Zinc II Terpyridine Polymers

The synthesis of 2,2 6, 2 -terpyridine was first described by Morgan and Burstall in 1931.19 Subsequently, metal-terpyridine complexes have become an area of active research. In 1995, Constable proposed a concept of using terpyridine functionalized monomer to give coordination polymers upon addition of metal ions (Fig. I).8 

Ru(II) based terpyridine polymers were prepared by Houston et al. in 2003.21 The polymers were prepared by reacting /h.v-terpyridine monomers with Ru(DMSO)4Cl2 in hot ethanol (Fig. 3). To improve the solubility of the coordination polymers, pinene moieties are attached to the monomers. The degree of polymerization of the polymers was studied by elemental analyses, gel permeation chromatography, vapor phase osmometry, STM and electrospray ionization mass spectrometry. However, no precise result was obtained, and the size of the polymers was estimated to be between 40 and 60 repeating units. 

In 2002, Wiirthner et al. reported a Zn(II) terpyridine polymer showing a rod-like structure (Fig. 4).11 The polymer exhibits a strong fluorescence, with an emission quantum yield of 0.61 in DMF solution. 

FIGURE 1. Terpyridine coordination polymer proposed by Constable.  

FIGURE 2. Synthetic scheme for self-assembled Fe(II) terpyridine polymer.20 

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In 2003, we first used self-assembled zinc(II) terpyridine polymers in the fabrication of PLEDs.22 We synthesized a series of monomers containing two terpyridine as chelating units, which are at opposite ends and separated by a spacer group (R) (Fig. 5). The self-assembled zinc(II) coordination polymers were easily prepared by the addition of Zn(OAc)2 into solution of monomers. The product yields were up to 80%, and the inherent viscosities of the polymers were in the range of 0.48 to 1.21 dL/g, as determined using an Ubbelohde viscometer in NMP at 30 0.1 °C. 

Spectroscopic data of la—li, monomers of la—li, and model compound 1 (zinc(II) fe(V-phenyl salicylaldiminato) are listed in Table 2. The absorption spectra of these zinc(II) terpyridine polymers are similar, with absorption maxima at 286—290 and 320—391 nm. PL of these polymers span violet, blue, green, and yellow color. The PL quantum yields ( I>PL) range from 25% for lb and le to 77% for If in DM Ac. The PL values of the polymer thin films were determined using integrating sphere,23 which were found to vary from 0.15 to 0.55 0.05 (Table 2). 

The spacer between terpyridine units affects PL of the zinc(II) polymers. The emission colors are blue (Tpl,film =0.20), green (0.50), green (0.20), green (0.55), and yellow (0.40) with Xmax at 450, 546, 530, 535, and 567 nm, respectively. The difference in the emission colors could be attributed to the difference in conjugation length of the terpyridine ligands. 

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