Interaction between pyrazine and halomethanes. II. Effects of halomethanes on the phosphorescence of pyrazine at 77 K.

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The phosphorescence and phosphorescence excitation spectra of pyrazine have been studied at 77 K in various mixed solvents of cyclohexane and halomethanes such as carbon tetrachloride, chloroform, and dichloromethane. When the halomethane concentration was low (below 10−2 mol dm−3), only the cyclohexane-phase phosphorescence was observed at 77 K; the lower energy components of the well-known doublet band series in the cyclohexanephase phosphorescence were found to decrease in intensity relative to the higher energy components, with increasing concentration of halomethane. When the halomethane concentration was raised to a range higher than 10−1 mol dm−3, where the complex formation between pyrazine and the halomethanes was confirmed through absorption spectroscopy, new phosphorescence bands appeared in the 373–375 nm region for all the systems studied. With further increase of the halomethane concentration, the new bands decreased in intensity or disappeared, and the phosphorescence approached to those in the pure halomethanes. Comparison of the absorption and phosphorescence excitation spectra shows that the new phosphorescence bands result from the 1 : 1 pyrazine–halomethane complexes. The possibility of hydrogen-bonding species formation with chloroform and dichloromethane has been discussed in the light of the phosphorescence spectra studied in the ethanol–cyclohexane mixed solvents.
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