Electronic structure and excited state dynamics of chromium (III) complexes

Interest in fundamental aspects of transition metal photophysics and photochemistry stems from potential application of such systems to technologies such as solar cells, photocatalysts and molecular machines. Chromium(III) offers a convenient platform for the fundamental study of transition metal photophysics due to its relatively simple ligand-field electronic structure. The work presented in this dissertation deals with understanding the ground and excited state electronic structure and dynamics of chromium(III) complexes, ranging from high-symmetry derivatives of tris(acetylacetonato)chromium(III) (Cr(acac)3) to low symmetry chromium-semiquinone complexes of the form [(tren)Cr(III)-SQ]+2 (where tren is tris(2-aminoethyl)amine, a tetradentate amine capping ligand enabling only one moity of the orthosemiquinone (SQ) to chelate to the chromium(III) ion). This effort can be thought of in terms of building up the additional interactions (lowered symmetry and spin exchange) in a piecewise fashion by first considering the electronic structure and dynamics of high-symmetry systems, then lowering the symmetry while maintaining the quartet spin nature of the high-symmetry system by studying the chromium(III)-catechol systems. Finally, spin exchange can be introduced via the chromium(III)-semiquinone system. In general, these complexes represent dramatic changes from the high-symmetry complexes in several ways: 1) the local symmetry of the chromium(III) ion is reduced from high-symmetry, pseudo-octahedral ligation to a C2v- like N4O2 coordination, effectively breaking the degeneracy of the ligand field T and E states; 2) unpaired spin of the semiquinone ligand interacts via Heisenberg spin-exchange with the unpaired spins of the chromium(III) ion, resulting in substantial changes in the absorption spectrum indicative of radically different electronic structure of both the ground and excited states. Studies of the excited-state dynamics were first carried out on derivatives of the archetypal complex Cr(acac)3 to gain an understanding of correlations between electronic structure, geometry, and excited state dynamics. These studies revealed an empirical correlation between low-frequency modes of the molecule and the rate of ultrafast intersystem crossing in the ligand field manifold. Efforts on the lower symmetry catechol and semiquinone complexes are focused mainly on synthesis and characterization of the electronic structure. The ground states of these systems are characterized primarily using electron paramagnetic resonance techniques, revealing the rich nature of these spin systems. For these studies, gallium(III)-semiquinones are employed as a structural analog to study spin density distribution in the absence of the chromium(III) ion. The concepts learned from these studies provide a useful backdrop to the eventual study of the excited state dynamics of the aforementioned chromium(III)-catechol and -semiquinone complexes.