Irradiation charge-transfer

Irradiation of coordination compounds in the charge-transfer spectral region can often enhance redox reactions. The quantum yields are variable. 

Though thermally stable, rhodium ammines are light sensitive and irradiation of such a complex at the frequency of a ligand-field absorption band causes substitution reactions to occur (Figure 2.47) [97]. The charge-transfer transitions occur at much higher energy, so that redox reactions do not compete. 

Isomerization and elimination reactions of alkyls and aryls Isomerizations of mono-alkyls and aryls have been widely studied [107] many ds-Pt(PR3)2ArCl undergo rapid isomerization in the presence of free phosphine, a reaction inhibited by Cl- with a mechanism believed to involve a 3-coordinate Pt(PR3)2Ar+ intermediate that is then attacked by Cl-. The cis- and trans-isomers of Pt(PEt3)2(Ph)Cl undergo reversible isomerization when irradiated at the wavelength of charge-transfer transitions (254 and 280 nm). 

The radical anions of dialkyl sulfoxides (or sulfones) may be obtained by direct capture of electron during y-irradiation. It was shown that electron capture by several electron acceptors in the solid state gave anion adducts 27. It was concluded276 that these species are not properly described as radical anions but are genuine radicals which, formed in a solid state cavity, are unable to leave the site of the anions and exhibit a weak charge-transfer interaction which does not modify their conformation or reactivity appreciably, but only their ESR spectra. For hexadeuteriodimethyl sulfoxide in the solid state, electron capture gave this kind of adduct 278,28 (2H isotopic coupling 2.97 G is less than 3.58 G normally found for -CD3). 

The stable triphenylcyclopropenium cation (81) undergoes an electron-transfer reaction when photolyzed in acidic medium (van Tamelen et al., 1968, 1971). Irradiation of 81 for 4 hours in 10% aqueous sulfuric acid resulted in a 49% yield of hexaphenylbenzene (82). The reaction is presumed to proceed by initial charge transfer to produce the cyclopropenyl radical 83, which then couples to give 84. This compound in... 

The photoelectrochemical properties of 283 colloids prepared by chemical solution growth [193] have been demonstrated by carrying out oxidation and reduction processes under visible light irradiation. Charged stabilizers such as Nation were found to provide an effective microenvironment for controlling charge transfer between the semiconductor colloid and the redox relay. 

Irradiation of an ITIES by visible or UV light can give rise to a photocurrent, which is associated with the transfer of an ion or electron in its excited state. Alternatively, the photocurrent can be due to transfer of an ionic product of the photochemical reaction occurring in the solution bulk. Polarization measurements of the photoinduced charge transfer thus extend the range of experimental approaches to... 

Photo-oxidation or reduction is often found if the complex is irradiated in the charge-transfer bands (see above) photo-oxidation of the metal occurring if the transition is M - -L. Thus the photochemical generation from Ir(IV)Cl6 of a species active in forcing filaments of E. coli may well involve the photoreduction of Ir(IV) to Ir(III) since the intense bands in the visible spectrum of Ir(IV)Clcharge-transfer bands. A report has appeared of the photo-aquation of IrCl -(43). 

If the EDA and CT pre-equilibria are fast relative to such a (follow-up) process, the overall second-order rate constant is k2 = eda c e In this kinetic situation, the ion-radical pair might not be experimentally observed in a thermally activated adiabatic process. However, photochemical (laser) activation via the deliberate irradiation of the charge-transfer absorption (hvct) will lead to the spontaneous generation of the ion-radical pair (equations 4, 5) that is experimentally observable if the time-resolution of the laser pulse exceeds that of the follow-up processes (kf and /tBet)- Indeed, charge-transfer activation provides the basis for the experimental demonstration of the viability of the electron-transfer paradigm in Scheme l.21... 

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

Thermal or photochemical activation of the [D, A] pair leads to the contact-ion pair D+, A-, the fate of which is critical to the overall efficiency of donor/acceptor reactivity as described by the electron-transfer paradigm in Scheme 1 (equation 8). In photochemical reactions, the contact ion pair D+, A- is generated either via direct excitation of the ground-state [D, A] complex (i.e., CT path via irradiation of the charge-transfer (CT) absorption band in Scheme 13) or by diffusional collision of either the locally excited acceptor with the donor (A path) or the locally excited donor with the acceptor (D path). 

Photoinduced methyl transfer. The yellow mixture of 4-phenylpyridinium cation (with relatively low reduction potential) and tetramethylborate anion in tetrahydrofuran persists for 24 h without any reaction (in the dark). However, the deliberate irradiation of the charge-transfer band (at /exc = 370 nm)... 

Electron-transfer mechanism for nucleophilic addition. In accord with Mulliken theory, irradiation of the charge-transfer band of [Py+, BMeT] directly affords the radical pair via one-electron transfer (equation 46). 

X-ray crystallographic analysis of the crystalline [bicumene, NO+] charge-transfer salt confirms that the charge-transfer color arises from a close approach of NO+ to the centroid of the phenyl moiety (see Fig. 10) with a non-bonded contact to an aromatic carbon of 2.63 A.194 The orange solution of bicumene bleaches slowly over a long period in a thermal reaction at room temperature (in the dark) or rapidly via irradiation of the CT band at low temperature. In both cases, l,l,3-trimethyl-3-phenylindane is obtained as the principal organic product (equation 63). 

Charge-transfer activation by irradiation at the CT absorption band (/jvct) bleaches the yellow color within 20 min to afford a trans-tetralin adduct (t-DTT) in quantitative yields207 (equation 70). 

Similarly, the CT irradiation of the isomeric [r-DBC, TCNE] charge-transfer complex leads to the c/ j-tetralin adduct (c-DTT) as the sole product207 (equation 71). 

Homobenzvalene (HB) is an electron-rich donor (IP = 8.02 eV) owing to the presence of a strained ring system, and thus readily forms a charge-transfer complex with TCNE. Charge-transfer irradiation of the [HB, TCNE] complex leads to rapid bleaching of the yellow color, and the formation of a mixture of isomeric cycloadducts208 (equation 73). 

As described above, the charge-transfer irradiation of the [HB, TCNE] complex produces the contact ion pair, in which the strained HB+ undergoes multiple bond cleavages to afford three isomeric cation radicals depicted in Scheme 19, which then undergo coupling with TCNE- to form a mixture of isomeric cycloadducts (A, B, and C) in equation (73).208... 

Photochemical osmylation. The irradiation of the charge-transfer bands (Fig. 13) of the EDA complex of 0s04 with various benzenes, naphthalenes, anthracenes, and phenanthrene yields the same osmylated adducts as obtained in the thermal reactions. For example, irradiation of the purple solution of anthracene and 0s04 in dichloromethane at k > 480 nm yields the same 2 1 adduct (B) together with its syn isomer as the sole products, i.e.,... 

Note that the charge-transfer irradiation of the same anthracene/Os04 complex in hexane solution yields only a small amount of the adduct (B) together with considerable amounts of anthraquinone. The latter probably arises from osmylation at the (9,10) positions followed by decomposition of the unstable osmium adduct.218... 

Electron-transfer activation. Time-resolved spectroscopy establishes that irradiation of the charge-transfer band (hvCj) of various arene/0s04 complexes directly leads to the contact ion pair. For example, 25-ps laser excitation of the [anthracene, 0s04] charge-transfer complex results in the ion-radical pair instantaneously, as shown in Fig. 14218 (equation 76). 

Time-resolved spectroscopy establishes that the 25-ps laser irradiation of the relatively persistent charge-transfer complex of p-bromoanisole with iodine monochloride generates the contact ion pair (see Fig. 15b) in which the metastable ICP undergoes mesolytic fragmentation to form the reactive triad, i.e.,... 

Nitration versus alkylation. Upon the CT irradiation of an orange solution of the charge-transfer complex, the color bleaches rapidly, and either an aromatic nitration product (i.e. 3-nitro-4-methoxytoluene) or an aromatic alkylation product (i.e. 3-trinitromethyl-4-methoxytoluene) is obtained in high yield depending on the reaction conditions summarized in Scheme 22.4lc... 

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