Photo-stationary state

Similarly to the case of non-dendritic 4a, the trans form of dendritic 4b, upon irradiation of a CHC13 solution at 340 nm, isomerizes to the cis form, which undergoes a backward isomerization to the trans form upon irradiation at 450 nm or heating. On the other hand, when the dendritic wedges are irradiated at 280 nm, cis-4b undergoes an isomerization to the trans form, to reach a photo-stationary state. In contrast, nondendritic 4a does not undergo such a ds-to-trans isomerization under similar conditions. Relative to these observations, a large but nonsymmetrically substituted azodendrimer such as 4c in the cis form,... 

Naphthopyran mero-forms have been studied using H-NMR and NOE studies [78,79]. It has been concluded that there are two isomeric forms that are possible for the mero-form when the terminal methine carbon atom (Cl in Scheme 1 and C42 in Fig. 10) is symmetrically substituted, as shown in Scheme 1. A fourth species has been identified in photo-stationary states of naphthopyran and its mero-forms however, this has not yet been clearly identified. The two main mero-isomers are not in rapid exchange, as two peaks can be clearly identified in the H-NMR spectrum. 

The explanation of this observation is obscure. However, photolysis does effect the transformation of the conformation from trans.trans to cis.trans. No fluorescence has been observed from solutions of this azaallyl ion. A further difference from the diphenylallyl system is that the photo-stationary state is insensitive to temperature. 

The conversion from the open- to the closed-ring isomer in the photo-stationary state under irradiation with 405 nm light decreased with increasing solvent polarity. The cyclization quantum yield decreased to 0.097 in THF, 0.029 in ethanol and 0.013 in acetonitrile, respectively. 

Fig. 9 Fluorescence emission spectra (n-hexane) of photo-stationary states of c/s-17a and trans-17b in 90 10 and 30 70 ratios (A.U. = arbitrary units, relative intensities). Inset modulated emission signal during alternating irradiation at 365 and 435 nm (excitation 300 nm, irradiation time 60s).

Photoisomerizations were carried out using a 450 watt Hanovia medium pressure mercury source, and each reaction was followed to the photo-stationary state by gas chromatographic determination of the cis/trans ratios. Twice recrystallized trans-stilbene (Aldrich), spectrophotometric grade cyclohexane, and dry, high purity carbon dioxide were used in these experiments. 

The absorption cross sections for NO2 and the corresponding quantum yields are given in Table 8 and 9, respectively. The photolysis of NO2 has been investigated intensively over the last 40 years because of its critical role in the formation of ozone in the polluted tropospheric boundary layer [56-63]. The three reactions of Eqs. 33 and 34 form the basis for the photochemical production of ozone. If one considers only these three reactions, then the photo-stationary state (or photochemical steady-state approximation) can be invoked around the oxygen atom as follows ... 

The latter equation predicts the concentration vs. time profile for the attainment of the photo-stationary state for nitrite, since nitrite is both produced and consumed during the photolytic process. On the other hand, the initial loss of nitrate vs. time can be described by the basic first-order photolysis equation. 

Equation 11 indicates a photo stationary state (pss) when dc jdt = dc fdt — 0. Then... 

Figure 9 Simulation of reaction of Scheme 10 with the photo stationary state at 10%. (From Ref. 117. Copyright The Royal Society of Chemistry.)...

Both of these methods are subject to uncertainty that arises from the manner of termination, i.e., by recombination or by disproportionation. This latter question, if not resolved, can introduce an error of 2 into the estimates of / . Assuming that all of these problems have been satisfactorily resolved, the study of photo-stationary-state kinetic systems gives the ratio of rate constants kp ki>. Within these limitations the results of various laboratories, obtained by the use of different techniques, show reasonably good agreement, usually within 10 to 20 per cent for a given monomer-solvent system. ... 

There is one report of a concentration dependence of the 313 nm photo-stationary state of azobenzene and 4-methoxyazobenzene in cyclohexane— not in benzene or CCI2F-CCIF2—in the literature.A bimolecular excimer intermediate was postulated. Further work is needed to elucidate whether the absorption coefficients or the quantum yields are concentration-dependent, for instance by ground or excited-state association (cf. Equation 1.3). 

The procedure for determining QYs is summarized as follows. On the one hand, both trans and cis isomers may be excited by the same irradiation and interconverted, and an equilibrium of the two isomers, called the photo-stationary state is reached. On the other hand, thermal cis->trans isomerization moves this equilibrium in favor of the trans isomer. So, the first part of the experiment consists of eliminating the effect of thermal isomerization on the equilibrium by determining the absorbance of hypothetical photo-stationary states of the photoisomerization reactions only, by extrapolating the irradiating intensity to infinity for combinations of two irradiation and analysis wavelengths (Rau s method). In the second part of the experiment, the obtained extrapolated values of the absorbance are used (in Fisher s method) to determine the extinction coefficients e i.e., the absorbance spectrum, of the cis isomer, and the determination of the quantum yields is straightforward. 

Among these systems, polyimide with azobenzene chromophores in the backbone was first studied by Agolini and Gay in 1970 [30]. They reported 0.5% contraction of the polymer film at 200 °C taking place under ultraviolet irr liation. No correlation, however, was reported between the degree of isomerization and film contraction. At the high temperature they studied, the cis isomer content in the photo-stationary state would be very low, because a rapid thermal reverse reaction from the cis to the trans form should occur. Thus, the isomerization is not the sole origin of the contraction of the system. 

The reaction, which is wavelength dependent reaches a photo stationary state in which a new polyene distribution is formed which depends on the wavelength of the irradiating light and... 

This expression, resulting from the steady-state analysis of reactions 1-3, has been named the photo stationary state relation. We note that the steady-state ozone concentration is proportional to the [N02]/[NO] ratio. We now need to compute [N02] and [NO]. These are obtained from conservation of nitrogen... 

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