Benzobisoxazole cruciforms: Heterocyclic fluorophores with spatially separated frontier molecular orbitals

Article abstract of DOI:10.1021/jo202107w

We report the synthesis of nine conjugated cruciform-shaped molecules based on the central benzo[1,2-d:4,5-d′]bisoxazole nucleus, at which two conjugated currents intersect at a ～90° angle. Cruciforms' substituents were varied pairwise among the electron-

Full text of DOI:10.1021/jo202107w

Article
pubs.acs.org/joc
Benzobisoxazole Cruciforms: Heterocyclic Fluorophores with
Spatially Separated Frontier Molecular Orbitals
†
†
‡
,†
̌
Jaebum Lim, Thomas A. Albright, Benjamin R. Martin, and Ognjen S. Miljanic*
́
†Department of Chemistry, University of Houston, 136 Fleming Building, Houston, Texas 77204-5003, United States
‡Department of Chemistry, Texas State UniversitySan Marcos, 601 University Drive, San Marcos, Texas 78666, United States
S
* Supporting Information
ABSTRACT: We report the synthesis of nine conjugated
cruciform-shaped molecules based on the central benzo[1,2-
d:4,5-d′]bisoxazole nucleus, at which two conjugated currents
intersect at a ∼90° angle. Cruciforms’ substituents were varied
pairwise among the electron-neutral phenyl groups, electron-
rich 4-(N,N-dimethylamino)phenyl substituents, and electron-
poor pyridines. Hybrid density functional theory calculations
revealed that the highest occupied molecular orbitals
(HOMOs) are localized (24−99%) in all cruciforms, in contrast to the lowest unoccupied molecular orbitals (LUMOs)
which are strongly dependent on the substitution and less localized (6−64%). Localization of frontier molecular orbitals (FMOs)
along different axes of these cruciforms makes them promising as sensing platforms, since analyte binding to the cruciform
should mandate a change in the HOMO−LUMO gap and the resultant optical properties. This prediction was verified using
UV/vis absorption and emission spectroscopy: cruciforms’ protonation results in hypsochromic and bathochromic shifts
consistent with the preferential stabilization of HOMO and LUMO, respectively. In donor−acceptor-substituted systems, a two-
step optical response to protonation was observed, wherein an initial bathochromic shift is followed by a hypsochromic one with
continued acidification. X-ray diffraction studies of three selected cruciforms revealed the expected ∼90° angle between the
cruciform’s substituents, and crystal packing patterns dominated by [π···π] stacking and edge-to-face [C−H···π] contacts.
Modification of substitutents along these arms was the chief
strategy for altering both cruciforms’ electronic and molecular
recognition properties and relied mostly on the introduction of
substituents with resonance and/or inductive donor or acceptor
properties. Relatively little attention had been paid to the
influence that central core could exert onto the properties of
the cruciform. This omission is unusual, as chemical
modification of the cruciform’s center could dramatically and
divergently affect its geometry, bias the electronic properties of
the “arms”, and perhaps introduce an additional molecular
recognition motif.
Here, we present the synthesis and detailed computational,
spectroscopic, and crystallographic investigations of a family of
heterocyclic cruciforms with spatially separated FMOs. Fully
conjugated cruciform-shaped compounds 1−9 (Chart 1) are
characterized by the central heterocyclic benzobisoxazole13,14
nucleus; emanating from this nucleus are the horizontal
benzobisoxazole axis and a vertical bis(arylethynyl)benzene
axis. The identity of substituents along each axis was varied
from an electron-neutral phenyl group, through an electron-rich
4-(N,N-dimethylamino)phenyl substituent, to an electron-poor
pyridyl group, with the intention of exploring how electronic
effects influence orbital separation and the associated optical
characteristics.
INTRODUCTION
■
Optical and electronic properties of conjugated π-systems, as
well as their reactivity, are profoundly influenced by their
frontier molecular orbitals (FMOs).1 In principle, these
important characteristics could be predictably engineered
through modulation of molecule’s highest occupied (HOMO)
and lowest unoccupied (LUMO) molecular orbitals. In reality,
the situation is not quite as simple: in most conjugated
molecules, FMOs are delocalized and extensively overlap
with each other, thus making their independent modification
difficult. Recently, several molecular platforms have been
designed with spatially separated FMOs which can be autono-
mously addressed. Among these, a dominant geometric motif is
that of cruciforms:2 X-shaped fully conjugated molecules in
which two π-circuits intersect at a central core. Some general
cruciform structures include tetrakis(arylethynyl)benzenes,3
distyrylbis(arylethynyl)benzenes,4 tetrastyrylbenzenes,5 tetraki-
salkynylethenes,6 and biphenyl-based “swivel”-cruciforms.7 Not
unexpectedly, proposed applications of cruciforms aim to utilize
their modular optical and electronic properties, and these
systems were suggested as viable fluorescent sensors for a variety
of analytes,3,4,8 organic light-emitters,9 components of dye-
sensitized solar cells,10 elements for molecular electronics,7,11,12
and nonlinear optics materials.3,5
In all previously studied cruciform systems, the central core
of the molecule served as an important, but rather inert,
conjugated connector for the two arms of the cruciform.
Received: October 10, 2011
Published: November 11, 2011
© 2011 American Chemical Society
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Chart 1. Benzobisoxazole-Based Cruciform-Shaped Compounds Discussed in This Study
Our work was motivated by the pioneering reports of
modular synthesis of benzobisoxazole cruciform by Nuckolls
and co-workers,15 who used these systems as components of a
molecular electronics toolkit.16 We have recently demonstrated
that benzobisoxazole cruciforms substituted with ester groups
form remarkably ordered two-dimensional sheets in the crystal
state.17 In this paper, we present the synthesis of 1−9,
computational investigation of their FMOs, their UV/vis
absorption and emission properties, the response of those
properties to protonation, and crystal structures of cruciforms
1, 2, and 8.
assisted20 Sonogashira couplings21 with three different terminal
alkynes, phenylacetylene (17), 4-ethynyl-N,N-dimethylaniline
(18), and 4-ethynylpyridine (19), to give nine final cruciforms
1−9. The yields of the final Sonogashira coupling varied from
poor to excellent and were probably compromised by (a) low
solubilities of starting benzobisoxazoles 14−16, (b) possible
coordination of pyridine-containing cruciforms to the Pd and
Cu catalysts,3e and (c) electronic mismatch between some
alkyne and aryl bromide coupling partners. Pure cruciforms
were obtained as yellow to dark red powders following column
chromatography and/or crystallization and appear to be stable
indefinitely both as solids and in solution. At 25 °C, cruciforms
1−9 are soluble in preparatively useful concentrations in
chlorinated solvents, and sparingly, but sufficiently for
absorption and emission studies (vide infra), in majority of
other common organic solvents.
RESULTS AND DISCUSSION
■
Synthesis. Cruciforms 1−9 were prepared using a variation
of our previously reported17 two-step protocol (Scheme 1).
Starting with 2,5-diamino-3,6-dibromobenzene-1,4-diol (10),18
dehydrative condensation with carboxylic acids19 11−13
produced sparingly soluble intermediates 14−16, in which
the electronic character of the substituents along the horizontal
axis was either neutral (14, phenyl), electron-rich (16, 4-
(dimethylamino)phenyl), or electron-poor (15, pyridyl). Each
of the three (crude) intermediates was subjected to microwave-
Computational Studies. In order to evaluate the spatial
separation of FMOs in cruciforms 1−9 and attempt the
prediction of their optical response to acidic analytes, we
performed molecular orbital calculations on these systems.
Calculations were done using the Gaussian 09W22 software
package and its accompanying graphical interface program
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Scheme 1. Synthesis of Cruciforms 1−9
GaussView 5.0. The B3LYP hybrid density functional23 and a
standard 3-21G basis set were used for the geometry
optimizations. Compounds 6 and 8 were initially optimized
within a Cs symmetry constraint and both converged to
structures with C2h symmetry. Accordingly, the other cruciform
structures were optimized within a C2h symmetry constraint.
An orbital analysis of the parent bis(ethynyl)benzobisoxazole
20 (Figure 1) revealed that its HOMO is antisymmetric
energy) is another orbital of bg symmetry (denoted as
HOMO−1 in Figure 1). It has very large amplitudes at the
oxazole carbons where the horizontal groups are substituted.
Therefore, strong electron-donating groups destabilize this
orbital so that it becomes the HOMO. In contrast to 4 and 6,
cruciform 5 has four 4-(N,N-dimethylamino)phenyl substitu-
ents, along both the horizontal and vertical axes, but still
localizes its HOMO along the vertical axis since the electron
donating substituents destabilize both the HOMO and
HOMO−1 by approximately the same amount. Analysis of the
LUMO distributions is somewhat more complex, as this orbital
communicates freely with both the horizontal and vertical axes
of the molecule. The substitution is thus critical in determining
whether the LUMO will be localized and, if so, on which
portion of the molecule. To put our analysis of orbital
localization onto a quantitative footing, we performed a simple
numeric analysis of corresponding orbital coefficients. Each
cruciform was divided into its horizontal (shaded in light red,
illustrated on compound 9 in Figure 2) and vertical (shaded in
light blue, illustrated on compound 9 in Figure 2) axis. The
atoms of the central benzene ring were ignored because they
belong to both axes, and all cruciforms show significant FMO
overlap in that region. Then, for both the HOMO and the
LUMO, squared orbital coefficients for “vertical” atoms were
summed, divided by the sum of squared orbital coefficients for
all atoms in the cruciform (excluding the central benzene ring),
and multiplied by 100. The resulting number represents the
percentage of HOMO/LUMO density along the vertical axis of
the molecule (%v, Figure 2). The analogous process was
repeated for the horizontal axis (%h, Figure 2); for each
individual orbital, the percentage distributions on the vertical
and horizontal axes added up to 100%. The corresponding axis
localization percentages are shown next to the individual orbital
representations in Figure 2. Finally, each orbitals’ localization is
described by a single number (Figure 2, numbers in squares) which
is defined as localization percentage and is equal to |%h − %v|.24
Several general trends can be observed. In all cruciforms, the
HOMO is more localized than the LUMO, and, regardless of
the extent of localization, the HOMO and the LUMO are
mostly positioned along different axes of the molecule, except
in cruciform 3. Cruciform 6 has both the most localized
HOMO (99%) and the most localized LUMO (64%) and, thus,
has the highest spatial separation of FMOs. Also strongly
Figure 1. Selected molecular orbitals of the parent bis(ethynyl)-
benzobisoxazole 20.
(bg symmetry in C2h) and LUMO symmetric (au symmetry in
C2h) with respect to the molecule’s inversion center. Both
orbitals have significant density in the central benzene ring, on
the oxazole heteroatoms, and along the CC bond. The key
difference between the two is the orbital density along the
oxazole C−H bonds: the HOMO has very little density in that
area, but the LUMO does. This means that while both the
HOMO and the LUMO can delocalize across the vertical axis
of the cruciform, only the LUMO can effectively “spill” over the
horizontal axis as well.
This qualitative feature of the parent system translates onto
all cruciforms (Figure 2): HOMOs of 1, 2, 3, 5, 7, 8, and 9 have
virtually no density in the peripheral rings of the horizontal axis.
Significant density in the HOMO along the horizontal axis is
found only in cruciforms 4 and 6, which are both substituted
with electron-donating 4-(N,N-dimethylamino)phenyl groups.
Energetically just below the bg HOMO in 20 (0.84 eV lower in
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Figure 2. FMOs for cruciforms 1−9. HOMOs shown above the corresponding cruciform’s number, LUMOs below it. The two percentages next to
each orbital’s representation denote their relative densities along the vertical (%v) and horizontal (%h) axis of the corresponding molecule (for axes
definition, see molecule 9). Number in the square next to each orbital’s representation indicates that orbital’s relative localization (0%, completely
delocalized; 100%, completely localized).
separated are the FMOs of 2, 8, and 7 (to a somewhat
attenuated degree). Cruciforms 4 and 5 have strongly localized
HOMOs (80 and 66%, respectively), but rather delocalized
LUMOs (6 and 14%, respectively). Finally, FMOs of
cruciforms 1, 3, and 9 are relatively delocalized (6−52%
localization). These trends are caused by the combination of
(1) predisposition of the bis(ethynyl)benzobisoxazole skeleton
to localize its HOMO orbital along the vertical axis, and (2) the
donor/acceptor substitution pattern, which tends to localize the
HOMO along the electron-richer and the LUMO along the
electron-poorer axis of the molecule. The latter factor is general
and has precedence in Bunz’s4b,c,j−l,p,q and Haley’s3a,e,g systems,
but the former effect of the benzobisoxazole core is unique, and
it introduces an additional level of control over the orbital
separation in cruciform systems.
the horizontal pyridine-bearing axis, leading to a stabilized
LUMO and a corresponding red (bathochromic) shift. Donor−
acceptor-substituted compounds 6 and 8 present interesting
These computational results suggest what the sensing
response of individual cruciforms should be. Cruciforms 2, 6,
7, and 8 would bode well as sensors for protons (and possibly
metals), since analyte binding to their basic sites should
dominantly affect one of the FMOs and, thus, alter the
HOMO−LUMO gap. Protonation of cruciform 2 should occur
at its basic dimethylaniline sites; thus, the HOMO should be
stabilized, leading to a blue (hypsochromic) shift in absorption
(Figure 3). In contrast, cruciform 7 should be protonated along
Figure 3. Anticipated optical shifts accompanying analyte binding to
different portions of cruciforms with spatially separated FMOs.
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cases because of their two possible nonequivalent protonation
sites. Protonation at their pyridine-based LUMOs would cause
a red shift, while protonation along the dimethylaniline-
centered HOMO would cause a blue shift. With a sufficient
amount of acid, both shifts could occur in a sequence, but it is
hard to predict which one would be first: tabulated pKa values
for parent dimethylaniline and pyridine are virtually identical
(5.15 and 5.25, respectively) and can vary by as many as four
pKa units depending on the substituent in the 4-position.25 In
the next two sections, we will explain how optical spectroscopy
was used to establish the order of protonation.
Cruciforms 4 and 5 have only one of their frontier orbitals
localized. In 4, protonation along the horizontal axis should
affect both FMOs but would exhibit significantly larger
stabilizing effect on HOMO, thus presumably leading to a
blue shift analogous to that in 2. In 5, basic groups are
positioned along both the horizontal and the vertical axes.
Protonation of “vertical” nitrogens would lead to a blue shift
(stabilized HOMO), while protonation of “horizontal” basic
groups should lead to a (smaller) red shift. Again, order of
protonation is difficult to predict a priori and had to be
elucidated experimentally.
donor−acceptor systems (e.g., 8, Figure 4C) also show a strong
solvent dependence, suggesting a charge-separated excited
state.3d,e,4p
All prepared cruciforms were analyzed by UV/vis absorption
and emission spectroscopies in dilute CH2Cl2 solutions. Their
normalized absorption and emission spectra are shown in
Figure 5, grouped into three sets of three spectra each, that
correspond to cruciforms with an identical horizontal axis. In
general, electronic absorption spectra of cruciforms 1−9 are
characterized by two broad bands: the lower energy band
appearing between approximately 350 and 500 nm and the
higher energy absorption between approximately 300 and 400 nm.
UV/vis spectra of 1 and 3 are almost superimposable; in
contrast, the lowest energy absorption of 2 is bathochromically
shifted by 75 nm, indicating possible intramolecular charge
transfer (Figure 5, left).3d,e,4p The orbital pictures are consistent
with this explanation. The FMOs of 2 are spatially significantly
more separated than the delocalized FMOs of 1 and 3. This
situation also occurs in the spectra of 7−9 (Figure 5, right).
The absorption spectra of 7 and 9 are almost superimposable,
but in their emission spectra, λmax for 7 is slightly bathochromi-
cally shifted, consistent with a greater degree of donor−
acceptor character (relative to 9). A 100 nm shift to lower
energies in the spectrum of 8 (with spatially separated orbitals)
is indicative of charge-transfer. In compounds 4−6, qualitative
differences in UV/vis spectra are the largest, but their λmax
values are quite similar, in the 430−457 nm range. As is
expected, compound 6 with significantly separated FMOs and
presumably the strongest charge-transfer, has the highest λmax
among the three. The normalized emission spectra (Figure 5)
parallel these observations very closely.
Finally, cruciforms 1, 3, and 9 are expected to show
uneventful optical response to protonation, as any change
would likely affect both the HOMO and the LUMO to a
comparable extent.
Computations also produced HOMO−LUMO energy gap
values (Table 1), which roughly correlate with λmax values
obtained from UV/vis spectra (vide infra).
Table 1. Optical Properties and Calculated HOMO−LUMO
Gaps for Cruciforms 1−9
Optical Response to Protonation. Since cruciforms 1−9
carry basic pyridyl, 4-(N,N-dimethylamino)phenyl, and oxazole
groups, our next series of experiments examined their optical
(UV/vis absorption and emission) response to protonation
with trifluoroacetic acid (TFA). All of these experiments were
performed in dilute CH2Cl2 solutions and their results have
been summarized in Figure 6 for absorption spectra and Figure 7
for emission spectra. As predicted by the orbital analysis, the
absorption spectra of cruciforms with poorly localized FMOs, 1,
3, and 9 showed negligible response to protonation. However,
the changes in their emission spectra were more dramatic.
Compound 1 showed the quenching of fluorescence at high
TFA concentrations, which we tentatively attribute to the
eventual protonation of oxazole nitrogen atoms. In the
emission spectra of compounds 3 and 9, a significant red
shift was observed around −log[TFA] of 3.71, consistent with
pyridine protonation.26
absorption
λmax (nm)
emission
λmax (nm)
Stokes shift calcd HOMO−LUMO
compd
(cm−1
)
gap (eV, nm)
1
2
3
4
5
6
7
8
9
367
443
368
437
421
458
367
468
367
407
529
419
497
486
544
439
597
409
2678
3670
3308
2763
3177
3452
4469
4617
2798
3.16, 392
2.47, 502
3.24, 383
2.98, 416
2.87, 432
2.79, 444
3.01, 412
2.44, 508
3.17, 391
Optical Properties. Dilute solutions of cruciforms 2, 4−6,
and 8 are strongly colored (Figure 4A). Under a hand-held UV
The spectra of cruciforms 4 and 5, which have partially
separated FMOs, are quite diagnostic. Protonation of 4 causes a
pronounced blue shift in both absorption and emission spectra,
consistent with stabilization of its HOMO. In 5, a slight red
shift in both absorption and emission, most apparent at
−log[TFA] = 2.31, is followed by a more pronounced blue shift
with continued acidification. Such behavior is consistent with
initial protonation along the horizontal dimethylamino groups,
which should stabilize the LUMO, and secondary protonation
along the vertical axis, which should strongly stabilize the more
localized HOMO.
Figure 4. (A) Colors of CH2Cl2 solutions of cruciforms 1−9 under
visible light. (B) Fluorescence colors of CH2Cl2 solutions of 1−9
under light from a hand-held UV lamp (λexc = 365 nm). (C) Strong
fluorescence solvatochromic behavior of 8 (λexc = 365 nm).
Cruciform 2, with its strongly separated FMOs, is protonated
at its dimethylamino groups positioned along the vertical axis.
Upon protonation, stabilization of the HOMO is apparent from
blue shifts in both absorption and emission spectra. As in 1, the
lamp (λexc = 365 nm), all of the prepared cruciforms also
exhibit bright emission (Figure 4B). Emission wavelengths of
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Figure 5. Normalized UV/vis absorption (top) and emission (bottom) spectra of benzobisoxazole cruciforms 1−9 in CH2Cl2. Excitation
wavelengths used were 320 (1), 350 (2 and 4), 335 (3), 341 (5), 356 (6), 377 (7), 417 (8), and 325 (9) nm.
blue-shifted emission is partially quenched in very acidic
solutions, which we attribute to partial protonation of the
oxazole rings. Cruciform 7 exhibits a rather moderate red shift
in absorption and quenching of fluorescence upon protonation;
the former effect is consistent with dominant protonation
along its LUMO-bearing horizontal axis, but is puzzlingly
unpronouncedgiven the strong spatial separation of calcu-
lated FMOs. Together with the results on protonation of 5, this
observation would suggest that both pyridyl and dimethylamino
groups are more basic when positioned along the cruciforms’
horizontal axes. This effect is probably caused by the influence
of the HOMO−1 orbital which donates electron density to the
substituents positioned along the horizontal axes, and is
especially relevant in the case of electron-donating 4-(N,N-
dimethylamino)phenyl substituents.
All observed protonation-induced optical changes could be
fully reversed by addition of a base such as NEt3.
Crystallographic Studies. Before this study, there were
only three reported crystal structures of benzo[1,2-d:4,5-d′]-
bisoxazoles: one published by Nuckolls,15a and two from our
previous work.17,28 We deemed crystallographic investigations
of compounds 1−9 interesting as their regular geometries and
highly localized FMOs could lead to the formation of donor−
acceptor stacks in the solid state. All prepared cruciforms are
solids, but obtaining single crystals of sufficiently high quality
for X-ray diffraction proved challenging. After extensive
experimentation, we were able to grow single crystals of
cruciforms 1 and 2 by slowly diffusing hexane into their
solutions in CHCl3, and those of cruciform 8 by slowly
diffusing hexane into its CH2Cl2 solution.15a,29
Cruciform 1 crystallizes in the R3 space group with nine
̅
Finally, spectra of donor−acceptor cruciforms 6 and 8 reveal
a distinct two-step optical response to protonation. Initially,
protonation causes a red shift in absorption spectra of both
compounds, most pronounced at −log[TFA] = 3.01.
Quenching of emission from both compounds suggests that
the initial protonated species is nonfluorescent. The red shift is
explained by the initial protonation of the pyridine nitrogen
atoms in both cruciforms, regardless of their positioning on
horizontal or vertical axis. These results stand in contrast to
those of Bunz4l and Haley,3e who observed that N,N-
dibutylanilines protonate before pyridines. This difference
may be caused by the stronger stabilization of protonated
pyridines through conjugation with the 4-substituent (vs
protonated anilines).27 With continued acidification both 6
and 8 recover their fluorescence, which is now blue-shifted.
Absorption maxima are also blue-shifted, indicative of a second
protonation along the electron-rich HOMO-bearing axis.
molecules of 1 in the unit cell. The molecules of 1 are
organized into stacks which are tilted relative to the
crystallographic c-axis. Six of these stacks (which alternate in
their tilting orientation) surround each 3-fold rotation axis.
While the atoms comprising the molecules of 1 could be
modeled without disorder, numerous electron density peaks
(presumably due to disordered molecules of CHCl3) were
found along the rotation axes, lying in the tunnels generated by
the stacked molecules. As these peaks could not be rigorously
modeled, we used the PLATON/SQUEEZE routine30 to
subtract the delocalized electron density from the structure.
The equivalent of 33 electrons/unit cell were identified by the
routine (corresponding to approximately 0.6 molecules of
CHCl3 per unit cell). The crystal structure of 1 (Figure 8A)
reveals an essentially planar molecule, with minimal distortions
of peripheral phenyl rings relative to the average plane of the
benzobisoxazole nucleus. The phenyl rings oriented along the
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Figure 6. Changes in the UV/vis absorption spectra of benzobisoxazole cruciforms 1−9 in CH2Cl2 in response to protonation with trifluoroacetic
acid (TFA).
horizontal plane are at a 5.7° angle relative to the central plane,
while the “vertical” phenyl rings define a 5.9° angle with the
molecule’s center. As expected, the four valences on the
benzobisoxazole nucleus define angles31 very close to ideal
90°alternating between 88.1 and 91.9°. The triple bonds of 1
are essentially undeformed, with CC−C angles of 178.9 and
179.6°. Molecules of 1 stack in an offset arrangement with
average interplanar distances of 3.44 Å, consistent with [π···π]
interactions.32 Within each molecule, hydrogen in the para-
position of the “horizontal” phenyl rings also establishes short
(2.58 Å)33 contacts with the nitrogen of the benzobisoxazole
nucleus in the neighboring set of stacked molecules. The
interdigitated stacks with short [C−H···π] contacts in the
overall structure parallel those observed by Nuckolls.15a
Cruciform 2 crystallizes in the C2/c space group, with four
molecules of 2 in the unit cell. As in the structure of 1,
significant disorder of the solvent molecules (CHCl3, located in
disk-shaped cavities) was treated using the PLATON/
SQUEEZE routine (134 electrons were identified, equivalent
to an average of 2.3 molecules of disordered CHCl3 per unit
cell).30 Its crystal structure (Figure 8B) is notably different
from that of 1: an essentially planar benzobisoxazole axis and
almost perpendicular positioning of the substituents along the
bisethynylbenzene axis, with an 82.6° angle between the two
planes. The angles between the four valences of the
benzobisoxazole alternate between 89.2 and 90.8°, with
negligible deformations in the CC bonds (179.3 and
179.6°). The difference in molecular structures between 1
(essentially coplanar) and 2 (divided between two perpendic-
ular planes) is translated into different supramolecular
organizations for the two cruciforms. Thus, while offset face-
to-face [π···π] contacts dominate the crystal structure of 1,
molecules of 2 associate primarily through three distinct sets of
[C−H···π] interactions34 established between (1) the hydro-
gens on the vertical 4-(N,N-dimethylamino)phenyl rings of one
molecule and the central benzobisoxazole π-system of another
(2.48 Å), (2) hydrogens in the para-position of the “horizontal”
phenyl rings of one molecule and the CC bond of another
(2.86 Å), and (3) hydrogens in the meta- and ortho-positions
of the “horizontal” phenyl rings of one molecule and the
horizontal phenyl rings of adjacent molecules (2.82 Å).33 Face-
to-face stacking is evident only in the 3.65 Å distance between
the peripheral 4-(N,N-dimethylamino)phenyl rings on adjacent
molecules.
Finally, the donor−acceptor cruciform 8 crystallizes in the
C2/c space group, with four molecules of 8 in the unit cell, and
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Figure 7. Changes in the UV/vis emission spectra of benzobisoxazole cruciforms 1−9 in CH2Cl2 in response to protonation with trifluoroacetic acid
(TFA). Excitation wavelengths used were 320 (1), 350 (2 and 4), 335 (3), 341 (5), 356 (6), 377 (7), 417 (8), and 325 (9) nm. Note that the
wavelength axis of the spectrum of 8 is shifted relative to those of other cruciforms.
Figure 8. Segments of crystal structures of cruciforms 1 (left), 2 (center), and 8 (right). Solvent molecules removed for clarity. Molecules
represented by faded space-filling models indicate [π···π] stacking relationships, while wireframe models indicate individual short and [C−H···π]
contacts (contacts are designated by dashed green lines). In the structure of 1, all four short contacts are identical2.58 Å; for 2, contact 1 is 2.70 Å,
contact 2 is 2.90 Å, and contact 3 is 2.61 Å; for 3, both indicated short contacts are identical and 2.65 Å long.
one (ordered) molecule of CH2Cl2 for each molecule of 8.
Analogously to 1, cruciform 8 (Figure 8C) is essentially planar
in the solid state: pyridyl rings are at a 3.5° angle relative to the
benzobisoxazole ring plane, while the electron-rich 4-
(dimethylamino)phenyl rings define a 9.4° angle with the
benzobisoxazole core. While still highly symmetric, cruciform 8
is the most deformed of the three: angles between benzoxazole
valences vary between 86.9 and 93.1°, while the triple bonds
deform to CC−C angles of 173.1 and 178.6°. Slipped [π···π]
stacks are apparent (interplanar distance 3.32 Å) and
10214
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characterized by the positioning of the pyridine rings over
benzobisoxazole nuclei (and vice versa). Within the overall
structure, interdigitated stacking is observed along the b axis,
with short contacts established between the hydrogen atoms of
the methyl groups in one molecule of 8 and the
benzobisoxazole’s nitrogen atoms in another (2.65 Å).
separation of FMOs, perhaps entirely independent of
substitution. We are actively pursuing several of these directions
and the results of these studies will be reported in due course.
EXPERIMENTAL SECTION
■
General Experimental Methods. Reagents and solvents were
purchased from commercial suppliers and used without further
Interestingly, none of the three cruciforms crystallographi-
cally characterized here resembles the all-parallel organization
of carbonyl- and cyano-substituted benzobisoxazole cruciforms
previously observed.17 Also, and somewhat surprisingly, no
significant donor−acceptor interactions were noticeable in the
crystal structures of either 2 or 8, despite strong computational
evidence supporting spatial separation of their FMOs. Together
with our previous results, these three crystal structures suggest
that there is no general packing pattern characterizing
benzobisoxazole cruciforms. As highly geometrically regular
molecules, cruciforms 1, 2, and 8 unsurprisingly form closely
packed structuresthey simply “fit” well together. However,
the specifics of the supramolecular structural organization are
highly sensitive to the presence of individual functional groups
and details of the molecular geometry. Additional details
regarding crystallographic characterization can be found in the
Supporting Information.
36
purification. Compounds PdCl2(PPh3)2 and 2,5-diamino-3,6-dibro-
mohydroquinone (10)18a were prepared according to literature
procedures. Microwave-assisted reactions were performed in a Biotage
Initiator 2.0 microwave reactor, producing monochromatic microwave
radiation with the frequency of 2.45 GHz. NMR spectra were obtained
1
on spectrometers with working frequencies for H nuclei of 400 and
500 MHz. All 13C NMR spectra were recorded with simultaneous
decoupling of 1H nuclei. 1H NMR chemical shifts are reported in ppm units
relative to the residual signal of the solvent (CDCl3, 7.26 ppm; DMSO-d6,
2.50 ppm; THF-d8, 1.73 ppm). All NMR spectra were recorded at 25 °C.
Column chromatography was carried out on silica gel 60, 32−63 mesh.
Analytical TLC was performed on aluminum-backed silica gel plates.
Synthesis of 2,6-Diphenyl-4,8-dibromobenzo[1,2-d;4,5-d′]-
bisoxazole (14). Benzoic acid (11, 902 mg, 7.38 mmol) and
compound 10 (1.00 g, 3.36 mmol) were mixed with polyphosphoric
acid (14.0 g) in a 100 mL round-bottom flask equipped with a
Friedrich condenser. The mixture was heated at 150 °C for 25 h and
was then cooled, neutralized with aqueous 1 M NaOH solution, and
filtered. The dark solid was washed with H2O, EtOH, and then Et2O.
The solid was air-dried to give 1.45 g (92%, 3.08 mmol) of crude 14
(mp >350 °C, with decomposition). Because of its extremely low
solubility, this material was only partially characterized and then used
without further purification. 14. IR (neat): 1608 (w), 1564
CONCLUSION
■
In summary, we have synthesized and exhaustively charac-
terized a family of benzobisoxazole-based cruciform-shaped
conjugated molecules. These materials are characterized by
modular FMOs, which can be localized onto different portions
of the molecule by design. While this behavior parallels that
observed previously in Bunz’s distyrylbis(arylethynyl)benzenes
and Haley’s tetrakis(arylethynyl) benzenes, benzobisoxazole
cruciforms have at least three additional features that warrant
their further study. The horizontal and vertical axes of these
cruciforms are electronically quite different, and their arrange-
ment enforces a certain degree of frontier orbital separation
that is independent of substitution. This “inherent” FMO
separation in 1−9 is quite moderate, and intervention of
substituents is still needed to produce systems with highly
localized HOMOs and LUMOs. The rigid geometry of
benzobisoxazole cruciforms is characterized by the 90° angle
between the four benzobisoxazole valences, which is a rather
unique arrangement in organic chemistry. This highly regular
structure should facilitate their incorporation into solid-state
extended structures with crystallographic order. Finally, the
heterocyclic central benzobisoxazole core could act as more
than just an “innocent bystander”in contrast to the benzene
center of previously described systemsserving perhaps as an
additional recognition motif.
(m, vO−CN), 1490 (w), 1453 (w), 1319 (m), 1223 (m), 1054 (m),
̃
1025 (m), 974 (m), 912 (m), 873 (m), 773 (m), 719 (m), 693 (s),
683 (s) cm−1. 1H NMR (CDCl3, 500 MHz): δ 8.37 (m, 4H), 7.58 (m,
6H). 13C NMR (CDCl3, 125 MHz): δ 164.8, 147.2, 139.9, 132.6,
129.3, 128.4, 126.4, 92.1. HRMS (ESI/[M + H]+): calcd for
C20H11Br2N2O2 468.9187, found 468.9181.
Synthesis of 2,6-Bis[p-(N,N-dimethylamino)phenyl]-4,8-
dibromobenzo[1,2-d;4,5-d′]bisoxazole (15). In a 100 mL
round-bottom flask equipped with a Friedrich condenser, compound
10 (1.00 g, 3.36 mmol) and 4-(N,N-dimethylamino)benzoic acid (12,
1.22 g, 7.38 mmol) were mixed with polyphosphoric acid
(14.0 g) and stirred for 24 h at 160 °C. The mixture was then
cooled, neutralized with aqueous 1 M NaOH solution, and filtered.
The dark solid was washed with H2O, EtOH, and then Et2O. After air-
drying, the solid was dissolved in 200 mL of a CH2Cl2/MeOH mixture
(9:1) and then passed through a short silica gel column. The solvent
was removed in vacuo to give 1.68 g (90%, 3.02 mmol) of 15 (mp >350
°C, with decomposition). 15. IR (neat): 1612 (s), 1514 (s, v
)
̃
O−CN
1372 (m), 1316 (m), 1224 (m), 1185 (s), 1055 (m), 904 (m), 870
1
3
(m), 812 (m) cm−1. H NMR (CDCl3, 400 MHz): δ 8.19 (d, JH−H
=
9.16 Hz, 4H), 6.77 (d, 3JH−H = 9.16 Hz, 4H), 3.10 (s, 12H). Because of
the low solubility of 15, a satisfactory 13C NMR spectrum could not be
obtained. HRMS (ESI/[M + H]+): calcd for C24H21Br2N4O2 555.0031,
found 555.0026.
Our results open several avenues of future investigation.
Synthetically, incorporation of different donor and acceptor
groups, as well as modification of their arrangement around
cruciforms’ axes would be of interest.3d,e In addition, more
sophisticated functionalities could be included in the structures
of these molecules to render their molecular recognition
properties specific for individual cationic, anionic, and neutral
analytes. Cruciforms 1−9 are potentially appealing as materials
with nonlinear optical properties.35 Finally, integration of
geometrically regular benzobisoxazole cruciforms into robust
crystalline structures could pave the way for their use as
practical solid-state sensors. On a conceptual note, other
substructures could be designed as central cores of future
molecular cruciforms that would exhibit even larger inherent
Synthesis of 2,6-Dipyridin-4-yl-4,8-dibromobenzo[1,2-d;4,5-d′]-
bisoxazole (16). In a 100 mL round-bottom flask equipped with a
Friedrich condenser, compound 10 (2.45 g, 8.22 mmol) and iso-
nicotinic acid (13, 3.07 g, 24.65 mmol) were mixed with polyphos-
phoric acid (20.0 g). The mixture was heated to 170 °C and stirred at
that temperature for 25 h. After cooling, the suspension was
neutralized with cold aqueous 1 M NaOH solution and filtered. The
dark solid was washed with H2O, EtOH, and then Et2O. After air-
drying, the solid was dissolved in 200 mL of a CH2Cl2/MeOH mixture
(9:1) and then passed through a short silica gel column. Product was
eluted using a CH2Cl2/MeOH/Et3N mixture (18:2:1). Solvent was
removed in vacuo to give 2.09 g (54%, 4.42 mmol) of 16 (mp >350
°C, with decomposition). 16. IR (neat): 1608 (w), 1576 (m), 1548
(m), 1490 (w), 1414 (s), 1357 (w), 1322 (m), 1279 (w), 1230 (s),
1060 (s), 990 (m), 971 (w), 920 (w), 876 (m), 830 (s), 740 (w),
10215
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723 (w), 697 (s), 687 (s) cm−1. 1H NMR (THF-d8, 500 MHz): δ 8.86 (d,
3JH−H = 5.73 Hz, 4H), 8.18 (d, 3JH−H = 5.73 Hz, 4H). 13C NMR (THF-d8,
125 MHz): δ 163.9, 152.2, 148.6, 141.3, 134.0, 121.9, 93.6. HRMS (ESI/
[M + H]+): calcd for C18H9Br2N4O2 470.9092, found 470.9087.
The entire amount of 19 (prepared as above-described) was added
to a thick-walled microwave pressure vial that contained a mixture of
compound 14 (300 mg, 0.638 mmol), PdCl2(PPh3)2 (90 mg, 0.13
mmol), CuI (24 mg, 0.13 mmol), NEt3 (5 mL), and MeCN (5 mL).
The vial was sealed and exposed to microwave irradiation for 3 h at
90 °C. After cooling, solvents were removed under reduced pressure,
and the crude solid was purified by column chromatography, eluting
first with pure CH2Cl2, and then successively with CH2Cl2/MeOH
mixtures in 97:3, 19:1, and 9:1 ratios. The solvent was removed under
reduced pressure to give 202 mg (62%, 0.39 mmol) of 3 (mp >350 °C,
with decomposition). 3. UV−vis (CH2Cl2): λmax (log ε) = 302 (6.52),
Synthesis of Cruciform 1. Phenylacetylene (17, 391 mg, 3.83
mmol) was added to a thick-walled microwave pressure vial that
contained a mixture of compound 14 (300 mg, 0.638 mmol),
PdCl2(PPh3)2 (90 mg, 0.13 mmol), CuI (24 mg, 0.13 mmol), NEt3
(5 mL), and MeCN (5 mL). The vial was sealed and exposed to
microwave irradiation for 3 h at 100 °C. After cooling, solvents were
removed under reduced pressure, and the crude solid was purified by
column chromatography, eluting first with pure CH2Cl2 and then
successively with CH2Cl2/MeOH mixtures in 97:3, 19:1, and 9:1
ratios. The solvent was removed under reduced pressure to give
270 mg (83%, 0.53 mmol) of 1 (mp 305 °C, with decomposition). 1.
UV−vis (CH2Cl2): λmax (log ε) = 304 (6.62), 367 (6.90) nm. IR
315 (6.52), 368 (6.76) nm. IR (neat): 2927 (w, v ), 2854 (w, vC−H),
̃
̃
C−H
2220 (w, v
), 1606 (s), 1591 (s), 1561 (m, vO−CN), 1538 (w),
̃
̃
C C
1486 (m), 1451 (m), 1407 (m), 1340 (m), 1280 (m), 1224 (m), 1210
(m), 1177 (w), 1160 (w), 1079 (m), 1064 (m), 1028 (m), 999 (w),
982 (m), 918 (m), 821 (s), 795 (m), 780 (m), 724 (m), 700 (s), 688
(s), 627 (m) cm−1. H NMR (CDCl3, 400 MHz): δ 8.72 (d, JH−H
=
1
3
(neat): 3067 (w, v ), 2221 (w, v
), 1609 (w), 1562 (m, vO−CN),
̃
̃
̃
C−H
CC
3
1488 (m), 1451 (m), 1383 (w), 1340 (m), 1278 (m), 1079 (w), 1062
(m), 1027 (m), 977 (m), 918 (m), 777 (m), 755 (m), 723 (m), 697
(s), 687 (s) cm−1. 1H NMR (CDCl3, 500 MHz): δ 8.42 (m, 4H), 7.80
(m, 4H), 7.58 (m, 6H), 7.46 (m, 6H). 13C NMR (CDCl3, 125 MHz):
δ 164.8, 149.0, 141.1, 132.4, 132.3, 129.4, 129.2, 128.7, 128.4, 126.8,
123.0, 100.5, 98.9, 80.0. LRMS (ESI/[M + H]+): calcd for
C36H21N2O2 513.15, found 513.33. HRMS: calcd for C36H21N2O2
513.1603, found 513.1601. Anal. Calcd. for C36H20N2O2: C, 84.36; H,
3.93; N, 5.47. Found: C, 84.01; H, 3.52; N, 5.39.
4.12 Hz, 4H), 8.38 (m, 4H), 7.63 (d, JH−H = 5.95 Hz, 4H), 7.58 (m,
6H). 13C NMR (CDCl3, 100 MHz): δ 165.2, 150.1, 149.0, 141.4,
132.6, 130.9, 129.2, 128.4, 126.4, 126.1, 98.4, 97.5, 84.0. LRMS (ESI/
[M + H]+): calcd for C34H19N4O2 515.14, found 515.2. HRMS: calcd
for C34H19N4O2 515.1508, found 515.1500. Anal. Calcd for
C34H18N4O2·1/4CH2Cl2: C, 76.78; H, 3.48; N, 10.46. Found: C,
77.19; H, 3.67; N, 10.46.
Synthesis of Cruciform 4. Phenylacetylene (17, 33 mg, 3.24 mmol)
was added to a thick-walled microwave pressure vial that contained a
mixture of compound 15 (300 mg, 0.54 mmol), PdCl2(PPh3)2 (76 mg,
0.11 mmol), CuI (21 mg, 0.11 mmol), NEt3 (5 mL), and MeCN
(5 mL). The vial was sealed and exposed to microwave irradiation for
3 h at 100 °C. After cooling, solvents were removed under reduced
pressure, and the crude solid was purified by column chromatography,
eluting first with pure CH2Cl2, and then successively with CH2Cl2/
MeOH mixtures in 97:3, 19:1, and 9:1 ratios. The solvent was
removed under reduced pressure to give 232 mg (72%, 0.39 mmol) of
cruciform 4 (mp >350 °C, with decomposition). 4. UV−vis (CH2Cl2):
λmax (log ε) = 335 (6.62), 378 (6.53), 437 (6.67) nm. IR (neat): 2904
Synthesis of Cruciform 2. In a nitrogen-flushed round-bottom
flask, anhydrous K2CO3 (1.06 g, 7.66 mmol) was added to a solution
of 2-(4-(N,N-dimethylamino)phenyl)trimethylsilylethyne37 (832 mg,
3.83 mmol) in a mixture of MeOH (5 mL) and THF (5 mL). After
stirring for 30 min, the reaction mixture was filtered through Celite.
The solvent was removed under reduced pressure to yield crude 4-
ethynyl-N,N-dimethylaniline (18), which was used without purifica-
tion in the next step. To minimize manipulations of this compound,
we assumed a 95% yield for this reaction.
The entire amount of 18 (prepared as above-described) was added
to a thick-walled microwave pressure vial that contained a mixture of
compound 14 (300 mg, 0.638 mmol), PdCl2(PPh3)2 (90 mg, 0.13
mmol), CuI (24 mg, 0.13 mmol), NEt3 (5 mL), and MeCN (5 mL).
The vial was sealed and exposed to microwave irradiation for 3 h at
100 °C. After cooling, solvents were removed under reduced pressure,
and the crude solid was purified by column chromatography, eluting
first with pure CH2Cl2, and then successively with CH2Cl2/MeOH
mixtures in 97:3, 19:1, and 9:1 ratios. The solvent was removed under
reduced pressure to give 338 mg (88%, 0.56 mmol) of pure 2 (mp
>350 °C, with decomposition). 2. UV−vis (CH2Cl2): λmax (log ε) =
279 (6.51), 317 (6.66), 342 (6.69), 356 (6.70), 374 (6.71), 443 (6.67)
(w, v ), 2220 (w, vC C), 1607 (s), 1585 (m), 1509 (s), 1487 (w),
̃
̃
C−H
1444 (m), 1436 (m), 1368 (m), 1355 (m), 1337 (m), 1281 (w), 1185
(s), 1065 (m), 976 (m), 944 (w), 910 (m), 815 (m), 762 (m), 741
(m), 691 (m) cm−1. 1H NMR (CDCl3, 400 MHz): δ 8.25 (d, 3JH−H
=
3
9.16 Hz, 4H), 7.79 (m, 4H), 7.43 (m, 6H), 6.79 (d, JH−H = 9.16 Hz,
4H), 3.10 (s, 12H). 13C NMR (CDCl3, 100 MHz): δ 165.6, 152.7,
148.8, 140.8, 132.4, 129.8, 129.0, 128.6, 123.4, 113.9, 111.7, 99.5, 97.3,
80.7, 40.4. LRMS (ESI/[M + H]+): calcd for C40H31N4O2 599.24,
found 599.33. HRMS: calcd for C40H31N4O2 599.2447, found
599.2444. Anal. Calcd for C40H30N4O2·1/3CH2Cl2: C, 77.26; H,
4.93; N, 8.94. Found: C, 77.67; H, 4.62; N, 8.78. Despite repeated
attempts, satisfactory elemental analysis for C could not be obtained.
Synthesis of Cruciform 5. Compound 18 (261 mg, 1.80 mmol)
was added to a thick-walled pressure vessel that contained a mixture of
compound 15 (100 mg, 0.18 mmol), PdCl2(PPh3)2 (25 mg, 0.036 mmol),
CuI (7 mg, 0.036 mmol), i-Pr2NH (2 mL), and MeCN (2 mL). The
vessel was sealed, and the mixture was heated for 7 days at 120 °C.
After cooling, solvents were removed under reduced pressure, and the
crude solid was purified by column chromatography, eluting first with
pure CH2Cl2 and then successively with CH2Cl2/MeOH mixtures in
97:3, 19:1, and 9:1 ratios. The solvent was removed under reduced
pressure to give 51 mg (41%, 0.074 mmol) of cruciform 5 (mp >350 °C,
with decomposition). 5. UV−vis (CH2Cl2): λmax (log ε) = 310 (6.76),
nm. IR (neat): 2890 (w, v ), 2853 (w, v ), 2801 (w, vC−H), 2211
̃
̃
̃
C−H
C−H
(w, v
), 1606 (s), 1563 (m, vO−CN), 1543 (m), 1512 (m), 1489
̃
̃
C C
(m), 1452 (m), 1366 (m), 1353 (m), 1341 (m), 1315 (w), 1280 (m),
1227 (m), 1184 (s), 1169 (m), 1028 (m), 976 (m), 946 (m), 917 (m),
813 (s), 779 (m), 722 (m), 698 (s), 686 (s) cm−1. 1H NMR (CDCl3,
3
500 MHz): δ 8.42 (m, 4H), 7.67 (d, JH−H = 9.16 Hz, 4H), 7.56 (m,
6H), 6.74 (d, 3JH−H = 9.16 Hz, 4H), 3.06 (s, 12H). 13C NMR (CDCl3,
125 MHz): δ 164.4, 150.8, 148.8, 141.5, 133.6, 132.0, 129.0, 128.3,
127.0, 111.9, 109.7, 102.9, 102.3, 78.5, 40.4. LRMS (ESI/[M + H]+):
calcd for C40H31N4O2 599.24, found 599.33. HRMS: calcd for
C40H31N4O2 599.2447, found 599.2440. Anal. Calcd. for C40H30N4O2:
C, 80.25; H, 5.05; N, 9.36. Found: C, 79.13; H, 4.52; N, 9.10. Despite
three separate attempts, satisfactory elemental analysis for C and H
could not be obtained.
334 (6.67), 421 (7.08) nm. IR (neat): 2910 (w, vC−H), 2212 (w,
̃
v
CC), 1607 (s), 1586 (w), 1540 (w), 1509 (s), 1445 (w), 1364 (m),
̃
Synthesis of Cruciform 3. In a nitrogen-flushed round-bottom
flask, anhydrous K2CO3 (1.06 g, 7.66 mmol) was added to a solution
of 2-(4-(pyridinyl))trimethylsilylethyne38 (671 mg, 3.83 mmol) in a
mixture of MeOH (5 mL) and THF (5 mL). After being stirred for
30 min, the reaction mixture was filtered through Celite. The solvent
was removed under reduced pressure to yield crude 4-ethynylpyridine
(19), which was used without purification in the next step. To
minimize manipulations of this somewhat sensitive compound, we
assumed a 95% yield for this reaction.
1339 (m), 1284 (w), 1230 (w), 1184 (m), 1066 (w), 975 (w), 945
(w), 911 (w), 814 (m), 741(w), 695 (w) cm−1. 1H NMR (CDCl3, 500
3
3
MHz): δ 8.25 (d, JH−H = 9.16 Hz, 4H), 7.66 (d, JH−H = 8.59 Hz,
3
3
4H), 6.79 (d, JH−H = 8.59 Hz, 4H), 6.73 (d, JH−H = 9.16 Hz, 4H),
3.09 (s, 12H), 3.04 (s, 12H). Because of the low solubility of cruciform
5, a satisfactory 13C NMR spectrum could not be obtained. LRMS
(ESI/[M + H]+): calcd for C44H41N6O2 685.32, found 685.50. HRMS:
calcd for C44H41N6O2 685.3291, found 685.3289. Anal. Calcd for
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C44H40N6O2·2CH2Cl2: C, 64.64; H, 5.19; N, 9.83. Found: C, 64.62; H,
5.41; N, 9.74.
obtained. LRMS (ESI/[M + H]+) calcd for C38H28N6O2 601.23, found
601.30. HRMS: calcd for C38H29N6O2 601.2352, found 601.2350.
Anal. Calcd for C38H28N6O2·CH2Cl2: C, 68.32; H, 4.41; N, 12.26.
Found: C, 69.67; H, 4.04; N, 12.46. Inclusion of dichloromethane was
confirmed by an X-ray crystal structure. Despite repeated attempts,
satisfactory elemental analysis for C could not be obtained.
Synthesis of Cruciform 9. Compound 19 (262 mg, 2.54 mmol)
was added to a thick-walled microwave pressure vial that contained a
mixture of compound 16 (300 mg, 0.64 mmol), PdCl2(PPh3)2 (89 mg,
0.13 mmol), CuI (24 mg, 0.13 mmol), i-Pr2NH (4 mL), and MeCN
(4 mL). The vial was sealed and exposed to microwave irradiation for
4 h at 115 °C. After cooling, solvents were removed under reduced
pressure, and the crude solid was purified by column chromatography,
eluting first with pure CH2Cl2, and then successively with CH2Cl2/
MeOH mixtures in 97:3, 19:1, and 9:1 ratios. The solvent was
removed under reduced pressure to give 28 mg (8.5%, 0.054 mmol) of
pure 9 (mp >350 °C, with decomposition). 9. UV−vis (CH2Cl2): λmax
(log ε) = 304 (6.18), 315 (6.18), 367 (6.34) nm. IR (neat): 3040 (w,
Synthesis of Cruciform 6. Compound 19 (56 mg, 0.54 mmol)
was added to a thick-walled microwave pressure vial that contained a
mixture of compound 15 (100 mg, 0.18 mmol), PdCl2(PPh3)2 (25 mg,
0.036 mmol), CuI (7 mg, 0.036 mmol), NEt3 (2 mL), and MeCN
(2 mL). The vial was sealed and exposed to microwave irradiation for
3 h at 120 °C. After cooling, solvents were removed under reduced
pressure, and the crude solid was purified by column chromatography,
eluting first with pure CH2Cl2, and then successively with CH2Cl2/
MeOH mixtures in 97:3, 19:1, and 9:1 ratios. The solvents were
removed under reduced pressure to give 32 mg (30%, 0.053 mmol)
of compound 6 (mp >350 °C, with decomposition). 6: UV−vis
(CH2Cl2): λmax (log ε) = 347 (6.92), 458 (6.81) nm. IR (neat): 2907
(w, v ), 2224 (w, vC C), 1608 (s), 1593 (m), 1508 (s), 1488 (w),
̃
̃
C−H
1448 (w), 1434 (w), 1408 (w), 1365 (m), 1337 (m), 1282 (w), 1224
(w), 1192 (m), 1170 (w), 1074 (m), 980 (m), 944 (w), 913 (m), 820
(s), 743 (m), 698 (w) cm−1. 1H NMR (CDCl3, 500 MHz): δ 8.71 (d,
3JH−H = 5.73 Hz, 4H), 8.23 (d, 3JH−H = 9.16 Hz, 4H), 7.64 (d, 3JH−H
=
v
), 2221 (w, vCC), 1591 (s), 1575 (m), 1545 (m), 1486 (w), 1409
̃
̃
C−H
5.73 Hz, 4H), 6.80 (d, 3JH−H = 9.16 Hz, 4H), 3.11 (s, 12H). 13C NMR
(CDCl3, 125 MHz): δ 166.0, 153.0, 150.1, 147.6, 141.1, 137.6, 129.9,
126.1, 113.4, 111.7, 85.0, 40.3 (one peak is missing; poor solubility of
this compound precluded the collection of a better 13C NMR
spectrum). LRMS (ESI/[M + H]+): calcd for C38H29N6O2 601.23,
found 601.30. HRMS: calcd for C38H29N6O2 601.2352, found
601.2343. Anal. Calcd. for C38H28N6O2·1/3CH2Cl2: C, 73.20; H,
4.59; N, 13.36. Found: C, 73.02; H, 4.17; N, 13.26.
(m), 1384 (w), 1339 (m), 1318 (w), 1283 (w), 1228 (w), 1207 (w),
1061 (m), 992 (m), 984 (m), 922 (m), 829 (s), 817 (s), 740 (m), 726
(m), 699 (m), 689 (m) cm−1. 1H NMR (CDCl3, 400 MHz): δ 8.91 (d,
3JH−H = 5.95 Hz, 4H), 8.76 (d, 3JH−H = 5.95 Hz, 4H), 8.25 (d, 3JH−H
=
5.95 Hz, 4H), 7.65 (d, JH−H = 5.95 Hz, 4H). 13C NMR (CDCl3, 100
MHz): δ 163.2, 151.2, 150.3, 149.3, 141.8, 133.4, 130.4, 126.0, 121.6,
99.6, 98.5, 83.1. LRMS (ESI/[M + H]+): calcd for C32H17N6O2
517.13, found 517.20.
3
Synthesis of Cruciform 7. Phenylacetylene (17, 95 μL, 0.85
mmol) was added to a thick-walled microwave pressure vial that
contained a mixture of compound 16 (100 mg, 0.21 mmol),
PdCl2(PPh3)2 (30 mg, 0.042 mmol), CuI (8 mg, 0.042 mmol), NEt3
(2 mL), and MeCN (2 mL). The vial was sealed and exposed to
microwave irradiation for 3 h at 100 °C. After cooling, solvents were
removed under reduced pressure, and the crude solid was purified by
column chromatography, eluting first with pure CH2Cl2 and then
successively with CH2Cl2/MeOH mixtures in 97:3, 19:1, and 9:1
ratios. The solvent was removed under reduced pressure to give 28 mg
(26%, 0.054 mmol) of pure 7 (mp >350 °C, with decomposition). 7.
UV−vis (CH2Cl2): λmax (log ε) = 311 (6.40), 367 (6.51) nm. IR
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional experimental and computational details, copies of
NMR spectra for all new compounds, and crystallographic
information files (CIFs) for 1, 2, and 8. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: miljanic@uh.edu.
(neat): 3061 (w, v ), 2214 (w, vCC), 1597 (m), 1576 (m), 1546
̃
̃
C−H
(m), 1489 (m), 1442 (m), 1414 (m), 1394 (w), 1339 (m), 1283 (m),
1216 (w), 1064 (m), 1028 (w), 991 (m), 981 (m), 924 (m), 831 (m),
ACKNOWLEDGMENTS
1
755 (s), 691 (s) cm−1. H NMR (CDCl3, 500 MHz): δ 8.89 (br s,
■
3
This research was financially supported by the grants made to
O.S.M. by the Welch Foundation (Grant No. E-1768), the
4H), 8.25 (br d, JH−H = 4.01 Hz, 4H), 7.80 (m, 4H), 7.47 (m, 4H).
Because of the poor solubility of 7, a satisfactory 13C NMR spectrum
could not be obtained. HRMS (ESI/[M + H]+): calcd for C34H19N4O2
515.1508, found 515.1502. Anal. Calcd for C34H18N4O2·CH2Cl2: C,
70.12; H, 3.36; N, 9.35. Found: C, 70.17; H, 2.96; N, 7.80. Despite
three separate attempts, satisfactory elemental analysis for N could not
be obtained.
̌
donors of the American Chemical Society Petroleum Research
Fund (ACS-PRF), the University of Houston (UH), and its
grant to Advance and Enhance Research, and the Texas Center
for Superconductivity at UH. Ms. Thao Shirley Nguyen and
Dr. Karolina Osowska (UH) are acknowledged for assistance
with synthesis of 1. We thank Prof. Rigoberto Advincula (UH)
for access to his fluorimetry equipment, and Jacob Armitage
(Texas State University) for his help with X-ray data collection.
Crystallographic equipment at Texas State University was
acquired through support from the National Science
Foundation (CHE-0821254).
Synthesis of Cruciform 8. Compound 18 (157 mg, 1.06 mmol)
was added to a thick-walled microwave pressure vial that contained a
mixture of compound 16 (100 mg, 0.21 mmol), PdCl2(PPh3)2 (30 mg,
0.042 mmol), CuI (8 mg, 0.042 mmol), NEt3 (2 mL), and MeCN
(2 mL). The vial was sealed and exposed to microwave irradiation for
3 h at 110 °C. After cooling, solvents were removed under reduced
pressure, and the crude solid was purified by column chromatography,
eluting first with pure CH2Cl2, and then successively with CH2Cl2/
MeOH mixtures in 97:3, 19:1, and 9:1 ratios. The solvent was
removed under reduced pressure to give 27 mg (21%, 0.045 mmol) of
compound 8 (mp >350 °C, with decomposition). 8. UV−vis (CH2Cl2):
λmax (log ε) = 277 (6.28), 332 (6.52), 350 (6.60), 368 (6.57), 468
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1
(27) Tentatively, H NMR spectroscopy supports this assignment of
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