Charge-transfer absorption band different electron donors

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

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 main question concerning the primary step in photosynthesis is as follows how the excited state of the primary electron donor P in reaction centers (Res) is converted into a charge transfer state (see / / for review) Is the latter state formed within P directly from its excited state P or does a neutral P donate an electron to neighboring bacteriochlorophyl1 (BL) and bacteriopheophytin (HL) monomers located in L protein subunit (revealed by x-ray studies /2/) Femtosecond measurements yielded contradictory conclusions /3, / The first measurements /5"7/ of holes burned at absorption band of P have not shown any narrow bands for P+Q.A"formation, yet narrow-band features have been observed for different reactions in Res /8,9/. Here we show that in open R.vi ridi s Res the holes burned in the region of 0-0 transition of P are broad (>50 cm ) while in closed Res a narrow hole is observed with weak electron-phonon coupling /10,11/ in agreement with the Stokes shift between absorption and fluorescence spectra at 1.7 K. 

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