Modeling of charge-transfer transitions and excited states in d6 transition metal complexes by DFT techniques

The state of art of the DFT description of charge-transfer electronic excited states of (mostly) d(6) transition metal complexes is presented and discussed. A brief theoretical background places DFT amongst quantum-chemical techniques and discusses the approximations involved. The time-dependent DFT (TD-DFT) treatment of electronic transitions is introduced, with emphasis on the challenges presented by long-range charge separation. Various ways how to characterize excited states in chemically relevant terms are discussed. Several detailed case studies demonstrate how DFT describes charge-transfer excited states of Re-I or Ru-II carbonyl-diimine complexes and interprets their photophysics and photochemistry. This "tutorial" section is followed by an overview of DFT and TD-DFT applications to electronic spectroscopy and excited-state properties of metal carbonyls, strongly emissive organometallics, Ru-II photosensitizers, luminescent "light-switches" and isonitrile complexes of Re-I and Ru-II. Effects of the computational procedure on the quality of the results and the type of information obtained are emphasized. It follows that the most accurate charge-transfer transition energies and descriptions of excited states of low-valent d6 metal complexes are obtained when using hybrid functionals and calculating the molecule in the actual solvent. A rather delocalized picture of charge-transfer states of these complexes emerges, whereby the electron density is excited from the metal atom and part of its coordination sphere to the electron-accepting ligand. (c) 2006 Elsevier B.V. All rights reserved.