Date of Award

5-1976

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Supervisor

J. D. Laposa

Abstract

The fluorescence, and phosphorescence spectra of phenylacetylene-H₆, -D₁, -D₅ and -D₆ as dilute (10⁻³ or less) solutions at 77°K are reported.

Vibrational analyses of the fluorescence and phosphorescence spectra of these compounds in polycrystalline methylcyclohexane indicate that the geometries of the emitting singlet (S₁) and triplet (T₁) states are different. The fluorescence spectra are interpreted in terms of a weak allowed component and a strong forbidden one, the latter based on one quantum of non-totally symmetric b₂ vibrations. The main progression in up to five quanta of the ring breathing fundamental is explained to signify a planar, slightly expanded regular ring geometry of S₁. The phosphorescence spectra show a relatively long progression in the totally symmetric C-C ring stretching mode indicating a planar, non-regular-hexagonal ring in the T₁ state.

Polarization of the fluorescence and phosphorescence spectra of phenylacetylene-H₆ in a rigid glass at 77ºK are used to assign the orbital symmetries of the T₁ state as well as the symmetries of the major bands of these spectra.

The phosphorescence lifetimes of -H₆, -D₁, -D₅ and -D₆ in rigid glasses at 77°K show that deuterium substitution for the single acetylenic hydrogen has almost the same effect as deuterium substitution for all five ring hydrogens in altering the radiationless deactivation of T₁. These result are interpreted on the basis of a current theory of radiationless transitions.

The effect of halo-substitution in the ring on radiative and non-radiative rates is investigated for seven monohalophenylacetylenes. The phosphorescence lifetimes (in 10⁻³ glassy solutions) decrease markedly with increasing mass of the halogen, and the effect is greatest for the para position. Heavy atom perturbation affects the ∫(d values of the S₁ ← S₀ and S₁ ← S₀ absorptions only slightly. Intersystem crossing yields are also found to increase with halogen mass. The vibrational analyses of the phosphorescence spectra of the halophenylacetylenes (in rigid solutions at 77ºK) indicate the presence of two subspectra. Subspectrum I consists of the 0,0 band and bands due to in-plane modes (mainly totally-symmetric) while Subspectrum II arises from out-of-plane modes. Subspectrum II is found to increase relative to Subspectrum I with halogen substitution. Polarization measurements of the main bands of the phosphorescence spectra are interpreted in terms of contributions from several spin-orbit mechanisms.

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