Conformational isomers from rotation of diacetylenic bond in an ethynylpyrene-substituted molecular hinge

Article abstract of DOI:10.1021/jo7024724

(Chemical Equation Presented) The first example of isolation and X-ray crystallographic structural characterization of two conformers arising from rotation along a diacetylenic bond is reported. In both the conformers extensive π-π interactions are observ

Full text of DOI:10.1021/jo7024724

SCHEME 1. Interconversion of Open (1a) and Closed (1b)
Conformers by Rotation of the Diacetylenic Bond
Conformational Isomers from Rotation of
Diacetylenic Bond in an
Ethynylpyrene-Substituted Molecular Hinge
Sethuraman Sankararaman,*,†
Gandikota Venkataramana,† and Babu Varghese‡
Department of Chemistry, Sophisticated Analytical Instrument
Facility, Indian Institute of Technology Madras,
Chennai 600036, India
sanka@iitm.ac.in
ReceiVed NoVember 18, 2007
the arrow should not have any barrier, and the open (1a) and
closed forms (1b) are two extreme conformers arising from such
a rotation. In the open form (1a), the two ethynylpyrene units
are far away from each other, whereas in the closed conforma-
tion (1b) they can come in contact within the van der Waals
distance (Scheme 1). Although entropic factor might favor the
open form, the more organized closed form could derive
stabilization from the π-stacking interaction between the two
pyrene units. Although a very weak interaction (<2 kcal mol-1),
π-π interaction could lead to the stabilization of certain
molecular conformations. From a different perspective, 1 is
useful for the understanding of the rotation of the triple bond.
Restricted rotation of acetylenic bonds is important from the
point of view of molecular chirality of exploded biphenyls.
Earlier examples of restricted rotation of acetylenic bond in
diphenylethyne derivatives deal with steric (repulsiVe) interac-
tions among the bulky silyl and aryl substituents placed in the
ortho positions of the phenyl groups. Vollhardt has reported
hindered rotation in a exploded biphenyl bearing bulky dim-
ethylthexylsilyl as the end groups.4 Toyota has demonstrated
hindered rotation of the acetylenic bond in the substituted
derivatives bis(9-anthryl)ethyne and bis(9-triptycyl)ethyne.5
Rotational barrier of 15-18 kcal mol-1 has been reported for
the C(sp3)-C(sp) bond. Moore has reported molecular turnstiles
with freely rotating and conformationally locked spindles
connected by acetylenic bonds.6 The question of whether or not
acetylenic bond rotation be restricted by weak attractiVe forces
such as π-π or hydrogen-bonding interactions remains to be
The first example of isolation and X-ray crystallographic
structural characterization of two conformers arising from
rotation along a diacetylenic bond is reported. In both the
conformers extensive π-π interactions are observed in the
solid state. VT-NMR and fluorescence spectroscopic studies
in solution suggest that the closed and open conformers are
in equilibrium and that the closed conformer is the predomi-
nant species at room temperature.
Construction of molecular assemblies in the solid state
through face-to-face π-stacking interaction of aromatic units is
important in the field of organic molecular electronics and
photonics.1 Among the aromatics pyrene is capable of undergo-
ing π-stacking interaction in the ground and excited states.2
Some of the derivatives of pyrene have been shown to be
potentially useful as molecular electronics and photonics materi-
als.3 Herein, we report the synthesis of a simple butadiynyl
bridged molecular hinge (1) bearing pendent ethynylpyrene
units. In 1, rotation along the diacetylenic axis as indicated by
(3) For recent sensor applications, see: (a) Suzuki, I.; Ui, M.; Yamauchi,
A. J. Am. Chem. Soc. 2006, 128, 4498-4499. (b) Schazmann, B.;
Alhashimy, N.; Diamond, D. J. Am. Chem. Soc. 2006, 128, 8607-8614.
(c) Choi, J. K.; Kim, S. H.; Yoon, J.; Lee, K.-H.; Bartsch, R. A.; Kim, J.
S. J. Org. Chem. 2006, 71, 8011-8015 and refs 11, 13, and 14 cited therein.
(d) Dale, T. J.; Rabek, J., Jr. J. Am. Chem. Soc. 2006, 128, 4500-4501.
For electronics and photonics applications, see: (e) Zhang, H.; Wang, Y.;
Shao, K.; Lin, Y.; Chen, S.; Qiu, W.; Sun, X.; Qi, T.; Ma, Y.; Yu, G.; Su,
Z.; Zhu, D. Chem. Commun. 2006, 755-757. (f) Jia, W.-L.; McCormick,
T.; Lin, Q.-D.; Fukutani, H.; Motala, M.; Wang, R.-Y.; Tao, Y.; Wang, S.
J. Mater. Chem. 2004, 14, 3344-3350. (g) Ogino, K.; Iwashima, S.;
Inokuchi, H.; Harada, Y. Bull. Chem. Soc. Jpn. 1965, 38, 473-477.
(4) (a) Miljanic, O. S.; Holmes, D.; Vollhardt, K. P. C. Org. Lett. 2005,
7, 4001-4004. (b) Miljanic, O. S.; Han, S.; Holmes, D.; Schaller, G. R.;
Vollhardt, K. P. C. Chem. Commun. 2005, 2606-2608.
† Department of Chemistry.
‡ Sophisticated Analytical Instrument Facility.
(1) (a) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004,
104, 4891-4945. (b) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.;
Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491-1546. (c) Watson, M.
D.; Ja¨ckel, F.; Severin, N.; Rabe, J. P.; Mu¨llen, K. J. Am. Chem. Soc. 2004,
126, 1402-1407. (d) Sokolov, A. N.; Friscic, T.; MacGillivray, R. J. Am.
Chem. Soc. 2006, 128, 2806-2807. (e) Dickey, K. C.; Anthony, J. E.; Loo,
Y.-L. AdV. Mater. 2006, 18, 1721-1726. (f) Payne, M. M.; Parkin, S. R.;
Anthony, J. E. J. Am. Chem. Soc. 2005, 127, 8028-8029.
(2) Winnik, F. M. Chem. ReV. 1993, 93, 587.
(5) (a) Toyota, S.; Yamamori, T.; Makino, T. Tetrahedron 2001, 57,
3521-3528. (b) Toyota, S.; Makino, T. Tetrahedron Lett. 2003, 44, 7775-
7778.
(6) Bedard, T. C.; Moore, J. S. J. Am. Chem. Soc. 1995, 117, 10662-
10671.
10.1021/jo7024724 CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/16/2008
2404
J. Org. Chem. 2008, 73, 2404-2407

SCHEME 2. Synthesis of Target 1
FIGURE 1. ORTEP representation of the structure of 1a (left) and
1b (right) in the crystal. 1a has a center of symmetry (achiral), and 1b
has a C2 axis of symmetry (chiral).
addressed. Molecular hinges of the type 1 might be useful as a
probe to investigate this interesting phenomenon. Recently, we
have reported a tetrakis(ethynylpyrene)-substituted molecular
hinge wherein the pyrene units are interlocked in π-π interac-
tions leading to restricted rotation of the central acetylenic
molecular axis.7
Synthesis of 1. Target 1 was synthesized in six steps starting
from 2-bromo-4-tert-butylbenzaldehyde (3) (Scheme 2). Sono-
gashira coupling of 3 with trimethylsilylacetylene gave 4.
Removal of the trimethylsilyl group from 4 followed by coupling
with 1-iodopyrene resulted in the formation of 5. Direct
Sonogashira coupling of 3 with 1-ethynylpyrene gave 5 only
in poor yield along with the oxidative dimer of 1-ethynylpyrene.
Aldehyde 5 was converted to the corresponding dibromovinyl
derivative 6, which on treatment with LDA furnished the desired
terminal acetylene precursor 2. Oxidative dimerization of 2
yielded the target molecule 1 as a yellow crystalline solid
(Scheme 2).
X-ray Crystallographic Studies. Crystallization of 1 from
hexane yielded two types of crystals, colorless triclinic crystals
corresponding to the open conformer (1a) and pale yellow
orthorhombic crystals corresponding to the closed conformer
(1b) (Figure 1). Crystals of 1a were of good quality and suitable
for single-crystal X-ray diffraction studies. Due to intermolecular
π-stacking and C-H‚‚‚π interactions, 1a forms a one-
dimensional chain in the crystal (Figure 2). Only one of the
pyrene rings shows π-stacking interactions with a pyrene ring
FIGURE 2. Packing of 1a in the crystal. The short contacts are shown
as dotted lines. The mean distance between the π-stacking planes of
pyrene, a ) 3.397 Å, and C-H‚‚‚π distance, b ) 2.761 Å. View
perpendicular to the pyrene planes showing lengthwise displaced parallel
π-stacking (bottom).
of the neighbor. The other pyrene ring shows only a C-H‚‚‚π
interaction. Crystallization of the closed conformer 1b was very
difficult. Crystals of 1b were consistently of poor quality and
small size.8 Although the gross structure obtained is clearly
consistent with the closed conformer (Figure 1), the quality of
the X-ray reflections for higher angles were very poor. In 1b,
the two pyrene rings show intramolecular π-stacking interactions
along with intermolecular π-π interaction between the neigh-
boring molecules (see the Supporting Information). In both
structures, the π-stacking pyrene rings are nearly parallel but
displaced lengthwise (Figure 2) with respect to each other.9 The
length of the butadiynyl bridge is optimal for such a displaced
π-stacking interaction between the pyrene units.
Precursor 2 with only one pyrene unit, and hence devoid of
intramolecular π-stacking, was used as a model compound to
compare the electronic absorption and emission properties and
intramolecular π-stacking behavior of 1.10 A comparison of the
UV-vis spectra of 1 and 2 (Figure 3) clearly revealed that the
(7) Nandy, R.; Subramoni, M.; Varghese, B.; Sankararaman, S. J. Org.
Chem. 2007, 72, 938-944.
(8) After repeated attempts, crystals of 1b of reasonable size for the
structure determination could be obtained. The examination of the crystals
with the polarizing microscope showed the crystals to have a poor degree
of crystallinity (poor extinction). The quality of the reflections for higher
angles was very poor, and there were only 1388 reflections with I > 2σ(I)
out of 3678 unique reflections. There are 301 least squares parameters and
199 restraints in the refinement. An R factor of 0.117 for reflections I >
2σ(I) is reasonable for such data.
(9) (a) Desiraju, G. R. Crystal Engineering. The Design of Organic Solids;
Elsevier: Amsterdam, 1989; pp 92-96. (b) For the crystal structure of
pyrene, see: Robinson, J. M.; White, J. G. J. Chem. Soc. 1947, 358-368.
(10) See the Supporting Information for the comparison of the 1H NMR
spectra of 1 and 2.
J. Org. Chem, Vol. 73, No. 6, 2008 2405

FIGURE 5. Temperature-dependent 1H NMR spectra of 1 in CDCl2-
CDCl2.
FIGURE 3. Absorption spectra of 2 (A) and 1 (B) in cyclohexane (1
× 10-5 M) and excitation spectrum of 1 (C) (λem 483 nm) in
cyclohexane.
spectrum was invariant with respect to excitation wavelength
as well as concentration in the range of 10-4-10-8 M, indicating
that the emission corresponds to the intramolecular excimer state
(see the Supporting Information). These results suggest that at
room temperature 1 exists predominantly in the closed form
(1b) in cyclohexane. Alternatively, 1 could predominantly exist
as 1a in the ground state and convert to 1b in the excited state
before emission could take place. In such a case also, excimer
emission would be expected from 1. However this would require
a substantial geometry change in the excited state on going from
1a to 1b. In such a case, the excitation spectrum is not expected
to resemble that of the absorption spectrum. The excitation
spectrum of 1 clearly resembled its absorption spectrum (Figure
3). The emission maximum in the solid state (in powder and in
thin film samples of 1) is bathochromically shifted (15 nm),
FIGURE 4. Fluorescence emission spectra of 2 (A, λex 367 nm) and
1 (B, λex 333 nm) in cyclohexane (1 × 10-5 M) and of 1 in the solid
state as powder (C, λex 333 nm).
and also the fwhm of the emission band is smaller (2333 cm-1
)
compared to that in cyclohexane (3300 cm-1). It is perhaps due
to a more ordered π-stacked structure of 1 in the solid state.
The fluorescence quantum yield of 1 in cyclohexane is 0.32.14
Variable-Temperature Spectral Studies. The absorption and
emission spectra were measured in the temperature range of
278-343 K (see the Supporting Information). The λmax, shape,
and intensity of the absorption bands did not change significantly
with increasing temperature. However, there was a small gradual
blue shift of the emission maximum from 487 nm at 278 K to
480 nm at 343 K. It is concluded that the intramolecular
π-stacking in 1b is not significantly disturbed within the
temperature range studied. The changes in the equilibrium in
Scheme 1 might be small, and hence, it is not manifested in
the absorption and emission spectra.
absorption bands were broader in the case of 1 and were
bathochromically shifted.11
The absorption cutoff was very sharp at 410 nm for 2,
whereas in 1 it was broad and extended up to 440 nm. The
bathochromic shift is expected for 1 due to extensive conjugation
in comparison to 2. The broadness of the bands might also be
due to many possible conformers of 1 in solution of which 1a
and 1b represent two extreme cases. It is also well-known that
π-stacking of pyrene units leads to broadening as well as
bathochromic shift of the absorption bands.7,11 The absorption
spectrum of 1 in the solid state (as a thin film) was nearly
identical to that in cyclohexane. The fluorescence spectra of 1
and 2 (Figure 4) were dramatically different. The emission
spectrum of 2 showed vibrational fine structures [λmax 401, 423,
445 (sh) nm], and it resembled the emission spectra of several
ethynylpyrene derivatives.12 It corresponds to the emission from
the monomer excited state.13
However, strong evidence for the equilibrium in Scheme 1
1
comes from H NMR spectral studies of 1 in the temperature
range of 263-413 K in CDCl2CDCl2. The proton chemical shift
is very sensitive to even small changes in the π-stacking
interactions of the aromatic rings.15 Compared to the model
compound 2, the pyrene proton resonances in 1 are spread out
(Figure 5).10 The individual resonances of the pyrene ring
In the case of 1, the fluorescence emission appeared as a broad
featureless band both in cyclohexane (λmax 483 nm) and in the
solid state (λmax 498 nm), a very characteristic feature of
emission from the excimer state.7,11,13 In cyclohexane, the
1
protons could be assigned on the basis of the H-1H COSY
spectrum of 1. π-Stacking interactions among aromatic units
result in shielding of the protons due to the ring current effect.11
With increasing temperature, the equilibrium is shifted toward
(11) For comparison with [2,2]-2,7-pyrenophane, see: Umemoto, T.;
Satani, S.; Sakata, Y.; Misumi, S. Tetrahedron Lett. 1975, 3159-3162.
(12) (a) Venkataramana, G.; Sankararaman, S. Eur. J. Org. Chem. 2005,
4162-4166. (b) Yang, S.-W.; Elangovan, A.; Hwang, K.-C.; Ho, T.-I. J.
Phys. Chem. B 2005, 109, 16628-16635.
(13) (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd
ed.; Kulwer Academic: New York, 1999; pp 9 and 611. (b) Berlman, I. B.
Handbook of Fluorescence Spectra of Aromatic Molecules; Academic
Press: New York, 1965; p 173.
(14) Hamal, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83-89.
(15) (a) Venkataramana, G.; Sankararaman, S. Org. Lett. 2006, 8, 2739-
2742. (b) Fechtenko¨tter, A.; Saalwa¨chter, K.; Harbison, M. A.; Mu¨llen, K.;
Spiess, H. W. Angew. Chem., Int. Ed. 1999, 38, 3039-3042. (c) Hynninen,
P. H.; Lo¨tjo¨nen, S. Biochem. Biophys. Acta 1993, 1083, 374-380.
2406 J. Org. Chem., Vol. 73, No. 6, 2008

[freshly prepared from 2.95 mmol each of n-BuLi (1.83 mL of 1.6
M solution in hexane) and diisopropylamine (0.4 mL) at -78 °C
in THF] was added and stirring continued for 1 h. Upon completion
of the reaction, it was quenched with saturated NH4Cl at -78 °C.
The reaction mixture was extracted with CH2Cl2 (30 mL). The
organic layer was washed with water (2 × 40 mL) and dried over
Na2SO4. Solvent was evaporated to dryness at rt under reduced
pressure. The crude product was purified by column chromatog-
raphy on silica gel using hexane to yield 1 as a pale yellow solid
(0.20 g, 71%): mp 85-87 °C; IR (KBr) 2950, 2189 cm-1; 1H NMR
(400 MHz, CDCl3) δ 8.99 (d, J ) 9.3 Hz, 1H), 8.34-8.06 (m,
8H), 7.87 (d, J ) 1.9 Hz, 1H), 7.67 (d, J ) 8.3 Hz, 1H), 7.48 (dd,
J ) 8.3, 1.9 Hz, 1H), 3.62 (s, 1H), 1.48 (s, 9H); 13C NMR (100
MHz, CDCl3) δ 152.0, 132.5, 132.0, 131.2, 131.1, 130.9, 129.6,
128.8, 128.1, 127.1, 126.2, 126.1, 125.9, 125.5, 124.4, 124.4, 124.3,
124.1, 121.6, 117.7, 94.0, 92.2, 83.0, 80.6, 34.8, 31.0; MS (EI, 70
eV) m/z 383 (30, 382 (100, M+), 326 (25), 170 (95); HRMS calcd
for C30H22 382.17215, found 382.17165.
FIGURE 6. Effect of temperature on the chemical shift of various
protons on the pyrene ring of 1 in CDCl2CDCl2.
1a because π-stacking is a weak interaction.7,15 As a conse-
quence, all of the pyrene protons are shifted to higher δ values
and that of the phenyl protons remained unaffected with
increasing temperature. From Figure 6, it is clear that the effect
of shifting the equilibrium is more dramatic on H-2 and H-3
compared to the other protons. It is due to the displaced parallel
π-stacking of the pyrene rings which brings H-2 and H-3 in
the shielding zone in 1b. Hence, with increasing temperature
they show a pronounced increase in their δ values. From the
VT-NMR study, the equilibrium constant is estimated to be 4.5
( 0.5 at 300 K (Scheme 1).16
1. Cu(OAc)2‚H2O (0.163 g, 0.82 mmol) was dissolved in a
mixture of acetonitrile (12 mL) and pyridine (3 mL), and 2 (0.125
g, 0.327 mmol) was added. The reaction mixture was stirred at rt
for 4 h. It was neutralized with 5% HCl and extracted with CH2-
Cl2 (25 mL). The organic layer was washed with water (30 mL)
and dried over Na2SO4, and solvent was evaporated at rt under
reduced pressure to dryness. The crude product was purified by
column chromatography on silica gel using hexane to yield the
dimer (1) as a yellow solid (0.106 g, 85%): temperature for onset
1
of decomposition 255-259 °C; IR (KBr) 2957, 2196 cm-1; H
NMR (400 MHz, CDCl3) δ 8.55 (d, J ) 9.2 Hz, 2H), 8.0 (d, J )
8.8 Hz, 2H), 7.80-7.95 (m, 6H), 7.70 (m, 4H), 7.61 (d, J ) 7.8
Hz, 2H), 7.46 (d, J ) 8.8 Hz, 2H), 7.35-7.40 (m, 4H), 7.27 (d, J
) 8.3 Hz, 2H), 1.37 (s, 18H); 13C NMR (100 MHz, CDCl3) δ 152.6,
133.0, 131.4, 130.9, 130.8, 130.3, 128.8, 128.6, 128.5, 127.3, 126.7,
125.6, 125.4, 124.7, 123.5, 123.4, 123.1, 121.5, 116.7, 93.6, 93.4,
82.4, 78.0, 35.0, 31.1; MS (EI, 70 eV) m/z 762 (100, M+); HRMS
calcd for C60H42 762.32865, found 762.32740.
In conclusion, we have demonstrated π-stacking in a novel
pyrene derivative (1) in solution as well as in solid state. The
two conformers 1a and 1b were crystallized and their structures
determined by X-ray crystallography. Variable-temperature
NMR studies established the equilibrium between 1a and 1b in
solution, wherein 1b appears to be the predominant species at
room temperature.
Acknowledgment. We thank DST (New Delhi) for financial
support, SAIF for spectral data, and IIT Madras for a fellowship
to G.V.
Experimental Section
1-(5-tert-Butyl-2-ethynylphenylthynyl)pyrene (2). An oven-
dried Schlenk flask was charged with 6 (0.4 g, 0.74 mmol) and
dry THF (30 mL). It was cooled to -78 °C. LDA (2.95 mmol)
Supporting Information Available: Synthesis, spectroscopic
data for 1-6, VT-NMR, absorption and fluorescence spectra for
1, and crystallographic data (CIF) for 1a and 1b. This material is
available free of charge via the Internet at http://pubs.acs.org.
(16) The δ value of H-10 in 2 was taken as that of pure 1a due to absence
of π-stacking in 2, and the δ value of H-10 at 263 K (Figure 5) was taken
as approximately equal to that of pure 1b to calculate the molar ratios 1a
and 1b. Therefore, the equilibrium constant is only an approximate estimate.
JO7024724
J. Org. Chem, Vol. 73, No. 6, 2008 2407