Excimer emission and energy transfer in cofacial boradiazaindacene (BODIPY) dimers built on a xanthene scaffold

Article abstract of DOI:10.1016/j.tet.2005.12.021

Using a rigid xanthene scaffold, a series of boradiazaindacene derivatives were synthesized. In some of these compounds, two boradiazaindacene derivatives were placed cofacially, resulting in significant inter-chromophoric interactions, including excimer

Full text of DOI:10.1016/j.tet.2005.12.021

Tetrahedron 62 (2006) 2721–2725
Excimer emission and energy transfer in cofacial
boradiazaindacene (BODIPY) dimers built on a xanthene scaffold
Neslihan Saki,a Tarik Dincb and Engin U. Akkayab,
*
aDepartment of Chemistry, Kocaeli University, TR-41380 Izmit, Turkey
bDepartment of Chemistry, Middle East Technical University, TR-06531 Ankara, Turkey
Received 8 September 2005; revised 24 November 2005; accepted 8 December 2005
Available online 28 December 2005
Abstract—Using a rigid xanthene scaffold, a series of boradiazaindacene derivatives were synthesized. In some of these compounds, two
boradiazaindacene derivatives were placed cofacially, resulting in significant inter-chromophoric interactions, including excimer emission.
A simple modification of boradiazaindacene structure leads to formation of an ICT dye, which has distinct spectral properties. Energy
transfer between two BODIPY dyes was demonstrated as well. In addition, the spectral properties of ICT dye can be modulated by the
addition of the acid leading to an acid switchable energy transfer cassette.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
on a suitable scaffold. Here, there is just one possible
transition dipole, which might lead to a very clearly
understandable experimental results. The xanthene unit
seems to provide such a structural feature because
functionalization at positions 4 and 5 looks straightforward
(Fig. 1). In recent literature, there are reports of cofacially
arranged porphyrin and perylenediimide dyes,8 but no
examples of cofacial boradiazaindacene dimers were found.
In this paper, we present the synthesis, energy transfer and
acid switching of spectral properties of novel boradiazain-
dacene dimers built on a xanthene scaffold.
Boradiazaindacenes (a.k.a, BODIPY dyes, BDPs, difluor-
obora-dipyrromethenes, etc.) are well known fluorescent
dyes with remarkable spectral properties like high quantum
yields, large extinction coefficients and narrow emission
bands.1 These properties facilitated their application in
many fields, such as fluorescent labeling of biomolecules,2
ion sensing and signaling,3 energy transfer cassettes,4 light
harvesting systems5 and fluorescent stains.1 Especially
considering the relative ease of derivatization, it should
not be difficult to predict further diversification of
applications in the near future. Dimeric boradiazaindacenes
should be particularly interesting as a new subclass of these
dyes. It is well known that when biomolecules are labeled
with boradiazaindacene dyes at relatively large dye/protein
ratios, two boradiazaindacene moieties can come very close
to each other and interactions yield quenching of the
emission and/or formation of a bathochromically shifted
excimer band.6 The existence of two structurally distinct
dimers were proposed and based on these observations, a
BODIPY dimer obtained by the labeling of diaminocyclo-
hexane was studied.7 However, in all of these dimeric
systems, boradiazaindacene units are highly flexible and in
principle can adopt a number of different excited state
structures. Thus, it would be very interesting to assemble
two boradiazaindacene units in a rigid cofacial arrangement
R7
8
R1
R2
5
R5
1
R3
7
4
O
9
6
3
2
6
4
B
N
N
3
5
7
2
8
1
R6
R4
F
F
I
II
Figure 1. Structures of 2,7-di-tert-butyl-9,9-dimethyl-9H-xanthane I and
difluorobora-dipyrromethene II.
2. Results and discussion
The synthesis of the dyes 4, 5 and 9 starts with the
chloromethylation of the t-butylated xanthene derivative 1
(Scheme 1). Paraformaldehyde and concd HCl was used to
carry out this transformation. Following a standard work-up,
the product was usually obtained in satisfactory purity and
Keywords: BODIPY dyes; Energy transfer; Multichromophoric systems;
Molecular switches.
Corresponding author. Tel.: C90 312 210 5126; fax: C90 312 210 1280;
e-mail: akkayaeu@metu.edu.tr
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.12.021

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N. Saki et al. / Tetrahedron 62 (2006) 2721–2725
Cl
Cl
N
H
N
O
O
(a)
H
F
H
F
F
F
F
F
F
F
2
B
B
H
1
B
B
N
N
N
N
N
N
N
N
(a)
(e)
(b)
O
O
H
O
H
O
O
H
O
O
4
9
Scheme 2. Synthesis of BODIPY dye 9. Reagents and conditions:
(a) piperidine, CH3COOH, N,N-dimethyl-4-aminobenzaldehyde.
6
3
(c)
(c)
The normalized absorption spectra of compounds 4 and 7 in
THF are shown in Figure 2. There are remarkable changes
in the absorption spectrum. In the cofacial dimer 4, the
absorption peak is blue shifted to 478 nm with a shoulder at
504 nm. Compound 5, as expected, shows two peaks, one
for the standard boradiazaindacene absorption (455 nm) and
one for the extended conjugation chromophore 575 nm.
When a few drops of TFA is added, the long wavelength
ICT absorption shifts to shorter wavelengths (530 nm)
reflecting lower electron donor characteristics of the
protonated dialkylamino groups (Fig. 3). Also, the
absorption spectra of compounds 5, 8 and 9 are shown
separately in Figure 4. Extension of conjugation, as
expected, results in a broad longer wavelength absorption
at 571 nm. This is in accordance with the internal charge
transfer (ICT) character of the dyes. The emission spectra
are even more interesting: while monochromophoric 7
yields a very strong emission at 500 nm when excited at
480 nm, 4 results in highly quenched emission with two
F
F
F
F
F
F
B
B
B
N
N
N
N
N N
O
O
4
7
(d)
(d)
N
H
N
H
F
H
F
B
F
F
F
H
F
B
B
N
N
N
N
N
N
O
O
1
8
5
0.8
Scheme 1. Synthesis of BODIPY dyes 4, 5, 7 and 8. Reagents and
conditions: (a) HCl, (CH2O)n, D, CH3COOH, o-phosphoric acid;
(b) DMSO, NaHCO3, D; (c) 2,4-dimethylpyrrole, TFA, Et3N, BF3:OEt2,
rt; (d) piperidine, CH3COOH, N,N-dimethyl-4-aminobenzaldehyde;
(e) HMTA, TFA, reflux.
0.6
0.4
0.2
0
4
7
used directly in the next step, which is the oxidation to bis-
aldehyde 3 using DMSO. This aldehyde was then utilized in
the construction of two boradiazaindacene units on the
xanthene scaffold. This was done in analogy to the literature
procedures, using 2,4-dimethylpyrrole as the source for the
pyrrole unit. Following the purification steps, a bright green
fluorescent dye 4 was obtained in 24% yield. Due to the
acidity of the methyl groups at positions 3 and 5 of the
boradiazaindacene heterocyclic system, extension of conju-
gation is possible by the condensation with aromatic
aldehydes.3e,9 We chose p-dimethylaminobenzaldehyde as
the aldehyde in this reaction, because the electron donor
capacity of the dialkylamino group can be modified by the
protonation/deprotonation equilibrium, which could result
in significant spectral changes. Thus, compounds 5 and 9
were obtained by refluxing the aldehyde together with the
dye 4 in a Dean–Stark apparatus (Scheme 2). The second
methyl group in each boradiazaindacene unit is presumably
less reactive. In order to accurately assess the inter-
chromophoric interactions including resonance energy
transfer, we also synthesized xanthene derivatives carrying
just one boradiazaindacene unit (compounds 7 and 8).
600
450
500
550
400
Wavelength (nm)
Figure 2. Normalized absorption spectra of compounds 4 and 7.
Normalized (performed at the maxima (478 and 504 nm, respectively)) to
an arbitrary value of 0.8.
0.5
0.4
5
0.3
5 + acid
0.2
0.1
0
375
425
475
525
575
625
Wavelength (nm)
Figure 3. Absorption spectra of compound 5 and its acidified form. Upon
acidification with a few drops of TFA, a significant blue shift is observed.

N. Saki et al. / Tetrahedron 62 (2006) 2721–2725
2723
further improved by the protonation of the dialkyl-
aminogroups by the addition of a few drops of TFA. The
increase in intensity is probably in part due to the increased
spectral overlap caused by the blue shift in the absorption
spectrum of compound 5.
5
0.6
0.5
0.4
0.3
0.2
0.1
0
8
9
5
3. Conclusion
500 550 600
Wavelength (nm)
700
400 450
650
We have synthesized and characterized xanthene deri-
vatives with boradiazaindacene units attached orthogonally
and in a very rigid arrangement. The cofacial chromophores
were separated from each other only by a distance of
Figure 4. Normalized absorption spectra of compounds 5, 8 and 9.
Normalization was done at the long wavelength peaks to an arbitrary
value of 0.3.
˚
approximately 4.5 A, so both energy transfer and excimer
formation can be observed in these systems. These
derivatives can be excellent models to study the role of
orientation of dipole moments during excitation. A careful
inspection of the structure of compound 9 reveals that syn–
anti stereoisomerism is possible. Although isolation of both
isomers would be very useful in the study relative
chromophore orientations in energy transfer, we were able
to isolate only one isomer, most likely the anti-isomer.
Based on our modeling studies, and considering the steric
requirements of the condensation reaction, the anti-isomer
is expected to be the major, if not the sole product.
peaks one at 505 nm and a broader excimer emission with a
peak at 590 nm (Fig. 5). This is in accordance with the
earlier observations made with less rigid boradiazaindacene
dimers and organized media like micelles. In Figure 6,
emission characteristics of 4, 5 and 8 were compared. When
the absorptions at the excitation wavelength (480 nm) for 4
and 5 were adjusted to 0.1, the emission spectra give more
quantitative information about the quantum yields and the
efficiency of the energy transfer relative emission. Very
clearly, in compound 5, the emission from the green
emitting boradiazaindacene unit is further quenched, and
some emission at long wavelength can be seen, this is due to
energy transfer between the donor and the acceptor
chromophore. The absorption of compound 8 was also
adjusted at the long wavelength peak (near 550 nm) to that
of compound 5. But, since this chromophore does not absorb
at 480 nm effectively, the emission is very weak. When
analyzed together, it becomes obvious that in compound 5
there is efficient energy transfer. The efficiency can be
The bichromophoric systems described here, have some
potential as energy transfer cassettes, as well. In principle,
these compounds can be excited at 480 nm, and the
emission can be collected at 650 nm. As novel fluorophore
systems, it is also likely that water soluble derivatives can be
used in labeling biomolecules and in novel fluorescent
chemosensors for cations or anions.
4. Experimental
4.1. General
1000
800
7
600
The compounds were characterized and analyzed by
nuclear magnetic resonance spectroscopy (NMR), UV/vis
400
4
1
spectroscopy, and fluorescence spectroscopy. H and 13C
200
nuclear magnetic resonance spectra of all compounds were
recorded in CDCl3 with Bruker Gmbh DPX-400,
400 MHz High Performance Digital FT-NMR Spec-
trometer. UV/vis spectra were recorded by Varian Bio
100 UV/vis Spectrophotometer. Fluorescence spectra were
recorded using Varian Cary Eclipse Fluorescence Spec-
trophotometer. All solvents were distilled over CaCl2
before use. 2,7-Di-tert-butyl-9,9-dimethyl-xanthane and
2,4-dimethylpyrrole were obtained from Aldrich. Hexa-
methylenetetramine was purchased from MERCK-Schu-
chardt. Merck Silica Gel 60 F254 TLC Aluminum sheets
were used in monitoring reactions by thin-layer chromato-
graphy. Merck Silica Gel 60 (particle size 0.040–
0.0963 mm, 230–400 mesh ASTM) used in column
chromatography.
0
480
580
Wavelength (nm)
780
680
Figure 5. Emission spectra of compounds 4 and 7. The absorption values at
the excitation wavelength (480 nm) was adjusted to 0.1.
5 + acid
140
120
5
8
4
100
80
60
40
20
0
480
680
Wavelength (nm)
780
580
4.1.1.
2,7-Di-tert-butyl-4,5-bis(chloromethyl)-9,9-
dimethyl-9H-xanthene 2. A mixture of 2,7-di-tert-butyl-
9,9-dimethyl-9H-xanthene (500 mg, 1.55 mmol), p-formal-
dehyde (200 mg), orthophosphoric acid (0.14 mL), HCl
(0.6 mL) and acetic acid (8.2 mL) were heated in a pressure
Figure 6. Emission spectrum of 4, 5, 8. Absorption values of the dyes 4 and
5 were set at 0.1, whereas the absorption peaks at the long wavelength
region (near 550 nm) were set to be comparable for the dyes 5 and 8.

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N. Saki et al. / Tetrahedron 62 (2006) 2721–2725
tube at 85 8C for overnight. The reaction mixture was then
diluted with CHCl3 and the solution was washed with
saturated NaHCO3. Then, the organic phase was dried over
anhydrous Na2SO4 and solvent was evaporated under
reduced pressure to give bis(chloromethyl)xanthene 2 as
white powder (585 mg, 90%). Used in the following steps
solution containing the crude product was concentrated
under reduced pressure and purified by silica gel column
chromatography (1:4 ethylacetate/hexane) in 75% yield. IR
(KBr) nmax: 3093, 3036, 1172, 1095 cmK1 1H NMR
.
(400 MHz, CDCl3) d 1.20 (unresolved singlets, 18HC
12H, C(CH3)3Cpyr-CH3), 1.57 (s, 6H, C(CH3)2), 2.34 (s,
3H, pyr-CH3), 2.39 (s, 3H, pyr-CH3), 2.41 (s, 3H, pyr-CH3),
3.0 (s, 6H, N(CH3)2), 5.20 (s, 1H, pyr-H), 5.51 (s, 1H, pyr-
H), 5.55 (s, 1H, pyr-H), 5.65 (s, 1H, pyr-H), 6.63–6.65 (m,
2H), 6.88–7.05 (m, 4H), 7.4–7.45 (m, 4H); 13C NMR
(100 MHz, CDCl3) d 31.4, 33.3, 33.7, 34.7, 119.8, 120.7,
112.4, 123.2, 123.3, 125.2, 125.5, 128.7, 130.9, 131.5,
136.7, 140.5, 141.8, 143.9, 144.1, 153.6, 156.9. Elemental
analysis: Found: C, 73.43; H, 7.07; N, 7.29.
C58H65N5OB2F4 requires C, 73.66; H, 6.93; N, 7.40.
without further purification. Mp 201–202 8C. IR (KBr) nmax
:
3022, 1110 cmK1. H NMR (400 MHz, CDCl3) d 1.27 (s,
18H, C(CH3)3), 1.57 (s, 6H, C(CH3)2), 4.75 (s, 4H, CH2),
7.17 (s, 2H, Ar-H), 7.28 (s, 2H, Ar-H); 13C NMR (100 MHz,
CDCl3) d 31.9, 32.0, 32.8, 34.9, 42.5, 123.9, 124.4, 125.9,
130.0, 146.1, 146.4.
1
4.1.2. 2,7-Di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-
dicarbaldehyde 3. 2,7-Di-tert-butyl-4,5-bis(chloro-
methyl)-9,9-dimethyl-9H-xanthene (1.2 g, 2.86 mmol) and
NaHCO3 (600 mg, 7.14 mmol) were heated in DMSO
(200 mL) for 3 days. The reaction mixture was then diluted
with CHCl3, and the solution was washed with water until
all the DMSO was removed. Then the mixture was dried
over Na2SO4. The solvent was evaporated, and the residue
was purified by silica gel column chromatography (CH3OH/
CHCl3 1:99) to give 3 (324.7 mg, 30%) as a white solid. Mp
4.1.5. 2,7-Di-tert-butyl-9,9-dimethyl-9H-xanthene-4-
carbaldehyde 6. A mixture of 2,7-di-tert-butyl-9,9-
dimethyl-9H-xanthene (967 mg, 3 mmol), hexamethylene–
tetramine (840 mg, 6 mmol) and CF3COOH (6 mL) were
refluxed for 24 h. The acid was removed under reduced
pressure and the residue was then subjected to silica gel
column chromatography. (CH3OH/CHCl3 1:99) to yield
compound 6 (540 mg, 51%). Mp 158–160 8C. IR (KBr)
nmax: 3039, 1719, 1114 cmK1. 1H NMR (400 MHz, CDCl3)
d 1.27 (s, 18H, C(CH3)3), 1.62 (s, 6H, C(CH3)2), 6.8 (d,
1
248–249 8C. IR (KBr) nmax: 3031, 1721, 1128 cmK1. H
NMR (400 MHz, CDCl3) d 1.27 (s, 18H, C(CH3)3), 1.62 (s,
6H, C(CH3)2), 7.64 (d, JmetaZ2.38 Hz, 2H, Ar-H), 7.74 (d,
JmetaZ2.39 Hz, 2H, Ar-H), 10.6 (s, 2H, CHO); 13C NMR
(100 MHz, CDCl3) d 31.7, 32.6, 32.8, 35.1, 62.4, 123.8,
124.4, 129.8, 130.9, 147.0, 149.9, 189.2. Elemental
analysis: Found: C, 79.13; H, 8.05. C25H30O3 requires C,
79.33; H, 7.99.
JorthoZ8.5 Hz, 1H, Ar-H), 7.06 (dd, JorthoZ8.5 Hz, Jmeta
Z
2.3 Hz, 1H, Ar-H), 7.3 (d, JmetaZ2.3 Hz, 1H, Ar-H) 7.52 (d,
JmetaZ2.2 Hz, 1H, Ar-H), 7.62 (d, JmetaZ2.3 Hz, 1H, Ar-
H), 10.5 (s, 1H, CHO); 13C NMR (100 MHz, CDCl3) d 21.4,
31.5, 32.8, 34.7, 41.4, 62.4, 116.0, 121.6, 122.9, 123.5,
124.4, 129.8, 130.9, 136.5, 148.5, 189.2. Elemental
analysis: Found: C, 82.45; H, 8.58. C24H30O2 requires C,
82.24; H, 8.63.
4.1.3. Bis-(boradiazaindacenyl)-derivatized xanthene 4.
2,4-Dimethylpyrrole (456 mg, 4.8 mmol) was added to a
solution of 2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-
dicarbaldehyde (400 mg, 1.05 mmol) in argon bubbled
CH2Cl2 (750 mL). Then a drop of CF3COOH was added
and the solution was allowed to stir for 4 h at room
4.1.6. Boradiazaindacenyl-xanthene derivative 7. 2,4-
Dimethylpyrrole (540 mg, 5.14 mmol) was added to a
solution of 2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4-
carbaldehyde (540 mg, 2.57 mmol) in argon bubbled
CH2Cl2 (750 mL). Then a drop of CF3COOH was added
and the solution was allowed to stir for 4 h at room
temperature. Then, tetrachloro-1,4-benzoquinone (258 mg,
2.57 mmol) in dry CH2Cl2 (50 mL), Et3N (4 mL) and
BF3:OEt2 (4 mL) were added in that order to the solution,
and stirred at room temperature overnight. The solution was
concentrated under reduced pressure and washed with water
several times, then dried over Na2SO4. The solvent was
evaporated and the crude product was purified by column
chromatography (CH3OH/CHCl3 1:99) to obtain reddish
product 7 (350.4 mg, 24%). IR (KBr) nmax: 3024, 1180,
temperature.
Then
tetrachloro-1,4-benzoquinone
(258.3 mg, 1.05 mmol) in absolute CH2Cl2 (50 mL), Et3N
(4 mL) and BF3:OEt2 (4 mL) were added in order to the
solution and stirred for overnight at at room temperature for
overnight. The solution was concentrated under reduced
pressure and washed with water several times and dried over
Na2SO4. The solvent was evaporated and the crude product
was purified by column chromatography (CH3OH/CHCl3
1:99) to obtain reddish product 4 (162.9 mg, 20%). IR (KBr)
nmax: 3027, 1178, 1133 cmK1. 1H NMR (400 MHz, CDCl3)
d 1.25–1.29 (overlapping singlets, 18HC12H, C(CH3)3C
pyr-CH3), 1.57 (s, 6H, C(CH3)2), 2.03 (s, 12H, pyr-CH3),
5.21 (s, 4H, pyr-H), 7.17 (d, JmetaZ1.96 Hz, 2H, Ar-H),
7.33 (d, JmetaZ2.0 Hz, 2H, Ar-H); 13C NMR (100 MHz,
CDCl3) d 13.2, 14.7, 31.5, 33.5, 34.7, 62.5, 120.1, 122.3,
123.3, 125.1, 128.7, 131.2, 136.8, 141.4, 143.8, 147.1,
155.1. Elemental analysis: Found: C, 72.14; H, 6.74; N,
6.99. C49H56N4OB2F4 requires C, 72.25; H, 6.93; N, 6.88.
1
1130 cmK1. H NMR (400 MHz, CDCl3) d 1.24 (s, 18H,
C(CH3)3), 1.34 (s, 6H, C(CH3)2), 1.59 (s, 6H, pyr-CH3),
2.52 (s, 6H, pyr-CH3) 5.87 (s, 2H, pyr-H), 6.77 (d, 1H,
Ar-H, JorthoZ8.6 Hz), 7.01 (d, 1H, Ar-H, JmetaZ2.6 Hz),
7.06 (dd, 1H, Ar-H, JorthoZ8.5 Hz, JmetaZ2.3 Hz), 7.3 (d,
1H, Ar-H, JmetaZ2.3 Hz), 7.4 (d, 1H, Ar-H, JmetaZ2.3 Hz);
13C NMR (100 MHz, CDCl3) d 21.4, 31.7, 32.8, 62.4, 123.8,
124.4, 129.8, 130.9, 147.0, 149.9, 189.2. Elemental
analysis: Found: C, 76.23; H, 7.81; N, 4.78.
C36H43N2OBF2 requires C, 76.05; H, 7.62; N, 4.93.
4.1.4. Bichromophoric xanthene derivative 5. Compound
4 (110 mg, 1.35 mmol) and N,N-dimethyl-4-aminobenzal-
dehyde (143.1 mg, 1.35 mmol) in a mixture of benzene
(18 mL), acetic acid (510 mL) and piperidine (560 mL).
Water formed during the reaction was removed azeotropi-
cally by heating in a Dean–Stark apparatus for 3 h. The
4.1.7. Extended conjugation boradiazaindacenyl
xanthene derivative 8. Compound
7
(100 mg,

N. Saki et al. / Tetrahedron 62 (2006) 2721–2725
2725
0.176 mmol) and N,N-dimethyl-4-aminobenzaldehyde
References and notes
(25.6 mg, 0.176 mmol) in a mixture of benzene (18 mL),
acetic acid (506 mL) and piperidine (557 mL). Any water
formed during the reaction was removed azeotropically by
heating in a Dean–Stark apparatus for 3 h. The reaction
mixture was concentrated under reduced pressure and then
subjected to silica gel column chromatography (1:4
ethylacetate/hexane) to yield the desired product 8 in
50% yield (61 mg). IR (KBr) nmax: 3090, 3032, 1177,
1. Handbook of Fluorescent Probes and Research Products;
Haugland, R. P., Ed. 9th ed.; Molecular Probes: Eugene, OR,
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2. (a) Yee, M.; Fas, S. C.; Stohlmeyer, M. M.; Wandless, T. J.;
Cimprich, K. A. J. Biol. Chem. 2005, 280, 29053–29059. (b)
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Carlo, A.; Prodi, L.; Montalti, M.; Zaccheroni, N.; Smith, K. M.
Tetrahedron 2004, 60, 1099–1106.
1
1101 cmK1. H NMR (400 MHz, CDCl3) d 1.24 (s, 18H,
3. Recent examples: (a) Turfan, B.; Akkaya, E. U. Org. Lett. 2002,
4, 2857–2859. (b) Ulrich, G.; Ziessel, R. J. Org. Chem. 2004,
69, 2070–2083. (c) Gabe, Y.; Urano, Y.; Kikuchi, K.; Kojima,
H.; Nagano, T. J. Am. Chem. Soc. 2004, 126, 3357–3367. (d)
Rurack, K.; Kollmannsberger, M.; Resch-Genger, U.; Daub, J.
J. Am. Chem. Soc. 2000, 122, 968–969. (e) Coskun, A.;
Baytekin, B. T.; Akkaya, E. U. Tetrahedron Lett. 2003, 44,
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2004, 45, 4947–4949. (g) Coskun, A.; Akkaya, E. U. J. Am.
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C(CH3)3), 1.35 (s, 3H, pyr-CH3), 1.39 (s, 3H, pyr-CH3),
1.59 (s, 6H, C(CH3)2), 2.5 (s, 3H, pyr-CH3), 3.01 (s, 6H,
N(CH3)2), 5.87 (s, 1H, pyr-H), 6.49 (s, 1H, pyr-H), 6.66 (d,
2H, JZ8.7 Hz), 7.04–7.10 (m, 4H), 7.31 (d, JZ2.3 Hz, 1H),
7.43–7.46, (m, 4H); 13C NMR (100 MHz, CDCl3) d 31.5,
32.2, 32.4, 34.5, 34.7, 116.3, 122.1, 122.2, 123.0, 124.4,
125.2, 128.7, 129.1, 130.4, 145.8, 146.5, 148.1. Elemental
analysis: Found: C, 77.45; H, 7.92; N, 5.99. C47H56N3OBF2
requires C, 77.57; H, 7.76; N, 5.77.
4. (a) Burghart, A.; Thoresen, L. H.; Chen, J.; Burgess, K.;
˚
Bergstrom, F.; Johansson, L.B.-A. Chem. Commun. 2000,
4.1.8. Extended conjugation bis-(boradiazaindacenyl)-
xanthene derivative 9. Compound 4 (110 mg, 0.176 mmol)
and N,N-dimethyl-4-aminobenzaldehyde (52.6 mg,
0.352 mmol) in a mixture of benzene (18 mL), acetic acid
(506 mL) and piperidine (557 mL). Any water formed during
the reaction was removed azeotropically by heating in a
Dean–Stark apparatus for 3 h. The reaction mixture was
concentrated under reduced pressure and then subjected to
silica gel column chromatography (1:4 ethylacetate/hexane)
to yield the desired product 8 in 40% yield (65 mg). IR
¨
¨
2203–2204. (b) Wan, C.-W.; Burghart, A.; Chen, J.; Bergstrom,
˚
F.; Johansson, L.B.-A.; Wolford, M. F.; Kim, T. G.; Topp,
M. R.; Hochstrasser, R. M.; Burgess, K. Chem. Eur. J. 2003, 9,
4430–4441. (c) Ulrich, G.; Goze, C.; Guardigli, M.; Roda, A.;
Ziessel, R. Angew. Chem., Int. Ed. 2005, 44, 3694–3698.
5. Maas, H.; Calzaferri, G. Angew. Chem., Int. Ed. 2002, 41,
2284–2288.
6. Dahim, D.; Mizuno, N. K.; Li, X.-M.; Momsen, W. E.;
Momsen, M. M.; Brockman, H. L. Biophys. J. 2002, 83,
1511–1524.
(KBr) nmax: 3087, 3033, 1173, 1099 cmK1 1H NMR
.
(400 MHz, CDCl3) d 1.24–1.30 (two unresolved singlets,
18HC12H), 1.59 (s, 6H), 2.55 (s, 6H), 3.01 (s, 12H,
N(CH3)2) 5.85 (s, 2H), 6.50 (s, 2H), 6.64 (d, 4H, JZ8.7 Hz),
6.78–6.82 (m, 2H), 7.02–7.25 (m, 8H), 7.35 (d, 2H).
Elemental analysis: Found: C, 74.53; H, 6.92; N, 7.69.
C67H74N6OB2F4 requires C, 74.72; H, 6.93; N, 7.80.
¨ ¨
7. (a) Bergstrom, F.; Mikhalyov, I.; Hagglof, P.; Wortmann, R.;
¨
˚
Ny, T.; Johansson, L.B.-A. J. Am. Chem. Soc. 2002, 124,
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196–204. (b) Mikhalyov, I.; Gretskaya, N.; Bergstrom, F.;
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Johansson, L.B.-A. Phys. Chem. Chem. Phys. 2002, 4,
5663–5670.
8. (a) Jokic, D.; Asfari, Z.; Weiss, J. Org. Lett. 2002, 4,
2129–2132. (b) Yagi, S.; Yonekura, I.; Awakura, M.; Ezoe,
M.; Takagishi, T. Chem. Commun. 2001, 557–558. (c) Giaimo,
J. M.; Gusev, A. V.; Wasielewski, M. R. J. Am. Chem. Soc.
2002, 124, 8530–8531. (d) Chang, C. J.; Loh, Z. H.; Deng,
Y. Q.; Nocera, D. G. Inorg. Chem. 2003, 42, 8262–8269.
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Patent 4,774,339, 1988. (b) Rurack, K.; Kollmannsberger, M.;
Daub, J. Angew. Chem., Int. Ed. 2001, 40, 385–387.
Acknowledgements
The authors gratefully acknowledge support from
TUBITAK (TBAG 104T350) and Turkish Academy of
Sciences.