Spectroscopic and travelling-wave lasing characterisation of tetraphenylbenzidine and di-naphtalenyl-diphenylbenzidine

The organic light emitting diode (OLED) hole transport molecules N,N,N',N'-tetraphenylbenzidine (TPB, triphenylamine dimer TAD or TPD) and N,N'-bis(2-naphtalenyl)-N,N'-bis(phenylbenzidine) (beta-NPB, naphtyl-diphenylamine dimer beta-NPD), dissolved in tetrahydrofuran (THF) and as neat film, are characterized by optical absorption and emission spectroscopy. The absorption and stimulated emission cross-section spectra, the fluorescence quantum distributions, fluorescence quantum yields, degrees of fluorescence polarization, and fluorescence lifetimes are determined. The lasing behaviour is studied by picosecond laser pulse excitation (excitation wavelength 347.15 nm, pulse duration 35 ps). 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 of the dye 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 was obtained. No laser action was achieved for beta-NPB because of small S-1-S-0 stimulated emission cross-section, and the presence of excited-state absorption in the fluorescence wavelength region. The TPB and beta-NPB results are compared with the corresponding spectroscopic and lasing behaviour of the related methyl-substituted triphenylamine dimers, 3-methyl-TPD and 4-methyl-TPD, which are well established OLED hole transport materials.