ReviewPhotophysics in bipyridyl and terpyridyl platinum(II) acetylides☆
Introduction
Since the original description of Pt(phen)(CCPh)2 by Che and co-workers in 1994 [1], the number of related square planar platinum(II) acetylides bearing a lone polyimine ligand has enormously expanded. Such molecules have demonstrated promise in diverse applications including optical power limiting [2], [3], electroluminescence [4], singlet oxygen photosensitization [5], [6], photocatalytic hydrogen production [7], cation sensors [8], [9], [10], vapochromism [11], medicinal chemistry [12], and as extrinsic luminescent probes [13]. In general, these chromophores are rich in terms of their photophysics, combining properties of traditional coordination compounds and organometallics within the same structure. This amalgamation has inspired creativity in molecular design, uncovering new and sometimes unexpected photophysical properties in seemingly straightforward platinum(II) complexes. The current review will limit its focus to the photophysical properties of mononuclear PtII chromophores bearing a combination of one or more alkyl- or arylacetylide ligands in concert with a lone polyimine (2,2′-bipyridine or 2,2′:6′,2″-terpyridine) fragment.
Section snippets
Synthetic approaches to bipyridyl and terpyridyl platinum(II) acetylides
The syntheses of compounds which are the subject of this review require relatively facile procedures. The synthesis generally departs either directly from K2PtCl4 [14] (acidic aqueous reaction conditions) or from Pt(DMSO)2Cl2 [15] (organic solvents) where the polyimine ligand displaces appropriate labile ligands, producing the corresponding Pt(LL)Cl2 or [Pt(LLL)Cl]+ species. The introduction of the acetylide unit(s) to the platinum(II) center is readily accomplished through a
Ground state electronic spectra, electrochemistry, and electronic structure calculations
Pt(LL)(CCR)2 and [Pt(LLL)(CCR)]+ molecules display characteristic low energy absorption bands that span wavelengths from approximately 350 nm to beyond 600 nm, depending upon the nature of the polyimine and acetylide ligands. In molecules where acetylide-localized π–π* transitions reside at high energies, the low energy absorption bands have been proposed to originate predominately from metal-to-ligand charge transfer (MLCT) transitions [4], [18], [19], [20]. Cyclic voltammetry data generally
Photoluminescence properties
Fig. 3 displays the photoluminescence spectra of compounds 1–4 measured in CH2Cl2 at room temperature under optically dilute conditions. Upon photoexcitation into their respective charge transfer transitions, complexes 1–4 exhibit broad and structureless photoluminescence over a region spanning 500–750 nm. The large Stokes shift, relatively long lifetime and susceptibility to quenching by dioxygen, suggest that the photoluminescence emanates from a triplet charge transfer excited state [18], [19]
Supra-nanosecond spectrometry
While luminescence spectroscopy has been widely applied to the study of Pt(II) polyimine acetylides, there have only been a handful of transient absorption investigations [2], [3], [19], [26], [27], [28], [33], [38], [39], [40] along with a single time-resolved infrared study [19]. The latter can be easily understood in terms of the low IR-absorption cross-sections uniformly observed for acetylide stretching frequencies, rendering their detection rather difficult. Regardless, a few Pt(II)
Concluding remarks
Although the primary motivation for research in this area largely stems from the potential applications of these chromophores across a variety of disciplines, the fact remains that there are many interesting questions remaining and several lines of investigation worth further exploration. This contribution was intended to outline current work in the field related predominately to the photophysical processes in bipyridyl and terpyridyl platinum(II) acetylides. Even within this seemingly small
Acknowledgements
We gratefully acknowledge the NSF (CAREER Award CHE-0134782), the AFOSR (FA9550-05-1-0276), the ACS-PRF (44138-AC3 and 36156-G6), and Bowling Green State University (Technology Innovation Enhancement Award) for their generous support of our own research projects relevant to this review. All transient absorption measurements described in this review were performed in the Ohio Laboratory for Kinetic Spectrometry at BGSU. We thank Dr. Albert Okhrimenko for acquiring the near-IR emission spectrum
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