M. S. de Groot

Paramagnetic resonance in phosphorescent aromatic hydrocarbons

Electron paramagnetic resonance spectra are given of glassy solutions of triphenylene, 1, 3, 5-triphenylbenzene, coronene and naphthalene excited into their lowest triplet state by ultra-violet irradiation at liquid-nitrogen temperature. For each substance two spectra were measured: one with the r.f. magnetic field parallel to the constant field, the other with the fields at right angles. A method is suggested for analysing the spectra of those molecules having a trigonal symmetry axis and values of the zero-field splitting parameter are derived. A complete treatment for molecules of lower symmetry has not been given, but it is shown that our results on a glassy solution of naphthalene are in agreement with those obtained by Hutchison and Mangum with single crystals.

Paramagnetic resonance in phosphorescent aromatic hydrocarbons. I: Naphthalene

Recently Hutchison and Mangum have observed electron spin resonance in the lowest triplet state of naphthalene. An analysis of their results is given on the basis of spin-spin interaction between the two unpaired electrons. In addition to the lines observed by these authors another transition, which corresponds to Δm = ±2 in the atomic case, must occur at low field strength. This transition has been observed in a mixed crystal as well as in rigid glass solution.

Vibronic interaction in the lower electronic states of benzene: I. Review of the static problem and solution of the pseudo-cylindrical vibronic equations

In the present paper the pseudo Jahn-Teller interaction between the four lower electronic states of benzene is considered, with special reference to the mixing of the 3 B 1u and 3 E 1u states through the e 2g carbon-carbon stretching mode. First a review is given of the pioneering work of Liehr on the static problem, where the ‘electronic Hamiltonian’ is solved as a function of fixed nuclear coordinates. Results of a specific calculation are presented. Subsequently, the equations for the vibronic problem are set up by generalizing the methods proposed by Moffitt et al. and Longuet-Higgins et al. for the dynamic Jahn-Teller effect. These equations are then solved in the first (‘pseudo-cylindrical’) approximation in which only the strong coupling linear in the displacements between the B 1u and E 1u states is considered. The results are discussed with reference to known properties of the phosphorescent state of benzene.

Phosphorescence and spin polarization

Intersystem-crossing in aromatic molecules is caused by spin-orbit coupling. This mechanism is expected to be highly selective for a specific spin-component of each triplet state. Experimental evidence is given for quinoxaline proving that spin polarization indeed occurs when this molecule is optically excited into its lowest triplet state. A remarkable coherence phenomenon, reminiscent of level-crossing situations in atomic spectroscopy, seems to be observed following flash excitation in a magnetic field.

Optical pumping in an organic crystal: quinoxaline in durene

Experiments have been performed to determine the path of entry into and exit from the phosphorescent triplet state T 0 of quinoxaline in a durene host. First of all the decay of phosphorescence after flash excitation was followed at 4·2 and 1·34°k. It was found that for both perdeutero- and perhydroquinoxaline the lifetime is shortened by a factor of about three when the temperature is lowered from 4·2 to 1·34°k. At 1·34°k relaxation between the spin components (i.e. re-orientation of the triplet spin angular momentum) is slow relative to the decay, and the observed reduction in lifetime indicates that entry into and exit from T 0 are through the same spin component. Similar decay experiments were then carried out at 1·34°k in a 10 kg magnetic field or in a somewhat weaker field so chosen that the effect of microwave saturation of one of the E.S.R. transitions between the components could be observed. From the results it follows that on intersystem crossing the molecules enter the manifold T 0 through the...

The pressure broadening of the depolarized Rayleigh line in pure gases of linear molecules

Abstract The pressure broadening of the depolarized Rayleigh line at room temperature is investigated for the gases normal H2, para H2, HD, normal D2, N2, CO, CO2 and OCS. It is shown that the line shape associated with the broadening of the depolarized Rayleigh line deviates in general from the lorentzian profile. This deviation is attributed to a dependence of the molecular re-orientation probability on the rotational state. The results of these experiments appear to be useful for testing general kinetic theories which have been used for the description of phenomena such as, e.g., the Senftleben-Beenakker effect for the viscosity and nuclear magnetic relaxation.

The effect of deuterium and chlorine substitution on triplet → singlet transition probabilities in naphthalene

The effect of substitution in aromatic molecules on the radiative transition probability from the triplet phosphores cent state (T) to the ground (S/sup 0/) state, was measured by irradiating a glassy solution of the phosphorescent substance, at 77 deg K, in a microwave cavity. The relative concentration of molecules in their triplet state was determined from the (integrated) intensity of the magnetic resonance signal. After the exciting light was switched off the corresponding phosphorescence intensity and decay time were measured by means of a photomultiplier and a recording instrument. Preliminary results on naphthalene, deuteronaphthalene, and beta - chloronaphthalene are tabulated. (L.N.N.)

Paramagnetic resonance of phosphorescent tetramethylpyrazine

Electron paramagnetic resonance spectra have been obtained of tetramethylpyrazine in its lowest (ππ*) triplet state in several rigid matrices. Some unusual observations were made : 1. (1) In a durene crystal host the in-plane zero-field principal axes are not parallel to the molecular symmetry axes. 2. (2) In rigid glass solution the zero-field parameters are strongly dependent on solvent polarity. Both effects are discussed in terms of pseudo Jahn-Teller interaction in conjunction with a crystal-field influence. In a durene matrix the zero-field parameters are The relatively small value of , as compared with that of the corresponding aromatic hydrocarbon, probably results from vibronic mixing of the lowest 3ππ* with the near-by 3nπ* state. Hyperfine structure was observed during measurements at 1·95 K ; it is in agreement with the ππ* character of the lowest triplet state and is due to coupling between the unpaired electrons and protons in the non-rotating methyl groups.

The two-quantum transition in the electron resonance spectrum of phosphorescent aromatic hydrocarbons

Synopsis The simultaneous absorption of two r.f. quanta has been observed in the electron resonance spectrum of phenanthrene in its lowest triplet (phosphorescent) state. An expression is derived which relates the corresponding magnetic field for resonance to the mean-square zero-field splitting parameter D*. The value obtained for D* agrees well with those obtained by a detailed analysis of the spectrum.

The study of aldehyde photochemistry with the spectrophone technique

Abstract Measurements with a spectrophone-type system have provided evidence for the occurrence of a hitherto unsuspected reactive intermediate in the gas-phase photochemistry of aldehydes.