Ground-state charge-transfer complexes

Both t-1 and c-1 form ground state charge-transfer complexes with strong electron acceptors (72-79). Equilibrium constants and absorption data for their complexes with several electron-poor alkenes are given in Table A. The absorption maxima of a family of charge-transfer complexes can be related to the donor ionization potential (IPp) and acceptor electron affinity (EA ) using eq. 16 (79). 

Oxetane formation is presumed to occur via the singlet exciplex however, excitation of the ground state charge-transfer complex may be necessary in order for the formation of 39 to compete with the rapid isomerization of c-1. The factors which favor oxetane versus cyclobutane formation in this reaction are not understood. 

There is no evidence for ground state charge-transfer complex formation between stilbenes and neutral amines. Amine cations and dications are powerful electron acceptors and can form ground state complexes in which t-1 serves as the electron donor. Complex formation between t-1 and the organic dication methyl viologen is responsible for quenching of the fluorescence of surfactant stilbenes in organized assemblies (112). 

Excitation of a ground-state charge-transfer complex between arene and alkene (Sec. II)... 

II. ORTHO PHOTOCYCLOADDITION VIA EXCITATION OF A GROUND-STATE CHARGE-TRANSFER COMPLEX BETWEEN ARENE AND ALKENE... 

Ortho photocycloadditions proceeding via excitation of a ground-state charge-transfer complex have been reported for the combination of benzene and alkyl-benzenes with maleic anhydride. The reaction was discovered by Angus and... 

Maleimide also forms 2 1 adducts with acceptor compounds such as benzonitrile, acetophenone, and methyl benzoate [46], This behavior contrasts with that of maleic anhydride, which neither exhibits charge-transfer absorption with nor photoadds to benzenes bearing strong electron-acceptor substituents. It clearly demonstrates that formation of a ground-state charge-transfer complex is not essential in the formation of 2 1 adducts from maleimide and benzene derivatives. 

If the alkene, from which the ortho photocycloadduct is derived, is a powerful dienophile, a thermal Diels-Alder reaction between adduct and alkene may take place during the irradiation so that a 2 1 alkeneiarene adduct is formed. Such adducts are quite stable and can easily be purified. Their structure contains all the regiochemical and stereochemical information of the primary 1 1 ortho adduct. This Diels-Alder reaction occurs spontaneously with the alkenes maleic anhydride, maleimide, and their derivatives, as discussed in the sections of this chapter that deal with photoreactions of ground-state charge-transfer complexes (maleic anhydrides) and reactions of excited alkenes with ground-state arenes (maleimides). Ortho adducts formed from other alkenes have often been identified via their reaction with a good dienophile. 

Another extensively investigated system involves the interaction of two alkenes, each capable of geometric isomerization, viz., the system stilbene-dicyanoethylene, which also illustrates the involvement of ground-state charge-transfer complexes. Excitation of the ground-state complex results in efficient Z - E isomerization of the stilbene exclusively, because the stilbene triplet state lies below the radical ion pair, whereas the dicyanoethylene triplet state lies above it (Fig. 11) [163-166]. 

Kochi studied the selective excitation of preformed ground-state charge transfer complexes [85]. Irradiation of the long wavelength band of these species results... 

While the meta photocycloaddition occurs at the n,n singlet state of the arene [4], different cases must be distinguished for the ortho cycloaddition depending on the structure of the substrates [5] (a) excitation of a ground state charge-transfer complex (b) excitation of the alkene (or alkyne) reaction partner (c) excitation of the arene partner and reaction at the singlet state and (d) excitation of the arene followed by intersystem crossing and reaction with the alkene at the triplet state. 

Incorporation of triethylamine into the reaction medium produced more reduction product presumably due to electron transfer from the triethylamine to the excited alkyl halide. This results in a weakly-bound amine-alkyl halide pair [57]. The alkyl halide radical anion releases X- (Scheme 17). In a related example, it is known that solutions of aliphatic amines in CC14 are unstable to light quickly forming white crystalline precipitates [60]. The initial reaction is formation of a singlet radical pair via excitation of a ground state charge-transfer complex. 

Naphthalene-photocatalyzed [4- -2]-cycloaddition between indole and cyclohexadiene based on selective irradiation of naphthalene-indole ground-state charge-transfer complex in the presence of 1,3-cyclohexadiene, has been published <20070L453>. 

Sulfur- and selenium-donor ligands stabilize several polynuclear zinc complexes, for example, the [E4Zii4(SPh)i6] (E = S or Se) anions, prepared as their Mc4N+ salts.The complex [Zn4(SPh)io] forms a ground-state charge transfer complex with methyl viologen. These... 

Photoactivated Copolymerization. Although polymerization and copolymerization generally involve the addition of a monomer to a reactive chain end, the ground state charge transfer complex generated by the interaction of an electron donor monomer and a strong electron acceptor monomer, acts as a single unit and, upon excitation of the complex, both monomers enter the chain. 

Complexation of methyl methacrylate and acrylonitrile with triethylaluminum converts these poor electron accepting monomers into stronger electron acceptors. Ground state charge transfer complexes are generated when styrene is added to these monomers in the presence of triethylaluminum. Photoexcitation of the... 

Ichimura et al. [76] have described photoresists based on soluble polymers bearing methacrylated side-chain groups which are photocross-linked by a free radical mechanism using a diphenyliodonium salt as initiator, with a p-dimethylaminobenzylidine sensitizer. These investigators found spectroscopic evidence for in situ ground state charge transfer complex formation between the sensitizer and initiator, and that such complexes are involved both in the photocross-linking process as well as in the thermal instability of the photoresist films. 

The viscosity dependence of emission from the charge-transfer complexes of tetracyanoethylene with benzene and a number of alkylbenzenes has been studied. At high viscosities the fluorescence spectra of alkylbenzenes have a double-band character and the two sub-bands change in different ways with changing viscosity. These observations and the dependence of the excitation spectrum on observation wavelength are discussed in terms of different orientational isomers of the ground-state charge-transfer complexes.1 7... 

Krongauz and Chawla, reported on the effeet of aromatie thiols on kinetics of acrylate radical photopolymerization in the presenee and absence of photoinitiators. They observed that aromatic thiols at concentrations, < 0.5% (-0.05 M), can accelerate radical photopolymerization. Initiation of radical photopolymerization, by some aromatic thiols in the absence of conventional photoinitiators was also observed. On the other hand, there was also an unexpected inhibition of photopolymerization at higher concentration of the aromatic thiols due to chain transfer. Also, a ground state charge-transfer complex formation between thiols and benzoin based photoinitiators was detected. 

For charge transfer leading to a fluorescent exciplex, aU rate constants can be evaluated from the fluorescence decays, but particular attention should be paid to the possibility of occurrence of (1) transient effects, (2) the harpoon mechanism [70] (the electron goes first and then the exciplex is formed) and (3) ground-state charge-transfer complexes. All these phenomena lead to deviations from doubleexponential decays and/or differences between Stem-Volmer plots obtained from time-resolved (tq/t vs [Q]) and steady-state (Iq// vs [Q]) measurements. 

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