Electrothermal manipulation of individual chromophores in single conjugated polymer chains: controlling intrachain FRET, blinking, and spectral diffusion.

Abstract

Single molecule spectroscopy of individual chains of a conjugated polymer opens up deep insight into electronic localization phenomena, which govern the underlying optical properties of these complex and disordered materials. We explore the nature of a single chromophore arising in a delocalized pi-electron system by applying periodic electrothermal perturbations at low temperatures. Brief heating of the chromophore leads to a dramatic increase in the transition line width and is generally accompanied by a random jump of the emission energy. This observation demonstrates that chromophores on a polymer chain are not only defined by structural disorder but also by the subtleties of the local dielectric environment. The effect of thermal perturbation becomes more complex when long polymer chains are considered, which can potentially support the formation of multiple chromophores. Here, a momentary increase in temperature can promote intrachain energy transfer to quenching sites, leading to a strong modulation of emission intensity with temperature. Unexpectedly, such energy transfer can serve to either raise or lower the transition line width and quantum yield of the ensemble with increasing temperature, depending on the specific energetics of the chromophores in the system, which in turn vary with time. The controlled perturbation of both the emission spectrum and the intensity by brief heating of the polymer chain offers insight into possible microscopic origins of fluorescence blinking and spectral diffusion, which ultimately impact on the efficiency and spectral purity of devices.