Phototochemical and photophysical studies of aryl isocyanide complexes of rhenium(I) and ruthenium(II)
Abstract
Previous studies of aryl isocyanide complexes of tungsten and chromium have shown the possibility of an associative mechanism for photosubstitution. This was based on the very low quantum yield seen and the lack of spectral evidence for any excited states except those based on metal-to-ligand charge transfer (MLCT) transitions. The study discussed in this dissertation tests this proposition for the isocyanide complexes of rhenium (I) and ruthenium (II). The rhenium (I) aryl isocyanide complexes were irradiated in nonaqueous solutions of methylene chloride containing 10 equivalents of either chloride, bromide or iodide ion. The quantum yields obtained for these systems were 0.30, 0.24 and 0.21 respectively. In all three cases clean photochemistry was seen, as shown by the presence of isosbestic points throughout the reactions. The very high quantum yields obtained, however, indicate that the substitution in the rhenium complexes is due to the presence of a metal centered (MC) state that is thermally populated from the spectrally observed MLCT state. To further test this, complexes of ruthenium (II) were also studied. The more highly charged ruthenium (II) complexes were chosen in an attempt to isolate the metal centered states, by raising the energy of the MLCT transitions. The photophysical studies, however, indicated that the LEES is ligand centered (LC). This complicated the picture, since the photochemistry seen for the complexes could be due to either a LC state or to a MC state thermally populated from a lower lying LC state. The complex photochemistry seen for the complexes led to the development of a computer program for the computation of photochemical quantum yield without the isolation of the product spectrum. This program was shown to give accurate results for both quantum yield and predicted product spectrum. The program was run in combination with other photophysical studies to obtain rate constants for the decay pathways in the rhenium (I) complexes. The unusual combination of a very low radiative rate constant and a high lifetime led to the postulation of the presence of a twisted intramolecular charge transfer (TICT) state in these complexes.
Degree
Ph.D.
Advisors
McMillin, Purdue University.
Subject Area
Chemistry
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