Photoisomerization interfaces

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

The photoisomerization reaction gives a valuable tool to investigate the molecular friction at the interface. Depending on the exact location of the reaction path, whether predominantly in one or the other phase, the time scale of the rate of the reaction is severely modified. The photoisomerization reaction of the dyes malachite green and 3,3 -dieth-yloxadicarbocyanine iodide (DODCI) has been reported at several interfaces, the air-... 

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. 

Studies by Nishiyama and Fujihara [149] utilizing azobenzene derivative (27) as isomerizable chromophores have demonstrated the importance of reaction cavity free volume in L-B films. The L-B films of amphiphilic derivative 4-octyl-4 -(3-carboxytrimethyleneoxy)-azobenzene (27) upon irradiation was found to be stable, no geometric isomerization of the azobennzene moiety occurred. This compound forms L-B films with water soluble polyallylamine 28 at an air-water interface. Reversible cis-trans photoisomerization occurs in the film containing 28. The reversible photoisomerization reaction in polyion complexed films is thought to occur because of the increased area per molecule provided in the film. The cross sections of molecule 27 in the pure film and in film containing 28 were estimated to be 0.28 and 0.39 nm2. Such an increased area per molecule... 

In non-viscous organic solvents the fluorescence efficiency of most SNA derivatives is comparable to that of TS as are the yields of trans to ois photoisomerization (33,3A). The double chain derivatives, MSNA and MSN, show substantially higher fluorescence yields (0.2 0.02) in non-viscous solvents with a corresponding decrease in the trans to ois isomerization efficiency (3A). In the sensitized isomerization, the MSN derivatives investigated thus far show only photoisomerization but there is an increase in the triplet lifetime by a factor of 2 however there is no change in the decay ratio of the isomerization precursor and the sum of the benzophenone sensitized isomerization efficiencies of ASA is approximately unity. When water insoluble SNA or MSNA molecules are spread as a film, either pure or in mixtures with insoluble fatty acids, at the air-water interface, it is found that the films show comparable behavior on compression to those of pure fatty acid with an indicated area per molecule of the surfactant stilbenes very close to that of a linear fatty acid (oa. 202 /molecule) (35). This suggests strongly that the TS chromophore in these molecules offers suitable molecular dimensions to pack into a crystalline-like array of linear parafin chains. 

Figure 18.5 Control of mass transfer through an azobenzene-modified film, (a) Structure and photoisomerization of azobenzene (b) structures of ferrocene derivatives used to monitor anodic current at the film-electrode interface and (c) current as a function of light exposure for FDM and FDMDG redox probes.57 (Reprinted with permission from N. Liu et al., Nano Lett. 2004, 4, 551-554. Copyright 2004 American Chemical Society.)...

Electrochemical and photochemical processes are the most convenient inputs and outputs for interfacial supramolecular assemblies in terms of flexibility, speed and ease of detection. This chapter provides the theoretical background for understanding electrochemical and optically driven processes, both within supramolecular assemblies and at the ISA interface. The most important theories of electron and energy transfer, including the Marcus, Forster and Dexter models, are described. Moreover, the distance dependence of electron and energy transfer are considered and proton transfer, as well as photoisomerization, are discussed. 

Cyclic photoisomerization of the electron-acceptor between the trans- and cis-states permitted reversible piezoelectric transduction of the formation of the complexes with trans-A,A -bNA, and trans-3,3 -bN A at the monolayer interface, and their dissociation upon photoisomerization to czs-4,4 -bNA and c/s-3,3 -bNA. 

The extent of the isomerization hindrance depends on the structure of the azobenzene amphiphile. When the azobenzene is located in the middle of the alkyl chain (i.e., 7), the trans to cis photoisomerization in LBK films is severely hindered but not completely suppressed.However, when the azobenzene moiety is located directly at the head group, as in 12, the isomerization in the monolayer at the air/water interface is completely blocked. ... 

These photoresponsive peptides form stable monolayers at the air/water interface,the structure of which depends on the isomerization of the azobenzene. Photoisomerization, therefore, induces changes in the structure, and with that an increase in the areal requirements at constant surface pressure. As confirmed by UV-vis spectroscopy, LBK films of these peptide mono-layers deposited on quartz glass also retain the ability to isomerize. 

The structural changes in the LBK films photoisomerization of the azobenzene moieties are accompanied with changes in the optical properties. This can be proven employing surface plasmon resonance speetroscopy. If a glass/metal interface is irradiated, the light is not reflected for all incident... 

Steinem, C., Janshoff, A., Vollmer, M. S., and Ghadiri, M. R. Reversible Photoisomerization of Self-Organized Cylindrical Peptide Assemblies at Air-Water and Solid Interfaces. Langmuir 15, 3956 (1999). 

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