Laser photolysis charge separation

The ionic field of micells increases the efficiency of photoinduced charge separation. Laser flash photolysis showed a longer lifetime of the e formed by irradiation of a donor molecule (D pyrene, perylene etc.) solubilized in anionic micells such as sodium lauryl sulfate (SDS) than in a non-micell systems 19b). This is why the e is repulsed by the anionic field at the micellar surface into the bulk solution (Eq. (10)). 

Laser Photolysis. In most donor-acceptor compounds, the rate of the photoinduced forward ET is faster than the translational diffiision of a dilute electrolyte. Therefore, it can be assumed that in the absence of pre-association with the ground state of the probe, the free charge-separated species is formed... 

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-... 

Porphyrin cation radical species as intemiediates in ET can be directly detected by laser-flash photolysis or ESR spectroscopy. In particular, transient absorption spectra of porphyrin-acceptor assembly systems excited by laser pulses often show the characteristic broad band assigned to porphyrin cation radical species at 500-800nm," so it is possible to determine the forward and/or back ET rate constants from the formation and decay profiles of the intermediate absorption. In noncovalent ET model systems, sufficient affinity between the porphyrin and the acceptor is required to obtain an appropriate inten,sity of transient absorption derived from porphyrin cation radical species. However, it is difficult to monitor the intermediate when charge recombination processes occur faster than the charge-separation process. 

M. Fujitsuka, O. Ito, T. Yamashiro, Y. Aso and T. Otsuho, Solvent polarity dependence of photoinduced charge separation in a tetrathiophene-Cgo dyad studied by pico- and nanosecond laser flash photolysis in the near-lR region, J. Phys. Chem. A, 104, 4876-4881 (2000). 

Moore, Gust, and coworkers synthesized the quinone-porphyrin-carotenoid (Figme 5) triad molecule. Upon excitation of the porphyrin moiety, initial charge separation occurred between porphyrin and quinone. Hole shift from porphyrin to carotenoid formed the final charge-separated state, that is, quinone radical anion and carotene radical cation, with a lifetime of 170 ns. These processes were confirmed by means of the picosecond and nanosecond laser flash photolysis. Their covalent bonding system was extended to tetrad and pentad using similar chromophores. 

Itaya, A., Yamada, T., and Masuhara, H., Laser photolysis study of photoinduced charge separation in poly(Ai-vinylcarbazole) thin films, Chem. Phys. Lett., 174, 145, 1990. 

Light excitation in the CT absorption bands formally leads to the transfer of an electron from the donor to the acceptor component (optical electron transfer). As a consequence, particularly when this process leads to formation of charges of the same sign in the two components (Fig. 8), one can expect destabilization of pseudorotaxane structures, followed by dethreading. In practice, however, this simple approach does not work because the back electron-transfer process is much faster than the separation of the molecular components, a process which requires extended nuclear motions and solvent rearrangement. In a particular case [24], laser flash photolysis experiments have suggested that a small fraction of the irradiated pseudorotaxane may undergo dissociation. 

The charge generation and charge transport efficiency of a polymer can also be measured separately by other techniques. For example, laser flash photolysis can measure the charge generation efficiency directly, if the generated electron or hole (anion or cation) has a distinct absorp-... 

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