Novel dual BODIPY-carbazole conjugates with various linkers

Article abstract of DOI:10.1071/CH16704

Four dual BODIPY-carbazole conjugates (BDPa-d, BODIPY is 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene), with various π bridges, including none, phenyl, thiophene, and furan, were designed and synthesized. The results suggest that the π bridges have significant effect on the thermal, photophysical, and electrochemical properties of the conjugates. BDPc and BDPd, with a five-membered heterocycle as a π bridge possessing more coplanar molecular geometry, exhibit broader and red-shifted absorption with an obvious charge transfer shoulder peak, as well as red-shifted emission. UV-visible absorption spectroscopy and cyclic voltammetry results show that the extension of the π-conjugated system leads to a reduction in the optical gap with a decrease of the LUMO level. All conjugates display remarkable Stokes shifts (107-216 nm) and low fluorescence quantum yields. BDPc and BDPd, which essentially possess broad and intense absorption, and suitable HOMO-LUMO energy levels, are potential candidates for light-harvesting and photovoltaic applications.

Full text of DOI:10.1071/CH16704

CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH16704
Novel Dual BODIPY-Carbazole Conjugates
with Various Linkers
A B
,
B C
,
B
B
Qiao Zong,
Junxu Liao,
Hongbin Zhao,
Weinan Zhou, Wentao Zhang,
B C
,
A C
,
and Nianfa Yang
A
College of Chemistry, Xiangtan University, Xiangtan 411105, China.
B
College of Chemical Engineering and Energy Technology, Dongguan University
of Technology, Dongguan 523808, China.
C
Corresponding authors. Email: zhaohbhanlf@163.com; liaojx83@126.com;
nfyang@xtu.edu.cn
Four dual BODIPY-carbazole conjugates (BDPa–d, BODIPY is 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene), with
various p bridges, including none, phenyl, thiophene, and furan, were designed and synthesized. The results suggest
that the p bridges have significant effect on the thermal, photophysical, and electrochemical properties of the conjugates.
BDPc and BDPd, with a five-membered heterocycle as a p bridge possessing more coplanar molecular geometry, exhibit
broader and red-shifted absorption with an obvious charge transfer shoulder peak, as well as red-shifted emission.
UV-visible absorption spectroscopy and cyclic voltammetry results show that the extension of the p-conjugated system
leads to a reduction in the optical gap with a decrease of the LUMO level. All conjugates display remarkable Stokes shifts
(107–216 nm) and low fluorescence quantum yields. BDPc and BDPd, which essentially possess broad and intense
absorption, and suitable HOMO–LUMO energy levels, are potential candidates for light-harvesting and photovoltaic
applications.
Manuscript received: 11 December 2016.
Manuscript accepted: 20 January 2017.
Published online: 27 February 2017.
two-photon fluorescence properties.[35] Zhu and coworkers
reported the synthesis of a BODIPY-carbazole polymer exhi-
biting near-infrared emission.[36] Sharma et al. have reported
high-efficiency organic solar cells devices designed with
carbazole-modified BODIPY derivatives as the donor materials;
that study marked one of the significant advances in the appli-
cations of this dye family.[26] Very recently, our group explored
a novel BODIPY-carbazole conjugate that exhibit high absor-
bance in the wavelength range of 400–900 nm, revealing the
potential of these dyes for panchromatic material applica-
tion.[37] However, such studies are scarce and scattered. Thus,
the construction of more novel molecules and systematic studies
on their structure–property relationships are of great important.
To the best of our knowledge, small molecular carbazole-based
symmetric dual BODIPY-carbazole conjugates with different
aromatic p bridges have not been reported. Developing an
understanding of this type of compound and obtaining insight
into its potential applications has aroused our interest.
Inspired by the excellent results mentioned above, in
the present contribution, we designed and synthesized four
BODIPY-carbazole conjugates (BDPa–d, Scheme 1), featuring
a carbazole motif as the central core, BODIPY as terminal
groups, and/or several types of aromatic groups, including
phenyl, thiophene, and furan, as p bridges. The photophysical
properties, electrochemical behaviour, and theoretical calcula-
tion of these complexes were systematically investigated with
the aim of understanding structure–property correlations. By
Introduction
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) deri-
vatives have proved to be highly versatile dyes owing to their
remarkable photophysical properties including strong ground-
state absorption, outstanding redox activity, intense fluorescent
emission, high photoluminescence quantum yield, and excellent
chemical stability.[1,2] Furthermore, BODIPY can be easily
tailored by chemical transformation and its photophysical
properties can be readily tuned by structural modifications at the
periphery and at the boron atom of the 4,4-difluoro-4-bora-
3a,4a-diaza-s-indacene skeleton.[3–6] Owing to these excellent
advantages, BODIPY derivatives have been widely used
in various fields such as fluorescence imaging,[7–11] cancer
photodynamic therapy,[12–14] photocatalysis,[15,16] electro-
chemiluminescence,[17] laser dyes,[18] transistors,[19] artificial
light-harvesting antenna,[20,21] hydrogen production,[22] organic
photovoltaics, and dye-sensitized solar cells.[23–28] Carbazole is
popular among chemists due to its facile functionalization,
strong electron-donating ability, and stability. Its derivatives
exhibit good absorption and efficient emissive properties,[29–32]
and have been used in various photovoltaic and photosensitizer
applications.[33,34] The combination of BODIPY with carbazole
moiety to construct donating–accepting-type BODIPY-
carbazole conjugates for optoelectronic applications has
attracted interest in recent years. Wang et al. have designed a
type of alkynyl-bridged BODIPY-carbazole conjugate, which
exhibits good cell permeability, non-toxicity, and excellent
Journal compilation ꢀ CSIRO 2017
www.publish.csiro.au/journals/ajc

B
Q. Zong et al.
C8H17
N
C8H17
N
H
N
O
O
O
O
C8H17Br,
NaOH
B B
Linker
OHC
Br
DMSO
PdCl2(dppf),
KOAc, DMF
Pd(PPh3)4,
Na2CO3,
Toluene/H2O
Br
Br
Br
B
O
B
O
O
Br
O
3
1
2
1) n-BuLi, THF, Ϫ78ЊC
2) DMF, Ϫ78ЊC to 20ЊC
For 4a
None linker
C8H17
C8H17
N
N
C8H17
N
1) TCQ, CH2Cl2
Pyrrole
InCl3, rt
·
2) Et3N, BF3 Et2O
N
N
N
B
N
H
H
O
O
NH
HN
N
N
F
B
F
F
F
4a–d
5a–d
BDPa–d
Linker ϭ None
S
O
a
b
c
d
Scheme 1. Synthetic routes of BDPa–d.
this means, we want to reveal whether and how p bridges
influence the photophysical and electrochemical properties of
these conjugates. Furthermore, we expect to develop some novel
optoelectronic materials based on these studies.
experiments were performed in CH2Cl2 solution using 0.1 M
tetrabutylammoniumhexafluorophosphate (n-Bu4NPF6) as sup-
porting electrolyte. The solution was deoxygenated at room
temperature under nitrogen protection.
Experimental
Synthesis
Materials
3,6-Dibromo-9-octyl-9H-carbazole (2)
3,6-Dibromo-9H-carbazole, 4,4,40,40,5,5,50,50-octamethyl-2,20-
di(1,3,2-dioxaborolane), 1-bromooctane, 4-bromobenzaldehyde,
n-butyllithium (n-BuLi), pyrrole, 2,3,5,6-tetrachlorocyclohexa-2,
5-diene-1,4-dione (TCQ), [1,10-bis(diphenylphosphino)ferro-
cene]dichloropalladium(II) (PdCl2(dppf)) and tetrakis(triphenyl-
phosphine)palladium (Pd(PPh3)4) were purchased from Aldrich.
5-Bromothiophene-2-carbaldehyde and 5-bromofuran-2-carbal-
dehyde were prepared according to the literature procedures.[38,39]
Pyrrole was distilled from CaH2 before use. DMF and dichlor-
According to the literature procedure,[40] to a solution of 3,6-
dibromo-9H-carbazole (3.25 g, 10 mmol) and benzyltriethyl-
ammoniumchloride (0.2 g, 0.84 mmol) in DMSO (80 mL) was
added aqueous sodium hydroxide (20 mL, 2 M), and the mixture
was stirred for 30 min. Then, 1-bromooctane (2.1 g, 11 mmol)
was added dropwise. The resulting mixture was stirred for 8h.
After adjusting the solution pH value to 7 by addition of
hydrochloric acid solution, the mixture was extracted with ethyl
acetate, dried over anhydrous MgSO4, and concentrated under
vacuum. The residue was purified by silica gel column chroma-
tography using petroleum ether to give the desired 2 as a white
solid (3.5 g, 80 %). dH (CDCl3, 600 MHz) 8.08 (2H, s), 7.52 (2H,
d, J 8.7), 7.21 (2H, d, J 8.7), 4.15 (2H, t, J 7.3), 1.82–1.72 (2H,
m), 1.37–1.11 (10H, m), 0.88 (3H, t, J 7.1). dC (CDCl3,
151 MHz) 139.25, 128.97, 123.39, 123.20, 111.91, 110.36,
43.30, 31.80, 29.34, 29.19, 28.87, 27.25, 22.64, 14.13.
˚
omethane were dried under 4 A molecular sieves. THF and tolu-
ene were dried and distilled over sodium and benzophenone. All
other reagent-grade chemicals were purchased and used without
further purification. Column chromatography was performed on
silica (300–400 mesh).
Instrumentation
1H NMR and 13C NMR spectra were recorded on a Bruker
Avance spectrometer (600 and 151 MHz, respectively) with
tetramethylsilane (TMS) as the internal standard. Matrix-assisted
laser desorption/ionization time-of-flight mass spectrometry
(MALDI-TOF-MS) patterns were recorded on a Bruker ultraflex-
II spectrometer using a-cyano-4-hydroxycinnamic acid matrix.
Absorption spectra were collected on a SHIMADZU UV-2550
UV-visible spectrophotometer and emission spectra were recor-
ded on an Hitachi F-4500 instrument. Emission spectra were fully
corrected by reference to a standard lamp. Cyclic voltammetry
(CV) analysis was carried out on a CHI660E electrochemical
workstation using the three-electrode configuration consisting ofa
gold electrode (working electrode), platinum wire (auxiliary
electrode), and Ag/AgCl electrode (reference electrode). The
9-Octyl-3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-9H-carbazole (3)
According to the literature,[41] to a solution of 3,6-dibromo-
9-octyl-9H-carbazole 2 (2.19 g, 5.0 mmol) and 4,4,40,40,5,5,50,
50-octamethyl-2,20-bi(1,3,2-dioxaborolane) (2.8 g, 11.0 mmol)
in DMF (80 mL) were added PdCl2(dppf) (367 mg, 0.5 mmol)
and potassium acetate (2.9 g, 30 mmol), and the mixture was
stirred at 958C under argon atmosphere for 24h. After cooling to
room temperature, the mixture was poured into water (300 mL)
and extracted with ethyl acetate (3 ꢀ 100 mL). The organic
extract was dried over anhydrous MgSO4 and concentrated
under vacuum. The residue was purified by silica gel column
chromatography using petroleum ether and ethyl acetate (15 : 1,

Dual BODIPY-Carbazole Conjugates
C
v/v) as eluent to give the desired 3 as a white solid (2.0 g, 76 %).
dH (CDCl3, 600 MHz) 8.66 (2H, s), 7.90 (2H, d, J 9.1), 7.39
(2H, d, J 8.2), 4.30 (2H, t, J 7.3), 1.91–1.78 (2H, m), 1.39
(24H, s), 1.36–1.15 (10H, m), 0.85 (3H, t, J 7.1). dC (CDCl3,
151 MHz) 142.64, 131.97, 128.08, 122.82, 108.14, 83.54, 43.15,
31.79, 29.72, 29.36, 29.15, 28.92, 27.24, 24.95, 22.61.
5,50-(9-Octyl-9H-carbazole-3,6-diyl)bis(furan-2-
carbaldehyde) (4d)
Compound 4d was prepared using the same synthetic
procedure as that used for 4b. 5-Bromofuran-2-carbaldehyde
was used instead of 4-bromobenzaldehyde. The desired 4d was
obtained as a yellow solid in 71 % yield. dH (CDCl3, 600 MHz)
9.64 (2H, s), 8.53 (2H, s), 7.89 (2H, d, J 8.5), 7.37 (4H, t, J 5.6),
6.85 (2H, d, J 3.6), 4.23 (2H, t, J 7.2), 1.93–1.74 (2H, m), 1.36–
1.19 (10H, m), 0.86 (3H, t, J 7.0). dC (CDCl3, 151 MHz)
176.73, 160.76, 151.60, 141.51, 123.84, 123.09, 120.69,
118.03, 109.48, 106.39, 43.40, 31.75, 29.29, 29.15, 28.97,
27.23, 22.59, 14.07. MALDI-TOF-MS m/z 467.244; [M]þ
requires 467.210.
9-Octyl-9H-carbazole-3,6-dicarbaldehyde (4a)
To the solution of 3,6-dibromo-9-octyl-9H-carbazole
2 (875 mg, 2.0 mmol) in dry THF (30 mL) was added n-butyl-
lithium (5 mL, 5.0 mmol, 1.0 M in THF) dropwise at ꢁ788C
under argon atmosphere. The mixture was stirred at this temper-
ature for 2h. After addition of DMF (0.4 mL, 5.2 mmol), the
resulting mixture was allowed to warm to room temperature and
was stirred for 3h. Then, water (30 mL) was added to quench the
reaction. The suspension was extracted with ethyl acetate
(3 ꢀ 50 mL). The organic extract was washed with water and
brine, dried over anhydrous MgSO4, and concentrated under
vacuum. The residue was purified by silica gel column chroma-
tography using petroleum ether and ethyl acetate (10 : 1, v/v) as
eluent to give the desired 4a as a yellow solid (483 mg, 72 %). dH
(CDCl3, 400 MHz) 7.96 (4H, s), 7.86 (2H, s), 7.31 (4H, d, J 9.3),
6.68 (4H, s), 6.16 (4H, s), 5.96 (4H, s), 5.63 (2H, s), 4.27–4.23
(2H, m), 1.83 (2H, d, J 5.4), 1.24 (10H, s), 0.86 (3H, s). dC
(CDCl3, 101 MHz) 139.95, 133.45, 132.74, 126.52, 122.73,
120.10, 117.16, 109.02, 108.37, 107.10, 43.99, 43.33, 31.85,
29.44, 29.22, 29.10, 27.37, 22.66, 14.15.
3,6-Bis(di(1H-pyrrol-2-yl)methyl)-9-octyl-9H-
carbazole (5a)
According to the literature procedure,[42] a mixture of pyrrole
(34 mL, 500 mmol) and 9-octyl- 9H-carbazole-3,6-dicarbaldehyd
4a (671 mg, 2.0mmol) was treated with InCl3 (44 mg, 0.2 mmol)
at room temperature under argon. After stirring for 1h, powdered
NaOH (0.4g, 10mmol) was added to terminate the reaction.
Pyrrole was removed under reduced pressure, and the residue was
purified by silica gel column chromatography using petroleum
ether and ethyl acetate (8: 1, v/v) as eluent to give the desired 5a
as a pale brown solid (976mg, 86 %). dH (CDCl3, 400MHz) 8.39
(2H, s), 7.96 (4H, d), 7.80 (2H, d, J 8.3), 7.62 (2H, d, J 8.5), 7.01
(4H, d, J 3.2), 6.58 (4H, s), 4.46 (2H, t, J 7.1), 2.05–1.98 (2H, m),
1.44–1.39 (2H, m), 1.28 (8H, m, J 13.2), 0.89–0.86 (3H, m). dC
(CDCl3, 101 MHz) 148.17, 143.41, 142.58, 135.24, 131.63,
129.53, 125.82, 123.62, 122.70, 118.39, 109.28, 43.85, 31.77,
29.33, 29.16, 29.08, 27.35, 22.59, 14.03.
4,40-(9-Octyl-9H-carbazole-3,6-diyl)dibenzaldehyde (4b)
A solution of 3 (1.06 g, 2.0 mmol) and 4-bromobenzaldehyde
(814 mg, 4.4 mmol) in toluene (40 mL) was degassed before
addition of Pd(PPh3)4 (160 mg, 0.16 mmol) and subsequent
degassing for 1h. Na2CO3 solution (20 mL, 2M in water) was
added, and the reaction mixture was heated at 958C under argon
atmosphere for 24h. After cooling to room temperature, the
reaction was quenched with water and extracted with ethyl
acetate. The extract was dried over anhydrous MgSO4 and
concentrated under vacuum. The residue was purified by silica
gel column chromatography using petroleum ether and ethyl
acetate (10 : 1, v/v) as eluent to give the desired 4b as a bright
yellow solid (761 mg, 78 %). dH (CDCl3, 600 MHz) 10.08 (2H,
s), 8.45 (2H, s), 7.99 (4H, d, J 8.2), 7.90 (4H, d, J 8.2), 7.81 (2H,
dd, J 8.5, 1.7), 7.53 (2H, d, J 8.5), 4.37 (2H, t, J 7.2), 1.97–1.91
(2H, m), 1.37–1.20 (10H, m), 0.87 (3H, t, J 7.0). dC (CDCl3,
151 MHz) 191.93, 147.99, 141.15, 134.62, 131.06, 130.40,
127.58, 125.67, 123.58, 119.42, 109.56, 43.50, 31.78, 29.36,
29.16, 29.05, 27.32, 22.60, 14.05. MALDI-TOF-MS m/z
487.310; [M] þ requires 487.251.
3,6-Bis(4-(di(1H-pyrrol-2-yl)methyl)phenyl)-9-octyl-9H-
carbazole (5b)
Compound 5b was prepared using the same synthetic proce-
dure as that used for 5a. 4,40-(9-Octyl-9H- carbazole-3,6-diyl)
dibenzaldehyde 4b was used instead of 9-octyl-9H-carbazole-
3,6-dicarbaldehyde. The desired 5b was obtained as a yellow
solid in 93 % yield. dH (CDCl3, 600 MHz) 8.32 (2H, s), 8.01 (4H,
s), 7.70 (2H, d, J 8.4), 7.66 (4H, d, J 8.1), 7.46 (2H, d, J 8.5), 7.31
(4H, d, J 8.1), 6.72 (4H, d, J 1.4), 6.19 (4H, d, J 2.9), 5.99 (4H, s),
5.54 (2H, s), 4.32 (2H, t, J 7.2), 1.92–1.87 (2H, m), 1.34–1.22
(10H, m), 0.86 (3H, t, J 6.9). dC (CDCl3, 151 MHz) 140.77,
140.42, 140.25, 132.58, 131.94, 128.86, 127.47, 125.27, 123.47,
118.80, 117.25, 109.13, 108.48, 107.25, 62.22, 43.69, 31.81,
29.40, 29.20, 29.08, 27.36, 20.84, 14.09. MALDI-TOF-MS m/z
718.781; [Mþ] requires 719.399.
3,6-Bis(5-(di(1H-pyrrol-2-yl)methyl)thiophen-2-yl)-9-
octyl-9H-carbazole (5c)
5,50-(9-Octyl-9H-carbazole-3,6-diyl)bis(thiophene-2-
carbaldehyde) (4c)
Compound 5c was prepared using the same synthetic proce-
dure as that used for 5a. 5,50-(9-Octyl-9H- carbazole-3,6-diyl)
bis(thiophene-2-carbaldehyde) 4c was used instead of 9-octyl-
9H-carbazole-3,6-dicarbaldehyde. The desired 5c was obtained
as a yellow solid in 90 % yield. dH (CDCl3, 600 MHz) 8.23 (2H,
s), 8.12 (4H, s), 7.64 (2H, d, J 10.2), 7.32 (2H, d, J 8.5), 7.17 (2H,
d, J 3.6), 6.86 (2H, d, J 3.4), 6.71 (4H, s), 6.20 (4H, d, J 2.9), 6.12
(4H, s), 5.74 (2H, s), 4.23 (2H, t, J 7.1), 1.87–1.79 (2H, m), 1.32–
1.19 (10H, m), 0.85 (3H, t, J 7.0). dC (CDCl3, 151 MHz) 144.78,
143.96, 140.37, 131.98, 126.53, 125.83, 124.16, 123.17, 121.44,
117.66, 117.49, 109.18, 108.50, 107.09, 62.23, 43.32, 31.79,
29.36, 29.18, 29.03, 27.29, 20.85, 14.10.
Compound 4c was prepared using the same synthetic proce-
dure as that used for 4b. 5-Bromothiophene-2carbaldehyde was
used instead of 4-bromobenzaldehyde. The desired 4c was
obtained as a yellow solid in 73 % yield. dH (CDCl3, 600 MHz)
8.41 (2H, s), 7.81 (2H, d, J 8.5), 7.77 (2H, d, J 3.8), 7.47 (2H, d,
J 3.8), 7.44 (2H, d, J 8.5), 4.31 (2H, t, J 7.2), 1.93–1.84 (2H, m),
1.39–1.22 (10H, m), 0.86 (2H, t, J 6.9). dC (CDCl3, 151 MHz)
182.59, 155.71, 141.57, 137.78, 126.47, 125.16, 124.89, 123.26,
123.15, 118.69, 109.78, 43.54, 31.75, 29.31, 29.13, 29.00,
27.26, 22.58, 14.04. MALDI-TOF-MS m/z 499.198; [M]þ
requires 499.164.

D
Q. Zong et al.
3,6-Bis(5-(di(1H-pyrrol-2-yl)methyl)furan-2-yl)-9-octyl-
9H-carbazole (5d)
(4H, d, J 4.0), 6.62 (4H, d, J 4.0), 4.37 (2H, t, J 7.2), 1.96–1.91
(2H, m), 1.43–1.23 (10H, m), 0.87 (3H, t, J 6.9). dC (CDCl3,
151 MHz) 152.97, 143.15, 141.43, 139.59, 135.06, 134.01,
132.94, 130.99, 124.94, 124.82, 123.69, 123.37, 118.52,
118.25, 109.89, 43.60, 31.76, 29.34, 29.15, 29.05, 27.29,
22.59, 14.05. MALDI-TOF-MS m/z 823.283; [M]þ requires
823.277.
Compound 5d was prepared using the same synthetic proce-
dure as that used for 5a. 5,50-(9-Octyl-9H- carbazole-3,6-diyl)
bis(furan-2-carbaldehyde) 4d was used instead of 9-octyl-9H-
carbazole-3,6-dicarbaldehyde. The desired 5d was obtained as a
yellow solid in 91 % yield. dH (CDCl3, 600 MHz) 8.41 (2H, s),
8.16 (4H, s), 7.77 (2H, d, J 9.7), 7.37 (2H, d, J 8.6), 6.73 (4H, d,
J 1.3), 6.63 (2H, d, J 3.1), 6.24 (4H, d, J 2.8), 6.22 (2H, d, J 3.1),
6.13 (4H, s), 5.62 (2H, s), 4.25 (2H, t, J 5.9), 1.89–1.84 (2H, m),
1.33–1.25 (10H, m), 0.91 (3H, t, J 7.0). dC (CDCl3, 151 MHz)
154.61, 153.28, 140.25, 130.33, 123.08, 122.55, 122.42, 117.73,
115.92, 109.29, 109.17, 108.37, 107.04, 104.09, 62.27, 43.27,
31.84, 29.41, 29.22, 29.05, 27.31, 20.86, 14.15. MALDI-TOF-
MS m/z 699.570; [Mþ] requires 699.357.
10,100-(5,50-(9-Octyl-9H-carbazole-3,6-diyl)bis(furan-
5,2-diyl))bis(5,5-difluoro-5H-dipyrrolo[1,2-c:20,10-f]
[1,3,2]diazaborinin-4-ium-5-uide) (BDPd)
BDPd was prepared using the same synthetic procedure as
that used for BDPa. Compound 5d was used instead of 5a. The
desired BDPd was obtained as a purple solid in 83 % yield. dH
(CDCl3, 600 MHz) 8.59 (2H, s), 8.00 (2H, d, J 10.2), 7.90 (4H,
s), 7.61 (4H, d, J 4.0), 7.56 (2H, d, J 8.6), 7.47 (2H, d, J 3.7), 7.08
(2H, d, J 3.7), 6.63 (4H, d, J 5.9), 4.39 (2H, t, J 7.3), 1.97–1.93
(2H, m), 1.43–1.23 (10H, m), 0.87 (3H, t, J 6.8). dC (CDCl3,
151 MHz) 153.89, 147.78, 141.45, 138.86, 134.52, 131.90,
129.48, 128.65, 125.00, 123.00, 120.60, 120.29, 118.00,
110.13, 109.63, 43.35, 31.94, 29.79, 29.33, 29.08, 27.28,
22.60, 14.13. MALDI-TOF-MS m/z 791.368; [M]þ requires
791.323.
10,100-(9-Octyl-9H-carbazole-3,6-diyl)bis(5,5-difluoro-
5H-dipyrrolo[1,2-c:20,10-f][1,3,2]diazaborinin-4-ium-
5-uide) (BDPa)
According to the literature,[42] to a solution of dipyrrome-
thane 5a (568 mg, 1.0 mmol) in CH2Cl2 (100 mL) was added
TCQ (568 mg, 2.5 mmol). The mixture was stirred at room
temperature under argon atmosphere for 3h. Then, Et3N
(4 mL, 28 mmol) and BF3ꢂEt2O (4 mL, 30 mmol) were added
in order, and the result mixture were stirred at this temperature
for 8h. The reaction was quenched with water and extracted with
CH2Cl2. The extract was washed with water and brine, dried
over anhydrous MgSO4, and concentrated under vacuum. The
residue was purified by silica gel column chromatography using
petroleum ether and ethyl acetate (5 : 1, v/v) as eluent to give the
desired BDPa as a purple solid (567 mg, 86 %), mp 303–3048C.
dH (CDCl3, 600 MHz) 8.39 (2H, s), 7.95 (4H, s), 7.80 (2H, d,
J 8.4), 7.62 (2H, d, J 8.5), 7.01 (2H, d, J 3.2), 6.58 (4H, s), 4.44–
4.48 (2H, t, J 7.1), 1.99–2.02 (2H, t, J 7.0), 1.26–1.41 (10H, m),
0.87–0.90 (3H, t, J 5.8). dC (CDCl3, 151 MHz) 148.47, 143.71,
142.88, 135.54, 131.93, 129.83, 126.12, 123.92, 123.00, 118.71,
109.46, 44.15, 32.07, 29.63, 29.46, 29.38, 27.65, 22.89, 14.33.
MALDI-TOF-MS m/z 640.428; [M – F]þ requires 640.302.
Results and Discussion
Synthesis and Characterization
The basic strategies employed for synthesizing these dual
BODIPY-carbazole conjugates, BDPa–d, are based on the
classical
condensation–oxidation–chelation
reaction
sequence.[43] To this end, the corresponding dialdehyde pre-
cursors were prepared first. Scheme 1 outlines the synthetic
route for making these dialdehydes 4a–d and the target dual
BODIPY-carbazole conjugates. Specifically, alkylation of
compound 1 with 1-bromooctane readily afforded the key
intermediate 2 according to the literature procedure. The dia-
ldehydes 4a–d were prepared from compound 2. Specifically,
dialdehyde 4a was obtained by a Br/Li-exchange with n-BuLi
followed by reaction with DMF, whereas dialdehydes 4b–d
were obtained via a Miyaura–Suzuki coupling reaction
sequence. The dialdehydes, 4a–d, were achieved with high yield
and high purification through the listed routes. Conversion of the
prepared dialdehydes to the desired BDPa–d was achieved by a
condensation–oxidation–chelation reaction sequence. First,
condensation of 4a–d with pyrrole in the presence of InCl3
according to the mild procedure we had reported previously
afforded the dipyrromethanes 5a–d in satisfactory yields.[43]
Second, oxidation of these dipyrromethanes with TCQ provided
their corresponding dipyrromethenes. Finally, chelation of
the dipyrromethenes with BF3ꢂEt2O in CH2Cl2 solution in the
presence of Et3N achieved the target complexes BDPa–d.
The total yields for this three-step synthesis are in the range of
73–78 %. The results indicated that the synthetic strategies for
these dual BODIPY-carbazole conjugates are efficient. All
BDPa–d are air-stable and soluble in CH2Cl2, CHCl3, acetone,
THF, and DMF. All these intermediates and the final dual
BODIPY-carbazole conjugates were characterized and confirmed
by 1H NMR and 13C NMR spectroscopy and MALDI-TOF-MS.
10,100-((9-Octyl-9H-carbazole-3,6-diyl)bis(4,1-
phenylene))bis(5,5-difluoro-5H-dipyrrolo[1,2-c:20,10-f]
[1,3,2]diazaborinin-4-ium-5-uide) (BDPb)
BDPb was prepared using the same synthetic procedure as
that used for BDPa. Compound 5b was used instead of 5a. The
desired BDPb was obtained as a purple solid in 84 % yield. dH
(CDCl3, 600 MHz) 8.49 (2H, s), 7.97 (4H, s), 7.91 (4H, d, J 8.0),
7.84 (2H, d, J 8.5), 7.71 (4H, d, J 8.0), 7.57 (2H, d, J 8.5), 7.07
(4H, d, J 3.7), 6.58 (4H, d, J 2.7), 4.40 (2H, t, J 7.1), 1.98–1.95
(2H, m), 1.48–1.23 (10H, m), 0.88 (3H, t, J 6.7). dC (CDCl3,
151 MHz) 147.43, 144.68, 143.89, 141.02, 134.94, 132.02,
131.51, 131.33, 127.11, 125.50, 123.65, 119.18, 118.49, 110.00,
109.62, 43.53, 31.80, 29.71, 29.20, 29.11, 27.36, 22.62, 14.07.
MALDI-TOF-MS m/z 792.371; [M – F]þ requires 792.364.
10,100-(5,50-(9-Octyl-9H-carbazole-3,6-diyl)bis
(thiophene-5,2-diyl))bis(5,5-difluoro-5H-dipyrrolo
[1,2-c:20,10-f][1,3,2]diazaborinin-4-ium-5-uide) (BDPc)
BDPc was prepared using the same synthetic procedure as
that used for BDPa. Compound 5c was used instead of 5a. The
desired BDPc was obtained as a purple solid in 81 % yield. dH
(CDCl3, 600 MHz) 8.47 (2H, s), 7.94 (4H, s), 7.85 (2H, d, J 8.5),
7.66 (2H, d, J 3.8), 7.57 (2H, d, J 3.8), 7.50 (2H, d, J 8.5), 7.45
Thermal Stability
The thermal properties of the four dual BODIPY-carbazole
conjugates were evaluated by thermogravimetric analysis
(TGA). The recorded TGA curves are shown in Fig. 1. The

Dual BODIPY-Carbazole Conjugates
E
105
100
95
90
85
80
75
70
65
60
55
50
45
(a)
0.5
0.4
0.3
0.2
0.1
0
BDPa
BDPb
BDPc
BDPd
BDPa
BDPb
BDPc
BDPd
50 100 150 200 250 300 350 400 450 500 550
800
300
400
500
600
700
Temperature [ЊC]
Wavelength [nm]
Fig. 1. TGA curves of BDPa–d.
(b)
1.0
0.8
0.6
0.4
0.2
0
BDPa
BDPb
BDPc
BDPd
corresponding data are summarized in Table S1, available as
Supplementary Material. The decomposition temperatures (Td)
at 5 % weight loss of the dual BODIPY-carbazole conjugates
were observed at 3088C for BDPa, 3018C for BDPb, 3158C for
BDPc, and 3018C for BDPd. These results indicate that these
conjugates all essentially possess good thermal stability and can
be used in organic optoelectronic devices.
UV-Visible Absorption Spectroscopy of BDPa–d
The UV-visible absorption spectra of the dual BODIPY-
carbazole conjugates in CH2Cl2 solution and in thin films are
shown in Fig. 2a and 2b, and the corresponding optical data are
summarized in Table 1. The dual BODIPY-carbazole con-
jugates show two absorption bands – the absorption band
observed at the shorter wavelength range of 280–400 nm is
ascribed to aromatic p–p* electronic transitions of the carba-
zole unit, whereas the absorption band observed at the longer
wavelength range of 420–660 nm is attributed to the p–p*,
S0–S1 transition. BDPa with two BODIPYs directly appended in
the terminal of central carbazole showed a typical sharp
absorption peak centred at 500 nm, which can be assigned to the
BODIPY segment. This result indicates that no effective con-
jugates were formed between the carbazole electron-donating
unit and BODIPY electron-accepting units. When introducing
the aromatic ring bridge groups between the central carbazole
and the terminal BODIPY of BDPa, the resulting BDPb–d
presented different absorption characteristics. BDPb displayed
an absorption profile similar to that of BDPa, implying that the
phenyl bridging groups did not help in extending the intra-
molecular conjugated length. However, BDPc and BDPd,
containing a five-membered heterocyclic, thiophene, and furan as
p bridges exhibited significant red-shifted and broaderabsorption
with maxima at 512 and 525 nm, and obvious shoulders, which
could be ascribedtothe enlargedp-conjugationsystem, as wellas
the intramolecular charge transfer (ICT) between electron-rich
carbazole donor and electron-deficient BODIPY acceptor. The
results indicate thatthe bridginggroupsplayimportant rolesinthe
absorption properties of these molecules. The remarkable red-
shifted absorption, originating from a lower p–p* transition
energyora lower bandgap, observedinBDPcand BDPd could be
explained asBDPcand BDPd possessing morefavourable spatial
configurations than the other conjugates. It is believed that
the planar conjugated structure is beneficial for enlarging
300
400
500
600
700
800
900
Wavelength [nm]
Fig. 2. (a) Absorption spectra of BDPa–d recorded in CH2Cl2 solution and
(b) normalized absorption spectra of BDPa–d recorded in films.
p conjugation and facilitating ICT, which in turn leads to the
broadened absorption spectrum and red shift. It is worth noting
that BDPd, which employs furan as a p bridge, presented more
intense shoulder band and broader absorption profile than BDPc,
thereby implying a more coplanar molecule structure relative to
BDPc. The relationships between molecular structures and
absorption properties will be discussed in detail in the Theoretical
Calculations of BDPa–d section. The molecular extinction
coefficients (e) of BDPa–d are 3.68 ꢀ 104, 1.76 ꢀ 104,
4.30ꢀ 104, and 1.46 ꢀ 104 Mꢁ1 cmꢁ1, respectively (Table 1).
Thiophene-linked dual BODIPY-carbazole conjugate BDPc
exhibited the strongest peak absorbance, which may due to strong
electron-donating ability of thiophene, which is consistent with
the observed relative planar molecular geometry. In contrast,
furan-linked BDPd showed a relatively lower e, which may be
caused by the higher degree of p conjugation.
The normalized absorption of BDPa–d in thin films are
shown in Fig. 2b. When compared with the absorption spectra
measured in CH2Cl2, the absorption bands of BDPb–d mea-
sured in films were all red-shifted with a substantially broadened
bandwidth, which can be attributed to the J-type p–p packing
and intermolecular interaction present in the solid state for these
conjugates. In contrast, BDPa displayed a blue-shifted absorp-
tion, indicating that H-type aggregation dominates the p–p
packing in the solid state for BDPa, BDPc and BDPd, with a
more rigid configuration, exhibited more obvious red-shifted

F
Q. Zong et al.
absorption, resulting in a more extended absorption profile. The
broad and intense absorption properties of BDPc and BDPd
make them good candidates for the light-harvesting and photo-
voltaic applications.
conjugates are suitable for use in organic optoelectronic materials
applications.
Electrochemical Properties of BDPa–d
To investigate the electrochemical behaviours of the dual
BODIPY-carbazole conjugates, the electronic states of BDPa–d
were studied by CV in CH2Cl2 solution. The CV curves are
depicted in Fig. 4, and the results are summarized in Table 2.
The electrochemical study reveals that the redox peaks are
reversible in nature. BDPa showed one oxidation and two
reduction waves corresponding to the formation of cation and
dianion radicals respectively, whereas BDPb and BDPc
exhibited two oxidation and two reduction waves corresponding
to the formation of dication and dianion radicals, respectively.
In BDPd, the formation of cation and anion radicals were
Fluorescence Spectroscopy of BDPa–d
The fluorescence properties of the dual BODIPY-carbazole
conjugates, BDPa–d, were studied in different solvents with
various polarities such as ethyl acetate, acetone, and dichlor-
omethane at room temperature (concentration ¼ 1 ꢀ 10ꢁ7 M,
Fig. S1, Supplementary Material). All the conjugates were
excited by the lmax determined from their corresponding
absorption spectra. Minor solvatochromic effects were observed
in the different types of solvents studied. This result indicates
that the fluorescence emission is not caused by solvent effect or
aggregation effect, but due to the localized excitation emission
or intramolecular charge transfer emission.
(a)
The normalized fluorescence spectra of the dual BODIPY-
carbazole conjugates in CH2Cl2 solution (concentration ¼ 1 ꢀ
10ꢁ7 M) and in films are shown in Fig. 3a, and the corresponding
data are listed in Table 1. Unlike previous studies reporting that
BDOIPY derivatives typically exhibit small Stokes shifts, the
conjugates BDPa–d exhibited significant Stokes shifts ranging
from 107 to 209 nm, resulting in maximum emission peaks at
607, 717, 721, and 719 nm (Table 1 and Fig. 3a). Such properties
make them potential candidates for near-infrared emission
material applications. When compared with BDPa, significant
emission bathochromic shifts of were observed for BDPb–d.
This result indicates that BDPb–d possess more appropriate
molecular structures in the excited state than BDPa, which
facilitate the intermolecular energy transfer of the excitons from
the BODIPY units to the central carbazole segments. As we can
see in Table 1, a gradual decrease in fluorescence quantum yield
(Ff) from 0.21 to 0.01 was observed for BDPa–d in the order of
BDPa.BDPb.BDPc.BDPd. This phenomenon could be
attributed to the basis of a photo-induced energy transfer from
the BODIPY p–p* excited state to the lower-lying singlet
excited state of carbazole and/or to the existence of new non-
radiative pathways from the BODIPY p–p* excited state to the
ground state, and this process increased phonon coupling.[44]
The remarkable Stokes shifts coupled with the low fluorescence
quantum yields suggest that the emission observed belong to
ICT emission. As shown in Fig. 3b, in the solid state, BDPa and
BDPb exhibited similar emission as those in solution. On
contrast, no emission were observed for BDPc and BDPd,
which could be explained by the more coplanar molecular
structure and longer effective conjugate skeleton in BDPc and
BDPd, when compared with BDPa and BDPb. Such structural
properties result in more intense intramolecular exciton dissoci-
ation (Fig. 3b). Photoluminescence (PL) quenching provides
direct evidence for the intramolecular exciton dissociation. The
PL characteristics of BDPc and BDPd suggest that the latter
BDPa
BDPb
BDPc
BDPd
1.0
0.8
0.6
0.4
0.2
0
500
550
600
650
700
750
800
850
900
Wavelength [nm]
(b)
BDPa
400
300
200
100
0
BDPb
BDPc
BDPd
900
500
600
700
800
Wavelength [nm]
Fig. 3. Normalized fluorescence spectra of BDPa–d measured in
(a) CH2Cl2 and (b) films.
Table 1. Summary of optical properties of BDPa–d
Conjugate
l
max [nm]A
e[ꢀ 104 Mꢁ1 cmꢁ1
]
l
max[nm]B
Egopt [eV]
2.25
l
em [nm]C
l
em [nm]D
S–S [nm]E
Ff
BDPa
BDPb
BDPc
BDPd
500
501
512
525
3.68
1.76
4.30
1.46
489
607
667
702
–
107
216
209
196
0.21
0.15
0.02
0.01
515
528
551
2.24
1.91
1.84
717
721
719
–
A,CMeasured in CH2Cl2 solution.B,D Measured in CH2Cl2 solution.E Stokes shift.

Dual BODIPY-Carbazole Conjugates
G
observed. As shown in Fig. 4, BDPa–d presented different first
onset oxidation potential (Eoxonset) values at 1.25, 1.11, 1.21, and
1.32 V, respectively, indicating that the aromatic bridges have
non-fullerene electron-acceptor materials. This result suggests
that the prepared conjugates have potential in organic solar cells
applications.[45–48] Furthermore, the lower-lying HOMO energy
levels can ensure that small molecular organic solar cell devices
can generate high open-circuit voltage values.
significant effects on the Eoxonset. The reduction potentials (Ered
)
calculated from EoxonsetꢁEgopt approximately matched those of
opt
the experimental values obtained from the reduction waves
(Eg refers to optical band gap). By using ferrocene as an
Theoretical Calculations of BDPa–d
internal standard, the HOMO and LUMO levels are determined
using the following equations:
To better understand the variation in the photophysical and
electrochemical properties among BDPa, BDPb, BDPc, and
BDPd, density functional theory calculations were applied to
the electronic structures of the conjugates. The optimized
ground-state geometries and electronic distribution in the
HOMO–LUMO levels are presented in Figs 5 and 6, respec-
tively. All the calculations were carried out with the Gaussian 09
program suite using the B3LYP method and 6–31 G* basis set.
As observed from Fig. 5, the calculated dihedral angle between
the carbazole motif and flanked BODIPY was 488 for BDPa.
The dihedral angle between the central carbazole and phenyl
linker and the dihedral angle between the BODIPY framework
and phenyl linker were computed to be 718 and 378, respectively,
for BDPb. Obviously, the dihedral angle between the carbazole
and phenyl bridge was too large to form effective conjugation,
which is detrimental to ICT transitions. This observation is
reflected in the absorption spectra of BDPa and BDPb, which
displayed relatively narrow and no significant red-shifted
absorption bands when compared with the spectrum of the
BODIPY core (,500 nm). The values for BDPc and BDPd were
calculated to be 448/68 and 278/28, respectively, as consistent
with the higher extent of coplanarity of BDPc and BDPd when
compared with BDPa and BDPb. The calculated results indicate
that the structures of BDPc and BDPd are favourable for ICT
transitions and reducing the HOMO–LUMO energy gap,
resulting in red-shifted absorption. This speculation matches
well with the UV-visible absorption spectral observations.
Especially, the furan-linked BDPd displaying the best degree of
coplanarity should exhibit the smallest energy gap and the
broadest absorption profile. These are clearly evident from the
absorption spectra (Fig. 2a).
HOMO ¼ ½ꢁðEoxonset ꢁ 0:52Þ ꢁ 4:8ꢃ eV
opt
LUMO ¼ HOMO þ Eg
where 0.52 V is the value for ferrocene versus Ag/AgCl and
4.8 eV is the energy level of Fc/Fcþ relative to the vacuum
energy level. The values obtained are listed in Table 2. BDPc
and BDPd featured lower LUMO levels (ꢁ3.57 eV and
ꢁ3.75 eV, respectively) than BDPa (ꢁ3.27 eV) and BDPb
(ꢁ3.14 eV). This disparity may be difficult to explain and is
most likely due to the fact that BDPc and BDPd have more
extended conjugated systems. Such types of systems have a
narrowed band gap energy, which is typically accompanied by
the lowering of LUMO and/or rising of HOMO energetics.
Indeed, the HOMO energy levels of the dual BODIPY-carbazole
conjugates were relatively low, and the differences among their
HOMO energy level are not as large as those among their LUMO
energy levels (Table 2). It should be noted that the HOMO and
LUMO levels of these conjugates are favourable match to those
of n-type semiconductor such as fullerene derivatives and other
BDPa
BDPb
BDPc
BDPd
Fig. 6 depicts the frontier molecular orbitals of BDPa–d. In
BDPa, HOMOs are located on one BODIPY unit, whereas
LUMOs are located on both BODIPY terminals, implying the
weak donor–acceptor interaction in the entire molecule due to
the twisted molecular structure. It is observed that the electron
distributions of BDPb in the HOMOs are mainly localized on
the carbazole segments, whereas their LUMOs are delocalized
over the entire molecule backbone, thus enabling effective
orbital interactions among the stacked p-systems. This in turn
indicates that the twisted molecular configurations are opti-
mized in the LUMOs, as consistent with the PL observation that
BDPb exhibits the largest Stokes shift. In BDPc and BDPd, the
HOMOs are almost localized at the electron-donating carbazole
Ϫ1.5
Ϫ1.0
Ϫ0.5
0
0.5
1.0
1.5
Potential [V] vs Ag/AgCl
Fig. 4. Cyclic voltammograms of BDPa–drecorded in CH2Cl2, containing
0.1 M Bu4NPF6 as the supporting electrolyte, at a scan speed of 100 mVsꢁ1
.
Table 2. Solution electrochemical properties of BDP a–d
Conjugate
Egopt[eV]A
Eoxonset[eV]B
Eoxp[eV]C
Ered[eV]D
HOMO [eV]
LUMO [eV]E
BDPa
BDPb
BDPc
BDPd
2.25
2.24
1.91
1.84
1.25
1.11
1.21
1.32
1.41
1.22
1.33
1.45
ꢁ1.00
ꢁ1.13
ꢁ0.70
ꢁ0.52
ꢁ5.52
ꢁ5.38
ꢁ5.48
ꢁ5.59
ꢁ3.27
ꢁ3.14
ꢁ3.57
ꢁ3.75
D
opt
.
AEgopt=1240/lonset
.
BEoxonset, onset oxidation potential. CEoxp, oxidation peak potential. Ered, reduction potential, was calculated as Eoxonset ꢁ Eg
EHOMO = [ꢁ(Eoxonset ꢁ 0.52)ꢁ4.8]; LUMO = HOMO þ Eg
opt
.

H
Q. Zong et al.
71.097
Ϫ36.465
BDPa
BDPb
44.393
1.852
27.017
5.536
BDPc
BDPd
Fig. 5. Optimized structures of BDPa–d.
LUMO
LUMO
LUMO
LUMO
Gap ϭ 1.944 eV
Gap ϭ 1.728 eV
Gap ϭ 1.630 eV
Gap ϭ 1.618 eV
HOMO
HOMO
HOMO
HOMO
BDPa
BDPb
BDPc
BDPd
Fig. 6. Electron distribution in HOMO and LUMO levels of BDPa–d.

Dual BODIPY-Carbazole Conjugates
I
fragment with some antibonding characters on the bridged
thiophene and furan. In contrast, the electron clouds in both
conjugates are widely spread over the molecule for the LUMOs.
This indicates that this planar structural feature facilitates the
delocalization of p electrons over the entire conjugated system,
which is beneficial to stabilizing the LUMO and thus reducing
the band gap, causing a red-shift of the p–p*/ICT band as
observed in the absorption measurements. Fig. 6 presents the
HOMO–LUMO energy differences (energy band gaps, calcu-
lated Egcal) of BDPa–d. The trend of the predicted Egcal values
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opt
(1.62–1.94 eV) is consistent with that of the estimated Eg
(calculated from the ground-state absorption, 1.84–2.25 eV) in
Table 1.
Conclusions
Four novel dual BODIPY-carbazole conjugates (BDPa–d)
featuring a carbazole motif as the central core and BODIPY as
the terminal group, containing none, phenyl, thiophene, and
furan as p-bridges, were successfully synthesized. The
absorption spectra, electrochemical curves, and theoretical
calculations clearly demonstrated that the pbridges play
important roles in their optoelectronic properties.
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In BDPa, carbazole and BODIPY fragments present inde-
pendent spectral absorption and no obvious ICT characteristics
due to the large dihedral angle between the two units. Phenyl
bridges induces more distortion in the molecular conjugate
skeleton and cannot form effective extended molecular conju-
gation in BDPb. In contrast, the thiophene and furan-linked
conjugates, BDPc and BDPd, exhibit more coplanar molecular
structures, leading to extended and red-shifted absorption bands.
The luminescence of BDPa–d can be observed in the near-
infrared region with large Stokes shifts. BDPa and BDPb
present moderate fluorescence quantum yields, whereas the
p-conjugation extended BDPc and BDPd display very low
values in both solution and films. The HOMO and LUMO
levels, as investigated by CV, are approximately ꢁ5.5 eV and
ꢁ3.6 eV, respectively. The results suggest that BDPa and BDPb
are suitable for fluorescence-based applications due to their near-
infrared emission, whereas BDPc and BDPd are potential candi-
dates for p-type semiconductor materials in organic solar cells.
The results also demonstrate that in addition to the electron-
donating unit and electron-accepting unit control, the choice of
suitable p bridges should be taken into account in the molecule
design as well to meet different application requirements.
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Supplementary Material
1H NMR and 13C NMR spectra and mass spectra of all unknown
compounds are available on the Journal’s website.
Acknowledgements
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We greatly acknowledge financial support from the Nature Science Founda-
tionofGuangdongProvince(Grant nos 2016A030313131, 2016A030313128,
2014A030313630) and the National Nature Science Foundation of China
(Grant nos 51476036, 21342003).
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