Surface Chelation Effects

Frei et al. [34[ examined the surface chelation of phenylfluorene on Ti02 using FT-IR and laser photolysis techniques. The surface chelate has its visible absorption band maximum located at 476 nm (e = 3.6x10 M cm ). Electronic excitation in the visible absorption band results in extremely rapid and efficient injection in the conduction band of the semiconductor. A lower limit for the rate constant of interfacial electron transfer was determined as 10 s l and the back electron transfer was found to occur with a specific rate of 2.8x10 s l. The injected electrons in the conduction band readily reduce electron acceptors such as methyl viologen efficiently in the same manner as when they are produced by bandgap excitation of the semiconductor. 

In a related study [35], surface complexation of colloidal Ti02 by aryl carboxylic acids (benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, salicylic acid and catechol) has been found to obey the Langmuir Isotherm. Surface chelation is accompanied by a decrease of the point of zero zeta potential (PZZP) and enhanced electron transfer rates from the conduction band and acceptors in solution. The enhanced electron transfer rates observed upon surface chelation (of cobaltocenium dicarboxylate, phenylfluorene and aryl carboxylic acids) suggest that trapping of electrons by Ti(IV) surface states (equation 3) can take place efficiently and removal of such traps by complexation improves the overall performance of the system. 

Direct evidence for the trapping process has been obtained from recent EPR studies with colloidal Ti02. In acidic solution all the trapped electrons could be identified with Ti + surface states. Impedance measurements on single 

Deposition of insoluble monolayer of the dye by the Langmuir-Blodgett technique has also been studied as means of assuring close contact of the chromophore and the semiconductor. For example, Arden and Fromherz prepared a thin film electrode by evaporation of indium-tin-oxide and covered it with a bilayer of a lipid containing a cyanine dye in direct contact with the electrode [36,37]. Quantum yield for the transfer of electrons from the excited chromophore in the membrane structure to the semiconductor was found to be quite high (O.S).Based on the fluorescence lifetime of the dye in solution (= 3ns) and the quantum yield of the emission in the lipid bilayer, the electron transfer rate from the singlet excited state has been estimated to be =10lO s.  

Sensitization of Polycrystalline Ti02-based Photoelectrochemical Cells 

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Figure 1 Schematic illustration of the surface chelate effect.

II. ENHANCED COORDINATION BY PREORGANIZATION. SURFACE CHELATE EFFECT AND SIGNALING... 

This thermodynamic effect of the enhancement of classical recognition features is basically due to entropic factors related to the preorganization of the coordination sites on the surface that reduce the conformational flexibility of the receptors and increase their effective concentration at the surface. This results in a remarkable improvement of the recognition characteristics. From a molecular viewpoint, surface functionalization creates a multidentate coordination environment that displays a statistical advantage in relation to coordination when compared to the molecular receptor. This effect has also been called the surface chelate effect and is not a unique feature of surfaces functionalized with binding sites. It has also been reported for coordinating dendrimers that show a positive dendritic effect, which is related to... 

Most of the reported examples involve the use of MCM-41, MCM-48, hydrothermally stable mesoporous molecular sieves (SBA-15) or silica nanotube matrices in which certain selective probe molecules are covalently anchored to the pore walls. In addition, the very low detection limits observed for some of these systems suggest that the surface chelate effect also plays a role here. Following this approach, chemosensors for the signaling of Cy2+ 34 36 pg3+37 2+ 35,38 describcd. Optical pH sensors using... 

The second approach consists of synthesizing first the complex MLra 1(L X) with the desired ratio (L )/(M) this complex bears the reactive fragment X which then reacts with the surface of the silica. This method is of limited interest, because the synthesis and isolation of these functionalized complexes is not straightforward. One of the successful examples concerns the synthesis of nickel carbonyl complexes anchored to the surface via two bonds in an attempt to increase the stability through a sort of chelate effect. Initial attempts to achieve this by the methods described in Equation(5) (initial functionalization of silica) and Equation(6) (initial functionalization of complex) failed, as demonstrated by 29Si and 31P CP MAS NMR spectroscopies.51... 

Fig. 5.5c illustrates the effects of various ligands upon the dissolution rate, and that a surface chelate (ring structure of the ligand bound to the metal center at the surface) is more efficient in enhancing the dissolution rate. Furthermore, the acceleration increases in the series Salicylate > oxalate > malonate > phtalate > succinate which indicates that a 5-ring chelate is more efficient than a 6-ring or 7-ring chelate. 

Effect of surface chelation on the kinetics of electron transfer from the conduction band of Ti02 to methylviologen (MV2+). Oscillograms showing the time-dependent growth of the MV+ absorption at 630 nm after laser excitation (at 355 nm) of aqueous solutions (pH 4.85) containing colloidal Ti02 (1 g/e) and 10 3 M MV2+ ... 

Ensembles 600 Enterokinase 480 Enthalpy 55 activation 56, 545-547 protein folding 509 -512 specific heat effects 511, 545 - 547 Enthalpy-entropy compensation 346 Enthalpy versus entropy in protein folding 509-512, 587, 599 Entropy 55, 68-72 activation 56, 545 -547 binding 324, 345 Boltzmann equation 510 chelate effect 345 configurational 510 configurational entropy of loops 535 effective concentration 68-72 equilibria on enzyme surface 118 hydrogen bond 338 hydrophobic bond 332, 510 importance in enzyme catalysis 72 importance in enzyme-substrate binding 72... 

Karim, A. and S. Kumar Polymer Surfaces, Interfaces and Thin Films, World Scientific Publishing Company, Inc., River Edge, NJ, 1999 Lisenskey, G.C., et al. Electro-Optical Evidence for the Chelate Effect at Semiconductor Surfaces, Science, 840 (May 18. 1990). 

Concerning 2-(aminomethyl)-cyclopentylamine formation, some dehydrogenation reaction of amino capronitrile can occur on the catalytic surface. Stereoselectivity can be interpreted in terms of chelating effect leading to the cis isomer. 

SAMs [48-50] provide a convenient way to produce surfaces with specific chemical functionalities that allow the precise tuning of surface properties. Previously, SAMs have successfully been used to demonstrate that the sensing process is feasible at the monolayer-solution interface [24,26,44,51-53]. The advantages of SAMs for surface-confined sensing are ease and reproducibility of synthesis [51], the introduction of additional chelating effects from the preorganization of the surface platform and fast response times [54]. 

The chelating effect of the surface appears to be the driving force for the formation of the surface cis-octahedral complex. Equation 17, which is the sum of eqs 15 and 16, is accompanied by an entropy increase with the release of water molecules and the disappearance of charged species leading to a subsequent disordering of nearby solvent molecules. This aspect is well documented in solution coordination chemistry [68, 69]. 

The chelate effect is distinct from multivalent binding modes that require receptor clustering [59]. Since the cell membrane is fluid, receptors are free to diffuse in two dimensions this mobility can lead to clustering of receptors. In such a system, the unliganded state of the saccharide-binding protein may not have the same distribution on the cell surface as the bound species. This clustering occurs with an entropy cost, but it does not require that the multidentate saccharide display match precisely the display of receptors on the cell surface. 

G. Redmond D. Eitzmaurige M. Graetzel, Effect of surface chelation on the energy of an intraband surface state of a nanocrystalline Ti02 film. J. Phys. Chem. 1993, 97, 6951-6954. 

Generally, the protonation of Al sites promotes the dissolution process with increasing H+ activity in acid solution (A1203, kaolinite, muscovite), whereas the rate of silica dissolution even decreases or remains constant (pH < 3). Obviously (lie more Al centers are exposed per unit surface area, the higher the proton-promoted dissolution rate and the more effective are surface chelates in catalyzing the weathering process. 

Probably no single causal mechanism functions in the calcification process of neointima-lined or smooth surface polyurethanes. Rather, surface calcification is most likely a result of the combination and interaction of mechanical and surface chemical effects at the blood-surface interface. Mechanical damage to or physical imperfections on the polymeric substrate in smooth surface devices or the neointima lining of textured bladders may be capable of inducing a deposition and mineralization process. Calcification of tissue valve leaflets has been proposed to result from the diffusion of blood elements into mechanically disrupted tissue (10), thus providing a site for mineralization to occur. Likewise, deposits of calcium-chelating proteins or lipids in defects in neointimal tissue or the polymer substrate may act as precursor binding sites for the observed mineralization. 

It is difficult to evidence pure cases of inner sphere complex formation between surface groups and transition metal ions (also called "grafting") since other phenomena are usually occurring in parallel. The clearest instances are observed when "spectator" ligands are inert to substitution, either because of chelate effects (ds- [Ni(en)2(H20)2p on various supports ) or because of high crystal field activation energies ([Co(NH3)5(RO)] + on TiAl2C)3 RO= OH, H2O or alcohol). 

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