Electron-Vibrational Dynamics of Photoexcited Polyfluorenes

The highly polarizable π-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 (∼20 fs) and slow (∼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.