3,5-diaryl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes: Synthesis, spectroscopic, electrochemical, and structural properties

Article abstract of DOI:10.1021/jo990796o

This research was undertaken to obtain new 'BODIPY' dyes that fluoresce at relatively long wavelengths. The title compounds 1a-e were prepared via a divergent route involving Suzuki couplings of arylboronic acids to N-tert- butoxycarbonyl-4-bromopyrrole 2, condensation of the products with an acid chloride, and incorporation of the boron difluoride entity. Two alkyl- substituted systems 7a and 7b were also prepared for comparison; the critical difference between structures 1 and 7 is that the former have an aryl group attached to each pyrrole nucleus whereas the latter only have alkyl substituents on that same ring. UV absorption and fluorescence emission data were compared for compounds 1 and 7. Absorption and fluorescence emission maxima for compounds 1 occur at higher wavelengths than for compounds 7, and the Stokes shifts for the aryl-substituted compounds 1 are larger than for the alkyl-substituted compounds 7. Fluorescence quantum yields measured for compounds 1 are less than for compounds 7, and possible reasons for this are outlined. Other physical data for the compounds were also collected. Oxidation and reduction potentials of the systems were obtained from cyclic voltammetry experiments, and a single-crystal X-ray structure determination was performed for the bisnaphthyl-substituted compound 1b.

Full text of DOI:10.1021/jo990796o

J . Org. Chem. 1999, 64, 7813-7819
7813
3,5-Dia r yl-4,4-d iflu or o-4-bor a -3a ,4a -d ia za -s-in d a cen e (BODIP Y)
Dyes: Syn th esis, Sp ectr oscop ic, Electr och em ica l, a n d Str u ctu r a l
P r op er ties
Armin Burghart, Heejin Kim, Mike B. Welch, Lars H. Thoresen, J oe Reibenspies, and
Kevin Burgess*
Texas A & M University, Chemistry Department, P.O. Box 30012, College Station, Texas 77842
Fredrik Bergstro¨m and Lennart B.-A° . J ohansson*
Department of Physical Chemistry, Umea˚ University, S-90 187 Umea˚, Sweden
Received May 14, 1999
This research was undertaken to obtain new “BODIPY” dyes that fluoresce at relatively long
wavelengths. The title compounds 1a -e were prepared via a divergent route involving Suzuki
couplings of arylboronic acids to N-tert-butoxycarbonyl-4-bromopyrrole 2, condensation of the
products with an acid chloride, and incorporation of the boron difluoride entity. Two alkyl-substituted
systems 7a and 7b were also prepared for comparison; the critical difference between structures 1
and 7 is that the former have an aryl group attached to each pyrrole nucleus whereas the latter
only have alkyl substituents on that same ring. UV absorption and fluorescence emission data
were compared for compounds 1 and 7. Absorption and fluorescence emission maxima for compounds
1 occur at higher wavelengths than for compounds 7, and the Stokes shifts for the aryl-substituted
compounds 1 are larger than for the alkyl-substituted compounds 7. Fluorescence quantum yields
measured for compounds 1 are less than for compounds 7, and possible reasons for this are outlined.
Other physical data for the compounds were also collected. Oxidation and reduction potentials of
the systems were obtained from cyclic voltammetry experiments, and a single-crystal X-ray structure
determination was performed for the bisnaphthyl-substituted compound 1b.
In tr od u ction
a project was initiated to synthesize these materials5 and
investigate their salient physical and spectroscopic prop-
erties. A full account of that work is given here.
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene dyes1 are
extremely fluorescent materials2 that have found numer-
ous applications in biochemistry and molecular biology.
3
The trade name for these “BODIPY” dyes is associated
with Molecular Probes (Eugene, OR),4 and much of the
literature on them is patent material designed to restrict
unlicensed commercial uses. BODIPY dyes tend to be sold
in small quantities for biochemical experiments, and
amounts typically required for synthetic organic chem-
istry are prohibitively expensive. As part of an effort to
make new fluorescent tags for DNA sequencing, we
required a series of compounds for which the emission
maxima would be higher than approximately 520 nm and
easily varied via minor modifications in a synthetic
pathway. It was reasoned that BODIPY dyes 1 with 3,5-
diaryl substituents might have the desired characteris-
tics. Synthetic schemes that allowed for incorporation of
variable substitution on the 3,5-diaryl units could afford
a series of dyes with tunable fluorescent characteristics,
and the added conjugation should shift their absorption
maxima to longer wavelengths than the corresponding
alkyl-substituted materials (i.e., above ca. 520 nm).
Computerized structure search routines indicated that
molecules of type 1 had not been reported. Consequently,
Resu lts a n d Discu ssion
Syn th esis. Scheme 1 depicts the synthetic route that
was used for preparation of compounds 1a -e. Suzuki
coupling6,7 of N-tert-butoxycarbonyl-4-bromopyrrole 28,9
with five different arylboronic acids10 gave the protected
2-arylpyrroles 3a -e. Removal of the BOC-protecting
(5) Thoresen, L. H.; Kim, H.; Welch, M. B.; Burghart, A.; Burgess,
K. Synlett 1998, 1276-8.
(1) Treibs, A.; Kreuzer, F.-H. Liebigs Ann. Chem. 1968, 718, 208-
23.
(6) Miyaura, N.; Ishiyama, T.; Sasaki, H.; Ishikawa, M.; Satoh, M.;
Suzuki, A. J . Am. Chem. Soc. 1989, 111, 314-21.
(7) Sato, M.; Miyaura, N.; Suzuki, A. Chem. Lett. 1989, 1405-8.
(8) Chen, W.; Cava, M. P. Tetrahedron Lett. 1987, 28, 6025-6026.
(9) Chen, W.; Stephenson, E. K.; Cava, M. P.; J ackson, Y. A. Org.
Synth. 1991, 70, 151-156.
(2) Karolin, J .; J ohansson, L. B.-A.; Strandberg, L.; Ny, T. J . Am.
Chem. Soc. 1994, 116, 7801-6.
(3) Wagner, R. W.; Lindsey, J . S. Pure Appl. Chem. 1996, 68, 1373-
80.
(4) Haugland, R. P. Handbook of Fluorescent Probes and Research
Chemicals, 6th ed.; Molecular Probes: Eugene, OR, 1996.
(10) Martina, S.; Schluter, A. D. Macromolecules 1992, 25, 3607-
3608.
10.1021/jo990796o CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/30/1999

7814 J . Org. Chem., Vol. 64, No. 21, 1999
Burghart et al.
Ta ble 1. Ma xim u m Mola r Absor p tivities (Em a x), th e
Wa velen gth of Em a x (λa bs.m a x), a n d th e Stok es Sh ifts (th e
la tter va lu e r ep or ted is th e d iffer en ce betw een
w a velen gth s for th e m a xim u m of a bsor p tion a n d
cor r ected flu or escen ce sp ectr a )
Sch em e 1. Syn th eses of Com p ou n d s 1a -e
a
ꢀmax (abs)
(M-1 cm-1
λabs.max
(nm)
Stokes shift
(nm)
solvent
EtOH
compound
)
1a
1b
1c
1d
1e
7a
7b
1a
1b
1c
1d
1e
7a
7b
53300
48300
54100
42100
30700
46300
75800
52400
49000
54100
42200
30500
47100
75900
558
550
585
559
549
504
528
555
542
582
555
545
500
524
34
63
44
33
52
10
13
33
65
44
35
53
10
13
CHCl3
a
Estimated errors in ꢀmax are within 500 M-1 cm-1
.
Sp ectr oscop ic P r op er ties. Absorption spectra of the
aryl-BODIPY derivatives 1a-1e have similar band shapes,
but they are different to those observed for 7a and 7b.
For 7a , the absorption and fluorescence wavelength
maxima are almost identical to the BODIPY 8,2,4 indicat-
ing that the 4-iodobenzene entity connected to carbon C-8
has little impact on the spectral band shapes of these
compounds. Qualitatively, the fluorescence spectra of all
the compounds 1, 7, and 8 (presumably a S1 f S0
transition) show an approximate mirror symmetry with
their absorption spectra. For compounds 7 and 8 the
mirror symmetry is nearly perfect, but for compounds 1
it is less so.
group gave the derivatives 4.11 We favor this synthesis
of arylpyrroles over some existing routes,12-14 for reasons
already outlined,5 i.e., convenience and overall yields.
Pyrroles 4 can be converted directly to the target materi-
als 1 in a one-pot, two-step procedure, but superior
overall yields were obtained when the intermediate
dipyrromethenes 5 were isolated. That isolation process
was best done using a deactivated alumina column;
extensive decomposition occurred when silica gel was
used as a support. In one particular experiment, a 10%
yield was obtained on silica, but over 90% on alumina.
Some notable points concerning the syntheses of these
compounds are as follows. The route is divergent from
compound 2. Attempts to couple 2-pyrroleboronic acid
with aryl halides were explored, but gave much less
satisfactory results. Yields of the dipyrromethenes 5a and
5b were the lowest in the series, presumably because of
incomplete reactions. In fact, ketones 6a and 6b are
intermediates in these transformations, and these were
formed in significant quantities in those particular
syntheses. The products 1a -e were isolated as dark
violet crystalline materials. In solution they are beauti-
fully colored. For instance in chloroform 1a is brick red,
1b is red, 1c is violet, 1d is fuchsia, and 1e is dark violet.
Two alkyl-substituted systems 7 were also prepared15 to
provide closely related molecules without aryl pyrrole
substituents for spectroscopic comparisons.
Compound 7b had the highest molar absorptivity (ꢀmax
) 75 000 M-1 cm-1) of dyes 1 and 7, being similar to that
for the unsubstituted derivative 8 (ꢀmax ) 80 000 M-1
cm-1). For compounds 1 and 7a , however, the maximum
molar absorptivities are lower and range from 30 000 to
55 000 M-1 cm-1 (Table 1). The Stokes shifts measured
for compounds 1a -1e were 3-6 times larger than for the
compounds 7. Moreover, these Stokes shifts do not vary
much when the solvent is changed from ethanol to
chloroform. These solvents have markedly different
polarities hence this observation implies that the large
Stokes shift observed are not caused by significantly
different permanent dipole moments in the electronic
ground and excited states.
Fluorescence quantum yields were calculated and
determined experimentally for compounds 1 and 7, and
the data are given in Table 2. Radiative lifetimes (τ0) were
calculated from the absorption spectra using the modified
Strickler-Berg equation.16 The fact that the τ0 values are
similar for ethanol and chloroform reflects the fact that
the molar absorptivities and refractive indices are very
(11) Hasan, I.; Marinelli, E. R.; Lin, L. C. C.; Fowler, F. W.; Levy,
A. B. J . Org. Chem. 1981, 46, 157-164.
(12) Trofimov, B. A. Adv. Heterocycl. Chem 1990, 51, 177-301.
(13) Boukou-Poba, J . P.; Farnier, M.; Guilard, R. Tetrahedron Lett.
1979, 19, 1717-1720.
(15) Thoresen, L. H.; Kim, H.; Burghart, A.; Gibbs, R. A.; Burgess,
K. Unpublished results.
(16) Birks, J . B. Photophysics in aromatic molecules; Wiley: New
York, 1970.
(14) Katrizky, A. R.; Li, J .; Gordeev, M. F. Synthesis 1994, 93-96.

BODIPY Dyes
J . Org. Chem., Vol. 64, No. 21, 1999 7815
Ta ble 2. Exp er im en ta l a n d Ca lcu la ted F lu or escen ce
Qu a n tu m Yield s (Φexp a n d Φca lc) Obta in ed for
Com p ou n d s 1 a n d 7. th e Mea su r ed F lu or escen ce Lifetim e
(τ), th e Ca lcu la ted Ra d ia tive Lifetim e (τ0), a n d th e
Ca lcu la ted F o1r ster Ra d iu s (R0) Ar e P r esen ted
τ
solvent compound
Φexp
Φcalc (ns)a
τ0 (ns)
R0 (Å)
EtOH
1a
1b
1c
1d
1e
7a
7b
1a
1b
1c
1d
1e
7a
7b
0.15 ( 0.01 0.15 1.1
0.20 ( 0.02 0.16 1.1
0.33 ( 0.01 0.35 2.4
0.15 ( 0.01 0.10 0.8
6.9 ( 0.7 35.8 ( 0.9
6.8 ( 0.1 31.5 ( 0.4
6.8 ( 0.6 41.8 ( 0.8
8.6 ( 0.5 34.5 ( 0.5
0.05 ( 0.01 0.03 0.3 10.5 ( 0.3 24.6 ( 0.3
0.55 ( 0.04 0.54 3.0
0.75 ( 0.03 0.57 5.0
0.20 ( 0.03 0.18 1.2
0.38 ( 0.03 0.30 2.1
0.42 ( 0.02 0.37 2.6
0.22 ( 0.01 0.13 1.1
0.08 ( 0.01 0.06 0.6 10.6 ( 0.7 28.4 ( 0.1
0.64 ( 0.02 0.53 3.0
0.78 ( 0.03 0.52 4.8
5.5 ( 0.4 46.3 ( 0.9
8.7 ( 0.3 45.8 ( 0.9
6.5 ( 0.1 38.2 ( 0.8
7.1 ( 0.3 36.3 ( 0.6
7.3 ( 0.4 43.8( 0.9
8.6 ( 0.5 37.9 ( 0.8
CHCl3
F igu r e 1. Cyclic voltammogram of 1b in acetonitrile (at 100
mV s-1).
5.7 (0.6 47.7( 0.9
9.1 ( 0.3 46.4 ( 0.9
substituted BODIPYs display irreversible oxidation waves
and reversible reduction waves. The reduction and oxida-
tion potentials are not large, so the compounds are quite
easily reduced and also quite easily oxidized. Electro-
chemical data have been reported for compound 918 and
that data serves as a useful point of reference for the
current study.
a
Errors in the measured fluorescence lifetimes are within 0.1
ns.
Ta ble 3. Electr och em ica l Da ta for 1a -e in Aceton itr ile
(sca n r a te 100 m V s-1
)
Eox,pa (V)
Ered,1/2 (V)
Ar
vs ferrocene
vs ferrocene
1a
1b
1c
1d
1e
Ph
1-Nap
4-MeOC6H4
4-FC6H4
+0.91
+0.98
+0.68
+0.93
+0.84
-1.15
-1.15
-1.20
-1.11
-1.19
2-MeOC6H4
similar in the two solvents. Time-resolved fluorescence
experiments were used to measure the actual fluores-
cence decay. The lifetimes τ recorded in this way were
found to be monoexponential for all compounds studied.
Comparison of the calculated and measured lifetimes
shows that the experimental values recorded were always
shorter. This implies that nonradiative relaxation pro-
cesses that are not accommodated in the calculations
were operative. Fluorescence quantum yield calculated
according to Φcalcd ) τ/τ0 agreed well with the experimen-
tal fluorescence quantum yields, except for 1b, 1d , and
7b in ethanol, and except for 1b, 1d , 7a , and 7b in
chloroform.
With the exception of compound 1c, the compounds
shown in Table 3 have higher oxidation potentials than
9. This may be attributed to inductive destabilization of
the cationic form by the 4-iodo substituents on the meso
aromatic ring and/or to lowering of the oxidation poten-
tials by the alkyl substituents of compound 9. The fact
that the oxidation waves for compounds 1 are irreversible
implies that the radical cations generated on the anode
react rapidly to form other products. Electrochemical
polymerization of pyrroles (a well-known industrial
process)19,20 may be operative for the BODIPY systems.
Reversible oxidation waves for compound 9 may reflect
retardation of pyrrole-pyrrole coupling processes by the
alkyl substituents. Peak potentials for compounds in the
series 1a -e vary over a 0.3 V range wherein the
methoxy-substituted compounds 1c and 1e exhibit the
lowest potentials. This is probably due to mesomeric
stabilization of the charges by the methoxy substituents.
The fact that the 2-methoxy compound has a higher
oxidation potential than the 4-methoxy derivative indi-
cates steric effects in the former case disfavor an ideal
alignment of orbitals. Similar stereoelectronic effects may
be operative for the naphthyl compound 1b, which has a
higher oxidation potential than the phenyl compound 1a .
Further evidence for congestion in the naphthyl com-
pound is seen on the proton NMR of this compound (vide
supra). In contrast to the oxidative processes, electro-
chemical reduction of compounds 1a -e was shown to be
Fo¨rster radii17 for compounds 1 and 7 were calculated
from absorption and corrected fluorescence spectra. The
values obtained range between 25 and 48 Å and show a
modest solvent dependence. These values are consider-
ably lower than the Fo¨rster radius for 8, i.e., less than
57 ( 1 Å.2
Finally, various NMR parameters were recorded for
dyes 1 and 7, but no striking differences were observed.
For instance, chemical shifts in the 11B NMR spectra
reveal that the different aryl substituents do not impart
any significant influences; the values obtained fall within
the relatively narrow range of -1.26 to -1.92 ppm.
Proton NMR signals for the naphthyl-substituted BO-
DIPY 1b were broad at ambient temperature, but sharpen
when the sample is heated to 80 °C in toluene. This
phenomenon is attributed to restricted motion of the
naphthyl groups.
Electr och em ica l P r op er ties. Cyclic voltammetry
was used to probe the electronic effects of the various
aryl substituents on the BODIPY electrophore. Table 3
shows salient details form this study. All new aryl-
(18) Kollmannsberger, M.; Gareis, T.; Heinl, S.; Breu, J .; Daub, J .
Angew. Chem., Int. Ed. Engl. 1997, 36, 1333-5.
(19) Schmittel, M.; Burghart, A. Angew. Chem., Int. Ed. Engl. 1997,
36, 2551-89.
(20) Diaz, A. F. In Organic Electrochemistry; Lund, H., Baizer, M.
M., Eds.; Marcel Dekker: New York, 1991.
(17) Fo¨rster, T. Ann. Physik 1948, 2, 55-75.

7816 J . Org. Chem., Vol. 64, No. 21, 1999
Burghart et al.
eliminate this degree of rotational freedom should in-
crease the fluorescence quantum yields, but this point
was not tested here. Such modifications could also be
designed to hold the aryl substituents into the BODIPY
plane. The electrochemical studies here demonstrate that
electron-withdrawing and electron-releasing aryl sub-
stituents have less impact on the oxidation and reduction
potentials than expected, consistent with twisting of the
aryl substituents out of the BODIPY plane. Such defor-
mations were observed in the solid-state structural
analysis of compound 1b.
There are several potential applications of the com-
pounds reported in this study. They could aid the design
of fluorescent chemosensors,21 act as reporter molecules
for dynamics, structure, and function of biomolecules,2
provide light-harvesting systems for artificial photosyn-
thesis,22 and for multipigment arrays in molecular pho-
tonic devices.23
Exp er im en ta l Section
F igu r e 2. Chem3D representation of compound 1b (data from
solid-phase crystal structure determination).
Gen er a l Exp er im en ta l. Compounds 1c and 3-5c were
prepared as previously described.5 Procedures for the prepara-
tion of compounds 7a and 7b will be reported elsewhere.15 All
chemicals were obtained from commercial suppliers and used
without further purification. THF and toluene were distilled
from Na/benzophenone prior to use, acetonitrile used for the
electrochemical studies was distilled from CaH2, and 1,2-
dichloroethane was distilled from CaH2.
reversible and to occur over a relatively small range (0.1
V). This may be due to reduction centered at the meso
position and stabilized by the relatively electron poor aryl
iodide. Aryl substituents at the 3- and 5-position have
less pronounced effects although, predictably, the 3,5-di-
(4-fluoroaryl) BODIPY has the least negative reduction
potential in the series.
Solid Sta te Str u ctu r e of Com p ou n d 1b. A single-
crystal X-ray analysis of compound 1b (Figure 2) showed
that the molecule is sterically congested. All three aryl
substituents were shown to be twisted relative to the
BODIPY plane. The torsional angle formed between the
8-p-iodophenyl group and the BODIPY core unit was
measured as 71°; this is presumably due to interaction
of the BODIPY 2- and 7-protons with the ortho-protons
of the 8-p-iodophenyl group. Conversely, it appears to be
that the fluorine atoms prevent the naphthyl groups from
adopting orientations coplanar with the BODIPY core.
The two naphthalene-BODIPY torsional angles mea-
sured were around 50° and 60°, respectively.
All NMR spectra were recorded on a Varian instrument at
200 and 300 MHz (1H) as well as 75 MHz (13C) and 64 MHz
(11B). NMR chemical shifts are expressed in ppm relative to
internal solvent peaks, and coupling constants were measured
in hertz. For 11B-NMR, BF3‚Et2O has been used as a reference.
For recording the cyclic voltammograms, a BAS-100A elec-
troanalyzer has been used.
N-ter t-Bu tyloxyca r bon yl-2-p h en ylp yr r ole (3a ). N-tert-
Butyloxycarbonyl-2-bromopyrrole8,9 (4.61 g, 18.7 mmol), phen-
ylboronic acid (2.28 g, 18.7 mmol), tetrakis(triphenylphosphine)-
palladium (432 mg, 0.2 mmol), Na2CO3 (18.7 mL, 37.4 mmol,
2.0 M aqueous solution), PhMe (78 mL), and MeOH (16 mL)
were added to a Schlenk tube and freeze-thaw-degassed three
times and then heated to 80 °C for 14 h. The reaction mixture
was filtered through a plug of silica gel on Celite using EtOAc
as eluant. After concentration, the crude product was purified
via flash chromatography using a gradient of 100% hexanes
to 5% EtOAc/hexanes as eluant giving 3a as a clear oil (4.49
g, 99% yield). Rf 0.25 (5% EtOAc in hexanes); MS (FAB+, NBA
) 4-nitrobenzoic acid) m/z (%) 243 (M+); HRMS calcd [M +
H+] 243.1259, found [M + H+] 243.1253;1H NMR (CDCl3, 300
MHz) δ 7.42-7.32 (m, 6 H), 6.29-6.22 (m, 2 H), 1.39 (s, 9 H);
13C NMR (CDCl3, 75 MHz) δ 149.3, 134.9, 134.4, 129.1, 127.4,
127.0, 122.4, 114.3, 110.4, 83.3, 27.5.
2-P h en ylp yr r ole (4a ).14 A suspension of sodium methoxide
(594 mg, 11.0 mmol) in MeOH (3.0 mL) was added to a stirred
solution of N-tert-butyloxycarbonyl-2-phenylpyrrole 3a (895
mg, 3.67 mmol) in THF (10.5 mL) and stirred at 25 °C for 15
h. The reaction was diluted with Et2O (150 mL) and extracted
with H2O (2 × 50 mL) and brine (2 × 50 mL), dried (Na2SO4),
filtered, and concentrated. The crude reaction mixture was
crystallized from hexanes, giving the 4a as white plates (339
mg, 65% yield). Rf 0.20 (10% EtOAc/hexanes); 1H NMR (CDCl3,
300 MHz) δ 8.42 (bs, 1 H), 7.46 (d, J ) 7.5 Hz, 2 H), 7.35 (t, J
) 7.5 Hz, 2 H), 7.18 (d, J ) 7.2 Hz, 1 H), 6.85-6.82 (m, 1 H),
Con clu sion
The new aryl-substituted BODIPYs described here are
accessible via a straightforward route. As expected,
substitution of aryl groups onto the pyrrole rings of the
BODIPY framework causes the corresponding wave-
lengths of the fluorescence emission maxima (denoted by
λ
em.max) to be red-shifted relative to the corresponding
alkyl-substituted compounds. The Stokes shifts for com-
pounds 1 are also higher than their alkyl counterparts,
a factor that also helps shift their fluorescence λem.max
values to higher wavelength. However, the fluorescence
quantum yields for the aryl-substituted compounds tend
to be lower than the corresponding alkyl-substituted
ones. This may be attributed to nonradiative energy loss
due to spinning motions about the C-aryl single bonds.
Indeed, it may not be a coincidence that the second
highest fluorescence quantum yield in the series 1a -e
was for the naphthyl derivative 1b for which rotation of
the aryl groups was slow enough to be observed on the
NMR time scale. Molecular modifications that totally
(21) Isgor, Y. G.; Akkaya, E. U. Tetrahedron Lett. 1997, 38, 7417-
20.
(22) Gust, D. Nature 1997, 386, 21-2.
(23) Wagner, R. W.; Seth, J .; Yang, S. I.; Kim, D.; Bocian, D. F.;
Holten, D.; Lindsey, J . S. J . Org. Chem. 1998, 63, 5042-9.

BODIPY Dyes
J . Org. Chem., Vol. 64, No. 21, 1999 7817
6.52 (bs, 1 H), 6.29 (q, J ) 2.7 Hz, 1 H); 13C NMR (CDCl3, 75
MHz) δ 132.7, 132.0, 128.8, 126.1, 123.8, 118.8, 110.0, 105.9.
(4 mL) were heated to reflux for 19 h, during which time the
solution gradually acquired a purple hue. Without workup the
crude product was purified via chromatography on basic
alumina using 20% ethyl ether/hexanes (Rf 0.45) as eluant
giving the title compound 5b as a dark purple solid (70 mg,
29% yield). mp 177-178 °C; MS (FAB+, NBA) m/z (%) 599
(M+), HRMS calculated [M+] 599.0984, found [M+] 599.1039;
1H NMR (CDCl3, 300 MHz) δ 8.75 (d, J ) 7.2 Hz, 2 H), 7.94-
7.82 (m, 8 H), 7.54-7.31 (m, 8 H), 6.89 (d, J ) 4.2 Hz, 2 H),
6.80 (d, J ) 4.2 Hz, 2 H); 13C NMR (CDCl3, 75 MHz) δ 154.7,
141.5, 138.3, 137.0, 134.1, 132.6, 131.3, 130.9, 129.3, 128.9,
128.5, 127.4, 126.9, 126.7, 126.0, 125.2, 119.6, 95.0.
6-(4′′-Iod op h en yl)-5,5′-d ip h en ylp yr r om et h en e
(5a ).
2-Phenylpyrrole 4a (1.06 g, 7.4 mmol), 4-iodobenzoyl chloride
(2.0 g, 7.4 mmol), and 1,2-dichloroethane (37 mL) were heated
to reflux for 45 h, during which time the solution gradually
acquired a purple hue. The reaction mixture was diluted with
CH2Cl2 (50 mL) and extracted with H2O (50 mL), 1 M HCl (50
mL), 1 M NaOH (50 mL), and brine (50 mL), dried (Na2SO4),
filtered, and concentrated. The crude product was purified via
flash chromatography on alumina using 30% CH2Cl2/hexanes
as eluant, giving the title compound 5a as a dark purple solid
(1.94 g, 49% yield). Rf 0.63 (10% EtOAc/hexanes); mp 177-9
°C;1H NMR (CDCl3, 300 MHz) δ 7.90 (d, J ) 7.2 Hz, 4 H),
7.79 (d, J ) 8.1 Hz, 2 H), 7.48 (t, J ) 6.9 Hz, 4 H), 7.39 (d, J
) 7.2 Hz, 2 H), 7.26 (d, J ) 8.4 Hz, 2 H), 6.82 (d, J ) 4.5 Hz,
2 H), 6.65 (d, J ) 4.2 Hz, 2 H); 13C NMR (CDCl3, 75 MHz) δ
138.3, 137.6, 136.9, 132.5, 131.7, 130.4, 129.1, 128.4, 126.2,
125.2, 121.3, 115.9, 109.0.
4,4-Diflu or o-8-(4′-iodoph en yl)-3,5-diph en yl-4-bor a-3a,4a-
d ia za -s-in d a n cen e (1a ). Boron trifluoride etherate (3.4 g, 24
mmol) was added to a mixture of compound 5a (1.94 g, 3.6
mmol), NEt3 (1.8 g, 18 mmol), and PhMe (36 mL). The
resulting solution was stirred at 25 °C for 10 min and then
heated to 80 °C for 20 min. Fluorescence appeared 2 min after
addition of BF3 at 25 °C. The crude reaction mixture was
filtered through a plug of SiO2 on Celite and then concentrated.
Purification via chromatography on basic alumina and a
gradient of 10 to 20% CHCl3/hexanes as eluant gave the title
compound 1a as a dark purple solid (750 mg, 36% yield). Rf
0.27 (40% CHCl3/hexanes); mp 157-158 °C; MS (FAB+, NBA)
m/z (%) 546 (M+); HRMS calculated [M + Na+] 546.0580, found
[M + Na+] 546.0619; IR (neat) ν (cm-1) 1560, 1542, 1284, 1144;
1H NMR (CDCl3, 300 MHz) δ 7.88-7.82 (m, 6 H), 7.41-7.38
(m, 6 H), 7.30 (d, J ) 8.4 Hz, 2 H), 6.83 (d, J ) 4.2 Hz, 2 H),
6.60 (d, J ) 4.5 Hz, 2 H); 13C NMR (CDCl3 75 MHz) δ 159.3,
137.6, 136.0, 133.8, 132.5, 132.1, 130.5, 129.6, 129.4, 128.2,
121.1, 96.6; 11B NMR (CDCl3, 64 MHz) δ -1.58 (t, J ) 31 Hz).
N-ter t-Bu tyloxyca r bon yl-2-(1′-n a p h th yl)p yr r ole (3b).
N-tert-Butyloxycarbonyl-2-bromopyrrole (4.00 g, 16.3 mmol),
1-naphthylboronic acid (2.80 g, 16.3 mmol), tetrakis(tri-
phenylphosphine)palladium (370 mg, 0.33 mmol), Na2CO3
(16.3 mL, 32.5 mmol, 2.0 M aqueous solution), PhMe (88 mL),
and MeOH (12 mL) were added to a Schlenk tube and heated
to 80 °C for 18 h. The two layers were then separated, and
the aqueous layer was back-extracted with ethyl ether (2 ×
40-50 mL). The combined organic layers were washed with
water (70 mL) and dried over MgSO4. After concentration, the
crude product was purified via flash chromatography using
1:7 EtOAc/hexanes as eluant giving the title compound 3b (Rf
0.75) as colorless crystals (4.45 g, 93% yield). mp 78-79 °C;
MS (FAB+, NBA) m/z (%) 293 (M+); HRMS calculated [M+]
293.1416, found [M+] 293.1418; 1H NMR (CDCl3, 300 MHz) δ
7.92-7.84 (m, 2 H), 7.62-7.41 (m, 6 H), 6.39 (t, J ) 3.8 Hz, 1
H), 6.31 (dd, J ) 3.8, 2.2 Hz, 1 H), 0.88 (s, 9 H); 13C NMR
(CDCl3, 75 MHz) δ 144.9, 129.4, 128.7, 128.6, 127.9, 123.5,
122.9, 121.6, 121.4, 121.2, 120.6, 117.4, 110.6, 106.2, 78.5, 22.5.
2-(1′-Na p h th yl)p yr r ole (4b). A suspension of sodium
methoxide (0.59 g, 10.8 mmol) in MeOH (3.0 mL) was added
to a stirred solution of N-tert-butyloxycarbonyl-2-(1′-naphthyl)-
pyrrole 3b (1.05 g, 3.6 mmol) in THF (11 mL) and stirred at
25 °C for 12 h. The reaction was diluted with Et2O (150 mL)
and washed with H2O (2 × 50 mL), dried (MgSO4), filtered,
and concentrated. The product was purified by flash chroma-
tography using 1:4 EtOAc/hexanes as eluant (Rf 0.55) giving
the title compound 4b as a colorless oil which crystallizes on
standing (550 mg, 78% yield). mp 172-173 °C; 1H NMR
(CDCl3, 300 MHz) δ 8.10-8.15 (m, 1 H), 7.60-7.75 (m, 2 H),
7.35-7.14 (m, 4 H), 6.65-6.68 (m, 1 H), 6.35-6.39 (m, 1 H),
6.23-6.26 (m, 1 H). The analytical data are in accord with
those reported.14
4,4-Diflu or o-8-(4′-iod op h en yl)-1,7-b is(1′-n a p h t h yl)-4-
bor a -3a ,4a -d ia za -s-in d a n cen e (1b). Compound 5b (30 mg,
0.05 mmol), boron trifluoride etherate (36 mg, 0.25 mmol),
NEt3 (15 mg, 0.15 mmol), and anhydrous PhMe (3 mL) were
heated to 80 °C for 20 min. The crude reaction mixture was
subjected to flash chromatography (Rf 0.38) on silica using 50%
CHCl3/hexanes as eluant, giving the title compound 1b as a
dark purple solid (29 mg, 90% yield). mp 165-166 °C (dec);
MS (FAB+, NBA) m/z (%) 646 (M+); HRMS calculated [M+]
646.0895, found [M+] 646.0910; 1H NMR (CDCl3, 300 MHz) δ
7.99 (d, J ) 7.2 Hz, 2 H), 7.89-7.76 (m, broadened by
coalescence, 8 H), 7.50 (d, J ) 7.2 Hz, 2 H), 7.54-7.38 (m,
broadened by coalescence, 6 H), 7.02 (d, J ) 4.2 Hz, 2 H), 6.65
(d, J ) 4.2 Hz, 2 H); in d8-toluene at 80 °C, the broad naphthyl
signals are sharp multiplets. 13C NMR (CDCl3, 75 MHz) δ
157.78, 143.1, 137.7, 135.3, 133.8, 133.3, 132.1, 131.9, 130.0,
129.7, 129.6, 128.4, 128.4 (br), 126.3, 125.9, 124.9 (br), 122.8,
96.8; 11B NMR (CDCl3, 64 MHz) δ -1.92 (t, J ) 30.3 Hz).
N-ter t-Bu toxyca r bon yl-2-(4′-flu or op h en yl)pyr r ole (3d ).
A Schlenk tube was charged with N-tert-butyloxycarbonyl-2-
bromopyrrole (2.00 g, 8.13 mmol), 4-fluorophenylboronic acid
(1.14 g, 8.13 mmol), and tetrakis(triphenylphosphine)pal-
ladium (190 mg, 0.16 mmol). The vessel was evacuated and
then refilled with nitrogen three times. Freshly distilled
toluene (44 mL) and degassed methanol (6 mL) were added
via syringe. Degassed 2 M Na2CO3(aq) (8.13 mL, 16.26 mmol)
was added, and then the mixture was stirred under a nitrogen
atmosphere at 80 °C for 20h. Two layers formed. These were
separated, and the organic layer was dried (Na2SO4) and then
evaporated. The resulting oil was purified by flash chroma-
tography eluting with 5% EtOAc/hexanes to yield 2.1 g of the
compound 3d (99% yield) as a clear oil. MS (FAB+, NBA) m/z
(%) 261 (M+); HRMS calculated [M+] 261.1165, found [M+]
1
261.1190; H NMR (CDCl3, 300 MHz) δ 7.37 (dd, J ) 3.3 Hz,
1.8 Hz, 1 H), 7.32 (dd, J ) 8.9 Hz, 5.4 Hz, 1 H), 7.05 (t, J )
8.9 Hz, 1 H), 6.23 (t, J ) 3.3 Hz, 1 H), 6.17 (dd, J ) 3.3, 1.8
Hz, 1 H), 1.40 (s, 9 H); 13C NMR (CDCl3, 75 MHz) δ 162.1 (d,
J ) 245 Hz, CF), 149.2, 133.8, 130.8 (d, J ) 8.0 Hz, CH), 122.4,
114.5 (d, J ) 4.5 Hz, CH), 114.2, 110.4, 83.6, 27.5.
2-(4′-F lu or op h en yl)p yr r ole (4d ). A suspension of sodium
methoxide (1.11 mg, 6.50 mmol) in MeOH (5 mL) was added
to a stirred solution of N-tert-butyloxycarbonyl-2-(4′-fluoro-
phenyl)pyrrole 3d (1.11 g, 19.51 mmol) in THF (25 mL) and
stirred at 25 °C for 2 h. The reaction was diluted with Et2O
(30 mL) and extracted with H2O (2 × 20 mL) and brine (2 ×
20 mL), dried (Na2SO4), filtered, and concentrated. The crude
reaction mixture was purified on silica gel via flash chroma-
tography using 10% EtOAc/hexanes as eluant. Compound 4d
was isolated as white plates (940 mg, 98% yield). mp 119-
120 °C; Rf 0.38 (10% EtOAc/hexanes);1H NMR (CDCl3, 300
MHz) δ 7.39 (m, 2 H), 7.02-7.08 (m, 2 H), 6.83-6.85 (m, 1 H),
6.43-6.46 (m, 1 H), 6.27-6.30 (m, 1 H); 13C NMR (CDCl3, 75
MHz) δ 161.5 (d J ) 244 Hz), 130.3 (d, J ) 155 Hz), 125.5 (d,
J ) 8 Hz), 121.2, 118.8, 115.8 (d, J ) 21 Hz), 110.1, 105.8.
6-(4′′-I o d o p h e n y l)-5,5′-b is (4-flu o r o p h e n y l)p y r r o -
m eth en e (5d ). 2-(4′-Fluorophenyl)pyrrole 4d (1.00 g, 6.17
mmol), 4-iodobenzoyl chloride (3.78 g, 14.2 mmol), and 1,2-
dichloroethane (20 mL) were heated to reflux for 2 d, during
which time the solution gradually changed to purple. The
reaction mixture was diluted with CH2Cl2 (50 mL), extracted
with H2O (50 mL), dried (Na2SO4), filtered, and concentrated.
The crude product was purified via flash chromatography
using 20% EtOAc/hexanes as eluant giving 5d as a dark purple
6-(4′′-Iod op h en yl)-5,5′-b is(1-n a p h t h yl)p yr r om et h en e
(5b). 2-(1′-Naphthyl)pyrrole 4b (153 mg, 0.79 mmol), 4-iodo-
benzoyl chloride (422 mg, 1.58 mmol), and 1,2-dichloroethane

7818 J . Org. Chem., Vol. 64, No. 21, 1999
Burghart et al.
solid (1.35 g, 82% yield). mp 168-179 °C (dec); Rf 0.71 (10%
EtOAc/hexanes); 1H NMR (CDCl3, 300 MHz) δ 7.79-7.87 (m,
4 H), 7.26 (d, J ) 8.4 Hz, 4 H), 7.17 (t, J ) 8.7 Hz, 4 H), 6.77
(d, J ) 4.2 Hz, 2 H), 6.65 (d, J ) 4.2 Hz, 2 H); 13C NMR (CDCl3,
75 MHz) δ 185.6, 164.9, 153.4, 141.6, 137.0, 132.5, 129.8, 129.5,
127.9, 127.8, 116.3, 116.0, 115.7.
13C NMR (CDCl3, 75 MHz) δ 157.3, 152.6, 140.6, 137.5, 136.7,
132.7, 129.8, 129.0, 128.3, 122.4, 120.9, 118.6, 111.6, 95.6, 55.9.
4,4-Diflu or o-8-(4′-iod op h en yl)-3,5-bis-(2-m eth oxyp h en -
yl)-4-bor a -3a ,4a -d ia za -s-in d a n cen e (1e). Compound 5e (100
mg, 0.179 mmol), NEt3 (0.075 mL, 0.537 mmol), and PhMe (5
mL) were stirred at 25 °C for 10 min. Boron trifluoride etherate
(0.113 mL, 0.896 mmol) was added, and the reaction mixture
was heated to 80 °C for 20 min. The crude mixture was purified
on basic alumina using 10% EtOAc/hexanes to yield 1e as a
dark purple solid (102 mg, 93.9%). Rf 0.2 (10% EtOAc/hexanes);
mp 109-110 °C; MS (FAB+, NBA) m/z (%) 606 (M+); HRMS
calculated [M+] 606.0883, found [M+] 606.0901; 1H NMR
(CDCl3, 300 MHz) δ 7.91 (d, J ) 8.1 Hz, 2 H), 7.75 (dd, J )
7.6, 1.2 Hz, 2 H), 7.38 (d, J ) 8.4 Hz, 2 H), 7.34-7.39 (m, 2
H), 7.02 (td, J ) 7.6, 1.2 Hz, 2 H), 6.95 (d, J ) 8.4 Hz, 2 H),
6.85 (d, J ) 4.2 Hz, 2 H), 6.63 (d, J ) 4.2 Hz, 2 H), 3.80 (s, 6
H); 13C NMR (CDCl3, 75 MHz) δ 157.5, 155.7, 143.2, 137.4,
135.1, 134.0, 132.0, 131.8, 130.6, 129.3, 122.5, 121.8, 120.2,
110.9, 96.3, 55.7; 11B NMR (CDCl3, 64 MHz) δ -1.26 (t, J )
30.6 Hz).
Sp ectr oscop ic Mea su r em en ts. Absorption spectra were
recorded on a GBC 912 (GBC Scientific Equipment Pty, Ltd,
Australia) absorption spectrometer. The steady-state fluores-
cence emission and excitation spectra were obtained by using
a SPEX Fluorolog 112 instrument (SPEX Industries, Metuchen
NJ ). The fluorimeter was calibrated by using a standard lamp
from the Swedish National Testing and Research Institute
(Borås, Sweden). All fluorescence spectra were corrected. The
fluorescence quantum yield (Φexp) was calculated from eq 1.24
4,4-Diflu or o-8-(4′-iod op h en yl)-3,5-bis-(4′-flu or oph en yl)-
4-bor a -3a ,4a -d ia za -s-in d a n cen e (1d ). Compound 5d (590
mg, 1.1 mmol), NEt3 (0.462 mL, 3.3 mmol), and PhMe (30 mL)
were stirred at 25 °C for 10 min. Boron trifluoride etherate
(0.696 mL, 5.5 mmol) was added, and the reaction mixture
was heated to 80 °C for 20 min and then cooled to 25 °C. The
crude mixture was purified on basic alumina using 10%
EtOAc/hexanes as eluant to yield 1d as a dark purple solid
(634 mg, 99%). Rf 0.2 (10% EtOAc/hexanes); mp 88-90 °C; MS
(FAB+, NBA) m/z (%) 582(M+); HRMS calculated [M+] 582.0389,
found [M+] 582.0460; 1H NMR (CDCl3, 300 MHz) δ 7.83 (m, 6
H), 7.30 (d, J ) 8.1 Hz, 2 H), 7.1 (t, J ) 8.6 Hz, 4 H), 6.85 (d,
J ) 4.2 Hz, 2 H), 6.59 (d, J ) 4.5 Hz, 2 H); 13C NMR (CDCl3,
75 MHz) δ 163.6 (d, J ) 251 Hz, CF), 158.1, 142.7, 137.6, 136.0,
133.6, 132.0, 131.5 (d, J ) 4 Hz, CH), 130.7, 128.5 (d, J ) 3
Hz, C), 121.0, 115.5 (d, J ) 22 Hz, CH), 96.8; 11B NMR (CDCl3,
64 MHz) δ -1.64 (t, J ) 31.7 Hz).
N-ter t-Bu t oxyca r b on yl-2-(2′-m et h oxyp h en yl)p yr r ole
(3e). A Schlenk tube was charged with N-tert-butyloxycarbo-
nyl-2-bromopyrrole (1.50 g, 6.10 mmol), 2-methoxyphenylbo-
ronic acid (0.93 g, 6.10 mmol), and tetrakis(triphenylphosphine)-
palladium (0.35 g, 0.31 mmol). The vessel was evacuated and
then refilled with nitrogen three times. Freshly distilled
toluene (32 mL) and degassed methanol (4 mL) were added
via syringe. Degassed 2 M Na2CO3 (aq) (6.1 mL, 12.196 mmol)
was added. The mixture was stirred under a nitrogen atmo-
sphere at 80 °C (oil bath) for 20 h. The layers were separated,
and the organic layer was dried (Na2SO4) and then evaporated.
The resulting oil was purified by flash chromatography, eluting
with 5% EtOAc/hexanes to yield 1.65 g of the desired com-
pound 3e (99% yield) as a clear oil. Rf 0.25 (5% EtOAc/
F{1 - exp(-Aref ln 10)}n2
Fref{1 - exp(-A ln 10)}nr2ef
Φexp ) Φref
(1)
Here, F denotes the integral of the corrected fluorescence
spectrum, A is the absorbance at the excitation wavelength,
and n is the refractive index of the medium. The reference
systems used were fluorescein (Φref ) 0.9225 and 0.9326),
rhodamine 101 methyl ester (Φref ) 1.0),27 and N,N′-bis(1-
hexylheptyl)-3,4:9,10-perylenebis(dicarboxyimide) (Φref ) 1.0).28
Radiative lifetimes (τ0 in ns) were calculated from the
modified Strickler-Berg (eq 2).16
1
hexanes); H NMR (CDCl3, 300 MHz) δ 7.4 (dd, J ) 3.3, 1.8
Hz, 1 H), 7.32 (tdd, J ) 3.0, 1.8 Hz, 2 H), 7.00 (td, J ) 7.4, 0.9
Hz, 1 H), 6.90 (dd, J ) 8.0, 0.9 Hz, 1 H), 6.28 (t, J ) 3.3 Hz,
1 H), 6.18 (dd, J ) 3.3, 1.8 Hz, 1 H), 3.79 (s, 3 H), 1.36 (s, 9
H); 13C NMR (CDCl3, 75 MHz) δ 157.3, 149.4, 130.2, 128.9,
124.2, 121.8, 124.2, 121.8, 120.2, 113.8, 110.3, 109.9, 82.7, 27.5.
2-(2′-Met h oxyp h en yl)p yr r ole (4e).14 A suspension of
sodium methoxide (625 mg, 11.0 mmol) in MeOH (3.0 mL) was
added to a stirred solution of N-tert-butyloxycarbonyl-2-(2′-
methoxyphenyl)pyrrole 3e (1.0 g, 3.66 mmol) in THF (15 mL)
and stirred at 25 °C for 6 h. The reaction was diluted with
Et2O (30 mL), extracted with H2O (2 × 20 mL) and brine (2 ×
20 mL), dried (Na2SO4), filtered, and concentrated. The crude
reaction mixture was purified on silica gel via flash chroma-
tography using 10% EtOAc/hexanes, giving 4e as white plates
(580 mg, 91.5% yield). Rf 0.37 (10% EtOAc/hexanes); 1H NMR
(CDCl3, 300 MHz) δ 7.72 (d, J ) 7.5 Hz 1 H), 7.21 (t, J ) 7.5
Hz, 1 H), 7.00-7.06 (m, 2 H), 6.90-6.93 (m, 1 H), 6.67-6.69
(m, 1 H), 6.31-6.35 (m, 1 H), 4.0 (s, 3 H); 13C NMR (CDCl3, 75
MHz) δ 154.7, 129.8, 126.7, 126.6, 121.4, 121.1, 117.8, 111.7,
108.9, 106.1, 55.6.
6-(4′′-Iod op h e n yl)-5,5′-b is(2-m e t h oxyp h e n yl)p yr r o-
m eth en e (5e). 2-(2′-Methoxyphenyl)pyrrole 4e (430 mg, 2.48
mmol), 4-iodobenzoyl chloride (1.12 g, 4.97 mmol), and 1,2-
dichloroethane (10 mL) were heated to reflux for 3 d, during
which time the solution gradually turned deep green. The
reaction mixture was diluted with CH2Cl2 (50 mL) and
extracted with H2O (50 mL), dried (Na2SO4), filtered, and
concentrated. The crude product was purified via flash chro-
matography using 20% EtOAc/hexanes as eluant, giving 5e
as a dark purple solid (450 mg, 65% yield). Rf 0.43 (20% EtOAc/
hexanes); mp 177-178 °C; MS (FAB+, NBA) m/z (%) 599 (M+);
HRMS calculated [M+] 599.0906, found [M+] 599.0883; 1H
NMR (CDCl3, 300 MHz) δ 8.07 (dd, J ) 1.8, 7.8 Hz, 2 H), 7.83
(d, J ) 8.4 Hz, 2 H), 7.34-7.40 (m, 2 H), 7.30 (d, J ) 5.7 Hz,
2 H), 7.09 (d, J ) 7.5 Hz, 2 H), 7.04 (d, J ) 8.1 Hz, 2 H), 6.97
(d, J ) 4.5 Hz, 2 H), 6.42 (d, J ) 4.5 Hz, 2 H), 3.89 (s, 6 H);
fl.spec.F(ν˜)dν˜
∫
1
τ0
) 2.88 × 10-9 n2
abs.spec.ꢀ(ν˜)ν˜-1dν˜ (2)
∫
fl.spec.F(ν˜)ν˜-3dν˜
∫
Here F(ν˜), ν˜, and ꢀ(ν˜) denote the corrected fluorescence
spectrum, the wavenumber of light (cm-1), and the molar
absorptivity, respectively. The refractive indices were mea-
sured and found to be n ) 1.33 for buffer solution (pH ) 9), n
) 1.36 in ethanol, n ) 1.42 for methylene chloride, and n )
1.45 for chloroform.
Fo¨rster radii (R0)17 were determined as follows. The cor-
rected fluorescence spectrum and the measured molar absorp-
tivity (in units of M-1 cm-1) were used to calculate the Fo¨rster
radius from:
9000 ln 10 κ2 ΦJ
1/6
Rof
)
(3a)
128π5n4NA
{
}
where
(24) Lipset, F. R. Prog. Dielectr. 1967, 7, 217.
(25) Melhuish, W. H. J . Phys. Chem. 1960, 64, 762-4.
(26) Weber, G.; Teale, F. W. J . Trans. Faraday Soc. 1958, 54, 640-
8.
(27) Drexhage, K. H. In Topics in Applied Physics; Scha¨fer, F. P.,
Ed., 1977; Vol. 1, pp 147-52, 171-2.
(28) Langhals, H.; Karolin, J .; J ohansson, L. B.-A. J . Chem. Soc.,
Faraday Trans. 1998, 94, 2919-22.

BODIPY Dyes
J . Org. Chem., Vol. 64, No. 21, 1999 7819
Cr ysta l d a ta (1b): C35H22BF2IN2, M ) 646.26, monoclinic,
a ) 12.278(3), b ) 14.203(3), c ) 16.332(3) Å, R ) 90°, â )
109.20(3)°, γ ) 90°, V ) 2689.6(9) Å3, space group P2(1)/n, Z
) 4, Dcalcd ) 1.596 g cm-3, abs coeff ) 1.234 mm-1, F(000) )
1288, Mo KR λ ) 0.71073 Å. The data were collected by ω
scanning techniques, at 298 K on a Siemens R3 X-ray diffrac-
tometer in the range 2.64 < Θ < 25.08°; 9702 reflections were
collected, corrected for Lorentzian polarization and absorption
effects, of which 4740 were independent reflections [R(int) )
0.1196]. The structure solution was made using Direct Meth-
ods29 and L.S. refinement of 370 parameters30 on F2 yielded
final R indices [I > 2σ(I)]: R(F) ) 0.0676wR(F2) ) 0.1291; R
indices (all data): R(F) ) 0.1776; wR(F2) ) 0.1638. Hydrogens
were placed on ideal positions, and the structural factors used
were taken from The Tables for X-ray Crystallography (Vol.
A).
J ) ꢀ(ν˜)f(ν˜)ν˜-4dν˜
(3b)
(3c)
∫
and
F(ν˜)
f(ν˜) )
F(ν˜)dν˜
∫
Here, NA is the Avogadro constant, and the mean value of
the orientational part of a dipole-dipole interaction is denoted
by κ2 . In the calculation of the Fo¨rster radius, it is convenient
to choose κ2
)
/
as a reference state. The Fo¨rster radius
using this reference value is denoted by R0.
2
3
The time-correlated single-photon-counting experiments
were performed on a PRA 3000 system (Photophysical Re-
search Assoc. Inc., Ontario, Canada). The excitation source was
a thyratron-gated flash lamp (Model 510C, PRA) filled with
deuterium gas and operated at ca. 30 kHz. Excitation wave-
lengths were selected by interference filters (Omega/Saven AB,
Sweden) centered at 500 nm (HBW ) 12.1 nm) and 550 nm
(HBW ) 40 nm). The emission wavelengths were selected by
an interference filter centered at 550 nm (HBW ) 40 nm) and
a long-pass filter of λ > 610 nm (Schott, Germany). The
instrument response function was determined by using a light-
scattering solution (LUDOX).
Ack n ow led gm en t. Financial support was provided
via NIH grant HG01745 and The Robert A. Welch
Foundation, and K.B. thanks the NIH for a Research
Career Development Award and the Alfred P. Sloan
Foundation for a fellowship. A.B. thanks the Deutsche
Forschungsgemeinschaft for a fellowship.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data for compound 1b; absorption and fluorescence emission
spectra for compounds 1a -e, 7a and 7b. This material is
available free of charge via the Internet at http://pubs.acs.org.
Cyclic Volta m m etr y. These experiments were conducted
in anhydrous acetonitrile at room temperature. The potentials
are reported vs ferrocene as internal standard using a scan
rate of 100 mV s-1, glassy carbon working electrode, Ag/AgNO3
reference electrode, platinum counterelectrode, and a support-
ing electrolyte 0.1 M tetrabutylammonium hexafluorophos-
phate.
J O990796O
(29) Sheldrick, G. M. In SHELX86 Program for Crystal Structure
Determination; University of Gottingen, Federal Republic of Germany,
1986.
(30) Sheldrick, G. M. University of Gottingen, Federal Republic of
Germany, 1993.