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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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quinone ligated to the zinc porphyrin. A ditopic binding structure was
determined by 'H NMR spectroscopy (Afa=l.l x 107M 1 in CH2C12). According
to a time-resolved fluorescence study, the ET rate constant was estimated
to be 1.6 x 10K)s " 1 within the conformationally restricted system.
R = -C6H4-/so-propyl
The construction of energy-transfer model systems by use of noncovalently
linked donor-acceptor pairing is also of great interest in that it helps
the understanding of biological photochemical behavior. In 1990, Hamilton
and coworkers presented the first example of noncovalently linked donor-
46 / Porphyrins and Metalloporphyrins as Receptor Models
acceptor complex formed by multiple hydrogen bonds.10-5 They prepared the
porphyrin 107 as a photodonor having a barbiturate derivative group which
formed complementary hydrogen bonds with a bis(diaminopyridine) receptor
bound to a fluorescent dansyl (dimethylamino-naphthalene-sulfonyl) group
with an association constant of
1.0 x I06M_l in CHiCL. According to their time-resolved fluorescence
study, the dancyl receptor showed monoexponential fluorescence decay with
a lifetime of
16,3 ns, while the addition of the porphyrin induced a biphasic decay
derived from two components (t, = 16.4 ns, ts = 0.4 ns), which were
assignable to the lifetimes of free and bound dancyl receptors.
Furthermore, addition of the excess of simple barbital (1000 equiv) to
the complex solution resulted in observation of the single exponential
decay of dancyl fluorescence, suggesting that the simple barbital
inhibited the hydrogen-bonded donor-acceptor pairing. These results
supported the conclusion that energy transfer occurred between two
chromophores within the well-organized assembly.
Harriman, Sessler and coworkers have prepared several energy-transfer
systems in dichloromethane or chloroform in which a donor and acceptor
were associated via hydrogen bonds by nucleobase subunits.161162 They
monitored the singlet and triplet energy transfers from zinc poiphyrin
146 to free-base porphyrin 147 by use of time-resolved fluorescence
studies below 5 mM of porphyrins. Fluorescence from the zinc porphyrin
showed biexponential decay profiles; the longer and shorter lifetimes
were assigned to the unperturbed zinc porphyrin and the zinc porphyrin
derived from porphyrin-base conjugates, respectively. The rate constants
for energy transfer and the association constants derived from the
fractional amplitude of the shorter-lived component are shown in Table 9.
This evidence supported their conclusion that the photoinduced energy
transfer between the chromophores occurs smoothly within the complex
formed by Watson-Crick nueleobase-pairing interactions.
Table 9. Rate Constants for Energy Transfer and Association Constants
Within the Porphyrin-Nucleic-Acid Base Conjugates 146 and/or 147 in CHCI,
at 22
Base pair , (ns) ts (ns) k* (s ') Ms ') ')
cyt-cvt 1.58 1.16 2.3 > 10s ihI 51
sua-gua 1.41 0.82 5.1 v 10" nd 24
cyt-gua 1.47 0.87 4.7 10" 16.5 x 10 s ",2S
gua-cyt 1.52 0.86 5.1 x 10" 12.8 x 190
In a second series of somewhat similar representative aggregates 148-
149 and 148-150, rigid and noncovalent systems for the study of
photoinduced energy-transfer processes were also reported by Sessler,
Harriman and coworker.163 The singlet energy transfer within the
hydrogen-bonded aggregate across a 22.5 A center-to-center separation
occurred with a rate constant of about 8-9 x 108 s _1 in dichloromethane
at 23 CC and with 60% quantum yield. This value supported the notion that
the singlet energy-transfer dynamics are consistent with a Forster-type
process. In contrast, the rate constants of triplet energy transfer were
about 1 x 106M'' s_l with quantitative efficiency, suggesting that the
energy transfer occurred with a Dexter-type process via a through-bond
pathway involving hydrogen bonds.
In addition, Sessler and coworkers have presented another approach to
constructing a noncovalent assembly linked by salt-bridge
interactions.161 A carboxyl-bearing porphyrin photodonor 151 can interact
with monoproto-nated sapphyrin 152 with an association constant of 2.6x
103M~' in CDnCb. Time-resolved fluorescence studies indicated singlet-
singlet energy transfer from the porphyrin to sapphyrin with a rate
constant of 1.8 x 10 4 s 1 and with a quantum yield of 0.96. This model
supported the hypothesis that the fast singlet-singlet energy transfer
also occurred in the electrostatically face-to-edge assembled donor-
acceptor conjugate.
R, R' = -222 or - -0
M = H2 or Zn
Ogoshi et al.
The next stage in the intracomplex ET study linked by noncovalent weak
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