Structure of 9,10-di(9,9-dioctylfiuorenyl)anthracene in the solid and in solution

Article abstract of DOI:10.1021/jo010617e

A new example of magnetic nonequivalence of chemically equivalent atoms is identified from the proton and carbon resonance spectra of 9,10-di(9,9-dioctylfluorenyl)anthracene with the aid of its conformation in the crystalline state. Molecular modeling suggests that it has a similar conformation in solution.

Full text of DOI:10.1021/jo010617e

J . Org. Chem. 2002, 67, 663-667
663
Str u ctu r e of 9,10-Di(9,9-d ioctylflu or en yl)a n th r a cen e in th e Solid
a n d in Solu tion
Sydney K. Brownstein,* J ianfu Ding, Gilles Robertson, and J acques Roovers
Institute for Chemical Process and Environmental Technology, National Research Council of Canada,
Ottawa, ON, Canada K1A0R6
Gary Enright
Steacie Institute for Molecular Sciences, National Research Council of Canada,
Ottawa, ON, Canada K1A0R6
Sydney.Brownstein@nrc.ca
Received J une 18, 2001
A new example of magnetic nonequivalence of chemically equivalent atoms is identified from the
proton and carbon resonance spectra of 9,10-di(9,9-dioctylfluorenyl)anthracene with the aid of its
conformation in the crystalline state. Molecular modeling suggests that it has a similar conformation
in solution.
In tr od u ction
After the synthesis of 9,10-di(9,9-dioctylfluorenyl)-
anthracene (I), following a procedure to be discussed
later, a proton magnetic resonance spectrum was ob-
tained in order to get an estimate of the purity. More
lines were found than could be explained in a straight-
forward manner, but the compound appeared to be pure.
To explain this apparent contradiction, the compound
was investigated by proton and carbon nuclear magnetic
resonance in solutions of varying concentration and
solvent, by single-crystal X-ray crystallography in the
solid, and by the technique of molecular modeling. The
combination of these techniques made it possible to
explain all the results.
Resu lts a n d Discu ssion
The numbering employed for discussion purposes is
shown in Figure 1, where for the sake of simplicity,
hydrogen atoms have not been included. For purposes of
discussion, they will have the same number as the carbon
atom to which they are bonded. Some disorder was
observed for one of the n-octyl chains in the solid state,
and this is also reflected in the numbering for the X-ray
data, where these atom numbers are followed by the
F igu r e 1. Numbering system used for discussion of I.
of the two half molecules. The fluorene rings are oriented
such that the n-octyl groups face away from each other
(trans). The aromatic rings of adjacent molecules are
separated by too large a distance for any significant π-π
interaction.1
Although only a single conformation is observed for the
molecules in the solid state, it is possible that the
relaxation of packing constraints in solution might allow
other conformations to be present. Therefore, a molecular
mechanics study was undertaken to determine the
potential energy (in kcal/mol) of the molecule as a
function of the dihedral angle including carbon atoms 7,
8, 15, and 16. The results are plotted in Figure 3, where
the zero of energy is an arbitrary value determined by
letter A; however, the disorder is ignored for the spec-
troscopic data. Figure 2 illustrates the conformations and
packing in the unit cell of I. Once again, hydrogen atoms
have been deleted from the figure for the sake of clarity.
Several features of this figure will be mentioned because
they are significant for the interpretation of results to
be discussed later. The anthracene ring system and the
fluorene ring systems are planar, and the fluorene ring
systems are essentially coplanar but not coplanar with
the anthracene ring system. The unit cell contains one
molecule and two half molecules. The dihedral angle
including carbons 7, 8, 15, and 16 is -107.9(1)° for the
single molecule and -91.4(1)° for the corresponding angle
* To whom correspondence should be addressed. Fax: (613)-991-
2384.
(1) Interlayer spacing in graphite is 3.354 Å.
10.1021/jo010617e CCC: $22.00 Published 2002 by the American Chemical Society
Published on Web 01/15/2002

664 J . Org. Chem., Vol. 67, No. 3, 2002
Brownstein et al.
F igu r e 2. Arrangement of the molecules of I in the unit cell.
F igu r e 4. Proton resonance spectrum of the aromatic region
of a 0.024 M CDCl3 solution of I.
positioned trans. The one at 120° has the fluorene rings
coplanar, while the one at 60° has them tilted at opposite
angles relative to the anthracene ring system. The
difference between the calculated minimum energy con-
formation for the isolated molecule and that observed for
the structure found via X-ray crystallography (29.611)
is 3 kcal/mol.
This difference could reasonably arise from two sources.
One is the energy requirement for the most efficient
packing in the solid as compared with an isolated
molecule. The second is the neglect of effects of entropy
in the molecular mechanics calculations, which give a
potential energy rather than a free energy. Nevertheless,
we can conclude from both the X-ray and molecular
mechanics results that the most stable conformation in
solution is one in which the two fluorene ring systems
are coplanar and at an angle of about 120° relative to
the plane of the anthracene ring.
F igu r e 3. Potential energy of I as a function of the dihedral
angle 7,8,15,16.
The potential energy barrier for rotation between the
cis and trans conformations is calculated as 12 kcal/mol.
We can conclude that interconversion between cis and
trans configurations would be sufficiently rapid on the
NMR time scale at room temperature that the very small
chemical shifts and sharp lines that are found could not
be due to separate cis and trans isomers because only
averaged lines would be observed.
The proton resonance spectrum of the aromatic region
of a 0.024 molar, degassed solution of I in CDCl3 is shown
in Figure 4, and the chemical shift assignments and spin
couplings are listed in Table 1. These assignments were
confirmed, as much as possible, by the internal consis-
tency of spin couplings, by selective decoupling, and by
a COSY2 spectrum. The carbon resonance spectrum of
the aromatic region, for the same solution, is shown in
Figure 5, and the chemical shift assignments are listed
the molecular mechanics program. The corresponding
opposite dihedral angle including carbon atoms 2, 1, 28,
and 29 remained approximately constant at from -41 to
-73°. Considerable hysteresis was found in the calcula-
tion of energy minima for the various angles, so the 7, 8,
15, and 16 dihedral angle had to be restrained at a
random variety of angles and the energy minimized to
get the global minimum for any given dihedral angle. A
value of 0° for the dihedral angle 7, 8, 15, and 16
corresponds most closely to the cis conformation, while
values near 180° correspond to the trans conformation.
The energies are not symmetric about 0° for the dihedral
angle 7, 8, 15, and 16 because the dihedral angle for 2,
1, 28, and 29 remains in the vicinity of -55°. The very
broad minimum about -120° corresponds to the situation
where the two fluorene ring systems are coplanar and
in the cis conformation. The two minima of 33.269 at 60°
and 29.611 at 120° have the two fluorene ring systems
(2) Aue, W. P.; Bartholdi, E.; Ernst, E. E. J . Chem. Phys. 1976, 64,
2229-2246.

9,10-Di(9,9-dioctylfluorenyl)anthracene
J . Org. Chem., Vol. 67, No. 3, 2002 665
Ta ble 1. P r oton a n d Ca r bon Ch em ica l Sh ifts (p p m fr om TMS) a n d Sp in Cou p lin gs (Hz) for I in a 0.024 M
CDCl3 Solu tion , Excep t a s Noted
atoms
chemical shifts
spin couplings
atoms
H22,H35 (a)
H22,H35 (b)
C22,C35
chemical shifts
spin couplings
C1,C8,C15,C28
C2,C7,C9,C14
H3,H6, or
137.56
129.99
7.806 or
7.799
7.884 or
7.880
8.121 or
8.118 or
8.113 or
8.110
126.99
7.336 or
7.328
not resolved
not resolved
127.12
J 3-4 ) 7.0 or
H10,H13
J 10-11 ) 7.0
H3,H6 or (a)a
H10,H13
same as above
H23,H36
not resolved
not resolved
H3 or (b)b
H6 or
J 3-4 ) 7.2 or
J 6-5 ) 6.8 or
J 10-11 ) 6.8 or
J 13-12 ) 6.8
H23,H36 (a)
H10 or
H13
C3,C6,C10,C13
H4,H5 or
H11,H12
H4,H5 or (a)
H11,H12
H23,H36 (b)
C23,C36 or
C21,C34
not resolved
122.84
J 4-3 ) 7.0 or
J 11-10 ) 7.0
same as above
7.884 or
7.880
C24,C37
150.87
H4,H5,H11,H12 (b)
not resolved
C25 or
C38
55.29 or
55.25
C4,C5,C11,C12
124.90
C26 or
C39
H27 or
H40
H27 or (a)
H40
H27 or (b)
H40
150.76 or
150.75
7.484 or
7.441
7.562 or
7.516
H16 or
H29
H16 or (a)
H29
H16 or (b)
H29
7.492 or
7.452
7.555 or
7.524
7.513 or
7.459
J 16-17 ) 7.6, J 16-27 ) 1.6 or
J 29-30 ) 7.6, J 29-40 ) 1.2
J 16-17 ) 7.2, J 16-27 ) 1.6 or
J 29-30 ) 6.0, J 29-40 ) 1.6
J 16-17 ) 7.4, J 16-27 ) 1.4
J 29-30 ) 7.4, J 29-40 ) 1.6
7.548 or
7.483
C16,C29 or
C27,C40
H17 or
126.16 or
126.14
7.949 or
7.943
8.001
7.742 or
7.736
C27,C40 or
C16,C29
H41,H51,H61,H71
129.78 or
129.72
1.99
J 17-16 ) 7.7, J 17-27 ) 0.4
J 30-29 ) 7.7, J 30-40 ) 0.4
J 17-16 ) 7.2, J 17-2 ) 0.4
J 17-16 ) 7.4 or
H30
H17,H30 (a)
H17 or (b)
H30
H41,H51,H61,H71 (a)
H41,H51,H61,H71 (b)
2.075
2.036
J 30-29 ) 7.4
C17,C30
C18,C31
C19 or
C32
H20,H33
119.56
140.44
140.88 or
140.87
7.834
C41,C51,C61,C71
C42,C52,C62,C72
C43,C53,C63,C73 or
C46,C56,C66,C76
C44,C54,C64,C74 or
C45,C55,C65,C75
H47,H57,H67,H77
H47,H57,H67,H77 (a)
40.51
24.08
31.89 or
30.14
29.41 or
29.29
1.11
J 20-21 ) 6.8
H20,H33 (a)
H20 or (b)
H33
7.876
7.724 or
7.718
J 20-21 ) 6.8
J 20-21 ) 7.2 or
J 33-34 ) 7.2
1.244
C20,C33
119.73
H47,H57,H67,H77 (b)
C47,C57,C67,C77
H48,H58,H68,H78
H48,H58,H68,H78 (a)
H48,H58,H68,H78 (b)
1.168
22.68
0.805
0.867
0.839 and
0.835
H21,H34
H21,H34 (a)
H21,H34 (b)
C21,C34 or
C23,C36
not resolved
not resolved
not resolved
126.80
J 48-47 ) 7.0
J 48-47 ) 7.2
J 48-47 ) 6.8
H22,H35
not resolved
C48,C58,C68,C78
14.18
a
b
(a) A 0.10 M CDCl3 solution. (b) A C6D6 solution.
in Table 1. These assignments were made with the aid
of DEPT3 and HETCOR4 pulse sequences.
one is closest to one side of the anthracene, the other is
closest to the second side, which is not equivalent. The
authors are not aware of another example of this asym-
metry involving the anthracene ring system.
The chief feature of interest in these spectra is the
doubling of the chemical shift for certain chemically
equivalent but magnetically nonequivalent nuclei. The
magnetic nonequivalence arises from the fact that the
plane of the fluorene rings is not perpendicular to that
of the anthracene ring. Therefore, in a given conforma-
tion, atoms 2 through 7 will be different from atoms 9
through 14. When the fluorene rings flip to the other
corresponding angle, the nonequivalence of the an-
thracene atoms is reversed but they remain nonequiva-
lent. This is formally similar to the magnetic nonequiv-
alence of the CH2 protons bonded to an adjacent
asymmetric carbon atom.5 For the same reason, the two
fluorene ring systems are nonequivalent because when
The carbon chemical shifts seem much less sensitive
to this asymmetry than the proton chemical shifts. Thus,
H3 and H6 are shifted from H10 and H13 by 0.007 ppm,
while there is no discernible splitting of the carbon
resonances. The same situation is found for H4,H5 and
H11,H12 where the difference is 0.008 ppm without
splitting of the corresponding carbon resonances. A
similar situation prevails with the first phenyl ring of
the fluorene moiety where H16 and H29 are magnetically
nonequivalent by 0.040 ppm and H17 and H30 are split
by 0.006 ppm while the corresponding carbon atoms are
not shifted. This is also observed for H27 and H40 with
a chemical shift difference of 0.043 ppm. Splitting of the
(3) Bendall, M. R.; Doddrell, D. M.; Pegg, D. T. J . Am. Chem. Soc.
1981, 103, 4603-4605.
(4) Freeman, R.; Morris, G. A. J . Chem. Soc., Chem. Commun. 1978
684-686.
(5) Drysdale, J . J .; Phillips, W. D. J . Am. Chem. Soc. 1957, 79, 319-
322.

666 J . Org. Chem., Vol. 67, No. 3, 2002
Brownstein et al.
F igu r e 5. Carbon resonance spectrum of the aromatic region of a 0.024 M CDCl3 solution of I.
F igu r e 7. Proton resonance spectrum of the aromatic region
of a C6D6 solution of I.
the anthracene ring. The proton signals, which are
significantly altered in the 0.10 M solution, are those of
H17,H30, which now have the same chemical shift.
H3,H6, or H10,H13, are now separated by 0.004 ppm,
and for H16,H29, the chemical shift difference is reduced
from 0.049 to 0.031 ppm. We postulate that these changes
arise from possible overlap of the outer phenyl rings of
the fluorene moiety of adjacent molecules. This hypoth-
esis was tested by obtaining the proton resonance spec-
trum of I in deuteriobenzene, which is shown in Figure
7 and partially listed in Table 1. The most significant
change is the large downfield shift of H3, H6, H10, and
H13. Each of these protons now has a resolvably different
chemical shift. This is consistent with solvation of
F igu r e 6. Proton resonance spectrum of the aromatic region
of a 0.10 M CDCl3 solution of I.
chemically equivalent C19 and C32 is 0.01 ppm, and the
same splitting is found for C26 and C39.
In a 0.10 molar solution of I, we have a proton
resonance spectrum that appears rather different. It is
shown in Figure 6, and the significant proton chemical
shifts are also listed in Table 1. The carbon chemical
shifts are essentially identical with those of the 0.024 M
solution. This is consistent with the observed lack of
sensitivity, in the present case, of the carbon chemical
shifts to the magnetic nonequivalence of the two sides of

9,10-Di(9,9-dioctylfluorenyl)anthracene
J . Org. Chem., Vol. 67, No. 3, 2002 667
SMART CCD diffractometer using Mo KR radiation and an
ω-scan mode. The unit cell constants at -100 °C were a )
9.080(2) Å, b ) 17.153(4) Å, c ) 29.692(7) Å, R ) 102.27(2)°,
â ) 95.32(2)°, and γ ) 94.85(2)°. The empirical formula for
the asymmetric unit (Z ) 2) was C109H135 with Dc ) 1.073 g/cm3
and µ ) 0.06 mm-1. No diffraction spots could be observed at
2θ angles greater than 40°. A Wilson plot of the reduced
intensities confirmed the absence of meaningful data above
this angle. The structure was solved in space group P1h using
the NRCVAX program9 and refined (on F2) using the SHELX-
TL program with full matrix least squares on all data. One of
the n-octyl groups (C71-C78) is disordered (each of C73-C78
is disordered over two sites, denoted by A and B, with
complementary occupancy). All the ordered, non-hydrogen
atoms were refined with anisotropic thermal parameters.
Hydrogen atoms were refined in calculated positions. The final
residual values R1(wR2) ) 0.061(0.1552) for I > 2σ(I) and
0.1847(0.1802) for all data (6703 unique reflections, 972
parameters).
The molecular modeling was done with the commercial
program Hyperchem 4.5.10 The force field MM+, a variant of
MM2,11 was employed. Energy minimization was done with a
Polak-Ribiere conjugate gradient procedure. The torsion angle
was constrained by putting a high energy barrier to change
from the constrained value. These calculations were confirmed
for the minimum energy conformation with the program
TINKER12 using the MM3 parameter set13 and a nonlinear
conjugate gradient method.
primarily one of the fluorene ring systems of I. There is
also an upfield shift of H17,H30, and they are nonequiva-
lent, as in the dilute CDCl3 solution. H20 and H33 have
become magnetically nonequivalent. There are also non-
equivalent signals from two of the four CH3 groups.
It was mentioned that the proton resonance spectra
were obtained on degassed and sealed samples. If the
samples were not degassed and the solution was exposed
to the air, the spectra gradually changed. This change
was greatly accelerated if the samples were also exposed
to light. It is presumed that the oxide is formed under
these conditions.6 The proton and carbon chemical shifts
of the oxide in CDCl3 solution are normal with no
indication of unusual magnetic nonequivalence. Since
oxide formation changes the no-longer aromatic an-
thracene ring into a boat form, hindrance to rotation
about the bonds attaching the fluorene moieties is
removed.
Exp er im en ta l Section
Syn th esis of I. 2-Bromo-9,9-dioctylfluorene was synthe-
sized from 2-bromofluorene (Aldrich) and 1-bromooctane (Al-
drich) in a manner similar to that reported.7 The product was
purified by chromatography with hexane to obtain a colorless
oil and was further treated with n-butyllithium, triisopropyl
borate in ether, and then 2 N HCl to give 9,9-dioctylfluorene-
2-boronic acid (II) at a total yield of 60%. The Suzuki coupling8
of II and dibromoanthracene (Aldrich) was done in a mixture
of toluene and 2 M Na2CO3 (3:2, v/v) in the presence of tetrakis-
(triphenylphosphine)palladium(0) (2.0 mol % of monomer). The
solution was purged with Ar and refluxed with vigorous
stirring for 48 h in the dark. I was obtained in 92% yield after
twice recrystallizing from hexane.
NMR spectra were obtained with a Varian Unity Inova
spectrometer operating at 100 MHz for C13, with a sweep width
of 25 000 Hz and 60 000 data points, and 400 MHz for H1, with
a sweep width of 6000 Hz and 30 000 data points. Samples (5
mm) were used in a wide bore magnet with the standard
programs of the manufacturer.
Ack n ow led gm en t. This report was issued as NRCC
No. 44374.
X-r a y Diffr a ction . Although the majority of the crystals
were extremely fine, colorless needles, a large rectangular-
shaped (0.04 × 0.06 × 0.22 mm) crystal was chosen for single-
crystal X-ray diffraction. The data were collected on a Siemens
J O010617E
(9) Gabe, E. J .; Lee, F. L.; LePage, Y.; White, P. S. J . Appl.
Crystallogr. 1989, 22, 384-387.
(10) Hyperchem, Inc., 115 North West 4th St., Gainesville, FL 32601.
(11) Allinger, N. L. J . Am. Chem. Soc. 1977, 99, 8127-8134.
(12) Pappu, R. V.; Hart, R. K.; Ponder, J . W. J . Phys. Chem. B 1998,
102, 9725-9742.
(13) Allinger, N. L.; Yuh, Y. H.; Lii, J . H. J . Am. Chem. Soc. 1989,
111, 8551-8566.
(6) Sinigersky, V.; Mullen, K.; Klapper, M.; Schopov, I. Adv. Mater.
2000, 12, 1060-1063.
(7) Woo, E. P.; Shiang, W. R.; Inbasekaran, M.; Roof, G. R. U.S.
Patent US5962631, 1999.
(8) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457-2483.