CTTS charge transfer to solvent

With some metal complexes, e.g. Fe(CN)6", where a clear CTTS (charge transfer to solvent) band is evident, photoexcitation can cause direct photoionisation and the creation of the solvated electron. 

A pulse radiolysis of Ag solution was studied, and the behavior of formed silver atom (Ag ) and dimer cation (AgJ) was measured [80]. The absorption band for dimer shows a significant red shift with increasing temperatures, which implies to the CTTS (charge transfer to solvent) character of the band. 

The preferentially solvating component is indicated by an asterisk ( ). K is for the formation of a solvated species containing the second mentioned solvent component. CTTS = charge transfer to solvent. 

Implications of ejected electrons in conventional photochemistry may be expected with some anions showing a CTTS (charge transfer to solvent) transition (as for ferricyanide anion) and giving fast and monophotonic ejected... 

This earliest as well as the simplest of all the meth s volves photolysis of transition metal ions such as Cu, Fe, Eu and others in their CTTS (Charge Transfer To Solvent) bands in acidic media. Systems of this type for photoproduction of reported till date are collected in Table II. General features of this type of systems can be summarised as follows ... 

Finally, as already mentioned, it seems clear that the solvent molecules can be involved in the charge-transfer process. Detailed discussions of charge-transfer spectra in transition metal complexes quite often label the corresponding bands quite separately, giving them the label CTTS—charge transfer to solvent. So, the fact that Fel3 has recently been prepared in non-aqueous media suggests that the solvent—water—is not always the mere spectator that it was implicitly assumed to be above. 

Since ligands are directly exposed to the solvent it is not surprising that electrons which reside in high-energy ligand orbitals can be ejected to the solvent. The formation of solvated electrons is thus a typical reaction of MLCT states. In some cases this reaction seems to be induced by CTTS (charge transfer to solvent) excitation [6,37,38,121]. However, CTTS absorptions are often difficult to identify and to distinguish from other bands. In any case, the formation of solvated electrons can be frequently related to the presence of MLCT states. Solvated electrons are detected by ESR or visible spectroscopy. They are stable in low-temperature matrices... 

The absorption spectra of anions are very sensitive to the composition of solvents in which they are embedded. In general, they are solvated, i.e. they are surrounded by a solvent shell. The molecules composing the solvent shell constantly exchange position with those in the bulk of the solvent. In these transitions, an electron is ejected not into the orbitals of a single molecule, but to a potential well defined by the group of molecules in the solvation shell. Such transitions are known as charge- transfer-to-solvent (CTTS) transitions. 

This simple oxidoreduction reaction involves complex OH - water molecules interactions whose the spectral signatures are assigned to Charge-Transfer-To-Solvent states (CTTS states). Indeed, the anionic precursor of the hydrated OH radical represents an interesting system for the direct investigation of elementary redox events in a protic molecular solution. 

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...

Photooxidation of the central atom Os(II) in hexacoordinated porphyrin complexes is supposed to start with the ejection of an electron from an charge-transfer to solvent excited state, CTTS, of the complexes. A complicated set of elimination, addition and redox steps involving radicals terminates in the formation of the complexes OsIV(Por)Cl2. Solvent molecules (CC14, CHC13, CH2C12) served as a source of chlorine atoms [92, 192]. 

Outer-sphere photoredox reactions are often interpreted as a consequence of ion-pair charge-transfer, IPCT [168] or charge-transfer to solvent, CTTS [92] excited states. In principle, however, any kind of excited state can be involved in such processes. 

A photoprocess rather common with inorganic compounds is the formation of solvated electrons, e [ in organic solvents and eat in aqueous solutions.43,44 The photoprocess is most commonly observed with anions whose absorption spectrum exhibits a characteristic charge transfer to solvent, CTTS, band in the ultraviolet. It is the typical photoprocess of the halide anions shown in Equations 6.89 and 6.90 where X = Cl, Br, and I-. 

The CTTS band can also be found in the absorption spectrum of some polyatomic anions together with transitions to the excited states described above43 44 In the case of SCN, an intense absorption band with 2max = 225 nm (s = 3.5 x 103 M 1 cm-1) has been assigned to a charge transfer to solvent transition. The wavelength-dependent photochemistry of SCN induces, however, the formation of solvated electrons according to Equation 6.89 and the detachment of S (Equation 6.91) in a parallel process. 

Another important type of electronic transition is the charge transfer to solvent (CTTS) transition. Recently, it has also been found5 that electronic transitions may occur between MO s which are localized on different ligands of the same complex. 

Under special conditions still further transitions can be observed as (1) metal-to-metal (MMCT) or intervalence (IT) charge transfers characteristic for polynuclear complexes and (2) charge transfer to solvent (CTTS) or ion-pair-charge-transfer (IPCT) detectable in cases of fairly strong external interactions [3]. 

Special cases of charge-transfer spectra are the so-called charge-transfer-to-solvent (CTTS) spectra [17, 68]. In this type of CT transitions, solute anions may act as electron-donors and the surrounding solvent shell plays the role of the electron-acceptor. A classical example of this kind of CTTS excitation is the UV/Vis absorption of the iodide ion in solution, which shows an extreme solvent sensitivity [68, 316]. Solvent-dependent CTTS absorptions have also been obtained for solutions of alkali metal anions in ether or amine solvents [317]. Quantum-mechanical molecular simulations of the CTTS spectra of halide ions in water are given in reference [468]. 

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