Stabilization of Charge-separated States

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. 

De la Escosura A, Martinez-Diaz MV, Guldi DM et al (2006) Stabilization of charge-separated states in phthalocyanine-fullerene ensembles through supramolecular donor-acceptor interactions. J Am Chem Soc 128 4112-4118... 

More complex triad porphyrin compounds containing a carotenoid or chlorophyll-like substituent in addition to the quinone have also been synthesized. One example is given in Scheme 7.2.9. Light-induced charge separations are fast in such molecules (10 °-10" s ), but the recombination rates are almost equally fast (10 -10 ° s ). Neither carotenes nor chlorophylls help in the stabilization of charge-separated states. Laser flash photolysis of chloro-... 

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... 

Increasing the distance between the donor and acceptor sites and, in particular, the introduction of a solid substrate charge clearly stabilizes the charge-separated state. 

The second is a complex of sulfenium cation and superoxide and explains the near equivalence of the spin density on the two peroxyl oxygens in thiyl peroxyls. Both valence structures display an exposed electron-deficient sulfur that makes it susceptible to nucleophilic solvents. Complexing with a lone pair donor is expected to stabilize the charge-separated state increasing its contribution to the ground state. A partial radical-cationic character of the sulfur could also make three-electron bonding with some solvents contribute to the charge separation. 

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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