Molecular compasses and gyroscopes. II. Synthesis and characterization of molecular rotors with axially substituted bis[2-(9-triptycyl)ethynyl]arenes

Article abstract of DOI:10.1021/ja012550i

We have developed a simple convergent procedure for the synthesis of molecular rotors consisting of a central aromatic group coupled with two axially positioned ethynyltriptycenes. Molecular rotors with 1,4-phenylene (1), 1,4′-1,1′-biphenylene (2), 9, 10-anthracenylene (3), and 2,7-pyrenylene (4) groups were prepared by Pd(0)-catalyzed coupling of ethynyl triptycenes with the corresponding dibromoarenes. Although compounds 1-4 were not expected to have free rotation in the solid state, the rotational potentials of 1 and 3 were analyzed by semiempirical methods and the crystal packing of 1 was analyzed to design the structures most likely to yield a functional rotor in the solid state. Semiempirical PM3 calculations predict compounds 1, 2, and 4 to have frictionless internal rotation even at temperatures as low as 25 K, while compound 3 is expected to have a barrier of ca. 4 kcal/mol.

Full text of DOI:10.1021/ja012550i

Published on Web 04/05/2002
Molecular Compasses and Gyroscopes. II. Synthesis and
Characterization of Molecular Rotors with Axially Substituted
Bis[2-(9-triptycyl)ethynyl]arenes
Carlos E. Godinez,† Gerardo Zepeda,‡ and Miguel A. Garcia-Garibay*,†
Contribution from the Department of Chemistry and Biochemistry, The UniVersity of California,
Los Angeles, California 90095-1569, and Departamento de Quimica Organica,
Escuela Nacional de Ciencias Biologicas, IPN, Mexico, D.F. 11340
Received November 15, 2001. Revised Manuscript Received February 13, 2002
Abstract: We have developed a simple convergent procedure for the synthesis of molecular rotors consisting
of a central aromatic group coupled with two axially positioned ethynyltriptycenes. Molecular rotors with
1,4-phenylene (1), 1,4′-1,1′-biphenylene (2), 9,10-anthracenylene (3), and 2,7-pyrenylene (4) groups were
prepared by Pd(0)-catalyzed coupling of ethynyl triptycenes with the corresponding dibromoarenes. Although
compounds 1-4 were not expected to have free rotation in the solid state, the rotational potentials of 1
and 3 were analyzed by semiempirical methods and the crystal packing of 1 was analyzed to design the
structures most likely to yield a functional rotor in the solid state. Semiempirical PM3 calculations predict
compounds 1, 2, and 4 to have frictionless internal rotation even at temperatures as low as 25 K, while
compound 3 is expected to have a barrier of ca. 4 kcal/mol.
1. Introduction1
Recent advances in communication technologies have stimu-
lated much interest in the field of photonics.2,3 The need for
materials with tunable transmittance, refraction, polarization,
and color poses new challenges and opportunities for organic
materials chemistry.2,4 While most research in this field falls in
the areas of polymer chemistry5 and liquid crystals,6,7 we have
recently begun a strong effort toward the realization of a new
concept for the fabrication of photonics materials based on
dipolar units that can reorient rapidly under the influence of
electric, magnetic, and optical stimuli. The desired compounds
will take advantage of novel molecular architectures expected
Figure 1. Space-filling model (left) and line formula (center) of a 1,4-
bis[(9-triptycyl)ethynyl]-2,3-difluorobenzene illustrating the homeomor-
phism of the molecule with a macroscopic gyroscope shown on the right.
to have a function analogous to those of macroscopic compasses
and gyroscopes (Figure 1).
The structural elements required for a functional molecular
compass, or molecular gyroscope, can be illustrated in the triply
bridged 1,4-bis[(9-triptycyl)ethynyl]-2,3-difluorobenzene shown
in Figure 1. In lieu of the magnetic dipole of a compass needle,
or the moment of mass of a gyroscope rotor, the desired
molecular rotors will have polar or polarizable groups that can
respond to external fields.8 This is illustrated with a 2,3-difluoro-
1,4-phenylene rotor, shown in red, with fluorine atoms in green.
We expect that rotation of aromatic rotors will be facilitated
by the cylindrical symmetry of the two axial alkyne bonds,
which will act as an axle. Since fast dynamics in the solid state
will require shape- and volume-conserving motions, the reori-
enting phenylene should be sterically shielded from contact with
other molecules in the environment. Shielding will be provided
* Address correspondence to this author. E-mail: mgg@chem.ucla.edu.
† The University of California.
‡ Escuela Nacional de Ciencias Biologicas.
(1) Part I: Dominguez, Z.; Strouse, H.; Garcia-Garibay, M. A. J. Am. Chem.
Soc. 2002, 124, 2393-2399.
(2) Salech, B. E. A.; Teich, M. C. Fundamentals of Photonics; Wiley-
Interscience: New York, 1991.
(3) Photonics has been defined as “The technology of generating and harnessing
light and other forms of radiant energy whose quantum unit is the photon”.
See: Laurin, T. C. The Photonics Ditionary, 45th ed.; Lauring Publishing
Co.: Pittsfield, MA, 1999; p D-107.
(4) Burns, G. Solid State Physics; Academic Press: New York, 1985.
(5) (a) Nalwah, H. S. Ferroelectric Polymers; Marcel Dekker: New York,
1995. (b) Hide, F.; Diaz-Garcia, M. A.; Schwartz, B. J.; Heeger, A. J. Acc.
Chem. Res. 1997, 30, 430-436.
(6) For some examples of recent research on crystalline photonics materials
please see: (a) Liu, C.-Y.; Bard, A. J. Acc. Chem. Res. 1999, 32, 235-
245. (b) Iverson, I. K.; Tam-Chang, S.-W. J. Am. Chem. Soc. 1999, 121,
5801-5802. (c) Wang, S. J.; Oldham, W. J.; Hudack, R. A.; Bazan, G. C.
J. Am. Chem. Soc. 2000, 122, 5695-5709.
(7) (a) Collings, P. J.; Hird, M. Introduction to Liquid Crystals; Taylor and
Francis, Ltd.: London, 1997. (b) Blinov, L. M. Electrooptic and Magneto-
Optical Properties of Liquid Crystals; Wiley: New York, 1983. (c)
Guymon, G. A.; Dougan, L. A.; Martens, P. J.; Clarck, N. A.; Walba, D.
M.; Bowman, C. N. Chem. Mater. 1998, 10, 2378-2388. (d) Walba, D.
M.; Dyer, D. J.; Sierra, T.; Cobben, P. L.; Shao, R.; Clark, N. A. J. Am.
Chem. Soc. 1996, 118, 1211-1212.
(8) The collective behavior of a lattice of reorienting dipoles depends on
symmetry, dimensions, temperature, and the magnitudes of the dipole and
the external field. See: (a) Adams, D. J.; McDonald, I. R. Mol. Phys. 1976,
32, 931. (b) Papazyan, A.; Maroncelli, M. J. Chem. Phys. 1991, 95, 9219-
9241.
9
10.1021/ja012550i CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 4701-4707
4701

A R T I C L E S
Godinez et al.
1,4-Bis[2-(9-triptycyl)ethynyl]benzene (1): Alcohol 6 (0.20 g, 0.59
mmol) and 1,4-diiodobenzene (0.99 g, 0.30 mmol) were dissolved in
46 mL of deaerated toluene and 1.6 mL of Et3N. Copper iodide (0.11
g, 0.58 mmol), KOH (0.81 g, 14.4 mmol), (Bu)4NI (1.1 g, 2.98 mmol),
PPh3 (0.18 g, 0.69 mmol), and (PPh3)2PdCl2 (0.05 g, 0.071 mmol) were
added under Ar at room temperature. The reaction mixture was
magnetically stirred and heated in an oil bath at 100 °C for 24 h. The
crude mixture was washed with brine and the organic layer was dried
over MgSO4. Solvent was removed under vacuum and the product
purified by column chromatography (hexanes:CH2Cl2 3:1 by volume)
to afford pure 1 in 20% yield. Coupling reactions carried out with alkyne
7 proceeded in 84% yield. The main byproduct under those conditions
was identified as 1,4-bis(9-triptycyl)butadiyne.11 Anal.: mp >400 °C
1
Figure 2. Space-filling models of phenylene-, biphenylene-, anthra-
cenylene-, and pyrenylene-based molecular rotors 1-4.
dec; H NMR (500 MHz, C2D2Cl4, TMS) δ 5.51 (s, 2H, bridgehead
H), 7.14 (m, 12H, triptycyl-Ar), 7.47 (dd, J ) 6.0, 1.2 Hz, 6H, triptycyl-
Ar), 7.86 (dd, J ) 7.0, 0.9 Hz, 6H, triptycyl-Ar), 7.92 (s, 4H, Spacer-
phenyl); 13C NMR (125 MHz, C2D2Cl4, TMS) δ 53.3, 53.8, 85.9, 92.7,
123.0, 123.5, 124.2, 125.6, 126.3, 132.6, 144.5, 144.7; IR (KBr) 3070.5,
3015.9, 2956.5, 1455.6, 1332.0, 745.0 cm-1; MS (70 eV) m/z (%) 630.2
(34, M+), 525.0 (5), 252.1 (6), 252.1, (20); HRMS (EI) calcd for C50H30
630.2348, found 630.2337.
by a triply bridged framework, as shown in the bis(triptycyl)
structure in Figure 1, or by frameworks with open topologies
with bulky substituents.9 To realize and optimize the desired
molecular assemblies, one will need convenient synthetic
procedures, a detailed analysis of their packing preferences, and
the implementation of practical strategies to determine their
solid-state rotational dynamics. In this paper, we report the
synthesis and characterization of four model compounds with
benzene, 1,1′-biphenyl, anthracene, and pyrene rotors (1-4,
Figure 2). Although the aromatic groups of compounds 1-4
are not expected to have rapid rotation in the solid state, we
have chosen this set to test a simple convergent procedure, to
analyze their gas-phase rotational potential, and to explore their
crystallization behavior and thermal properties.
1,4′-Bis[2-(9-triptycyl)ethynyl]-1,1′-biphenyl (2): Rotor 2 was
obtained as shown above for rotor 1 in 17% isolated yield. Anal.: mp
1
>400 °C dec; H NMR (500 MHz, CDCl3/CS2, TMS) δ 5.39 (s, 2H,
bridgehead H), 7.03 (m, 12H, triptycyl-Ar), 7.37 (dd, J ) 7.1, 0.97
Hz, 6H, triptycyl-Ar), 7.76 (d, J ) 8.3 Hz, 4H, biphenyl), 7.78 (dd, J
) 8.3, 1.2 Hz, 6H, triptycyl-Ar), 7.91 (m, 4H, biphenyl); 13C NMR
(125 MHz, C2D2Cl4/CS2, TMS) δ 53.4, 53.5, 85.1, 92.4, 122.4, 122.5,
123.4, 125.1, 125.6, 127.1, 132.7, 140.4, 144.1, 144.2; IR (KBr) 3069.1,
3001.0, 2956.9, 1494.7, 1455.4, 822.6, 744.9, 640.8 cm-1; MS (70 eV)
m/z (%) 706.3 (5, M+), 317.0 (20), 217.0, (28), 162 (32), 124 (100);
HRMS (EI) calcd for C56H34 706.2661, found 706.2657.
2. Experimental Section
9,10-Bis[2-(9-triptycyl)ethynyl]anthracene (3): Rotor 3 was pre-
pared as shown above for rotor 1 in 12% yield. Anal.: mp >400 °C
dec; 1H NMR (500 MHz, CDCl3, TMS) δ 5.53 (s, 2H, bridgehead H),
7.11 (m, 12H, triptycyl-Ar), 7.74 (dd, J ) 6.8, 3.1 Hz, 6H, triptycyl-
Ar), 8.03 (d, J ) 7.3 Hz, 6H, triptycyl-Ar), 9.00 (dd, J ) 6.6, 3.2, Hz,
6H, triptycyl-Ar); 13C NMR (125 MHz, C2D2Cl4, TMS) δ 56.7, 57.6,
92.4, 100.0, 121.6, 126.6, 127.0, 127.2, 128.5, 129.7, 147.6, 147.6; IR
(KBr) 3067.8, 2923.7, 2852.7, 1455.6, 750.2 cm-1; MS (70 eV) m/z
(%) 730.3 (5, M+), 629.0 (25), 510.2 (45), 252.1, (100); HRMS (EI)
calcd for C58H34 730.2661, found 730.2670.
General. IR spectra were acquired on a Perkin-Elmer Paragon 1000
FT-IR instrument. The 1H[13C] NMR spectra were obtained on a Bruker
1
NMR spectrometer operating at 500 MHz for H and at 125 MHz for
13C in CDCl3 or C2D2Cl4 with TMS as internal standard. Gas
chromatography (GC) analyses were recorded on a Hewlett-Packard
5890 Series II capillary instrument equipped with a flame ionization
detector. Melting points were determined with a Fisher-Johns melting
point apparatus.
4-(9-Anthryl)-2-methyl-3-butyn-2-ol (5): Compound 5 was pre-
pared by Pd(PPh3)2Cl2-catalyzed coupling of 9-bromoanthracene and
2-methyl-3-butyne-2-ol in refluxing piperidine as described in a recent
communication.10
2,7-Bis[2-(9-triptycyl)ethynyl]pyrene (4): Rotor 4 was prepared as
1
shown above for rotor 1 in 36% yield. Anal.: mp >400 °C (dec); H
4-(9-Triptycyl)-2-methyl-3-butyn-2-ol (6): 4-(9-Anthryl)-2-methyl-
3-butyn-2-ol 5 (0.14 g, 0.397 mmol) was dissolved in 25 mL of benzene
in a three-neck round-bottom flask and brought up to reflux. Anthranilic
acid (0.32 g, 2.3 mmol) dissolved in 4 mL of 1,2-dichloroethane and
isoamyl nitrite (0.3 mL, 2.2 mmol) dissolved in 4 mL of 1,2-
dichloroethane were added dropwise and simultaneously. At the end
of the addition and after cooling, the reaction mixture was washed with
saturated NaHCO3 (5 × 20 mL) and brine (2 × 20 mL) and dried over
MgSO4. Solvent was removed under vacuum and the product purified
by column chromatography (hexanes:ethyl acetate 9:1 by volume) to
afford 0.09 g (67%) of 6 as a white solid. Anal.: mp 248.7-250.3 °C
(uncorrected); 1H NMR (500 MHz, CDCl3, TMS) δ 1.89 (s, 6H, -CH3),
2.26 (s, 1H, OH), 5.40 (s, 1H, bridgehead H), 7.03 (m, 6H, Ar), 7.37
(m, 3H, Ar), 7.66 (dd, J ) 7.2, 1.5 Hz, 3H, Ar); 13C NMR (125 MHz,
CDCl3, TMS) δ 32.1, 52.6, 53.1, 65.8, 97.8, 122.4, 123.3, 125.0, 125.6,
144.1, 144.3; IR (KBr) 3771.2, 3443.0, 3068.4, 2978.1, 1456.6, 1332.0,
1165.6, 748.2 cm-1; MS (70 eV) m/z (%) 336.1508 (100, M+), 303.1
(57), 276.1 (31), and 321.1 (24).
NMR (500 MHz, CDCl3/CS2, TMS) δ 5.47 (s, 2H, bridgehead H), 7.08
(td, J ) 7.2, 1.0 Hz, 6H, triptycyl-Ar), 7.13 (td, J ) 7.3, 0.99 Hz, 6H,
triptycyl-Ar), 7.43 (d, J ) 7.1 Hz, 6H, pyrene), 7.93 (d, J ) 7.5 Hz,
6H, triptycyl-Ar), 8.19 (s, 4H, pyrene), 8.60 (s, 4H, pyrene); 13C NMR
(125 MHz, C2D2Cl4, TMS) δ 53.4, 53.9, 84.6, 93.5, 121.1, 123.2, 124.2,
124.4, 125.7, 126.3, 128.2, 129.0, 131.7, 144.6, 144.7; IR (KBr) 3039.3,
1945.3, 1915.8, 1602.9, 1455.1, 882.7, 749.6, 639.9 cm-1; MS (70 eV)
m/z (%) 754.3 (55, M+), 579.0 (70), 525.0, (35), 151 (100); HRMS
(EI) calcd for C60H34 754.2661, found 754.2659.
9-(2-Phenylethynyl)triptycene (8): Compound 8 was prepared as
shown above for rotor 1 in 70% yield. Anal.: mp 297.5-300.0 °C
1
(uncorrected); H NMR (500 MHz, CDCl3/CS2, TMS) δ 5.45 (s, 1H,
bridgehead H), 7.06 (m, 6H, triptycyl-Ar), 7.40 (m, 3H, triptycyl-Ar),
7.48 (m, 3H, -Ph), 7.81 (m, 3H, triptycyl-Ar), 7.83 (m, 2H, -Ph);
13C NMR (125 MHz, C2D2Cl4, TMS) δ 53.3, 53.5, 83.8, 92.7, 122.5,
123.0, 123.4, 125.2, 125.7, 128.5, 128.7, 132.1, 144.4, 144.4; IR (KBr)
3063.5, 1455.7, 757.8, 640.9 cm-1; MS (70 eV) m/z (%) 354.1 (100,
M+), 276.1 (25), 252.1, (35), 175.1 (13).
(9) Variations in the framework structure will help introduce steric barriers
when hindered rotation is desirable.
(10) Dang, H.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2001, 123, 355-356.
(11) Akiyama, S.; Ogura, F.; Masazumi, N. Bull. Chem. Soc. Jpn. 1971, 44.
9
4702 J. AM. CHEM. SOC. VOL. 124, NO. 17, 2002

Molecular Compasses and Gyroscopes
A R T I C L E S
Scheme 1 a
Scheme 3 a
a Key: (i) KOH, Bu4Nl, Ph-H, 2% Pd(PPh3)Cl2, Et3N, Cul, PPh3.
Scheme 4 a
a Key: (i) Pd(PPh3)2Cl2, piperidine, reflux; (ii) isoamyl-ONO, DCE,
reflux; (iii) KOH, Bu4Nl, Ph-H, reflux; (iv) KOH, Bu4NI, Ph-H, Pd(PPh3)2Cl2,
Et3N, Cul, PPh3, reflux.
Scheme 2
a Key: (i) Pd/C(mol 10%), H2 45 psi, 6 days, 97% yield; (ii) Br2/H2O,
9% yield; (iii) Br2/CS2, quantitative.
compound 8 were prepared with commercial 4-(4-bromophen-
yl)bromobenzene, 9,10-dibromoanthracene, and bromobenzene
(Scheme 3), respectively. Samples of 2,7-dibromopyrene for the
synthesis of rotor 4 were prepared from pyrene by partial
reduction, bromination, and rearomatization, as reported in the
literature (Scheme 4).17
3. Results and Discussion
3.1. Synthesis and Spectral Characterization. Molecular
rotors 1-4 were prepared from 9-bromoanthracene by using
the three-step procedure shown for the synthesis of 1,4-
phenylene rotor 1 in Scheme 1. Initially, the preparation of the
acetone-protected 9-anthrylacetylene 5 by Sonogashira coupling
with copper cocatalyst12 led to the unexpected formation of an
isomeric aceanthrylene in 20-30% yield (Scheme 2).13 How-
ever, reactions carried out with 10% (PPh3)2PdCl2 in refluxing
piperidine (Scheme 1a) yielded alcohol 5 in 70-75% isolated
yields.
The preparation of 4-(9-triptycyl)-2-methyl-3-butyn-2-ol (6)
was accomplished in 67% isolated yield via Diels-Alder
addition of benzyne to anthracene 5 (Scheme 1b). Benzyne was
formed in situ by diazotization of anthranilic acid with isoamyl
nitrite.14 Alcohol 6 is a stable, white solid with mp 249-250
°C that was characterized by conventional spectroscopic pro-
cedures. The structure of 6 was also confirmed by chemical
correlation with the terminal alkyne 7 previously prepared by
Akiyama7 and by Stevens.15 The deprotection of 6 was carried
out in 87% yield with hydroxide ion in the presence of a phase-
transfer catalyst in refluxing benzene (Scheme 1c).
The complete disappearance of the starting materials under
tandem deprotection and double Pd(0)-catalyzed coupling
conditions was observed after 24-36 h. Pure samples of rotors
1-4 were obtained after repeated column chromatography in
silica gel with a mixture of CH2Cl2:hexanes (3:2) as the eluant.
The isolated yields of compounds 1, 2, 3, and 4 under these
conditions were 20%, 17%, 12%, and 36%, respectively. Besides
reflecting the outcome of three consecutive reactions, these
yields also reflect the solubility and ease of handling of each
of the final products. We established that preparation of rotor 1
by stepwise deprotection and coupling reactions proceeds in
higher overall yield (ca. 73%). Deprotection of 6 to give the
terminal alkyne occurred in 87% yield and the double coupling
reaction carried out with 2.5 equiv of the terminal alkyne 7
produced 1 in 84% yield. A drawback of the stepwise procedure
was the formation of the nearly inseparable 1,4-bis(9-triptycyl)-
butadiyne, which is formed by oxidative coupling of two
terminal alkynes.11 With only one coupling reaction and having
a significantly higher solubility, compound 8 was obtained in
70% yield from the protected alkyne 6 and bromobenzene.
Rotors 1-4 and compound 8 were prepared from alkynyl
triptycene 6 by in situ deprotection using conditions similar to
those reported by Pugh and Percec.16 The reaction was carried
out in refluxing benzene in the presence of KOH and Bu4NI.
The double coupling reaction proceeded with 2 equiv of 6 with
respect to the dihaloarene using 10% Pd(PPh3)2Cl2 in the
presence of CuI, Et3N, and PPh3. Rotor 1 was prepared with
1,4-diiodobenzene (Scheme 1c). Molecular rotors 2 and 3 and
1
Rotors 1-4 were characterized by H and 13C NMR, EI-
HRMS, and FT-IR spectroscopies. Although highly insoluble
solids were obtained in all cases, only crystals of 1 grown from
m-xylene were suitable for X-ray diffraction analysis (see
below). All four solid samples decomposed without melting
above 400 °C. High-resolution mass spectra obtained by electron
impact ionization gave relatively abundant parent ions. As
expected from the low polarity across the triple bond, infrared
spectra exhibited unobservable (1-3) or extremely weak alkyne
(12) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 4467-
4470.
(13) Reaction conditions could be optimized to obtain aceanthrylene in better
than 80% yield, see ref 10 and: Dang, H.; Levitus, M.; Garcia-Garibay,
M. A. J. Am. Chem. Soc. 2002, 124, In press.
(14) Friedman, L.; Logullo, F. M. J. Org. Chem. 1969, 34, 3089-3092.
(15) Stevens, A. M.; Richards, C. J. Tetrahedron lett. 1997, 38, 7805-7808.
(16) Pugh, C.; Percec, V. Polym. Bull. 1990, 23, 177-184.
(17) (a) Coonor, D. M.; Allen, S. D.; Collar, D. M.; Litta, D. M.; Shiraldi, D.
A. J. Org. Chem. 1999, 64, 6888-6890. (b) Lee, H.; Harvey, R. G. J.
Org. Chem. 1986, 51, 2847-2848.
9
J. AM. CHEM. SOC. VOL. 124, NO. 17, 2002 4703

A R T I C L E S
Godinez et al.
Table 1. 1H NMR Chemical Shifts of Rotors 1-4 and Several Model Compoundsa
signal
1
2
3
4
8
6
H bridge
H2, H3
Hexo
5.51
7.14
7.47
7.86
7.92
5.39
7.03
7.37
7.76
5.53
7.11
7.46
8.03
5.47
7.10
7.43
7.93
5.45
7.058, 7.060
7.40
7.81
7.48, 7.83
5.40
7.03
7.37
7.66
Hendo
b
Haryl
7.78, 7.91
7.70, 9.00
8.19, 8.60
diethynylarylene
(9a) 7.34c
(10a) n.a.d
(11a) 7.67, 8.59e
(12a) 8.00, 8.22f
a Chemical shifts in C2D2Cl4 except 2, which was measured in a mixture of C2D2Cl4:CS2 ) 3:1. b Hydrogens in aromatic rotor. c MacBride, J. A. H.;
Wade, K. Synth. Commun. 1996, 26, 2309-2316. d Wu, X.; Dirlikov, S. K. Polym. Mater. Sci. Eng. 1989, 60, 762-766; NMR data not reported. e Schmieder,
K.; Levitus, M.; Garcia-Garibay, M. A. J. Phys. Chem. 2002, 106, 1551-1556. f Inouye, M.; Itoh, M.-A. S.; Nakazumi, H. J. Org. Chem. 1999, 64, 9393-
9398.
Scheme 5
another are within 0.1 ppm. Analysis of the data shown in Table
1 shows that magnetic anisotropic effects from the alkyne and
ring currents of the central aromatic rotor have a deshielding
effect on the Hendo signal.18 Assuming linearly additive effects,
and based on the difference between the endo- and exo-
hydrogens in ethynyl triptycene 6 (Scheme 5), one may conclude
that the magnetic anisotropy of the triple bond alone causes a
shift of -0.29 ppm. The ring current effect of the aromatic
groups facing the triptycyl endo-hydrogens can be estimated
by subtracting the chemical shift of the endo-hydrogen of 6 (δ
7.66 ppm) from that of the endo-hydrogen in each arylethynyl-
triptycene. For example, the field effect of a phenyl group in 1
can be estimated as δHendo(1) - δHendo(6) ) 7.86 - 7.66 )
0.2 ppm. Similar arithmetics for 3 and 4 give values of 0.37
and 0.27 ppm, respectively. The larger field effects of anthryl
Scheme 6
and pyrenyl groups in 3 and 4 probably reflect their greater
π-surface. The signals of the aromatic groups of 1-4 are also
deshielded by the reciprocating field effect of the triptycenes.
The value obtained by subtracting the chemical shift of the
aromatic signals in 1, 3, and 4 from those of model arylalkynes
9a, 11a, and 12a is 0.59 in all cases (Table 1). This suggests
that the triptycyl group exerts the same field effect on all the
aromatic rotors.
The 13C NMR spectra of compounds 1-4 obtained at 125
MHz at ambient temperature were also consistent with a
dynamically averaged structure. Triptycyl signals were also
assigned by correlation with those of compound 8. Carbon atoms
in the two nonequivalent halves of the triptycenes were separated
by ∼0.5 ppm. Bridgehead carbons in 8 resonate at 53.55 and
53.29 ppm. Aromatic signals corresponding to C2 and C3 occur
at 125.19 and 125.68 ppm. The signals corresponding to Cendo
and Cexo appear at 122.53 and 123.42 ppm, respectively.
Nonprotonated aromatic triptycyl signals resonate at 144.43 and
144.35 ppm. Similar shifts were observed for compounds 1-4.
3.2. Aryl Group Rotation. The space-filling models in Figure
2 suggest that the conformational space of molecular rotors 1,
2, and 4 should be uniquely determined by the energetics of
rotation about sp-sp3 and sp-sp2 single bonds. In contrast,
because of the close proximity between the two peripheral
triptycenes and the central anthracene, some steric hindrance
may be expected in the case of compound 3. In contrast to the
well-documented rotation about single bonds involving sp3- and
sp2-hybridized carbon atoms,19 little is known about the energet-
ics of rotation about single bonds involving sp-hybridized
carbons. A handful of experimental and computational studies
stretching bands (4, 2230 cm-1). The NMR spectra were
acquired in C2D2Cl4 for 1, 3, and 4 and in mixtures of C2D2-
Cl4:CS2 (3:1) in the case of 2 (Scheme 5 and Table 1). Fast
rotation about the aryl-alkyne single bonds in solution was
deduced from the dynamically averaged 3-fold symmetry of the
triptycyl groups and the 2-fold symmetry of the four aromatic
rotors in all cases. Chemical shift assignments for compounds
1-4 were carried out by analogy with those of compound 8.
Hydrogen signals in the triptycyl group were assigned by 2D-
NOESY taking advantage of the easily recognized bridgehead
hydrogen (Scheme 6).
Carbon resonances were assigned by a 2D HMQC experiment
by taking advantage of the previously assigned hydrogen signals.
The alkyne substituents at the bridgehead position of the
triptycene core change the spin system from AA′BB′ in the
parent hydrocarbon to a more asymmetric ABMX pattern. The
bridgehead hydrogen of 8 has a chemical shift of 5.45 ppm and
signals corresponding to hydrogens at positions 2 and 3 of the
triptycene core (H2 and H3 in Scheme 6) appear at δ 7.05.
Signals corresponding to Hexo occur at δ 7.46 and signals of
Hendo are deshielded to δ 7.83 (Scheme 6). Although a solvent
effect is responsible for a systematic 0.1 ppm shielding in all
1
the signals of 2, a comparison of the H chemical shifts of
(18) Gu¨nther, H. NMR Spectrosocopy. Basic Principles, Concepts, and Ap-
plications in Chemistry, 2nd ed.; John Wiley & Sons: New York, 1995;
Chapter 4.
(19) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; John
Wiley & Sons: New York, 1994.
molecular rotors 1-4 indicates that significant differences occur
only for the aromatic rotors and the endo-hydrogens. Variations
in the chemical shifts of H2,3 and Hexo from one compound to
9
4704 J. AM. CHEM. SOC. VOL. 124, NO. 17, 2002

Molecular Compasses and Gyroscopes
A R T I C L E S
Scheme 7
Figure 3. Changes in energy (AM1, kcal/mol) of rotor 1 as a function of
phenylene rotation in a framework of eclipsed triptycyls. The minimum
corresponds to E(0) and the maximum to E(30) (Scheme 7).
reported in the past few years suggest that rotation of groups
attached to carbon-carbon triple bonds should be essentially
frictionless.20 To gain some insight into the ground-state
rotational potential of axially substituted diethynyl arenes, we
analyzed compounds 1 and 3 with the semiempirical AM1
method.21
The conformational motions of compounds 1-4 can be
defined in terms of the dihedral angles formed by the planes of
the two triptycyls with respect to each other, and with respect
to the plane of the central arylene group (Scheme 7). The relative
position of the two triptycyls can be illustrated by projections
along an axis connecting the two bridgehead carbons. These
are shown in Scheme 7 with the tryptycene framework
represented in blue and the arylene group in red. These
projections reveal structures that are analogous to those of
ethane. The triptycyl groups may be eclipsed E(Θ), staggered
S(Θ), or have an infinite number of structures in between. One
can define the orientation of the central rotor by a dihedral angle
Θ, given by the planes of the arylene rotor and the plane of a
benzene ring in the reference triptycene.
Calculations with the AM1 method in the case of compound
1 predicted remarkably flat energy surfaces. Minimizations that
started with arbitrary dihedral angles between triptycyl and
phenylene groups ended locally with energy differences of less
than 0.05 kcal/mol. Varying the relative orientation of the two
triptycyl groups made no difference on the calculated energies.
Conformations E(0) and S(0) are predicted to be within 0.004
kcal/mol. The energy profile obtained by rotation of the
phenylene group with a framework that has the two triptycenes
eclipsed, E(Θ), is shown in Figure 3 along with marks that
indicate the value of RT for T ) 300 K. While the accuracy
and resolution of these results must be questioned, rotation of
the triptycyl and phenylene groups in the ground state should
be essentially frictionless. These calculations predict that
compound 1 should be a free rotor at temperatures as low as
25 K. Similar results were obtained for compounds 2 and 4.
In contrast to compounds 1, 2, and 4, which have an
essentially flat rotational potential, AM1 minimizations with
compound 3 lead to two structures with the triptycyl groups
eclipsed [E(0), E(30)] and one with the triptycyls staggered
[S(30)]. The energies of conformers S(30) and E(30) (Figure
4a,b) are indistinguishable from each other (374.77 kcal/mol),
and only 0.74 lower in energy than that of conformer E(0)
Figure 4. Space-filling models of rotamers of compound 3: (a) S(30), (b)
E(30), (c) S(0), and (d) E(0). Please see Scheme 7 for notation.
[Figure 4d]. The maximum point in the rotational surface, 3.93
kcal/mol above S(30) and E(30), is given by the structure of
S(0) [Figure 4c]. This rotamer has the plane of the anthracene
group aligned with one of the aromatic planes of the each of
the periferal triptycyls, and experiences close contacts with both
of them. These results are in good qualitative agreement with
1
dynamically averaged H NMR spectra measured in solution,
which indicate a rapid conformational equilibrium.
3.3. X-ray Studies. Although molecular rotors 1-4 crystallize
readily from a wide variety of solvents, only crystals of 1 have
been of suitable quality for single-crystal X-ray diffraction
analysis. The X-ray structure of 1 was solved and analyzed to
obtain some insight into the packing preferences of molecular
rotors with triptycene frameworks. Crystallographic parameters
and refinement data are included in Table 2. ORTEP diagrams
of the molecular and packing structures are included in Figures
5 and 6, and details of close-packing interactions are shown in
Figure 7. Crystals grown from m-xylene incorporated solvent
molecules in a 1:1 stoichiometry. Although crystals of 1 resisted
melting up to 400 °C, thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) indicated loss of
m-xylene between 120 and 160 °C. The structure of the solvate
was solved in the triclinic space group P1h with two molecules
per unit cell. Compound 1 crystallizes with the two triptycyls
in a nearly eclipsed conformation with the plane of the phen-
ylene group closely aligned with one of the aromatic planes of
the flanking triptycenes. The point group of 1 deviates very
slightly from C2V to C1 symmetry. Instead of being perfectly
eclipsed, the aromatic planes of the two triptycyls form a
dihedral angle of 0.6° and the two alkyne bonds deviate from
collinearity by 1.1° (Figure 5).
(20) (a) Saebo, S.; Almolof, J.; Boggs, J. E.; Stark, J. G. J. Mol. Struct.
(THEOCHEM) 1989, 200, 361-373. (b) Abramenkov, A. V.; Almenningen,
A.; Cyvin, B. N.; Cyvin, S. J.; Jonvik, T.; Khaikin, L. S.; Rommingin, C.;
Vilkov, L. V. Acta Chem. Scand. 1988, A42, 674-678. (c) Sipachev, V.
A.; Khaikin, L. S.; Grikina, O. E.; Nikitin, V. S.; Traettberg, M. J. Mol.
Struct. 2000, 523, 1-22.
(21) Spartan, V2-4.0; Wavefunction, Inc.: 18401 von Karman Ave., Suite 210,
Irvine, CA 92715.
9
J. AM. CHEM. SOC. VOL. 124, NO. 17, 2002 4705

A R T I C L E S
Godinez et al.
Table 2. Crystallographic Parameters for Phenylene Rotor 1
empirical formula
formula weight
crystal system
C50H30‚C8H10
630‚106
triclinic
space group
P1h
Z
2
size, mm3
0.1 × 0.2 × 0.45
colorless prism
100(2)
color, morphology
temperature, K
unit cell dimensions
a, Å
b, Å
c, Å
11.955(2)
12.723(3)
14.685(3)
65.016(4)
77.582(4)
84.540(4)
1977.3(7)
1.193
R, deg
â, deg
γ, deg
V, Å3
Dc, Mg/m3
total no. of reflcns
no. of independent reflcns
[R(int) ) 0.0691], [I > 2σ (I)]
R1
13165
9143
0.0705
0.1615
wR2
Figure 7. (a) Close packing interactions around the phenylene group of
rotor 1 (in red) shown by a projection along the principal molecular axis.
A solvent molecule is shown in turquoise and triptycyl groups in blue. (b)
Top view of space-filling models showing close packing interactions
between the phenylene group in 1 and its close neighboring molecules A,
B, and C, as labeled in the top diagram. The reference molecule is labeled
R.
Scheme 8
Figure 5. Ortep diagram of the asymmetric unit of rotor 1, which includes
one molecule of m-xylene.
Figure 6. Packing diagram of rotor 1 illustrating a unit cell. All molecules
of rotor 1 are aligned in the same direction.
As expected for a rigid, rodlike structure with a length-to-
width ratio of ca. 2.2, the packing structure of rotor 1 is
characterized by having all molecules aligned in the same
direction (Figure 6). Adjacent molecules fit their voids and
protuberances, or interdigitate very tightly, with solvent mol-
ecules acting as spacers (Figures 6 and 7). Although rotation in
solution and in the gas phase may be essentially frictionless
(vide supra), rotation of the phenylene group in the solid state
is very strongly hindered. As illustrated by a projection along
the molecular long axes in Figure 7a and by top space-filling
views in Figure 7b, the rotational freedom of a reference
phenylene R (shown in red in Figure 7) is restricted by the
triptycyl groups from three close neighbors, labeled A, B, and
C (all in blue), and the methyl group of a m-xylene molecule
(in turquoise). The four close contacts of the reference rotor
may be described as follows: (i) The plane of the phenylene
group of R bisects the angle formed by two benzene rings of a
triptycene in B. (ii) Phenylene R interdigitates with a triptycene
of molecule C by aromatic face-to-face and edge-to-face
contacts. (iii) The left flank of the phenylene group in Figure
7a is covered by the edge of a triptycyl group in molecule A
and (iv) by the methyl group of a m-xylene. The top views in
Figure 7b illustrate the location of the solvent molecules, acting
as spacers, and the relative displacement of molecules A, B,
and C with respect to the long axis of the reference molecular
rotor (R), reproduced in the same position to analyze each pair.
The close packing interactions that most severely hinder the
rotation of the phenylene group come from tight interdigitation
of adjacent molecules. Interdigitation occurs as the protruding
triptycenes fill in the voids left by the smaller phenylene rotors.
It is worth noting that this interaction is accompanied by close
contacts between hydrogens at C2, C3, C6, C7, C12, and C13
of adjacent triptycenes (Scheme 8). This observation suggests
that analogous compounds with bulky substituents at those
positions should not interdigitate as efficiently. At the same time,
9
4706 J. AM. CHEM. SOC. VOL. 124, NO. 17, 2002

Molecular Compasses and Gyroscopes
A R T I C L E S
as illustrated in Scheme 8, bulky substituents at those positions
should help prevent the inclusion of solvent molecules near the
phenylene rotor. The synthesis of these compounds by the
convergent procedure described in this paper will require the
use of 2,3,6,7-tetrasubstiuted anthracenes and 3,4-disubstituted
anthranylic acids.
two flanking triptycenes. As expected from its rigid, rodlike
structure, compound 1 packs in a low symmetry space group
(P1h) with all molecules aligned in the same direction. Rotation
of the phenylene group of 1 in the solid state is prevented by
interdigitation of adjacent molecules and by close contacts with
the solvent of crystallization. Analysis of the packing structure
of 1 suggests that interdigitation may be prevented by substit-
uents at C2, C3, C6, C7, C12, and C13. Work in progress is
directed to the synthesis of such compounds and to the
characterization of their rotational dynamics in solution and in
crystals. A photophysical characterization of compounds 1-4,
including their UV-vis dichroism and fluorescence depolar-
ization, is also in progress.
4. Conclusions
We have developed a simple convergent procedure for the
synthesis of molecular rotors consisting of aromatic groups with
two axial alkynes linked to two bulky triptycenes. The molecular
rotors reported here, with 1,4-phenylene (1), 1,4′-1,1′-biphen-
ylene (2), 9,10-anthracenylene (3), and 2,7-pyrenylene groups
(4), were fully characterized by conventional spectroscopic
methods. Semiempirical calculations with the AM1 method
indicate that rotation about tryptycene-alkyne and aryl-alkyne
single bonds should be essentially frictionless in the gas phase.
Rapid rotation in solution was deduced in all cases from a
dynamically averaged 1H and 13C NMR spectra. Although
compounds 1-4 formed insoluble crystals, only those of
compound 1, grown from m-xylene, were suitable for single-
crystal X-ray diffraction analysis. Compound 1 crystallized in
a near C2V conformation with the plane of the phenyl group
eclipsed with one of the three aromatic planes of each of the
Acknowledgment. This work was supported by NSF grants
DMR9988439, CHE9871332 (X-ray), and DMR9975975 (Solid
State NMR).
Supporting Information Available: Stereoscopic data of
compounds 1-4 and X-ray data tables for rotor 1 (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA012550I
9
J. AM. CHEM. SOC. VOL. 124, NO. 17, 2002 4707