Donor-acceptor amphiphilic 2,2'-bipyridine chromophores: Synthesis, linear optical, and thermal properties

Article abstract of DOI:10.1016/j.tetlet.2010.10.120

Two symmetrical and one unsymmetrical 'push-pull' amphiphilic 2,2'-bipyridine chromophores have been synthesized through Horner-Wordsworth- Emmons and Knoevenagel reaction mechanized synthetic protocols and characterized by spectroscopy. Linear optical properties and thermal stability of the synthesized chromophores have been investigated.

Full text of DOI:10.1016/j.tetlet.2010.10.120

Tetrahedron Letters 51 (2010) 6906–6910
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Donor–acceptor amphiphilic 2,20-bipyridine chromophores: synthesis, linear
optical, and thermal properties
⇑
Tanmay Chatterjee, Monima Sarma, Samar K. Das
School of Chemistry, University of Hyderabad, PO Central University, Hyderabad 500 046, A.P., India
a r t i c l e i n f o
a b s t r a c t
Two symmetrical and one unsymmetrical ‘push–pull’ amphiphilic 2,20-bipyridine chromophores have
been synthesized through Horner–Wordsworth–Emmons and Knoevenagel reaction mechanized syn-
thetic protocols and characterized by spectroscopy. Linear optical properties and thermal stability of
the synthesized chromophores have been investigated.
Article history:
Received 1 September 2010
Revised 15 October 2010
Accepted 25 October 2010
Available online 30 October 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
‘Push–pull’ molecule
Chromophore
Fluorescence
Knoevenagel reaction
Horner–Wordsworth–Emmons reaction
There is a considerable research interest on the electron donor–
acceptor (EDA) molecules because of their widespread applica-
tions, especially in the areas of non-linear optics (NLO)1 and
optoelectronics2 etc. 2,20-Bipyridines are excellent chelating ligands
in forming coordination complexes due to their potential metal ion
binding capability.3 The electronic blending of 2,20-bipyridine with
suitable donor groups attached to the 4- or 4,40-positions through
bipyridyl compounds with other co-ligands.10 Oligophenylenevin-
ylene (OPV) derivatives are highly conjugated organic molecules
and are often registered as organic light-emitting diodes (OLED).2
p
-conjugated-2,20-bipyridines can also act as OLED if suitable donor
groups are attached to the specified positions in the
p-backbone
of 2,20-bipyridines.11 Therefore, it would be interesting to
investigate the optical properties of the LB films, fabricated from
mixed-ligand coordination complexes of the appropriate transition
metals, featuring both the amphiphilic and suitably functionalized
bipyridine chromophores. We have recently reported the synthesis
and photo-physical properties of OPV functionalized bipyridines
bearing methoxy donors.5f In this communication, we wish to re-
port three amphiphilic bipyridine chromophores (TM 1–3, see
Fig. 1 for their structural representations) along with their detailed
photo-physical and thermal behaviors.
p
-linker (transmitter), results in the construction of 4- or 4,40-
p-
conjugated-2,20-bipyridines, in which the electron drifting nature
of the pyridine rings makes them archetypal examples of ‘push–
pull’ or EDA molecules. These molecules are synthetically flexible
and offer easy optical tuning (absorption, emission etc.) through
variation/simple modification of their
p-backbones. Since the last
two decades, several research groups, especially, Bozec, Abbotto,
Beer, and others have reported a number of such chromophores
and interesting properties of their transition metal complexes.4–6
Another motivating feature of these molecules is their metal
ion templated macroscopic assemblies and these coordination
complexes exhibit potential applications toward octupolar
nonlinearity7 and solar energy harvation.8 Langmuir–Blodgett (LB)
technique, on the other hand, is the most efficient technique for ul-
tra-thin monolayered deposition of amphiphilic compounds with
precise and accurate thickness.9 LB films, formed from coordina-
tion complex based surfactants, are still less explored in literature.
A recent publication by Bolink et al. describes dual-emitting nature
of the LB films built from Ru(II) or Ir(III) complexes of amphiphilic
The bipyridine chromophores TM 1–3 (Fig. 1) have been ob-
tained in gram quantities (2–3 g) through condensation of suitable
bipyridine precursors and functional benzaldehydes. The C10 and
C14 alkyl chain containing aldehydes (Ald 1–2) have been synthe-
sized using the reaction protocol as described in Scheme 1. In the
first step, the diol (1) has been mono-brominated using an analo-
gous procedure reported by Chong et al.12 The product (1a) has
then been converted to its tetrahydropyranyl ether (THP) by over-
night stirring of a THF solution of 1a containing 1.5 equiv of 3,4-
dihydro-2H-pyran (DHP) and 5 mol % of p-toluenesulfonic acid
(PTSA). The successive N-alkylation of N-methyl aniline (2) has
then been performed with 1b in refluxing THF. After an acid med-
iated cleavage of the OTHP functionality, the hydroxy containing
N,N-dialkyl aniline 2a has been obtained in a moderate yield
⇑
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.2010.10.120

T. Chatterjee et al. / Tetrahedron Letters 51 (2010) 6906–6910
6907
even selective bromination (mono- or di-) of 4 under similar reac-
tion condition.14 However, in the present case, the chlorinated ana-
log, 4,40-bis-chloromethyl-2,20-bipyridine (5) has been used that
has been synthesized following an efficient procedure described
by Fraser and co-workers.15 An Arbuzov reaction of 5 with tri-
ethylphosphite has converted it to the corresponding bis-phospho-
nate (6). Finally, the HWE condensations between 6 and the
appropriate functional benzaldehydes (Ald 1–2) at room tempera-
ture have afforded the symmetrically substituted bipyridine chro-
mophores (TM 1–2) in good yields (Scheme 2). TM 1, containing
hydroxy functionalization has required an acid-hydrolysis of its
corresponding THP ether (TM 10). The dissymmetrically substituted
bipyridine chromophore (TM 3) has been synthesized via con-
trolled Knoevenagel type condensation between mono-deproto-
nated 4 and Ald 2 followed by dehydration of the resulting
mono-ol (7) in presence of catalytic amount of pyridinium-p-tolu-
ene-sulfonate (PPTS) in refluxing toluene.
Figure 1. Molecular structure of the synthesized
idine chromophores (TM 1–3).
p-conjugated amphiphilic bipyr-
(51%). The hydroxy group has then been almost quantitatively pro-
tected by acetyl functionalization in dichloromethane using acetic
anhydride in presence of catalytic amount (10 mol %) of 4-dimeth-
ylamino pyridine (DMAP). The next step involves the classical Vils-
meier–Haack formylation of the dialkyl aniline 2b followed by
cleavage of the acetyl group to form the aldehyde 2c, the hydroxy
group of which has then been converted to its THP ether (Ald 1).
The synthesis of the aldehyde Ald 2 involves an analogous syn-
thetic route as described for the synthesis of Ald 1 (Scheme 1).
The synthesized chromophores have been characterized by
spectroscopy including successful elemental analyses.16–18 The
conformation of the vinylic bonds of TM 1–3 have unequivocally
been designated as E on the basis of JHH vinylic coupling constant
(ca. 16 Hz).
The steady state absorption and emission properties of the
p-
conjugated bipyridines, TM 1–3, have been investigated in dichlo-
romethane at room temperature. The relevant spectra have been
presented in Figure 2 and the concerned optical data have been
tabularized in Table 1. The absorption spectra of the chromoph-
ores, TM 1–3 have been characterized by a broad structureless
absorption band in the near-visible region (kmax = 390–395 nm).
This band unambiguously corresponds to the intra-ligand charge
transfer (ICT) from the dialkylamino donors to the pyridinic acceptor
subunits and this is in fact evident from the following observa-
The p
-functionalized-2,20-bipyridine compounds (TM 1–3) have
been synthesized following two different synthetic protocols viz.
(i) Knoevenagel type condensation between 4,40-dimethyl-2,20-
bipyridine (4) and the appropriate aldehyde, and (ii) Horner–
Wordsworth–Emmons (HWE) reaction of the bis-phosphonate
precursor (6) with suitable aldehydes. The resulting bipyridine
chromophores (TM 1–3) are sufficiently soluble in common organ-
ic solvents thereby allowing easy chromatographic purification.
The tricky step of the synthesis is the halogenation of 4,40-di-
methyl-2,20-bipyridine (4). Attempt to prepare the brominated
precursor 4,40-bis-bromomethyl-2,20-bipyridine by a radical mech-
anized bromination of 4,40-dimethyl-2,20-bipyridine (4) with N-
bromosuccinimide (NBS) in refluxing CCl4 (in presence of radical
initiator benzoyl peroxide or AIBN) has resulted in multiple num-
ber of products (closely spaced multiple spots in TLC) from where,
chromatographic separation of the desired product (4,40-bis-bro-
momethyl-2,20-bipyridine) was difficult. This is in accord with
some literature reports, which comment about poorer performance
of this reaction,13 although there are some reports that document
tions: (i) the absorption is broad and rather intense (
e
ꢀ35,000–
42,000 Lꢁcmꢂ1ꢁmolꢂ1), (ii) the position of the band maxima is
sensitive to the polarity of the fluid medium (vide Table 2). The
presence of the ICT band clearly demonstrates the ‘push–pull’
electronic grouping between the donor and acceptor subunits of
the title chromophores. The molar absorptivity of the ICT band in
the spectrum of TM 3 is relatively lower than that of the other
two compounds, presumably due to the presence of only one donor
sub-chromophore in TM 3 compared to two in TM 1 and TM 2.
However, the position of the concerned ICT band remains almost
Scheme 1. Synthesis of the dialkylamino benzaldehydes.

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T. Chatterjee et al. / Tetrahedron Letters 51 (2010) 6906–6910
Scheme 2. Synthetic routes to obtain the p-conjugated bipyridine chromophores (TM 1–3).
Table 2
Summary of the linear optical data for the compounds TM 1–3 in various solvents at
298 K, k (nm)
Solvent
TM 1
TM 2
TM 3
kmax
kem
kmax
kem
kmax
kem
Hexane
PhMe
THF
EtOAc
DCM
DMF
MeCN
MeOH
—
—
380
390
390
389
393
397
383
390
428
440
477
473
493
524
515
535
372
382
385
381
390
392
386
392
426
445
475
476
490
518
521
533
385
391
386
395
400
385
398
445
476
476
495
524
521
530
tation spectra with the original absorption spectra of the respective
compounds. The fluorescent spectra of the chromophores in DCM
exhibit broad structureless blue emission (kem = 490–495 nm) with
ca. 100 nm Stokes’ shift between the emission and the lowest en-
ergy absorption maxima. The photoluminescent quantum yields
of these compounds have been measured in DCM using quinine
sulfate as the reference substance19 in 1 N H2SO4 (298 K) and have
been found to be 0.138, 0.135 and 0.096 for the compounds TM 1–
3, respectively. Thus, it is obvious that introduction of the hydroxy
functionality has not induced any significant change in the elec-
tronic properties of the bipyridine chromophores. This observation
is consistent with the similar chromophores reported by Bozec and
coworkers.4a
Solvatochromism has been observed for all the title bipyridine
chromophores (TM 1–3). The corresponding optical data have been
summarized in Table 2 and the relevant spectra have been pre-
sented in the section of Supplementary data. Broad structureless
absorption and emission bands are featured in all the solvents, ex-
cept in hexane, which exhibits structured bands. An irregular trend
in the position of the absorption and emission band maxima is ob-
served in the Reichardt’s ET(30) solvent polarity scale (see Fig. 3).20
However, Stokes’ shift of the absorption and emission maxima evi-
dently demonstrates the polarity-dependent nature of the ground
and the excited electronic states of the relevant chromophores
(TM 1–3). Compared to the absorption spectra, the effect of med-
ium on the position and/or shape of the emission bands in the cor-
responding fluorescent spectra have been found to be profound.
The change in dipole moment on electronic excitation of the chro-
mophores is calculated using the Lippert–Mataga equation21 and
Figure 2. Normalized steady-state electronic absorption and emission spectra of
the compounds TM 1–3 in dichloromethane at 298 K. Concentration of the solutions
has been adjusted such that OD ꢀ0.05 in the relevant absorption spectra. Emission
spectra have been recorded after exciting the solutions at the corresponding lowest
energy absorption band maxima.
Table 1
Summary of the linear optical (dichloromethane, 298 K) and thermal data for TM 1–3
a
b
kabs (nm)
e
(Lꢁcmꢂ1ꢁmolꢂ1
)
kem (nm)
Uem
Td10 (°C)
TM 1
TM 2
395
250
393
320
247
390
285
246
42,274
20,685
43,585
25,142
24,228
35,114
23,742
26,257
495
0.138
0.135
375
493
490
367
TM 3
0.096
389
a
Measured using quinine sulfate as the reference in 1 N H2SO4 (Uem = 0.546).
10% weight loss temperature.
b
invariant in the absorption spectra of all the compounds. The chro-
mophores, TM 1–3, fluoresce in fluid medium at room temperature
(see Fig. 2 and Table 1). Origination of the emission bands in all the
cases has been crosschecked by comparing the corresponding exci-

T. Chatterjee et al. / Tetrahedron Letters 51 (2010) 6906–6910
6909
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Figure 3. Variation of Stokes’ shift (mabs
ꢂ mem) of TM 1–3 with solvent polarity
parameter ET(30). The absorption and emission intensities have been measured at
298 K. Concentration of the solutions have been adjusted such that OD ꢀ0.05. All
the excitations have been performed at the lowest energy absorption maxima. The
solvents are: hexane (32.4), toluene (33.9), THF (37.4), EtOAc (38.1), DCM (40.7),
DMF (43.2), MeCN (45.6), MeOH (55.4). Values in the parenthesis indicate the
ET(30) values of the solvents.20
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are estimated to be around 2.63 D, 3.20 D, and 1.27 D for TM 1–3,
respectively (see Supplementary data).
The spectroscopic properties of the synthesized chromophores,
TM 1–3, are found to remain unchanged over prolonged storage in
air, as indicated by the NMR and UV–visible spectroscopy, reveal-
ing their chemical inertness toward air or moisture. Thermo-gravi-
metric analyses have been performed on the compounds TM 1–3
under nitrogen and the respective thermogravimetric plots (see
Supplementary data) exhibit high thermal stability of these chro-
mophores. The thermal decomposition temperature of these com-
pounds (10% weight loss temperature) has been reported in
Table 1. These values are effectively higher indicating thermal
robustness of these chromophores.
In conclusion, three amphiphilic bipyridine chromophores have
been synthesized using appropriate reaction protocols and their
optical and thermal properties have been demonstrated. The posi-
tion of the absorption or emission band maxima is not found to be
dependent on the alkyl chain lengths. The optical properties of LB
films based on the amphiphilic and OPV mixed ligand coordination
complex surfactants will be reported in near future.
Acknowledgments
15. (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.
16. Characterization data for TM 1: This compound was obtained as a yellow solid
after acid-hydrolysis of the chromatographed Horner–Wordsworth–Emmons
product TM 10. Yield: 71%; mp: 122 °C (DTA); IR (KBr, cmꢂ1): 3371.87 (O–H),
2922.42–569.05 (multiple bands); 1H NMR (400 MHz, CDCl3): d = 8.62 (d,
J = 4 Hz, 2H), 8.48 (s, 2H), 7.45 (d, J = 8 Hz, 4H), 7.41 (d, J = 16 Hz, 2H), 7.35 (d,
J = 8 Hz, 2H), 6.91 (d, J = 16 Hz, 2H), 6.69 (d, J = 8 Hz, 4H), 3.65 (t, 4H, –CH2–OH),
3.35 (t, 4H, –N–CH2–), 2.99 (s, 6H, –N–CH3), 1.59–1.55 (unresolved, 8H, –N–
CH2–CH2–, –CH2–CH2–OH), 1.30 (unresolved, 28H, –CH2–); 13C NMR (100 MHz,
CDCl3): d = 156.4, 149.6, 149.3, 146.8, 133.6, 128.5, 123.8, 121.0, 120.5, 117.9,
111.8, 63.0, 52.6, 38.3, 32.8, 29.5, 27.1, 26.8, 25.8; LC–MS (positive mode) m/z:
731 (M+H)+; Anal. Calcd for. C48H66N4O2 (730.52): C, 78.86; H, 9.10; N, 7.66%.
Found: C, 78.83; H, 9.06; N, 7.71%.
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 at University of Hyderabad by DST, Govt. of India is
highly acknowledged. T.C. and M.S. thank CSIR and UGC, India,
respectively, for their fellowships. We thank Mr. S. Ghanta for help-
ing us while calculating the optimized structure of the chromoph-
ores. Useful discussion with Prof. T.P. Radhakrishnan is highly
acknowledged.
17. Characterization data for TM 2: Yellow solid; yield: 86%; mp: 110 °C (DTA); 1H
NMR (400 MHz, CDCl3): d = 8.63 (d, J = 4 Hz, 2H), 8.49 (s, 2H), 7.46–7.35 (m,
10H), 6.91 (d, J = 16 Hz, 2H), 6.69 (d, J = 8 Hz, 4H), 3.35 (t, 4H, –N–CH2–), 3.02 (s,
6H, –N–CH3), 1.60 (unresolved, 4H, –N–CH2–CH2–), 1.32 (unresolved, 48H, –
CH2–), 0.90–0.87 (unresolved, 6H, –CH2–CH3); 13C NMR (100 MHz, CDCl3):
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.10.120.

6910
T. Chatterjee et al. / Tetrahedron Letters 51 (2010) 6906–6910
d = 156.4, 149.6, 149.3, 146.8, 133.7, 128.5, 123.9, 121.0, 120.5, 117.9, 111.8,
d = 156.4, 156.2, 149.6, 149.3, 148.9, 148.1, 146.8, 133.6, 128.5, 124.7, 123.8,
122.1, 121.0, 120.5, 117.8, 111.8, 52.6, 38.3, 32.0, 29.7, 29.4, 27.2, 26.8, 22.7,
21.2, 14.2; LC–MS (positive mode) m/z: 498 (M+H)+; Anal. Calcd for C34H47N3
(497.38): C, 82.04; H, 9.52; N, 8.44%. Found: C, 82.00; H, 9.50; N, 8.50%.
19. Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107–1114.
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Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465; (c) Mataga, N. Bull. Chem. Soc.
Jpn. 1963, 36, 654.
52.6, 38.4, 32.0, 29.6, 29.4, 27.2, 26.8, 22.7, 14.2; LC–MS (positive mode) m/z:
812 (M+H)+; Anal. Calcd for. C56H82N4 (810.65): C, 82.91; H, 10.19; N, 6.91%.
Found: C, 82.89; H, 10.16; N, 6.95%.
18. Characterization data for TM 3: Yellow solid; yield: 78%; mp: 124 °C (DTA). 1H
NMR (400 MHz, CDCl3): d = 8.58 (d, J = 4 Hz, 2H), 8.46 (s, 1H), 8.25 (s, 1H), 7.45
(d, J = 8 Hz, 2H), 7.39 (d, J = 16 Hz, 1H), 7.34 (d, J = 4 Hz, 1H), 7.15 (d, J = 4 Hz,
1H), 6.90 (d, J = 16 Hz, 1H), 6.68 (d, J = 8 Hz, 2H), 3.35 (t, 2H, –N–CH2–), 2.98 (s,
3H, –N–Me), 2.45 (s, 3H, 4–py–Me), 1.59 (unresolved, 2H, –N–CH2–CH2–), 1.27
(unresolved, 22H, –CH2–), 0.88 (t, 3H, –CH2–Me); 13C NMR (400 MHz, CDCl3):