Synthesis and electronic properties of meso-furyl boron dipyrromethenes

Article abstract of DOI:10.1016/j.ica.2011.11.017

Four meso-furyl boron-dipyrromethenes (BODIPYs) were synthesized and characterized. The X-ray structures solved for three meso-furyl BODIPYs indicated the presence of an intramolecular hydrogen bond between meso-furyl 'O' and 'H' of boron-dipyrromethene core resulting in decrease of dihedral angle between the meso-furyl group and boron-dipyrromethene core leading to better electronic interaction. However, the hydrogen bonding is absent in solution as confirmed by NMR studies in different solvents. The presence of meso-furyl group alters the electronic properties of BODIPY which reflected in the downfield shifts in ¹H NMR, bathochromic shifts in absorption and emission bands compared to the meso-tolyl BODIPY. The electrochemical studies indicated that the meso-furyl BODIPYs are easier to reduce compared to meso-tolyl BODIPYs. DFT studies showed that the HOMO-LUMO energy gap is decreased in meso-furyl BODIPYs compared to meso-tolyl BODIPY which is in agreement with the experimental observations.

Full text of DOI:10.1016/j.ica.2011.11.017

Inorganica Chimica Acta 383 (2012) 257–266
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and electronic properties of meso-furyl boron dipyrromethenes
Tamanna K. Khan a, Sunit K. Jana a, M. Rajeswara Rao a, Mushtaque S. Shaikh b, M. Ravikanth a,
⇑
a Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400076, India
b Department of Pharmaceutical Chemistry, Bombay College of Pharmacy, Santacruz (E), Mumbai 400098, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 March 2011
Received in revised form 11 October 2011
Accepted 8 November 2011
Available online 19 November 2011
Four meso-furyl boron-dipyrromethenes (BODIPYs) were synthesized and characterized. The X-ray struc-
tures solved for three meso-furyl BODIPYs indicated the presence of an intramolecular hydrogen bond
between meso-furyl ‘O’ and ‘H’ of boron-dipyrromethene core resulting in decrease of dihedral angle
between the meso-furyl group and boron-dipyrromethene core leading to better electronic interaction.
However, the hydrogen bonding is absent in solution as confirmed by NMR studies in different solvents.
The presence of meso-furyl group alters the electronic properties of BODIPY which reflected in the down-
field shifts in 1H NMR, bathochromic shifts in absorption and emission bands compared to the meso-tolyl
BODIPY. The electrochemical studies indicated that the meso-furyl BODIPYs are easier to reduce com-
pared to meso-tolyl BODIPYs. DFT studies showed that the HOMO-LUMO energy gap is decreased in
meso-furyl BODIPYs compared to meso-tolyl BODIPY which is in agreement with the experimental
observations.
Keywords:
Meso-furyl boron dipyrromethene
Hydrogen bonding
Dihedral angle
Stoke’s shift
Easier reductions
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
fluoride atoms which in turn would alter the electronic properties
of boron-dipyrromethene [30]. Cohen and co-workers [30] showed
Boron-dipyrromethene fluorophores (BODIPYs) have received
much interest in various research fields as labeling reagents, fluo-
rescent switches, chemosensors, light-harvesting systems and dye
sensitized solar cells owing to their advantageous photophysical
properties such as photostability, high absorption coefficients
and high fluorescent quantum yields [1–3]. BODIPY dyes are ame-
nable to modifications, and the properties can be fine-tuned by
introduction of suitable substituents at appropriate positions of
the boron-dipyrromethene framework. Various approaches have
been attempted by different research groups to create new BODI-
PYs with altered electronic properties that include (1) introduction
of electron-donating moieties [4–12], (2) replacement of meso C
atom by a N atom [13–16], (3) rigidification of rotatable moieties
that the two amide substituents at 3,5-positions in BODIPY II in-
volved in intramolecular hydrogen bonding with boron-bound
fluoride atoms help in rigidifying the chromophore resulting in sig-
nificant alteration in electronic properties. Dehaen and co-workers
[31,32] also showed the presence of intramolecular hydrogen
bonding in their 3-anilino BODIPY III and 3,5-dianilino BODIPY
IV compounds. These are few 3-monosubstituted or 3,5-disubsti-
tuted BODIPYs where the presence of intramolecular hydrogen-
bonding alters the electronic properties of boron-dipyrromethenes.
Interestingly, there is no report on BODIPY in which the meso-sub-
stituent is involved in intramolecular hydrogen-bonding with any
atom on boron dipyrromethene core. In this paper, we report the
examples of intramolecular hydrogen bonded meso-furyl dipyrro-
methenes 2–5 (Chart 2) in which the meso-furyl ‘‘O’’ is involved
in hydrogen bonding with pyrrole ‘‘H’’ of boron-dipyrromethene
core in solid state. While our study was in progress, Churchill
and co-workers [33] reported the effect of five membered hetero-
aryl groups such as furyl, thienyl and selenyl at meso-position on
structural and electronic properties of BODIPY. Interestingly, they
did not discuss about the intramolecular hydrogen bonding pres-
ent in 2 in solid state. Our study indicated that in meso-furyl BOD-
IPYs 2, 3 and 4, the presence of intramolecular hydrogen bonding
along with the small size of furyl group helps in reducing the dihe-
dral angle between meso-furyl group and boron-dipyrromethene
unlike meso-phenyl boron-dipyrromethene [35] in which the six
membered phenyl group is relatively more out-of-plane orienta-
tion with the boron-dipyrromethene core. However, the hydrogen
bonding is not present in solution for meso-furyl BODIPYs 2–5
[17–20], (4) extension of the
p-conjugation [21–23], and (5) the
extension of the -conjugation by the fusion of aryl moieties
p
[24,25]. Although all the above mentioned approaches have
brought substantial changes in the electronic properties of the
BODIPY dyes, there are relatively few reports on BODIPY dyes con-
taining five membered heteroaryl moieties such as thienyl, furyl,
pyrrole, imidazole etc in their framework [26,6,27,28]. Umezawa
et al. [29] reported recently that the furan fused BODIPY dye I
exhibited significant red-shifts in absorption and fluorescence
spectral bands with very high fluorescence yields (Chart 1). Re-
cently, it is reported that the substituents present at the 3,5-posi-
tions would be involved in intramolecular hydrogen bonding with
⇑
Corresponding author. Tel.: +91 22 5767176; fax: +91 22 5723480.
E-mail address: ravikanth@chem.iitb.ac.in (M. Ravikanth).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.11.017

258
T.K. Khan et al. / Inorganica Chimica Acta 383 (2012) 257–266
methene 3 was prepared in two steps (Scheme 1) by following our
earlier reported method [37]. The compounds 4 and 5 were
prepared in decent yields by treating compound 3 with phenyl-
2-boronic acid and thienyl-2-boronic acid, respectively in the pres-
ence of Pd(PPh3)4 in water/THF/toluene (1:1:1) at 80 °C. All
reactions worked smoothly and compounds 2–5 are freely soluble
in common organic solvents. The compounds 2–5 were confirmed
by HR-MS mass spectral analysis and characterized by using vari-
ous spectroscopic techniques. The compounds 2, 3 and 4 were also
characterized by X-ray crystallography.
The compounds 2–5 were characterized in detail by 1D, 2D,
19F and 11B NMR and the selected NMR data for compounds 2–
5 along with compound 1 are presented in Table 1. The compar-
ison of 1H NMR spectra of compounds 1 and 2 in selected region
is shown in Fig. 1a and 1H–1H COSY NMR spectrum recorded for
compound 2 is shown in Fig. 1b. The extensive NMR studies
including 1H, 13C, 19F and 11B NMR for all compounds along with
1H–1H COSY, HSQC and HMBC NMR for compounds 2 and 4
helped in identifying the protons such as b-pyrrole protons repre-
sented as Ha, Hb and Hc and meso-furyl protons represented as Hd,
He and Hf (Supporting information). The introduction of five
membered furyl group at meso-position in place of six membered
aryl group alter the electronic properties of boron-dipyrrometh-
ene core which clearly reflected in the downfield shift of certain
protons [33]. In 1H NMR spectrum of 2, the three pyrrole protons
Ha, Hb and Hc were observed as set of three signals at 7.89, 6.58
and 7.47 ppm, respectively and the three furyl protons Hd, He and
Hf were also observed as set of three signals at 7.83, 6.71 and
7.20 ppm, respectively. A close inspection of Fig. 1a and data in
Table 1 indicates that the pyrrole protons Ha and Hb experienced
slight shifts in 2 compared to 1. However, the Hc proton of com-
pound 2 experienced ca. ꢁ0.5 ppm downfield shift relative to
compound 1. Similarly, the corresponding carbons of Ha, Hb and
Hc in compound 2 also exhibited downfield shifts compared to
1 and maximum downfield shift was noted for carbon which pos-
sess Hc proton as judged by 13C NMR, HSQC and HMBC analysis.
(Supporting information) Similar observations were made re-
cently by Churchill and co-workers [33]. These results indicated
Chart 1. Molecular structures of BODIPYs I, II, III and IV.
that the p-delocalization is increased in 2 compared to 1 resulting
in downfield shifts of pyrrole protons. This is because the meso-
furyl group is smaller in compound 2 hence more in plane (as
confirmed by X-ray crystallography) with the BODIPY core result-
ing in the extension of
p-electron delocalization of BODIPY core
into the meso-furyl group. On the other hand, the larger meso-to-
lyl group in compound 1 is more out-of-plane with the BODIPY
core thus preventing the extension of p-delocalization into
meso-tolyl group. The significant shift of Hc proton in 2 is because
of its proximity to the furan ‘‘O’’ atom which provides more elec-
tron rich environment to Hc proton compared to Ha and Hb pro-
tons [33]. This is also clearly evident in 3,5-disubstituted meso-
furyl boron-dipyrromethenes 3–5 which showed significant shifts
for Hc proton compared to Ha and Hb protons (Table 1). However,
the 19F and 11B NMR showed slight downfield shifts for com-
pound 2 compared to compound 1 unlike Cohen’s compound II
which showed 16 ppm downfield shift in 19F NMR due to the
presence of strong intramolecular hydrogen bonding between
fluoride and amide group present at the 3,5-position of BODIPY.
However, the 1H NMR spectra of compound 2 recorded in differ-
ent solvents such as deuterated acetone, methanol and acetoni-
trile indicated that there is no intramolecular hydrogen bonding
between meso-furyl ‘‘O’’ and the pyrrole Hc (H3 and H7) proton
of boron dipyrrin unit in solution (Supporting information). The
19F and 11B NMR of compounds 3, 4 and 5 showed significant
downfield shifts compared to 2 which is due to presence of sub-
stituents at 3,5-positions such as bromo, phenyl and thienyl
groups, respectively.
Chart 2. Molecular structures of Boron dipyrromethene dyes 1–5.
unlike Cohen’s BODIPY II in which strong intramolecular hydrogen
bonding is present in solution.
2. Results and discussion
The meso-tolyl boron-dipyrromethene 1 was prepared by fol-
lowing the literature procedure [36]. The meso-furyl boron-dip-
yrromethene 2 was prepared [33] as shown in Scheme 1. The
required precursor, meso-furyl dipyrromethane 6 was prepared in
63% yield by treating one equivalent of furan-2-aldehyde with 25
equivalents of pyrrole in the presence of catalytic amount of
BF3ꢀOEt2 at room temperature for 15 min [37]. In the next step,
the compound 6 was first oxidized with DDQ in CH2Cl2 at room
temperature and then treated with triethylamine followed by
BF3ꢀOEt2. After standard work-up, the crude compound was puri-
fied by silica gel column chromatography and afforded 2 as dark
red fluorescent solid in 26% yield. The 3,5-dibromo boron-dipyrro-

T.K. Khan et al. / Inorganica Chimica Acta 383 (2012) 257–266
259
Scheme 1. Synthesis of 3,5-disubstituted meso-furyl BODIPYs 2–5.
information). The X-ray structure of compound 2 and its packing
diagram are shown in Fig. 2; the structures of 3 and 4 are shown
in Fig. 3 and the relevant data of compounds 2, 3, 4 along with
the reported meso-phenyl boron-dipyrromethene 8 [35] are pre-
sented in Table 2. The X-ray structures of 2, 3 and 4 revealed that
the dipyrrin unit and the central six-membered ring containing the
boron atom are almost coplanar with small deviations associated
with either the attractive secondary interaction between furyl
‘‘O’’ or the repulsive interaction to minimize steric strain involving
adjacent C-H’s on furyl ring and dipyrromethene. The plane de-
fined by the F–B–F atoms is almost perpendicular to that of the
boron-dipyrromethene core which is in line with the previously re-
ported BODIPY 8 [35].
Table 1
Comparison of 1H, 11B and 19F NMR chemical shift values (in ppm) of compounds 1–5
recorded in CDCl3.
Compound
1H NMR
Ha
19F NMR
11B NMR
Hb
Hc
1
2
3
4
5
7.94(s)
7.89(s)
–
–
–
6.55(d)
6.58(d)
6.68(d)
6.86(d)
6.58(d)
6.89(d)
7.47(d)
7.43(d)
7.48(d)
7.33(d)
ꢂ145.27(q)
ꢂ145.94(q)
ꢂ146.64(q)
ꢂ132.99(q)
ꢂ139.56(q)
0.50(t)
0.45(t)
0.75(t)
1.57(t)
1.86(t)
The orientation of meso-furyl group towards the plane of boron-
dipyrromethene core is clearly evident in the crystal structures of
compounds 2, 3 and 4. The single crystals of compounds 2, 3 and
4 were obtained from n-hexane/dichloromethane on slow evapora-
tion over a period of one week and crystallized in the monoclinic
space groups P21/c, P21/c and P21/n, respectively (Supporting
The most remarkable feature of X-ray crystal structures of com-
pounds 2, 3 and 4 compared to the reported compound 8 is the sig-
nificant decrease in the dihedral angle (C6–C5–C10–C11) between
the meso-furyl group and the plane defining the various dipyrrin
atoms. In compound 8, the meso-phenyl group is more out-of-plane

260
T.K. Khan et al. / Inorganica Chimica Acta 383 (2012) 257–266
Fig. 1. (a) Comparison of 1H NMR spectra of compounds 1 and 2 in selected region recorded in CDCl3. (b) 1H–1H COSY spectrum for compound 2 recorded in CDCl3.
with dipyrrin unit (dihedral angle of 52°) and the meso-phenyl
group and dipyrrin core are almost non-interactive [35]. However,
in compounds 2, 3 and 4, the dihedral angle between meso-furyl
group and the boron dipyrrin frame-work are 25.1(3)°, 30.3(7)°
and 31.0(3)°, respectively. Churchill and co-workers [33] also
solved the crystal structure for compound 2 and observed the de-
crease in dihedral angle between meso-furyl group and the boron-
dipyrrin framework (ꢁ26.6°). These results indicate that the
meso-furyl group is more in the plane of dipyrrin unit resulting in
strong interaction between the meso-furyl group and the boron
dipyrrin core in compounds 2–4. The significant decrease in dihe-
dral angle in compounds 2, 3, and 4 is attributed to the small size
of furyl group and also to the presence of intramolecular hydrogen
bonding between meso-furyl ‘‘O’’ and the pyrrole Hc (H3 and H7)
proton of dipyrrin unit (O1. . .H3) as observed in their crystal struc-
tures. The meso-furyl ‘‘O’’ is engaged in hydrogen bonding with Hc
proton of the two pyrrole groups in dipyrrin unit thus preventing
the free rotation of meso-furyl group leading to the decrease of
dihedral angle between meso-furyl group and dipyrrin frame-work.
Furthermore, as evident in the packing diagram of 2 presented in
Fig. 2b, one of the hydrogen atoms of the meso-furyl group is en-
gaged in weak intermolecular hydrogen bonding with fluoride
atom of another molecule forming a polymeric chain. In addition
to hydrogen bonding, the two BODIPY molecules in asymmetric
unit are arranged in such a way that there is a possibility of
p–p
stacking interaction between the pyrrole of one unit with the other
and the distance between them is in the range of 3.4–3.8 Å [34].
Interestingly, Churchill and co-workers [33] did not discuss about
intra and intermolecular hydrogen bonding and p–p stacking inter-
actions noted here for meso-furyl boron dipyrromethenes. The
bond-lengths such as B–F, B–N, C–N and bond angles such as N1–
B1–N2, C4–C5–C6 in compounds 2, 3 and 4 are also slightly altered
compared to 1 which is attributed to the alteration of electronic
properties due to the presence of furyl group at meso-position in
compounds 2-4.
The absorption, electrochemical and fluorescence properties of
BODIPYs 2–5 were studied to understand the effect of meso-furyl
group on the electronic properties of the boron-dipyrromethene

T.K. Khan et al. / Inorganica Chimica Acta 383 (2012) 257–266
261
Fig. 2. Crystal structure of (a) compound 2 and (b) its packing diagram. Hydrogen bonds are denoted by dashed lines.
Fig. 3. Crystal structures of compounds (a) 3 and (b) 4 with hydrogen bonds denoted by dashed lines.
core. The absorption properties of compounds 1, 2, 4 and 5 were
studied in six different solvents of varying polarity. The compari-
son of normalized absorption spectra of compounds 1, 2, 4 and 5
recorded in toluene is shown in Fig. 4 and the data for compounds
1, 2, 4 and 5 recorded in six different solvents are presented in Ta-
ble 3. Generally, the meso-aryl substituted BODIPYs such as 1 show
two absorption bands [35]: (1) a strong S0 ? S1 transition with a
maximum appearing at 502 nm and (2) the second maximum or
shoulder on the high energy side, centered at about 480 nm, which
is attributed to the 0 ? 1 vibrational transition. In addition, a con-
siderably weaker, broad absorption band at 375 nm corresponding
to S0 ? S2 transition is also present. The absorption properties of
meso-furyl boron-dipyrromethene 2 showed some similarities
and differences compared to 1. The absorption spectrum of 2 is
similar in shape as that of 1. It exhibits a strong absorption band
at 525 with a shoulder at 485 nm and relatively more intense band
at 402 nm. However, as compared to compound 1, the absorption
spectrum of 2 exhibited 873 cmꢂ1 bathochromic shift in S0 ? S1
transition. Furthermore, the absorption band of 2 is very broad
with full-width at half maximum (FWHM) of 1819 cmꢂ1 compared
to 1 which show relatively narrow band with FWHM of 1166 cmꢂ1
in toluene. The absorption spectrum of 2 is not affected much by
solvent polarity and the maximum being slightly shifted hypso-
chromically when the solvent is changed from toluene (525 nm)
to acetonitrile (516 nm) which is consistent with the general
behavior of BODIPY chromophores [38–45]. This small shift reflects
the polarizability of the solvent. In all solvents, the compound 2
experienced similar bathochromic shift of S0 ? S1 band with
change in FWHM compared to 1. The S0 ? S2 band did not show
any variation in peak maxima and band width on changing the
polarity of the solvent. Upon introduction of aryl substituents at
3,5-positions such as phenyls in 4 and thienyls in 5 resulted in fur-
ther bathochromic shift in S0 ? S1 transition with more FWHM and
the maximum effect was observed for compound 5. This is

262
T.K. Khan et al. / Inorganica Chimica Acta 383 (2012) 257–266
Table 2
0
Selected Bond Distances (AÅ) and Bond Angles (°) for compounds 2, 3 and 4 along with 8.
Bond length/torsion angles
8
2
3
4
B1–F1
B1–N1
N1–C1
1.381(10)
1.547(9)
1.345(8)
1.389(8)
1.385(3)
1.531(3)
1.345(3)
1.393(3)
2.38
1.380(6)
1.551(7)
1.331(6)
1.382(6)
2.38
1.3763(19)
1.544(2)
1.3594(16)
1.3975(16)
2.48
N1–C4
O1ꢀ ꢀ ꢀHca
–
C3–H3
0.95
0.95
0.95
0.95
C2–C3
C5–C10
O(1)–C(10)
O(1)–C(13)
F1–B1–F2
1.387(10)
1.47
1.374(3)
1.451(3)
1.364(2)
1.382(3)
110.0(2)
110.21
106.16(18)
120.45(17)
105.41(18)
25.0(3)
1.367(7)
1.439(6)
1.374(6)
1.370(6)
110.0(4)
110.95
106.3(4)
120.1(4)
106.2(4)
26.4(6)
30.3(7)
3.3(7)
1.369(2)
1.4544(19)
1.282(2)
1.362(5)
110.76(12)
108.93
106.84(11)
120.58(12)
107.5(3)
35.4(3)
–
–
110.29(12)
–
106.39(8)
120.17(4)
–
O1–H3ꢀ ꢀ ꢀC3
N1–B1–N2
C(4)–C(5)–C(6)
C(10)–O(1)–C(13)
C(4)–C(5)–C(10)–O(1)
C(6)–C(5)–C(10)–C(11)
C(10)–C(5)–C(6)–C(7)
C(3)–C(4)–C(5)–C(10)
–
52.5(3)
6.2(4)
3.5(4)
25.1(3)
2.4(3)
1.4(3)
31.0(3)
8.8(3)
2.7(2)
7.6(7)
a
Hc is H3/H7.
probably due to further enhancement of conjugation in BODIPY
unit because of the presence of aryl substituents at 3,5-positions.
Compounds 4 and 5 also exhibited similar hypsochromic shifts
on changing the polarity of the solvent.
The electrochemical properties of BODIPYs 1, 2, 4 and 5 were
investigated by cyclic voltammetry at a scan rate of 50 mV/s using
4
5
1
2
1.0
0.8
0.6
0.4
0.2
tetrabutylammonium perchlorate as supporting electrolyte.
A
comparison of reduction waves of compounds 1, 2, 4 and 5 is
shown in Fig. 5 and redox potential data are presented in Table 4.
The BODIPYs generally show one irreversible oxidation, one revers-
ible reduction and one quasi-reversible reduction. As clearly re-
vealed from Fig. 5 and data in Table 4 that the first reduction
potential of the meso-furyl boron dipyrromethene 2 is shifted to-
wards less negative compared to meso-tolyl boron dipyrromethene
1 by ꢁ165 mV. This suggests that the compound 2 is much easier
to reduce as compared to compound 1. However, the compound
4 containing two phenyl groups at 3,5-positions is difficult to re-
duce than compound 2 whereas the compound 5 containing two
0.0
400
500
600
700
800
Wavelength (nm)
Fig. 4. Comparison of normalized absorption spectra of compounds 1, 2, 4 and 5
recorded in toluene.
Table 3
Photophysical data of BODIPY compounds 1, 2, 4 and 5 in different solvents. Concentration used was 5 ꢃ 10ꢂ6 M.
Compound
Solvent
kabs (nm)
e
FWHM (cmꢂ1
)
kem (nm)
D
m
st (cmꢂ1
)
U
1
Toluene
CHCl3
THF
Ethyl acetate
MeOH
MeCN
502
501
502
497
496
495
4.68
4.73
4.68
4.74
4.76
4.75
1166
1085
1379
1200
1226
1240
520
517
521
514
513
513
690
618
626
665
668
709
0.053
0.042
0.023
0.017
0.017
0.010
2
4
5
Toluene
CHCl3
THF
Ethyl acetate
MeOH
MeCN
525
523
520
518
516
516
4.40
4.50
4.48
4.49
4.49
4.50
1819
1703
1628
2015
2058
1352
573
568
569
563
562
564
1596
1515
1656
1543
1587
1650
0.058
0.057
0.036
0.050
0.032
0.027
Toluene
CHCl3
THF
Ethyl acetate
MeOH
MeCN
580
577
576
576
569
566
4.72
4.72
4.73
4.77
4.61
4.68
2154
2335
2464
2177
2223
2223
648
647
645
635
626
624
1810
1876
1858
1887
1600
1642
0.025
0.021
0.033
0.021
0.016
0.020
Toluene
CHCl3
THF
Ethyl acetate
MeOH
MeCN
647
645
641
636
634
632
4.61
4.55
4.64
4.61
4.62
4.48
1985
2303
1975
1993
2235
2428
684
680
675
674
673
672
836
798
786
887
914
942
0.012
0.016
0.042
0.037
0.055
0.041

T.K. Khan et al. / Inorganica Chimica Acta 383 (2012) 257–266
263
Table 4
Electrochemical redox data (V) of compounds 1, 2, 4 and 5 in dichloromethane
containing 0.1 M TBAP as supporting electrolyte.
Compound
Oxidation
Reduction
I
II
1
2
4
5
–
ꢂ0.788
ꢂ0.622
ꢂ0.736
ꢂ0.628
ꢂ1.81
ꢂ1.74
ꢂ1.63
ꢂ1.56
1.41
1.36
1.14
The fluorescence properties of 1, 2, 4 and 5 are also studied in
six solvents (Table 3). The comparison of normalized fluorescence
spectra of compounds 1, 2, 4 and 5 recorded in toluene is shown in
Fig. 6a. The compounds 2, 4 and 5 exhibited significant differences
in fluorescence properties compared to 1 which are outlined as fol-
lows: (1) the emission band of compound 2 exhibited 1778 cmꢂ1
red shift compared to 1. However, among meso-furyl boron-dip-
yrromethenes 2, 4 and 5, the compound 5 showed maximum red
shift in emission maxima; (2) the fluorescence band of 2, 4 and 5
is very broad compared to 1 (Fig. 6a); (3) compound 2 exhibited
large Stokes shift compared to 1 (Fig. 6b and c). Among compounds
2, 4 and 5, compound 5 showed less Stokes shift and 4 showed
maximum Stokes shift; (4) the quantum yield of 2 and 1 is almost
same whereas the compounds 4 and 5 showed lower quantum
yield compared to 2. The observations were similar in all six sol-
vents. The lower quantum yield of compound 4 requires detailed
investigation. The large red-shifts observed for meso-furyl BODIPYs
Fig. 5. Comparison of reduction wave of cyclic voltammograms of compounds 1, 2,
4 and 5 recorded in CH2Cl2 containing 0.1 M TBAP as the supporting electrolyte
recorded at a scan speed of 50 mV/s.
thienyl groups at 3,5-positions is equally easier to reduce like com-
pound 2. Thus, the electrochemical study clearly indicated that
meso-furyl BODIPYs 2-5 were easier to reduce compared to the
meso-tolyl BODIPY 1.
2, 4 and 5 are attributed to the enhancement of p-electron conju-
gation due to increased co-planarity of meso-furyl group with
(a)
4
5
2
1
1.0
0.8
0.6
0.4
0.2
0.0
550
600
650
700
750
800
Wavelength (nm)
50 (nm)
(c)
(b)
12 (nm)
1.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
450
500
550
600
650
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 6. (a) Comparison of normalized emission spectra of compounds 1, 2, 4 and 5 recorded in toluene using kex = 488 nm. (b) Comparison of absorption (—) and emission
(----) spectra of 1 in toluene. (c) Comparison of absorption (—) and emission (----) spectra of 2 in toluene.

264
T.K. Khan et al. / Inorganica Chimica Acta 383 (2012) 257–266
cence properties. It was found through, both the experimental
facts and computational approach that on replacement of six mem-
bered meso-aryl group with five membered furyl group alter the
electronic properties of the boron-dipyrromethene dye signifi-
cantly. The introduction of meso-furyl group in place of meso-phe-
nyl group lead to down field shifts in the chemical shifts of pyrrole
protons, bathochromic shifts in absorption and emission bands
with increase in full width at half maximum, relatively large
Stoke’s shifts and easier reductions. These changes in meso-furyl
BODIPYs were attributed to in-plane orientation of meso-furyl
group with the plane of the boron-dipyrromethene and assisting
the enhancement of
p-conjugation unlike meso-tolyl boron-dip-
yrromethene in which the six membered tolyl group is not in plane
with the boron-dipyrromethene and preventing the
as confirmed by their X-ray crystal structures.
p-conjugation
4. Experimental
4.1. General
Fig. 7. Orbital-correlation diagram of HOMO and LUMO (FMO’s) for compounds 1,
2, 4 and 5.
The known compounds 2, 3, 6 and 7 were prepared by following
the reported procedures [33,37]. THF and toluene were dried over
sodium benzophenone ketyl and chloroform, ethyl-acetate, meth-
anol, acetonitrile were dried over calcium hydride and distilled
prior to use. BF3ꢀOEt2 and 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none (DDQ) obtained from Spectrochem (India) were used as ob-
tained. All other chemicals used for the synthesis were reagent
grade unless otherwise specified. Column chromatography was
performed on silica (60–120 mesh).
boron-dipyrromethene unit and also partly because of the pres-
ence of substituents at 3,5-positions. The large Stokes shifts noted
for 2 and 4 in different solvents support the enhancement of
Franck-Condon factor due to substantial structural reorganization
in the excited state.
In order to understand the differences between the electronic
properties of compounds 1, 2, 4 and 5, quantum chemical calcula-
tions were performed using DFT method. A qualitative analysis of
the electronic distribution in HOMO and LUMO states (Fig. 7) on
these molecules suggested that there were very minute differences
between compounds 1 and 2. These compounds were found to be
very similar in electronic distribution in HOMO and LUMO states
particularly in the main BODIPY nucleus. A close examination of
the LUMO states of compounds 1 and 2 revealed that electrons
were more delocalized in 2-furyl ring as compared to p-tolyl group.
Such a comparison clearly revealed that the 2-furyl group is in-
volved in FMOs (HOMOs and LUMOs) to a greater extent to bring
down the gap in the energies through some electronic interaction.
For compounds 4 and 5, the significant changes were observed in
the HOMO and LUMO electronic distribution. For these com-
pounds, the electronic distribution in the HOMO and LUMO states
was delocalized away from the BODIPY nucleus. Such an effect can
4.2. Instrumentation
1H NMR spectra (d in ppm) were recorded using Varian VXR 300
and 400 MHz and Bruker 400 MHz NMR spectrometer. 13C NMR
spectra were recorded on Varian and Bruker spectrometer operat-
ing at 100.6 MHz. 19F NMR spectra were recorded on Varian spec-
trometer operating at 282.2 MHz. 11B NMR spectra were recorded
on Varian spectrometer operating at 96.3 MHz. TMS was used as an
internal reference for recording 1H (of residual proton; d 7.26) and
13C (d 77.0 signal) in CDCl3. Absorption and steady-state fluores-
cence spectra were obtained with Perkin–Elmer Lambda-35 and
PC1 Photon Counting Spectrofluorometer manufactured by ISS,
USA instruments, respectively. Fluorescence spectra were recorded
at 25 °C in a 1 cm quartz fluorescence cuvette. The fluorescence
quantum yields (Uf) were estimated from the emission and
absorption spectra by comparative method at the excitation wave-
length of 488 nm using Rhodamine 6G (Uf = 0.88) [31] as standard.
Cyclic voltammetric (CV) and differential pulse voltammetric
(DPV) studies were carried out with electrochemical system utiliz-
ing the three electrode configuration consisting of a glassy carbon
(working electrode), platinum wire (auxiliary electrode) and satu-
rated calomel (reference electrode) electrodes. The experiments
were done in dry dichloromethane using 0.1 M tetrabutylammo-
nium perchlorate as supporting electrolyte. Half wave potentials
were measured using DPV and also calculated manually by taking
the average of the cathodic and anodic peak potentials. MALDI-TOF
spectra were obtained from Axima-CFR manufactured by Kratos
Analyticals.
be attributed to be arisen through extension of p-conjugation. The
quantitative differences were found among the compounds 1 and 2
for their HOMO, LUMO energies and their gaps. It is interesting to
note that the values for HOMO–LUMO gap of compound 2 is less
than 1 indicating the existence of some stabilizing force. Com-
pounds 4 and 5 were found to have least energy gaps between
the FMOs. As shown in Fig. 7, the gaps in the energies of HOMOs
and LUMOs follow the order as 1 > 2 > 4 > 5. The decrease in the
HOMO–LUMO energy gap is in agreement with the observed red
shifts of absorption and emission bands. The decrease in the reduc-
tion potential of compounds 2, 4 and 5 compared to 1 is clearly re-
flected through the stabilization of LUMO states of these
compounds. Precisely speaking, for compounds 4 and 5, this effect
was solely observed due to elevation of HOMO energies (Table S2
in Supporting information). Thus the quantum mechanics based
studies of HOMO and LUMO states of these molecules were found
to be in agreement with the experimental results.
4.3. X-ray diffraction studies
3. Conclusions
The single crystals of compounds 2 (CCDC-768790), 3 (CCDC-
768788) and 4 (CCDC-768789) were obtained on slow evaporation
of n-hexane/dichloromethane over a period of one week. The
intensity data collection for compounds 2, 3 and 4 have been
We described the synthesis of four meso-furyl boron-dipyrro-
methene dyes and their absorption, electrochemical and fluores-

T.K. Khan et al. / Inorganica Chimica Acta 383 (2012) 257–266
265
carried out on a Nonius MACH3 four circle diffractometer at 293 K.
Acknowledgment
Structure solutions for the compounds 2, 3 and 4 were obtained
using direct methods (SHELXS-97) [46] and refined using full-ma-
trix least-squares methods on F2 using SHELXL-97 [47]. Com-
pounds 2 and 3 were crystallized with two molecules in the
asymmetric unit. There are no statistically significant differences
in the metrical parameters for the two molecules. In each mole-
cule, the furyl ring is disordered over two conformations.
M.R. thanks CSIR and DST for financial support. T.K.K. thanks In-
dian Institute of Technology for fellowship. We thank the DST
funded National Single Crystal X-ray Diffraction Facility for diffrac-
tion data. We also thank Dr. Evans Coutinho (Bombay College of
Pharmacy) for providing computational facility.
Appendix A. Supplementary material
4.4. Computational studies
1H, 1H–1H COSY, HSQC, HMBC, 13C, 19F and 11B NMR spectra and
mass spectra of all compounds. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.11.017.
The computational studies were performed with GAUSSIAN 03 [48]
installed on a windows operating system. The structures were ex-
tracted from the X-ray diffraction output files generated. Initial
structural rectifications, atom and bond typing and addition of
hydrogen atoms were done with Chem3D utility in Chemoffice.
For these studies, some conformers representing two possible rota-
tions of meso-furyl ring around BODIPY nucleus were also gener-
ated from the structures as templates. The structures were
energy optimized using quantum mechanics with density func-
tional theory (DFT) and B3LYP gradient corrected correlation func-
tional method in conjugation with standard 6-31G (p,d) basis set
and parameters. For the optimized structures, we carried out com-
plete full population analyses.
References
[1] R. Ziessel, G. Ulrich, A. Harriman, New J. Chem. 31 (2007) 496.
[2] A. Loudet, K. Burgess, Chem. Rev. 107 (2007) 4891.
[3] G. Ulrich, R. Ziessel, A. Harriman, Angew. Chem., Int. Ed. 47 (2008) 1184.
[4] M. Kollmannsberger, T. Gareis, S. Heinl, J. Daub, J. Breu, Angew. Chem., Int. Ed.
36 (1997) 1333.
[5] K. Rurack, M. Kollmannsberger, U. Resch-Genger, J. Daub, J. Am. Chem. Soc. 122
(2000) 968.
[6] T. Matsumoto, Y. Urano, T. Shoda, H. Kojima, T. Nagano, Org. Lett. 9 (2007)
3375.
[7] S. Zrig, P. Remy, B. Andrioletti, E. Rose, I. Asselberghs, K. Clays, J. Org. Chem. 73
(2008) 1563.
[8] A.C. Benniston, G. Copley, A. Harriman, D.B. Rewinska, R.W. Harrington, W.
Clegg, J. Am. Chem. Soc. 130 (2008) 7174.
4.5. 3,5-Diphenyl-8-(2-furyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (4)
[9] E. Lager, J. Liu, A. Aguilar-Aguilar, B.Z. Tang, E. Peña-Cabrera, J. Org. Chem. 74
(2009) 2053.
[10] X. Yin, L. Yongjun, L. Yuliang, Y. Zhu, X. Tang, H. Zheng, D. Zhu, Tetrahedron 65
(2009) 8373.
[11] R. Ziessel, P. Retailleau, K.J. Elliott, A. Harriman, Chem. Eur. J. 15 (2009) 10369.
[12] M.R. Rao, K.V.P. Kumar, M. Ravikanth, J. Organomet. Chem. 695 (2010) 863.
[13] A. Coskun, M.D. Yilmaz, E.U. Akkaya, Org. Lett. 9 (2007) 607.
[14] A. Loudet, R. Bandichhor, K. Burgess, A. Palma, S.O. McDonnell, M.J. Hall, D.F.
O’Shea, Org. Lett. 10 (2008) 4771.
Two neck 100 mL round bottom flask fixed with a reflux con-
denser was degassed with N2 for few minutes. Samples of 3
(100 mg, 0.60 mmol), phenyl-boronic acid (387 mg, 3.04 mmol)
and Na2CO3 (256 mg, 2.44 mmol) in water/THF/toluene (1:1:1)
15 ml was stirred under N2 for 5 min. Pd(PPh3)4 (68 mg, ꢁ5–
10 mol%), was added and the reaction mixture was refluxed at
80 °C. After completion of the reaction as judged by TLC analysis,
the reaction mixture was diluted with water (5 ml) and extracted
with diethyl ether. The combined organic layers were washed with
water, brine and dried over MgSO4. The solvent was evaporated
and the crude product was purified on a silica gel column using
petroleum ether/ethyl acetate 95:5 to afford 3,5-diphenyl-8-(phe-
nyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene 4 in 75% yield. 1H
NMR (400 MHz, CDCl3, d in ppm): 6.68 (d, J = 4.27 Hz, 2H, pyrrole),
6.73 (s, 1H, furan), 7.09 (d, J = 3.36 Hz, 1H, furan), 7.40–7.44 (m, 8H,
phenyl), 7.83 (s, 1H, furan), 7.86 (d, J = 6.72 Hz, 4H, pyr-
role + phenyl). 13C NMR (100 MHz, CDCl3, d in ppm): 113.0,
118.8, 121.0, 128.3, 129.5, 130.3, 132.9, 134.0, 146.7, 149.0,
158.5, 168.2. 19F NMR (282.2 MHz, CDCl3, d in ppm): ꢂ132.99 (q,
JB-F = 64.0 Hz). 11B NMR (96.3 MHz, CDCl3, d in ppm): 1.57 (t,
JB-F = 32.0 Hz). MALDI-TOF: m/z = 408.5 [Mꢂ1]+. HRMS calcd for
(C25H17BF2N2O): 391.1418 [Mꢂ19]+ found 391.1399 [Mꢂ19]+.
[15] A. Palma, M. Tasior, D.O. Frimannsson, T. Vu, R. Méallet-Renault, D.F. O’Shea,
Org. Lett. 11 (2009) 3638.
[16] J. Murtagh, D.O. Frimannsson, D.F. O’Shea, Org. Lett. 11 (2009) 5386.
[17] B.A. Trofimov, Adv. Heterocycl. Chem. 51 (1990) 177.
[18] A.B. Zaitsev, R. Meallet-Renault, E.Y. Schmidt, A.I. Mikhaleva, S. Barde, C.
Dumas, A.M. Vasil’tsov, N.V. Zorina, R.B. Pansu, Tetrahedron 61 (2005) 2683.
[19] K. Rurack, U. Resch-Genger, Chem. Soc. Rev. 31 (2002) 116.
[20] Y. Mei, P.A. Bentley, W. Wang, Tetrahedron Lett. 47 (2009) 2447.
[21] A. Burghart, H. Kim, M.B. Welch, T. Joe Reibenspies, K. Burgess, J. Org. Chem. 64
(1999) 7813.
[22] L. Bonardi, G. Ulrich, R. Ziessel, Org. Lett. 10 (2008) 2183.
[23] M. Bröring, R. Krüger, S. Link, C. Kleeberg, S. Köhler, X. Xie, B. Ventura, L.
Flamigni, Chem. Eur. J. 14 (2008) 2976.
[24] T. Kowada, S. Yamaguchi, K. Ohe, Org. Lett. 12 (2010) 296.
[25] A. Nagai, Y. Chujo, Macromolecules 43 (2010) 193.
[26] S.H. Choi, K. Kim, J. Lee, Y. Do, D.G. Churchill, J. Chem. Crystallogr. 37 (2007)
315.
[27] E. Peña-Cabrera, A. Aguilar-Aguilar, M.G. Domínguez, E. Lager, R.Z. Vázquez,
J.G. Vargas, F.V. Garcia, Org. Lett. 9 (2007) 3985.
[28] S.H. Choi, K. Kim, J. Jeon, B. Meka, D. Bucella, K. Pang, S. Khatua, J. Lee, D.G.
Churchill, Inorg. Chem. 47 (2008) 11071.
[29] K. Umezawa, Y. Nakamura, H. Makino, D. Citterio, K. Suzuki, J. Am. Chem. Soc.
130 (2008) 1550.
[30] J.A. Jacobsen, J.R. Stork, D. Magde, S.M. Cohen, Dalton Trans. 39 (2010) 957.
[31] W. Qin, V. Leen, T. Rohand, W. Dehaen, P. Dedecker, M. Van, K. der Auweraer, L.
Robeyns, D. Van Meervelt, B. Beljonne, J.N. Van Averbeke, K. Clifford, K.
Driesen, N. Binnemans, N. Boens, J. Phys. Chem. A 113 (2009) 439.
[32] 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 113 (2009) 11731.
[33] K. Kim, C. Jo, S. Easwaramoorthi, J. Sung, D.H. Kim, D.G. Churchill, Inorg. Chem.
49 (2010) 4881.
[34] A. Nagai, J. Miyake, K. Kokado, Y. Nagata, Y. Chujo, J. Am. Chem. Soc. 130 (2008)
15276.
4.6. 3,5-Di(2-thienyl)-8-(2-furyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (5)
Compound 5 was prepared by following the same procedure as
for compound 4 using thiophene-2-boronic acid. The crude com-
pound was purified on silica column using petroleum ether as an
eluent and obtained the pure compound 5 in 33% yield. 1H NMR
(400 MHz, CDCl3,
d in ppm): 6.67 (s, 1H, furan), 6.86 (d,
J = 4.58 Hz, 2H, pyrrole), 6.98 (d, J = 2.74 Hz, 1H, furan), 7.18 (m,
2H, thiophene), 7.32 (d, J = 3.97 Hz, 2H, thiophene), 7.48 (d,
J = 4.27 Hz, 2H, pyrrole), 7.76 (s, 1H, furan), 8.18 (d, J = 3.05 Hz,
2H, thiophene). 19F NMR (282.2 MHz, CDCl3, d in ppm): ꢂ139.68
(q, JB-F = 64.0 Hz). 11B NMR (96.3 MHz, CDCl3, d in ppm): 1.86 (t,
JB-F = 32.07 Hz). MALDI-TOF: m/z = 420.6 [Mꢂ1]+. HRMS calcd for
(C21H13BF2N2OS2): 403.0546 [Mꢂ19]+ found: 403.0548 [Mꢂ19]+.
[35] H.L. Kee, C. Kirmaier, L. Yu, P. Thamyongkit, W.J. Youngblood, 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 109 (2005) 20433.
[36] R.W. Wagner, J.S. Lindsey, Pure Appl. Chem. 68 (1996) 1373.
[37] T.K. Khan, M.R. Rao, M. Ravikanth, Eur. J. Org. Chem. (2010) 2314–2323.
[38] M. Kollmannsberger, K. Rurack, U. Resch-Genger, J. Daub, J. Phys. Chem. A 102
(1998) 10211.
[39] F. Amat-Guerri, Chem. Phys. Lett. 299 (1999) 315.

266
T.K. Khan et al. / Inorganica Chimica Acta 383 (2012) 257–266
[40] K. Rurack, M. Kollmannsberger, J. Daub, Angew. Chem., Int. Ed. 40 (2001) 385.
[41] R.Y. Lai, A.J. Bard, J. Phys. Chem. B 107 (2003) 5036.
[46] G.M. Sheldrick, SHELXS-97: Program for Crystal Structure Solution, University
of Göttingen, Germany, 1997.
[42] F. López Arbeloa, J. Bañuelos Prieto, V.M. Martinez, T. Arbeloa López, I. López,
Chem Phys Chem. 5 (2004) 1762.
[47] G. M. Sheldrick, SHELXL-97: Program for crystal structure refinement;
University of Göttingen, Germany, 1997.
[43] B. Ventura, G. Marconi, M. Bröring, R. Krüger, L. Flamigni, New J. Chem. 33
(2009) 428.
[48] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M. Robb, J.R. Cheeseman,
et al., GAUSSIAN03, Gaussian, Inc., Wallingford CT, 2004.
[44] K. Guzow, K. Kornowska, W. Wiczk, Tetrahedron Lett. 50 (2009) 2908.
[45] K. Cies´lik-Boczula, K. Burgess, L. Li, B. Nguyen, L. Pandey, W.M. De Borggraeve,
M. Van der Auweraer, N. Boens, J. Photochem. Photobiol. Sci. 8 (2009) 1006.