Polymorphism of dehydrobenzo[14]annulene possessing two methyl ester groups in noncentrosymmetric positions

Article abstract of DOI:10.1021/cg201075z

We revealed that the π-conjugated cyclic compound octadehydrotribenzo[14]annulene ([14]DBA), with two methyl ester groups in noncentrosymmetric positions, formed five polymorphic crystals: forms I (plate, P2₁/c), II (block, P2₁/c), II′ (block P1), III (needle, P2₁/c), and IV (needle, C2/c), which is the largest number for polymorphic crystals of the DBA family. Forms I, III, and IV can be obtained selectively upon the crystallization conditions applied. The present polymorphism was provided by conformational flexibility and varied CH/O interaction methods of the methyl ester groups. The thermal stability, fluorescence properties, and photoconductivity of the polymorphs were investigated. Such properties are affected by the polymorphic supramolecular structure. Furthermore, we demonstrate that the polymorphs exhibit physical or chemical defects depending on the magnitude of the π-overlap of the DBA planes and that the fluorescence and electronic properties are strongly affected by the defects. Particularly, form I shows a significant fluorescence band at 530 nm, probably due to defects in the crystals.

Full text of DOI:10.1021/cg201075z

ARTICLE
pubs.acs.org/crystal
Polymorphism of Dehydrobenzo[14]annulene Possessing Two Methyl
Ester Groups in Noncentrosymmetric Positions
Ichiro Hisaki,*,† Eriko Kometani,† Hajime Shigemitsu,† Akinori Saeki,‡,§ Shu Seki,‡ Norimitsu Tohnai,†,§ and
Mikiji Miyata*,†
†Department of Material and Life Science, and ‡Division of Applied Chemistry, Graduate School of Engineering, Osaka University,
Suita, Osaka 565-0871, Japan
§PRESTO Japan Science and Technology Agency (JST), Japan
S Supporting Information
b
ABSTRACT: We revealed that the π-conjugated cyclic compound
octadehydrotribenzo[14]annulene ([14]DBA), with two methyl ester
groups in noncentrosymmetric positions, formed five polymorphic crystals:
forms I (plate, P21/c), II (block, P21/c), II0 (block P1), III (needle, P21/c),
and IV (needle, C2/c), which is the largest number for polymorphic crystals
of the DBA family. Forms I, III, and IV can be obtained selectively upon the
crystallization conditions applied. The present polymorphism was pro-
vided by conformational flexibility and varied CH/O interaction methods
of the methyl ester groups. The thermal stability, fluorescence properties,
and photoconductivity of the polymorphs were investigated. Such proper-
ties are affected by the polymorphic supramolecular structure. Furthermore, we demonstrate that the polymorphs exhibit physical or
chemical defects depending on the magnitude of the π-overlap of the DBA planes and that the fluorescence and electronic properties
are strongly affected by the defects. Particularly, form I shows a significant fluorescence band at 530 nm, probably due to defects in
the crystals.
’ INTRODUCTION
In this study, we planned to obtain polymorphs of π-con-
jugated molecules by taking advantage of the conformational and
interactional flexibility of methyl ester groups (Figure 1c). The
intended polymorph preparation based on large planar π-con-
jugated molecules is still an important challenge,15 because the
consequent polymorphs were expected to show varied arrange-
ments concerning π-overlap geometries and coplanarity of
adjacent π-planes, which affect the optical and electrical proper-
ties of the bulks (Figure 1d).16,17
As a π-conjugated molecule to obtain polymorphs, we
applied octadehydrotribenzo[14]annulene ([14]DBA).18 The com-
pound was first reported by Vollhardt, Youngs, and co-workers as
a monomer of topochemical polymerization.19 Haley and co-
workers synthesized a number of its derivatives, such as a series of
octadehydro[14]annulenes annelated by 0 to 3 benzene ring-
(s)20 and [14]annulene-based hybrid systems annelated by
dihydropyrene21 and paracyclophene,22 to investigate a mag-
netic-induced ring current on the annulene rings. They also
reported [14]DBA-based donorꢀacceptor systems23 and ex-
panded systems,24 which show significant optical properties.
However, systematical investigation of [14]DBA-based supra-
molecular structures in the solid state has not been reported.
The functionality of solid state materials based on π-conju-
gated molecules is crucially affected by their molecular arrange-
ments.1 Therefore, understanding of the relationships between
structures and properties is significantly important. Polymorphic
crystals2 are exactly the appropriate systems to reveal the above-
mentioned relationships clearly because their varied supramole-
cular structures are just composed of a unique molecule or
component. Indeed, a number of structure-dependent properties
have been reported on the basis of polymorphs: those include
conductivity,2c,3 magnetic properties,2c,4 solid state reactions,2c,5
photochromism,2c,6 and fluorescence emission.2c,7
As pointed out by McCrone, every compound essentially has
the potential to give polymorphic crystals.8,9 However, to obtain
polymorphs more effectively, conformational flexibility and multi-
functional groups providing versatile intermolecular interactions
are generally required for molecules, i.e., conformational polymor-
phism (Figure 1a)10 and synthon polymorphism (Figure 1b),11
respectively. For example, Yu and co-workers revealed seven
polymorphic crystal structures of the molecule so-called ROY,
which are provided due to its versatile conformers,12 and Nangia
and co-workers reported 19 symmetry-independent conformers of
4,4-diphenyl-2,5-cyclohexadienone in its polymorphic crystals.13
Co-crystals composed of multifunctionalized compounds also yield
polymorphs frequently due to a heterosynthon, that is a hydrogen
bonded recognition unit between dissimilar molecules.14
Received: August 17, 2011
Revised:
October 16, 2011
Published: October 18, 2011
r
2011 American Chemical Society
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Scheme 2. Conformational (a) and Interactional (b) Flex-
ibility of [14]DBA 1 with Methyl Ester Groupsa
Figure 1. Schematic representation of strategies for affirmative emer-
gence of polymorphs: conventional systems with (a) conformational
flexibility and (b) multifunctional groups and (c) the present system
with methyl esters groups at noncentrosymmetric positions, which is
expected to yield varied stacking geometries concerning π-overlap and
coplanarity of adjacent π-planes (d).
a Part a shows three conformers of 1: the ver-ver, hor-ver, and hor-hor
conformations, which are classified according to the direction (vertical
or horizontal) of the carbonyl groups in the ester. Part b is a schematic
representation of the versatile CH/O interactions expected for 1: the
single, complementary, and branched CH/O interactions.
Scheme 1. Octadehydrotribenzo[14]annulene Derivative
moieties and the naphthalene derivative with two methyl ester
groups yielded three and two polymorphs, respectively.25 On the
basis of these results, we propose an empirically derived hypoth-
esis that noncentrosymmetric introduction of methyl ester
groups into the periphery of a π-plane tends to provide poly-
morphs, due to the following aspects of the group: (1) Rotational
flexibility of the methyl ester can provide conformers, i.e. the ver-
ver, hor-ver, and hor-hor conformers (Scheme 2a), where ver and
hor denote the vertical and horizontal direction of the carbonyl
group, bringing about significant changes of the dipole and
quadrupole moments of the molecule and electrostatic potential
surface on the molecule.25a (2) The carbonyl oxygen atom in the
group can involve weak, less directional, and, therefore, geome-
trically- and topologically versatile CH/O interactions26 with
electrostatically positive hydrogen atoms such as those of aro-
matic rings and methyl groups (Scheme 2b), allowing the
π-conjugated core to pack into various arrangements. (3) The
compounds functionalized by the group show better solubility
for organic solvents than functional groups such as carboxylic
acid and urea, allowing the compound to be subjected to various
crystallization conditions, i.e., solvents and temperatures.
[14]DBA 1 was synthesized according to Scheme 3. Pd-
catalyzed cross-coupling reaction of iodobenzene derivative 217
and diethynylbenzene 3 gave acyclic tetrayne 4. Desilylation
followed by Cu-mediated oxidative intramolecular cyclization
gave 1 in good yield.
Therefore, construction of various molecular arrangements of
[14]DBA in the solid state might lead to development of
significant optical and electrical materials and topochemically
polymerized functional materials.
Herein, we describe the preparation and crystallographic
characterization of five polymorphs obtained from [14]DBA 1
with two methyl ester groups at the syn-positions (Scheme 1).
The parent compound of 1 has just one crystalline form, while
the five polymorphs are the largest numbers for polymorphic
crystals in the DBA family.19 This fact indicates that the
noncentrosymmetric introduction of a methyl esters in conju-
gated planar molecules can be a useful strategy to obtain
polymorphs affirmatively. Furthermore, the thermal stability,
fluorescent, and electronic properties of the polymorphs were
investigated. Such properties are dependent on the polymorphs.
Interestingly, we revealed that the polymorphs exhibited struc-
tural defects depending on the degree of π-overlap of the DBA
planes and that the fluorescence and electronic properties were
strongly affected by the defects. Particularly, form I shows a
significant fluorescence band at 530 nm, probably due to defects
in the crystals.
Crystal Structures of Polymorphs. Crystallization under
various conditions yielded totally five polymorphic crystals as
shown in Figure 2: forms I (plate, P21/c), II (block, P21/c), II0
(block P1) III (needle, P21/c), and IV (needle, C2/c), where the
crystal morphologies and the space groups are in parentheses.27
’ RESULTS AND DISCUSSION
Molecular Design and Synthesis. Previously, we reported
that the octadehydro[12]annulene derivative with methyl phthalate
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Scheme 3. Synthesis of [14]DBA 1a
II, II0, and III exhibits the ver-ver conformation, while that in form
IV exhibits the hor-ver conformation. No hor-hor conformation
was observed in the present polymorphs, although the confor-
mation was preliminarily observed in the solvate of 1 with 1,2,4-
trichlorobenzene.
Because of the isosceles triangular shape of 1, the molecules in
the each polymorph form parallelogramꢀdimeric pairs with the
inversion center as expediential motifs of the crystals, where
directional interaction was not necessarily observed in the motif.
The dimeric motifs of forms I, II, and II0 have similar coplanar
geometries, while those of forms III and IV have uneven parallel
geometries, as shown in Figure 3(ii).
a Reagents and reaction conditions: (a) Pd(PPh3)4, CuI, i-Pr2NH, THF,
RT, 22 h, 78%; (b) TBAF, THF, RT, 1 h; (c) CuCl, pyridine, MeOH,
RT, 2 h, 69% for 2 steps. TBAF, tetrabutylammonium fluoride; TMS,
trimethylsilyl.
As shown in Figure 3a, the motif in form I is aligned one-
dimensionally to form a tapelike structure. The tape is slipped-
stacked to form a layer parallel to the ab plane. The adjacent
layers (green and yellow ones) are staggered by 45.1°. Two types
of CH/O interactions are observed among the layers; one is
between the carbonyl and the methoxy group (type S1): C30-
(A)ꢀH O3 (C O distance, 3.14 Å; CꢀHꢀO angle,
3 3 3
3 3 3
111.2°); the other is a branched interaction involving the carbonyl,
methoxy, and phenyl groups (type-B1): C28(B)ꢀH O1
3 3 3
(C O distance, 3.33 Å; CꢀHꢀO angle, 122.5°) and C16-
3 3 3
(C)ꢀH O1 (C O distance, 3.51 Å; CꢀHꢀO angle,
3 3 3
3 3 3
143.8°).
In form II (Figure 3b), the dimeric motif aligns into an offset
arrangement to form a planar sheet parallel to the (1 0 4) plane.
The sheet is slipped-stacked to form a coplanar structure.
[14]DBA 1 participates in two in-plane CH/O interactions
between the carbonyl and phenyl groups (type S2): C17-
(A)ꢀH O3 (C O distance, 3.19 Å; CꢀHꢀO angle,
3 3 3
3 3 3
Figure 2. Photos of the polymorphs under ambient light (top) and UV
light with wavelength of 365 nm (bottom): forms I (a, e), II and II0 (b, f),
III (c, g), and IV (d, h). Scale bars: 200 μm. Forms II and II0 are obtained
as a mixture, where a typical crystal shape of form II0 is cubic (inset),
while that of form II shows an irregular mosaic morphology.
132.3°) and C15(B)ꢀH O1 (C O distance, C
3 3 3
O
3 3 3
3 3 3
distance, 3.60 Å; CꢀHꢀO angle, 178.6°).
Interestingly, a sheet motif of form II0 (Figure 3c) parallel to
the (1 ꢀ1 6) plane is quite similar to that of form II. The CH/O
interactions are also observed in similar way: C17(A)ꢀH O3
3 3 3
(C O distance, 3.19 Å; CꢀHꢀO angle, 132.7°) and
3 03 3
These crystals are generally obtained concomitantly and depend-
ing on the crystallization batches (Table S1). The appearance
frequency of the forms is in the following order: I, III, II ∼ II0, IV.
However, careful consideration of the crystallization conditions
achieved almost selective formation of individual forms except
for forms II and II0: form I was solely obtained by slow
evaporation (SE) of ethyl acetate/dichloromethane solution at
ambient temperature (AT). Form III was obtained by SE from
tetrahydrofuran solution at AT. Sonication of the gelly materials
obtained from SE of a toluene solution at ꢀ19 °C also gave
crystalline bulk of form III. Form IV appeared only by cooling of
supersaturated 1,2-dichloroethane solution at ca. ꢀ19 °C. A
mixture of forms II and II0 (abbreviated as form II/II0) was
formed by SE of a chloroform and 1-propanol solution at AT.
However, despite many attempts, forms II and II0 were difficult to
obtain separately. Structural uniformity of the obtained crystal-
line bulks was confirmed by consistency of the experimental and
simulated powder X-ray diffraction patterns (see Figure S3).
Since the block crystals of form II exhibited high mosaicity and
the needle crystals had width no more than 10 μm, determination
of crystal structures was performed by synchrotron X-ray diffrac-
tion spectroscopy as well as with general equipment.28 Figure 3
shows the crystal structures of the polymorphs with their
hierarchical interpretation29 in the following order: (i) molecular
conformations, (ii) expediential dimeric motifs, (iii) the motif’s
arrangements, and (iv) crystal structures. [14]DBA 1 in forms I,
C15 ꢀH O1 (C O distance, 3.65 Å; CꢀHꢀO angle,
3 3 3
3 3 3
176.0°).30 The only significant difference between forms II and
II0 is emerged in the stacking of the sheet motifs: their slipped-
stacking directions are opposite, as shown by the blue arrows in
Figure 3 (also see, Figure 4). Because of the slight difference,
these two forms are difficult to obtain in complete solitude.
In form III (Figure 3d), 1 forms the slipped-stacked columnar
assembly with large π-overlap along the c axis. The adjacent pairs
colored in green and yellow are staggered by 50.7°, while those in
the same color are parallel. Two types of CH/O interactions are
observed: one is the B1-type branched interactions: C30-
(A)ꢀH O3 (C O distance, 3.52 Å; CꢀHꢀO angle,
3 3 3
3 3 3
162.4°) and C17(B)ꢀH O3 (C O distance, 3.31 Å;
3 3 3
3 3 3
CꢀHꢀO angle, 131.3°). The other is the self-complementary
CH/O interaction involving the aromatic hydrogen atoms (type
C-1): C7ꢀH O1(C) and C7(C)ꢀH O1 (C O dis-
3 3 3
3 3 3
3 3 3
tance, 3.27 Å; CꢀHꢀO angle, 161.4°). Contrary to the ver-ver
conformation in forms IꢀIII, 1 in form IV exhibits the hor-ver
conformation. As shown in Figure 3e, the carbonyl group with
the horizontal conformation leads to a complementary CH/O
interaction to form the uneven parallel dimeric motif:
C29ꢀH O3(B) and C29(B)ꢀH O3 (C O distance,
3 3 3
3 3 3
3 3 3
3.30 Å; CꢀHꢀO angle, 125.9°). The carbonyl oxygen O3 also
contacts with a methoxy group to form a CH/O bond: C17-
(C)ꢀH O3 (C O distance, 3.39 Å; CꢀHꢀO angle,
3 3 3
3 3 3
140.8°). The carbonyl group with the vertical conformation
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Figure 3. Comparison of crystal structures of forms I (a), II (b), II0 (c), III (d), and IV (e) based on hierarchical interpretation in the following order: (i)
molecular conformation, (ii) dimeric motif, (iii) the motif‘s arrangement, and (iv) crystallographic packing diagram. (Right column) CH/O interactions
of the carbonyl oxygen atoms in the polymorphs. Totally, four kinds of CH/O interaction patterns (S-1, S-2, B-1, and C-1) were observed. The dimeric
motifs of forms I, II, II0, and IV are described in thermal ellipsoids with 50% probability, while that of form III is drawn in ball-stick due to a problem of
data quality. In the motif’s arrangements, the molecules are schematically described with green and yellow isosceles triangles. The molecules in the
packing diagrams are also colored yellow and green for clarity. Form II0 has two nonsymmetrical DBA molecules, A and B. Symmetry code for form I: (A)
1
1 ꢀ x, 1/2 + y, 3/2 ꢀ z; (B) 2 ꢀ x, ꢀ /2 + y, 3/2 ꢀ z; (C) x, 5/2 ꢀ y, 1/2 + z. For form II: (A) x, 1 + y, z; (B) ꢀx, 1/2 + y, 3/2 ꢀ z. For form II0: (A) ꢀ1 + x, ꢀ1
+ y, z. For form III: (A) 2 ꢀ x, 1/2 + y, 3/2 ꢀ z; (B) x, 3/2 ꢀ y, 1/2 + y; (C) 1 ꢀ x, 1 ꢀ y, 1 ꢀ z. For form IV: (A) 1/2 ꢀ x, 1/2 ꢀ y, 1 ꢀ z; (B) 1 ꢀ x, 1 ꢀ y,
2 ꢀ z; (C) 1/2 + x, 1/2 ꢀ y, 1/2 + z.
forms the C-1 type self-complement CH/O interaction:
layers of DBA molecules in the polymorphs. Forms III and IV
exhibit larger overlaps of the DBA planes compared with forms I,
II, and II0.
A carbonyl oxygen atom can accept one or two hydrogen
atoms to make CH/O interactions. In the present systems, the
carbonyl oxygen atoms participate totally in four types of CH/O
C17ꢀH O1(A) and C7(A)ꢀH O1 (C O distance,
3 3 3
3 3 3
3 3 3
3.30 Å; CꢀHꢀO angle, 160.4°). The adjacent dimeric pairs
colored in green and yellow are staggered by 51.0°. The
molecules packed into a slipped-stacked columnar assembly with
large π-overlap are similar to that of form III. Figure 4 shows two
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Table 1. Geometric Parameters of [14]DBA in Crystalline
States
form
R1,4 (Å) d (Å) γ (deg)
T (°C)
ΔH° (kJ/mol)
1 (form I)
5.68
7.20
6.87
7.06
4.47
4.49
3.92
8.92
10.13
9.88
23.2
31.3
29.5
28.3
69.8
76.3
35.5
254 (dec)c
314
1 (form II)
1 (form II0)a
252 (dec)c,d
359
10.12
3.87
1 (form III)
1 (form IV)
parent
240 (dec)c
245 (dec)c
145 (polyn)e
250 (dec)c
312
293
3.81
6.31
compoundb
a Two values for each parameter were provided due to two crystal-
lographically independent molecules in the cell (Z0 = 2). b Reference 19.
c Decomposition temperature. d A mixture of forms II and II0 was used.
e Polymerization temperature.
polymorphs are brought from the different circumstances around
the reactive butadiyne units of 1, as previously reported.25a
Butadiyne contained [14]DBA is an attractive monomer for
topochemical polymerization, as demonstrated by Vollhardt,
Youngs, and co-workers.19 For successful polymerization, spe-
cific orientations of the butadiyne moieties are required.31 The
present systems, however, did not meet the condition, judging
from the geometrical parameters of the butadiyne: stacking
distance of butadiynes (d), a neighboring C1 C4 distance of
3 3 3
the 1,3-butadiyne moieties (R1,4), and the angle between the
molecular and stacking axis (γ), as shown in Table 1. [14]DBAs
in forms I, II, and II0 are stacked with d values of 8.92ꢀ10.13 Å,
while those in forms III and IV have d values of 3.87 and 3.81 Å.
For all cases, the R1,4 value is too long for the reaction. Indeed,
insoluble black materials yielded after thermal decomposition
showed no significant PXRD pattern, indicating disappearance of
the stereoregular structure.
Figure 4. Stacking geometry of the DBA plans in the polymorphs. The
selected packing diagrams are viewed from directly above the DBA
planes.
FT-IR Spectroscopy of Forms IꢀIV. In Figure 5a, observed
FT-IR spectra of the polymorphs are shown. The bands of the
CdO stretching for forms I, II/II0, III, and IV appeared at 1716,
1722, 1729, and 1726 cmꢀ1, respectively. However, no signifi-
cant correlation was observed between the vibration wavenum-
bers and CH/O interaction distances or geometries. The CdC
stretching region, on the other hand, shows clear differences
between the spectra of forms IꢀIII and that of form IV. The weak
but unambiguous bands at 1544 and 1335 cmꢀ1 are only present
in the spectrum of form IV. Additionally, the band of form IV at
1402 cmꢀ1 is weaker than those of the others. These bands
characteristic for form IV are well reproduced in the theoretical
spectrum calculated for the hor-ver conformer but not for the ver-
ver one (Figure 5b). The bands at 1335 and 1402 cmꢀ1 are
attributed to the asymmetric CdC stretching vibration of the
two benzene rings bearing the ester, with accompanying the in-
plane CꢀH bending, while that at 1544 cmꢀ1 to the symmetrical
CdC stretching vibration of the three aromatic rings with
accompanying the in-plane CꢀH bending (Figure S5 of the
Supporting Information). These results demonstrated that the
observed IR spectra of the polymorphs clearly reflect conforma-
tions of the molecules.
interactions: namely, the CH/O interaction with the methoxy
group (S-1) or phenyl group (S-2), the branched interactions
involving the methoxy and phenyl groups (B-1), and the
complement interaction (C-1) as shown in Scheme 2. Tolerance
of methyl ester groups for CH/O interaction, i.e. the versatile con-
formations and interaction ways, as well as the shape of the
annulene core, enabled the molecules to pack into diverse
arrangements.
Thermal Reactivity of Forms IꢀIV. To investigate the
thermal behavior of the polymorphs, the obtained crystals were
subjected to differential scanning calorimetry (DSC) analysis. In
the following analyses, a mixture of forms II and II0 (II/II0) was
used because of a separation problem. As shown in Figure S4 of
the Supporting Information, the polymorphs showed no peaks
ascribable to structural exchanges among the polymorphs or
melting points. Therefore, the relative stability of each poly-
morph was not estimated, though the crystal density implies that
the forms are stable in the order III (1.385 g cmꢀ3 at 100 K), IV
(1.37ꢀ53g cmꢀ3 at 100 K), I (1.331 g cmꢀ3 at 213 K and 1.359
g cm at 100 K; see the Supporting Information), II (1.349
g cmꢀ3 at 100 K) ∼ II0 (1.328 g cmꢀ3 at 213 K). On the other
hand, forms I, II/II0, III, and IV show irreversible exothermic
peaks at 254, 252, 240, and 245 °C with ΔH° values of 290ꢀ
360 kJ/mol (Table 1). These temperature differences among the
Fluorescence Properties of Forms IꢀIV. Compared with the
fluorescence spectrum of 1 in solution (λmax = 400 nm, ϕF = 0.40,
τ = 10.4 ns) (Figures S1 and S2 of the Supporting Information),
those of the polymorphs present in the lower energy region
depend slightly on the polymorphic supramolecular structures
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Figure 7. Photographs of two representative crystals of form I exhibit-
ing (a) green and (b) blue fluorescence colors under UV light with
wavelengths of 365 nm (right) and ambient light (left). Scale bars
200 μm. (c) Normalized fluorescence (solid line) and excitation (dotted
line) spectra of the G-rich (green) and B-rich (blue) crystalline bulks.
The excitation spectrum of the G-rich crystal at 430 nm (dotted green
line) and that at 530 nm (dotted red line) show the same spectral profile.
Figure 5. Experimental and theoretical IR spectra of [14]DBA (1). (a)
Experimental IR spectra of the polymorphs: forms I (i), II (ii), III (iii),
and IV (iv). (b) Theoretical IR spectra of the conformers of [14]DBA
(1): the ver-ver (v), hor-ver (vi), and hor-hor (vii) conformers. The
oscillation intensities of selected peaks are listed in the parentheses.
Figure 8. Fluorescence quantum yield of the polymorphic crystals:
forms I(G-rich), I(B-rich), II/II0, III, and IV.
It is noteworthy that only form I exhibits a significant emission
band at around 530 nm, as shown in Figure 6i. At first, we
anticipated that the band was attributed to a new excited state
with a low energy level due to species such as an excited dimer
and oligomers specifically generated in form I. Later, however, we
realized that the intensity of the band depended on the crystal-
lization batches of form I. This observation cannot be explained
by the excimer emission. As shown in Figure 7a and b, two
crystals, both of which have the form I structure and explore the
(1 0 0) face, exhibit either green or blue fluorescence. When these
crystals are dissolved in a solvent, the resultant solutions show
exactly the same UVꢀvis, fluorescence, and excitation spectra
(Figure S6 of the Supporting Information). On the other hand,
the crystalline bulk of form I with a green-rich fluorescence color
[form I(G-rich)] obviously showed an emission band at around
530 nm, despite its excitation spectrum similar with that of the
blue-rich crystal [form I(B-rich)], as shown in Figure 7c. Further-
more, the excitation spectrum of form I(G-rich) at 530 nm also
shows good agreement with that at 430 nm, indicating that
energy transfer occurs in the crystals.
Figure 6. Normalized fluorescence (solid line) and excitation (dotted
line) spectra of (i) forms I (G-rich), (ii) II, (iii) III, and (iv) IV.
(Figure 6). Forms I and II/II0 show a fluorescence maximum
wavelength (λemmax) at 431 and 432 nm, respectively. Forms III
and IV do at 441 and 440 nm, respectively, which are red-shifted
by 10 nm compared with those of the formers. These results
agree with the fact that the molecules in forms III and IV stack
with larger π-planes overlap than those in forms I, II, and II0,
although the structure-dependent difference is much smaller
than our expectation.
To understand the optical properties of the polymorphs in
more detail, the fluorescence quantum yield (ΦF) and lifetime
(τ) were determined for forms II/II0, III, and IV, as well as for
form I(B-rich) and form I(G-rich). As shown in Figure 8, the ΦF
value is strongly dependent on the polymorphs: The ΦF values
are increasing in the order forms I, II/II0, III, and IV, and that of
form IV is three times larger than that of form I. Usually, it is
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Table 2. Fluorescence Lifetimes for Forms IꢀIV
em
max
form
λ
(nm)
τ at λemmaxb (ns)
τ at lower energy regionb,c (ns)
I
431
0.58 ( 0.03(0.36)
1.85 ( 0.04(0.51)
5.45 ( 0.10(0.13)
0.60 ( 0.06(0.13)
2.22 ( 0.05(0.59)
5.20 ( 0.06(0.28)
0.79 ( 0.06(0.09)
4.05 ( 0.25(0.44)
10.1 ( 0.1(0.47)
2.20 ( 0.07(0.35)
4.99 ( 0.03(0.65)
3.17 ( 0.07(0.30)
7.73 ( 0.03(0.70)
0.75 ( 0.02(ꢀ0.08)d
4.66 ( 0.39(0.27)
7.21 ( 0.04(0.81)
0.39 ( 0.04(ꢀ0.03)d
3.73 ( 0.09(0.47)
8.12 ( 0.04(0.56)
1.53 ( 0.15(ꢀ0.05)d
6.47 ( 0.11(0.83)
13.4 ( 0.1(0.22)
(G-rich)
I
431
432
(B-rich)
a
II/II0
III
IV
441
440
4.00 ( 0.05(0.50)
9.64 ( 0.05(0.50)
4.42 ( 0.08(0.38)
9.66 ( 0.04(0.62)
a Forms II and II0 were measured as a mixture. b χ2 values for forms I(B-
rich), I(G-rich), II/II0, III, and IV are 1.167, 1.026, 1.232, 1.320, and
1.150 at 440 nm and 1.241, 1.038, 1.391, 1.216, and 1.112 at 540 nm.
Relative amplitudes are shown in the parentheses. c The value was
determined at 530 nm for forms I and II/II0 and 520 nm for III and IV.
d A negative sign of the amplitude indicates that energy transfer occurs.
Figure 9. Photoconductivity of polymorphic crystals: forms I (red),
II/II0 (blue), III (orange), and IV (green). (a) Transient curves. The inset
shows the partially expanded transient curves. (b) Maximum values of
the photoconductivity (ϕΣμ) of forms I (red), II/II0 (blue), III (orange),
and IV (green).
recognized that stacking of a π-molecule with a large π-overlap
decreases the ΦF value. In the present system, however, the trend
is not in agreement with this idea. Probably these ΦF values are
given on subtle balances among the π-overlap degree and other
causes, such as (1) loose molecular packing, which promotes
nonradiative vibrational deactivation of the excitation state,32,33
and (2) the distorted DBA ring from the planar conformation,
which also has an influence on the rate of nonradiative
deactivation.34
Electrical Properties of the Polymorphs. To investigate
structure-dependent charge carrier dynamics in the crystals of
[14]DBA 1, the obtained polymorphic crystals were subjected
to flash photolysis time-resolved microwave conductivity (FP-
TRMC) measurements.36 The TRMC technique, which can
predict the nanometer-scale mobility of charge carriers generated
by laser pulse irradiation under a low oscillating microwave
electric field, has been applied to assess the intrinsic mobility
of π-conjugated polymers and πꢀπ-stacked discotic
materials,36,37 because the peak-value is not significantly affected
by chemical or physical defects in the material or the organic/
metal-electrode interface. Irradiation of the laser flash brings rise
and decay profiles of TRMC signals (ϕΣμ), where ϕ denotes
the photocarrier generation yield and Σμ the dose sum of the
mobilities of the generated charge carriers; in the case of the DBA
substituted by carboxyl groups or esters, the carrier is a hole.17
In the present systems, the ϕ value was not determined because
of the difficulty in spectroscopic evaluation of the crystals.
Therefore, the ϕΣμ transients were applied to compare the
charge carrier dynamics of the polymorphs.
Time dependent fluorescence decay of the polymorphs was
measured to determine fluorescence lifetimes at ca. 430 nm and
ca. 530 nm (Figures S7ꢀS11 of the Supporting Information).
Forms I and II/II0 exhibit triexponential fluorescence decays as
shown in Table 2 (τ1 < 1 ns, τ2 = 2ꢀ4 ns, τ3 = 5ꢀ10 ns) at
430 nm and (τ1 < 2 ns, τ2 = 4ꢀ6 ns, τ3 = 7ꢀ13 ns) at 530 nm,
while forms III and IV exhibit biexponential ones (τ1 = 2ꢀ3 ns,
τ2 = 5ꢀ8 ns) at 440 nm and (τ1 = 4 ns, τ2 = 10 ns) at 520 nm.
These indicate that several excited states are involved in the fluores-
cence process. It is important that form I(G-rich) shows the
lifetimes within 10 ns, which is too short for that of excimer
emission.35 This indicates that the fluorescence band at 530 nm is
brought not from the excimer state but from another excited
state. Furthermore, form I(G-rich) has the lifetime with a
negative signed amplitude at 530 nm (τ1 = 0.75 ns). The negative
sign denotes delay of the fluorescence decay. The delay and the
fact that forms I(B-rich) and I(G-rich) show almost the same ΦF
values (0.06) strongly indicate effective energy transfer inflowed
from another excited state, probably from that possessing the τ
value of 0.60 ns at 430 nm. This is consistent with the unique
excitation spectra at 430 and 530 nm, as described above.
The negative delay is also observed for form II/II0, indicating
the presence of the defects in form II/II0, although the fluores-
cence band at around 530 nm was not observed.
For crystals of forms I, III, and IV, the microwave electric field
was applied for unisotropically aligned crystalline bulks, and the
photoconductivity was measured along the b axis, which is
corresponding to the columnar axis of the π-stacked [14]DBA
molecules. On the other hand, for form II/II0, an isotropically laid
crystalline powder was used for the measurement because of a
technical problem caused by crystal size and shape.
Figure 9a shows the transient photoconductivities of the
polymorphs observed by FP-TRMC. Surprisingly, the maximum
values of ϕΣμ for the polymorphs were significantly small
(Figure 9b); even for forms III and IV (1.6 ꢁ 10ꢀ5 and 2.0 ꢁ
On the basis of these results, we conclude that the broad band
at around 530 nm originates not from a specific molecular
arrangement of form I, such as excimer formation, but from
structural defects. The defects prefer to present specifically in
form I and, subsequently, in form II/II0 crystals.
10ꢀ5 cm2 Vꢀ1 ꢀ1, respectively), which have largely π-over-
s
lapped columnar structures, the value is 2 orders of magnitude
less than that of the slipped stacked [12]DBA systems (2 ꢁ
10ꢀ3 cm2 Vꢀ1 sꢀ1) we previously reported.17 Comparison of the
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ϕΣμ values among the polymorphs indicates that form IV shows
slightly large conductivity, although they have quite similar
values. The lifetime of the charge carrier, on the other hand, is
significantly dependent on the molecular arrangements. Namely,
in the crystals of forms III and IV, the charge carrier is still alive at
8 μs after its generation, while the carrier in the crystals of forms I
and II/II0 was almost extinct at that time (Figure 9a). The
observed fast decay in the latter crystals suggests that forms I and
II/II0 contain a large number of trapping sites.38 These results
correspond with those of fluorescence lifetime measurements.
positions and refined as rigid atoms with the relative isotropic
displacement parameters.
Fluorescence Measurements. Emission spectra in solid
states were measured on a JASCO FP-6500 spectrofluorometer
with an accessory and a cell for solid samples from JASCO. The
fluorescence decay time and lifetime were obtained using HOR-
IBA FluoroCube and the software (Data Station v.2.4 and DAS
6) supplied with the apparatus.
Charge Carrier Mobility Measurement. Nanosecond laser
pulses from a Nd:YAG laser (third harmonic generation, THG
(355 nm) from Spectra Physics, INDY, fwhm 5ꢀ8 ns) were used
as excitation sources. The incident photon density of the laser
was set at 1.4 ꢁ 1016 photons/cm2/pulse. For time-resolved
microwave conductivity (TRMC) measurements, the microwave
frequency and power were set at ∼9.1 GHz and 3 mW,
respectively, so that the motion of the charge carriers was not
disturbed by the low electric field of the microwaves. The TRMC
signal picked up by a diode (rise time <1 ns) is monitored with a
digital oscilloscope. All the above experiments were carried out at
room temperature. The photoconductivity is given by the
equation ϕ∑μ = (1/eAI0Flight)(ΔPr/Pr), where ϕ, ∑μ, e, A, I0,
Flight, Pr, and ΔPr denote photocarrier generation yield, the sum
of the charge carrier mobilities, the unit charge of a single
electron, the sensitivity factor (Sꢀ1 cm), the incident photon
density of an excitation laser (photon cmꢀ2), the filling factor
(cmꢀ1), and the reflected microwave power and its change,
respectively.
Theoretical Study. The DFT calculations were performed
with the Gaussian 03 program package.43 The geometries of the
conformers were optimized by using the B3LYP method in
combination with the 6-31G* basis set. The theoretical IR spectra
of the conformers were calculated for their optimized geometries.
The wavelength of the resulting spectrum was scaled by 0.9614.44
Syntheses. Acyclic Tetrayne 4. 1,2-Bis(trimethylsilylethynyl)-
benzene (768 mg, 2.84 mmol) and anhydrous K2CO3 (3.93 g, 28.4
mmol) in methanol/ether (51/13 mL) were stirred at room
temperature for 1.5 h. The mixture was extracted with CH2Cl2
and washed with water and brine. The organic layer was dried over
anhydrous MgSO4 and concentrated in vacuo to yield 3 as a yellow
oil. The oil 3 was added to a mixture of 216 (2.24 g, 6.25 mmol),
Pd(PPh3)4 (164 mg, 142 μmol), CuI (54.1 mg, 284 μmol), and
iPr2NH (2.0 mL) in THF (25 mL). After it was stirred for 22 h at
room temperature, the reaction mixture was concentrated in vacuo.
The mixture was extracted with CH2Cl2 and washed with water and
brine. The organic layer was dried over anhydrous MgSO4 and
concentrated in vacuo. Purification by column chromatography
(silica gel, CH2Cl2) and preparative HPLC gave 4(1.29 g, 78%) asa
yellow solid. mp 130.5ꢀ133 °C, 1H NMR (270 MHz, CDCl3): δ
8.19 (d, J = 1.4 Hz, 2H, ArH), 7.92 (dd, J1 = 1.6 Hz, J2 = 8.1 Hz, 2H,
ArH), 7.65ꢀ7.58 (m, 4H, ArH), 7.39ꢀ7.36 (m, 2H, ArH), 3.93
(s, 6H, OCH3), 0.23 (s, 18H, SiCH3) ppm. 13C NMR (67.5 MHz,
CDCl3): δ 165.9, 133.4, 132.1, 132.1, 130.2, 129.6, 128.9, 128.6,
125.9, 125.6, 102.4, 100.0, 94.8, 91.9, 52.4, ꢀ0.1 ppm. HR-MS
(FAB) m/z calcd for [M + H]+ C36H35O4Si2 587.2074, found
587.2088.
’ CONCLUSION
In this paper, we demonstrated that the octadehydrotribenzo-
[14]annulene derivative with two methyl ester groups in non-
centrosymmetric positions ([14]DBA, 1) gave five polymorphic
crystals successfully: forms I (plate, P21/c), II (block, P21/c), II0
(block P1), III (needle, P21/c), and IV (needle, C2/c). This is the
largest number for polymorphic crystals of the DBA family.
These versatile forms were provided by conformational and
interactional versatility of the methyl ester group: the present
systems contain two conformers (ver-ver and hor-ver) and four
kinds of CH/O interactions. DBA core shape also played a role to
achieve versatile molecular arrangements. The properties of the
polymorphs (thermal stability, IR absorption, fluorescence prop-
erty, and charge carrier dynamics) were investigated. Such
properties depend on the polymorphs. Furthermore, more
fatally, structural defects have an influence on the optical and
electrical properties. We demonstrated that the polymorphs
exhibited structural defects depending on the magnitude of π-
overlap of the DBA planes (I > II/II0 > III ∼ IV), and the
fluorescence and charge carrier properties were strongly affected
by the defects. Particularly, form I shows a significant fluores-
cence band at 530 nm due to defects in the crystals.
The present molecular design to obtain polymorphic crystals
of planar π-conjugated molecules is expected to contribute to the
understanding not only of the relationships between structures
and physical properties but also of the degree of molecular order
in crystals, and to advanced design of DBA-based functional
materials.
’ EXPERIMENTAL SECTION
X-ray Diffraction Measurements. Powder X-ray diffraction
data were collected on a Rigaku RINT-2000 using graphite-
monochromatized Cu Kα radiation (λ = 1.54187 Å) at room
temperature. Single crystal diffraction: Diffraction data were
collected on a Rigaku R-AXIS RAPID diffractometer with a
2-D area detector using graphite-monochromatized Cu Kα
radiation (λ= 1.54187 Å) for forms I and II and by using the synchro-
tron radiation (λ = 0.7000 Å) at the BL38B1 in the SPring-8 with
approval of JASRI (proposal nos. 2009B2115 and 2010A1427)
for forms II, III, and IV. The cell refinements were performed
with HKL2000 software39 for forms II, III, and IV. Direct
methods (SIR-2004) were used for the structure solution of all
polymorphs.40 All calculations were performed with the observed
reflections [I > 2σ(I)] with the program CrystalStructure crystal-
lographic software packages,41 except for refinement, which was
performed using SHELXL-97.42 All non-hydrogen atoms, except
for the case of form IV, were refined with anisotropic displace-
ment parameters, and hydrogen atoms were placed in idealized
[14]DBA 1. A 1.0 M solution of TBAF in THF (0.29 mL) was
added dropwise to a solution of 4 (171 mg, 0.291 mmol) in THF
(8 mL) at room temperature. After it was stirred for 1 h, the
mixture was extracted with CHCl3 and washed with water and
brine. The organic layer was dried over anhydrous MgSO4 and
concentrated in vacuo to yield a light brown solid. The resulting
material was used for the following step without further
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Crystal Growth & Design
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purification. A drop of the material obtained in the reaction above
in pyridine (20 mL) was added into CuCl (289 mg, 2.91 mmol)
in pyridine/methanol (150/150 mL). After it was stirred for 2 h
at room temperature, the reaction mixture was concentrated in
vacuo. The mixture was extracted with CHCl3 and washed with
water and brine. The organic layer was dried over anhydrous
MgSO4 and concentrated in vacuo. Purification by column
chromatography (silica gel, CH2Cl2) gave 1 (88.7 mg, 69% for
2 steps) as a light yellow solid. 1H NMR (270 MHz, CDCl3): δ
8.25 (d, J = 1.4 Hz, 2H, ArH), 8.12 (dd, J1 = 1.8 Hz, J2 = 8.1 Hz,
2H, ArH), 7.93ꢀ7.89 (m, 4H, ArH), 7.52ꢀ7.46 (m, 2H, ArH),
3.96 (s, 6H, OCH3) ppm. 13C NMR (67.5 MHz, CDCl3): δ
165.8, 136.4, 133.4, 133.1, 130.4, 129.5, 129.3, 128.6, 123.2,
122.8, 96.4, 92.7, 85.2, 80.8, 52.5 ppm. HR-MS (FAB) m/z calcd
for [M]+ C30H16O4 440.1049, found 440.1052.
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’ ASSOCIATED CONTENT
S
Supporting Information. Optical properties of 1 in a
b
(9) Comprehensive reports for recent polymorphs, see:Brittain,
H. G. J. Pharm. Sci. 2007, 96, 705–728. 2008, 97, 3611–3636. 2009,
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solution, crystallization conditions, fluorescence decay profiles,
PXRD patterns, theoretical IR vibration modes, DSC curves, 1H
and 13C NMR spectra of new compounds, and X-ray crystal-
lographic data for the five polymorphs. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Bourne, S. A.; Caira, M. R. Chem. Commun. 2011, 47, 1530–1532.
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’ AUTHOR INFORMATION
Corresponding Author
*Telephone and fax: +81-6-6879-7406. E-mail: (I.H.) hisaki@
mls.eng.osaka-u.ac.jp; (M.M.) miyata@mls.eng.osaka-u.ac.jp.
’ ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid for Scientific
Research (A 21245035), for Scientific Research on Innovative
Areas (22108517), and for Challenging Exploratory Research
(22651042) by MEXT (Japan). We are grateful to Prof. Dr. T.
Kawai, Dr. T. Nakashima, and Mr. Y. Okajima at NAIST for time-
resolved spectroscopy and to Dr. K. Miura, Dr. S. Baba, and Dr.
N. Mizuno for crystallographic data collection at Spring-8
(2009B2115 and 2010A1427). I.H. thanks FRC, Osaka Univer-
sity, for funding. A.S. and N.T. thank JSPS for a Research
Fellowship for Young Scientists.
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copper phthalocyanine and pentacene, have been investigated, for
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(27) Crystal data for form I: C30H16O4, Mw = 440.45, a =
11.5094(2), b = 8.92224(16), c = 21.9131(4) Å, α = 90, β =
102.4710(9), γ = 90°, V = 2197.16(7) Å3, T = 213 K, monoclinic,
space group P21/c (No. 14), Z = 4, Fcalcd = 1.331 g cmꢀ3, 3969 unique
reflections, the final R1 and wR2 values 0.051 (I > 2.0σ(I)) and 0.144 (all
data), respectively.Crystal data for form II: C30H16O4, Mw = 440.45, a =
10.1300(1), b = 13.8586(2), c = 16.1692(3) Å, α = 90, β = 107.1536(6),
γ = 90°, V = 2168.98(6) Å3, T = 100 K, monoclinic, space group P21/c
(No. 14), Z = 4, Fcalcd = 1.349 g cmꢀ3, 4550 unique reflections, the final
R1 and wR2 values 0.062 (I > 2.0σ(I)) and 0.178 (all data), respectively.
Crystal data for form II0: (C30H16O4)2, Mw = 880.91, a = 9.88125(18),
b = 10.12115(18), c = 22.1855(4) Å, α = 95.3050(9), β = 93.7500(9),
γ = 91.7710(10)°, V = 2203.01(7) Å3, T = 213 K, triclinic, space group
P1 (No. 2), Z = 2, Fcalcd = 1.328 g cmꢀ3, 7759 unique reflections, the final
R1 and wR2 values 0.065 (I > 2.0σ(I)) and 0.158 (all data), respectively.
Crystal data for form III: (C30H16O4), Mw = 440.45, a = 25.991(4), b =
3.8710(3), c = 21.352(4) Å, α = 90, β = 100.522(7), γ = 90°, V =
2112.1(5) Å3, T = 100 K, monoclinic, space group P21/c (No. 14), Z = 4,
Fcalcd = 1.385 g cmꢀ3, 2291 unique reflections, the final R1 and wR2
values 0.168 (I > 2.0σ(I)) and 0.454 (all data), respectively.Crystal data
for form IV: C30H16O4, Mw = 440.45, a = 46.1390(8), b = 3.85580(10), c
= 28.7157(7) Å, α = 90, β = 123.6138(9), γ = 90°, V = 4254.38(17) Å3,
T = 100 K, monoclinic, space group C2/c (No. 15), Z = 8, Fcalcd = 1.375
g cmꢀ3, 4693 unique reflections, the final R1 and wR2 values 0.070 (I >
2.0σ(I)) and 0.203 (all data), respectively.CCDC 814048(form I), 814049-
(form II), 831522(form II0), 814050(form III), and 814051 (form IV)
contain the supplementary 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.
(44) Scott, A. T.; Radom, L. J. Phys. Chem. 1996, 100, 16502–16513.
(28) The crystal structure of form III was resolved isotropically
because some anisotropic displacement parameters were nonpositive
due to the high mosicity caused by the plasticity of the thin needle
crystals. The resultant structure, however, is reliable because the
maximum laest-square shift error converged to zero.
(29) Hierarchical interpretation of organic crystals, see:(a) Tanaka,
A.; Inoue, K.; Hisaki, I.; Tohnai, N.; Miyata, M.; Matsumoto, A. Angew.
Chem., Int. Ed. 2006, 45, 4142–4145. (b) Miyata, M.; Tohnai, N.; Hisaki,
I. Acc. Chem. Res. 2007, 40, 694–702.
(30) The crystal of form II0 has two crystallographically independent
molecules. However, Figure 3c shows the CH/O interactions for only
one of them.
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dx.doi.org/10.1021/cg201075z |Cryst. Growth Des. 2011, 11, 5488–5497