Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers

Femtosecond transient absorption measurements were carried out on p-conjugated supramol. assemblies in a high-pump-fluence regime. Oligo(p-phenylenevinylene) monofunctionalized with ureido-s-triazine (MOPV) self-assembles into chiral stacks in dodecane soln. below 75 Deg at a concn. of 4 * 10-4 M. Exciton bimol. annihilation was obsd. in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of 11Bu-exciton spectral signatures and by the sublinear fluence dependence of time- and wavelength-integrated photoluminescence (PL) intensity. These two characteristics are much less pronounced in MOPV soln. where the phase equil. is shifted significantly away from supramol. assembly, slightly below the transition temp. A mesoscopic rate-equation model is applied to ext. the bimol. annihilation rate const. from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimol. annihilation rate is high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramol. nanostructures have electronic properties that are intermediate between mol. aggregates and polymeric semiconductor