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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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via double hydrogen bonding. Upon addition of 2,6-dichloro-
3,5-dicyano-/?-benzoquinone, the fluorescence of zinc porphyrin was
considerably quenched in benzonitrile,
46 / Porphyrins and Metalloporphyrins as Receptor Models
317
suggesting complex formation between quinone and hydro-quinone. The time-
resolved fluorescence decay of the zinc porphyrin led to two or three
fluorescence lifetimes, although D'Souza did not mention any kinetic
aspects in the report.
The direct evaluation of photoinduced ET rates in noncovalently linked
donor-acceptor systems via hydrogen bonding is so far quite limited.
Nocera and coworkers reported photoinduced ET mediated by hydrogen
bonding within the zinc-porphyrin 132-dinitrobenzoic array.1-47 The
hetero-association constant of carboxylic acids was estimated by IR
spectroscopy to be 522 M while the association constants were also
derived from static quenching of fluorescence ( = 698 M -1 and 316 M ~1
for 132-H in o-dichlorobenzene and 132-D in 2, respectively).
Picosecond laser flash photolysis showed the singlet state and cation
radical absorption of 132 in the presence of the benzoic acid. The
transient spectra on the picosecond time scale led to the forward ET rate
constant of 5.0 x 1010 s 1 and the charge recombination
rate constant of
1.0 x 10 10 s - 1 for 132-H, in which the isotope effects of kH/k" were
1.7 and 1.6, respectively. These findings indicated that the intracomplex
ET mediated by hydrogen bonding also depends upon the nature of the bond
linkage. Nocera and coworkers have prepared the similar representative
aggregate 133, formed by amidinium-carboxylate asymmetrical hydrogen-
bonding interaction, to compare the rate of a proton-coupled ET with
model 132 having the symmetrical double hydrogen bond between two
carboxylate substituentsl4S An estimated rate constant from the decay
kinetics was 7.5 x I08s 1 in CH2C12, suggesting that the ET rate constant
of the donor-acceptor system via asymmetrical interface is considerably
smaller than that of the symmetrical interface system and the covalently
linked
system.144 Nocera et al.'s systems 132 and 133 could be good models for
supporting Cukier's theoretical study about proton-coupled ET
reactions.1''0
Another approach for evaluating noncovalently linked ET was reported
by Therien and coworkers.1''1-1''2 They prepared three porphyrin-
porphyrin arrays 134-136 to investigate the relative magnitudes of
electronic coupling provided by hydrogen, <r-, and -bonds. They
monitored the photoinduced ET from zinc porphyrin to iron porphyrin by
time-correlated single-photon counting spectroscopy and the forward ET
rate constants were determined to be
8.1 x 104 s 1 for 134, 4.3 x 109 s 1 for 135 and
8.8 x I04s_1 for 136. Interestingly, the ET rate in hydrogen-bonded
system 134 was faster than that in the system 135 linked by <r-symmetry
bonds, and comparable to that in the -bond system 136. Thus, the
comparison of the ET reaction in their systems predicts that the
hydrogen-bonded interface in proteins could mediate ET as efficiently as
a covalent bridge.
E. NONCOVALENTLY LINKED DONOR-ACCEPTOR PAIRINGS VIA VAN DER WAALS
CONTACTS
Cyclodextrin-appended porphyrins are one of the suitable types of
compound for ET models, since cyclodextrin (CD) has a hydrophobic cavity
able to capture a small nonpolar molecule such as a quinone in aqueous
solution. The first example of a porphyrin-/?-CD photochemical system 137
was reported by Bolton, Weedon and coworkers in 1984.1-They monitored the
photoinduced ET reaction in a frozen mixture of porphyrin 137 and ;;-
benzoquinone by ESR spectroscopy. Irradiation of the solution showed the
characteristic single ESR signal due to the generation of porphyrin
cation radical and quinone anion radical species. The signal intensities
in the ESR spectra, using a variety of quinones, indicated that the
efficiency of the ET depended upon the reduction potential of the quinone
acceptors.
In 1993, Kuroda and coworkers reported a unique porphyrin-quinone
system using cyclodextrin-sandwiched porphyrin 138 as a photodonor.1''4
In this model, CD is fixed above the porphyrin plane, and the electron
acceptor receives an electron from the photoexcited porphyrin in
132*dinitrobenzoic acid
134
135
136
O-H.....
133*dimlrobenzoic acid
in
R, , R2 = 3,4,5-tnmethoxyphenyl
R, = phenyl, R? = 4-methoxyphenyl
Ri = phenyl R? = 4-methoxyphenyl
318
Ogoshi et al.
p-CD
Table 8. Fluorescence Lifetimes and ET Rate Constants of the Porphyrin
138-Quinone Pairing Through /J-Cyclodextrin Bound to Porphyrin at 23 in
pH 9 Phosphate Buffer
Quinone
rL(ns)
rs(ns)
Ka(M ')
Naphthoquinone (NQ) Anthraquinone-2-sulfonate (AQ) Adamantane-linked
quinone (AdQ) None
3.2 0.3
3.2 0.2 3.7 0.3
3.2 0.3
0.5 0.1 0.7 0.1
0.7 0.2
5.0 x 10 ' 2.5 x I04
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