π-Fused bis-BODIPY as a candidate for NIR dyes

Article abstract of DOI:10.1039/c2ob25930c

Benzene-fused bis-(borondipyrromethene)s (bis-BODIPYs) were synthesized by retro-Diels-Alder reaction of the corresponding bicyclo[2.2.2]octadiene-fused (BCOD-fused) bis-BODIPYs, which were, in turn, prepared from 4,8-ethano-4,8-dihydropyrrolo[3,4-f]isoindole derivatives. The π-fused bis-BODIPY chromophores were designed to show intensive absorption and strong fluorescence in the near-infrared region and not to have any strong absorption in the visible region. A 6,10-dibora-5a,6a,9a,10a-tetraaza-s-indaceno[2,3-b:6,5- b′]difluorene derivative (syn-bis-benzoBODIPY) obtained by a thermal retro-Diels-Alder reaction of the corresponding BCOD-fused BODIPY dimer has strong absorption and emission bands at 775 and 781 nm, respectively. The absolute quantum yield is 0.36. The absorption is more than 5.0 times stronger than other absorptions observed in the visible region. In the case of 6,15-dibora-5a,6a,14a,15a-tetraaza-s-indaceno[2,3-b:6,7-b′]difluorene derivatives (anti-bis-benzoBODIPY), the absorption and emission maxima exceed 840 nm.

Full text of DOI:10.1039/c2ob25930c

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Volume 10 | Number 34 | 14 September 2012 | Pages 6805–7016
ISSN 1477-0520
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Toshi Nagata, Hidemitsu Uno et al.
π-Fused bis-BODIPY as a candidate for NIR dyes
1477-0520(2012)10:34;1-A

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PAPER
π-Fused bis-BODIPYas a candidate for NIR dyes†
Mitsunori Nakamura,a Hiroyuki Tahara,a Kohtaro Takahashi,a Toshi Nagata,*b Hiroki Uoyama,a
Daiki Kuzuhara,d Shigeki Mori,c Tetsuo Okujima,a Hiroko Yamadad and Hidemitsu Uno*a
Received 15th May 2012, Accepted 28th June 2012
DOI: 10.1039/c2ob25930c
Benzene-fused bis-(borondipyrromethene)s (bis-BODIPYs) were synthesized by retro-Diels–Alder
reaction of the corresponding bicyclo[2.2.2]octadiene-fused (BCOD-fused) bis-BODIPYs, which were, in
turn, prepared from 4,8-ethano-4,8-dihydropyrrolo[3,4-f]isoindole derivatives. The π-fused bis-BODIPY
chromophores were designed to show intensive absorption and strong fluorescence in the near-infrared
region and not to have any strong absorption in the visible region. A 6,10-dibora-5a,6a,9a,10a-tetraaza-
s-indaceno[2,3-b:6,5-b′]difluorene derivative (syn-bis-benzoBODIPY) obtained by a thermal retro-Diels–
Alder reaction of the corresponding BCOD-fused BODIPY dimer has strong absorption and emission
bands at 775 and 781 nm, respectively. The absolute quantum yield is 0.36. The absorption is more than
5.0 times stronger than other absorptions observed in the visible region. In the case of 6,15-dibora-
5a,6a,14a,15a-tetraaza-s-indaceno[2,3-b:6,7-b′]difluorene derivatives (anti-bis-benzoBODIPY),
the absorption and emission maxima exceed 840 nm.
commonly robust in spite of abundant π electrons. This phenom-
enon is essential because cyclic chromophores have degenerated
or nearly degenerated HOMOs and LUMOs due to the existence
of a certain kind of symmetry. The porphyrin chromophore is a
typical example; there are two major absorption bands, namely
Soret and Q bands, in the UV-vis region mainly derived from
transitions between HOMOs and LUMOs.7 When these chromo-
phores are fused, not only the absorption bands mainly due to
the transitions between enlarged HOMOs and LUMOs but also
other bands in the visible region due to monomeric transitions
are usually observed.8 On the other hand, linear chromophores
show relatively simple absorptions. For example, boron-dipyrro-
methene (BODIPY)9 and cyanine10 dyes are well known to have
simple strong absorption and emission bands in the visible
region. When the BODIPY chromophore is fused with other
chromophores in the meso direction, however, the absorption
spectra become complicated due to the multi-directional tran-
sitions.11 Recently, a German group successfully prepared selec-
tive NIR dyes. They used pyrrolopyrrole12 as the cyanine-
connecting unit and succeeded in the preparation of bis- and
tris-cyanine dyes,13 which had not only intense absorptions in
the NIR region and good transparency in the visible region, but
also very high fluorescence quantum yields.13c In this paper, we
discuss our approach toward the preparation of selective NIR
dyes by the fusion of BODIPY chromophores (Chart 1).
Introduction
Increasing interest has been paid towards chromophores with
absorptions and emissions in the red or near-infrared (NIR)
region. Organic compounds with such chromophores are thought
to be promising materials for a broad range of applications in
material science. Their applications such as photo-dynamic
therapy dyes,1 microscopic imaging agents,2 semiconductors in
NIR light emitting diodes,3 dyes for NIR cut-filters,4 and photo-
sensitizers in NIR-utilizing solar cells5 have been reported. In
particular these materials are essential for the success of high
performance organic solar cells.6 The chromophores must
consist of large conjugated π systems and are classified into two
types: linear and cyclic chromophores. Generally, the latter chro-
mophores show rather complicated absorption bands due to the
existence of plural transitions between molecular orbitals with
different symmetries near the HOMO and LUMO levels. These
chromophores have rather large HOMO–LUMO gaps and are
aDepartment of Chemistry and Biology, Graduate School of Science and
Engineering, Ehime University, Matsuyama 790-8577, Japan.
E-mail: uno@ehime-u.ac.jp; Fax: +81-89-927-9610
bNational Institutes for Natural Sciences (NINS), Institute for Molecular
Science (IMS), 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan
cIntegrated Center for Sciences, Ehime University, Matsuyama
790-8577, Japan
dGraduate School of Materials Science, Nara Institute of Science and
Technology, Ikoma 630-0192, Japan; CREST, JST, Chiyoda-ku,
102-0075, Japan
Results and discussion
†Electronic supplementary information (ESI) available: Other exper-
imental data, X-ray data, and data indicated in the text. CCDC 865408,
864644–864652. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2ob25930c
We started the synthesis by preparation of BCOD-fused bis-pyr-
roles
2
from ethyl 4,7-dihydro-4,7-ethano-2H-isoindole-1-
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Chart 1 Skeletal nomenclature of bis-BODIPYs.
carboxylate (1)14 according to the literature procedure.8b
Addition of phenylsulfenyl chloride followed by oxidation with
mCPBA and a modified Barton–Zard reaction15 gave bis-pyr-
roles syn-2 and anti-2 as a diastereomeric mixture (4 : 3 ratio) in
a total yield of 63%.16 Chromatographic separation of the
mixture afforded syn-2 and anti-2 in pure form (Scheme 1).
Although reduction of bis-pyrroles syn-2 and anti-2 with LiAlH4
in refluxing THF gives the corresponding dimethyl derivatives in
good yields,17 the dimethyl bis-pyrroles are rather labile toward
acid and readily decompose under the acidic conditions
employed in the preparation of dipyrromethanes. Therefore, the
more stable di-esters syn-2 and anti-2 under acidic conditions
were directly used for the dipyrromethane synthesis. Double
dipyrromethane formation of syn-2 and anti-2 with ethyl 5-acet-
oxymethyl-3,4-diethylpyrrole-2-carboxylate (3a)18 under acidic
conditions (p-TsOH, AcOH) gave bis-dipyrromethane tetra-
esters syn-4a and anti-4a in 54% and 73% yields, respectively.
Exhaustive reduction of syn-4a and anti-4a with LiAlH4 in
refluxing THF afforded tetramethyl derivatives syn-5a and anti-
5a in respective yields of 96% and 93%. Oxidative complexation
of the tetramethyl derivatives and BF3·OEt2 with the aid of
chloranil gave BCOD-connected bis-BODIPYs syn-6a and anti-
6a in 36% and 53% yields, respectively. A similar synthesis
using BCOD-fused acetoxymethylpyrrole 3b19 in the first bis-
dipyrromethane formation was carried out to give syn and anti
bis-BODIPYs syn-6b and anti-6b bearing three BCOD moieties,
two of which were fused at the distal pyrrole rings. The overall
yields of syn-6b and anti-6b were 28% and 48%, respectively.
The thermal behavior of the BCOD-fused bis-BODIPYs 6
was next examined by the thermogravimetric (TG) analysis. The
measurements were conducted at a rate of 10 °C min−1 and the
TG curves obtained are illustrated in Fig. 1. In all cases, theoreti-
cal weight is lost due to the extrusion of ethylene moieties
except for a small quantity of solvent included in the samples. In
the cases of syn- and anti-6a, the weight loss due to the retro-
Diels–Alder reaction connecting the BODIPY chromophores
started from 180 °C and ceased around 240 °C and no obvious
weight decrease was observed below 300 °C. In the cases of syn-
and anti-6b, there was an inflection point around 200 °C in the
weight loss started around 150 °C. Double the amount of weight
was lost in the lower temperature range before the inflection
Scheme 1 Reagents, conditions, and yields: (i) ref. 11. (ii) 3, p-TsOH,
AcOH, rt; syn-4a, 54%; syn-4b, 61%; anti-4a, 73%; and anti-4b, 72%.
(iii) LiAlH4, THF, reflux; syn-5a, 96%; syn-5b, 65%; anti-5a, 93%; and
anti-5b, 82%. (iv) chloranil, BF3·OEt2, (i-Pr)2EtN, CH2Cl2, rt; syn-6a,
36%; syn-6b, 70%; anti-6a, 53%; and anti-6b, 82%.
Fig. 1 TG curves (10 °C min−1) of syn-6a (grey broken line), anti-6a
(grey line), syn-6b (black broken line), and anti-6b (black line).
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point and the higher temperature range corresponded to the
temperature for the retro-Diels–Alder reaction connecting the
BODIPY chromophores.
deviations of twelve BODIPY atoms from their mean planes lie
between 0.021 Å in anti-6b·2CHCl3 and 0.103 Å in one mol-
ecule of anti-6a·PhCl (Table 1). A maximum deviation of 0.186(2)
Å was observed in the boron atom of syn-6a as observed in
crystal structures of the recently prepared mother BODIPY.20 The
dihedral angles between the mean planes of BODIPY chro-
mophores vary from 122.54(3)° in syn-6a to 134.76(4)° in another
molecule of anti-6a·PhCl. These values are similar to those of bis-
porphyrins fused with BCOD.8b The observed large widening is
ascribed to the crystal packing effect. In fact, the dihedral angles of
BCOD skeletons in these bis-BODIPYs are similar to those
reported for BCOD-connected dipyrroles 2,16 although the dihedral
angles of connecting pyrroles are still a little larger. In the case of
π-fused bis-BODIPY 7, the whole chromophores are almost flat
(Fig. 3).
According to the TG experiments, the bulk thermal conver-
sion of BCOD-fused bis-BODIPYs was conducted at 250 °C for
1 h under a reduced pressure and the π-fused bis-BODIPYs 7
and 9 were obtained (Fig. 2). Due to poor solubility in common
solvents and instability in solution, spectroscopic identification
of 9 could not be done except for UV spectroscopy. When
syn-6b and anti-6b were treated at a temperature (160 °C) below
the inflation point for 1 h in vacuo, BCOD-fused π-expanded
BODIPYs syn-8 and anti-8 were obtained in good yields.
Structures of the BCOD- and benzene-fused bis-BODIPYs
were determined by an X-ray crystallographic analysis. Suitable
single crystals of syn-6a, anti-6a·PhCl, syn-6b·CHCl3, anti-
6b·2CHCl3, syn-7·C7H8, and anti-7·2C10H7Cl were obtained.
All crystals except for syn-6a contained co-crystallized solvent
molecules. The crystal of anti-6a·PhCl consists of two crystallo-
graphically independent bis-BODIPY molecules. In all cases,
the BODIPY chromophores are almost flat and the mean
Electronic spectra of the bis-BODIPYs were next examined.
The UV-vis and fluorescence spectra in chloroform are shown in
Fig. 4 and 5, and the data are summarized in Table 2 (also see
Fig. S1–S9 in ESI†). Quite different spectra were obtained for
syn- and anti-BCOD-connected BODIPYs (syn-6a,b vs. anti-6a,
b). The anti-dimers show strong and sharp absorption bands at
544–545 nm with ε = 2.34–2.37 × 105 M−1 cm−1 and their fluor-
escence bands were observed at 550–551 nm with high quantum
yields (0.97 for anti-6a and 0.69 for anti-6b). Although the
absorption bands are slightly red-shifted compared with those of
fully alkyl-substituted BODIPYs and the Stokes shift was quite
small (6 nm), these molar extinction values are almost twice as
large as those of the common BODIPY dyes.9,21 On the other
hand, splitting of the absorption bands is observed in the case of
BCOD-fused syn-bis-BODIPYs. syn-6a shows a split absorption
band at 503 and 550 nm with ε = 7.52 × 104 and 1.51 ×
105 M−1 cm−1, respectively. The molar extinction values are
similar to those of the common BODIPYs.9,21 Similarly, syn-6b
shows a band at 504 (5.76 × 104) and 553 (1.09 × 105 M−1 cm−1
)
nm. Fluorescence emission of syn-6a and syn-6b is observed at
560 and 561 nm with quantum yields of 0.92 and 0.82, respect-
ively. The difference in the absorption bands between the
syn- and anti-isomers is rationalized by taking the molecular
geometry into an account (vide infra).
BCOD-connected bis-benzoBODIPYs 8 show a red shift of
24 nm compared to the corresponding syn- and anti-6b (Fig. 5)
and this bathochromic shift is less than a half of those observed
in the doubly benzo-expanded BODIPYs.19a,19b,22 A very effec-
tive bathochromic shift was achieved by fusion of the BODIPY
Fig. 2 Structures of π-expanded bis-BODIPYs by thermal treatment.
Table 1 Deviation from BODIPY mean plane
BODIPY deviations (Å)
Plane torsion angles
Compound
Mean
BODIPYs
Pyrroles
BCOD
syn-6a
anti-6a
0.060
0.052
0.103
0.053
0.030
0.095
0.054
0.054
0.067
0.021
C8, 0.126(2)
B1, 0.118(3)
B1, 0.183(2)
C2, 0.111(9)
C3, 0.051(3)
B2, 0.186(2)
C11, 0.106(2)
B2, 0.097(3)
B2, 0.16(2)
122.54(3)°
134.76(4)°
132.66(4)°
132.8(1)°
130.22(5)°
4.77(7)°
129.08(6)°
133.94(8)°
130.71(8)°
130.4(3)°
132.1(1)°
4.5(1)°
124.02(9)°
124.2(1)°
123.6(1)°
122.7(5)°
123.9(2)°
—
syn-6b
anti-6b
syn-7
C17, 0.042(4)
0.055
0.026
B2, 0.219(5)
B, 0.044(6)
B2, 0.223(4)
anti-7
0°
0°
—
16/CHCl3
16/PhCl
17
0.024
0.017
0.036
0.075
0.027
0.033
B1, 0.045(4)
C4, 0.032(2)
C9, 0.085(3)
124.71(5)°
119.78(3)°
130.73(3)°
123.1(1)°
124.00(8)°
129.59(8)°
122.5(2)°
122.9(1)°
123.1(1)°
C12, 0.050(2)
B2, 0.124(2)
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chromophores with benzene and the fusion mode was important.
BODIPY anti-7 with C2h symmetry reaches 758 (1.90 ×
105 M−1 cm−1) nm which is very close to the NIR region. These
bis-BODIPYs syn-7 and anti-7 still have strong fluorescence at
703 (Φ = 0.72) and 774 (Φ = 0.32) nm, respectively. Similar Φ
values of NIR fluorescence are reported for 8-aza-BODIPY
bearing coplanar anisyl-substituents at 3,5-positions,23 dibenzo-
BODIPYs with aryl groups at 3,5-positions,24 Keio Fluos,25 and
their thia analogs.26 When the benzoBODIPY chromophores are
fused with benzene, both the absorption and fluorescence
maxima of C2v and C2h isomers syn-9 and anti-9 are observed in
the NIR region: syn-9 absorbs at 775 (1.41 × 105 M−1 cm−1) nm
and emits at 781 (Φ = 0.36) nm and anti-9 absorbs at 848 (2.04
× 105 M−1 cm−1) nm and emits at 868 (Φ = 0.04) nm. These
NIR absorptions are 3.7 to 5.2 times larger than the correspond-
ing visible absorptions.
Benzene-fused syn-bis-BODIPY syn-7 with C2v symmetry
shows the absorption maximum at 692 (2.32 × 105 M−1 cm−1
)
nm, while the absorption maximum of benzene-fused bis-
The benzene-fused bis-benzoBODIPYs are labile under
aerobic conditions. When syn-9 was dissolved in CHCl3 under
aerobic conditions, a strong visible absorption peak at 645 nm
with a shoulder (618 nm) appeared at the expense of the 775 nm
Fig. 3 Ortep drawings (50% probability) of top (a, c) and side (b, d)
views of syn-7 and anti-7. Disordered atoms with smaller occupancy,
hydrogen atoms, and co-crystallized solvent molecules are omitted for
clarity.
Fig. 4 UV-vis (black) and fluorescence (grey, scaled by arbitrary unit)
spectra of syn-6a (broken lines in a), syn-7 (solid lines in a), anti-6a
(broken lines in b), and anti-7 (solid lines in b).
Fig. 5 UV-vis spectra of syn derivatives (a), syn-6b: solid, syn-8:
broken, and syn-9: bold; and anti derivatives (b), anti-6b: solid, anti-8:
broken, and anti-9: bold.
Table 2 UV-vis and fluorescence spectra of bis-BODIPYs in CHCl3
UV-vis (nm)
Fluorescence
Compound
(ε/104 M−1 cm−1
)
/nma
Φ
syn-6a
anti-6a
syn-6b
anti-6b
syn-7
anti-7
syn-8
anti-8
syn-9
anti-9
16
387
388
397
403
510
390
400
322
551
615
403
328
613
(2.15)
(1.86)
(2.15)
(3.17)
(3.42)
(2.40)
(1.88)
(3.13)
(2.70)
(5.56)
(2.51)
(2.30)
(0.68)
503
504
504
509
641
691
524
535
655
762
511
532
756
(7.52)
(4.65)
(5.76)
(6.85)
(4.86)
(5.25)
(8.84)
(8.33)
(2.44)
(3.37)
(5.69)
(4.98)
(2.54)
550
544
553
545
692
758
577
569
775
848
543
570
843
(15.10)
(23.70)
(10.90)
(23.40)
(23.20)
(19.00)
(12.60)
(28.20)
(14.10)
(20.40)
(23.50)
(19.20)
(15.80)
560
550
561
551
703
774
591
583
781
868
549
580
861
(10)
(6)
(8)
0.92
0.97
0.82
0.69
0.72
0.32
0.87
0.88
0.36b
0.04
0.71b
0.32b
0.03b
(6)
(11)
(16)
(14)
(14)
(6)
(20)
(6)
17
10
(10)
(18)
a Numerals in the parentheses are Stokes shifts. b In CH2Cl2.
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Fig. 6 UV-vis spectra of syn-9 (a) after 0 h (broken) and 20 h (solid);
and anti-9 after 0 h (broken) and 6 h (solid).
NIR peak, which completely disappeared after 20 h (Fig. 6). The
visible peak of 551 nm in syn-9 also disappeared and a new peak
at 562 nm appeared. On the other hand, when anti-9 was dis-
solved in CHCl3 under air, the visible absorption at 615 nm
increased, as the NIR peak of 848 nm decreased. After 6 h, there
was no peak over 700 nm. Although we could not identify the
decomposition products, the absorption peaks at 645 nm in syn-
9 and at 615 nm in anti-9 are imputed to be decomposed com-
pounds formed during the sample preparation for the UV
measurements. On the other hand the peak of 551 nm in the
visible region of the syn-9 spectrum is thought to be due to syn-9
itself.
Scheme 2 . (i) 4-tert-Butylbenzoic acid, TFAA, TFA, rt; 77%; NaBH4,
THF–MeOH, rt; 95%; Ac2O, DMAP, CH2Cl2, rt; 99%. (ii) anti-2,
pTSA, AcOH, rt; 99%; (iii) LiAlH4, THF, reflux; 86%. (iv) DDQ,
CH2Cl2, rt; BF3·OEt2, (i-Pr)2EtN, CH2Cl2, rt; 75%. (v) 160 °C, 30 min,
93%. (vi) 235 °C, 30 min, 94%.
In order to get spectroscopic evidence that the final retro-
Diels–Alder reaction proceeded smoothly, we planned to prepare
the more soluble anti-bis-benzoBODIPY 10 (Scheme 2). Etha-
noisoindole 1 was reacted with p-tert-butylbenzoic acid under
acidic dehydration conditions to give 3-(p-tert-butylbenzoyl)-
ethanoisoindole 11, which was then treated with NaBH4 fol-
lowed by Ac2O to afford acetoxy derivative 13.27 Acidic conden-
sation of anti-2 with 13 provided bis-dipyrromethane
tetracarboxylate 14 in good yield. Exhaustive reduction of 14
with LiAlH4 in refluxing THF gave all-methyl derivative 15,
which was oxidatively reacted with BF3·OEt2 to provide bis-
BODIPY 16. Thermogravimetric analysis of 16 revealed three
marked weight losses around 100, 170, and 230 °C, which corre-
sponded to evaporation of co-crystallized solvent, extrusion of
ethylene from distal ends of BCOD moieties, and extrusion of
ethylene from the center BCOD moiety, respectively. In fact,
when 16 was heated at 175 and 235 °C in vacuo for 1 h, BCOD-
connected bis-benzoBODIPY 17 and benzene-connected bis-
benzoBODIPY 10 were successfully isolated in 93% and 94%
yields, respectively. Benzene-connected bis-benzoBODIPY 10
Fig. 7 UV-vis spectra of 16 (solid), 17 (broken), and 10 (bold).
dimers. The TD-DFT results of anti-6a predict a strong band at
430 nm (oscillator strength, f = 0.9828), which corresponds to
the observed band at 544 nm. On the other hand, the results of
syn-6a predict three strong bands at 433, 417, and 399 nm,
which correspond to the observed bands between 503 and
550 nm. The relevant molecular orbitals of BODIPY and its
BCOD-connected dimers are shown in ESI.† The four frontier
molecular orbitals of the dimer (HOMO − 1 to LUMO + 1) can
be approximately described by symmetric and anti-symmetric
linear combinations of BODIPY HOMO and LUMO. We denote
these orbitals as HOMO(s), HOMO(a), LUMO(s), and LUMO
(a). The TD-DFT results revealed that the most prominent peak
in the electronic spectra consists of LUMO(a) ← HOMO(s) and
LUMO(s) ← HOMO(a) transitions, and that the next prominent
peak consists of LUMO(s) ← HOMO − 2. As apparent from the
1
was unambiguously determined by H NMR analysis. The UV-
vis spectral shapes of bis-BODIPYs 10, 16, and 17 are almost
the same as the corresponding meso-unsubstituted derivatives
(Fig. 7 and Table 2) except that the visible absorption of 10 at
613 nm is quite weak.
In order to understand the electronic spectra, TD-DFT calcu-
lations on bis-BODIPYs (RB3LYP/6-31G(d)) were carried out.28
The calculated spectra are shown in Fig. 8 and 9. First we
discuss the results for BODIPY itself and BCOD-connected
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Fig. 8 TD-DFT results of syn-6a (upper) and anti-6a (lower) with UV-
vis spectra.
Fig. 10 The calculated frontier orbitals of syn-7 (left) and anti-7
(right).
are similar for syn-7 and anti-7, whereas the energies of LUMO
and LUMO + 1 are markedly different; the LUMO of anti-7 is
lower by 0.186 eV than the LUMO of syn-7, while the LUMO +
1 of anti-7 is higher by 0.28 eV than the LUMO + 1 of syn-7.
The former difference causes the smaller HOMO–LUMO gap in
anti-7 than syn-7, leading to the electronic transition at longer
wavelength.
The extra stabilization of LUMO in anti-7 results from the
delocalization of the LUMOs of the two BODIPY moieties
through the benzo bridge. The AO coefficients on carbons 6 and
13 (see Chart 1) are significant enough to cause delocalization.
On the other hand, the LUMO in syn-7 has zero AO coefficients
on these carbons, thereby showing no interaction between the
LUMOs of the two BODIPY moieties. Actually, these character-
istics can be observed not only in the DFT results, but also in the
simple Hückel-type calculation of the π-orbitals (ESI†). There-
fore, the presence and absence of the interaction between the two
BODIPY LUMOs in these isomers are controlled purely by the
different topology of the π-electron framework.
Fig. 9 TD-DFT results of syn-7 (upper) and anti-7 (lower) with UV-vis
spectra.
orbital isosurfaces, the LUMO(a, s) ← HOMO(s, a) transitions
affect all four pyrrole rings in the molecule, whereas the LUMO(s)
← HOMO − 2 transition only affects the inner two pyrrole rings.
Therefore, the former transitions have longer transition dipole
moment, and thus longer oscillator strength.
The difference between the anti and syn isomers is that the
terminal pyrrole rings are more distant in the anti isomer than in
the syn isomer. This causes the LUMO(a, s) ← HOMO(s, a)
transitions in the anti isomer to be even more prominent com-
pared with the LUMO(s) ← HOMO − 2 transition. On the con-
trary, the LUMO(a, s) ← HOMO(s, a) transitions are less
prominent in the syn isomer, which results in a clearer obser-
vation of the LUMO(s) ← HOMO − 2 transition.
In the cases of benzene-fused bis-BODIPYs 7, frontier orbitals
with nicely separated energy are obtained (Fig. 10). Therefore,
the sharp and strong absorptions with the lowest energy mostly
consist of simple HOMO–LUMO transitions in both cases:
566 nm (LUMO ← HOMO, 72%, f = 1.2907) in syn-7 and
639 nm (LUMO ← HOMO, 70%, f = 1.0350) in anti-7. A
similar trend in energy diagram is observed in benzene-fused
bis-benzoBODIPYs 9, and the energy gaps are narrower (ESI†).
Fig. 10 shows the relevant molecular orbitals of syn-7 and anti-
7, together with their orbital energies. The energies of HOMO
Conclusion
We succeeded in the preparation of BODIPY dimers fused at
β,β-positions with benzene, namely benzene-fused bis-
BODIPYs, which are proved to have sharp and strong absorption
and emission bands in the red to NIR region. As the bis-
BODIPYs keep good transparency in the other visible region,
the dyes are thought to be promising candidates for selective
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 6840–6849 | 6845

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NIR dyes, although the stability is rather low. Stabilization of
bis-BODIPY dyes is the next subject for investigation.
mixture was extracted with chloroform. The organic extract was
washed with aqueous saturated NaHCO3, water, and brine, dried
over Na2SO4, and concentrated in vacuo. The residue was chro-
matographed on silica gel (CHCl3 or 30% EtOAc–hexane).
Experimental section
Diethyl 3,7-bis(5-ethoxycarbonyl-3,4-diethylpyrrol-2-ylmethyl)-
4,8-dihydro-4,8-ethanopyrrolo[3,4-f]isoindole-1,5-dicarboxylate
(anti-4a). The reaction of 3a (0.321 g, 1.18 mmol), and diethyl
4,8-dihydro-4,8-ethanopyrrolo[3,4-f]isoindole-1,5-dicarboxylate
(anti-2; 0.165 g, 0.50 mmol) provided 0.273 g (73%) of the title
compound as a white powder: mp, 147–149 °C; 1H NMR: δ
9.10 (br s, 2H), 8.79 (br s, 2H), 4.66 (m, 2H), 4.20–4.30 (m,
8H), 3.92 (m, 4H), 2.71 (m, 4H), 2.42 (m, 4H), 1.71 (m, 4H),
1.31 (t, 6H, J = 7.1 Hz), 1.25 (t, 6H, J = 7.1 Hz), 1.14 (t, 6H, J
= 7.6 Hz), 1.04 (t, 6H, J = 7.6 Hz); 13C NMR: δ 161.70, 161.28,
137.22, 133.58, 128.93, 127.87, 124.44, 123.18, 116.92, 113.19,
60.00, 59.70, 31.55, 30.44, 28.32, 23.07, 18.34, 17.01, 16.21,
General
Melting points are uncorrected. Unless otherwise specified,
NMR spectra were obtained with a JEOL JNM AL-400 spec-
trometer at ambient temperature by using CDCl3 as a solvent and
tetramethylsilane as an internal standard for H and 13C. Mass
1
spectra (EI and FAB) were measured with an MStation spec-
trometer (JEOL MS-700). MALDI-TOF mass spectra were
measured on a Voyger DE Pro (Applied Biosystems) in VBL,
Ehime University by using sinapinic acid as matrix. IR spectra
were measured with a Horiba FT-720 spectrophotometor. UV-vis
and fluorescence spectra were measured on JASCO V-570 and
HITACHI F-4500, respectively. Absolute quantum yields were
measured on a Hamamatsu Photonics C9920-02. TG analysis
was done with an SII Exstar 600 TG/DTA 6200. Elemental
analysis was performed on a Yanaco MT-5 elemental analyzer.
DMF was distilled under reduced pressure and then stored over
MS 4 Å. Pyridine and DMSO were distilled from CaH2 and
stored over MS 4 Å. Other dry solvents were purchased
from Kanto Chemical Co. Ethyl isocyanoacetate was kindly
provided by The Nippon Synthetic Chemical Industry Co. Ltd.
5-(Acetoxymethyl)pyrrole-2-carboxylates18,19 and bis-pyrroles
28 were prepared according to the literature.
15.78, 14.36; IR (KBr): 3334, 1701, 1647, 1444, 1263 cm−1
;
MS (FAB+): m/z 743 (M+ + 1), 742 (M+), 741 (M+ − 1); HRMS
(FAB+): C42H54N4O8 + H+: 743.4020. Found: 743.4019. Anal.
Calcd for C42H54N4O8 + 1/8C6H14: C, 68.13; H, 7.46; N, 7.43.
Found: C, 67.90, H, 7.33, N, 7.54.
General procedure for reduction of the ester groups. To a
stirred solution of bis-dipyrromethane 4 (1.0 mmol) in dry THF
(25 mL) was added a 1 M solution of LiAlH4 (6.0 mL,
6.0 mmol) in ether at 0 °C. The mixture was allowed to warm up
to room temperature and then the mixture was refluxed for 3 h.
After the mixture was cooled to room temperature, the reaction
was quenched with water. The resulting suspension was filtered
through a Celite pad and the filtrate was extracted with EtOAc.
The organic extract was washed with brine, dried over Na2SO4,
and concentrated in vacuo. The residue was passed through a
short silica-gel column. This material was used without further
purification due to its labile nature under air.
X-ray crystal structure analysis
Single crystals were prepared by vapor diffusion of isopropanol
or methanol into a solution of bis-BODIPYs in CHCl3, chloro-
benzene, toluene or 1-chloronaphthalene. The crystals were
taken with a Cryoloop and mounted. Determination of cell par-
ameters and collection of reflection intensities were performed
by the CrystalClear or RAPID AUTO software.29 The data were
corrected for Lorentz, polarization, and absorption effects. The
structures were solved by SIR200430 and expanded using the
Fourier technique.31 Hydrogen atoms were placed in calculated
positions and refined by using riding models. All calculations
were performed by using the CrystalStructure crystallographic
software package.32 Shelxl-9733 was used for structure refine-
ment. The data were validated by the Platon program.34 All crys-
tallographic data and the Platon validation results of syn-6a,
anti-6a·PhCl, syn-6b·CHCl3, anti-6b·2CHCl3 syn-7·PhMe, anti-
7·2C10H7Cl, 16·2CHCl3, 16·PhCl, and 17·PhCl·1/2i-C3H7OH
are shown in the ESI.†
1,5-Dimethyl-3,7-bis(3,4-diethyl-5-methylpyrrol-2-ylmethyl)-
4,8-dihydro-4,8-ethanopyrrolo[3,4-f]isoindole (anti-5a). anti-
Bis-dipyrromethane tetra ester anti-4a (0.372 g, 0.50 mmol) was
reduced to give 0.239 g (93%) of the title compound as a pale
1
yellow powder: H NMR: δ 7.30 (br s, 2H), 6.84 (br s, 2H),
3.77–3.87 (m, 6H), 2.37–2.45 (m, 8H), 2.10 (s, 6H), 2.08 (s,
6H), 1.65 (m, 4H), 1.11 (t, 6H, J = 7.6 Hz),1.10 (t, 6H, J = 7.6
Hz); 13C NMR: δ 126.69, 125.71, 122.04, 121.01, 120.22,
119.82, 116.58, 115.71, 30.07, 28.80, 22.83, 17.61, 17.53,
16.80, 16.38, 11.10, 11.06; IR (KBr): 3384, 2962, 2931, 2868,
cm−1; MS (FAB+): m/z 511 (M+ + 1), 510 (M+), 509(M+ − 1);
HRMS (FAB+): C34H46N4 + H+, 511.3801. Found: 511.3795.
General procedure for the oxidative complexation with
BF3. To a stirred solution of bis-dipyrromethane 5 (0.1 mmol)
in dry CH2Cl2 (5 mL) was added a suspension of chloranil
(50 mg, 0.2 mmol) in CH2Cl2 (5 mL) under an inert atmosphere
in the dark and the mixture was stirred for 1 h at room tempera-
ture. To the mixture were added successively diisopropylethyl-
amine (0.2 mL, 1.2 mmol) and BF3·OEt2 (0.16 mL, 1.3 mmol)
and the mixture was stirred at room temperature for 2 h. The
mixture was quenched with water and then filtered through a
Celite pad. The filtrate was washed with aqueous saturated
Materials
General procedure for the condensation of acetoxymethyl-
pyrrole-1-carboxylate 3 and dipyrroledicarboxylate 2. To a
stirred solution of 3 (2.0 mmol), and dipyrroledicarboxylate 2
(1.0 mmol) in acetic acid (80 mL) was added p-toluenesulfonic
acid monohydrate (122 mg, 0.78 mmol) under an inert atmos-
phere in the dark. After the solution was stirred for 3 h at room
temperature, the reaction was quenched with water and the
6846 | Org. Biomol. Chem., 2012, 10, 6840–6849
This journal is © The Royal Society of Chemistry 2012

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6,6,10,10-Tetrafluoro-5,7,9,11-tetramethyl-8,17-dihydro-8,17-
ethano-6,10-dibora-5a,6a,9a,10a-tetraaza-s-indaceno[2,3-b:
6,5-b′]difluorene (syn-8). syn-Bis-BODIPY syn-6b (5 mg,
0.006 mmol) was heated at 160 °C to give syn-8 quantitatively
as a purple powder: mp, 250 °C (decomp.); 1H NMR: δ
7.72–7.80 (m, 4H), 7.52 (m, 2H), 7.31–7.39 (m, 4H), 4.21 (m,
1H), 4.10 (m, 1H), 2.93 (s, 6H), 2.55 (s, 6H), 1.78 (m, 4H); 13C
NMR: δ 155.75, 151.96, 146.34, 143.96, 142.05, 137.85,
135.07, 131.76, 130.54, 125.68, 122.77, 119.18, 116.52, 48.79,
46.09, 31.82, 29.62, 13.06, 12.35; IR (KBr): νmax 2946, 1604,
1473, 1226, 1118 cm−1; UV-vis (CHCl3): λmax (ε/104 M−1
cm−1), 577 (12.6), 524 (8.84), 400 (1.88) nm; MS (FAB+): m/z
NaHCO3, water, and brine, dried over Na2SO4, and concentrated
in vacuo. The residue was chromatographed on silica gel.
1,2,8,9-Tetraethyl-4,4,11,11-tetrafluoro-3,5,10,12-tetramethyl-
6,13-dihydro-6,13-ethano-4,11-dibora-3a,4a,10a,11a-tetraaza-
benzo[1,2-a:4,5-a′]-s-indacene (anti-6a). anti-Bis-dipyrromethane
anti-5a (49 mg, 0.095 mmol) was reacted to give 30.5 mg (53%)
1
of the title compound as pink crystals: H NMR: δ 7.03 (s, 2H),
4.38 (m, 2H), 2.60 (q, 4H, J = 7.6 Hz), 2.55 (s, 6H), 2.51 (s,
6H), 2.39 (q, 4H, J = 7.6 Hz), 1.64–1.78 (m, 4H), 1.21 (t, 6H,
J = 7.6 Hz), 1.09 (t, 6H, J = 7.6 Hz); 13C NMR: δ 158.18,
148.24, 145.66, 144.64, 134.60, 132.94, 132.20, 126.14, 119.27,
30.93, 28.88, 17.89, 17.30, 16.94, 15.01, 12.96, 12.72; IR
(KBr): 2962, 2931, 2871, 1602, 1174 cm−1; UV-vis (CHCl3):
λmax (ε/104 M−1 cm−1), 544 (23.7), 504 (4.65), 388 (1.86) nm;
MS (FAB+): m/z 603 (M+ + 1); HRMS (FAB+): C34H40B2F4N4
591 (M+
+
1), 590 (M+); HRMS (FAB+): Calcd for
C34H28B2F4N4 + H+, 591.2514. Found: 591.2521.
6,6,15,15-Tetrafluoro-5,7,14,16-tetramethyl-8,17-dihydro-8,17-
ethano-6,15-dibora-5a,6a,14a,15a-tetraaza-s-indaceno[2,3-b:6,7-
b′]difluorene (anti-8). Bis-BODIPY anti-6b (5 mg, 0.006 mmol)
was heated at 160 °C for 30 min and the title compound was
obtained in quantitative yield. anti-8: purple powder; mp,
250 °C (decomp.); 1H NMR: δ 7.72–7.81 (m, 4H), 7.53 (m,
2H), 7.33–7.38 (m, 4H), 4.41 (m, 2H), 2.93 (s, 6H), 2.57 (s,
6H), 1.77 (m, 4H); 13C NMR δ 158.171, 145.68, 141.63,
135.33, 133.40, 131.74, 130.61, 129.27, 125.71, 124.58, 122.72,
+
H+, 603.3453. Found: 603.3459. Anal. Calcd for
C34H40B2F4N4 + 1/3CHCl3: C, 64.22; H, 6.33; N, 8.73. Found:
C, 64.32, H, 6.23, N, 8.44.
General procedure for thermal conversion of bis-BODIPYs
with BCOD moieties. A bis-BODIPY compound was weighed
in a small test tube and the test tube was placed in a vessel. The
vessel was evacuated and placed in a pre-heated glass tube oven
at the indicated temperature. After 30 min, the vessel was taken
out and cooled to room temperature. The product in the test tube
was directly subjected to the analyses.
119.30, 116.25, 31.94, 30.70, 29.71, 29.50; IR (KBr): νmax
,
,
2938, 1508, 1511, 1227, 1002 cm−1; UV-vis (CHCl3): λmax
(ε/104 M−1 cm−1), 569 (28.2), 535 (8.33), 322 (3.13) nm;
MS (FAB+): m/z 591 (M+ + 1), 590 (M+): HRMS (FAB+): Calcd
for C34H38B2F4N4 + H+, 591.2514. Found: 591.2521.
1,2,10,11-Tetraethyl-4,4,8,8-tetrafluoro-3,5,7,9-tetramethyl-4,8-
dibora-3a,4a,7a,8a-tetraazabenzo[1,2-a:5,4-a′]-s-indacene (syn-7).
syn-Bis-BODIPY syn-6 (10.0 mg, 0.017 mmol) was converted at
250 °C to afford 9.2 mg (97%) of the title compound as blue
6,6,10,10-Tetrafluoro-5,7,9,11-tetramethyl-6,10-dibora-5a,6a,-
9a,10a-tetraaza-s-indaceno[2,3-b:6,5-b′]difluorene (syn-9). Bis-
BODIPY syn-8 (4 mg, 0.007 mmol) was heated at 250 °C for
30 min and the title compound was obtained in quantitative
yield. syn-9: black powder; mp, >350 °C; 1H NMR: δ 7.63–7.73
(m, 4H), 7.52–7.57 (m, 4H), 7.39 (m, 2H), 3.01 (s, 6H), 2.90 (s,
6H); UV-vis (CHCl3): λmax (ε/104 M−1 cm−1), 775 (14.1), 655
(2.44), 551 (2.70) nm; MS (FAB+): m/z 563 (M+ + 1), HRMS
(FAB+): Calcd for C32H24B2F4N4 + H+, 563.2201. Found:
563.2200.
1
crystals: H NMR: δ 8.08 (s + s, 2H), 7.29 (s, 2H), 2.99 (s, 6H),
2.62 (q, 4H, J = 7.6 Hz), 2.51 (s, 6H), 2.42 (q, 4H, J = 7.6 Hz),
1.23 (t, 6H, J = 7.6 Hz), 1.12 (t, 6H, J = 7.6 Hz); 13C NMR: δ
157.34, 149.43, 139.97, 129.77, 129.62, 129.45, 127.76, 118.93,
114.02, 134.76, 108.50, 17.86, 17.24, 17.15, 15.34, 12.98,
12.27; IR (KBr): 1589, 1209, 1186, 1095 cm−1; UV-vis
(CHCl3): λmax (ε/104 M−1 cm−1), 692 (23.2), 641 (4.86), 510
(3.42) nm; MS (FAB+): m/z 574 (M+); HRMS (FAB+):
C32H36B2F4N4 + H+, 575.3140. Found: 575.3127. Anal. Calcd
for C32H36B2F4N4 + H2O: C, 64.89; H, 6.47; N, 9.46. Found: C,
64.56; H, 5.96; N, 9.24.
6,6,15,15-Tetrafluoro-5,7,14,16-tetramethyl-6,15-dibora-5a,6a,-
14a,15a-tetraaza-s-indaceno[2,3-b:6,7-b′]difluorene (anti-9). Bis-
BODIPY anti-8 (4 mg, 0.007 mmol) was heated at 250 °C for
30 min and the title compound was obtained in quantitative
yield. anti-9: black powder; mp, >350 °C; UV-vis (CHCl3): λmax
(ε/104 M−1 cm−1), 848 (20.4), 762 (3.37), 615 (5.56) nm; MS
(FAB+): m/z 562 (M+), HRMS (FAB+): Calcd for C32H24B2F4N4
1,2,8,9-Tetraethyl-4,4,11,11-tetrafluoro-3,5,10,12-tetramethyl-
4,11-dibora-3a,4a,10a,11a-tetraazabenzo[1,2-a:4,5-a′]-s-indacene
(anti-7). The title compound (9.3 mg, 98%) was obtained from
anti-bis-BODIPY anti-6a (10 mg 0.017 mmol) at 250 °C as
+
H+, 563.2201. Found: 563.2217. Anal. Calcd for
1
green crystals: H NMR: δ 8.15 (s, 2H), 7.27 (s, 2H), 3.03 (s,
C32H24B2F4N4 + 1/2H2O: C, 67.29; H, 4.41; N, 9.81. Found: C,
67.59; H, 4.71; N, 9.45. Due to the low solubility, other spectro-
scopic analyses could not be done.
6H), 2.65 (q, 4H, J = 7.6 Hz), 2.52 (s, 6H), 2.43 (q, 4H, J = 7.6
Hz), 1.25 (t, 6H, J = 7.6 Hz), 1.12 (t, 6H, J = 7.6 Hz); 13C
NMR: δ 155.87, 149.82, 140.27, 132.65, 131.44, 129.85,
129.49, 128.54, 113.57, 113.52, 17.91, 17.27, 17.15, 15.35,
13.06, 12.32; IR (KBr): 1606, 1213, 1184, 1095 cm−1; UV-vis
(CHCl3): λmax (ε/104 M−1 cm−1), 758 (19.0), 691 (5.25), 390
(2.40) nm; MS (FAB+): m/z 575 (M+ + 1), 574 (M+); HRMS
(FAB+): C32H36B2F4N4 + H+, 575.3140. Found: 575.3146.
Anal. Calcd for C32H36B2F4N4 + H2O: C, 64.89; H, 6.47; N,
9.46. Found: C, 64.67; H, 6.34; N, 9.23.
5,9-Bis(4-tert-butylphenyl)-6,6,15,15-tetrafluoro-5,7,14,16-tetra-
methyl-1,4,8,10,13,17-hexahydro-1,4:8,17:10,13-triethano-6,15-
dibora-5a,6a,14a,15a-tetraaza-s-indaceno[2,3-b:6,7-b′]difluorene
(16). The title compound was obtained from 15 (0.351 g,
0.429 mmol) in 75% yield (0.293 g) as a diastereomer mixture:
red crystals; mp, 157 °C (decomp.); 1H NMR: 7.56 (m, 4H),
7.31 (m, 4H), 6.38 (m, 2H), 6.08 (m, 2H), 3.81 (m, 2H),
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 6840–6849 | 6847

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2.76 (m, 2H), 2.72 (m, 2H), 2.51 (s, 6H), 2.22 and 2.20 (a pair
of singlets, 6H), 1.45 (s, 18H), 1.4–1.1 (m, 12H); 13C NMR
(typical signals): δ 152.52, 152.50, 151.18, 151.04, 149.74,
149.66, 148.92, 146.25, 139.52, 138.62, 138.51, 135.71, 135.60,
133.82, 131.61, 128.93, 128.90, 128.61, 128.58, 128.41, 128.11,
127.75, 127.14, 125.30, 125.19, 124.66, 124.63, 124.58, 124.56,
35.26, 35.23, 34.96, 32.96, 32.93, 31.59, 31.54, 31.47, 27.78,
27.70, 27.63, 26.50, 26.44, 25.94, 25.87, 12.71, 12.10, 12.05;
UV-vis (CHCl3): λmax (ε/104 M−1 cm−1), 543 (23.5), 511 (5.69),
403 (2.51) nm; MS (FAB+): m/z 911 (M+ + 1); HRMS (FAB+):
C58H60B2F4N4 + H+, 911.5018. Found: 911.5013. Anal. Calcd
for C58H60B2F4N4 + CHCl3 + C3H7OH: C, 68.30; H, 6.38; N,
5.14. Found: C, 68.36; H, 6.69; N, 5.28.
5,9-Bis(4-tert-butylphenyl)-6,6,15,15-tetrafluoro-5,7,14,16-tetra-
methyl-8,17-dihydro-8,17-ethano-6,15-dibora-5a,6a,14a,15a-tetra-
aza-s-indaceno[2,3-b:6,7-b′] difluorene (17). Bis-BODIPY 16
(17.4 mg, 0.019 mmol) was heated at 175 °C for 2 h and
15.1 mg (93%) of the title compound was obtained: black
1
powder; mp, >223 °C; H NMR: δ 7.7–7.6 (m, 6H), 7.38 (m,
4H), 7.19 (t, 2H, J = 8.0 Hz), 7.12 (t, 2H, J = 8.0 Hz), 6.39 (d,
2H, J = 8.0 Hz), 2.93 (s, 6H), 2.66 (br s, 2H), 2.23 (s, 6H),
1.6–1.4 (m, 4H), 1.41 (s, 18H); UV-vis (CHCl3): λmax (ε/104
M−1 cm−1), 570 (19.2), 532 (4.98), 328 (2.30) nm; MS (FAB+):
855 (M + 1); HRMS (FAB+): Calcd for C54H52B2F4N4 + H+,
855.4392. Found: 855.4391. Anal. Calcd for C54H52B2F4N4 +
1/5CHCl3: C, 74.10; H, 5.99; N, 6.38. Found: C, 74.05, H,
5.82; N, 6.13.
15 K. D. Johnstone, W. A. Pearce and S. M. Pyke, J. Porphyrins Phthalo-
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5,9-Bis(4-tert-butylphenyl)-6,6,15,15-tetrafluoro-5,7,14,16-tetra-
methyl-6,15-dibora-5a,6a,14a,15a-tetraaza-s-indaceno[2,3-b:6,7-
b′]difluorene (10). Bis-BODIPY 16 (13.2 mg, 0.014 mmol) was
heated at 235 °C for 2 h and 11.3 mg (94%) of the title com-
1
pound was obtained: black powder; mp, >350 °C; H NMR: δ
7.67 (m, 4H), 7.51 (d, 2H, J = 8.0 Hz), 6.98 (t, 2H, J = 8.0 Hz),
6.87 (t, 2H, J = 8.0 Hz), 6.30 (d, 2H, J = 8.0 Hz), 6.22 (s, 2H),
2.83 (s, 6H), 2.53 (s, 6H), 1.53 (s, 18H); UV-vis (CHCl3): λmax
(ε/104 M−1 cm−1), 843 (15.8), 756 (2.54), 613 (0.68) nm; MS
(FAB+): 827 (M + 1); HRMS (FAB+): Calcd for C52H48B2F4N4
+ H+, 827.4079. Found: 827.4064.
22 Z. Shen, H. Röhr, K. Rurack, H. Uno, M. Spieles, B. Schulz, G. Reck
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Acknowledgements
25 K. Umezawa, Y. Nakamura, H. Makino, D. Citterio and K. Suzuki,
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This work is partially supported by Grant-in-Aids for Scientific
Research (23350020, 23655044, and 23108714 (π-space)) from
the Japanese Ministry of Education, Culture, Sports, Science and
Technology. Some calculations were done at IMS under the
cooperative research program (Nanonet).
28 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin,
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