Synthesis, mesomorphism and fluorescence of triphenylene-Bodipy dyads

Article abstract of DOI:10.1016/j.tetlet.2016.09.082

Four novel triphenylene-Bodipy dyads were synthesized in yields of 32–38% and characterized by MS, NMR, elemental analysis. They exhibit good columnar liquid crystal behaviors at room temperature. The alkyl substitution on the Bodipy unit does not cause the disruption of the mesophase. They possess excellent fluorescence properties with high quantum yields in solution and the weak fluorescence in film. The alkyl substitutions on the Bodipy unit increase the fluorescence intensity and the introduction of triphenylene units reduce the aggregation-caused quenching of orderly stacking of Bodipy units.

Full text of DOI:10.1016/j.tetlet.2016.09.082

Tetrahedron Letters 57 (2016) 4939–4943
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, mesomorphism and fluorescence of triphenylene-Bodipy
dyads
Xiaoting Fang a, Hongyu Guo a, Jianrong Lin a, Fafu Yang a,b,
⇑
a College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou 350007, PR China
b Fujian Key Laboratory of Polymer Materials, Fuzhou 350007, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel triphenylene-Bodipy dyads were synthesized in yields of 32–38% and characterized by MS,
NMR, elemental analysis. They exhibit good columnar liquid crystal behaviors at room temperature.
The alkyl substitution on the Bodipy unit does not cause the disruption of the mesophase. They possess
excellent fluorescence properties with high quantum yields in solution and the weak fluorescence in film.
The alkyl substitutions on the Bodipy unit increase the fluorescence intensity and the introduction of
triphenylene units reduce the aggregation-caused quenching of orderly stacking of Bodipy units.
Ó 2016 Elsevier Ltd. All rights reserved.
Received 10 August 2016
Revised 21 September 2016
Accepted 26 September 2016
Available online 28 September 2016
Keywords:
Triphenylene
Bodipy
Synthesis
Mesophase
Fluorescence
In recent years, columnar discotic liquid crystals have been paid
considerable attention for their various potential applications
including organic field-effect transistors, organic photovoltaic cells,
organic light-emitting diodes, gas sensors, etc.1–4 Lately, the
columnar liquid crystals with excellent fluorescence, attracted
much research interests due to the broad application in the novel
liquid crystalline materials possessing unique properties.5–10 4,4-
Difluoro-4-borata-3a,4a-diaza-s-indacene (abbreviated as Bodipy)
is a class of highly fluorescent dye that are attracting increasing
attention based on their intense fluorescence, photochemical sta-
bility and energy- and electron-transfer capabilities.11–16 Investi-
gations have also been carried out to obtain the fluorescent
Bodipy liquid crystal. There are two synthetic methods to attain
this target up to now. One method was the bridging of a long alkyl
chain and a rigid Bodipy unit together (as shown in Fig. 1(a)),
which resulted in the nematic liquid crystals in most cases.17,18
Another method was the bridging of multiple alkyl chains (such
as gallic unit with three alkyl chains) onto one Bodipy unit (as
shown in Fig. 1(b)).19–24 However, this kind of liquid crystal using
Bodipy unit as columnar core might produce an antinomy between
the mesomorphic range and the fluorescence quantum yield, i.e.
the increasing alkyl substitution on the Bodipy fluorophore causes
the restriction of the rotation of the 8-phenyl ring resulting in an
increase in fluorescence intensity, but leads to a disruption of the
mesophase.24 To avoid this antinomy, in theory, if the Bodipy unit
is not the only core stacking units for columnar liquid crystal, but is
staggered with other columnar core to reduce the aggregation
quenching effect (as shown in Fig. 1(c)), the obtained columnar liq-
uid crystal might possess both good mesomorphic property and
good fluorescence. However, such Bodipy liquid crystal has not
been presented so far.
It is well known that triphenylenes are the popular discotic
liquid crystals. They were typical columnar mesophase in most
cases because of the strong p–p interactions for the flat, rigid aro-
matic core structures.25–29 Indeed, synthetic advances have real-
ized a wide range of symmetrical and asymmetrical substituted
triphenylene derivatives possessing various functional groups on
the side-chain to be prepared with columnar mesophases based
on the strong
p–p
interactions of triphenylene core.30–35 Some
triphenylene liquid crystal with good fluorescence groups on
side-chain also were reported.34,35 Thus, when the Bodipy unit
is introduced onto the side-chain of triphenylene derivatives (as
shown in Fig. 1(c)), is this triphenylene-Bodipy dyad possible to
have both good columnar phase and excellent fluorescence prop-
erty? Herein, we wish to report the first synthesis of triph-
enylene-Bodipy dyads and investigate their mesomorphic
properties and fluorescence properties. The results showed that
these triphenylene-Bodipy dyads have both good columnar phase
and excellent fluorescence property, avoiding successfully the
inverse relationship between the mesomorphic range and the
fluorescence quantum yield.
⇑
Corresponding author. Tel./fax: +86 591 83465225.
E-mail address: yangfafu@fjnu.edu.cn (F. Yang).
http://dx.doi.org/10.1016/j.tetlet.2016.09.082
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

4940
X. Fang et al. / Tetrahedron Letters 57 (2016) 4939–4943
other discotic core
BODIPY uint
BODIPY core
alkyl chains
Cr
BODIPY unit
alkyl chain
Col
alkyl chains
Iso
n
n
5d
5d
cooling
heating
cooling
heating
cooling
Cr
Cr
Col
Iso
Col
Col
Col
Iso
n
n
5c
5c
5b
n
Cr
n
Iso
Iso
n
n
Cr
Cr
liquid crystal with
BODIPY core
(b)
nematic liquid crystal
(a)
Bodipy columnar liquid crystal
staggered with other discotic core
(c)
5b
heating
Iso
Iso
Col
Col
Cr
Cr
5a
5a
Figure 1. Three kinds of Bodipy liquid crystals. (a) and (b) had been reported, and
cooling
heating
150
(c) was not presented.
Iso
Col
0
50
100
The synthetic routes were illustrated in Scheme 1. In order to
study the influences of alkyl substitution on mesomorphic and flu-
orescence properties, triphenylene-Bodipy dyads 5a–d with or
without methyl groups on Bodipy were designed. According to
the published procedures,29,36,37 1,2-bispentyloxybenzene 1 was
treated with FeCl3 (5 equiv) to give monohydroxytriphenylene 2,
which was converted to triphenylene derivatives 3a and 3b by
reacting with excess 1,3-dibromopropane or 1,6-dibromohexane.
Further treating 3a and 3b with 4-hydroxy benzaldehyde in
K2CO3/MeCN system afforded triphenylene derivatives 4a and 4b
in yields of 75% and 78% after column chromatography, respec-
tively. Then, according to the typical procedure for synthesis of
Bodipy, the triphenylene-Bodipy dyads 5a–d were prepared by
treating compound 4 with pyrrole (or 2,4-dimethylpyrrole) via
sequential condensation, oxidation, and complexation reactions
in the moderate yields of 30–40% after columnar chromatography.
In their 1H NMR spectra, all obvious peaks were assigned well. The
MS, 13C NMR, FT-IR and elemental analysis also supported their
structures distinctly.
Temperature (oC)
Figure 2. The DSC traces of compounds 5a–d on heating and cooling (scan rate
10 °C minꢀ1). Cr = crystalline, Col = columnar, Iso = isotropic.
Table 1
Phase transfer temperatures (°C) and enthalpy changes (kJ/mol in parentheses) of
5a–d
Dyads
Phase transitiona
T(D
H) heating Scan
T(DH) cooling scan
5a
Cr-Col
Col-Iso
Cr-Col
Col-Iso
Cr-Col
Col-Iso
Cr-Col
Col-Iso
26.4(5.35)
50.6(23.28)
17.9(9.82)
52.7(10.33)
25.1(8.68)
50.6(9.86)
27.1(7.31)
58.5(21.69)
20.4(5.98)
41.3(24.61)
16.6(12.06)
43.7(8.24)
19.4(8.85)
44.2(10.01)
23.8(7.44)
52.4(17.85)
5b
5c
5d
a
Cr = crystalline, Col = columnar, Iso = isotropic.
The mesomorphic properties of triphenylene-Bodipy dyads
5a–d were investigated by differential scanning calorimetry (DSC),
polarizing optical microscopy (POM) and X-ray diffraction (XRD).
Figure 2 and Table 1 showed the DSC traces, phase transfer temper-
atures and associated enthalpy changes. All dyads 5a–d displayed
typical liquid crystalline melting behavior. Two phase transitions
of solid state-mesophase and mesophase-isotropic phase on heat-
ing and cooling were observed. Moreover, dyads 5a–d possessed
similar phase transitions temperatures and mesomorphic ranges
although they have different substitutions on the Bodipy units.
These results were different from the Bodipy liquid crystals with
Bodipy units as cores, in which the alkyl substitution on Bodipy
caused a disruption of the mesophase.24 These phenomena could
be explained by that the columnar triphenylene units of dyads
5a–d were similar and the small changes of the structures on Bod-
ipy influenced little on mesophase. Their wide Iso-Col transitions
and the wide range of transition enthalpies might be attributed
to their big molecular weights and such viscous materials in LC
phase, which were usually observed for triphenylene columnar liq-
uid crystals.38–51 Further observation under POM also suggested
the two phase transitions of solid state-mesophase-isotropic phase
and the phase transfer temperatures agreed with the peaks of DSC
approximately, respectively. They exhibited typical fan-like tex-
ture of mesophase (in Fig. 3). These textures were similar to the
known textures of triphenylene columnar liquid crystal.38–51
The XRD data of dyads 5a–d further confirmed their columnar
liquid crystal behaviors as illustrated in Figure 4. Some obvious
peaks at small angles and wide angles were observed. Due to com-
pounds 5a–d possess similar triphenylene columns, their positions
of reflection peaks were similar. In small angle, a strong reflection
and two weak reflections appeared at 2h = 5.03–5.19°, 8.74–9.02°
and 10.02–10.42°, respectively. These degrees indicated the dis-
tances of 17.01–17.65 Å, 9.80–10.13 Å, and 8.49–8.78 Å, respec-
tively. These data were in agreement with the ratios of
OR
OR
OR
Br
OR
RO
RO
RO
RO
OR
OR
HO
CHO
K2CO3, MeCN
nBr
FeCl3
H2SO4
n
Br
K2CO3, MeCN
OH
O
1
2
3
OR
OR
OR
OR
OR
RO
RO
OR
N
(1) CF3COOH,
R'
R'
RO
RO
p
p
1:1/ 3:1/ 4 approximately for (100), (110) and (200) reflection
peaks, suggesting the hexagonal columnar mesophase for com-
pounds 5a–d. The reflections at about 5° suggested 19.6–20.26 Å
for the lattice parameter ɑ. The diameters of compounds 5a–d
including both triphenylene units and Bodipy units, neglecting
the length of the spacer, were approximately 21 Å based on the
CPK model. These values were close to the lattice parameter ɑ.
The reflections at 2h = 16–25° (3.56–5.53 Å, broad halo) and
O
B
O
n
(2) DDQ
OR
(3) TEA, BF3Et2O
5
O
O
n
OR
R'
R'
4
5a: R = C5H11, n = 3, R' = H
5b: R = C5H11, n = 3, R' =CH3
5c: R = C5H11, n = 6, R' = H
5d: R = C5H11, n = 6, R' = CH3
N
N
CHO
R'
R'
F
F
Scheme 1. Synthesis of triphenylene-Bodipy dyads 5a–d.

X. Fang et al. / Tetrahedron Letters 57 (2016) 4939–4943
4941
esting to study the changes of their fluorescence properties. Their
absorption spectra and fluorescence spectra were illustrated in
Figures 6 and 7. Due to the triphenylene has no obvious absorption
and fluorescence when the wavelength is bigger than 500 nm,52
their OH-Bodipy precursors A53 and B54 with or without methyl
groups, respectively, were used as reference for data comparison.
Table 2 displayed the photophysical data collected for dyads
5a–d and their precursors A and B. The absorption wavelength
and emission wavelengths of 5a–d were at 503 nm and 520 nm
approximately, which were similar with that of the precursors A
or B due to their similar structures of Bodipy units. In Figure 7
and Table 2, both the fluorescence intensity and the fluorescence
quantum yield of compounds 5b and 5d were far higher than that
of compounds 5a and 5c, suggesting that the alkyl substitution on
the Bodipy increased fluorescence obviously. These phenomena
0.12
A
OH
OH
0.10
0.08
0.06
0.04
0.02
0.00
5b
Figure 3. The textures of dyads 5a–d under POM on cooling at 35 °C (ꢁ400).
N
N
F
N
N
F
5d
5c
B
B
F
F
8000
7000
A
B
6000
5.04
5000
21.44
5a
5.03
21.31
21.42
4000
3000
2000
1000
0
5.20
5.06
5a
400
500
600
5b
5c
21.26
Wavelength / nm
Figure 6. Adsorption spectra of dyads 5a–d in toluene (10ꢀ6 M).
-1000
5d
10
20
30
40
Scattering angle
(degree)
1200
Figure 4. XRD traces of compounds 5a–d at 35 °C.
1000
800
600
400
200
0
5b in toluene
2h = 21.26–21.44° (4.14–4.18 Å) were assigned to the average dis-
tance of the molten alkyl chains and the intracolumnar order,
respectively. Thus, based on the above XRD analysis, it was reason-
able to propose the schematic representation of the columnar lay-
ered molecular arrangement of dyads 5a–d as shown in Figure 5.
However, it should also be noticed that Figure 5 was just the
approximation of the most likely molecular stacking due to the
broad reflection peaks suggesting a broad diversity of distances
among the layers and cylinders. The hexagonal columnar meso-
phase was composed of triphenylene moiety and Bodipy moiety
concurrently. All these XRD data suggested that the dyads 5a–d
possessed hexagonal columnar mesophase and the alkyl substitu-
tion on the Bodipy did not cause the disruption of the mesophase.
As dyads 5a–d possessed similar liquid crystal behaviors
regardless of the different structure of Bodipy moiety, it was inter-
5d in toluene
A
5b (thin film)
5c in toluene
5a in toluene
5b (thin film)
5c (thin film)
5a (thin film)
400
500
600
700
Wavelength (nm)
Figure 7. Fluorescence spectra of dyads 5a–d in toluene (10ꢀ6 M). All of com-
pounds were excited at 470 nm.
d100
d110
column
Z
RO
OR
†
R'
R'
F
N
B
Y
F
RO
RO
O
n
O
N
d001
R'
R'
OR
X
Figure 5. The proposed schematic representation of the columnar layered molecular arrangements of dyads 5a–d.

4942
X. Fang et al. / Tetrahedron Letters 57 (2016) 4939–4943
Table 2
Absorption and fluorescence data for the dyads 5a–d in toluene at 298 K
Dyads
kmax/nm (log
e)
kem/nm
UF
(sF/ns) in toluene
kem/nm in thin film
UF in thin film
5a
5b
5c
5d
Aa
Bb
503(4.40)
504(4.97)
503(4.76)
503(4.89)
503(4.99)
501(/)
523
516
525
516
514
518
0.29(2.6)
0.97(4.7)
0.38(3.0)
0.95(4.6)
0.94(4.3)
0.08(/)
582
589
597
594
No
0.02
0.05
0.03
0.04
0
No
0
a
The data of compound A were cited in Ref. 53.
The data of compound B were cited in Ref. 54.
b
were similar with the literature report,24 which could be attributed
to additional rotation restriction of the alkyl groups on Bodipy
units.
Team in Science and Technology in Fujian Province University were
greatly acknowledged.
On the other hand, the fluorescence intensity of dyads 5b and
5d were almost same or strong comparing with their precursors
A, indicating no electronic or energy transfer between triphenylene
and Bodipy. The quantum yields of 5b and 5d were as high as the
precursor A and were more excellent than the other Bodipy liquid
crystals (their values of UF < 0.7 in most cases).19–24 The quantum
yields of dyads 5a and 5c were also far higher than that of their
precursor B. Moreover, photoluminescence properties of dyads
5a–d as thin film were measured. Their emission spectra were
shown in Figure 7 and the quantum yields listed in Table 2. The
peaks were obviously red-shifted and boardening indicating the
existence of aggregation-caused quenching. However, in contrast
to their precursors A and B which showed no solid fluorescence
due to the strong aggregation-caused quenching, dyads 5a–d
exhibited corresponding solid fluorescence although the quantum
yields were low (0.02–0.05). The reinforced fluorescence in solu-
tion and solid state of dyads 5a–d could be attributed to the addi-
tional aggregation restriction of the triphenylene groups for Bodipy
units resulting in the decreases of the aggregation-caused quench-
ing. The low quantum yield in film might indicate that triph-
enylene units and Bodipy units stacked randomly in film. Some
alternate stacking of triphenylene units and Bodipy units was
favorable for fluorescence but the other stacking of Bodipy units
only resulted in aggregation-caused quenching. All these data sug-
gested that the fluorescence properties of dyads 5a–d were more
excellent than that of precursors A and B, respectively. The triph-
enylene unit was favorable not only for producing columnar liquid
crystal but also for reinforcing fluorescence. Combing both the
mesomorphic and photophysical studies of dyads 5a–d, it could
be concluded that dyads 5a–d possessed both good columnar liq-
uid crystal and fluorescence properties. These results made impor-
tant progress in comparison with the literature that the antinomy
existed for the relationship of the mesomorphic property and the
fluorescence property.24
Supplementary data
Supplementary data associated with this article can be found, in
theonlineversion, at http://dx.doi.org/10.1016/j.tetlet.2016.09.082.
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