Photochemical Switching Reaction

Thiophenes continue to play a major role in commercial applications as well as basic research. In addition to its aromatic properties that make it a useful replacement for benzene in small molecule syntheses, thiophene is a key element in superconductors, photochemical switches and polymers. The presence of sulfur-containing components (especially thiophene and benzothiophene) in crude petroleum requires development of new catalysts to promote their removal (hydrodesulfurization, HDS) at refineries. Interspersed with these commercial applications, basic research on thiophene has continued to study its role in electrocyclic reactions, newer routes for its formation and substitution and new derivatives of therapeutic potential. New reports of selenophenes and tellurophenes continue to be modest in number. 

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]. 

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-... 

An interesting photochemical switching reaction utilizing ESI-MS was reported by Arakawa and coworkers [47] (Scheme 5.7). This process involved a cation (e.g., K" ",... 

Schemes. Photochemical switching reaction with Na and l< as an off or on mode of a cation binding host. The screening was performed with substrate concentrations around 0.2 mmol L in acetonitrile as solvent.

Fig. 13.2. Kinetic curve of the chain reaction for switching-off (t = 0) and switch-ing-on the light (the moment is marked by arrow) in the photochemical aftereffect method.

One strategy in limiting the formation of ozone and other photochemical oxidants has been the use (in the past) of low reactivity fuels in internal combustion engines. More recently, alternate fuels (methanol, for instance) have been proposed for regions that suffer from elevated levels of photochemical air pollution. The effect of switching to such a low-reactivity fuel may be seen in Equation E2 for methanol, which has a simple atmospheric reaction mechanism. 

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). 

At low pH values, dimerization must involve the combination of two neutral carbon radicals since the same ( ) / meso ratio is obtained as from the photochemical reaction of the carbonyl compound in methanol [26], a process which also involves neutral radicals. The switch in isomer ratio to that characteristic of alkaline media occurs in the region of pH close to the value of pKj for the neutral radical. Dimerization then occurs in a fast reaction between the radical-anion and the neutral radical. In strongly alkaline solutions where the pH pK the major reactive species formed at the potential of the first reduction wave is the radical-anion. Reaction between two radical-anions is relatively slow due to coulorobic repulsion so that dimerization in strongly alkaline solution still occurs by reaction... 

Figure 6.2 Steering of photochemical reactions by coherent control of ultrafast electron dynamics in molecules by shaped femtosecond laser pulses. Ultrafast excitation of electronic target states in molecules launches distinct nuclear dynamics, which eventually lead to specific outcomes of the photochemical reaction. The ability to switch efficiently between different electronic target channels, optimally achieved by turning only a single control knob on the control field, provides an enhanced flexibility in the triggering of photochemical events, such as fragmentation, excited state vibration, and isomerization.

R. A. Marcus I have a question for Prof. Rice Do most industrial photochemical processes involve chain reactions If so, one suspects that they will be less susceptible to coherent control. Are there some industrial photochemical processes not involving free radical chains I gather that you hold the view, which seems reasonable, that the opportunities for coherent control are greater in devices such as electronic switches and generally in telecommunications ... 

Figure 7. IR spectra in the combination/overtone region showing the photochemical reaction of Cr(CO)6 in scC2H4 at ambient temperature. The first spectrum shown in the Figure was recorded just before the UV lamp was switched on, and subsequent spectra were taken at ca. 10 min intervals. The bands labelled are those of Cr(CO)6 and those labelled A show the growth of the final photoproduct Cr(CO)4(C2H4)2. The steady state concentration of the intermediate species Cr(CO)5(C2H4) is relatively low, hence the overtone/combination bands of this species are not observed in this experiment.

---