Intramolecular π-stacking interaction in a rigid molecular hinge substituted with 1-(pyrenylethynyl) units

Article abstract of DOI:10.1021/jo062173y

(Chemical Equation Presented) Synthesis of a tetrakis(1-pyrenylethynyl)- substituted rigid hinge-like molecule (1) is described. The intramolecular π-stacking interaction of the pyrene units is studied by ¹H NMR and fluorescence spectroscopy. D

Full text of DOI:10.1021/jo062173y

Intramolecular π-Stacking Interaction in a Rigid Molecular Hinge
Substituted with 1-(Pyrenylethynyl) Units‡
Ritesh Nandy,† Mahadevan Subramoni,§ Babu Varghese,§ and Sethuraman Sankararaman*,†
Department of Chemistry and Sophisticated Analytical Instrument Facility, Indian Institute
of Technology Madras, Chennai 600036, India
sanka@iitm.ac.in
ReceiVed October 19, 2006
Synthesis of a tetrakis(1-pyrenylethynyl)-substituted rigid hinge-like molecule (1) is described. The
1
intramolecular π-stacking interaction of the pyrene units is studied by H NMR and fluorescence
spectroscopy. Due to intramolecular π-stacking interactions, chemical shifts of the pyrene protons in 1
are highly shielded in the NMR spectrum. Fluorescence from the static excimer state is observed due to
π-stacking interactions among the pyrene units in the ground state of 1. Based on the spectroscopic
evidence, conformations and dynamics of 1, arising from the hindered rotation of the major axis, are
proposed.
Introduction
and emission at longer wavelengths.6 π-Stacking interactions
of aromatic pendent units attached to a polymer backbone have
been studied in many systems.7 Supramolecular aggregation
through intermolecular π-π interactions is well-known in many
polycyclic aromatic hydrocarbons and superacenes, resulting in
Among weak molecular interactions, the π-π interaction is
very important. In biomolecules, π-stacking interactions among
aromatic units play a major role in DNA stacking,1 protein
folding,2 and molecular recognition.3 In chemistry, π-stacking
interactions of aromatic π systems influence the supramolecular
assembly of molecules in solution and solids4 and at the
interfaces.5 Electronic properties of organic molecules are
affected by π-stacking interactions, resulting in efficient inter-
molecular electron transfer, improved electrical conductivity,
(4) (a) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. ReV. 2005, 105, 1491-1546. (b) Claessens, C. G.;
Stoddart, J. F. J. Phys. Org. Chem. 1997, 10, 254-272. (c) Watson, M. D.;
Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV. 2001, 101, 1267-1300. (d)
Sokolov, A. N.; Friscic, T.; MacGillivray, L. R. J. Am. Chem. Soc. 2006,
128, 2806-2807. (e) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem.
ReV. 2004, 104, 4891-4945.
(5) Dou, R.-F.; Ma, X.-C.; Xi, L.; Yip, H. L.; Wong, K. Y.; Lau,
W. M.; Jia, J.-F.; Xue, Q.-K.; Yang, W.-S.; Ma, H.; Jen, A. K.-Y. Langmuir
2006, 22, 3049-3056 and references cited therein.
(6) (a) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am.
Chem. Soc. 2001, 123, 9482-9483. (b) Watson, M. D.; Ja¨ckel, F.; Severin,
N.; Rabe, J. P.; Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 1402-1407.
(c) Wu¨rthner, F. Chem. Commun. 2004, 1564-1579. (d) Wu¨rthner, F.;
Thalacker, C.; Diele, S.; Tschierske, C. Chem.sEur. J. 2001, 7, 2245-
2253.
(7) (a) Reitzel, N.; Greve, D. R.; Kjaer, K.; Howes, P. B.; Jayaraman,
M.; Savoy, S.; McCullough, R. D.; McDevitt, J. T.; Bjornholm, T. J. Am.
Chem. Soc. 2000, 122, 5788-5800. (b) Rivera, E.; Belletete, M.; Zhu,
X. X.; Durocher, G.; Giasson, R. Polymer 2002, 43, 5059-5068. (c) de
Melo, J. S.; Costa, T.; Miguel, M. D.; Lindman, B.; Schillen, K. J. Phys.
Chem. B 2003, 107, 12605-12621.
‡ Dedicated to Professor Alfred Fischer on the occasion of his 75th birthday.
* To whom correspondence should be addressed. Phone: +91 44 2257 4210.
Fax: +91 44 2257 0545.
† Department of Chemistry.
§ Sophisticated Analytical Instrument Facility.
(1) Saenger, W. Principles of Nucleic Acid Structure; Springer-Verlag:
New York, 1984; pp 261-265.
(2) (a) McGaughey, G. B.; Gagne, M.; Rappe, A. K. J. Biol. Chem. 1998,
272, 15458-15463. (b) Burley, S. K.; Petsko, G. A. AdV. Protein Chem.
1988, 39, 125-192. (c) For π stacking in lipid bilayers, see: Bhosale, S.;
Sisson, A. L.; Sakai, N.; Matile, S. Org. Biomol. Chem. 2006, 4, 3031-
3039.
(3) (a) Hunter, C. A. Chem. Soc. ReV. 1994, 101-109. (b) Inouye, M.;
Fujimoto, K.; Furusyo, M.; Nakazumi, H. J. Am. Chem. Soc. 1999, 121,
1452-1458. (c) Lehn, J.-M. Supramolecular Chemistry: Concepts and
PerspectiVes; VCH: Weinheim, Germany, 1995.
10.1021/jo062173y CCC: $37.00 © 2007 American Chemical Society
Published on Web 01/11/2007
938
J. Org. Chem. 2007, 72, 938-944

Intramolecular π-Stacking Interactions in a Molecular Hinge
SCHEME 1. Synthesis of 1a
a Reagents and conditions: (a) Pd(PPh3)2Cl2, CuI, PPh3, Et3N, 60 °C, 2.5 h, 63%; (b) CBr4, PPh3, CH2Cl2, rt, 16 h, 95%; (c) LDA, THF, -78 °C, 20 min,
75%; (d) 1-iodopyrene, Pd(PPh3)4, CuI, Et3N, THF, 48 h, 25%.
discotic liquid crystalline properties in certain systems.8 Many
theoretical studies have been undertaken to understand the nature
of orbital interactions, stabilization arising from weak π-π
interactions, and the potential energy surface of such systems.9
Pyrene is a prototypical polycyclic aromatic fluorescent
molecule. Emission of excited-state pyrene from the monomer
and excimer states is fairly well understood.10 π-Stacking
interactions between two pyrene units either in the ground state
or in the excited state result in the excimer emission upon
photoexcitation. Intramolecular interaction of two pyrene units
in the ground state through π-π stacking has been observed
when pyrene units are attached to a conformationally rigid
molecular framework.11 In molecules with conformational
flexibility, metal-ion-mediated conformational changes have
resulted in bringing two pyrene units close to undergo a
π-stacking interaction.12 Such molecules find application in
fluorescence-based sensors. Recently, we have reported a novel
pyrene octaaldehyde that exhibited interesting cooperative π-π
and C-H‚‚‚O interactions, resulting in aggregation in the
solution and solid states.13 Herein, we report the synthesis of a
rigid acetylenic molecular hinge to which four 1-pyrenylethyne
units are attached. The intramolecular π-stacking interaction and
hindered rotation along the major axis are studied by variable-
1
temperature fluorescence and H NMR spectroscopy.
Results and Discussion
Synthesis. We have designed and synthesized a molecular
hinge (1) that has a rigid rod-like central unit to which four
1-pyrenylethynyl units are attached. It was synthesized in four
steps as shown in Scheme 1. The 2-fold Sonogashira coupling
of dialdehyde 2 and 1,4-diethynylbenzene (3) gave tetraaldehyde
1
4 in 63% yield. The H NMR spectrum of tetraaldehyde 4
showed four singlets in the integration ratio of 1:1:1:4.5,
consistent with its structure. Treatment of 4 with CBr4 and Ph3P
yielded octabromide 5 in 95% yield as a pale-orange crystalline
solid. It was characterized by various spectroscopic methods,
and the structure was confirmed by single-crystal X-ray crystal-
(8) (a) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-1122. (b) Hayer,
A.; de Halleux, V.; Kohler, A.; El-Garoughy, A.; Meijer, E. W.; Barbera,
J.; Tant, J.; Levin, J.; Lehmann, M.; Gierschner, J.; Cornil, J.; Geerts,
Y. H. J. Phys. Chem. B 2006, 110, 7653-7659. (c) Wu, J.; Watson, M. D.;
Zhang, L.; Wang, Z.; Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 177-186.
(9) (a) Tsuzuki, S.; Uchimaru, T. Curr. Org. Chem. 2006, 10, 745-
762. (b) Bernstein, E. R.; Sun, S. J. Phys. Chem. 1996, 100, 13348-13366.
(c) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-
5534.
(10) (a) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-
Interscience: London, 1970. (b) Winnik, F. M. Chem. ReV. 1993, 93, 587-
614. (c) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.;
Kluwer Academic: New York, 1999; pp 9 and 611.
(11) Chou, T.-C.; Hwa, C.-L.; Lin, J.-J.; Liao, K.-C.; Tseng, J.-C. J. Org.
Chem. 2005, 70, 9717-9726.
(12) (a) Yuasa, H.; Miyagawa, N.; Izumi, T.; Nakatani, M.; Izumi, M.;
Hashimoto. Org. Lett. 2004, 6, 1489-1492. (b) Lee, S. H.; Kim, S. H.;
Kim, S. K.; Jung, J. H.; Kim, J. S. J. Org. Chem. 2005, 70, 9288-9295.
(c) For anion-mediated π stacking, see: Suzuki, I.; Ui, M.; Yamauchi, A.
J. Am. Chem. Soc. 2006, 128, 4498-4499. (d) Kim, S. K.; Bok, J. H.;
Bartsch, R. A.; Lee, J. Y.; Kim, J. S. Org. Lett. 2005, 7, 4839-4842. (e)
Yang, J.-S.; Lin, C.-S.; Hwang, C.-Y. Org. Lett. 2001, 3, 889-892.
(13) Venkataramana, G.; Sankararaman, S. Org. Lett. 2006, 8, 2739-
2742.
J. Org. Chem, Vol. 72, No. 3, 2007 939

Nandy et al.
SCHEME 2. Dynamics of 1 Leading to π Stacking and Its
Disruption at Higher Temperatures (tert-butyl groups are
not shown)
FIGURE 1. 1H NMR spectra of 1 in CDCl2CDCl2, 7, and 8 (in CDCl3)
at 25 °C.
chemically identical environment. The resolution of the signals
of various pyrene protons of 1 in comparison to those of 7 and
8 is due to a π-stacking interaction and the anisotropic effect
arising from the ring current of the pyrene rings. In the absence
of any strong electronic effects due to functional groups, it is
not possible to explain the spread of the aromatic proton signals
in 1. The spread of the chemical shifts to give a near first-order
spectrum for all nine protons of the pyrene ring can only be
due to the π-stacking interactions of pyrene rings in the ground
state. Otherwise, polycyclic aromatic hydrocarbons generally
show complex second-order multiplets that are poorly resolved
(7 and 8 in Figure 1). Moreover, the chemical shifts of the
pyrene protons in 1 are relatively more shielded (7.2 to 8.7 ppm)
compared to that in other ethynyl-substituted pyrene derivatives
(typically 8.0 to 8.5 ppm). It is well-established that π-stacking
interactions between aromatic rings result in the shielding of
the protons due to the anisotropy of the ring current effect.13,14
For example, the pyrene protons of [2,2]-2,7-pyrenophane are
more shielded than that of 2,7-dimethylpyrene.15 In the present
case, π-stacking interactions of two pyrene rings on either side
of the rigid hinge are not exactly parallel-superimposed (A in
Figure 2), it is rather a parallel-displaced type of π stacking.
With respect to the pyrene rings, it is the lengthwise displace-
ment (C in Figure 2) which puts the H-7 proton of one pyrene
lographic data (Supporting Information). In the 1H NMR
spectrum of 5, the vinylic protons appeared as a singlet at δ
7.77 ppm. Reaction of octabromide 5 with excess LDA yielded
1
tetrayne 6 in 75% yield as a colorless solid. The H NMR
spectrum of tetrayne 6 showed four singlets in the integration
ratio of 1:1:1:4.5, and the acetylenic protons appeared as a sharp
singlet at δ 3.37 ppm, confirming the smooth conversion of 5
to 6. The target molecule 1 was obtained in 25% yield by the
4-fold Sonogashira coupling of tetrayne 6 with 1-iodopyrene.
The tetrakis(1-pyrenylethynyl) derivative 1 was obtained as a
1
bright-yellow fluorescent solid. From the H NMR spectral
integration of 1, it is evident that the relative ratio of the two
different types of phenyl protons (Ha and Hb) (Scheme 2) and
the pyrene protons is 1:1:9, confirming the presence of four
pyrene units in the molecule. The MALDI-TOF MS of 1
confirmed its molecular weight.
1H NMR Spectrum of 1. The 1H NMR spectrum of 1
recorded in 1,1,2,2-tetrachloroethane-d2 at 25 °C is shown in
1
Figure 1. For comparison, the H NMR spectra of 1-(phenyl-
ethynyl)pyrene (7) and 1-(2-ethynylphenylethynylpyrene) (8)
recorded in CDCl3 are also shown. The protons on the phenyl
rings (Ha and Hb) (Scheme 2) in 1 appeared as two singlets of
equal intensity at δ 8.01 and 7.86 ppm, respectively. The signals
due to the protons on the pyrene rings in 1 are very well resolved
in comparison to that in 7 and 8. Assignment of the individual
proton resonance in 1 is based on the 1H-1H COSY spectrum,
which established the coupling partners. It is also evident that,
for all of the protons in the four pyrene rings in 1, there is only
one set of signals observed in the spectrum. This is due to the
highly symmetrical structure of 1 in which the proton of a
particular position in each of the pyrene unit is under a
(14) (a) Ref 6b. (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) Shetty, A. S.; Fischer, P. R.; Storck, K. F.; Bohn, P. W.; Moore,
J. S. J. Am. Chem. Soc. 1996, 118, 9409-9414. (d) For an NMR study of
π-π stacking of pheophytin, see: Hynninen, P. H.; Lo¨tjo¨nen, S. Biochem.
Biophys. Acta 1993, 1083, 374-380.
(15) For the spectral data of [2,2]-2,7-pyrenophane, see: Umemoto, T.;
Satani, S.; Sakata, Y.; Misumi, S. Tetrahedron Lett. 1975, 3159-3162.
940 J. Org. Chem., Vol. 72, No. 3, 2007

Intramolecular π-Stacking Interactions in a Molecular Hinge
FIGURE 2. Various modes of π-stacking interactions between two
pyrene units. (A) Parallel-superimposed. (B) Parallel-displaced breadth-
wise. (C) Parallel-displaced lengthwise.16
ring on the shielding zone of the ring current of the other pyrene
ring.16 The chemical shift of proton H-7 is unusually low,
appearing at δ 7.24 ppm. Assignment of this signal at δ 7.24
ppm to proton H-7 is straightforward because H-7 is the only
proton that has two ortho protons (H-6 and H-8) which couple
and, hence, a triplet signal (J ) 7.6 Hz). The most deshielded
signal is a doublet at δ 8.75, and it corresponds to H-10.
Typically, in ethynyl-substituted pyrene derivatives, proton H-10
is highly deshielded, and it appears at δ 8.8-9.0 ppm due to
the anisotropic effect of the ethynyl group in the peri position.17
The H-10 in 1 is located in the deshielding zone of the two
ethynyl units. Due to the π-stacking interactions, the H-10 proton
is relatively more shielded in 1 than it is in ethynylpyrene
derivatives such as 7 and 8 in which the π-stacking interaction
is absent (Figure 1).
Variable-Temperature 1H NMR Spectral Studies on 1. The
effect of temperature on the chemical shift of various protons
in 1 was investigated in the temperature range of 25 to 135 °C
(Figure 3). With increasing temperature, the chemical shift of
the protons (Hb) on the phenyl rings bearing the pyrene
substituents was unaffected. However. protons (Ha) on the
phenyl ring at the core of the molecule were gradually shifted
to a lower δ value, from 8.03 ppm at 25 °C to 7.76 ppm at 135
°C. The H-2 and H-3 protons on the pyrene rings were also
unaffected by temperature. However, there was a significant
increase in the chemical shifts of the H-6, H-7, and H-8 protons
with increasing temperature (Figure 4). There was a slight
increase in the chemical shift of H-10 from 8.75 ppm at 25 °C
to 8.81 ppm at 135 °C. π Stacking, being a weak interaction,
can be broken at higher temperatures.18 A rotation along the
major axis can disrupt the π-stacking interaction in 1, and this
process is facilitated with increasing temperature. During the
rotation along the major axis, the interplanar distances between
the pyrene units become longer, resulting in varying degrees
of the π-stacking interaction. As a result, the protons on the
pyrene ring are gradually deshielded in going from conformer
I to conformer II (Scheme 2). Conformer II is one extreme case
where the pyrene rings are farthest away from each other.
Further rotation along the major axis will bring the pyrene closer.
1
FIGURE 3. Variable temperature H NMR spectra of 1 in CDCl2-
CDCl2.
FIGURE 4. Effect of temperature on the chemical shift of various
protons on the pyrene ring in 1.
Therefore, along the major axis, the molecule can only undergo
an oscillatory motion and not a full rotation. In other words,
there is a restricted rotation of the major acetylenic axis in 1
due to stabilizing π-stacking interactions between the pyrene
units at room temperature. The pyrene units are interlocked due
to π-stacking interactions. Earlier examples of hindered rotation
of the triple bond of diphenylacetylene derivatives deal with
repulsive steric interactions of bulky substituents present in the
ortho positions of the aromatic ring.19 Among the various
protons, the ones at the core of the ring current effect will be
relatively more deshielded (H-6, H-7, and H-8) as compared to
the other protons (H-2 and H-3) which are located outside of
the ring current effect of the pyrene rings. The experimental
(16) Intermolecular π-π interactions in benzene and naphthalene can
occur through a T-shaped geometry. Calculations show that the minimum
energy configuration is a T-shape. However, in polycyclic aromatic
hydrocarbons such as pyrene and coronene and porphyrins, it is the displaced
parallel stacking that is observed in the solid state. In such systems, the
T-shaped geometry does not contribute to the π-stacking interactions. See
refs 3a and 9c. See also: Desiraju, G. R. Crystal Engineering. The Design
of Organic Solids; Elsevier: Amsterdam, 1989; pp 92-96.
(17) (a) Gu¨nther, H. NMR Spectroscopy; John Wiley: New York, 1980;
p 73. (b) Venkataramana, G.; Sankararaman, S. Eur. J. Org. Chem. 2005,
4162-4166. (c) Bernhardt, S.; Kastler, M.; Enkelmann, V.; Baumgarten,
M.; Mu¨llen, K. Chem.sEur. J. 2006, 12, 6117-6128.
(18) The binding energy of the homo π dimers for benzene and
naphthalene is calculated to be 3-10 kcal/mol depending upon the geometry
of the dimers; see: Sato, T.; Tsuneda, T.; Hirao, K. J. Chem. Phys. 2005,
123, 104307-1-104307-10.
J. Org. Chem, Vol. 72, No. 3, 2007 941

Nandy et al.
observations described support this hypothesis. At room tem-
perature, free rotation of the p-phenylene ring at the core of the
molecule is perhaps not possible due to steric hindrance.
Nevertheless, a sharp singlet is observed for all four protons
(Ha) at 8.03 ppm at 25 °C. Upon increasing the temperature,
the signal gradually shifted to lower δ values (7.76 ppm at
135 °C). This observation suggests that the p-phenylene ring at
the core is perpendicular to the end phenyl rings at 25 °C, in
the sterically least-hindered conformation. With increasing
temperature, free rotation of the p-phenylene ring along the
major axis is made possible, and as a result, the protons on this
ring experience the ring current effect of the pyrene rings,
causing the shielding of the Ha protons. From the detailed
1
analysis of the H NMR spectrum of 1 and its changes with
temperature, we conclude that the molecule (1) exists in
conformation I (Scheme 2) at room temperature and lower
temperatures. Conformer I is stabilized by the π-stacking
interactions between the pyrene rings. At higher temperatures,
partial rotation along the major axis, as well as free rotation of
the core p-phenylene ring, is possible, resulting in a change of
conformation to II; perhaps a rapid equilibrium exsits at
135 °C, the highest temperature reached in this study. The
spectrum at 135 °C resembles that of 7 and 8 in the aromatic
region for the pyrene protons, suggesting that the π stacking is
disrupted at higher temperatures. Although rotation along the
minor axis can also disrupt the π stacking, such a process does
not appear to be a major one because only one set of signals is
observed for all of the pyrene protons, even at higher temper-
atures. Thus, the overall molecular symmetry must be retained.
Moreover, the fact that the Hb protons are relatively insensitive
to temperature supports this claim.
FIGURE 5. (A) UV-vis spectrum of 1 in CH2Cl2 (1 × 10-5 M). (B)
Excitation spectrum in CH2Cl2 (λem ) 463 nm, the y-axis in arbitrary
scale). The spectrum of 9 (C) (1 × 10-5 M) is shown for comparison.
The excitation spectrum of 1 is nearly identical to that of its
absorption spectrum in CH2Cl2. The fluorescence emission in
these two solvents was broad and devoid of any vibrational fine
structure. The fluorescence emission was independent of the
wavelength of excitation in both of these solvents. The broad,
featureless emission bands at λmax 460 nm in CH2Cl2 and at
485 nm in cyclohexane correspond to emission from the excimer
state. The monomer emission of pyrene and its derivatives
invariably shows vibrational fine structure.10,12 The fluorescence
spectrum of the model compound (9) in which the π-stacking
interaction is absent shows vibrational fine structure, and its
emission maxima are 400 and 421 nm in cyclohexane. A large
Stokes shift of 55-77 nm for 1 is consistent with the emission
being from the excimer state. Alternatively, the observed red
shift of the emission spectrum of 1 and its broad, featureless
nature compared to that of the model system 9 could be due to
the extensive through-bond π conjugation of the four pyrene
units in the molecule and not due to through-space π-π
interactions. However, this explanation can be ruled out because
Absorption and Emission Spectra of 1. The pyrene chro-
mophore is highly fluorescent. The fluorescence emission from
the monomer and excimer states of pyrene is significantly
different. Emission from the monomer state shows a vibrational
fine structure and occurs at shorter wavelengths (λem 380 nm)
in comparison to the broad, featureless emission of the excimer
state that occurs at longer wavelengths (λem 480 nm).20 In
nonpolar solvents such as CH2Cl2 and cyclohexane, compound
1 showed several absorption bands with high extinction coef-
ficients in the region of 200-410 nm (Figure 5). For compari-
son, 1-(5-tert-butyl-2-ethynylphenylethynyl)pyrene (9) is chosen
as a model compound, and its absorption spectrum is also shown
(C, Figure 5). It is evident from Figure 5 that the band intensities
(and hence the extinction coefficients) of 1 are approximately
four times that of the model compound 9 under identical
concentrations. It is also evident that the absorption bands of 1
are broader and more red shifted by 14 nm than that of 9,
indicating the presence of π-stacking interactions in 1.15 In fact,
the absorption spectrum of 1 corresponds to the π-stacked
pyrene dimer chromophore rather than a pyrene monomer
chromophore.
1
the H NMR spectrum of 1 very clearly indicates a π-π-
stacking interaction in the ground state itself (vide supra).
Otherwise, the spread of the NMR signals, the unusual shielding
of H-7, and the observed gradual deshielding of H-6, H-7, and
H-8 with increasing temperature cannot be explained satisfac-
torily. The Stokes shift in 1 is smaller than that of the pyrene
excimer emission (100 nm). The emission maximum (and hence
the Stokes shift) will depend on the extent of the π-stacking
interaction. In case of pure pyrene, the π-stacking interaction
is of the type A (Figure 2). However, in a constrained system
such as 1, type A π stacking is not possible. The excimer
emission arises from type C π stacking in 1, and hence, the
excimer emission maximum is blue shifted compared to that of
the pure pyrene excimer emission. An excimer emission
maximum in the range of 460-480 nm is consistent with earlier
reports for various pyrene derivatives.12 In compound 1, an
excimer emission arises due to the intramolecular π-stacking
interactions of the pyrene rings in the ground state. Such an
excimer is termed as a static excimer as opposed to a dynamic
excimer, which is formed by the diffusion of an excited-state
molecule and a ground-state molecule.21 The fluorescence
emission maximum in CH2Cl2 (λmax 460 nm) was invariant with
the concentration of 1 in the range of 10-5-10-9 M, indicating
that the emission is arising from a static excimer due to the
intramolecular π stacking and is not arising from a dynamic
(19) For hindered rotation of a triple bond, see: (a) Toyota, S.;
Yamamori, T.; Makino, T. Tetrahedron 2001, 57, 3521-3528. (b) Toyota,
S.; Makino, T. Tetrahedron Lett. 2003, 44, 7775-7778. (c) Toyota, S.;
Iida, T.; Kunizane, C.; Tanifuji, N.; Yoshida, Y. Org. Biomol. Chem. 2003,
1, 2298-2302. (d) Miljanic, O. S.; Holmes, D.; Vollhardt, K. P. C. Org.
Lett. 2005, 7, 4001-4004. (e) Miljanic, O. S.; Han, S.; Holmes, D.; Schaller,
G. R.; Vollhardt, K. P. C. Chem. Commun. 2005, 2606-2608.
(20) (a) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules; Academic Press: New York, 1965; p 173. (b) See also ref 10b.
(21) (a) Choi, J. K.; Kim, S. H.; Yoon, J.; Lee, K.-H.; Bartsch, R. A.;
Kim, J. S. J. Org. Chem. 2006, 71, 8011-8015. (b) Jun, E. J.; Won, H. N.;
Kim, J. S.; Lee, K.-H.; Yoon, J. Tetrahedron Lett. 2006, 47, 4577-4580.
942 J. Org. Chem., Vol. 72, No. 3, 2007

Intramolecular π-Stacking Interactions in a Molecular Hinge
study of 1. Both in solution as well as in the solid state, 1
exhibits an excimer-type broad fluorescence emission. On the
basis of all of this spectroscopic evidence, we have suggested
that 1 exists in conformation I. At higher temperatures, rotation
along the major axis of the hinge results in a change of
conformation to II, resulting in equilibration between these two
conformations. All of the experimental evidence presented
herein fits this model well. Since π-stacking architecture is
emerging as a powerful method to produce functional organic
molecules that find application in sensors, molecular electronics,
and photonics, study of molecules such as 1 will help in
understanding the nature of π-π interactions.
Experimental Section
Tetraaldehyde 4. A Schlenk flask was charged with bromodi-
aldehyde24 2 (0.83 g, 3.1 mmol), Pd(PPh3)Cl2 (0.056 g, 0.08 mmol),
PPh3 (0.042 g, 0.16 mmol), CuI (0.017 g, 0.16 mmol), and degassed
Et3N (20 mL). The mixture was stirred and heated to 60 °C. A
solution of diyne 3 (0.2 g, 1.59 mmol) in THF (5 mL) was added
dropwise. After an hour, a copious amount of precipitate was
observed and stirring was continued for another 1.5 h. The reaction
mixture was cooled to rt and filtered. The yellow precipitate was
dissolved in CH2Cl2 (100 mL), and the solution was washed with
5% aqueous HCl. The organic layer was dried over Na2SO4, and
solvent was removed under reduced pressure at rt. The crude
product was purified by column chromatography on silica gel using
an ethyl acetate-hexane mixture (15:85 v/v) to yield the tetraal-
dehyde (4) as a yellow solid (0.5 g, 63%): mp 310 °C dec; IR
FIGURE 6. Fluorescence spectra of 1 (1 × 10-5 M, λex ) 353 nm) in
cyclohexane (A), CH2Cl2 (B), and the solid state (C) (λex ) 347 nm).
The fluorescence spectrum of 9 in cyclohexane (D) (1 × 10-5 M,
λex ) 367 nm) is shown for comparison.
excimer due to intermolecular π stacking. Similar behavior was
observed in cyclohexane with respect to a change of concentra-
tion from 10-5 to 10-9 M. In view of the results obtained from
the variable-temperature 1H NMR of 1, the effect of temperature
on the fluorescence emission was investigated in the temperature
range from -10 to +70 °C in CHCl2CHCl2. The shape of the
emission band and the intensity of fluorescence emission did
not change significantly with increasing temperature. However,
there was a gradual shift of the emission maximum from
467 nm at -10 °C to 457 nm at +70 °C. The observed blue
shift of the pyrene excimer emission with increasing temperature
is an indication of the π stacking being disturbed at higher
temperatures.8b,22 The blue shift of the excimer band and the
distinct absence of any band due to monomer emission up to
+70 °C indicates that there is rapid conformational equilibrium
between I and II (Scheme 2). It is concluded that, within this
temperature range, there is no significant change in the
π-stacking behavior of 1, resulting in a very minimal change
in the emission behavior. The quantum efficiency of fluores-
cence emission of 1 was 0.8 ((0.03), measured using 9,10-
diphenylanthracene as a reference.23 Compound 1 is fluorescent
in the solid state as well (Figure 6, D). In the solid state, the
emission spectrum was broad and devoid of fine structures. The
excimer emission maximum in the solid state is 512 nm (Stokes
shift of 104 nm), and the emission maximum is independent of
the excitation wavelength. In the solid state, besides intramo-
lecular π stacking of the pyrene units, it is also possible that
extensive intermolecular π stacking exsits in 1.
1
(KBr) 2211, 1687 cm-1; H NMR (CDCl3, 400 MHz) δ 10.66 (s,
4H), 8.18 (s, 4H), 7.58 (s, 4H), 1.33 (s, 18H); 13C NMR (CDCl3,
100 MHz) δ 190.6, 153.1, 136.5, 131.9, 129.9, 126.1, 125.8, 122.8,
101.5, 35.4, 31.0; MALDI-TOF MS m/z 475 (M + 1 - CO), 503
(M + 1), 976 (dimer - CO), 1004 (dimer); ESI Q-TOF MS m/z
502 (10), 503 (55), 504 (20); HRMS calcd for C34H31O4 [M +
H+], 503.2222; found, 503.2222.
Octabromide 5. A Schlenk flask was charged with CBr4 (1.05
g, 3.2 mmol), PPh3 (0.84 g, 3.2 mmol), and Zn powder (0.21 g,
3.2 mmol), and the mixture was dissolved in CH2Cl2 (70 mL) and
stirred at rt for 4 h. Tetraaldehyde 4 (0.10 g, 0.2 mmol) was added
in one lot, and stirring was continued at rt for 16 h. The mixture
was filtered, and solvent was removed under reduced pressure in
the rotary evaporator to obtain the crude product as an orange solid.
It was purified by column chromatography on silica gel using 30%
CH2Cl2-hexane (vol %) to yield 5 as a pale-orange crystalline solid
(0.22 g, 95%): mp 210-212 °C; IR (KBr) 2962, 2917, 2849, 1593,
1
846, 764 cm-1; H NMR (CDCl3, 400 MHz) δ 7.84 (s, 4H), 7.77
(s, 4H), 7.56 (s, 4H), 1.35 (s, 18H); 13C NMR (CDCl3, 100 MHz)
δ 151.2, 137.4, 135.9, 131.6, 125.6, 123.1, 118.8, 99.9, 91.9, 87.4,
35.2, 31.0. Anal. Calcd for C38H30Br8: C, 40.54; H, 2.68. Found:
C, 41.37; H, 2.91.
Tetrayne 6. (Caution: During the determination of the melting
point of 6, violent decomposition with the formation of black soot
was observed at 153 °C. Care should be taken not to heat this
compound in large amounts). To a solution of octabromide 5 (0.1
g, 0.09 mmol) in THF (20 mL) stirred at -78 °C was added a
solution of LDA [freshly prepared from diisopropyl amine (0.18
mL, 1.3 mmol) and n-BuLi (0.61 mL of 1.4 M solution in hexane,
1.3 mmol) in THF (10 mL) at -78 °C]. The mixture was stirred
for 20 min at -78 °C and then quenched by adding a degassed
saturated NH4Cl solution (20 mL). It was allowed to attain rt. The
organic layer was removed, and the aqueous layer was extracted
with CH2Cl2 (2 × 20 mL). The combined organic layer was dried
over Na2SO4, and solvent was removed under reduced pressure in
a rotary evaporator. The crude product thus obtained was purified
Conclusions
We have designed and synthesized a molecular hinge (1) with
four 1-pyrenylethynyl groups attached to it. Evidence from 1H
NMR and fluorescence spectroscopic studies suggests that there
is an intramolecular π-stacking interaction among the pyrenyl
units in 1. The π-stacking interaction can be disrupted at higher
temperatures, as is evident from the observed changes in the
chemical shift of the pyrene protons in 1. A similar conclusion
is arrived at based on the temperature-dependent fluorescence
(22) A similar blue shift of the pyrene excimer emission with increasing
temperature has been reported earlier; see: (a) Lou, J.; Hatton, T. A.;
Laibinis, P. E. Anal. Chem. 1997, 69, 1262-1264. (b) See also ref 8b.
(23) Hamal, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83-89.
(24) Narayanan, V.; Sankararaman, S.; Hopf, H. Eur. J. Org. Chem. 2005,
2740-2746.
J. Org. Chem, Vol. 72, No. 3, 2007 943

Nandy et al.
by column chromatography on silica gel using 2% ethyl acetate in
hexane (vol %). The tetrayne 6 was obtained as a colorless solid
(0.032 g, 75%): mp 153 °C (violent decomposition); IR (neat) 2213
(CDCl2CDCl2, 400 MHz, 25 °C) δ 8.75 (d, 4H, J ) 8.8 Hz), 8.26
(d, 4H, J ) 7.8 Hz), 8.03 (s, 4H), 7.95 (d, 4H, J ) 8.3 Hz), 7.83
(s, 4H), 7.80 (s, 8H), 7.64 (d, 4H, J ) 7.8 Hz), 7.44 (m, 8H), 7.23
(t, 4H, J ) 7.8 Hz), 1.56 (s, 18H); 13C NMR (CDCl2CDCl2, 100
MHz, 25 °C) δ 151.6, 133.0, 132.3, 131.7, 130.8, 130.6, 130.2,
129.8, 129.1, 128.7, 127.2, 127.0, 126.0, 125.9, 125.6, 124.8, 124.5,
124.3, 123.9, 119.5, 117.5, 97.2, 94.6, 93.6, 90.9, 35.4, 31.6;
MALDI-TOF MS m/z calcd for C102H62, 1287; found, 1287. Anal.
Calcd for C102H62: C, 95.15; H, 4.85. Found: C, 89.22; H, 4.46.25
1
cm-1; H NMR (CDCl3, 400 MHz) δ 7.57 (s, 4H), 7.55 (s, 4H),
3.37 (s, 4H), 1.32 (s, 18H); 13C NMR (CDCl3, 100 MHz) δ 151.3,
131.7, 130.2, 125.9, 124.7, 123.4, 96.7, 88.6, 82.2, 80.9, 34.7, 30.9.
Anal. Calcd for C38H30: C, 93.79; H, 6.21. Found: C, 92.89; H,
6.00.25
Tetrapyrenyl Derivative 1. A Schlenk flask was charged with
iodopyrene7b (0.32 g, 0.95 mmol), Pd(PPh3)4 (19 mg, 0.016 mmol),
CuI (6 mg, 0.032 mmol), degassed Et3N (10 mL), and THF (10
mL). The mixture was stirred at 55 °C for 10 min. A solution of
tetrayne 6 (80 mg, 0.16 mmol) in THF (5 mL) was added drop-
wise for 45 min, and stirring was continued for 2 days. After cooling
to rt, the mixture was filtered, and yellow solid was repeatedly
washed with CH2Cl2. It was recrystallized from a hot 1,1,2,2-
tetrachloroethane: Yield 53 mg (25%); mp 292 °C; IR (neat) 2195
cm-1; UV-vis (CH2Cl2) (log ꢀ) 232 (5.268), 290 (5.145), 301
Acknowledgment. We thank SAIF, IIT Madras for the
spectral data; CSIR, New Delhi for financial support; Dr. M.
Vairamani, IICT for MALDI-TOF MS of 1; and Mr. G.
1
Venkataramana for the H NMR spectrum of 8 and the UV-
vis spectrum of 9. We are grateful to one of the reviewers for
thought-provoking comments and the revisions suggested.
1
(5.136), 347 (sh, 5.005), 378 (5.125), 405 nm (5.116); H NMR
Supporting Information Available: 1H NMR, electronic
absorption and fluorescence spectroscopic data of 1, crystallographic
(25) In spite of repeated purification by recrystallization, elemental
analysis of compounds 6 and 1 always gave lower values for the carbon
content, presumably due to incomplete combustion of these carbon-rich
molecules. Formation of black soot during the violent decomposition of 6
was evident during its melting point determination.
1
data of 5 in CIF format, H and 13C NMR spectra of 1-6. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
JO062173Y
944 J. Org. Chem., Vol. 72, No. 3, 2007