Charge separated states energy

We have seen that, in photosynthetic bacteria, visible light is harvested by the antenna complexes, from which the collected energy is funnelled into the special pair in the reaction centre. A series of electron-transfer steps occurs, producing a charge-separated state across the photosynthetic membrane with a quantum efficiency approaching 100%. The nano-sized structure of this solar energy-conversion system has led researchers over the past two decades to try to imitate the effects that occur in nature. 

Fig. 1. The Marcus parabolic free energy surfaces corresponding to the reactant electronic state of the system (DA) and to the product electronic state of the system (D A ) cross (become resonant) at the transition state. The curves which cross are computed with zero electronic tunneling interaction and are known as the diabatic curves, and include the Born-Oppenheimer potential energy of the molecular system plus the environmental polarization free energy as a function of the reaction coordinate. Due to the finite electronic coupling between the reactant and charge separated states, a fraction k l of the molecular systems passing through the transition state region will cross over onto the product surface this electronically controlled fraction k l thus enters directly as a factor into the electron transfer rate constant...

The inverted region was initially predicted by Marcus and the decrease in the electron transfer rate constant with —AG° has been observed experimentally many times.18 This is an important and remarkable result both for natural and artificial photosynthesis and energy conversion it predicts that, following electron transfer quenching of the excited A -B, the back electron transfer in the inverted region for the charge-separated state A + -B becomes slower as the energy stored increases. 

Finally, an effective influence of the environment (solvent polarity) on the wavefunctions and energies of the low-lying excited states of the complex dative bond will be qualitatively considered. An influence of a polar solvent stabilizing the charge-separated state might play a key role for the process considered. An example of an extreme strong... 

The donor B as well as the acceptor A subunits of the system typically have locally excited states that have to be considered in addition to the charge-separated states which we have been discussing. Locally excited states of corresponding ions might also have relatively low energies. Therefore, as the starting point several zero-order states have to be considered, for example,... 

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. 

The important determining factor for the relative energy of the charge-separated state A B to the hole-pair state AB is certainly the nature of the environment (polar solvent). Note that, although the emission from the TICT state has been observed in the gas phase, most of the observations have been made in polar solvents. We can again use the... 

In summary, if the good combination of donor-acceptor subunits can be found so that the charge-separated state does not lie substantially higher than the locally excited states and if the polar solvent is adequately chosen to lower the energy of the charge-separated state, TICT state will be responsible for the anomalous fluorescence emission. 

Figure 10 Potential energy diagram for emission from normal planar (NP) and twisted intramolecular charge transfer (TICT) excited states. The charge-separated state is stabilized by twisting and by the polarity of the environment.

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