From low to very high birefringence in bis(2-pyridylimino)isoindolines: Synthesis and structure-property analysis

Article abstract of DOI:10.1002/chem.201103421

A series of new substituted 1,3-bis(2-pyridylimino)isoindolines-1,3-bis(2- pyridylimino)-5,6-bis(2,6-diisopropylphenoxy)isoindoline (2 b), 1,3-bis(2-pyridylimino)-5,6-bis(4-tert-butylphenyl)isoindoline (2 c), and 1,3-bis(2-pyridylimino)-5-tert-butylisoindoline (2 d)-were synthesized and structurally characterized by single-crystal X-ray diffraction. The birefringence (Δn) of the crystals of unsubstituted 1,3-bis(2- pyridylimino)isoindoline (2 a), 2 b, 2 c, and 2 d were measured and found to vary greatly, with Δn values of 0.0654(3), 0.0629(17), 0.588(10), 0.701(12), respectively. A structure-property relationship for the birefringence values of 2 a-2 d was outlined and indicated that the anisotropy of the polarizability of the molecules plays a crucial role in the birefringence of the crystals. The greatest birefringence values are achieved when the molecules are oriented in a face-to-face configuration intermolecularly, and along the crystallographic face being measured. Copyright

Full text of DOI:10.1002/chem.201103421

FULL PAPER
DOI: 10.1002/chem.201103421
From Low to Very High Birefringence in Bis(2-pyridylimino)isoindolines:
Synthesis and Structure–Property Analysis
Edwin W. Y. Wong, Jeffrey S. Ovens, and Daniel B. Leznoff*[a]
Abstract: A series of new substituted
1,3-bis(2-pyridylimino)isoindolines—
1,3-bis(2-pyridylimino)-5,6-bis(2,6-di-
pyridylimino)isoindoline (2a), 2b, 2c,
and 2d were measured and found to
vary greatly, with Dn values of
property relationship for the birefrin-
gence values of 2a–2d was outlined
and indicated that the anisotropy of
the polarizability of the molecules
plays a crucial role in the birefringence
of the crystals. The greatest birefrin-
gence values are achieved when the
molecules are oriented in a face-to-face
configuration intermolecularly, and
along the crystallographic face being
measured.
A
0.0654(3),
0.701(12), respectively.
0.0629(17),
0.588(10),
A structure–
bis(2-pyridylimino)-5,6-bis(4-tert-butyl-
phenyl)isoindoline (2c), and 1,3-bis(2-
pyridylimino)-5-tert-butylisoindoline
(2d)—were synthesized and structural-
ly characterized by single-crystal X-ray
diffraction. The birefringence (Dn) of
the crystals of unsubstituted 1,3-bis(2-
Keywords: birefringence
·
crystal
engineering · optical properties ·
structure–property relationships
supramolecular chemistry
·
Introduction
design coordination polymers with very high birefringence
values.[21–24] These high values arise from a polymer frame-
work that supports a set of anisotropic, highly polarizable
Birefringent materials (the refractive index of which de-
pends on crystallographic direction) have a wide range of
applications such as optical filters,[1,2] waveplates,[3] and
liquid-crystal displays.[4–6] Inorganic minerals, such as calcite
and quartz, have been particularly widely employed for
these purposes due to a high birefringence value in the case
of calcite (Dn=0.172), and the ability to cut large crystals of
these materials for applications.[7–10]
Organic crystalline solids are also known to be birefrin-
gent. For instance, crystals of urea have a substantial value
of Dn=0.118.[11,12] For many organic crystals, the face-de-
pendent refractive indices (and therefore the birefringence)
were measured in the early-to-mid 1900s by using refractive-
index-matching oils and/or prism coupling methodologies,[13]
and over 2000 measurements have been tabulated and com-
piled.[11] The values are generally below Dn=0.2; a very
small number are greater than 0.5.[14,15] However, despite
this plethora of measurement data and a good understand-
ing of molecular polarizabilities, a chemical and crystallo-
graphic perspective regarding the 3D structural features that
influence the birefringence properties has received very
little attention.[16,17]
lig
build upon one another. Using the highly anisotropic
2,2’:6’,2’’-terpyridine (terpy) ligand on
Pb/[Au(CN)2]ꢀ
ACHTUNGTRENaNUNG nds, such that their individual polarizability anisotropies
a
framework, we were able to achieve values approaching
Dn=0.5.[22,23] Such a high value arises from the face-to-face
arrangement of the terpy ligands, organized in this fashion
by the coordination polymer framework. Terpy itself does
not form good quality crystals for either of its two poly-
morphs, and neither polymorph shows the alignment re-
quired for a high birefringence value.[25,26]
In order to expand on this methodology, we began to ex-
plore bis(2-pyridylimino)isoindoline, or “lobsterate” (Lb)-
based ligands (so named by us because of their resemblance
to a lobster). Much like terpy, these are flat ligands built up
of aromatic substituents, and would be expected to have a
very high anisotropic polarizability as well. Furthermore, their
transparency above 450 nm facilitates optical applications.
However, these isoindoline molecules, unlike terpy, do
form good quality crystals. Depending on the R-group sub-
stituent, the majority have their molecules aligned face-to-
face in their crystal structure, thus fulfilling one condition
needed for high birefringence. Thus, the synthesis, struc-
tures, and birefringence measurements on a series of lobster-
ate molecules is outlined below and the birefringence corre-
lated to the 3D packing array of the materials.
At the interface of the inorganic salts and organic com-
pounds lies the realm of coordination polymers, which pro-
vide a Legoꢀ building-block approach to designing new ma-
terials.[18–20] Recently, we have described how to rationally
[a] E. W. Y. Wong, J. S. Ovens, D. B. Leznoff
Department of Chemistry, Simon Fraser University
8888 University Drive, Burnaby, BC, V5A 1S6 (Canada)
Fax : (+1)778-782-3765
Results
Synthesis and structure: The synthesis of a series of 5,6-sub-
E-mail: dleznoff@sfu.ca
stituted bis(2-pyridylimino)isoindolines was accomplished by
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
1
&
ÞÞ
These are not the final page numbers!

using
a
one-pot reaction of 4,5-substituted phthaloni-
absorption band. The lowest energy absorption bands of 2b
and 2c are slightly red-shifted to 411 and 412 nm, respec-
tively. Overall, consistent with these spectra, the compounds
appear yellow in solution and in the solid state. This lack of
absorption in the majority of the visible spectrum is impor-
tant with respect to the solid-state optical birefringence
measurements below.
trile,[27,28] 2-aminopyridine, and calcium chloride in refluxing
1-butanol or 1-hexanol for 18–19 h (Figure 1).[29,30]
Compounds 2b–2d were also structurally characterized by
using single-crystal X-ray diffraction. The solid-state struc-
tural data are consistent with the NMR spectra and elemen-
tal analyses. The single-crystal X-ray structure of 2a was re-
ported previously[31] and all bond lengths are within the ex-
pected ranges.[32] The metrical parameters of compounds 2c
and 2d are comparable to those previously reported for 2a:
all of the bond lengths are within 0.02 ꢂ and bond angles
are within 2.58 of the reported values for 2a.[31] Unfortunate-
ly, 2b exhibits significant disorder in the pyridyl rings and
the bond lengths of 2b show a larger deviation from the
values reported for 2a.[31] Nevertheless, the identity and con-
nectivity of 2b is unambiguous (Figure 3, top left).
Figure 1. Synthetic procedure for a series of bis(2-pyridylimino) isoindo-
lines 2b–2d. Compound 2a in which R=R’=H was synthesized accord-
ing to a literature procedure.[31]
Upon cooling to room temperature, crude 2b or 2c pre-
cipitated out of solution, was isolated by filtration and puri-
fied by recrystallization by layering hexanes onto a solution
of the crude product in chloroform. However, 2d remains in
solution upon cooling of the reaction mixture and a crude
solid was isolated by removing the solvent in vacuo and pu-
rified by silica gel column chromatography. Compounds 2b–
2d were isolated in low to moderate yields, which is partially
due to the tendency of the phthalonitrile starting materials
to form phthalocyanine impurities in addition to the desired
products.
1
The identity and purity of 2b–2d were determined by H
and 13C NMR spectroscopy and from the elemental analysis
1
(C, H, N). The H NMR spectra of 2b–2d all show broad
ꢀ
singlets for the isoindoline N H proton at d=13.63, 13.96,
and 13.88 ppm in CD2Cl2, respectively. The remaining reso-
nances in the spectra are consistent with the structure and
are unremarkable.
Unsubstituted bis(2-pyridylimino)isoindoline (2a) and
2b–2d all absorb strongly in the near-UV and violet regions
in chloroform with extinction coefficients on the order of
104 mꢀ1 cmꢀ1 (Figure 2). The lowest energy absorption band
for 2a has an absorption maximum at 409 nm. Compound
2d has an absorption spectrum that is similar to 2a and also
has an absorption maximum of 409 nm for the lowest energy
Figure 3. Molecular structures of 2b (top left), 2c (top right), and 2d
(bottom). Thermal ellipsoids are set at 30% probability. Hydrogen atoms
have been removed for clarity.
Compound 2a is not planar and the pyridyl rings are
tilted out of the isoindoline backbone mean plane by 108.
The disorder of the pyridyl rings in 2b makes it difficult to
provide a more detailed description of the tilting of the
rings. The diisopropylphenoxy substituents, however, are ori-
ented approximately perpendicular to the isoindoline core
of 2b. Compound 2c crystallizes with two molecules in the
asymmetric unit. The 4-tert-butylphenyl substituents of both
molecules are tilted out of the mean plane formed by the
nearly planar isoindoline moieties by 44–538 due to steric re-
pulsion between adjacent phenyl rings (Figure 3, top right).
The pyridine units are also tilted out of the mean plane
formed by the isoindoline backbone, but to a lesser degree
(8–158). Presumably, the tilting of the pyridine rings in 2a–
Figure 2. UV/Vis absorption spectra of 2a–2d in CHCl3.
&
2
&
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Birefringence in Bis(2-pyridylimino)isoindolines
FULL PAPER
2c is due to packing effects in the solid state, since the pyri-
dine rings are equivalent in solution for all three compounds
1
according to their H and 13C NMR spectra. Compound 2d
is unique in that it is planar in the solid-state (Figure 3,
bottom).
Overall, the molecular solid-state structures of 2a–2d are
unremarkable. However, the birefringence of the crystals is
determined not just from the anisotropy of the molecular
polarizability but, critically, the intermolecular solid-state
packing; this packing is, of course, influenced by the molecu-
lar conformation and overall planarity.
Intermolecular packing and birefringence: When examining
the birefringence of molecular crystals, such as those dis-
cussed in this paper, it is very important to take into consid-
eration both the shape of the individual molecules as well as
how they pack and orient themselves with respect to one an-
other in the overall crystal structure; these are standard con-
cepts in organic crystal engineering.[33–35] These considera-
tions can be aided by examining the Lorentz–Lorenz equa-
tion, which states that the refractive index in a particular di-
rection increases with the increase of either the polarizabili-
ty in that direction, or how densely the molecules pack in
the crystal structure.[3,8–10] Thus, by introducing a high aniso-
tropy in the polarizability, the anisotropy of the refractive
index (and hence birefringence) will be high. For instance,
in the case of 2,2’:6’,2’’-terpyridine (terpy), the individual
molecules are flat and contain highly polarizable aromatic
rings, which suggests a very high anisotropy in polariza-
tion,[36] a feature key to the enhancement of birefringent
properties. However, when its crystal structure is viewed
down the c axis, the molecules are oriented in a herringbone
pattern, which means overall, their anisotropy is lost, result-
ing in a nearly isotropic arrangement overall, and a very low
birefringence, whereas a face-to-face arrangement would
result in a high value. For the isoindoline molecules de-
scribed above, the birefringence structure–property relation-
ship is probed below by measuring their birefringence
values and interpreting them with respect to the 3D packing
of the molecules.[37]
Figure 4. View of the crystal structure of 2a in the (001) plane (top) and
(010) plane (bottom). The slice of the optical indicatrix measured is
shown. N atoms are shown in blue and C atoms in black.
b axis experiences the maximum polarizability of a molecule
of 2a, while the component along the a axis experiences
a smaller, but still significant projection of the moleculeꢃs
polarizability, thus a small birefringence value is observed.
If the component of the field along the a axis felt the full
polarizability of the molecule (i.e., if the molecule were ori-
ented 908 to the c axis, not 458), then the birefringence in
the ab plane would likely approach zero and Dn in the ac or
bc planes would be much higher. Hence, ideally, to maxi-
mize the birefringence, the molecules should be at 08 to
(i.e., aligned with) the direction of view. These considera-
tions are consistent with the 2D slice of the optical indicatrix
(an ellipsoidal plot of the value of the refractive index in
3D) shown in Figure 4.
Birefringence of 2b: In the solid state, molecules of 2b form
head-to-tail chains along the b axis that stack along the
a axis with intermolecular p–p distances of 6.78 ꢂ. The
much larger p–p distance in comparison to 2c and 2d (dis-
cussed below) is likely enforced by the steric bulk of the di-
AHCTUNGTREGiNNUN sopropylphenoxy substituents. Unlike 2a, 2b crystallizes in
a monoclinic space group with (110) as the primary face,
which means that none of the principle birefringence values
are measureable. Instead, an oblique slice of the optical in-
dicatrix is observed. The birefringence of this slice is there-
fore denoted as Dn(110) =n’ꢀn’’, in which n’ and n’’ are the
major axes of this slice of the indicatrix. For 2b, this value
was found to be Dn(110) =0.0629(17), with n’=1.713(2) and
n’’=1.651(2). Figure 5 (top) presents a view of this face.
What is seen here is very similar to 2a: a set of layered mol-
ecules oriented at an angle to the primary face. From this
figure, the birefringence can already be expected to be
small. When the structure is viewed perpendicularly to this
direction (Figure 5, bottom), it is revealed that the mole-
cules are oriented 358. This angle being slightly smaller than
that in 2a, one would expect a higher birefringence value,
but upon closer inspection, it is seen that the molecules are
also slightly out of alignment in the n’ direction as well.
This, in combination with the polarizability contribution of
the nearly perpendicular iPrPhO groups is consistent with
Birefringence of 2a: Since 2a crystallizes in an orthorhom-
bic space group, with the (001), or (ab), plane as the primary
face, one of the principle birefringence values (i.e., Dnab)
was determined. The birefringence of 2a was found to be
a modest, but still significant Dnab =0.0654(3) in this plane.
The polarizabilities in each direction are important, but in
particular those in the plane perpendicular to the direction
of view will determine Dn down that crystal face. Accord-
ingly, Figure 4 (top) presents a view of the crystal structure
as viewed perpendicularly to the (001) plane (i.e., along the
c axis). Here, the orientation of the individual flat molecules
of 2a is difficult to examine, therefore when the crystal
structure is viewed along the b axis (perpendicular to the di-
rection of measurement), it can be seen that the molecules
are tilted 458 from the c axis, as shown in Figure 4 (bottom).
Thus, the component of the lightꢃs electric field along the
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
3
&
ÞÞ
These are not the final page numbers!

D. B. Leznoff et al.
tion of highest polarizability is in the n’ direction. This align-
ment is what likely gives rise to the very large birefringence
value observed. A clearer picture of this system is shown in
the lower panel of Figure 6. This presents a perpendicular
view to that shown in the upper panel of Figure 6. As
shown, the molecules of 2c are aligned at 258 to the direc-
tion of view. Although this is much shallower than in 2a,
which is consistent with its much higher birefringence value,
the difference between the angles of this compound and 2b
is not large, yet a huge jump in birefringence is seen. It is
clear that other factors must be involved in this leap. For in-
stance, the bulky iPrPhO groups create a larger spacing be-
tween molecules (as seen in comparing the p–p distances),
resulting in a lower density, one of the key factors in refrac-
tive index value. It is likely that this, in combination with
the slight tilt in 2b discussed above results in a seemingly
low value for 2b, compared with 2c.
Birefringence of 2d: Molecules of 2d align themselves to
form parallel layers. Within these layers, each molecule
stacks to form columns along the c axis with short intermo-
lecular p–p distances of 3.39 ꢂ (Figure 7). As with 2a, com-
Figure 5. View of the crystal structure of 2b in the (110) plane (top) and
a perpendicular plane (bottom). The slice of the optical indicatrix mea-
sured is shown. N atoms are shown in blue, C atoms in black, and O
atoms in red.
the observation of a birefringence value similar to that for
2a.
Birefringence of 2c: Molecules of 2c pack in such a way as
to align themselves in parallel layers as well as stack in col-
umns along the c axis. These stacks form slanted columns
with average intermolecular p–p distances of 3.85 ꢂ. Com-
Figure 7. View of the crystal structure of 2d in the (010) plane. The slice
of the optical indicatrix measured is shown. N atoms are shown in blue
and C atoms in black.
¯
pound 2c crystallizes in a triclinic space group, with (011) as
the primary face, and a very high value of Dn(011) =0.588(10)
¯
was determined for the birefringence in this plane. When
¯
viewed perpendicularly to (011), as in Figure 6 (top), a lay-
pound 2d crystallizes in an orthorhombic space group, but
with (010) as the primary face. However, the 2d molecules
align themselves with the direction of view (i.e., at 08),
which, along with the short p–p distance implies that the
primary birefringence value, Dnac, should be very high.
Indeed, when measured, the value is found to be Dnac =
0.701(12), the highest of those reported among 2a–2d. Note
that 2a–2d have nearly identical UV/Vis spectra and thus
the wide range of Dn values, including the very large value
for 2d, is not attributable to apparent Dn inflations due to
measurements near absorption bands.[15]
ered structure can be seen, in which the molecules of 2c are
aligned nearly perpendicularly to this plane, and the direc-
Discussion
In lobsterate 2a and the three new lobsterate-based mole-
cules (2b–2d) synthesized here, we have illustrated how
heavily the orientation and arrangement of individual units
in a molecular crystal impact the birefringence. By varying
one aspect of the molecule, and hence how the molecules
collectively pack in the solid state, birefringence values
ranging across an order of magnitude (0.06–0.7) were ob-
tained. This trend is also seen in the literature, with birefrin-
¯
Figure 6. View of the crystal structure of 2c in the (011) plane (top) and
a perpendicular plane (bottom). The slice of the optical indicatrix mea-
sured is shown. N atoms are shown in blue and C atoms in black.
&
4
&
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Birefringence in Bis(2-pyridylimino)isoindolines
FULL PAPER
gence values for solid materials ranging from isotropic
(Dn=0) to very high (Dn>0.6),[11,38–45] However, only
a handful have a value greater than 0.3 (with many of them
less than 0.1), despite containing highly polarizable aromatic
moieties. The fact that these values vary so wildly stresses
the importance of the crystallographic arrangement when
a high value of birefringence is desired. For example, picric
acid is a planar aromatic compound, which should have
a very high polarizability in the plane and low polarizability
perpendicular to the plane, however, its birefringence value
is an unremarkable Dnac =0.06.[46] Much like 2a and 2b
(which have similar values), the picric acid molecules orient
themselves in a herringbone pattern in their crystal struc-
ture, and are not near parallel to the direction of view. How-
ever, ammonium picrate (the anion of which has the same
molecular polarizability anisotropy as picric acid) has
a much larger birefringence value of Dnac =0.419.[46,47] The
addition of the ammonium cations causes this material to
favor a much better aligned structure, in which the picrate
molecules are oriented near face-to-face, and are near paral-
lel to the direction of view, similar to 2c. This type of analy-
sis suggests that modern crystal-engineering concepts could
be harnessed to maximize preferred supramolecular orienta-
tions for generating high Dn materials.
ly aids in generating the high values of 2c and 2d. The
impact on Dn of controlling density in a systematic fashion
through crystal engineering deserves further attention.
Through these analyses, an improved understanding of
the impact of the solid-state 3D structure on birefringence
values was targeted, thereby providing a basis for interpret-
ing the large body of organic crystal birefringence data[11] at
the molecular packing level, as well as facilitate molecular
and crystal-engineering design principles[33–35] to generate
highly birefringent materials. With all of these features in
mind, we have started incorporating these ligands in combi-
nation with a coordination polymer framework with the
goal of reaching higher birefringence values.
Experimental Section
General: Anhydrous calcium chloride was stored, weighed out, and trans-
ferred to reaction flasks under a dinitrogen atmosphere in a MBraun
Labmaster 130 glovebox. All reflux reactions were carried out under a di-
nitrogen atmosphere using standard Schlenk techniques. Isolation of the
products was conducted in air.
Compounds 1b[27] and 1c[28] were synthesized according to reported pro-
cedures. Compound 2a was synthesized using a literature procedure.[31]
Compounds 2b–2d were synthesized using procedures adapted from pub-
lished literature methods.[29,30] Anhydrous calcium chloride (ꢁ96.0%),
anhydrous 1-hexanol (ꢁ99%), anhydrous 1-butanol (99.8%) and all
other reagents were obtained from commercial sources and used as re-
ceived.
Conclusion
1H and 13C NMR spectra were collected on a Bruker AVANCE II digital
NMR spectrometer with a 5 mm QNP cryoprobe operating at 600 MHz
for 1H spectra. Chemical shifts for 1H NMR spectra were referenced to
residual 1H NMR resonances of the deuterated solvent and 13C NMR
spectra were referenced to 13C NMR resonances of the deuterated sol-
vent. All chemical shifts are reported relative to tetramethylsilane.
This structure–birefringence property analysis of 2a and its
derivatives has clearly demonstrated the importance of sev-
eral factors which affect the birefringence values of solids.
These include:
All UV/Vis absorption spectra were recorded on a Varian Cary 5000
Spectrophotometer in a 0.1 cm quartz cell at room temperature. UV/Vis
samples were prepared as 0.33 mm stock solutions (50 mL) in chloroform.
1) The anisotropic polarizability of the molecules.
2) Supramolecular orientation of the molecules in the unit
cell.
Elemental analyses (C, H, N) were performed by Mr. Farzad Haftbara-
daran at Simon Fraser University on a Carlo Erba EA 1110 CHN ele-
mental analyzer.
3) The primary growth face.
Optical transparency at the wavelength of measurement is
also an important consideration. The first point above is il-
lustrated especially well in 2c and 2d, for which a highly
anisotropic polarizability is the critical factor. Without this
feature, the relative orientation of the molecules would have
little effect. This alone, however, is insufficient for a high bi-
refringence value. All of the molecules examined here have
similar molecular polarizability anisotropies, yet the differ-
ences in birefringence vary widely. Ideally, for aromatic lig-
Synthesis of 1,3-bis(2-pyridylimino)-5,6-bis(2,6-diisopropylphenoxy)isoin-
doline (2b): A mixture of 4,5-bis(2,6-diisopropylphenoxy)phthalonitrile
ACHTUNGTRENNUNGands, such as these lobsterates, the molecules would be
aligned face-to-face (the second point, above). This is the
case for 2c and 2d, hence their very high Dn values. Finally,
a crystal may have a very large birefringence in one plane,
but it may not be observable if the crystals do not grow with
that face as the primary (largest) face.
One other factor to consider is density. With a lower
packing density, the refractive indices will also decrease (as
per the Lorentz–Lorenz equation). This is likely another im-
portant factor leading to the low value for 2b, and potential-
(2.8711 g, 5.973 mmol), 2-aminopyridine (1.2928 g, 13.74 mmol), and
CaCl2 (0.3315 g, 2.987 mmol) was heated to reflux in 1-butanol (20 mL)
for 18 h to give a purple reaction mixture. The reaction mixture was
cooled to room temperature and a yellow solid with green and purple im-
purities was collected by vacuum filtration, washed with hexanes, and air
dried. The crude product was dissolved in chloroform, filtered through
celite, and recrystallized by layering hexanes over the chloroform solu-
tion in test tubes. Yellow crystals of 2b were collected by vacuum filtra-
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
5
&
ÞÞ
These are not the final page numbers!

D. B. Leznoff et al.
tion, washed with hexanes, and air dried. A second small batch of crystals
can be obtained by repeating the crystallization procedure on the filtrate.
Yield: 2.587 g (66.44%); m.p. >3008C; 1H NMR (600 MHz, CD2Cl2,
298 K): d=1.24 (brm, 26H; CHMe2), 3.21 (septet, J=7 Hz, 4H;
CHMe2), 6.98 (s, 2H; H-8), 7.08 (m, 2H; H-2), 7.29 (m, 2H; H-4), 7.36
(m, 6H; H-11 and H-12), 7.71 (m, 2H; H-3), 8.55 (m, 2H; H-1),
13.63 ppm (brs, 1H; NH); 13C{1H} NMR (151 MHz, CD2Cl2, 298 K): d=
22.9 (C-15), 24.6 (C-15), 28.0 (C-14), 107.7 (C-8), 120.5 (C-2), 123.4 (C-
4), 125.4 (C-12), 127.0 (C-11), 130.2 (C-7), 138.6 (C-3), 142.1 (C-13),
148.3 (C-1), 149.1 (C-10), 152.0 (C-9), 153.7 (C-6), 161.1 ppm (C-5); ele-
mental analysis calcd (%) for C42H45N5O2: C 77.39, H 6.96, N 10.74;
found: C 77.06, H 6.88, N 10.51.
129.77 (C-9’), 133.85 (C-7’), 136.48 (C-7), 138.65/138.67 (C-3 and C-3’),
148.41/148.45 (C-1 and C-1’), 154.09 (C-6’), 154.39 (C-6), 156.49 (C-9),
161.14/161.19 ppm (C-5 and C-5’); elemental analysis calcd (%) for
C22H21N5: C 74.34, H 5.96, N 19.70; found: C 74.51, H 5.82, N 19.40.
X-ray crystallography: Crystallographic data for 2b–2d are listed in
Table 1. A crystal suitable for data collection was selected and mounted
in Paratone oil on a MiTeGen head, then placed in the cold stream
(100 K) of the diffractometer. Data were collected on
a Bruker
SMART6000 diffractometer with CuKa radiation using w and f scans for
complete coverage. The data were then processed using SAINT (Bruker
AXS, v6.26 A) and corrected for absorption using SADABS (Bruker
AXS v2.03).
Synthesis of 1,3-bis(2-pyridylimino)-5,6-bis(4-tert-butylphenyl)isoindoline
(2c):
A mixture of 4,5-bis(4-tert-butylphenyl)phthalonitrile (1.996 g,
Table 1. Crystallographic data for 2b, 2c, and 2d.
2b
2c
2d
formula
formula weight
crystal system
space group
a [ꢂ]
b [ꢂ]
c [ꢂ]
a [8]
b [8]
C42H45N5O2
651.85
monoclinic
C2/c
14.6097(2)
13.1038(2)
19.5557(3)
90
92.6030(10)
90
3739.93(10)
4
C38H37N5
563.74
C22H21N5
355.44
orthorhombic
Pbcm
13.4118(3)
20.0522(4)
6.7768(2)
90
triclinic
¯
P1
11.8199(4)
16.8456(5)
18.3027(5)
111.7740(10)
94.227(2)
108.673(2)
3128.49(18)
4
5.085 mmol), 2-aminopyridine (1.101 g, 11.69 mmol), and CaCl2 (0.282 g,
2.54 mmol) was heated to reflux in 1-hexanol (20 mL) for 18 h to give
a green mixture. A yellow solid precipitated out of the reaction mixture
as it cooled to room temperature. The yellow solid was collected by
vacuum filtration, washed with hexanes, and air dried. The crude product
was dissolved in chloroform, gravity filtered and then recrystallized by
layering hexanes over the chloroform solution in test tubes. Yellow crys-
tals of 2c were collected by vacuum filtration, washed with hexanes, and
air dried. The solvent was removed from the filtrate under reduced pres-
sure and a second batch of yellow crystals was obtained by repeating the
recrystallization process. Yield: 1.119 g (39.04%); m.p. 2988C; 1H NMR
(600 MHz, CD2Cl2, 298 K): d=1.31(s, 18H; tBu), 7.15 (m, 2H; H-2), 7.18
(m, 4H; H-11), 7.30 (m, 4H; H-12), 7.42 (m, 2H; H-4), 7.79 (m, 2H; H-
3), 8.06 (s, 2H; H-8), 8.65 (m, 2H; H-1), 13.96 ppm (brs, 1H; NH);
13C{1H} NMR (151 MHz, CD2Cl2, 298 K): d=31.6 (C-15), 34.9 (C-14),
120.8 (C-2), 123.8 (C-4), 125.0 (C-8), 125.4 (C-12), 130.0 (C-11), 135.2 (C-
7), 138.6 (C-10), 138.7 (C-3), 145.1 (C-9), 148.5 (C-1), 150.7 (C-13), 154.0
(C-6), 161.1 ppm (C-5); elemental analysis calcd (%) for C38H37N5: C
80.96, H 6.62, N 12.42; found: C 80.69, H 6.59, N 12.25.
90
90
g [8]
V [ꢂ3]
Z
1822.53(8)
4
T [K]
100(2)
1.158
0.565
3324
3218
0.0811
0.0893
1.0994
100(2)
1.197
0.550
10852
10152
0.0480
0.0542
1.0898
100(2)
1.295
0.627
1812
1714
0.0426
0.0683
0.8964
1calcd [gcmꢀ3
]
m [mmꢀ1
]
reflns collected
significant reflns
R [Io ꢁ2.5s(Io)]
Rw [Io ꢁ2.5s(Io)]
goodness of fit
The structures were solved using direct methods (SIR92)[48] and refined
by least-squares procedures using CRYSTALS.[49] Hydrogen atoms were
placed in idealized geometric positions and linked to their respective
carbon atoms using a riding model during refinement. The isotropic tem-
perature factor of each hydrogen atom was initially set to 1.2 times that
of the carbon atom it is bonded to and then the temperature factors of
groups of similar hydrogen atoms were linked during refinement. Com-
pound 2b exhibited disorder of the entire molecule over two positions as
well as significant disorder of the pyridyl moieties in multiple conforma-
tions that could not be accounted for, which resulted in irregular ellipsoid
shapes throughout the entire molecule. Crystal structure diagrams were
generated using ORTEP-3 for Windows (v. 2.02)[50] and rendered using
POV-Ray (v. 3.6.1c).[51]
Synthesis of 1,3-bis(2-pyridylimino)-5-
tert-butylisoindoline (2d): A mixture
of 4-tert-butylphthalonitrile (1.658 g,
9.000 mmol), 2-aminopyridine (1.788 g,
19.00 mmol), and CaCl2 (0.499 g,
4.50 mmol) was heated to reflux in 1-
butanol (25 mL) for 19 h to give a pale
green mixture. The solvent was re-
moved by vacuum distillation and the
green oil was dissolved in chloroform
and washed with water. The organic
layer was collected and concentrated
under reduced pressure. The green im-
CCDC-848830 (2b), 848829 (2c), and 848828 (2d) contain the supple-
mentary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Birefringence measurements: Optical retardation and crystal thickness
measurements were made on plate-shaped crystals of 2a, 2c and 2d by
means of polarized light microscopy using an Olympus BX60 microscope
with a U-CTE Berek compensator (2a) and a U-CTB thick Berek com-
pensator (2c and 2d) at l=546 nm at room temperature. The birefrin-
gence was calculated by dividing the retardation by the thickness. The
orientation of the slice of the optical indicatrix in the viewing plane was
determined using single-crystal X-ray diffraction techniques.
purity was separated from the product by silica gel column chromatogra-
phy (50:1 CHCl3/CH3OH followed by 40:1 CHCl3/CH3OH then 25:1
CHCl3/CH3OH). X-ray quality crystals were obtained by layering water
under a concentrated ethanol solution of the purified product. The
yellow crystals of 2d were collected by vacuum filtration, washed with
water, and dried under vacuum at 808C. Yield: 0.875 g (27.4%); m.p.
1278C; 1H NMR (600 MHz, CD2Cl2, 298 K): d=1.45 (s, 9H; tBu), 7.14
(m, 2H; H-2 and H-2’), 7.43 (m, 2H; H-4 and H-4’), 7.73 (m, 1H; H-9’),
7.79 (m, 2H; H-3 and H-3’), 7.95 (m, 1H; H-8’), 8.08 (m, 1H; H-8), 8.62
(m, 2H; H-1 and H-1’), 13.88 ppm (brs, 1H; NH); 13C{1H} NMR
(151 MHz, CD2Cl2, 298 K): d=31.70 (C-11), 36.01 (C-10), 119.78 (C-8),
120.65/120.70 (C-2 and C-2’), 122.58 (C-8’), 123.61/123.62 (C-4 and C-4’),
The refractive index, and its dependence on crystal orientation, of 2b
was determined by using a Metricon Model 2010/M Prism Coupler at l=
1552 nm. The refractive index was measured as the crystal was rotated
about the normal to the measurement plane from q=0–1208 in 108 incre-
&
6
&
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Birefringence in Bis(2-pyridylimino)isoindolines
FULL PAPER
ments (collection of further data points was precluded by crystal destruc-
tion). The data were fit to Equation (1), in which n(q) is the orientation-
dependent refractive index, 2A=Dn(hkl) is the difference between the
maximum and minimum refractive indices (i.e., the birefringence in the
(hkl) plane), d is the relative phase between the instrument and crystal
axes and n¯ is the average refractive index.
[20] Design and Construction of Coordination Polymers (Eds.: M.-C.
Hong, L. Chen), Wiley, New Jersey, 2009.
[21] J. S. Ovens, A. R. Geisheimer, A. A. Bokov, Z.-G. Ye, D. B. Leznoff,
Inorg. Chem. 2010, 49, 9609.
[22] M. J. Katz, D. B. Leznoff, J. Am. Chem. Soc. 2009, 131, 18435.
[23] M. J. Katz, H. Kaluarachchi, R. J. Batchelor, A. A. Bokov, Z.-G. Ye,
D. B. Leznoff, Angew. Chem. 2007, 119, 8960; Angew. Chem. Int.
Ed. 2007, 46, 8804.
[24] N. D. Draper, R. J. Batchelor, B. C. Sih, Z.-G. Ye, D. B. Leznoff,
Chem. Mater. 2003, 15, 1612.
ꢀ
ꢁ
q
ꢀ
nðqÞ ¼ A cos
ꢀ d þ n
ð1Þ
p
[25] C. A. Bessel, R. F. See, D. L. Jameson, M. R. Churchill, K. J. Takeu-
chi, J. Chem. Soc. Dalton Trans. 1992, 3223.
[26] K. F. Bowes, I. P. Clark, J. M. Cole, M. Gourlay, A. M. E. Griffin,
M. F. Mahon, L. Ooi, A. W. Parker, P. R. Raithby, H. A. Sparkes, M.
Towrie, CrystEngComm 2005, 7, 269.
[27] N. B. McKeown, S. Makhseed, K. J. Msayib, L.-L. Ooi, M. Helliwell,
J. E. Warren, Angew. Chem. 2005, 117, 7718; Angew. Chem. Int. Ed.
2005, 44, 7546.
Acknowledgements
The authors gratefully acknowledge financial support provided by the
Natural Sciences and Engineering Research Council of Canada, Natural
Resources Canada, and Simon Fraser University. We thank Dr. James F.
Britten and Dr. Hilary Jenkins at McMaster University for assistance
with X-ray crystallography.
[28] S. Eu, T. Katoh, T. Umeyama, Y. Matano, H. Imahori, Dalton Trans.
2008, 40, 5476.
[29] W. O. Siegl, J. Org. Chem. 1977, 42, 1872.
[30] B. Langlotz, H. Wadepohl, L. Gade, Angew. Chem. 2008, 120, 4748;
Angew. Chem. Int. Ed. 2008, 47, 4670.
[31] W. Schilf, J. Mol. Struct. 2004, 691, 141.
[1] P. Velasquez, M. del Mar Sꢄnchez-Lꢅpez, I. Moreno, D. Puerto, F.
Mateos, Am. J. Phys. 2005, 73, 357.
[2] S. Saeed, P. J. Bos, Z. Li, Jpn. J. Appl. Phys. Part 1 2001, 40, 3266.
[3] S. G. Lipson, H. Lipson, D. S. Tannhauser, Optical Physics, 3rd ed.,
Cambridge University Press, Cambridge, U.K., 1995.
[4] S.-T. Wu, C.-S. Wu, Appl. Phys. Lett. 1996, 68, 1455.
[5] M. Schadt, Jpn. J. Appl. Phys. 2009, 48, 03B001.
[6] H. L. Ong, Appl. Phys. Lett. 1991, 59, 155.
[7] M. J. Weber, Handbook of Optical Materials, CRC Press, Boca
Raton, FL, 2003.
[32] F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen,
R. Taylor, J. Chem. Soc. Perkin Trans. 2 1987, 12, S1.
[33] K. Biradha, C.-Y. Su, J. J. Vittal, Cryst. Growth Des. 2011, 11, 875.
[34] C. B. Aakerçy, N. R. Champness, C. Janiak, CrystEngComm 2010,
12, 22.
[35] D. Das, G. Desiraju, CrystEngComm 2006, 8, 674.
[36] S. B. Bulgarevich, D. V. Bren, D. Y. Movshovich, S. E. Filippov, E. P.
Olekhnovich, I. V. Korobka, Russ. J. Gen. Chem. 2002, 72, 1446.
[37] R. R. Tykwinski, U. Gubler, R. E. Martin, F. Diederich, C. Bosshard,
P. Gꢆnter, J. Phys. Chem. B 1998, 102, 4451.
[38] R. N. Castle, N. F. Witt, C. F. Poe, J. Am. Chem. Soc. 1949, 71, 228.
[39] R. N. Castle, Mikrochim. Acta 1951, 38, 92.
[40] R. N. Castle, Mikrochim. Acta 1953, 40, 196.
[41] R. N. Castle, J. Am. Pharm. Assoc. 1952, 41, 143.
[42] R. N. Castle, Mikrochim. Acta 1955, 43, 761.
[43] J. A. Biles, Mikrochim. Acta 1952, 39, 69.
[8] R. E. Newnham, Structure–Property Relations, Springer, New York,
1975.
[9] R. E. Newnham, Properties of Materials: Anisotropy, Symmetry,
Structure, Oxford University Press, New York, 2005.
[10] E. Hecht, Optics, 4th ed., Addison Wesley, San Francisco, 2002.
[11] A. N. Winchell, The Optical Properties of Organic Compounds, Aca-
demic Press, New York, 1954.
[12] E. T. Wherry, J. Wash. Acad. Sci. 1918, 8, 277.
[13] R. G. Hunsperger, Integrated Optics: Theory and Technology,
Springer, New York, 2009.
[14] Most of the very high birefringence values are for crystals that are
highly colored dyes, and since the refractive index of a material at
a wavelength close to an absorption band changes enormously
versus the transparent region, these measurements should be viewed
with a great deal of caution; see reference [15].
[44] W. Bryant, Microscope 1976, 24, 261.
[45] W. Bryant, Microscope 1992, 40, 153.
[46] J. Mitchell Jr., W. M. D. Bryant, J. Wash. Acad. Sci. 1943, 33, 128.
[47] H. E. Merwin, J. Wash. Acad. Sci. 1919, 9, 429.
[48] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J. Appl.
Crystallogr. 1993, 26, 343.
[49] P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout, D. J.
Watkin, J. Appl. Crystallogr. 2003, 36, 1487.
[50] L. J. Farrugia, J. Appl. Crystallogr. 1997, 30, 565.
[51] Persistance of Vision Pty., Persistance of Vision (TM) Raytracer, Per-
sistance of Vision Pty., Williamstown, Victoria, Australia, 2004.
[15] S. H. Simpson, R. M. Richardson, S. Hanna, J. Chem. Phys. 2007,
127, 104901.
[16] B. Kahr, J. Freudenthal, E. Gunn, Acc. Chem. Res. 2010, 43, 684.
[17] W. Kaminsky, K. Claborn, B. Kahr, Chem. Soc. Rev. 2004, 33, 514.
[18] S. R. Batten, S. M. Neville, D. R. Turner, Coordination Polymers:
Design, Analysis and Application, RSC, Cambridge (UK), 2009.
[19] Metal–Organic Frameworks: Design and Application (Ed.: L. R.
MacGillivray), Wiley, New Jersey, 2010.
Received: October 31, 2011
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!

D. B. Leznoff et al.
Birefringence
E. W. Y. Wong, J. S. Ovens,
D. B. Leznoff* ................ &&&& — &&&&
From Low to Very High Birefringence
in Bis(2-pyridylimino)isoindolines:
Synthesis and Structure–Property
Analysis
Lobsterates! Three bis(2-pyridylimi-
no)isoindoline (lobsterate) derivatives
were synthesized and structurally char-
acterized. The birefringence (Dn)
values of these (including the parent
lobsterate) were measured, yielding
Dn values of 0.06–0.7. These are corre-
lated with the orientation and 3D
packing of the molecules in their crys-
tal structure, providing a framework
for interpreting birefringence data of
known organic molecules.
&
8
&
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!