Bridged bis-BODIPYs: Their synthesis, structures and properties

Article abstract of DOI:10.1039/c5dt01925g

A series of bis-BODIPYs 1-6 bridged via thiophene, furan, N-alkylcarbazole, triphenyl-amine, para- and meta-phenylene groups have been synthesized and characterized by various spectroscopic techniques. The change in the spectroscopic properties of bis-BODIPYs upon varying the size of spacers was studied. X-ray crystal structures of three bis-BODIPYs containing triphenylamine, para- and meta-phenylene bridges were solved. Intermolecular C(H)...π and π...π stacking interactions were observed in solid state structures of three bis-BODIPYs. The dihedral angles between the spacer unit and two boron-dipyrrin units were lower in all three compounds as compared to their corresponding monomers. This suggests increased interactions between the two boron-dipyrrin units in molecules which are in turn reflected in the anodic shifts in their reduction potentials. DFT studies indicated effective electronic interactions between spacers and two boron dipyrrin units in all the bis-BODIPYs. The calculated HOMO-LUMO gap was found to be lower for bis-BODIPY having bulky carbazole spacers and higher for bis-BODIPY having smaller furan spacers. Changing the spacer size clearly affected the spectroscopic properties of the bis-BODIPYs and red shifted absorption and emission maxima were observed for bis-BODIPYs with furan and thiophene spacers as compared to bis-BODIPYs with phenylene or bulky aromatic spacers.

Full text of DOI:10.1039/c5dt01925g

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DOI: 10.1039/C5DT01925G
TOC Graphic:
TOC Abstract:
The synthesis and studies of bridged bis-BODIPYs having spacers like thiophene, furan, N-
butylcarbazole, phenylene and triphenylamine are reported.

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DOI: 10.1039/C5DT01925G
Bridged Bis-BODIPYs: Synthesis, Structures and Properties
Praseetha E. Kesavan,†Sudipta Das,†Mohsin Y. Lone,‡Prakash C. Jha,‡ Shigeki Mori§and Iti Gupta*†
†*Correspondence to: Dr. Iti Gupta, Indian Institute of Technology Gandhinagar, VGEC Campus, Chandkheda,
Ahmedabad-382424, India. Phone: +91-9925479623, Fax: +91-23972324, E-mail: iti@iitgn.ac.in
‡School of Chemical Sciences, Central University of Gujarat, Gandhinagar382030, Gujarat, India
§Integrated Centre for Sciences, Ehime University, Matsuyama 790-8577, Japan
Abstract
A series of Bis-BODIPYs 1-6 bridged via thiophene, furan, N-alkylcarbazole, triphenyl-
amine, para- and meta-phenylene groups have been synthesized and characterized by various
spectroscopic techniques. The change in spectroscopic properties of Bis-BODIPYs upon
varying the size of spacer was studied. X-ray crystal structures of three Bis-BODIPYs
containing triphenylamine, para- and meta-phenylene bridges were solved. Intermolecular
C(H)----and----stacking interactions were observed in solid state structures of three Bis-
BODIPYs. The dihedral angles between the spacer unit and two borondipyrrin units were
lower in all three compounds as compared to their corresponding monomers. This suggests
increased interactions between the two boron-dipyrrin units in molecules which in turn
reflected in the anodic shifts in their reduction potentials. DFT studies indicated effective
electronic interactions between spacers and two boron dipyrrins units in all the Bis-
BODIPYs. The calculated HOMO-LUMO gap was found to be lower for Bis-BODIPY
having bulky carbazole spacer and higher for Bis-BODIPY having smaller furan spacer. The
change in spacer size clearly affected the spectroscopic properties of the Bis-BODIPYs and
red shifted absorption and emission maxima were observed for Bis-BODIPYs with furan and
thiophene spacers as compared to Bis-BODIPYs with phenylene or bulky aromatic spacers.
Key Words: Bis-BODIPYs, BODIPY Dimers, X-ray structures, DFT studies, Bridged
BODIPYs
Introduction
Covalently linked pairs of chromophores have been a matter of interests for the scientific
community from long time. Such chromophore dimers have been found in biology, such as
photosynthetic bacterial proteins and natural light harvesting antenna systems.1-3 Numerous
artificial fluorescent dyes have been used in biosciences for cell imaging and investigation of
in-vivo biological process. The boron dipyrromethenes (BODIPYs) are well known class of
1

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highly emissive molecules and they have been found extremely useful for biological
DOI: 10.1039/C5DT01925G
applications, e.g. biochemical labelling,4-6 photosensitizers for PDT (Photo Dynamic
Therapy)7 and fluorescent probes.8-10 The electronic and photophysical properties of
BODIPYs can be fine-tuned for different applications by structural modification of boron-
dipyrrin core or by making their dimers and oligomers.11 The substitution at meso positions of
the boron dipyrrin core with bulky electron rich heterocycles could produce large Stokes
Shifts of 100-120 nm.12 The - linked dimers, meso-meso linked dimers and cofacial
BODIPY dimers have been synthesised and studied.13-15 Such artificial BODIPY dimers have
increased Stokes Shifts, broad absorption and red shifted emission for efficient light
harvesting ability for solar cells and biological applications. Recently Akkaya et al. have
demonstrated that meso-meso and meso-beta linked BODIPY dimers can be used as singlet
oxygen photosensitizer in nonpolar organic media.16 Li and co-workers have synthesized
linked BODIPY dimers with triphenylsilylphenyl groups for increased solid state
emission.17 Bithiophene bridged BODIPY dimers have been prepared by Ziessel and co-
workers with red shifted absorption and emission for future applications as NIR fluorescent
probes.18 The interesting symmetry breaking ICT (intramolecular charge transfer) properties
have been observed for p-phenylene bridged -alkylated BODIPY dimer.19 Nishihara and co-
workers have reported p-diethynylphenyl bridged BODIPY dimer and meso-alkynyl
BODIPY with red shifted emission maxima.20 Liu et al. have synthesized triphenylamine
BODIPY monomer, bridged dimer and trimer in one pot reaction and their lasing properties
in organic media were studied.21 BODIPY dimers having bipyridine spacers with interesting
ECL (electrogenerated chemiluminescence) properties have been reported by Bard and co-
workers.22 Benniston and co-workers have synthesized cofacial BODIPY dimers containing
dibenzothiophene and dibenzofuran moieties; such dimers formed intramolecular excimer in
emission studies.23 Saki et al. have designed cofacial BODIPY dimers based on xanthene
scaffold with efficient energy transfer between the two boron dipyrrins units.24 The reports
on covalently linked BODIPYs dimers are on rise but still there is a need to study the
photophysical properties of meso-phenylene and heterocycle bridged BODIPY dimers. In this
article we present the synthesis, crystal structures, photophysical and electrochemical
properties of thiophene, furan, N-butyl carbazole, triphenylamine, p- and m-phenylene
bridged Bis-BODIPYs. Also, DFT studies were carried out for all six Bis-BODIPYs and the
HOMO-LUMO energy gap was calculated.
Results and Discussion
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The stepwise synthesis of Bis-BODIPYs 1-6 is depicted in Schemes 1-3. The corresponding
DOI: 10.1039/C5DT01925G
key precursors like bis-aromatic aldehydes were prepared by Vilsmeier Haack formylation
reaction and bis-dipyrranes 7-12 were synthesized as per the reported procedure.25
Terephthalaldehyde and isophthalaldehyde were condensed with pyrrole in the presence of
catalytic amount indium chloride to form bis-dipyrranes 7 and 8 respectively (Scheme 1).
Crude dipyrranes 7 and 8 were purified by silica gel column chromatography using 1:2:4
mixture of ethyl acetate/dichloromethane/petroleum ether in 29% and 46% yields
respectively. The corresponding bis-aldehydes for bis-dipyrranes 9 and 10 were prepared by
Vilsmeier Haack formylation reaction as per the reported procedure25 (ESI). Bis-dipyrranes 9
and 10 were synthesized by indium chloride method25 followed by silica gel column
purification to yield 55% and 41% respectively (Scheme 2). The bis-dipyrranes 7-10 were
oxidized by 2.4 equivalents of DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) and the
resulting bis-dipyrrins were then complexed with BF3.OEt2 by doubling the equivalents of
reagents required for the standard BODIPY synthetic method.26
Scheme 1. Synthetic route for Bis-BODIPYs 1 and 2.
Scheme 2. Synthetic route for Bis-BODIPYs 3 and 4.
3

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DOI: 10.1039/C5DT01925G
Scheme 3. Synthetic route for Bis-BODIPYs 5 and 6.
Silica gel column chromatographic purification afforded Bis-BODIPYs 1-4 as solid powders
in 8% to 34% yields (Schemes 1, 2). In order to synthesize Bis-BODIPYs 5 and 6, bis-formyl
derivative of thiophene and furan were prepared (Scheme 3) by dilithiation of thiophene and
furan followed by addition of dry DMF (N,N-dimethylformaldehyde). The bis-
formylthiophene and furan were subjected to acid catalysed condensation with pyrrole to
obtain the corresponding bis-dipyrranes 11 and 12. The crude bis-dipyrrane 11 and 12 were
directly used further for the next step without any purification due to their instability in
column conditions. The bis-dipyrranes 11 and 12 were reacted with DDQ followed by the
complexation with BF3.OEt2 in presence of TEA (triethylamine, Scheme 3). Silicagel column
chromatographic purification of the crude reaction mixture in 60-70%
dichloromethane/petroleum ether mixture yielded 8% of Bis-BODIPY 5 as dark orange
powder and 10% of Bis-BODIPY 6 as dark brown powder.
The bis-dipyrranes 7-10 were characterized by HRMS and 1H NMR spectroscopy. All
1
Bis-BODIPYs 1-6 were characterized by HRMS, H, 13C, 19F and 11B spectroscopy (ESI).
Molecular ion peaks in HRMS spectra of bis-dipyrranes 7-10 and bis-BODIPYs 1-6
confirmed the formation of these compounds (ESI). Also, single crystal X-ray structures were
1
solved for Bis-BODIPYs 1, 2 and 4. Typically in the H NMR of bis-dipyrranes 7 to 10 the
four N-H protons appeared around 7.8-7.9 ppm as one singlet. The four -pyrrole protons
showed up between 6.65-6.83 ppm, whereas eight -pyrrole protons gave two separate
signals around 5.92-6.14 ppm. The two protons attached to sp3 hybridized carbon (C-H
protons) appeared around 5.45-5.29 ppm as one singlet (ESI). The appearance of
characteristic signals for the aromatic protons present in bridging units also confirmed the
4

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1
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formation of bis-dipyrranes 7-10. The formation of Bis-BODIPY 1-6 was evidenDtOiIn: 10th.10e3i9r/C5HDT01925G
1
NMR spectra. The representative H, 19F and 11B NMR of Bis-BODIPY 6 is presented in
Figure 1. The NMR spectra (a) 1H, (b) 19F and (c) 11B of Bis-BODIPY 6 in CDCl3.
Figure 1. Compared to their corresponding dipyrranes proton NMRs, two protons (C-H)
attached to sp3 hybridized carbon and four N-H protons were missing in all the Bis-BODIPYs
1-6. In the proton NMR of Bis-BODIPY 6 (Figure 1a) all the eight -pyrrole protons
appeared as two doublets at 6.64 and 7.68 ppm and the four -pyrrole protons appeared as a
singlet at 7.99 ppm. The two -furan protons were showed up as a singlet at 7.32 ppm. As
compared to the corresponding bis-dipyrranes 7-10, the four -pyrrole protons signal was
downfield shifted and appeared around 7.93-8.01 ppm in the Bis-BODIPYs. In all Bis-
BODIPYs the eight -pyrrole protons showed up as two separate signals between at 6.61 -
7.02 ppm. In all the Bis-BODIPYs the aromatic protons of bridging spacers showed up in the
range of 7.29 -7.59 ppm. The 19F NMR of all Bis-BODIPYs 1-6 were recorded in CDCl3 and
fluorine resonance signals splitted into quartets due to the coupling with the adjacent boron
19
19
atom (11B, I= 3/2).6 In F NMR analysis of all Bis-BODIPYs 1-6, the F quartets appeared
around -145 ppm and only one 19F signal was observed due to symmetrical nature of the
11
11
molecules (Figure 1b). B NMR of all Bis-BODIPY 1-6 exhibited a triplet for B between
0.18 and 0.36 ppm (Figure 1c).
5

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DOI: 10.1039/C5DT01925G
Single crystal X-ray diffraction studies
The X-ray structures of Bis-BODIPYs 1, 2 and 4 were determined. ORTEP views of Bis-
BODIPY 1, Bis-BODIPY 2 and Bis-BODIPY 4 are given in Figure 2. The single crystals of
three Bis-BODIPYs were obtained by slow evaporation of the chloroform /n-heptane solution
over a period of two weeks. The crystal structure and data refinement parameters for 1, 2 and
4 are displayed in ESI (Table S38). Bis-BODIPY 1 (CCDC 1006185) gave red prism shaped
crystals after solvent evaporation. Bis-BODIPY 1 was crystallised in monoclinic crystal
system with P21/c space group. The values of N-B-N and F-B-F angles were 106.03 o(12) and
109.12 o(13) respectively. Here two boron dipyrrin units were aligned in the same plane and
the bridging phenylene ring was in a different plane. The dihedral angle between the boron
dipyrrin units and the phenylene ring (C4-C5-C11-C12) was 54.00o(18). The dihedral angle
was comparatively lower than the meso-phenyl BODIPY27 (reported value was 60.80o). Bis-
BODIPY 2 (CCDC 1006186) gave an orange rectangle shaped crystals on crystal growth.
The crystals were monoclinic type with I2/a(#15) space group.
(a)
(b)
(c)
Figure 2. ORTEP views of (a) Bis-BODIPY 1; (b) Bis-BODIPY 2 and (c) Bis-BODIPY 4.
Displacements are drawn at 50% probability level.
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Table 1. Details of intermolecular interactions in the crystal structures of Bis-BODIPYsD1O,I:210a.1n0d394/C.5DT01925G
No. Interaction
Distance (Ao)
Angle (o)
141.73
132.79
151.41
177.11
160.80
173.93
_
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
9
10
11
1
2
3
4
5
6
7
2.69
C(1)-H(1)---  (C(12) phenyl))
C(2)-H(2)--- (C(7) pyrrolic)
C(3)-H(3)---B(1)
2.84
3.09
C(3)-H(3)---F(1)
2.42
C(7)-H(7)---F(1)
2.39
2.71
C(10)-H(10)--- (C(3) pyrrolic)
(C(1) pyrrolic) (C(9) pyrrolic)
(C(2) pyrrolic) (C(7) pyrrolic)
C(1)-H(1)---(C(6) pyrrolic)
C (1)-H(1)---N(2)
3.56(3)
3.55(3)
2.68
_
166.51
149.63
164.40
138.83
_
2.64
C(3)-H(3)--- F(1)
2.30
C(3)-H(3)---B(1)
3.06
C(7)---F(1)
3.11(17)
2.58
C(8)-H(8)---F(2)
117.53
_
C(8)---F(2)
3.13(17)
2.64
C(12)-H(12)---F(2)
143.28
_
3.54(3)
3.59(3)
3.48(18)
3.26(4)
2.59
(C(1) pyrrolic) (C(8) pyrrolic)
(C(1) pyrrolic) (C(8) pyrrolic)
(C(2) pyrrolic) (C(13) phenyl)
(C(1)Pyrrolic)  (C(11) phenyl)
C(8)-H(8)---F(2)
_
_
_
134.23
169.24
131.48
_
C(11)-H(11)---F(1)
2.30
C(12)-H(12)---F(2)
2.55
3.31(3)
3.33(3)
2.79
(C(14)Phenyl)  (C(15) phenyl)
(C(15)Phenyl)  (C(15) phenyl)
C(19)-H(19) (C(14) phenyl)
_
146.83
All three units in Bis-BODIPY 2 (two boron dipyrrin units and one phenylene unit) were
aligned indifferent planes. The dihedral angle between C4-C5-C11-C10 was 52.01o(17) and
that between C6-C5-C11-C12 was 54.11o (18). This suggested that the inward angle was
lesser than the outward angle and thus the two boron-dipyrrin units are trying to come closer
to each other. The values of N-B-N and F-B-F angles were 105.54o(12) and 109.42o(12)
respectively. Bis-BODIPY 4 (CCDC 1028232) gave orange needle shaped crystals. The
crystals were again monoclinic type with C2/c space group. The values of N-B-N and F-B-F
o
o
angles were 106.21 (15) and 109.53 (17) respectively. The dihedral angles between the
7

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boron dipyrrins units and the triphenylamine unit were 44.60o(3) for C4-C5-CD1O0I-: C10.11013V9iea/wCn5AdDrtiTc0le1O9n2l5inGe
45.50o(3) for C6-C5-C10-C15. These values were much lower than that of the Bis-BODIPYs
Figure 3. Packing diagram (above) and  stacking diagram (below) of Bis-BODIPY 1.
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DOI: 10.1039/C5DT01925G
Figure 4. Packing diagram (above) and  stacking diagram (below) of Bis-BODIPY 2.
1 and 2. The N-B-N and F-B-F angles in all three Bis-BODIPYs were very similar to the
reported meso-aryl BODIPY monomer. All three Bis-BODIPYs 1, 2 and 4 exhibited
supramolecular interactions in their packing diagrams and the distances and angles of the
interactions in the packing mode are presented in Table 1. Bis-BODIPY 1 has formed layered
structure in 3D framework through weak hydrogen bonding (C-H---F) and C-H---
interactions (Table 1). In crystal packing, one layer was connected to the other via a mutual
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C(2)-H (2)--- (pyrrolic) and C(10)-H (10)--- (pyrrolic) interactions (Figure 3)D. OIIn: 10p.1a0c3k9/iCn5gDT01925G
diagram of Bis-BODIPY 2, molecules were connected through an antiparallel head to tail
interaction via two mutual C(7)---F(1) interactions. Two boron dipyrrins units were
connected to each other through C(3)-H(3)---F(1) and C(3)-H(3)---B(1) type weak
intermolecular interactions to form stacked 3D network (Figure 3). One layer was also
connected to other through C(1)-H(1)---(pyrrolic), C(1)- H(1)---N(2) and C(8)-H(8)---F(2)
interactions (Figure 4). The metric parameters for the interactions in Bis-BODIPY 1 and 2
were, C-H--- (pyrrolic) distances 2.69 to 2.84 Å and C-H--- (pyrrolic) angles 132.79o and
173.93o respectively (Table 1). In the crystal packing mode of Bis-BODIPY 4, it formed a
cluster of five molecules with the help of several intermolecular interactions (Figure S37,
ESI). It included weak ---interactions, hydrogen bonding and C-H---interactions. These
clusters were connected through C(11)-H(11)---F(1) to form the 3D structure of Bis-BODIPY
4.
Photophysical properties
UV-vis absorption and fluorescence studies
The UV/vis absorption, fluorescence spectra and fluorescence lifetimes of Bis-BODIPY 1-6
were examined in five different solvents. The absorption spectra of compounds 1-6 are
presented in Figure 5 and photophysical properties are summarized in Table 2. Bis-BODIPYs
1-4 showed slight polarity dependence and exhibited a strong absorption band around at 500
nm with a shoulder around at 480 nm. The strong absorption at 500 nm is attributed to a
strong S0-S1 electronic transition and the shoulder band corresponds to S0-S1 vibrational
1.0
Bis-BODIPY 1
0.8
0.6
0.4
0.2
0.0
Bis-BODIPY 2
Bis-BODIPY 3
Bis-BODIPY 4
Bis-BODIPY 5
Bis-BODIPY 6
400
500
600
Wavelength (nm)
Figure 5. Comparison of normalized absorption spectra of compounds 1-6 in chloroform.
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-6 M.
Photophysical data of compounds1-6 in different solvents. Concentration used was 2.4 xD1O0I: 10.1039/C5DT01925G
View Article Online
Table 2.
Compound
Solvent
fwhmabs
(cm-1)
fwhmem
(cm-1)
kr
knr
(109 s-1)
abs
(nm)
log
emi
vst
f

(ns)
(109 s-1)
(cm-1)
(cm-1)


Bis-BODIPY
Toluene
Dichloromethane
Chloroform
Tetrahydrofuran
Acetonitrile
506
504
506
503
498
1510
1485
1516
1581
1610
4.63
4.46
4.59
4.69
4.75
539
531
534
532
526
1675
1618
1517
1731
1680
1210
1009
1036
1084
1069
0.014
0.012
0.015
0.0085
0.0009
0.08
0.07
0.08
0.06
0.02
0.18
0.17
0.19
0.14
0.045
10.75
14.11
12.31
16.53
49.95
1
Bis-BODIPY
Toluene
Dichloromethane
Chloroform
Tetrahydrofuran
Acetonitrile
503
502
504
501
496
1497
1503
1534
1638
1650
4.82
4.34
4.89
4.52
4.75
529
525
527
525
520
1502
1335
1311
1388
1428
977
873
866
912
931
0.051
0.042
0.048
0.031
0.009
0.27
0.26
0.20
0.23
0.04
0.19
0.16
0.24
0.14
0.23
3.51
3.68
4.76
4.21
24.78
2
Bis-BODIPY
Toluene
Dichloromethane
Chloroform
Tetrahydrofuran
Acetonitrile
--
--
--
--
--
--
--
--
--
--
3
498
499
497
--
1630
1724
1637
--
4.66
4.74
4.91
--
581
517
569
--
3337
2115
3235
--
2869
698
2546
--
0.173
0.196
0.18
--
2.90
1.08
0.07
--
0.06
0.18
0.07
--
0.29
0.74
0.30
--
Bis-BODIPY
Toluene
Dichloromethane
Chloroform
Tetrahydrofuran
Acetonitrile
503
501
502
500
495
1659
1724
1666
1769
1844
4.88
4.69
4.80
4.98
4.60
585
680
633
667
--
1953
2457
2472
3981
--
2787
5254
4123
5007
--
0.41
0.018
0.135
0.025
--
3.43
2.18
4.12
1.65
--
0.12
0.008
0.033
0.015
--
0.17
0.38
0.21
0.59
--
4
Bis-BODIPY
Toluene
Dichloromethane
Chloroform
Tetrahydrofuran
Acetonitrile
523
520
522
517
513
1796
1863
1772
1895
1416
4.28
4.23
4.30
4.42
4.32
531
521
529
524
526
3423
1977
2943
2393
1993
288
37
253
528
482
0.012
0.009
0.025
0.005
0.0002
--
0.03
--
--
--
--
0.3
--
--
--
--
33.03
--
--
--
5
Bis-BODIPY
Toluene
Dichloromethane
Chloroform
Tetrahydrofuran
Acetonitrile
542
537
539
535
529
5313
5235
5612
4983
4831
4.18
4.15
4.03
4.33
4.05
633
627
610
576
577
--
--
--
--
--
2652
2673
2159
1330
1573
0.0018
0.00087
0.0019
0.0016
0.00038
--
0.06
--
--
--
--
0.0145
--
--
--
--
16.65
--
--
--
6
kr = f and knr = (1f
transition. The broader and weaker band centred at 350 nm in Bis-BODIPYs 1-2 and around
380 nm in Bis-BODIPYs 3-4 was assigned to the higher energy S0-S2 electronic transition.
The wavelength of major absorption band in Bis-BODIPYs 1-2 in toluene was comparable
with that of the meso-phenyl BODIPY monomer.28 Also, the absorption maxima varied
slightly (2-8 nm) upon changing the solvent polarity from toluene to acetonitrile (ESI),
reflecting the typical behavior of BODIPY chromophores.11 The wavelength of major
absorption band of Bis-BODIPYs 3 and 4 was similar to the corresponding meso-carbazole
BODIPY (497 nm)12 and meso-triphenylamine substituted BODIPY (500 nm).29 Bis-
BODIPY 5 and 6 exhibited 17 nm red shifted absorption band with reduced extinction
coefficient as compared to the corresponding meso-thienyl- and meso-furyl-BODIPY
monomers.30,31 shifted by 18 nm in the former. The fwhmabs (full width at half maxima of
major absorption band) of all the compounds 1-6 were calculated in different solvents (Table
11

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2). Solvent polarity affected the broadening of the absorption band. Thus fwhmDabOsI:v10a.l1u03e9s/Co5DfT01925G
Bis-BODIPY 1-6 were appeared as lower in non-polar solvent (toluene) and higher in polar
solvents (acetonitrile, tetrahydrofuran) for our compounds. This trend was in agreement with
the meso-aryl BODIPYs. Compared to Bis-BODIPYs 1-4, Bis-BODIPYs 5 and 6 showed
more solvent polarity dependence. As polarity of the solvent is increased, the absorption
maxima were shifted to blue region. Among all compounds, Bis-BODIPY 6 showed
significantly large fwhmabs values, which suggests that significant changes occurred in bond
lengths / bond angles upon excitation.32
1.0
0.8
Bis-BODIPY 1
Bis-BODIPY 2
0.6
Bis-BODIPY 3
Bis-BODIPY 4
0.4
0.2
0.0
Bis-BODIPY 5
500
550
600
650
700
750
800
Wavelength (nm)
Figure 6. Comparison of normalized emission spectra of compounds 1-5 in chloroform (ex = 488
nm).
A comparison of emission spectra of 1-5 is shown in Figure 6 and solvatochromism data is
given in Table 2. The typical mirror-image relationship was observed for the lowest energy
absorption band and emission spectra for Bis-BODIPYs 1-5. The emission maxima for
compounds 1 and 2 were blue shifted by 41 and 34 nm respectively as compared to that of
meso-phenyl BODIPY (in chloroform). The fwhmem (full width at half maxima of emission
band) and quantum yields of all the compounds 1-6 were calculated in different solvents
(Table 2). Solvent polarity affected the broadening of the emission bands and typically
fwhmem values were lower in non-polar solvent (toluene) and higher in polar solvents for our
compounds. The fluorescence quantum yields for Bis-BODIPY 1-2 were lower than meso-
phenyl BODIPY (0.06 in toluene).28 Usually the meso-aryl BODIPYs showed 10-15 nm
12

Dalton Transactions
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Stokes shifts whereas these values were 26-33 nm for Bis-BODIPYs 1 and 2. In DpOoI:l1a0r.10m39e/dC5iDaT01925G
red shifted emission maxima reflects larger dipoles of the CT excited states, which was in
agreement with the report by N. Boens et al.32 The fluorescence quantum yields for Bis-
BODIPY 3 and 4 were higher than the corresponding meso-carbazole- and meso-
29
triphenylamine-BODIPY monomers.12,
The Stokes shifts values were highest for
compound 4 among all the Bis-BODIPYs 1-6. The emission maxima were red shifted for
Bis-BODIPY 5 and 6 as compared to the corresponding meso-thienyl- and meso-furyl-
BODIPY monomer. However, the quantum yields were lower for both the Bis-BODIPYs 5
and 6 as compared to their corresponding BODIPY monomers.30, 31
Bis-BODIPY 1
1000
Bis-BODIPY 2
Bis-BODIPY 3
Bis-BODIPY 4
100
10
1
0
20
40
60
Time (ns)
Figure 7. Fluorescence decay profiles of compounds 1-4 in chloroform. The excitation wavelength
was 515 nm, collected at their respective emission maxima.
Among all the Bis-BODIPYs, compounds 3 and 4 showed higher quantum yields than 1-2
and 5-6. Higher quantum yield indicates the possibility of restricted rotation of the two
boron-dipyrrin units in Bis-BODIPYs 3 and 4 due to bulky spacer; it was supported by the
fact that much lower rate constant values (knr, see Table 2) were observed for non-radiative
decay processes in these two compounds. The lower quantum yields for compounds 1, 2, 5
and 6 suggests that other non-radiative decay processes (such as internal conversion or
intersystem crossing from S1 to T1) might be more feasible in these compounds.27 Also, the
quantum yields of bis-BODIPYS 5 and 6 were much lower than other compounds could be
associated with the electron donating ability of the heteroatom present in the bridging unit.30
The quantum yields of Bis-BODIPYs 1-6 were higher in non-polar solvent (toluene) and
lower in polar solvents (tetrahydrofuran); this is due to the large dipole moment difference
13

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between CT excited state and ground state which in turn facilitate internal conversion in polar
DOI: 10.1039/C5DT01925G
media.32 The time-correlated single photon counting technique was used to measure the
singlet state life times  of Bis-BODIPYs 1-6. The fluorescence decay of compounds 1-4
were fitted to a single exponential decay (Figure 7). However, in the case of Bis-BODIPYs 5
and 6 decay profiles fitted to three exponential decays (ESI). The bridging phenylene unit is
more rigid in bis-BODIPY 2 as compared to bis-BODIPY 1 (Table 2), thus the non-radiative
decay processes are less prominent in 2. It has been reported that the meso-mesityl-BODIPY
showed higher quantum yields and singlet state lifetimes than meso-phenyl BODIPY,
because the hindered rotation of bulky mesityl group leads to drastic decrease in internal
conversion process.27 Therefore, the quantum yields and singlet state lifetimes of bis-
BODIPY 2 are higher than the bis-BODIPY 1; this was again supported by the lower value of
non-radiative decay constant for 2 than 1 (Table 2). For Bis-BODIPYs 1-4 singlet state life
times , were lower than their corresponding BODIPY monomers For example Bis-BODIPY
3 has life time of 1.9 ns in CHCl3, whereas meso-carbazole BODIPY has 3.37 ns in same
solvent.12 The decrease in radiative decay constant kr and the increase in non-radiative decay
constant knr were in line with the quantum yield data.
Computational details
The ground state geometry optimization of the molecules 1-6 were performed with density
functional theory (DFT) at B3LYP/6-31G (d) level,33-35 using Gaussian 09 program
package.36 Frequency calculations were performed to guarantee the optimized geometriesare
at energy minima on the potential energy surface. The vertical excitation energies were
calculated employing time dependent density functional theory (TD-DFT) approach at the
same functional and basis set.
Comparison of geometrical parameters
It is imperative to compare the vital structural parameters obtained as a result of geometry
optimization of the molecules 1, 2 and 4 with the X-ray crystallography results (ESI). To be
more precise, the largest deviations in the bond lengths of the molecule 1, 2 and 4 are 0.017
Å, 0.025 Å and 0.021 Å respectively. The deviations from the X-ray geometry is reflected
from bond angles of the molecules (1, 2 and 4) and are found to be 2.59°, 2.29° and 1.43°
respectively. Moreover, the maximum deviations of optimized dihedral angle in 1, 2 and 4
are 6.14°, 12.54° and 8.87° respectively. The computed results compare well with the
14

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experimental data, thereby providing reliable support for the use of selected levDeOlI:o1f0.1t0h3e9/oCr5yDT01925G
for structural and spectroscopic studies of the rest of molecules.
Absorption studies
To get detailed insight into the roots of absorption bands the singlet-singlet transition of
BODIPY molecules were simulated using TD-DFT/B3LYP level of theory at valence double-
ζ6-31G(d) basis set.The gas phase determined electronic spectra exhibit number of absorption
peaks and the peaks with maximum wavelength absorption (λmax) were selected for the
comparison.The effect of solvent polarities has not being taken into account as it is obvious
from experimental results. Ineffectiveness of the solvent polarity on λmax excludes any ground
state intramolecular charge transfer (ICT) process of the molecules under consideration.37
The lowest energy absorption maxima of molecules 1-6 are summarised in Table 3. The λmax
of 1, 2, 3 and 4 were observed at around 500 nm and for 5 and 6, the red-shifted bands at 522
and 539 nm respectively indicate lower HOMO-LUMO gap, which is supported by DFT
studies. The delocalization of frontier molecular orbitals (FMO’s) of the compounds 1-6
indicated the significant electronic conjugation between the boron dipyrrin units and the
spacers. Interestingly the HOMO-LUMO gap for 6 (2.513 eV) is less than that of 1 (2.877
eV), 2 (3.040 eV), 3 (3.050 eV), 4 (2.812 eV) and 5 (2.770 eV). Though DFT produced
HOMO-LUMO gaps do not directly corroborate the experimentally observed (UV-Vis)
energy gap values but, almost same trend of band gap increment (3 > 2> 1 > 4 > 5 >6) is
reflected. For Bis-BODIPY 3, in-spite of highest HOMO energy level, the most destabilized
LUMO resulted in its highest HOMO-LUMO gap. On the other hand for Bis-BODIPY 6 the
destabilization of the LUMO is considerably less because of approximately planar
arrangement of the furan spacer increasing the level of effective conjugation, resulting in a
HOMO-LUMO gap that was lower than rest of the molecules 1, 2, 3, 4 and 5.
Table 3. Absorption data of Bis-BODIPYs 1-6
Experimental
λmax(nm)
506
Calculated
λmax(nm)
472.02
logϵ
4.59
4.89
4.74
4.80
4.30
4.03
(F)
0.0631
0.0029
0.2490
0.0411
0.0390
0.0440
Bis-BODIPY 1
Bis-BODIPY 2
Bis-BODIPY 3
Bis-BODIPY 4
Bis-BODIPY 5
Bis-BODIPY 6
504
499
502
522
539
466.98
445.05
520.17
493.65
541.08
ϵ, is molar extinction coefficient and F, is oscillator strength.
15

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DOI: 10.1039/C5DT01925G
BODIPY
LUMO+1
1
2
3
4
5
6
LUMO
HOMO
HOMO-1
Figure 8. Frontier molecular orbitals of Bis-BODIPYs 1-6 at B3LYP/6-31G(d) level for all the atoms.
The frontier molecular orbitals of the molecules 1-6 are displayed in Figure 8. The
distribution of HOMO and LUMO reflects the strong donor-acceptor interactions. In Bis-
BODIPY 1, 2, 3 and 5, the HOMO was localized on the BODIPY moiety and the LUMO was
shifted to the corresponding spacer groups. On the other hand in 4 and 6, the HOMO was
localized on the spacer moieties whereas the LUMO was budged to the BODIPY moiety. The
major absorption band in the electronic absorption spectra of molecule 1-6 is related to the
BODIPY absorption which reflects that the origin of major absorption peaks is from HOMO
energy levels. It can be seen that the TD-DFT calculations were in good agreement with the
experimental results and unveil that the main transitions for 1, 2, 3, 4, 5 and 6 are from
HOMO → LUMO, HOMO → LUMO+1, HOMO → LUMO+1, HOMO → LUMO, HOMO
→ LUMO and HOMO → LUMO+1 respectively.
Electrochemical Studies
The cyclic voltammetry measurements of Bis-BODIPYs 1-6 were carried out at the scan rate
of 50 mV/s using tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The
redox potential data and a comparison of reduction waves are summarised in the Table 4 and
Figure 9 respectively. All the Bis-BODIPYs showed only reduction peaks. Bis-BODIPY 1
16

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showed two reduction peaks. The first reduction peak was chemically reversible at -0.49V
DOI: 10.1039/C5DT01925G
and the other one was chemically irreversible around -1.75V. Bis-BODIPY 2 showed three
reduction waves, two reversible and one irreversible.
Table 4. Electrochemical redox data (V) of compounds 1-6 in dichloromethane, containing 0.1 M
TBAP as supporting electrolyte recorded at 50 mV/s scan speed.
Ered(V vs SCE)
I
II
-1.75
-0.67
-1.72
-1.36
-
III
-
-1.67
-
-
-
-
Bis-BODIPY 1
Bis-BODIPY 2
Bis-BODIPY 3
Bis-BODIPY 4
Bis-BODIPY 5
Bis-BODIPY 6
-0.49
-0.55
-0.66
-0.64
-0.17
-0.17
-
Bis BODIPY 2
Bis BODIPY 1
0.0
-0.4
-0.8
-1.2
-1.6
0.0
-0.4
-0.8
Potential (V)
-1.2
-1.6
Potential (V)
Bis BODIPY 4
Bis BODIPY 3
-0.4
-0.8
-1.2
-1.6
0.0
-0.4
-0.8
-1.2
-1.6
Potential (V)
Potential (V)
Bis BODIPY 6
Bis BODIPY 5
0.0
-0.2
-0.4
-0.6
-0.8
0.0
-0.2
-0.4
-0.6
-0.8
Potential (V)
Potential (V)
Figure 9. Comparison of cyclic voltammograms of compounds 1-6 in dichloromethane, containing 0.1
M TBAP as supporting electrolyte recorded at 50 mV/s scan speed (V vs. SCE).
17

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DOI: 10.1039/C5DT01925G
The reported meso-tolyl BODIPY monomer, showed two reduction peaks at -0.788 and -1.81
V. As compared to the reduction potentials of meso-tolyl BODIPY,31 the first reduction
potential of both Bis-BODIPY 1 and 2 exhibited anodic shift; i.e. they were much easier to
reduce compared to the monomer meso-tolyl BODIPY. Bis-BODIPY 3 also showed two
reduction waves. The first reduction potential of Bis-BODIPY 3 was shifted towards the less
negative side by 50 mV as compared to its corresponding monomer.12 This reflects that the
compound 3 was much easier to reduce compared to the monomer. Bis -BODIPY 4 also
showed two reduction waves with a reduction potential at -0.64 V and -1.36 V. Bis-BODIPY
5 and 6 showed single reduction waves with a reduction potential around -0.17 V. The
reported first reduction potential for monomeric meso-furyl BODIPY was -0.66 V.31 The
electrochemical study of compound 1-6 suggests that compared to their corresponding
BODIPY monomers, the reduction potentials of 1-6 were low and they were very easy to
reduce.
Conclusion
In conclusion, we have synthesized and characterized six boron-dipyrrins containing bridging
arene or heterocyclic ring. Their absorption, emission, electrochemical and time resolved
florescence studies were carried out. By changing the spacer size from bulky carbazole to
phenylene to smaller thiophene and furan rings affected the spectroscopic properties of Bis-
BODIPYs. The absorption and emission maxima were blue shifted for Bis-BODIPYs having
bulky aromatic spacers and red shifted for Bis-BODIPYs having smaller thiophene/furan
spacers. Also, the X-ray crystal structures of three Bis-BODIPYs were solved. The reduced
dihedral angle between the bridging ring and dipyrrin core in all three crystal structures
indicated better interactions between the two constituting boron dipyrrin units. Also, phenyl
bridged Bis-BODIPYs showed C-H--- (pyrrolic) and --- interactions in 3D structures.
DFT calculations showed largest HOMO-LUMO gap for 3 and lowest for 6. Also, gas phase
absorption maxima calculated for 1-6 were closer to the experimental values. The observed
anodic shifts in the reduction potentials of all six Bis-BODIPYs suggested increased
electronic interactions between two boron dipyrrin units within the molecule. Also, as
compared to their corresponding monomer BODIPYs all six Bis-BODIPYs were easier to
reduce.
EXPERIMENTAL
18

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DOI: 10.1039/C5DT01925G
General Methods and Reagents
Unless otherwise mentioned, all the reagents and solvents were purchased from Aldrich,
Acros Organics or Merck and used without further purification. Pyrrole was distilled under
vacuum prior to use. Silica gel (60-120 mesh size) used for column chromatography were
procured from Merck. The solution NMR spectra of compounds were recorded with Bruker
Avance III 500 MHz NMR spectrometer at IIT Gandhinagar. Absorption spectra were
recorded with Shimadzu UV-1700 and Fluorescence emission studies were performed
using Horiba-Jobin Yvon Fluolorog-3 Spectrometer at IIT Gandhinagar. The fluorescence
quantum yields (f) were estimated from the emission and absorption spectra by a
comparative method at the excitation wavelength of 488 nm using Rhodamine B (f= 0.49)
in ethanol as the standard. The time-resolved fluorescence decay measurements were carried
out at IIT Gandhinagar using a pico-second-diode-laser-based, time-correlated, single-photon
counting (TCSPC) fluorescence spectrometer from Edinburgh Instruments Ltd., LifespecII,
(Livingston, UK). Cyclic voltammetric (CV) studies were carried out with an electrochemical
system utilizing the three electrode configuration consisting of a glassy carbon (working
electrode), platinum wire (auxiliary electrode) and saturated calomel (reference electrode)
electrodes. The experiments were done in dry dichloromethane using 0.1 M
tetrabutylammonium perchlorate as supporting electrolyte. All potentials were calibrated vs.
saturated calomel electrode by the addition of ferrocene as an internal standard, taking E ½
(Fc/Fc+) = 0.51 V vs. SCE.
Synthesis of bis-dipyrrane 7
To a stirred solution of terephthaldehyde (500 mg, 3.72 mmol) and pyrrole (38.79 mL,
559.14 mmol), acid catalyst (InCl3, 82.49 mg, 0.37 mmol) was added at room temperature.
The reaction mixture was allowed to stir for 2 h at room temperature under N2 atmosphere.
After stirring for 2 h, excess pyrrole was removed by distillation under reduced pressure. The
crude residue was dissolved in small amount of dichloromethane and purified by column
chromatography on silica gel (ethyl acetate/ dichloromethane/ hexane = 1:2:4) to give 7 as
1H NMR (500 MHz,
CDCl , δ ppm):
pale yellow powder. Yield: 29 % (400 mg).
7.921 ( bs,
3
4H), 7.170 (s, 4H), 6.695- 6.692 (d, J = 1.5 Hz, 4H), 6.160- 6.143 (m, 4H), 5.914 (s, 4H),
HRMS [ESI]: calcd for C H N [(M-H)+]: m/z 365.1766 Obsvd: m/z 365.1769.
5.45 (s, 2H).
24 21
4
19

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DOI: 10.1039/C5DT01925G
Synthesis of bis-dipyrrane 8
Isophthalaldehyde (600 mg, 4.47 mmol) and pyrrole (150 eq., 46.5 mL, 670 mmol) were
taken in an 100 mL round bottom flask and stirred for 5 minutes under nitrogen at room
temperature. After stirring for 5 min InCl3 (0.1 eq., 98.91 mg, 0.44 mmol) was added and
the reaction mixture was allowed to stir for another 2h
. By vacuum distillation, the excess
pyrrole was emo ed. The crude mixture was subjected to silica gel column
r
v
chromatography (ethyl acetate/ dichloromethane/ hexane = 1:2:4) to isolate the desired
in 4 6 % (750 mg).1H NMR (500 MHz,
CDCl , δ
compound
8 as a sticky yellow solid
3
ppm): 7.862 ( bs, 4H), 7.263-7.233 (t, J = 6 Hz 1H), 7.121 (s, 1H), 7.089-7.074 (dd, 2H),
6.654-6.651 (d, J = 1.5 Hz, 4H) 6.140-6.123 (m, 4H), 5.864 (s, 4H), 5.367 (s, 2H). HRMS
[ESI]: calcd for C24H21N4 [(M-H)+]: m/z 365.1766 Obsvd: m/z 365.1769.
Synthesis of bis-dipyrrane 9
N-butylcarbazolebis-aldehyde (500 mg, 1.78mmol) was dissolved in pyrrole (150 eq., 18.63
mL, 268.51 mmol) under
mmol) was added and the reaction mixture was allowed to be stirred for 2h
pyrrole was removed by vacuum distillation The desired compound was isolated via
a
nitrogen atmosphere. After 5 min InCl3 (0.1eq. 39.49 mg, 0.18
.
Excess
9
column chromatography (ethyl acetate/dichloromethane/hexane = 5:20:75) in
(550 mg).1H NMR (500 MHz,
CDCl , δ ppm):
55%
7.94 (s, 4H), 7.32 (s, 4H), 6.68 (s, 4H),
3
6.16 (d, J= 3 Hz, 4H), 5.96 (s, 4H), 5.62 (s, 2H), 4.26 (t, J = 7 Hz, 2H), 1.82 (m, 2H), 1.41
HRMS [ESI]: calcd for C H N [(M-H)+]: m/z 510.2658 Obsvd: m/z
(m, 2H), 0.93 (m, 3H).
34 32
5
510.2657.
Synthesis of bis-dipyrrane 10
To a stirred solution of triphenylaminebis-aldehyde (700 mg, 2.32 mmol) and pyrrole (24.12
mL, 347.68 mmol), InCl3 (51 mg, 0.23 mmol) was added. The reaction mixture was protected
from light and kept for stirring under nitrogen for 8 h at room temperature. After verifying
the formation of the compound by TLC, excess pyrrole was vacuum distilled and the crude
compound was subjected to silica gel column chromatography. The desired compound 10
1
,
was eluted with 60% dichloromethane/hexane. Yield: 41 % (510 mg). H NMR (500 MHz
CDCl
3,δ ppm):
7.93 (s, 4H), 7.07 (m, 7H), 7.00 (d, J= 8 Hz, 6H), 6.70 (s, 4H), 6.16 (d, J =
HRMS [ESI]: calcd for C H N [(M+H)+]: m/z
2.5 Hz, 4H), 5.93 (s, 4H), 5.41 (s, 2H).
36 32
5
534.2658 Obsvd: m/z 534.2664.
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DOI: 10.1039/C5DT01925G
Synthesis of Bis-BODIPY 1
Bis-dipyrrane 7 (200 mg 0.54 mmol) was dissolved in dichloromethane (75 mL) and
oxidized with DDQ (297 mg, 1.3mmol 2.4 eq.) at RT under air. The reaction mixtur was
allowed to stir at room temperature for 1h Triethylamine (6.1 mL, 80 eq.) followed by
BF ·Et O (6.80mL, 100 eq.) as added to the reaction mixture successively, without any
e
.
w
3
2
time delay. The stirring was continued at room temperature for additional 30 min., the
reaction mixture was evaporated and the crude product was purified by silica gel
column chromatography and eluted with a ethyl acetate/dichloromethane/petroleum
(5:15:80) mixture to afford B i s - BODIPY 1 as orange powder in
8% yield (20 mg).
ether
1
, CDCl
3, δ ppm): 8.008 (s, 4H), 7.761 (s, 4H), 7.002-6.994 (d, J = 3.5
H NMR (500 MHz
13
Hz, 4H), 6.615-6.609 (d, J = 3 Hz). C NMR (125.7 MHz, CDCl3, ppm): 145.44, 144.96,
136.15, 134.77, 131.47, 130.49, 119.03. 19F NMR (470.4 MHz, CDCl3, ppm): -145.03 (q,
4F). 11B NMR (160 MHz, CDCl3, δ ppm): 0.29 (t, 2B). HRMS [ESI]: calcd for C24H16B2F3N4
[(M-F)+]: m/z 439.1513 Obsvd: m/z 439.1520.
Synthesis of Bis-BODIPY 2
Bis-dipyrrane 8 (200 mg 0.54 mmol) was dissolved in dichloromethane (75 mL) and
oxidized with DDQ (297 mg, 1.3mmol 2.4 eq.) at RT under air. The reaction mixtur
allowed to stir at room temperature for 1h Triethylamine (6.1 mL, 80 eq.) and BF ·Et O
(6.8mL, 100 eq.) ere added to the reaction mixture without any time delay. The stirring
e was
.
3
2
w
was continued at room temperature for another 30 min, after this reaction mixture was
evaporated and the crude product was purified by silica gel column chromatography.
(5:15:80)
The column was eluted with ethyl acetate/dichloromethane/petroleum ether
1H NMR (500
mixture to afford
2 as orange powder in
20% yield (50 mg).
Bis-BODIPY
, CDCl
3, δ ppm): 7.988 (s, 4H), 7.817-7.790 (t, J = 7.5Hz, 3H), 7.745-7.715 (d, J =
MHz
7.5 Hz, 1H), 6.944-6.938 (d, J = 3 Hz, 4H), 6.593-6.589 (d, J = 2 Hz 4H). 13C NMR
(125.7 MHz, CDCl3, ppm): 145.22, 145.01, 134.84, 134.27, 132.42, 131.87, 131.31,
128.76, 119.11. 19F NMR (470.4 MHz, CDCl3, ppm): -145.01 (q, 4F). 11B NMR (160 MHz,
CDCl3, δ ppm): 0.26 (t, 2B). HRMS [ESI]: calcd for C24H16B2F3N4 [(M-F)+]: m/z 439.1513
Obsvd: m/z 439.1520.
21

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DOI: 10.1039/C5DT01925G
Synthesis of Bis-BODIPY 3
Bis-Dipyrrane 9 (200 mg 0.39 mmol) was dissolved in dichloromethane (54 mL) and
oxidized with DDQ (213 mg, 0.94 mmol, 2.4 eq.) at RT under air. The reaction mixtur
was allowed to stir at room temperature for 1h Triethylamine (4.3 mL, 80 eq.) followed by
BF ·Et O (4.91 mL, 100 eq.) as added to the reaction mixture without any time delay.
e
.
w
3
2
The stirring was continued at room temperature for additional 30 min, the reaction
mixture was evaporated and the crude product was purified by silica gel column
(80:20) mixture to
chromatography and eluted with a dichloromethane/petroleum ether
1
,
afford B i s - BODIPY 3 as orange powder in
33% yield (80 mg).
H NMR (500 MHz
CDCl
3, δ ppm): 8.385 (s, 2H), 7.954 (s, 4H), 7.804-7.787 (d, J = 8.5 Hz 2H), 7.637-7.620 (d,
J = 8.5Hz, 2H), 7.012-7.006 (d, J = 3 Hz, 4H), 6.579-6.575 (d, J = 2 Hz, 4H) 4.487-4.459 (t,
J = 7 Hz, 2H), 2.016-1.987 (t, J = 7.5 Hz, 2H), 1.422-1.393 (t, J = 7 Hz, 2H), 1.067-1.038 (t,
J = 7 Hz, 3H). 13C NMR (125.7 MHz, CDCl3, δ ppm): 143.40, 142.56, 135.19, 131.66,
19
129.53, 125.78, 123.63, 118.44, 109.29, 43.22, 31.21, 20.65, 13.89. F NMR (470.4 MHz,
11
CDCl3, δ ppm): -145.10 (q, 4F). B NMR (160 MHz, CDCl3, δ ppm): 0.36 (t, 2B). HRMS
[ESI]: calcd for C34H27B2F4N5 [(M-F)+]: m/z 584.2405 Obsvd: m/z 584.2444.
Bis-BODIPY 4
Synthesis of
Bis-dipyrrane 10 (500 mg, 0.94 mmol) was dissolved in 75 mL of dichloromthane and
oxidised with DDQ (511 mg, 2.27 mmol, 2.4 eq.) at room temperature under air. The
reaction mixture was allowed to stir at RT for 1h. After 1h triethylamine (10.4 mL, 75
mmol, 80 eq.), followed by
BF ·Et O (8.13 mL, 94 mmol, 100 eq.)
was added to the
3
2
reaction mixture one by one without much time lag. The reaction mixture was allowed to
stir at room temperature for 30 min, after this solvent was evaporated and the crude
B i s -
product was purified by silica gel column chromatography. Desired compound
BODIPY 4
(15:85) mixture in
30% yield (176 mg,
was eluted with ethyl acetate/ hexane
, CDCl
reddish powder).1
3, δ ppm): 7.937 (s, 4H), 7.557-7.540 (d, J = 8.5
H NMR (500 MHz
Hz, 4H), 7.467-7.436 (t, J = 8.5 Hz, 2H), 7.306-7.275 (m, 7H), 7.063-7.057 (d, J = 3 Hz, 4H),
6.577-6.573 (d, J = 2 Hz, 4H). 13C NMR (125.7 MHz, CDCl3, δ ppm): 149.65, 146.89,
145.88, 143.51, 134.71, 132.30, 131.19, 130.19, 128.32, 126.94, 125.96, 122.55, 118.32. 19F
11
NMR (470.4 MHz, CDCl3, δ ppm): -145.13 (q, 4F). B NMR (160 MHz, CDCl3, δ ppm):
0.29 (t, 2B). HRMS [ESI]: calcd for C36H25B2F3N5 [(M-F)+]: m/z 606.2248 Obsvd: m/z
606.2269.
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DOI: 10.1039/C5DT01925G
Bis-BODIPY 5
Synthesis of
Bis-dipyrrane 11 (400 mg, 0.28 mmol) and pyrrole (30 mL, 428 mmol) were stirred under
nitrogen for 5 min. Added InCl3 (6 mg, 0.03 mmol) and allowed to stir for further 2 h. after
2h
excess pyrrole was vacuum distilled and the crude reaction mixture dissolved in 50 mL of
dichloromethane and oxidised with DDQ (292 mg) and allowed to stir for 1h at RT. After 1h
triethylamine (10.4 mL, 75 mmol, 80 eq.) and
BF ·Et O (8.13 mL, 94 mmol, 100 eq.) were
3
2
added to the reaction mixture. The stirring was continued at room temperature for
further 30 min, then reaction mixture was evaporated and crude product was subjected
to silica gel column chromatography. Desired compound was eluted in
dichloromethane/hexane (70:30) mixture in 8% yield (25 mg, dark orange powder). 1H NMR
, CDCl
3, δ ppm): 7.999 (s, 4H), 7.689 (s, 4H), 7.331-7.323 (d, J = 4 Hz, 4H),
(500 MHz
6.652-6.645 (d, J = 3.5 Hz, 4H). 13C NMR (125.7 MHz, CDCl3, δ ppm): 145.16, 139.38,
137.26, 134.21, 132.86, 131.43, 119.22. 19F NMR (470.4 MHz, CDCl3, δ ppm): -145.13 (q,
4F). 11B NMR (160 MHz, CDCl3, δ ppm): 0.20 (t, 2B). HRMS [ESI]: calcd for
C22H14B2F3N4S [(M-F)+]: m/z 445.1077 Obsvd: m/z 445.1107
Bis-BODIPY 6
Synthesis of
The synthesis of Bis-BODIPY 6 was performed by following similar procedure used for
Bis-
1
, CDCl
H NMR (500 MHz
5. Yield: 10% (25 mg)
3, δ ppm):7.976 (s, 4H), 7.452-
BODIPY
13
7.445 (d, J = 3.5 Hz, 4H), 7.413 (s, 2H), 6.642-6.637 (d, J = 2.5 Hz, 4H). C NMR (125.7
MHz, CDCl3, δ ppm): 151.75, 144.70, 130.84, 121.23, 119.37. 19F NMR (470.4 MHz,
11
CDCl3, δ ppm): -145.49 (q, 4F). B NMR (160 MHz, CDCl3, δ ppm): 0.18 (t, 2B). HRMS
[ESI]: calcd for C22H14B2F3N4O [(M-F)+]: m/z 429.1306 Obsvd: m/z 429.1277
Acknowledgements
IG gratefully acknowledges DST (New Delhi), CSIR, (New Delhi) and IIT Gandhinagar for
financial support. IG thanks IIT Bombay, Chemistry Department for CV studies and Prof. H.
Furuta, Kyushu University for X-ray measurements. PEK and SD thank IIT Gandhinagar for
their fellowship. PCJ acknowledges CUG for providing basic computational facilities and
UGC, New Delhi for start-up grants. MYL thanks UGC for fiscal assistance.
23

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DOI: 10.1039/C5DT01925G
Supporting Information
1
The synthetic procedures of some starting materials, characterization data like HRMS, H,
19
11
13C, F, B NMR spectra, fluorescence decay profiles, DFT calculation and X-ray structure
details of reported compounds are available as supporting information.
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