Photophysics of a fluorene co-polymer in solution and films Original Research Article

The photophysical properties of a new light-emitting co-polymer of fluorene and di-alkoxyphenylene are reported. A range of optical spectroscopies were applied to both solutions and solid films, and comparing the results gives information about the excited states in each phase. Several important differences are observed. The peak of the absorption spectrum is at 350 nm in both solutions and films, but the films show additional absorption features at 450 and 560 nm. The luminescence of the solution phase has a maximum at 420 nm while the films exhibit two maxima with the stronger one located at 420 nm and the weaker one at 540 nm. The photoluminescence quantum yield of the solutions was found to be 45–60%, depending upon the solvent used, and was reduced to 7% for films. Time-resolved luminescence of the solutions revealed a dominant 500 ps decay time upon detection at 420 nm. For redder detection, the excited state decay became bi-exponential with an additional strong nanosecond component that was sensitive to solvent polarity. Time-resolved luminescence of films revealed more complex excited state dynamics. In the blue, the decay is dominated by a ∼40 ps phase followed by weaker 400 ps and 3 ns components. These observations point to the formation of stronger electronic inter-chain interactions aggregate, when going from the liquid to the solid phase.