Photochemistry and Photophysics of Coordination Compounds

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124 S. Campagna et al. 3.2 Deactivation of Upper Excited States As mentioned in the Introduction, the upper excited states of transition metal complexes usually undergo radiationless deactivation to the lowest excited state. For [Ru(bpy)3] 2+ the lowest excited state (or, better, the cluster of lowest excited states) is relatively long livedm and its formation and disappearance can be easily monitored by flash spectroscopy and luminescence decay. The absorption spectrum of the lowest excited state is shown in Fig. 6 [36–39]. Fig. 6 Electronic absorption spectrum of the lowest excited state of [Ru(bpy)3] 2+ in alcoholic solution The risetime of the lowest excited state upon excitation of spin-allowed excited states was initially estimated to be ≪ 1 ns for [Ru(bpy)3] 2+ [40, 41]; successively, available ultrafast spectroscopic techniques demonstrated that intersystem crossing occurs in the subpicosecond timescale (see later). The efficiency of formation of the lowest excited state, Φ( 3 MLCT) (and thus, the efficiency of intersystem crossing from the upper singlets obtained by excitation to the lowest triplet, ηisc), is essentially unity [36, 37, 42–44]. 3.3 Emission Properties Excitation of [Ru(bpy)3] 2+ in any of its absorption bands leads to a luminescence emission (Fig. 7) whose intensity, lifetime, and energy position are more or less temperature dependent. Detailed studies on the temperature dependence [1, 4, 6, 45–49] of the luminescence lifetime and quantum yield in the temperature range 2–70 K showed that luminescence originates from a set of three closely spaced levels (∆E, 10, and 61 cm –1 )inther-
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 125 Fig. 7 Emission spectrum of ∗ [Ru(bpy)3] 2+ in alcoholic solution at 77 K(solid line) and at room temperature (dashed line) mal equilibrium. This cluster of luminescent, closely spaced excited states will be indicated in the following by ∗ [Ru(bpy)3] 2+ or as the 3MLCT state. ∗ [Ru(bpy)3] 2+ has a substantial triplet character and a single ligand localized excitation. In rigid glass at 77 K the emission lifetime of ∗ [Ru(bpy)3] 2+ is ∼ 5 µs and the emission quantum yield is ∼ 0.4 [1, 6, 8]. Taken together with the unitary intersystem crossing efficiency, these figures yield a value of ∼ 13 µs for the radiative lifetime. Values of this order of magnitude have been found for MLCT excited states of other transition metal complexes [50–53]. LC excited states of transition metal complexes usually exhibit radiative lifetimes in the millisecond range [1, 6, 49, 50, 53–60]. Fig. 8 Temperature dependence of the emission lifetime of ∗ [Ru(bpy)3] 2+ in nitrile solution
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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258 Author Index Volumes 251-280 Be
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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