Poly(fluorene-co-thiophene)-based ionic transition-metal complex polymers for solar energy harvesting and storage applications

Abstract

This paper describes the synthesis, electrochemical and photophysical properties of two poly(fluorene-co-thiophene)-based polymers featuring pendant Ru(II) polypyridyl-based ionic transition metal complexes. These systems combine long-lived excited states of the Ru(II) chromophores and the large optical cross-sections of the conjugated backbones, potentially allowing for polymer-assisted solar radiation-harvesting and storage functions via ultrafast energy and charge transfer processes. Modified azide–alkyne “click” cycloaddition chemistry conditions are demonstrated as an effective approach to obtain quantitatively functionalized conjugated polymer–ionic transition metal complex assemblies. Introduction of variable fractions of electron-rich thiophene units into these architectures induces a bathochromic shift in the polyfluorene absorption maxima, while simultaneously stabilizing the oxidized states of the conjugated scaffolds, as determined by electrochemical and in situ spectroelectrochemical experiments. The hybrid assemblies exhibit ultrafast energy flow (700 fs to 4.8 ps) and photoinduced charge-separation (1.8–2.0 ps) between the conjugated backbone and the Ru(II) moieties. Notably, the chemical composition of the main-chain repeat unit determines the dominant pathway for decay of the conjugated backbone excited states with the fraction of electron transfer increasing from 25% to 75% upon incorporation of an additional thiophene heterocycle.