EPA glass

Irradiation of a diazonium compound in EPA glass at 77°K and monitoring the reaction by UV revealed the presence of new absorption bands, which disappeared upon warming to room temperature. Little or no evolution of nitrogen occurred in these experiments. Hence the low-temperature intermediate has been proposed to be a rather stable triplet state of the diazonium compound. 

Maloney and co-workersstudied diphenoxyphosphorylnitrene by direct observational techniques. Irradiation (75 s, 254 nm) of 25 in EPA glass at 77 K produced persistent triplet EPR signals with zero-field parameters D/hc = 1.5148 cm and E/hc = 0.00739 cm which are rather similar to those of triplet carboethoxy-... 

Figure 11.2. The absorption spectrum of triplet phenylnitrene in EPA glass at 77 K. The computed positions and oscillator strengths (f, right-hand axes) of the absorption bands are depicted as sohd vertical hues. For very small oscillator strength, the value multiphed by 10 is presented (/ x 10). [Reproduced with permission from N. P. Gritsan, Z. Zhu, C. M. Hadad, and M. S. Platz, J. Am. Chem. Soc. 1999, 121, 1202. Copyright 1999 American Chemical Society]...

Benzonitrile sulfide has previously been trapped by the same solvent as a product of the thermal decomposition of 5-phenyl-1,3,4-oxathiazol-2-one.28 Spectral studies of this elusive substance can be made when photolyzing either thiatriazole or the other precursors mentioned in an EPA glass at 85°K.20... 

Since nitrene intermediates appear to be ruled out, the photochemical generation of isothiocyanate could either take place via other types of intermediates or directly from a singlet excited state of the thiatriazole. As possible intermediates thiobenzoyl azide, benzonitrile sulfide, and phenylthiazirine were considered.20 Since isothiocyanate is formed from the thiatriazole at 85°K but not from 8 at 85°K where benzonitrile sulfide is stable, the latter was ruled out as a precursor for phenyl isothiocyanate. Thiobenzoyl azide is assumed to absorb in the visible but could not be observed when the thiatriazole was irradiated in an EPA glass at 85 °K, and thus no evidence for its intermediacy was obtained. In conclusion it appears that the isothiocyanate is formed directly from the singlet excited state of the thiatriazole, although the intermediacy of phenylthiazirine cannot be directly excluded. 

Closed shell anions were also examined and fluorene carbanions substituted in the 9 positions shown to photoeject electron under UV light in EPA glasses at 77 K this process was demonstrated as biphotonic and strongly dependent on the ion pair status slight modifications in the EPA glasses composition modifies the reaction efficiency [57, 58]. 

A formation of thiirene by secondary photolysis of thioketene, which is generated by photolysis of 1,2,3-thiadiazole, has been reported <92ACS482>. Irradiation of [4-13C]-l,2,3-thiadiazole together with diethylamine in an EPA glass (diethyl ether-isopentane-ethanol (5 5 2)) at 77 K gave N,N-diethylthioacetamide with 37% carbon randomization in the thioketene (Scheme 9). Experiments with [4-13C]-l,2,3-thiadiazole at room temperature demonstrate lack of carbon randomization in the thioacetyl group of the trapping product. Therefore, the reaction mechanism of photolysis of... 

With one exception there are essentially no spectroscopic data available for oxathiiranes. The one exception is the report by Carlsen et al. <76JCS(P1)1404> in which they conclude that an intermediate formed in the photolysis of thiobenzophenone sulfine (13) is the oxathiirane, 3,3-diphenyloxathiirane (14). The intermediate is formed when (13) is photolyzed in an EPA glass at 77 K and has a long wavelength absorption at 390 nm. 

Figure 4. The absorption, fluorescence, and fluorescence excitation spectra of triplet phenyl nitrene in EPA glass at 77 K.

This point was investigated by Leyva et al. [69], They were able to demonstrate that there is a weak emission from the species produced by brief photolysis of phenyl azide in EPA glass at 77 K. As mentioned earlier the emission has a mirror image relationship with the absorption spectrum, and significantly, the excitation spectrum of the emission is identical to that of the absorption spectrum (Figure 4). Thus, barring unlikely coincidences Reiser s spectrum must be due to a single species. 

Leyva photolyzed azides 4 and 5 in EPA glass at 77 K assuming that only the triplet nitrene would be formed in the low temperature glass. As shown in Figures 5 and 6 the absorption, emission, and fluorescence excitation spectra obtained by brief photolysis of 4 and 5 are very similar to the low temperature spectra obtained from phenyl azide. This of course implies that brief photolysis of phenyl azide at 77 K produces only triplet phenyl nitrene as a UV-VIS active species. 

Figure 7. A comparison of the spectra of the 1-naphthylaminyl radical and triplet 1-naphthyl nitrene. The spectrum of the radical was produced by laser flash photolysis of di-tcrt-butyl peroxide containing 1-naphthylamine. The spectrum of the nitrene was produced by photolysis of 1-naphthyl azide in EPA glass at 77 K.

EPA glass at 77 K, the spectrum of the azide disappeared and was replaced by a series of new bands in the UV-VIS region. The new spectrum was attributed to triplet 1-naphthyl nitrene on the basis of Wasserman s [18] EPR studies and the similarity of the matrix spectrum to the absorption spectrum of the 1-naphthylaminyl radical [83] (Figure 7). Triplet 1-naphthyl nitrene is very sensitive to light and its matrix UV-VIS spectrum is readily bleached by over irradiation. 

Phosphorescence of 2-methylisoindole is weak. It can be observed in an EPA glass at 77K and occurs at 526 nm (0p = 0.08 + 0.05, tp = 0.4 sec). Benzophenone is an efficient triplet sensitizer in this system and hence enhances this emission. Triplet-triplet absorption spectra have been measured in flash-photolysis studies the use of sensitizers and quenchers confirms the triplet energy as 227 kJ mol-1.94... 

Fig. 4. Comparison of absorption and fluorescence spectra of hexadecaheptaene (7a), a C30 spheroidene (7b), and /3-apo-12 -carotene (7c). Spectra were obtained at 77 K in an EPA glass (ether/isopentane/ethanol, 5/5/2, v/v/v). Electronic origins are designated by asterisks.

Figure 5.13 The absorption spectrum of triplet phenylnitrene in EPA glass at 77 The computed positions and oscillator...

A fluorimeter suitable for two-photon excitation using a tunable dye laser has been described. Spectra of diphenylbutadiene in an EPA glass at 77K were used for illustration. Two-photon excited fluorescence was also observed for 0s04 and UFg with excitation using a CO2 and Raman frequency-shifted dye laser, respectively. Polarization effects on two-photon excitation have been examined. ... 

Bennett and Birge have determined a two-photon excitation spectrum of d -trans 1,4-diphenylbuta-1,3-diene in EPA glass at 77 K. The system origin of the lowest A nn state is observed at 27 900 20 cm , 130cm " below the system origin of the strongly allowed state. The short intrinsic lifetime of... 

Table 7 Deuterium isotope effect on triplet-state radiative lifetimes (rx). All data in EPA glass at 77 ...

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