Peripherally fused porphyrins via the Scholl reaction: Synthesis, self-assembly, and mesomorphism

Article abstract of DOI:10.1021/ja210991f

Oxidative coupling of activated aryl groups attached to β-positions of the porphyrin ring provides convenient access to derivatives containing peripherally fused phenanthrene and benzo[g]chrysene units. Tetra(benzochryseno) porphyrin, reported here for the first time, contains a nonplanar, sterically locked π system and shows very intense electronic absorptions in the Q range of the electronic spectrum. Tetraphenanthroporphyrins show a tendency to aggregate in solution. In one case, a discrete dimer is formed, whose structure was investigated spectroscopically and theoretically. Derivatives bearing long alkyl chains are mesomorphic and exhibit columnar phases (tetraphenanthroporphyrins) and a monoclinic 3D phase (tetrabenzochrysenoporphyrin). The symmetry of column packing in the columnar phases is dependent on the number of alkyl chains per molecule. X-ray diffraction measurements show that, in spite of their nonplanarity, the aromatic cores in the mesophases are tightly stacked within the column. The corresponding stacking patterns were derived from the structure of the dimer, on the basis of geometrical analysis and molecular modeling.

Full text of DOI:10.1021/ja210991f

Article
pubs.acs.org/JACS
Peripherally Fused Porphyrins via the Scholl Reaction: Synthesis,
Self-Assembly, and Mesomorphism
Damian Mysliwiec,† Bertrand Donnio,‡ Piotr J. Chmielewski,† Benoît Heinrich,‡ and Marcin Stepien*,†
́
́
̨
†Wydział Chemii, Uniwersytet Wrocławski, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland
‡Institut de Physique et Chimie des Mater
́ ́
iaux de Strasbourg, CNRS-Universite de Strasbourg (UMR7504), BP 43, F-67034
Strasbourg Cedex 2, France
S
* Supporting Information
ABSTRACT: Oxidative coupling of activated aryl groups
attached to β-positions of the porphyrin ring provides
convenient access to derivatives containing peripherally fused
phenanthrene and benzo[g]chrysene units. Tetra-
(benzochryseno)porphyrin, reported here for the first time,
contains a nonplanar, sterically locked π system and shows
very intense electronic absorptions in the Q range of the
electronic spectrum. Tetraphenanthroporphyrins show a
tendency to aggregate in solution. In one case, a discrete
dimer is formed, whose structure was investigated spectro-
scopically and theoretically. Derivatives bearing long alkyl chains are mesomorphic and exhibit columnar phases
(tetraphenanthroporphyrins) and a monoclinic 3D phase (tetrabenzochrysenoporphyrin). The symmetry of column packing
in the columnar phases is dependent on the number of alkyl chains per molecule. X-ray diffraction measurements show that, in
spite of their nonplanarity, the aromatic cores in the mesophases are tightly stacked within the column. The corresponding
stacking patterns were derived from the structure of the dimer, on the basis of geometrical analysis and molecular modeling.
which the expansion of the porphyrin π-electron system is
achieved by fusing adjacent β substituents (route D). In spite of
its apparent simplicity, this route remains largely unexplored.9
Here we show that appropriately designed β-aryl porphyrins
afford peripherally fused derivatives when subjected to Scholl-
type oxidations. This strategy provides access to large, fused
aromatic cores, which display promising chromophore proper-
ties and are easily functionalized on the periphery. Peripheral
functionalization of these macrocycles provides a means of
controlling their π-stacking self-assembly both in solution and
the liquid-crystalline state.10
INTRODUCTION
■
Organic structures with extensive, two-dimensional (2D) π-
conjugation play an increasingly important role in materials
chemistry.1 Several classes of motifs, such as hexa-peri-
benzocoronenes,1a porphyrins,1b,c and phthalocyanines,1c,d
have been investigated as electroactive cores for organic
semiconductors and photovoltaic devices.2 On the molecular
level, properties of these materials, such as charge carrier
mobilities,2b,d n- vs p-type semiconductivity,2c etc., can be
tailored in two ways: by designing new core motifs and by
controlling the self-assembly of the molecules in the bulk. A
high level of structural control is frequently achieved through
self-organization into well-defined, usually columnar meso-
phases,3 which facilitate fabrication of large-area thin devices
and are solution processable.2a
The electronic properties of core motifs strongly depend on
the extent of π-conjugation, and increasing the size of the π
system is advantageous in many potential applications. A
number of useful methods of “supersizing” the π-electron
system have evolved in the context of porphyrinoid chemistry.
The most successful strategies include macrocycle expansion,4
construction of multiporphyrin arrays,5 and fusion of additional
rings on the periphery of the macrocycle.6−8 To date,
peripherally fused macrocycles have been made by condensa-
tion of fused monopyrroles (Scheme 1, route A),6,8e−k by
aromatization of peripheral rings in a preformed macrocycle
(route B),8a−d or by oxidative fusion of meso substituents
(route C).5a−e,7 A complementary strategy can be envisaged, in
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Tetraphenanthro-
porphyrins. We focused our initial synthetic efforts on
2,3,7,8,12,13,17,18-octakis(3,4-dialkoxyphenyl)porphyrins 2a−
c-H2, containing, respectively, none, 8, and 16 dodecyloxy
chains per molecule (Scheme 2). The alkoxy groups were
introduced as convenient functionalization sites and as a means
of activating the aryl substituents toward oxidative coupling
reactions. Porphyrins 2a−c-H2 were prepared by condensing
appropriate monopyrroles 1a−c,11 which in turn were
synthesized via a modified Zard−Barton reaction12 (see
Received: November 22, 2011
Revised: December 18, 2011
Accepted: January 9, 2012
Published: January 9, 2012
© 2012 American Chemical Society
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Supporting Information). Scholl oxidations were performed on
zinc complexes 2b-Zn and 2c-Zn using 12 equiv of anhydrous
ferric chloride in dichloromethane, yielding alkoxy-substituted
tetraphenanthroporphyrins 3b-H2 and 3c-H2 in 74% and 97%
isolated yield, respectively.
Scheme 1. Synthetic Routes to Peripherally Fused
Porphyrins
a
The starting zinc complexes 2b-Zn and 2c-Zn are
demetalated in the course of the oxidation, most likely because
of the acidic reaction conditions. Even though the oxidation can
be performed on the free bases 2b-H2 and 2c-H2, the use of
zinc complexes offers higher yields and shorter reaction times.
In contrast, because of the very low solubility of 2a-Zn, the all-
methoxy derivative 3a-H2 was more conveniently prepared by
reacting the free base 2a-H2 with FeCl3 in o-dichlorobenzene.
The free bases 3a−c-H2 are easily metalated to the
corresponding Zn complexes under standard conditions.
Scholl-type oxidative couplings are known to proceed cleanly
for many types of aromatic rings.1a In some cases though, the
yields of these reactions are compromised by low regioselec-
tivity and unwanted side reactions (competition between inter-
and intramolecular coupling, oligomerization, quinone for-
mation, etc.)13 In the present work, the 3,4-dialkoxyphenyl
substituents in 2a−c-H2 are activated toward oxidative coupling
at both ortho positions (2 and 6), and mixtures of products
corresponding to both 2−2′ and 2−6′ coupling (Scheme 2)
could potentially be observed. In the reactions described above,
tetraphenanthroporphyrins are obtained as pure regioisomers,
suggesting that the 2−6′ coupling route is strongly disfavored.
a
“π” denotes a π-conjugated ring or ring system. (A) From fused
monopyrroles. (B) Aromatization of peripheral rings. (C) Fusion of
meso-substituents. (D) Fusion of β substituents (this work)
a
Scheme 2. Synthesis of Tetraphenanthro- and Tetra(benzochryseno)porphyrins Using the Scholl Reaction
a
Reagents and conditions: (1) LiAlH4, THF; (2) CHCl3, BF3·Et2O, then DDQ or chloranil; (3) FeCl3, o-dichlorobenzene; (4) (a) Zn(OAc)2,
CHCl3/MeOH; (b) FeCl3, CH2Cl2; and (5) BF3·Et2O, DDQ, CH2Cl2
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Figure 1. van der Waals models of 3a−c-H2 and 6b-H2 based on DFT optimized geometries (B3LYP/6-31G**).
Figure 2. 1H NMR spectrum of 3c-Zn (top, 600 MHz, toluene-d8, 380 K) and its dimer (3c-Zn)2 (bottom, 600 MHz, chloroform-d, 230 K). Blue
and red lines correlate signals of the monomer with those of the dimer. Green lines connect diastereotopic pairs of α-CH2 signals. The numbering
scheme is shown in the inset.
The identity of phenanthroporphyrins and their complexes
was confirmed by 1H and 13C NMR spectroscopy, high-
resolution mass spectrometry, and electronic absorption
spectroscopy. Proton magnetic resonance spectra of 3a−c-H2
and 3a−c-Zn show signal patterns consistent with the
formation of phenanthrene moieties. Specifically, the low-field
region of the spectrum contains three singlets, corresponding to
the meso protons and the two sets of protons attached to
phenanthrene units. The significant downfield shift of the meso
signal (∼11.9 ppm) is likely caused by the deshielding effect of
the flanking phenanthrenes.
Three-dimensional structures of the free base tetraphenan-
throporphyrins 3a−c-H2 were modeled using density functional
theory (B3LYP/6-31G**, Figure 1). Initially, it was recognized
that the macrocyclic core of 3a-H2 cannot be planar because of
steric interactions between the neighboring phenanthrene units.
Consequently, geometries of the four distinct conformers of 3a-
H2 were optimized (Supporting Information). Remarkably,
relative energies of the four conformers span a range of only 2
kcal/mol. The conformer with lowest energy, shown in Figure
1, has a saddle-type distortion corresponding to the effective
D2d symmetry in the limit of fast tautomerization. This
structure qualitatively resembles geometries predicted for
meso-substituted derivatives,8i except that the degree of out-
of-plane deformation is much smaller. The molecule of 3a-H2 is
roughly disk-shaped with the van der Waals diameter of ca. 25
Å. This value increases to ca. 53 Å for 3b-H2 and 3c-H2, both of
which were modeled with fully outstretched dodecyl chains.
The nonplanarity of 3b−c-H2 is not observed in the
1
experimental H NMR spectra recorded at room temperature,
which correspond to effective D4h symmetry. Any nonplanar
structure should exhibit diastereotopic differentiation of α-CH2
groups. The absence of this feature indicates that nonplanarity
in tetraphenanthroporphyrins is averaged out by conforma-
tional motions in solution.
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At room temperature, 1H NMR spectra of alkoxy-substituted
phenanthroporphyrins and their complexes show a varying
degree of line broadening, which is dependent on sample
concentration, temperature, and solvent. This effect is
attributable to π-stacking aggregation in solution,14 which is
likely promoted by the extended π-surface of the macrocycle.
Indeed, no similar line broadening is observed for the
unoxidized species 2a−c-H2 and 2a−c-Zn, in which the β-
aryl substituents are not coplanar with the macrocycle, thus
likely suppressing aggregation. The line broadening effect
becomes very strong for the zinc(II) complexes 3b-Zn and 3c-
Zn. A well-resolved 1H NMR spectrum of the latter species was
nevertheless recorded in toluene-d8 at 380 K (Figure 2). In the
case of 3c-Zn, π-stacking interactions lead to the equilibrium
formation of a well-defined aggregate, as can be inferred from
Initial screening of several structures showed that the
interaction energy in the dimer strongly depends on the extent
of overlap between the two π systems. High stabilization was
observed for dimers consisting of two saddle-shaped (∼D2d)
conformers combined into a laterally displaced sandwich
(Figure 3). Two modes of interaction, C2h and C2, could be
1
the H NMR spectra recorded in chloroform-d over a range of
temperatures. At 300 K, the spectrum contains broadened
signals corresponding to the fully symmetric monomer and to a
species of lower symmetry, which yields exchange correlations
with the monomer signals in the NOESY spectrum. As the
temperature is lowered, the molar fraction of the low-symmetry
species increases, and at 230 K, the monomer is no longer
observed (Figure 2). In dichloromethane-d2, the low-symmetry
species prevails, and signals of the monomer are not observed
even at 290 K. In the latter solvent, lowering the temperature
below 260 K causes dramatic broadening of all signals, and no
decoalescence is observed down to 190 K.
Figure 3. Optimized structure of [3a-Zn]2 (ωB97XD/6-31G**). (A)
Top and (B) side views and (C) side view cross section revealing the
stacking interaction between the cores. Red and blue colors denote the
two interacting components of the dimer. For definitions of z, x, and
φ, see footnote of Table 1.
distinguished for [3a-H2]2. Each of these modes provides a
large contact area between the two π surfaces but differs in the
in-plane alignment of the two molecules. Locally, the separation
between monomer molecules in the DFT and MM models can
be as small as 3.3 Å, which is a typical π−π stacking distance.
The two curved π-systems are oriented in such a way that the
convex side of a phenanthrene unit in one molecule is placed
above the saddle-shaped center the other molecule. The C2h
structure of [3a-H2]2 is consistent with the symmetry of the 1H
NMR spectrum of [3c-Zn]2 and is predicted to be more stable
than [3a-H2]2-C2 by ca. 8.7 kcal/mol (Table 1). The energy gap
between C2h and C2 structures is relatively small when
compared with the dimerization energies.
Even though the low-symmetry spectra observed in
chlorinated solvents are quite broad at all accessible temper-
atures, partial assignment of signals was obtained from the
analysis of correlation spectra, providing evidence consistent
with the formation of a dimer, [3c-Zn]2. The effective
symmetry of a single macrocyclic subunit in the proposed
dimer structure is reduced from D4h to Cs, with the plane of
symmetry passing through two opposite phenanthrene
1
moieties. Consequently, the H NMR spectrum of [3c-Zn]2
contains two meso signals, eight phenanthrene signals (denoted
1 and 4 in Figure 2), and eight diastereotopic pairs of α-CH2
signals. On the basis of the observed spectral symmetry, it can
be assumed that the two macrocyclic subunits are not stacked
directly one above the other but displaced laterally. In such a
structure, the diastereotopic differentiation of α-methylene
signals is consistent with the two faces of the macrocycle being
nonequivalent. The signals of the meso and phenanthrene
protons in [3c-Zn]2 show consistent upfield relocations (up to
ca. 3 ppm) relative to their positions in the monomer. Such an
effect is expected for two stacked aromatic rings, which
experience mutual shielding caused by their respective ring
currents.14c
The [3c-Zn]2 dimer can be considered a structural prototype
for larger π-stacked assemblies of tetraphenanthroporphyrins,
including those involved in the formation of discotic
mesophases (see below). Unfortunately, systematic theoretical
exploration of the dimerization process is a complex task, given
the size of the system, the number of degrees of freedom, and
the necessary level of theory. For an initial qualitative analysis,
the problem can be simplified by considering structures
containing only methoxy groups, namely [3a-H2]2 and [3a-
Zn]2. These structures were modeled using a high-level
dispersion-corrected DFT method (ωB97XD/6-31G**) and,
for comparison, with a general-utility molecular mechanics
force field (MM+, Hyperchem).15
Table 1. Calculated Energies and Geometrical Parameters of
Dimers [3a-H2]2 and [3a-Zn]2
a
b
species
method
E [kcal/mol]
z [Å]
x [Å]
φ [°]
c
[3a-H2]2-C2h
DFT
76.23
3.97
3.86
4.11
4.14
3.57
5.49
4.49
5.68
5.44
4.40
90
90
d
MM
[3a-H2]2-C2
DFT
MM
DFT
67.55
83.89
107
105
90
[3a-Zn]2-C2h
a
Dimerization energies relative to optimized monomers (gas phase,
b
counterpoise-corrected DFT energies). z is the stacking distance
calculated as the distance between the N4 centroid of one molecule
and the N4 mean plane of the other molecule; x is lateral offset,
calculated as x = (r2 − z2)1/2, where r is the distance between the two
N4 centroids; φ is the approximate angle necessary to superimpose the
molecules after translating one of them onto the other along the z and
c
d
x vectors. ωB97XD/6-31G**. MM+ force field (Hyperchem).
For the sake of subsequent discussion of mesophase
structures, it is convenient to quantify the interaction geometry
in the dimers using three parameters: the separation of mean
N4 planes of the two macrocycles (z), the lateral offset of ring
centers (x), and the rotation angle φ (Table 1, Figure 3). The
overlap of the two π systems is larger in the Zn complex as
evidenced by the smaller value of the lateral offset x. In line
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1
Figure 4. H NMR spectrum of tetra(benzochryseno)porphyrin 6b-H2 (600 MHz, chloroform-d, 325 K). The assignment of signals was obtained
from correlation spectroscopy. For additional NMR data, see Supporting Information.
with this observation, the gas-phase interaction energies
calculated by DFT are 83.89 and 76.23 kcal/mol for [3a-
Zn]2-C2h and [3a-H2]2-C2h, respectively. These values are
significantly larger than the dimerization energies calculated for
dimers of porphine,15c reflecting the expansion of the aromatic
core in tetraphenanthroporphyrins and, possibly, the presence
of alkoxy substituents, which can be involved in additional
stabilizing nonbonded interactions. Interestingly, the geo-
metries of [3a-H2]2-C2h and [3a-H2]2-C2 optimized using the
MM+ force field are in fairly good agreement with the DFT
structures.
throporphyrins, 6b-H2 is sterically “locked” because the
benzochrysene subunits eclipse each other to a greater extent
than phenanthrene rings in compounds 3a−c-H2. This
eclipsing arrangement of aromatic rings in 6b-H2 causes
substantial shielding of one of the methoxy groups, which
resonates at 3.00 ppm.
To gain further insight into the three-dimensional structure
of tetra(benzochryseno)porphyrins, a number of conformers of
the all-methoxy variant, 6a-H2 (not synthesized) were
optimized at the B3LYP/6-31G** level of theory. The lowest
energy conformer was found to have the expected alternating
arrangement of fused subunits similar to that found in
tetraphenanthroporphyrins. Each of the subunits is internally
twisted so as to reduce congestion in the fjord regions,18
resulting in a highly nonplanar but rather tightly packed
aromatic structure. The preferred conformation has the S4 point
symmetry (in the limit of fast tautomerization), in line with the
observed NMR spectral pattern. Reoptimizing the structure
with dodecyl chains attached provides a plausible 3D model of
6b-H2 (Figure 1). As a consequence of the out-of-plane
distortion, the aromatic core of 6b-H2 is significantly thicker
than the cores of tetraphenanthroporphyrins. However, the
diameter of the molecule (55.5 Å, including the dodecyl chains)
is only slightly larger than those of 3b−c-H2 in spite of the
additional fused benzene rings.
Electronic Structure. Expansion of the π-conjugated
system by peripheral ring fusion strongly affects the electronic
absorption profiles of tetraphenanthro- and tetra-
(benzochryseno)porphyrins and their zinc(II) complexes
(Figure 5). With the increasing size of the π system, both
Soret and Q bands are shifted bathochromically relative to their
positions in the spectra of parent porphyrins (Table 2). The
spectra of tetraphenanthroporphyrins, which show no signifi-
cant dependence on the alkyl substitution pattern, are
qualitatively similar to those reported earlier for differently
substituted species.8e In particular, λmax increases by ca. 60 nm
on going from porphyrin 2c-H2 to tetraphenanthroporphyrin
3c-H2, which contains 12 fused benzenoid rings). Interestingly,
further extension of the π system in 6b-H2, which involves
fusion of eight additional rings, leads to a smaller bathochromic
shift of 25 nm relative to 3c-H2. This observation shows that
when peripheral ring fusion occurs at a distance from the
porphyrinic core, it becomes less efficient at decreasing the
Tetra(benzochryseno)porphyrin. The oxidative coupling
strategy described above for octaarylporphyrins is also
applicable to porphyrins bearing polycyclic aromatic substitu-
ents, such as 5b-H2 (Scheme 2). This porphyrin, which bears
four phenyl and four 9-phenanthryl groups, was obtained from
the corresponding monopyrrole 4b in 34% yield. The
condensation step proceeds without observable scrambling
(reversible dissociation of oligopyrrole intermediates16),
yielding just one regioisomer. However, 5b-H2 is isolated as a
mixture of atropisomers resulting from the restricted rotation of
the bulky phenanthryl substituents.14c In an initial synthetic
attempt, this isomeric mixture was converted into the zinc(II)
complex 5b-Zn, which was in turn subjected to oxidative
coupling with FeCl3. After workup, tetra(benzochryseno)-
porphyrin17 6b-H2 was isolated in 42% yield. A more efficient
synthesis was therefore sought: It was found that the free base
5b-H2 would produce the desired benzochryseno- derivative in
yields of up to 78% when treated with four equivalents of DDQ
in the presence of boron trifluoride etherate. 6b-H2 is easily
metalated to provide the zinc complex 6b-Zn.
1
The H NMR spectrum of 6b-H2 is consistent with the
presence of 4-fold symmetry in the molecule, providing
additional indication that the parent porphyrin 5b-H2 used
for oxidation was a pure etio I-type isomer (Figure 4). The
alkoxy-substituted benzo[g]chrysene system fused to each
pyrrole ring gives rise to six singlets in the aromatic region,
which are accompanied by signals corresponding to three
nonequivalent methoxy groups and three dodecyloxy chains.
The α-methylene groups within the chains exhibit marked
diastereotopicity, indicating that the macrocycle is not planar
and the nonplanarity is not averaged out by conformational
motions in solution. Apparently, in contrast to tetraphenan-
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While the red shifts observed in 3a−c-H2 and 6b-H2 are
much smaller than those reported for the best meso-arene-
fused porphyrins,7f the expansion of the π system leads to a
substantial increase in the intensity of Q bands.20 The
maximum Q-band intensity increases from 24 100 (2c-H2,
522 nm), through 80 300 (3c-H2, 650 nm), up to 141 000
M−1cm−1 (6b-H2, 686 nm). The latter value compares
favorably with those reported for Q bands of cyclo[8]-
pyrroles21,10c (∼1.3 × 105 M−1cm−1) and a quadruply
anthracene-fused porphyrin7f (∼1.2 × 105 M−1cm−1). A similar
trend is observed for zinc complexes, with the principal Q-band
in 6b-Zn reaching ε = 183 000 M−1cm−1
Kohn−Sham orbitals obtained from DFT calculations for the
all-methoxy-substutited structures 3a-H2 and 6a-H2 are shown
in Figure 6. In each case, the range of orbitals included covers
those principally involved in the strongest electronic transitions,
as predicted by TD-DFT. In the case of tetraphenanthropor-
phyrin 3a-H2, the two lowest-lying excited states, assumed to
represent experimental Q bands, are mixtures of excitations
involving the four orbital range (364−367, Figure 6),
corresponding to the Gouterman model.22 For higher-energy
states, including those responsible for the Soret absorption, the
range of occupied levels involved is larger, leading to
considerable mixing of excitations. In the case of tetra-
(benzochryseno)porphyrin 6a-H2, the most intense Q-band
excited states are mixtures of excitations from levels 533, 532,
and 528. It is noteworthy that the MO coefficients on the extra
benzene rings fused to 6a-H2 are rather small in the frontier
orbitals (especially in LUMO and LUMO+1). This might
explain the small bathochromic shift in tetra(benzochryseno)-
porphyrin relative to tetraphenanthroporphyrin.
Figure 5. Electronic spectra of porphyrin 2c-H2, tetraphenanthropor-
phyrin 3c-H2, tetra(benzochryseno)porphyrin 6b-H2, and their
zinc(II) complexes (dichloromethane, 293 K).
Table 2. Electronic Properties of Porphyrins and Their
Analogues Discussed in the Text
Mesomorphism. The thermal phase behavior of porphyr-
ins 2b−c-M, tetraphenanthroporphyrins 3b−c-M (M = 2H,
Zn) and zinc tetra(benzochryseno)porphyrin 6b-Zn was
investigated using small-angle X-ray diffraction (XRD, Table
3), differential scanning calorimetry (DSC, Figure 7), and
polarizing optical microscopy (POM).23 With the exception of
3b-H2,24 all of the materials analyzed form well-defined liquid
crystalline phases, often exhibiting broad ranges of thermal
stability. Three distinct types of 2D and 3D phases were
identified, whose structural diversity results from variations of
the substitution pattern and the nature of the core motif.
Compounds 2b-H2 and 2b-Zn, both containing eight
dodecyl chains per molecule, exhibit structurally similar Colh
phases. The hexagonal symmetry of these phases was assigned
unequivocally on the basis of the XRD patterns, containing one
or two sharp reflections in the small-angle region. Close
inspection of the wide-angle scattering revealed the presence of
three overlapping signals: (i) a large diffuse halo associated with
molten aliphatic chains (hch, correlation length ξ = 10 Å),
showing the largest temperature shift,25 (ii) the typical h0 signal
at 3.6 Å, resulting from π−π stacking of aromatic rings, and (iii)
an additional semidiffuse reflection at ca. 4.7 Å (ξ ≈ 50 Å),
denoted hpor. The occurrence of these three diffuse wide-angle
signals strongly suggests a double-segregation process, leading
to one-dimensional stacking of porphyrins into columns, with
the peripheral aromatic systems segregated from both the
aliphatic continuum and the central porphyrin cores. The hpor
signal, clearly identifiable for 2b-H2 and 2b-Zn, is in good
agreement with interplanar distances between stacked macro-
cyclic rings, observed in the solid-state structures of β-aryl-
substituted porphyrins.26 Furthermore, the average molecular
thickness hM of 2b-H2 and 2b-Zn, calculated as the ratio of
absorptions [nm]
HOMO−LUMO gap [eV, V]
a
b
c
species
Soret
λmax
UV−vis
CV
DFT
2c-H2
3c-H2
6b-H2
2c-Zn
3c-Zn
6b-Zn
427
474
493
431
475
483
639
698
724
589
664
679
1.94
1.78
1.71
2.11
1.87
1.83
2.27
1.66
1.77
2.18
1.76
1.75
2.10
1.92
1.82
2.22
1.96
1.86
a
b
Calculated from λmax [eV]. Eox − Ered, measured by cyclic
c
voltammetry [V]. B3LYP/6-311G**//B3LYP/6-31G** [eV]; calcu-
lated for all-methoxy-substituted structures (2a-H2, 3a-H2, etc.). For
complete UV−vis, electrochemistry, and DFT data, see the Supporting
Information.
highest occupied−lowest unoccupied molecular orbital
(HOMO−LUMO) gap. This conclusion is confirmed by
electrochemistry data and DFT calculations (Table 2). Cyclic
voltammetry measurements reveal that expansion of the π
system in the sequence 2c-M, 3c-M, 6b-M (M = 2H or Zn)
results in gradual anodic shifting of the first reduction potential.
Interestingly, the change is not systematic in the case of the first
oxidation potential, which decreases in the sequence 2c-M >
6b-M > 3c-M. As a consequence, the electrochemically
determined HOMO−LUMO gaps for tetra(benzochryseno)-
porphyrins are comparable to or slightly larger than those of
their tetraphenanthro- couterparts. The calculated HOMO−
LUMO gaps are overestimated by ca. 0.1 eV, which is an
expected deviation at the employed level of theory,19 but the
decrease of the gap with increasing conjugation is well
reproduced.
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Figure 6. Highest occupied and lowest unoccupied Kohn−Sham orbitals of tetraphenanthroporphyrin (3a-H2, top) and tetra(benzochryseno)-
porphyrin (6a-H2, bottom) calculated at the B3LYP/6-311G**//B3LYP/6-31G** level of theory (cutoff level 0.009 au).
Table 3. Geometrical Parameters of Mesophases Obtained from XRD
ab
a
,
lattice parameters
core parameters
c
e
g
h
i
j
j
j
k
l
d
f
T
a × b × c
[Å]
qhex
hM
VM
Vc
Sc
σc
ψc
[°]
Dc
dc
dc/Dc
Σc
qch
species [°C]
type
[Å] [103 Å3]
[103 Å3]
[Å2]
[Å2]
[Å]
[Å]
[Å2]
2b-H2
2b-Zn
3b-Zn
2c-H2
235
190
140
120
160
100
120
100
150
Colh
Colh
Colh
Colr1
Colr2
Colr1
Colr1
Colr1
Mon
34.5
34.8
34.5
1
5.3
5.1
5.0
7.9
8.3
7.9
7.9
7.1
5.7
5.45
5.32
5.16
7.67
7.86
7.58
7.66
7.57
7.69
2.19
2.16
2.11
2.11
2.14
2.09
2.09
2.08
3.07
4.16
4.24
4.21
2.67
2.59
2.66
2.67
2.93
5.36
416
424
421
424
424
424
421
421
0
23.0
23.2
23.2
23.2
23.2
23.2
23.2
23.2
28.0
1
381
372
364
476
492
473
472
441
472
0.97
0.97
0.98
1.30
1.31
1.31
1.29
1.22
0.84
1
0
1
1
0
1
53.1 × 36.6
63.4 × 30.0
52.6 × 36.6
52.5 × 37.2
56.0 × 38.1
68.2 × 39.0 × 81.0
0.84
1.22
0.83
0.82
0.85
1.01
51.0
52.4
51.2
50.7
45.9
29.0
14.6
14.2
14.6
14.7
16.1
24.4
0.63
0.61
0.63
0.63
0.70
2c-Zn
3c-H2
3c-Zn
6b-Zn
m
n
614
−
b
a
See Figure 8 for definitions of relevant parameters and Table S6, Supporting Information for complete data and formulas used. Lattice symmetries
c
are p6mm (2D, Colh), c2mm (2D, Colr1 and Colr2), and P21/m (3D, Mon). Sublattice compression ratio relative to an ideal hexagonal lattice; qhex
a′/(b′√3) and for Colr phases a′ = a, b′ = b and for the Mon phase b′ = b, a′ = d001. Columnar slice thickness. Molecular volume at the given
temperature. Partial volume of the molecular core (alkyl chains and methyl groups excluded from the calculation). Effective cross-section of the
=
d
e
f
g
h
i
core perpendicular to the column axis, Sc = Vc/hM. Cross-section of the core in the absence of tilt. Sc = σc for nontilted phases. Tilt angle of cores
j
relative to column axis. Maximum (Dc) and minimum (dc) diameters of the elliptical cross-section Sc; dc/Dc ratio is the ellipticity of the core.
k
l
m
Interface area between the core and the aliphatic periphery. Chain packing ratio; for explanation see text. β = 98.0°, and V = 2.13 × 105 Å3.
n
Parameters in italics are calculated for hypothetical tilted packing of column segments.
molecular volume VM and columnar cross-section S (Table 3),
is only slightly larger than hpor, indicating that the porphyrin
rings are perpendicular to the columnar axis. Small off-axis tilts
and lateral shifts of the macrocycles, resulting from thermal
fluctuations, likely account for the small discrepancies between
hM and hpor values.
Zinc tetraphenanthroporphyrin 3b-Zn forms a well-defined
Colh−p6mm phase between 110 and 180 °C, the identity of
which was confirmed by XRD and DSC. The structural
parameters of this phase (Table 2) closely resemble those
determined for the corresponding porphyrin complex 2b-Zn.
This similarity is not surprising because molecular volumes of
2b-Zn and 3b-Zn are nearly identical, but it does not
necessarily imply exact correspondence of the packing pattern.
It should be noted that in 2b-Zn, the core consists of a planar
macrocycle surrounded by eight aryl groups, whereas in 3b-Zn,
the core comprises a single saddle-shaped π-system. In the
latter case, the central part of the π system (porphyrin) and the
peripheral “wings” (phenanthrene units) can still give rise to
separate scattering signals (hpor and h0, respectively). In contrast
Figure 7. Thermal data for liquid-crystalline porphyrins, tetraphenan-
throporphyrins, and tetra(benzochryseno)porphyrin obtained from
differential scanning calorimetry. Symbols: Cr, crystalline or
pseudocrystalline; Col, columnar phases; and I, isotropic liquid.
*Transition detected by polarizing optical microscopy.
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Figure 8. Packing diagrams for selected discotic mesophases. Effective core dimensions are indicated as colored ellipses. Alkyl chain continuum is
shown in light blue, and a′ and b′ denote rectangular axes of the column packing sublattice; qhex = a′/(b′√3). In the diagram of the Mon phase
(right), the extent of the disruption zone along the c axis is arbitrary. Standard cell setting for the P21/m space group is shown in red.
to porphyrins, these two signals are not well resolved in the
wide-angle scattering pattern observed for 3b-Zn.
but it does not include the effect of thermal motions. In the
actual Colh phase, the helical packing is likely effective only over
short distances, combining sections of opposite helicity. The
expected variability of the x, z, and φ parameters will contribute
to the overall disorder within the stack.
In the Colh mesophase of 3b-Zn, the stacking distance hM =
5.0 Å is significantly smaller than the van der Waals thickness of
the distorted tetraphenanthroporphyrin ring (ca. 8 Å, Figure 1).
Consequently, the aromatic cores in the Colh phase must be
packed quite tightly, with curvatures of adjacent π systems in
the stack matched in a similar fashion as in the dimer [3a-Zn]2.
Thus the mean plane separations predicted for dimers [3a-H2]2
and [3a-Zn]2 (z = 3.6−4.1 Å, Figure 3) constitute the lower
limit of stacking height. In an actual stack the steric
requirements imposed by neighboring molecules and peripheral
substitution will likely be different than in the dimers, leading
to an increase of the z distance. Curvature matching between
consecutive molecules in the stack implies lateral displace-
ments, analogous to that defined by the x vector in the [3a-
Zn]2 dimer. Experimental evidence for such displacements is
provided by the effective core diameter Dc (23.2 Å), which is
larger than the geometrical diameter of the tetraphenanthro-
porphyrin nucleus (19 Å, oxygen−oxygen distance in DFT
models). An important observation, which follows from the
earlier discussion of the dimer structure, is that consecutive x
vectors within the stack cannot be collinear but have to be
rotated by the angle φ (Figure 3) with respect to the column
axis. Additionally, to make the stack grow along the column
axis, the sense of rotation has to be the same for all x vectors,
resulting in a helical arrangement of molecules. The φ angles
are determined by the saddle-like shape of the π surface and
should approximate the values obtained for the dimer models
(Table 1). An idealized representation of the stacking paradigm
outlined above can be obtained from molecular mechanics
calculations. The helical stack shown in Figure 9 consists of 16
molecules of 3a-Zn, yielding an average stacking distance of 4.5
Å. This value is smaller than the columnar slice thickness hM,
Porphyrins 2c-M and tetraphenanthroporphyrins 3c-M (M =
2H, Zn), containing 16 dodecyl chains per molecule, form
liquid crystalline phases at relatively low temperatures. These
mesophases show excellent ranges of thermal stability and were
fully characterized using XRD, DSC, and POM. For all systems
of the c series, diffractograms showed a pair of fundamental
reflections, (11) and (20), accompanied by several higher order
peaks in the small-angle region. These features enabled
unequivocal assignment of rectangular lattices with c2mm
symmetry and determination of lattice parameters. In contrast
to the b series, the reflections constituting the wide-angle
diffuse signals are not as easily separable because of the much
stronger contribution of the hch scattering (the volume fraction
of the aliphatic continuum is doubled in the c series), and the
smaller expected shift between hch and hpor at low temperature.
For similar reasons, the h0 peak cannot be precisely extracted.
Porphyrin 2c-H2 exhibits a transition between two rectangular
phases of c2mm symmetry, clearly evidenced by XRD and DSC
data, that is not observed for other compounds in the c series.
The two phases differ in the ordering of fundamental
reflections: the low-temperature phase, denoted Colr1, is
characterized by d11 > d20, similarly to other compounds in
the c series. The order of reflections is inverted in the high-
temperature phase Colr2, which consequently has different
lattice parameters. The qhex ratio (Table 3, Figure 8), which
expresses the distortion of the Colr lattices relative to the
hexagonal lattice, takes values of ca. 0.8 for all Colr1 phases,
indicating that the lattice is compressed along the a direction
(only the d20 fundamental periodicity is shrunken with respect
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to the fundamental reflection of the Colh phase of 2b-H2). For
the unique Colr2 phase of 2c-H2, qhex equals 1.2, corresponding
to a compression of the lattice along the b direction (here, the
reduced periodicity corresponds to two fundamental reflections
d11 and d1,−1).
parameters are underestimated relative to experimental data,
indicating the influence of heavy peripheral substitution in the c
series on the packing geometry within the columns.
In contrast to porphyrins and tetraphenanthroporphyrins
described above, which display columnar phases with 2D
ordering, zinc tetra(benzochryseno)porphyrin 6b-Zn forms an
unusual three-dimensional (3D) phase, denoted Mon, which is
stable from ca. 130 to 232 °C. Formation of three-dimension-
ally ordered phases from discoid molecules is a rare
phenomenon, typically involving bicontinuous phases with
cubic symmetry.29 XRD patterns recorded on cooling at three
different temperatures contain up to 10 sharp, small-angle
reflections, whose indexation converges toward a monoclinic
lattice (Figure S20 and Tables S3 and S5, Supporting
Information).30 Given the presence of the (100) and (201)
reflections, five space groups are compatible with the proposed
indexation: groups P2, Pm, and P2/m with no reflection
conditions and the groups P21 and P21/m, where the
occurrence of a 21 screw axis parallel to the b direction sets
the condition k = 2n for the (0k0) reflections.30 The absence of
the (010) reflection makes the groups with screw axis more
likely. Unit cell parameters determined at 100 °C yield a cell
volume of 2.13 × 105 Å3, corresponding to approximately 28
molecules of 6b-Zn per unit cell.
The hM values determined for the Colr1 and Colr2 phases are
in the range of 7.1 to 8.3 Å, indicating that the macrocyclic
cores must be tilted within the columns. The direction of the
tilt coincides with the compressed axis in the rectangular lattice
(Figure 8).27 The Colr1 and Colr2 phases can be viewed as
discotic analogs of smectic phases SmF and SmI, respectively.
In the Colr1 and SmF phases, the molecules are tilted toward
the edge of the pseudohexagonal network, whereas in the Colr2
and SmI phases the tilt is directed toward the nearest vertex of
the lattice.28 Geometrical parameters of the core in the Colr
phases, namely its effective cross-section σc, maximum and
minimum diameter Dc and dc and the tilt angle ψc can be
calculated using data of the nontilted Colh phases as a reference
(Figure 8 and Table 3, see the Supporting Information for
details of these calculations). A small divergence of the qhex ratio
(Table 2) with respect to the core ellipticity (dc/Dc ratio) can
be noticed for the investigated Colr phases. This effect
evidences the blurring out of the elliptical shape by the
aliphatic continuum, possibly resulting from limited tilt
correlation within the lattice plane.
The proposed organization of the Mon phase, shown in
Figure 8, is analogous to a structure described recently in a
series of semifluorinated dendrimers.31 The mesophases formed
by this family of dendrimers contained columns of segregating
dendritic moieties and an aliphatic periphery. However, for the
highest generation dendrimer, the columns disrupted into
strings of elongated micelles. The relative positions of
disruption zones in neighboring strings generated the P21/m
space group symmetry. In the Mon phase of 6b-Zn, the 3D
organization is generated with segments of rigid columns,
instead of micelles, which lie along the c axis of the monoclinic
lattice and are separated by disruption zones. As the P21/m unit
cell contains two such segments, each of them will contain an
average of 14 discoid molecules, corresponding to a slice
thickness hM of ca. 5.7 Å, which is not much larger that the hM
values determined for the Colh phases. The actual mean
separation between discogens will be even smaller in the Mon
phase, because the calculated hM does not take into account the
height of the disruption zone (the lower stacking limit,
estimated for 6a-H2 using molecular modeling is ca. 4.8 Å).
The small hM value additionally indicates that the discogens
should not show any systematic tilting within the column
segment. In contrast to micelles, which are easily deformable,
the envelope of the columnar segments cannot so easily deviate
from cylindrical shape. The rigidity of these segments might
explain the persistence of the near-perfect hexagonal symmetry
of the columnar lattice (qhex ≈ 1), which is perpendicular to the
c axis of the monoclinic cell (Figure 8). With respect to this
sublattice plane (identifiable as d001), the [ab] plane is tilted by
an angle of 8° (i.e., β − π/2), which corresponds to a vertical
shift of the second neighboring row of columns of about 10 Å
along the c direction (a × sin(β − π/2) = 9.5 Å). This shift is
relatively small compared to the cell dimensions, making the
resulting organization quite similar to either an orthorhombic
or a 3D hexagonal lattice, in which consecutive rows of column
sections are shifted by half the periodicity along the c direction.
The symmetry breaking toward a monoclinic symmetry,
observed for 6b-Zn, is indicative of a subtle divergence of
columns from the perfect cylindrical shape. In the absence of
As in the case of the Colh phases, the curvatures of
neighboring molecules in the stack will also have to be aligned
in the tilted rectangular phases. The simplest arrangement of x
vectors giving rise to a tilted stack is a zigzag pattern in which
consecutive φ angles take opposite signs. Zigzag stacks of this
kind constructed from the all-methoxy tetraphenanthropor-
phyrin molecules 3a-H2 yield a columnar slice thickness hM of
ca. 6 Å and a tilt angle ψc of approximately 45° (Figure 9). Both
Figure 9. Models of helicoidal (left) and zigzag (right) columnar
packing obtained for the all-methoxy tetraphenanthroporphyrin 3a-H2
(MM+).
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tilting, this divergence has to be explained by other structural
features, such as correlated off-axis shifts or dissymmetric ends
of column segments.
Formation of the unique 3D mesophase of tetra-
(benzochryseno)porphyrin shows that peripherally fused
porphyrin derivatives provide a potentially general class of
core motifs for construction of structurally nontrivial liquid-
crystalline materials. Further extension of this work may help in
tailoring mesophase structures of large nonplanar aromatics so
as to meet specific needs of molecular electronics applications.
The evolution of mesomorphism in the family of compounds
described above can be explained by comparing the interface
area between the discoid core and the aliphatic continuum with
the requirements imposed by the pattern of peripheral
substitution.32 Interface areas Σc (Table 3) depend on the
dimensions of the core and on column packing and were
calculated using reported dilatometric data.25 On the basis of
the interface area, the chain packing ratio can be defined as qch
= (Σc/Nc)/σch, where Nc is the number of peripheral functional
groups (16 for porphyrins and tetraphenanthroporphyrins, 24
for tetra(benzochryseno)porphyrin) and σch is the effective
cross-section of a stretched alkyl chain at the temperature of
measurement. In the c series, qch is approximately 1.3,
indicating that the 16 dodecyl chains would exceed the
available interface area in the case of an untilted arrangement
and that the area matching is achieved by tilting the cores
relative to the column axis. For Colh phases of the b series, the
presence of smaller methyl substituents enables other
mechanisms of area matching, such as expulsion of sub-
stitutents from the interface by fluctuations in the columnar
stacking. In the case of 6b-Zn neither of the above mechanisms
is apparently sufficient to overcome steric crowding of alkyl
chains at the interface, because of the packing requirements of
24 peripheral groups attached to the macrocycle exceed the
available interface area (qch ≈ 0.84 for a theoretical tilted
arrangement). The surface area is however expanded by folding
some of the chains over the ends of the columnar segments
(Figure 8), leading to the formation of disruption zones
between them. This mechanism of surface matching is
potentially more efficient than it could be for porphyrins and
tetraphenanthroporphyrins because of the larger effective cross-
section of the macrocyclic core of 6b-Zn.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures, analytical data, and computational details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
marcin.stepien@chem.uni.wroc.pl
ACKNOWLEDGMENTS
■
Financial support from the Ministry of Science and Higher
Education (decision 105/10/E-344/M/2011) is gratefully
acknowledged. We thank the Foundation for Polish Science
for financial support at an early stage of this project. Quantum
chemical calculations were performed in the Wrocław Center
for Networking and Supercomputing. We thank Piotr
Stefanowicz for recording mass spectra. B.D. and B.H. thank
the CNRS and University Strasbourg for support and D. Burger
for the TGA measurements (IPCMS, le service commun
́
d′analyse de l′Universite de Strasbourg).
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