Molecular photochemical switch

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]. 

S. L. Gilat, S. H. Kawai, and J.-M. Lehn, Light-triggered molecular devices photochemical switching of optical and electrochemical properties in molecular wire-type diarylethene species, Chem. Eur. J1, 275-284 (1995). 

The fabrication of LB FETs and photochemical switching devices is encouraging to researchers working in the field of molecular electronics. The active parts of devices really consist of LB films. The latter could be considered a molecular device. 

Aromatic modified oligonucleotides have been used in photochemical reactions, in one case to act as a photochemical switch, in other cases for cleavage reactions. Azobenzene derivatives in ODNs have been used as a molecular switch by taking advantage of the cistrans isomerisation by UV light. The azobenzene moiety has now been introduced into ODNs on an enantioselective-... 

Molecular hyperpolarizabilities of the two isomers of different photochromes are presented in Tables 10,1 and 10.2. As Disperse Red 1 (DRl) is probably the most studied photochrome due to its very interesting NLO properties in polymers, its nonlinear coefficients have also been tabulated. The spiropy-ran/photomerocyanine group has also been investigated in detail in polymers and in solutions as well, and the NLO properties have been described already.More recently, Atassi et al. have shown that it was possible to observe NLO response with a furylfulgide system," " " and Lehn et al. have shown that some diarylethene compounds can photochemically switch between a low and a high level of NLO response. The first two systems are... 

Fig. 5. A Schematic representation of a locking molecular memory. B A locking molecular memory based on photochemical switching and electrochemical locking. C Cyclic voltammograms of the electrochemically inactive open -state 8 and the electrochemically active closed -state 9, which can be reversibly oxidized to the quinoid locked -state 10. Recorded in THF with Bu4NC104 (0.1 M) and a potential scan rate of 100 mV s-1. D Absorbance spectra of the three states of the lockable molecular memory system. C and D are adapted from [25] with permission...

The great demand for miniaturization of components in electrotechnical, medicinal or material applications has led to the development of a highly multidisciplinary scientific and technological field called nanotechnology to produce devices with critical dimensions within the range 1 100 nm. The ultimate solution to miniaturization is logically a functional molecular machine, an assembly of components capable of performing mechanical motions (rotation or linear translation) upon external stimulation, such as photoactivation.1103,1104,1239-1244 This motion should be controllable, efficient and occur periodically within an appropriate time-scale therefore, it involves photochromic behaviour discussed in the Special Topic 6.15. Such devices can also be called photochemical switches (Special Topics 6.18 and 6.15). Here we show two examples of molecular machines a molecular rotary motor and a molecular shuttle. 

A stereospecific photochemical switching process provided a useful basis for chiroptical molecular switches and molecular memory elements (07CC1745). 

Seefeldt B, Kasper R, Beining M et al (2010) Spiropyrans as molecular optical switches. Photochem Photobiol Sci 9 213-220... 

Figure 14.58. Molecular structure of amphiphiles for multiple photochemical switching devices. (Reproduced by permission of the American Institute of Physics from ref. 368.)...

Another synthetic strategy is based on self-assembly driven by molecular recognition between complementary TT-donors and 7T-acceptors. Examples include the synthesis of catenanes and rotaxanes that can act as controUable molecular shuttles (6,236). The TT-donors in the shuttles are located in the dumb-beU shaped component of the rotaxane and the 7T-acceptors in the macrocycHc component, or vice versa. The shuttles may be switched by chemical, electrochemical, or photochemical means. 

Let us consider molecular switches based on intramolecular electronic transition. Generally, transfer of energy or an electron within a molecule proceeds in femtoseconds. The aim is to produce molecular electronic devices that respond equally rapidly. Molecular switches that employ optically controlled, reversible electron-transfer reactions sometimes bring both speed and photostability advantages over molecular switches which are usually based on photochemical changes in their molecular structure. Important examples are the molecnlar switches depicted in Scheme 8.3 (Debreczeny et al. 1996). 

Pina, F. et al.. Thermal and photochemical properties of 4, 7-dihydroxyflavylium in water-ionic liquid biphasic systems a write-read-erase molecular switch, Angew. Chem. Int. Ed., 43,1525,2004. 

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