Photochemistry and Photophysics of Coordination Compounds

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132 S. Campagna et al. whose potential E 0 (Y/Y – )ismorenegativethan–0.86 V (see also Fig. 10). [Ru(bpy)3] + +X→ ∗ [Ru(bpy)3] 2+ +X – (11) [Ru(bpy)3] 3+ +Y – → ∗ [Ru(bpy)3] 2+ +Y (12) ∗ [Ru(bpy)3] 2+ → [Ru(bpy)3] 2+ + hν . (13) A variety of oxidants (e.g., S2O8 2– [125, 126]) and reductants (e.g., e – aq [127], hydrazine and hydroxyl anion [128], oxalate ion [129, 130]) have been used in these chemiluminescent processes. In some cases (e.g., with OH – ), the reaction mechanism cannot be a simple outer sphere electron transfer reaction and the emitting species could be a slightly modified (on the ligands) complex. It should also be pointed out that minor amounts of oxidizing and reducing impurities are sufficient to produce luminescence in chemiluminescence and electrochemiluminescence experiments [131]. The most interesting way [132] to obtain chemiluminescence from [Ru(bpy)3] 2+ solutions is probably to produce the oxidized and/or reduced form of the complex “in situ” by electrochemical methods. Three classical experiments of this type can be performed: (a) To pulse the potential applied to a working electrode between the oxidation and reduction potentials of [Ru(bpy)3] 2+ in a suitable solvent [132, 133]. In such a way the reduced and oxidized forms produced in the same region of space can undergo a comproportionation reaction where enough energy is available to produce an excited state and a ground state (see also Fig. 10): [Ru(bpy)3] 2+ +e – → [Ru(bpy)3] + (14) [Ru(bpy)3] 2+ –e – → [Ru(bpy)3] 3+ (15) [Ru(bpy)3] 3+ + [Ru(bpy)3] + → ∗ [Ru(bpy)3] 2+ + [Ru(bpy)3] 2+ . (16) (b) To reduce [Ru(bpy)3] 2+ in the presence of a strong oxidant (reductive oxidation). For example, luminescence is obtained upon continuous reduction of [Ru(bpy)3] 2+ at a working electrode in the presence of S2O8 2– [125, 126]. This oxidant in a first one-electron oxidation reaction generates the very powerful oxidant SO4 – that can either oxidize [Ru(bpy)3] + to ∗ [Ru(bpy)3] 2+ (Eq. 18) or [Ru(bpy)3] 2+ to [Ru(bpy)3] 3+ (Eq. 19), which then reacts with [Ru(bpy)3] + (Eq. 16) to yield the luminescent excited state: [Ru(bpy)3] 2+ +e – → [Ru(bpy)3] + (14) [Ru(bpy)3] + +S2O8 2– → [Ru(bpy)3] 2+ +SO4 – +SO4 2– (17) [Ru(bpy)3] + +SO4 – → ∗ [Ru(bpy)3] 2+ +SO4 2– (18) [Ru(bpy)3] 2+ +SO4 – → [Ru(bpy)3] 3+ +SO4 2– (19) [Ru(bpy)3] + + [Ru(bpy)3] 3+ → ∗ [Ru(bpy)3] 2+ + [Ru(bpy)3] 2+ . (16)
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 133 (c) To oxidize [Ru(bpy)3] 2+ in the presence of a strong reductant (oxidative reduction). For example, light is generated upon continuous oxidation of [Ru(bpy)3] 2+ at a working electrode in the presence of C2O4 2– [129, 130]. This reductant in a first one-electron reaction generates the strongly reducing CO2 – radical that can reduce [Ru(bpy)3] 3+ to the excited ∗ [Ru(bpy)3] 2+ [Ru(bpy)3] 2+ –e – → [Ru(bpy)3] 3+ (15) [Ru(bpy)3] 3+ +C2O4 2– → [Ru(bpy)3] 2+ +CO2 +CO2 – (20) [Ru(bpy)3] 3+ +CO2 – → ∗ [Ru(bpy)3] 2+ +CO2 . (21) These chemiluminescent electron transfer reactions are quite interesting from an applicative [134–136] as well as from a theoretical viewpoint. Actually, method a is at the basis of electroluminescent materials, such as organic light-emitting diodes (OLEDs) and similar devices, which are receiving increasing interest for practical applications [137–141]. 4 Some Important Features of Ru(II) Polypyridine Complexes 4.1 Nonradiative Decay Rate Constants and Emission Spectral Profiles of Ru(II) Polypyridine Complexes Radiationless decay from MLCT states of metal polypyridine complexes occurs with energy release into medium-frequency (polypyridyl-based) modes and, to a lower degree, low-frequency modes and solvent [4, 142–149]. Averaging the medium-frequency modes which mainly promote the transition and combining low-frequency modes, including solvent, into a single mode, treated classically, the rate constant for radiationless decay knr is predicted to follow the so-called energy gap law [150–154]. Most of the work to define this topic has been made by using Ru(II) polypyridine complexes as models; however, the approach also applies to any MLCT emitter, as largely demonstrated for Os(II) [146, 147, 155] and Re(I) polypyridine [147, 149, 156] complexes. Actually, the energy gap law can be expressed by Eq. 22, where β0 includes the vibrationally induced electronic matrix element and F(calc) is the vibrational overlap factor (the quantity 1s in Eq. 22 is used to give unitless expression): ln(knr · 1s)=lnβ0 +ln[F(calc)] . (22) In a simplified version, F(calc) can be expressed as in Eq. 23 [157]: � � – γ E0 F(calc) ∝ �ω � � E0 γ =ln –1. SM�ω (23)
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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