Excitation transfer 608 Subject

The deep violet color of pentaphenylbismuth and certain other pentaarylbismuth compounds has been the subject of considerable speculation. It has been shown by x-ray diffraction (173) that the bismuth atom in pentaphenylbismuth is square—pyramidal. WeU-formed crystals are dichromic, appearing violet when viewed in one plane but colorless in another plane. The nature of the chromophore has been suggested to be a charge-transfer transition by excitation of the four long equatorial bonds ... 

Large excitation energies and long lifetimes of RGMAs (see Table 5.1) make these atoms suitable subjects to study transfer of EEP excitation energy to a solid body. 

A further technique exists for the determination of triplet energy levels. This technique, called electron impact spectroscopy, involves the use of inelastic scattering of low-energy electrons by collision with molecules. The inelastic collisions of the electrons with the molecules result in transfer of the electron energy to the molecule and the consequent excitation of the latter. Unlike electronic excitation by photons, excitation by electron impact is subject to no spin selection rule. Thus transitions that are spin and/or orbitally forbidden for photon excitation are totally allowed for electron impact excitation. 

A variety of transition metal complexes including organometallics was subjected to an ac electrolysis in a simple undivided electrochemical cell, containing only two current-carrying platinum electrodes. The compounds (A) are reduced and oxidized at the same electrode. If the excitation energy of these compounds is smaller than the potential difference of the reduced (A ) and oxidized (A ) forms, back electron transfer may regenerate the complexes in an electronically excited state (A+ + A A + A). Under favorable conditions an electrochemiluminescence (eel) is then observed (A A + hv). A weak eel appeared upon electrolysis o t]jie following complexes Ir(III)-(2-phenylpyridine-C, N ) [Cu(I)(pyridine)i],... 

The formation of charge transfer complexes between N,N-dimethylaniline or N,N-diethylaniline and Cspectroscopic studies, also in view of their potential optical and electronic applications. Even if the spectroscopic properties of Cgo, C70 are complicated by the presence of aggregates in room temperature solutions, the emissions from the excited state charge transfer complexes between fullerenes and iVjV-dialkylani lines are strongly solvent-dependent exciplet emissions are observed in hexane, but in toluene they are absent145. 

In general, carbonyl compounds that are reactive in the photocycloaddition reaction are also reduced upon irradiation in isopropyl alcohol.17 Subject to the limitation of triplet-triplet transfer to the olefin mentioned previously, the converse is also true. That is, carbonyl compounds that are photoreduced in isopropyl alcohol can form oxetanes unless their triplet energies are high enough for the olefins to act as quenchers. Thus, the two reactions are characteristic of the same type of excited state. (This is not an exclusive generalization.) The quenching experiments mentioned on pp. 308-311 provide evidence that the reactive state can be the triplet and, in some cases, only the triplet. Evidence for this state being n,ir comes from the fact that carbonyl compounds which are reactive usually emit from the n,n triplet, while those which are unreactive emit from some other excited state. 

A remarkable number of organic compounds luminesce when subjected to consecutive oxidation-reduction (or reduction-oxidation) in aprotic solvents1-17 under conditions where anion radicals are oxidized or cation radicals are reduced. In many instances, the emission is identical with that of the normal solution fluorescence of the compound employed. In these instances the redox process has served to produce neutral molecules in an excited electronic state. These consecutive processes which result in emission are not special examples of oxidative chemiluminescence, but are more properly classified as electron transfer luminescence in solution since the sequence oxidation-reduction can be as effective as reduction-oxidation.8,10,12 A simple molecular orbital diagram, although it is a zeroth-order approximation of what might be involved under some conditions, provides a useful starting... 

Electron transfer rates are naturally subject to molecular-scale electric fields. Therefore, ion binding to a molecule is an effective way of controlling PET within it. Since PET is an excited state deactivation pathway, the competing radiative route, i.e. luminescence, also becomes exposed to ionic manipulation. Under favorable conditions, PET rates can be much faster (lO s" ) than luminescence (10 -10 ° s ). At the other extreme, conditions can be arranged under which PET is effectively non-existent. Therefore, luminescence can be ionically switched between off and on states representing digital action. ... 

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