Glassy matrices

Although the photochemistry of the thietane ring has been studied to some degree in the gas phase, the study of its decomposition in solution or in glassy matrices has not been extensive. Biradicals are postulated as intermediates in order to account for the differences in stereochemistry, abiding by the Woodward-Hoffmann rules for concerted reactions. Biradi-... 

Reiser and co-workers published an important series of papers beginning in 1965. They were the first to observe the low-temperature UV-Vis spectrum of triplet phenylnitrene. Later studies in low-temperature glassy matrices by Leyva et alP would reveal an additional long-wavelength band in the spectrum of PN and that the spectrum of PN, originally reported by Reiser et al. was contaminated by the presence of ketenimine K. The difficulty is that PN is extremely light sensitive and, upon excitation at 77 K, rapidly isomerizes to the isomeric ketenimine. ... 

It is interesting to note that the efficiency of radical reactions in solid glassy matrices appears to be uninhibited by the presence of the polymer matrix and in fact in many cases we have observed that the efficiency of radical escape when at least one of the radicals is a small molecule is just as great in polymeric glasses as it is in solution. One can deduce from this that polymeric glasses are not particularly good at trapping radical species unless they are cooled to very low temperatures. 

Entrapment in amorphous (glassy) matrices achieved by rapid cooling or by drying. 

In the past few years the ESR technique has been applied to demonstrate the triplet nature of many highly reactive organic biradicals and of various metastable photoexcited states, to estimate the rates of their decay, and to evaluate their electronic distribution. These metastable states are normally produced and observed either frozen in glassy matrices at 77°K or aligned in a host crystal. Spectra have also been obtained of triplet species dissolved in a translucent plastic. 

Hutchison reported the first ESR spectrum of a metastable phosphorescent state by study of naphthalene oriented in durene crystals.4 Since then, similar spectra have been recorded for several other polynuclear aromatics both oriented in host crystals and randomly suspended in glassy matrices. D values for all these ir,n excited states are quite low, indicating little interaction between the unpaired electrons. Interestingly, D for the quinolines equals 0.10cm"1 just as in naphthalene,197 indicating that the presence of a heteroatom does not necessarily change the ir,w nature of the lowest triplet state very much. A similar conclusion has been reached from a comparison of the ESR spectra of fluorene, carbazole, dibenzofuran, and dibenzothiophene.198... 

It should also be noted that Leach and Migirdician37 have produced adducts of hexatriene in mixtures of benzene with certain other molecules in glassy matrices at low temperatures. Thus, along with benzene isomers which retain the empirical formula of the monomer, there may be formed photochemically other products which result from a rupture of the ring. Just possibly some of the cuprenelike polymer formed at very short wavelengths20 may be made in this way. We will return to this point in later sections. 

According to Bolshakov and Tolkachev [1976] and Zaskulnikov et al. [1981], the kinetics of radical conversion in glassy matrices consists of an initial part determined by the rate constant k T) described above and a subsequent conversion of the remaining molecules which obeys the empirical Kolrausch law ... 

Electron spin resonance studies have many uses—for example, to investigate a large number of substances (e.g., polycrystalline material), to characterize the dynamics of molecules of biological interest, and to study the diffusion of paramagnetic ions in glassy matrices. 

Kevan L. (1981) Solvated electron structure in glassy matrices. Acc Chem Res 14 138-145. 

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