Photoisomerization Isomerization

Park N S and Waldeck D FI 1989 Implications for multidimensional effects on isomerization dynamics photoisomerization study of 4,4 -dimethylstilbene in / -alkane solvents J. Chem. Phys. 91 943-52... 

A kinetic scheme and a potential energy curve picture ia the ground state and the first excited state have been developed to explain photochemical trans—cis isomerization (80). Further iavestigations have concluded that the activation energy of photoisomerization amounts to about 20 kj / mol (4.8 kcal/mol) or less, and the potential barrier of the reaction back to the most stable trans-isomer is about 50—60 kJ/mol (3). 

Photochromism Based on Geometric Isomerism. The simplest examples of a photochromic reaction involving reversible cis-trans isomerization is the photoisomerization of azobenzene [103-33-3] C22H2QN2 (16). 

Photoisomerization of perfluoro(4,5-diisopropylpyridazine) is postulated to proceed through Dewar diazabenzenes (25) and (26) to perfluoro(2,5-diisopropylpyrazine) (27), which is in equilibrium with the isomeric perfluoro(2,6-diisopropylpyrazine) (28) after prolonged irradiation in the liquid phase (Scheme 9) (75JCS(P1)1130). Benzo-fused pyridazines do not isomerize readily under photochemical conditions. An exception is perfluorocinnoline which rearranges to perfluoroquinazoline. 

The systems discussed here are aromatic systems which undergo a variety of isomerizations on irradiation. Irradiation of imidazoles led to a scrambling of substituents, whereas such scrambling has not been observed in the pyrazoles which undergo photoisomerization to imidazoles. 

The first, and to this writing still only case of a ketone a-cleavage-recombi-nation sequence in the steroid field was reported by Butenandt, who found that 17-ketones epimerize at C-13. Ultraviolet irradiation of either stereoisomer produces an equilibrium mixture in which the thermodynamically more favored 13a-compound cf. (15)] with cw-fusion of rings C and D predominates at room temperature. As ultraviolet absorption energies and intensities of the two isomeric ketones are practically identical, the equilibrium composition depends largely on the rate of the competing recombination process from (14). For further examples of the photoisomerization at C-13 of 17-ketosteroids, see ref. 8, 12, 15 and 43. 

The intervention of mesoionic intermediates is suggested by the facile transformation of steroidal dienones, and by a number of acid-catalyzed nonphotolytic reactions which either parallel the photoisomerizations or correlate photoproducts from reactions in protic and aprotic solvents. The isomerization (175) -> (176) -l- (177) has also beeen achieved in the dark by acetic and formic acid catalysis and clearly involves the conjugate acid of the proposed mesoionic intermediate (199) in the dark reaction. Further,... 

The irradiation of 2- and 3-cyanothiophene gave interesting results in agreement with the scheme described above (Scheme 19). The photoisomerization reaction involved only the excited singlet state and Dewar thiophenes were isolated when the reactions were carried out at -68°C and shown to be intermediates in the isomerization reactions [79JCS(CC)881 79JCS(CC)966]. 

The first fully unsaturated 1,4-oxazepines 3 were prepared by the photoisomerization of 3-oxa-6-azatricyclo[3.2.0.02,4]hept-6-enes l.29,30 The reaction proceeds via the bicyclic intermediates 2 which undergo valence isomerization. 

The photochemical extrusion of S02 from a-phenylsulfonyl-substituted enone systems, to give the analogous /J-phenylenones in modest yield, has also been reported60. Where Z, -photoisomerization is possible, however, for example in compounds such as 28 or 29, photoequilibration of the two isomeric sulfones is the dominant process observed61,62. 

The most reasonable mechanism for this transformation, in accord with that suggested by van Tamelen et al. for the dilute acid photolysis, is initial photoisomerization to 2 followed in this case by thermal conversion of the Dewar tropylium ion to 7. The isomerization of 2 to 7 has been reported to be very rapid at temperatures below —60°, and it has been shown, in addition that, in nucleophilic solvents, capture of 2 competes very efficiently with isomerization (Lustgarten et al., 1967). 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

It has been found that cyclohexa-2,4- and -2,5-dienones undergo a light-induced valence isomerization reaction in strong acid analogous to the alkylbenzenes, to yield 2-hydroxy-bicyclo[3,l,0]hexenyl cations. The hydroxybenzenium ion (34), for example, underwent a clean photoisomerization to 35 at temperatures below — 60° (Paxrington and Childs, 1970). Cation 35 was also produced upon similar irradiation of 36. 

Category 4. Photoisomerization. The most common reaction in this category is photochemical cis-trans isomerization." For example, cw-stilbene can be... 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

As discussed in Section II.A, Eisenthal and coworkers have studied the related problem of isomerization at liquid-solid interfaces. They used time-resolved second harmonic generation to investigate the barrierless photoisomerization of malachite green at the silica-aqueous interface using femtosecond time-resolved second harmonic generation [26]. They found that the photoisomerization reaction proceeded but was an order of magnitude slower at the water-silica interface than in bulk solution. 

Recently, a photoisomerization reaction of azoferrocene was found to proceed in polar solvents such as benzonitrile and DMSO through both a 7t it transition of the azo-group with a UV light (365 nm) and the MLCT transition with a green light (546 nm) (Fig. 6) (Scheme 1) (153). The quantum yields of the photo-isomerization reaction at 365 nm and 546 nm were estimated to be 0.002 and 0.03, respectively. The transformation into the cis form causes the higher field shift of Cp protons in the 1H-NMR spectrum and an appearance of u(N = N) at 1552 cm-1. The cis form is greatly stabilized in polar media, and dilution of the polar solution of cis-25 with less polar solvents resulted in a prompt recovery of the trans form. 

Both Z and E isomeric enol lactones undergo photoisomerization to yield mixtures of isomers (5,14,87) in which the thermodynamically more stable one prevails. It is the Z form in hydrastine series (5) and the E isomer in the more hindered narcotine series (87). Relative stabilities of isomeric enol lactones (98 versus 99 and 101 versus 102) were determined by comparing their rates of methanolysis. Keto esters of type 126 were formed (87). It turned out that both ( )-N-methylhydrastine enol lactone (99) and (Z)-narceine enol lactone (101) solvolyzed faster than their geometric partners. 

The less stable isomers were obtained from the more stable ones by photoisomerization. (Z)-Fumaridine (113) when exposed to sunlight was isomerized to a separable 3 2 mixture of geometric isomers (5). The Z form of narceine imide (116) is unstable and in daylight equilibrates easily to a mixture of Z and E forms (123). 

A comparison of the rate constant for photoisomerization of the unsubstituted 3-phenyl derivative (kT = 3 x 1010 sec-1) to that of the 3-(p-methoxy phenyl) derivative (kr = 1.5 x 1010 sec-1) indicates that the presence of the p-methoxy groups imparts no special stability to the intermediate responsible for isomerization even though cleavage of a cyclopropane bond is predominant. Clearly these results are inconsistent with an intermediate possessing electron-poor or electron-rich species such as would be obtained from heterolytic cleavage of the cyclopropane. On the other hand, the results are consistent with a biradical species as intermediate. Further evidence consistent with this conclusion was obtained in a study of trans-3-p-cyanophenyl-/ra w-2-phenyl-1 -benzoylcyclopropane,<82)... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

Photo-isomerism is also found in this group of compounds. Cis-Pt(NH3)2(H20)l+ photo-isomerizes to trans with a quantum yield of about 0.1 at 363 nm (40). Photoisomerism of Pt (glycine) 2 from cis to trans, but not its reverse is also reported. 

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