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ABSTRACT: The authors observed a narrow emission from a 500-nm-thick codeposited organic thin film, 6 wt % 4,4′-bis[(N-carbazole)styryl]-biphenyl (BSB-Cz) doped into a 4,4′-bis(N-carbazole)-biphenyl host, under continuous-wave excitation. Although they observed no clear threshold characteristics on the spectral narrowing, they found that the emission centered at a wavelength of λASE = 462 nm, corresponding to the zero to one transition of BSB-Cz, has a transverse electric mode with a narrow full width at half maximum of λFWHM = 3.4 nm, indicating the occurrence of amplified spontaneous emission. No excited state absorptions of singlet and triplet excited states of BSB-Cz are confirmed around λASE, leading to large net gain for the light amplification.
Applied Physics Letters 06/2007; 90(23):231109-231109-3. · 3.84 Impact Factor
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ABSTRACT: We studied the generation and decay dynamics of triplet excitons in tris-(8-hydroxyquinoline) aluminum (Alq3) thin films by using transient absorption spectroscopy. Absorption spectra of both singlet and triplet excitons in the film were identified by comparison with transient absorption spectra of the ligand molecule (8-hydroxyquinoline) itself and the excited triplet state in solution previously reported. By measuring the excitation light intensity dependence of the absorption, we found that exciton annihilation dominated under high-density excitation conditions. Annihilation rate constants were estimated to be gammaSS = (6 +/- 3) x 10(-11) cm3 s(-1) for single excitons and gammaTT = (4 +/- 2) x 10(-13) cm3 s(-1) for triplet excitons. From detailed analysis of the light intensity dependence of the quantum yield of triplet excitons under high-density conditions, triplet excitons were mainly generated through fission from highly excited singlet states populated by singlet-singlet exciton annihilation. We estimated that 30% of the highly excited states underwent fission.
The Journal of Physical Chemistry A 09/2006; 110(34):10173-8. · 2.95 Impact Factor
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ABSTRACT: The effect of aggregation on the excited-state electronic structure of perylene was studied through transient absorption measurements of isolated molecules, excimers, monomeric crystals (beta-perylene), and dimeric crystals (alpha-perylene). Changes of electronic state were clearly identified from the changes in transient absorption spectra. A detailed investigation was made by combining the obtained results with previous measurements of ground-state absorption and fluorescence spectra. The energy level of the ion-pair state in alpha-perylene crystals was estimated, and the results are compared with previous photoconductivity results. Moreover, the relaxation processes of excited states in alpha-perylene crystals were studied by femtosecond transient absorption measurements.
The Journal of Physical Chemistry A 06/2006; 110(20):6465-71. · 2.95 Impact Factor
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ABSTRACT: We investigated the dynamics of photoinduced ultrafast electron injection from the excited state of cis-di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) (N3 dye) into the conduction band of nanocrystalline In2O3 films by measuring transient absorption of N3-sensitized In2O3 films in the wavelength region from 600 nm to 4 μm at 100–250-fs temporal resolutions, and the results were compared with those for N3-sensitized nanocrystalline ZnO, SnO2, and TiO2 films. Although the reaction kinetics could not be simply described by a single exponential function and the near-IR transient absorption spectral shapes were dependent on the environmental conditions, the predominant injection time for the In2O3 films was 5–10 ps. The SnO2 and In2O3 films showed similar injection dynamics, and the injection times for these films were much shorter than the 150-ps injection time reported for ZnO films. The predominant injection time for the TiO2 film was within 100 fs, which is in agreement with reported values. The differences in the injection times are qualitatively explained in terms of the density of acceptor states in the conduction bands at the LUMO level of N3 dye.
Journal of Photochemistry and Photobiology A: Chemistry.
