Photochemistry and Photophysics of Coordination Compounds

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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
Photochemistry and Photophysics of Coordination Compounds: Chromium 39 and 4 T1g , respectively, with the former level being the more stable. For almost all octahedral Cr(III) complexes studied, the two lowest-lying excited states are the 2 T1g and 2 Eg (doublet) levels of (t2g) 3 electronic configuration, where spin-pairing has occurred within the t2g subshell. A qualitative orbital energy level diagram depicting the electron occupation of the pertinent electronic ground and excited states discussed above is provided in Fig. 1. A representative state energy level diagram for octahedral Cr(III) compounds was presented in Chap. 1 of this volume (Fig. 5b). Consistent with this diagram, for the preponderance of Cr(III) complexes the 2 Eg state has the lower energy of the two doublet excited states [1, 2]. However, exceptions occur occasionally for mixed ligand systems such as trans-[CrN4F2] + of D4h symmetry, where splitting of the 2 T1g (Oh)excitedstate(into 2 A2g and 2 Eg in D4h symmetry) results in its 2 Eg (D4h) component lying lower in energy than the components of the 2 Eg (O h) level [10, 11]. The generic Jablonski diagram shown in Chap. 1 of this volume (Fig. 6) has been modified in Fig. 2 of this chapter for the specific case of octahedral Cr(III) complexes. Fig. 1 The d orbital diagram for the electronic ground and excited states for d 3 Cr(III) complexes assuming Oh symmetry Fig. 2 Jablonski state energy level diagram for Oh Cr(III) complexes, showing the principal processes that may occur subsequent to vertical excitation to a 4 T2g or 4 T1g Franck–Condon excited state. Full arrows represent radiative processes (absorption or emission), whereas wavy arrows designate radiationless deactivation processes
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