Dynamics and mechanism of spin-state interconversion in transition metal complexes. (Volumes I and II)

Abstract

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.