Nonlinear optical spectroscopy of excited states in polyfluorene

We used a variety of nonlinear optical (NLO) spectroscopies to study the singlet excited-state order and primary photoexcitations in polyfluorene, an important blue-emitting ∏-conjugated polymer. The polarized NLO spectroscopies include ultrafast pump-probe photomodulation in a broad spectral range of 0.2-2.6 eV, two-photon absorption in the range of 3.2-4.2 eV, and electroabsorption covering the spectral range of 2.8-5.0 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, and a strong stimulated emission band that 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 and two strongly coupled even symmetry states, namely, mAg at 3.7 eV and kAg at 4.7 eV.