Azobenzene configuration

Photoisomerization of the monolayer to the c/s-azobenzene configuration removes the threaded Fc- i-CD to a spatially separated position which retards the electron transfer rate. Therefore, the assembly functions as a molecular optoelectronic system which records optical information and transduces it into an electronic signal. 

The decrease of the maximum conversion degree concerning the CM-azobenzenic configuration, obtained as a result of UV irradiation, can be explained for the other samples by the decrease of the free volume value, induced by the possibility of the nucleobases to generate H-bonding by themselves. The possibility to form such H-bonds is a known process, especially in the case of thymine [26, 27]. 

Gold NPs with azobenzene terminated alkane thiol chains chemisorbed on the surface and complexed with a-CD gave a photoresponsive suspension in water, in which the azobenzene photosomerization was as efficient as that of the free molecule. Preferential complexation of the allgrl chain in both trans- and ds-azobenzene configurations favoured the water solubility of the alkyl-aromatic moiety and the chemisorption process and reduced the interaction between the azobenzene units on the particle surface with positive effects on the efficiency of the photoisomerization. 

The photoswitchable RGD peptide-SAM supported cell adhesion in the trans-azobenzene configuration, while in the cis form a few cells adhered to the surface. 

Vogtle et al. reported the first example of a photoswitching dendrimer [94] with six azobenzene moieties attached to a derivative of 1,3,5-trisubstituted benzene as the central core. Irradiation of the all (F)-isomer at 313 nm led to a photostationary equilibrium where most of the azobenzene units were switched to the (Z)-configuration. Conversely, irradiation of this species again at a lower energy frequency (436 nm) led to a second photostationary equilibrium where the (F)-form was dominant however, it was not proven as to how many azobenzene units isomerized after irradiation. 

Fig. 8-2.—The planar configuration of cis-azobenzene, drawn to scale using 1.0 A for the van der Waals radius for hydrogen. Steric interactions of hydrogen atoms prevent the assumption of this configuration.

We now report the photochromic reaction of an azobenzene in the nanoporous silica film. Since the photochromic behavior is environmentally sensitive, photochromism of organic substances in solid matrices has been investigated to understand as well as to modify the photochromic behavior.[21] Photochromism of azobenzene and its derivatives due to cis-trans isomerization (Scheme I) has widely been investigated. Photocontrol of chemical and physical functions of various supramolecular systems has vigorously been studied by using photochemical configurational change of azobenzene derivatives.[22,23]... 

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

Heilbroner and co-workers found the UV spectra of A-protonated 1,2-diazocine 8 and M-protonated (Z)-azobenzene were very similar. This and other spectroscopic and chemical evidence indicated that upon protonation (Z)- and ( )-azobenzene retain their configuration and are classical rather than bridged or nonclassical cations (60HCA1890). Haselbach and co-workers studied protonated 8 (and other azo compounds) using electron spectroscopy for chemical analysis (ESCA) and concluded that the cation has classical structure 69 as opposed to a nonclassical structure (70) (72HCA705). 

Fig. 5.21 A POPAM dendrimer with peripheral azobenzene units, on the left in the E-configuration, on the right in the Z-configuration (schematic). The E- and Z-isomers interact differently with eosin as guest...

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