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substituted porphyrins by incorporating them as "guest" molecules in a
mesogenic matrix of "clipper" host molecules (Figure 11). Two smectic
phases were observed between 15 and 150 °C for the 5,10,15,20-
tetrakis(3,5-dihydroxyphenyl)porphyrin complex. Mesomorphic beha-
Figure 10. Orientation of H2(TPP)and Neumann's mesogenic o-amino-
substituted tetraphenylporphyrin in nematic liquid-crystal phase.
Reprinted with permission from Michaeli, S.; Hugerat, Ì.; Levanon, H.;
Bernitz, Ì.; Natt, A.; Neumann, R. j. Amer. Chem. Soc. 1992, 714, 3612. (c)
1992 American Chemical Society.
Chou et al.
N4-N o=< T >=o
Figure 11. Structure of Nolte's mesomorphic inducing "clipper" molecule
and schematic representation of a tetra-substituted porphyrin guest being
held by the "clipper" molecule. Reprinted with permission from van Nunen,
J. L. Ì.; Folmer, B. F. B.; Nolte, R. J. M. j. Amer. Chem. Soc. 1997,
119, 283. (c) 1997 American Chemical Society.
vior was not observed for 2,6-substituted-dihydroxyphenyl-porphyrins due
to steric restrictions which prevented the "host" complex from binding in
the orr/?o-positions." Patel and Suslick recently reported the synthesis
and properties of a series of mesogenic porphyrins with liquid-
crystal phases stable over an extremely wide temperature range, which
greatly increases their potential utility. In addition, they isolated
novel metalloporphyrin mesogens that permitted axial ligation to the
metal, including one novel phase possessing a permanent dipole moment.
Hexagonal columnar discotic liquids crystals of the n-alkyl esters of
free-base 5,10,15,20-tetrakis(3,5-dicarboxyphe-nyl)porphyrin, H2[T(3,5-
C02R)PP], R = n-CnH2n+|, n = 10,12,14,16,18,20,22), displayed a narrowing
range of mesogenic behavior as the alkyl chain length increased (Figure
12). These porphyrins feature pocketed sites on both faces of the
porphyrin macrocyclic ring due to the relative rotation of the phenyl
groups (Figure 13). The presence of these protected pockets for axial
ligation was utilized to construct a unique five-coordinate vanadyl
derivative with dodecyloxyl chains. This approach produced both a liquid-
crystalline range of over 246 °C and a substantial dipole moment
perpendicular to the porphyrin plane.23
An early report of mesogenic behavior in a porphyrin species was
originally presented by Gaspard in 1984. In an attempt to discover the
minimum number of pendant long alkyl chains need to induce a liquid-
crystalline phase in a porphyrin system, derivatives of 5-phenalkoxy-
10,15,20-tritolylporphyrin and 5,15-di(phenalkoxy)-2,3,7,8,12,13, 17,18-
octamethylporphyrin were explored. The free-base compounds proved not to
be mesogenic. This was attributed to the nonplanar orientation of the
tolyl and phenoxyl groups relative to the porphyrin macrocyclic ring.
However, mixtures of the copper(II) derivatives of 5-phenyl-n-dodecyloxy-
10,15,20-tritolylporphyrin with one single long alkoxyl chain were
reported to display metastable meso-phases in combination with paraffin
or halogenated long alkyl chains.24
3. Di-substituted Porphyrins
Due to the disc-like geometry of the porphyrin macrocycle, discotic
mesophases have generally been the most common for porphyrin liquid-
crystalline materials. The first porphyrin liquid crystals featuring
"rod-like" calamitic mesophases were described by Bruce and coworkers
using substituted 5,15-di(phenyl) zinc porphyrin complexes as the basic
molecular building block.25 24 The first materials
Figure 12. Histogram of DSC results for T(3,5-COOR)PP materials: black =
solid phase, white = mesophase, gray = isotropic.
Figure 13. Structure of 3,5-di-carboxyphenyl porphyrin.
41 / Porphyrin Materials
Figure 14. Structure of ether-substituted di-phenyl porphyrins (R_CnH?n ,
i, n ¦= 8, 10, 12, 14, 16).25-2°
studied were derivatives of 5,15-(4-/i-alkoxyphenyl)por-phyrins,
Í^ÍÎÑöÍò,, i i)DPP|./i = 8, 10. 12, 14, 16), which feature long alkoxyl
chains directly attached in the para-phenyl position (Figure
14).Mesogenic compounds of the ë-octyloxy and "-decyloxy porphyrin
systems have relatively high melling points. Liquid crystalline materials
with shorter alkoxyl chains (R = R' = h n ~ 8, 10) were
characterized as having smectic Â phases, whereas, (he less-ordered
smectic F or smectic FJ phases were observed in the longerchain materials
(R = R' =CnHi" : ,,/i = 12, 14, 16).2:1
Subsequent extensions of this niolif incorporated a cvclohexane ester
or phenyl ester spacer between the n-alkoxyl groups and the /w.vo-phenyl
group on a single side.