Irradiation of Charge-Transfer Complexes

The CT complexes are characterized by a new absorption band which is usually red-shifted as compared to local excitation bands [47-49], According to the Mulliken formulation the CT-exdtation corresponds to an electronic transition from the HOMO of the donor to the LUMO of the acceptor, i.e. it accomplishes full electron transfer [47], The transition is instantaneous, producing two intermediates (ions) in a direct contact but in a non-equilibrium, Franck-Condon state. The relaxation of the pair competes with BET, diminishing the quantum yield for ion generation [49], This process is believed to take 

The formation of a detectable quantity of weak CT complexes usually requires a large excess of one component, or high concentrations of both components. Such a situation may lead to mechanistic complications. Thus, 1 2 complexes may form in the ground state [54], or the photogenerated ion pairs may be rapidly (subnanosecond or nanosecond time scale) intercepted to form triplexes [55] or ionic dimers [56]. It may be expected that the chemical behavior of these aggregates will crudely resemble the reactivity of normal ion pairs. 

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Detailed analyses of each of these major classes of photopolymer sensitization will be presented below. A related process which is sensitive to visible light, which will not be discussed, is irradiation of charge transfer complexes, exemplified by the initiating complex between isoquinoline and bromine. See ref. 13. 

Hammond and coworkers (147,349,374) have developed a rotating sector method to measure by ESR the bimolecular decay of radicals produced by photolysis. Calvin and coworkers (148) have also studied the kinetics of ESR signal decay by intermittent UV-irradiation of charge-transfer complexes. A CAT was used by both groups to... 

Cationic polymerization of epoxides by irradiation of charge-transfer complexes has been mentioned in the literature Fluorinated alkanesulfonic acid salts chromates and dichromates of alkali metals, alkaline earth metals and ammonium phototropic o-nitrobenzyl esters iodocyclohexene unsaturated nitrosamines and carbamates have been reported to act as cationic photoinitiators. 

Spontaneous Crosslinking. In the absence of TCPA, evaporated solutions of XVIII can be completely redissolved in fresh solvent in a few minutes. And solutions of 0.05 M XVIII containing 0.007 M TCPA in acetonitrile are stable indefinitely (at least three weeks) at room temperature. However, on evaporation of the solvent under vacuum, the mixture forms a very tough film that is insoluble in acetonitrile or hot DMF, does not melt up to 300°, and is resistant to attack by chromic acid. The IR spectrum is identical to that of XVIII crossiinked BF3 or by irradiation in the presence of TCPA. Apparently as the solution becomes highly concentrated, crosslinking occurs by thermal excitation of charge-transfer complexes of TCPA and XVIII. The IR evidence and the insolubility behavior appear to discount an alternative possibility of simple association of polymer chains facilitated by the presence of the acceptor compound. The spontaneous crossiinking is also observed with the acceptors chloranil and 1,3,5-trinitrobenzene, and presumably would occur with others. 

The co-condensation of Mn with PX at 77 K yields complexes of two types charge transfer complexes analogous to those of Mg-PX and d-n complexes of quinoid PX involving d orbitals of Mn and n orbitals of PX [36, 42]. Polymerization at 77 K under irradiation is accompanied by the destruction of d-n complexes and transformation of charge transfer complexes to organoman-... 

Maleic anhydride and hexamethylbenzene form a 1 1 charge-transfer complex in methylcyclohexane solvent with a UV maximum at 340 tx. Ultraviolet irradiation of these solutions brings about the formation of carbon dioxide, pentamethylbenzylsuccinic anhydride, and resinous products (see Chapter 6). Analytical data suggest the polymeric material has a decarboxylated MA building block. Charge-transfer complexes were not detected in solid-state mixtures of MA and pentamethyl-benzene. Ultraviolet irradiation of these solid mixtures gives cyclic dimer, 1,2,3,4-cyclobutanetetracarboxylic anhydride 2, and resinous products. Pentamethylbenzylsuccinic anhydride, due to the possible absence of charge-transfer complexes, was not isolated from the solid-state photoreaction. 

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. 

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. 

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). 

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... 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

DCA in such a photooxygenation Similar oxygenations of more electron-poor substrates 123 (e.g., Ar = Ar = Ph) give the cyclic peroxides 124 in poor yields (15-30%), and the corresponding benzophenones as the major product . Alternatively, 3,3,6,6-tetraphenyl-l,2-dioxane and related tetraaryl-analogues 124 can be effectively prepared in 80-90% yield by irradiation of pre-formed charge transfer complexes of 123 with SbCls in methylene chloride at —78°C in the presence of oxygen ... 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

Recently chloromethylated polystyrene (CMS), a highly sensitive, high resolution electron resist with excellent dry etching durability, was developed. Very recently reactive intermediates in irradiated polystyrene, which is a starting material of CMS, have been studied and the transient absorption spectra of excimer (2-4), triplet states (2,5), charge-transfer complexes, and radical cations (6) of polystyrene have been measured. The present paper describes the cross-linking mechanism of the high sensitivity CMS resist and compares it to that of polystyrene on the basis of data on reactive intermediates of polystyrene and CMS. 

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