Chyongjin Pac

STRUCTURE-REACTIVITY RELATIONSHIPS IN REDOX-PHOTOSENSITIZED SPLITTING OF PYRIMIDINE DIMERS AND UNUSUAL ENHANCING EFFECT OF MOLECULAR OXYGEN *

Abstract Redox photosensitization using the phenanthrene‐p‐dicyanobenzene pair in acetonitrile has been applied to the respective four isomeric dimers of N.N′‐dimethylthymine (DMT) and N,N′‐dimethyluracil (DMU) as well as to several related cyclobutane compounds. The head‐to‐head (syn) dimers of both DMT and DMU can undergo photosensitized splitting in the following order of efficiency: cis, syn dimer of DMT > cis, syn dimer of DMU > trans, syn dimer of DMT. On the other hand, the head‐to‐tail (anti) dimers are totally unreactive and have higher oxidation potentials than the corresponding syn dimers. It is suggested that the key mechanistic pathway is the formation of π complexes between the dimers and the photo‐generated cation radical of phenanthrene by way of which splitting of the cyclobutane ring catalytically occurs without the formation of the discrete cation radical of the dimers. Structure‐reactivity relationships are interpreted in terms of through‐bond interactions between the n orbitals of N(l) and N(l′) involving the C(6)‐C(6′) bond, as well as in terms of steric repulsion. It was found that aeration of solution greatly enhances the quantum yields of photosensitized splitting; the limiting quantum yield for splitting of the cis, syn dimer of DMT is 100.

Violet emission observed from phthalocyanines

Spectral features of violet emission observed from phthalocyanines in addition to the well-known strong red emission are presented. The origin of the violet emission from the present and reported phthalocyanines is argued based upon the observed and estimated features.

Electron-transfer organic and bioorganic photochemistry.

In recent years, electron transfer has been a central focus of general interest in a variety of fields of chemistry, particularly in organic photochemistry, since photoexcitation of organic molecules is a convenient way to generate the ion radicals which are very reactive to undergo versatile chemical reactions of mechanistic and synthetic significance. A recent Yearly Review covers the literature on photochemical electron-transfer reactions and exciplexes from 1983 to the end of June 1984 (Creed and Caldwell, 1985). Other reviews have appeared on general aspects of electron transfer in photochemistry (Kavarnos and Turro, 1986), on proton transfer of ion-radical pairs following photoinduced electron transfer (Lewis, 1986), on photoaddition reactions of aromatic compounds by way of exciplexes and electron transfer (McCullough, 1987). on structures and chemical reactivities of photogenerated organic cation radicals studied by means of CIDNP techniques (Roth, 1987), on organic photochemistry involving charge-transfer and electron-transfer events (Mattay, 1987), on electron transfer of specific molecules (Balzani, 1987), and on general electron-transfer photochemistry (Fox and Chanon, 1988). Several reviews are also mentioned in the appropriate sections below. This review covers the literature on electrontransfer organic and bioorganic photochemistry mainly from 1985 to April 1988, whereas earlier works are also included as complementary references. However, this review does not describe general and comprehensive aspects of electron-transfer photoreactions of organic molecules but specialized topics on photoinduced electron transfer in homogeneous solution, because the authors wish to avoid duplication of earlier reviews. The topics which have been excluded are photooxidations, polymer and polymerization photochemistry, and photochemistry in organized media and on solid surfaces, even though electron transfer is involved as an essential mechanistic pathway.

Effect of excitation wavelength on photoreduction of metal-free and copper(II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanines with triethanolamine

Abstract Irradiation of 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine (H 2 Pc(OBu) 8 ) and its copper derivative (CuPc(OBu) 8 ) in degassed ethanol solutions containing triethanolamine with red light at 656 nm exciting their Q band did not induce any reaction but the irradiation with UV-violet light at 313–425 nm exciting their Soret band led to their reduction. The reciprocal quantum yields for photoreduction of H 2 Pc(OBu) 8 and CuPc(OBu) 8 on 365 nm excitation increased with increasing reciprocal concentration of triethanolamine. Among the excitation light wavelengths, 360 nm light was the most effective for photoreduction. On the basis of these and other findings, the mechanism of the reaction is discussed and the participation of a higher excited state of the phthalocyanines is suggested.

Photochemical reactions of aromatic compounds XLV: controlling factors for reactivities and mechanisms in photoelectron transfer reactions of some selected diarylcyclobutanes with electron acceptors

Abstract We have attempted to explore mechanistic aspects of the photosensitized ring-cleavage reactions of cis -1,2-diphenylcyclobutane ( 1 ), cis-transoid-cis -cyclobutal[1,2- a :4,3- a′ ] diindene ( 2 ) and r -1, c -2-dimethyl- t -3, t -4-di(4-methoxyphenyl)cyclobutane ( 3 ) by electron acceptors (A) in acetonitrile. The experimental results demonstrate that the ring cleavage of 1 and 2 occurs as a consequence of the rapid geminate recombination of ion-radical pairs occurring at a rate of well over 10 9 s −1 without ionic dissociation. In the case of 3 , however, the photoreactions proceed by way of a chain-reaction mechanism involving the free cation radical of 3 which undergoes ring cleavage at much less than 10 7 s −1 . The rapid ring cleavage of 1 + and 2 + is attributed to significant perturbations of the cyclobutane ring by the population of positive charge on the orbital array of the two π-electron systems and the cyclobutane-ring σ framework because of strong through-bond couplings. It is presumed that the cyclobutane ring of 3 + is much less distorted since the positive charge is mostly localized on the aryl group. The rapid geminate recombination of the A −− − 1 + and A −− − 2 + pairs is discussed in terms of a very efficient transition from the “distorted” and “ring-opened” minima of the A −− −D + surface to the A–D surface. In the case of 3 , this mechanism cannot be expected to operate in the geminate recombination.

Redox-photosensitized chain monomerization of cis-syn-dimer of dimethylthymine; unusual effect of molecular oxygen☆

Abstract cis , syn -Dimer of dimethylthymine was monomerized by means of a chain reaction upon the selective photoexcitation of phenanthrene in aerated acetonitrile in the presence of p -dicyanobenzene. It was found that the dimer forms a CT-complex with O 2 .

PHOTOSENSITIZED REACTIONS BY [Re(bpy)(CO) 3 X]

It was found that [ fac -Re(bpy)(CO) 3 X] (bpy = 2,2′-bipyridine; X = Br or Cl) undergoes the following versatile photoreactions in the presence of triethylamine or triethanolamine; (1) the quantitative one-electron photoreduction of 1-benzyl-3-carbamoylpyridinium cation (an NAD + model) and pyridine-4-carboxaldehyde, (2) the ethylation of the bpy ligand at C-5 with triethylamine, (3) the formation of CO 2 -incorporated Re(I) complexes during the photoreduction of CO 2 in N, N -dimethylformamide, (4) the selective photoevolution of H 2 in tetrahydrofuran, and (5) the efficient photosubstitution of both the Br - and CO ligands with P(OR) 3 to give a bisphosphito complex, [Re(bpy)(CO) 2 {P(OR) 3 } 2 ] + Br - , in quantitative yield.

Photochemical reactions of aromatic compounds. Part 49. An exciplex chain mechanism for the 1,4-dicyanonaphthalene-photosensitised isomerisation of hexamethyl-(Dewar benzene)

Mechanistic studies have been performed on the isomerisation of hexamethyl-(Dewar benzene)1 to hexamethylbenzene 2 photosensitised by 1,4-dicyanonaphthalene (DCN). Analysis of the emission spectra of the DCN–1 and DCN–2 pairs demonstrates that the adiabatic isomerisation of the DCN–1 exciplex to that of DCN–2 occurs with the probabilities 0.53 in cyclohexane and 0.40 in dibutyl ether. The limiting quantum yields for the net isomerisation are 1.5 in cyclohexane and 0.77 in dibutyl ether. Extensive kinetic analyses suggest that the isomerisation proceeds mostly through the adiabatic pathway from the DCN–1 exciplex to DCN–2 followed by sensitisation of the 1 isomerisation by the product (DCN–2) exciplex via the putative DCN–2–1 triplex, while the participation of the non-adiabatic isomerisation from the DCN–1 exciplex is not important.

The photochemical reaction of benzophenone with diphenylamine

The photochemical reaction of benzophenone with diphenylamine affords 4-(N-phenylamino)phenyldiphenylmethanol (I) in a good yield, and an ion-pair intermediate is suggested.

Photoreaction of 2-naphthonitrile and vinyl ethers; a novel photorearrangement of the primary cycloadduct

Irradiation of the dihydrocyclobutanaphthalene adduct (I), obtained by photocycloaddition of 2-naphthonitrile to alkyl vinyl ethers, affords the isomeric dihydro-cyclobutanaphthalene (II) and the dihydrobenzocyclooctene (III); pyrolysis of (II) gives (III), whereas irradiation of (III) gives the dihydrocyclobutanaphthalene (IV) quantitatively.