MLCT state

Figure C 1.2.9. Schematic representation of photo induced electron transfer events in fullerene based donor-acceptor arrays (i) from a TTF donor moiety to a singlet excited fullerene and (ii) from a mthenium excited MLCT state to the ground state fullerene.

Fe(6-Mepy)2(py)tren] (004)2 Doped in PSS. Magnetic susceptibilities measured for a microcrystalline sample of the complex produce a magnetic moment value = 0.36 pg at 10 K and 0.61 pg at 150 K, followed by a gradual increase to Peff = 2.80 pe at 311 K [138]. Thus 26% of the complexes are in the HS state at 300 K if a magnetic moment of 5.1 Pe is assumed for the pure HS compound. On the other hand, the complex doped into a polystyrene sulfonate (PSS) film does not provide any evidence for a thermal population of the HS state up to 340 K as demonstrated by variable-temperature UV-VIS and Mossbauer spectra. In fact, all the complexes doped into the PSS film are in the LS state at temperatures below 340 K. However, if irradiated by a single pulse of a Q-switched Nd/YAG laser (532 mp), the complex is excited from the LS ground state to the HS J2 states via an intermediate MLCT state and the metal states. The subsequent back relaxation from the excited T2 state to the... 

A typical ligand capable of generating a dendritic structure is 1,4,5,8,9,12-hexaazatriphenylene (HAT). Photophysical studies of trinuclear species based on HAT have been reported [14 a, 49]. Representative example of complexes of this type are 26, 27, and 28. For some of these complexes, the luminescence, originating from MLCT levels involving the central HAT ligand, was found to decay with multiexponential kinetics. Furthermore, the vibrational modes responsible for the nonradiative decay of the luminescent MLCT states are different in the polynuclear dendritic edifices with respect to the mononuclear [M(L)2(HAT)]2+ compounds [14a]. 

We have reported the first direct observation of the vibrational spectrum of an electronically excited state of a metal complex in solution (40). The excited state observed was the emissive and photochemically active metal-to-ligand charge transfer (MLCT) state of Ru(bpy)g+, the vibrational spectrum of which was acquired by time-resolved resonance Raman (TR ) spectroscopy. This study and others (19,41,42) demonstrates the enormous, virtually unique utility of TR in structural elucidation of electronically excited states in solution. 2+... 

At least two possibilities for the structure of the MLCT state exist. It may be formulated as Ru(III)(bpy) (bpyr) +, which has maximum symmetry of C2, or the heretofore commonly presumed Ru(III) (bpy l ) which may have D3 symmetry. We shall refer to the former structure as the "localized" model of the excited state, and the latter as the "delocalized" model. The experimental details of this study are presented elsewhere (19). 

The TR spectrum of the MLCT state comprising the predominant fraction of excited Ru(bpy)3+ species averaged over the timescale of our laser excitation pulse (7 ns) is shown in Figure 5 (lower trace). This predominant MLCT species has been variously denoted "triplet charge-transfer" ( CT) (45), "dTr " (43), or simply Ru(bpy)g+ (44). We adopt the latter nomenclature. It is the emissive and photochemically active state which has a lifetime of ca. 600 ns (room temperature, aqueous solution). 

For the metal, A increases on descending a column in the periodic table. In addition, CT state energies are affected by the ease ofoxidation/reduction of the ligands and metal ion. For MLCT transitions, more easily reduced ligands and more easily oxidized metals lower the MLCT states. 

Figure 4.6. Lowest triplet state orderings for different metal complexes showing the relative positions of the d-d, n-n, and MLCT states as a function of metal, ligand, and effective crystal field strength. (Reprinted with permission from Ref. 7. Copyright 1991 American Chemical Society.)...

An interesting point can arise with Re(I) complexes (e.g., fac-Re+L(CO)3X where X is a halide, pyridine, nitrile, or isonitrile) where the n-n and MLCT states can be very nearly isoenergetic. This near-degeneracy allows the lowest state to be rather easily switched back and forth between n-n and MLCT by suitable choices of ligands and in some cases merely by altering the temperature 22 23 ... 

Lastly, photochemically unstable ligands should be avoided. Re(bpy)(CO)3Cl shows a moderately efficient MLCT emission at room temperature (R. M. Ballew, unpublished results from our laboratory). However, the apparently closely related Re(dpk)(CO)3Cl (dpk = 2,2 -dipyridyl ketone) shows a benzophenone like phosphorescence at 77K indicating that the n-n excited state of the ketone in complex is the lowest state of the complex. No luminescence is seen at room temperature, and even at 77K the dpk triplet state is such a powerful hydrogen atom extractor that it removes protons from alcohol glasses as seen by the formation of the intense blue color of the keto free radical. The absence of an MLCT emission is caused by the greater difficulty of reducing dpk relative to bpy, which pushes the MLCT states above the dpk ligand states. 

The photophysics and photochemistry of Ru(II) complexes have been extensively studied and good reviews are av able on this subject [1,86,87]. The type of reactivity associated with Ru(II) polypyridyl complexes in the excited state depends on the nature of this excited state and consequently on the different possible photophysical pathways controlling the luminescence lifetimes. For most polypyridyl Ru(II) complexes (for example Ru(bpy), Rufphenls", Ru(bpz)3. .. Fig. 2) [1, 86, 87], population of the excited singlet MLCT state is followed by crossing to the triplet MLCT state ( MLCT) with a quantum yield... 

In this Section, two types of photo-electron transfer processes with the MLCT state of complexes will be successively discussed. We will first introduce the direct photo-electron transfer from a DNA base to the excited complex Sect. 4.3.1. Afterwards we will coiKider the electron transfer between an excited... 

Fig. 12. Rate constants of luminescence quenching by GMP for a series of complexes as a function of the reduction potential of their MLCT states, [adapted from Lecomte J-P. (1992) Ph.D. thesis, Brussels, Belgium] From the left to the right Ru(HAT) , RuCHATl TAP , Ru(TAP)2HAT ", Ru(TAP)i Ru(bpz)i +, RuidiCHjTAP) RufHATjjphen RufHATljbpy, Ru(TAP)2bpy2+, Ru(phen)2HAT Ru(bpy)2HAT"+...

On the basis of the reduction potential of Rh(phen) (Eo = — 0.75 V/SCE) and of its nn energy (2.75 eV), Rh(phen)3 in the nn state is expected to be a very powerful oxidising agent (with a reduction potential of 2.0 V/SCE [133]), making it a stronger oxidant than the MLCT states of the Ru(II) complexes discussed above. Electron transfer from aromatic amines [134] or di-and tri-methoxybenzenes [135] to excited Rh(III) polypyridyl complexes have indeed been observed. 

Co-free PAE). In PAE-CoCpl, the fluorescence quantum yield is only 18% of that observed for Co-free PAE, even though the quencher substitutes less than 0.1% of the aryleneethynylene units. The fluorescence in solution disappeared in PAE-CoCp4, where every fifth unit is a cyclobutadiene complex. The mechanism by which this quenching occurs is via the cobalt-centered MLCT states [82,83], conferred onto the polymer by the presence of cyclobutadiene complexes. Even in the solid state the polymers PAE-CoCpl-2 are nonemissive. It was therefore shown that incorporation of CpCo-stabilized cyclobutadiene complexes into PPEs even in small amounts leads to an efficient quenching of fluorescence in solution and in the solid state. Quenching occurs by inter- and intramolecular energy transfer [84]. 

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