Spectroscopic Properties, Excitation, and Electron Transfer in an Anionic Water-Soluble Poly(fluorene-alt-phenylene)-perylenediimide Copolymer

Abstract

An anionic fluorene-phenylene poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl}-based copolymer containing on-chain perylenediimine (PDI) chromophoric units, PBS-PFP-PDI, was synthesized and its photophysical properties studied as aggregates and isolated chains in water and dioxane/water (1:1) solution. UV-vis and emission spectroscopy measurements, time-correlated single photon counting, and wide field imaging have been employed to investigate the excited-state behavior of the PBS-PFP-PDI copolymer, including the effect of environment on the energy and electron transfer to the on-chain PDI chromophore. Although the Förster overlap integral is favorable, no evidence is found for intramolecular singlet excitation energy transfer in isolated copolymer chains in solution. Fluorescence is suggested to involve an interchain process, thus revealing that isolated copolymer chains in solution do not undergo efficient intramolecular energy transfer. However, quenching of the PBS-PFP excited state by PDI is observed in aqueous media and ultrafast pump-probe studies in water or dioxane-water solutions show that electron transfer occurs from the phenylene-fluorene units to the PDI. The extent of electron transfer increases with aggregation, suggesting it is largely an interchain process. The interaction of the negatively charged PBS-PFP-PDI copolymer with the positively charged surfactant hexadecyltrimethylammonium bromide (CTAB) in solution has also been studied. The copolymer PBS-PFP-PDI aggregates with the surfactant already at concentrations below the critical micelle concentration (cmc) and the nonpolar environment allows intermolecular energy transfer, observed by the weak emission band located at 630 nm that is associated with the emission of the PDI chromophore. However, the fact that the PDI photoluminescence (PL) lifetime (~1.4 ns) obtained in the presence of CTAB is considerably shorter than that of the nonaggregated chromophore (~5.4 ns) suggests that even in this case there is considerable PL quenching, possibly through some charge transfer route. The increase of the PBS-PFP-PDI photoluminescence intensity at surfactant concentrations above the cmc indicates deaggregation of polyelectrolyte within the initially formed polyelectrolyte-surfactant aggregates.

Effects of Charge Density on Photophysics and Aggregation Behavior of Anionic Fluorene-Arylene Conjugated Polyelectrolytes

“…The sums of three discrete exponential functions were needed to fit the fluorescence decays of these three oligomers in 1:1 methanol-water. This contrasts with previous studies with PBS-PFP in 1:1 dioxane-water, in which the fluorescence decays could be fitted with two exponentials [47]. The reasons for this difference are not clear.…”

Synthesis of a PPV‐fluorene derivative: Applications in luminescent devices

“…The photoluminescence quantum yield (in solution) was measured to be 65%, using 9,10‐diphenylanthracene as the standard. This value is similar to those reported for polyfluorenes derivatives …”

Synergistic photoluminescence enhancement in conjugated polymer-di-ureasil organic–inorganic composites

“…In solution, poly(fluorene-phenylene) polyelectrolytes often exhibit complex decay curves, which can be resolved into three exponential components: τ CPE1 ∼ 10–60 ps, assigned to fast intra-/inter-chain energy migration and/or conformational relaxation along the polymer backbone, 55 , 56 τ CPE2 ∼ 400 ps, attributed to radiative relaxation of aggregated polymer clusters, 57 , 58 and τ CPE3 ∼ 800 ps, due to radiative decay of isolated polymer chains. 56 , 57 The relative contribution of τ CPE2 and τ CPE3 to the total decay profile is thus dependent on the extent of polymer aggregation.…”