Synthesis of fluorene-based di-BODIPY dyes containing different aromatic linkers and their properties

Article abstract of DOI:10.1016/j.tetlet.2015.11.023

Three novel fluorene-based symmetric di-BODIPY dyes (BDPa-c) featuring a fluorene motif as the central core, BODIPY as the terminal groups, and several types of aromatic groups, including phenyl, thiophene, and furan, as linkers were synthesized. The optical and electrochemical properties of these BODIPY dyes were investigated, and DFT theoretical calculations were performed. All these dyes show absorption bands in the region of 300-650 nm with high extinction coefficients. Furan-linked dye BDPc exhibit a more coplanar molecular structure, which increases the efficient π-conjugation length and causes intensive intramolecular charge transfer (ICT), resulting in broadened and red-shifted absorption bands. The luminescence of these dyes is located in near-infrared regions with large Stokes shifts (1013-4862 cm^-1). The HOMO/LUMO energy levels, as investigated by CV are approximately 5.5 eV and 3.6 eV, respectively.

Full text of DOI:10.1016/j.tetlet.2015.11.023

Tetrahedron Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of fluorene-based di-BODIPY dyes containing different
aromatic linkers and their properties
a
a
a
b
a
Hongbin Zhao a, Junxu Liao a,b, , Min Peng , Yucai Wang , Weinan Zhou , Bohong Li , Songping Shen ,
⇑
Zechuan Xie a
a College of Chemistry and Environmental Engineering, Dongguan University of Technology, Dongguan 523808, People’s Republic of China
b College of Energy and Chemical Engineering, Dongguan University of Technology, Dongguan 523808, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel fluorene-based symmetric di-BODIPY dyes (BDPa–c) featuring a fluorene motif as the central
core, BODIPY as the terminal groups, and several types of aromatic groups, including phenyl, thiophene,
and furan, as linkers were synthesized. The optical and electrochemical properties of these BODIPY dyes
were investigated, and DFT theoretical calculations were performed. All these dyes show absorption
bands in the region of 300–650 nm with high extinction coefficients. Furan-linked dye BDPc exhibit a
Received 15 August 2015
Revised 2 November 2015
Accepted 9 November 2015
Available online xxxx
more coplanar molecular structure, which increases the efficient
p-conjugation length and causes
Keywords:
BODIPY
Fluorene
Linkers
Photoelectric properties
DFT calculation
intensive intramolecular charge transfer (ICT), resulting in broadened and red-shifted absorption bands.
The luminescence of these dyes is located in near-infrared regions with large Stokes shifts
(1013–4862 cmÀ1). The HOMO/LUMO energy levels, as investigated by CV are approximately 5.5 eV
and 3.6 eV, respectively.
Ó 2015 Elsevier Ltd. All rights reserved.
Derivatives of borondipyrromethenes (BODIPYs) form an
important class of dyes subject to considerable interest owing to
a unique combination of facile synthesis, stability, high absorption
coefficient, and high photoluminescence efficiency.1,2 Additionally,
the spectroscopic and photophysical properties of BODIPYs can be
fine-tuned by attachment of ancillary residues at the appropriate
positions of the BODIPY core3–6 by carrying out various synthetic
reactions on BODIPYs. The benefit of these appealing characteris-
tics is that they have been shown as promising for a variety of
applications including biological labeling7–9 as electroluminescent
devices, as tunable laser dyes,10 as potential candidates for
solid-state solar concentrators,11–15 as fluorescent switches16–19
and fluorophores in sensors, and as potential photosensitizers in
photodynamic therapy of cancer.20–23 Recently, increased efforts
have led to numerous BODIPY derivatives using this flexible
functional materials for organic light-emitting diodes,29 solar
cells,30 photosensitizers,31 fluorescence microscopy,32 and so on.
Consequently, the combination of BODIPY dyes with fluorene
moieties to construct D–A type fluorene–BODIPY conjugates for
optoelectronic applications has attracted interest. Recently, a few
researchers have prepared several novel fluorene–BODIPY
conjugates, containing both small-molecules and polymers, and
studied their basic optoelectronic properties and discussed their
potential applications in NIR emissions,33 solar cells,34 and
panchromatic materials.35 However, such studies are scarce, so
the construction of more novel molecules based on fluorene and
BODIPY with structural variety and the systematic and in-depth
study of their potential properties seem important. To the best of
our knowledge, the small-molecular fluorene-based symmetric
di-BODIPY dyes with different aromatic linkers have not been
reported. Developing an understanding of this type of compound
and obtaining insight into its potential applications has aroused
our interest.
platform. On the other hand, electronic p-systems containing fluo-
rene rings represent an attractive class of aromatic compounds to
build various molecular structures.24 Fluorene based systems pos-
sess unique photophysical properties such as high fluorescent
quantum yield, large optical nonlinearities, large photostability,
and excellent hole-transporting properties.25–28 Due to these
properties fluorene derivatives have been used extensively as
In this contribution, three novel fluorene-based di-BODIPY con-
jugates featuring a fluorene motif as core, BODIPY as terminal
groups, and several types of aromatic group, including phenyl,
thiophene, and furan, as linkers were synthesized. The synthetic
routes and the structures of target BODIPY dyes are outlined in
Scheme 1. The photophysical and electrochemical properties and
theoretical calculations of these complexes were systematically
⇑
Corresponding author. Tel.: +86 158 1831 8389; fax: +86 769 2286 1233.
E-mail address: liaojx83@126.com (J. Liao).
http://dx.doi.org/10.1016/j.tetlet.2015.11.023
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhao, H.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.11.023

2
H. Zhao et al. / Tetrahedron Letters xxx (2015) xxx–xxx
C8H17
C8H17
C8H17
C8H17
O
O
O
O
B
B
C8H17Br,
NaOH
DMSO
O
O
O
O
Linker
OHC
Br
Br
Br
Br
Br
B
B
Pd(dppf)2Cl2,
KOAc, DMF
Pd(PPh3)4,
Na2CO3,
3
1
2
toluene/H 2
O
C8H17
C8H17
C8H17
C8H17
NH
NH
HN
HN
1) TCQ, CH2Cl2
Pyrrole
InCl3, rt
Linker
Linker
Linker
Linker
Linker
OHC
CHO
.
2) Et3N, BF3 Et2O
4a-c
5a-c
C8H17
C8H17
N
N
N
N
F
F
F
F
B
Linker
Linker
B
=
,
,
S
O
a
b
c
BDP a-c
Scheme 1. The synthetic route of BODIPY dyes BDPa–c.
investigated with the aim of understanding the structure–property
correlations and developing novel opto-electronic materials.
For the construction of these fluorene-based di-BODIPY conju-
gates, we utilized the classical condensation–chelation reaction
sequence to execute the conversion from the dialdehydes to the
The normalized UV–vis absorption spectra of these BODIPY
dyes BDPa–c in CH2Cl2 solution (2 Â 10À5 mol/L) are shown in
Figure 1, and the corresponding optical data are summarized in
Table 1. All of these BODIPY dyes show two extinct absorption
bands. In BDPa, the more energetic one ranging 280–350 nm is
desired dyes, where fluorene-based dialdehyde
4
should be
assigned to the
overlapping with the S0–S2 transition, while the less energetic
one located at 450–530 nm is attributed to the
p⁄, S0–S1 transi-
p–
p⁄ transition, derived from fluorene residues
prerequisite for the synthesis (Scheme 1). Thus, our synthetic
approach started with the preparation of 4 from commercially
p–
available 2,7-dibromo-9H-fluorene
1
through
a
three-step
tion of the indacene unit with a vibronic structure due to the minor
red-shifted absorption profile compared to the meso-phenyl sub-
stituted BODIPY. In comparison with BDPa, BDPb and BDPc exhibit
significant bathochromic-shifted absorption profiles with peaks at
515 and 530 nm, respectively. This could be explained as BDPb and
BDPc possess more excellent spatial configurations, which result in
sequence involving alkylation, bis Miyaura–Suzuki cross coupling
reaction. In particular, alkylation of 1 with 1-bromooctane could
readily gain compound 2. For introducing the aromatic ring linker,
the Suzuki–Miyaura cross-coupling reaction was employed to
complete this conversion. The bis-borate ester 3 was prepared
from 2 via a Miyaura coupling reaction. Subsequently, Suzuki cou-
pling reaction was performed to give the desired aromatic ring
linked dialdehydes 4a–c. With the key intermediate 4, the conden-
sation reaction was performed between 4a–c and pyrrole in the
presence of InCl3 according to the mild procedure we had reported
previously. Readily afforded dipyrromethanes 5a–c in satisfactory
yields. Finally, the desired dyes BDPa–c were obtained by
executing the chelation reaction after oxidation with TCQ. All these
intermediates and the final fluorene-based di-BODIPY dyes were
characterized by a range of spectroscopic techniques, including
1H NMR, 13C NMR, and MALDI-TOF-MS.
efficient
p-extension in these covalent molecules. In BDPb and
BDPc, the more intensive absorption band regions of 422–
610 nm and 430–650 nm, respectively, should be ascribed to a
mixture of
p–
p⁄ and intramolecular charge transfer (ICT) charac-
ters due to their more coplanar conjugated structures. Compared
with BDPa, in BDPb and BDPc, the less intensive absorption band
region of 300–430 nm originating from the p–
p⁄ transition of flu-
orene residue also shows some degree of bathochromic-shift. All
of these dyes show relatively high extinction coefficients, ranging
from 3.7 Â 104 to 7.8 Â 104 cmÀ1 M-1 in solution, which ensure
their excellent light harvesting ability. The energy band gaps
(Eogpt) of these dyes, calculated from their absorption spectra are
2.36, 2.03, and 1.88, respectively. To be noted that all of these dyes
exhibit perfect solubility and film-forming ability, which are fasci-
nating features for organic material-based electronic devices.
The photoluminescence of dyes BDPa–c in different solvents
were investigated at room temperature (see Fig. 22 in ESI). Rho-
damine B (=0.71) was used as standard in methanol to estimate
the fluorescence quantum yield.36 Their emission spectra in CH2Cl2
solution at a concentration of 1 Â 10À7 mol/L were illustrated in
Figure 2 and the corresponding emission data were listed in
Table 1. The excitations of these compounds at their respective
Table 1
The basic photophysical data of BDPa–c
Eogpt
Entry kmax
emax
kem
(nm)
S–
Uf
(nm)
MÀ1 cmÀ1
Sa(cmÀ1
)
(eV)
BDPa 503
BDPb 515
BDPc 530
63,700
78,200
36,600
2.36
2.03
1.88
530
623
714
1013
3366
4862
0.42
0.08
0.07
a
Stokes-shift.
Figure 1. Normalized absorption spectra of BDPa–c in CH2Cl2.
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H. Zhao et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
Figure 2. Fluorescence spectra of BDPa–c in CH2Cl2 solution, c = 1 Â 10À7 mol/L.
Figure 3. Cyclic voltammograms of BDPa–c in CH2Cl2.
absorption band maxim at room temperature result in near-
infrared emissive BODIPY dyes with emission maxima of 550,
623, and 714 nm, respectively. This can be explained as being
due to a photo-induced energy transfer from the BODIPY
excited state to the lower lying singlet excited state of the donor
Table 2
Solution electrochemical properties of BDPa–c
Eopx
Eogpt
red
onset
Entry
HOMO
(eV)
LUMOd
(eV)
ox
onset
a
b
c
p–
p⁄
E
E
(V)
(V)
(V)
(eV)
moieties and/or to the existence of new non-radiative pathways
from the BODIPY
BDPa 2.36
BDPb 2.03
BDPc 1.88
1.19
1.28
1.34
À1.0
À0.85
À0.78
À0.56
À5.46
À5.55
À5.61
À3.10
À3.52
À3.73
p–
p⁄ excited state to the ground state. BDPb–c
À1.16
À1.08
exhibit red-shifted emission bands compared to BDPa. This should
be attributed to BDPb–c inherently possessing more coplanar
structures resulting in increased effective conjugation lengths,
which facilitate the intramolecular charge transfer. The fluores-
cence spectra of each dyes display minor differences in different
solvents, including ethyl acetate, dichloromethane, and acetone
(Fig. 22 in ESI). Additionally, the excitation spectra match the
absorption spectra for each of BDPs (Fig. 23 in ESI). These observa-
tions indicate that these emissions are not caused by solvent effect
or/and aggregation effect. All of these BDPs present red-shifted
emission bands compared to the meso-phenyl substituted BODIPY,
associated with the large Stokes shifts ranging from 1013 cmÀ1 to
4862 cmÀ1 implying that the fluorescence spectra of these BDPs
are typical for ICT emissions. It is also worth noting that BDPa
has a 42% fluorescent quantum yield, while BDPb–c have relatively
low values. These could be attributed to the increased internal
conversion according to the energy gap law that states the non-
radiative deactivation probability of S0–S1 increases as the energy
gap of S0–S1 decreases in an extended conjugating system.
In order to investigate the electrochemical behaviors of BDPa–c,
the electronic states of these dyes were studied through cyclic
voltammetry in CH2Cl2 solution at a scan rate of 100 mV sÀ1 using
n-Bu4NPF6 as supporting electrolyte. Potentials were standardized
with ferrocene/ferrocenium (Fc/Fc+) couple as internal reference
versus Ag/AgCl. The cyclic voltammograms of BDPa–c are illus-
trated in Figure 3, and their electrochemical properties are summa-
rized in Table 2. As shown in Figure 3, dyes BDPa–c present
reversible onset oxidation potential of 1.19, 1.28, and 1.34 V,
a
b
c
ox
E
, onset oxidation potential.
onset
Eopx, oxidation peak potential.
red
onset
E
, reduction potential.
EHOMO = [À(EoonxsetÀ0.53)À4.8] eV, ELUMO = EHOMO + Egopt eV.
d
heterojunction (BHJ) solar cell devices.37,38 The reduction potential
versus NHE (Ered), which corresponds to the lowest unoccupied
molecular orbital (LUMO versus NHE), can be obtained from E
ox
onset
and Eg, where Eg is estimated from the absorption thresholds from
absorption spectra in solution. The results are shown in Table 2.
The HOMO/LUMO levels and optical properties of these dyes
suggest that they can potentially be applied as favorable n-type
semiconductors, which are a research focus in non-fullerene
organic BHJ solar cells.39,40
For a better understanding of the structural and electronic fea-
tures of these novel dyes, the DFT theoretical calculations were fur-
ther performed, and the optimized structures and the electronic
distribution in HOMO/LUMO levels are presented in Figures 4
and 5. All calculations were carried out with the Gaussian 09 pro-
gram suite by using the B3LYP method and 6–31 G⁄ basis set. From
the optimized ground-state geometries of the dyes in Figure 4, we
observed that the dihedral angle between the central fluorene and
phenyl linker and the dihedral angle between the BODIPY frame-
work and phenyl linker are computed to be 105.5° and 38.3°,
respectively, for BDPa. The values for BDPb and BDPc are
43.5°/19.8° and 29.4°/12.2°, respectively. In general, a more copla-
nar structure leads to a larger extended p-conjugation in covalent
respectively. This indicates that the aromatic linkers have obvious
molecules, deservedly resulting in bathochromic shift in the
absorption and emission spectra. These calculated results indicate
that BDPb and BDPc have higher degrees of molecular coplanarity
than BDPa, which is in accordance with the UV–vis absorption
spectra observed in solution. Compared with BDPb, the furan-
linked BDPc exhibits more coplanarity, which suggests that furan
is more suitable for the construction of
thiophene. The electron distributions of BDPa–c indicate that the
-electrons in the HOMOs are mainly localized on the fluorene seg-
ments and linkers, while their LUMOs are delocalized over the
ox
effects on the E
. The reduction potentials (Ered) calculated from
onset
ox
E
ÀEg are approximately matched with the experimental values
onset
obtained from reduction waves. The highest occupied molecular
orbital (HOMO) energy levels of BDPa–c were calculated (5.46,
5.55, and 5.61 eV, respectively) from the equation EHOMO
=
ox
[À(E
À0.52)À4.8] eV, where 0.52 V is the value for ferrocene
onset
p-extended molecules than
versus Ag/AgCl and 4.8 eV is the energy level of Fc/Fc+ relative to
the vacuum energy level. These dyes are actually located at
very low energy levels, which is an attractive property in bulk-
p
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4
H. Zhao et al. / Tetrahedron Letters xxx (2015) xxx–xxx
BDPa
BDPb
BDPc
Figure 4. Optimized structures of BDPa–c.
LUMO
Gap
1.819
1.652
1.563
HOMO
BDPa
BDPb
BDPc
Figure 5. Electron distribution in HOMO and LUMO levels and energy gaps of BDPa–c.
entire molecule backbone, enabling effective orbital interactions
among the stacked -systems (Fig. 5). Thus, the HOMO–LUMO
transition, that is, the dominant contributing configuration to the
S1 state in BDPa–c should be ascribed to a mixture of
p⁄ and
approximately 5.5 eV and 3.6 eV, respectively, making them
fascinating candidates for n-type semiconductor materials in
non-fullerene organic solar cells.
p
p
–
ICT characters, which is consistent with their absorption assign-
ments. On the other hand, the calculated results clearly demon-
strate that a strongly electron-donating substituent with a more
coplanar geometry decreases the HOMO–LUMO gap, causing a
Acknowledgments
We greatly acknowledge the financial support from the Nature
Science Foundation of Guangdong Province (2014A030313630),
the National Nature Science Foundation of China (51476036,
21342003), Project for High-level Personnel of University in
Guangdong Province ([2013]246), the Production-Study-Research
Combinational Project of Guangdong Province and Ministry of Edu-
cation (2012B091100296), the University Science and Technology
Innovation Key Project of Guangdong Province, (cxzd1148) and
the University and Scientific Research Institution Key Project of
Dongguan City (2012108101005).
red-shift of the
p–
p⁄/ICT band. This trend follows that observed
from the UV–vis absorption measurement. Figure 5 presents the
HOMO–LUMO energy differences (energy band gaps, calculated
Ecgal) of BDPa–c. We could see that the trend of the predicted Ecgal
values (1.82–1.56 eV) is consistent with that of the estimated Egopt
(calculated from the ground-state absorption, 2.36–1.88 eV) in
Table 1.
In summary, three novel fluorene–BODIPY dyes (BDPa–c) fea-
turing a fluorene motif as the central core, BODIPY as the terminal
groups, and several types of aromatic groups, including phenyl,
thiophene, and furan, as linkers were successfully synthesized.
Absorption studies indicate that the aromatic linkers play an
important role in this series of BODIPY dyes. Furan and thio-
phene-linked dyes exhibit more coplanar molecular structures
thus leading to extended and red-shifted absorption band regions
in 300–650 nm and higher extinction coefficients. The lumines-
cence of these dyes is located in near-infrared regions with large
Stokes shifts. BDPa has a 42% fluorescent quantum yield, while
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.11.
023.
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