Monolayer photochemical reactions

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Monolayers discussed in Section 4.2 also introduce order preorganizing molecules. An analogous photochemical reaction making use of this phenomenon was carried out in diacetylene monolayers on gold [47b]. At present most one-pot reactions are of no practical importance. However, their occurence in prostaglandin synthesis [48] indicates that they will find their applications leading to more environmentally clean technologies. 

Organic Photochemical Reactions in Monolayers and Monolayer Systems... 

Organic photochemical reactions in monolayer organi-zates are strongly influenced by the restricted molecular mobility in these systems. Reactions at the air-water interface where molecular relaxation is possible, can be followed by measuring the enhanced light reflection in the spectral range of the absorption band of the involved species. In monolayer systems, photoinduced electron transfer processes have been studied by fluorescence techniques. 

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of rru/i -azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979 Quina et al, 1976 Quina and Whitten, 1977). This contrasts with what is found when the fra/jj-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octa-ethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979 Janzen and Bolton, 1979). 

Quina, F H., and Wliitten, D. G. Photochemical Reactions in Organized Monolayer Assemblies. 4. Photodimerization, Photoisomerization, and Excimer Formation with Surfactant Olefins and Dienes in Monolayer Assemblies, Crystals, and Micelles. J. Am. Chem. Soc. 99, 877 (1977). 

Photochemical reactions in monolayers present the novel characteristic that the quantum efficiency, 4>, may be varied over wide ranges according to the orientation taken up by the absorbing links at any par-... 

Organized and constrained media may provide cavities and surfaces, sometimes called microreactors or nanoreactors,171 that can control the selectivity of photochemical reactions of reactants. There are many types of microreactors, for example, molecular aggregates of micelles or monolayers, macrocyclic host cavities of crown ethers or cyclodextrins and microporous solid cavities and/or surfaces of zeolites, silica or... 

The volume contains 16 chapters dealing with calculations on organogold compounds, physical and spectroscopic properties (NMR, ESR, PES, Mossbauer spectra), thermochemical and analytical properties, the synthesis and uses of the title compounds and their reactions such as rearrangements, pyrolysis and photochemical reactions. The medicinal use of organogold compounds and the increased use of gold-thiol monolayers are also summarized. 

Alternative approaches that have been explored for control of photochemical reactions include irradiation in liquid crystals [7], micelles [8], inclusion complexes [9], and zeolites [10]. In addition, photochemistry of monolayers [11] and on surfaces [12] (e.g., alumina, silica, clays, and semiconductors) has received considerable attention. Each of these methods has potential for regiocontrol and/or stereocontrol of certain photochemical processes. However, reactions between different groups (e.g., intermolecular photocycloadditions) are difficult to modulate with most of these approaches. 

Another interesting photochemical reaction that occurs with the monolayers is dimerization. This is exempUfled by the photochemical behaviour of the SAM of 7-(10-thiodecoxy)coumarin (52) on polycrystaUine gold. Irradiation at 350 nm results in the (2 -f 2)-cycloaddition of the coumarin moieties. The photodimerization is a reversible process by irradiating at 254 nm. Better regioselectivity in the cycloaddition is obtained when the solid monolayer is irradiated rather than when it is in contact with benzene. The dimer formed is the yn-head-to-head dimer identified as 53 . Self-assembled monolayers of cis- and frani-4-cyano-4 -(10-thiodecoxy)stilbene (54) are also photochemically reactive. Irradiation of a thin film in benzene solution using A, > 350 nm results in the formation of a photostationary state with 80% of the cis-isomer present. Irradiation in the solid shows that cis.trans isomerism occurs but that trans.cis-isomerism fails. Prolonged irradiation brings about (2 - - 2)-cycloaddition of the stilbene units to afford cyclobutane adducts. Such dimerization is a well established process . The influence of irradiation at 254 nm or 350 nm of self-assembled monolayers of 10-thiodecyl 2-anthryl ether on polycrystaUine... 

Other applications of photochemistry include the development of sensitive fluorescent chemosensors for analysis of dilute solutions of inorganic cations and anions and the study of the diffusion of individual molecules in solution at room temperature. Fluorescent compoimds have been used as replacements for radioisotopes in the analysis of biological compoimds and the study of biologically active compounds and living systems. Photochemical reactions also offer alternative probes for the characterization of the microenvironments in diverse solid and liquid media, including crystals, zeolites, alumina, silica and clay surfaces, semiconductor surfaces, liquid crystals and host-guest inclusion complexes, polymer films, monolayers and supported multilayers of surfactant molecules, mi-celles, and dendrimers. ... 

An impressive quantity of articles dealing with other applications of porphyrins and phthalocyanines can be found in the literature. For example, the utilization as optical antennas for plasmonic enhancement of photochemical reactions, still a challenge for scientists. In theory, the concept is very simple but its practical realization have demonstrated to be tricky showing that there are many still unknown aspects to be unveiled. Dceda et al. [245] compared the photocurrent generation efficiency with and without optical antennas on analogous porphyrin-based molecular monolayers, where they found out that the overall reaction efficiency is strongly dependent on the electrochemical properties of the optical antennas. 

The polyion complex LB technique is the most precise fabrication method of layer-by-layer polymer assemblies because the lateral molecular ordering and layer structure (X-, Y-, and Z-type deposition and heterogeneous deposition) of the polymer films can be controlled. The molecular assembly in polyion complex monolayers can be switched by pH change [105] or photochemical reaction [106]. Film uniformity of the polyion complex monolayer was improved by in situ ther-... 

Kell, A. J., Stringle, D. L. B., and Workentin, M. S., Norrish type II photochemical reaction of an aryl ketone on a monolayer-protected gold nanocluster development of a probe of conformational mobihty, Org. Lett., 2, 3381, 2000. 

Photochemical Generation of an Interfacial Shock Wave. Both the an-thocyanidine and the thioindigo monolayers showed a decrease in surface pressure at constant area during the photoisomerization reaction. A different behavior is observed with mixed monolayers of the surface active spiropyran SP and octadecanol (OD), molar ratio SP 0D = 1 5, on illumination with UV radiation. The isomerization of the spiropyran to the merocyanine MC causes an increase in surface pressure at constant area (5, 14). This is shown in Figure 4, where the sudden rise in surface pressure it upon repeated 0.5 s exposures (as indicated by the arrows) can be seen to occur in a wide surface pressure range (15). The kinetics of the relaxation process following the surface pressure increase depends on the surface pressure. 

Photochemical processes in monolayers at the air-water interface can be controlled externally by variation of the various parameters like matrix composition, subphase composition, temperature and surface pressure. When the product of the reactions has a different area per molecule, the surface pressure may change at constant monolayer area. An interfacial shock wave has been generated in this way. This technique permits the investigation of the kinetics of reorganization processes and the transmission of mechanical signals in monolayers. 

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