Excited states intraligand

The excited states which are responsible for the eel of the previous examples are of the CTML type or involved in metal-metal bonding of polynuclear complexes. Photoluminescence, or eel in our case, can also originate from intraligand (IL) excited states provided these states are the lowest excited states of such complexes. IL emissions are characteristic for many transition metal... 

Intraligand (IL) excited states of coordination compounds arise from electronic transitions between molecular orbitals primarily localized on a coordinated ligand. It is difficult, a priori, to predict the reactivity of this type of state. While it is logical to expect ligand-centered reactions, the influence of the metal on such processes can be substantial and result in net photochemistry which differs from that of the free ligand. A few examples should serve to illustrate the range of IL photoreactions reported to date. 

Figure 4.75 Schematic representation of the charge transfer in various excited states of a metal complex. M is the metal centre and L stands for a ligand. LF is a ligand field transition, CTs are the charge transfer transitions, LL is an intraligand transition, and CTTS is a charge transfer to solvent...

The first two processes are common reactions for complexes in their intraligand singlet or triplet (it, 7t ) excited states. Since excited molecules are both better reductants and better oxidants than those in the ground state [1], the same complex can be oxidized to its ic-cation radical (when quinones, nitroaromatics, halocarbons, viologenes or other oxidizing agents are present) or reduced to its n-anion radical (in the presence of a reductant, e.g. NAHD or l-ascorbic acid). The radicals formed are usually unstable species the anion radical [SbO(UroP )]6- formed at a photolysis of SbO(UroP)5- in alkaline solutions is one of the rare exceptions [216]. 

A case in question is the ac electrolysis of the complex Re(trans-SP)2(C0)3C1 (SP = 4-styrylpyridine) (59). It was shown before that the coordinated ligand SP undergoes a photochemical trans/cis isomerization (60). The reactive excited state is the lowest mr intraligand (IL) state, which is not luminescent. The ac electrolysis leads also to the trans/cis isomerization of the coordinated ligand (59). Hence it is a reasonable assumption that the electrolysis proceeds via the generation of the ttu IL state ... 

The absorption spectra of [Pt(tpy)Cl]+ salts in solution show two sets of bands. Intense, structured bands occur at wavelengths less than about 350 nm, attributed to intraligand n-n bands similar to those shown by Zn(tpy)Cl2 [68]. The somewhat weaker bands in the 350-450 nm range are assigned to 1M LCT transitions they have no counterpart in the zinc complex, which acts as a model in which the metal center cannot be oxidized and where low-energy d-7r excited states are thus not possible. 

The fact that the intraligand transition is lowest in energy in ClRe(CO)3-(fnms-4-styrylpyridine)2 also allows a direct assessment of the effect of the coordination on the decay properties of the intraligand excited state. Data for both the photoisomerization by direct irradiation, Table 19, and the triplet sensitized isomerization, Table 20, of ClRe(CO)3(fra s-4-styrylpyridine)2 and frans-styrylpyridine... 

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