Ultrafast Excited State Dynamics of a Fluorene Derivative Conjugated Polymer

A detailed photophysical study of a polymer based on a fluorene derivative is performed through ultrafast nonlinear spectroscopy. The polymer chains are studied in solution, where absorption and emission occur from a disordered distribution of isolated chains, and in solid state as a film, where interchain interactions promote efficient energy transfer. Coherent photoexcitation produces delocalized excitons, which relax and localize on the lowest energy chromophores on the polymer chains on a 100 fs time scale. This is revealed by ultrafast transient absorption spectral evolution, anisotropy relaxation, and excited state specific high time resolution fluorescence spectroscopy. The photophysical behavior of the polymer in solution and as a film is significantly different suggesting a near absence of self-collapsed chain conformations in solution. In the film, the excited state relaxes an order of magnitude faster when compared to solution, indicating that interchain interactions are strong, promoting efficient energy transfer. The time scales of exciton dynamics revealed in this study will support further theoretical modeling of these polymer structures and are useful for designing blends for use in optoelectronic and electro-optical devices.