Multiethynyl corannulenes: Synthesis, structure, and properties

Article abstract of DOI:10.1021/ja802334n

Syntheses, crystal structures, ab initio density functional theory computations, and photophysical properties of 1,6-di-, 1,2,5,6-tetra-, and 1,3,5,7,9-pentaethynyl-substituted corannulenes (classes 3, 4, and 5, respectively) are reported. Classes 3 and 4 were prepared from the corresponding corannulenyl bromides and terminal alkynes in excellent yields (nine examples, with yields of 57-92%) using the Sonogarshira reaction. Class 5 was prepared from 1,3,5,7,9-pentacholorocorannulene and trimethylalkynylstannanes using a modification of Nolan's procedure (8 examples, with yields of 45-93%). The molecular packing in crystals of 1,6-diphenylethynyl-2,5-dimethylcorannulene (3-Ph₂) displays a polar columnar structure with all of the molecule bowls oriented in the same direction. Similarly, 1,2,5,6-tetrakis(3,5- dimethylphenylethynyl)corannulene [4-Ar(c)₅] and 1,3,5,7,9- pentakis(3,5-dimethylphenylethynyl)corannulene [5-Ar(c)₅] form columnar structures, but the bowls are oriented in opposing directions. Additionally, the number of attached alkynyl arms is correlated with an increase in bowl depth of the corrannulene nucleus. Most of the aryl derivatives displayed high-quantum-efficiency solution luminescence and variable emission wavelengths that were dependent on the nature of the substitution.

Full text of DOI:10.1021/ja802334n

Published on Web 07/19/2008
Multiethynyl Corannulenes: Synthesis, Structure, and
Properties
Yao-Ting Wu,†,‡ Davide Bandera,† Roman Maag,† Anthony Linden,†
Kim K. Baldridge,*,† and Jay S. Siegel*,†
Institute of Organic Chemistry, UniVersity of Zurich, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland, and Department of Chemistry, National Cheng-Kung UniVersity,
No.1 Ta-Hsueh Road, Tainan, Taiwan
Received March 31, 2008; E-mail: jss@oci.uzh.ch
Abstract: Syntheses, crystal structures, ab initio density functional theory computations, and photophysical
properties of 1,6-di-, 1,2,5,6-tetra-, and 1,3,5,7,9-pentaethynyl-substituted corannulenes (classes 3, 4, and
5, respectively) are reported. Classes 3 and 4 were prepared from the corresponding corannulenyl bromides
and terminal alkynes in excellent yields (nine examples, with yields of 57-92%) using the Sonogarshira
reaction. Class 5 was prepared from 1,3,5,7,9-pentacholorocorannulene and trimethylalkynylstannanes
using a modification of Nolan’s procedure (8 examples, with yields of 45-93%). The molecular packing in
crystals of 1,6-diphenylethynyl-2,5-dimethylcorannulene (3-Ph2) displays a polar columnar structure with
all of the molecule bowls oriented in the same direction. Similarly, 1,2,5,6-tetrakis(3,5-dimethylphenylethy-
nyl)corannulene [4-Ar(c)5] and 1,3,5,7,9-pentakis(3,5-dimethylphenylethynyl)corannulene [5-Ar(c)5] form
columnar structures, but the bowls are oriented in opposing directions. Additionally, the number of attached
alkynyl arms is correlated with an increase in bowl depth of the corrannulene nucleus. Most of the aryl
derivatives displayed high-quantum-efficiency solution luminescence and variable emission wavelengths
that were dependent on the nature of the substitution.
Chart 1. Corannulene and Some of Its Derivatives
Introduction
The shape of corannulene (1), a C5V, bowl-shaped fragment
of C601 and a potential end cap for a single-walled [10,10] carbon
nanotube,2 evokes the image of an aggregate columnar con-
struction, resembling stacked bowls in a cupboard. Also, the
mere fact that 1 is a polynuclear aromatic hydrocarbon with a
surface area comparable to that of pyrene suggests that 1 should
display exceptional photophysical and electrochemical proper-
ties. In contrast to these expectations, 1 is a relatively feeble
fluorophore3 and, along with a number of its derivatives, packs
in the crystal phase without displaying any columnar order.4
The story changes, however, for ethynyl derivatives of 1 (2-5;
Chart 1), which constitute a particularly efficient set of fluo-
† University of Zurich.
‡ National Cheng-Kung University.
(1) (a) Wu, Y. T.; Siegel, J. S. Chem. ReV. 2006, 106, 4843–4867. (b)
Sygula, A.; Rabideau, P. W. In Carbon-Rich Compounds: From
Molecules to Materials; Haley, M., Tykwinski, R., Eds.;Wiley-VCH:
Weinheim, Germany, 2006; p 529. (c) Hirsch, A. Top. Curr. Chem.
1999, 199, 1–65. (d) Scott, L. T. Pure Appl. Chem. 1996, 68, 291–
300. (e) Seiders, T. J.; Siegel, J. S. Chem. Br. 1995, 31, 307–312.
(2) (a) Dresselhaus, M. D.; Avouris, P. In Carbon Nanotubes: Synthesis,
Structure, Properties, and Applications; Dresselhaus, M. D., Dressel-
haus, R. S., Avouris, P., Eds.; Springer: Heidelberg, Germany, 2001;
p 1. (b) Baldridge, K. K.; Siegel, J. S. Theor. Chim. Acta. 1997, 97,
67–71.
(3) (a) Verdieck, J. F.; Jankowski, W. A. Mol. Lumin. Int. Conf. 1969,
829–836. (b) Argentine, S. M.; Francis, A. H.; Chen, C.-C.; Lieber,
C. M.; Siegel, J. S. J. Phys. Chem. 1994, 98, 7350–7354. (c) Dey, J.;
Will, A. Y.; Agbaria, R. A.; Rabideau, P. W.; Abdourazak, A. H.;
Sygula, R.; Warner, I. M. J. Fluoresc. 1997, 7, 231–236. (d) Mack,
J.; Vogel, P.; Jones, D.; Kaval, N.; Sutton, A. Org. Biomol. Chem.
2007, 5, 2448–2452.
rophores. For example, 1,2-dicorannulenylethyne (2-Cor) dis-
plays a quantum yield of 0.57 and absorption and emission λmax
values similar to those of (phenylethynyl)corannulene (2-Ph).5
9
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 10729–10739 10729

A R T I C L E S
Wu et al.
Scheme 1. Synthesis of Di- and Tetraalkynylcorannulenes
Table 1. Synthesis of Class 3, 4, and 5 Multiethynyl-Substituted
Corannulenes
The photophysical properties of 5-H5 and pentaarylcorannulenes
(6-Ar5; Chart 1)6 raise the question of whether it is possible to
prepare highly fluorescent materials with columnar arrangements
in the solid phase through the use of aryl and alkynyl substituents
and a corannulene core.
Results and Discussion
Synthesis. The diethynylcorannulenes 3-R2 and tetraethynyl-
corannulenes 4-R4 were synthesized according to published
coupling procedures.7 Reaction of 1,6-dibromo-2,5-dimethyl-
corannulene (7) or 1,2,5,6-tetrabromocorannulene (8) with
terminal alkynes under Sonogashira conditions8 [catalytic
amounts of PdCl2(PPh3)2 and CuI in NEt3] afforded products
in good-to-excellent yields (Scheme 1 and Table 1).
The previously reported preparation of 1,3,5,7,9-pentakis(tri-
methylsilylethynyl)corannulene (5-TMS5) from sym-pentachlo-
rocorannulene (10) using Eberhard’s method with the pincer
catalyst 11 required a large excess (50 equiv) of trimethyl-
silylethyne and high temperature (165 °C).6 Recently, improved
methods have been reported for Sonogashira-,9 Suzuki-,10 and
Stille-type11 couplings involving aryl chlorides. Nolan’s protocol
is especially attractive.12 Therefore, a modification of Nolan’s
protocol was applied for the general preparation of the penta-
ethynylcorannulenes 5-R5 from 10 and the corresponding
trimethyltin-substituted alkynes (12-R) in reasonable-to-excellent
yields (Scheme 2 and Table 1).13 In comparison with our
previous procedure,6 the present synthetic method establishes
(4) (a) Hanson, J. C.; Nordman, C. E. Acta Crystallogr. 1976, B32, 1147–
1153. (b) Petrukhina, M. A.; Andreini, K. W.; Mack, J.; Scott, L. T.
J. Org. Chem. 2005, 70, 5713–5716.
(5) Jones, C. S.; Elliott, E. L.; Siegel, J. S. Synlett 2004, 187–191.
(6) Grube, G. H.; Elliott, E. L.; Steffens, R. J.; Jones, C. S.; Baldridge,
K. K.; Siegel, J. S. Org. Lett. 2003, 5, 713–716.
(7) (a) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 2000, 122, 6323–
6324. (b) Xu, G.; Sygula, A.; Marcinow, Z.; Rabideau, P. W.
Tetrahedron Lett. 2000, 41, 9931–9934. (c) Sygula, A.; Xu, G.;
Marcinow, Z.; Rabideau, P. W. Tetrahedron 2001, 57, 3637–3644.
(8) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
6, 4467–4470. (b) Sonogashira, K. In Metal-Catalyzed Cross-Coupling
Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH, Weinheim,
Germany, 1998; pp 203-229. (c) Brandsma, L.; Vasilevsky, S. F.;
Verkruijsse, H. D. Application of Transition Metal Catalysts in Organic
Synthesis; Springer: Berlin, 1999; pp 179-225.
(9) For Sonogashira-type methods, see:(a) Me´ry, D.; Heuze´, K.; Astruc,
D. Chem. Commun. 2003, 1934–1935. (b) Remmele, H.; Ko¨llhofer,
A.; Plenio, H. Organometallics 2003, 22, 4098–4103. (c) Ko¨llhofer,
A.; Pullmann, T.; Plenio, H. Angew. Chem., Int. Ed. 2003, 42, 1056–
1058. (d) Choudary, B. M.; Madhi, S.; Chowdari, N. S.; Kantam,
M. L.; Sreedhar, B. J. Am. Chem. Soc. 2002, 124, 14127–14136. (e)
Eberhard, M. R.; Wang, Z.; Jensen, C. M. Chem. Commun. 2002,
818–819.
milder conditions that require a smaller amount of alkyne and
could be used in a large-scale synthesis.
The amount of alkyne required in this procedure appears to
depend upon its purity; crude stannanes 12-Ar(a,c-g) required
2 equiv per halide site, whereas 1.5 equiv sufficed for the purer
stannylacetylenes 12-TMS and 12-Ph (see the Supporting
Information). Commercial tri-n-butyl(phenylethynyl)stannane,
under the same conditions, did not deliver the desired pen-
ta(arylethynyl)corannulene coupling product 5-Ph5.
(10) For Suzuki-type methods, see: Fu¨rstner, A.; Leitner, A. Synlett 2001,
290–292.
(11) For Stille-type methods, see:(a) Shirakawa, E.; Yamasaki, K.; Hiyama,
T. J. Chem. Soc., Perkin Trans. 1997, 1, 2449–2450. For another
method, see: (b) Nishihara, Y.; Ikegashira, K.; Hirabayashi, K.; Ando,
J.-i.; Mori, A.; Hiyama, T. J. Org. Chem. 2000, 65, 1780–1787.
(12) Grasa, G. A.; Nolan, S. P. Org. Lett. 2001, 3, 119–122.
(13) This procedure has been applied in the preparation of 2,3-diethynyl-
corannulene derivatives. See: Wu, Y. T.; Hayama, T.; Baldridge, K. K.;
Linden, A.; Siegel, J. S. J. Am. Chem. Soc. 2006, 128, 6870–6884.
The pentachloride 10 is known to be sparingly soluble. After
the coupling partners associate, the solubilities of the reaction
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Multiethynyl Corannulenes
A R T I C L E S
Scheme 2. Synthesis of the Pentaethynyl Corannulene Derivatives
5-R5
structure (Figure 2) was solved in the polar orthorhombic space
group Pca21, but here the translational element along the c axis
moves two steps along the column of stacked bowls. The hub
rings in each molecular pair are oriented in opposite directions
about their centroids. The bowl direction along the stack is the
same, save for a distinct tilting of the hub plane with respect to
the stacking axis (i.e., the normal to the hub plane forms an
angle of 12.2° with the c axis). Neighboring columnar stacks
have the same bowl direction. The molecular order in the ab
plane places the body of one molecule next to the arms of the
adjacent ones. The arylalkynyl arms fill the voids between
corannulenyl units and adopt conformations that optimize
arene-arene T interactions (Figure 3).
Because of the low solubilities of 4-Ph4 and 5-Ph5 in most
common organic solvents (e.g., CH2Cl2, toluene, etc.), we could
not prepare crystals suitable for X-ray structure analysis.
Therefore, more-soluble derivatives having short side chains on
the phenyl rings were studied. Crystals of the compounds
4-Ar(c)4 and 5-Ar(c)5 were obtained by slow evaporation of
the CH2Cl2/CHCl3/MeOH solvent mixture at ambient temper-
ature. Diffraction data for 4-Ar(c)4 and 5-Ar(c)5 were recorded
at 273 and 260 K, respectively, instead of the usual 160 K
because it was found that these crystals crack at the lower
temperature.
The structure of 4-Ar(c)4 (Figure 4) was solved in the space
group Pnma with one molecule sitting on a special position of
crystallographic Cs symmetry. After the corannulene core had
been modeled, it was found that the unit cell contained two
remaining voids, each with a volume of 173 Å3. Difference
Fourier maps revealed four unconnected peaks distributed on
and about a mirror plane within each void. It was assumed that
these voids represented water molecules, i.e., that the formula
was C60H42 ·1.5H2O. Attempts to include the water molecules
in the model did not produce satisfactory results, although it
was clear that the sites were only partially occupied. Subse-
quently, the contribution of the solvent molecules to the intensity
data was removed using the SQUEEZE19 routine of the
PLATON20 program.
In the structure of 4-Ar(c)4, translation along the a axis
coincides with moving two steps along the column of stacked
bowls. The hub rings in each molecular pair are oriented in
opposite directions about their centroids. The bowl direction
along the stack is the same, save for a small tilting of the hub
plane with respect to the stacking axis (i.e., the normal to the
hub plane forms an angle of 4.8° with the a axis). Neighboring
columnar stacks along the c axis have the same bowl direction;
adjacent columns along the c axis define the columnar channel
housing the disordered solvents. These like-pointing columnar
stacks define a layer in the ac plane. The bowl directions in
neighboring planes are antiparallel; the interface between layers
fits without any appreciable voids or channels (Figure 4).
The structure of 5-Ar(c)5 (Figure 5) was solved in the
monoclinic space group P21/n. There are two 5-Ar(c)5 molecules
in the asymmetric unit. Each molecule must sit on a general
position because the C5 molecular symmetry is incompatible
with any space group. The achiral space group indicates that a
racemic mixture of left- and right-handed bowls must be present.
intermediates increase, enhancing the subsequent coupling
reactions. In general, the yields within series 5 tended to be
very good overall (70-95%), with single-site coupling efficien-
cies of >90%. The lower yield (45%) for 5-Ph5 than for
5-Ar(a,c-f)5 may reflect the extent to which the addition of
side chains imparted increased solubility to the intermediates
and products of 5-Ar(a,c-f)5 during the synthesis reactions.
X-ray Crystallographic Structures of 3-Ph2, 8, 4-Ar(c)4,
and 5-Ar(c)5. X-ray-quality crystals of 3-Ph2, 8, 4-Ar(c)4, and
5-Ar(c)5 were obtained and their structures determined as
representatives of the corresponding classes (Table 2). All show
the characteristic bowl structure of corannulene and internal
geometries consistent with their constituent substructures (arene,
alkyne, and corannulene). The striking feature in this family of
structures is the stacking order. All four compounds adopt
essentially columnar packings in either an orthorhombic or a
monoclinic unit cell with one particular stacking axis.
Although columnar packing is essentially unobserved in
simple corannulene derivatives, the tetrabromide 8 crystallizes
in the polar orthorhombic space group Pna21 (Figure 1). Each
cell translation along the c axis is a step along a column of
perfectly stacked bowls (i.e., the normal to the plane of the hub
ring is collinear with the stacking axis).14 Within the stack, the
bowls overlap with identical orientations of the hub ring.
Neighboring columns are shifted in register but have identical
bowl directions; the rotational orientation appears to come from
fitting of the bromines. More generally, this kind of unidirec-
tional columnar molecular packing is observed in large non-
planar polycyclic compounds, such as circumtrindene (C36H12),15
trigonal crystals of hemibuckminsterfullerene (C30H12),16 diin-
deno[1,2,3,4-defg;1′,2′,3′,4′-mnop]chrysene (C26H12),17 and semi-
buckminsterfullerene (C30H12).18
A crystal of 3-Ph2 was obtained by slow evaporation of the
CH2Cl2/MeOH solvent mixture at ambient temperature. The
(14) Work on modeling of the relative packing of 7, 8, and 1,2,5,6-
tetramethylcorannulene is ongoing: Wu, Y. T.; Maag, R.; Potier, Y.;
Ciurash, J. G.; Baldridge, K. K.; Hardcastle, K. I.; Linden, A.;
Gavezzotti, A.; Siegel, J. S. Unpublished results, 2008.
(15) Forkey, D. M.; Attar, S.; Noll, B. C.; Koerner, R.; Olmstead, M. M.;
Balch, A. L. J. Am. Chem. Soc. 1997, 119, 5766–5767.
(16) Petrukhina, M. A.; Andreini, K. W.; Peng, L.; Scott, L. T. Angew.
Chem., Int. Ed. 2004, 43, 5477–5481.
In the standard setting of the monoclinic crystal system, the
b axis is normal to the ac plane. Translation along the b axis
(17) Bronstein, H. E.; Choi, N.; Scott, L. T. J. Am. Chem. Soc. 2002, 124,
8870–8875.
(19) van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194–201.
(20) Spek, A. L. PLATON, Program for the Analysis of Molecular
Geometry; University of Utrecht: Utrecht, The Netherlands, 2005.
(18) Sygula, A.; Marcinow, Z.; Fronczek, F. R.; Guzei, I.; Rabideau, P. W.
Chem. Commun. 2000, 2439–2440.
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A R T I C L E S
Wu et al.
Table 2. Crystal Structure Data for the Corannulene Derivatives 3-Ph2, 8, 4-Ar(c)4, and 5-Ar(c)5
3-Ph2
8
4-Ar(c)4
5-Ar(c)5
formula
molecular weight
solvent
C38H22
478.59
CH2Cl2/MeOH
160(1)
C20H6Br4
565.88
toluene
160(1)
C60H45O1.5
790.01
CH2Cl2/CHCl3/MeOH
273(1)
C70H50
891.16
CH2Cl2/CHCl3/MeOH
260(1)
T (K)
crystal system
space group
Z
orthorhombic
Pca21
4
orthorhombic
Pna21
4
orthorhombic
Pnma
4
monoclinic
P21/n
8
a (Å)
b (Å)
c (Å)
15.4564(6)
22.190(1)
7.4161(2)
90
17.8356(5)
22.9372(7)
3.8032(1)
90
7.4846(5)
43.341(3)
13.684(1)
90
33.414(1)
7.8392(3)
41.096(2)
109.8771(8)
10123.5(7)
ꢀ (deg)
V (Å3)
2543.6(2)
1555.88(8)
4438.9(5)
coincides with moving two steps along the column of stacked
bowls. The hub rings of each molecular pair are oriented in
opposite directions about their centroids. The pairs have the
same handedness, and this combines with the opposite orienta-
tions of the hub rings to place their arms in a staggered
relationship to one another. The bowl direction along the stack
is the same, save for a small tilting of the hub plane with respect
to the stacking axis (i.e., the normal to the hub plane forms an
angle of 6.6° with the b axis). Neighboring columnar stacks
along the a axis have the same bowl direction, whereas those
along the c-axis have antiparallel bowl directions. The arms of
the aggregated columns embrace, forming a space-filled network
with no channels or voids (Figure 5).
All four structures establish that columnar stacking is
attainable in substituted corannulenes. These observations reopen
the question of controlled stacking in mixtures of arylalkynyl-
corannulenes and their perfluoroarylalkynyl analogues. The
issues with respect to series 5 would involve control of the
stacking order, rotational orientation, bowl direction, and
chirality of the individual component bowls (Figure 6).
Molecular Geometries of 3-Ph2, 8, 4-Ar(c)4, and 5-Ar(c)5. The
molecular geometry of a corannulene derivative contains three
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Multiethynyl Corannulenes
A R T I C L E S
Figure 1. Columnar packing in the crystal structure of 8.
Figure 2. Crystal packing in 3-Ph2 along the (top left) a, (top right) b, and (bottom) c axes.
atom types [hub (H), spoke (S), and rim (R)] that are connected
by four core bond types [H, S, flank (F), and R] and about which
five descriptive angles [five-ring hub (h5), six-ring hub (h6), six-
ring flank (f6), exo flank (fx), and six-ring rim (r6)] are defined.
The typical structural motif follows the pattern of a [5]radialene
with five rim double bonds (H and F lengths > S and R lengths)
and angles determined by symmetry and atom pyramidality or
bowl depth (h5 ) 108°; h6 ) 120-126°; f6 ) 115°, fx ) 130°;
r6 g 120°). The bulk of the pyramidal character is seen in the
hub atoms, as gauged by π-orbital axis vector (POAV) angles21
(flat ) 0; corannulene ) 8°; C60 ) 13°). The bowl depth is
typically measured as the distance between the rim and hub
mean planes or as the average of the distances of the hub atoms
from the rim mean plane (flat ) 0 Å; corannulene ) 0.87 Å;
C60 ) 1.1 Å). This set of parameters forms a useful taxonomic
set for the characters of corannulene bowl fragments ranging
from flat to fullerene (nanotube cap).
All three of the arylalkynylcorannulenes 3-Ph2, 4-Ar(c)4, and
5-Ar(c)5 follow the standard geometry in their core fragments
(Table 3). Correlative trends involving the H6 angle, bowl depth,
and POAVhub angle can be observed. The bowl depth is
positively correlated with the POAVhub angle and negatively
correlated with the H6 angle, as would be expected if the hub
atoms are the principal sites of out-of-plane distortion. Specific
comparison reveals bowl depths of 0.846(7), 0.884(8), and
0.936(5) Å for 3-Ph2, 4-Ar(c)4, and 5-Ar(c)5, respectively, and
corresponding POAVhub and h6 angles of 8.00(14), 8.2(3), and
8.7(2)° and 123.1(2), 122.9(3), and 122.6(12)°, respectively.
Alkyl rim substituents on corannulene flatten the bowl and
decrease the inversion barrier.22 In contrast, for arylalkynyl
substituents, the crystal structure analyses of 3-Ph2, 4-Ar(c)4,
and 5-Ar(c)5 showed that the bowl depth increases with the
number of attached alkynyl groups. It is striking that 5-Ar(c)5
(21) (a) Haddon, R. C. Science 1993, 261, 1545–1550. (b) Haddon, R. C.
J. Am. Chem. Soc. 1990, 112, 3385–3389. (c) Haddon, R. C. Acc.
Chem. Res. 1988, 21, 243–249. (d) Haddon, R. C.; Scott, L. T. Pure
Appl. Chem. 1986, 58, 137–142.
(22) The bowl depths and inversion barriers for 1,6-dimethylcorannulene (0.866
Å and 9.0 kcal/mol, respectively), 2,5-dimethylcorannulene (0.865 Å and 9.0
kcal/mol), and 1,3,5,7,9-pentamethylcorannulene (0.850 Å and 8.7 kcal/mol)
are smaller than those for corannulene (calcd 0.877 Å and 9.2 kcal/mol; exptl
11.5 kcal/mol). For details, see: Seiders, T. J.; Baldridge, K. K.; Grube, G. H.;
Siegel, J. S. J. Am. Chem. Soc. 2001, 123, 517–525.
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A R T I C L E S
Wu et al.
Table 3. Bond Distances (Å), Angles (deg), Bowl Depths (Å), and Hub POAV Angles (deg) for 1, 3-Ph2, 4-Ar(c)4, and 5-Ar(c)5
1a
3-Ph2
4-Ar(c)4
5-Ar(c)5
exptl
calcdc
exptl
calcdc
exptl
calcdc
exptl
calcdc
bond lengthsb,d
R
F
H
1.380(2)
1.444(2)
1.414(2)
1.378(2)
1.392 [1.384]
1.450 [1.448]
1.419 [1.419]
1.387 [1.380]
1.388(13)
1.449(10)
1.410(6)
1.374(7)
1.397 [1.388]
1.445 [1.449]
1.418 [1.417]
1.386 [1.379]
1.387(19)
1.450(7)
1.415(8)
1.372(5)
1.405 [1.393]
1.453 [1.449]
1.418 [1.417]
1.385 [1.379]
1.390(5)
1.435(7)
1.409(5)
1.390(7)
1.409 [1.396]
1.439 [1.439]
1.418 [1.417]
1.386 [1.379]
S
anglesb,d
h5
h6
f6
fx
108.0(1)
122.9(2)
114.3(2)
130.1(4)
122.0(2)
0.875(2)
8.3(2)
108.0 [108.0]
123.0 [122.9]
114.3 [114.5]
130.1 [129.8]
122.0 [122.0]
0.873 [0.887]
8.2 [8.3]
108.0(2)
123.1(2)
114.4(7)
130.0(5)
121.8(8)
0.846(7)
8.00(14)
108.0 [108.0]
123.0 [122.9]
114.4 [114.6]
130.1 [129.6]
121.9 [121.8]
0.861 [0.879]
8.1 [8.3]
108.0(4)
122.9(3)
114.8(5)
129.0(6)
121.6(6)
0.884(8)
8.2(3)
108.0 [108.0]
123.1 [122.9]
114.6 [114.8]
129.8 [129.1]
121.6 [121.6]
0.848 [0.888]
8.0 [8.4]
108.0(4)
122.6(12)
114.8(6)
128.8(6)
121.9(8)
0.936(5)
8.7(2)
108.0 [108.0]
123.1 [122.8]
114.6 [114.9]
129.7 [129.0]
121.6 [121.5]
0.853 [0.892]
8.0 [8.4]
r6
bowl depth
POAVhub anglee
a Data taken from ref 4a,b. b All values are averages over bond type. Underlined values identify closer agreement between experiment and theory.
c B3LYP/cc-pVDZ [M06-2X/cc-pVDZ]. d Bond types: H ) Hub; S ) Spoke; F ) Flank; R ) Rim. Angle types: h5 ) five-ring hub; h6 ) six-ring hub;
f6 ) six-ring flank; fx ) exo flank; r6 ) six-ring rim. e Pyramidality measure; see ref 21.
case of bond lengths, all of the values predicted using M06-2X
fell within 0.01 Å of the experimental values, whereas B3LYP
overestimated all of the rim and spoke bond lengths across the
series by >0.01 Å, except for the spoke of 5-Ar(c)5. Of these
two methods, only M06-2X predicted the positive correlation
between bowl depth and POAVhub that was observed experi-
mentally, although the calculated effect was much less dramatic
(Table 3).
Photophysical Properties. Now that the possibility of stacking
corannulene derivatives in the solid state has been addressed,
the second point, related to producing enhanced photophysical
properties, remains. Previously, the photophysical properties of
1, 2-TMS, and 2-Ph were examined in cyclohexane, and these
compounds were found to be modest fluorophores. The present
measurements offer a more extensive database of the photo-
physical properties of classes 2, 3, 4, and 5 in dichloromethane
(Table 4) that can be employed to elucidate the effects of
arylalkynyl substitution as well as general aryl effects.
Figure 3. Orientation of corannulenyl units and adjacent arylalkynyl arms
in the stacking of 3-Ph2. Arene-arene T interactions are prevalent. Arrows
define the stacking axes (bold for foreground, dashed for background).
Curves represent corannulene, and bars represent arylalkynyl arms.
has a deeper bowl [0.936(5) Å] and is more pyramidal at the
hub carbons [8.7(2)°] than the parent corannulene [0.875(2) Å,
8.3(2)°], which would suggest that the bowl-inversion process
would be slower for such derivatives. A general investigation
of sym-penta-X-corannulenes could reveal whether this effect
is general for electronegative substituents.
Having a well-defined set of parameters and a chemically
reasonable trend makes this set of compounds a good testing
ground for computational methods. Recent advances in density
functional theory (DFT) treatments, and in particular, incorpora-
tion of dispersive effects, are in need of scrutiny. Here, results
obtained with the cc-pVDZ basis set using both the classical
DFT functional B3LYP, which has been broadly employed in
previous investigations of other corannulene-based structures
and coupled with second-order Moller-Plesset perturbation
theory (MP2) for the accurate determination of bowl dynamics,
and the new functional M06-2X are compared to the current
X-ray crystallographically derived data.
For the purpose of discussion, the present classes of molecules
are viewed as having a central corannulene fragment and a
perturbing substituent field. The principal subsymmetries of C5V
are a C5 proper rotation axis and five mirror planes (σ)
individually representing Cs symmetry. The symmetry perturba-
tions due to arylalkyne substitution for classes 2, 3, 4, and 5
are C1, Cs, Cs, and C5, respectively. The nodal structures and
symmetries of the frontier orbitals manifest the substituent
effects.
The parent corannulene is C5V-symmetric and has a quadruply
degenerate HOMO (2 × E1, 2 × E2; see Figure 7); the highest
occupied orbital of A-type symmetry is the HOMO-4 orbital.
The virtual-orbital space begins with a doubly degenerate set
(E2) followed by an orbital of A-type symmetry. The principal
transitions are π-to-π*, with several transitions that are cryptic
because of small extinction coefficients.
Examination of the B3LYP/cc-pVDZ and M06-2X/cc-pVDZ
geometries in direct comparison to the experimental data shows
remarkable agreement for the average bond angles: correspond-
ing values computed using the two methods never deviated from
each other or the experimental data by more than 0.9°. In the
In the case of class 5, where the C5 symmetry allows for
effective interaction between the substituent field and the orbital
of A symmetry in the corannulene core, the resultant orbital of
9
10734 J. AM. CHEM. SOC. VOL. 130, NO. 32, 2008

Multiethynyl Corannulenes
A R T I C L E S
Figure 4. Crystal packing in 4-Ar(c)4 along the (left) a and (right) c axes.
Figure 5. Crystal packing in 5-Ar(c)5: (left) the two-molecule asymmetric unit; (right) view along the c axis.
transition moment because of symmetry. The first transition with
any significant absorptivity appears at 360 nm [358 nm (log ε
5.0) exptl]. As is observed in benzene, the frontier orbitals with
mutually exclusive nodal structures lead to poor Franck-Condon
factors for HOMO-to-LUMO transitions.
For classes 3 and 4, the intersection of the Cs symmetry of
the field and the C5V symmetry of the core results in a molecular
symmetry of Cs. Although the orbitals are formally of the A′
and A′′ types, their nodal structures and relative energies indicate
their origins. In class 3, the quasi-diametrically opposed
substituents at the 1 and 6 positions on corannulene fall
essentially along the nodal planes of two of the E-type orbitals
(Figure 8). As a result, there is no core-to-field perturbation,
and these oribtals remain at essentially their “non-interacting”
energies. In contrast, the other two E-type orbitals are well-
suited spatially and by symmetry to interact with the field
orbitals, and HOMO and HOMO-1 are the resultant antibonding
combinations of this core-to-field interaction. Thus, even though
the four orbitals should be energetically separated according to
symmetry arguments, the HOMO-2 and HOMO-3 orbitals are
effectively degenerate. The longest-wavelength principal absorp-
tion (377 nm exptl) is predicted to be a strong HOMO-to-LUMO
transition with a calculated wavelength of 368 or 396 nm
(depending on the method used; see Table 4).
Figure 6. Comparison of the stacking order parameters for 3-Ph2, 8,
4-Ar(c)4, and 5-Ar(c)5. Arrows define the stacking axes, and curves represent
corannulene fragments.
A symmetry (HOMO-4) represents an antibonding interaction
between the core and the field. The two sets of E-type orbitals
(i.e., 4 orbitals total) remain essentially degenerate, forming a
complex HOMO-to-HOMO-3 orbital manifold. The computa-
tions predicted a manifold of π-to-π* transitions among
degenerate HOMO and LUMO levels that have essentially no
The four substituents in class 4 set up a symmetry field similar
to that in class 3 but present a different spatial distribution of
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A R T I C L E S
Wu et al.
Table 4. Photophysical Properties of Multiethynyl-Substituted Corannulene Derivativesa
c
entry compound
λa [nm] (log ε [M-1 cm-1])
λcalcd
λf (nm)
Φf
τ (ns)
1
2
3
4
5
6
7
8
9
1
320sh (4.0), 288 (4.7), 254 (5.0)
423
435
0.03
0.07
7.3
8.0
6.6
2-TMS 297 (4.6)
2-Ph
400sh, 363-345 (4.1), 298br (4.7)
340, 313, 272 [375, 315, 292]
368, 315, 275 [396, 347, 313]
434, 415 0.20
7
292 (4.8), 259br (5.0)
446, 426 0.029 7.8
3-TMS2 416sh (3.3), 371sh (4.0), 353sh (4.1), 307 (4.6)
455, 442sh 0.25
469, 446 0.75
5.5
4.8
3-Ph2
4-TMS4 331 (4.8)
4-Ph4
425sh, 378-359 (4.4), 320sh, 298 (4.6)
475, 449 0.17 11.1
450weak, 400sh, 356 (4.9), 305 (4.9), 288
399, 363, 328, 314, 286 [429, 399, 352/344, 321, 285]d 500, 473 0.83 10.3
4-Ar(a)4 370 (4.8)
529
504, 476 0.58
506 0.71
527, 501 0.73
468, 452 0.05
0.67
7.9
9.1
6.5
6.0
9.5
10 4-Ar(b)4 458sh (3.8), 361 (4.9), 332 (4.9), 312sh (4.9)
11 4-Ar(d)4 381 (4.9), 361sh (4.9), 319sh (4.9)
12 4-Ar(e)4 382 (4.9), 331 (4.9)
13 5-TMS5 339 (4.7), 303 (4.9)
14 5-Ph5
358 (5.0), 324 (5.0), 284 (4.9)
365, 332, 286 [360, 348/325, 290]
485, 465 0.12 10.5
15 5-Ar(a)5 376 (5.0), 332sh (4.9)
542
503
0.37
0.24
8.4
5.8
8.9
16 5-Ar(d)5 383 (5.0), 354sh (4.9), 345 (5.0), 306 (5.0), 287 (5.0)
17 5-Ar(e)5 385 (5.0), 345sh (5.0), 321 (5.0)
18 5-Ar(f)5 364 (4.9), 328 (4.9)
499, 485 0.44
491, 465 0.16 11.0
498, 479 0.26 10.2
19 5-Ar(g)5 374 (5.0), 334 (4.9), 322sh (4.8)
20 13
21 14
22 14b
335 (4.6), 319 (4.6)
305 [346]
333 [377, 333]
357, 340 0.89
391, 370 0.94
0.75
0.65
0.67
363sh (4.7), 346 (4.9), 317sh (4.6)
362sh, 347 (4.9)
387, 368
1
a All measurements were for samples in degassed dichloromethane. For compounds 3-R2, 4-R4 and 5-R5, only absorption bands with λa > 300 nm are
given. The excitation wavelength was 306 nm. Quantum yields are relative to 9,10-diphenylanthracene (Φf ) 0.95) in ethanol. Bold values correspond
to those used for comparison of experiment and computation. b Data from ref 23; experiments were performed in dioxane. c M06-2X/6-311+G(d,p)//
M06-2X/cc-pVDZ [ZINDO//M06-2x/cc-pVDZ].
nm exptl) is predicted to be a HOMO-to-LUMO transition with
a calculated wavelength of 363 or 399 nm (Table 4).
The trends in the absorption wavelengths can also be
qualitatively rationalized using this picture. The longer-
wavelength absorptions for class 4 compared to class 3 come
from stronger interactions between filled field and core orbitals
(to give the HOMO) and between unfilled field and core orbitals
(to give the LUMO). Stronger antibonding and bonding interac-
tions, respectively, between the core and the field lead to higher-
lying HOMOs and lower-lying LUMOs. The longer-wavelength
absorptions for class 5 are seen computationally but with
nominal transition probabilities, which is reflected by the fact
that these transitions are experimentally cryptic. The first
experimentally observed transition in 5-Ph5 is at 358 nm, which
corresponds to a HOMO-to-LUMO manifold transition with a
calculated wavelength of 365 or 360 nm (Table 4).
Figure 7. Nodal planes for the degenerate orbitals HOMO to HOMO-3
and for the HOMO-4 orbital (A2 symmetry) in corannulene.
orbital densities and phases. Interaction of the core with all of
the E-type orbitals is possible and gives HOMO through
HOMO-3 as the resultant antibonding core-to-field interactions
(Figure 9). Unlike the ones in 3-Ph2, these four orbitals are
energetically separated in practice as well as by symmetry
arguments. The longest-wavelength principal absorption (400
9
10736 J. AM. CHEM. SOC. VOL. 130, NO. 32, 2008

Multiethynyl Corannulenes
A R T I C L E S
Figure 8. HOMO to HOMO-3 (left to right) for 3-Ph2. HOMO and HOMO-1 represent antibonding core-to-field interactions, while HOMO-2 and HOMO-3
are effectively degenerate core orbitals.
Figure 9. HOMO to HOMO-3 (left to right) for 4-Ph4. All represent antibonding core-to-field interactions.
Figure 10. HOMOs of 3-Ph2, 14, and 4-Ph4 (left to right). All have a common nodal number and structure.
Figure 11. Absorption and emission spectra of the phenylethynyl corannulenes 2-Ph, 3-Ph2, 4-Ph4, and 5-Ph5.
This extremely simplified model leads one to presume a
similarity in character between compounds of classes 2 and 3
and those of mono- and disubstituted fluorenes or biphenyls,
which are important chromophores in OLED materials. Indeed,
the orbital structure and photophysical behavior of 13 relative
to those of 14 serve as a good analogue for the behavior of
2-Ph relative to 3-Ph2.23 The HOMOs of 3-Ph2, 14, and 4-Ph4
show a common orbital density and phase pattern (Figure 10).
This similarity naturally raises the question of whether these
compounds exhibit similar fluorescence behavior.
Computational work on the details of corannulene fluores-
cence is still ongoing, but we can present here some general
empirical trends extracted from the compilation of experimental
fluorescence spectra, quantum yields and lifetimes in Table 4.
In general, having more substituents leads to longer emission
wavelengths (λf), with classes 4 and 5 showing the longest λf
(23) Birckner, E.; Grummt, U.-W.; Go¨ller, A. H.; Pautzsch, T.; Egbe,
D. A. M.; Al-Higari, M.; Klemm, E. J. Phys. Chem. A 2001, 105,
10307–10315.
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A R T I C L E S
Wu et al.
Figure 12. Absorption and emission spectra of the 1,2,5,6-tetraethynyl-substituted corannulenes 4-R4.
Figure 13. Absorption and emission spectra of the 1,3,5,7,9-pentaethynyl-substituted corannulenes 5-R5.
for a given aryl type. The Stokes shifts for the series 2-Ph, 3-Ph2,
4-Ph4, and 5-Ph5 increase slightly (by 40, 45, and 50 nm) for
the first three but then jump significantly (>100+) for the last;
a possible explanation for this might be that the emission from
5-Ph5 is coming from the reverse of the HOMO-LUMO state,
which is effectively blocked to absorption by a poor transition
moment. A change in the molecular geometry of the excited
state could result in better overlap between the states.
quenching due to an activated process and/or intersystem
crossing. This supposition was supported by the observation of
phosphorescence of 3-phenethynylphenanthroline at low tem-
perature. Preliminary low-temperature work on 2-Ph hints at
similar phosphorescence behavior, but additional work is
required before we can ascertain the cause of the low quantum
yield at room temperature.
The low quantum yield of 5-Ph5 is somewhat surprising. There
are conflicting reports in the literature regarding the effect of
symmetry on quantum yield. A study by Nizhegorodov24 described
an increase in quantum yield with symmetry due to an increase in
radiative states. More commonly cited is the Ham effect,25 in which
high symmetry leads to forbidden transitions between the ground
and excited states, as is observed in benzene. Vibrational, envi-
ronmental, or substitutional perturbation of the molecular symmetry
allows previously forbidden states to be accessed and thereby
increases absorption and emission properties.26
The lifetimes (τ) for the series 2-Ph, 3-Ph2, 4-Ph4, and 5-Ph5
are in the range 5-10 ns, with specific values 6.6, 4.8, 10.3,
and 10.5 ns, respectively; all of these are a factor of 10 longer
than for the model compounds 13 and 14. The fluorescence
quantum yields (Φf) for the series 2-Ph, 3-Ph2, 4-Ph4, and 5-Ph5
vary from modest values of 20 and 12% (for 2-Ph and 5-Ph5,
respectively) to respectable values of 75 and 83% (for 3-Ph2
and 4-Ph4, respectively). The high quantum yields for 3-Ph2
and 4-Ph4 parallel the virtually quantitative yield observed for
14. A contrast between 2-Ph and 13 is observed. The modest
value for 2-Ph parallels more closely the behavior of 3-phen-
ethynylphenanthroline, which has been presumed to suffer
(24) Nizhegorodov, N. I. Zh. Prikl. Spektrosk. 1992, 57, 477–483.
9
10738 J. AM. CHEM. SOC. VOL. 130, NO. 32, 2008

Multiethynyl Corannulenes
A R T I C L E S
The study most closely paralleling the situation in 5-Ph5 describes
an increase in quantum yield upon desymmetrization of a carbon-
based nanotube.27 This would correctly predict that in the present
case, the lower-symmetry molecules 3-Ph2 and 4-Ph4 should lead
to materials with higher quantum yields. Furthermore, since the
lifetimes of 4-Ph4 and 5-Ph5 are essentially identical, the lifetime
of the fluorescence state is not the differentiating factor. One
hypothesis would be that the symmetry of the ground state of 5-Ph5
allows for various nonradiative relaxation modes and thus results
in a lower quantum yield by virtue of an increased rate of
nonradiative decay.
More than symmetry must be at play in the quantum-yield
behavior of corannulene derivatives. Although the low quantum
yield (7%) for the parent, corannulene, seems consistent with the
symmetry picture, the value for the lower-symmetry molecule
cyclopentacorannulene (1%) reported by Warner and co-workers3c
does not. One supposition is that bowl depth and its relation to
bowl strain is another factor. Warner and co-workers noted that
the radiative decay of cyclopentacorannulene is slower than that
of corannulene and hypothesized that this could result from greater
strain and/or loss of oscillator strength due to greater out-of-plane
bowl distortion. There appear to be many interesting and tunable
aspects to the photophysical properties of corannulene derivatives.
Clearly, if corannulene-based materials are to find their way into
photonic applications, an even better understanding of their
photophysical behavior is needed.
Direct comparison of the experimental absorption and emission
spectra (Figures 11–13) reveals their complex nature. The details
for the series 2-Ph, 3-Ph2, 4-Ph4, and 5-Ph5 have been discussed
above. Here it suffices to say that the emission spectra (Figure 11)
show that 4-Ph4 has the longest-wavelength emission; the drop in
quantum yield can also be assessed qualitatively from the picture.
Among the 4-R4 series, 4-Ar(a)4, whose arene is the electron-rich
tris(dodecyloxy)phenyl group, shows the longest emission (Figure
12); this behavior is characteristic of the occurrence of some
electron transfer from the arylalkynyl field to the corannulene core.
If one follows the derivatives as a function of electron-rich character
of the arene, the series reorganizes to 4-Ar(b) < 4-Ph < 4-Ar(e)
< 4-Ar(d) < 4-Ar(a), and the least-electron-rich compounds show
the greatest amount of spectral structure and the shortest Stokes
shifts. Thus, the emission wavelength in series 4 appears to be
tunable within a modest range. In series 5, the trend tends to be
the same (Figure 13), but all of the quantum yields are somewhat
smaller, as explained above.
their potential utility as photophysically active materials. This
combination of materials properties bodes well for the development
of photoactive and/or polar liquid-crystalline materials based on
substituted corannulenes of high symmetry.
Theoretical Section
The conformational analyses of the molecular systems described
in this study, including structural and orbital arrangements as well as
property calculations, were carried out using the Gaussian 9828 and
GAMESS29 software packages. Structural computations on all of the
compounds were performed using hybrid DFT (HDFT) methods. The
HDFT method employed Becke’s three-parameter functional30 in
combination with nonlocal correlation provided by the Lee-Yang-Parr
expression31,32 including both local and nonlocal terms (B3LYP) as
well as the new M06-2X functional of Zhao and Truhlar.33 Dunning’s
correlation-consistent cc-pVDZ basis set, which is a [3s2p1d] contrac-
tion of a (9s4p1d) primitive set, and the double-ꢁ polarized sets denoted
DZ(2d,p) and DZ+(2d,p), were also employed.34 Full geometry
optimizations were performed and uniquely characterized via second-
derivative (Hessian) analysis in order to determine the number of
imaginary frequencies (0 ) minimum; 1 ) transition state). From the
fully optimized structures, single-point energy computations were
performed using the MP235 dynamic correlation treatment; these
provided more accurate energy barriers as well as single-point time-
dependent absorption characteristics.36 These levels of theory have
previously been shown to be reliable for structural and energetic
determinations in these types of compounds. ZINDO computations39
were carried out for comparative purposes. Molecular orbital contour
plots used as an aid in the analysis of the results were generated and
depicted using the programs 3D-PLTORB37 and QMView.38
Acknowledgment. This work was supported by Swiss National
Science Foundation grants to K.K.B. and J.S.S. We are grateful to
Don Truhlar for granting us access to the use of the M06-2X
functional and computational time on the facilities of the Minnesota
Supercomputing Institute.
Supporting Information Available: Complete experimental
procedures for preparing all of the compounds and intermediates
mentioned in this work, complete ref 28, and crystal structure
data in CIF format for 8, 3-Ph2, 4-Ar(c)4, and 5-Ar(c)5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA802334N
(28) Frisch, M. J. Gaussian 98, revision A.6; Gaussian, Inc.: Pittsburgh,
PA, 1998.
Conclusion
(29) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.;
Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus,
T. L.; Elbert, S. T. J. Comput. Chem. 1993, 14, 1347–1363.
(30) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(31) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(32) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
157, 200–206.
(33) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241.
(34) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007–1023.
(35) Moller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618–622.
(36) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998,
109, 8218–8224.
(37) 3D-PLTORB, 3D version; San Diego, 1997.
(38) Baldridge, K. K.; Greenberg, J. P. J. Mol. Graphics 1995, 13, 63–66.
(39) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta 1979, 53, 21–54.
Here we have provided simple ways to prepare multiethynyl-
substituted corannulenes from halo precursors in good-to-excellent
yields. These alkynyl derivatives pack in the crystal as columnar
stacks and in some cases display polar unit cells. The high
fluorescence quantum yields seen for these derivatives demonstrate
(25) Ham, J. S. J. Chem. Phys. 1953, 21, 756.
(26) (a) Nakajima, A. Bull. Chem. Soc. Jpn. 1971, 44, 3272–3277. (b) Dong,
D. C.; Winnik, M. A. Photochem. Photobiol. 1982, 35, 17–21.
(27) (a) Shavar, J.; Kono, J.; Portugall, O.; Krstic’, V.; Rikken, G. L. J. A.;
Miyauchi, Y.; Maruyama, S.; Perebeinos, V. Nano Lett. 2007, 7, 1851–
1855. (b) Sun, Y.-P. Supramol. Photochem. 1997, 1, 325–390.
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