Charge separated state formation

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

A similar supramolecular approach, in which both n-n stacking stacking of pyrene on the SWNT surface and alkyl ammonium-crown ether interactions were used in the self-assembly process of a fullerene derivative with SWNTs, was recently reported (Scheme 9.22).72 The nanohybrid integrity was probed with various spectroscopic techniques, , and electrochemical measurements. Nanosecond transient absorption studies confirmed electron transfer as the quenching mechanism of the singlet excited state of C60 in the nanohybrid resulting in the formation of SWNT"1"/ Pyr-NH3 + /crown- charge-separated state. 

After the TICT minimum is reached, the transition moment between charge-separated state and the ground state represented by A-B and AB, respectively, is expected to be fairly small owing to almost no overlap between part A and B. Therefore, the fluorescence intensity will be small and significant contributions most probably stem from neighboring geometries (6 90°) for which the emission from admixed locally excited states can occur. The return from Sj or Tj minimum to S0, which can proceed in radiative or radiationless manner, usually does not lead to formation of cis-trans isomers as one would expect from the assumed energy surfaces. This is due most probably to rapid thermal cis-trans interconversion in the S() state. So far in this Section electronic properties of the free molecules have been addressed. 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

Fig. 9.52 Calculated heats of formation, AHf, of the charge-separated states of < xTTF-oFL -Cgo in vacuo and different solvents with increasing polarity n = 1 red, n = 2 orange, n = 3 pink...

Unfortunately, turnover control of PSII is more complicated than the above description would indicate. Because turnover of the S states is achieved via a photochemical reaction, the yield of the reaction depends on both the electron donors and the electron acceptors. The overall picture of electron transfer in PSII is shown in Figure 2 (II). Light induces a series of electron-transfer reactions that lead to the formation of progressively more stable charge-separated states. The dominant reaction under physiological conditions leads to a one-step advancement of the S state and reduction of the secondary quinone electron acceptor (Qb). In purified PSII preparations, however, the quinones are depleted and the QB site will mostly be unoccupied unless exogenous quinones are added. 

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