Triplet exciton phenomena in benzene crystals

Citation

Nieman, George Carroll (1965) Triplet exciton phenomena in benzene crystals. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-04172003-162535

Abstract

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Various aspects of the triplet exciton problem are investigated. By studying three-component isotopic mixed crystals of benzene it is shown that trap-to-trap triplet excitation migration is very rapid compared to singlet excitation migration. The closely related phenomenon of triplet-triplet annihilation is also discussed. Triplet-triplet annihilation accounts for the production of delayed fluorescence in molecular crystals and for the quenching of all phosphorescence from pure crystals of n-electron molecules.

By studying the small quasiresonance shifts introduced in the C[subscript 6]H[subscript 6] phosphorescence by changing the degree of deuteration of the benzene host crystal, the nearest neighbor "pairwise" triplet exciton matrix element ([beta]) for benzene is determined to be 12[...]1 cm[superscript -1]. This compares very well with the rough value of 9 cm[superscript -1] determined from excitation transfer studies and is only slightly less than that (18 cm[superscript -1] found for the singlet exciton interaction. These investigations also imply a new interpretation of the factor group splitting of the first singlet-singlet transition in crystalline benzene.

Optical evidence is presented which shows that benzene in its lowest triplet state is not a regular hexagon. Rather detailed phosphorescence spectra of various deuterated benzenes are also presented.

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