous emission. Weak amplification of spontaneous emission is observed for m-MTDAB in THF around the fluorescence maximum (370 to 405 nm). For TPA in THF and m-MTDAB neat film attenuation of spontaneous emission occurs over the whole fluorescence region (excited-state absorption stronger than stimulated emission).udThe molecules N,N,N�,N�-tetraphenylbenzidine (TPB, triphenylamine dimer TAD) and N,N�-bis(2-naphtalenyl)-N,N�-bis(phenylbenzidine) (beta-NPB, naphtyl-diphenylamine dimer beta-NPD) dissolved in tetrahydrofuran (THF) and as neat films are studied. The excited-state absorption at the pump laser wavelength is determined by saturable absorption measurement. Low-Q laser oscillation of TPB in THF is achieved by transverse pumping the solution in a cell. The excited-state absorption of TPB in THF at the laser wavelength is extracted from the laser threshold. In TPB neat films wave-guided travelling-wave lasing is achieved. No laser action is obtained for -NPB because of very small S1-S0 stimulated emission strength, and the presence of the excited-state absorption in the fluorescence wavelength region.udThe oligomeric molecule 1,4-bis(9-ethyl-3carbazovinylene)-2-methoxy-5-(2�-ethyl-hexyloxy)-benzene (abbreviated 2CzV-MEH-B) dissolved in tetrahydrofuran (THF) and as neat film is investigated upon its optical spectroscopic properties and its lasing properties. Again the excited-state absorption at the pump-laser wavelength is determined by saturable absorption measurements. Laser oscillation of the dye in THF in a rectangular cell is achieved by transverse pumping. From the emission behaviour around laser threshold the excited-state absorption cross-section spectrum in the laser active spectral region of the material is extracted. The wave-guided travelling-wave lasing behaviour of the emitter as neat film is studied by analysis of the amplification of the transverse pumped spontaneous emission. Surface emitting distributed-feedback lasing is achieved with a neat film on corrugated second-order periodic gratings.udThe luminescent polymer, poly[2,2�-bipyridine-5,5�-diyl-(2,5-dihexyl-1,4-phenylene)] (abbreviated PPBpy) dissolved in tetrachloroethane (TCE) and as neat film is characterized optically by absorption, emission spectroscopy and saturable absorption measurements. Low-Q laser oscillation of the polymer in TCE is achieved and used to determine the effective stimulated emission cross-section at the lasing wavelength. Travelling-wave lasing of a neat film of the polymer is achieved. Surface-emitting distributed-feedback lasing is obtained with a neat film on corrugated second-order periodic gratings.udThe investigated electroluminescent molecules are applied in organic light emitting devices (OLEDs). The investigated polymer is a potential candidate for polymer light emitting devices. For their application in light emitting devices a high quantum yield of electroluminescence is sufficient. But for the application of the electroluminescent materials in laser devices it is indispensible that the stimulated emission cross-section is higher than the excited state absorption cross-section. Good OLED performance of an electroluminescent material is a precondition for good organic lasers, but in each case the effective stimulated emission cross-section (stimulated emission cross-section�excited state absorption cross-section) has to be determined to get information about its lasing ability as is done in this work.udThe oligomeric emitter 2CzV-MEH-B, the triphenylamine dimer TPB, and the phenylene-bipyridine polymer PPBpy turned out to be good organic active media for laser action in the violet and blue spectral region.