Synthesis and fluorescence properties of novel pyrazine-boron complexes bearing a β -iminoketone ligand

Article abstract of DOI:10.1021/ol202819w

A novel fluorescence dye based on pyrazine-boron complexes bearing a β -iminoketone ligand has been synthesized by using a simple two-step reaction. Synthesized complexes exhibited fluorescence in solution (F_max: 472-604 nm) and in the solid st

Full text of DOI:10.1021/ol202819w

ORGANIC
LETTERS
2011
Vol. 13, No. 24
6544–6547
Synthesis and Fluorescence Properties of
Novel PyrazineꢀBoron Complexes
Bearing a β-Iminoketone Ligand
Yasuhiro Kubota,* Hiroshi Hara, Syunki Tanaka, Kazumasa Funabiki, and
Masaki Matsui*
Department of Materials Science and Technology, Faculty of Engineering,
Gifu University, Yanagido, Gifu 501-1193, Japan
kubota@gifu-u.ac.jp; matsuim@gifu-u.ac.jp
Received October 21, 2011
ABSTRACT
A novel fluorescence dye based on pyrazineꢀboron complexes bearing a β-iminoketone ligand has been synthesized by using a simple two-step
reaction. Synthesized complexes exhibited fluorescence in solution (Fmax: 472ꢀ604 nm) and in the solid state (Fmax: 496ꢀ624 nm). These
complexes showed a larger Stokes shift (3690ꢀ4900 cmꢀ1) than well-known boron dipyrromethene dyes (400ꢀ600 cmꢀ1, in most cases).
Organoboron complexes are one of the most important
fluorescent dyes. Fluorescent organoboron complexes such
as pyrromethene,1 diketone,2 pyridomethene,3 subphth-
alocyanine,4 subporphyrin,5 azobenzene,6 and others7
have been reported. In particular, boron dipyrromethene
(BODIPY) is known to show excellent optical properties
ꢁ
(4) (a) Claessens, C. G.; Gonzalez-Rodrıguez, D.; Torres, T. Chem.
Rev. 2002, 102, 835. (b) Zhu, H.; Shimizu, S.; Kobayashi, N. Angew.
Chem., Int. Ed. 2010, 49, 8000. (c) Xu, S.; Chen, K.; Tian, H. J. Mater.
Chem. 2005, 15, 2676.
(5) (a) Inokuma, Y.; Kwon, J. H.; Ahn, T. K.; Yoo, M.-C.; Kim, D.;
Osuka, A. Angew. Chem., Int. Ed. 2006, 45, 961. (b) Hayashi, S.;
Inokuma, Y.; Osuka, A. Org. Lett. 2010, 12, 4148.
(1) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891. (b)
Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47,
1184. (c) Tram, K.; Yan, H.; Jenkins, H. A.; Vassiliev, S.; Bruce, D. Dyes
Pigm. 2009, 82, 392. (d) Arroyo, I. J.; Hu, R.; Merino, G.; Tang, B. Z.;
Pena-Cabrera, E. J. Org. Chem. 2009, 74, 5719. (e) Schmitt, A.;
Hinkeldey, B.; Wild, M.; Jung, G. J. Fluoresc. 2009, 19, 755.
~
(6) (a) Yoshino, J.; Furuta, A.; Kambe, T.; Itoi, H.; Kano, N.;
Kawashima, T.; Ito, Y.; Asashima, M. Chem.;Eur. J 2010, 16, 5026.
(b) Yoshino, J.; Kano, N.; Kawashima, T. Chem. Commun. 2007, 559.
(7) (a) Job, A.; Wakamiya, A.; Kehr, G.; Erker, G.; Yamaguchi, S. Org.
Lett. 2010, 12, 5470. (b) Zhou, Y.; Kim, J. W.; Kim, M. J.; Son, W.-J.; Han,
S. J.; Kim, H. N.; Han, S.; Kim, Y.; Lee, C.; Kim, S.-J.; Kim, D. H.; Kim,
J.-J.; Yoon, J. Org. Lett. 2010, 12, 1272. (c) Li, H.-J.; Fu, W.-F.; Li, L.; Gan,
X.; Mu, W.-H.; Chen, W.-Q.; Duan, X.-M.; Song, H.-B. Org. Lett. 2010,
12, 2924. (d) Wu, L.; Burgess, K. J. Am. Chem. Soc. 2008, 130, 4089. (e)
Zhou, Y.; Xiao, Y.; Chi, S.; Qian, X. Org. Lett. 2008, 10, 633. (f) Yoshino,
J.; Kano, N.; Kawashima, T. J. Org. Chem. 2009, 74, 7496. (g) Feng, J.;
Liang, B.; Wang, D.; Xue, L.; Li, X. Org. Lett. 2008, 10, 4437. (h) Zhou, Y.;
Xiao, Y.; Li, D.; Fu, M.; Qian, X. J. Org. Chem. 2008, 73, 1571. (i) Xia, M.;
Wu, B.; Xiang, G. J. Fluor. Chem 2008, 129, 402. (j) Wakamiya, A.; Mori,
K.; Yamaguchi, S. Angew. Chem., Int. Ed. 2007, 46, 4273. (k) Fischer,
G. M.; Daltrozzo, E.; Zumbusch, A. Angew. Chem., Int. Ed. 2011, 50, 1406.
(2) (a) Nagai, A.; Kokado, K.; Nagata, Y.; Arita, M.; Chujo, Y.
J. Org. Chem. 2008, 73, 8605. (b) Maeda, H.; Mihashi, Y.; Haketa, Y. Org.
Lett. 2008, 10, 3179. (c) Manaev, A. V.; Okhrimenko, I. N.; Lyssenko, K. A.
Traven, V. F. Russ. Chem. Bull., Int. Ed. 2008, 57, 1734. (d) Zhang, G.;
Lu, J.; Sabat, M.; Fraser, C. L. J. Am. Chem. Soc. 2010, 132, 2160. (e)
Poon, C.-T.; Lam, W. H.; Wong, H.-L.; Yam, V. W.-W. J. Am. Chem.
ꢀ
Soc. 2010, 132, 13992. (f) Stefane, B. Org. Lett. 2010, 12, 2900.
€
(3) (a) Basting, D.; Schafer, F. P.; Steyer, B. Appl. Phys. 1974, 3, 81.
(b) Douglass, J. E.; Barelski, P. M.; Blankenship, R. M. J. Heterocycl.
€
Chem. 1973, 10, 255. (c) Scheibe, G.; Daltrozzo, E.; Worz, O.; Heiss, J. Z.
Phys. Chem. N.F. 1969, 64, 97. (d) Kubota, Y.; Tsuzuki, T.; Funabiki,
~
K.; Ebihara, M.; Matsui, M. Org. Lett. 2010, 12, 4010. (e) Banuelos, J.;
ꢁ
Lopez Arbeloa, F.; Martinez, V.; Liras, M.; Costela, A.; Garcıa Moreno,
ꢁ
I.; Lopez Arbeloa, I. Phys. Chem. Chem. Phys. 2011, 13, 3437.
r
10.1021/ol202819w
Published on Web 11/16/2011
2011 American Chemical Society

such as high fluorescence quantum yield, sharp absorption
and fluorescence emission spectra, and high photo- and
chemical stability. Therefore, BODIPY dyes have played
increasingly important roles in many fields involving mole-
cular probes,8 photodynamic therapy,9 laser dyes,10 and
solar cells.11 However, most BODIPY dyes have a very
small Stokes shift (400ꢀ600 cmꢀ1, in most cases) because
they have a rigid molecular structure with minimal differ-
ences between the ground- and excited-state structures.12
Such a small Stokes shift makes it difficult for optical filters
to cut the excitation light, making it difficult to read the
fluorescence signal over the noise in a bioassay.12a A small
Stokes shift also induces the reabsorption of its own fluor-
escence, which causes a reduction in fluorescence intensity.
Also, most BODIPY dyes have high planarity, which in-
creases intermolecular interactions, causing concentration
quenching in the solid state.13 Thus, they hardly exhibit
fluorescence in the solid state.14 Recently, we reported on a
pyridometheneꢀBF2 complex that is an analog of a BOD-
IPY dye.3d Although this complex exhibited fluorescence
in the solid state, the Stokes shift was very small in solution
(250ꢀ400 cmꢀ1). In this paper, we report the synthesis and
fluorescence properties of a novel pyrazineꢀboron com-
plex that exhibits solid-state fluorescence and has a large
Stokes shift (3690ꢀ4900 cmꢀ1).
of diketone with triphenylborane, which gives the BPh2
complex.15 The structures of 4 and 7 were confirmed by
X-ray crystallography (Figures 1, S1).
Scheme 1. Synthesis of PyrazineꢀBoron Complexes
The THF solution of 2,5-dimethylpyrazine and ethyl
benzoate was refluxed in the presence of sodium hydride
to yield a tautomeric mixture of iminoketone 1a and
iminoenol 1b (Scheme S1, Table S1). The reaction of the
tautomeric mixture 1 with trifluoroborane gave pyrazine-
containing BF2 complex 4 (Scheme 1). Furthermore, 1 was
easily reacted with triphenylborane in THF at rt to give
BPh2 complex 7. Although this result is different from the
reaction of pyrromethene with triphenylborane, which
does not yield the BPh2 complex, it is consistent with that
(8) (a) Shao, J.; Guo, H.; Ji, S.; Zhao, J. Biosens. Bioelectron. 2011, 26,
3012. (b) Tachikawa, T.; Wang, N.; Yamashita, S.; Cui, S.-C.; Majima, T.
Angew. Chem., Int. Ed. 2010, 49, 8593. (c) Kobayashi, H.; Ogawa, M.;
Alford, R.; Choyke, P. L.; Urano, Y. Chem. Rev. 2010, 110, 2620. (d)
Gonc-alves, M. S. T. Chem. Rev. 2009, 109, 190.
Figure 1. ORTEP view of 7. Hydrogen atoms have been omitted
for clarity.
(9) (a) Lovell, J. F.; Liu, T. W. B.; Chen, J.; Zheng, G. Chem. Rev. 2010,
110, 2839. (b) Ozlem, S.; Akkaya, E. U. J. Am. Chem. Soc. 2009, 131, 48.
The absorption and fluorescence spectra of 1, 4, and 7 in
dichloromethane are shown in Figure 2. Compound 4
exhibited a maximum absorption wavelength (λmax) at 403
nm, which is more bathochromic than 1 (364 nm). Although
1 exhibited no fluorescence, 4 showed blue fluorescence at
480 nm. The λmax of 7 (427 nm) was red-shifted compared to
that of 4 (402 nm), and the molar absorption coefficient (ε)
of 7 (14000) was lower than that of 4 (24000). The max-
imum fluorescence wavelength (Fmax) of 7 (520 nm) also
exhibited a red shift compared to that of 4 (480 nm). As a
result, 7 exhibited yellow-green fluorescence. The absolute
fluorescence quantum yields (Φf) of 4 and 7 in dichloro-
methane were 0.57 and 0.70, respectively. Interestingly, 4
(3980 cmꢀ1) and 7 (4190 cmꢀ1) exhibited a larger Stokes
shift than a fluorescent boron complex such as a BODIPY
dye (400ꢀ600 cmꢀ1) and a pyridometheneꢀBF2 complex
(250ꢀ400 cmꢀ1). This large Stokes shift may be due to
the flexible structure of the pyrazineꢀboron complex
bearing a β-iminoketone ligand at the excited state.12
ꢁ
(10) (a) Gomez-Duran, C. F. A.; Garcıa-Moreno, I.; Costela, A.;
Martin, V.; Sastre, R.; Banuelos, J.; Lopez Arbeloa, F.; Lopez Arbeloa,
ꢁ
~
ꢁ
ꢁ
~
I.; Pena-Cabrera, E. Chem. Commun. 2010, 5103. (b) Ortiz, M. J.;
Garcia-Moreno, I.; Agarrabeitia, A. R.; Duran-Sampedro, G.; Costela,
ꢁ
~
ꢁ
A.; Sastre, R.; Lopez Arbeloa, F.; Banuelos Prieto, J.; Lopez Arbeloa, I.
Phys. Chem. Chem. Phys. 2010, 12, 7804.
(11) (a) Ertan-Ela, S.; Yilmaz, M. D.; Icli, B.; Dede, Y.; Icli, S.;
Akkaya, E. U. Org. Lett. 2008, 10, 3299. (b) Rousseau, T.; Cravino, A.;
Bura, T.; Ulrich, G.; Ziessel, R.; Roncali, J. Chem. Commun. 2009, 1673.
(12) (a) Ito, F.; Nagai, T.; Ono, Y.; Yamaguchi, K.; Furuta, H.;
Nagamura, T. Chem. Phys. Lett. 2007, 435, 283. (b) Qin, W.; Rohand,
T.; Baruah, M.; Stefan, A.; Van der Auweraer, M.; Dehaen, W.; Boens,
N. Chem. Phys. Lett. 2006, 420, 562.
(13) (a) Matsui, M.; Ikeda, R.; Kubota, Y.; Funabiki, K. Tetrahedron
Lett. 2009, 50, 5047. (b) Park, S.-Y.; Ebihara, M.; Kubota, Y.; Funabiki,
K.; Matsui, M. Dyes Pigm. 2009, 82, 258. (c) Park, S.-Y.; Kubota, Y.;
Funabiki, K.; Shiro, M.; Matsui, M. Tetrahedron Lett. 2009, 50, 1131.
(d) Yoshida, K.; Ooyama, Y.; Miyazaki, H.; Watanabe, S. J. Chem.
Soc., Perkin Trans. 2 2002, 700. (e) Ooyama, Y.; Nabeshima, S.;
Mamura, T.; Ooyama, H. E.; Yoshida, K. Tetrahedron 2010, 66, 7954.
(14) (a) Kubota, Y.; Uehara, J.; Funabiki, K.; Ebihara, M.; Matsui,
M. Tetrahedron Lett. 2010, 51, 6195. (b) Ozdemir, T.; Atilgan, S.; Kutuk,
I.; Yildirim, L. T.; Tulek, A.; Bayindir, M.; Akkaya, E. U. Org. Lett.
2009, 11, 2105.
(15) Kersten, L.; Roesner, S.; Hilt, G. Org. Lett. 2010, 12, 4920.
Org. Lett., Vol. 13, No. 24, 2011
6545

The λmax and Fmax of 4 and 7 were slightly affected
by solvent polarity. In hexane, the spectra of 4 and 7
showed vibrational transitions due to exclusion of solvation
relaxation15 (Figures S2ꢀS5 and Tables S2, S3).
the BPh2 complexes 8 (429 nm) and 9 (474 nm) are more
bathochromic than those of the corresponding BF2 com-
plexes 5 (402 nm) and 6 (466 nm). The ε of the BPh2 com-
plexes 8 (17000) and 9 (35000) are lower than those of the
corresponding BF2 complexes 5 (27 000) and 6 (46 000).
Methoxycarbonyl-group-substituted derivatives 5 and 8
exhibited similar absorption and fluorescence properties
compared to the corresponding nonsubstituted derivatives
4 and 7. On the other hand, owing to the pushꢀpull effect,
dimethylamino derivatives 6 and 9 exhibited a spectral red
shift and an increase in the ε in the absorption spectra. The
Φf of 6 and 9 were relatively lower (Table 1).
Two crystallographically independent conformers, 4A
and 4B, are identified in the unit cell of 4 (Z0 = 2), whereas
one conformer is found in that of 7 (Z0 = 1) (Figure S1).
The geometrical structures of 4A and 4B are almost same.
In the crystal structures of 4A and 4B, the six-membered
ring consisting of the boron atom (B1 or B2) and the
iminoenolate (N1ꢀC4ꢀC5ꢀC6ꢀO1 or N3ꢀC17ꢀC18ꢀ
C19ꢀO2) is almost planar. Yet, in the case of 7, the six-
membered ring that contains atoms N1, C4, C5, C6, and
O1 does not form a plane. B1, C6, and O1 atoms are out of
˚
the plane of N1ꢀC4ꢀC5; B1 is 0.23 A above the plane; and
Table 1. Optical Properties of PyrazineꢀBoron Complexes
˚
C6 and O1 are 0.24 and 0.39 A, respectively, below the
˚
˚
plane (Figure S6). The B1ꢀN1 (1.64 A) andB1ꢀO1 (1.48 A)
distances of 7 are greater than those of 4A and 4B (B1ꢀN1
˚
˚
and B2ꢀN3 are 1.59 A; B1ꢀO1 and B2ꢀO2 are 1.43 A). As
a result, the molecule of 7 is slightly bent. The spectral red
shift and decrease in the extinction coefficient of 7 may be
attributed to this molecular bend.17 The dihedral angle of
7 between the C5ꢀC6ꢀO1 plane and the phenyl ring is
1.4°. The corresponding dihedral angles of 4A and 4B are
6.7° and 13.6°, respectively. Therefore, the phenyl group is
considered to contribute to the expansion of π conjugation.
a Measured at a concentration of 1.0 ꢁ 10ꢀ5 mol dmꢀ3 at 25 °C.
b Measured using a FluoroMax-4 spectrofluorometer.
To obtain theoretical results for the absorption proper-
ties of 4ꢀ9, DFT calculations were performed with the
Gaussian 09 package.18 The geometries of 4ꢀ9 were opti-
mized at the DFT/B3LYP level using a 6-31G (d,p) basis
set. TDDFT calculations were also performed using the
B3LYP/6-311þþG(d,p) method. The calculated λmax, the
main orbital transition, and oscillator strength f are listed
in Table S4. These calculated λmax values are consistent
with the experimental ones. In all the cases, the first and
second absorptions are mostly attributed to the transitions
from HOMO to LUMO and the transitions from HOMO
Figure 2. UVꢀvis absorption and fluorescence spectra of 1, 4,
and 7 in dichloromethane measured at a concentration of 1 ꢁ
10ꢀ5 mol dmꢀ3 of substrate at 25 °C. Solid and dotted lines
represent absorption and fluorescence spectra, respectively.
Compounds 2, 3, 5, 6, 8, and 9 were obtained by a similar
reaction (Schemes 1 and S1). The absorption and fluores-
cence spectra of 1ꢀ3 in dichloromethane are shown in
Figure S7. Although 1 and 2 did not show fluorescence, 3
exhibited weak blue fluorescence at 390 nm (Φf = 0.03).
The absorption and fluorescence spectra of 4ꢀ9 in
dichloromethane are shown in Figures S8 and S9. As in
the case of phenyl derivatives 1 and 4, the absorptions of
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,
J. A. Gaussian 09, revision A. 02; Gaussian, Inc.: Wallingford, CT, 2009.
(16) Kudo, K.; Momotake, A.; Kanna, Y.; Nishimura, Y.; Arai, T.
Chem. Commun. 2011, 3867.
(17) (a) Brooker, L. G. S.; White, F. L.; Sprague, R. H.; Dent, S. G;
Van Zandt, G. Chem. Rev. 1947, 41, 325. (b) Longuet-Higgins, H. C.
J. Chem. Phys. 1950, 18, 265.
6546
Org. Lett., Vol. 13, No. 24, 2011

to LUMOþ1, respectively. All of the HOMO and LUMO
orbitals are delocalized over the entire set of molecules
(Figure S10). In the case of dimethylamino derivatives 6
and 9, calculations indicate that the charge transfer (CT)
transition might have occurred from the (dimethylamino)-
phenyl group to the pyrazine moiety. The CT transition is
considered to cause the red-shifted absorptions of 6 and 9.
Interestingly, all of the synthesized pyrazineꢀboron com-
plexes showed fluorescence in the solid state (Figure S11).
The fluorescence spectra and Φf of 4ꢀ9 in the solid
state are shown in Figure S12 and Table 1, respectively.
The kind of substituent groups on the boron atom and the
phenyl ring has an effect on the solid-state fluorescence
properties. As in the case of the solution, the solid-state
fluorescence spectra of 6 and 9 exhibited greater bathochro-
mic shifts and lower Φf. To explain the difference in the Φf,
we conducted X-ray crystallographic studies of 4, 6, 7, and9.
In the crystal packing of 4, 4A and 4B formed independent
stacking columns (Figure S13). In the column of 4A, gray
and orange molecules formed dimers by intermolecular CH/
of 9. Thus, networks of strong πꢀπ interactions induced
fluorescence quenching in the solid state of 9 (Φf = 0.03).
The ORTEP drawing and crystal packing of 6 are shown
in Figures S17 and S18, respectively. In the case of 6, al-
˚
though networks of π/π interactions (CꢀC: 3.52ꢀ3.55 A)
were observed, the interactions were weaker than those of
˚
9 (CꢀC: 3.22ꢀ3.57 A). Because compound 6 had weak
intermolecular interactions, 6 exhibited relatively weak
fluorescence in the solid state (Φf = 0.11).
˚
π interactions (CꢀC: 3.51 A) and CH/F interactions (CꢀF:
Figure 4. (a) Top view of crystal structure of 9. (b) Side view of
crystal structure of 9. Hydrogen atoms have been omitted for
clarity. The blue, black, and red dotted lines show intermole-
cular πꢀπ interactions (CꢀC), π/π interactions (CꢀN), and
CH/π interactions, respectively.
˚
3.37 A). Furthermore, networks of CH/F interactions
˚
(CꢀF: 3.30ꢀ3.37 A) were observed for the dimers. Simi-
˚
larly, networks of CH/F interactions (CꢀF: 3.23ꢀ3.36 A)
were observed in the column of 4B. It has been known that
strong and continuous intermolecular interaction between
neighboring fluorophores causes fluorescence quenching in
the solid states.13 Due to the strong networks of CH/F
interactions, 4 exhibited low Φf (0.04) in the solid state.
Yet, in the crystal packing of 7, although dimers
were formed between orange and gray molecules by CH/
In conclusion, we have shown that pyrazineꢀboron
complexes bearing a β-iminoketone ligand can be easily
synthesized from methylpyrazine and benzoate derivatives.
All synthesized complexes exhibited fluorescence in dichloro-
methane. The Fmax (472ꢀ604 nm) and Φf (0.28ꢀ0.71) were
affected by the kind of substituents on the boron atom and
the phenyl group. Unlike common fluorescent boron
complexes such as BODIPY dyes, pyrazineꢀboron com-
plexes exhibited a large Stokes shift (3690ꢀ4900 cmꢀ1).
Although BODIPY dyes have a rigid fluorophore, pyr-
azineꢀboron complexes have a relatively flexible fluoro-
phore owing to the molecular rotation of the aryl group.
Since the flexible fluorophore causes large differences
between the ground- and excited-state structures, pyrazineꢀ
boron complexes may show large Stokes shifts. Further-
more, these complexes exhibited fluorescence in the solid
state (Φf: 0.03ꢀ0.32). Relationships between the solid-state
Φf and molecular packing of pyrazineꢀboron complexes
were obtained by X-ray crystallographic analysis. The Φf
of 4 (0.04) and 9 (0.03) were low owing to networks of CH/
F interactions and strong π/π interactions, respectively. The
relatively lower Φf of 6 (0.11) was caused by networks of
weak π/π interactions. Compound 7 exhibited a relatively
high Φf (0.27) due to the prevention of network interactions.
˚
π (CꢀC: 3.50 A) and π/π interactions (CꢀC: 3.45ꢀ
˚
3.55 A), there was no interaction between adjacent dimers
(Figures 3 and S14). Because network interactions were
avoided by the substitution of fluorines to phenyl groups,
7 exhibited relatively high Φf (0.27).
Figure 3. (a) Top view of crystal structure of 7. (b) Side view of
crystal structure of 7. Hydrogen atoms have been omitted for
clarity. The blue and red dotted lines show intermolecular π/π
interactions and CH/π interactions, respectively.
The ORTEP drawing and crystal packing of 9 are shown
in Figures 4, S15, and S16. Similar to 7, in the crystal packing
of 9, orange and gray molecules formed dimers by CH/π
Acknowledgment. This work was supported by Grant-
in-Aid for Young Scientists (B) (23750228).
˚
˚
(CꢀC: 3.53 A) and π/π interactions (CꢀC: 3.22ꢀ3.57 A).
Supporting Information Available. Details of experi-
mental procedures and copies of 1H, 13C NMR, and 2D
spectra for new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
˚
However, unlike the case of 7, π/π interactions (CꢀN: 3.40 A)
were observed between neighboring dimers. As a result, net-
works of π/π interactions were formed in the crystal packing
Org. Lett., Vol. 13, No. 24, 2011
6547