Browsing by Author "Williams, John O."

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Acridine: an investigation of its molecular and crystalline photophysical behaviour
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (03), 1977) Williams, John O.; Clarke, Brian P.
The absorption spectrum of acridine in various solvents together with the fluorescence spectra and lifetimes in a rigid matrix confirms the presence of a weak, intrinsic π–π* emission in this material with its origin at 26 300 cm–1 akin to a similar emission in its aromatic analogue anthracene. The measured “molecular” acridine lifetime is ∼ 8 ns at 298 K. The temperature dependence of the emission spectra and lifetimes of several crystallographic modifications of acridine are also reported. The properties of acridine V are characteristic of a “monomeric” structure whereas acridine III, known to possess discrete, antiparallel molecular pairs, exhibits “excimeric” emission. Two different acridine molecular pairs comprising the acridine II structure also display excimeric emission with their respective maxima appreciably red-shifted from the molecular emission level, and exhibiting different polarisation behaviour. Triplet states are believed to offer facile non-radiative pathways following thermal activation from both “monomeric” and “excimeric” crystalline states.
Ionic Conduction in Ammonium Perchlorate Doped with Divalent Ions
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (11), 1974) Owen, Gari P.; Thomas, John M.; Williams, John O.
D.c. electrical measurements are reported for sulphate-ion-doped and barium-ion-doped ammonium perchlorate. The extension of an impurity-dominated regime in sulphate-ion-doped ammonium perchlorate, and the fact that barium-ion-doped ammonium perchlorate displays no marked difference in electrical behaviour from that of the “pure” salt strongly suggests that the conduction process in ammonium perchlorate is controlled by interstitial ammonium ions. Measurements of surface conduction reveal no evidence for proton conduction at or near the surface where such a process would be expected to dominate via sublimation. The dominant point defect structure deduced from electrical measurements, i.e. interstitial NH+4 ions would seem to favour electron transfer from ClO–4 to NH+4 ions as the first step in the thermal decomposition of the solid, since this mechanism requires the existence of interstitial NH+4 ions.
Luminescence of 9-cyanoanthracene. Identification of new excimeric species
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (06), 1978) Ebeid, El-Zeiny M.; Morsi, Salah E.; El-Bayoumi, M. Ashraf; Williams, John O.
From studies of the emission spectra of 9-cyanoanthracene (9 CNA) solutions, glasses, polycrystalline films and single crystals at 298 and 77 K a centrosymmetric excimeric species with emission maximum at ≈ 490 nm has been observed. The presence of “incipient-dimer” pairs giving rise to such emission has been confirmed by investigations of the photolytic and thermal cleavage of the photodimer in 9-cyanoanthracene single crystals and polycrystalline matrices. Luminescence decay time measurements substantiate our views. The emission spectrum of 9 CNA single crystals is asymmetric, with a maximum at 490 nm at room temperature, but upon cooling to 77 K the peak becomes symmetrical and occurs at lower energy. The energy difference of ≈ 2000 cm–1 is appreciably higher than the value predicted on the basis of the “frequency effect” and often used to explain the temperature shift of an excimeric emission in both “pair” and “stack” structures. At the higher temperatures in 9 CNA single crystals at least two excimeric forms exist in thermal equilibrium but at the lower temperatures emission occurs from the mirror-symmetric form.