Approximation photostationary state

O3] Predicted tram Photostationary State Approximation as a Function of Initial [N02f... 

Deuteration has been previously shown to cause an increase in the lifetime of triplet free-base porphyrins ( 7). This has been attributed to the strong coupling of N-H tautomerism with nonradiative decay. In the case of mesoporphyrin IX the increase upon deuteration is approximately two-fold ( ) As indicated in Table III deuteration of the picket fence porphyrin results in little change in the photostationary state composition but an almost twofold increase in the quantum yield of 4,0 -> 3>1. As stated above there is no measurable deuterium isotope effect on the thermal reaction the proportionate increase in quantum yield and triplet lifetime upon deuteration of the picket fence porphyrin is thus completely consistent with the adiabatic mechanism described above. Although the evidence amassed does not completely rule out other possibilities, it seems that the photoatropisomerization is to date best described by the adiabatic pathway in which the porphyrin ground and excited state potential surfaces are modified much as illustrated in Figure 3. 

The importance of such a redox property is also illustrated in the photodissociation (59) of Co2(CO)6(PBu3)2 in the presence of both Q+ PF6" and PBu3. Separately, neither Q+ nor PBu3 has any perceptible effect on the photostationary state of Co2(CO)6(PBu3)2 on irradiation of the a,a band (60) at wavelengths of approximately 380 nm. However, when both are present, essentially quantitative yields of Co(CO)3(PBu3)2+ PF6 are obtained by a photoredox process that can be described as... 

A much more sophisticated spectroscopical method was proposed by Fischer. This method allows the calculation of photostationary states in systems containing substances A and B when only A is known. The only assumption made in this method is that the ratio of the quantum yields < A/(/>B does not change with the wavelength. The approximate validity of this assumption has been shown. Further requirements of this method are that two photostationary states must be able to be achieved by irradiation at different wavelengths and that the changes in the optical spectrum must be due only to the changes in the cisitrans ratio. In solutions of azobenzene derivatives, these requirements are fulfilled in most cases. In the solid state. 

A patent has been lodged dealing with the photochemical trans-cis isomerization of the cyanocyclopropane 191. The photostationary state reached in these photolyses consists of approximately 65% cis Both direct and sensitized irradiation of the cyanocyclopropane 192 leads to the formation of the cw-isomer 193. Two rearrangement products, 194 and 195, are also formed in this reaction. The biradical intermediate 196 is proposed as the... 

This is a relatively crude approximation. However, a graph of the change in absorbance versus time gives a linear slope for the beginning of the reaction under these conditions with a deviation less than 1% in comparison to the correct evaluation. These factors can be calibrated by other actinometers as for example the Parker solution [121]. The values given in Table 5.4 can be used to determine some of the lines of a mercury arc. The dashed line in the table indicates that for the measurement of the 254 nm line the other band of the azobenzene solution has to be taken. A trans solution must be preirradiated at 313 nm to bring the solution into the photostationary state. 

Combined determination of reaction constants and absorption coefficients In many cases the absorption coefficients of the photo-product cannot be determined, since B cannot be isolated in a pure form. For these conditions quite a few methods have been proposed in the literature [156-160]. If the conditions required for these approximations do not fit, the thermal reverse reaction is used to shift the photostationary state by variation of temperature. The possibilities of such an approach are demonstrated by the photoreaction of dihydroindolizines [49,161]. Using eq. (5.119) and the approximation at the photostationary state... 

Irradiation of cis- and /rans-cyclooctenes yields approximately equal amounts of both isomers in a photostationary state because of their almost equal extinction coeflftcients. Both these isomers can be isolated [5]. 

The triplet excitation energy of benzophenone is approximately 69 k cal/mol and makes the cis- isomer in dominance, from this fact it is concluded the energy required for excitation of trans- isomer is less than that for cis- isomer. Also the sensitiser having triplet energy in the range of 52-58 k cal/mol, selectively excites the trans- isomer. Since the rate of trans to cis conversion increased, the photostationary state is enriched with cis- isomer. 

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