Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers

TL;DR

This study investigates exciton bimolecular annihilation in supramolecular nanostructures of conjugated oligomers using femtosecond transient absorption, revealing high annihilation rates due to fast exciton diffusion and energy transfer.

Contribution

It provides the first detailed analysis of exciton annihilation dynamics in supramolecular conjugated oligomer assemblies, combining experimental measurements with a mesoscopic rate-equation model.

Findings

High bimolecular annihilation rate constant extracted

Annihilation exhibits square-root time dependence

Fast exciton diffusion and resonance energy transfer are key mechanisms

Abstract

We present femtosecond transient absorption measurements on $\pi$-conjugated supramolecular assemblies in a high pump fluence regime. Oligo(\emph{p}-phenylenevinylene) monofunctionalized with ureido-\emph{s}-triazine (MOPV) self-assembles into chiral stacks in dodecane solution below 75$^{\circ}$C at a concentration of $4\times 10^{-4}$ M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of $1^1$B$_{u}$-exciton spectral signatures, and by the sub-linear fluence dependence of time- and wavelength-integrated photoluminescence (PL) intensity. These two characteristics are much less pronounced in MOPV solution where the phase equilibrium is shifted significantly away from supramolecular assembly, slightly below the transition temperature. A mesoscopic rate-equation model is applied to extract the bimolecular annihilation rate constant from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimolecular annihilation rate is very high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramolecular nanostructures studied here have electronic properties that are intermediate between molecular aggregates and polymeric semiconductors.