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text Brown, Theodore L. McCusker, James Kenneth 2011-05-07T12:29:07Z 2011-05-07T12:29:07Z 10000-01-01 1992 The kinetics of the $\Delta$S = 2 spin-state interconversion in d$\sp6$ transition metal complexes are examined. A series of compounds having a polypyridyl-based ligand framework are described which allow for systematic modification of molecular geometry. A nanosecond time-resolved laser spectrometer was constructed to measure the kinetics of spin-state interconversion as a function of temperature in an attempt to probe the molecular mechanism of $\rm\sp5T\sb2$ $\to$ $\rm\sp1A\sb1$ relaxation in Fe$\sp{\rm II}$ complexes. The data establish an empirical correlation between the flexibility of the ligand framework and the activation energy for the process, suggesting a smaller barrier to spin-state interconversion for those molecules having a greater tendency to distort along torsional coordinates. In addition, a correlation is noted that suggests conformational preferences of the ligand structure may influence the intrinsic rate of spin-state interconversion by modulating the extent to which a system proceeds along the reaction coordinate. The importance of torsional modes in controlling the kinetics of $\rm\sp5T\sb2$ $\to$ $\rm\sp1A\sb1$ relaxation is further supported by theoretical analyses. Fitting of variable-temperature kinetic data to classical, semi-classical, and quantum-mechanical theories of electron transfer give identical results, indicating that the quantum nature of the vibrational mode coupled to spin-state interconversion does not manifest itself in the 160-300 K range. This observation is inconsistent with coupling to a metal-ligand stretching mode and strongly suggests the involvement of low-frequency mode(s) $($10$\sp{12}$ s$\sp{-1}$ is interpreted in terms of direct $\rm\sp1MLCT$ $\to$ $\rm\sp5T\sb2$ conversion from the Franck-Condon state following excitation. The excited-state dynamics of several Co$\sp{\rm III}$ complexes are also discussed. The biphasic kinetics observed are attributed to decay from singlet and quintet ligand-field states, suggesting a $\sp1$LMCT state lifetime of $<$1 ps and internal conversion rates that are kinetically competitive with intersystem crossing. Made available in DSpace on 2011-05-07T12:29:07Z (GMT). No. of bitstreams: 2 license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5) 9305618.pdf: 18087581 bytes, checksum: 035668f3580715186a4907feabc19b50 (MD5) Previous issue date: 1992 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding (hding2@illinois.edu) on 2011-05-07T14:41:38Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:18:02-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: ETDs are only available to UIUC Users without author permission ETDs are only available to UIUC Users without author permission U of I Only AAI9305618 (UMI)AAI9305618 http://hdl.handle.net/2142/20110 eng Copyright 1992 McCusker, James Kenneth Chemistry, Inorganic Chemistry, Physical Dynamics and mechanism of spin-state interconversion in transition metal complexes. (Volumes I and II) Chemistry Chemistry University of Illinois at Urbana-Champaign Dissertation Ph.D.