Effect of ring fusion on the amplified spontaneous emission properties of oligothiophenes

We report the observation of amplified spontaneous emission (ASE) in oligothienoacenes, oligothiophenes composed solely of fused thiophene rings, doped into polystyrene films. The effect of ring fusion on the ASE properties has been investigated by comparing the performance of a pentathienoacene derivative, 2,6-bis-triisopropylsilanyl-alpha-pentathienoacene (BT-PTA), with that of its corresponding non-fused oligothiophene, 5,5 ''-bis-triisopropylsilanyl-alpha-terthiophene (BT-alpha 3T), whose synthesis is also reported here. The spectral properties of both systems have been studied by UV-vis optical absorption and photoluminescence in solution at both room and low temperature and interpreted by means of density functional theory (DFT) and time dependent DFT quantum chemical calculations. The dependence of ASE in terms of threshold, linewidth and emission wavelength on the concentration of active material in the films has been investigated in detail, allowing for optimization of the ASE performance. The best results were obtained for oligomer concentrations between 10 and 30 wt%. BT-PTA-doped films show ASE thresholds as low as 10 kW.cm(-2), one order of magnitude lower than those of films doped with BT-alpha 3T. These results indicate that the materials are good candidates as active laser materials and that their performance can be improved by adjusting the concentration of material in the film.