Functionalized luminescent platinum(II) molecular hosts

: synthesis, spectroscopic characterization and sensing applications

Abstract

New families of luminescent platinum(II) molecular host complexes supported by tridentate polypyridine and tetradentate Schiff base ligands have been synthesized and characterized by spectroscopic techniques. A novel series of ditopic Pt(II) terpyridine hosts have been prepared, which feature a hydrophobic cavity created by incorporating a rigid xanthene or dibenzofuran ‘backbone’. Interestingly, selected terpyridyl hydrogen atoms in these complexes display significant 1H NMR perturbations in the presence of different anions, and 1H NMR titration experiments have been performed to examine this. A Pt(II) terpyridine complex bearing a hydrophobic cavity has been characterized by X-ray crystallography. Their absorption spectra and fluid luminescence at 77 K (n-butyronitrile and MeOH:EtOH glass) and 298 K have been investigated and compared with those for analogous Pt-terpyridine systems. All xanthene-based complexes are luminescent in solution at room temperature and exhibit a structureless 3MLCT emission at λmax ~625 nm, while dibenzofuran congeners are very weakly emissive. The 77 K glassy emissions for all complexes are highly structured bands with vibronic progressions of 1050–1200 cm-1, plus, xanthene derivatives exhibit excimeric emission, as indicated by the bandshape, second-order lifetime decays, and concentration dependence of the emission band. The design and preparation of phosphorescent monodispersed oligo(phenylene-ethynylene)s (OPE) and the corresponding poly(phenylene-ethynylene)s (PPE) linked by a Pt(II) Schiff base module have been undertaken, which constitute the initial step towards the development of phosphorescent salen-based conjugated materials as tunable sensory materials. Photophysical properties that are desirable in sensing applications, such as solvent-sensitive fluid phosphorescence at room temperature and excited states with prominent MLCT character, have been observed. The transient absorption-difference (TA) spectrum for the excited state of a OPE-Pt(II) Schiff base complex has been obtained, which show far-red absorptions that are not observed in the TA spectra of previously reported Pt-Schiff base analogues and are reminiscent of the OPE ππ* state. The PPE-Pt(II) Schiff base polymer is emissive in the red region (298 K solid state emission at λmax 680 nm) Strategies to modify the recognition characteristics of the Pt(II) host complexes in a rational fashion have been developed, and applications in luminescent sensing have been investigated. Binding studies with analytes of biomolecular and environmental interest have been performed by spectrofluorometric titrations. Substantial emission changes (including enhancement) have been detected that are dependent on the complementarity between host and guest molecules, and good selectivity for selected guests have been observed. The nature and mechanism of the host-guest interaction has been examined and modeled.

Date of Award	16 Feb 2009
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Michael Chi Wang CHAN (Supervisor)