Synthesis and photophysical properties of 3,5-bis(oxopyridinyl)- and 3,5-bis(pyridinyloxy)-substituted boron-dipyrromethenes

Article abstract of DOI:10.1002/ejoc.200901460

Nucleophilic substitution reactions of 2-, 3- and 4-hydroxypyridines with 3,5-dibromo meso-aryl and meso-furyl borondipyrromethenes (BODIPYs) resulted in the formation of the corresponding 3,5-bis(oxopyridinyl)-BODIPYs and 3,5-bis(pyridinyloxy)-BODIPYs in decent yields. The effect of a pyridone versus an oxypyridine at the 3- and 5-positions on the spectral, electrochemical and photophysical properties were studied as a function of solvent. The 3, 5-bis(oxopyridinyl)-BODIPYs exhibit: broad, red-shifted absorption and emission bands, decreased quantum yields and lifetimes, displayed large Stokes shifts and easier reductions than did the 3, 5-bis(pyridinyloxy)-BODIPYs. The differences in the properties of these two classes of BODIPY dyes are attributed to the extension of π-delocalization associated with the electron-deficient nature of the pyridone groups.

Full text of DOI:10.1002/ejoc.200901460

FULL PAPER
DOI: 10.1002/ejoc.200901460
Synthesis and Photophysical Properties of 3,5-Bis(oxopyridinyl)- and
3,5-Bis(pyridinyloxy)-Substituted Boron-Dipyrromethenes
Tamanna K. Khan,[a] M. Rajeswara Rao,[a] and M. Ravikanth*[a]
Keywords: Boron / Nitrogen heterocycles / Dyes/pigments / Fluorescence / Fluorescent probes / Photophysics / Nucleophilic
substitution
Nucleophilic substitution reactions of 2-, 3- and 4-hydroxy-
pyridines with 3,5-dibromo meso-aryl and meso-furyl boron-
dipyrromethenes (BODIPYs) resulted in the formation of the
corresponding 3,5-bis(oxopyridinyl)-BODIPYs and 3,5-bis-
(pyridinyloxy)-BODIPYs in decent yields. The effect of a pyr-
idone versus an oxypyridine at the 3- and 5-positions on the
spectral, electrochemical and photophysical properties were
studied as a function of solvent. The 3,5-bis(oxopyridinyl)-
BODIPYs exhibit broad, red-shifted absorption and emission
bands, decreased quantum yields and lifetimes, displayed
large Stokes shifts and easier reductions than did the 3,5-
bis(pyridinyloxy)-BODIPYs. The differences in the properties
of these two classes of BODIPY dyes are attributed to the
extension of π-delocalization associated with the electron-de-
ficient nature of the pyridone groups.
tions. Boron-dipyrromethene (BODIPY) dyes are interest-
ing fluorescent dyes with valuable properties such as high
Introduction
2-, 3- and 4-Hydroxypyridines exhibit keto-enol tautomeriz- chemical and photostability and relatively high absorption
ation between the pyridone and hydroxypyridine form and coefficients and fluorescence quantum yields.[2] Further-
exhibit interesting chemistry due to the presence of two, more, BODIPY dyes can be optically excited with visible
different, reactive, nucleophilic centres.[1] The hydroxypyr- light, and their absorption and emission properties can be
idines would undergo nucleophilic substitution reactions by fine tuned by introducing appropriate substituents onto the
reacting either through the phenolic oxygen or the pyridine BODIPY framework.[3] The substituents can be introduced
nitrogen, depending on the substrate and reaction condi- onto any position of the BODIPY framework, although the
Scheme 1. Structures of 1–6.
electronic properties of BODIPYs are more significantly al-
[a] Department of Chemistry, Indian Institute of Technology,
tered upon introducing substituents directly at the pyrrole
C atoms than they are by adding substituents onto the
meso-aryl group. The meso-aryl group and the BODIPY
chromophore interact weakly since the two moieties are al-
Powai,
Mumbai 400076, India
E-mail: ravikanth@chem.iitb.ac.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200901460.
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Fluorescent Boron-Dipyrromethenes
most perpendicular to each other. Hence, any modifications cent yields (Scheme 2). The 3,5-dibromo dipyrromethanes 8
to the meso-aryl group do not alter electronic properties and 11 were then subjected to a two-step, one-pot reaction
effectively.[4] A more effective way to modulate the proper- by treating them with 1 equiv. of 2,3-dichloro-5,6-dicyano-
ties of the BODIPY chromophore is to introduce aryl 1,4-benzoquinone (DDQ) followed by BF3·OEt2 at room
groups at the pyrrole carbon atoms (i.e. the 3- and 5-posi- temperature. The progress of the reaction was judged by
tions). Boens, Dehaen and coworkers[5] recently showed TLC analysis at regular intervals. The crude reaction mix-
that it is possible to introduce substituents onto a BODIPY tures were subjected to silica gel column chromatographic
chromophore by nucleophilic substitution at the 3- and 5- purification and afforded the fluorescent 3,5-dibromo-sub-
positions.[5] They used 3,5-dichloro-BODIPY dyes[6] as pre- stituted BODIPYs 9 and 12 in 25 and 42% yield, respec-
cursors and prepared a wide range of oxygen-, nitrogen-, tively. Compounds 9 and 12 were confirmed by their molec-
sulfur- and carbon-centred nucleophiles substituted at the ular-ion peaks in their mass spectra and elemental analysis,
1
3- and 5-positions of BODIPY dyes under very simple reac- which matched the expected composition. In the H NMR
tion conditions.[5] These studies showed that the 3,5-substit- spectra, unlike the 3,5-unsubstituted BODIPY, which shows
uents alter the electronic properties of BODIPY dyes signif- three sets of signals corresponding to three sets of pyrrole
icantly, and the properties of the BODIPY dyes depend on protons, 9 and 12 showed only two sets of signals, support-
the kind of substituents introduced at the 3- and 5-posi- ing the substitution of the 3- and 5-positions with bromines.
tions.[7] In this paper, we report the synthesis of a new type The absorption spectra of 9 and 12 showed a strong absorp-
of 3,5-disubstituted BODIPY, containing pyridone and tion band at 520–540 nm, corresponding to the S0ǞS1 tran-
oxypyridine substituents (1–6, Scheme 1) and their photo- sition, with a shoulder on the higher-energy side due to the
physical properties investigated in various solvents. Al- 0Ǟ1 vibrational transition. In addition, a broad and weak
though there are several recent reports on 3,5-disubstituted absorption band at Ͻ400 nm, corresponding to the S0ǞS2
BODIPYs,[7] the oxopyridinyl- and pyridinyloxy-containing transition, was also present. All these features were in
BODIPYs are distinctive because they contain hydroxypyr- agreement with the reported 3,5-unsubstituted BODIPY
idines bonded by nitrogen and oxygen, respectively, at the dyes.[3] The introduction of bromines at the 3- and 5-posi-
3- and 5-positions, which alter their photophysical charac- tions resulted in an ≈ 20 nm red-shift in their absorption
teristics significantly, as shown in this paper.
bands compared to those of the 3,5-unsubstituted
BODIPYs. Furthermore, the 12 showed a 25 nm batho-
chromic shift in the S0ǞS1 absorption band compared to
that of 9, which is attributed to the enhancement of π-delo-
calization in 12 because the meso-furyl group[9] and BOD-
Results and Discussion
To synthesize 3,5-bis(oxopyridinyl)- and 3,5-bis(pyridin- IPY lie in the same plane, unlike in 9, in which the p-meth-
yloxy)-substituted BODIPYs 1–6, we needed access to 3,5- oxyphenyl group is perpendicular to the BODIPY plane.[4]
dibromo-meso-(4-methoxyphenyl)-BODIPY (9) and 3,5-di-
In the next step, the 3,5-dibromo-BODIPYs 9 and 12
bromo-meso-furyl-BODIPY (12), which were prepared as were subjected to nucleophilic substitution reactions by tre-
shown in Scheme 2. The meso-aryl dipyrromethane[8a] 7 and ating them with nucleophiles such as 2-, 3- and 4-hydroxy-
meso-furyl dipyrromethane[8b] 10 were treated with 2 equiv. pyridines. The 2- and 4-hydroxypyridines generally exist as
of N-bromosuccinimide in THF at –78 °C followed by flash the 2-pyridone and 4-pyridone, respectively, whereas the 3-
column chromatographic purification on silica to afford hydroxypyridine exists in the enol form and prefers to react
3,5-dibromo-meso-aryl dipyrromethane (8) and 3,5-di- by the phenolic oxygen.[1] Compounds 1–3 were prepared
bromo meso-furyl dipyrromethane (11), respectively, in de- by reacting 1 equiv. of 9 with 3 equiv. of 2-hydroxypyridine,
Scheme 2. Synthesis of 9 and 12.
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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T. K. Khan, M. R. Rao, M. Ravikanth
FULL PAPER
Scheme 3. Synthesis of 3,5-bis(oxopyridinyl)- and 3,5-bis(pyridinyloxy)-substituted BODIPYs 1–5.
3-hydroxypyridine and 4-hydroxypyridine, respectively, in protons appeared as a doublet at δ = 6.81 ppm in 2 and
MeCN in the presence of Cs2CO3 at reflux for 1 h. The experienced a downfield shift to 7.04 in 1 and 7.12 ppm in
progress of the reaction was followed by TLC analysis, 3. Similarly, the 2,6-pyrrole protons, which appeared as a
which indicated the formation of the desired compounds as doublet at δ = 5.74 ppm in 2, experienced an downfield shift
the sole products. The crude reaction mixtures were sub- in 1 and 3 of about 0.85 ppm (Table 1). The pyridyl protons
jected to silica gel column chromatography and afforded of 1 and 3 appeared upfield of those of 2, supporting the
1–3 in 66–75% yield (Scheme 3). These reactions worked existence of the pyridine in the pyridone form in 1 and 3
efficiently with 3 equiv. of nucleophile rather than with and the oxypyridine form in 2. The pyridone and oxypyrid-
2 equiv., which gave mixtures of products. Similarly, the 4 ine forms in 1–5 were also differentiated by using 13C NMR
and 5 were prepared by reacting 12 with 2-hydroxypyridine and IR spectroscopic techniques. The pyridone compounds
and 3-hydroxypyridine, respectively, under reaction condi- showed a signal at 165–180 ppm in the 13C NMR and a
tions similar to those used for the synthesis of 1–3. How- peak at ca. 1680 cm–1 in the IR spectra, confirming the
ever, when we treated 12 with 4-hydroxypyridine under sim- presence of a carbonyl carbon. Similar differences in the
ilar reaction conditions, although the reaction appeared chemical shifts of the pyrrole and pyridyl protons were also
successful, as judged by absorption spectroscopy and TLC observed for 4 and 5. However, due to the presence in 4
analysis, 6 was very unstable and decomposed on the silica and 5 of the meso-furyl group, which is more in-plane with
gel column. Several attempts to prepare 6 under various the BODIPY unit, the pyrrole and pyridyl protons of 4 and
modified reaction conditions failed due to the unstable na- 5 were shifted downfield relative to those of the correspond-
ture of 6 at room temperature.
ing meso-aryl analogues, in which the aryl group is perpen-
Compounds 1–5 were characterized by mass, NMR, ab- dicular to the BODIPY ring. All these above observations
sorption and fluorescence spectroscopy and electrochemical clearly suggest that 3,5-bis(oxopyridinyl) substituents alter
and elemental analysis. The molecular-ion peak in the mass the π-delocalization of the BODIPY unit more effectively
spectra and the matching of the elemental analysis with the than do 3,5-bis(pyridinyloxy) substituents. These results
1
expected compositions confirmed the identity of 1–5. H, were also supported by 19F and 11B NMR spectra. Com-
13C, 19F and 11B NMR spectroscopy and the colour of the pounds 1–5 showed characteristic 19F and 11B NMR sig-
solutions under UV light were used to differentiate BOD- nals[10] with a quartet in the 19F NMR and a triplet in the
IPY compounds containing oxypyridines from BODIPY 11B NMR (Figure 1). In the 19F NMR spectra, 2 showed a
compounds containing pyridones at the 3- and 5-positions. typical quartet at δ = –149.1 ppm, which was shifted down-
Under UV light, the 3,5-bis(pyridinyloxy)-BODIPYs are field for 1 and 3, appearing at –142.1 ppm in 1 and δ =
more fluorescent than the 3,5-bis(oxopyridinyl)-BODIPYs –137.3 ppm in 3 (Figure S13). Similarly, downfield shifts
1
(Figure S19). A comparison of H and 11B NMR spectra were also noted in the 11B NMR spectra of 1 and 3 relative
in a selected region for 1–3 is presented in Figure 1, and to those of 2 (Table 1). Thus, the NMR study revealed that
selected NMR spectroscopic data for 1–5 are presented in the 3,5-bis(oxopyridinyl)- and 3,5-bis(pyridinyloxy)-
Table 1. As is clear from Figure 1 and the data in Table 1, BODIPYs showed clear differences in chemical shifts,
the NMR chemical shifts are very useful in differentiating which is attributed to the differences in the π-delocalization
3,5-bis(pyridinyloxy)-BODIPYs 2 and 5 from 3,5-bis(oxo- of the BODIPY unit, caused by pyridone and oxypyridine
pyridinyl)-BODIPYs 1, 3 and 4. For example, the 1,7-pyrrole substituents present at the 3- and 5-positions.
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Fluorescent Boron-Dipyrromethenes
1
Figure 1. Comparison of H and 11B NMR spectra of 1–3 in a selected region, recorded in CDCl3.
1
Table 1. Comparison of H, 11B and 19F NMR chemical shift values (in ppm) of 1–5, recorded in CDCl3.
Entry
Compound
1H NMR
2,6-pyrrole
19F NMR
11B NMR
1,7-pyrrole
1
2
3
4
5
1
2
3
4
5
6.59 (d, J = 3.97 Hz)
5.74 (d, J = 4.27 Hz)
6.56 (d, J = 4.27 Hz)
6.64 (m)
7.04 (d, J = 4.27 Hz)
6.81 (d, J = 3.97 Hz)
7.12 (d, J = 3.66 Hz)
7.56 (d, J = 3.97 Hz)
7.62 (m)
–142.10 (q)
–149.12 (q)
–137.29 (q)
–142.79 (br. s)
–149.26 (q)
0.61 (t)
0.73 (t)
0.67 (t)
0.56 (t)
0.64 (t)
7.30 (d, J = 3.97 Hz)
The absorption properties of 1–5 were studied in six dif- ring and restricts conjugation, (3) the absorption band of
ferent solvents of varying polarity. A comparison of the 3,5-bis(3-pyridinyloxy)-BODIPYs 2 and 5 is narrower with
normalized absorption spectra of 1–3 in toluene is shown full-width at half-maximum (FWHM) values in the range
in part a of Figure 2; 2 and 5 are shown in Figure 2 (b), of 800–1405 cm–1 in toluene, whereas the absorption band
and the data for 1–5 in different solvents are presented in of 3,5-bis(2/4-oxopyridinyl)-BODIPYs 1, 3 and 4 is very
Table 2. Compounds 1–5 showed absorption features typi- broad, with FWHM values of 1450–2530 cm–1 in toluene,
cal of BODIPYs[4a] such as a strong S0ǞS1 transition at (4) the meso-furyl analogues exhibit broader absorption
about 520 nm with a vibronic transition on the higher-en- bands than do the meso-aryl analogues and (5) the absorp-
ergy side as a shoulder and an ill-defined, weak band corre- tion of 1 in different solvents shown in Figure 3 indicates
sponding to the S0ǞS2 transition at about 420 nm. The that 1–5 exhibit a blueshift in the absorption band and an
data presented in Table 2 and Figure 2 reveal the following: increase in the FWHM with increasing solvent polarity,[11]
(1) the 3,5-bis(4-oxopyridinyl)-BODIPY exhibited an red- and maximum solvent effects were noted for 3,5-bis(oxopyr-
shift of about 50 nm in the S0ǞS1 absorption band relative idinyl)-BODIPYs. Thus, the absorption studies reveal that
to that of 3,5-bis(2-oxopyridinyl)- and 3,5-bis(3-pyridinyl- 3,5-bis(oxopyridinyl)-BODIPYs showed broad, red-shifted
oxy)-BODIPYs. For example, 1 and 2 showed absorption S0ǞS1 absorption bands, whereas the 3,5-bis(pyridinyloxy)-
bands at 522 and 518 nm, respectively, whereas 3 showed a BODIPYs exhibited narrow, well-defined absorption bands
50 nm red-shift in the absorption band and appeared at with absorption features matching other BODIPYs re-
569 nm in toluene. This is attributed to the enhancement of ported in the literature.[3] Furthermore, the meso-furyl-
π-delocalization in 3,5-bis(4-oxopyridinyl)-BODIPY 3 com- BODIPYs showed broad and red-shifted absorption bands
pared to that of 3,5-bis(2-oxopyridinyl)- and 3,5-bis(3-pyr- compared to those of the meso-anisyl analogues.
idinyloxy)-BODIPYs 1 and 2, respectively, (2) the meso-
The electrochemical properties of 3,5-bis(2/4-oxopyridin-
furyl analogues showed absorption bands at longer wave- yl)- and 3,5-bis(3-pyridinyloxy)-BODIPYs 1–5 were deter-
lengths compared to that of the corresponding meso-aryl mined by cyclic voltammetry at a scan rate of 50 mV/s using
analogues (Figure 2, b). For example, the 3,5-bis(pyridin- tetrabutylammonium perchlorate (TBAP) as the supporting
yloxy) meso-(4-methoxyphenyl)-BODIPY 2 showed an ab- electrolyte. A comparison of the reduction waves of 1–3 is
sorption band at 518 nm in toluene, which is red-shifted in shown in Figure 4 (a), and the reduction waves of 2 and 5
the corresponding meso-furyl analogue 5 and appeared at are shown in Figure 4 (b). The redox potential data for 1–
540 nm. This is due to the in-plane orientation of the meso- 5 are presented in Table 3. Compounds 1–5 showed one oxi-
furyl group and BODIPY unit, which helps increase π-con- dation and two reductions occurring at the BODIPY unit.
jugation, unlike the scenario of the meso-aryl analogues, in The oxidation and one of the reduction are irreversible and
which the meso-aryl group is perpendicular to the BODIPY did not follow any definite trend. However, the first re-
Eur. J. Org. Chem. 2010, 2314–2323
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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T. K. Khan, M. R. Rao, M. Ravikanth
FULL PAPER
Figure 3. Comparison of normalized absorption spectra of 1, re-
corded in (a) toluene; (b) THF; (c) EtOAc; (d) CHCl3; (e) MeCN
and (f) MeOH.
duction is quasireversible and exhibited a systematic trend
with the substituents present at the 3- and 5-positions as
well as at the meso-position. An inspection of Figure 4 (a,
b) and the data presented in Table 3 reveal the following:
(1) the first reduction potentials of 3,5-bis(oxopyridinyl)-
BODIPYs were less negative than those of 3,5-bis(pyridinyl-
oxy)-BODIPYs by 200–400 mV, indicating that 3,5-bis(oxo-
pyridinyl)-BODIPYs are easier to reduce than 3,5-bis(pyr-
idinyloxy)-BODIPYs, (2) the 3,5-disubstituted meso-furyl-
BODIPYs are easier to reduce than the corresponding 3,5-
disubstituted meso-aryl-BODIPYs and (3) among 3,5-bis-
(2/4-oxopyridinyl)-BODIPYs, the 3,5-bis(4-oxopyridinyl)-
BODIPY 3 exhibited a less negative reduction potential
Figure 2. Comparison of normalized absorption spectra of (a) 1–3
and (b) 2 and 5, recorded in toluene.
Table 2. Photophysical properties of BODIPYs 1–5 in different solvents. * Broad, ill-defined band.
Entry Compd. Solvent
λabs
[nm]
λem
[nm]
FWHM (abs.)
[cm–1]
∆νst
ε
φ
τ [ns]
kf108
knr109
[cm–1]
1
2
3
4
5
6
7
8
1
2
3
4
5
toluene
CHCl3
THF
EtOAc
MeOH
MeCN
toluene
CHCl3
THF
EtOAc
MeOH
MeCN
toluene
CHCl3
THF
EtOAc
MeOH
MeCN
toluene
CHCl3
THF
EtOAc
MeOH
MeCN
toluene
CHCl3
THF
EtOAc
MeOH
MeCN
522
510
518
512
501
503
518
516
514
512
510
510
569
548
555
551
485
534
550
536
543
539
528
525
540
538
535
533
530
531
569
567
566
560
537
549
531
531
529
528
523
526
600
584
591
587
535
576
625
624
632
621
607
600
589
583
583
580
575
578
2530
1870
1910
1980
1570
1450
810
1090
1000
950
1570
1640
1650
1680
1310
1650
450
510
550
570
460
4.48
4.45
4.48
4.49
4.61
4.62
4.94
4.91
4.87
4.93
4.89
4.79
4.69
4.69
4.63
4.7
4.39
4.67
4.12
4.47
4.21
4.51
4.52
4.46
4.79
4.56
4.84
4.72
4.82
4.79
0.063
0.027
0.019
0.028
0.0036
Ͻ0.001
0.45
0.58
0.33
0.286
0.23
0.25
0.77
0.46
0.30
0.26
0.12
–
0.81
0.58
0.63
1.07
0.30
–
1.21
2.11
3.21
3.73
8.3
–
2.13
2.58
1.30
1.46
1.37
1.08
1.43
1.01
0.59
0.58
0.18
0.22
1.37
1.07
0.50
0.47
–
–
2.9
3.3
2.4
2.27
2.21
2.14
2.11
2.24
2.53
1.95
1.67
2.31
2.09
1.73
0.93
1.34
1.22
1.13
0.19
0.21
0.16
0.13
–
7.8
1.6
5.1
4.8
5.6
6.9
4.8
8.1
1.6
1.58
5.4
4.4
0.71
0.91
1.98
2.11
–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
950
1070
1930
1980
1700
1930
4270*
2210
2240
2107
2177
2104
1932
1819
1405
1206
1409
1292
1217
1290
570
910
0.30
1130
1110
1110
1900
1370
2181
2646
2606
2449
2465
2381
1541
1449
1553
1523
1480
1543
0.175
0.055
0.078
0.022
0.025
0.027
0.023
0.0083
0.0063
0.002
0.0013
0.169
0.15
–
–
0.58
0.45
0.48
0.44
0.50
0.55
2.8
2.5
3.6
3.9
4.0
4.1
0.115
0.10
0.111
0.119
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Fluorescent Boron-Dipyrromethenes
than that of 3,5-bis(2-oxopyridinyl)-BODIPY 1, indicating Table 3. Electrochemical redox data [V] of 1–5 in CH2Cl2 contain-
ing 0.1  TBAP as the supporting electrolyte.
that 3 is much easier to reduce than 1. Thus, the electro-
chemical study clearly indicated that the meso-furyl-
BODIPYs were easier to reduce than the corresponding
meso-anisyl analogues. Furthermore, the electron-with-
drawing 2- and 4-pyridone substituents at the 3- and 5-posi-
tions render the reduction of these compounds more facile
than that of 3,5-bis(3-pyridinyloxy)-BODIPYs. The 3,5-
bis(4-oxopyridinyl)-BODIPY is much easier to reduce than
the 3,5-bis(2-oxopyridinyl)-BODIPY, suggesting that the
Entry Compound
Oxidation
Reduction
I
II
1
2
3
4
5
1
2
3
4
5
–
–0.626
–0.913
–0.504
–0.460
–0.789
–1.71
–1.53
–1.54
–1.50
–1.69
1.28
1.48
1.66
1.25
3,5-bis(4-oxopyridinyl)-BODIPYs are more stable towards idinyloxy)-BODIPYs. For example, among 1–3, the 3,5-bis-
reduction than are the 3,5-bis(2-oxopyridinyl)-BODIPYs.
(pyridinyloxy)-BODIPY 2 showed an emission band at
531 nm, and the 3,5-bis(2/4 pyridone)-BODIPYs 1 and 3
showed red-shifted emission bands at 569 and 600 nm,
respectively [Figure 5 (a)], (3) the 3,5-bis(pyridinyloxy)-
BODIPYs showed narrow emission bands, whereas the
emission band of 3,5-bis(2/4 pyridone)-BODIPYs are much
broader, (4) among 3,5-bis(oxopyridinyl) derivatives such as
1 and 3, 3 showed a broad and red-shifted emission band
compared to that of 1. This is attributed to an extension of
π-delocalization in 3, (5) the meso-furyl-BODIPYs exhib-
ited broad and red-shifted emission bands compared to
those of the corresponding meso-aryl-BODIPYs, (6) the
quantum yields of 3,5-bis(pyridinyloxy)-BODIPYs are rela-
tively higher than those of the 3,5-bis(oxopyridinyl)-BODI-
PYs, and the meso-furyl derivatives showed lower emission
yields compared to that of their corresponding meso-anisyl
derivatives (Table 2), (7) 1–5 showed slight hypochromic
shifts with a reduction in the quantum yield upon increas-
ing solvent polarity, which is consistent with the general
behaviour of BODIPY compounds[11] and (8) the 3,5-bis-
(pyridinyloxy)-BODIPYs showed a slight Stokes shift
(12 nm), as shown in part b of Figure 5 for 2, whereas the
3,5-bis(2/4-oxopyridinyl)-BODIPYs exhibited much larger
Stokes shift (ca. 50 nm), as shown in Figure 5 (c) for 1.
Moreover, the Stokes shifts of each compound do not vary
much as a function of the solvent. These observations indi-
cate that the structures of 3,5-bis(oxopyridinyl)-BODIPYs
in the ground state and excited state are different from each
other unlike with the 3,5-bis(pyridinyloxy)-BODIPYs,
which may possess the same structure in the ground and
excited states. Thus, the Stokes shift data indicate that 3,5-
bis(oxopyridinyl)-BODIPYs undergo structural reorganiza-
tion in the excited state.
Time-resolved studies were carried out on 1–5 to under-
stand their fluorescence properties in detail, and a represen-
tative fluorescence decay of 2 is shown in Figure 5 (d).
Compounds 1–5 generally showed a single exponential de-
cay in all solvents, and the lifetimes parallel the fluorescence
Figure 4. Comparison of reduction waves of cyclic voltammograms
of (a) 1–3 and (b) 2 and 5 in CH2Cl2 containing 0.1  TBAP as
the supporting electrolyte recorded at a scan speed of 50 mV/sec.
The fluorescence properties of 1–5 were studied in six quantum yields. The 3,5-bis(pyridinyloxy)-BODIPYs exhib-
different solvents by steady-state and time-resolved fluores- ited longer singlet-state lifetimes than that of the 3,5-bis-
cence techniques (Table 2). The steady-state fluorescence (oxopyridinyl)-BODIPYs. These observations are consistent
studies on 1–5 reveal the following: (1) all the compounds with the observed fluorescence yields. The decrease in kf
(1–5) showed one single emission band, and the band width and increase in knr are in agreement with the shorter life-
depended on the kind of substituents present at the 3- and times of the 3,5-bis(oxopyridinyl)-BODIPYs compared to
5-positions as well as at the meso-position, (2) the 3,5-bis- that of the 3,5-bis(pyridinyloxy)-BODIPYs. Thus, the rela-
(oxopyridinyl)-BODIPYs showed emission bands batho- tively low fluorescence yields observed for 3,5-bis(oxopyrid-
chromically shifted with respect to those of the 3,5-bis(pyr- inyl)-BODIPYs may be due to the decreased S1 state life-
Eur. J. Org. Chem. 2010, 2314–2323
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2319

T. K. Khan, M. R. Rao, M. Ravikanth
FULL PAPER
Figure 5. (a) Comparison of normalized emission spectra of 1–3, recorded in toluene, at λex = 488 nm (b) Comparision of normalized
absorption (Ϫ) and emission (---) spectra of 3,5-bis(pyridinyloxy) 2 in toluene. (c) Comparision of normalized absorption (Ϫ) and
emission (---) spectra of 3,5-bis(oxopyridinyl) 1 in toluene. (d) Fluorescence decay profile and weighted, residual, distribution fit of 2 in
chloroform. The excitation wavelength used was 406 nm and emission was detected at 531 nm.
time and possibly due to the increased S1ǞT1 intersystem and singlet-state lifetimes relative to those of the 3,5-bis-
crossing and increased S1ǞS0 internal conversion rates. The (pyridinyloxy)-BODIPYs. All these results suggest that the
large Stokes shifts observed for the 3,5-bis(oxopyridinyl)- electronic properties of a BODIPY dye depend on the kind
BODIPYs support the enhancement of the Franck–Con- of substituents (pyridine versus oxypyridine) present at the
don factor due to substantial structural reorganization in 3- and 5-positions. We believe that the remarkable differ-
the excited state, resulting in the increased nonradiative de- ences in the spectral, electrochemical and photophysical
cay rates.
properties between 3,5-bis(oxopyridinyl)-BODIPYs and
3,5-bis(pyridinyloxy)-BODIPYs will lead to a wide range of
applications in biological and molecular sensors.
Conclusions
We have described the synthesis of 3,5-bis(oxopyridinyl)-
BODIPYs and 3,5-bis(pyridinyloxy)-BODIPYs by reacting
the corresponding 3,5-dibromo-BODIPYs with 2-, 3- and
4-hydroxypyridines in MeCN in the presence of Cs2CO3.
The absorption studies indicated that the 3,5-bis(oxopyrid-
inyl)-BODIPYs exhibit broad, red-shifted absorption bands
relative to those of the 3,5-bis(pyridinyloxy)-BODIPYs. The
presence of the meso-furyl group also induces red-shifts in
the absorption bands relative to those of the meso-anisyl
analogues. The electrochemical study revealed that the 3,5-
bis(oxopyridinyl)-BODIPYs are easier to reduce than the
3,5-bis(oxypridine)-BODIPYs, supporting the electron-de-
ficient nature of pyridone groups. The fluorescence study
indicated that the 3,5-bis(oxopyridinyl)-BODIPYs exhibit a
Experimental Section
Chemicals: THF and toluene were dried with sodium benzophen-
one ketyl, and CHCl3, EtOAc, CH3OH and MeCN were dried with
calcium hydride and distilled prior to use. BF3·OEt2 and DDQ were
obtained from Spectrochem (India) and used as obtained. All other
chemicals used for the synthesis were reagent-grade unless other-
wise specified. Column chromatography was performed on silica
(60–120 mesh).
Instrumentation: 1H NMR spectra (δ values, ppm) were recorded
using Varian VXR 300 and 400 MHz spectrometers. 13C NMR
spectra were recorded with a Varian spectrometer operating at
100.6 MHz. 19F NMR spectra were recorded with a Varian spec-
trometer operating at 282.2 MHz. 11B NMR spectra were recorded
with a Varian spectrometer operating at 96.3 MHz. Tetramethylsil-
1
broad, red-shifted, emission bands with low quantum yields ane (TMS) was used as an external reference for recording H (of
2320
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 2314–2323

Fluorescent Boron-Dipyrromethenes
residual proton; δ = 7.26 ppm) and 13C (δ = 77.0 ppm) spectra in
CDCl3. Absorption and steady-state fluorescence spectra were ob-
tained with a Perkin–Elmer Lambda-35 spectrometer and a PC1
Photon Counting Spectrofluorimeter (ISS, USA). Fluorescence
spectra were recorded at 25 °C in a 1-cm quartz fluorescence cu-
vette. The fluorescence quantum yields (Φf) were estimated from
the emission and absorption spectra by the comparative method at
the excitation wavelength of 488 nm using Rhodamine 6G (Φf =
0.88)[7h] as the standard. The time-resolved fluorescence decay mea-
surements were carried out at the magic angle using a pico-
second-diode-laser-based, time-correlated, single-photon-counting
(TCSPC) fluorescence spectrometer[9] from IBH, UK. All the de-
cays were fitted to a single exponential.
tiate the reaction, and the reaction mixture was stirred for 15 min
at room temperature. The reaction mixture was diluted with
CH2Cl2 and washed thoroughly with aqueous NaOH (0.1 ) and
water. The organic layers were combined, dried with Na2SO4 and
concentrated in a rotary evaporator. The crude compound was sub-
jected to silica gel column and afforded pure meso-(2-furyl)dipyrro-
methane 10 using petroleum ether/EtOAc (95:5) as a white solid in
1
63% yield (1.2 g); m.p. 120 °C. H NMR (400 MHz, CDCl3): δ =
5.49 (s, 1 H, furan), 5.98 (s, 2 H, pyrrole), 6.12–6.15 (m, 2 H, pyr-
role), 6.31–6.33 (m, 1 H, furan), 6.67 (d, J = 1.52 Hz, 2 H, pyrrole),
7.38 (s, 1 H, furan), 8.04 (br. s, 2 H, NH) ppm. 13C NMR
(100 MHz, CDCl3): δ = 37.7, 37.8, 106.7, 106.8, 106.9, 108.1, 108.3,
110.3, 117.6, 117.7, 129.9, 142.0, 154.4 ppm. LC-MS: m/z = 211.2
[M]+. C13H12N2O (212.25): calcd. C 73.56, H 5.70, N 13.20; found
C 73.92, H 5.95, N 13.56.
Cyclic voltammetric (CV) and differential pulse voltammetric
(DPV) studies were carried out with an electrochemical system uti-
lizing a three-electrode configuration consisting of a glassy carbon
(working) electrode, platinum wire (auxiliary) electrode and a satu-
rated calomel (reference) electrode.[9] The experiments were per-
formed in dry CH2Cl2 using 0.1  TBAP as the supporting electro-
lyte. Half-wave potentials were measured using DPV and also cal-
culated manually by taking the average of the cathodic and anodic
2,10-Dibromo-meso-(2-furyl)dipyrromethane (11): meso-(2-Furyl)di-
pyrromethane 10 (1.0 g, 4.71 mmol) was treated with N-bromosuc-
cinimide (1.67 g, 9.4 mmol) in THF at –78 °C under an argon at-
mosphere for 1 h, and the resulting crude compound was purified
by silica gel column chromatograhy using petroleum ether to afford
1
pure 11 as a dark solid in 73% yield (1.4 g); m.p. 118–119 °C. H
peak potentials. MALDI-TOF spectra were obtained with an Ax- NMR (400 MHz, CDCl3): δ = 5.39 (s, 1 H, CH), 5.91 (s, 2 H,
ima-CFR spectrometer, manufactured by Kratos Analyticals. LC- furan), 6.09 (s, 2 H, pyrrole), 6.16 (s, 1 H, pyrrole), 6.35 (s, 1 H,
MS spectra were obtained with a Varian Pro Star 500 MS.
pyrrole), 7.41 (s, 1 H, furan), 7.99 (s, 1 H, furan), 8.27 (br. s, 2 H,
NH) ppm. 13C NMR (100 MHz CDCl3): δ = 29.7, 36.7, 38.0, 90.3,
97.8, 107.5, 109.0, 110.5, 110.6, 130.8, 142.6, 149.5, 178.8 ppm. LC-
MS: m/z = 375.6 [M]+. C13H10Br2N2O (370.04): calcd. C 42.20, H
2.72, N 7.57; found C 42.43, H 2.89, N 7.92.
2,10-Dibromo-meso-(4-methoxyphenyl)dipyrromethane (8): meso-(4-
Methoxyphenyl)dipyrromethane (7, 1.0 g, 4.23 mmol) in dry THF
(75 mL) was cooled to –78 °C under an argon atmosphere and
treated with N-bromosuccinimide (1.6 g, 9.0 mmol) for 1 h. The re-
action mixture was brought to room temperature, and the solvent
was removed in a rotary evaporator under vacuum. The crude com-
pound was purified by silica gel column chromatography using pe-
troleum ether and afforded pure 8 as a red solid in 75% yield
(1.27 g); m.p. 118–119 °C. 1H NMR (400 MHz, CDCl3): δ = 3.9 (s,
3 H, -OCH3), 5.29 (s, 1 H, -CH), 5.82 (m, 2 H, pyrrole), 6.06 (m,
3,5-Dibromo-4,4-difluoro-8-(2-furyl)-4-bora-3a,4a-diaza-s-indacene
(12): Compound 11 (0.500 g, 1.35 mmol) in CH2Cl2 was first oxid-
ized with DDQ (306 mg, 1.35 mmol) at room temperature for 1 h.
The reaction mixture was then treated with a small amount of Et3N
(6.6 mL) followed by BF3·OEt2 (9.3 mL, 67.5 mmol), and the mix-
ture was stirred for an additional 1 h at room temperature. The
2 H, pyrrole), 7.06 (d, J = 7.8 Hz, 2 H, Ar), 7.13 (d, J = 7.8 Hz, 2 solvent was removed in a rotary evaporator, and the resultant crude
H, Ar) ppm. C16H14Br2N2O (410.11): calcd. C 46.86, H 3.44, N
6.83; found C 47.02, H 3.53, N 6.95.
compound was purified by silica gel column chromatography using
CH2Cl2 as the eluent. Pure 12 was obtained as dark-red solid in
1
42% yield (375 mg). H NMR (400 MHz, CDCl3): δ = 6.58 (d, J
3,5-Dibromo-4,4-difluoro-8-(4-methoxyphenyl)-4-bora-3a,4a-diaza-
s-indacene (9): Compound 8 (500 mg, 1.31 mmol) in CH2Cl2
(30 mL) was oxidized with DDQ (300 mg, 1.31 mmol) at room tem-
perature whilst being stirred for 1 h. Et3N (6.4 mL) was added, fol-
lowed by BF3·OEt2 (8.20 mL, 65.8 mmol), and stirring was contin-
ued at room temperature for an additional 1 h. The reaction mix-
ture was washed successively with aqueous NaOH (0.1 ) and
= 3.97 Hz, 2 H, pyrrole), 6.70 (m, 1 H, furan), 7.10 (d, J = 3.66 Hz,
1 H, furan), 7.33 (d, J = 4.27 Hz, 2 H, pyrrole), 7.83 (s, 1 H, furan)
ppm. 13C NMR (100 MHz CDCl3): δ = 113.5, 120.3, 122.7, 129.2,
131.0, 131.6, 133.2, 147.5, 147.8 ppm. LC-MS: m/z = 397.0 [M –
19]+. C13H8BBr2F2N2O (416.83): calcd. C 37.46, H 1.93, N 6.72;
found C 37.78, H 2.01, N 6.89.
water thoroughly. The combined organic layers were dried with General Synthetic Procedure for the Preparation of 3,5-Dipyridone
Na2SO4 and filtered, and the solvents were evaporated. The crude
product was purified by silica gel column chromatography using
CH2Cl2 and afforded 9 as an orange-red powder in 25 % yield
(208 mg); m.p. 210–211 °C. H NMR (400 MHz, CDCl3): δ = 3.9
(s, 3 H, -OCH3), 6.52 (d, J = 4.28 Hz, 2 H, pyrrole), 6.82 (d, J =
4.28 Hz, 2 H, pyrrole), 7.01 (d, J = 7.9 Hz, 2 H, Ar), 7.42 (d, J =
and 3,5-Dioxypyridine-substituted Boron-dipyrromethenes 1–5:
Compounds 1–5 were synthesized by treating the corresponding
3,5-dibromo dipyrromethene 9 or 12 (1 equiv.) with 2-, 3- or 4-
hydroxypyridine (3 equiv.) in MeCN in the presence of Cs2CO3
(2.5 equiv.) under an inert atmosphere at reflux for 1 h. The pro-
gress of the reaction was followed by TLC analysis, which showed
1
7.9 Hz, 2 H, Ar) ppm. 13C NMR (100 MHz, CDCl3): δ = 55.63, the disappearance of the starting compound and appearance of a
114.27, 122.56, 124.87, 131.60, 131.87, 132.33, 135.48, 148.42, new spot corresponding to the desired compound. The solvent was
162.22 ppm. 19F NMR (282.2 MHz, CDCl3): δ = –147.09 (q, JB,F removed in a rotary evaporator under vacuum, and the crude com-
= 57.8 Hz) ppm. 11B NMR (96.3 MHz, CDCl3): δ = 0.88 (t, JB,F
pound was subjected to silica gel column chromatography using
CH2Cl2/2–5% CH3OH.
= 2 8 . 2 H z ) p p m . M A L D I - T O F : m / z = 4 5 4 . 5 [ M ] +
.
C16H11BBr2F2N2O (455.89): calcd. C 42.15, H 2.43, N 6.14; found
C 42.87, H 2.62, N 6.45.
3,5-Bis(1,2-dihydro-2-oxopyridin-1-yl)-4,4-difluoro-8-(4-methoxy-
phenyl)-4-bora-3a,4a-diaza-s-indacene (1): Compound 1 was ob-
1
meso-(2-Furyl)dipyrromethane (10): In a 250 mL, round-bottomed
flask, freshly distilled furfural (1 g, 8.97 mmol) and pyrrole
(15.5 mL, 0.22 mmol) were dissolved in CH2Cl2 (50 mL) under an
argon atmosphere. BF3·OEt2 (123 µL, 0.89 mmol) was added to ini-
tained as a red solid in 75% yield. H NMR (400 MHz, CDCl3): δ
= 3.92 (s, 3 H, -OCH3), 6.17 (t, J1 = J2 = 6.72 Hz, 2 H, pyridine),
6.54 (m, 2 H, pyridine), 6.59 (d, J = 3.97 Hz, 2 H, pyrrole), 7.04
(d, J = 4.27 Hz, 2 H, pyrrole), 7.07 (d, J = 8.55 Hz, 2 H, Ar), 7.33
Eur. J. Org. Chem. 2010, 2314–2323
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2321

T. K. Khan, M. R. Rao, M. Ravikanth
FULL PAPER
(m, 2 H, pyridine), 7.42 (d, J = 6.72 Hz, 2 H, pyridine), 7.55 (d, J
= 8.55 Hz, 2 H, Ar) ppm. 13C NMR (100 MHz, CDCl3): δ = 55.53,
102.98, 114.07, 124.45, 125.31, 127.65, 128.36, 130.80, 132.04,
142.32, 146.83, 161.54, 162.64 ppm. 19F NMR (282.2 MHz,
CDCl3): δ = –142.10 (q, JB,F = 60.9 Hz) ppm. 11 B NMR
(96.3 MHz, CDCl3): δ = 0.61 (t, JB,F = 28.9 Hz) ppm. IR (KBr
Acknowledgments
M. R. thanks the Council of Scientific & Industrial Research
(CSIR) and Department of Science & Technology (DST) for finan-
cial support. T. K. K. thanks the Indian Institute of Technology,
Bombay for a fellowship.
film): ν = 1681 cm–1. MALDI-TOF: m/z = 463.8 [M – 19]+.
˜
C26H19BF2N4O3 (484.27): calcd. C 64.49, H 3.95, N 11.57; found
C 65.12, H 4.09, N 11.96.
[1] A. Kocak, S. Kurbanli, Synth. Commun. 2007, 37, 3697–3708
and references cited therein.
4,4-Difluoro-8-(4-methoxyphenyl)-3,5-bis(pyridin-3-yloxy)-4-bora-
[2] For recent reviews, see: a) R. Ziessel, G. Ulrich, A. Harriman,
New J. Chem. 2007, 31, 496–501; b) A. Loudet, K. Burgess,
Chem. Rev. 2007, 107, 4891–4932; c) G. Ulrich, R. Ziessel, A.
Harriman, Angew. Chem. Int. Ed. 2008, 47, 1184–1201.
[3] a) M. Kollmannsberger, K. Rurack, U. Resch-Genger, J. Daub,
J. Phys. Chem. A 1998, 102, 10211–10220; b) T. López Arbeloa,
F. López Arbeloa, I. López Arbeloa, I. García-Moreno, A. Co-
stela, R. Sastre, F. Amat-Guerri, Chem. Phys. Lett. 1999, 299,
315–321; c) K. Rurack, M. Kollmannsberger, J. Daub, Angew.
Chem. Int. Ed. 2001, 40, 385–387; d) B. Cunderlikova, L. Siku-
rova, Chem. Phys. 2001, 263, 415–422; e) R. Y. Lai, A. J. Bard,
J. Phys. Chem. B 2003, 107, 5036–5042; f) W. Qin, M. Baruah,
A. Stefan, M. Van der Auweraer, N. Boens, ChemPhysChem
2005, 6, 2343–2351; g) W. Qin, M. Baruah, M.
Van der Auweraer, F. C. De Schryver, N. Boens, J. Phys. Chem.
A 2005, 109, 7371–7384; h) T. Rohand, W. Qin, N. Boens, W.
Dehaen, Eur. J. Org. Chem. 2006, 4658–4663; i) M. Baruah,
W. Qin, C. Flors, J. Hofkens, R. A. L. Vallée, D. Beljonne, M.
Van der Auweraer, W. M. De Borggraeve, N. Boens, J. Phys.
Chem. A 2006, 110, 5998–6009; j) A. C. Benniston, G. Copley,
K. J. Elliott, R. W. Harrington, W. Clegg, Eur. J. Org. Chem.
2008, 2705–2713; k) M. Bröring, R. Krüger, S. Link, C. Klee-
berg, S. Köhler, X. Xie, B. Ventura, L. Flamigni, Chem. Eur.
J. 2008, 14, 2976–2983; l) W. Qin, M. Baruah, M. Sliwa, M.
Van der Auweraer, W. M. De Borggraeve, D. Beljonne, B.
Van Averbeke, N. Boens, J. Phys. Chem. A 2008, 112, 6104–
6114; m) B. Ventura, G. Marconi, M. Bröring, R. Krüger, L.
Flamigni, New J. Chem. 2009, 33, 428–438; n) K. Guzow, K.
Kornowska, W. Wiczk, Tetrahedron Lett. 2009, 50, 2908–2910;
o) E. Fron, E. Coutiño-Gonzalez, L. Pandey, M. Sliwa, M.
Van der Auweraer, F. C. De Schryver, J. Thomas, Z. Dong, V.
Leen, M. Smet, W. Dehaen, T. Vosch, New J. Chem. 2009, 33,
1490–1496.
3a,4a-diaza-s-indacene (2): Compound 2 was obtained as a green
solid in 70% yield. H NMR (400 MHz, CDCl3): δ = 3.89 (s, 3 H,
1
-OCH3), 5.74 (d, J = 4.27 Hz, 2 H, pyrrole), 6.81 (d, J = 3.97 Hz,
2 H, pyrrole), 7.02 (d, J = 8.55 Hz, 4 H, Ar), 7.36 (q, J = 4.58 Hz,
2 H, pyridine), 7.46 (d, J = 8.55 Hz, 2 H, Ar), 7.63 (d, J = 7.02 Hz,
2 H, pyridine), 8.52 (s, 2 H, pyridine), 8.66 (s, 2 H, pyridine) ppm.
13C NMR (100 MHz, CDCl3): δ = 55.89, 114.32, 114.83, 119.02,
124.93, 133.16, 139.26, 148.70, 152.02, 163.26, 179.28 ppm. 19F
NMR (282.2 MHz, CDCl3): δ = –149.12 (q, JB,F = 55.0 Hz) ppm.
11B NMR (96.3 MHz, CDCl3): δ = 0.73 (t, JB,F = 27.4 Hz) ppm.
MALDI-TOF: m/z = 483.7 [M]+. C26H19BF2N4O3 (484.27): calcd.
C 64.49, H 3.95, N 11.57; found C 65.17, H 4.12, N 11.92.
3,5-Bis(1,4-dihydro-4-oxopyridin-1-yl)-4,4-difluoro-8-(4-methoxy-
phenyl)-4-bora-3a,4a-diaza-s-indacene (3): Compound 3 was ob-
tained as a red fluorescent solid in 66% yield. 1H NMR (400 MHz,
CDCl3): δ = 3.95 (s, 3 H, -OCH3), 6.45 (d, J = 7.94 Hz, 4 H, pyr-
idine), 6.56 (d, J = 4.27 Hz, 2 H, pyrrole), 7.12 (m, 4 H, Ar, pyr-
role), 7.57 (d, J = 8.55 Hz, 2 H, Ar), 7.70 (d, J = 7.63 Hz, 4 H,
pyridine) ppm. 13C NMR (100 MHz, CDCl3): δ = 55.8, 114.3,
114.8, 119.0, 124.9, 133.1, 139.2, 148.7, 152.0, 163.2, 179.2 ppm.
19F NMR (282.2 MHz, CDCl3): δ = –137.29 (q, JB,F = 60.9 Hz)
ppm. 11B NMR (96.3 MHz, CDCl3): δ = 0.67 (t, JB,F = 28.9 Hz)
ppm. IR (KBr film): ν = 1640 cm–1. MALDI-TOF: m/z = 484.5
˜
[M]+. C26H19BF2N4O3 (484.27): calcd. C 64.49, H 3.95, N 11.57;
found C 65.15, H 4.04, N 11.69.
3,5-Bis(1,2-dihydro-2-oxopyridin-1-yl)-4,4-difluoro-8-(2-furyl)-4-
bora-3a,4a-diaza-s-indacene (4): Compound 4 was obtained as a red
solid in 46% yield. 1H NMR (400 MHz, CDCl3): δ = 6.18 (m, 2
H, pyridine), 6.57 (d, J = 9.77 Hz, 2 H, pyridine), 6.64 (m, 2 H,
pyrrole), 6.78 (s, 1 H, furan), 7.35 (m, 5 H, pyridine, furan), 7.56
(d, J = 3.97 Hz, 2 H, pyrrole), 7.91 (d, J = 1.83 Hz, 1 H, furan)
[4] a) H. L. Kee, C. Kirmaier, L. Yu, P. Thamyongkit, W. J. Young-
blood, M. E. Calder, L. Ramos, B. C. Noll, D. F. Bocian, W. R.
Scheidt, R. R. Birge, J. S. Lindsey, D. Holten, J. Phys. Chem.
B 2005, 109, 20433–20443; b) E. Lager, J. Liu, A. Aguilar-Agui-
lar, B. Z. Tang, E. Peña-Cabrera, J. Org. Chem. 2009, 74, 2053–
2058.
[5] a) M. Baruah, W. Qin, R. A. L. Vallée, D. Belijonne, T. Ro-
hand, W. Dehaen, N. Boens, Org. Lett. 2005, 7, 4377–4380; b)
T. Rohand, M. Baruah, W. Qin, N. Boens, W. Dehaen, Chem.
Commun. 2006, 266–268.
ppm. 19F NMR (282.2 MHz, CDCl3): δ = –142.79 (br. s) ppm. 11
B
NMR (96.3 MHz, CDCl3): δ = 0.56 (t, JB,F = 28.9 Hz) ppm.
MALDI-TOF: m/z = 423.9 [M – 19]+. C23H15BF2N4O3 (444.20):
calcd. C 62.19, H 3.40, N 12.61; found C 62.53, H 3.56, N 12.75.
4,4-Difluoro-8-(2-furyl)-3,5-bis(3-pyridin-3-yloxy)-4-bora-3a,4a-diaza-
s-indacene (5): Compound 5 was obtained as green, crystalline solid
in 62% yield. 1H NMR (400 MHz, CDCl3): δ = 5.79 (d, J =
4.27 Hz, 2 H, pyridine), 6.65 (m, 1 H, furan), 6.97 (d, J = 3.66 Hz,
1 H, furan), 7.30 (d, J = 3.97 Hz, 2 H, pyrrole), 7.36 (q, J =
4.88 Hz, 2 H, pyridine), 7.62 (m, 2 H, pyrrole), 7.75 (s, 1 H, furan),
8.52 (d, J = 4.58 Hz, 2 H, pyridine), 8.65 (d, J = 2.44 Hz, 2 H,
pyridine) ppm. 13C NMR (100 MHz, CDCl3): δ = 102.8, 112.4,
117.2, 124.18, 124.5, 126.0, 127.3, 127.7, 130.2, 142.1, 142.5, 146.0,
146.7, 151.4, 162.2 ppm. 19F NMR (282.2 MHz, CDCl3): δ =
–149.26 (q, JB,F = 55.0 Hz) ppm. 11B NMR (96.3 MHz, CDCl3): δ
= 0.64 (t, JB,F = 27.4 Hz) ppm. MALDI-TOF: m/z = 443.8 [M]+.
C23H15BF2N4O3 (444.20): calcd. C 62.19, H 3.40, N 12.61; found
C 62.76, H 3.62, N 12.89.
[6] M. Baruah, W. Qin, N. Basaric´, W. M. De Borggraeve, N.
Boens, J. Org. Chem. 2005, 70, 4152–4157.
[7] a) W. Qin, M. Baruah, M. Van der Auweraer, F. C.
De Schryver, N. Boens, J. Phys. Chem. A 2005, 109, 7371–7384;
b) W. Qin, T. Rohand, M. Baruah, A. Stefan, M.
Van der Auweraer, W. Dehaen, N. Boens, Chem. Phys. Lett.
2006, 420, 562–568; c) W. Qin, T. Rohand, W. Dehaen, J. N.
Clifford, K. Driesen, D. Beljonne, B. Van Averbeke, M.
Van der Auweraer, N. Boens, J. Phys. Chem. A 2007, 111, 8588–
8597; d) T. Rohand, J. Lycoops, S. Smout, E. Braeken, M.
Sliwa, M. Van der Auweraer, W. Dehaen, W. M. De Borggra-
eve, N. Boens, Photochem. Photobiol. Sci. 2007, 6, 1061–1066;
e) Z. Li, R. Bittman, J. Org. Chem. 2007, 72, 8376–8382; f) Z.
Ekmekci, M. D. Yilmaz, E. U. Akkaya, Org. Lett. 2008, 10,
461–464; g) L. Li, J. Han, B. Nguyen, K. Burgess, J. Org. Chem.
2008, 73, 1963–1970; h) W. Qin, V. Leen, T. Rohand, W. De-
haen, P. Dedecker, M. Van der Auweraer, K. Robeyns, L.
Supporting Information (see also the footnote on the first page of
this article): 1H, 13C and 19F NMR spectra and mass spectra of all
compounds.
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Eur. J. Org. Chem. 2010, 2314–2323

Fluorescent Boron-Dipyrromethenes
Van Meervelt, D. Beljonne, B. Van Averbeke, J. N. Clifford, K.
Driesen, K. Binnemans, N. Boens, J. Phys. Chem. A 2009, 113,
439–447; i) W. Qin, V. Leen, W. Dehaen, J. Cui, C. Xu, X.
Tang, W. Liu, T. Rohand, D. Beljonne, B. Van Averbeke, J. N.
Clifford, K. Driesen, K. Binnemans, M. Van der Auweraer, N.
Boens, J. Phys. Chem. C 2009, 113, 11731–11740; j) K. Cies´lik-
Boczula, K. Burgess, L. Li, B. Nguyen, L. Pandey, W. M.
De Borggraeve, M. Van der Auweraer, N. Boens, Photochem.
Photobiol. Sci. 2009, 8, 1006–1015.
Chem. Commun. 2008, 4593–4595; c) T. W. Hudnall, F. P.
Gabbaï, Chem. Commun. 2008, 4596–4597; d) J. A. Jacobsen,
J. R. Stork, D. Magde, S. M. Cohen, Dalton Trans. 2010, 39,
957–962.
a) A. Costela, I. García-Moreno, C. Gomez, R. Sastre, F.
Amat-Guerri, M. Liras, F. López Arbeloa, J. Bañuelos Prieto,
I. López Arbeloa, J. Phys. Chem. A 2002, 106, 7736–7742; b)
J. Bañuelos Prieto, F. López Arbeloa, V. Martínez Martínez, T.
Arbeloa López, F. Amat-Guerri, M. Liras, I. López Arbeloa,
Chem. Phys. Lett. 2004, 385, 29–35; c) F. López Arbeloa, J.
Bañuelos Prieto, V. Martínez Martínez, T. Arbeloa Lopez, I.
López Arbeloa, ChemPhysChem 2004, 5, 1762–1771.
Received: December 14, 2009
[11]
[8] a) C.-H. Lee, J. S. Lindsey, Tetrahedron 1994, 50, 11427–11440;
b) G. Santosh, M. Ravikanth, Tetrahedron 2007, 63, 7833–
7844.
[9] S. Rai, G. Gayatri, G. N. Sastry, M. Ravikanth, Chem. Phys.
Lett. 2008, 467, 179–185.
[10] a) R. Ziessel, L. Bonardi, G. Ulrich, Dalton Trans. 2006, 2913–
2918; b) C. Bonnier, W. E. Piers, M. Parvez, T. S. Sorensen,
Published Online: March 15, 2010
Eur. J. Org. Chem. 2010, 2314–2323
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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