Photoisomerization In Solution Research Articles

Photochemical Behavior of Some p-Styrylstilbenes and Related Compounds: Spectral Properties and Photoisomerization in Solution and in Solid State

The spectral properties and Z,E-photoisomerizations of three 4-styrylstilbenes, a 4,4'-bis(P-methylstyryl)benzene and a 4,4'-distyrylstilbene were investigated in solution and in the solid state. Some notable features of the absorption and fluorescence spectra due to the structures and the phases (solution or solid) were observed, especially the Stokes shifts. Interesting photochemical behaviors in solution and in the solid state were also found. While the (E,Z)-4-styrylstilbenes undergo a one-way photoisomerization to their E,E-isomers with almost equal rates in solution, their quantum yields in the solid state decrease with increasing substituent size at the terminal aromatic rings. On the other hand, the 4,4'-bis(beta-methylstyryl)benzene undergoes a mutual photoisomerization in solution, but its E,Z-isomer isomerizes in a one-way manner to the E,E-isomer in the solid state. Additionally, the (Z,E,Z)-4,4'-styrylstilbene was found to undergo a one-way photoisomerization to the E,E,E-isomer via the E,E,Z-isomer in solution and the crystalline E,E,Z-isomer to the E,E,E-isomer in the solid state.

Monitoring photoswitching of azobenzene-based self-assembled monolayers on ultrathin platinum films by UV/Vis spectroscopy in the transmission modeElectronic supplementary information (ESI) available: synthesis and characterization of 1 and its photoisomerization in solution. See http://www.rsc.org/suppdata/jm/b3/b314710j/

The reversible trans ↔ cis photoisomerization of azobenzene-based self-assembled monolayers on ultrathin platinum films has been monitored for the first time by UV-Vis transmission spectroscopy with a conventional spectrophotometer.

Monolayers of tetrachloro-thioindigo and thioindigo in the STM: orientational disorder and the absence of photochromism

Monolayers of the organic dye molecules thioindigo and 4,4′,7,7′-tetrachloro-thioindigo on graphite and MoS 2 have been investigated by scanning tunneling microscopy operating in air and in solution. In monolayers of tetrachloro-thioindigo, orientational disorder with two clearly discernable orientations has been observed for the first time. In contrast, the unsubstituted thioindigo forms perfectly ordered monolayers with three molecules per unit cell. Thioindigo molecules are known to show trans→ cis photoisomerization in solution. In well-ordered monolayers however this trans→ cis transition seems to be hindered as the STM experiments show.

Synthesis and photochemical properties of octacinnamoylsubstituted tetraazaporphyrins

New metal-free and metal-containing 2,3,7,8,12,13,17,18-octakis[(4-methoxycinnamoyl)oxyalkylthio]tetraazaporphyrins with different polymethylene chain lengths (3,6,8 or 11 methylene groups) were synthesized by esterification of 2,3,7,8,12,13,17,18-octakis(hydroxyalkylthio)tetraazaporphyrins, obtained from 1,2-dicyano-1,2-bis(hydroxyalkylthio)ethylenes, with 4-methoxycinnamic acid. The structures of the compounds were confirmed by UV–VIS, IR, 1H and 13C NMR spectroscopies. None of the derivatives shows liquid crystalline behaviour; their solid phase is of amorphous or microcrystalline consistency. On UV irradiation the dominating processes of the cinnamoyl groups are E,Z photoisomerization in solution and cross-linking reactions via the (CC) bonds in spin-coated films, whereas photodegradation is the more (in solution) or less (in films) effective reaction of the tetraazaporphyrin chromophore. Further irradiation of the spin-coated films results in insoluble transparent films with a high content of tetraazaporphyrin moieties. All of the processes determined show a dependence upon the length of the polymethylene chains. The efficiency of the fluorescence, the photoisomerization and the cross-linking reactions of the cinnamoyl group are lower with decreasing chain length, whereas the photodegradation of the tetraazaporphyrin is increased. This contrary behaviour suggests the existence of energy-transfer processes from the cinnamoyl chromophore to the tetraazaporphyrin chromophore. In solution, the dependence of the efficiency on the distance with 1/R6 indicates a long-range dipole–dipole mechanism.

Photoisomerization of 3‐benzoylacrylates on a silica gel surface: Effect of reaction environment on selectivity

AbstractOn irradiation on a dry silica gel surface, 2‐(N, N‐dialkylamino)ethyl 3‐benzoylacrylates undergo selective E‐Z isomerization. Competing processes, such as remote‐hydrogen abstraction via a charge‐transfer state, with the photoisomerization in solution are completely suppressed on the surface. The yield of the Z ‐ from the E‐ isomer increases with increase in the coverage of the E‐isomer on the surface, reaching a limiting value. Thermal isomerization of the Z‐ to the E‐ isomer occurs easily on the silica gels surface when the alkyl group on the nitrogen atom is small. Benzoylacrylates without any amino group also undergo selective photoisomerization.

ChemInform Abstract: Rotational Photoisomerization of a Thioamide, N‐5‐Trifluoromethyl‐6‐methoxy‐1‐thionaphthoyl‐N‐methylglycine.

AbstractThe thioamide (I), which exerts aldose reductase inhibitory activity and is useful for the treatment of diabetic complications, undergoes trans‐cis photoisomerization in solution.

Photochemical transformation of the DDT and methoxychlor degradation products, DDE and DMDE, by sunlight

DDE and DMDE, degradation products of the pesticides DDT and methoxychlor, rapidly undergo an unusual photoisomerization in solution when exposed to sunlight. The isomerization involves the exchange of a vinyl chlorine and an ortho aromatic hydrogen. Other photoproducts identified were corresponding benzophenones and 1,1-diaryl-2-chloroethylenes. Quantum yields for the reactions were measured and then used to compute sunlight photolysis half-lives for DMDE and DDE. Although both compounds absorb only the short-wavelength ultraviolet component of sunlight, their photolysis was found to be surprisingly rapid. During summer at latitude 40 degrees N, the photolysis half-lives near the surface of a water body are one hour and one day for dissolved DMDE and DDE, respectively. Photolysis of the DDE photoisomers is about an order of magnitude slower than that of DDE, suggesting that they may accumulate under environmental conditions. The DDE photoisomers photocyclize to form chlorinated dibenzofulvene and dichlorofluorenone. Neither DDE nor its photoisomers photoreact in solution to form PCB's. The environmental significance of these results is discussed, and its is suggested that the persistence of DDE in inland surface waters may be related to its tendency to sorb onto sediments and biota where not light is present.