Isopentane glass

Evidence for the role of a singlet excimer in the formation of (78) was provided by the observation of excimer fluorescence from dilute solutions of coumarin in methylcyclohexane and isopentane glasses at 77°K.(93b)... 

Judeikis and Siegel236 irradiated naphthalene in an isopentane glass containing dissolved NO at 77 and 87°K. They followed the triplet... 

Figure 4. Photograph of recorder trace showing decay of ESR signal after ultraviolet irradiation at 77° K. of isopentane glass containing 7 mole % TEA and 0.02 mole % TMPD. The experimental technique used to obtain the trace has been described in Ref. 14...

Okoyuma et al. have reported that the strained Dewar paracyclophanes (98) can be converted into the benzenoid parent compounds (99) by irradiation of the compounds in a diethylether-isopentane glass at 77 K using 365 nm light. Interestingly when (99, R = CN, = H) is allowed to thaw to room temperature there is a thermal reaction that converts it back into the Dewar form which provides the first observation of such a thermal cyclization. Irradiation at 254 nm of the tetraene (100), a hk-Dewar benzene, at 77 K in a matrix results in its conversion into the paracyclophane (101). Continued irradiation transforms (101) into the (4+4)-adduct (102). The products from this low temperature... 

As the ether-isopentane-ethanol glass containing oriented Wurster s blue ions is allowed to warm up from liquid air temperatures, the... 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

The author has found methylcyclohexane a particularly useful solvent, either on its own or (in low temperature glass studies) mixed with isopentane presumably this is because of poor solvent molecule packing. 

An alternative photoinduced pathway has been identified in the photolysis of the meso-ionic oxathiazolone (335). Irradiation in ethanol in the presence of oxygen yields ethyl phenylglyoxylate (340 65%), ethyl benzoate (3%), and benzonitrile (21%). The postulated intermediates (Fig. 5) have been spectroscopically identified by photolysis in ether-isopentane-ethanol-glass at 85°K. ... 

In a careful study of cyclopentenone photocycloaddition reaction, DeMayo and coworkers215 have noted that ketone sensitizers of triplet energy less than 71.2 kcal did not sensitize cycloaddition to cyclohexene. Triphenylene (Et = 66.6 kcal) and acenaphthene (ET = 59.3 kcal) were exceptional since they resulted in quantum yields of 0.10 and 0.21, respectively. This behavior, as well as the fact that 0.1 M cyclopentenone quenched acenaphthene fluorescence by 90% in an EPA (ether-isopentane-alcohol) glass at 77°K strongly implicate singlet energy transfer. 

For aromatic hydrocarbons, the quantum efficiencies of fluorescence and phosphorescence in low temperature glasses (Appendix H) such as EPA (ether isopentane ethyl alcohol in the ratio 2 2 5) add up to unity. This suggests that direct nonradiative decay from Si —> S0 is of very low probability. All the nonradiative paths to the ground state are coupled via the triplet state. The sequence of transfers is ... 

EtOH isopentane Et20) glasses at 77 K, four products 256, 257, 258 and 259 were obtained in overall >95% yield. The major product 256 was shown by isotopic labeling studies to have been derived from the silirene 260 formed by intramolecular coupling of the bis-carbene 261, and pathways for the formation of the other products were proposed. The photolysis of the bis-diazo compound 255 was studied in detail at 405 nm and it was found that the diazirine 262 was formed this on photolysis at 305 nm also gave 260 on the pathway to 256. The chemistry is shown in Scheme 46. 

FIG. 14. Histograms for the distribution of the number of pairs of neighbors, in arbitrary units, for different values of mIM for (a) isopentane liquid at 301 K and (b) glass at 30 K, with and without annealing. (From Yashonath and Rao (71).)... 

Differences in low temperature emission spectra of benzil in methylcyclohexane and in isopentane have been ascribed to inhibition of the conformational changes involved in the skewed to planar relaxation in isopentane 33). Emission spectra were identical in both solvents at temperatures above the glass-forming temperature. Preference for an s-cis conformation in ethylene glycol solution has been suggested 33b) to account for anomalous emission spectra of benzil in that medium. Other aspects of benzil emission have been examined 33c). 

The luminescent cyclometalated Pt(II) complexes 5, with absorption maxima around 520 nm, exhibit similarly structured emission bands, independent of the nature of the /3-diketonato ligand. A representative complex of this type (R = H, R = Me) in a frozen EPA (ether-isopentane-alcohol) glass at 77 K has two sets of vibrationally structured bands (460-560 nm, 570-800 nm), due to the presence of two emitting isomeric species. 

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