Photochemistry and Photophysics of Coordination Compounds

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134 S. Campagna et al. In Eq. 23, the energy gap E0 is related to the energy separation of the two coupled surfaces the �ω term describes, assuming a single configurational coordinate model, the average energy of the medium-frequency vibrational (accepting) modes that couple the MLCT and ground states, that is, the vibrational spacing of the ground state, and SM is the electron-vibrational coupling constant (Huang–Rhys factor). E0, �ω, andSM are expressed in cm –1 ,aswell as ∆ν1/2, the “classical” bandwidth that takes into account the low-frequency modes and is present in the detailed expression for F(calc) [146, 147], not shown here. If β0, the electronic term, remains roughly constant for a series of related complexes, Eq. 22 yields a straight line with intercept ln β0 [146]. It is interesting to note that the parameters E0, �ω, SM, and∆ν1/2 also define the emission spectral profile, so they can be obtained by a single-mode Franck–Condon analysis of the emission spectra, using Eq. 24: 5� ��E0 �3 � – x�ω Sx � M I(ν)= E0 x! x=0 � � � � �� 2 ν – E0 + x�ω exp –4ln2 . ∆ν1/2 (24) In Eq. 24, I(ν) is the relative emission intensity at energy ν (in cm –1 ), E0 is the energy of the zero–zero transition (i.e., the energy of the emitting 3 MLCT state), �ω is the average of medium-frequency acceptor modes coupled to the MLCT transition, x is the quantum number of such an averaged mediumfrequency mode which serves as the final vibronic state (note that x is usually limited to 5), ∆ν1/2 is the half-width of the individual vibronic bands, and SM is the Huang–Rhys factor. Application of the above equations to Ru(II) polypyridine complexes allows important information to be obtained on the excited-state properties. It should be considered, however, that Eqs. 22–24 are based on several assumptions, the most important being the following. (1) For radiationless decay, the thermal population of higher-energy excited states is neglected; when such an activated route cannot be disregarded, Eq. 22 only gives a contribution to the observed radiationless rate constant, the one related to k0 of Eq. 1. (2) The Franck–Condon analysis of the emission spectral profile here shown is based on a single coordinate; when more coordinates need to be considered, fitting of the spectral profile following Eq. 24 can give uncorrected parameter values. A simple refinement of Eq. 24 requires inclusion of a second (low energy) frequency acceptor mode due to solvent contributions [158–161]. More sophisticated theoretical methods to analyze the emission spectra of Ru(II) complexes have been introduced [162–166]. In particular, these methods allow for the detailed characterization of the high-frequency vibronic contributions to the emission spectra and the dependence of such contributions on various factors, such as the energy gap between ground and excited states. Extensive discussion can be found in the cited references.
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 135 Emission spectral profiles calculated by equations such as Eq. 24 have also been used, along with theoretical quantum mechanical expressions and experimentally determined rate constant values, to estimate the electronic coupling matrix element for such intercomponent processes for photoinduced energy transfer in dinuclear Ru(II) polypyridine complexes [160]. 4.2 Ultrafast Time-Resolved Spectroscopy and Localization/Delocalization Issues In the last 20 years, ultrafast pump–probe spectroscopy has become accessible to several research laboratories, including research groups interested in Ru photochemistry. The possibility of investigating the excited-state dynamics at very short time delays after the excitation pulse allowed clarification of several problems and shed light on many aspects of ruthenium photochemistry. New features, sometimes unexpected, have also been revealed and new questions and research topics have emerged. For example, as discussed in other parts of this chapter, it was found that singlet MLCT states can be involved in electron transfer and energy transfer processes, even before intersystem crossing and/or thermal relaxation. This is the case of photoinduced electron injection in semiconductors [167] and energy transfer/migration between Ru subunits of large, strongly coupled dendriticshaped systems [168–170]. Fluorescence from ruthenium complexes has also been detected [171]. Powered by the availability of ultrafast techniques, the long-term issue of localization/delocalization of MLCT states has also been revitalized. As previously stated, the general view is that the emissive state is localized on a single ligand, even for homoleptic species. However, open questions remain concerning the nature of the Franck–Condon state and the early-time dynamics which leads to the emissive state. As for the early-time dynamics, it is largely accepted that in [Ru(bpy)3] 2+ and analogous homoleptic species light excitation in the MLCT singlet manifold initially produces a Franck–Condon state that is delocalized, which is where the promoted electron is shared by all the polypyridine ligands. Then on the timescale of tens of femtoseconds, the promoted electron becomes localized on a single ligand, due to coupling with local solvent dipoles. Intersystem crossing then takes place in about 100 fs, producing a localized triplet state. The triplet MLCT state becomes “randomized” by interligand hopping on the timescale of 10 ps, the same scale of thermal (including vibrational and solvent reorganization) relaxation of the 3 MLCT state. This general figure is schematized in Fig. 11, and is based on results dealing with many Ru(II) polypyridine complexes, taking advantage of various experimental techniques (transient absorption anisotropy, time-resolved resonance Raman, pump–probe femtosecond transient absorption spectroscopy).
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280 Topics in Current Chemistry Edi
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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