The characteristic line fluorescence of trivalent europium is excited in certain organoeuropium compounds by irradiation with light absorbed only by the organic part of the compound. The efficiency of excitation varies greatly with the nature of the compound, temperature, and solvent. Under optimum conditions, i.e., solution of a covalent compound at liquid-air temperature, almost unit efficiency has been obtained. The decay time of the fluorescence is independent of quenching; quenching prevents excitation of the europium ion. There seems to be a steady gradation of efficiency of transfer from covalent to ionic compounds.

Electron spin echo observations of the photoexcited triplet state of 0.1 mol % pentacene-h14 and -d14 in p-terphenyl crystals at room temperature are presented. Theory is presented for calculation of the echo envelope modulations for an S = 1, I = 1/2 spin system including zero field splittings in the high field limit. Echo envelope modulations due to proton and deuteron hyperfine interactions in the pentacene molecule have been observed. The echo decay data are used to calculate triplet state decay parameters.

Phosphinidenes have been proposed as short-lived intermediates in several reaction systems. ' We have reported products from the photodecomposition of 1 -mesitylphosphirane that are in accord with the formation of free mesitylphosphinidene (2) as a reactive intermediate.2 Product structures suggest that mesitylphosphinidene adds to ?r-bonds in a fashion similar to that for carbenes and silylenes.*J But, heeding Mathey's warning regarding the invocation of phosphinidene intermediates without physical evidence: it was recognized that spectroscopic studies were n d e d in order to fully establish the formation of phosphinidenes.Theoretical studies predict that many phosphinidenes have triplet ground electronic statesS and that the ground triplet state of arylphosphinidenes is lower in energy by more than 40 kcal/ mol than the lowest excited (singlet) states6 It thus seemed likely that a triplet arylphosphinidene could be detected by ESR spectroscopy, as in the case of carbenes and nitrenes.' While ESR experiments on the photolysis of proposed phosphinidene precursors in frozen matrices have been reported previously,* the (1) (a) Schmidt,U. Angew. Chem., Int. Ed. Engl. 1975,14,523. (b) Weber, B.; Reaitz. M. Houben-Wevl, p 34: "The effective generation of these transient phosphinidenes from appropriate precursors is deduced from the isolation of products whose formation can be explained by a mechanism involving such intermediates. However, in almost every case as will be demonstrated later, it is possible to conceive alternate mechanisms which do not involve phosphinidenes. Thus, only a thorough physiwchemicalstudy of the generating systems can provide a firm basis for the actual formation of phosphinidenes." ( 5 ) For H P (a) Nguyen, M. T. Mol. Phys. 1986, 59, 547. (b) Droege, A.T.; Engelking, P. C. Scheme 1 CH3( cCH3 1. MesP(H)LI, THF TsO' OTs Z.wBuLl,MF -H * .C-cFH I Scheme 2 1 2 3 2 4detection by ESR of a triplet phosphinidene was not achieved. We report here the observation of triplet mesitylphosphinidene. trans-2,3-Dimethyl-1 -mesitylphosphirane (1) was synthesized from MesPHz9 and (*)-2,3-butanediol ditosylatelo in 52% yield as shown in Scheme 1.11Compound 1 was found to decompose more easily and cleanly than the previously reported l-mesitylphosphirane.Z Room temperature photolysis of compound 1 in the presence of 3-hexyne gave a 64% yield of 2,3-diethyl-1 -mesitylphosphirene (4)12 (Scheme 2).When frozen solutions of compound 1 (0.01-0.1 M) in methylcyclohexane were irradiated at 77 K with a 254 nm light source, the methylcyclohexane glass became yellow. After thawing, trimesitylcyclotriphosphine (3) was observed in 30% yield in the absence of trapping agent;l3 in the presence of 3-hexyne, 2,3-dimethyl-1-mesitylphosphirene (4) was formed in yields of 2240% based on the initial amount of compound 1. The formation of these products strongly suggests that mesitylphosphinidene (2) was formed in the matrix.A methylcyclohexane solution of compound 1 (0.1 M, 0.6 mL) was placed in a quartz ESR tube, degassed, and se...

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The pulsed EPR free induction decay (FID) signals of the photo-excited pentacene triplet state are reported for three mixed crystals at room temperature: pentacene-h14 in p-terphenyl, pentacene-h14 in benzoic acid, and pentacene-d14 in p-terphenyl. The recorded FID signals have relatively long decay times of about four microseconds, presumably due to the reduced hyperfine interactions in the zero magnetic field. The time domain FID signals transform to spectral components typically narrower than 500 kHz, allowing us to determine the pentacene triplet zero field splitting parameters to better accuracy than previously reported. Further, a new experimental technique using the high speed magnetic field jumping capability enables us to examine the anisotropic hyperfine and quadrupole interactions.

Urea has a resonance energy of approximately 41 kcal.,28 and at least in the solid state is a completely planar molecule.29 This planarity is destroyed when the activated complex is formed. Thus the activation process should require enough energy to compensate for the loss of resonance energy involved in activated complex formation. This resonance energy loss will be given by the difference in resonance energy between reactant and complex. If it is assumed that the activated complex involved in the urea decomposition possesses resonance energy approximately equal to that of an alkyl isocyanate, i.e., about 8 kcal.28; then the loss in resonance energy should be 41 -8 = 33 kcal.30

The electron spin resonance spectra of four isotopically labeled biradicals have been observed. The hyperfine splittings demonstrate independence of the halves.