Luminescence and other excited states

It is the 4-coordinate square-planar geometry that makes Pt(II) complexes very different from those of most of the other metal ions familiar to the inorganic photochemist, including Cr(III), Ru(II), Os(II), Rh(III), Ir(III) (almost always 6-coordinate octahedral), copper(I) (4-coordinate tetrahedral), and lanthanides (8 or 9 coordinate). The square planar conformation is responsible for many of the key features that characterize the absorption, luminescence and other excited state properties of platinum(II) complexes. 

Trigonal ML3 metal complexes exist as optically active pairs. The complexes can show enantiomeric selective binding to DNA and in excited state quenching.<34) One of the optically active enantiomers of RuLj complexes binds more strongly to chiral DNA than does the other enantiomer. In luminescence quenching of racemic mixtures of rare earth complexes, resolved ML3 complexes stereoselectively quench one of the rare earth species over the other. 35-39 Such chiral recognition promises to be a useful fundamental and practical tool in spectroscopy and biochemistry. 

In case of co-facial quinone-capped porphyrins (P and Q are linked by four tetraamidophenoxy bridges and are located at a distance of 10 A from each other), the quantum yield of charge separation is much bigger and reaches 30% for short distances between P and Q [53, 54]. Luminescence quenching via electron transfer from P to Q is observed for both singlet- and triplet-excited states of the porphyrin fragment of P-Q. The appearance of the additional channel for luminescence decay via electron transfer manifests itself in the biphase character of P-Q luminescence decay kinetics. 

It is also possible for species that are created in ECL reactions to interact with each other in ways that interfere with the generation of ECL and partially, if not completely, quench the emission. For example, one difficulty in direct sensing of coreactants is that the coreactant may also quench the luminescence of the excited state generated in the annihilation process. This difficulty was recognized several years ago by Bard and coworkers in the examination of the [(bpy)3Ru]2+/S20g system [24], Luminescence arises upon reduction of the Ru(II) complex and reduction of S20g mediated by the Ru(I) complex formed. The intermediate SOJ ion formed is a powerful oxidant and annihilation with the Ru(I) complex will yield the excited state of Ru (II) complex [Eq. (13d)]. However, the persulfate ion is an effective quencher of the MLCT excited state of the Ru(II) complex. Figure 9 shows the observed ECL intensity for this system... 

In the sections which follow, the principles discussed above will be used in exploring the properties of a range of platinum(II) complexes. The emphasis of the chapter will be on emission—luminescence—from Pt(II) complexes, on the features and properties of molecules that tend to favor emission over other non-radiative processes. In other words, photophysics, as opposed to photochemistry, is our main subject here, but we also consider other excited state processes in selected systems, such as electron transfer and photooxidation. 

It seems to be surprising that only a few examples of luminescent LMCT states are known. This lack could have two explanations. Many complexes with prominent LMCT states have other excited states such as MC states at lower energies. These provide a facile access for radiationless deactivation of LMCT states. In other cases LMCT states are quite reactive and photoreactions compete successfully with the luminescence. 

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