الفهرس 
Chapter IOnly 14 pages are availabe for public view
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Abstract
The cyclometalation reaction has been widely studied¹⁻² because it represents probably one of the mildest routs for activating strong C-H and C-R bonds. The tendency of transition metal salts to undergo cyclometalation reaction, and, in particular, other-metalation reaction, with heteroaromatic ligands (mostly including nitrogen donors, but oxygen-sulfur-and phosphorus-containing ligands have also been cyclometalated ) to give five-membered metallocycles has been demonstrated with various metals, including, for instance, Re (I), Pt (II), Pd(II).
In comparison with Pd (II), which is by far the most used metal in cyclometalation reactions, the cyclometalation reaction is not so intensively studied and not very easy to accomplish (cycloplatination reactions which took about four weeks or required relatively forcing conditions, eg. refluxing toluene with boor yields, have been reported ) . However, it is possible to increase the yields and reduce the time of reaction by using different starting materials such as bis ( ή3- allyl ) –di-μ-chlorodiplatinum (II) or PtCl2(DMSO)2, etc. although K2PtCl4 or [Pt2Me4 (μ-SMe2) 2)] are commonly used to yield cyclometalated species.
The Pt(II) ions adopt a square-planar geometry, being part of the major group of exceptions to the otherwise very successful model of Kepert. They show coordinative unsaturation which can allow different interactions such as: excimer formation, chemical quenching, interactions with lewis bases, and oxidative addition.
The single-crystal X-ray diffraction is a very powerful technique for characterization of cyclometalated Platinum (II) complexes and strong correlations between the structure and luminescence properties can be made as, for example, the solid-state emission is greatly influenced by the crystal packing and the presence of Pt-Pt or π-π interactions is favored by the coordinative unsaturation. The most important structural types of cyclometalated Pt (II) complexes that have been investigated by X-ray diffraction with an emphasis on their luminescence properties. Cyclometalated Pt (II) complexes have been extensively investigated in the past years because of their interesting photophysical properties and several reviews are dealing with this topic ³⁻⁸.The luminescence of Pt (II) complexes is assigned to either ligand-centered (LC) or metal-to-ligand charge transfer (MLCT) states or a mixed of both.
The influence of cyclometalated ligands on the photophysical properties of square-planar Pt(II) complexes was clearly seen when the Pt(II) complex with 2,2`- bipyridine (bpy) is compared to the cyclometalated Pt(II) complex with 2-phenylpyridine (ppy). While the [Pt(bpy)²]²⁺is almost non-emissive at room temperature, the Pt(II) cyclometalated complex [Pt(bpy) (ppy)] ⁺ shows emission with a quantum yield nearly 0.02 at 298 K ⁹⁻¹ᶿ.
The cyclometalated Platinum (II) complexes involve in number of applications demonstrated by phosphorescent materials in the areas of organic light emitting diode (OLEDs) ¹¹.Chemical sensing ¹², biological labeling and imogin ¹³and these can appear by developing oligonvilutid-based label- free detection method for nanomolar quantities of Hg⁺² and Ag⁺ ions by employing luminescent Platinum (II) metallointercalators.¹⁴⁻¹⁵.
Some of the other applications are the photocatalysis ¹⁶, luminescent efficiency is one of the key factors controlling the utility of the luminescent materials, this is due to very high phosphorescence efficiencies exhibited by this class of complexes, described as tridentate N^C^N- coordinated platinum complexes. The cyclometalated platinum complex based on 1,3-
2
dipyridyl benzene was first reported by Cardenas et al ¹⁷, also the divergent Behavior of Palladium II and Platinum II in the metalation of 1,3 di –(2- pyridyl ) benzene ¹⁸⁻²ᶿ, the highly efficient room- temperature phosphorescence emitted from this complex was not recognized until its photophysical properties were studied by Williams et al ²¹ The luminescence properties of Platinum II complex of 1,3 di (2- pyridyl) benzene acting as a tridentate NCN- coordinating ligand cyclometalated at C2 of the benzene ring had been investigated ²² .
The use of this compound as an efficient triplet dopant in organic light emitting diode (OLEDs)²³, the research on this important class of cyclometalated Platinum complexes had been expanded significantly by Williams and other research groups focusing on photophysical properties ²⁴, the Platinum complexes of these ligands are amongst the most emissive ever reported with quantum yields excess of 60% and the color of emission can be turned from green to red according to the substituents on the ligand ²⁵.
The application of inorganic light emitting diode devices ²⁶on five–membered coordinated Platinum (II) complexes as phosphorescent emitters in high performance organic light-emitting devices where their luminescence properties were used as a phosphorescence- emitting dopants in a blended host matrix as the emitting layer resulting in very high electroluminescence efficiencies because of the high phosphorescence quantum yields of these Platinum complexes and the efficient energy transfer from both singlet and triplet excited states of the host to the emitting guest. Tuning of the electroluminescence spectra from the yellow to the green- bluish region of the chromaticity diagram is obtained simply by changing the substituents at the central 5-position of the cyclometalating ligand²⁷. The use of tridentate Platinum complexes is also involve in noninvasive imaging and mapping of living cells ²⁸, where Platinum (II) complexes with time –resolved imagine in time-resolved emission imaging microscopy (TERM) [which uses longer times scales-hundreds nanoseconds to microseconds] developed other essential criteria for (TERM) agents including chemical and photochemical stability, low cytotoxicity and synthetic versatility for potential specific targeting.
The square planar geometry of the Platinum complexes bearing a conjugated ligand usually displays interesting photophysical and photochemical properties and these properties results from the axial Pt-Pt or π-π ; interactions where there is dx2-dx2 orbital interactions between the Pt-Pt metals and the π-π interaction is between the bipyridine in the neighboring planar complexes, and they will be like a linear chain group ²⁹. Cross-quenching and self-quenching ( concentration quenching); where the three fundamental emission parameters are intensity, wavelength and lifetime the emission lifetimes are normally independent of the concentration³ᶿ. The cyclometalated Platinum (II) complexes which based on 1,3-dipyridil benzene [HL(1)], incorporating aryl substituents at the central 5 position of the ligand, all are intensely luminescent in the degassed solution at 298K in dichloromethane . The introduction of the aryl substituents leads to a red shift in the lowest energy, intense charge-transfer absorption band compared to [PtL(1)] in dichloromethane in the order H< mesityl < 2- pyridyl < 4- tolyl < 4- biphenyl < 2- thienyl < 4- (dimethyl amino).
Phenyl is 431nm in dichloromethane which correlates with the decrease order of oxidation potentials. Similar order is observed in the emission maxima ranging from 491 nm for [PtL(1)cl] to 988 nm for the 4-(dimethyl amino) phenyl-substituted complex, The emission spectra of all the complexes except for the amino-substituted compound are highly structured in diluted solution.