Fluorescence coordination polymers

A red fluorescent coordination polymer (32) was obtained by our research group from a tpy-functionalized perylene bisimide chromophore by complexation with zinc triflate [85]. The polymer 32, which is readily soluble in chloroform/methanol mixtures and DMF, retains the excellent fluorescence properties of the free ligand and shows reversible binding, thus, the chain length decreases upon addition of an excess amount of Zn . The polymeric structure was estabhshed by NMR using a dimer model compound as reference as well as by DOSY NMR, UV/vis spectroscopy, fluorescence anisotropy measurements, and AFM [86]. Further superstructures were ob-... 

Zinc carboxylate interactions have been exploited as part of a fluorescent molecular sensor for uronic acids. The sensors feature two interactions coordination of the carboxylate to the zinc and a boronic acid diol interaction.389 Photoluminescent coordination polymers from hydrothermal syntheses containing Zn40 or Zn4(OH)2 cores with isophthalate or fumarate and 4,4 -bipyridine form two- and three-dimensional structures. Single X-ray diffraction of both dicarboxylates identified the network structure.373... 

Figure 13.9 Scheme for the complexes Na[Ln(L14)(H20)4]-2H20 , and increase in luminescence intensity of a 100 xM solution of Eu-14 in H2O upon addition of Ag+ [37]. (Reproduced with permission from W.S. Liu etal., Lanthanide coordination polymers and their Ag+ -modulated fluorescence, Journal of the American Chemical Society, 126, 2280-2281, 2004. 2004 American Chemical Society.)... 

A triazole-bridged cadmium coordination polymer, based on 3,5-dimethyl-4-amino-1,2,4-triazole, has been synthesized under hydrothermal conditions [17]. This material exhibits a strong blue fluorescence emission band in the solid state at ambient temperature. 

This concept was extended by Che et al. for a series of terpyridine containing chromophores [88]. The respective zinc coordination polymers obtained thereby were investigated by optical spectroscopy, NMR and viscosimetry. These coordination polymers exhibit fluorescence ranging from violet to yellow and two such polymeric compoxmds coifld be successfully incorporated into an electroluminescent device. Whereas in polymer 33 the green fluorescence originates from the fluorene unit located between the tpy ligands, the blue fluorescence in polymer 34 stems from the 4 -phenyl-substituted tpy ligand itself, so that in the latter case tpy acts as both the structural and the functional unit. 

Zhang C, Che Y, Zhang Z et al (2011) Fluorescent nanoscale zmc(II)-carboxylate coordination polymers for explosive sensing. Chem Commun 47 2336-2338... 

Yanai N, Kitayama K, Hijikata Y, Sato H, Matsuda R, Kubota Y, Takata M, Mizuno M, Uemura T, Kitagawa S (2011) Gas detection by structural variations of fluorescent guest molecules in a flexible porous coordination polymer. Nat Mater 10 787-793 Hirai K, Furukawa S, Kondo M, Meilikhov M, Sakata Y, Sakata O, Kitagawa S (2012) Targeted functionalisation of a hierarchically-structured porous coordination polymer crystal enhances its entire function. Chem Commun 48 6472-6474... 

Fig. 32 Effect of analyte coordination in (a) a traditional molecular chemosensor system, (b) receptors wired in series in a conjugated polymer, (c) chemosensors grafted onto the surface of a nanoparticle, and (d) a fluorescent dendrimer. The curved arrows indicate the active...

The sensor covalently joined a bithiophene unit with a crown ether macrocycle as the monomeric unit for polymerization (Scheme 1). The spatial distribution of oxygen coordination sites around a metal ion causes planarization of the backbone in the bithiophene, eliciting a red-shift upon metal coordination. They expanded upon this bithiophene structure by replacing the crown ether macrocycle with a calixarene-based ion receptor, and worked with both a monomeric model and a polymeric version to compare ion-binding specificity and behavior [13]. The monomer exhibited less specificity for Na+ than the polymer. However, with the gradual addition of Na+, the monomer underwent a steady blue shift in fluorescence emission whereas the polymer appeared to reach a critical concentration where the spectra rapidly transitioned to a shorter wavelength. Scheme 2 illustrates the proposed explanation for blue shift with increasing ion concentration. 

Similarly, this amphiphilic polymer micelle was also used to dismpt the complex between cytochrome c (Cc) and cytochrome c peroxidase (CcP Sandanaraj, Bayraktar et al. 2007). In this case, we found that the polymer modulates the redox properties of the protein upon binding. The polymer binding exposes the heme cofactor of the protein, which is buried in the protein and alters the coordination environment of the metal. The exposure of heme was confirmed by UV-vis, CD spectroscopy, fluorescence spectroscopy, and electrochemical kinetic smdies. The rate constant of electron transfer (fc°) increased by 3 orders of magnimde for the protein-polymer complex compared to protein alone. To establish that the polymer micelle is capable of disrupting the Cc-CcP complex, the polymer micelle was added to the preformed Cc-CcP complex. The observed for this complex was the same as that of the Cc-polymer complex, which confirms that the polymer micelle is indeed capable of disrupting the Cc-CcP complex. 

Polyphenylene and polyfluorene have been extensively used as fluorescence-based sensors, and several chromogenic forms of these polymers have been reported. Incorporation of monomers with additional coordination sites into these polymers has led to the development of a variety of different anion sensors, mostly for halide ions (Lee et al. 2004 Zhou et al. 2005 Vetrichelvan et al. 2006 Kim et al. 2007). Extension of these materials toward recognition of more complex analytes should be possible. 

---