Ultrafast dynamics of ligand-field excited states

Ultrafast time-resolved spectroscopic methods have been used to probe the dynamics associated with ligand-field excited states in complexes of Cr-III and Fe-II. In the former case, Cr(acac)(3) (where acac is the monodeprotonated form of acetylacetonate) serves as a prototype for studying the T-4(2) -> E-2 conversion endemic to complexes of this ion. The data reveal that formation of the 2 E state occurs faster than the instrumental time resolution of ca. 100 fs, implying a rate constant for intersystem crossing of k(ISC) > 10(13) s(-1). Subtle changes observed in the differential absorption spectra were attributed to vibrational cooling in the E-2 state having a time constant of 1.1 +/- 0.1 ps. The role of low-lying ligand-field states as highly efficient deactivation pathways for higher-lying charge-transfer states was demonstrated in low-spin Fe-II polypyridyl complexes. Following (1)A(1) -> (MLCT)-M-1 excitation, the ligand-field manifold is accessed with a time constant of similar to 100 fs. This pattern was observed for several different complexes, suggesting it is a general feature of this class of molecules. Dynamics subsequent to populating the ligand-field manifold are ill-defined at present but are presumed to consist of a barrierless evolution through a series of highly mixed ligand-field states, ultimately leading to the formation of the T-5(2) state as the lowest-energy excited state on the sub-picosecond time scale. This observation has important implications for the use of such complexes as sensitizers in photovoltaic applications. (c) 2006 Elsevier B.V. All rights reserved.