Thickness dependence of the fluorescence lifetime in films of bisfluorene-cored dendrimers

The effect of film thickness on fluorescence lifetime, quantum yield, and exciton diffusivity is studied in first-generation bisfluorene-cored dendrimers with E-stilbenyl and biphenyl-based dendrons. A decrease of the fluorescence lifetime and quantum yield is observed in films thinner than 50 nm spin-coated on fused silica substrates. The radiative decay of the singlet excited-state and singlet exciton diffusion rates are independent of the film thickness within a 20% uncertainty. The fluorescence lifetime of dendrimers dispersed in an inert host is also independent of film thickness. The nonradiative decay rate increases by a factor of 4 in thin films of dendrimers with first-generation E-stilbenyl dendrons. The same effect is observed on different glass and sapphire substrates, and it is much weaker in the dendrimer with first-generation biphenyl dendrons, which allows us to rule out quenching by substrate. The data can be explained by long-range (dipole - dipole) energy transfer to quenching sites concentrated at the surface of thin films. The results contribute to understanding of the thin film photophysics and will help development of light-emitting devices.