MLCT excited state

The first photophysical investigation performed on stereochemically pure metal-based dendrimers having a metal complex as the core is that concerning the tetranuclear species based on a [Ru(tpphz)3]2+ core (tpphz=tetrapyrido[3,2-a 2, 3 -c 3",2"-h 2",3"j]phenazine) [67]. Dendrimer 45 is an example of this family. In this compound, two different types of MLCT excited states, coupled by a medium- and temperature-dependent photoinduced electron transfer, are responsible for the luminescence behavior. However, the properties of all the optical isomers of this family of compounds are very similar. This finding is also in... 

The intense colors in 2,2/-bipyridyl complexes of iron(II), ruthenium(II), and osmium(II) are due to excitation of an electron from metal t2g orbitals to an empty, low-lying ir orbital of a conjugated 2,2 bipyridyl ligand. The photoexcitation of this MLCT excited state can lead to emission as the excited state collapses back to the ground state. However, not all complexes are... 

In order to illustrate the approach suggested above, it is of value to consider a specific case. Visible or near-UV excitation of the complex RuCbpy results in excitation and formation of the well-characterized metal to ligand charge transfer (MLCT) excited state Ru(bpy)32+. The consequences of optical excitation in the Ru-bpy system in terms of energetics are well established, and are summarized in eq. 1 in a Latimer type diagram where the potentials are versus the normal hydrogen electrode (NHE) and are... 

Eqs. 9 and 10 make clear predictions about the dependence of quenching rate constants on the free energy change in the quenching step. One way of testing the theory is to observe the quenching of the excited state by a series of related quenchers where the parameters kq(0), K, and k j) should remain sensibly constant and yet where the potentials of the quenchers as oxidants or re-ductants can be varied systematically. Such experiments have been carried out, most notably with the MLCT excited state, Ru(bpy)3 + (1). The experiments have utilized both a series of oxidative nitroaromatic and alkyl pyridinium quenchers, and a series of reductive quenchers based on aniline derivatives. From the data and known redox potentials for the quenchers, plots of RTlnk q vs. 

According to eq. 14, if variations in XQ are relatively small through a series of solvents, plots of lnknr vs. Eem should be linear for a single excited state. It has been shown that for a series of polar organic solvents, the prediction is borne out for several of the (phen)0sL 2+-type MLCT excited states, including Os(phen>32+ (18). It is especially striking that the slopes of the plots are the same within experimental error as in the experiments described above where L was varied. 

Time-resolved IR difference spectra for the MLCT excited states in the range 1,400-1,625 cm have been recorded for [Ru(bpy)3]. By comparisons with ground-state spectroscopic data and data for the electrochemically generated [Ru(bpy)3] and [Ru (bpy )(bpy)2], it has been possible to assign the new spectra the results provide evidence for a localized [Ru (bpy )(bpy)2] on a 100 ns timescale. ... 

A number of cyano-bridged complexes are included here even though they strictly do not fall in the general family-type defined for the section. The syntheses and photophysical properties of [(NC)(bpy)2Ru(/r-NC)Cr(CN)5] and [(NC)5Cr(Ai-CI Ru(bpy)2(M-NC)Cr(CN)5] have been described. Absorption of visible light by the Ru(bpy)2 unit results in phosphorescence from the Cr(CN)g luminophore, and the results evidence fast intramolecular exchange energy transfer from the MLCT state of the Ru(bpy)2 chromophore to the doublet state of the Cr -based unit. Time-resolved resonance Raman and transient UV-vis absorption spectroscopies have been employed to investigate the MLCT excited states of [(NC)(bpy)2Ru(//-CN)Ru (bpy)2(CN)], [(NC)(bpy)2Ru(//-CN)Ru(phen)2(CN)]+, [(NC)(phen)2Ru(//-CN)Ru (bpy)2(CN)]+, [(NC)(bpy)2... 

Dynamic quenching of the MLCT excited state of [Ru(phen)2(dppz)] " " by H" " transfer in MeCN solution occurs for proton donors with pAa values in the range 4.7-15.7. Comparisons of the quenching have been made in the presence and absence of DNA. " The addition of Cu " " to DNA-bound [Ru(bpy)2L] " " (L is the phenazine derivative (167)) leads to luminescence quenching. This is explained in terms of complexation of Cu " " with the vacant coordination site of L in [Ru(bpy)2L] " ". Formation of the [Ru(bpy)2L] " "/Cu " " complex in the presence of DNA is proposed to place one metal center in the major groove and one in the minor groove. " ... 

Figure 2. Schematic representation of some relevant ground and excited-state properties of Ru(bpy)j. MLCT and MLCT are the spin-allowed and spin-forbidden metal-to-ligand charge transfer excited states, responsible for the high intensity absorption band with = 450 nm and the luminescence band with = 615 nm, respectively. The other quantities shown are intersystem crossing efficiency energy (E°°) and lifetime (x) of the MLCT state luminescence quantum yield ( ) quantum yield for ligand detachment (O,). The reduction potentials of couples involving the ground and the MLCT excited states are also indicated.

The versatility of the structural types possible for 1,10-phenanthroline ligands is exemplified in a review article on metal-to-ligand charge-transfer (MLCT) excited states of copper(ll) bis-phenanthroline coordination compounds, where 14 different 1,10-phenanthroline-based ligands were discussed (Figure 2) <2000CCR243>. 

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