A non-planar crystal polymorph of 1,2-bis(9-anthracenyl)ethyne

Article abstract of DOI:10.1016/j.tetlet.2015.05.119

1,2-Bis(9-anthracenyl)ethyne was synthesized and a new crystal polymorph was discovered. The previously reported structure was completely planar with a torsional angle between the two anthracenyl rings of 0.0°, whereas the new polymorph had a torsional angle of 66.6°. The new polymorph also stacks differently between molecules and has greater π-electron overlap. In solution, the spectroscopic characteristics of both polymorphs were identical, but in the solid-state, results of red-shifted spectra for the co-planar polymorph indicate a strikingly simple example of the effect on UV-vis and fluorescence spectra of extending π-conjugation wrapped up in one molecule with two polymorphs. It was also determined that the co-planar polymorph is favored with rapid crystallization, whereas the twisted polymorph prefers slow crystallization. This may indicate that the co-planar polymorph is kinetically favored and the twisted polymorph is thermodynamically favored.

Full text of DOI:10.1016/j.tetlet.2015.05.119

Tetrahedron Letters 56 (2015) 4574–4577
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Tetrahedron Letters
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A non-planar crystal polymorph of 1,2-bis(9-anthracenyl)ethyne
a,
⇑
a
a
b
b
Barrett Eichler , Jeremy Erickson , Joseph Keppen , Andrew Sykes , Grigoriy Sereda
a
Augustana College, 2001 S. Summit Ave., Sioux Falls, SD 57197, United States
University of South Dakota, 414 E. Clark St., Vermillion, SD 57069, United States
b
a r t i c l e i n f o
a b s t r a c t
Article history:
1,2-Bis(9-anthracenyl)ethyne was synthesized and a new crystal polymorph was discovered. The
previously reported structure was completely planar with a torsional angle between the two anthracenyl
rings of 0.0°, whereas the new polymorph had a torsional angle of 66.6°. The new polymorph also stacks
differently between molecules and has greater p–electron overlap. In solution, the spectroscopic charac-
teristics of both polymorphs were identical, but in the solid-state, results of red-shifted spectra for the
Received 16 March 2015
Revised 28 May 2015
Accepted 29 May 2015
Available online 11 June 2015
co-planar polymorph indicate a strikingly simple example of the effect on UV–vis and fluorescence
Keywords:
Crystal polymorph
Stille coupling
Pi-conjugation
Aromatic compounds
spectra of extending
p-conjugation wrapped up in one molecule with two polymorphs. It was also
determined that the co-planar polymorph is favored with rapid crystallization, whereas the twisted
polymorph prefers slow crystallization. This may indicate that the co-planar polymorph is kinetically
favored and the twisted polymorph is thermodynamically favored.
Ó 2015 Elsevier Ltd. All rights reserved.
Discussion
bianthryl structures show significant
p
-stacking between mole-
0
cules. Adding the ethynyl linker decreases this H1/H1 and
0
1
,2-Bis(9-anthracenyl)ethyne (1, Fig. 1) and its derivatives have
attracted increased interest in recent years because of a number of
factors. The conjugated -electronic structure of 1 relates to
H8/H8 repulsion, which allows rotation and therefore torsional
angles of less than 80°, which resulted in the co-planar Becker
structure of compound 1.
p
1
organic electronics and it has been investigated in organic
field-effect transistors (OFETs).1 Compound 1 is rigid within its
anthracene moieties and along the ethynyl axis, but is free to
Compound 1 was synthesized in 83% yield by Pd-catalyzed
Stille cross-coupling with bis(tri-n-butylstannyl)ethyne and two
f
6
equivalents of 9-bromoanthracene in a manner similar to that
7
rotate along that same axis, giving it
a
paddlewheel-like
reported previously (Fig. 3). An unusual result occurred when this
structure, which has become important for the field of rotational
isomers and their application in nanoscale machinery.2 Here we
report a new polymorph of compound 1.
compound was first synthesized; illumination with a hand-held
UV lamp on the crystals resulted in some crystals glowing red-or-
ange, while others glowed yellow-green. It was determined
through spectroscopic means that neither of the crystals was a
starting material, and it was determined by X-ray crystallography
that the red-orange crystals were the co-planar structure of 1 pre-
An X-ray crystal structure of 1 has been known since 1985,
3
when it was reported by Becker et al. This structure showed that
two anthracene moieties were co-planar, which can be rationalized
3
by maximizing both the
the –electron interactions between molecules in the solid state.
A similar compound without the ethynyl linker is 9,9 -bianthryl,
(Fig. 2), and due to van der Waals forces between the hydrogens
p-conjugation within the molecule and
viously determined by Becker. A yellow-green fluorescent crystal
p
was also subjected to X-ray crystal diffraction at low temperatures
and was determined to be a polymorph of the co-planar Becker
structure of 1.
The new polymorph of 1 shows that the anthracene moiety and
one carbon from the ethyne compose a centrosymmetric, asym-
metric unit. Whereas the previous structure of this molecule shows
0
2
0
0
on opposing rings (H1/H1 and H8/H8 ), the anthracene planes
were relieved of the repulsion by compromising to a torsional
angle of and its polymorph 80.54 (2)°. The study of another bian-
thryl derivative, 10,10 -dibromo-9,9 -bianthryl, shows similar
structures, including two independent structures with torsional
angles of 89.42(9)° and 82.36(7)° within the same crystal. All
4
0
0
5
a completely planar p-system, the two anthracene groups here are
twisted at an average torsion angle of 66.6° (Fig. 4). The ethynyl
0
bond distance, C15–C15 , equals 1.205(3) Å, compared to
3
1
.193(10) Å in the co-planar polymorph, showing minimal bond
length difference. In the co-planar structure, the
ethynyl group overlaps with the central (C5–C10) aromatic ring
p-system of the
⇑
Corresponding author. Tel.: +1 (605)274 4814; fax: +1 (605)274 4492.
E-mail address: barrett.eichler@augie.edu (B. Eichler).
http://dx.doi.org/10.1016/j.tetlet.2015.05.119
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0

B. Eichler et al. / Tetrahedron Letters 56 (2015) 4574–4577
4575
1
Figure 1. 1,2-Bis(9-anthracenyl)ethyne.
H1'
H8'
H8
H1
2
0
Figure 2. 9,9 -Bianthryl. The anthracenes are almost perpendicular to each other
0
0
due to repulsions between H1/H1 and H8/H8 .
Figure 5. Packing diagram for the twisted polymorph of 1 (30%).
structure, the torsional angle corresponding to the minimum of
energy is different from the angle observed in the crystal.
However, the calculated minimum energy is 1.5 kcal/mol less than
the energy of the coplanar conformation, and just by 0.5 kcal/mol
less than the energy of the perpendicular conformation (Fig. 6).
Therefore, the rotation barrier around the triple bond is very small.
In agreement with a previous calculation by Seideman et al.,10
the energy minimum was calculated to be 36.5°. This result illus-
[
Pd(PPh3)4], PPh3
2
Br + Bu3Sn
SnBu3
p-dioxane, Δ, 18 h
1
Figure 3. The synthesis of 1,2-bis(9-anthracenyl)ethyne.
trates the clash between co-planarity (0°) to maximize
p
-conjuga-
0
tion and the repulsion between the hydrogens at H1/H1 and
0
H8/H8 , and 36.5° appears to be the happy medium between the
two factors. Becker rationalized that the co-planarity was an
attraction that held the hydrogen atoms close to each other
(
2.1 Å observed), even though they were within the van der
Waals radius of hydrogen 2.4 Å, which now seems counterintuitive
as they should repel each other. Interestingly, small changes in the
calculated conformational energy correspond to significant
changes in the HOMO–LUMO gap for 1, which is consistent with
the observed different UV–vis spectra for the polymorphs made
up by different conformers of 1 (Fig. 7). The HOMO–LUMO gaps
calculated in vacuum are blue-shifted with respect to the lowest
energy absorption maxima observed in solution or in the
crystalline state, which is an expected effect of intermolecular
interaction. However, the calculated (53 nm) and observed
Figure 4. Thermal ellipsoid diagrams (30%) of the twisted polymorph of 1, showing
the structure of the molecule and the 66.6 degree twist between the two
anthracenyl rings.
of a neighboring anthracene group. In the new polymorph, only
dimers are formed between neighboring anthracene groups with
typical contacts averaging 3.7 Å (Fig. 5), which is at the outer
limits of the sum of their van der Waals radii (3.70 Å ). The reduced
conjugation of the -system in this polymorph (as observed by the
spectroscopic blue shifts in kmax) is energetically compensated for
by the involvement of the second -component of the carbon–car-
bon triple bond and change of the lattice energy, for instance
because of the greater degree of -stacking between two anthra-
p-
(
43 nm) differences of the HOMO–LUMO values between the
co-planar and twisted polymorphs are close, which indicate
the appropriate modeling of the conformational effects on
the UV–vis absorption wavelength.
p–p
8
p
p
Relaꢀve energy vs. angle
p
1
1
1
1
0
0
0
0
.60
.40
.20
.00
.80
.60
.40
.20
cene units versus only one anthracene and one ethyne in the co-
planar structure (Fig. 5). The anthracene plane is virtually planar
(
1.32° dihedral angle between C1–C6 and C9–C14 rings) in the
structure, maximizing intermolecular interactions. Although
crystal structures of aromatic hydrocarbons can have significant
contributions from both and C–H– interactions, in the struc-
ture of 1 it appears that the long range structure is dominated by
interactions, with only minimal C–H– -interactions. There is
p–p
p
–p
p
p
–p
p
8
0.00
one interaction, between H3 and C8 (3.072 Å ), that approaches
the van der Waals radius sum (3.05 Å), although cannot be consid-
ered ‘close contact’ as it is less than the sum. In most cases, the
hydrogen atoms point toward voids in the structure. We per-
0
20
40
60
80
100
torsional angle (°)
Figure 6. Relative energy of a molecule of gas-phase 1 versus the torsional angle
between the anthracene rings.
9
formed calculations that show that for the single gas-phase

4
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B. Eichler et al. / Tetrahedron Letters 56 (2015) 4574–4577
polymorphs, as well as simple visual observation under UV light by
HOMO-LUMO gap
4
4
4
4
4
3
3
80.0
60.0
40.0
20.0
00.0
80.0
60.0
raising the temperature to the melting point of the twisted poly-
morph (Fig. S7). Results from both methods indicate that there is
no transformation of one polymorph to another up to the melting
points. The DSC results show no major exo- or endothermic peaks
up to 300 °C and visual inspection of the crystals indicated that
they neither decomposed/cracked, nor did they change their
fluorescence properties.
As stated earlier, both polymorphs grew out of the same reac-
tion mixture. Attempts to preferentially grow one polymorph over
the other by changing the recrystallization solvent offered little
repeatable success. For example, only the co-planar crystals (dark
red needles) were formed from a starting mixture of the two poly-
morphs when grown from hot toluene, but another attempt under
the same conditions resulted in a mixture of both polymorphs.
Attempts were made to determine if either coordinating solvents,
like THF or 1,4-dioxane crystallized one polymorph over the other,
but mixtures of both polymorphs were frequently obtained from
heating the solvent to the boiling point and cooling to room
temperature. The same was tried with aromatic solvents (toluene,
benzene) and halogenated solvents (dichloromethane, chloro-
form), but they produced the same results.
Finally, two time- and temperature-related experiments were
attempted. In the first, a mixture of polymorphs was dissolved in
boiling toluene. Upon cooling in less than 30 min, only the dark
red-orange co-planar polymorph was obtained. After 24 h at room
temperature, the supernatant was removed from the sample, fil-
tered and allowed to crystallize slowly. Almost exclusively, the
orange-yellow twisted polymorph was obtained. The other exper-
iment involved seed crystals and dichloromethane. A mixture of
crystals was dissolved in room temperature dichloromethane until
the solution was saturated. The supernatant was filtered, and
placed in three different, loosely capped vials—one with three crys-
tals of the red-orange co-planar polymorph, one with three crystals
of the orange-yellow twisted polymorph, and one control with
nothing else in the vial. The one with the co-planar polymorph
gave a small amount of the co-planar polymorph at the beginning
of the crystallization, but by the time all of the dichloromethane
had evaporated, the majority of the crystals were the twisted poly-
morph. The other two samples gave exclusively the twisted poly-
morph. To summarize, rapid crystallization from hot solvents
cooling quickly gives only the coplanar polymorph and slow evap-
oration at room temperature gives only the twisted polymorph,
exclusive of the identity of the solvent.
0
20
40
60
80
100
torsional angle (°)
Figure 7. The HOMO–LUMO energy gap based on the torsional angle between the
anthracene rings.
The two polymorphs are spectroscopically identical in solution
1
13
by UV–vis, fluorescence, and H and C NMR. However, solid-state
UV–vis and fluorescence spectra of the crystals (finely ground in a
mortar and pestle) differed and the data demonstrate a very sim-
ple, yet striking, example of the effects of extending
within a molecule (Table 1). The co-planar polymorph has the
more extended -conjugation and displays the expected red-
shifted peaks in its UV–vis absorption and fluorescence spectra
than those for the twisted polymorph.
The solid state fluorescence spectra show that in the twisted
polymorph, the p–p stacking interactions in the dimers do not lead
to the significant quenching of fluorescence observed for the co-
planar polymorph (Fig. 8).
The observed melting point for the twisted polymorph is just
°C higher than the melting point for the co-planar polymorph
Table 1), which is consistent with the observed slightly higher
thermodynamic stability of the twisted polymorph. However, the
similarity of these melting points could result from the conversion
of one polymorph to another upon heating and subsequent melting
at the same temperature. To investigate this, we looked at the dif-
ferential scanning calorimetry (DSC) plots (Figs. S5 and S6) of both
p-conjugation
p
4
(
Table 1
Spectroscopic and melting point data for 1
Material
UV–vis (kmax) (nm) Fluorescence (kmax
nm)
)
Mp (°C)
(
We believe that rapid crystallization produces the kinetically
favored polymorph—the coplanar one. When given more time to
arrange themselves, the molecules of compound 1 prefer the
twisted polymorph and it could be concluded that the twisted form
is favored thermodynamically.
Solution
435
500
—
(
acetone)
Co-planar solid
Twisted solid
580
537
650
570
307
311
In summary, 1,2-bis(9-anthracenyl)ethyne was synthesized by
Stille coupling and the reaction mixture gave two different crystal
polymorphs. One was the structure reported by Becker et al., and
had a torsional angle of 0.0° between the anthracene ring planes,
but the other was a previously uncharacterized polymorph that
had a torsional angle of 66.6°. These were spectroscopically
identical in solution, but in the solid-state they clearly illustrated
Solid-state fluorescence
6
5
4
3
2
1
0
.00E+05
.00E+05
.00E+05
.00E+05
.00E+05
.00E+05
.00E+00
twisted co-planar
spectroscopic characteristics related to extended
The co-planar polymorph (longer -conjugation) was red-shifted
in the UV–vis and fluorescence spectra compared to that of the
twisted polymorph (shorted -conjugation). Many crystallization
p-conjugation.
p
p
attempts gave mixtures of both polymorphs, but it was determined
that fast crystallization favored the co-planar polymorph, but slow
crystallization favored the twisted polymorph, illustrating kinetic
versus thermodynamic factors are involved. The much stronger
influence of the molecular shape of bisanthrylethynes on their
optical properties than on the conformational and crystal lattice
4
80.0
530.0
580.0
630.0
nm
680.0
730.0
780.0
Figure 8. The solid-state fluorescence spectra of the twisted and co-planar
polymorphs.

B. Eichler et al. / Tetrahedron Letters 56 (2015) 4574–4577
4577
energies makes this class of compounds potential fluorescent sen-
sors for pressure or mechanical strain in the industrial materials.
3. Becker, H.-D.; Skelton, B. W.; White, A. H. Aust. J. Chem. 1985, 38, 1567–1570.
4
5
.
.
(a) Kyzioł, J. B.; Zaleski, J. Acta Crystallogr. 2007, E63, o1235–o1237; (b) Langer,
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Acknowledgments
6. Into a Schlenk flask was placed 9-bromoanthracene (10.00 g, 38.89 mmol,
2
.1 equiv), Pd(PPh
)
3 4
(0.67 g, 0.58 mmol, 3.1 mol %), and PPh
3
(0.10 g,
0
.38 mmol, 2.1 mol %). These contents were dissolved in 1,4-dioxane
The authors would like to thank NSF-EPSCoR for funding from
the ‘Photoactive Nanosystems’ (NSF Award # 0903804) Grant.
We would like to thank Brandon Gustafson for assistance with
photography and NMR, Dr. Zhenqiang (Rick) Wang for his assis-
tance with differential scanning calorimetry and Dr. Haoran Sun
for his helpful discussion about the X-ray crystal structure.
(
100 mL), giving a light brown solution. To this solution was added bis(tri-n-
butylstannyl)ethyne (9.82 mL, 11.3 g, 18.6 mmol) dropwise via syringe. The
reaction mixture was heated using a boiling water bath and was left to react
overnight. After reacting, dioxane was removed by vacuum and remaining solid
was partially dissolved in boiling toluene. After crude crystallization, solid was
collected by vacuum filtration and washed with cold hexanes, giving an orange
solid (1, 5.84 g, 15.4 mmol, 83%). Mp, UV–vis and fluorescence results reported
within the text. 1H NMR (400 MHz, CDCl
): d 8.91 (d, J = 9 Hz, 4H), 8.52 (s, 2H),
3
13
8
3
.08 (d, J = 9 Hz, 4H), 7.61 (m, 8H). C NMR (100 MHz, CDCl ): d 132.89,
Supplementary data
131.44, 128.95, 128.11, 127.03 (2 overlapped peaks), 125.88, 117.77, 97.49.
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8
9
.
.
.
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Quantum chemistry calculations were performed with the Gaussian 09
computational package ( Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria,
G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
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F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Vreven, T.; Montgomery, J. A.; Peralta, J. E., Jr.; Ogliaro, F.; Bearpark, M.;
Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.;
Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;
Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.;
Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian 09, Revision B.01; Gaussian: Wallingford CT, 2010. ) First, the geometry
of each conformer was partially optimized with the dihedral angle frozen at the
array of values at the B3LYP/3-21G level (split basis set), and the HOMO–LUMO
gaps were calculated by a single point run for each optimized geometry at the
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.05.
1
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