The observation of emission from benzophenone1•2 and acetophenone2 in fluorocarbon1 and hydrocarbon2 solvents represents a substantial breakthrough in the study of photochemical processes in solution. In the following we show that the emission from several aromatic ketones is composed of delayed fluorescence and phosphorescence components.Sources of materials and purification methods will be described in the full paper. Solutions of the ketones in carbon tetrachloride or benzene were outgassed to < 10~6 mm using 5-7 freeze-pump-thaw cycles. Emission spectra were recorded using a Hitachi Model MPF-2A spectrophotometer (Phototube, Hitachi -06). Corrected spectra and emission quantum yields were determined using quinine bisulfate as standard, ¥ = 0.56. [3][4][5] The emission spectrum of benzophenone in carbon tetrachloride at 23°is typical (Figure 1). It agrees well with the spectra obtained in benzene, perfluoromethylcyclohexane,1 and isooctane.2 The positions of phosphorescence 0-0 bands and the corresponding triplet excitation energies are given in Table I. These energies are 1-2 kcal/mol lower than those obtained from measurements in hydrocarbon glass at 77°K.6 For the benzophenones a plot of £T against Hammett's is (1) C. A. Parker, Chem. Commun., 749 (1968).

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As an extension of an earlier investigation (J. Med. Chem. 1984, 27, 1431), we prepared a series of 3-substituted 5-[(hydroxyimino)methyl]-1,2,4-oxadiazoles and the corresponding 5-thiocarbohydroximic acid 2-(N,N-dialkylamino)ethyl S-esters. The compounds were evaluated in vitro as reactivators of phosphonylated electric eel and human erythrocyte (RBC) acetylcholinesterases (AChE). The compounds were characterized with respect to (hydroxyimino)methyl acid dissociation constant, nucleophilicity, octanol/buffer partition coefficient, reversible AChE inhibition, and kinetics of reactivating ethyl methylphosphonylated AChE. One compound was also tested for effectiveness in preventing AChE phosphonylation. All of the tested compounds significantly reactivate ethyl methylphosphonylated AChE: the 3-n-octyl- and 3-(1-naphthyl)-substituted aldoximes are as reactive (within a factor of 5-10) toward the inhibited enzymes as the benchmark pyridinium reactivators, 2-PAM and HI-6. All of the substituted thiocarbohydroximic acid S-esters are powerful reversible inhibitors of AChE's: the 3-n-octyl- and 3-(1-naphthyl)-substituted thiocarbohydroximates inhibit eel AChE to 50% initial activity at concentrations less than 5 microM. When added to an eel AChE solution at concentrations between 5 and 50 microM, the 3-phenyl-substituted thiocarbohydroximate effectively antagonizes AChE inhibition by ethyl p-nitrophenyl methylphosphonate (EPMP), suggesting the potential utility of this compound for preventing anti-AChE-agent poisoning.

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