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. 

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. 

The photodynamics of electronically excited indole in water is investigated by UV-visible pump-probe spectroscopy with 80 fs time resolution and compared to the behavior in other solvents. In cyclohexane population transfer from the optically excited La to the Lb state happens within 7 ps. In ethanol ultrafast state reversal is observed, followed by population transfer from the Lb to the La state within 6 ps. In water ultrafast branching occurs between the fluorescing state and the charge-transfer-to-solvent state. Presolvated electrons, formed together with indole radicals within our time resolution, solvate on a timescale of 350 fs. 

From the steady state fluorescence spectrum of indole in water a fluorescence quantum yield of about 0.09 is determined. Since the cation appears in less than 80 fs a branching of the excited state population has to occur immediately after photo excitation. We propose the model shown in Fig. 3a). A fraction of 45 % experiences photoionization, whereas the rest of the population relaxes to a fluorescing state, which can not ionize any more. A charge transfer to solvent state (CITS), that was also introduced by other authors [4,7], is created within 80 fs. The presolvated electrons, also known as wet or hot electrons, form solvated electrons with a time constant of 350 fs. Afterwards the solvated electrons show no recombination within the next 160 ps contrary to solvated electrons in pure water as is shown in Fig. 3b). 

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...

In charge transfer to solvent transition the bound electron in the spectroscopically observed first excited state interacts strongly with the solvent oriented in the field of the ion. Will the formation of e aq occur also when excitation results in an internal molecular transition For instance, on illumination will aqueous solutions of aromatic molecules evolve H2 through preceding e aq formation ... 

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. 

The formation of the transient species Ag° by reduction of Ag+ from hydrated electrons using a double flash photolysis technique has been observed.199 This species may be photodissociated at 315 nm, probably via charge-transfer to solvent, to produce Ag+ and solvated electrons. 

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]. 

The outer-sphere CT processes result in the photoinduced electron transfer (PET) modes, which are mostly the consequence of charge-transfer-to-solvent... 

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