Nonlinear Optical Spectroscopy of Excited States in Polyfluorene

We used a variety of nonlinear optical (NLO) spectroscopies to study the singlet excited states order, and primary photoexcitations in polyfluorene; an important blue emitting πconjugated polymer. The polarized NLO spectroscopies include ultrafast pump-probe photomodulation in a broad range of 0.2 to 2.6 eV, two-photon absorption in the range of 3.2 4.2 eV, and electroabsorption covering the spectral range of 2.8 5.3 eV. For completeness we also measured the linear absorption and photoluminescence spectra. We found that the primary photoexcitations in polyfluorene are singlet excitons with ~ 100 ps lifetime that have a characteristic photomodulation spectrum comprising of two photoinduced absorption (PA) bands, PA1 at 0.55 eV and PA2 at 1.65 eV, respectively, and a strong stimulated emission band peaked at ~ 2.5 eV. The two-photon absorption and electroabsorption spectra identify the exciton PA bands with optical transitions between the lowest lying odd symmetry 1Bu exciton at 3.1 eV into two strongly coupled even symmetry states, namely mAg at 3.7 eV and kAg at 4.7 eV, respectively. The excited states manifold also contains a strongly coupled odd symmetry exciton, nBu at 4.1 eV. A polarization memory of ~ 2.2 typical to π-conjugated polymer chains characterizes all three NLO spectra reflecting the highly anisotropic third-order NLO coefficient χ of the polymer chains. The four essential excited states, namely the 1Bu, mAg, nBu and kAg were used to satisfactorily fit all three nonlinear optical spectra using the summation over states model. The combination of the three NLO spectra and the model fit conclusively show that the band model typical to inorganic semiconductors cannot describe the PFO polymer. On the contrary, a strongly bound exciton with intrachain binding energy of ~ 0.9 eV dominates the linear and NLO spectra of this polymer.