Competition between Ultrafast Energy Flow and Electron Transfer in a Ru(II)-Loaded Polyfluorene Light-Harvesting Polymer

Abstract

This Letter describes the synthesis and photophysical characterization of a Ru(II) assembly consisting of metal polypyridyl complexes linked together by a polyfluorene scaffold. Unlike many scaffolds incorporating saturated linkages, the conjugated polymer in this system acts as a functional light-harvesting component. Conformational disorder breaks the conjugation in the polymer backbone, resulting in a chain composed of many chromophore units, whose relative energies depend on the segment lengths. Photoexcitation of the polyfluorene by a femtosecond laser pulse results in the excitation of polyfluorene, which then undergoes direct energy transfer to the pendant Ru(II) complexes, producing Ru(II)* excited states within 500 fs after photoexcitation. Femtosecond transient absorption data show the presence of electron transfer from PF* to Ru(II) to form charge-separated (CS) products within 1-2 ps. The decay of the oxidized and reduced products, PF(+•) and Ru(I), through back electron transfer are followed using picosecond transient absorption methods.

Light Harvesting and Charge Separation in a π-Conjugated Antenna Polymer Bound to TiO2

“…The 400 nm bleach and the 580 nm absorption are both attributed to oxidized PF polymer (PF + ) on the basis of spectroelectrochemical observations. 6 The absence of these two features in the transient spectra for PF-Ru-A//ZrO 2 (Fig. S5) indicate that formation of PF + is a consequence of charge injection, most likely due to hole transfer from Ru(III), produced by charge injection, to the PF backbone (Scheme 1, step 4).…”

“…10,11 We previously reported the synthesis and photophysical study of a polyfluorene (PF)-based Ru(II) polypyridyl assembly (PF-Ru, Chart 1), where selective photoexcitation of the PF backbone gives rise to a kinetic competition between ultrafast energy and electron transfer to the pendant Ru(II) sites, producing a charge-separated state that persists for approximately 6 ns. 6 In the present investigation, we describe an approach that anchors a structurally similar PF-based assembly through ionic carboxylate functionalized Ru(II) chromophores to metal oxide (TiO 2 ) films. When bound to TiO 2 , the polymer exhibits multifunctional characteristics in which light 4 absorption is coupled with energy transport and charge separation.…”

“…A feature corresponding to absorption of the PF backbone (λ < 425 nm) is weaker than expected in the photocurrent spectrum, indicating that excitations on the polymer are less efficient in charge injection compared to excitations on the Ru chromophores 18. In these polymer assemblies, PF excitation decays through competitive PF* to Ru(II) energy and charge transfer pathways 6. Energy transfer from PF* to Ru(II)* should give the same photocurrent (on a per absorbed photon basis) as direct photoexcitation of a surface bound Ru(II) unit.…”

Ultrafast energy transfer from rigid, branched side-chains into a conjugated, alternating copolymer

“…We also use short linkers, relative to previous studies of energy transfer in conjugated polymers coupled to absorbers. [49][50][51][52] Our rigid linking structure also creates a welldefined spatial relationship between the chromophores, although the degree to which side chains rotate about the linking structure has not been determined due to the size of the system. A longer, more flexible linker, such as those employed in previous studies of energy transfer between conjugated polymers and optically active side chains, 49, 50 creates a wide variety of interchromophore distances and orientations within any experimentally measured ensemble.…”

“…This approach differs from previous designs that have created energy flow from the polymer into the side chains. 49,50 We seek to use the side chains as covalently linked, light harvesting antennae. Similar strategies have achieved light harvesting by antenna moieties using dye molecules hydrogen-bonded to conjugated polymers 51 or macrocycles encircling the polymer backbone.…”