Photoisomerization solid matrices

Gegiou et found only a very slight viscosity effect, both in the n-Ti and in the jt-jc absorption bands on the isomerization quantum yield. They used glycerol as a viscous solvent, but the result may also be transferred to polymer matrices. In solid matrices, several photoisomerization modes are observed (see the preceding section on the influence of temperature), A com parison between azobenzene isomerization in liquid methylmethacrylate and the slow mode in poly (methylmethacrylate) showed that the difference in quantum yields on Si (0.17) and S2 excitation (0.03) is retained in the solid matrix. The fast process is not observed in n —> n excitation. These data are important in relation to the use of the azobenzene isomerization method for the determination of the free volume in a polymer. 

The kinetics of spiropyran and azobenzene photoisomerization deviate from first order when these dyes are entrapped in a solid matrix below Tg.24-34 This behavior has been attributed to the presence of a distribution of free volume within the matrix, as shown in Table 3.11 .35 When the probe is located in sites of free volume Vf greater than the critical volume for isomerization Vfc, the reaction proceeds at the same rate as in solution. For sites of Vf < Vfc, the reaction is retarded, since it becomes controlled by the matrix molecular motions. At low temperature, the local molecular motions are frozen and fluctuations of local free volume become increasingly small as the temperature decreases. Consequently the fraction of sites where Vf < Vk increases. 

Under unconstrained conditions (e.g., in fluid solutions or gaseous conditions), the conventional one-bond-flip (OBF) process is the dominant process with the HT being an undetectable higher energy (AG ) process. Under confined conditions (whether in a solid matrix or solution or in a protein binding cavity), the additional viscosity-dependent barrier makes OBF a less favorable process, allowing the volume-conserving HT to be the dominant process for photoisomerization. 

In the case of the Ag° isomer in solid Ar, the spectroscopic and photochemical results indicate the operation of an efficient Ag3 - Agj + Ag photodissociative-recombination process localized in the matrix cage.However>for the Ag3 isomer in solid Kr and Xe, the available data leans heavily in favour of a photoisomerization to the Ag° structural form within a deformable matrix cage. The observation of thermal and photolytic reversibility,amongst other things,argues in favour of a photoisomerization rather than a photodissociation or photoionization process for the Ag3 isomer in Kr and Xe matrices.These photoprocesses for Ag3 and Ag3 in rare gas solids are summerized in Scheme III. 

The cis form (85) is predicted to be more stable than the trans form (86) by 12-17kJmol by quantum chemical calculations. Tetramethylammoniumperoxonitrite crystallizes in the cis form (85) (d(0=N) 116 pm, d(N-O) 135 pm, d(N-O) 141 pm) and is nearly flat with a torsion angle of 22°. In a solid argon matrix, both conformers show a reversible photoisomerism. 

Recently, Moore and Pimentel have photolyzed diazirine isolated in a solid nitrogen matrix. They found that the photolysis did produce diazomethane. In order to decide whether the diazomethane arose by a photoisomerization reaction or by the reaction of methylene with the nitrogen of the matrix, experiments were carried out in a nitrogen matrix enriched in N. On the basis of these experiments they conclude that the photolysis of the diazirine produces methylene and nitrogen, and that the diazomethane arises from the reaction of the methylene with the matrix and not by photoisomerization. Photolysis of diazomethane does not... 

Jacox and Rook (1982) photolyzed CH3ONO at 14 K in a solid Ar matrix and observed the products by infi aFed spectroscopy. They found that the threshold for photodecomposition lies near 370 nm (77.3 kcal/mole), with no evidence for photoisomerization at longer wavelengths. Thus the photodissociation threshold is at a very much greater energy than the dissociation energy of 684 nm (41.8 kcal/mole) reported by Batt et al. (1974). 

Reactions in amorphous polymer solids are first characterized in terms of influences of molecular motion of matrix polymers and non-homogeneity of reaction sites. Specific features of photophysical processes, photoisomerization, photodimerization, chain scission and crosslinking reactions in polymer solids are then discussed separately. 

The redistribution of free voliunes also influences the sub-glass transition temperatures Tp and T observed for photoisomerization reactions in polymer solids. T, Tp and T are frequency-dependent, and the response of any process to the transitions at these temperatures depends on the time scale. The time scale of photoprocesses may not be equal to those of DSC or dynamic mechanical methods, which are of the order of 10 to 1( Hz. However, for photodecoloration of the merocyanine form of spiro-bepzopyran in polycarbonate film under steady-state irradiation of 560 nm light after laser-single-pulse induced coloration, it was found that the Arrhenius plot of the apparent rate coefficient broke at T (150 °C), Tp (20 C), and T (—120 °Q of the matrix polycarbonate these temperatures are the ones determined by dynamic mechanical measurements. The excited state lifetime of the merocyanine form in polycarbonate was 1.8 ns . Hence, the decolorating isomerization during the lifetime proceeded only in a small fraction of the molecules surrounded by a sufficient amount of free volume. Thus, it is likely that the temperature dependence of the apparent rate coefficient reflecting the relative quantum yield is controlled by the frequency of redistribution of free volumes, which may be comparable with the frequency determined by dynamic mechanical measurements. 

As has been outlined in Chapt 3, the isomerization reactions in amorphous polymer solids are appreciably influenced by local mobility and heterogeneity of reactive sites, often leading to the deviation of reaction profiles from first-order kinetics. However, this situation allows us to obtain an insist into the microstructure of amorphous polymer solids, e.g. distribution of local free volume, by using photoisomerization reactions as molecular probes. Since the photochromic phenomena in polymer solids were reviewed by Smets in 1983, our discussion below will be limited to more recent advances, putting emphasis on the explanation of non-homo-geneous progress of reactions in terms of the distribution of local free volume in matrix polymers. 

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