CHEMICAL MODELS OF ENZYMES - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 180

, 2000). In these compounds, the phenyl bridge was substituted
at the 2 and 5 positions for small Me or methoxy groups to yield ANI-diMe-NI and ANI-
diMeO-NI and these molecules differ only in the energy levels of the bridge molecular
orbitals. The rate constants for charge separation and charge recombination within ANI-
diMeO-NI in toluene were 32 and 1400 times larger, respectively, than the
corresponding rate constants for ANI-diMe-NI. Solvents of higher polarity diminish
these differences in rate constants. The authors suggested that the reaction
via a double electron-transfer process (Fig. 6.4).

synthesized (Miller et al .

There are investigation of charge separated process in a donor-acceptor pair, based
on a tetracene donor linked to a pyromellitimide acceptor via a PPV oligomeric bridge of
variable length and on zinc porphyrins linked to a perylene-3,4:9,10-diimide acceptor
(PDI) via a series of Ph bridges, that progressively restrict rotational motion of the
porphyrin relative to that of PDI (Wasielewski et al., 2000). The temperature dependence
of the rates of electron transfer in these molecules did not obey the predictions of ET
theories based upon the Condon approximation. These results revealed the importance of
bridge dynamics in electron transfer in donor-bridge-acceptor molecules. A series of
derivatives of green chromophore, -yl)-3,4:9,10-perylene-

bis(dicarboximide) (5PDI), that exhibits photophysical and redox properties similar to
those of chlorophyll a (Chl a) has been synthesized (Lukas et al., 2002a.b). It was shown
that 5PDI is both oxidized and reduced in at 0.57 V and -0.76 V vs SCE,
respectively, making it a facile electron donor or acceptor. Rod-like covalent electron
donor-acceptor pairs were prepared by linking the imide group of the 5PDI donor to
pyromellitimide (PI),

1 ,7-bis(pyrrolidin- 1

,7-bis(3,5-di-
tert-butylphenoxy)-3,4:9,10-perylene-bis(dicarboximide) (PDI) acceptors via an N-N
bond. Measuring the formation and decay of their excited and radical ion pair states
monitored by the femtosecond transient absorption spectroscopy, indicated high yield of
photoseparated charges only in 5PDI-NI and 5PDI-PDI pairs but not in 5PDI-PI. This
difference was explained by estimation of the ionic radii of the photogenerated ions,
which for perylene chromophores 5PDI and PDI (strong electron acceptor) are
whereas those of the PI and NI (weak electron acceptors) are

1 ,8:4,5-naphthalenebis (dicarboximide)(NI), and 1