Photoisomer

Scheme 4 summarizes the photoisomers obtained from 4-methyl-2-phenylthiazole. involving bending of the bonds adjacent to sulfur. 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

From N-oxides of aromatic bases oxaziridines were obtained only at very low temperatures, but oxaziridines were often postulated as intermediates in the photoconversion of such N-oxides (Section 5.08.3.1.2). Isolation of the more stable photoisomers of nitrones also causes some problems due to their thermal and photochemical instability leading to acid amides, e.g. (69TL2281), or, by fragmentation, to carbonyl compounds and products of stabilization of nitrenes, e.g. from (260) (69ZN(B)477). 

Compound A is photochromic that is, it becomes colored on exposure to light. The process is reversible, giving back the starting material in the dark. Suggest a structure for the colored photoisomer. 

Photoketone (118) has served as the key intermediate in a relatively simple transformation of 3-keto-10/ -steroids to 3-keto-lOa-isomers without the assistance of other functional groups suitably situated next to the ring junction—a task that appears difficult to attempt by other methods. Optimal yields of (118) are achieved by catalytic hydrogenation of the unsaturated ketone (175), the photoisomer of 1-dehydrotestosterone acetate (see section III-C). In this way, a 6-step conversion of 1-dehydrotestosterone acetate (174) to IOa-testosterone acetate (127 acetate) is achieved in good yield. ° ... 

The presentation of the rearranging intermediates as mesoionic ground-state species (196) and (199) has proved an adequate tool for the prediction of structural changes in this field. The steric factors which direct the partly selective rearrangement paths of the bicyclohexenone and dienone photoisomers of (174) and their corresponding alkylated homologs have been extensively discussed in a recent review by this author. ... 

Photochemical cyclohex-2-enoneH> bicyclo (3.1.0) hexan-2-one rearrangement irradiation of testosterone acetate, 322 Photochemical rearrangements of cross-conjugated cyclohexadienones and their photoisomers... 

That the photoreactive species is the carbonium ion and not the corresponding alcohol is clearly indicated by the relative concentrations of the two species present. The calculated equilibrium constant for 5% aqueous sulfuric acid implies an alcohol content of 2 7 x 10 %, much too low to account for any detectable photoreaction from this covalent species. In addition, when the tropylium salt is irradiated in the absence of acid, neither 3 nor 4 is detected as a product, but rather ditropyl (5) and its photoisomer (6) are observed. 

Baillet, A., Corbeau, L., Rafidson, R, and Ferrier, D., Separation of isomeric compounds by reversed-phase high-performance liquid chromatography using Ag+ complexation. Application to cis-trans fatty acid methyl esters and retinoic acid photoisomers, /. Chromatogr., 634, 251, 1993. 

Irradiation of acetone or methanol solutions of this compound at — 50°C, however, yields the cis photoisomer, which upon warming (0°C), decomposes to yield photoelimination products. This is taken as evidence that the major path at room temperature involves photoisomerization of the trans to cis isomer and rapid thermolysis of the latter. Similar behavior was observed for azobis(isobutyronitrile), azobis(cyanocyclohexane), and azobis(2-methyl-propyl acetate).(4>... 

Technical chlordane is stable under ultraviolet (UV) light, although some components, such as chlordene, heptachlor, cis-chlordane, and /ram-chlordanc, will form photoisomers under high-intensity UV light in the presence of sensitizers, such as ketones (NRCC 1975 Menzie 1978). Several compounds were measured in alfalfa grown on soils treated with chlordane, including 1,2-... 

Podowski, A.A., B.C. Baneijee, M. Feroz, M.A. Dudek, R.L. Willey, and M.A.Q. Khan. 1979. Photolysis of heptachlor and civ-chlordanc and toxicity of their photoisomers to animals. Arch. Environ. Contam. Toxicol. 8 509-518. 

Kasatani K, Kawasaki M, Sato H (1984) Lifetime shortening of the photoisomer of a cyanine dye by inclusion in a cyclodextrin cavity as revealed by transient absorption spectroscopy. J Phys Chem 88 5451-5453... 

Cyclophanes are naturally suited for MMPI (15b) calculations. The results ofsuch calculations regarding the structures and electronic spectra of the [m] paracyclophanes (n = 5-10) agreed well with the experimental data (169). Attempted X-ray analyses of [2.4]- and [2.5](9,10)-anthracenophanes (46) encountered serious disorder in the ahphatic bridges. MMPI calculations of all possible conformers of these molecules revealed four and six energy minima for 46a and 46b, respectively. Comparison of the calculated C10 C10 distances and bridge conformations with X-ray information unambiguously identified two conformations each for 46a and 46b as the final solutions. These and the calculated structures of photoisomer 47 were highly useful in the interpretation of fluorescence spectra and photoisomerization processes of 46 (170). 

Heptachlor can be converted to its photoisomer, photoheptachlor, in the presence of sunlight or ultraviolet light. This photolysis can take place on plant leaves. Despite the use of a small number of... 

Georgacakis, E. and Khan, M.A.Q. Toxicity of the photoisomers of cyclodiene insecticides to freshwater animals. Nature... 

Rordorf, B.F. Prediction of vapor pressures, boiling points and enthalpies of fusion for twenty-nine halogenated dibenzo-p-dioxins and fifty-five dibenzofurans by a vapor pressure correlation method. Chemosphere, 18(l-6) 783-788,1989. Rosen, J.D. and Carey, W.F. Preparation of the photoisomers of aldrin and dieldrin. 7 Agric. Food Chem., 16(3) 536-537,1968. Rosen, J.D. and Strusz, R.F. Photolysis of 3-(p-bromophenyl)-l-methoxy-l-methylurea, 7 Agric. Food Chem., 16(4) 568-573, 1968. 

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