IPCT band

Upon irradiation of an IPCT band in degassed condition (Xex> 365 nm), the colour of both LB films changed from pale yellow to blue. The UV/vis absorption spectrum after irradiation is shown in Figure 25, which is characteristic of 4,4 -bipyridinium radical cation monomer[108]. Coloured species photogenerated in mixed LB films of AV2+/AA or HV2+/AA systems decayed almost exponentially in the dark in vacuo with a lifetime of about 4 h at 20 °C [93,94]. The lifetime of 4,4 -bipyridinium radical cations in LB films was almost... 

Immediately upon excitation of an IPCT band with a fs laser at 400 nm, transient absorption was observed for both salts in solutions with a peak at about 600 nm, characteristic of 4,4/-bipyridinium radical cations. Figure 20 shows the transient absorption spectra of PV2+(I )2 in methanol solution. A marked increase in the absorbance of the 4,4/-bipyridinium radical cations took place within 1 ps after excitation. 4,4/-Bipyridinium radical cations were thus formed in a fs time scale by the photoinduced electron transfer from a donor I- to an acceptor 4,4/-bipyridinium upon IPCT excitation [48], The time profiles of transient absorption at 600 nm are shown in Fig. 21 for (a) PV2+(I )2 in a film cast from DME and (b) PV2+(TFPB )2 in DME solutions. Both of them showed a very rapid rise in about 0.3 ps, which was almost the same as the time resolution of our fs Ti sapphire laser measurement system with a regenerative amplifier. Similar extremely rapid formation of 4,4/-bipyridinium radical cations was observed for PV2+(I )2 salts in methanol and dimethylsulfoxide solutions upon IPCT excitation, respectively. These results demonstrated that the charge separated 4,4/-bipyridinium radical cations were formed directly upon IPCT excitation because of the nature of IPCT absorption bands (that the electrons correlated with the IPCT band are transferred partially at the ground state and completely at the excited state). Such a situation is very different from usual photochromism which is caused by various changes of chemical bonds mainly via the excited singlet state. No transient absorption was observed for PV2+(I )2... 

The total reorganization energy >. in classical Marcus theory was estimated by the method described below. A plot of the maximum of the IPCT band in solution (T op) against the driving force of electron transfer within the contact... 

A similar but, perhaps, more interesting case is that of the ion pairs between Co(sep)3+ and oxalate ions. Excitation of the ion pair in the IPCT band leads to the formation of the Co(II) complex and of an oxalate radical which undergoes a fast decomposition reaction. Thus, the back electron transfer reaction is prevented and Co(sep)2+, which is a good reductant, can accumulate in the system. When colloidal... 

Application of Hush theory to the observed IPCT bands yielded information about the relationship between optical and thermal ET in these systems. The redox potentials of both the metal dithiolene donors and the viologen acceptors can be systematically varied, which, in turn, tunes the thermodynamic driving force for electron transfer. The researchers found that the IPCT band energy increases linearly with more positive free energy AG for ET, and that the reorganization energy (x) remains constant with variation in the metal or cation redox potentials (66, 67). 

The ion-pair approach to the design of photosensitizers for electron transfer processes has been followed also in the case of [Co" (sep)] -oxalate system. In a deoxygenated solution, the excitation in the IPCT band of [Co" (sep)] +-HC204 causes the reduction of [Co" (sep)] + to [Co"(sep)] " and the oxidation of oxalate to carbon dioxide. The [Co"(sep)] + complex is a sufficiently strong reductant to reduce H+ to H2 at moderately acidic pH values. Thus, when the photoreaction is carried out in the presence of colloidal platinum catalyst, such a reaction indeed occurs, and H2 evolves from the solution in addition to carbon dioxide. Under such conditions, the overall reaction is the oxidation of oxalate, which plays the role of sacrificial agent, combined with the reduction of water to yield carbon dioxide and dihydrogen, according to Eq. 8. 

Diphenyliodonium salts of transition metal cyanometallates, for example, Ru"(CN)r, Molv(CN)f , Wlv(CN)f, and Fe"(CN) -, exhibit broad absorptions in the visible assigned to outer sphere ion pair charge transfer (1PCT) interactions (107). Irradiation into these IPCT bands leads to efficient long wavelength photolysis of the onium cation coupled with oxidation of the cyanometallate anion. 

Charge transfer band absorption occurs whenever inter- or intramolecular complexes between donor and acceptor are formed. In these complexes, anions may be, among the most effective donors. When an ion pair shows such an absorption band, the electronic transition is named ion-pair charge-transfer (IPCT). Ion pairs of coordination compounds are often concerned by IPCT and... 

According to Mulliken [200], ion radical pairs arise from excitation of a CT band this corresponds to a vertical optical electron transfer from the donating to the accepting moiety of the complex. The CT ion pair so obtained is considered to be akin to a contact ion pair [201], A radical pair is thus expected from IPCT. 

In a solid salt of viologen MV2+(TFPB")2 where the anion is not the usual chloride but tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB-), an IPCT absorption is found. When excited in this band in vacuo or under argon, MV + is observed to accumulate and to reversibly disappear when heated or dissolved [208, 209]. This system has also been observed in degassed organic solutions [210] showing that the extraordinarily bulky and stable TFPB- anion works as a reversible electron donor. The MV2+-TFPB- ion pair is also the only example for which ion pair luminescence (optical back electron transfer) is observed at room temperature [211]. 

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