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The cis-trans-isomerism of the WITTIG hydrocarbon was investigated in solid state and solution by means of fluorescence spectroscopy. The fluorescence behavior of both isomers in 2-methyltetrahydrofurane was determined as a function of concentration, temperature, and wavelength of exciting radiation. Furthermore, irradiation experiments were undertaken with light of various wavelengths.
The results obtained are in agreement with the assumption that the WITTIG hydrocarbon behaves with regard to the cis-trans-isomerism like a 1,3-butadien derivative, i.e. a thermal but no photochemical cis-trans-isomerisation can be detected. The enthalpy difference between the two isomers was estimated to ΔΗ = 250 ± 50 cal/mole. It could be shown that the fluorescence of the cis-isomer is quenched by the trans-isomer. This quenching occurs probably according to the resonance energy transfer mechanism.
The photochemical cleavage of the endoperoxides of anthradichromene and benzodixanthene into the parent hydrocarbon and oxygen is shown to be an adiabatic photoreaction originating from an upper excited singlet state. This photochemical behaviour is described by a theory for the photochemistry of endoperoxides developed by Kearns and Khan.
The rates of formation of three mono- and three dicarbenium ions of the triphenylmethyl type from the corresponding carbinols in trifluoro acetic acid - toluene mixtures were investigated using stopped flow techniques.
The reaction mechanisms are discussed in view of the obtained dependencies of the rate constants on acid concentration and temperature.
The association of Schlen k’s hydrocarbon was studied by means of osmometric and magnetic measurements. The mixed chain-ring-association can be explained satisfactorily assuming that two different dimers and four monomer species participate in the equilibria, including a monomeric diamagnetic ring. The equilibria existing between the different species are discussed. For the equilibria between the monomer and dimer species, which can be detected in solutions of normal viscosity by means of ESR-measurements, the unexpected values of ΔH=0 for the enthalpie of association and ΔS= +19.7 e. u. for the entropie of association were found.
The influence of temperatur and pressure on the fluorescence quantum yield of N-methylacridone (9,10-dihydro-9-oxo-10-methyl-acridine) in toluene in the range of 283-313 K and 1 bar to 2.5 kbar, respectively, has been investigated. Treatment of the data in terms of the Eyring transition-state theory leads to a consistent interpretation of the observed effect. The unusually large increase of the quantum yield with increasing pressure is attributed to a positive volume of activation, ⊿V≠, for the thermally activated S1-T2 intersystem crossing which is known to be the only deactivation process (of the Si-state) competing with fluorescence. Comparison of the values for ⊿H≠, the activation enthalpy of this process, determined at various pressures, indicates a decrease in ⊿H≠ at elevated pressures. Since ⊿H≠ can be associated with the S1-T2 energy gap involved in intersystem crossing, this result further confirms the conclusion that the change in Franck-Condon factors alone cannot account for the decrease in the intersystem crossing rate with increasing pressure.