D-π-A-A-π-D prototype 2,2′-bipyridine dyads exhibiting large structure and environment-sensitive fluorescence: Synthesis, photophysics, and computation

Article abstract of DOI:10.1021/jo202015m

A series of 4,4′-π-conjugated-2,2′-bipyridine chromophores (MS 1-8) were synthesized, and their photophysical and thermal properties were investigated. The title "push-pull' chromophores", except MS 1, were integrated with both alkoxy and alkylamino donor functionalities that differ in their donation capabilities. The oligophenylenevinylene (OPV) chromophores MS 4-8 are associated with a π-extended backbone in which the position and the number of alkoxy donors were systematically varied. All of the studied systems possess a D-π-A-A-π-D dyad archetype in which the A-A is the central 2,2′-bipyridine acceptor core that is electronically attached with the donor termini through π-linkers. The fluorescence quantum yields of the synthesized chromophores are found to be sensitive to the molecular archetype and the solvent medium. Out of the eight fluorescent compounds reported in this article, the compound MS 5 exhibits fluorescence in the solid state also. The modulating effect of the nature, position, and number of donor functionalities on the optical properties of these classes of compounds has further been comprehended on the basis of DFT and TD-DFT computation in a solvent reaction field.

Full text of DOI:10.1021/jo202015m

Article
pubs.acs.org/joc
D-π-A-A-π-D Prototype 2,2′-Bipyridine Dyads Exhibiting Large
Structure and Environment-Sensitive Fluorescence: Synthesis,
Photophysics, and Computation
Monima Sarma,† Tanmay Chatterjee,† Susanta Ghanta, and Samar K. Das*
School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad 500 046, Andhra Pradesh, India
S
* Supporting Information
ABSTRACT: A series of 4,4′-π-conjugated-2,2′-bipyridine
chromophores (MS 1−8) were synthesized, and their photo-
physical and thermal properties were investigated. The title
“push−pull’ chromophores”, except MS 1, were integrated
with both alkoxy and alkylamino donor functionalities that
differ in their donation capabilities. The oligophenylenevinyl-
ene (OPV) chromophores MS 4−8 are associated with a
π-extended backbone in which the position and the number of alkoxy donors were systematically varied. All of the studied
systems possess a D-π-A-A-π-D dyad archetype in which the A-A is the central 2,2′-bipyridine acceptor core that is electronically
attached with the donor termini through π-linkers. The fluorescence quantum yields of the synthesized chromophores are found
to be sensitive to the molecular archetype and the solvent medium. Out of the eight fluorescent compounds reported in this
article, the compound MS 5 exhibits fluorescence in the solid state also. The modulating effect of the nature, position, and
number of donor functionalities on the optical properties of these classes of compounds has further been comprehended on the
basis of DFT and TD-DFT computation in a solvent reaction field.
the pyridine rings offers introduction of an assorted class of
donor end-capping functionalities to tune the optical properties
of the relevant 2,2′-bipyridine based dyads. Over the past two
decades, many research groups including Le Bozec, Beer,
Abbotto, and others have accounted for the diverse end-capped
2,2′-bipyridine chromophores with moderate to strong emis-
sion responses.13−15 Recently, Ajayaghosh and co-workers have
exemplified the chemo-sensing properties of the 2,2′-bipyridine
based luminophores.16 Transition metal complexes of the 2,
2′-bipyridine based DA systems are of topical interest due to
their potential applicability in octupolar nonlinearity.17 Hetero-
leptic bis-thiocyanato ruthenium complexes, bearing a TiO2
anchoring 2,2′-bipyridine ligand along with another auxiliary
2,2′-bipyridine ligand (known as antenna), have proved to be
very promising photosensitizers for building high-performance
dye-sensitized solar cell modules.18
The exciting photophysical responses of the 2,2′-bipyridine
based DA systems and their transition-metal complexes have
drawn our recent attention.19 The bipyridine based chromophores
reported so far are either symmetrically (point group = Ci) or
dissymmetrically (point group = C1) substituted with alike
donor functionalities (for example, alkylamino, etc.), whereas
the photophysical properties of the associated heterodonor
systems are less explored. In this article, we wish to report a
series of styryl- and bistyryl-2,2′-bipyridine luminophores (MS
2−8, Chart 1) functionalized with both the alkoxy and amino
functionalities in Ci symmetrical fashion and their photophysical
INTRODUCTION
■
The phenomenon of fluorescence has turned out to be an
indispensable analytical technique in the various branches of
science, most prominently, in the fields of analytical, biological,
medical sciences, etc.1 Among the diverse classes of organic
π-systems, the materials that absorb electromagnetic radiation
by virtue of an intramolecular charge transfer (ICT) and emit
from the corresponding photoexcited state are most fascinating
because of their notable applications in the field of molecular
electronics, integrated photonic devices, non-linear optics
(NLO),2 etc. The elegant fabrication of an electron donor−
acceptor (DA) or “push−pull” architecture can be carried out
via the electronic union between the donor and acceptor
mesomeric units in a chromophore, which is in turn associated
with spontaneous charge redistribution between the function-
alities (ICT). Consequently, much research has been con-
ducted in designing and synthesizing diverse classes of DA-type
fluorescent probes, which are associated with brilliant photo-
physical behaviors, some typical examples of such fluorescent
probes being acridine,3 fluorescein,4 cyanine,5 rhodamine,6
coumarin,7 BODIPY,8 squarines,9 oligophenylenevinylenes
(OPVs),10 etc.
The disconnection approach is a beautiful tool for organic
chemists to devise a wide variety of chromophores wherein the
nitrogen-containing heterocycles act as very promising building
blocks to synthesize diverse classes of strongly emissive
materials.11 2,2′-Bipyridine derivatives are endowed with an
extensive coordination/supramolecular chemistry.12 However,
in photoscience, it is advantageous to employ this N-hetero-
biaryl compound because of the fact that easy derivatization of
Received: October 7, 2011
Published: November 7, 2011
© 2011 American Chemical Society
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a
Chart 1. Molecular Structure of the Synthesized and the Parent π-Conjugated-2,2′-bipyridine Chromophores (MS 1−8)
a
The two phenyl rings of the OPV chromophores were named as A and B for easy discussion in the text.
Scheme 1. Convenient Synthetic Protocols To Access the Symmetrical Bipyridine Chromophores
properties. The prototype of the synthesized dyads is D-π-A-A-
π-D (D = donor, A = acceptor) in which the bipyridine moiety
acts as the central acceptor core to join the terminal donor func-
tionalities through vinylene linkers. The Horner−Wordsworth−
Emmons reaction (HWE) was exclusively used so as to
introduce electron-donating groups into the bipyridine central
acceptor core unit. The photophysical properties of the syn-
thesized chromophores (MS 1−8, Chart 1) were compared with
four reported dyes, namely, (a) the blue OLED dye 4,4′-bis(2,5-
dimethoxystyryl)-2,2′-bipyridine (known as N945L) reported by
solvent reaction field was performed to scrutinize the effect of
donor positions on the geometrical and electronic parameters of
the reference and the synthesized chromophores. All of the
synthesized π-conjugated molecules MS 1−8 fluoresce at room
temperature with large Stokes shift, while the emissive behavior
of the OPV chromophores MS 4−8 epitomizes a large
sensitivity to the solvent polarity. Among eight fluorescent
compounds reported in this article, the compound, MS 5 is
emissive in the solid state also.
Nazeeruddin and Gratzel;15a (b) 4,4′-bis(4-dibutylaminostyryl)-
̈
RESULTS AND DISCUSSION
■
2,2′-bipyridine (named as HLB 1 in this article)13d and 4,4′-
bis(4-(4-dibutylaminostyryl)styryl)-2,2′-bipyridine (named as
HLB 2 in this article)13h reported by Le Bozec; and (c) 4,4′-
bis(4-(2,5-dimethoxystyryl)styryl)-2,2′-bipyridine (named as TM
6) reported by us19a (see Chart 1). A pragmatic observation
points out that the introduction of the amino donor
functionalities to N945L/TM 6 or alkoxy donors to HLB 1/
HLB 2 have induced a significant alteration of the photonic
responses in the present chromophores. In addition, the position
of the alkoxy functionalities attached to the conjugated backbone
of the OPV derivatives, MS 4−8, greatly influences their
photophysical properties. Furthermore, a thorough computational
analysis at the level of density functional theory (DFT) in a
Synthesis and Characterization. The bench-top synthe-
ses of the 4,4′-π-conjugated-2,2′-bipyridine chromophores were
accomplished via highly efficient synthetic protocol (see Scheme 1).
The symmetrically substituted 2,2′-bipyridine derivatives (point
symmetry = Ci) can be easily derived either through (a) a
Knoevenagel-type reaction between the doubly deprotonated
4,4′-dimethyl-2,2′-bipyridine (1) and suitable aromatic aldehydes
or (b) a Horner−Wordsworth−Emmons (HWE) reaction
between the bis-phosphonate (2) and aromatic aldehydes. The
Knoevenagel-type reaction triggers at the acidity of the 4-picolyl
protons of the bipyridine starting precursor (1). A two-step
synthetic approach that has been developed entails the deprotona-
tion of 1 by a strong base, e.g., lithium diisopropylamide (LDA)
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at low temperature followed by nucleophilic addition of the
resulting carbanion to an aromatic aldehyde to form a
secondary alcohol. The resulting alcohol is then dehydrated,
usually by pyridinium p-toluene sulfonate (PPTS), to convert it
to the corresponding alkene (Scheme 1).14a An alternative one-
step protocol directly yields the desired alkenes when a mixture
of 1 and an aromatic aldehyde is heated in presence of a strong
and hindered base, e.g., KOBut, in DMF.13d
executed via the appropriate selection of the starting materials.
The usage of an alkoxy derivatized phosphonate precursor (9)
allows the introduction of the alkoxy functionalities in the
formylated phenyl rings. At the outset, the HWE reaction be-
tween the appropriate phosphonate (8−9) and the alkylami-
nobenzaldehydes (5a−b, 7a−b) affords the corresponding
bromostilbenes (10a−15a) in excellent yield with E-selectivity
of the CC bonds. Subsequent lithium−halogen exchange
reaction followed by electrophilic quenching with dimethyl-
formamide converts the bromostilbenes (10a−15a) to the
corresponding benzaldehydes (10b−15b) in moderate to good
yields (Table 1).
The donor end-capping functionalities in the present
bipyridine chromophores (MS 1−8) were introduced by the
HWE protocol due to its better performance (high yield,
E-selectivity, etc.) in comparison with the two other method-
ologies as described in Scheme 1. Also, the conventional Wittig
pathway was avoided because of the undesired trouble with the
coproduct, triphenylphosphine oxide, during workup and
purification. The key intermediate of the HWE pathway is
the phosphonate 2, which has been extensively used in liter-
ature to synthesize a diverse class of symmetrically substituted
2,2′-bipyridine chromophores. The concerned intermediate (2)
is easily derived from the corresponding halomethyl derivatives
through an Arbuzov reaction. An initial attempt to brominate 1
through a radical mechanized reaction route (NBS/benzoylper-
oxide or AIBN) was unsatisfactory for obtaining 4,4′-bis-bro-
momethyl-2,2′-bipyridine. However, the corresponding chloro-
methyl derivative, 1a, was used throughout the present work and
was synthesized following an efficient synthetic protocol
developed by Fraser and co-workers. The concerned two-step
approach involves deprotonation of 1 with LDA and trapping of
the resulting carbanion with chlorotrimethylsilane, followed by
subsequent chlorination with hexachloroethane in presence of a
dry fluoride source, cesium fluoride (CsF).20 The alde-
hydes used in the present study are not commercially accessi-
ble and thus were synthesized as described in the following
sections.
The molecular structures of all of the synthesized bipyridine
chromophores (MS 1−8) were unambiguously determined
through NMR (1H and 13C) and mass (LC−MS and MALDI−
1
TOF/TOF) spectroscopy. The presence of only one set of H
and 13C signals in the NMR spectra evidently demonstrates the
1
symmetrical structure of the chromophores. The H NMR
resonances of the pyridine rings and that of the vinylic protons
for the chromophores MS 1−8 are summarized in Table 2. All
of the vinylic CC bonds are found to be in E-geometry as
indicated by the splitting of each of the CH resonances by the
neighboring protons with a 3JHH coupling constant of ca. 16 Hz.
However, no trace of the Z-isomer was signified in the relevant
1
1H NMR spectra. A comparison between the H NMR signals
due to the aromatic protons of the open chain amino donor
end-capped chromophore MS 2 (dibutylamino donor) and its
cyclic analogue, i.e., MS 3 (pyrrolidine donor) reveals that the
protons attached ortho to the amino functionalities have a larger
chemical shift in the case of MS 2 in comparison with MS 3
(see Supporting Information). The same trend is observed for
the other open chain/cyclic amino donor pairs, viz., MS 4/MS
5 and MS 7/MS 8. This finding is indicative of a greater
donation capability of the cyclic pyrrolidine compared with the
corresponding open chain amino donor, dibutylamine.
The anilines (3a−b) were alkylated in presence of the
suitable electrophiles in a N-methylpyrrolidinone (NMP)−
DMF solvent system to obtain the desired N,N-dialkyl anilines
(4a−b, 6a−b) in good yield (Scheme 2). Subsequently, the
One Photon Absorption and Emission Properties: (a)
Nature of Substituents. The steady-state one photon
absorption and emission properties of the synthesized
chromophores MS 1−8 were investigated in four different
solvents (see Table 3 and Figure 1). As shown in Figure 1,
broad structureless bands of the absorption and emission
spectra in the visible region are the chief characteristic features
of all of the chromophores in the present study. The molar
extinction coefficient (ε) of the lowest energy absorption band
of all of the chromophores is fairly high (see Table 3). The low
solubility of MS 1 has precluded measurement of the molar
absorptivity with a better accuracy. The origination of the
lowest energy band in the absorption spectra of MS 1−8,
however, is due to an intramolecular π → π* ICT from the
donor-based molecular orbitals to the acceptor (pyridine)-
based molecular orbitals in the relevant dipolar chromophores,
which in fact was firmly affirmed owing to the sensitivity of the
relevant absorption band on the solvent polarity (see Table 3).
In addition, the ICT band maxima were found to undergo a
bathochromic shift by ca. 15 nm for the pyrrolidine end-capped
(cyclic amino donor) chromophores as compared to those of
the dibutylamino (open chain amino donor) analogues, viz.,
MS 2/MS 3, MS 4/MS 5, and MS 7/MS 8 in the same solvent
medium (Table 3). The rationale for this observation might be
due to the greater donation capability of the cyclic pyrrolidine
donor compared to the open chain dibutylamino donor (vide
supra).
Scheme 2. Synthesis of the Alkylamino Benzaldehydes
Vilsmeier−Haack formylation of the N,N-dialkyl anilines (4a−b,
6a−b) converts them to the corresponding benzaldehydes
(5a−b, 7a−b) with excellent para-selectivity.
The π-conjugated benzaldehydes bearing different substitu-
tions were obtained through a two-step synthetic approach as
depicted in Table 1. The introduction of different donor
functionalities in the corresponding aldehydes (10b−15b) was
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Table 1. Synthesis of the 4-Styrylbenzaldehydes
Upon excitation at the lowest energy absorption maxima, the
chromophores MS 1−8 exhibit bright fluorescence at room
temperature with a large Stokes shift from the relevant absorp-
tion maxima (see Table 3). The positions of the corresponding
emission maxima are found to be independent of the excitation
wavelength. Likewise, an excellent overlapping of the excitation
spectra with the relevant lowest energy absorption maxima for
the title chromophores MS 1−8 was observed (see Figure 1a
and 1b), thereby signifying the involvement of this photo-
excited state for the occurrence of fluorescence. The effect of
solvent polarity on the emission property of the present
chromophores was found to be pronounced as compared to the
absorption spectra (see Table 3). This observation is indicative
of the fact that the photo−excited state of the concerned lumino-
phores is markedly polar than the ground electronic state. The
effect of solvent polarity on the emission properties is even more
profound in case of the OPV derivatives (MS 4−8) compared
to the styryl-analogues MS 1−3 (see Table 3 and Figure 1c).
The variation in the photoluminescent quantum yield of all of
the title fluorophores was examined with the same set of
solvents and is compiled in Table 3. The OPV derivatives, MS
4−8, exhibit brighter fluorescence in comparison with the first
homologues, MS 1−3 (see Table 3). The relative quantum
yield of the title chromophores is, however, strongly dependent
on the fluid medium, and an irregular alteration of the same
with the solvent polarity is observed (see Table 3).
A comparison of the photophysical parameters of MS 2 with
that of its parents HLB 113d and N945L15a shows that the
ground state property of MS 2 is comparable with that of HLB
1. Presumably, owing to the greater donation capability of the
amino donor functionality in comparison with the methoxy
donor, it has a dominant contribution in the respective ICT
transition. However, a significant variation in the emission color
was observed for MS 2 in comparison with that of HLB 1 and
N945L. The chromophore MS 2 exhibits a deep green emis-
sion at 533 nm in DCM that occurs at considerably longer
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a
1
Table 2. Selected H NMR Chemical Shifts for MS 1−8
Table 3. Summary of Optical Data of the Synthesized
Chromophores (MS 1−8)
b
c
d
compound Py−H6,6
′
Py−H3,3
′
Py−H5,5
′
CHCH
b
c
e
λmax ε ( 500−1000)
compound solvent (nm) (L mol−1 cm−1
λem
Φem
Δν
̅
MS 1
MS 2
MS 3
MS 4
8.625 (d)
8.635 (d)
8.605 (d)
8.695 (d)
8.48 (s)
7.35−7.34
7.44−7.43
7.43−7.41
7.42−7.40
7.41 (d), 6.91 (d)
7.75 (d), 7.05 (d)
7.75 (d), 6.98 (d)
a
d
)
(nm) ( 0.1)
(cm−1
)
8.48 (d)
8.46 (s)
8.56 (s)
MS 1
MS 2
toluene 390
448
485
498
472
507
533
550
480
516
543
560
540
615
644
585
552
630
662
603
538
575
598
641
550
591
648
650
556
627
660
640
0.05
0.09
0.09
0.09
0.17
0.20
0.03
0.10
0.18
0.17
0.04
0.55
0.64
0.37
0.07
0.85
0.72
0.21
0.17
0.66
0.81
0.65
0.19
0.63
0.62
0.35
0.03
0.59
0.61
0.23
0.07
3320
4827
5172
4324
5529
6301
7007
3677
5010
5738
6653
5931
8190
8685
7597
5303
7601
8150
6781
4348
5492
5902
7494
4966
6281
7556
7871
4486
6523
7170
7740
THF
393
396
7.50 (d), 7.47 (d),
7.14 (d), 7.00 (d)
DCM
MS 5
MS 6
MS 7
MS 8
8.680 (d)
8.675 (d)
8.675 (d)
8.675 (d)
8.56 (s)
8.52 (s)
8.51 (s)
8.51 (s)
7.42−7.40
7.46−7.45
7.50 (d), 7.47 (d),
6.94 (d)
toluene 392
THF
396
399
397
7.80 (d), 7.28 (d),
7.20 (d), 7.10 (d)
DCM
MeCN
60000
84000
not resolved 7.80 (d), 7.37 (d),
7.21 (d)
MS 3
MS 4
MS 5
MS 6
MS 7
MS 8
toluene 408
THF
410
414
408
not resolved 7.80 (d), 7.49 (d),
7.32 (d), 7.20 (d)
DCM
MeCN
a
All spectra were recorded at 400 MHz working frequency in CDCl3
at 298 2 K. The resonances are reported in ppm with respect to the
toluene 409
114000
100000
95000
b
c
3
TMS signal. JHH = 4 Hz except in MS 5 for which J = 8 Hz. Could
THF
409
413
405
d
be either doublet or doublet of doublet. Coupling constant (3JHH) ≈
DCM
MeCN
16 Hz, for MS 5 and MS 6 one signal is overlapped with others and
could not be separately detected. MS 1−3 contain only two vinylic
protons in their molecular structure.
toluene 427
THF
426
430
428
DCM
MeCN
122000
wavelength than the emission due to HLB 1 (497 nm) or
N945L (450 nm). Addition of the amino functionality to
N945L or two methoxy donors to HLB 1 generates MS 2,
which exhibits a significant weaker fluorescence compared to
that of N945L, though the fluorescence quantum yields of
MS 2 and HLB 1 are reasonably comparable. Hence, it can be
said that presence of both the amino and the methoxy func-
tionalities in same chromophore has a more profound effect
on the emitting state compared to the ground state of the
aforementioned luminophores.
The absorption and emission behavior of the OPVs HLB 2,
TM 6, and MS 4 follow an almost similar trend as the first
homologues, viz., HLB 1, N945L, and MS 2, respectively. For a
convenient discussion, the two phenyl rings of the OPV
chromophores were named as ring A and ring B (see Chart 1).
The hybrid chromophore MS 4, which comprises both the
-NBu2 and the -OMe functionalities in the same positions as in
HLB 213h and TM 6,19a absorbs at 413 nm (in DCM) due to
an ICT and exhibits an orange-red emission (λem = 644 nm)
with a very large Stokes shift (8685 cm−1). Thus, like the first
homologues, coexistence of the methoxy donors along with the
dibutylamino groups in MS 4 has very little influence on the
absorption wavelength. The influence of methoxy donors is
found to be rather dramatic on the fluorescence behavior of the
relevant OPV chromophores. Compared to HLB 2 and TM 6,
the fluorophore MS 4 emits at a considerably longer wave-
length but with almost half photoluminescent quantum yield. In
other words, both the approaches, viz., (a) attaching methoxy
donors to HLB 2 at ring B or (b) introducing amino donor to
TM 6 at ring B, render a significant bathochromic shift of the
emission wavelength in the resultant chromophore but with the
concomitant diminution of the fluorescence brightness.
toluene 436
116000
99000
95000
THF
437
442
433
DCM
MeCN
toluene 432
100400
96000
87000
THF
431
435
430
DCM
MeCN
toluene 445
131000
100000
98500
THF
445
448
428
DCM
MeCN
a
ET(30) values of the used solvents are as follows: toluene (33.9),
THF (37.4), DCM (40.7), MeCN (45.6).21 Average of five data. In
b
c
cases where no data are reported, this is due to low solubility. The
solutions were excited at the corresponding lowest energy absorption
d
maxima. Fluorescence relative quantum yield of the compound MS 1
was measured using quinine sulfate (in 1 N H2SO4) as the reference
(Φem = 0.545),22 and those of the rest of the compounds (MS 2−8)
were determined using DCM−Pyran as the reference in MeOH
(Φem = 0.435).23 Comparable result was obtained using fluorescein (in
0.1 N NaOH) as the standard substance also. Optically matched
solutions (OD ≈ 0.05) of the samples and of the standards were
excited at identical operating condition. Data is presented as an
e
average of two measurements. Stokes shift Δν = νabs − νem.
̅
̅
̅
(Chart 1). The alkoxy donors were moved to ring A (2,5-
positions) in case of MS 6 for which two butyloxy donors were
used to aid in solubility instead of two methoxy donors as in
MS 4, the total number of donor units being same for both the
chromophores. The alkyl chain length in the alkoxy donors is
not expected to alter the +I effect to a very significant extent.
The absorption due to an intramolecular charge transfer in MS
6 (442 nm in DCM) is shifted to longer wavelength by ca.
30 nm compared to that in MS 4 (413 nm in DCM), and the
reason behind this might probably be due to the greater extent
of donor(alkoxy)−acceptor(pyridine) interaction in MS 6 due
to a smaller separation between the concerned functionalities.
On the contrary, the emission spectrum of MS 6 exhibits an
almost 45 nm hypsochromic shift compared to MS 4 in DCM,
One Photon Absorption and Emission Properties: (b)
Position of Substituents. A careful analysis of the optical
data presented in Table 3 reveals some interesting features. The
OPV chromophores MS 4, MS 6, and MS 7 were designed and
synthesized in order to investigate the outcome of varying the
position and number of alkoxy donors on the absorption and
emission properties of the relevant chromophores. In MS 4,
two methoxy donors were attached with ring B at the 2,5-
positions along with the dibutylamino donor at the 4-position
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bear the highest number of donor units among all of the
compounds discussed in this article. Most surprisingly, despite
attaching extra alkoxy donors with the ring B of MS 6, a further
shift of the ICT absorption to the longer wavelength has not
been induced but instead a hypsochromic shift of ca. 7 nm of
the relevant band position is observed for MS 7. The emission
properties of the luminophores MS 4 and MS 7 are almost
similar except in MeCN in which the former exhibits yellow
emission but the latter exhibits red emission (see Table 3 and
Figure 1c).
Solid-State Emission of MS 5. Among all of the studied
“push−pull” chromophores in the present work (MS 1−8),
compound MS 5 fluoresces even in the solid state. The solid-
state absorption and emission spectra of the pertinent
compound were recorded in dilute KBr matrix, and the
relevant spectra are presented in Figure 2. The absorption
Figure 2. Solid-state absorption (black) and emission (red) spectra of
MS 5 at room temperature (KBr pellet, arbitrary concentration). The
inset shows photographs of the relevant KBr pellet and bulk sample
illuminated with a 365 nm UV lamp.
spectrum is characterized by a broad band centered at 442 nm
that is slightly red-shifted with respect to the absorption
maxima of the relevant dyad in the dissolved media (see Table 3).
The bathochromic shift of the absorption maximum in
condensed phase as compared to that of the solution state is
presumably due an aggregation. When excited at the relevant
absorption maximum, the chromophore MS 5 exhibits orange
emission with the maximum intensity at 614 nm (see Figure 2).
Computational Analysis. To gain insight into the effect
of various substituents on the geometrical and electronic out-
come of the 2,2′-bipyridine chromophores, density functional
theory (DFT) and time-dependent DFT (TD-DFT) were ap-
plied on the synthesized compounds MS 1−8 along with HLB
1, N945L, HLB 2, TM 6, and styryl- and bistyryl-derivatives
without any donor substituent using the Gaussian09 program
package.24 Although the DFT analysis of N945L has already
been reported,15a we have reproduced the same with our own
computational setup for the purpose of ready comparison. A
semiempirical calculation on HLB 2 was earlier reported
Figure 1. Normalized absorption (solid lines, concentration of the
samples being ∼1 × 10−5 M), emission (dashed lines), and excitation
spectra (overlapped with the absorption spectra) together with the
computed vertical excitation energies (stick lines) of (a) MS 1−3 and
(b) MS 4−8 in DCM at 298 2 K. All of the solutions were excited at
the lowest energy absorption maxima. Heights of the vertical lines
were adjusted so as to obtain viewing lucidity; the relative heights are
however in scale. (c) Photographs showing photoluminescence
behavior of the MS 1−8 in four different solvents illuminated under
an UV lamp (excitation 365 nm) in the dark.
by Hernan
́
dez and co-workers (named as SY187).25 The
geometry of all of the studied systems was optimized using
the CAM-B3LYP exchange-correlation hybrid functional
together with the 6-31+g(d) basis set imposing Ci symmetry
constraint. All of the butyl chains of the OPV derivatives
(MS 4, MS 6, and MS 7) were truncated to methyl groups in
and furthermore, the fluorescence quantum yield of MS 6 is
considerably larger than that of MS 4 (Φem = 0.65 for MS 6 and
0.37 for MS 4 in DCM). The chromophores MS 7 and MS 8
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an attempt to reduce the number of basis functions and
consequently to increase the computational speed. The relevant
geometrical structures were modeled by applying the self-
consistent reaction field (SCRF) under the polarizable con-
tinuum model (C-PCM) incorporating DCM as the solvent.
For the purpose of ready comparison between the various
computed parameters, an identical theoretical setup was
maintained throughout the course of the computational investi-
gations. The DFT-computed HOMO and LUMO frontier
molecular orbitals (FMOs) of the chromophores MS 1−8 are
presented in Figure 3. A scrutiny on the computer-modeled
and out-of-phase π-bonding combination and consists of major
coefficients at the phenyl rings containing the donor sub-
stituents and at the CC vinylic bonds. On the contrary, the
LUMO of the model structures stems from the π*-type
combination of the molecular orbitals (MOs), mainly
contributed by the CC bonds and the two pyridine rings
along with a sizable coefficient on the C−C bond joining the
two pyridine rings. The modulation of energy for the four
occupied (HOMO−3, HOMO−2, HOMO−1, HOMO) and
four virtual (LUMO, LUMO+1, LUMO+2, LUMO+3) molec-
ular orbitals along with the corresponding computed HOMO−
LUMO gap (Eg) upon alteration of the core architecture in the
studied systems are presented in the Supporting Information.
The HOMO−1 and HOMO occupied MOs of all of the
studied systems constitute an almost degenerate couple. Such
pairs in 4,4′-bis(styryl)-2,2′-bipyridine and 4,4′-bis(4-styryl)-
2,2′-bipyridine, which are devoid of any donor functionality, are
most stabilized among all of the model structures and are
computationally estimated at −7.534/−7.480 and −6.936/−
6.909 eV, respectively. A varying degree of destabiliza-
tion of the HOMO−1 and HOMO occupied levels along
with alteration of the HOMO−LUMO gap was observed upon
introduction of the donor substituents to the 2,2′-bipyridine
core skeleton (see Supporting Information). The consequence
of conjugation length on the electronic structure of the
bipyridine dyads is evidently comprehended in the computer-
modeled structures. For example, expansion of conjugation
from MS 3 to MS 5 results in destabilization of the HOMO by
0.108 eV and a sizable shrinking of the HOMO−LUMO gap by
0.407 eV with respect to MS 3.
The various vertical Franck−Condon electronic excitations
were computed by the TD-DFT formalism in order to
comprehend the nature of electronic transitions responsible
for appearance of the absorption bands in the relevant
electronic spectra. Some representative excited states for MS
1−8 and the computer-generated absorption spectra of the
studied systems are presented in the Supporting Information.
The computed vertical excitations for MS 1−8 in DCM are
shown in Figure 1a and b. An initial attempt to compute the
excited states (TD-DFT) using the B3LYP hybrid functional
has resulted in large overestimation of the absorption bands,
mostly for the OPV analogues. In particular, the well-celebrated
TD-DFT method is often known to produce flawed results in
the case of computation on the excited states of molecules with
extended π-systems and those that involve a charge transfer
character.26 This method (TD-DFT) is also known to be
sensitive to the functional to gain the correct long-range 1/R
dependence on the donor−acceptor distance in the π-expanded
systems.26 Configuration Interaction Singles (CIS) is a pro-
mising method to compute the excited states of such systems,
but our attempts to calculate the absorption spectra of the
present OPV chromophores at this level have not improved the
situation. This has prompted us to use the recently developed
coulomb-attenuated long-range corrected version of B3LYP,
i.e., CAM-B3LYP, hybrid functional, which has turned out to be
very promising as this functional recovers the long-range 1/R
behavior.27
Figure 3. Isodensity plots of HOMO and LUMO frontier molecular
orbitals of the bipyridine chromophores MS 1−8 as computed by the
CAM-B3LYP/6-31+g(d) level of theory (isodensity value = 0.02). The
butyl chains of MS 4 and MS 6−8 were truncated to methyl groups.
structures of the studied systems reveals some common
features such as almost coplanar structure of the systems that
are devoid of any alkoxy functionality, out-of-plane displace-
ment of the phenyl rings bearing the alkoxy substituents from
the planes containing the pyridine ring and the CC vinylic
bonds, etc. Hence, these matters have not been individually
addressed in this article.
An investigation on the electronic structures reveals that an
almost similar type of localization and nature of HOMO and
LUMO are retained among the individual homologues (see
Figure 3), the only difference being in the energy of these FMOs,
which in turn depend on the system architecture. The HOMO
of the relevant chromophores is associated with the in-phase
A fair agreement between the computed vertical excitations
and the experimentally observed absorption maxima is observed
for the studied systems (see Figure 1a and b). The lowest
energy intense absorption band in all of the cases is due to the
involvement of two excited states, viz., HOMO−1 → LUMO
and HOMO → LUMO+1 excitations with high oscillator
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strength (f > 1) indicating a π → π* ICT from the donor-based
molecular orbitals to the pyridine (acceptor)-centered molec-
ular orbitals. The HOMO(Au) → LUMO(Au) excitation in all
of the studied systems is symmetrically forbidden and does not
contribute to the electronic absorption spectra in the relevant
bipyridine dyads. The scenario of the computed electronic
excitations thereby essentially points out the “push−pull”
architecture of the pertinent systems. Unfortunately, ∼15 nm
bathochromic shift of the ICT absorption maximum of MS 3 in
comparison with MS 2 could not be reproduced in our
computational setup. Interestingly, the ∼18 nm red-shift of the
ICT absorption maximum for MS 6 was computed in
comparison with MS 4, very similar to the experimentally
observed ∼30 nm bathochromic shift upon identical alteration
of the donor position (see Table 3). However, the experi-
mentally observed ∼7 nm hypsochromic shift of the ICT band
maximum upon increment of the number of alkoxy donors
from MS 6 to MS 7 could not be reproduced.
emitting state of the fluorophores is substantially more polar
than the ground electronic state, which was affirmed by the
large solvent-sensitive photoluminescent behavior of the syn-
thesized compounds. The absorptive and emissive nature of the
present chromophores are enormously dependent on the
conjugation backbone, i.e., the nature, position, and number of
donor functionalities, the parameters which act as the function
of their photophysical outcome. The computational analyses of
the absorption properties of the synthesized chromophores
clearly depict an alteration of the various occupied and virtual
energy levels with the structure of the synthesized chromo-
phores and is in fair agreement with the variation trend of the
experimentally observed absorption spectra.
In this work, we have demonstrated optical properties of 4,4′-
π-conjugated-2,2′-bipyridine dyes that were fabricated with
heterodonor functionalities. We have shown how simple modi-
fication of the π-skeleton of these dyes modulates their
fluorescence behavior. For example, out of the eight dye stuffs
described in this article, the compound, MS 5 fluoresces even in
the condensed phase ,which is one of the prerequisites for
possible application in light-emitting devices. Thus, changing
the amino donor from open chain dibutylamino in MS 4 to the
cyclic pyrrolidine donor in MS 5 results in this differing
photophysical behavior. Although the 4,4′-π-conjugated-2,2′-
bipyridine derivatives were extensively studied in terms of
devising nonlinear optical and solar energy harvesting materials,
our systematic study on the photophysical properties of the
heterodonor systems in comparison with the parent chromo-
phores bearing alike donor functionalities approaches a
structure−function relationship that would be useful for
choosing the proper material for practical applications.
Accordingly, the findings of the present study in conjunction
with the photophysics of the earlier reported parent dye
molecules construct a library of such systems in which the
present investigation finds some notable differences in the
fluorescence responses compared to the parent molecules
functionalized with only one kind of donor subchromophores.
The present work reveals that the coexistence of both the
alkoxy and amino functionalities in the same phenyl ring
modulates the emitting state of MS 2 and MS 4, which exhibit
considerable bathochromic shifted fluorescence with respect to
the reported chromophores having either alkoxy (N945L, TM
6) or amino (HLB 1-2) donor groups. Furthermore, moving
the alkoxy donors from ring B (MS 4) to ring A (MS 6) results
in a large bathochromic shift of the absorption wavelength
presumably due to the enhanced participation of the alkoxy
groups in the charge transfer process when placed in ring A
rather than in ring B. The introduction of additional alkoxy
donor functionalities does not induce further bathochromic
shift of the absorption maxima. Furthermore, it is firmly
believed that the transition metal complexes of the present
neutral bipyridine chromophores would exhibit exciting linear
and nonlinear optical responses, and this work is presently
under progress in our laboratory.
Thermal Stability. Thermal durability is one of the most
significant and essential parameters for custom-tailored appli-
cations of the bench-top-synthesized compounds. The thermal
decay/weight loss of compounds MS 1−8 was examined by
heating the solid samples in an analyzer under a flow of
nitrogen. Thermal weight loss behavior of the chromophores
MS 3, MS 5, and MS 6−8 are shown in Figure 4, which depicts
Figure 4. Thermogravimmetric Behavior of MS 3, MS 5, and MS 6−8.
very high thermal stability of the relevant “push−pull”
chromophores. The 10% weight loss temperature (Td10) for
these chromophores is evidently high (∼350 °C) as shown in
Figure 4. No significant weight loss occurs even up to 250 °C.
However, the Td10 value for compounds MS 1−2 and MS 4 are
found to be relatively low (ca. 150 °C).
CONCLUSION
■
In summary, a series of 4,4′-π-conjugated-2,2′-bipyridine dyads
having a D-π-A-A-π-D architecture were synthesized wherein
the 2,2′-bipyridine heterocycle acts as the central acceptor core
to join the donor termini through olefinic spacers. The photo-
physical properties (absorption and emission) of the syn-
thesized chromophores are governed by the intramolecular
charge separation from the donor end-capping functionalities to
the pyridine acceptor heterocycle. This observation is in line
with the photophysics of previously reported related dyes. The
EXPERIMENTAL SECTION
■
N,N-Dibutylaniline (4a). A mixture of aniline (19 mL, 200 mmol),
1-bromo butane (65 mL, 600 mmol), and Na2CO3 (84 g, 800 mmol)
in 100 mL of 90:10 v/v DMF/N-methylpyrrolidinone (NMP) was
heated at 120 °C for 24 h. The reaction mixture was then cooled
to room temperature and filtered to remove the insoluble material.
The precipitate was washed with ethyl acetate, and the combined
filtrate was evaporated to dryness. Water was then added, and the
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aqueous phase was extracted with ethyl acetate. After a silica gel
filtration using hexane as the mobile phase, the pure compound
was obtained as a pale yellow liquid. Yield: 35.3 g (86%). H NMR
(400 MHz, CDCl3): δ 7.25−7.21 (unresolved, 2H), 6.68
(unresolved, 3H), 3.29 (t, 4H), 1.58 (unresolved, 4H), 1.41−1.35
(m, 4H), 1.00 (t, 6H). 13C NMR (100 MHz, CDCl3): δ 129.2, 115.1,
111.7, 50.8, 29.4, 20.4, 14.0. LC−MS (positive mode): m/z 206
(M + H)+. Anal. Calcd for C14H23N: C, 81.89; H, 11.29; N, 6.82.
Found: C, 81.83; H, 11.33; N, 6.84.
LC−MS (positive mode): m/z 148 (M + H)+. Anal. Calcd for C10H13N:
C, 81.59; H, 8.90; N, 9.51. Found: C, 81.48; H, 9.04; N, 9.48.
1
1-(2,5-Dimethoxyphenyl)pyrrolidine (6b). This compound
was prepared using the same procedure as described for the synthesis
of compound 6a; 7.6 g (50 mmol) of 2,5-dimethoxy aniline was used
instead of aniline, and the reaction mixture was heated at 120 °C for
48 h. Silica gel filtration using EtOAc/Hexane 5:95 v/v as the mobile
phase afforded the pure compound 6b as a yellow light-sensitive liquid.
Yield: 7.9 g (76%). 1H NMR (400 MHz, CDCl3): δ 6.78 (d, J = 8 Hz,
1H), 6.38 (d, 4JHH = 4 Hz, 1H), 6.34−6.31 (dd, J = 8 Hz, 4JHH = 4 Hz,
1H), 3.79 (s, 3H), 3.78 (s, 3H), 33.2 (unresolved, 4H), 19.4
(unresolved, 4H). 13C NMR (100 MHz, CDCl3): δ 154.5, 144.8,
141.1, 113.0, 103.1, 101.6, 56.6, 55.5, 50.4, 24.5. LC−MS (positive
mode): m/z 208 (M + H)+. Anal. Calcd for C12H17NO2: C, 69.54; H,
8.27; N, 6.76. Found: C, 69.41; H, 8.23; N, 6.73.
4-(Pyrrolidin-1-yl)benzaldehyde (7a). This compound was syn-
thesized using the same procedure as described for 5a. 1-Phenyl-
pyrrolidine (6a, 7.35 g, 50 mmol) was used instead of N,N-dibutyl
aniline (4a). Reaction time at 90−95 °C: 6−7 h. Column chromato-
graphic (silica gel, 100−200 mesh) purification of the crude product
using EtOAc/Hexane 20:80 v/v as the eluent afforded the desired
product 7a as a white crystalline solid. Yield: 7.26 g (83%). Mp:
81−83 °C. 1H NMR (400 MHz, CDCl3): δ 9.72 (s, 1H), 7.73 (d, J = 8
Hz, 2H), 6.57 (d, J = 8 Hz, 2H), 3.41−3.37 (m, 4H), 2.07−2.04 (m,
4H). 13C NMR (100 MHz, CDCl3): δ 190.2, 152.0, 132.1, 124.9,
111.2, 47.7, 25.4. IR (KBr, cm−1): υmax 1674 (>CO). LC−MS
(positive mode): m/z 176 [M + H]+. Anal. Calcd for C11H13NO: C,
75.40; H, 7.48; N, 7.99. Found: C, 75.45; H, 7.39; N, 8.07.
2,5-Dimethoxy-4-(pyrrolidin-1-yl)benzaldehyde (7b). This
compound was synthesized using the same procedure as described
for 5a. 1-(2,5-Dimethoxyphenyl)pyrrolidine (6b, 10.35 g, 50 mmol)
was used instead of N,N-dibutyl aniline (4a). Reaction time at 90−
95 °C: 7−8 h. The crude product was purified by silica gel (100−
200 mesh) column eluting with EtOAc/Hexane 20:80 v/v as the
mobile phase to obtain the pure aldehyde 7b a light-sensitive cream
colored solid. Yield: 10.34 g (88%). Mp: 120−121 °C. 1H NMR
(400 MHz, CDCl3): δ 10.13 (s, 1H), 7.23 (s, 1H), 6.00 (s, 1H), 3.85 (s,
3H), 3.76 (s, 3H), 3.55 (unresolved, 4H), 1.93 (unresolved, 4H). 13C
NMR (100 MHz, CDCl3): δ 186.7, 159.5, 146.7, 142.9, 113.8, 110.1,
96.2, 56.4, 55.8, 50.7, 25.5. IR (KBr, cm−1): υmax 1639 (>CO). LC−
MS (positive mode): m/z 236 [M + H]+. Anal. Calcd for C13H17NO3: C,
66.36; H, 7.28; N, 5.95. Found: C, 66.28; H, 7.25; N, 6.11.
N,N-Dibutyl-2,5-dimethoxyaniline (4b). This compound was
synthesized using the same procedure as described for compound 4a.
2,5-Dimethoxy aniline (3b, 30.6 g, 200 mmol) was used instead of
aniline (3a), and the reaction time was 72 h at 120 °C. The black
crude liquid was subjected to silica gel filtration using hexane as the
eluant to obtain the pure product 4b as a light-sensitive pale yellow
1
liquid. Yield: 38.7 g (73%). H NMR (400 MHz, CDCl3): δ 6.76 (d,
4
J = 8 Hz, 1H), 6.54 (d, JHH = 4 Hz, 1H), 6.46−6.43 (dd, J = 8 Hz,
4JHH = 4 Hz, 1H), 3.80 (s, 3H), 3.78 (s, 3H), 3.08 (t, 4H), 1.45
(p, 4H), 1.28 (sextet, 4H), 0.88 (t, 6H). 13C NMR (100 MHz,
CDCl3): δ 153.9, 147.9, 141.3, 112.8, 108.7, 104.7, 56.2, 55.5, 52.4,
26.9, 20.5, 14.1. LC−MS (positive mode): m/z 266 (M + H)+. Anal.
Calcd for C16H27NO2: C, 72.41; H, 10.25; N, 5.28. Found: C,
72.47; H, 10.11; N, 5.32.
4-(Dibutylamino)benzaldehyde (5a). POCl3 (5.5 mL, 60 mmol)
was slowly added to a DMF (20 mL, excess) solution of N,N-dibutyl
aniline (4a, 10.25 g, 50 mmol) with cooling in an ice bath under an
inert atmosphere. The resulting orange-red viscous liquid was then
stirred at this temperature for 15 min, slowly warmed to room
temperature, and then heated at 90−95 °C for 7 h. The dark reaction
mixture was then cooled in an ice bath and carefully quenched with
water, followed by neutralization with aqueous Na2CO3 solution. The
aqueous layer was extracted with dichloromethane. The combined
organic layer was washed with water and brine, dried (anhydrous
Na2SO4), and evaporated. Chromatographic purification of the crude
mixture in a silica gel (100−200 mesh) column eluting with EtOAc/
Hexane 15:85 v/v gave the product 5a as a yellow viscous liquid. Yield:
11.16 g (95%). 1H NMR (400 MHz, CDCl3): δ 9.69 (s, 1H), 7.69 (d,
J = 8 Hz, 2H), 6.64 (d, J = 8 Hz, 2H), 3.35 (t, 4H), 1.60 (p, 4H), 1.36
(hextet, 4H), 0.97 (t, 6H). 13C NMR (100 MHz, CDCl3): δ 189.9,
152.6, 132.2, 124.5, 110.7, 50.8, 29.3, 20.3, 20.25, 13.9. IR (neat, cm−1):
υmax 1680 (>CO). LC−MS (positive mode): m/z 234 [M + H]+.
Anal. Calcd for C15H23NO: C, 77.21; H, 9.93; N, 6.00. Found: C,
77.27; H, 9.84; N, 6.12.
4-(Dibutylamino)-2,5-dimethoxybenzaldehyde (5b). This
compound was synthesized using the same procedure as described
for 5a. N,N-Dibutyl-2,5-dimethoxyaniline (2b, 13.25 g, 50 mmol) was
used instead of N,N-dibutyl aniline (2a). Reaction time at 90−95 °C:
4 h. The product 5b was isolated as an orange-yellow viscous liquid
after silica gel (100−200 mesh) filtration of the crude with EtOAc/
Hexane 20:80 v/v as the mobile phase. Yield: 12.60 g (86%). 1H NMR
(400 MHz, CDCl3): δ 10.20 (s, 1H), 7.25 (s, 1H), 6.27 (s, 1H), 3.85
(s, 3H), 3.79 (s, 3H), 3.30 (t, 4H), 1.55 (p, 4H), 1.31 (hextet, 4H),
0.90 (t, 6H). 13C NMR (100 MHz, CDCl3): δ 187.3, 158.6, 147.9,
145.3, 115.9, 110.1, 100.6, 55.98, 55.90, 52.2, 29.8, 20.4, 14.0. IR
(neat, cm−1): υmax 1660 (>CO). LC−MS (positive mode): m/z 294
[M + H]+. Anal. Calcd for C17H27NO3: C, 69.59; H, 9.28; N, 4.77.
Found: C, 69.51; H, 9.33; N, 4.70.
(E)-4-(4-(Pyrrolidin-1-yl)styryl)bromobenzene (10a). Solid
potassium tert-butoxide (1.68 g, 15 mmol) was added at once to an
ice-cooled THF solution (30 mL) containing 3.68 g of phosphonate 8
(12 mmol) and 1.75 g of aldehyde 7a (10 mmol) under an inert
atmosphere. The resulting slurry was warmed to room temperature
and stirred for 60 min before the reaction was quenched with water.
The precipitated yellow solid was collected by filtration, washed
several times with water, and dried in air. It was re-dissolved in
dichloromethane (ca. 600 mL), and the solvent was rotavaporated to
ca. 15 mL and filtered to obtain the pure product 10a as yellow solid.
1
Yield: 2.85 g (87%). Mp: >200 °C. H NMR (400 MHz, CDCl3): δ
7.43 (d, J = 8 Hz, 2H), 7.39 (d, J = 8 Hz, 2H), 7.33 (d, J = 8 Hz, 2H),
7.03 (d, J = 16 Hz, 1H), 6.81 (d, J = 16 Hz, 1H), 6.55 (d, J = 8 Hz,
2H), 3.33 (unresolved, 4H), 2.02 (unresolved, 4H). 13C NMR was not
possible because of low solubility. LC−MS (positive mode): m/z 328
(M)+, 330 (M + 2H)+. Anal. Calcd for C18H18BrN: C, 65.86; H, 5.53;
N, 4.27. Found: C, 65.69; H, 5.48; N, 4.30.
(E)-4-(4-(Dibutylamino)-2,5-dimethoxystyryl)bromobenzene
(11a). Under nitrogen, solid potassium tert-butoxide (1.68 g, 15 mmol)
was added all at once to a THF solution (30 mL) containing 3.68 g
of phosphonate 8 (12 mmol) and 2.93 g of aldehyde 5b (10 mmol)
pre-cooled at 0 °C. The ice bath was removed, and the reaction mixture
was allowed to stir at room temperature for 60 min. It was sub-
sequently quenched with water and then extracted with dichloro-
methane. The combined organic phase was washed with brine, dried
over anhydrous Na2SO4, and evaporated to dryness. Purification of
the crude product through column chromatography (silica gel, 100−
200 mesh) using EtOAc/Hexane 5:95 v/v as the eluent afforded the
1-Phenylpyrrolidine (6a). A mixture of aniline (4.6 mL, 50 mmol),
1,4-dibromobutane (6.5 mL, 55 mmol), and K2CO3 (28 g, 200 mmol)
in 90:10 v/v DMF/N-methylpyrrolidinone (NMP) was heated at 120 °C
for 24 h after which it was cooled to room temperature and filtered. The
precipitated solid was washed with ethyl acetate, and the combined
filtrate was evaporated to dryness. Water was then added, and the
aqueous phase was extracted with ethyl acetate. The crude product was
purified on short silica gel column eluting with EtOAc/Hexane 1:99 v/v
to afford the pure product 6a as a yellow light-sensitive liquid. Yield: 4.7
g (62%). 1H NMR (400 MHz, CDCl3): δ 7.29−7.25 (unresolved, 2H),
6.70−6.61 (m, 3H), 3.32 (unresolved, 4H), 2.04 (unresolved, 4H). 13C
NMR (100 MHz, CDCl3): δ 148.2, 129.2, 115.6, 111.8, 47.7, 25.5.
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product 11a as dark yellow solid. Yield: 4.06 g (91%). Mp: 76−
56.5, 50.5, 31.5, 25.1, 19.4, 19.3, 14.0. LC−MS (positive mode):
m/z 532 (M)+, 534 (M + 2H)+. Anal. Calcd for C28H38BrNO4: C,
63.15; H, 7.19; N, 2.63. Found: C, 63.21; H, 7.22; N, 2.68.
1
78 °C. H NMR (400 MHz, CDCl3): δ 7.46−7.37 (m, 5H), 7.06
(s, 1H), 6.89 (d, J = 16 Hz, 1H), 6.50 (s, 1H), 3.87 (s, 3H), 3.85
(s, 3H), 3.16 (t, 4H), 1.51 (p, 4H), 1.33 (m, 4H), 0.90 (t, 6H). 13C NMR
(100 MHz, CDCl3): δ 151.8, 147.4, 141.4, 137.4, 131.6, 127.7, 125.0,
124.1, 120.4, 118.4, 110.3, 105.2, 52.3, 29.3, 20.6, 14.1. LC−MS (positive
mode): m/z 446 (M)+, 448 (M + 2H)+. Anal. Calcd for C24H32BrNO2:
C, 64.57; H, 7.23; N, 3.14. Found: C, 64.43; H, 7.16; N, 3.16.
(E)-4-(2,5-Dimethoxy-4-(pyrrolidin-1-yl)styryl)bromo-
benzene (12a). This compound was prepared using the same
procedure as described for 11a using 3.68 g of phosphonate 8 (12 mmol),
2.35 g of aldehyde 7b (10 mmol), and 1.68 g of potassium tert-
butoxide (15 mmol) in 30 mL of THF. Column chromatographic
purification on silica gel (100−200 mesh) using EtOAc/Hexane
20:80 v/v as the mobile phase afforded the product 12a as fluores-
cent yellow needles. Yield: 3.65 g (94%). Mp: 128−130 °C. 1H NMR
(400 MHz, CDCl3): δ 7.45−36 (m, 5H), 7.07 (s, 1H), 6.85 (d, J = 16
Hz, 1H), 6.29 (s, 1H), 3.86 (s, 3H), 3.84 (s, 3H), 3.41 (unresolved,
4H), 1.96 (unresolved, 4H). 13C NMR (100 MHz, CDCl3): δ 152.5,
144.1, 141.1, 137.7, 131.6, 127.6, 124.2, 123.5, 120.0, 115.3, 110.8,
99.6, 56.9, 56.4, 50.5, 25.1. LC−MS (positive mode): m/z 388 (M)+,
390 (M + 2H)+. Anal. Calcd for C20H22BrNO2: C, 61.86; H, 5.71; N,
3.61. Found: C, 62.01; H, 5.64; N, 3.55.
(E)-4-(4-(Pyrrolidin-1-yl)styryl)benzaldehyde (10b). A solu-
tion of bromostilbene 10a (0.98 g, 3 mmol) in 110 mL of THF
was cooled to −40 °C, and then 2.2 mL of 1.6 M nBuLi (3.6 mmol)
was added dropwise. The heterogeneous mixture was stirred at this
temperature for 60 min, and 0.4 mL of dry DMF (5 mmol) was
subsequently added followed by stirring at this temperature for
additional 60 min. The cooled reaction mixture was then slowly
warmed to room temperature, quenched with saturated ammonium
chloride solution, and extracted with dichloromethane. The organic
layer was washed twice with water and then with brine, dried
(Na2SO4), and evaporated to dryness. The crude product was purified
by column chromatography on silica gel (100−200 mesh) using
dichloromethane as the mobile phase to obtain the pure aldehyde 10b
as an orange yellow solid. Yield: 0.66 g (80%). Mp: >200 °C. IR (KBr,
1
cm−1): υmax 1691 (>CO). H NMR (400 MHz, CDCl3): δ 9.96 (s,
1H), 7.83 (d, J = 8 Hz, 2H), 7.60 (d, J = 8 Hz, 2H), 7.43 (d, J = 8 Hz,
2H), 7.22 (d, J = 16 Hz, 1H), 6.92 (d, J = 16 Hz, 1H), 6.57 (d, J =
8 Hz, 2H), 3.33 (unresolved, 4H), 1.58 (unresolved, 4H). 13C NMR
(100 MHz, CDCl3): δ 191.7, 148.1, 144.8, 134.3, 132.9, 130.3, 129.4,
128.4, 126.2, 121.9, 111.8, 47.6, 25.5. LC−MS (positive mode): m/z
278 (M + H)+. Anal. Calcd for C19H19NO: C, 82.28; H, 6.90; N, 5.05.
C, 88.23; H, 6.88; N, 5.12.
(E)-2,5-Dibutoxy-4-(4-(dibutylamino)styryl)bromobenzene
(13a). This compound was prepared using the same procedure
as described for 11a using 2.3 g of phosphonate 7 (5 mmol), 1.1 g
of aldehyde 5a (4.8 mmol), and 0.7 g of potassium tert-butoxide
(6 mmol) in 25 mL of THF. The product 13a was isolated as fluo-
rescent yellow solid after column chromatography on silica gel (100−
200 mesh) using EtOAc/Hexane 5:95 v/v as the mobile phase. Yield:
(E)-4-(4-(Dibutylamino)-2,5-dimethoxystyryl)benzaldehyde
(11b). This compound was synthesized by the same procedure as
described for aldehyde 10b using bromostilbene 11a (1.34 g, 3 mmol),
1.6 M nBuLi (2.2 mL, 3.6 mmol), and DMF (0.4 mL, 5 mmol) in
20 mL of THF was used instead. Chromatographic purification using
silica gel stationary phase (100−200 mesh) and EtOAc/Hexane 10:90
v/v mobile phase rendered the aldehyde 11b as a vermillion-red
gummy mass. Yield: 0.81 g (68%). IR (KBr, cm−1): υmax 1695 (>C
1
2.34 g (92%). Mp: 84−86 °C. H NMR (400 MHz, CDCl3): δ 7.39
(d, J = 8 Hz, 2H), 7.16 (d, J = 16 Hz, 1H), 7.12 (s, 1H), 7.06 (s, 1H),
7.02 (d, J = 16 Hz, 1H), 6.54 (d, J = 8 Hz, 2H), 4.05 (t, 2H), 3.95
(t, 2H), 3.30 (t, 4H), 1.82 (p, 4H), 1.63−1.52 (m, 8H), 1.36 (t, 4H),
1.03−0.96 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 150.7, 149.9,
147.9, 129.6, 127.8, 124.9, 117.8, 111.6, 111.2, 110.3, 70.0, 69.3, 50.8,
31.5, 29.5, 20.4, 19.4, 14.0. LC−MS (positive mode): m/z 531 (M)+,
533 (M + 2H)+. Anal. Calcd for C30H44BrNO2: C, 67.91; H, 8.36; N,
2.64. Found: C, 68.02; H, 8.25; N, 2.69.
1
O). H NMR (400 MHz, CDCl3): δ 9.97 (s, 1H), 7.84 (d, J = 8 Hz,
2H), 7.65 (d, J = 8 Hz, 2H), 7.61 (d, J = 16 Hz, 1H), 7.09 (s, 1H),
7.01 (d, J = 16 Hz, 1H), 6.49 (s, 1H), 3.87 (unresolved, 6H), 3.18 (t,
4H), 1.52−1.47 (p, 4H), 1.33−1.26 (m, 4H), 0.91 (t, 6H). 13C NMR
(100 MHz, CDCl3): δ 191.7, 152.3, 147.8, 144.8, 142.2, 134.6, 130.2,
127.0, 126.5, 124.6, 117.6, 110.5, 104.6, 56.40, 56.35, 52.2, 29.4, 20.5,
14.1. LC−MS (positive mode): m/z 396 (M + H)+. Anal. Calcd for
C25H33NO3: C, 75.91; H, 8.41; N, 3.54. Found: C, 76.09; H, 8.35; N,
3.58.
(E)-4-(2,5-Dimethoxy-4-(pyrrolidin-1-yl)styryl)benzaldehyde
(12b). This compound was synthesized by the same procedure as
described for aldehyde 10b using bromostilbene 12a (1.16 g, 3 mmol),
1.6 M nBuLi (2.2 mL, 3.6 mmol), and DMF (0.4 mL, 5 mmol) in
20 mL of THF. Purification of the crude by column chromatography
using silica gel (100−200 mesh) and EtOAc/Hexane 10:90 v/v mobile
phase afforded the aldehyde 12b as a vermillion-orange solid. Yield:
0.71 g (70%). Mp: 120−122 °C. IR (KBr, cm−1): υmax 1689 (>CO).
1H NMR (400 MHz, CDCl3): δ 9.96 (s, 1H), 7.82 (d, J = 8 Hz, 2H),
7.64−7.59 (m, 3H), 7.09 (s, 1H), 6.95 (d, J = 16 Hz, 1H), 6.26 (s,
1H), 3.88 (s, 3H), 3.84 (s, 3H), 3.44 (unresolved, 4H), 1.96
(unresolved, 4H). 13C NMR (100 MHz, CDCl3): δ 191.7, 153.1,
145.1, 143.9, 141.8, 134.3, 130.3, 127.2, 126.3, 123.0, 114.6, 111.0,
99.1, 57.0, 56.3, 50.5, 25.2. LC−MS (positive mode): m/z 338 (M +
H)+. Anal. Calcd for C21H23NO3: C, 74.75; H, 6.87; N, 4.15. Found:
C, 74.62; H, 6.84; N, 4.13.
(E)-2,5-Dibutoxy-4-(4-(dibutylamino)-2,5-dimethoxystyryl)-
bromobenzene (14a). This compound was prepared using the
same procedure as described for 11a using 2.3 g of phosphonate 7
(5 mmol), 1.4 g of aldehyde 5b (4.8 mmol), and 0.7 g of potassium
tert-butoxide (6 mmol) in 25 mL of THF. The product 14a was
isolated as yellow solid after chromatographic purification on silica gel
stationary phase (100−200 mesh) using EtOAc/Hexane 5:95 v/v as
the mobile phase. Yield: 2.61 g (92%). Mp: 58−60 °C. 1H NMR (400
MHz, CDCl3): δ 7.41 (d, J = 16 Hz, 1H), 7.26 (d, J = 16 Hz, 1H), 7.15
(s, 1H), 7.10 (s, 1H), 7.07 (s, 1H), 6.51 (s, 1H), 4.05 (t, 2H), 3.96
(t, 2H), 3.86−3.85 (m, 6H), 3.15 (t, 4H), 1.85−1.78 (m, 4H), 1.60−
1.43 (m, 8H), 1.29 (m, 4H), 0.99 (t, 6H), 0.91 (t, 6H). 13C NMR (100
MHz, CDCl3): δ 151.7, 150.9, 149.9, 147.6, 141.1, 127.8, 123.9, 120.8,
119.4, 117.9, 111.7, 110.9, 110.5, 105.4, 70.0, 69.3, 56.5, 56.3, 52.4,
31.5, 29.4, 20.6, 19.4, 19.3, 14.0, 13.9. LC−MS (positive mode): m/z
590 (M)+, 592 (M + 2H)+. Anal. Calcd for C32H48BrNO4: C, 65.07;
H, 8.19; N, 2.37. Found: C, 65.10; H, 8.08; N, 2.43.
(E)-2,5-Dibutoxy-4-(2,5-dimethoxy-4-(pyrrolidin-1-yl)styryl)-
bromobenzene (15a). This compound was synthesized by the
same procedure as described for 11a using 2.3 g of phosphonate 7
(5 mmol), 1.1 g of aldehyde 7b (4.8 mmol), and 0.7 g of potassium
tert-butoxide (6 mmol) in 25 mL of THF. Chromatographic puri-
fication using silica gel stationary phase (100−200 mesh) and EtOAc/
Hexane 10:90 v/v as the mobile phase afforded the product 15a as
(E)-2,5-Dibutoxy-4-(4-(dibutylamino)styryl)benzaldehyde
(13b). This compound was synthesized by the same procedure as
described for aldehyde 10b using bromostilbene 13a (1.6 g, 3 mmol),
1.6 M nBuLi (2.2 mL, 3.6 mmol), and DMF (0.4 mL, 5 mmol) in
20 mL of THF. Chromatographic purification of the crude product on
silica gel (100−200 mesh) using EtOAc/Hexane 5:95 v/v as the eluent
afforded the aldehyde 13b as a thick red gum. Yield: 0.82 g (57%). IR
1
fluorescent yellow solid. Yield: 2.25 g (88%). Mp: 116−118 °C. H
NMR (400 MHz, CDCl3): δ 7.40 (d, J = 16 Hz, 1H), 7.20 (d, J = 16
Hz, 1H), 7.15 (s, 1H), 7.10 (s, 1H), 7.06 (s, 1H), 6.30 (s, 1H), 4.05 (t,
2H), 3.96 (t, 2H), 3.86 (s, 3H), 3.83 (s, 3H), 3.41 (unresolved, 4H),
1.95 (unresolved, 4H), 1.81 (p, 4H), 1.56 (m, 4H), 1.00 (t, 6H). 13C
NMR (100 MHz, CDCl3): δ 152.4, 150.8, 149.9, 144.2, 140.8, 128.0,
124.0, 119.2, 117.9, 116.3, 111.4, 110.8, 110.4, 99.8, 70.0, 69.4, 56.8,
1
(KBr, cm−1): υmax 1672 (>CO). H NMR (400 MHz, CDCl3): δ
10.44 (s, 1H), 7.43 (d, J = 8 Hz, 2H), 7.31 (s, 1H), 7.23 (d, J = 16 Hz,
1H), 7.16 (s, 1H), 6.65 (d, J = 8 Hz, 2H), 4.13 (t, 2H), 4.03 (t, 2H),
3.32 (t, 4H), 1.86−1.83 (m, 4H), 1.59−1.53 (m, 8H), 1.41−1.37 (m,
4H), 1.04−0.96 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 189.0,
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The Journal of Organic Chemistry
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156.5, 150.3, 148.3, 135.8, 132.7, 128.4, 124.4, 123.2, 117.4, 111.6,
110.0, 109.6, 68.9, 68.7, 50.8, 31.4, 29.5, 20.4, 19.4, 14.0. LC−MS
(positive mode): m/z 481 (M + H)+. Anal. Calcd for C31H45NO3: C,
77.62; H, 9.46; N, 2.92. Found: C, 77.51; H, 9.42; N, 3.00.
methanol/chloroform 5:95 v/v as the eluent to obtain the compound
MS 2 as a dark colored thick gum. Yield: 0.66 g (90%). H NMR
1
(400 MHz, CDCl3): δ 8.635 (d, J = 4 Hz, 2H), 8.48 (s, 2H), 7.75 (d,
J = 16 Hz, 2H), 7.435 (d, J = 4 Hz, 2H), 7.09 (s, 2H), 7.05 (d, J = 16
Hz, 2H), 6.50 (s, 2H), 3.88 (unresolved, 12H), 3.19 (t, 8H), 1.53−
1.47 (p, 8H), 1.34−1.26 (m, 8H), 0.91 (t, 12H). 13C NMR (100 MHz,
CDCl3, C-type based on DEPT-135 spectrum): δ 156.6 (C), 152.5
(C), 149.3 (CH), 147.1 (C), 146.9 (C), 142.3 (C), 128.2 (CH), 123.7
(CH), 120.3 (CH), 118.5 (CH), 117.4 (C), 111.0 (CH), 104.6 (CH),
56.3 (OMe), 52.2 (CH2), 29.4 (CH2), 20.5 (CH2), 14.0 (CH3). LC−
MS (positive mode): m/z 736 (M + H)+. Anal. Calcd for C46H62N4O4:
C, 75.17; H, 8.50; N, 7.62. Found: C, 75.23; H, 8.41; N, 7.56.
4,4′-Bis(2,5-dimethoxy-4-(pyrrolidin-1-yl)styryl)-2,2′-bipyri-
dine (MS 3). This compound was obtained using the same procedure
as described for MS 2. Aldehyde 7b (0.52 g, 2.2 mmol) was used
instead of aldehyde 5b. Column chromatographic purification (silica
gel, 100−200 mesh) using methanol/chloroform 5:95 v/v as the
mobile phase led to the isolation of the pure compound MS 3 as a
brown microcrystalline solid. Yield: 0.56 g (91%). Mp (DTA): 250 °C.
1H NMR (400 MHz, CDCl3): δ 8.605 (d, J = 4 Hz, 2H), 8.46 (s, 2H),
7.75 (d, J = 16 Hz, 2H), 7.42 (d, J = 8 Hz, 2H), 7.10 (s, 2H), 6.98 (d,
J = 16 Hz, 2H), 6.27 (s, 2H), 3.89 (s, 3H), 3.84 (s, 3H), 3.44
(unresolved, 8H), 1.95 (unresolved, 8H). 13C NMR (100 MHz,
CDCl3, C-type based on DEPT-135 spectrum): δ 153.3 (C), 149.1
(CH), 147.4 (C), 143.8 (C), 141.8 (C), 128.4 (CH), 122.1 (CH),
120.2 (CH), 118.4 (CH), 114.4 (C), 111.4 (CH), 99.1 (CH), 56.9
(OMe), 56.2 (OMe), 50.5 (CH2), 25.2 (CH2). LC−MS (negative
mode): m/z 618 (M−H)+. Anal. Calcd. For C38H42N4O4: C, 73.76; H,
6.84; N, 9.05. Found: C, 73.61; H, 6.78; N, 9.15.
(E)-2,5-Dibutoxy-4-(4-(dibutylamino)-2,5-dimethoxystyryl)-
benzaldehyde (14b). This compound was synthesized by the same
procedure as described for aldehyde 10b using bromostilbene 14a
(1.8 g, 3 mmol), 1.6 M nBuLi (2.2 mL, 3.6 mmol), and DMF (0.4 mL,
5 mmol) in 20 mL of THF. The crude product was subjected to column
chromatography on silica gel (100−200 mesh) using EtOAc/Hexane
5:95 v/v as the mobile phase to afford the aldehyde 14b as a thick red
1
gum. Yield: 0.76 g (47%). IR (KBr, cm−1): υmax 1674 (>CO). H
NMR (400 MHz, CDCl3): δ 10.42 (s, 1H), 7.58 (d, J = 16 Hz, 1H), 7.55
(d, J = 16 Hz, 1H), 7.30 (s, 1H), 7.19 (s, 1H), 7.12 (s, 1H), 6.50 (s, 1H),
4.12 (t, 2H), 4.02 (t, 2H), 3.86 (s, 6H), 3.18 (t, 4H), 1.83 (p, 4H), 1.58−
1.48 (m, 8H), 1.31 (m, 4H), 1.00 (t, 6H), 0.91 (t, 6H). 13C NMR (100
MHz, CDCl3): δ 189.1, 156.4, 152.2, 150.5, 147.2, 141.9, 135.7, 126.9,
123.5, 120.2, 118.5, 110.6, 110.0, 109.9, 104.7, 68.8, 68.7, 56.4, 56.2, 52.2,
31.4, 31.3, 29.4, 20.5, 19.5, 19.3, 14.1, 14.0, 13.9. LC−MS (positive
mode): m/z 541 (M + H)+. Anal. Calcd for C33H49NO5: C, 73.43; H,
9.15; N, 2.60. Found: C, 73.57; H, 9.09; N, 2.53.
(E)-2,5-Dibutoxy-4-(2,5-dimethoxy-4-(pyrrolidin-1-yl)styryl)-
benzaldehyde (15b). This compound was synthesized by the same
procedure as described for aldehyde 10b using bromostilbene 15a
(1.6 g, 3 mmol), 1.6 M nBuLi (2.2 mL, 3.6 mmol), and DMF (0.4 mL,
5 mmol) in 20 mL of THF. Purification of the crude product by silica
gel (100−200 mesh) column chromatography using EtOAc/Hexane
10:90 v/v as the mobile phase led to the aldehyde 15b as a vermillion-
orange solid. Yield: 0.72 g (50%). Mp: 103−105 °C. IR (KBr, cm−1):
1
υmax 1670 (>CO). H NMR (400 MHz, CDCl3): 10.42 (s, 1H),
4,4′-Bis(4-(2,5-dimethoxy-4-dibutylaminostyryl)styryl)-2,2′-
bipyridine (MS 4). Synthesis of this compound follows the same
procedure as described for MS 2. Aldehyde 11b (0.86 g, 2.2 mmol)
was used instead of aldehyde 5b. The crude product was purified
through column chromatography using silica gel (100−200 mesh)
stationary and methanol/chloroform 5:95 v/v as the mobile phases to
obtain the compound MS 4 as a thick red gum that solidified after
standing at room temperature for few days. Yield: 0.87 g (93%). Mp:
7.58 (d, J = 16 Hz, 1H), 7.30 (d, J = 16 Hz, 1H), 7.29 (s, 1H), 7.19 (s,
1H), 7.12 (s, 1H), 6.27 (s, 1H), 4.42 (t, 2H), 4.43 (t, 2H), 3.88 (s,
3H), 3.82 (s, 3H), 3.43 (unresolved, 4H), 1.95 (unresolved, 4H), 1.84
(p, 4H), 1.56 (m, 4H), 0.99 (t, 6H). 13C NMR (100 MHz, CDCl3): δ
156.5, 153.0, 150.4, 143.9, 141.5, 136.1, 127.1, 123.3, 118.6, 115.5,
111.1, 110.0, 109.8, 99.3, 68.9, 68.8, 56.9, 56.3, 50.4, 31.4, 25.2, 19.5,
19.4, 13.9. LC−MS (positive mode): m/z 483 (M + H)+. Anal. Calcd
for C29H39NO5: C, 72.32; H, 8.16; N, 2.91. C, 72.27; H, 8.19; N, 3.01.
4,4′-Bis(4-(pyrrolidin-1-yl)styryl)-2,2′-bipyridine (MS 1).
Under nitrogen, solid potassium tert-butoxide (0.45 g, 4 mmol) was
added at one time to a THF solution (50 mL) of the bis-phosphonate
(2, 0.46 g, 1 mmol) and the aldehyde (7a, 0.39 g, 2.2 mmol) at room
temperature, and the resulting heterogeneous reaction mixture was
stirred at this temperature for 3 h. The reaction mixture was
subsequently quenched with water (25 mL) and evaporated to
dryness, and 50 mL of methanol was added to cause precipitation of
the product. It was collected by filtration, washed several times with
water, methanol, and a small amount of ether, and dried in air. It was
re-dissolved in dichloromethane (ca. 500 mL), and the yellow solution
was concentrated to almost 20 mL. The bright yellow precipitated
material was collected by filtration and dried thoroughly. The
compound MS 1 was isolated as yellow microcrystalline solid. Yield:
1
not measured. H NMR (400 MHz, CDCl3): δ 8.695 (d, J = 4 Hz,
2H), 8.56 (s, 2H), 7.59−7.53 (m, 8H), 7.50 (d, J = 16 Hz, 2H), 7.47
(d, J = 16 Hz, 2H), 7.41 (d, J = 8 Hz, 2H), 7.16−7.11 (m, 2H), 7.00
(d, J = 16 Hz, 2H), 6.55 (s, 2H), 3.88 (s, 6H,), 3.86 (s, 3H), 3.17 (t,
8H), 1.54−1.46 (p, 8H), 1.36−1.26 (m, 8H), 0.91 (t, 12H). 13C NMR
(100 MHz, CDCl3, C-type based on DEPT-135 spectrum): δ 156.3
(C), 151.8 (C), 149.5 (CH), 147.5 (C), 146.0 (C), 138.9 (C), 134.8
(C), 133.2 (CH), 127.4 (CH), 126.7 (CH), 126.0 (C), 125.8 (CH),
125.3 (CH), 123.9 (CH), 121.1 (CH), 119.0 (C), 118.3 (CH), 110.3
(CH), 105.4 (CH), 56.5 (OMe), 56.3 (OMe), 52.4 (CH2), 29.3
(CH2), 20.5 (CH2), 14.1 (CH3). HRMS (MALDI−TOF/TOF,
positive mode): m/z calcd. 938.571 (M+), found 939.673 (M + H)+.
Anal. Calcd for C62H74N4O4: C, 79.28; H, 7.94; N, 5.96. Found: C,
79.35; H, 7.89; N, 5.88.
4,4′-Bis(4-(2,5-dimethoxy-4-(pyrrolidin-1-yl)styryl)styryl)-
2,2′-bipyridine (MS 5). Synthesis of this compound follows
the same procedure as described for MS 1. Aldehyde 12b (0.74 g,
2.2 mmol) was used instead of aldehyde 7a. The compound MS 5 was
isolated as orange microcrystalline solid. Yield: 0.72 g (87%). Mp
1
0.46 g (92%). Mp (differential thermal analysis, DTA): 357 °C. H
NMR (400 MHz, CDCl3): δ 8.615 (d, J = 4 Hz, 2H), 8.48 (s, 2H),
7.46 (d, J = 8 Hz, 4H), 7.41 (d, J = 16 Hz, 2H), 7.345 (d, J = 4 Hz,
2H), 6.90 (d, J = 16 Hz, 2H), 6.58 (d, J = 8 Hz, 4H), 3.35 (unresolved,
8H), 2.03 (unresolved, 8H). 13C NMR was not possible because of
low solubility. LC−MS (positive mode): m/z 499 (M + H)+. Anal.
Calcd for C34H34N4: C, 81.89; H, 6.87; N, 11.24. Found: C, 81.72; H,
6.81; N, 11.15.
4,4′-Bis(4-dibutylamino-2,5-dimethoxystyryl)-2,2′-bipyri-
dine (MS 2). Neat potassium tert-butoxide (0.45 g, 4 mmol) was
added at once to a mixture of the bis-phosphonate (2, 0.46 g, 1 mmol)
and the aldehyde (5b, 0.64 g, 2.2 mmol) dissolved in 50 mL of THF at
room temperature under an inert atmosphere, and the dark colored
reaction mixture thus obtained was stirred at this temperature for 3 h.
It was then quenched with water (25 mL), and the product was
extracted with dichloromethane. The organic layer was washed with
brine, dried over anhydrous Na2SO4, and subjected to chromato-
graphic purification over silica gel (100−200 mesh) column using
1
(DTA): 274 °C. H NMR (400 MHz, CDCl3): δ 8.68 (d, J = 8 Hz,
2H), 8.56 (s, 2H), 7.54 (s, 8H), 7.50 (d, J = 16 Hz, 2H), 7.47 (d, J =
16 Hz, 2H), 7.41 (d, J = 8 Hz, 2H), 7.16−7.11 (unresolved, 4H), 6.94
(d, J = 16 Hz, 2H), 6.31 (s, 2H), 3.88 (s, 6H), 3.86 (s, 6H), 3.41
(unresolved, 8H), 1.96 (unresolved, 8H). 13C NMR was not possible
because of low solubility. HRMS (MALDI−TOF/TOF, positive
mode): m/z calcd. 822.415 (M+), found 823.575 (M + H)+. Anal.
Calcd for C54H54N4O4: C, 78.80; H, 6.61; N, 6.81. Found: C, 78.96;
H, 6.54; N, 6.68.
4,4′-Bis(2,5-dibutoxy-4-(4-dibutylaminostyryl)styryl)-2,2′-bi-
pyridine (MS 6). Synthesis of this compound follows the same
procedure as described for MS 2. Aldehyde 13b (1.0 g, 2.2 mmol) was
used instead of aldehyde 5b. The crude product was purified by a short
silica gel column (100−200 mesh) eluting with methanol/chloroform
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5:95 v/v as the mobile phase. The compound MS 6 was isolated as a
thick red gel that solidified to an orange solid after standing overnight
at room temperature. Yield: 1.1 g (95%). Mp: not measured. 1H NMR
(400 MHz, CDCl3): δ 8.675 (d, J = 4 Hz, 2H), 8.52 (s, 2H), 7.80 (d,
J = 16 Hz, 2H), 7.455 (d, J = 4 Hz, 2H), 7.42 (d, J = 8 Hz, 4H), 7.28
(d, J = 16 Hz, 2H), 7.20 (d, J = 16 Hz, 2H), 7.15 (s, 2H), 7.14 (s, 2H),
7.10 (d, J = 16 Hz, 2H), 6.66 (d, J = 8 Hz, 4H), 4.11 (t, 4H), 4.07 (t,
4H), 3.31 (t, 8H), 1.93−1.87 (p, 8H), 1.66−1.57 (m, 16H), 1.41−1.36
(m, 8H), 1.08 (t, 12H), 0.99 (t, 12H). 13C NMR (100 MHz, CDCl3,
C-type based on DEPT-135 spectrum): δ 156.6 (C), 151.8 (C), 150.6
(C), 149.4 (CH), 147.9 (C), 146.6 (C), 129.8 (CH), 129.4 (C), 128.4
(CH), 128.0 (CH), 125.6 (CH), 125.1 (C), 124.3 (C), 120.3 (CH),
118.9 (CH), 118.2 (CH), 111.7 (CH), 111.1 (CH), 109.8(CH),
69.2 (CH2), 50.8 (CH2), 31.7 (CH2), 29.6 (CH2), 20.4 (CH2), 19.6
(CH2), 14.1 (CH3). HRMS (MALDI−TOF/TOF, positive mode):
m/z calcd. 1106.759 (M+), found 1107.748 (M + H)+. Anal. Calcd
for C74H98N4O4: C, 80.25; H, 8.92; N, 5.06. Found: C, 80.15; H,
8.78; N, 4.91.
4,4′-Bis(2,5-dibutoxy-4-(2,5-dimethoxy-4-dibutylamino-
styryl)styryl)-2,2′-bipyridine (MS 7). This compound was syn-
thesized following the same procedure as described for MS 2. Aldehyde
14b (1.2 g, 2.2 mmol) was used instead of aldehyde 5b. The crude
product was purified by a short silica gel column (100−200 mesh)
eluting with methanol/chloroform 5:95 v/v as the mobile phase. The
compound MS 7 was isolated as a red gum that solidified to a red solid
after standing for five days at room temperature. Yield: 1.1 g (90%). Mp:
not measured. 1H NMR (400 MHz, CDCl3): δ 8.675 (d, J = 4 Hz, 2H),
8.51 (s, 2H), 7.80 (d, J = 16 Hz, 2H), 7.51−7.46 (m, 4H), 7.37 (d, J =
16 Hz, 2H), 7.21 (d, J = 16 Hz, 2H), 7.18 (s, 2H), 7.14 (s, 4H), 6.53 (s,
2H), 4.12 (t, 4H), 4.08 (t, 4H), 3.88 (s, 6H), 3.87 (s, 6H), 3.17 (t, 8H),
1.92−1.86 (m, 8H), 1.66−1.59 (m, 8H), 1.54−1.47 (m, 8H), 1.34−1.27
(m, 8H), 1.08−1.01 (m, 12H), 0.92 (t, 12H). 13C NMR (100 MHz,
CDCl3, C-type based on DEPT-135 spectrum): δ 156.5 (C), 151.8 (C),
151.7 (C), 150.8 (C), 149.3 (CH), 147.6 (C), 146.6 (C), 140.9 (C),
129.3 (C), 128.4 (CH), 125.8 (CH), 124.8 (C), 124.0 (CH), 121.1
(CH), 120.4 (CH), 119.7 (C), 118.9 (CH), 111.2 (CH), 110.5 (CH),
110.3 (CH), 105.4 (CH), 69.1 (CH2), 56.4 (OMe), 56.2 (OMe), 52.4
(CH2), 31.7 (CH2), 31.6 (CH2), 29.4 (CH2), 20.6 (CH2), 19.6 (CH2),
14.1 (CH3). HRMS (MALDI−TOF/TOF, positive mode): m/z calcd.
1226.801 (M+), found 1227.884 (M + H)+. Anal. Calcd for
C78H106N4O8: C, 76.31; H, 8.70; N, 4.56. Found: C, 76.25; H, 8.78;
N, 4.46.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of all of the new compounds,
coordinates of all of the optimized geometries, tables and
figures related to the computational outputs. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: skdsc@uohyd.ernet.in.
Author Contributions
†These authors contributed equally to this work.
ACKNOWLEDGMENTS
■
The authors thank the Department of Science and Technology
(DST), India (Project No. SR/S1/IC-23/2007) and Centre for
Nanotechnology in the University of Hyderabad for funding.
The 400 MHz NMR facility at University of Hyderabad by
DST, Govt. of India is gratefully acknowledged. High perfor-
mance computational facility at the Centre for Modeling, Simu-
lation and Design (CMSD), University of Hyderabad Campus
is duly acknowledged. Useful suggestions and critical comments
by the anonymous referees are gratefully acknowledged. We
thank Ms. Monika Kannan for helping us to record the mass
(MALDI-TOF/TOF) spectra of compounds MS 4−8. T.C.
thanks CSIR and M.S. and S.G. thank UGC, India for their
fellowships. We thank Prof. S. Mahapatra, School of Chemistry,
University of Hyderabad, for useful discussions.
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Mp (DTA): 202 °C. H NMR (400 MHz, CDCl3): δ 8.675 (d, J =
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