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second-generation carboxylic acid dendrons to zinc tetrakis(3,5-
dihydroxy-phenyl (porphyrinate and zinc tetrakis(2,6-dihydroxyphe-
nyl)porphyrinate produced a series of dendrimer metalloporphyrins (Figure
141). Quantitative determination of the binding constants of nitrogenous
bases to the zinc were made using visible spectroscopy. The meta-
substituted dendrimer porphyrins interestingly show increased affinity
for the bases relative to unhindered Zn(TPP), a phenomenon due to
favorable interactions between the amines and the aromatic dendrimer
structure. The ortho-substituted dendrimer porphyrins, however, showed
excellent shape selectivity, as demonstrated by the log(ATeq) values
(Figure 142). Particularly striking for the ortho-substituted dendrimer
porphyrins is the difference in binding affinity between linear and
nonlinear amines, such as that between 4-phenylpyridine and quinine (Êù
difference > 105). This type of selectivity could potentially be
exploited in sensors for ligating species in either solution or the gas
41 / Porphyrin Materials Chemistry
Figure 141. Molecular models showing a side view of Ihe binding sites of
dendrimer porphyrins. Top-, meia-substituted H2(T(3',5'-G1P)P). Bottom-.
ortfrosubstituted dendiimer-porphyrin H.,[T(2';6 -G1 AP)PJ. Note the open
cavity of 1 0 A versus a narrow slit of ~ 5, respectively; in both cases,
lop access lo the porphyrin is completely blocked.
Because of their synthetic versatility and chemical robustness,
porphyrins and metalloporphyrins are proving to be very useful platforms
on which to tailor field-responsive and chemo-responsive materials.
Diverse applications of porphyrins and metalloporphyrins to materials
chemistry have been developed over the past decade, botii for their
optical properties and their applications as sensors.
Notably, porphyrins and metalloporphyrins have found applications as
lield-responsive materials, particularly lor optoelectronic applications,
including mesomorphic materi-
Figure 142. Binding of axial ligands to dendrimer porphyrins; log(/(t,q)
values for a series of nitrogenous bases with various shapes.
als and optical-limiting coatings. Porphyrins show interesting second-
and third-order NLO properties, due to their exceptionally large 7r-
eleetron conjugation length. The primary attention has focused on third-
order NLO properties and optical-limiting materials. Improvements in the
area of nonlinearity versus transparency trade-off should he possible via
molecular engineering. The molecular design of novel porphyrins
possessing desired physical, optical and nonlinear optical properties for
specific photonic applications remains a difficult, hut often rational,
challenge. Ingenious designs and newly developed synthetic methodologies
have also led to significant progress in the development of molecular
optoelectronic porphyrinic materials. Long-term stability against
photodegradation remains important, however, and may preclude effective
devices for many molecular electronic applications.
The development of chemo-responsive materials based on porphyrins as
highly porous, molecularly based molecular sieves or shape-selective
solid catalysts is currently underdevelopment. Porphyrins and
metalloporphyrins have also been examined for a variety of sensor
applications, which proves their importance as an emerging class of
chemo-responsive materials. The unique spectral characteristics and
synthetic versatility of porphyrins allow for a variety of sensing
applications. In the case of oxygen detection via luminescent porphyrins,
the sensing characteristics of Ihe porphyrin are well established, and
work on the immobilization matrix is the key to sensor performance. Other
areas, such as array-based detection of organic vapors and detection of
ionic species in solution, will depend upon continued development of
well-designed receptors (hat exhibit high specificities.
Chou et al.
The contributions from the Suslick research group reported herein have
been supported generously by the National Institutes of Health (HL25934),
the Department of Energy (DEFG0291ER45439), and the Department of Defense
(DAAG55-97-1-0126). We gratefully acknowledge the early efforts by Drs.
Chin-Ti Chen, Homer Chou, Christopher L. Hein, Philip A. Gorlin and Bimal
Patel in the development of this work.
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