Charge-transfer excitations

The metastable species involved in these reactions may be either excited charge-transfer complexes or biradicals, although a charge-transfer complex would be expected to result in a predominance of head-to-head dimers. Dipole effects, on the other hand, would favor formation of head-to-tail dimers, especially in relatively nonpolar solvents ... 

Fig. 5 Absorption and fluorescence emission spectra of the 3-hydroxychromone dye F4N1 in the absence (black) and presence (red) of a local electric field, which promotes the excitation charge transfer leading from the ground state to the N state. In the presence of the local electric field, the energy of the N state is reduced, causing a red shift of the N emission peak and an increase in its intensity relative to the T emission peak. The change in relative intensities of the N and T peaks reflects a shift in the excited state tautomeric equilibrium toward the N state...

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

The reaction is thought to proceed via an excited charge transfer complex to a thermal and prototropic equilibrium mixture of cycloadducts 6 and 7, wliich decompose to the products shown. 

The 02 ion appears to play an important role in a number of photooxidation reactions (see Section VI,C) for example, the photo-oxidation of alkenes over TiOz. However, it seems likely that OJ is not, in many cases, active in the oxidation step but further conversion occurs to give a mononuclear species, not detected directly, which then oxidizes the adsorbed hydrocarbons. Photo-oxidation of lattice oxygen in the M=0 systems (e.g., V2Os supported on PVG) gives rise to an excited charge transfer state such as V4 + -0 . This excited state can react as O- either by addition to a reactant molecule or by an abstraction reaction (see Section V of Ref. /). In the presence of oxygen, 03 is formed which then reacts further with organic molecules. 

Electronic absorption spectra have been recorded for a large number of oxomolybdenum(IV) species but, in addition to d-d transitions, there is the possibility of two-electron excitations, charge transfer bands and splitting of degenerate energy levels due to low symmetry. These factors make it difficult to interpret or even to compare spectra.5... 

Adams and Cherry (78) have investigated the effects of stilbene substitution on the behavior of their excited complexes with fumaronitrile and find that the rate constants for fluorescence and nonradiative decay are insensitive to substitution, but that the rate constant for intersystem crossing is increased by electron-donating substituents (lower stilbene oxidation potential). This trend is attributed to a decrease in the energy gap between the excited complex and locally excited 3t (Fig. 4). The observed energy gap dependence of the exciplex lifetime could also account for the absence of fluorescence (or cycloadduct formation, see Section IV-B) from the excited charge-transfer complexes of t-1 with stronger electron acceptors such as maleic anhydride (76) or tetracyanoethylene (85). 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

In this case, the ethylamine radical is formed by hydrogen abstraction from the solvent, instead of dissociation of the excited (charge-transfer) state of the amide ion pair. 

As a result, cycloaddition between excited aryl groups and simple alkyl-substituted olefins generally leads to the formation of meta cycloadducts, whereas ortho cycloaddition predominates from the singlet state when excited charge transfer is energetically favorable. In fact, direct irradiation of donor-acceptor bichromophores 206 and 207 affords no meta cycloadduct [261],... 

There are thus these two kinds of excited charge transfer states... 

These photoinduced ET can conceivably be accomplished by one of the following mechanisms (1) homolytic cleavage of the C-X bond (2) ET from the excited ArX to its ground state (3) ET from the excited nucleophile to the ArX, generating a radical anion that enters the cycle (4) ET within an excited charge transfer complex and (5) photoejection of an electron from the excited nucleophile. 

Since the excited charge-transfer complexes are photoactivated, weaker donor-acceptor combinations can be used than those in thermal polymerizations, including not only strong D/strong A, but also strong D/weak A and weak D/strong A. 

Electron Transfer from Excited Charge Transfer States. 135... 

Electronic Structures of Exciplexes and Excited Charge-Transfer Complexes. 

Three possible explanations for these observations are (a) a photo-chemical process, (b) a mobile defect, similar to the (CH) soliton and (c) photo-excited charge-transfer. The first of these can be eliminated since photochemistry even in lOH monomer requires u-v irradiation and the radicals produced in irradiated monomer ( ) and related matrix isolated species ( ) have spectra with strong hyperfine structure. 

EXCITED CHARGE-TRANSFER COMPLEXES - Complexes produced by excitation of ground state molecular complexes for which there is conclusive evidence for association in the ground state. 

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