Synthesis and photo-physical properties of methoxy-substituted π-conjugated-2,2′-bipyridines

Article abstract of DOI:10.1016/j.tetlet.2010.02.027

4,4′-Bis-vinyl-2,2′ bipyridines have been known in the area of materials chemistry since last few decades. Bipyridine molecules bearing donor substituents in their π backbone are examples of 'push-pull' molecules. Emissive properties of such ligands depen

Full text of DOI:10.1016/j.tetlet.2010.02.027

Tetrahedron Letters 51 (2010) 1985–1988
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and photo-physical properties of methoxy-substituted
p
-conjugated-2,20-bipyridines
*
Tanmay Chatterjee, Monima Sarma, Samar K. Das
School of Chemistry, University of Hyderabad, Hyderabad 500046, AP, India
a r t i c l e i n f o
a b s t r a c t
4,40-Bis-vinyl-2,20 bipyridines have been known in the area of materials chemistry since last few decades.
Bipyridine molecules bearing donor substituents in their backbone are examples of ‘push–pull’ mole-
cules. Emissive properties of such ligands depend on the position of the donor unit in their backbone.
The Horner–Wordsworth–Emmons reaction has been used extensively to synthesize such types of con-
jugated systems. Three newly synthesized highly conjugated 2,20 bipyridines (3a–c) have been described.
The molecules 3a–c are highly emissive in the visible region at room temperature.
Ó 2010 Elsevier Ltd. All rights reserved.
Article history:
Received 30 December 2009
Revised 31 January 2010
Accepted 5 February 2010
Available online 10 February 2010
p
p
Keywords:
2,20 Bipyridine
Wordsworth–Emmons reaction
Fluorescence
Push–pull molecules
2,20-Bipyridines are nitrogen-containing organo-materials with
enormous applications in materials chemistry. They readily form
complexes with transition metals in which they can interact with
and co-workers.2–7 Optical properties of such metal complexes
can be easily tuned through variation of substituents at the back-
bone of the bipyridine ligands. In this Letter, we have described
three newly synthesized highly conjugated 2,20-bipyridines (3a–c)
as shown in Schemes 1 and 2. The resulting molecules (that can
be used as potential ligands for metal coordination) are highly
emissive in the visible region.
Wordsworth–Emmons reaction is a well-known synthetic route
for the preparation of 4,40-bis-vinyl-2,20-bipyridines with predom-
inantly E-selectivity of the C@C vinyl bond as described by the
reactions shown in the second part of Scheme 1.15 The key precur-
sor of the Wordsworth–Emmons reaction is the phosphonate (2)
which has been prepared by means of an Arbuzov Reaction by
treating 4,40-bis-chloromethyl-2,20-bipyridine (1a) with tri-
ethylphosphite (93%). We have synthesized the required starting
molecule, 4,40-bis-chloromethyl-2,20-bipyridine (1a) using an effi-
cient procedure (of preparing 4,40-bis-halomethyl-2,20-bipyridines)
p
the metals via both
M.Os.1 The resulting complexes are very stable due to the formation
of a five-membered chelate. 4,40- -conjugated-2,20-bipyridines
r-donating nitrogen atoms and p-accepting
p
have been explored in materials chemistry since last two decades
due to their excellent performance in the areas of non-linear optics
(NLO),2–8 light-emitting diode devices,9 electrochemistry,10,11 solar
cell12–14 etc. 2,20-bipyridines bearing a chromophore or fluorophore
in their
p backbone constitute a ‘push–pull’ molecule, where the
donor moieties act as the pushing unit and the pyridine moieties
are the pulling unit. These systems are synthetically flexible, in
which the centrosymmetric or non-centrosymmetric push–pull
units are easily accessible via functionalization of 4,40-dimethyl-
2,20-bipyridine (1). The major synthetic strategies in synthesizing
such systems include either the classical Knoevenagel Condensa-
tion between the deprotonated 4,40-dimethyl-2,20-bipyridine and
an aromatic aldehyde or Wordsworth–Emmons Reaction between
the tetraethyl-2,20-bipyridine-4,40-diylbis(methylene)diphospho-
nate (2) and an aldehyde. The resulting highly conjugated mole-
cules are important in the sense that they can be aggregated in a
polymeric unit simply by a (transition) metal ion resulting in
coordination complexes that have potential to exhibit NLO activity
due to intraligand charge transfer (ILCT) or metal–ligand charge
transfer (MLCT) transition.7 Several homoleptic and heteroleptic
complexes exhibiting NLO activity have been reported by Bozec
Scheme 1. Synthesis scheme for 3a–c using Wordsworth–Emmons synthetic route.
Reagents and conditions: (i) LDA (2.4 equiv), THF, ꢀ70 °C, Me3SiCl (2.2 equiv), 15 s,
EtOH (95%); (ii) CsF (4 equiv), C2Cl6 (4 equiv), MeCN, 75 °C, 4 h (80%); (iii) (EtO)3P,
120 °C, 12 h (93%); (iv) KOBut, 8a–c, THF, rt. The structural drawings of 8a–c are
shown in Scheme 3.
* Corresponding author. Tel.: +91 40 2301 1007; fax: +91 40 2301 2460.
E-mail addresses: skdsc@uohyd.ernet.in, samar439@gmail.com (S.K. Das).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.027

1986
T. Chatterjee et al. / Tetrahedron Letters 51 (2010) 1985–1988
Scheme 2. Structural representations of the newly synthesized and highly emissive p-conjugated bipyridines (3a–c).
as described by J. Fraser et al.16 The synthesis involves deprotona-
tion of 1 by lithium diisopropylamide (LDA) followed by trapping
of the resulting di-anion with chlorotrimethylsilane (95%) and fi-
nally chlorination with a suitable commercially available electro-
phile, hexachloroethane in the presence of a dry fluoride source
(CsF) in acetonitrile (80%) as shown in the first part of Scheme 1.
Finally, the Wordsworth–Emmons reaction between 2 and the syn-
thesized methoxy-substituted 4-styryl benzaldehydes (8a–c, see
Scheme 2 for their structural drawing and preparation) in the pres-
ence of KOBut in THF at room temperature afforded the desired
conjugated 2,20-bipyridines (3a–c) in good yields.17
The methoxy-substituted 4-styryl benzaldehydes (8a–c) have
been synthesized via a three-step procedure starting from 4-bro-
mobenzylbromide (4). Treatment of 4 with triethylphosphite re-
sults in the corresponding phosphonate derivative (5) that
undergoes Wordsworth–Emmons reaction with three different
methoxy-substituted benzaldehydes (6a–c) in THF to produce
the corresponding conjugated bromobenzenes (7a–c) in good
yields.18 Subsequent lithiation in ether at lower temperature fol-
lowed by quenching with DMF affords the designed methoxy-
substituted 4-styryl benzaldehydes (8a–c).19 Scheme 3 depicts a
schematic representation of the syntheses of 8a–c.
All the synthesized materials have been characterized by NMR
spectroscopy including elemental and LC–MS analyses. E confor-
mations of the compounds 7a–c and 8a–c are clearly understood
from the JHH vinylic coupling constant of ca 16 Hz (see Supplemen-
tary data for relevant NMR spectra). Molecular structures of the
newly synthesized bipyridines (3a–c) have been determined by
1H NMR, LC–MS spectral studies including elemental analyses
and the knowledge of the precursors from which they have been
synthesized. The major problem associated with the characteriza-
tions of this kind of highly conjugated bipyridine materials is their
poor solubility in common organic solvents which precludes the
detailed solution NMR investigations (for example, 13C NMR stud-
ies). The low solubility is probably due to the stacking of concerned
bipyridine molecules in the solid state.
and 10ꢀ6 M) at 298 K. The nature of absorption of three compounds
3a–c is almost identical as shown in Figure 1. Compounds 3a, 3b
and 3c show absorption maxima at 365, 384 and 380 nm, respec-
tively (Fig. 1). These absorptions in the UV region are assigned to
intra-ligand charge transfer transitions within the molecules.2–5
As the molecules bear no chromospheres in their
p backbones,
no absorption band has been observed in the visible region. The
room temperature emission spectra of the compounds 3a–c in
dichloromethane revealed band maxima at 435 nm, 505 nm and
488 nm, respectively (Fig. 2, see Table 1), when the excitation
wavelengths were 365 nm, 384 and 380 nm, respectively. As a re-
sult, at kem = 435 nm (for 3a), 505 nm (for 3b) and 488 nm (for 3c),
the excitation spectra exhibit band maxima at 360 nm, 382 nm and
380 nm, respectively. The emission quantum yields (uem) of the
compounds 3a–c at room temperature (using quinine sulfate in
1 N H2SO4 as the reference, whose quantum yield is known to be
0.54520) were found to be 0.59, 0.70 and 0.75 respectively.
The molecules were excited at same optical densities where the
absorption curve of the sample intersects the same of quinine sul-
fate (see Supplementary data for relevant spectra). As shown in
Figure 2, compounds 3a–c show excellent emission in 400–
600 nm region. The lower homologue of 3c, namely, 4,40-di(2-
(2,5-dimethoxyphenyl)ethynyl)-2,20-bipyridine has been reported
by Nazeeruddin and Gratzel9 that exhibits emission band centred
at 450 nm with photoluminescence quantum yield of 0.43 and is
one of the highest blue organic light-emitting diode known in
literature. It is known that 4,40-bis-styryl-2,20-bipyridines become
more fluorescent when the two donor groups are in ortho and meta
positions on the phenyl ring.9 We have also observed that 4,40-
di(2-(3-methoxyphenyl)ethynyl)-2,20-bipyridine and 4,40-di(2-
(2,4-dimethoxyphenyl)ethynyl)-2,20-bipyridine (lower homo-
logues of 3a and b, respectively) have quantum yields of only
0.081 and 0.007; it is amazing that when we increase the conjuga-
tion by synthesizing 3a and 3b, it enhances the quantum yields
drastically (u3a = 0.59, u3b = 0.70). As expected, 3a–c show emis-
sion maxima that are red shifted compared to the emission max-
ima of their lower homologues. Spectroscopic data are
summarized in Table 1.
UV–visible and emission spectral studies of 3a–c have been per-
formed in dichloromethane solution (concentration range: ꢁ10ꢀ5

T. Chatterjee et al. / Tetrahedron Letters 51 (2010) 1985–1988
1987
Scheme 3. Synthesis of the methoxy-substituted 4-styryl benzaldehydes
Figure 1. UV–visible spectra of 3a–c in dichloromethane at 298 K.
Figure 2. Emission spectra of 3a–c in dichloromethane at 298 K normalized to 1.
In conclusion, we have succeeded in synthesizing three new
-conjugated ‘push–pull’ 2,20-bipyridines. The resulting com-
pounds are highly emissive in the visible range at room temper-
common organic solvents, they form soluble complexes with
the transition metal ions. Further investigation of these highly
emissive materials towards light-emitting devices is in progress
in our laboratory.
p
ature. Even though, the title compounds are less soluble in

1988
T. Chatterjee et al. / Tetrahedron Letters 51 (2010) 1985–1988
15. Carry, F. A.; Sundberg, Richerd J. Advanced Organic Chemistry: Part B, 5th ed.,
Table 1
2007, 164.
Summary of the photo-physical data
16. (a) Smith, A. P.; Lamba, J. J. S.; Fraser, C. L. Org. Synth. 2004, 10, 107–112; (b)
Fraser, C. L.; Anastasi, N. R.; Lamba, J. J. S. J. Org. Chem. 1997, 62, 9314–9317.
17. Typical procedure for the preparation of the bipyridine (3c): Solid potassium t-
butoxide (4 mmol, 4 equiv) was added at a time to a THF solution (20 ml) of
tetraethyl 2,20-bipyridine-4,40-diylbis(methylene)diphosphonate (1 mmol) (2)
and (E)-4-(2,5-dimethoxystyryl)benzaldehyde (8c) (1.5 mmol, 3 equiv) at
room temperature. The resulting yellow turbid reaction mixture was stirred
at this temperature until completion (monitored by TLC), which was then
quenched with water (1 ml). Subsequently, methanol was added to result in
the precipitation of the desired compound (3c), which was isolated by
filtration, washed with small amount of water, thoroughly with methanol
(until the filtrate did not show presence of the aldehyde), ether and dried
under vacuum. Orange yellow solid; mp: more than 200 °C; yield = 84%; IR
(KBr, cmꢀ1): 3026.58, 2945.57, 2825.97, 1583.70, 1541.26, 1494.97, 1458.32,
1417.81, 1375.37, 1338.72, 1221.05, 1103.38, 1041.65, 954.85, 848.76, 798.60,
709.87, 567.12, 509.25; 1H NMR (400 MHz, DMSO-d6): d 8.69 (d, J = 4.6 Hz, 2H),
8.58 (s, 2H), 7.63–7.65 (m, 12H), 7.42–7.48 (m, 4H), 2.26- 2.32 (m, 4H), 6.98 (d,
J = 8.8 Hz, 2H), 6.86 (d, J = 8 Hz, 2H), 3.81 (s, 6H), 3.76 (s, 6H); we could not
record 13C NMR because of the less solubility of the compound in common
organic solvents; LC–MS (positive mode): m/z = 686 (M++H)+; elemental Anal.
Calcd for C46H40N2O4: C, 80.68; H, 5.89; N, 4.09. Found: C, 80.72; H, 5.93; N,
4.12. UV–vis: kmax = 380 nm (in dichloromethane, ꢁ10ꢀ5 M concentration);
fluorescence: kem = 488 nm (in dichloromethane, ꢁ10ꢀ6 M concentration).
18. Typical procedure for the preparation of (E)-2-(4-bromostyryl)-2,5
dimethoxybenzene (7c): Solid sodium t-butoxide (1.92 gm, 20 mmol) was
Compound
kabs (nm)
kem (nm)
uem
3a
3b
3c
365
384
380
435
505
488
0.59
0.70
0.73
Acknowledgements
The authors thank Department of Science and Technology
(DST), India (Project No. SR/S1/IC-23/2007) and Centre for Nano-
technology at University of Hyderabad for funding. 400 MHz
NMR facility by DST, Govt. of India at University of Hyderabad is
highly acknowledged. Useful discussions with Dinesh Khara and
Ch. Gupta Chandaluri are appreciated. TC and MS thank CSIR and
UGC, India, respectively, for their fellowships.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.02.027.
added at
a
time to
a
50 ml THF solution of diethyl 4-
bromophenylphosphonate (5) (4 gm, 13 mmol) and 2,5 dimethoxy
benzaldehyde (6c) (10 mmol) at 0 °C. The resulting slurry was warmed up to
room temperature and stirred for 90 min. The reaction was quenched with
water and extracted with hexane. The combined organic layers were washed
with brine solution, dried over Na2SO4 and evaporated. The crude product was
purified through silica gel column using ethyl acetate/hexane = 5:95 (v/v) as
the mobile phase to afford the desired material as white solid in 92% yield. mp:
68–69 °C; 1H NMR (400 MHz, TMS, CDCl3): d 7.38–7.47 (m, 5H), 7.12 (s, 1H),
7.01 (d, 1H, 16 Hz), 6.79–6.85 (m, 2H), 3.84 (s, 3H), 3.81 (s, 3H); 13C NMR
(100 MHz, TMS, CDCl3): d 154.54, 152.30, 137.56, 132.50, 131.31, 128.91,
127.60, 124.86, 121.94, 114.80, 113.03, 112.50, 56.99, 56.59; LC–MS (positive
mode): m/z = 321 (M++2H)+; elemental Anal. Calcd for C16H15BrO2: C, 60.21; H,
4.74. Found: C, 60.28; H, 4.79.
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(8c): n-Butyl lithium (2.5 M in hexane) (4 ml, 10 mmol) was added dropwise to
a 50 ml ether solution of 7c (5 mmol) at ꢀ20 °C and stirred for 60 min at this
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(KBr, cmꢀ1): 3045.87, 3005.37, 2951.35, 2831.76, 2739.17, 1819.04, 1695.58,
1599.13, 1493.04, 1425.52, 1284.71, 1244.20, 1211.41, 1120.74, 1047.44,
1020.44, 958.71, 852.61, 792.81, 715.66, 505.40; 1H NMR (400 MHz, TMS,
CDCl3): d 9.97 (s, 1H), 7.84 (d, 2H, 8 Hz), 7.58–7.66 (m, 3H), 7.09–7.15 (m, 2H),
6.84 (s, 2H), 3.85 (s, 3H), 3.81 (s, 3H); 13C NMR (CDCl3): d 191.70, 153.75,
151.81, 143.98, 135.19, 130.21, 127.91, 127.00, 126.93, 126.35, 114.76, 112.30,
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