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Primary photophysical processes in hexafluorobiacetyl McIntosh, John Samuel Edward

Abstract

The phosphorescence and fluorescence quantum yields and spectra of hexafluorobiacetyl vapour have been studied at various exciting wavelengths between 290 and 440 nm. The relative spectral distribution of each luminescence is independent of pressure over the range 0.2 - 700 torr, but the phosphorescence-fluorescence ratio is a function of pressure and of exciting energy. Absolute emission quantum yield determinations at 250 torr of hexafluorobiacetyl are used to place, on an absolute basis, relative yields obtained as a function of pressure and exciting energy. The high pressure limiting yields are Ф°°[subscript f] = 0.078 and Ф°°[subscript f]= 4.9 x 10⁻³ The lifetime of the lowest triplet state is independent of pressure and temperature (over the range 27 to -57°C). However it is found to vary somewhat with absorbed intensity, indicating triplet-triplet annihilation occurs at high concentrations. From a combined first and second order kinetic treatment of the data, the first-order decay constant was found to be 476 ± 24 sec⁻¹. Electronic energy transfer between systems consisting of hexafluorobiacetyl-oxygen, hexafluorobiacetyl-hexafluoro-acetone and hexafluorobiacetyl-biacetyl were also investigated. In all but the first system, hexafluorobiacetyl acts as the energy acceptor. Bimolecular rate constants for energy transfer have been evaluated from Stern-Volmer treatments of the data. The rate constant for quenching of the triplet state of hexafluorobiacetyl by oxygen is (7.2 ± 1.4) x 10⁷ liters mole⁻¹ sec⁻¹. In the hexafluorobiacetyl-hexafluoro- acetone system, the rate constant for triplet-triplet energy transfer is (8.5 ± 1.5) x 10⁸ liter mole⁻¹ sec⁻¹ and for singlet-singlet electronic transfer is (6.0 ± 0.6) x 10⁹ liter mole⁻¹ sec⁻¹. Hexafluorobiacetyl was found to quench the biacetyl triplet state rather inefficiently, with a specific rate constant of (2.4 ± 0.5) x 10⁶ liters mole⁻¹ sec⁻¹. The data from the various investigations are combined to give a description of the primary photophysical events. From this information a mechanism for the primary process in hexafluorobiacetyl is proposed and estimates of the specific rate constants for some of the photophysical processes are given.

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