Photostationary Methods

The photostationary methods are only valid for AB systems without photodegradation. The only assumptions are that product A is thermally stable, that its spectmm (eA) is known, and that the Beer-Lambert law applies. 

In the case of a photochemically reversible, photochromic sytem of the AB, 2<)> type, the following data are required for application of the photostationary method ... 

The rate of recombination of bromine atoms is 2.4 x 10 l mole s in reasonably good agreement with the value of 3.6 x 10 found by using flash photolysis, and the photostationary method can thus be used to obtain reaction rates much greater than those obtained by other competitive methods. 

The triplet energy of thianaphthene 1,1-dioxide was determined by two indirect methods. The first involved the use of several sensitizers of decreasing triplet energy. The results summarized in Table 1 indicate that triplet lies between 53 and 49 kcal mol-1. The second method is more precise and involves the use of thianaphthene 1,1-dioxide as a sensitizer to establish a photostationary state of the a—methylstilbenes. The composition of the photostationary state of a-methylstilbene has been determined as a function of the triplet energy level of the sensitizer. The results indicate a triplet energy for thianaphthene 1,1-dioxide of 50 1 kcal mol-1. Quantum yields of the photodimerization of thianaphthene 1,1-dioxide were determined in benzene as a function of concentration. Oisc is 0.18. The product distribution as a function of solvent polarity demonstrates the ratio of the head-to-head to head-to-tail dimer (HH/HT) increases with the polarity of the solvents. This is consistent with preferential solvatation of the head-to-head transitions state. 

A widely used method for assessing supercritical fluid phenomena consists of comparing physical and chemical behavior above the critical point with corresponding behavior in the subcritical liquid. Because this approach (unrealistically) seeks to observe discontinuous behavior between states, the results of such experiments are often ambiguous. In the present study, we have compared the photoisomerization of trans-stilbene in subcritical and supercritical C02 and, to model liquid behavior, we have also carried out these isomer-izations in cyclohexane. In all three systems, the effects of temperature and concentration on the cis/trans ratio were compared and, for the C02 systems, the effect of pressure on this photostationary ratio was also probed. The results from these experiments are shown in Tables I and II and are plotted in Figures 4 through 6. 

By measuring the dark period time which corresponds to the time required for the radicals to decay, one obtains the average radical concentration (from the rate of loss of monomer) and finally the ratio Together with the photostationary measurements of kt lkp, this allows kt and kp to be determined separately. This method is useful only in systems in which termination and propagation have different orders with respect to radicals. Although it studies the nonstationary period of the chain reaction, it studies it in a periodic or stationary way. 

This method is valid for systems without thermal relaxation. Therefore, all data concerning the photostationary state were extrapolated to infinite flux. 

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. 

The time necessary to reach the photostationary c/s-state for the first time in LBK films of this azobenzene amphiphile transferred in the trans-state is considerably longer than for the subsequent cycles. This is explained by the time it takes to break up some aggregates in the dense packing of the LBK film. The extent of isomerization, determined by Brode s method,is given to be 30%. Reestimation employing electrochemical methods showed that the irradiation yielded a c/s-isomer count of only 19% ... 

Dynamic analysis is also a powerful method for the study of complex behavior. The description of an ideal ABC system with the possibility of multiple photostationary states illustrates the dynamic bistable behavior of TPID. 

The principle of the method (Fischer s method)8 89 is illustrated in Figure A3. The rate of conversion (3 in the photostationary state PS (corresponding to... 

The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. 

No effect of solvent polarity on the position of the photostationary state was found for 4,4 -CMS [85,153], in contrast with the case of 4,4 -NMS, despite the fact that the nitro and cyano groups are equally polar substituents. The dipole moments of 4,4 -CDS and 4,4 -NDS are similar in the ground state (7 Debye) and only slightly different in the lowest excited singlet state (20-29 and 25-32 Debye, respectively, according to the various methods employed [155-158]). 

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... 

Since the isomerisation product B reconverts thermally to the initial product, a method has been applied which allows a graphical evaluation for a linear relationship between the reciprocal absorbance difference between absorbance in the photostationary state and the beginning of the reaction plotted versus the reciprocal intensity of irradiation [160]. Applying eq. (5.133), the photochemical quantum yields (pf can be determined using the slope and (pf using the intercept, if... 

Compound 2c, like a crown-ether indolenine styryl dye lb, can be subjected to a reversible photochemical reaction of trans-cis isomerization around the central double C=C bond. The absorption spectra of cis-2c in acetonitrile was calculated by means of Fisher method [18] using the spectra of trans-isomer and photostationary states obtained from the irradiation of a 2b solution with light at 365 nm and 436 nm. 

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