Photochemistry and Photophysics of Coordination Compounds

38 N.A.P. Kane-Maguire
excited state relaxation, energy transfer, and photoactivated redox processes (both intermolecular
and intramolecular). Each of these sections begins with an overview of the
subject area, and then one or more representative papers from the recent literature are
selected for more detailed discussion.
Keywords Energy transfer · Photoredox · Photosubstitution ·
Thermal excited state relaxation · Ultrafast dynamics
1
Introduction
Investigations of the photobehavior of octahedral (O h) or pseudo-octahedral
chromium(III) complexes have played a pivotal role in the development of
transition metal photochemistry as a vital scientific discipline. Except for
the case of ruthenium(II), the photochemistry and photophysics of Cr(III)
systems have been explored more fully than those of any other transition
metal ion. Their photoactivity has been extensively reviewed previously, and
readers are directed to the coverage in three texts [1–3], and the excellent discussion
in the most recent comprehensive review of the topic by Kirk (which
covered the literature up to December 1998) [4]. Several shorter Cr(III) reviews
have since appeared, which have focused on a diverse range of more
specific topics such as the excited state chemistry of pentacyanochromate(III)
anions [5], intermediates in Cr(III) photochemistry [6], emission properties
of hexam(m)ine Cr(III) systems [7], the interaction of [M(diimine)3] n+
complexes of Ru(II) and Cr(III) with DNA [8], and thermal excited state relaxation
[9].
The Cr(III) field remains an active one, and the objective of the present
chapter is to provide an overview of some of the interesting developments in
the area from 1999 to December 2006.
2
State Energy Levels and General Excited State Behavior
The electronic configuration of the Cr 3+ ion is [Ar]3d 3 . In its octahedral (Oh)
complexes the degeneracy of the Cr d orbitals is lifted, resulting in two orbital
subsets of t2g and eg symmetry. The 4 A2g (quartet) ground state has the
electronic configuration (t2g) 3 ,withthed orbitals filled according to Hund’s
Rule. Two excited states with (t2g) 2 (eg) 1 electronic configuration result from
promotion of a t2g electron to an eg orbital while preserving electronic spin.
Six such spin-allowed promotions are possible, which in O h symmetry are
divided into two sets differing in the magnitude of the interelectronic repulsion
terms. The associated quartet excited states generated are labeled 4 T2g