Photoisomerization in Solutions

Femtosecond photoelectron spectroscopy was employed to study the excitation of trons-stilbene above the isomerization reaction barrier [82]. Apart from the contribution, evidence of a second electronic state was found on the basis of two different transients measured across the photoelectron spectrum. Time-dependent density functional theory calculations on So, Si, S2, and Do, tt ether with simulations of the electron energy distribution, supported the experimental findings for selective photoelectron energies of the So, Si. electronic states. The photoelectron spectra of trans-stilbene following the excitation with 266 nm laser pulses consisting of a pronounced three-peak structure were subjected to a substantia] broadening, due to the large number of closely spaced vibrational states involved in the excitation scheme. 

A series of four platinum acetylide complexes that contain 4-ethynylstilbene (4-ES) ligands have been subjected to a detailed photochemical and photophysical investigation [83]. Using absorption, variable temperature photoluminescence, and transient absorption spectroscopy, UV-vis absorption, and NMR spectroscopy, it was shown that these compounds undergo trans-cis photoisomerization from the triplet excited state. The obtained experimental data indicated that in all of the complexes, excitation led to a high yield of a 3n,n excited state that is localized on 

Water-soluble p-sulfonato calix[n]arenes (n = 8, la-lb and n = 6, 2a-2b) was employed as host to control the outcome of the photodimerization and 

The fluorescence of trans-stilbene and four methoxy-substituted stilbene derivatives 

The time-resolved fluorescence behavior of two derivatives of4-(dimethylamino)-4 -cyanostilbene (DCS) bearing a more voluminous (JCS) and less voluminous anilino group (ACS) has been investigated. 

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A second example of the use of ionic chiral auxiliaries for asymmetric synthesis is found in the work of Chong et al. on the cis.trans photoisomerization of certain cyclopropane derivatives [33]. Based on the report by Zimmerman and Flechtner [34] that achiral tmns,trans-2,3-diphenyl-l-benzoylcyclopropane (35a, Scheme 7) undergoes very efficient (0=0.94) photoisomerization in solution to afford the racemic cis,trans isomer 36a, the correspondingp-carboxylic acid 35b was synthesized and treated with a variety of optically pure amines to give salts of general structure 35c (CA=chiral auxiliary). Irradiation of crystals of these salts followed by diazomethane workup yielded methyl ester 36d, which was analyzed by chiral HPLC for enantiomeric excess. The results are summarized in Table 3. 

H. Hamaguchi I would like to comment on the stilbene photoisomerization in solution. We recently found an interesting linear relationship between the dephasing time of the central double-bond stretch vibration of Si franj-stilbene, which was measured by time-resolved Raman spectroscopy, and the rate of isomerization in various solutions. Although the linear relationship has not been established in an extensive range of the isomerization rate, I can point out that the vibrational dephasing time measured by Raman spectroscopy is an important source of information on the solvent-induced vibrational dynamics relevant to the reaction dynamics in solution. 

Angeloni, Chiellini, and their co-workers [116-118] reported the synthesis and characterization of both MCLC and SCLC polymers containing the 4,4 -bisalkoxyazobenzene chromophore. These interesting materials photoisomerized in solution but their photochemistry in an LC mesophase was not reported. 

Torsion about one of the formal double bonds is invariably the most efficient excited singlet state decay process of acyclic polyenes, and also often occurs efficiently in cyclic systems of moderate-to-large ring size- . E.Z-isomerization in the excited singlet state manifold takes place about only one of the double bonds per photon, as was initially demonstrated for 2,4-hexadiene (5) by Saltiel and coworkers and has since been shown to be quite general. Table 1 contains a summary of quantum yields for the direct E,Z-photoisomerization, in solution, of acyclic and cyclic polyenes 1, 42, 43, 5-18 bearing various substituents. For the most part, quantum yields for direct E,Z-photoisomerization of aliphatic dienes are not highly dependent on the structure of the system (i.e. acyclic, cyclic or exocyclic). 

The rate coefficient k(f) is equal to k , the apparent rate coefficient for photoisomerization in solution, for f > f. ... 

Cunningham, F. and Schiff, J., Photoisomerization of delta-carotene stereoisomers in cells of Euglena gracillis mutant W3BUL and in solution, Photochem. Photobiol. Sci. 42, 295, 1985. 

Photoisomerization of pyrazolo[l,2- ]benzotriazoles 28 was studied in argon matrices at 12 K and in solution at 190 K. On irradiation at 365 nm, 28a and its dimethyl derivative 28b undergo ring closure to yield the triazasemibullvalenes 29a and 29b, respectively, which were identified by NMR and IR spectroscopy. The cyclization is reversed on warming or by irradiation at 313 nm (Equation 2) <2002PPS38>. 

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of Cl Disperse Yellow 23 (3.161 X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161 X = N02, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

In solution, an initial photoequilibrium is established between the Z- and -isomers, while the rearrangement products 117 and 118 are formed along with traces of cyclohexadiene (CHD) over much longer irradiation times (equation 46). In solution, the major products are 3-vinylcyclobutene (117) and bicyclo[3.1.0]hex-2-ene (118) Z-l,2,4-hexatriene (119), which is a major product in the gas phase176,211, is formed in relatively low yields. The quantum yields for ,Z-photoisomerization of Z- and -l,3,5-hexatriene in pentane solution (265 nm excitation) are /, r = 0.034 and E—Z = 0.016, respectively188. 

Scheme 12 Photoisomerization of 1,3-diene compounds in the crystalline state and solution, a One-way isomerization to EE isomer in the crystalline state, b Usual isomerization in equilibrium observed in solution...

Table 3 Photoisomerization of (Z,Z)- and ( , )-muconates under irradiation with a high-pressure mercury lamp in the crystalline state or in solution...

Diphenyl-1,3-butadiene. The excited-state behavior of this diene differs significantly from stilbene and is the subject of a review. Unlike tS in which the lowest vertical excited singlet state is the 1 B state and S2 is the 2 Ag state in solution, these two excited states lie very close to each other in all-trans-1,4-diphenyl-1,3-butadiene (DPB). The additional carbon-carbon double bond introduces a new conformational equilibrium involving the s-trans and s-cis rota-mers. Most spectroscopic studies in solution have concluded that the l B state is S. The DPB compound has a low quantum yield for photoisomerization, so the use of DPB in time-resolved spectroscopic studies on photoisomerization, especially those that monitor only fluorescence decay, needs to be considered cautiously and critically. 

I 1 The nitro-nitrito photoisomerization occurs in solution through an Intramolecular mechanism involving the homolytic fission of Co—NOa ifeond and then cage recombination of the two fragments by means of a Co—ONO bond. This complex exhibits CT and CTLM character at 239 and 325 nm and a ligand field band at 458 nm. 

A remarkable series of papers has appeared over the last few years reporting the photoisomerization of substituted benzenes, largely in the condensed phase. One of the earliest observations of photoisomerization in benzenes was that of Van Tamelen and Pappas38 who irradiated an ether solution of 1,2,4-tri-I-butyl benzene (1) with a Hanovia Type L lamp and identified a product as the Dewar analog of tributyl benzene (2). This compound reverted thermally to the starting material... 

The formation of 1,2-dimethylcyclobutene (Formula 385) in the vapor phase irradiation of 2,3-dimethyl-l,3-butadiene (Formula 384) is not quenched by oxygen or nitric oxide (169). Addition of inert vapor (diethyl ether) increased the quantum efficiency in this reaction (169). The inert vapor presumably removes excess vibrational energy from the product cyclobutene thus stabilizing the product (169). Rate studies on the cis- and Jrans-isomers of 1,3-pentadiene in solution indicate that the iraras-isomer is the only source of 3-methylcyclobutene (169). The photoisomerization to 3-methylcyclobutene is faster than photoisomerization of trans- to m-l,3-pentadiene (169). 

The high diastereoselectivity in the addition of i-PrOH, t-BuOH and EtOH (at low concentration) suggests that E Z photoisomerization of (E)- or (Z)-16 does not occur in solution at room temperature or that the trapping of (E)- or (Z)-16 by alcohols proceeds faster than the E Z isomerization. In addition, the results show that proton transfer in the intermediate adduct formed by the disilenes and alcohols occurs much faster than rotation around the Si—Si bond. However, in the reaction with ethanol, an appreciable amount of the anti addition product was formed. Thus, the diastereoselectivity remarkably depended on the concentration of ethanol. 

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