Photochemical efficiency

A rough, but useful, comparison between typical sonochemical and photochemical efficiencies is shown in Table I. As shown, homogeneous sonochemistry is typically more efficient than photochemistry, and heterogeneous sonochemistry is several orders of magnitude better. Unlike photochemistry, whose energy inefficiency is inherent in the production of photons, ultrasound can be produced with nearly perfect efficiency from electric power. Still, a primary limitation of sonochemistry remains its... 

Simultaneous UV and MW irradiation of the sample Possibility of performing photochemistry at high temperature Good photochemical efficiency — the EDL is inside the sample Simplicity of the experiment and the low cost of the EDL Use of a commercially available microwave oven Wireless EDL operation... 

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

I. Akkerman, M. Janssen, J. Rocha, R. Wijffels (2002) Photobiological hydrogen production production photochemical efficiency and bioreactor design. Int. J. Hydr. Energy., 27 1195-1208... 

The effect of zearalenone on crop development may be connected to its influence on the status and functioning of the photosynthetic apparatus (Koscielniak et al. 2008). The after-effects of zearalenone on the growth of soybean and wheat plants, net photosynthesis and transpiration rates, stomatal conductance, photochemical efficiency of photosystem 2 and on final seeds yield were determined. Modifications in leaf area were more pronounced in soybean than in wheat, and this tendency increases in successive developmental phases. The net photosynthesis was stimulated during the juvenile phase and during that of the final one by about 13.6% (average) in soybean plants. Stimulation of transpiration was also observed after... 

Quantum yield A measure of a photochemical efficiency of a photochemical reaction, expressed as the ratio of the number of chemical events per unit time to the number of photons absorbed per unit time it is a unitless quantity. 

Functional dyes of many types are important photochemical sensitizers for chemical reactions involving oxidation, polymerization, (polymer) degradation. isomerization, and photodynamic therapy. Often, dye structures from several classes or materials can fulfill a similar technological need, particularly for laboratory or small-scale reactions where production efficiency may be of secondary importance. Commercial photochemical technology, however, is more selective and requires photochemical efficiency, ease of product separation, and lack of unwanted side reactions to an extent similar to that required by imaging processes. In addition, reusability of the spectral sensitizer is also preferred in commercial photochemical reactions. 

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

Utilizing a commercially available microreactor, fabricated from FOR-TURAN glass, Fukuyama et al. (2004) evaluated a series of [2 + 2] cycloadditions as a means of reducing the reaction times conventionally associated with the synthetic transformation (Table 27). Using a high-pressure mercury lamp (300 W), the reaction of cyclohex-2-eneone 179 with vinyl acetate 168 (Scheme 51), to afford the cycloadduct 180, was used to compare photochemical efficiency within the microreactor [1,000 pm (wide) x 500 pm (deep)] and a conventional batch reactor (10 ml). 

Fukuyama, Ryu and coworkers reported intermolecular [2 + 2]-type cycloaddition of various cyclohexenone derivatives and alkenes using a micro reactor made entirely of glass, which was supplied by Mikroglas (Scheme 4.26) [39]. The device was equipped with a heat exchanger channel system through which water flowed to maintain isothermal reaction conditions. The remarkable photochemical efficiency of this device was manifested in rapid cycloaddition of vinyl acetate to cyclohex-2-enone. With this device, the desired product was obtained in 88% yield after 2 h, whereas the same reaction carried out in a Pyrex flask was very sluggish (only 8%... 

Akkerman, I., Janssen, M., Rocha, J.M.S., Reith, J.H., and Wijffels, R.H. 2003. Photobiological hydrogen production Photochemical efficiency and bioreactor design. In, Biomethane Bio-hydrogen, (J. H. Reith, R. H. Wijffels, and H. Barten, eds.), Chapter 6, Dutch Biological Hydrogen Foundation, Petten, The Netherlands, pp. 124-145. 

Hallenbeck, P.C. and Benemann, J.R. 2002. Biological hydrogen production photochemical efficiency and bioreactor design. Ini. J. Hydrogen Energy 27, 1185-1194. 

Fv/Fm) depended on the amount of time the cells were left in Si-starved media. Recovery in fluorescence parameters was similar to recovery times seen for other nutrients such as N, P, and Fe (Berges et al., 1996 Geider et al., 1993). The rapid recovery (minutes) rules out the recovery of photochemical efficiency by a coupling to protein synthesis, which occurs on the order of hours (Werner, 1977), while the decoupling of Fv/Fm and optical density (division rates) suggest that the changes are not directly attributable to a reinitiating of the cell cycle. 

The total irradiance received by a sample can be determined by chemical actinometry, which uses a reaction of known photochemical efficiency. The ICH guideline proposes quinine actinometry as a standard based on the assumption that its increase in absorbance is proportional to the integrated UVA irradiance for a given lamp (22). The suitability of quinine as actinometer is, however, questioned in several different reports (10,23,24). The photolysis of quinine is sensitive to temperature, pH, and dissolved oxygen content. The physical characteristics of the quartz cell (e.g., cuvette dimension) are further shown to influence the result (24). 

Recently, telechelic photoinitiators with alkoxythiocarbonyl sulfide groups have been reported [132] to be active in the photoinitiated polymerization of MMA. In particular, the photochemical efficiencies of BPX, BMX and poly(MMA) terminated by one or two isc ropoxythiocarbonyl sulfide groups (PMMA-X) have been compared in the polymerization of MMA. The above systems, upon irradiation, give photodissociation reactions as depicted in Scheme 33 ... 

Castagnola and Dutta investigated the use of nanocrystalline zeolites with high surface area as hosts [18]. The strategy to increase photochemical efficiency was... 

These examples demonstrate that within the last few years, the sophistication for designing novel photochemical assemblies in zeolites has increased considerably and has resulted in our getting closer to architectures that provide permanent charge separation. The challenge continues to be in increasing the photochemical efficiency of the process. 

The Bhcmoc group was developed as a photochemically removable protective group for caged compounds. Among the series tested this one showed the highest photochemical efficiency in its release of an alcohol. ... 

Neither filter radiometers nor luxmeters can be used to obtain an absolute measurement of irradiance or to compare irradiance between sources unless they are calibrated specifically for each source (Tpnnesen and Karlsen, 1997). A spectro-radiometer is needed for a detailed estimate of the SPD but at present such equipment is not widely used on a regular basis because of cost and convenience. The total irradiance (i.e., actual number of photons) can be determined by chemical actinom-etry using a reaction of known photochemical efficiency (Chapter 3 and Chapter 6). The chemical actinometer listed in the ICH guideline (quinine hydrochloride) has its limitations and its suitability as actinometer has been questioned (Baertschi, 1997 Bovina et al., 1998 Drew, 1998). This actinometer is not suitable for calibration of option 1 radiation sources (Thatcher et al., 2001a, b). An alternative actinometer (2-nitrobenzaldehyde) has recently been discussed for calibration of option 1 (and other sources) for UV irradiance (Allen et al., 2000). 

---