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materials synthesized are potential light-harvesting antennas. Burrell
and coworkers described the synthesis of functionalized TPPs, building
blocks that have been successfully used to prepare a variety of porphyrin
arrays including trimers, pentamers and nonamers (Figures 48 and 49).
In the early 1990s, Osuka et al. reported the synthesis of linear and
stacked trimeric and pentameric porphyrins bridged by rigid aromatic
spacers.107 When the number of porphyrin rings increased, serious
solubility problems occurred which prevented study of higher homologues.
To circumvent this difficulty, peripheral substituents were placed on the
porphyrin in hopes of increasing the solubility so that larger porphyrin
arrays could be manipulated in common organic solvents. Using this
strategy, they reported linear 1,4-phenylene-bridged porphyrin arrays
including the dimer, trimer, pentamer, heptamer and nonamer by acid-
catalyzed condensation of formyl-substituted porphyrins with di(3-ethyl-
4-methylpyrrol-2-yl)methane.107 The molecular length of the nonamer is
estimated upwards of 122 A (Figure 50).
The same research group also reported the first bridgeless
5,10,15,20-linked porphyrin arrays which were synthesized by direct
oxidative coupling of zinc porphyrins.108 Treatment of zinc(II) 5,15-di(-
3,5-di-rm-butylphenyl)porphyrin with AgPFf, in acetonitrile for 5 hours
produced the dimer and the trimer in low yield. The proposed mechanism is
initial one-electron oxidation of the zinc porphyrin by the Ag(I)
followed by the nucleophilic attack of the neutral zinc porphyrin. The
dimer and the trimer were separated by size-exclusion chromatography.
Reaction of the dimer under similar conditions afforded the tetramer in
modest yield (Figure 51).
An extension of this work was reported for orthogonally arranged,
windmill-like porphyrin arrays, involving a
5,10,15,20-linked porphyrin dimer unit.109 The synthesis involved the
preparation of a symmetric, linear 1,4-phenylene-bridged porphyrin
trimer. The reaction of the trimer with two equivalents of AgPF6 produced
windmilllike porphyrin hexamer and nonamer systems (Figure 52).
Other groups are also pursuing light-harvesting porphyrin arrays. In
1993, Lindsey et al. reported a convergent strategy for the synthesis of
soluble covalently linked porphyrin arrays.110111 The porphyrins are
linked together via homogeneous Pd-catalyzed couplings of iodo- and
ethyne-substituted tetraarylporphyrins (Figure 53). Reaction of the iodo-
porphyrin and the ethyne-porphyrin in pyridine/ triethylamine at 100 °C
for 12 hours afforded a mixture of products including higher molecular
weight material, the pentamer, dimer, and unreacted starting materials.
Chromatography on silica gel yielded the pentamer in 50% yield. The
absorption spectrum of the pentamer is similar to the monomers with a
slight red-shift of the Soret band. Arrays with various metallation
states are readily available. The fluorescence spectrum of the system
with a central free-base
41 / Porphyrin Materials Chemistry
Figure 48. Porphyrin assemblies. (A) Functionalized M(TPP) as building
block, (B) Porphyrin trimer and (C) Porphyrin pentamer. Reprinted with
permission from Officer, D. L.; Burnell, A. K.; Reid, D. C. W. /. Chem.
Soc. Chem. Commun. 1996, 1657.
porphyrin and peripheral zinc porphyrins is dominated by the emission of
the free-base porphyrin. The yield of energy transfer from the zinc to
free-base porphyrin is estimated to be ~ 90%.
An excellent example of employing secondary pigment to enhance the
absoiption properties of the porphyrin was reported by Lindsey, Holten,
Bocian, Bilge and coworkers. They designed and synthesized light-
comprised of a central porphyrin with one, two, or eight boron-dipyrrin
(BDPY) pigments (Figure 54).''* In order to connect as many as eight
boron-dipyrrin pigments around a central porphyrin, the weta-position of
the 5, K), 15,20-aryl rings on the porphyrin was utilized. Instead of the
"porphyrin first, coupling second" approach, they decided to build the
BDPY unit into the aryl aldehyde unit at the 4- or 3,5- positions using a
Pd-catalyzed coupling reaction. The
Chou et al.
Figure 49. Porphyrin assemblies. (A) Functionalized M(TPP) bridged-dimer
as building block, (B) Porphyrin pentamer and (C) porphyrin monomer.
Reprinted with permission from Officer, D. L.; Burnell, A. K.; Reid, D.
C. W. /. Chem. Soc. Chem. Commurt. 1996, 1657.
BDPY-incorporated aldehyde is then condensed with pyrrole to afford the
desired BDPY-substituted porphyrin arrays. This strategy eliminated
successive Pd-catalyzed coupling reactions and relied on the robust
Lindsey porphyrin condensation. The synthetic routes of 4-(BDPY)-
benzaldehyde and 3,5-bis(BDPY)benzaldehyde are shown in Figure 54.
Porphyrin arrays containing one, two and eight PDPY dyes were prepared by