Electron-vibrational dynamics of photoexcited polyfluorenes

The highly polarizable pi-electron system of conjugated molecules forms the basis for their unique electronic and photophysical properties, which play an important role in numerous biological phenomena and make them important materials for technological applications. We present a theoretical investigation of the dynamics and relaxation of photoexcited states in conjugated polyfluorenes, which are promising materials for display applications. Our analysis shows that both fast (approximately 20 fs) and slow (approximately 1 ps) nuclear motions couple to the electronic degrees of freedom during the excited-state dynamics. Delocalized excitations dominate the absorption, whereas emission comes from localized (self-trapped) excitons. This localization is attributed to an inherent nonlinear coupling among vibronic degrees of freedom which leads to lattice and torsional distortions and results in specific signatures in spectroscopic observables. Computed vertical absorption and fluorescence frequencies as well as photoluminescence band shapes show good agreement with experiment. Finally, we demonstrate that dimerization such as spiro-linking does not affect the emission properties of molecules because the excitation becomes confined on a single chain of the composite molecule.