Triphenylpyridine-based star-shaped π-conjugated oligomers with triphenylamine core: Synthesis and photophysical properties

Article abstract of DOI:10.1016/j.tetlet.2012.01.116

Two novel star-shaped π-conjugated oligomers bearing a triphenylpyridine moiety as peripheral units and triphenylamine as a core have been synthesized via the threefold Heck/Sonogoshira coupling reaction protocol and their absorption and fluorescence prop

Full text of DOI:10.1016/j.tetlet.2012.01.116

Tetrahedron Letters 53 (2012) 1798–1801
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Triphenylpyridine-based star-shaped
p-conjugated oligomers with
triphenylamine core: synthesis and photophysical properties
⇑
Debabrata Jana, Binay K. Ghorai
Department of Chemistry, Bengal Engineering and Science University, 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:
Two novel star-shaped
p-conjugated oligomers bearing a triphenylpyridine moiety as peripheral units
Received 29 December 2011
Revised 23 January 2012
Accepted 25 January 2012
Available online 3 February 2012
and triphenylamine as a core have been synthesized via the threefold Heck/Sonogoshira coupling reac-
tion protocol and their absorption and fluorescence properties have been examined. These oligomers
showed excellent solubility in common organic solvents, emit light in blue and violet regions, and have
high thermal stability.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Triphenylamine
Triphenylpyridine
Star-shaped oligomer
Fluorescence
Star-shaped triphenylamine based
p
-conjugated systems con-
is highly electron rich and possesses a propeller-shaped structure8
and it can maintain uninterrupted conjugation between central
nitrogen lone pair electrons and the arms9 as well as function as
a strong electron donor to the conjugation system.
stitute one of the most extensively studied classes of organic com-
pounds.1 Research efforts in this field cover a broad spectrum of
topics ranging from purely fundamental investigations to explor-
ing the potential applications of individual compounds and their
ensembles in nanoelectronics and solar energy conversion.2 Tri-
phenylamine cored compounds have found to be used as wide
band gap energy transfer materials as well as promising applica-
tions in the area of hole-transporting.3 Specifically, the molecular
architecture and strong absorption and emission properties of this
type complexes separated by different lengths of linkers have ren-
dered these molecules useful as emitters in organic light-emitting
diodes (OLEDs)4, sensitizers in dye-sensitized solar cells (DSSCs),5
and photovoltaic cells.6 It has been found that the molecular and
optical properties of these materials can be controlled by modify-
ing the peripheral units.
The synthetic pathway leading to 2,6-diphenyl-4-(4-vinylphe-
nyl)pyridine (1) and 4-(4-ethynylphenyl)-2,6-diphenylpyridine (2)
is outlined from 4-(4-bromophenyl)-2,6-diphenylpyridine (3) in
Scheme 1. Diphenylpyridine 3 was obtained in a good yield by the
condensation of 3-(4-bromophenyl)-1-phenylpropenone and
acetophenone in the presence of ammonium acetate, using the liter-
ature procedure.7d Treatment of 3 with n-BuLi in THF at ꢀ78 °C for
2 h, followed by addition of N-formylpiperidine, gave the expected
aldehyde 4 in a 70% yield. Wittig condensation of aldehyde 4 and
methylphosphoniumbromide salts in the presence of n-BuLi at
ꢀ20 °C temperature afforded vinyl derivative 1 in a 65% yield. The
Sonogashira coupling reaction of 3 with 2-methyl-3-butyn-2-ol in
toluene at 90 °C for 12 h, afforded 4-[4-(2,6-diphenylpyridin-4-yl)-
phenyl]-2-methylbut-3-yn-2-ol (5) in an 85% yield. The desired
terminal alkyne 2 was obtained from 5 by removal of the acetone
with powdered KOH in a 76% yield.
Pyridine has been a key building block in constructing func-
tional materials in view of its outstanding mechanical and dielec-
tric properties.7 As part of our continuing interest in this area,
we were interested in the synthesis of 2,4,6-triphenylpyridine-
based
p
-conjugated oligomers with triphenylamine core. In this
In order to synthesize oligomers TPA-TPP1 and TPA-TPP2, two
different strategies have been explored which involve threefold
Heck/Sonogoshira coupling reaction (Scheme 2). The core molecule
tris-(4-iodophenyl)amine (6) was readily prepared from triphenyl-
amine in a good yield following the literature procedure.10 The
TPA-TPP1 was prepared using a threefold Heck coupling of 6 with
vinyl compound 1 [10% Pd(OAc)2, P(o-tol)3, Et3N, DMF, 90 °C] in a
75% yield. The TPA-TPP2 was synthesized utilizing threefold
Sonogoshira coupling of 6 with alkynyl derivative 2 in the presence
of CuI/Pd(PPh3)2Cl2/NEt3 in an 86% yield.
Letter, we wish to report the synthesis, characterization, and
photophysical properties of two representative new molecular
structures having triphenylamine as the core molecule with three
branches extending outward connected to 2,4,6-triphenylpyridine
units. The triphenylamine moiety is carefully chosen and intensely
employed in each structure as the core, since triphenylamine group
⇑
Corresponding author. Tel.: +91 33 26684561; fax: +91 33 26682916.
E-mail address: bkghorai@yahoo.co.in (B.K. Ghorai).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.116

D. Jana, B. K. Ghorai / Tetrahedron Letters 53 (2012) 1798–1801
1799
a
b
N
CHO
N
Br
70%
65%
4
1
N
3
c
d
N
N
OH
85%
76%
5
2
Scheme 1. Reagents and conditions: (a) n-BuLi (1.5 equiv), THF, ꢀ78 °C, 2 h, then N-formylpiperidine (2 equiv), rt, 12 h; (b) Ph3P+CH3Brꢀ, n-BuLi, THF, ꢀ20 °C; (c) 2-Methyl-3-
butyn-2-ol, Pd(PPh3)2Cl2, Et3N, PPh3, CuI, toluene, 90 °C, 12 h; (d) Powdered KOH, toluene, reflux, 6 h.
N
N
I
a
b
N
N
N
N
N
86%
75%
I
I
6
TPA-TPP2
TPA-TPP1
N
N
Scheme 2. Reagents and conditions: (a) 1 (3.5 equiv), Pd(OAc)2, P(o-tol)3, Et3N, DMF, 90 °C, 16 h; (b) 2 (3.5 equiv), Pd(PPh3)2Cl2, PPh3, CuI, Et3N, THF, rt, 24 h.
1.0
4.0x105
3.5x105
3.0x105
2.5x105
2.0x105
1.5x105
1.0x105
8x105
7x105
6x105
5x105
4x105
3x105
2x105
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
1x105
5.0x104
0
600
300
350
400
450
500
550
0.0
Wavelength (nm)
300
400
500
600
Wavelength (nm)
Figure 2. Absorption spectra (solid lines) and fluorescence emission spectra
(dashed lines) performed at kex = kmax (abs) of TPA-TPP2 in toluene (black) and in
tetrahydrofuran (red) in dilute solution (ꢁ10ꢀ6 M) at room temperature.
Figure 1. Absorption spectra (solid lines) and fluorescence emission spectra
(dashed lines) performed at kex = kmax (abs) of TPA-TPP1 in toluene (black) and in
tetrahydrofuran (red) in dilute solution (ꢁ10ꢀ6 M) at room temperature.
The UVꢀvis absorption and fluorescence properties of two
synthesized compounds TPA-TPP1 and TPA-TPP2 in toluene
and tetrahydrofuran are shown in Figures. 1 and 2. Their compre-
hensive photophysical characteristics are presented in Table 1.
The absorption spectra of the compounds TPA-TPP1/TPA-TPP2
are almost identical in toluene and tetrahydrofuran and exhibit
maximum absorption wavelengths at 410 nm and 378 nm,
respectively. The maximum absorption peak of the alkenyl
bridged oligomer TPA-TPP1 was red shifted by 32 nm with re-
spect to the alkyne bridged oligomer TPA-TPP2. The
gap (
E) of these oligomers was calculated from the UVꢀvis
absorption maximum. It was obvious that the E value of TPA-
p–
p⁄ energy
D
D
TPP1 could be reduced by the introduction of alkenyl spacers at
the backbone.

1800
D. Jana, B. K. Ghorai / Tetrahedron Letters 53 (2012) 1798–1801
Table 1
Photophysical properties of derivatives TPA-TPP1 and TPA-TPP2.
a
Compound
UV–vis (kabs, nm)
PL (kmax, nm, Ufl
)
Stokes shiftsb (103 cmꢀ1
Tol THF
)
D
Ec (eV)
Td (°C)
Tol
THF
Tol
THF
TPA-TPP1
TPA-TPP2
410
378
411
379
466 (0.14)
426 (0.16)
496 (0.08)
455 (0.12)
0.29
0.30
0.42
0.44
2.73
2.98
212
270
a
b
c
Fluorescence quantum yields, measured in solution using 9,10-diphenylanthracene (Ufl = 0.90, in CH2Cl2) as standard, excited at 365 nm.
Stokes shift = (1/kabs ꢀ 1/kem).
Determined from UV–vis absorption maximum.
The photoluminescence (PL) spectra of TPA-TPP1 (exited at
Acknowledgments
410 nm, Fig. 1) and TPA-TPP2 (exited at 378 nm, Fig. 2) in toluene
showed maximum emission bands at 466 and 426 nm, respec-
tively. In comparison to alkyne spacer TPA-TPP2, vinyl spacer
TPA-TPP1 showed a red shift of emission band. This is due to the
Financial support from UGC [No. 37-93/2009(SR)], Government
of India is gratefully acknowledged. We thank IICB, Kolkata for pro-
viding us MALDI-MS facility. The CSIR, New Delhi, is also thanked
for the award of Senior Research Fellowship to D.J.
much more delocalisation through vinyl
p-spacers when compared
to the alkyne -spacers. A significant positive solvatochromic shift
p
was also observed in the emission. The maximum emission peak of
TPA-TPP1 was red shifted ca. 30 nm in the tetrahydrofuran with
respect to the toluene and in the case of TPA-TPP2, it was
29 nm. These results indicated that there was some charge-transfer
character which is probably due to the electron-withdrawing effect
of the pyridine moiety and it is spacer independent. We also noted
an increase of the Stokes shifts with the solvent polarity. This dis-
tinct positive solvatochromic behavior suggests that significant
charge redistribution takes place upon excitation leading to highly
polar excited state. The fluorescence quantum yields of TPA-TPP1
and TPA-TPP2 in toluene solution are measured to be 0.14 and
0.16 (0.08 and 0.12 in tetrahydrofuran), respectively, by using
diphenylanthracene as a standard. The thermal stability of these
oligomers was checked by the TGA method, which showed that
both final products exhibit good thermal stabilities. Their decom-
position (5% weight loss) temperatures are found to be 212 °C for
TPA-TPP1 and 270 °C for TPA-TPP2.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.01.116.
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ing electrolyte at a scan rate 100 mV/s and at 25 °C. On the basis
ox
of the roughly evaluated onset oxidation potentials (E
), the
onset
HOMO energy levels of TPA-TPP1 and TPA-TPP2 are estimated
ox
onset
as ꢀ5.78 and ꢀ6.02 eV, respectively ½HOMO ¼ ꢀeðE
þ 4:66Þꢂ.
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and ꢀ3.04 eV respectively, calculated from the HOMO energy level
and
DE (LUMO = HOMO + DE). These results indicate that the
introduction of alkenyl
p-spacers between the core and peripheral
unit increases the electron donating ability of the compound and
resulted in the enhancement of the HOMO energy level. TPA-
TPP2 can be viewed as a suitable candidate for hole-blocking
material due to the lower HOMO energy level.
In summary, we have designed and efficiently prepared two
new star-shaped
p-conjugated oligomers bearing triphenylpyri-
dine moiety as peripheral units and triphenylamine as a core
through a convergent synthetic strategy by the way of the three-
fold Heck/Sonogoshira coupling reaction. These compounds have
excellent solubility in common organic solvents such as chloro-
form, tetrahydrofuran, toluene, and methanol. Their fluorescence
quantum yields 0.14 and 0.16, emit light in the blue and violet re-
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decomposition (5% weight loss) temperatures are 212 and 270 °C,
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