Photochemical pressure effect

Koda S, Sugimoto K. Pressure effect on the absorption and photodissociation of 02 near the dissociation threshold. J Photochem Photobiol C Photochem Rev 2003 4 215-26. 

Grosch B, Orlebar CN, Herdtweck E, Kaneda M, Wada T, Inoue Y, Bach T (2004) Enantioselective [4+2]-cycloaddition reaction of a photochemically generated o-quinodimethane Mechanistic details, association studies, and pressure effects. Chem Eur J 10 2179-2189... 

Reviews have appeared on the luminescence of inorganic solids, photophysics of metal complexes, pressure effects on photochemical reactions, photoredox reactions of mixed-valence compounds, photochemical electron treuisfer reactions, ... 

The two most common parameters measured in photochemistry are O, the quantum yield for a specific process, and t, the lifetime of the ES. For a photochemical reaction, the quantum yield is operationally defined as the moles of product formed (or starting species reacted) per einstein of light absorbed by the system at a particular wavelength of irradiation (Airr). In this context, the pressure effect on the quantum yield can be evaluated in... 

In this chapter, the focus will be on the application of high pressure techniques in the study of the photochemical behavior of transition metal complexes (coordination, organometallic and bio-inorganic) in solution. We will present a systematic treatment of pressure effects on the nature of excited states (ES) and on the photophysical and photochemical processes that lead to ligand substitution, electron or energy transfer and thermal reactions of reactive intermediates generated by ES reactions. Selected examples will be presented in detail to illustrate how pressure effects can provide valuable mechanistic insight when combined with other quantitative studies. 

Reversible electronic relaxation has been found in smaller molecules. In methylene (for which collision-induced intersystem crossing has been extensively studied for many years ) the reversible character of singlet-triplet transitions has been evidenced in a study of pressure effect on the CH2 radical prepared either in the singlet state (by photochemical decomposition of diazirine) or in the triplet state (by photofragmentation of... 

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

Mechanistic information from the effect of pressure on thermal and photochemical substitution reactions. R. van Eldik, Comments Inorg. Chem., 1986, 5,135 (33). 

Air or water cooled mercury discharge lamps find many uses, one of the more obvious of which is the study of photochemical reactions. These lamps are usually made of vitreous silica because of its low thermal expansion, high melting point and its transparency to ultraviolet radiation. Their operating pressure has a profound effect on the spectral distribution of the radiation produced and therefore it is important to consider the requirements in the design of such lamps. 

Table 14.1 Energy dependence of the secondary isotope effect ku/kc at zero pressure. Energies inkJ/mol (After Rabinovitch, B. S., Setser, D. W,Adv. Photochem. 3,1 (1964))...

The mby fluorescence method allows us to perform pressure measurements in a short time scale (1-10 s), providing a real-time access for pressure control comparing to the time scale of many solid-state chemical processes. As a matter of fact, real-time pressure measurements are necessary when studying kinetic processes [117], but it is also important to minimize the laser power used for measuring the mby fluorescence in order to avoid undesired photochemical effects on the sample, whenever these are possible. In the case of IR absorption studies, which are commonly used for kinetic purposes, the advantage of using the mby fluorescence method, once photochemical effects are prevented, with respect to the employment of vibrational gauges is that no additional absorption bands are introduced in the IR spectmm. 

ETEROAROMATics FURAN AND THIOPHENE. The chemical transformation of thiophene at high pressure has not been studied in detail. However, an infrared [441,445] study has placed the onset of the reaction at 16 GPa when the sample becomes yellow-orange and the C—H stretching modes involving sp carbon atoms are observed. This reaction threshold is lower than in benzene, as expected for the lower stability of thiophene. The infrared spectrum of the recovered sample differs from that of polythiophene, and the spectral characteristics indicate that it is probably amorphous. Also, the thiophene reaction is extremely sensitive to photochemical effects as reported by Shimizu and Matsunami [446]. Thiophene was observed to transform into a dark red material above 8 GPa when irradiated with 50 mW of the 514.5-nm Ar+ laser line. The reaction was not observed without irradiation. This material was hypothesized to be polythiophene because the same coloration is reported for polymeric films prepared by electrochemical methods, but no further characterization was carried out. 

The photochemical step could also be accomplished by adding Mo(CO)e, EtjN and EtgO to a photochemical immersion well, and irradiating under an inert atmosphere with a Hanovia medium pressure 450W mercury vapor lamp for 20-30 min. Commercially available immersion wells generally require > 800 mL of solvent in order to work effectively (so that the solvent is level with the lamp), and the submitters have found that the more concentrated solution reported here (ca. 450 mL) is more effective. 

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