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Subject: search.filters.subject.530Start date: 1966End date: 1979Author: search.filters.author.Schweitzer, Dieter

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Now showing 1 - 10 of 26
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    Kinetics of the triplet state of 2,3-dichloroquinoxaline from microwave-induced phosphorescence transients
    (1973) Schweitzer, Dieter; Zuclich, Joseph A.; Maki, August H.
    The phosphorescent state of 2,3-dichloroquinoxaline doped into single crystals of durene and 1,2,4,5-tetrachlorobenzene has been studied using several methods based upon optical detection of magnetic resonance (ODMR). Flash excitation and continuous optical pumping methods are described and analysed. Phosphorescent transient effects caused by spin-lattice relaxation and variable intersystem crossing rates are observed and described using first-order solutions of the appropriate rate equations. The effects of spatial polarization of the phosphorescence (anisotropic spatial distribution of phosphorescence intensity) of single crystals on ODMR signals is observed and discussed. The spatial polarization of emission can cause difficulties in determining relative radiative rate constants of the triplet sublevels in oriented samples, but can yield information about the linear polarization of the emission analogous to that obtained by conventional means using polarizers.
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    Nuclear magnetic shielding tensors for 1H, 13C, and 15N in organic solids
    (1975) Spiess, Hans Wolfgang; Haeberlen, Ulrich; Kempf, Jürgen; Schweitzer, Dieter
    A survey will be given of nuclear magnetic shielding tensors obtained by multiple pulse techniques for 1H in carboxylic acids and by high field NMR for 13C in carbonyl- and carboxyl groups of aromatic compounds and for 15N in pyridine and nitrobenzene.
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    13C nuclear magnetic relaxation studies at 62 MHz
    (1971) Jaeckle, H.; Haeberlen, Ulrich; Schweitzer, Dieter
    An apparatus suitable to measure 13C relaxation times T1 and T2 in liquids at 62 MHz is described. The required field of 58 kG is generated by a superconducting magnet. Results of T1 measurements on a variety of liquids, including benzene, derivatives of benzene, saturated rings, CS2, and others are reported. They are discussed in terms of dipole-dipole interactions, spin-rotation interactions and anisotropic chemical shifts, the latter of which turn out to play only a minor role. Intramolecular dipole-dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism, whenever protons are bound directly to the carbons under investigation, even when the samples contain dissolved oxygen.
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    Wavelength dependent triplet state populating mechanisms of naphthalene in heavy atom hosts
    (1973) Schweitzer, Dieter; Zuclich, Joseph A.; Maki, August H.
    We have made zero-field optically-detected magnetic resonance measurements on the triplet state of naphthalene-d8 incorporated in the heavy-atom host crystals p-dichlorobenzene, p-dibromobenzene, and s-tetrachlorobenzene. The dependence of the relative triplet sublevel populating rates on the exciting wavelength is interpreted in terms of the electronic excitation transfer mechanisms. We find that upon excitation of the host S1 ← S0 transition in p-dichlorobenzene, energy transfer to the guest T1 takes place via the guest S1, whereas in the other host crystals the intermediate is mainly the host T1 band. Upon host T1 ← S0 excitaton, trapping takes place from a spin-aligned excitor band in s-tetrachlorobenzene, whereas the initial alignment is lost prior to trapping in the dihalobenzene hosts.
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    Electronic properties of the triplet state of fluorene, carbazole, dibenzofuran and dibenzothiophene(X-traps)
    (1979) Goldacker, Wilfried; Schweitzer, Dieter; Zimmermann, Herbert
    The triplet state of X-traps in neat single crystals of fluorene, carbazole, dibenzofuran and dibenzothiophene was investigated bv the method of ODMR in zero field at 1.3 K. In addition to the intersystem crossing rates, steady state populations, decay rate constants and relative radiative rate constants of the sublevels, the spin-lattice relaxation (SLR)-rates were measured, which could not be neglected even at this low temperature. In the case of dibenzothiophene a strong intramolecular heavy-atom effect due to the sulphur atom was observed which allowed direct So → T1 excitation.
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    Energy relaxation and intervalley relaxation of hot electrons in n-type germanium
    (1966) Seeger, Karlheinz; Schweitzer, Dieter
    The average current density in n-type germanium subjected to superimposed strong ac and weak dc electric fields was both measured and calculated for frequencies of 9 and 35 GHz. Repopulation relaxation of the conduction band valleys and energy relaxation were taken into account. Experimental and theoretical result agree qualitatively.
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    Transanular interaction in [2.2]phanes: models for dimers?
    (1979) Goldacker, Wilfried; Hausser, Karl H.; Schweitzer, Dieter; Staab, Heinz A.
    The emission spectra of pseudo-ortho 3 and pseudo-geminal-4,7,12,15-tetramethoxy[2.2]-paracyclophane 3 were measured in glass matrices at 1.3 K. Furthermore, the zero field splitting parameters D and E and the decay rate constants ki of the excited triplet state were studied by optical detection of magnetic resonance in zero field. The results were compared with the corresponding monomer 1,4-dimethyl-2,5-dimethoxybenzene.
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    Investigation of the excited triplet states of [2.2]- and [3.3]paracyclophane
    (1979) Melzer, Guenter; Schweitzer, Dieter; Hausser, Karl H.; Colpa, Johannes Pieter; Haenel, Matthias
    We have studied the excited triplet states of [2-2]- and [3.3]paracyclophane by emission spectroscopy and by ODMR and have compared the results with the corresponding monomer paraxylene. The most striking result is that both phanes exhibit two sets of zero field splitting parameters indicating the existence of two nearly degenerate low lying excited triplet states. Furthermore, we find in distinction to expection a stronger reduction of the zero field splitting parameters as compared to the monomers for [3.3]paracyclophane than for [2.2]paracyclophane. The causes of these surprising experimental findings are discussed.
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    Optically detected magnetic resonance of the tryptophan phosphorescent state in native proteins
    (1973) Zuclich, Joseph A.; Schweitzer, Dieter; Maki, August H.
    The phosphorescent triplet state of tryptophan has been studied by the method of optically detected magnetic resonance (ODMR) at pumped helium temperatures in zero magnetic field. Only one of the triplet sublevels is found to be significantly radiative; the other two decay radiationlessly. Although the phosphorescence and ODMR decay lifetimes are influenced by spin-Ianice relaxation processes at T = 1·3°K. the lifetime of the radiative level can be estimated as approximately 2 s, whereas the lifetimes of the non radiative levels are in excess of 10 s. Comparison of the ODMR signals and the phosphorescence spectra has been made for tryptophans in native proteins with the following results: the ODMR signals of the two types of tryptophan sites in horse liver alcohol dehydrogenase can be resolved due to a shift in the D and E values of the respective triplet states; binding of the substrate tri-N-acetylglucosamine to hen lysozyme leads to a considerable narrowing of the phosphorescence peaks and ODMR signals as well as to a shift in the E value oflhe triplet state. The following tentative conclusions can be reached: the tryptophan triplet D and E values are measurably affected by the environment of the chromophore in the protein, as are the linewidths of the magnetic resonance transitions. The IEI value is reduced and the magnetic resonance linewidth is increased with increasing exposure of the tryptophan to hydroxylic solvent. Although a considerable part of the width of the magnetic resonance transition can be ascribed to a heterogeneity of environments in the sample, there appears to exist an intrinsic line-broadening process which at present is not understood.
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    ODMR of phanes and charge transfer phanes
    (1979) Schweitzer, Dieter; Goldacker, Wilfried; Hausser, Karl H.; Staab, Heinz A.
    Triplet zero field splitted parameters |D| and |E| and decay rates ki of the triplet sublevels of two stereoisomeric tetramethoxy [2.2] paracyclophanes as well as those of two CT [2.2] paracyclophanes in low concentration in glasses and small neat single ctystals as measured by ODMR in zero fields at 1,3 K are presented.