Photoreaction quantum yield

Photolysis of 4- and 3-nitrophenyl acetates (176 —> 177 178 —> 179) in neutral aqueous solution leads to the corresponding phenols with quantum yields 0.002 and O.OO6105 (equation 84). A greater difference in the photoreactivity (quantum yields of 0.002 and 0.129, respectively) is shown between 2-mcthoxy-4-nitrophenyl acetate 180 and 2-methoxy-5-nitrophenyl acetate 182. The nitro substituent clearly exhibits a meta-activating effect in the hydrolysis of phenyl acetates. 

In the condensed phase the close proximity of neighbouring molecules can have a considerable effect on the subsequent fate of the excited species. Solvation can reduce molecular energies, affect the selection rules, and greatly increase the number of collisions undergone by each molecule [23]. The last effect can increase the photophysical decay and decrease the photoreaction quantum yield. Moreover, the... 

Study of Ph3SnRe(CO)3(1,10-phenanthroline) resulted in the first direct determination of the rate constant for excited state cleavage of the M -M bond ( 7> 56). The key is that this complex is emissive from the reactive state under the conditions where the cleavage reaction also occurs. Measurement of the emission lifetime (1.8 x 10 6 s) and the photoreaction quantum yield (-0.23) give a rate constant of 1.3 x 10s s 1 at 298 K for... 

Emission Peak Uj,), Emission Quantum Yield (Emission Lifetime (te), and Photoreaction Quantum Yield ((Pr) of [ReCdiimineXCOJCCFsSOs) in CH2CI2 Under CO at Room Temperature. 

Figure 11. Stem-Volmer plots for the quenching of emission intensity ( ), emission lifetime ( ), and photoreaction quantum yield (O) of Pt(tpy)2 by anthracene. The inset shows a plot of r/° versus 7/7° or t/t°. From Ref. 117 with permission of American Chemical Society.

Let us first consider the situation where initial excitation is followed by relaxation to a bound LEES, which is then responsible for the ligand substitution chemistry. In accord with the above discussion, the quantum yield <1>S for ligand substitution from that state would be fl>lscfcst, where intersystem crossing from the state(s) initially formed, ks is the rate constant for ligand substitution from the LEES, and r = kd1 (kd being the sum of the rate constants for the decay of the LEES). The apparent activation volume for the photoreaction quantum yields is therefore defined as... 

Thus the activation volume AV for the rate constant kp of an individual ES reaction pathway can be evaluated if the pressure dependencies of the photoreaction quantum yield, of intersystem crossing and of the ES lifetime can be separately determined. However, such parameterization becomes considerably more complex if several different excited states are involved or if a fraction of the photosubstitution products are formed from states that are not vibrationally relaxed with respect to the medium. Currently, parameterization of pressure effects on photosubstitutions has been attempted for a limited number of metal complexes. These include certain rhodium(III) and chromium(III) amine complexes and some Group VI metal carbonyls, which will be summarized here. 

Pressure effects for both the emission and photoreaction quantum yields under comparable conditions have now been described for several Cr111 complexes in fluid solution including Cr(bpy) +, Cr(NH3) +, Cr(NH3)5(NCS)2 +, and the cis and trans isomers of Cr(cyclam)(NH3)2+ [28, 89-95, 99]. There is a long history and a rich and subtle literature regarding the photochemistry of hexacoordinate Cr111 complexes to which such pressure studies have contributed insight. However, in the interest of brevity, this chapter will confine further discussion to the photoreactions of the simplest of these species, the hexaammine ion Cr(NH3) + in fluid solutions, for example,... 

Coupling [Ru (p.-dpp)Ru(bpy)2 3] (dpp = 2,3-bis(2 -pyridyl)pyrazine) with Ru4(POM) has allowed the expansion to the useful wavelength region, thus maximizing the overlap with solar emission. An outstanding photoreaction quantum yield for oxygen production of 0.3 has indeed been calculated by irradiating at 550 nm. ... 

These studies have been performed in a variety of solvents, the most common being isooctane (2,2,4-trimethylpentane) or methanol. The photoreaction quantum yields are gathered in Table 4.6, from which their wavelength dependence is evident. 

Photoreaction quantum yield were determined by monitoring the changes in absorbance associated with the disappearance of the reactants or the formation of the products, at a wavelength where the absorbance could be related to the concentration of these species. 

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