Synthesis, rotational dynamics, and photophysical characterization of a crystalline linearly conjugated phenyleneethynylene molecular dirotor

Article abstract of DOI:10.1021/jo4004053

We report the synthesis, crystal structure, solid-state dynamics, and photophysical properties of 6,13-bis((4-(3-(3-methoxyphenyl)-3,3-diphenylprop-1- yn-1-yl)phenyl)ethynyl)-5,7,12,14-tetrahydro-5,14:7,12-bis([1,2]benzeno) pentacene (1), a molecular dirotor with a 1,4-bis((4-ethynylphenyl)ethynyl) benzene (BEPEB) chromophore. The incorporation of a pentiptycene into the molecular dirotor provides a central stator and a fixed phenylene ring relative to which the two flanking ethynylphenylene rotators can explore various torsion angles; this allows the BEPEB fluorophore dynamics to persist in the solid state. X-ray diffraction studies have shown that molecular dirotor 1 is packed so that all the BEPEB fluorophores adopt a parallel alignment, this is ideal for the development of functional materials. Variable temperature, quadrupolar echo ²H NMR studies have shown that phenylene rotator flipping has an activation energy of 9.0 kcal/mol and a room temperature flipping frequency of ～2.6 MHz. Lastly, with measurements in solution, glasses, and crystals, we obtained evidence that the fluorescence excitation and emission spectra of the phenyleneethynylene chromophores is dependent on the extent of conjugation between the phenylene rings, as determined by their relative dihedral angles. This work provides a promising starting point for the development of molecular dirotors with polar groups whose amphidynamic nature will allow for the rapid shifting of solid-state absorption, fluorescence, and birefringence, in response to external electric fields.

Full text of DOI:10.1021/jo4004053

Article
pubs.acs.org/joc
Synthesis, Rotational Dynamics, and Photophysical Characterization
of a Crystalline Linearly Conjugated Phenyleneethynylene Molecular
Dirotor
Melissa Hughs, Miguel Jimenez, Saeed Khan, and Miguel A. Garcia-Garibay*
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States
S
* Supporting Information
ABSTRACT: We report the synthesis, crystal structure, solid-state dynamics, and photo-
physical properties of 6,13-bis((4-(3-(3-methoxyphenyl)-3,3-diphenylprop-1-yn-1-yl)phenyl)-
ethynyl)-5,7,12,14-tetrahydro-5,14:7,12-bis([1,2]benzeno)pentacene (1), a molecular dirotor
with a 1,4-bis((4-ethynylphenyl)ethynyl)benzene (BEPEB) chromophore. The incorporation of
a pentiptycene into the molecular dirotor provides a central stator and a fixed phenylene ring
relative to which the two flanking ethynylphenylene rotators can explore various torsion angles;
this allows the BEPEB fluorophore dynamics to persist in the solid state. X-ray diffraction
studies have shown that molecular dirotor 1 is packed so that all the BEPEB fluorophores adopt
a parallel alignment, this is ideal for the development of functional materials. Variable
temperature, quadrupolar echo 2H NMR studies have shown that phenylene rotator flipping has
an activation energy of 9.0 kcal/mol and a room temperature flipping frequency of ∼2.6 MHz. Lastly, with measurements in
solution, glasses, and crystals, we obtained evidence that the fluorescence excitation and emission spectra of the
phenyleneethynylene chromophores is dependent on the extent of conjugation between the phenylene rings, as determined
by their relative dihedral angles. This work provides a promising starting point for the development of molecular dirotors with
polar groups whose amphidynamic nature will allow for the rapid shifting of solid-state absorption, fluorescence, and
birefringence, in response to external electric fields.
(BEPEB) chromophore (Figure 1). Based on the well-known
changes in the photophysical properties of these structures as a
function of aromatic group planarization and twisting,8,9 we
expect that changes in conjugation brought about by changes in
the orientation of the moving phenylene rotators will result in
solid-state spectral shifts that will be useful for the generation of
a new class of optical materials. As shown in the top portion of
Figure 1, the chromophore may vary from a fully coplanar 1,4-
bis(4-ethynylphenyl)ethynyl)benzene (BEPEB, shown in blue)
to structures that have one or two rings out of conjugation,
which result in chromophores analogous to 1,2-bis(4-
ethynylphenyl)ethyne (BEPE, shown in purple) and 1,4-
diethynylbenzene (DEB, shown in violet).
Linearly conjugated phenyleneethynylenes are remarkable
chromophores. Their three aromatic rings experience nearly
barrierless rotation in the ground state but have large energy
variations as a function of their dihedral angle in the excited
state.8,9,15 The difference between the ground- and excited-state
rotational potentials is the result of changes in bond order, as
the single and triple bonds linking the aromatic groups in the
ground state acquire the character of an extended cummulene
upon electronic excitation. It has been shown that coplanar
conformations of 1,4-bis(arylethynyl)benzene display red shifts
of ca. 20−30 nm in absorption and emission as compared to
“twisted” conformers where the plane of the central ring is
INTRODUCTION
■
Recent advances in the fields of functional materials and
artificial molecular machines1 have led to the development of
crystalline molecular machinery,2 which is intended to take
advantage of internal molecular motion, and/or chemical
reactivity, to influence mechanical properties,3 electric,4
magnetic,5 and/or optical functions6 in a highly controlled
and anisotropic manner. Over the past few years, our group has
emulated the structures of macroscopic gyroscopes, as they are
useful models for the design of molecules capable of forming
amphidynamic crystals with static elements and rapidly moving
parts.2 Like their macroscopic namesakes, molecular gyroscopes
consist of a central rotator linked by an axle to two shielding
stator groups that are responsible for self-assembly of an
ordered crystal lattice. The rotational dynamics of these
molecules in the solid-state depends on the nature of the
stator, the rotator size and symmetry, and the details of the
crystal packing.7 Using frequency-dependent dielectric spec-
troscopy as a function of temperature, we showed that
molecular gyroscopes with rotators bearing a permanent dipole
moment can be interfaced with external AC fields.4c In this
study we report a structural motif whose rotational dynamics
are expected to allow for the incorporation of optical functions
into the bulk crystalline material. Specifically we have modified
the common 1,4-diethynylbenzene rotator motif for one that
contains two rotators linearly conjugated with a phenyl ring at
the center, which is part of a pentiptycene stator. The desired
dirotor has a 1,4-bis((4-ethynylphenyl)ethynyl)benzene
Received: March 2, 2013
© XXXX American Chemical Society
A
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Shown in the bottom part of Figure 1 is an idealized crystal
of the desired molecular dirotors. The application of a strong
electric field is expected to influence the orientation and torsion
angle of the dipolar 2,3-difluorophenylenes in and out of the
plane of the pentiptycene central ring. While changes in the
direction along the preferred orientation have the potential of
breaking a centrosymmetric arrangement in the structure,
changes in the orientation of the rotators can affect the
conjugation of the chromophore, resulting in both nonlinear
optical effects and changes in color (absorption spectra), as well
as changes in dichroism and birefringence. While this behavior
is reminiscent of the function of a liquid crystal,10 suitable stator
and rotator combinations in the solid state should lead to
essentially barrierless rotation and optical switching at time
scales that are only limited by the moment of inertia of the
rotator (ca. 10−12 s), which is 6 to 9 orders of magnitude faster
than the time scale required for domain reorientation in a liquid
crystal.10 While we have yet to achieve a true inertial rotator, we
have previously measured rotation barriers as low as 1.48 kcal/
mol, with a room temperature rotation frequency in the range
of ∼1−4.3 GHz.11
In this Article we report the synthesis, solid-state dynamics,
and photophysical properties of the BEPEB molecular dirotor 1
shown in Scheme 1. Compound 1 was synthesized with a m-
Scheme 1
Figure 1. Top: Rotation of the phenylene rotators in a phenyl-
eneethynylene molecular dirotor reveals chromophores with varying
degrees of π-conjugation; the chromophore may vary from a fully
coplanar 1,4-bis((4-ethynylphenyl)ethynyl)benzene (BEPEB, shown
in blue) analogue to a partially conjugated 1,2-bis(4-ethynylphenyl)-
ethyne (BEPE, shown in purple) analogue or an unconjugated 1,4-
diethynyl benzene (DEB, shown in violet) analogue. Bottom: An
idealized crystal where polar (2,3-difluorophenylene) rotators are able
to reorient as a function of an external electric field to change the
optical properties of the material.
twisted with respect to the planes of the rings on the sides.8
Elegant studies as a function of temperature by Yang and co-
workers with a series of pentiptycene-derived oligo(p-
phenyleneethynylene)s with significant intrachain interactions
showed remarkable shifts as the emission changes from an
excited-state population of mainly twisted conformers at low
temperatures to one that consists of mainly coplanar structures
as the temperature increases.9 By incorporating two smaller
phenylene rotators with one bulky pentypticene and two
shielding trityl groups within the structure of a BEPEB
fluorophore, we expect two phenylene rings will preserve
some freedom of rotation in the solid state. Figure 1 (top)
shows the suggested pentiptycene structure with two central
2,3-difluorophenylene rotators flanked by two triphenylmethyl
stators. The hypothesized changes in excitation energy as a
function of planarization for the idealized chromophore are
qualitatively expected to resemble the BEPEB, BEPE, and DEB
chromophores highlighted in blue, purple, and violet,
respectively.
methoxy group in each of the two trityl stators to increase its
solubility. Samples of the isotopologue, 1-d8, with deuterated
phenylene rotators were obtained to allow for the determi-
nation of the phenylene rotational barrier in the solid state
2
using quadrupolar echo H NMR line-shape analysis experi-
ments.12,13 We confirmed that the relatively large aspect ratio of
the desired molecular structure yields crystals with all the
chromophores aligned in the same direction, which is ideal for
the preparation of functional materials addressable by external
electric fields. In addition, we showed that phenylene rotation is
facile at ambient temperature (298 K) with a phenylene 180°
jump average frequency of 2.6 MHz. Lastly, we showed that the
BEPEB chromophore displays a reasonably strong emission
that is homogeneous in a dilute toluene solution at room
temperature but becomes highly heterogeneous in a rigid glass
B
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Scheme 2
1
at 77 K. Clear evidence for the range of chromophores
suggested in Figure 1 was obtained by analyzing the emission
spectra as a function of excitation wavelength in the rigid glass,
which revealed spectra that are nearly identical to those
obtained from simple 1,4-diethynylbenzene (DEB), 1,2-bis(4-
ethynylphenyl)ethyne (BEPE), and 1,4-bis((4-ethynylphenyl)-
ethynyl)benzene (BEPEB). We confirmed that pentiptycene
dirotors provide a promising platform for the development of
solid-state functional materials with shifting absorption and
emission properties with the application of external electric
fields.
conserved across the five structures. In the H NMR spectrum
of 1, a singlet assigned to the methoxy groups is observed at
3.79 ppm, two doublet of doublet of doublets at ca. 6.70 and
6.90 ppm are assigned to the protons that are ortho and para to
the methoxy group (H3 and H5 in Scheme 1), respectively. A
doublet of doublets corresponding to H7 appears at 6.93 ppm
and an apparent triplet at 7.23 ppm that is assigned to H4
complete the description of the m-methoxyphenyl group. The
signals corresponding to the protons of the unsubstituted trityl
phenyl rings appear as an unresolved multiplet between 7.19
and 7.40 ppm. Hydrogen atoms of the phenylene rotator,
labeled H16 and H17 in Schemes 1 and 2, appear as a pair of
doublets at 7.65 and 7.74 ppm. Signals corresponding to the
pentiptycene moiety appear as a singlet at 5.87 ppm for the
bridgehead protons (H23) and a doublet of doublets at 6.96
ppm for H26, and the signal for H25 between 7.30 and 7.40
ppm is obscured beneath other aromatic signals between 7.30
and 7.40 ppm. In the asymmetric compound 8, the bridgehead
protons (H23 and H30) give two distinct signals at 5.83 and
5.82 ppm and the proton signals of the hexyl tail (H35−H40)
are present between 2.76 and 1.01 ppm. All the 13C NMR
spectra matched expectations from their structures. In the case
of 1, all 26 nonequivalent carbons are accounted for. Notably,
the chemical shifts of the four alkyne carbons (C14, C13, C19,
and C20) appear at 84.7, 86.5, 96.5, and 97.9 ppm, respectively.
The phenylene rotator carbons C16 and C17 occur at 131.5
and 131.7 ppm, respectively; however, these carbon signals are
not observed in deuterated rotators 1-d8 and 3-d4 due to signal
splitting caused by deuterium substitution. The pentiptycene
bridgehead carbons give signals at 52.2, the quaternary trityl
carbons at 56.2 ppm, and the methoxy methyl at 55.1 ppm.
Crystal Structure. X-ray diffraction quality single crystals of
molecular dirotor 1 were obtained by slow evaporation of a
saturated 1:4 toluene/dichloromethane (DCM) solution and
were used for X-ray diffraction data acquisition. Diffraction data
were obtained with a crystal of 1-d8, after the sample of origin
had been shown to have identical PXRD patterns to crystals of
natural abundance 1. Evaporation of DCM/diethyl ether,
DCM/benzene, and DCM/hexane also yielded crystals but
were not of sufficient quality for single crystal X-ray diffraction.
Although the crystals obtained from toluene/DCM were shown
to include the aromatic solvent (see below), evaporation from
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization. The
synthesis of the BEPEB molecular dirotor 1 was accomplished
in a convergent manner, relying on previously developed
procedures.14 Samples of 3-(m-methoxyphenyl)-3,3-diphenyl-
prop-1-yne 2 were obtained as described recently by our group.
The central 6,13-diethynyl-5,7,12,14-tetrahydro-5,14:7,12-bis-
([1,2]benzeno)pentacene 4 was prepared as reported by
Swager and Yang.15 As illustrated in Scheme 1, samples of 1-
(3-(m-methoxyphenyl)-3,3-diphenylprop-1-ynyl)-4-iodoben-
zene 3 and its 4-bromo-phenylene deuterated analogue 3d4
were prepared via a Pd(0)-catalyzed Sonogashira coupling of 2
with either 1,4-diiodobenzene or 1,4-dibromobenzene-d4 in
refluxing 2:1 THF:diisopropylamine. Another Pd(0)-catalyzed
coupling reaction using 3 equiv of 3 or 3-d4 per equivalent of 4
yielded BEPEB dirotors 1 and 1-d8, respectively. To have a
model chromophore for photophysical studies, 6-((4-(3-(3-
methoxyphenyl)-3,3-diphenylprop-1-yn-1-yl)phenyl)ethynyl)-
13-(oct-1-yn-1-yl)-5,7,12,14-tetrahydro-5,14:7,12-bis([1,2]-
benzeno)pentacene 8 was prepared as shown in Scheme 2.
Pentiptycene quinone 5 was reacted subsequently with 1-
octynyllithium and lithium TMS-acetylide to yield an asym-
metrical diol which was immediately rearomatized with
tin(II)chloride and deprotected with potassium hydroxide.
The resulting asymmetrical diyne 7 was coupled to 1-(3-(m-
methoxyphenyl)-3,3-diphenylprop-1-ynyl)-4-iodobenzene 3 via
a Pd(0)-catalyzed Sonogashira reaction.
Both the 1H and the 13C NMR spectra of compounds 1, 1-d8,
3, 3-d4, and 8 possess all the signals expected from a mono m-
methoxyphenyl trityl group with chemical shifts that are well
C
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pure benzene or toluene yielded no crystals. X-ray diffraction
data acquired at 100 K from the colorless plates obtained from
channels of toluene molecules, which display a high degree of
mobility or disorder as evidenced by their larger thermal
ellipsoids. Notably, the toluene channels are adjacent to the
phenylene rotators and may provide a locally “fluid” region in
the crystal which may allow for greater phenylene rotational
freedom.
The experimental X-ray powder patterns of 1 and 1-d8 are
identical to each other but do not have a perfect match to the
pattern simulated from the single crystal data (Figure S20,
Supporting Information). This may be attributed to the
disordered nature and variable content of the toluene clathrate,
which tends to escape faster in powder samples. The thermal
stability of the crystals was investigated by differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA).
The DSC shows an endotherm at 458 °C, which is immediately
followed by decomposition (Figure S22, Supporting Informa-
tion). The TGA curve shows the gradual loss of 8% of the total
mass between 70 °C and 180 °C, followed by a more rapid loss
of an additional 15% of the total mass between 400 °C and 455
°C (Figure S21, Supporting Information). Both are attributed
to the escape of toluene from the crystal lattice.
toluene/DCM were solved in the space group P1 with only one
̅
molecular dirotor and four molecules of toluene per unit cell.16
As required by the crystal space group, molecules of 1 have a
center of symmetry coincident with the one in the triclinic unit
cell. As a consequence of this, the two trityl stators have
opposing axial chiralities (M and P), and the two phenylene
rotators are equivalent (Figure 2). The alkynes in the structure
Figure 2. Molecular structure of compound 1 at 100 K with thermal
ellipsoids at 50% probability.
Solid-State Rotational Dynamics of the Phenylene
Group. To realize the electro-optical response depicted in
Figure 1, it will be essential to design molecular and crystal
architectures where the phenyl groups are able to rotate and
reorient. To determine whether or not fast phenylene
reorientation is possible within crystals of 1, we decided to
use quadrupolar echo 2H NMR spectroscopy with samples that
had the two phenylene groups 2H-labeled. It is well-known that
static 2H NMR is one of the most sensitive probes for dynamics
deviate significantly from linearity giving 1 an overall S-shape.
The torsion angle between rotator phenylenes and the central
pentiptycene phenyl ring is 34.2°.
As observed previously for crystals of molecular gyroscopes,
which tend to incorporate solvents in the lattice,17,18 crystals of
1 include four molecules of toluene per unit cell (Figure 3).
2
in the solid state. The H NMR spectrum is dominated by the
interaction between the nuclear spin and the electric quadru-
pole moment at the nucleus.12,13 The orientation dependence
of the quadrupolar coupling characterizes the spectra; each
deuteron provides a doublet with a splitting frequency Δν that
depends on the orientation angle β that the C−2H bonds make
with respect to the external field:
Δν =3/ (e2q Q /h)(3 cos2 β − 1)
4
zz
=3/ QCC(3 cos2 β − 1)
4
where Q represents the electric quadrupole moment of the
deuteron, e and h are the electric charge and Plank’s constant,
respectively, and qzz is the magnitude of the principal
component of electric field gradient tensor, which lies along
the C−2H bond. The combination of these quantities is
commonly called the quadrupolar coupling constant (QCC).
For aromatic C−2H bonds, QCC = 180 kHz. The peak
separation for static deuterons varies from ca. Δν = 270 kHz
when β = 0o, to Δν = 0 when the C−2H bond is at the magic
angle β = 54.7°. In powdered samples, all values of β are
represented, but they are not all equally populated; this gives
rise to a collection of doublets in all possible orientations,
which collectively give a broad symmetric spectrum with two
maxima and two shoulders known as a Pake pattern. When the
C−2H bonds experience reorientations that reduce the range of
their magnetic interactions by dynamic averaging, the shape of
the Pake pattern is altered in characteristic ways. It is possible
to determine the frequency of the reorientation from ca. 104 to
108 Hz through line-shape comparison analysis between
experimental and simulated spectra. Phenylene rotations in
solids are typically modeled in terms of a Brownian exchange
between degenerate sites related by 180° rotation, also known
Figure 3. Packing diagram of molecular dirotor 1 illustrating the unit
cell. Molecules of 1 are packed in a parallel manner and in layers, with
toluene molecules between the layers.
The packing coefficient, given by the van der Waals volume of
all the molecules in the unit cell in relation to the total volume
of unit cell, is 0.71. The unit cell shows that each phenylene
rotator is nested in the cleft of the central pentiptycene stator of
a neighboring molecule. It is noteworthy that 1 is packed in a
lamellar fashion, with layers of molecular dirotors separated by
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as 2-fold flips. Spectral changes that occur as a result of this 2-
fold phenylene flipping have been well studied in the
literature,19−21 and excellent programs for line-shape simulation
are now readily available.22,23
Measurements were carried out using a quadrupolar echo
pulse sequence on static crystalline powder samples of 1-d8.
Spectra were obtained at 46.07 MHz with a 90° pulse width of
2.5 μs, echo and refocusing delays of 50 and 42 μs, respectively,
a 5 s delay between pulses, and line broadening of 5000 Hz at
temperatures ranging from 170 K to 350 K. Some of the data
and the best simulations are illustrated in Figure 4. Notably, the
coplanarity between the aromatic rings and the extent of
fluorophore aggregation.8,9,25−27 Theoretical and experimental
studies have been performed on these systems to determine the
magnitude and the nature of the contribution of planarization.
It has been found that when freedom of rotation is allowed and
aggregation minimized (i.e., a dilute solution in a nonviscous
solvent), the excitation band is heterogeneously broadened
while the emission spectrum is vibrationally resolved and
relatively narrow. The broadness of the absorption spectrum is
determined by a large distribution of phenylene rotamers and
the well-resolved emission by a rapid conformational
equilibration in the excited state, which gives rise to the
dominant contribution from the lowest energy, fully planar
rotamer.8,25 When the rotation of the aryleneethynylenes is
restricted and the coplanar rotamer is dominant, as it occurs in
stretched poly(ethylene) films, the excitation and absorption
spectra become narrow and red-shifted.8,9 By contrast, when
aggregation is present, the red shift is more pronounced and an
additional low energy band is sometimes observed.9,26,28
A
single molecule study of a related poly(9,10-anthracenediyle-
thynylene-1,4-phenyleneethynylene) has further highlighted the
importance of conformation in phenyleneethynylene systems.29
In addition, density functional theory calculations of 2,5-
dimethoxy-p-phenyleneethynylene oligomers support the con-
clusion that twisting about the triple bond leads to a shortening
of conjugation length and a widening of the HOMO−LUMO
gap.30 A number of studies have been performed on
phenyleneethynylenes with pentiptycene units. These studies
have indicated that the incorporation of pentiptycene units into
the phenyleneethynylene backbone generally decreases fluo-
rophore aggregation and prevents the corresponding broad-
ening of the emission.15,28
To correlate qualitatively the phenylene twist geometries
with absorption and emission data, we performed TD-DFT
calculations at the B3LYP 6-31G(d) level of theory. The results
obtained using a simplified model system consisting of the
unadorned BEPEB chromophore are shown in Figure 5a and
plotted in Figure 5b. The model considers a frame of reference
given by the plane of the central phenylene and explores
variations in energy as a function of rotation of one or both
aromatic end groups. As shown in Figure 5b, changes in energy
for rotation about the alkyne bonds in the ground state are less
than 1.0 kcal/mol between the fully coplanar chromophore
(Figure 5b, 0° torsion) and that with the two outer phenylenes
at a 90° (or −90°) torsion angle. The corresponding energy
difference in the excited state is significantly larger, i.e., 9.9 kcal/
mol. However, the energy difference is much smaller, 2.7 kcal/
mol, when only one of the two outer phenylene rings is rotated
by 90° while the other two remain coplanar. The vertical
excitation energies calculated in this manner are in good
qualitative agreement with experimental data for analogous
compounds.31 We set out to qualitatively validate this trend by
comparing the spectra of the BEPEB chromophore containing
dirotor 1 with those of model compounds containing BEPE
and DEB chromophores in their structures: compounds 8 and
4, respectively.
Figure 4. Experimental (left) and simulated (right) solid-state 2H
NMR spectra of 1-d8. Arrhenius analysis of the rate and temperature
data resulted in an average activation energy of 9.0 kcal/mol with a
pre-exponential factor of 2.4 × 1013 s−1 (Figure S19).
experimental spectra were fitted poorly by simulations that
assume a single phenylene jump frequency. A good fit required
the use of a log-Gaussian distribution model with a width σ =
1.5, indicating that there is a distribution of activation energies
in the sample.24 It is likely that different environments form as a
result of solvent disorder and/or partial desolvation. An
Arrhenius plot of the data using the average 2-fold jumping
rate constant (Figure S19, Supporting Information) revealed an
activation energy of 9.0 kcal/mol with a pre-exponential factor
is 2.4 × 1013 s−1. The line-shape differences between the spectra
obtained below 210 K were negligible, indicating that motion at
these temperatures reaches the slow exchange regime.
The photophysical properties of the three fluorophores 1, 4,
and 8 are summarized in Table 1, and their fluorescence
excitation and emission spectra are shown in Figure 6. Included
in the table are the position of the λ0−0 bands for absorbance
and fluorescent emission, the molar extinction coefficient
(ε0−0), and the extinction coefficient at λ = 303 (ε303, the
wavelength used to measure fluorescence lifetimes), the
Photophysics. It is well-known that the fluorescence of
phenyleneethynylenes depends both on the degree of
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Figure 6. Fluorescence excitation and emission of dirotor BEPEB 1
and model chromophores BEPE 8 and DEB 4 in deoxygenated
methylcyclohexane solutions at room temperature.
yield while both BEPEB 1 and DEB 4 have significantly higher
yields.
To experimentally determine whether changes in fluoro-
phore planarization result in substantial changes in the emission
spectra, one needs conditions where the distribution of twist
angles and phenylene rotation is restricted. Considering the
relatively flat ground-state rotational energy surface of 1, one
should expect a statistical distribution of all possible phenylene
torsion angles to exist in solution, from perfectly planar to
perfectly orthogonal. Thus, by flash freezing these solutions it
should be possible to capture a “snapshot” of the torsion angle
population. To document this, a dilute solution (1.49 μM) of
molecular dirotor 1 was prepared in toluene, and its
fluorescence spectra were measured both at 298 K and at 77
K (Figure 7). The fluorescence emission spectrum of trimer 1
Figure 5. (a) Simplified structure used to calculate the ground and
vertical excited-state energies as a function of one and two phenylene
torsion angles. (b) Calculated ground-state and excited-state energy
surfaces at the TD-DFT B3LYP 6-31G(d) level of theory plotted
against one (dashed line) or two (dotted line) phenylene torsion
angles. Blue, purple, and violet arrows correspond to the excitation
energies of the corresponding BEPEB-, BEPE-, and DEB-like
chromophores proposed in Figure 1.
Table 1. Photophysical Properties of the BEPEB, BEPE, and
DEB Chromophores in Dirotor 1 and Model Compounds 8
and 4 in Dilute Methylcyclohexane Solutions at Room
Temperature
ε0−0 abs
λ0−0 fl λ0−0 abs
(M−1
ε303
compound (nm)
(nm)
cm−1
)
(M−1 cm−1
)
Φfl
τfl (ns)
a
BEPEB 1
BEPE 8
DEB 4
376
351
342
369
346
333
39437
20099
7033
25866
18329
2178
0.85
0.29
0.70
≤0.6
1.4
5.5
a
Fluorescence lifetimes were shorter than the time resolution of our
current instrument.
Figure 7. (Top) Fluorescence emission of BEPEB molecular dirotor 1
in frozen toluene at 77K excited at wavelengths that range from 295 to
350 nm, compared to (bottom) the emission spectra of model DEB 4
(violet), model BEPE 8 (purple), and BEPEB dirotor 1 (blue) in
toluene at room temperature.
quantum yield of fluorescence (Φfl), and the fluorescence
lifetime (τfl). As expected, both the excitation and emission
spectra are red-shifted as the number of conjugated aromatic
groups and alkyne linkages increases. A red-shift in fluorescence
emission of 9 nm was determined from DEB 4 to BEPE 8, and
a shift of 25 nm when comparing BEPE 8 to the full BEPEB
chromophore of 1. This result is in qualitative agreement with
the quantum mechanical calculations. The excited-state life-
times of the three chromophores decrease with increasing
conjugation length, while the molar extinction coefficient at
λ0−0 abs increases linearly with conjugation length. The
fluorescence quantum yields appear to be independent of
conjugation length, with BEPE 8 having the lowest quantum
measured at 298 K does not vary as a function of excitation
energies and consists of a strong 0−0 band followed by four
vibrationally resolved maxima with decreasing intensities. This
observation is consistent with a distribution of phenylene angles
in the ground state, which are able to equilibrate to the lowest
energy coplanar conformation during their excited-state
lifetime. Thus, mainly the emission of the lower energy
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coplanar fluorophore is observed, as illustrated by the spectrum
shown in cyan color in the bottom part of Figure 7. The spectra
corresponding to BEPE 8 and the DEB 4 are increasingly blue-
shifted, as shown by the spectra labeled in violet and purple.
The fluorescence emission spectrum of BEPEB 1 measured in
toluene glasses at 77 K showed the postulated heterogeneity. As
phenylene rotation is prevented by the frozen matrix, it is
possible to detect a broad range of rotamers by varying the
excitation energy. As illustrated in the top frame of Figure 7,
excitation at higher energy wavelengths (i.e., 295 nm)
preferentially addresses fluorophores that give higher energy
emission assigned to the doubly twisted structures that
approach the spectral properties of the model DEB 4. As also
shown in the top of Figure 7, a systematic shift in the excitation
wavelength from 295 to 350 nm results in a similar shift in the
onset of emission. While the spectra are highly heterogeneous
and it would be difficult to excite any given species selectively,
as the spectrum evolves, it acquires a qualitative resemblance to
the spectrum of model BEPE 8 and eventually to that of
BEPEB dirotor 1.
While the fluorescence results from dirotor 1 in solution and
in glassy matrix support a strong effect of aromatic torsion
angles in the absorption and emission spectra, it was also
important to confirm that the BEPEB dirotor 1 remains
emissive in the crystalline state. With that in mind, we carried
out fluorescence excitation and emission experiments by front-
face excitation and detection at 77 K and at 298 K with finely
powdered samples of 1 grown from toluene. As illustrated in
Figure 8, the solid-state excitation and emission spectra
change in the shape of the emission envelop where the band of
the 0−0 transition by ca. 385 nm disappears. Changes in going
from low to high temperature indicate that the high
temperature emission (red spectrum) is distorted by self-
absorption effects, and, considering the increase in population
of the species that absorbs at ca. 405 nm, it is likely that
emission occurs from this lower energy species after efficient
energy transfer.32
To account for the solid-state fluorescence observations, we
propose a model based on two distinct absorbing species with
equilibrium concentrations that change as a function of
temperature. It is reasonable to assume that the major
component is the one documented by single crystal X-ray
diffraction, with the external phenylene rotators adopting angles
of 34.2° with respect to the plane of the phenyl ring in the
central pentiptycene. The second and minor component shows
the characteristics of a chromophore that is more coplanar and
has a lower fluorescence excitation energy. The relative
intensities of the band at 420 nm suggest that the population
of the minor component increases as a function of temperature,
as expected by an increase in mobility within the crystal lattice.
In addition, changes in the emission spectrum as function of
temperature require that energy transfer from the twisted
component to the coplanar species is significantly more
favorable at higher temperature. Alternatively, these observa-
tions can also be understood as arising from a temperature-
dependent reabsorption of the higher energy emission by
neighboring chromophores, which could be associated with
transient conformational effects related to the high frequency
ring oscillations and rotation.
CONCLUSIONS
■
We have presented the synthesis, crystal structure, solid-state
dynamics, and photophysical properties of a phenyleneethyny-
lene molecular dirotor 1 and its deuterated isotopologue, 1-d8,
embedded in the structure of a pentiptycene stator. The role of
the bulky pentiptycene is to provide both a central shielding
stator and a fixed phenylene, so that the two flanking but
linearly conjugated phenylene rotators can explore a variety of
torsion angles to affect their excitation energies and photo-
physical properties in the solid state. X-ray diffraction studies
have shown that molecular dirotor 1 is packed in such a manner
that all the phenyleneethynylene chromophores are arranged in
parallel, so that they all share the same internal rotational axis,
as required for the development of functional materials.
2
Variable temperature quadrupolar echo H NMR studies have
shown that phenylene rotator 2-fold flipping is facile at room
temperature, with an activation energy of 9.0 kcal/mol and a
Brownian rotational frequency of approximately 2.6 MHz. This
energy barrier is lower than those previously shown to help
align phenylene flipping with external AC fields, indicating that
externally influencing phenylene rotation with an electric field
should be feasible in this system.4c We have also shown that the
phenyleneethynylene chromophore displays a remarkable
spectral heterogeneity in rigid glasses, as expected for a
chromophore that is capable of displaying significant
fluorescence changes as a function of its interphenylene torsion
angles. These results provide a very promising platform for the
development of molecular dirotors whose amphidynamic
nature allows for the rapid shifting of solid-state fluorescence
emission and optical properties with the application of an
electric field.
Figure 8. Fluorescence excitation (top) and emission (bottom)
spectra of phenyleneethynylene molecular dirotor 1 in solution
(black), in the crystalline state at 77 K (blue), and in crystalline state at
298 K (red). The arrow shown in the excitation spectra indicates the
thermal population of a red-shifted emissive species in the solid state,
which we assign to the fully coplanar conformation.
measured at 77 K (blue spectra) were red-shifted by ca. 10
nm when compared to those measured in solution (black
spectra). The solid-state excitation spectrum shows a strong 0−
0 transition by ca. 385 nm and an interesting new band at ca.
405 nm. The emission spectrum obtained upon excitation at
405 nm is red-shifted versus that obtained with a 385 nm
excitation wavelength (Figure S23, Supporting Information).
Notably, an increase in temperature to 298 K resulted in a
substantial increase of the 405 nm band and a remarkable
G
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evaporated under reduced pressure to yield 1 as a white powder with
strong violet fluorescence (134 mg, 33%). Crystallization of 1 by slow
evaporation of a 4:1 DCM:toluene mixture afforded small square
EXPERIMENTAL SECTION
■
2
Commercial reagents were used without additional purification. H
NMR spectra were simulated using the program Express 1.0.22 A 180°
jump model with a quadrupole coupling constant of 180 kHz and an
asymmetry parameter of 0.02 was used. A log-Gaussian distribution of
19 phenylene jump rates (σ = 1.5) centered around each nominal
phenylene jump rate was generated. A weighted average of the
simulated spectra were superimposed to generate the final simulated
spectra.
(3-(4-Iodophenyl)-1-(3-methoxyphenyl)prop-2-yne-1,1-
diyl)dibenzene 3. A three-neck flask containing 150 mL of THF and
75 mL of diisoproplyamine was purged with argon for 1 h. 2 (1.3 g, 4.4
mmol), 1,4-diiodobenzene (7.18 g, 21.8 mmol), bis-
(triphenylphosphine)palladium dichloride (185 mg, 0.264 mmol),
and copper iodide (50 mg, 0.528 mmol) were added to the flask, and
the reaction mixture was brought to reflux. The reaction mixture was
refluxed 12 h, cooled to room temperature, and quenched with
saturated aqueous ammonium chloride. The aqueous layer was
extracted 2× with DCM. The organic fractions were combined,
dried over magnesium sulfate, and evaporated under vacuum. The
product was purified by flash chromatography, first using hexanes to
elute excess diiodobenzene and switching to 2:1 hexanes:DCM to
elute 1.34 g (61%) of the colorless solid product: mp 120.6−121.4 °C;
1H NMR (300 MHz, CDCl3) δ 7.60 (d, J = 8.1, 2H), 7.27−7.15 (m,
1
crystals: mp (decomp); H NMR (CDCl3, 500 MHz) δ 7.74 (d, J =
8.1, 4H), 7.65 (d, J = 8.1, 4H), 7.4−7.26 (m, 30H), 6.98−6.93 (m,
12H), 6.87−6.85 (dd, J = 8.1, 1.7 Hz, 2H), 5.87 (s, 4H), 3.79 (s, 6H);
13C NMR (CDCl3, 125 MHz) δ 159.3, 146.6, 144.9, 144.7, 144.0,
131.7, 131.5, 129.1, 128.9, 128.0, 126.9, 125.2, 123.9, 123.7, 122.8,
121.7, 115.5, 114.6, 111.9, 97.9, 96.5, 86.5, 84.7, 56.2, 55.1, 52.2; IR
(DRIFT cm−1) 3062, 3020, 2968, 2930, 1598, 1488, 1458, 1290, 1248,
1181, 1035, 836, 751, 697; MS (ESI/APCI m/z) calculated for
C94H62O2+NH4 1240.5093, found 1240.5088.
1-d8: Diethynylpentiptycene 4 (207 mg, 0.43 mmol) was dispersed
in 75 mL of tetrahydrofuran and purged with argon in an addition
funnel for 1 h. 3-d4 (596 mg, 1.3 mmol), bis(triphenylphosphine)
palladium dichloride (30.2 mg, 0.043 mmol), and copper iodide (4.1
mg, 0.0215 mmol) were dissolved in 75 mL of tetrahydrofuran and 75
mL of diisopropylamine in a three-neck flask, purged with argon for 1
h, and heated to reflux. Diethynylpentiptycene 4 dispersion was added
over 24 h, and a 10 mL portion of THF was used to rinse residual
pentiptycene into the reaction flask. The reaction was allowed to
continue at reflux for another 24 h, and it was then quenched with
saturated ammonium chloride, extracted into dichloromethane, dried
over anhydrous magnesium sulfate, and evaporated under reduced
pressure. The residue was purified by column chromatography with a
60:40 hexanes:DCM eluent. The solvent was evaporated under
reduced pressure to yield 1-d8 as a white powder with strong violet
fluorescence (288 mg, 54%). Crystallization of 1-d8 by slow
evaporation of a 4:1 DCM:toluene mixture afforded small square
crystals. Anal.: mp decomposes; 1H NMR (CDCl3, 500 MHz) δ 7.39−
7.33 (m, 24H), 7.32−7.30 (m, 4H), 7.28 (t, J = 8.0 Hz, 2H), 6.99−
6.93 (m, 12H), 6.87 (dd, J = 8.5, 2.5, 2H), 5.87 (s, 4H), 3.79 (s, 6H);
13C (CDCl3, 125 MHz) δ 159.7, 146.9, 145.2, 144.9, 144.2, 129.4,
13H), 6.86−6.81 (m, 2H), 6.76 (d, J = 8.1, 2.4, 1H), 3.69 (s, 3H); 13C
NMR (75 MHz, CDCl3) δ 159.3, 146.6, 144.9, 137.4, 133.1, 129.1,
128.9, 128.0, 126.9, 123.0, 121.7, 115.5, 111.9, 97.0, 93.7, 84.2, 56.1,
55.1; IR (DRIFT cm−1) 3058, 3026, 2997, 2965, 2937, 2834, 1604,
1579, 1482, 1445, 1433, 1289, 1241, 1049, 816, 747, 695; MS (ESI/
APCI m/z) calculated for C28H21IO+H 501.0715, found 501.0710.
(3-(d4-4-Bromophenyl)-1-(3-methoxyphenyl)prop-2-yne-
1,1-diyl)dibenzene 3-d4. A three-neck flask containing 100 mL of
tetrahydrofuran and 50 mL of diisopropylamine was purged with
argon for 2 h. 2 (995 mg, 3.3 mmol), d4-1,4-dibromobenzene (3225
mg, 13.5 mmol), bis(triphenylphosphine)palladium dichloride (121
mg, 0.17 mmol), and copper iodide (157 mg, 0.82 mmol) were added
to the flask, and the flask was purged for another 10 min. The reaction
mixture was heated to reflux for 18 h, and it was then cooled and
quenched with saturated ammonium chloride. The crude product was
extracted into dichloromethane and dried over anhydrous magnesium
sulfate. The solvent was evaporated under reduced pressure, and the
residue was redissolved in dichloromethane and loaded onto a silica
gel column. The product was eluted with hexanes/dichloromethane,
with a gradient from 80:20 to 50:50. The solvent was evaporated, to
give a colorless solid (1.01 g, 60%). Excess d4-1,4-dibromobenzene
129.2, 128.3, 127.2, 125.5, 124.0, 122.9, 122.0 115.8, 115.0, 112.2, 98.2,
96.7, 86.8, 84.9, 56.5, 55.4, 53.6, 52.5; IR (DRIFT cm−1) 2955, 2922,
2853, 1599, 1457, 1251, 1051; MS (MALDI) calculated for
C94H54D8O2 1230.53, found 1230.53.
6-Ethynyl-13-(oct-1-yn-1-yl)-5,7,12,14-tetrahydro-5,14:7,12-
bis([1,2]benzeno)pentacene 7. Pentiptycene quinone 3 (50 mg,
0.11 mmol) is dissolved in dry THF in a flame-dried flask under argon.
To generate 1-octynyllithium solution, 1-octyne (103 mg, 0.93 mmol)
was dissolved in 8 mL of dry THF in a flame-dried flask under argon,
the solution was cooled to −78 °C, and n-butyllithium (0.5 mL, 1.6 M
in hexane, 0.80 mmol) was added. 1-Octynyllithium solution was
titrated into the pentiptycene quinone 3 solution at −78 °C until TLC
showed that pentiptycene quinone 3 was completely consumed. A
solution of trimethylsilylethynyllithium was prepared by adding n-
butyllithium (0.5 mL, 1.6 M in hexane, 0.80 mmol) to a solution of
ethynyltrimethylsilane (88 mg, 0.90 mmol) in 8 mL of dry THF at
−78 °C. Trimethylsilylethynyllithim solution was titrated into the
monoreacted pentiptycene quinone until it was consumed as shown by
TLC. The reaction mixture was warmed to room temperature,
acidified with 1 M HCl, extracted into DCM, and washed with water.
The organic layer was dried over magnesium sulfate and evaporated to
yield the crude asymmetric diol. This product dissolved in acetone and
was stirred overnight at room temperature with a solution of tin(II)
chloride hydrate (69 mg, 0.27 mmol) in 50% acetic acid to give the
rearomatized asymmetrical diyne. The reaction mixture was extracted
into DCM, the organic layer was washed with a saturated solution of
sodium bicarbonate, and the organic layer was dried over magnesium
sulfate and evaporated. The product mixture was purified by
preparatory TLC (80:20 hexane/DCM) and found to contain a 1:2
mixture of TMS-protected diyne 6 and deprotected terminal diyne 7.
Both products were redissolved in THF and stirred overnight at room
temperature with an excess of potassium hydroxide in water. The
reaction mixture was washed with brine, the brine layer was re-
extracted with DCM, and the combined organic layers were washed
with brine, dried over magnesium sulfate, and then evaporated to give
7 as a waxy yellow solid (29 mg, 48%): mp 109.6−112.3 °C; 1H NMR
(500 MHz, CDCl3) δ 7.36−7.32 (m, 8H), 6.94−6.93 (m, 8H), 5.81 (s,
1
(2.11 g) was recovered with high purity: mp 118.3−119.6 °C; H
NMR (CDCl3, 300 MHz) δ 7.32−7.24 (m, 10H), 7.22 (t, J = 8.0 Hz,
1H), 6.90 (m, 2H), 6.81 (ddd, J = 8.1, 2.4, 0.9 Hz, 1H), 3.73 (s, 3H);
13C (CDCl3, 75 MHz) δ 159.4, 146.7, 145.0, 129.1, 129.0, 128.1, 127.0,
122.3, 122.0, 121.8, 115.6, 111.9, 96.8, 84.0, 56.2, 55.2; IR (DRIFT
cm−1) 3085, 3065, 3045, 3026, 3015, 2955, 2932, 2903, 2835, 1598,
1578, 1482, 1446, 1432, 1287, 1255, 1050, 756, 736, 697; mass
(MALDI m/z) calculated for C28H17D4BrO 456.10, found 456.32.
6,13-Bis((4-(3-(3-methoxyphenyl)-3,3-diphenylprop-1-yn-1-
yl)phenyl)ethynyl)-5,7,12,14-tetrahydro-5,14:7,12-bis([1,2]-
benzeno)pentacene 1. Diethynylpentiptycene 4 (159 mg, 0.33
mmol) was dispersed in 75 mL of tetrahydrofuran and purged with
argon in an addition funnel for 1 h. 3 (500 mg, 1.0 mmol),
bis(triphenylphosphine)palladium dichloride (23.4 mg, 0.033 mmol),
and copper iodide (3.2 mg, 0.017 mmol) were dissolved in 75 mL of
tetrahydrofuran and 75 mL of diisopropylamine in a three-neck flask,
purged with argon for 1 h and heated to reflux. Diethynylpentiptycene
4 dispersion was added over 6 h, and a 10 mL portion of THF was
used to rinse residual pentiptycene into the reaction flask. The reaction
was allowed to continue at reflux for another 1 h at which point 4 had
been consumed by TLC. The reaction was then quenched with
saturated ammonium chloride and extracted into dichloromethane,
dried over anhydrous magnesium sulfate, and evaporated under
reduced pressure. The residue was purified by column chromatog-
raphy with a 70:30 to 60:40 hexanes:DCM eluent. The solvent was
H
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2H), 5.79 (s, 2H), 3.66 (s, 1H), 2.75 (t, J = 6.7 Hz, 2H), 1.87 (pent, J
= 7.2 Hz, 2H), 1.76 (pent, J = 7.2 Hz, 2H), 1.53−1.48 (m, 4H), 1.01
(t, J = 7.0 Hz, 3H); 13C (CDCl3, 125 MHz) δ 144.9, 144.8, 144.2,
143.6, 125.2, 125.1, 123.7, 123.6, 116.1, 112.6, 98.15, 83.9, 79.2, 75.9,
52.0, 51.9, 31.4, 29.0, 28.7, 22.7, 19.8, 14.1; IR (DRIFT cm−1) 3297,
3069, 3043, 3022, 2960, 2927, 2855, 2150, 1680, 1458, 1260, 1088,
(2) (a) Vogelsberg, C. S.; Garcia-Garibay, M. A. Chem. Soc. Rev.
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A. Acc. Chem. Res. 2006, 39, 413. (f) Garcia-Garibay, M. A. Proc. Natl.
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+
1017; mass (ESI/APCI m/z) calculated for C44H34+NH4 580.3004,
found 580.3023.
(3) (a) Irie, B.; Morimoto, M.; Irie, M. J. Am. Chem. Soc. 2010, 132,
14172. (b) Zhu, L.; Al-Kaysi, R. O.; Bardeen, C. J. J. Am. Chem. Soc.
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T.; Noro, S.-I.; Takeda, S.; Nakamura, T. Chem.Eur. J. 2011, 17,
14442. (b) Winston, E. B.; Lowell, P. J.; Vacek, J.; Chocholousova, J.;
Michl, J.; Price, J. C. Phys. Chem. Chem. Phys. 2008, 10, 5188.
(c) Horansky, R. D.; Clarke, L. I.; Winston, E. B.; Price, J. C.; Karlen,
S. D.; Jarowski, P. D.; Santillan, R.; Garcia-Garibay, M. A. Phys. Rev. B
2006, 74, 054306. (d) Horansky, R. D.; Clarke, L. I.; Price, J. C.;
Khuong, T.-A. V.; Jarowski, P. D.; Garcia-Garibay, M. A. Phys. Rev. B
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Hasegawa, T.; Nakamura, T.; Hosokoshi, Y.; Inoue, K.; Ikeuchi, S.;
Miyazaki, Y.; Saito, K. J. Am. Chem. Soc. 2005, 127, 4397.
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6-((4-(3-(3-Methoxyphenyl)-3,3-diphenylprop-1-yn-1-yl)-
phenyl)ethynyl)-13-(oct-1-yn-1-yl)-5,7,12,14-tetrahydro-
5,14:7,12-bis([1,2]benzeno)pentacene 8. A solution of 8 mL of
diisopropylamine in 16 mL of THF was purged with argon in a three-
neck flask. After 1 h, asymmetrical pentiptycene 7 (29 mg, 0.5 mmol),
half-rotor 3 (31 mg, 0.06 mmol), bis(triphenylphospnene)palladium
dichloride (3.5 mg, 0.005 mmol), and copper iodide (0.6 mg, 0.003
mmol) were added quickly to the purged solvents, and the mixture was
purged with argon for an additional 0.5 h. The reaction mixture was
stirred at reflux for 12 h under argon. The cooled reaction mixture was
washed with saturated ammonium chloride, the ammonium chloride
was extracted with DCM, and the combined organic layers were
washed with brine, dried over magnesium sulfate, and evaporated
under reduced pressure. The product was purified by preparatory TLC
by three subsequent elutions with 80:20 hexane/DCM. The product 8
1
was isolated as an off-white solid (13 mg, 27%): mp (decomp); H
NMR (500 MHz, CDCl3) δ 7.73 (d, J = 8.1 Hz, 2H), 7.64 (d, J = 8.1
Hz, 2H), 7.40−7.26 (m, 19H), 6.98 (t, J = 2.0 Hz, 1H), 6.96−6.93 (m,
9H), 6.85 (dd, J = 8.0, 2.5 Hz, 1H), 5.83 (s, 2H), 5.82 (s, 2H), 3.79 (s,
3H), 2.76 (t, J = 6.7 Hz, 2H), 1.88 (pent, J = 7.3 Hz, 2H), 1.77 (pent, J
= 7.3 Hz, 2H), 1.56−1.50 (m, 4H), 1.01 (t, J = 7.0 Hz, 3H); 13C
(CDCl3, 125 MHz) δ 159.4, 146.8, 145.1, 145.0, 144.9, 143.8, 143.7,
131.8, 131.6, 129.2, 129.0, 128.1, 127.0, 125.2 (two carbons with
coincident chemical shifts), 123.8, 123.8, 123.7, 123.1, 121.8, 115.9,
115.6, 113.7, 112.0, 98.3, 97.8, 96.0, 86.8, 84.8, 76.2, 56.3, 55.2, 52.3,
52.1, 31.6, 29.7, 28.8, 22.8, 20.0, 14.2; IR (DRIFT cm−1) 3063, 6022,
2960, 2921, 2851, 1723, 1598, 1584, 1458, 1258, 1084, 1023; mass
+
(ESI/APCI m/z) calculated for C72H54O+NH4 952.4518, found
952.4542.
ASSOCIATED CONTENT
* Supporting Information
■
S
(c) Khuong, T.-A.; Nunez, J.; Godinez, C.; Garcia-Garibay, M. Acc.
̃
Spectroscopic data (1H, 13C NMR, FTIR) for compounds 3, 3-
d4, 1,1-d8, 7, and 8. Crystallographic information file (cif) for
compound 1. DSC, TGA, PXRD, and solid-state fluorescence
plots. Computational details. This material is available free of
charge via the Internet at http://pubs.acs.org.
Chem. Res. 2006, 39, 413.
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F.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2002, 124, 8181.
(c) Schmieder, K.; Levitus, M.; Dang, H.; Garcia-Garibay, M. A. J.
Phys. Chem. A 2002, 106, 1551.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: mgg@chem.ucla.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSF IGERT: Materials Creation
Training Program (MCTP) − DEG-0654431 and the
California NanoSystems Institute and by grants
DMR1101934 and CHE0-844455.
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1, a = 10.562(2) Å, b = 12.190(2) Å, c = 18.671(4) Å, α = 95.819(2)o,
I
dx.doi.org/10.1021/jo4004053 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
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