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 $\pi$-conjugated polymer. The polarized NLO spectroscopies include ultrafast pump-probe photomodulation in a broad spectral range of $0.2–2.6\mathrm{eV}$, two-photon absorption in the range of $3.2–4.2\mathrm{eV}$, and electroabsorption covering the spectral range of $2.8–5.0\mathrm{eV}$. For completeness, we also measured the linear absorption and photoluminescence spectra. We found that the primary photoexcitations in polyfluorene are singlet excitons with $\sim 100\mathrm{ps}$ lifetime that have a characteristic photomodulation spectrum comprising of two photoinduced absorption (PA) bands, ${\mathrm{PA}}_1$ at $0.55\mathrm{eV}$ and ${\mathrm{PA}}_2$ at $1.65\mathrm{eV}$, and a strong stimulated emission band that peaked at $\sim 2.5\mathrm{eV}$. 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 $3.1\mathrm{eV}$ and two strongly coupled even symmetry states, namely, $m{A}_g$ at $3.7\mathrm{eV}$ and $k{A}_g$ at $4.7\mathrm{eV}$. The excited state manifold also contains a strongly coupled odd symmetry exciton $n{B}_u$ at $4.1\mathrm{eV}$, which identifies the continuum band threshold. A polarization memory of $\sim 2.1$, typical to films of $\pi$-conjugated polymers, characterizes all three NLO spectra, reflecting the highly anisotropic third-order NLO coefficient, ${\chi }^{\left(3\right)}$ of the polymer chains. The four essential states, namely, $1A_g$, $1B_u$, $m{A}_g$, and $n{B}_u$, were used to satisfactorily fit the ${\chi }^{\left(3\right)}$ 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 $\sim 1\mathrm{eV}$ dominates the linear and NLO spectra of this polymer.