The Complex Excited-state Behavior of a Polyspirobifluorene Derivative: The Role of Spiroconjugation and Mixed Charge Transfer Character on Excited-state Stabilization and Radiative Lifetime

In this study, we report on the unusual fluorescence decay of an alkoxy-substituted polyspirobifluorene. Excited state behavior has been probed as a function of time, using femtosecond photobleaching, single photon counting, and streak camera techniques. Unusually complex decay kinetics are observed, which strongly depend on solvent viscosity and polarity, featuring decay components in both the tens of picoseconds and in the nanosecond time domain. These findings are explained by the consequences of spiroconjugation in combination with excited-state conformational relaxation. We propose that exciton wave function delocalization into the spiro units effectively traps the exciton, allowing it to relax further into a highly emissive state with a very long lifetime as compared to non-spiroconjugated polymer analogues. Frontier molecular orbitals and exciton orbitals have been calculated using a first principles density functional theory (DFT) approach. These results confirm the importance of spiroconjugation as both the highest occupied molecular orbital (HOMO) and the (lowest) exciton level are not localized on the polymer backbone but strongly extend into the side fluorene groups of the spirobifluorene units. The results of our calculations are very sensitive to the substitution pattern on the spirobifluorene units, in particular when oxygen is included. This finding may lead to new materials of this kind with optimized charge carrier transport properties and high luminescence quantum yields.