Ground-state charge-transfer

In considering the origin of the results in Table III and the extraordinary rate exhibited by the HM/CHVE system, it is tempting to consider the role that ground-state charge transfer interaction between the donor and acceptor monomers in the... 

The ease of formation of porphyrin radicals and the existence of ground state charge transfer into the axial ligands (3 ) makes it necessary to keep in mind the possibility of ligand ejection accompanied by oxidation-reduction during the excited states of metal1oporphyri ns. 

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). 

An investigation with the electron donor 4-methoxybenzo[b]thiophene (35) and electron acceptor p-chloroacetophenone (36) and with the bichro-mophores 37 and 38, where the above donor and acceptor moieties are connected by an olefinic (unsaturated as well as saturated) spacer, was performed (02JPP(A)(152)41). The absorption spectra of the donor 35 in the presence of the electron acceptor 36 were measured in n-heptane and highly polar acetonitrile solutions. In both nonpolar and highly polar media, it was found that the spectra of the mixture of 35 and 36 are just the superposition of the absorption bands of the individual components. This observation excludes the possibility of formation of any ground state charge transfer (CT) complex. 

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... 

For now, it has been shown, that the presence of attractive interactions as they are present between the two redox-active moieties, exTTF and leads to aggregation phenomena in the high concentrations regime (i.e. FT 4 M 1). In turn, the photophysical response of the resulting intracomplex hybrids differs substantially from that found in the low concentrations regime, where only the monomeric form is present. In particular, the ground-state charge-transfer interactions result from a shift of electron density from exTTF to C o due to the short distance between donor and acceptor. 

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. 

The method used to represent the time dependent evolution of the solvent polarization that follows the transition between two different electronic states in the solute has been obtained as a generalization of the time-dependent model originally proposed to describe ground state charge-transfer phenomena within the PCM... 

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... 

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