科研之友
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摘要
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 pi-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 similar to 100 ps lifetime that have a characteristic photomodulation spectrum comprising of two photoinduced absorption (PA) bands, PA(1) at 0.55 eV and PA(2) at 1.65 eV, and a strong stimulated emission band that peaked at similar to 2.5 eV. The two-photon absorption and electroabsorption spectra identify the exciton PA bands with optical transitions between the lowest-lying odd symmetry 1B(u) exciton at 3.1 eV and two strongly coupled even symmetry states, namely, mA(g) at 3.7 eV and kA(g) at 4.7 eV. The excited state manifold also contains a strongly coupled odd symmetry exciton nB(u) at 4.1 eV, which identifies the continuum band threshold. A polarization memory of similar to 2.1, typical to films of pi-conjugated polymers, characterizes all three NLO spectra, reflecting the highly anisotropic third-order NLO coefficient, chi((3)) of the polymer chains. The four essential states, namely, 1A(g), 1B(u), mA(g), and nB(u), were used to satisfactorily fit the chi((3)) 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 in inorganic semiconductors, cannot properly describe the poly(9,9-dioctylfluorene) polymer. On the contrary, a strongly bound exciton with intrachain binding energy of similar to 1 eV dominates the linear and NLO spectra of this polymer.
- 出版日期2007-3
