Picosecond studies of excited states in conjugated polymers

Abstract

This thesis reports on the interplay between molecular structure and photophysics in light emitting conjugated polymers revealed by steady state and picosecond time-resolved fluorescence spectroscopy. The fundamental excited state relaxation of a polyfluorene derivative is compared to that of two oligofluorenes using isolated molecules in dilute solution. Their long-time time-dependent spectral dynamics are monitored by means of picosecond streak camera and single photon counting techniques. Excited state relaxation in oligofluorenes is entirely conformational and depends on solvent viscosity. The intrachain photophysics of the polyfluorene is dominated by fast excitation migration with a slow conformational component. A rigid ladder-type polymer exhibits only migrational relaxation. In analogy, the spectral dynamics of an alkoxy-substituted polyspirobifluorene are studied in dilute solution. Their qualitative dependence on solvent viscosity is elucidated by further femtosecond photobleaching measurements. Two excited states are proposed with the lower energy state involving strong spiroconjugation as confirmed by electronic structure calculations. Conversion between them occurs via conformational relaxation of the fluorene side groups. The sensitive reaction of these photophysics to the substitution pattern of the polymer suggests an easy chemical tunability of polyspirobifluorenes towards optimised charge carrier transport properties. Finally, the formation of the beta phase in amorphous polydioctylfluorene is investigated as a function of spin coating fabrication in the solid state. It is suggested that it forms by condensation at colloidal sites, which arise from incomplete solvation in the master solution. A further room temperature phase exists in the absence of these nuclei. The excited state relaxation after energy transfer from amorphous to beta phase is monitored via time-resolved spectroscopy. Within the beta phase, exciton migration is restricted as confirmed by steady state anisotropy data. This evidence for exciton confinement is an important step towards the application of the beta phase as a polymer laser.