Synthesis, characterization, and spectroscopy of 4,7,12,15-[2.2]paracyclophane containing donor and acceptor groups: Impact of substitution patterns on through-space charge transfer

Article abstract of DOI:10.1021/ja0121383

This paper reports the synthesis of 4,7,12,15-tetra(4′ -dihexylaminostyryl)[2.2]paracyclophane (1), 4-(4′-dihexylaminostyryl)-7,12,15-tri(4″-nitrostyryl) [2.2]paracyclophane (2), 4,7-bis(4′- dihexylaminostyryl)-12,15-bis(4″-nitrostyryl)-[2.2]paracyclophan

Full text of DOI:10.1021/ja0121383

Published on Web 04/10/2002
Synthesis, Characterization, and Spectroscopy of
4,7,12,15-[2.2]Paracyclophane Containing Donor and Acceptor
Groups: Impact of Substitution Patterns on Through-Space
Charge Transfer
Glenn P. Bartholomew and Guillermo C. Bazan*
Contribution from the Departments of Chemistry and Materials and Institute for Polymers and
Organic Solids, UniVersity of California, Santa Barbara, California 93106
Received September 6, 2001. Revised Manuscript Received February 8, 2002
Abstract: This paper reports the synthesis of 4,7,12,15-tetra(4′-dihexylaminostyryl)[2.2]paracyclophane (1),
4-(4′-dihexylaminostyryl)-7,12,15-tri(4′′-nitrostyryl)[2.2]paracyclophane (2), 4,7-bis(4′-dihexylaminostyryl)-
12,15-bis(4′′-nitrostyryl)-[2.2]paracyclophane (3), 4,7,12-tris(4′-dihexylaminostyryl)-15-(4′′-nitrostyryl)[2.2]-
paracyclophane (4), 4,15-bis(4′-dihexylaminostyryl)-7,12-bis(4′′-nitrostyryl)[2.2]paracyclophane (5), and 4,12-
bis(4′-dihexylaminostyryl)-7,15-bis(4′′-nitrostyryl)[2.2]paracyclophane (6). These molecules represent different
combinations of bringing together distyrylbenzene chromophores containing donor and acceptor groups
across a [2.2]paracyclophane (pCp) bridge. X-ray diffraction studies show that the lattice arrangements of
1 and 3 are considerably different from those of the parent chromophores 1,4-bis(4′dihexylaminostyryl)-
benzene (DD) and 1,4-di(4′-nitrostyryl)benzene (AA). Differences are brought about by the constraint by
the pCp bridge and by virtue of chirality in the “paired” species. The absorption and emission spectra of
1-6 are also presented. Clear evidence of delocalization across the pCp structure is observed. Further, in
the case of 2, 3, and 4, emission from the second excited state takes place.
Introduction
Optical signatures have been used to characterize the geometry
and phase transitions of gas-phase van der Waals clusters.10
Through-space delocalization between optically and electroni-
cally active organic molecules constitutes a broad area of
chemical research. In the area of optoelectronic materials, the
charge transport ability and optical performance are controlled
by the spatial relationship between molecules that form the bulk
material.1-3 Substantial efforts to control the distance and
orientation between molecular subunits4,5 have resulted in novel
functions and properties such as organic injection lasers6 and
superconductivity.7 The function of important biological pro-
cesses such as photosynthesis8 and oxidative DNA damage9
depends on energy transfer processes between organic subunits.
Defining the extent of molecule-molecule interactions in
conjugated molecules with precise dimensions,11 and controlling
these interactions, will also bear an influence on the area of
molecular electronics.12,13
Molecules built around the [2.2]paracyclophane (pCp) skel-
eton14 have proven useful for studying chromophore-chro-
mophore interactions.15 Some of these structures are designed
to mimic through-space delocalization in conjugated organic
materials.16,17 Others probe intramolecular charge transfer (ICT)
across the transannular gap and thus the properties of donor/
acceptor groups separated through space.18 In this context, 4-(4′-
dihexylaminostyryl)-16-(4′′-nitrostyryl)-[2.2]paracyclophane (4D-
16A-pCp) was recently synthesized, and its optical and nonlinear
optical properties were correlated against theoretical analysis.19
* To whom correspondence should be addressed. E-mail: bazan@
chem.ucsb.edu.
(1) Li, X.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti, S. C.; Feeder,
N.; Clegg, W.; Teat, S. J.; Friend, R. H. J. Am. Chem. Soc. 1998, 120,
2206.
(2) McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.; O’Regan, M. B.;
Bazan, G. C.; Srdanov, V. I.; Heeger, A. J. AdV. Mater. 1999, 11, 1349.
(3) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals;
Clarendon Press: New York, 1982.
(4) Desiraju, G. R. In ComprehensiVe Supramolecular Chemistry, Solid-State
Supramolecular Chemistry: Crystal Engineering; Atwood, J. L., Davies,
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1996; Vol. 6, pp 1-22 and references therein.
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10.1021/ja0121383 CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 5183-5196
5183

A R T I C L E S
Bartholomew and Bazan
Scheme 1
molecules may show multidirectional ICT states. Conceptually,
our molecular design involves bringing together distyrylbenzene
chromophores with their cofacial contact defined by the 4,7,-
12,15-tetrasubstitution pattern in pCp (Scheme 1). The three
parent distyrylbenzene chromophores are 1,4-bis(4′dihexylamino-
styryl)benzene (DD), 1-(4′-dihexylaminostyryl)-4-(4′′-nitrostryl)-
benzene (DA), and 1,4-di(4′-nitrostyryl)benzene (AA).
Unambiguous “through-space” charge transfer was evident by
the 3-fold increase of âEFISH for 4D-16A-pCp relative to the
additive parallel superposition of the corresponding substituted
stilbenes the pCp interconnects.20 This increase in nonlinear
efficiency was not accompanied by the usual red-shift in the
absorption spectrum commonly seen with typical D/A through-
bond coupling schemes.
Of the seven possible unique AA, DD, DA combinations,
six are presented in this contribution (Scheme 2). With four
donor groups, 4,7,12,15-tetra(4′-dihexylaminostyryl)[2.2]para-
cyclophane (1) can be viewed as the pairing of two DD
chromophores. For 4-(4′-dihexylaminostyryl)-7,12,15-tri(4′′-
nitrostyryl)[2.2]paracyclophane (2) and 4,7,12-tris(4′-dihexy-
laminostyryl)-15-(4′′-nitrostyryl)[2.2]paracyclophane (4), a di-
polar DA chromophore is matched with a symmetrical acceptor
(AA) and donor unit (DD), respectively. Molecule 4,7-bis-
(4′-dihexylaminostyryl)-12,15-bis(4′′-nitrostyryl)[2.2]para-
cyclophane (3) represents a close contact between AA and DD.
In 4,15-bis(4′-dihexylaminostyryl)-7,12-bis(4′′-nitrostyryl)[2.2]-
paracyclophane (5) and 4,12-bis(4′-dihexylaminostyryl)-7,15-
bis(4′′-nitrostyryl)[2.2]paracyclophane (6), we see two different
orientations for the overlap of two DA chromophores. The
seventh permutation, (4,7,12,15)tetra(4′-nitrostyryl)[2.2]para-
cyclophane, is excluded from the study due to its extraordinary
insolubility.
In addition to the synthesis of 1-6, we present a study of
the impact on the solid-state arrangements by the covalent
constraint introduced by pCp, relative to the individual chro-
mophores. The linear spectroscopy of 1-6 is also presented
and reveals competing relaxations between the first and second
excited states. The evaluation of the (4,7,12,15)-substitution
pattern in the design of multipolar nonlinear optical chro-
mophores will be published elsewhere.21
Within 4D-16A-pCp, the ICT is mediated by the π-π stack,
defined by the pCp core, and is unidirectional, from the
dihexylamino group to the nitro functionality. In this contribu-
tion, we turn our attention to pCp chromophores with more
complex combinations of donor and acceptor groups. These
(15) Chromophores studied in this manner include: (a) phenanthrenophane,
Schweitzer, D.; Hausser, K. H.; Haenel, M. Chem. Phys. 1978, 29, 181.
(b) anthracenophane, Ishikawa, S.; Nakamura, J.; Iwata, S.; Sumitami, M.;
Nagakura, S.; Sakata, Y.; Misumi, S. Bull. Chem. Soc. Jpn. 1979, 52, 1346.
(c) fluorenophane, Haenel, M. W. Tetrahedron Lett. 1976, 36, 3121. (d)
Colpa, J. P.; Hausser, K. H.; Schweitzer, D. Chem. Phys. 1978, 29, 187.
(e) pyrenophane and several isomers of naphthalenophane, Haenel, M.;
Staab, H. A. Chem. Ber. 1973, 106, 2190. Otsubo, T.; Mizogami, S.; Osaka,
N.; Sakata, Y.; Misumi, S. Bull. Chem. Soc. Jpn. 1977, 50, 1858-1862.
(f) stilbenophanes, Anger, I.; Sandros, K.; Sundahl, M.; Wennerstro¨m, O.
Cis-Trans Photoisomerization of Bis-stilbenes with Ethylene Bridges. J.
Phys. Chem. 1993, 97, 1920-1923. Tsuge, A.; Nishimoto, T.; Uchida, T.;
Yasutake, M.; Moriguchi, T.; Sakata, K. Synthesis of Small-Sized Stil-
benophanes and Their Transannular Delocalization. J. Org. Chem. 1999,
64, 7246-7248.
(16) (a) Bartholomew, G. P.; Bazan, G. C. Acc. Chem. Res. 2001, 34, 30. (b)
Oldham, W. J.; Miao, Y.-J.; Lachicotte, R. J.; Bazan, G. C. J. Am. Chem.
Soc. 1998, 120, 419. (c) Bazan, G. C.; Oldham, W. J.; Lachicotte, R. J.;
Tretiak, S.; Chernyak, V.; Mukamel, S. J. Am. Chem. Soc. 1998, 120, 9188.
(17) (a) Wang, S.; Bazan, G. C.; Tretiak, S.; Mukamel, S. J. Am. Chem. Soc.
2000, 122, 1289. (b) Verdal, N.; Goodbout, J. T.; Perkins, T. L.;
Bartholomew, G. P.; Bazan, G. C.; Myers-Kelly, A. Chem. Phys. Lett. 2000,
320, 95.
(18) (a) Staab, H. A.; Rebafka, W. Chem. Ber. 1977, 110, 3333. (b) Staab, H.
A.; Haffner, H. Chem. Ber. 1977, 110, 3358. (c) Staab, H. A.; Starker, B.;
Krieger, C. Chem. Ber. 1983, 116, 3831. (d) Staab, H. A.; Schanne, L.;
Krieger, C.; Weiser, J.; Tagliever, V. Chem. Ber. 1985, 118, 1204. (e)
Tashiro, M.; Koya, K.; Yamato, T. J. Am. Chem. Soc. 1983, 105, 6650. (f)
Machida, H.; Tatemitsu, H.; Sakata, Y.; Misumi, S. Tetrahedron Lett. 1978,
915. (g) Review: Keehn, P. M. In Cyclophanes; Keehn, P. M., Rosenfeld,
S. M., Eds.; Academic Press: New York, 1983. (h) Review: Schwartz,
M. H. J. Inclusion Phenom. Mol. Recognit. Chem. 1990, 9, 1.
(19) Zyss, J.; Ledoux, I.; Volkov, S.; Chernyak, V.; Mukamel, S.; Bartholomew,
G. P.; Bazan, G. C. J. Am. Chem. Soc. 2000, 122, 11956.
(20) For a discussion on hyperpolarizability as a sensitive measure of intramo-
lecular charge-transfer, see: Zyss, J. J. Chem. Phys. 1979, 71, 446.
(21) Bartholomew, G. P.; Ledoux, I.; Zyss, J.; Bazan, G. C., unpublished work.
9
5184 J. AM. CHEM. SOC. VOL. 124, NO. 18, 2002

Synthesis of 4,7,12,15-[2.2]Paracyclophane
A R T I C L E S
Scheme 2
Results and Discussion
sonicated for 5 days at 60 °C (eq 1). Sonication is thought to
counter the low solubility of the paracyclophane starting
material. This reaction leads to two isomers, specifically the
desired para-para (7) and the ortho-ortho ((4,5,12,13)tetra-
(bromomethyl)[2.2]paracyclophane). Successive triturations in
chloroform give the desired isomer in 20% yield. The (4,7,-
12,15)-substitution pattern was confirmed by single-crystal
X-ray diffraction (Supporting Information). Crystallographic
parameters can be found in Table 1. Compound 7 was readily
converted into 8 by heating to 120 °C in triethyl phosphite (eq
2).
Horner-Emmons Precursor for Tetra-Substituted Para-
cyclophane. Previous experience with tetra-substituted pCp has
shown that the palladium-mediated Heck reaction,22 often used
in the formation of phenylenevinylene linkages,23 results in
poor yields and a distribution of partly coupled products. Yields
can be as low as 18% when coupling a moderately soluble
and reactive vinyl styrene to (4,7,12,15)tetrabromo[2.2]para-
cyclophane24 under these conditions, even after careful optimi-
zation of solvent, temperature, and reaction time.17 One concern
is that nitrobenzene substitution tends to reduce solubility and
would inhibit complete substitution by the Heck route after
attachment of one or more of these groups.
To address this challenge, we examined a Horner-Emmons
coupling25 precursor to obtain the desired products, starting with
the diethylphosphonatemethyl-substituted paracyclophane pre-
cursor. This route has the following advantages: (1) the reaction
byproducts are water soluble and enable simpler purification,
(2) the phosphonate-substituted pCp is of excellent solubility
in typical solvents, and little precipitation of intermediates is
expected, and (3) these couplings are typically high yield and
produce mainly the E isomer.25
Attempts to use sodium hydride as a base for 8 in THF and
DMF were unsuccessful and led to a mixture of products.
Sodium ethoxide was equally unsatisfactory. Coupling of 4.2
equiv of 4-dihexylaminobenzaldehyde to 8 using potassium
t-butoxide as the base leads to 95% yield of 1 after column
chromatography of the crude (eq 3). Alternatively, liquid-liquid
extraction of the crude followed by successive recrystallizations
in hexanes lead to an 86% yield of bright orange cube-shaped
crystals that were of sufficient quality for a single-crystal X-ray
diffraction experiment. The results of this study are shown in
Figure 1. A complete description of the molecular strucure and
the crystal lattice can be found in the following section. When
compared to alternative protocols, the high yield and purity of
the product obtained by Horner-Emmons precursor route are
noteworthy.
The first synthetic objective was (4,7,12,15)tetra(bromomethyl)-
[2.2]paracyclophane (7) as the precursor to (4,7,12,15)-tetra-
(diethylphosphonatemethyl)[2.2]paracyclophane (8). A route to
7 was developed using sonication, instead of stirring, in the
bromomethylation of paracyclophane. Paracyclophane and an
excess of paraformaldehyde were sealed in a thick-walled
Schlenk bomb in a 33% HBr solution in acetic acid and
(22) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009.
(23) (a) Oldham, W. J., Jr.; Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc.
1998, 120, 2987. (b) Sengupta, S.; Sadhukhan, S. K. Tetrahedron. Lett.
1999, 40, 9157. (c) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed.
1999, 38, 1350.
(24) Reich, H. J.; Cram, D. J. J. Am. Chem. Soc. 1969, 91, 3534.
(25) Wadsworth, W. S. Org. React. 1977, 25, 73.
9
J. AM. CHEM. SOC. VOL. 124, NO. 18, 2002 5185

A R T I C L E S
Bartholomew and Bazan
Table 1. Crystal Data and Refinement Parameters for 7, DD, 1, and 3
crystal parameters
7
DD
1
3
chemical formula
C20H20Br4
580.00
triclinic
Ph1 (No. 2)
4
C46H68N2
649.02
monoclinic
P21/c
C96H140N4
1350.12
monoclinic
C2/c
C72H88N4O4
1073.46
triclinic
Ph1 (No. 2)
2
formula weight
crystal system
space group (No.)
Z
8
4
a, Å
b, Å
c, Å
8.6099(6)
22.4743(14)
11.6272(7)
31.793(2)
90
106.5030(10)
90
31.858(3)
10.201(3)
12.508(4)
24.799(8)
101.804
97.221(6)
96.463(7)
3039(2)
13.3474(10)
17.2548(13)
101.1140(10)
97.1880(10)
90.1080(10)
1929.7(2)
16.2284(13)
21.780(2)
90
131.1180(10)
90
R, deg
â, deg
γ, deg
volume, Å3
7965.7(9)
1.082
8483.1(12)
1.057
F
calc, Mg/m3
1.996
1.173
crystal dimens, mm
temp, Κ
0.40 × 0.40 × 0.04
0.53 × 0.27 × 0.10
0.53 × 0.40 × 0.13
0.11 × 0.08 × 0.08
293
202
202
202
2θ range for data
total reflections
2-25
17 068
2-50
68 503
2-50
28 312
2-50
25 164
independent reflections
no. obsd data
6766[Ri(F2) ) 0.0512]
6755 (I > 2_(I))
433
14 011[Ri(F2) ) 0.0709]
14 011 (I > 2_(I))
873
5204[Ri(F2) ) 0.0718]
5163 (I > 2_(I))
512
10 609[Ri(F2) ) 0.0862]
10 601 (I > 2_(I))
725
no. parameters varied
µ, mm-1
8.341
0.061
0.060
0.072
R1(F), wR2(F 2), (I > 2σ(I))
R1(F), wR2(F 2), all data
GOF on F 2
0.0709, 0.1561
0.1094, 0.1649
1.498
0.0428, 0.1002
0.1136, 0.1259
0.842
0.1037, 0.2375
0.1650, 0.2678
2.343
0.0759, 0.1619
0.1934, 0.1859
1.055
The reaction in eq 5 was analyzed by HPLC using a nitro-
derivatized silica column. Small amounts of 1 and several
unidentified products were formed in this reaction along with
one major fraction (68% yield with respect to 8 after HPLC
separation). NMR spectroscopy and mass spectrometry (MS)
of this fraction were consistent with a two-donor coupled
product. MS offered further insight into the absolute configu-
ration of this single regioisomer. Fragmentation of pCp under
mild electron ionization (EI) conditions shows predominately
p-xylylene ion radicals.26 A large ion peak at m/z ) 673 was
observed that is consistent with the two-donor coupled p-
xylylene ion. These data led to the assignment of the major
fraction from DMF as (4,7)bis(diethylphosphonatemethyl)-(12,-
15)bis(4′-dihexylaminostyryl)[2.2]paracyclophane (9). Further
reaction with an excess of 4-nitrobenzaldehyde and potassium
t-butoxide leads to 84% yield of 3 (eq 6). The substitution
assigned was fully confirmed by a single-crystal X-ray diffrac-
tion study of 3 (Figure 2).
Figure 1. ORTEP drawing of 1. Ellipsoids are shown at 20% probability,
and hydrogens are omitted for clarity.
Compound 2, a less symmetric target, was obtained by
stepwise addition of 1 equiv of potassium t-butoxide and
4-dihexylaminobenzaldehyde to 8 followed 24 h later by excess
4-nitrobenzaldehyde and 3 equiv of potassium t-butoxide (eq
4). Purification was achieved by HPLC separation performed
on a nitro-derivatized silica column providing a 68% yield. The
best results are obtained for this separation using a gradient of
chloroform in hexanes.
The simultaneous synthesis of the three regioisomers of two-
donor/two-acceptor coupling, specifically 3, 5, and 6, was
attempted starting with 8. Two equivalents of 4-dihexylami-
nobenzaldehyde were coupled to 8 with potassium t-butoxide
in DMF (eq 5).
The two-donor coupling to 8 in the less polar solvent THF
resulted in two fractions by HPLC, which could be confirmed
(26) Reich, H. J.; Cram, D. J. J. Am. Chem. Soc. 1969, 91, 3534.
9
5186 J. AM. CHEM. SOC. VOL. 124, NO. 18, 2002

Synthesis of 4,7,12,15-[2.2]Paracyclophane
A R T I C L E S
The latter condition would require carbanion exchange/
equilibration during the time scale of the reaction. Regardless
of the exact origin of the observed selectivity, this system
reflects a unique interplay of factors that allows for regiochemi-
cal control by choice of reaction solvent.
Compound 4, with three 4-dihexylaminostyryl and one
4-nitrostyryl group, was not obtained from the tetraphosphonate
precursor by successive couplings in a manner similar to 2.
HPLC analysis of attempts to couple 1 equiv of nitrobenzal-
dehyde to 8 in both THF and DMF revealed a broad distribution
of partially coupled products. Attempts to first couple 3 equiv
of dihexylaminobenzaldehyde invariably lead to a distribution
of intermediates along with a substantial amount of 1.
Bromo/Formyl Combinations for Site-Specific Coupling.
Limitations to the use of 8 for the synthesis of 4 and 5 lead us
to search for a complimentary tetrasubstitution precursor.
Monolithiation of (4,7,12,15)tetrabromo-[2.2]paracyclophane27
(12) followed by DMF quenching yields [4,7,12]tribromo-15-
formyl[2.2]paracyclophane (13) (eq 7). Flash chromatography
on silica support provided purification of the product for a yield
of 37%. The NMR and MS spectra were consistent with the
proposed structure, and fragmentation under EI conditions for
MS showed ions that are consistent with both a dibromo and
bromo-formyl substituted p-xylylene radicals (m/z ) 262 and
210, respectively).
Figure 2. ORTEP drawing of 3. Ellipsoids are shown at 30% probability,
and hydrogens are omitted for clarity.
Scheme 3
as two-donor coupling products by MS. The first fraction was
9 in 23% yield. Examination of the fragmentation of the second
fraction showed a large ion peak at m/z ) 540 which would be
consistent with the structure of (4,12)bis(diethylphosphonate-
methyl)-(7,15)bis(4′-dihexylaminostyryl)[2.2]paracyclophane (10,
in eq 5) or (4,17)bis(diethylphosphonatemethyl)-(7,12)bis(4′-
dihexylaminostyryl)[2.2]paracyclophane (11), where each pCp
ring is substituted with one donor styrene and a benzyl
diethylphosphonate. Results from a 1H NMR COSY experiment
for the second fraction were consistent with the structure shown
for 10 (see Supporting Information). Further reaction of 10 with
4-nitrobenzaldehyde and potassium t-butoxide afforded 6.
Interestingly, no fraction in either the DMF or the THF reaction
was found to correspond with 11.
Compound 13 was then subjected to Heck coupling with 6
equiv of 4-dihexylaminostyrene. The reaction mixture was
separated using flash chromatography resulting in a 64% yield
of the desired (4,7,12)tris(4′-dihexylaminostryl)-15-formyl[2.2]-
paracyclophane (14) (eq 8).
To further investigate the regioselectivity of 8 toward two-
donor coupling, quantitative analytical HPLC was performed
for this reaction in both DMF and THF using samples of 9 and
10 as external standards. As solvent polarity increases (i.e.,
DMF), selectivity for 9 increases (68% and 10% of 9 and 10,
respectively), while reducing polarity (THF) leads to 10 as the
major product (23% and 57% of 9 and 10, respectively). In an
attempt to understand the factors at work in this selectivity,
consider a situation in which the first coupling of donor
benzaldehyde is complete and the product distribution of the
reaction is determined solely by the second coupling step. The
three remaining reaction sites are shown and labeled in Scheme
3. Site III is ruled out on the basis of steric interference from
the dihexylaminostyryl group. Formation of 11 is thus not
observed. In the more polar solvent DMF, reaction at site I is
preferred, leading to the formation of 10. In THF, reaction at
site II is considerably enhanced, while site I remains somewhat
active. It is not clear whether this reversal in selectivity is due
to different kinetic preferences in the deprotonation step or to
differences in reactivity of the carbanion toward the aldehyde.
Compound 14 was then subjected to a Horner-Emmons
coupling with 4-nitrophenylmethanephosphonate using potas-
sium t-butoxide as base (eq 9). Again, this reaction could be
purified by flash chromatography for a yield of 81% for this
step.
(27) Reich, H. J.; Cram, D. J. J. Am. Chem. Soc. 1969, 91, 3527.
9
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Figure 3. A comparison of 2D-COSY proton spectra for compounds (a) 16 and (b) 17. The methylene regions of both COSY spectra which contain
resonances for the protons bound to the paracyclophane bridging carbons are displayed.
A similar approach was used to obtain 5 and 6. Compound
12 was treated with 2 equiv of n-BuLi followed by quenching
with DMF (eq 10). Surprisingly, only two isomers were
obtained. The fragmentation of these isomers under EI during
MS gave evidence that the two isomers obtained were 4,15-
dibromo-7,12-diformyl[2.2]paracyclophane (16) and 4,12-di-
bromo-7,15-diformyl[2.2]paracyclophane (17). The mass spec-
trum of 15 would contain major ion peaks at m/z ) 160 and
262, neither of which were observed for the two products.
Instead, large peaks at m/z ) 210 corresponding to the
bromoformyl-p-xylylene radical ion were recorded in the mass
spectra of the two products. The absence of 15 would be
consistent with the higher energy associated with two negative
charges created by dilithiation on a single benzene ring relative
to one charge on each ring of paracyclophane to yield 16 and
17. These two isomers could be further differentiated by
2-dimensional 1H NMR spectroscopy. Specifically, the C2 axis
of symmetry lies differently in both isomers with respect to the
bridging protons. For 16, the four protons on a single ethylene
bridge are inequivalent; therefore the COSY spectrum in Figure
3a shows correlation between all the bridging protons. In the
case of 17, due to the orientation of the C2 axis, there are only
two inequivalent protons on a single arm. As shown in Figure
3b, this relationship results in a much simpler 2-D spectrum.
The yields for 16 and 17 were 55% and 34%, respectively.
5 and 6, respectively. For example, the reaction of 16 with ex-
cess 4-dihexylaminostyrene under Heck conditions for 24 h leads
to a 93% yield of (7,12)-diformyl-(4,15)-(4′-dihexylaminostyryl)-
[2.2]paracyclophane (18) after flash chromatography (eq 11).
The final step in the synthesis of 5 requires coupling of 18
to 4-nitrophenylmethanephosphonate with potassium t-butoxide
as base in THF for a 79% yield (eq 12). Performing the reactions
shown in eqs 11 and 12, but starting with 17, provides access
to compound 6.
Synthesis of 1-(4′-Dihexylaminostyryl)-4-(4′′-nitrostryl)-
benzene (DA), 1,4-Di(4′-nitrostyryl)benzene (AA), and 1,4-
Bis(4′dihexylaminostyryl)benzene (DD). The parent chro-
mophores were synthesized according to previously reported
procedures.28-30 For example, DD was prepared by a coupling
of 2 equiv of 4-dihexylaminobenzaldehyde with R,R′-diphos-
phonate-p-xylene. The synthesis of DA consisted of a stepwise
coupling of 1 equiv of dihexylaminobenzaldehyde with R,R′-
Compounds 16 and 17 were both subjected to Heck coupling
with the donor styrene followed by a Horner-Emmons coupling
with the phosphonate precursor for the nitrostyryl group to yield
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Synthesis of 4,7,12,15-[2.2]Paracyclophane
A R T I C L E S
diphosphonate-p-xylene, followed by 1 equiv of 4-nitrobenzal-
dehyde. The crystal structure of DD was obtained from a crystal
grown from hexanes (Supporting Information). Crystallographic
parameters can be found in Table 1, and a full description of
the molecule and the lattice is found in the following section.
Crystallographic Packing Features. We have previously
reported the crystal structure and lattice properties of a wide
variety of distyrylbenzene derivatives.31,32 In this section, we
present structural data for “dimerized” chromophores 1 and 3
and compare the arrangement of molecules in the lattice against
the parent structures DD and AA. The “pairing” of chro-
mophores into a single pCp molecule creates a unique situation
in which the basic unit of the packing unit has been constrained
by covalent attachment. The features that are of greatest interest
are those that reflect the disposition of the conjugated distyryl-
benzene fragments relative to one another; thus we omit a
detailed description of the exact arrangement of aliphatic
solubilizing groups.
Intramolecular metrical parameters are normal for the DD
molecular structure, and the terminal rings deviate from planarity
with the central ring by ∼25°.33 The molecules stack at an
oblique angle, and neighboring stacks are oriented such that
the long axes of the molecules in neighboring stacks are roughly
perpendicular. This orientation leads to a criss-cross appearance
to the phenylenevinylene portion of the chromophore when
viewed along the a-axis (Figure 4). In this view of the lattice,
the hexyl chains are omitted. With this view, a layered structure
is revealed alternating between the distyrylbenzene portion of
the chromophore and the hexyl groups. Although no conspicuous
close contacts are present, the formation of layers between the
phenylenevinylene and alkyl regions reflects an advantage to
separation over a structure where they are more continuous.
The structure and lattice of AA‚2 DMF has been described
previously31 and will be covered only briefly here. One of the
most characteristic features of this structure is the overlapping
nitrostryl subunits of the molecule, where the NO2 group is
adjacent to the olefin of its neighbor. This overlap for AA‚2
DMF leads to a bricklike structure that forms channels in which
the two DMF molecules are situated.
Single cube-shaped crystals of 1 were grown from hexanes.
The methylene bridge of the pCp core is twisted with a torsion
angle for the ethyl bridge of 25.24(3)°. There is a crystallo-
graphically imposed C2 axis, with the terminal rings deflected
from planarity from the central ring by ∼10° and ∼14°. This
difference is a consequence of inequivalent environments for
the two ends of the parent chromophore due to the orientation
of one enantiomer relative to the other. In comparison with the
conformation suggested in Scheme 2, the actual structure
features the twisted core accompanied by a 180° rotation of
Figure 4. Packing diagram of DD viewed down the a-axis. The hexyl
carbons are not shown for clarity.
the pCp-olefin bond. This change brings the long axis of the
bound parent chromophores closer to a parallel arrangement than
the proposed structure.
The molecules of 1 are arranged in the lattice within
“interlocking” columns that run along the b-axis with the
terminal hexyl groups intertwined in a complex fashion.
Molecules in any one column are a single enantiomer with the
other enantiomer located in the closely neighboring column.
Figure 5 shows a view along the c-axis that features a
superposition of the enantiomeric columns. With this view, a
layered structure appears with interlocking chromophore regions
where the hexyl groups fill the void between layers.
Deep red crystals of 3 suitable for X-ray diffraction were
grown from acetone. Figure 6a shows the enantiomeric pair
without the hexyl groups for clarity. Again the pCp-based
compound displays a twisted core and an alignment of the two
parent chromophores closer to a parallel arrangement that one
might initially expect. Deviations from planarity for the styryl
groups relative to the central ring to which they are attached
are much more complex than those in 1. Angles range from
2.476(4)° for one nitrostyryl group to 28.578(4)° for the
dihexylaminostryl on the opposing face. Overall, these devia-
tions follow the pattern seen in 1 where styryl groups on one
face are more distorted than the other (2.476(4)° and 7.687(4)°
for one face, 28.578(4)° and 19.255(4)° for the other).
Further inspection of Figure 6a shows a reciprocal overlap
of nitro groups with the neighbor’s olefin groups, a configuration
(28) Wadsworth, D. H.; Schupp, O. E.; Seus, E. J., Ford, J. A., Jr. J. Org. Chem.
1964, 30, 680.
(29) Nakatsuji, S.; Matsuda, K.; Uesugi, Y.; Nakashima, K.; Akiyama, S.; Katzer,
G.; Fabian, W. J. Chem. Soc., Perkin Trans. 2 1991, 6, 861.
(30) Rumi, M.; Ehrlich, J. E.; Heikal, A. A.; Perry, J. W.; Barlow, S.; Hu, Z.;
McCord-Maughon, D.; Parker, T. C.; Ro¨ckel, H.; Thayumanavan, S.;
Marder, S. R.; Beljonne, D.; Bre´das, J.-L. J. Am. Chem. Soc. 2000, 122,
9500.
(31) Bartholomew, G. P.; Bazan, G. C.; Bu, X.; Lachicotte, R. J. Chem. Mater.
2000, 12, 1422.
(32) (a) Wu, G.; Jacobs, S.; Lenstra, A. T. H.; Van Alsenoy, C.; Geise, H. J. J.
Comput. Chem. 1996, 17, 1820. (b) Van Hutton, P. F.; Wildeman, J.;
Meetsma, A.; Hadziioannou, G. J. Am. Chem. Soc. 1999, 121, 5910.
(33) Torsional distortions in stilbenoid compounds are characterized by a
relatively flat potential energy surface, see: Hoekstra, A.; Meertins, P.;
Vos, A. Acta Crystallogr. 1975, B31, 2813.
9
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A R T I C L E S
Bartholomew and Bazan
smaller Stokes shift (20 nm) relative to the data from CHCl3
(135 nm). The absorption spectra of DA in hexanes are
characterized by two strong absorption bands (Figure 8) with
λmax ) 433 nm and a smaller band at 330 nm. The emission
spectrum is structured and has a maximum at 530 nm.
Figure 9 shows the emission and absorption spectra for 1 in
hexanes and CHCl3. Emission in hexanes is structured and
exhibits a relatively small Stokes shift (λmax(abs) ) 431 nm,
λmax(em) ) 477 nm). In CHCl3, the structure in the emission is
lost, and the Stokes shift increases (λmax(abs) ) 440 nm, λmax(em)
) 560 nm). This solvatochromatic shift is consistent with an
excited state that is more polarized than the ground state. The
excitation spectra in both solvents match the absorption spectra.
Note that the absorption maximum in hexanes is red-shifted by
30 nm relative to DD. This extended conjugation due to the
pCp junction has been observed in similar “criss-cross” delo-
calized pCp chromophores.17
Figure 10 contains the absorption and emission of 5 and 6 in
hexanes. These two chromophores correspond to regioisomers
of DA dimerization via pCp. The first thing to note is the red-
shift in absorption for these compounds relative to DA. The
absorption maxima for 5 and 6 are 470 and 460 nm, respectively,
which contrast to the maximum for DA at 433 nm. Cofacial
overlap of the central rings of the parent chromophores leads
to extended conjugation and additional charge transfer from
donor and acceptor combinations across the pCp bridge. The
red-shift for the absorption and emission of 5 relative to 6 is of
additional interest. This difference is likely a result of a greater
stabilization of the charge redistribution by the dipole orienta-
tions of the “parent” chromophores. The dipoles of the parent
chromophores should result in a larger net dipole for the
arrangement found in 5 relative to 6.
Figure 5. Packing diagram from the crystal structure of 1 viewed along
the c-axis demonstrating the segregation of the distyrylbenzene portions
from the alkyl regions in an interlocking layered strucure. The hexyl groups
are not shown for clarity.
seen in the parent AA. Figure 6b shows the lattice with a view
along the c-axis and reveals the dihexylaminostyryl groups
opposite each other. The nitrostyryl interactions along the a-axis
and the end-to-end packing of the dihexylaminostyryl groups
running along the b-axis result in channels that propagate along
the c-axis. These channels are, in fact, where the alkyl groups
are contained. In essence, chains of overlapped constituent AA
chromophores “cross-link” the segregated layers formed by end-
to-end interactions among the DD portion of 3. After pairing
of the two parent chromophores, elements of their individual
packings appear to coexist. The high level of distortion within
the individual molecules of 3 in this lattice may reflect the
competition of these packing motifs.
Spectra for 3, the pairing of DD with an AA chromophore,
are presented in Figure 11. The maximum for absorption in
hexanes occurs at 417 nm, with a weak and broad tail in the
475-600 nm region. Emission from hexanes shows two bands.
One is centered at approximately 500 nm, while the maximum
of the more intense band appears around 690 nm. The ratio of
the two bands does not depend on sample history. In CHCl3,
there appears to be a blue-shift in the emission (∼600 nm),
while absorption is slightly red-shifted (440 nm). The overall
fluorescence quantum efficiency is low; in hexanes it is 0.5 (
0.3% and in chloroform 3 ( 2%.
Emission spectra, exciting at 420 nm, in different ratios of
hexanes and CHCl3 with a constant concentration of 3 were
collected (for spectra see Supporting Information). At 20%
CHCl3, the band at 690 nm in hexanes is essentially quenched,
while the 500 nm band remains. This higher energy band
appears to gradually red-shift as the concentration of CHCl3 is
increased to 50%. The integrated intensity of the band at ∼600
nm in 100% CHCl3 rivals that of the 690 nm band in hexanes.
These data show that the band in chloroform is a normal
solvatochromic shift of the band at ∼500 nm in hexanes.
Optical Spectroscopy. To provide a baseline measure of the
changes in the optical spectroscopy of 1-6, the absorption and
emission spectra of DD, AA, and DA are presented first. Figure
7 contains the spectra of DD in hexanes and AA in CHCl3.
The absorption λmax of DD is ∼400 nm, and the emission
maximum occurs at 437 nm. The spectroscopy of this molecule
is characterized by structured emission with a normal Stokes
shift. Because of the poor solubility of AA in hexanes, the
solution spectra were collected in chloroform. In CHCl3, AA
exhibits an absorption maxima at 400 nm, while λmax(em) ) 535
nm, a 135 nm Stokes shift. Spectroscopic data for AA in dioxane
have been reported previously34 and showed an absorption
maxima at 395 nm and λmax(em) ) 415 nm. This is a considerably
Excitation spectra observing at different emission wavelengths
in both hexanes and CHCl3 were collected. Observing fluores-
cence at 690 nm, we see the absorption spectrum reproduced
in the excitation spectrum. Observing at 520 nm, the excitation
spectrum no longer exhibits the beginning of the 475-600 nm
tail. The absence of this tail in the excitation spectrum in CHCl3,
observing at 600 nm, reveals a common origin for both the band
(34) Nakatsuji, S.; Matsuda, K.; Uesugi, Y.; Nakashima, K.; Akiyama, S.; Katzer,
G.; Fabian, W. J. Chem. Soc., Perkin Trans. 2 1991, 6, 861.
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Synthesis of 4,7,12,15-[2.2]Paracyclophane
A R T I C L E S
Figure 6. The enatiomeric pair of molecules (a) and packing diagram (b) from the crystal structure of 3 viewed along the c-axis. Note the overlapping
nitrostyryl groups. The long molecular axis of the dihexylamino-substituted portion of 3 is parallel to the b-axis, while the long axis of the nitro-derivatized
portion runs parallel to the a-axis. The hexyl groups, which are not shown for reasons of clarity, are contained in the channels that run parallel to the c-axis.
Figure 7. Normalized absorption (a) and emission (b) (exciting at 380
Figure 8. Normalized absorption (a) and emission (b), exciting at 433 nm,
nm) of DD in hexanes and absorption (c) and emission (d) (exciting at 400
of DA in hexanes.
nm) of AA in CHCl3.
quenched, and the remaining emission results from S2 f ground-
at 500 nm in hexanes and the ∼600 nm band in CHCl3. All of
state relaxation. The observed participation in hexanes of S2 in
these experiments were performed using material that showed
the emission from S1 indicates that the rates of fluorescence
a single peak by analytical HPLC using different solvent
combinations.35
decay and internal conversion to S1 are within the same order
of magnitude and thus competitive.
The fact that two bands are observed indicates that the
emission at 500 nm is from the second excited state (S2). The
overall efficiency of this process is low, in the order of 0.05%.
Emission at 690 nm is likely from a lower-lying ICT state (S1).36
As solvent polarity is increased, the polar ICT excited state is
To account for the spectroscopy of 3, we propose the
excitation process outlined in Scheme 4. The oscillator strength
for S1 excitation is considerably smaller than the S2 counterpart.
After excitation to S2, the molecule may relax to S1 or decay
by radiative and nonradiative paths directly from S2. The
partition of the two processes depends on the ratio of the sum
total of the rates of fluorescence from S2 (kF2) and nonradiative
relaxation from S2 (kNR2) (excluding internal conversion to S1)
to the rate of internal conversion to S1 (kIC). The magnitude of
(35) Two emission bands are also observed by single molecule spectroscopy.
(a) Sommers, M.; Bartholomew, G. P.; Buratto, S. K.; Bazan, G. C., work
in progress. (b) Mason, M. D.; Sirbuly, D. H.; Carson, P. J.; Buratto, S. K.
J. Chem. Phys. 2001, 114, 8119.
(36) Quantum mechanical calculations show substantial charge-transfer character
to the S1 state: Ottonelli, M.; Mukamel, S., work in progress.
9
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A R T I C L E S
Bartholomew and Bazan
Figure 9. Normalized absorption (a) and emission (b) of 1 in hexanes and
absorption (c) and emission (d) in CHCl3. Both emission scans were
collected exciting at 420 nm.
Figure 12. Normalized absorption (a), emission (b), and excitation (c) of
2 in hexanes. Emission data were collected exciting at 419 nm, and the
excitation was monitored at 530 nm.
Figure 10. Normalized absorption of 5 (a) and 6 (b) and emission of 5 (c)
and 6 (d) in hexanes. Emission data were both collected exciting at 460
nm.
Figure 13. Normalized absorption (a), emission (b), and excitation (c) of
2 in CHCl3. Emission data were collected exciting at 440 nm, and the
excitation was monitored at 605 nm.
Scheme 4
parent, AA. The most notable feature is the penetration of the
absorption band well into the emission band. Specifically, the
emission band begins at ∼450 nm, while emission trails well
past 550 nm. The excitation spectrum of 2 indicates that the
broad shoulder in the absorption spectrum, located roughly in
the 475-550 nm region, does not give rise to emission. The
emission from hexanes is structured. Emission from CHCl3 is
red-shifted to ∼600 nm, and the structure is lost (Figure 13).4
Again the low energy shoulder region is missing in the excitation
Figure 11. Normalized absorption (a) and emission (b), exciting at 417
nm, of 3 in hexanes and absorption (c) and emission (d), exciting at 431
nm, in CHCl3.
these rates, in addition to the ratios of kF2/kNR2 and kF1/kNR1 in
hexanes, is such that emission from S1 and S2 is observed. That
emission from S2 occurs is an interesting violation of Kasha’s
rule.37 In CHCl3, emission from S1 is quenched, and only the
residual S2 fluorescence is observed.
The absorption, emission, and excitation spectra of 2 in
hexanes are presented in Figure 12. Compound 2 represents the
pairing of a DA chromophore with the symmetric acceptor
spectrum observed at λmax(em)
.
The absorption, emission, and excitation spectra of 4, the
combination of DA with DD, are similar to those of 2 and are
presented in Figure 14. Note again the penetration of the
absorption shoulder well into the emission band in hexanes, as
well as the absence of that shoulder in the excitation spectrum.
(37) Klessinger, M.; Michl, J. Excited States and Photochemistry of Organic
Molecules; VCH Publishers: New York, 1995.
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Synthesis of 4,7,12,15-[2.2]Paracyclophane
A R T I C L E S
side groups, even in the presence of additional interchromophore
interactions and constraints. These guiding principles may prove
useful in the design of organic solids with optimized interchro-
mophore delocalization and/or bulk ordering.
The disposition of donor and acceptor groups within the three-
dimensional conjugated fragment provided by the tetrastyryl-
pCp fragment gives rise to unexpected optical properties. For
1, 5, and 6, additional delocalization occurs, as expected on
the basis of previous work on through-space delocalized
paracyclophane molecules. However, in the case of 2, 3, and 4,
the relaxation from S2 to S1 by internal conversion is not
complete, and emission from S2 can be observed. Interestingly
for 3 in hexanes one observes emission from S2 and S1.
Changing solvents from hexanes to chloroform results in
quenching from S1, and the result is an anomalous blue-shift in
emission by switching to the more polar solvent. Scheme 4 can
be used to provide a simplified picture for these processes.
Molecules 2 and 4 operate similarly; however, no emission from
S1 has been observed under our experimental conditions.
Figure 14. Normalized absorption (a), emission (b), and excitation (c) of
4 in CHCl3. Emission data were collected exciting at 437 nm, and the
excitation was monitored at 550 nm.
Scheme 4 can also be used to account for the photophysics
of 2 and 4. Molecule 2 will be used to guide discussion.
Excitation in the region of 400-450 nm for 2 promotes the
molecule to S2. The low energy “shoulder” seen in the
absorption corresponds to S1 excitation. Nonradiative relaxation
dominates from S1, that is, kF1 , kNR1, and this is not an
emissive state. All of the emission thus originates from S2, and
kF2 + kNR2 are competitive (at least within 2 orders of
magnitude) with kIC.
Experimental Section
General Details. All synthetic manipulations were performed under
an inert atmosphere in a nitrogen filled glovebox or using Schlenk
techniques. All reagents were obtained from Aldrich and were used as
1
received. H NMR spectra were obtained using a Varian Unity 400
MHz spectrometer. 13C NMR and 2D-1H COSY spectra were obtained
using a Varian Unity INOVA 500 MHz spectrometer. High-resolution
mass spectra were performed on a VG-70SE double-focusing system
using electron ionization. 4-Dihexylaminostyrene40 and diethyl 4-ni-
trophenylmethanephosphonate28 were obtained by procedures described
earlier. Tetrahydrofuran used for reactions was dried by stirring with
Na metal overnight followed by degassing and vacuum transfer. HPLC
was performed using a Shimadzu instrument equipped with a photo-
diode array detector. Reaction monitoring and separation were achieved
with a nitro-derivatized silica HPLC column (ES Industries, catalog
no. 138213-NO2) with the exception of the dibromodiformyl[2.2]-
paracyclophane mixture which was separated on a standard silica HPLC
column. Quantitative HPLC analysis of the 2-fold donor coupling to 8
was performed by construction of 3-point calibration curves monitoring
at 400 nm.
Conclusion
In summary, the synthetic methodology to prepare molecules
1-6 is presented. These molecules correspond to the possible
regioisomers that result from bringing together distyrylbenzene
chromophores38 containing donor and acceptor groups across
the pCp bridge. The tetraphosphonate precursor 8 is a key
intermediate because of its versatility in generating molecules
1-3, and because it enables coupling with Horner-Emmons
procedures, which are of higher yields and easier to purify than
Heck protocols. For the preparation of 4, use of the monoformyl
tribromide derivative 13 is required. In the case of the
orientational isomers 5 and 6, we rely on 2-fold bromo-lithium
exchange, as shown in eq 10, which provides the requisite 16
and 17 precursors. The only missing combination of the series,
corresponding to four dinitrostyryl groups on pCp, is not
obtained because of its intrinsic low solubility in common
organic solvents.
By constraining the distance and orientation of two distyryl-
benzene chromophores by pCp, one observes lattice arrange-
ments different from those of the parent chromophores. The
principal reason is the intrinsic chirality of the dimers. Many
of the original crystal engineering “synthons” remain.39 In
particular, the alignment of electron-deficient nitro fragments
which prefer to rest adjacent to polarizable olefin groups is
retained. Additionally, the distrylbenzene chromophore portion
of these molecules readily segregates from the alkyl solubilizing
(4,7,12,15)-Tetra(bromomethyl)[2.2]paracyclophane (7). A 500
mL thick-walled Schlenk bomb with a wide-bore Teflon needle valve
was charged with 7.2 g (34.5 mmol) of [2.2]paracyclophane and 20.7
g (690 mmol in formaldehyde) of paraformaldehyde with ∼150 mL of
33% HBr in glacial acetic acid. After tightly sealing the vessel, the
mixture was sonicated at 60 °C for 5 days. The solids were collected
and then extracted in three portions with 500 mL of CHCl3 and washed
with 500 mL of 1.0 M CHCl3 in three portions. The crude yield was
44%. Purification was achieved by successive tritrations in warm CHCl3,
giving an off-white finely divided powder for a final yield of 4.1 g or
20%. 1H NMR (CDCl3): 6.596 (s, 4H), 4.324 (q, 8H), 3.441 (m, 4H),
2.932 (m, 4H). 13C (CDCl3): 138.463, 136.990, 133.765, 31.262,
31.066. HRMS-EI m/z ) 578.8290, ∆ ) 1.4 ppm.
(4,7,12,15)-Tetra(diethylphosphonatemethyl)[2.2]para-
cyclophane (8). A 50 mL 14/20 round-bottom flask was charged with
a Teflon-coated stir bar, 1.1 g (1.9 mmol) of 7, 1.37 mL (8 mmol) of
triethyl phosphite, and fitted with a short path distillation head with
receiving flask. Argon was passed through the thermometer port and
ran through an oil bubbler attached to the vacuum port. The reaction
was heated to 120 °C for 2 h. The reaction was then cooled to ∼50 °C
(38) (a) Heller, A. J. Chem. Phys. 1964, 40, 2839. (b) Tachelet, W.; Jacobs, S.;
Ndayikengurukiye, H.; Geise, H. J. Appl. Phys. Lett. 1994, 64, 2364. (c)
Yang, Z.; Geise, H. J.; Mehbod, M.; Debrue, G.; Visser, J. W.; Sonneveld,
E. J.; Van’t dack, L.; Gijbels, R. Synth. Met. 1990, 39, 137. (d) Strehmel,
B.; Sarker, A. M.; Malpert, J. H.; Strehmel, V.; Seifert, H.; Neckers, D. C.
J. Am. Chem. Soc. 1999, 121, 1226.
(39) (a) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311. (b)
Thalladi, V. R.; Weiss, H.-H.; Bla¨ser, D.; Boese, R.; Nangia, A.; Desiraju,
G. R. J. Am. Chem. Soc. 1998, 120, 8702.
(40) Oprea, S.; Still, R. H. J. Appl. Polym. Sci. 1976, 20, 639.
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A R T I C L E S
Bartholomew and Bazan
and fitted with a thermometer and placed under vaccuum. Excess
triethyl phosphite was removed under vacuum at 100 °C. The resulting
oil was washed with several portions of pentane and diethyl ether
affording a yellowish solid. The final yield was 98%. 1H NMR
(CDCl3): 6.459 (s, 4H), 3.895 (m, 16H), 3.482 (m, 4H), 3.072 (m,
4H), 2.770 (m, 8H), 1.175 (q, 24H). 13C (CDCl3): 140.142, 138.762,
134.191, 131.089, 62.307, 32.300, 31.514, 16.532. HRMS-EI: 809.3120,
∆ ) 1.0 ppm (for M + H)+.
mmol) of potassium t-butoxide dissolved in a minimal amount of dry
THF added again by syringe. The reaction was allowed to warm to
room temperature and to progress overnight. The reaction was quenched
with deionized water and extracted into 125 mL of CHCl3 and washed
with brine. The solvent was removed under vacuum, and the mixture
was separated using HPLC. The resulting yield was 23%. Alternatively,
reaction in DMF results in a 68% yield for the same procedure. The
regiochemistry was assigned by characteristic paracyclophane frag-
1
mentation during mass spectrometry. H NMR (CD2Cl2): 7.411 (d,
(4,7,12,15)-Tetra(4′-dihexylaminostyryl)[2.2]paracyclophane (1).
A 25 mL 14/20 round-bottom flask was charged with 153.4 mg (0.19
mmol) of 8, 275 mg (0.95 mmol) of 4-dihexylaminobenzaldehyde, a
Teflon-coated stirbar, and closed with a septum. Approximately 5 mL
of anhydrous DMF was added via syringe, and the mixture was stirred
under argon at 0 °C. Deprotonation of the phosphonate was achieved
with 96.5 mg (0.86 mmol) of potassium t-butoxide dissolved in a
minimal amount of dry DMF added again by syringe. The reaction
was allowed to warm to room temperature and stir overnight. The
resulting mixture was extracted with hexanes (250 mL) and washed
with three 250 mL portions of brine and then dried over MgSO4. After
filtration, the solvent was again reduced by rotary evaporation. Flash
chromatography eluting with 10% diethyl ether in hexanes produced
243 mg of the product as an orange powder for a yield of 95%.
Alternatively, the resulting orange oil from rotary evaporation was
recrystallized twice in hexanes and afforded orange cube-shaped crystals
of single-crystal X-ray diffraction quality for a final yield of 86% (214
mg). 1H NMR (CDCl3): 7.400 (d, 8H), 6.961 (m, 12H), 6.670 (d, 8H),
3.510 (m, 4H), 3.331 (t, 16H), 2.838 (m, 4H), 1.645 (m, 16H), 1.356
(m, 48H), 0.931 (m, 24H). 13C NMR (CDCl3): 147.774, 137.232,
136.781, 128.310, 128.106, 127.498, 125.641, 121.428, 111.872, 51.364,
33.316, 31.998, 27.574, 27.126, 22.945, 14.307. HRMS-FAB/NBA:
1349.1123, ∆ ) 3.3 ppm.
4-(4′-Dihexylaminostyryl)-7,12,15-tri(4′′-nitrostyryl)[2.2]-
paracyclophane (2). A 25 mL 14/20 round-bottom flask was charged
with 300 mg (0.37 mmol) of 8, 110 mg (0.37 mmol) of 4-dihexylami-
nobenzaldehyde, a Teflon-coated stirbar, and closed with a septum.
Approximately 5 mL of anhydrous DMF was added via syringe, and
the mixture was stirred under argon at 0 °C. Deprotonation of the
phosphonate was achieved with 45 mg (0.37 mmol) of potassium
t-butoxide dissolved in a minimal amount of dry DMF. The reaction
was allowed to warm to room temperature and stir overnight. Next,
170 mg (1.13 mmol) of 4-nitrobenzaldehyde in THF was added, and
the reaction was again cooled to 0 °C. Another 1.2 mmol of potassium
t-butoxide was added, and the reaction was allowed to proceed
overnight. The resulting mixture was extracted with CHCl3 (500 mL)
and washed with three 500 mL portions of brine and then dried over
MgSO4. After filtration, the solvent was again reduced by rotary
evaporation. Purification by HPLC afforded 235 mg of a black-metallic
solid for a yield of 68%. 1H NMR (CDCl3): 8.275 (m, 6H), 7.567 (m,
6H), 7.376 (m, 4H), 7.135 (s, 1H), 7.006 (m, 4H), 6.92-6.83 (bm,
4H), 6.691 (d, 3H), 3.652 (m, 4H), 3.364 (t, 4H), 2.920 (m, 4H), 1.646
(m, 4H), 1.357 (m, 12H), 0.941 (t, 6H). 13C NMR (CDCl3): 148.510,
147.02, 146.882, 144.304, 144.078, 143.976, 139.308, 138.911, 138.743,
137.145, 136.824, 134.494, 130.339, 130.004, 129.745, 129.446,
129.148, 128.966, 128.678, 128.365, 128.103, 128.487, 127.298,
126.974, 126.741, 125.419, 124.636, 124.461, 124.173, 119.574,
111.839, 51.328, 33.622, 33.429, 33.054, 31.987, 31.852, 29.922,
27.552, 27.322, 27.078, 26.933, 22.938, 22.861, 14.300. HRMS-FAB/
NBA: 934.4691, ∆ ) 2.3 ppm.
4H), 6.851 (m, 6H), 6.661 (d, 4H), 6.450 (s, 2H), 3.844 (m, 8H), 3.451
(m, 2H), 3.330 (m, 10H), 3.173 (m, 2H), 2.820 (m, 6H), 1.608 (t, 8H),
1.342 (m, 24H), 1.121 (m, 12H), 0.915 (m, 12H). 13C NMR (CD2Cl2):
148.423, 138.712, 138.067, 137.475, 135.101, 130.716, 128.804,
128.242, 127.423, 125.276, 121.141, 112.159, 62.474, 51.557, 33.046,
32.887, 32.310, 32.052, 27.804, 27.348, 23.267, 16.674, 14.390. HRMS-
ESI/TOF: 1079.71151, ∆ ) 1.2 ppm for M+4
.
(4,12)Bis(diethylphosphonatemethyl)-(7,15)bis(4′ -
dihexylaminostyryl)[2.2]paracyclophane (10). A 25 mL 14/20 round-
bottom flask was charged with 50 mg (0.06 mmol) of 8, 35 mg (0.12
mmol) of 4-dihexylaminobenzaldehyde, a Teflon-coated stirbar, and
closed with a septum. Approximately 2 mL of anhydrous THF was
added via syringe, and the mixture was stirred under argon at 0 °C.
Deprotonation of the phosphonate was achieved with 13.4 mg (0.12
mmol) of potassium t-butoxide dissolved in a minimal amount of dry
DMF added again by syringe. The reaction was allowed to warm to
room temperature and progress overnight. The reaction was quenched
with deionized water and extracted into 125 mL of CHCl3 washed with
brine. The solvent was removed under vacuum, and the mixture was
separated by HPLC. The resulting yield was 57% (37 mg). The
regiochemistry was assigned by characteristic paracyclophane frag-
mentation during mass spectrometry combined with 2D 1H NMR
(COSY). 1H NMR (CD2Cl2): 7.416 (d, 4H), 6.851 (m, 6H), 6.662 (d,
4H), 6.424 (d, 2H), 3.863 (m, 8H), 3.451 (m, 4H), 3.311 (t, 8H), 3.113
(m, 2H), 2.789 (m, 6H), 1.620 (m, 8H), 1.342 (m, 24H), 1.159 (m,
12H), 0.915 (t, 12H). 13C NMR (CD2Cl2): 148.461, 138.848, 138.120,
137.832, 135.245, 129.980, 129.873, 129.107, 128.258, 127.355,
125.193, 121.301, 112.151, 62.565, 62.497, 51.557, 33.008, 32.871,
32.318, 31.627, 27.804, 27.348, 23.267, 16.750, 16.689, 16.636, 14.390.
HRMS-ESI/TOF: 1079.71151, ∆ ) 1.8 ppm for M+4
.
4,7-Bis(4′-dihexylaminostyryl)-12,15-bis(4′′-nitrostyryl)[2.2]-
paracyclophane (3). A 25 mL 14/20 round-bottom flask was charged
with 0.5 mmol (540 mg) of 9, 2.2 mmol of 4-nitrobenzaldehyde, a
Teflon-coated stirbar, and was closed with a septum. Anhydrous DMF
was added, and the reaction was cooled to 0 °C. Potassium t-butoxide
(1.7 mmol) dissolved in a minimal amount of DMF was added via
syringe, and the reaction was allowed to proceed for 24 h, warming
slowly to room temperature. The resulting mixture was extracted with
CHCl3 (250 mL) and washed with three 250 mL portions of brine and
then dried over MgSO4. After filtration, the solvent was reduced by
rotary evaporation. Purification by HPLC afforded 453.4 mg of a
metallic-purple solid for a yield of 84%. 1H NMR (CDCl3): 8.252 (d,
4H), 7.589 (d, 4H), 7.363 (m, 6H), 7.121 (s, 2H), 7.001 (dd, 4H), 6.894
(s, 2H), 6.785 (d, 2H), 6.690 (d, 4H), 3.603 (m, 4H), 3.358 (t, 8H),
2.971 (m, 2H), 2.813 (m, 2H), 1.662 (m, 8H), 1.383 (m, 24H), 0.946
(t, 12H). 13C NMR (CDCl3): 147.892, 146.652, 144.136, 139.028,
136.753, 136.623, 136.530, 129.604, 128.439, 128.299, 127.814,
127.656, 127.003, 126.854, 124.496, 124.207, 120.143, 111.614, 51.102,
33.346, 32.954, 31.771, 27.325, 26.868, 22.720, 14.083. HRMS-EI:
1072.6818, ∆ ) 1.1 ppm.
(4,7)Bis(diethylphosphonatemethyl)-(12,15)bis(4′ -
dihexylaminostyryl)[2.2]paracyclophane (9). A 25 mL 14/20 round-
bottom flask was charged with 50 mg (0.06 mmol) of 8, 35 mg (0.12
mmol) of 4-dihexylaminobenzaldehyde, a Teflon-coated stirbar, and
closed with a septum. Approximately 2 mL of anhydrous THF was
added via syringe, and the mixture was stirred under argon at 0 °C.
Deprotonation of the phosphonate was achieved with 13.4 mg (0.12
4,7,12-Tribromo-15-formyl[2.2]paracyclophane (13). A 100 mL
24/40 round-bottom flask was charged with 500 mg (0.95 mmol) of
1226 and a Teflon-coated stirbar and fitted with a needle valve.
Approximately 50 mL of dry THF was transferred to the flask, and
the reaction mixture was cooled to -78 °C. To the flask was added 1
equiv of n-butyllithium (0.95 mmol) via syringe through the needle
9
5194 J. AM. CHEM. SOC. VOL. 124, NO. 18, 2002

Synthesis of 4,7,12,15-[2.2]Paracyclophane
A R T I C L E S
valve, which was allowed to react for 30 min, maintaining the reaction
at -78 °C. Next, 8 equiv (∼1 mL) of anhydrous DMF was added via
syringe, and the reaction was allowed to warm to room temperature.
The reaction was quenched with a dilute ammonium hydroxide solution,
and the THF was removed under vaccuum. The resulting solid was
extracted with CHCl3 (250 mL) and washed with three 250 mL portions
of brine and then dried over MgSO4. After filtration, the solvent was
reduced by rotary evaporation. The resulting solid was coated onto 1.5
g of chromatographic silica and purified by flash chromatography (40
g column) with 3:2 CHCl3:hexanes as eluent. A soapy white solid was
recovered (166 mg, 37% yield). 1H NMR (CDCl3): 9.866 (s, 1H), 7.612
(s, 1H), 7.304 (s, 1H), 7.174 (s, 1H), 6.461 (s, 1H), 3.38-2.8 (bm,
8H). 13C NMR (CDCl3): 191.735, 143.241, 141.132, 140.662, 139.766,
137.270, 136.208, 135.852, 135.761, 134.099, 133.462, 125.678,
125.291, 34.722, 32.939, 32.780, 30.838. HRMS-EI: 473.001, ∆ )
4.1 ppm.
GC/MS analysis showed two close peaks with the expected product
mass of 420. Both revealed a major fragment at 210, characteristic of
the isomers identified as 16 and 17. These were separated successfully
with HPLC to yield 55% of 16 and 34% of 17. None of 4,7-dibromo-
12,15-diformyl-[2.2]paracyclophane (15) was identified from GCMS.
1
The absolute spatial assignment was achieved by 2D H COSY. The
disposition of the C2 axis of symmetry for 16 allows for coupling
between all of the inequivalent methylene protons found on the
paracyclophane bridgeheads. Analogous coupling is not possible for
17 where the protons on the two bridging carbons on one side of the
molecule are related by symmetry. All proton resonances are coupled
in the COSY of 16, while there are uncoupled methylene resonances
for 17. For 16, 1H NMR (CDCl3): 9.802 (s, 2H), 7.286 (s, 2H), 6.894
(s, 2H), 3.950 (m, 2H), 3.499 (m, 2H), 3.050 (m, 4H). 13C NMR
(CDCl3): 191.332, 143.620, 140.290, 137.551, 136.087, 135.639,
133.189, 35.132, 30.679. HRMS-EI: 419.539, ∆ ) 1.2 ppm. For 17,
1H NMR (CDCl3): 9.910 (s, 2H), 7.592 (s, 2H), 6.608 (s, 2H), 4.051
(m, 2H), 3.393 (m, 2H), 3.189 (m, 2H), 2.918 (m, 2H). 13C NMR
(CDCl3): 191.469, 143.886, 139.956, 138.674, 135.579, 134.759,
133.644. HRMS-EI: 419.539, ∆ ) 0.4 ppm.
4,7,12-Tris(4′-dihexylaminostyryl)-15-(4′′-nitrostyryl)[2.2]-
paracyclophane (4). A 25 mL 14/20 round-bottom flask was charged
with 392.6 mg (0.83 mmol) of 13, 1.188 g (4.1 mmol) of 4-dihexy-
laminostyrene, 100 mg of tri-o-tolylphosphine, 60 mg of palladium
acetate, 0.5 mL of triethylamine, and ∼5 mL of DMF, and was equipped
with a Teflon-coated stirbar and a needle valve. The reaction mixture
was degassed and was allowed to react at 100 °C overnight. The reaction
was diluted with methylene chloride and the product extracted into
methylene chloride (∼125 mL) and washed with three 125 mL portions
of brine. The solution was dried with MgSO4, filtered, and the solvent
stripped. The resulting liquid was purified by flash chromatography
(90 g column with 3:2 CH2Cl2:hexanes as eluent) for a total of 583 mg
(64% yield) of 4-formyl-7,12,15-tris(4′-dihexylaminostyryl)[2.2]-
paracyclophane (14). A second 10 mL round-bottom flask was charged
with 338.6 mg (0.31 mmol) of 14, 84.7 mg (0.31 mmol) of 4-nitro-
phenylmethanephosphonate, ∼2 mL of THF, a stirbar, and closed with
a septum. The mixture was stirred and cooled to 0 °C. The phosphonate
was deprotonated with 50 mg (0.43 mmol) of potassium t-butoxide,
and the reaction was allowed to warm to room temperature and stir
overnight. The reaction was diluted with methylene chloride and the
product extracted into methylene chloride (∼125 mL) and washed with
three 125 mL portions of brine. The solution was dried with MgSO4,
filtered, and the solvent removed in vacuo. The resulting liquid was
purified by flash chromatography (90 g column with 3:7 CH2Cl2:
4,15-Bis(4′-dihexylaminostyryl)-7,12-bis(4′′-nitrostyryl)[2.2]-
paracyclophane (5) and 4,12-Bis(4′-dihexylaminostyryl)-7,15-bis-
(4′′-nitrostyryl)[2.2]paracyclophane (6). Because the preparations of
5 and 6 are identical, only the synthesis of 5 is reported as an example.
A 25 mL 14/20 round-bottom flask was charged with 110 mg (0.26
mmol) of 16, 290 mg (1.04 mmol) of 4-dihexylaminostyrene, 50 mg
of tri-o-tolylphosphine, 50 mg of palladium acetate, 0.5 mL of
triethylamine, and ∼3 mL of DMF, and was equipped with a Teflon-
coated stirbar and a needle valve. The reaction mixture was degassed
and was allowed to react at 100 °C overnight. The reaction was diluted
with methylene chloride and the product extracted into methylene
chloride (∼500 mL) and washed with three 500 mL portions of
deionized water. The solution was dried with MgSO4, filtered, and the
solvent removed. The resulting liquid was purified by flash chroma-
tography (90 g column with 75% CH2Cl2/25% hexanes as eluent) for
a total yield of 93% of 7,12-diformyl-4,15-bis(4-dihexylaminostyryl)-
[2.2]paracyclophane (18). A second 5 mL round-bottom flask was
charged with 97 mg (0.1 mmol) of 18, 68.3 mg (0.25 mmol) of
4-nitrophenylmethanephosphonate, ∼2 mL of THF, a stirbar, and closed
with a septum. The mixture was stirred and cooled to 0 °C. The
phosphonate was deprotonated with 28 mg (0.25 mmol) of potassium
t-butoxide, and the reaction was allowed to warm to room temperature
and stir overnight. The reaction was diluted with methylene chloride
and the product extracted into methylene chloride (∼500 mL) and
washed with three 500 mL portions of deionized water. The solution
was dried with MgSO4, filtered, and the solvent removed in vacuo.
The resulting liquid was purified by flash chromatography (90 g column
with 40% CH2Cl2/60% hexanes as eluent) for a total yield of 79% (85
mg) of 5. An alternate preparation of 6 is reaction of 10 with
4-nitrobenzaldehyde in a procedure identical to the preparation of 3
for a yield of 92%. For 5, 1H NMR (CDCl3): 8.268 (d, 4H), 7.549 (d,
4H), 7.398 (m, 6H), 6.964 (m, 10H), 6.676 (d, 4H), 3.604 (m, 4H),
3.350 (t, 8H), 2.842 (m, 4H), 1.655 (m, 8H), 1.363 (m, 24H). 13C NMR
(CDCl3): 148.353, 146.649, 144.694, 139.177, 138.656, 137.458,
134.687, 130.120, 129.865, 128.496, 128.277, 128.081, 126.999,
125.794, 124.465, 124.421, 120.044, 111.839, 51.328, 33.600, 33.145,
31.991, 27.559, 27.086, 22.938, 14.300. HRMS-FAB/NBA: 1072.6765,
∆ ) 3.8 ppm. For 6, 1H NMR (CDCl3): 8.239 (d, 4H), 7.565 (d, 4H),
7.372 (m, 6H), 6.958 (10H), 6.689 (d, 4H), 3.595 (m, 4H), 3.362 (t,
8H), 2.845 (m, 4H), 1.645 (m, 8H), 1.383 (m, 24H), 0.969 (t, 12H).
13C NMR (CDCl3): 148.266, 146.740, 144.584, 139.461, 139.097,
137.192, 134.403, 130.477, 130.411, 128.893, 128.500, 127.684,
126.682, 125.033, 124.752, 124.603, 120.160, 111.788, 51.388, 33.823,
33.021, 31.987, 27.555, 27.104, 22.934, 14.300. HRMS-FAB/NBA:
1072.6794, ∆ ) 1.1 ppm.
1
hexanes as eluent) for a total yield of 81% (302 mg) of 4. H NMR
(CDCl3): 8.235 (d, 2H), 7.577 (d, 2H), 7.44-7.32 (bm, 6H), 7.064 (s,
1H), 7.02-6.74 (bm, 11H), 6.72-6.64 (m, 6H), 3.62-3.48 (m, 4H),
3.332 (t, 12H), 2.98-2.74 (m, 4H), 1.653 (s, 12H), 1.360 (m, 48H),
0.924 (m, 18H). 13C NMR (CDCl3): 148.124, 148.004, 147.909,
146.514, 144.985, 139.297, 139.046, 137.534, 137.185, 136.897,
136.879, 136.879, 136.624, 134.614, 130.539, 130.047, 128.747,
128.416, 128.186, 128.099, 127.968, 127.844, 127.702, 127.542,
126.941, 125.419, 125.346, 125.047, 124.410, 120.973, 120.874,
120.605, 111.890, 111.847, 111.814, 51.353, 33.808, 33.327, 33.149,
31.998, 27.566, 27.111, 27.104, 22.942, 14.304. HRMS-FAB/NBA:
1210.8943, ∆ ) 0.1 ppm.
4,15-Dibromo-7,12-diformyl[2.2]paracyclophane (16) and 4,12-
Dibromo-7,15-diformyl[2.2]paracyclophane (17). A 100 mL 24/40
round-bottom flask was charged with 618.1 mg (1.18 mmol) of 12 and
a Teflon-coated stirbar and fitted with a needle valve. Approximately
50 mL of dry THF was transferred to the flask, and the reaction mixture
was cooled to -78 °C. To the flask was added 1.1 mL of a 2.3 M
n-butyllithium solution (2.48 mmol) via syringe through the needle
valve, which was allowed to react for 30 min. Next, 8 equiv (∼1 mL)
of anhydrous DMF was added via syringe, and the reaction was allowed
to warm to room temperature. The reaction was quenched with a dilute
ammonium hydroxide solution, and the THF was removed under
vacuum. The resulting solid was extracted with CHCl3 (250 mL) and
washed with three 250 mL portions of brine and then dried over MgSO4.
After filtration, the solvent was again reduced by rotary evaporation.
9
J. AM. CHEM. SOC. VOL. 124, NO. 18, 2002 5195

A R T I C L E S
Bartholomew and Bazan
Acknowledgment. Financial support from the National Sci-
ence Foundation (DMR 0097611) and the Office of Naval
Research is gratefully acknowledged. The authors would like
to thank Dr. Xianhui Bu for assistance with crystallography and
for useful discussions.
Supporting Information Available: Complete experimental
details of the X-ray crystallographic determination of 7, DD,
1, and 3 (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA0121383
9
5196 J. AM. CHEM. SOC. VOL. 124, NO. 18, 2002