Primary photoprocess

By changing from the simplest to larger aliphatic and cyclic ketones, structural factors may be introduced which favor alternative unimolecular primary photoprocesses or provide pathways to products not available to the simple model compound. In addition, both the increase in molecular size and irradiation in solution facilitate rapid vibrational relaxation of the electronically excited reactant as well as the primary products to thermally equilibrated species. In this way the course of primary and secondary reactions will also become increasingly structure-selective. In a,a -unsym-metrically substituted ketones, the more substituted bond undergoes a-cleavage preferentially. 

The 7T orbital extends over the entire enone system. As a consequence the C —CO bond is strengthened in the excited states and a-cleavage, the important primary photoprocess in nonconjugated ketones, is not observed. The most important unimolecular reaction types are processes which involve... 

Complete dissociation of the primary product (160) would result in the loss of the formyl fragment and in the stabilization of the remaining oxydiene radical by hydrogen abstraction from any suitable donor. As a competitive primary photoprocess bridging between the C-5 jS-carbon and the... 

Photolysis of chromium alkoxycarbenes with azoarenes produced 1,2- and 1,3-diazetidinones, along with imidates from formal azo metathesis (Eq. 21) [85, 86]. Elegant mechanistic studies [87-89] indicated the primary photoprocess was trans-to-cis isomerization of the azoarene followed by subsequent thermal reaction with the carbene complex. Because of the low yields and mixtures obtained the process is of little synthetic use. 

Different photoreceptor pigments may reasonably be expected to undergo different primary photoprocesses upon light absorption, aside from possessing different spectral characteristics. Specifically, light absorption in flavins fairly easily leads to electronic excitation of the molecule to the triplet state, while this process does not readily occur in carotenoids87). 

Since the last review in this series, a number of reports has been published to clarify the primary photoprocess and to show the application of aminium radical reactions in syntheses. 

The reaction volume (AF°) for Fe +aq FeOH "aq + H" "aq is +4.8cm mol (extrapolated to zero ionic strength A V° decreases significantly with increasing ionic strength, to reach a value of +1.6cm mol at 7=2.0moldm . Laser flash photolysis kinetics indicate the formation of hydroxyl radicals in the primary photoprocess of FeOH " "aq photolysis. ... 

Organo-Transition Metal Compounds, Primary Photoprocesses of (Bock and von Gustorf). 

For molecules containing atoms of high electron affinity the photocurrent at the first threshold may be due to a dissociation into ions. This phenomenon was already demonstrated in the early 1930 s by Terenin and Popov28 for TIHal vapors, which split into T1+ + Hal - as a primary photoprocess. Such was the first instance of the application of mass spectrometry to the study of the photoionization of gases. A similar process has been later shown by Morrison et al.8 for Br2 and I2, the first threshold, corresponding to a pre-ionization, accompanied by the dissociation into Hal+ + Hal-. 

Since the primary photoprocess is absorption of a photon to create a photoexcited molecule, photochemistry and spectroscopy are intimately related. Quantum mechanics has played a vital part in describing the energy states of molecules. 

The transient absorption spectroscopy of the denatured PYP is found to be similar to that of the native protein in the early stages of the excited-state deactivation. This behavior seems to indicate that the intrinsic properties of the chromophore play a determining role in the complex primary photoprocesses of the native PYP. 

When the enone chromophore of the diketone (148) is excited selectively using 2537 A-light, a smooth conversion to the two stereoisomeric cyclopropyl diketones (149) and (150) takes place exclusively.27,28,44 Experiments with the 4a-deuteriomethyl analog of (148) establish a stepwise reaction sequence with the diradicals (151) [- (149)] and (152) [-> (150)] as the most likely intermediates formed in the primary photoprocess. Total quenching with naphthalene indicates the triplet nature of the reaction. 

Previous work in our laboratory (3) and in others (4) has established that the primary photoprocess in a variety of excited carbanions involves electron ejection. This photooxidation will generate a reactive free radical if recapture of the electron is inhibited. Parallel generation of these same carbon radicals by electrochemical oxidation reveals an irreversible anodic wave, consistent with rapid chemical reaction by the oxidized organic species (5). Little chemical characterization of the products has been attempted, however (6). 

Iminium Salts. The occurrence of electron transfer as a primary photoprocess is well established in iminium ions (194). With olefins, for example, photolysis of iminium salts produces a radical-radical cation pair, eq. 64 (197) ... 

A mechanism that accounts for the oxidative addition of halocarbons has been proposed for the two d8-d8 dimers (Figure 4) (23). The mechanism involves the oxidative quenching of the triplet excited state of the metal dimer as the primary photoprocess. This gives a radical anion species that dissociates a halide, thereby producing an organic radical. The dissociated halide adds to the partially oxidized metal dimer to form a mixed valence Ir -Ir -X or Pt 1-Pt -X intermediate. This intermediate reacts further with the remaining organic radical (presumably in a second, thermal electron transfer step) to form the final d2-d2 dihalide dimer. 

Bimolecular quenching of the excited states of metal complexes generally involves electron transfer or energy transfer processes ( 1). Recently, however, Pt2(pop)4 " has been found to undergo a photochemical reaction involving atom abstraction as a primary photoprocess (.26). The reaction involves the catalytic conversion of isopropanol to acetone ... 

While [Pt2(pop)4 ] is a strong reductant, it is not sufficiently reducing to transfer one electron to an alcohol (27). Extraction of a hydrogen in a primary photoprocess would produce an isopropyl radical that could undergo disproportionation to yield acetone (28) ... 

The photochemistry of the isoelectronic series (tj6-benzene)Cr(CO)3, (17s -cyclo-pentadienyl)MN(CO)3, and (i74-cyclobutadienyl)Fe(CO)3 all involve CO photosubstitution as their principal primary photoprocess.2 ) None of these are thought to undergo substitution of the 6-electron donor w-system with high quantum efficiency as previously suspected. The similarity in reactivity shows the value of isoelectronic relationships in these complexes, but no detailed treatment of the electronic structure of d6 (arene)M(CO)3 complexes has appeared. 

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