Phosphorescence quenching of a platinum acetylide polymer by transition metal ions

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Abstract

A platinum acetylide-based polymer (p-Pt2BPyPh) that contains 2,2′-bipyridine units in the polymer backbone has been synthesized. A model complex (Pt2BPyPh2) has also been prepared that features the same structure as a single polymer repeat unit. The polymer and model complex exhibit moderately efficient and long-lived phosphorescence emission from a triplet excited state. The effect of addition of six different transition metal ions (Fe3+, Co2+, Zn2+, Ni2+, Cu2+ and Pd2+) on the absorption and phosphorescence spectra of p-Pt2BPyPh and Pt2BPyPh was examined. The results show that in every case, the metal ions bind to the bipyridine unit in the polymer (model) backbone, and metal ion binding induces a red-shift in the near-UV absorption band. Phosphorescence spectroscopy shows that for all of the metals (with the exception of Zn2+), metal ion binding is accompanied by phosphorescence quenching. For some of the metal ions (Cu2+ and Ni2+) quenching of the polymer is considerably more efficient than in the model, an observation consistent with an “amplified quenching effect” that is analogous to that observed for quenching of fluorescent conjugated polymers. A semi-quantitative analysis of the absorption and phosphorescence data provide a model consistent with the notion that triplet exciton hopping along the polymer chain is rapid compared to the triplet lifetime, and that the overall quenching efficiency for the different metal ions is controlled by the intrinsic rate of triplet quenching within the metal ion–bipyridine complex.

Introduction

In the past few decades, π-conjugated polymers (CPs) and oligomers have received significant research interest due to their unique electronic and optical properties. Potential applications for these materials include organic light emitting diodes [1], [2], organic photovoltaic devices [3], [4], [5], and chemical sensors [6], [7]. The sensory application is based on the discovery that CPs exhibit “amplified luminescence quenching” by charge neutral or ionic quenchers that are present at very low concentration [8], [9], [10], [11], [12]. This effect is due to delocalization and rapid diffusion of singlet excitons along the polymer backbone. Such properties suggest that CPs have the potential to be more sensitive chemical sensors than systems based on small molecular luminophores.

Due to environmental and biological concerns, selective and sensitive sensors capable of detecting trace concentrations of metal ions are needed [13]. Conjugated polymers featuring bipyridine or terpyridine units on the side chains have been shown to exhibit amplified fluorescence quenching effects when the metal complexing units bind to transition metal ions [14], [15]. In another approach to metal ion sensing, 2,2′-bipyridine units were incorporated into backbone of a poly(phenylene vinylene)-based CP. The absorption and emission properties of the material were altered significantly upon coordination of a variety of metal ions to the bipyridine units. It was shown that such change in photophysical behavior is due to (1) the extension of the conjugation length of the polymer backbone induced by coordination of metal ions to 2,2′-bipyridine units; and (2) the change in the HOMO-LUMO gap due to the interaction of the a positively charged metal ion with the bipyridine moiety [16].

Although fluorescent chemical sensors are well known, only a few examples of transition metal ion sensors based on phosphorescence quenching can be found in the literature [17], [18], [19]. Platinum acetylide-type CPs and oligomers have gained much attention due to their phosphorescent properties with potentials for a variety of applications [20], [21], [22], [23]. The previous work demonstrated that triplet excitons are confined within two or three repeat units of a Pt-acetylide conjugated backbone [24]. Although triplet excitons are much less delocalized in comparison to singlet excitons in the same π-conjugated system, their long lifetimes and potential for intra- or inter-chain triplet exciton migration lead to potential for sensitive chemosensing materials. In addition, the study of the interaction of platinum acetylide polymers with quencher ions can provide considerable insight concerning the extent of triplet exciton delocalization and its ability to migrate along the polymer backbone.

This report describes the synthesis and characterization of a novel platinum(II)-acetylide-based conjugated polymer that features 2,2′-bipyridine units incorporated into the backbone as transition metal ion binding sites (p-Pt2BPyPh, Scheme 1). This polymer exhibits strong phosphorescence at ambient temperature, and the luminescence can be quenched by addition of a variety of transition metal ions. In order to provide a model system to explore the ability of the polymer to amplify the quenching effect, we also synthesized and studied the quenching response of the model complex Pt2BPyPh2 (Scheme 1) which is essentially a single repeat unit of p-Pt2BPyPh.

Section snippets

Synthesis

The polymer p-Pt2BPyPh was synthesized by the multi-step reaction scheme shown in Scheme 2. Dichlorobis(tributylphosphine) platinum [25], 2,2′-bipyridine hydrobromide (2) [26], 5,5′-dibromo-2,2′-bipyridine (3) [26], and 5,5′-diethynyl-2,2′-bipyridine (4) [27] were synthesized according to the literature procedure. Polymerization was effected via a CuI-catalyzed Hagihara coupling protocol [28]. Initially, attempts were made to incorporate the monomer 2,5-bisbutyloxy-1,4-diethynylbenzene in the

Summary and conclusion

A novel platinum acetylide-based conjugated polymer that contains 2,2′-bipyridine moieties in every polymer repeat unit has been synthesized, along with a model complex that features a single bipyridine binding unit. The polymer and model complex display moderately efficient phosphorescence that emanates from the triplet excited state. Absorption and phosphorescence studies carried out in the presence of a series of six transition metal ions provide evidence that the metal ions complex to the

General synthetic procedures and source of starting materials

2,2′-Bipyridine, Ni(OAc)2, Pd(OAc)2, and FeCl3 were purchased from Aldrich Chemical Company. Co(OAc)2, Zn(OAc)2, Cu(OAc)2, spectroscopy grade THF and methanol were purchased from Fisher Scientific Company. Unless otherwise noted, chemicals were used without further purification. Dichlorobis(tributylphosphine) platinum [25], 2,2′-bipyridine hydrobromide (2) [26], 5,5′-dibromo-2,2′-bipyridine (3) [26], and 5,5′-diethynyl-2,2′-bipyridine (4) [27] were synthesized according to the literature

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