Hexaphenylbenzene end-capped tri(p-phenylenevinylenes): Synthesis and properties

Article abstract of DOI:10.1016/j.tetlet.2014.07.099

Three new hexaphenylbenzene end-capped tri(p-phenylenevinylenes) were synthesized by the Horner-Wadsworth-Emmons reaction in good yields (86-76%). All compounds have bright fluorescent emissions in the blue to blue-green region in solution (λ_max

Full text of DOI:10.1016/j.tetlet.2014.07.099

Tetrahedron Letters 55 (2014) 5203–5206
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Hexaphenylbenzene end-capped tri(p-phenylenevinylenes):
synthesis and properties
⇑
Debabrata Jana, Binay K. Ghorai
Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711 103, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new hexaphenylbenzene end-capped tri(p-phenylenevinylenes) were synthesized by the Horner–
Wadsworth–Emmons reaction in good yields (86–76%). All compounds have bright fluorescent emissions
in the blue to blue–green region in solution (kmax = 445–492 nm in tetrahydrofuran) and also high emis-
sion efficiency (Ufl = 0.51–0.72 in tetrahydrofuran). They exhibit good electrochemical reversibility, high
thermal stability, and have high HOMO value.
Received 25 June 2014
Revised 21 July 2014
Accepted 23 July 2014
Available online 30 July 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Hexaphenylbenzene
Horner–Wadsworth–Emmons reaction
Cyclic voltammetry
Photoluminescence
Blue chromophores
Conjugated organic materials such as oligo(p-phenylenevinyl-
ene) (OPV) have attracted much attention due to their unique opti-
cal and semiconducting properties.1 The OPV derivatives are
which arise from the mutual steric interactions of the phenyl
groups that hampers stacking interaction. In addition, hexa-
p–p
phenylbenzenes (HPBs) are direct precursors of hexabenzocoron-
ene (HBC) derivatives, which are potential candidates for organic
semiconducting materials.6 Now a days, several HPB derivatives
have emerged as attractive candidates for a variety of applications
such as organic emitting materials,7 synthetic graphene,8 crystal
engineering,9 liquid crystals,10 PET-chromophores,11 proton-trans-
port materials,12 and also chemo-sensors.13 However, so far, p-
phenylenevinylene derivatives with hexaphenylbenzene moieties
as end group have not at all been explored. In our recent publica-
characterized by an extended
p-conjugated system with different
chromophore-linked molecular architectures. The incorporation
of different organic chromophores or functional moieties into the
OPV backbone provides fine-tuning of their optical and electronic
properties. There is a continuous strive to develop new, high per-
formance organic chromophores which provide excellent thermal
and electrochemical stabilities, as well as desirable photolumines-
cent properties. In that case, hexaphenylbenzene (HPB) chromoph-
ores are regarded as promising candidates for organic light
emitting diodes (OLED) because of their several merits with regard
to both synthetic strategy and intrinsic characteristics, such as high
photoluminescence (PL) efficiency and high thermal stability.2 HPB
derivatives can be seen as the model compound for the preparation
of dendritic poly(p-phenylenes) (PPPs); another kind of important
blue-emitting conjugated polymers.3 Recently, Müllen and co-
workers4 synthesized hexaphenylbenzene based materials with
strong blue emission and high thermal robustness. Schenning
and co-workers,5 also reported the synthesis of oligo(p-phenylene-
vinylene) (OPV) dendrimer with hexaphenylbenzene as a core mol-
ecule and studied their self-assembly behavior. The important
characteristics of HPB oligomers are their unique photophysical
properties, their thermal stability and their propeller-like shape,
tions, the synthesis and optical properties of several p-conjugated
materials have been demonstrated.14 In the present study, we
report a successful synthesis of three blue emitting hexaphenyl-
benzene end-capped tri(p-phenylenevinylene) (TPV) derivatives
and their photophysical properties.
For the synthesis of HPB-modified TPV we have utilized the
Horner–Wadsworth–Emmons (HWE) reaction. The required HPB
aldehyde derivative (2/4/7) was prepared in two steps, as illus-
trated in Scheme 1. Sonogashira coupling of 4-bromobenzaldehyde
with phenylacetylene in the presence of a catalytic amount of
PdCl2(PPh3)2 in dimethylformamide followed by Diels-Alder cyclo-
addition reaction with tetraphenyl-cyclopentadienone in diphenyl
ether afforded 2 in 74% yield. HPB aldehyde derivatives 4 and 7
were synthesized from 3-iodo-4-methoxybenzaldehyde and 4-
[bis-(4-iodophenyl)amino]benzaldehyde (5)15 respectively, in
good yield, using same reaction protocol. The synthetic route for
targeted HPB-substituted trimers 9–11 is shown in Scheme 2.
⇑
Corresponding author. Tel.: +91 33 26684561; fax: +91 33 26682916.
E-mail address: binay@chem.becs.ac.in (B.K. Ghorai).
http://dx.doi.org/10.1016/j.tetlet.2014.07.099
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

5204
D. Jana, B. K. Ghorai / Tetrahedron Letters 55 (2014) 5203–5206
CHO
CHO
OHC
OMe
CHO
Br
CHO
CHO
i
ii
74%
iii
88%
ii
65%
85%
I
OMe
OMe
2
3
1
4
CHO
N
CHO
CHO
iii
62%
ii
60%
N
N
I
I
5
6
7
Scheme 1. Reagents and conditions: (i) phenylacetylene, Pd(PPh3)2Cl2, Ph3P, CuI, Et3N, DMF, 80 °C, 12 h; (ii) tetraphenylcyclopentadienone, Ph2O, reflux, 6 h; (iii) phenyl
acetylene, Pd(PPh3)2Cl2, Ph3P, CuI, Et3N, THF, rt, 12 h.
O C12H25
2, i
86%
C12H25
O
9
OMe
O C12H25
PO(OEt)2
OC12H25
4, i
C12H25
O
C12H25
O
MeO
82%
(EtO)2OP
10
8
O C12H25
N
N
C12H25
O
7, i
76%
11
Scheme 2. Synthesis of HPB modified
p
-conjugated trimers 9À11. Reagents and conditions: (i) tBuOK, THF, rt, 12 h; then catalytic I2, toluene, reflux, 48 h.
The HWE reaction was used for the formation of
p
-conjugated HPB
the cis isomers to all trans isomers. The newly synthesized HPB-
substituted conjugated trimers 9–11 were characterized by 1H
NMR and MALDI-MS, and were found to be in good agreement with
their structures. The dodecyloxy groups in the central unit
attached to the 2 and 5 positions of the benzene ring increase
the solubility in organic solvents. This also helps in the electron-
donating ability of the trimers.
based trimers. The reaction between HPB aldehyde derivatives (2/
4/7) and [4-(diethoxyphosphorylmethyl)-2,5-bisdodecyloxyben-
zyl]phosphonic acid diethyl ester (8) in the presence of KOtBu in
THF yielded 9–11, as a mixture of cis and trans isomers. In order
to get all trans products, a catalytic isomerization of the mixtures
was performed in the presence of iodide in toluene to convert

D. Jana, B. K. Ghorai / Tetrahedron Letters 55 (2014) 5203–5206
5205
4x106
3x106
2x106
The UV–vis absorption spectra of the synthesized compounds
9–11 were recorded in THF at room temperature as shown in
Figure 1. In the absorption spectra, trimers 9 and 10 display two
prominent bands around 328–332 nm (
e
ꢀ 4.58 Â 10À4 cmÀ1 MÀ1
ꢀ 3.48 Â 10À4 cmÀ1 MÀ1 for 10) and 398–402 nm
9 (excited at 402 nm)
10 (excited at 398 nm)
11 (excited at 436 nm)
for 9 and
e
(
e
ꢀ 6.48 Â 10À4 cmÀ1 MÀ1 for
9
and
e
ꢀ 7.14 Â 10À4 cmÀ1 MÀ1
for 10), while 11 exhibits similar absorption behavior with more
intense and bathochromically shifted bands located at 340 nm
(e
ꢀ 7.21 Â 10À4 cmÀ1 MÀ1) and 436 nm (
e
ꢀ 10.85 Â 10À4 cmÀ1 MÀ1).
Such bathochromic shifts may be a result of the enhancement of
an intramolecular charge transfer (ICT) process in the presence of
the electron-donating amine group in 11.
1x106
0
Strong fluorescence has been observed in all HPB-trimers 9–11.
The PL spectra of trimers 9–11 in THF (10À6 mol LÀ1) are shown in
Figure 2 and the spectral data are summarized in Table 1. The PL
spectra of 9 (exited at 402 nm) and 11 (exited at 398 nm) show a
peak at 453 nm (with a shoulder at 480 nm) and 445 nm (with a
shoulder at 470 nm), respectively. The PL emission of 11 (exited
at 436 nm) peaking at 492 nm (with a shoulder at 520 nm) was
39 nm red shifted compared to 9, which suggests that there was
450
500
550
600
650
Wavelength (nm)
Figure 2. Photoluminescence spectra of trimers 9–11 in tetrahydrofuran
(C = 1.0 Â 10À6 M).
more
p-delocalization due to the presence of amine moiety in
the trimer backbone. The color of the fluorescence of 9–11 ranges
from blue (9 and 10) to blue–green (11) in dilute solution. The tri-
mers 9–11 display moderate to high Ufl values between 51 and
72%. It is remarkable that 9 and 10 display high Ufl values of 72%
and 65%, respectively. Thus, they are good emitters, due to a com-
1.5x10-4
9
1.2x10-4
10
11
bination of high Ufl and large
e values.
The electrochemical properties of compounds 9À11 were stud-
ied in anhydrous dichloromethane using three-electrode electro-
chemical cell with tetrabutylammonium hexafluorophosphate
(Bu4NPF6, TBAF) (0.1 M) as an electrolyte and Ag/AgCl as a refer-
ence electrode (Fig. 3 and Table 1). Trimer 9 shows two quasi-
reversible oxidation peaks but trimers 10 and 11 exhibit three
quasi-reversible oxidation peaks. The onset oxidative potentials
for compounds 9À11 were measured as 0.78, 0.78, and 0.76 V,
respectively. All trimers have nearly same onset oxidation poten-
tial, which may be arising from the central 1,4-bis-dodecyloxyben-
9.0x10-5
6.0x10-5
3.0x10-5
0.0
-3.0x10-5
zene segment. On the basis of the equation, HOMO
=
À[Eoonxset + 4.66 eV],16 the HOMO energy levels for 9À11 were calcu-
lated as À5.44, À5.44, and À5.42 eV, respectively. Meanwhile, the
LUMO energy levels for compounds 9–11 were calculated as
À2.36, À2.33, and À2.58 eV, respectively. The HOMO-LUMO gaps
0.0
0.4
0.8
1.2
1.6
2.0
Potential (V)
Figure 3. Cyclic voltammetry of trimers 9–11 (ꢀ1 mM) measured in 0.1 M n-
(DE) of samples 9–11 are 3.08, 3.11, and 2.84 eV, respectively. It
Bu4NPF6 CH2Cl2, scan rate 100 mV/s.
is worth noting that the change of the substituent groups from
phenyl to anisole and then to amine moieties induces a decrease
in LUMO energy level but causes a little influence of HOMO energy
level, which results in the smaller band gap of compound 11 com-
pared with that of compounds 9 and 10. The lower band gap for 11
is probably attributable to intramolecular charge transfer from
0.15
amine ring to the central p-conjugated core.
The thermal properties of the HPB based trimers were analyzed
by thermogravimetric analysis (TGA) and are presented in Table1.
The thermal decomposition (Td) temperatures (5% weight loss in
nitrogen atmosphere) were observed in the range of 335–420 °C.
These compounds exhibited good thermal stabilities. The decom-
position temperatures with 5% weight loss under N2 atmosphere
(Td) for 9, 10, and 11 were 335, 345, and 420 °C, respectively. The
Td values increased with the increasing number of HPB unit in
the trimer.
9
10
11
0.10
0.05
0.00
In summary, three new hexaphenylbenzene modified p-conju-
gated trimers have been efficiently synthesized and characterized.
Significant features, such as strong blue/blue–green fluorescence
(Ufl = 51–72%) in solution, good thermal stability (335–420 °C, 5%
300
350
400
450
500
550
600
decomposition temperature), and higher HOMO level (À5.42 eV)
were observed. These properties suggest that HPB trimers can be
employed in light-emitting application.
Wavelength (nm)
Figure 1. Absorption spectra of trimers 9–11 in tetrahydrofuran (C = 1.0 Â 10À6 M).

5206
D. Jana, B. K. Ghorai / Tetrahedron Letters 55 (2014) 5203–5206
Table 1
Photophysical properties of compounds 9–11
a
d
Compd
UV-vis (nm)
PL (nm)
U%
Eox (onset) (eV)
HOMO/LUMOb (eV)
D
Ec (eV)
Td (°C)
9
10
11
332, 402
328, 398
340, 436
453, 480
445, 470
492, 520
72
65
51
0.78
0.78
0.76
À5.44/À2.36
À5.44/À2.33
À5.42/À2.58
3.08
3.11
2.84
335
380
420
a
b
c
Fluorescence quantum yields (
HOMO = À[Eox (onset) + 4.66 eV]; LUMO = HOMO +
E = HOMO-LUMO gap; estimated from the onset of the absorption spectra: 1240/konset
Decomposition temperature (Td, 5% weight loss) obtained from thermogravimetric analysis (TGA).
U
), measured in tetrahydrofuran solution using 9,10-diphenylanthracene (Ufl = 0.90, in cyclohexane) as standard, excited at 375 nm.
D
E.
D
.
d
´
5. Tomovic, Z.; van Dongen, J.; George, S. J.; Xu, H.; Pisula, W.; Leclère, P.;
Acknowledgments
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Financial support from CSIR [No. 02(0150)/13/EMR-II], New
Delhi is gratefully acknowledged. We are thankful to Professor
Anup Mondal, Department of Chemistry, Indian Institute of Engi-
neering Science and Technology, Shibpur, Howrah, India for helpful
discussion.
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