Amplified spontaneous emission spectra of poly(9,9-dioctylfluorenyl-2,7-diyl) under pulsed laser excitation

Abstract

Photophysical parameters such as absorption, fluorescence spectra, stokes’ shift, fluorescence quantum yield and amplified spontaneous emission (ASE) of a soluble conducting polymer poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) were measured in different solvents. The absorption spectra of PFO in tetrahydrofuran (THF) have shown two bands at 390 and 437nm. The results showed that the optical density of the band 437nm increased by increasing the concentration. This is a strong indication that PFO in solution aggregates at higher concentrations. On the other hand, at lower concentrations, the fluorescence spectra of PFO in THF have shown three bands; 422 (dominant), 445 (shoulder) and 470nm (hump). For the higher concentrations, the band at 422nm totally vanished while the band 470nm became a dominant. Therefore, the band at 470nm could be attributed to the dimer state. The ASE action was tested in a transverse pumping configuration where the conducting polymer was pumped by laser pulses from the third harmonic of an Nd:YAG laser (355nm). It could be seen that there were three ASE peaks at 425, 450 and 475nm. The spectral and ASE properties of PFO were compared with the conjugated polymer poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PDHF) under the same operating conditions. The most important features are: (1) the ASE intensity and the photochemical stability of PDHF were remarkably high compared with PFO, (2) PFO has an ASE action even under low concentrations (3) and the ASE emission from PFO is tunable in the wavelength region between 425, 450 and 475nm.