Photoinduced electron transfer hydrogen bonds

Bond distortion, spin crossover Sensitization, energy transfer Photoinduced electron transfer Hydrogen atom abstraction Photooxidation, photoreduction Photoaddition, photosubstitution Photodissociation, photo-cleavage Photoisomerizations Photodimerization, photo-pol3nnerization... 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

Co(CN)6]3 or [Fe(CN)6]3. 89 The double biimidazole-NCRu hydrogen bond is evident in both the solid state and in CDC13 solution, where binding constants of over 103 - 105 were measured. Interaction with hexacyanoferrate(III) brings about a photoinduced electron transfer process which entirely quenches the Ru(II) luminescence. 

In summary, the triplet (do po) excited states of the d -d metal dimers [Ir(p-pz)(C0D)]2 and Pt2(pop)4 " undergo a variety of photochemical reactions. Electron transfer to one-electron quenchers such as pyridinium cations or halocarbons readily occurs with acceptors that have reduction potentials as negative as -2.0 V. With the latter reagents, net two-electron, photoinduced electron transfer yields d -d oxidative addition products. Additionally, the triplet (da pa) excited state of Pt2(pop)4 apparently is able to react by extracting a hydrogen atom from a C-H bond of an organic substrate. 

The photoinduced electron transfer (PET) is especially important in the case of large or giant molecules (supermolecules), ie systems made up of molecular components in the same way as molecules are made up of atoms [11-19], As the systems are made up of a number of discrete components held together by different but not always exactly specified forces (covalent bonds, electrostatic interactions, hydrogen bonds, or other intermolecular interactions), the photoinduced electron transfer or energy transfer in these systems may be formally treated as intermolecular [20],... 

Similar to the deprotonation of enol radical cations, silyl enol ether radical cations can undergo loss of trialkylsilyl cations (most likely not as ionic silicenium ions [190]). Based on photoinduced electron transfer (PET), Gass-man devised a strategy for the selective deprotection of trimethylsilyl enol ethers in the presence of trimethylsilyl ethers [191]. Using 1-cyanonapthalene (1-CN) ( = 1.84 V) in acetonitrile/methanol or acetonitrile/water trimethylsilyl enol ether 93 ( j = 1.29 V) readily afforded cyclohexanone 64 in 60%. Mechanistically it was proposed that the silyl enol ether radical cation 93 undergoes O-Si bond cleavage, most likely induced by added methanol [192-194], and that radical 66 abstracts a hydrogen from methanol. Alternatively, back electron transfer from 1-CN - to 66 would yield the enolate of cyclohexanone which should be readily protonated by the solvent. 

A. Harriman, Y. Kubo, J.L. Sessler, Molecular Recognition via Base-Pairing - Photoinduced Electron-Transfer in Hydrogen-Bonded Zinc Porphyrin Benzoquinone Conjugates , J. Am. Chem. Soc., 114, 388 (1992)... 

Gust, Moore, Moore and coworkers covalent cartenoid-porphyrin-quinone molecular triads 55-60 contain a cyclized hydrogen bond within the quinone acceptor framework [143], The naphthaquinone moiety of 55 is fused to a norbomene system whose bridgehead position bears a carboxylic acid, which can hydrogen bond to an adjacent quinone. Photoinduced electron transfer from the porphyrin to the quinone leads to a marked p/fg increase of the latter, resulting in a fast proton transfer ( pt 10 s ) to form the semiquinone. Back electron transfer from the semiquinone is attenuated as a consequence of the proton-stabilized charge-separated species. This leads to a two-fold increase in the quantum yield of the charge-separated state of 55, as compared to those of the reference triads 56 and 57 (see Volume III, Part 2, Chapter 2). 

Figure 23. A molecular-level extension. Thread 31H " can undergo a hydrogen-bonding interaction with macrocycle 32 + and an acceptor-donor interaction with macrocycle 2 (MeCN, 298 K). Once the two connections have been established, photoinduced electron transfer occurs from the excited state of the Ru(bpy)3 + moiety of macrocycle 32 to the bipyridinium unit hosted in...

Photoinduced electron transfer reactions in hydrogen bonded complexes between electron donor and electron acceptor is an exciting field of investigation of the importance of hydrogen bonds in affecting and, in some cases, in facilitating the electron transfer induced by photon absorption166,167. 

Deactivation of 2-naphthylamine singlet state by pyridines in enhanced by dipole moment and the ability to form hydrogen bonds. Picosecond laser spectroscopy shows charge transfer from the excited amine. The fluorescence of 2-iV,A -dimethylaminopyridine induced by p-nitroaniline is also caused by exciplex formation. The latter enhances triplet population of p-nitroaniline. The quenching of the fluoresence of carbazole and some derivatives by trichloroacetic acid and related compounds in fluid solutions has been studied by Johnson.A charge-transfer interaction is involved and the basicity of carbazole and derivatives determined. Charge transfer is also involved by quenching of carbazole by halocarbons. The A -isopropylcarbazole-dimethylterephthalate exciplex has been observed in PMMA films.Photoinduced electron-transfer in the p-phenylenediamine-paraquat complex yields the paraquat cation. ... 

Photoinduced electron Transfer in hydrogen-bonded zinc porhyrin-benzoquinoe conjugates, J. Am. Chem. Soc., 114 388. 

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