Asymmetrically Substituted and π-Conjugated 2,2′-Bipyridine Derivatives: Synthesis, Spectroscopy, Computation, and Crystallography

Article abstract of DOI:10.1021/acs.joc.5b02345

A new series of monosubstituted styryl- and bistyryl-2,2′-bipyridine luminophores (compounds 16-23) have been synthesized via Horner-Wadsworth-Emmons reaction involving a monophosphonate and donor aromatic aldehydes. In the title chromophores, the amino donors are varied between acyclic and cyclic while the alkoxy donors are varied in terms of their number and position. The absorption maxima of these chromophores shift predominantly due to intramolecular charge transfer (ICT) between different donor and acceptor moieties. The title donor-acceptor molecules exhibit intense fluorescence in solution at room temperature, and their emissive behavior has been found to be highly sensitive to solvent polarity. The fluorescence spectra and quantum yields of all the chromophores were recorded in four different solvent media, and the chromophores 16, 17, 19, and 21 exhibit fluorescence in the solid state too. The influence of the nature and position of the donor functionalities in the conjugated backbone of the bipyridine moiety on the electronic absorption properties of the title chromophores (16-23) has been demonstrated, which has further been corroborated by DFT and TD-DFT computation both in gas phase and in solution phase. The crystal structure of compound 18 has been described as a representative member of the family (16-23).

Full text of DOI:10.1021/acs.joc.5b02345

Article
pubs.acs.org/joc
Asymmetrically Substituted and π‑Conjugated 2,2′-Bipyridine
Derivatives: Synthesis, Spectroscopy, Computation, and
Crystallography
Ramakrishna Bodapati, Monima Sarma, Arunkumar Kanakati, and Samar K. Das*
School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad 500 046, India
S
* Supporting Information
ABSTRACT: A new series of monosubstituted styryl- and bistyryl-2,2′-
bipyridine luminophores (compounds 16−23) have been synthesized via
Horner−Wadsworth−Emmons reaction involving a monophosphonate
and donor aromatic aldehydes. In the title chromophores, the amino
donors are varied between acyclic and cyclic while the alkoxy donors are
varied in terms of their number and position. The absorption maxima of
these chromophores shift predominantly due to intramolecular charge
transfer (ICT) between different donor and acceptor moieties. The title
donor−acceptor molecules exhibit intense fluorescence in solution at room
temperature, and their emissive behavior has been found to be highly
sensitive to solvent polarity. The fluorescence spectra and quantum yields
of all the chromophores were recorded in four different solvent media, and
the chromophores 16, 17, 19, and 21 exhibit fluorescence in the solid state
too. The influence of the nature and position of the donor functionalities in
the conjugated backbone of the bipyridine moiety on the electronic absorption properties of the title chromophores (16−23) has
been demonstrated, which has further been corroborated by DFT and TD-DFT computation both in gas phase and in solution
phase. The crystal structure of compound 18 has been described as a representative member of the family (16−23).
The 2,2′-bipyridine derivatives are endowed with an enriched
coordination chemistry and have received much attention
because they can readily form complexes with transition metals,
where they can bind with metals both by σ-molecular orbitals of
the electron-donating nitrogen atoms and by electron accepting
π-molecular orbitals.6 The resulting metal complexes of such
extended π-conjugated systems are very stable due to the
formation of a five membered ring and are known to be
excellent candidates for studying nonlinear optical properties. It
has been reported by various research groups that 2,2′-
bipyridine derivatives are known to complex metals in square-
planar, tetrahedral, and octahedral coordination geometries and
that the second-order nonlinear response of such π-conjugated
systems is increased on coordination to a metal center.7
Zyss et al. reported the octupolar nonlinearity of [Ru-
(bpy)3]2+ complex ion;8 later, abundant octupoles featuring the
4,4′-π-conjugated 2,2′-bipyridine derivatives were reported.9
Such π-conjugated systems can also be used as antenna
fragments in the TiO2 anchoring heteroleptic ruthenium
complexes, which come into competition with the silicon
junction semiconductors so as to harvest solar energy. In recent
years, there have been a growing number of reports containing
π-conjugated ligands and their coordination complexes. For
instance, Odobel and co-workers reported that heteroleptic
INTRODUCTION
■
In recent years, π-conjugated functional materials have attracted
a great deal of attention by virtue of their enormous
applications in organic light emitting diodes (OLEDs),1
nonlinear optics (NLOs),2 electrogenerated chemilumines-
cence (ECL),3 dye-sensitized solar cells (DSSCs),4 fluorescent
sensors,5 etc. Among the diverse classes of organic π-conjugated
systems, the push−pull chromophores with donor and acceptor
moieties have generated a lot of interest since their optical
properties can be tuned judiciously over a wide range simply by
varying the donor or acceptor moieties. The fundamental type
of interaction in such molecules generally occurs by an
intramolecular charge-transfer (ICT) between the donor (D)
and the acceptor (A), thereby tuning the HOMO−LUMO
energy gap. In this regard, the heteroaromatic DA-type
fluorescent probes can be considered as potential candidates,
and among the diverse classes of DA-type building blocks, 2,2′-
bipyridines act as very promising building blocks because of
their ability to tune their optical properties very easily either by
extending the conjugation or by introducing an assorted class of
donor end-capped functionalities, or via a metal complexation.
As the orbital energy of the lowest unoccupied molecular
orbital (LUMO) of pyridine is lower than that of benzene (due
to the presence of ring nitrogen atom), pyridine is a good
electron accepting mesomeric unit, and thus, suitable push−
pull chromophores can be designed and synthesized simply by
end-capping the pyridine ring with various donor moieties.
Received: October 9, 2015
© XXXX American Chemical Society
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Chart 1. Molecules Synthesized and Studied in the Present Work
copper−polypyridine complexes display impressive power
conversion efficiencies (PCEs) and thus could be used as
efficient sensitizers in dye-sensitized solar cells.10 Dragonetti et
al. showed that cyclometalated Ir(III) complexes bearing
different substituted 2-phenylpyridines act as interesting
second-order NLO chromophores.11 In a recent review,
Bozec and Guerchais have accounted how new chromophores
could be designed by combining dithienylethene (DTE)-based
bipyridine ligands with different metallic fragments and studied
both NLO and luminescence properties of the multi-
photochromophoric materials.12 Also, recently, the one- and
two-photon absorption and emission properties of an oligo-
(phenylenethienylene) series have been demonstrated by Nair
et al.; this study revealed that increasing oligomer length results
in only a slight shift of the two-photon absorption band with a
drastic increase of two-photon absorption cross section.13 Zhou
and co-workers reported that 2,2′-bipyridine derivatives
containing aza-crown ethers exhibit large two-photon absorp-
tion cross sections and their applicability for biomedical
imaging.14
However, the π-conjugated bipyridine derivatives reported to
date are substituted either symmetrically or unsymmetrically
with identical donor functionalities. In our previous report,15
we have discussed a series of symmetrically substituted
heterodonor systems, containing a series of styryl- and
bistyryl-2,2′-bipyridine luminophores, end-capped with alkoxy
and amino donor functionalities having D−π−A−A−π−D (D
= donor, A = acceptor) archetype. We have shown how simple
modification of the π-skeleton of such dye molecules modulates
their fluorescence behavior, and we have also described the
diverse photophysical aspects on changing the amino donor
from open chain dibutylamino (e.g., MS 4) to the cyclic
pyrrolidine donor (e.g., MS 5).15 All these reported compounds
were found to exhibit high fluorescence quantum yields in
solution state, and we also comprehended the modulating effect
of the position, nature, and number of donor functionalities on
the optical properties of such π-conjugated molecules by DFT
and TD-DFT computational studies. The results obtained with
the symmetrically substituted hetero-donor π-conjugated
bipyridine derivatives inspired us to explore the unsymmetri-
cally substituted heterodonor systems by functionalizing one of
the pyridine rings of the bipyridine moiety, keeping the methyl
group intact on the other pyridine ring. In this article, we have
described synthesis, characterization, and photophysical studies
of eight asymmetrically substituted 2,2′-bipyridine lumino-
phores (compounds 16−23, Chart 1). Though the target
molecules can be obtained via three different synthetic routes,
in the present work, the Horner−Wadsworth−Emmons
reaction (HWE) has been exclusively used to introduce
electron-donating groups onto the bipyridine acceptor core
unit because this protocol is known to give high yields in
addition to the E-selectivity. All the synthesized chromophores
have unsymmetrical A−A−π−D archetype, wherein the A−A
unit represents the 2,2′-bypyridine acceptor core, which is
attached to the different donor termini through π-conjugation
(see Chart 1). We have investigated the solvatochromic
behavior and all the chromophores (compounds 16−23)
exhibit bright fluorescence in solution at room temperature
with large Stokes shift; interestingly, chromophores 16, 17, 19,
and 21 exhibit solid-state emission too. We have also performed
computational analysis in the level of density functional theory
(DFT) in solvent reaction field to analyze the influence of
“nature” and “position” of donor functionalities on the
geometrical and electronic parameters of the synthesized
B
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Scheme 1. Synthetic Route of A−A−π−D Bipyridine Chromophores
chromophores. One of the title compounds (chromophore 18)
has unambiguously been characterized by single crystal X-ray
crystallography.
suitable electrophiles via an intermolecular mode (Scheme S1).
The corresponding cyclic (pyrrolidine) analogues (6a and 6b,
see Scheme S1) were synthesized by using 1,4-dibromobutane
as the double electrophiles. The usage of 10% N-methyl-
pyrrolidinone (NMP) in DMF as the solvent system affords the
corresponding N,N-dialkylanilines in high yields, and thereafter,
they were formylated using the Vilsmeier−Haack reaction
which yielded excellent para-selectivity. In addition, the π-
conjugated benzaldehydes (10b−15b, Scheme S2) with
different substituents were obtained via a two-step synthetic
approach. The synthesized pull−push compounds containing
the open donors (17, 19, 21, and 22) are highly viscous liquids,
while those with cyclic pyrrolidine ring donors (16, 18, 20, and
23) are solids.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The bipyridine phos-
phonate precursor and appropriate aldehydes and their
precursors were synthesized according to previous literature
reports,5−8,15,16 while the target molecules have been
synthesized using an efficient HWE reaction protocol (Scheme
1). The advantages of the HWE reaction pathway over the
conventional Wittig reaction are many-fold: the former has a
good response with the stabilized yields and predominantly
gives E-stereoselectivity of the olefinic double bond, generating
a water-soluble phosphate salt which can easily be removed
from the reaction mixture through an aqueous process. The
great difficulty of separation of the Wittig byproduct,
triphenylphosphine oxide, is thus largely ruled out in the
HWE reaction. Consequently, this particular synthesis protocol
has mainly been used for the synthesis of our target molecules.
At first, the monochloromethyl derivative was synthesized by a
method, developed by Smith and Fraser,17 that involves usage
of the base, LDA (instead of potassium tert-butoxide), which
deprotonates the starting material by crucially controlling the
amount of base used, thereby allowing in situ Knoevenagel-type
reactions to be executed in order to achieve the unsymmetri-
cally substituted chromophores. Thus, the proper selection of
reaction conditions may effortlessly fabricate materials with
desired symmetry. The HWE olefination, which enhances the
yield of the phosphonate ester 2 as the reactive intermediate
(Scheme 1), has received much acclamation for obtaining the
π-conjugated bipyridine chromophores.
The molecular structures of all the chromophores (16−23)
are determined through NMR (1H and 13C) and mass
spectroscopy including CHN analysis. The pyridine-H6,6′
protons, being largely deshielded by the adjacent electro-
negative nitrogen atom, resonate at the lowest field of the
spectra while the pyridine-H5,5′ protons, owing to their meta-
orientation with respect to the nitrogen atom, resonate in the
benzene region. The downfield shift of the pyridine-H3,3′
protons compared to the pyridine-H5,5′ protons can be
attributed to the transoid-arrangement of the two pyridine
rings in the relevant bipyridine chromophores.18 No indication
of the Z-isomer has been observed, and the vinylic CC bonds
are found to be in E-geometry, as indicated by the coupling
3
constant (ca. JHH = 16 Hz).
1
Comparison of the H NMR signals due to the aromatic
protons of the open chain amino donor end-capped
chromophore 17 (dibutylamino donor) and its cyclic analogue
18 (pyrrolidine donor) clearly shows that the signals due to the
cyclic donor are more upfield shifted (H11 nucleus resonate at δ
6.2) as compared to the open chain donor (H11 nuclei resonate
The open chain N,N-dialkylated anilines (4a and 4b) were
synthesized in good yields from anilines (3a and 3b) by using
C
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Figure 1. Difference in chemical shift of the proton adjacent to the amino donor (H11) in 17 and 18.
at δ 6.4), which is indicative of a more shielded environment
around the relevant protons in the cyclic amino donor end-
capped chromophores compared to open chain amino donor
end-capped chromophore as shown in Figure 1. The electron-
donating mesomeric effect of the amino groups makes the
corresponding ortho-substituted phenyl rings more electron-
rich, thereby exerting a greater shielding effect on the relevant
protons (H11). Thus, the better the delocalization of the amine
lone pair into the phenyl ring, the greater will be the shielding
effect on the relevant protons, thereby causing them to resonate
at the upfield region of the relevant spectrum. In other words, it
can be said that the pyrrolidine moiety has a greater electron-
donating capability than the corresponding open chain amino
donor, dibutylamine.
Optical Properties of Compounds (16−23) in Solution
State. The photophysical properties of the synthesized
compounds in solution phase were recorded in four different
solvents, namely, toluene, dichloromethane (DCM), tetrahy-
drofuran (THF), and acetonitrile (MeCN), at room temper-
ature, and the corresponding photophysical data are
summarized in Table 1. The nature and position of the
donor functionalities in the conjugated backbone of the
bipyridine moiety and the polarity of the solvent media in
which the compounds are solubilized have shown profound
influence on the absorption and emission properties of all the
investigated chromophores. The lowest energy absorption
bands in the range of 330−430 nm in all the chromophores
are due to an intramolecular π → π* electronic transition from
donor based molecular orbitals to acceptor molecular orbitals,
which is affirmed owing to its sensitivity to solvent polarity.
From the absorption spectra (Figure 2) and Table 1, it is quite
evident that the compounds with pyrrolidine donors 18, 20,
and 23 exhibit bathochromic shift by ≈10 nm compared to the
compounds containing dibutylamino donors 17, 19, and 22
respectively; however, this trend is reversed for the pair of
cyclic donor 20 (408 nm) and acyclic donor 19 (412 nm).
Thus, the cyclic pyrrolidine ring has a greater donating
capability compared to the open chain dibutylamino donor,
which is consistent with NMR spectroscopy (vide supra). All
the chromophores are highly fluorescent at room temperature,
and when excited at the lowest energy absorption maxima, they
exhibit excellent fluorescence behavior (Figure 2). The fact that
the excited state of all the chromophores is more polar than the
ground state is exemplified from the photophysical data
Table 1. Photophysical Data of All the A−A−π−D
Bipyridine Chromophores 16−23
a
c
λmax
(nm)
ε (M−1
λem
(nm)
Δv
̅
b
compound solvent
cm−1
)
Φem
(cm−1
)
16
17
18
19
20
21
22
23
toluene
DCM
THF
325
334
333
329
388
395
393
393
402
407
404
405
402
412
407
405
411
408
402
403
419
428
418
415
380
381
380
374
420
429
418
417
408
409
413
423
476
522
506
553
482
576
516
560
560
662
637
668
571
590
627
656
561
604
602
636
483
507
514
528
543
516
512
526
0.20
0.06
0.07
0.08
0.17
0.29
0.31
0.36
0.16
0.43
0.27
0.28
0.78
0.29
0.59
0.26
0.63
0.33
0.43
0.49
0.37
0.38
0.46
0.25
0.56
0.47
0.51
0.37
0.45
0.43
0.47
0.39
6259
5490
5816
6754
4764
6159
5682
7362
4128
7208
5372
6834
7018
9166
8871
9721
6817
7864
8926
9569
6041
6808
7312
8373
5611
6522
6860
7798
5393
3930
4392
4969
50000
49000
45000
31000
45000
45000
37000
MeCN
toluene
DCM
THF
MeCN
toluene
DCM
THF
MeCN
toluene
DCM
THF
MeCN
toluene
DCM
THF
MeCN
toluene
DCM
THF
MeCN
toluene
DCM
THF
MeCN
toluene
DCM
THF
34000
MeCN
a
b
ε were measured in DCM solution. Fluorescence: relative quantum
yield of the compounds 16−23 was measured by using quinine sulfate
c
(in 1 N H2SO4) as the reference (Φem = 0.545). Stokes shift Δv = vabs
̅
̅
− vem.
̅
obtained in various solvents (Table 1). On varying the solvent
polarity from low to high, the shift in emission bands is found
to be more profound than that in the absorption bands (Table
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The positions of the corresponding emission maxima are
found to be independent of the excitation wavelength, which is
in accordance with Kasha’s rule, and it has been observed that,
on increasing the conjugation length, the absorption and
emission maxima are bathochromically shifted. For instance,
when we compare the absorption and emission of 17 vs 19 or
18 vs 20 in DCM, it has been observed that the λabs and λem
increase with increasing the conjugation length.
In a bid to check the variation in photophysical properties of
the symmetrical chromophores15 and the unsymmetrical
chromophores (present study), we have also compared the
photophysical properties of MS 2 and 17, MS 3 and 18, MS 4
and 19, MS 5 and 20, MS 6 and 21, MS 7 and 22, and MS 8
and 23, and the concerned data have been summarized in Table
2 along with their computationally obtained HOMO−LUMO
Table 2. Comparison between Symmetrical15 and
Unsymmetrical (Present Work) Bipyridine Derivatives in
Dichloromethane (DCM)
λabs
λem
Δv
HOMO
(eV)
LUMO
(eV)
H−L
̅
compd (nm) (nm)
Φem (cm−1
)
(eV)
MS 2
17
399
395
414
407
413
412
430
408
442
428
435
381
448
429
533
522
543
576
644
662
662
590
598
604
648
507
660
526
0.20
0.29
0.17
0.43
0.37
0.29
0.21
0.33
0.65
0.38
0.35
0.48
0.23
0.40
6301
6159
5738
7208
8685
9166
8150
7864
5902
6808
7556
6522
7170
4969
−6.204
−6.139
−6.192
−5.943
−6.342
−6.013
−6.072
−5.818
−6.214
−5.944
−6.253
−5.893
−6.018
−5.718
−0.944
−0.46
−5.26
−5.67
−5.26
−5.53
−5.08
−5.17
−4.84
−4.99
−4.98
−5.13
−5.00
−5.07
−4.80
−4.92
MS 3
18
−0.934
−0.413
−1.262
−0.848
−1.229
−0.820
−1.231
−0.817
−1.247
−0.824
−1.220
−0.796
MS 4
19
MS 5
20
MS 6
21
MS 7
22
MS 8
23
gaps (the photophysical data of chromophores MS 2 to MS 8
have been taken from ref 15 in the present comparative
discussion of the following paragraph).
Comparison between the chromophores (symmetrical and
unsymmetrical) based on nature and position of the donor
functionalities gives us some insight into the photophysical
properties. In the case of cyclic donor systems, the absorption
and emission bands in the unsymmetrical chromophores are
blue-shifted with higher quantum yields compared to their
symmetrical counterparts (except 18 emission) as shown in
Table 2. However, in the acyclic donor systems, a different
scenario has been observed. Though the absorption bands are
blue-shifted on changing from symmetric to unsymmetric, there
is an irregular trend observed in the emission behavior and,
alternatively, solid-state fluorescence is observed in almost all of
acyclic unsymmetrical compounds (except 22). Furthermore, it
has been observed that for the unsymmetrical compounds
(present work), in which the alkoxy groups are attached to the
ring with no amino group, the quantum yield of the
chromophores is increased. For instance, in the acyclic systems,
when the alkoxy group is shifted from second phenyl ring (19)
to first phenyl ring (21), the quantum yield increases from 0.29
to 0.38 and the presence of two alkoxy groups in the system
(22) further increases the quantum yield to 0.48. Again, the
absorption band shows a bathochromic shift when the alkoxy
group is shifted from second ring (19) to first ring (21) but
Figure 2. Normalized UV−vis absorption and emission spectra of
compounds 16−18 (a) and compounds 19−23 (b), recorded in
dichloromethane at room temperature. (c) Normalized UV−vis
absorption and emission spectra of compound 17 (as representative
one) showing the solvatochromic effect.
1 and Figure 2). Generally, the dipole character is boosted in
the excited state S1, when the electrons are excited from the
highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO). As a result, the
solvents with high polarity tend to stabilize such polarized
excited states by reorienting the solvent molecules so as to
lower the energy of the system thereby leading to a red shift in
the emission spectra.19 This solvatochromic phenomenon is
spectroscopically shown with compound 17 in Figure 2c, as a
representative spectral change. The same solvatochromic
phenomena for the rest of the compounds are displayed in
Figures S2−S5.
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Figure 3. Thermal ellipsoidal plot of molecule 18 (30% probability). Hydrogen atoms have been omitted for clarity.
shows a hypsochromic shift with alkoxy groups on both rings;
but surprisingly, there is a hypsochromic shift on the emission
bands on altering the number and position of the alkoxy groups
(from 19 to 21 to 22).
Solid-State Emission of the Compounds. Of all the
investigated chromophores, the chromophores 16, 17, 19, and
21 show solid-state emission in addition to emission in solution
(Figure S6). Solid-state UV/vis absorption spectra were
recorded in diffuse reflectance mode. The samples were
prepared in the form of KBr pellet (for homogeneity), and
the reflectance spectra were converted to absorption spectra
using the Kubelka−Munk function. The pertinent compounds
absorb in the range 400−450 nm, and it is observed that there
is a bathochromic shift in the absorption maxima in going from
solution to solid state, which signifies the presence of
intermolecular interactions between the molecules (might be
due to J aggregation). This is indeed true because a
representative crystal structure (compound 18) shows J
aggregation through intermolecular supramolecular interactions
(vide infra, following section). When these chromophores are
excited at their absorption maxima (Figure S6a), they exhibit
emission at 618 nm, 628 nm, 536 nm, and 547 nm respectively
as shown in Figure S6b. Even though the molecule 18 does not
exhibit any emission at room temperature, it is one of the
analogous members of the same family (compounds 16−23)
which should have similar intermolecular interactions as other
members (which emit in the solid state) would have.
Figure 4. Molecular packing diagram in the crystal structure of
compound 18.
Theoretical Approach. In order to understand the trends
in the behavior of vertical excitation (absorption maxima)
shifts, HOMO−LUMO energy gaps etc., density functional
theory (DFT) and time-dependent DFT (TD-DFT) were
carried out on the compounds 16−23 using the Gaussian 09
program package.20 The calculations were performed both in
gas phase and solution phase using polarizable continuum
model (C-PCM) applying self-consistent reaction field (SCRF)
in DCM. The geometry of the target compounds was
optimized at the level of exchange-correlation hybrid functional
of CAM-B3LYP theory with the 6-31g(d,p) basis set. The
optimized structures, identified as global minimum as the
potential energy surfaces, were verified by the absence of any
imaginary frequencies. The HOMO and LUMO frontier
molecular orbitals (FMOs) of all the compounds 16−23 (via
DFT computation) are presented in Figure 5, which indicate
that an almost similar type of localization is observed for all the
chromophores. The HOMO is constituted with the π-type
combination of orbitals and is situated at the donor substituted
phenyl rings and at the CC bonds, while the LUMO is of π*-
type and is localized at the CC bonds and the pyridine ring
(see Figure 5). 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) molecular orbitals
are presented in Table 3 while the TD-DFT computed most
CRYSTALLOGRAPHY: CRYSTAL STRUCTURE OF
COMPOUND 18
■
After purification by column chromatography was done,
compound 18 solution in acetonitrile (MeCN) was kept for
direct evaporation at room temperature, whereby brown color
crystals, suitable for X-ray diffraction, were obtained after 2
days. Compound 18 crystallizes in monoclinic space group
P21/c. The concerned asymmetric unit consists of the full
molecule. The thermal ellipsoidal plot of the same is presented
in Figure 3. The relevant crystal data and structure refinement
parameters have been given in Table S1. The bond lengths and
bond angles are presented in Tables S4 and S5 respectively.
Interestingly in the crystal structure, the molecules undergo C−
H···O and C−H···N intermolecular hydrogen bonding
interactions leading to a three-dimensional supramolecular
structure (Figure 4). This intermolecular supramolecular
aggregation (J aggregation) justifies the bathochromic shift in
the absorption maxima in going from solution to solid state
(vide supra).
F
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Thus, from computation, we can predict that as the conjugation
length in the chromophores increases (e.g., 17 vs 19 and 18 vs
20), the HOMO gets destabilized and the HOMO−LUMO
gap decreases thereby shifting the absorption maxima bath-
ochromically (see Table 4). Also, in going from 19 to 21 to 22,
it is observed that destabilization of the HOMO by 0.069 and
0.051 eV respectively and a shrinking of the HOMO−LUMO
gaps by 0.04 and 0.06 eV respectively indicate the trend in
observed absorption in these compounds. However, the
experimentally found hypsochromic shift in λabs of compound
22 could not be explained on the basis of computation.
Potential of the Present System: Asymmetric versus
Symmetric. An important parameter for the comparison of
photophysical performances of diverse fluorophores is the
fluorescence quantum yield (Φem), which is the direct measure
of the efficiency of the conversion of absorbed light into
emitted light. We have already shown (in Table 2) that the
asymmetrical bipyridines (present work) perform better than
corresponding symmetrical bipyridines15 as far as measured
quantum yields are concerned. Thus, as shown in Table 2, the
present asymmetrical molecules are more efficient fluorophores
than the symmetrical ones.15 In MeCN solvent, the trend of
better photophysical efficiencies of asymmetric bipyridines over
symmetrical bipyridines is consistent (Table 2).
In the comparison of symmetry versus asymmetry, the
symmetrical fluorophores15 are larger than their corresponding
asymmetrical ones by “one side substituent” of the central
bipyridine unit (see Chart 1 and ref 15). In other words, the
asymmetrical bipyridines (present work) are of lower molecular
weights in comparison to their corresponding symmetrical
ones. It would be thus comparatively easier to crystallize the
asymmetrical bipyridines into their single crystals, as we have
unambiguously characterized the crystal structure of fluoro-
phore 18 as a representative one (vide supra) from this
asymmetrical series. Despite our enormous efforts, we could
not succeed in crystallizing the corresponding symmetrical
bipyridines into their single crystals.15
In recent time, there has been a rapid increase in research
activity in Ru(II)−bipyridine coordination complexes as
efficient photosensitizers in making dye-sensitized solar cells
(DSSCs) because of their low-lying metal ligand charge transfer
(MLCT) transition that can cover the red and near-infrared
region of the solar spectrum. Obtaining an optimal photo-
sensitizer, with an electronic absorption band extending up to
red region of the visible spectrum and high molar extinction
coefficient, is a challenging task in the area of visible light
induced photocatalytic reactions including water oxidation21 as
well as dye-sensitized solar cells.22 A recent report on 4,4′-
unsymmetrically substituted 2,2′-bipyridine Ru(II) coordina-
tion complexes demonstrates an improvement in the molar
extinction coefficient of all these sensitizers because of the
presence of π-conjugation in this system.23 Thus, the present
work dealing with a new series of asymmetrically substituted
and π-conjugated 2,2′-bipyridine derivatives demonstrates that
these asymmetrical bipyridines have potential to form efficient
photosensitizers (for example, with Ru2+ ion) in the context of
better/improved dye-sensitized solar cells and photocatalytic
water splitting.
Figure 5. Isodensity plots of HOMO and LUMO frontier molecular
orbitals of the synthesized molecules (16−23) as computed by the
CAM-B3LYP/6-31g(d,p) level of theory (isodensity value = 0.02).
relevant S0 → S1 vertical Franck−Condon electronic excitations
between these energy levels (CAM-B3LYP/6-31g(d,p) in
DCM) along with their associated oscillator strengths are
shown in Table 4.
It is quite evident from Tables 3 and 4 that, for almost all the
chromophores, two excited states are responsible for the
appearance of the lowest energy intense absorption band, the H
→ L transition being the predominant one thereby indicating a
π → π* intramolecular charge transfer (ICT) from donor
molecular orbitals to acceptor (pyridine) molecular orbitals.
CONCLUSION
■
In conclusion, a new family of 4-methyl-4′-π-conjugated-2,2′-
bipyridine derivatives with A−A−π−D architecture have been
designed and accomplished successfully. This system consists of
G
DOI: 10.1021/acs.joc.5b02345
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Table 3. Energies of Four Higher Occupied (H−3, H−2, H−1, H) and Four Lower Virtual (L, L+1, L+2, L+3) Molecular
a
Orbitals of the Studied Systems as Computed in CAM-B3LYP/6-31g(d,p) Level Theory (in DCM)
H−3
H−2
H−1
H
L
L+1
L+2
L+3
16
17
18
19
20
21
22
23
−8.367
−7.885
−7.877
−7.824
−7.818
−7.810
−7.556
−7.540
−8.034
−7.766
−7.747
−7.456
−7.442
−7.577
−7.399
−7.385
−7.765
−7.451
−7.441
−7.107
−7.067
−6.848
−6.764
−6.711
−6.184
−6.139
−5.943
−6.013
−5.818
−5.944
−5.893
−5.718
−0.404
−0.460
−0.413
−0.848
−0.820
−0.817
−0.824
−0.796
−0.100
−0.067
−0.101
−0.052
−0.027
−0.005
−0.020
0.000
−0.973
−0.986
−0.985
−0.457
0.502
−1.356
−1.468
−1.538
−0.993
1.005
0.576
1.028
0.522
1.012
0.566
1.029
a
H = HOMO, L = LUMO.
Table 4. TD-DFT Computed CAM-B3LYP/6-31g(d,p)
Representative Intense Vertical Excitations, λmax and
Oscillator Strength (f) of the Synthesized Molecules in
DCM
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out under ambient
conditions unless otherwise stated. Column chromatography was
performed on silica gel (100−200 mesh). TLC plates were visualized
in an iodine chamber (sometimes in a UV chamber). Unless stated
otherwise, all reagents were purchased from commercial sources and
used without additional purification. THF was freshly distilled over
molecule
transition
λmax (nm)
oscillator strength
16
H → L
340.86
253.78
356.47
255.16
271.32
367.76
255.65
382.84
301.29
267.55
393.36
274.95
393.29
264.04
400.15
268.68
273.41
409.12
275.24
1.4844
0.4862
1.2090
0.4780
0.1129
1.2906
0.4790
2.0977
0.2137
0.3201
2.0774
0.2555
2.0874
0.3902
2.0030
0.3262
0.1410
2.0238
0.3245
Na-benzophenone ketyl. Unless stated otherwise, H NMR and 13C
1
H−1 → L
H → L
17
NMR spectra were recorded either on a 400 or on a 100 mHz machine
in CDCl3 as solvent with TMS as reference. CHNS analyzer has been
used for elemental analyses. Infrared (IR) spectra were recorded by
using KBr pellets on an FT/IR spectrometer. HRMS (ESI-TOF)
equipment was used to record mass spectra for all the title compounds.
Absorbance spectra were recorded on a UV−visible spectrophotom-
eter, and fluorescence emission spectra were recorded on a
spectrofluorimeter.
Crystallography. Single crystal X-ray diffraction data of
compound 18 was measured at room temperature on a single crystal
X-ray diffratometer [λ (Mo Kα) = 0.7103 Å] with a graphite
monochromator. 2400 frames were recorded with an ω scan width of
0.3°, each for 10 s, and crystal−detector distance of 60 mm with
collimator of 0.5 mm.24 Data reduction was performed with the
SAINTPLUS software.24a Absorption correction was made using an
empirical method SADABS.24b Structure solution was performed using
the SHELXS-97 program,24c and it was refined using the SHELXL-97
program.24d Hydrogen atoms on the aromatic rings were introduced
on calculated positions and included in the refinement riding on their
respective parent atoms. CCDC 1430463 contains the supplementary
crystallographic data of compound 18. Relevant crystal data can be
obtained free of charge via http://www.ccdc.cam.atc.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, U.K.; fax, (+44) 1223-336-033;
or via e-mail, deposit@ccdc.cam.ac.uk.
H−2 → L
H−2 → L+1
H → L
18
19
H−2 → L
H → L
H−1 → L
H−3 → L
H → L
20
21
22
H−1 → L
H → L
H−3 → L
H → L
H−4 → L
H → L+2
H → L
23
H−3 → L
the 2,2-bipyridine heterocycle acting as the acceptor and the
fragment containing different donor functionalities. All the
chromophores are administered by the intramolecular charge
transfer from the donors to the acceptor units. The emissive
behavior of all the chromophores has been demonstrated with
four of the compounds showing solid-state emission that
provides a doorway for solid-state lighting processes. In
addition, the title compounds show large solvent sensitive
emissive behavior and their photophysical properties of the
compounds are highly reliant on the number, nature, and
position of the donor functionalities, which have also been
computationally corroborated by DFT calculations. In our
previous article, we reported symmetrical derivatives of 2,2′-
bipyridine, and herein, we report the unsymmetrical derivatives
of 2,2′-bipyridine. Thus, a complete library of compounds have
been designed and synthesized, and we strongly believe that the
unsymmetrical bipyridine derivatives along with their transition
metal complexes may also exhibit interesting nonlinear optical
behavior. This work is currently being undertaken in our
laboratory.
Synthesis of (E)-4-Methyl-4′-(4-(pyrrolidin-1-yl)styryl)-2,2′-bipyr-
idine (16). In the presence of nitrogen atmosphere, potassium tert-
butoxide (0.22 g, 2 mmol) was added at 0 °C to a solution of
monophosphonate (2, 0.320 g, 1 mmol) and the aldehyde (7a, 0.175
g, 1 mmol) in 20 mL of THF; the ice bath was subsequently removed,
and the reaction mixture was stirred at room temperature for 1 h. After
the completion of the reaction, the reaction mixture was quenched
with 10 mL of water, and the product was extracted with
dichloromethane. The resulting mixture was washed several times
with water and then with brine. The yellow colored material obtained
was washed with ether, dried in air, and then further purified on a silica
gel (100−200 mesh) column using methanol/dichloromethane 5:95
v/v as the eluent to obtain the compound 16 as a dark brown colored
solid. Yield: 0.23 g (72%). IR spectrum (ν/cm−1): 2970, 2926, 1738,
1
1366, 1229, 1217. H NMR (400 MHz, CDCl3): δ 8.56 (d, J = 5 Hz,
2H), 8.46 (s, 1H), 8.27 (s, 1H), 7.45 (d, J = 9 Hz, 2H), 7.40 (d, J = 16
Hz, 1H), 7.33 (d, J = 4 Hz, 1H), 7.15 (d, J = 5 Hz, 1H), 6.89 (d, J = 16
Hz, 1H), 6.56 (d, J = 9 Hz, 2H), 3.33 (t, J = 6 Hz, 4H), 2.45 (s, 3H),
2.02 (t, J = 6 Hz, 4H). 13C NMR (100 MHz, CDCl3): δ 155.1, 155.0,
148.1, 147.8, 147.2, 147.1, 145.9, 132.9, 127.5 (2C), 123.6, 122.5,
121.0, 119.5, 119.4, 116.7, 110.7 (2C), 46.5 (2C), 24.4 (2C), 20.1.
H
DOI: 10.1021/acs.joc.5b02345
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The Journal of Organic Chemistry
Article
HRMS (ESI/TOF-Q) m/z: (M + H)+, calcd for C23H24N3 342.1970,
found 342.1970. Anal. Calcd for C23H23N3: C, 80.90; H, 6.79; N,
12.30. Found: C, 80.72; H, 6.71; N, 12.41.
mmol) was used instead of aldehyde 7b. The crude product was
further purified on a silica gel (100−200 mesh) column using
methanol/dichloromethane 5:95 v/v as the eluent to obtain the
compound 20, which was isolated as gray microcrystalline solid. Yield:
0.39 g (77%). IR spectrum (ν/cm−1): 2947, 2934, 2926, 1736, 1586,
1519, 1453, 1359, 1209, 1040, 963, 825. 1H NMR (400 MHz, CDCl3):
δ 8.63 (d, J = 5 Hz, 1H), 8.58 (d, J = 5 Hz, 1H), 8.52 (s, 1H), 8.26 (s,
1H), 7.53 (s, 4H), 7.48 (d, J = 13 Hz, 1H), 7.44 (d, J = 13 Hz, 1H),
7.38 (d, J = 6 Hz, 1H), 7.16 (d, J = 4 Hz, 1H), 7.10 (t, J = 13 Hz 2H),
6.93 (d, J = 16 Hz, 1H), 6.31 (s, 1H), 3.87 (s, 3H), 3.85 (s, 3H), 3.41
(t, J = 6 Hz, 4H), 2.46 (s, 3H), 1.96 (t, J = 6 Hz, 4H). 13C NMR (100
MHz, CDCl3): δ 156.5, 155.9, 152.6, 149.4, 148.9, 148.1, 145.9, 144.0,
141.0, 139.2, 134.4, 133.1, 127.4, 126.4, 125.1, 124.8, 124.1, 122.0,
120.8, 118.1, 115.5, 110.7, 99.54, 56.9, 56.3, 50.4 (2C), 25.1 (2C),
21.2. HRMS (ESI/TOF-Q) m/z: (M + H)+, calcd for C33H34N3O2
504.2651, found 504.2650. Anal. Calcd for C33H33N3O2: C, 78.70; H,
6.60; N, 8.34. Found: C, 78.64; H, 6.71; N, 8.26.
Synthesis of (E)-N,N-Dibutyl-2,5-dimethoxy-4-(2-(4′-methyl-2,2′-
bipyridin-4-yl)vinyl)aniline (17). The monophosphonate (2, 0.32 g, 1
mmol) and the aldehyde (5b, 0.29 g, 1 mmol) were dissolved in 20
mL of dry THF. Then, potassium tert-butoxide (0.224 g, 2 mmol) was
added to the reaction mixture at 0 °C under nitrogen atmosphere; ice
bath was then removed and stirred at room temperature for 1 h. The
resulting dark colored solution was quenched with 10 mL of water,
and the product was extracted with dichloromethane. The organic
layer was washed several times with water and with brine and dried
over Na2SO4, and the crude product was purified by column
chromatography using silica gel (100−200 mesh) using methanol/
dichloromethane 5:95 v/v as the eluent to obtain compound 17, which
is a dark red colored thick gum. Yield: 0.36 g (78%). IR spectrum (ν/
1
cm−1): 2953, 2928, 2859, 1585, 1504, 1460, 1204, 1040, 965, 819. H
NMR (400 MHz, CDCl3): δ 8.59 (d, J = 5 Hz, 1H), 8.56 (d, J = 5 Hz,
1H), 8.45 (s, 1H), 8.24 (s, 1H), 7.73 (d, J = 16 Hz, 1H), 7.40 (d, J = 5
Hz, 1H), 7.13 (d, J = 5 Hz, 1H), 7.07 (s, 1H), 7.02 (d, J = 16 Hz, 1H),
6.48 (s, 1H), 3.86 (s, 6H), 3.17 (t, J = 8 Hz, 4H), 2.43 (s, 3H), 1.50
(quin, J = 8 Hz, 4H), 1.34 (sextet, J = 8 Hz, 4H), 0.90 (t, J = 7 Hz,
6H). 13C NMR (100 MHz, CDCl3): δ 156.4, 156.1, 152.4, 149.2,
148.9, 148.1, 147.0, 146.8, 142.2, 128.1, 124.6, 123.6, 122.0, 120.3,
118.4, 117.3, 110.8, 104.5, 56.2 (2C), 52.2 (2C), 29.3 (2C), 21.2, 20.5
(2C), 14.0 (2C). HRMS (ESI/TOF-Q) m/z: (M + H)+, calcd for
C29H38N3O2 460.2964, found 460.2963. Anal. Calcd for C29H37N3O2:
C, 75.78; H, 8.11; N, 9.14. Found: C, 75.62; H, 8.15; N, 9.23.
Synthesis of (E)-4-(2,5-Dimethoxy-4-(pyrrolidin-1-yl)styryl)-4′-
methyl-2,2′-bipyridine (18). This compound was synthesized using
the same pathway as described for compound 17. Aldehyde 7b (0.26 g,
1 mmol) was used instead of aldehyde 5b. That resulting dark gray
colored solid was subjected to chromatographic purification over silica
gel (100−200 mesh) using methanol/dichloromethane 5:95 v/v as the
eluent to obtain the compound 18 as a brown colored solid. The
compound was isolated as brown colored single crystals. Yield: 0.30 g
(76%). 1H NMR (400 MHz, CDCl3): δ 8.55 (t, J = 5 Hz, 2H), 8.42 (s,
1H), 8.22 (s, 1H), 7.72 (d, J = 16 Hz, 1H), 7.37 (d, J = 5 Hz, 1H),
7.09 (d, J = 5 Hz, 1H), 7.05 (s, 1H), 6.93 (d, J = 16 Hz, 1H), 6.21 (s,
1H), 3.83 (s, 3H), 3.79 (s, 3H), 3.39 (t, J = 6 Hz, 4H), 2.40 (s, 3H),
1.90 (t, J = 6 Hz, 4H). 13CNMR (100 MHz, CDCl3): δ 156.2, 156.0,
153.2, 149.0, 148.8, 148.0, 147.2, 143.7, 141.7, 128.3, 124.6, 122.0,
121.8, 120.1, 118.28, 114.1, 111.3, 98.9, 56.8, 56.1, 50.4(2C),
25.2(2C), 21.1. HRMS (ESI/TOF-Q) m/z: (M + H)+, calcd for
C25H28N3O2 402.2182, found 402.2181. Anal. Calcd for C25H27N3O2:
C, 74.79; H, 6.78; N, 10.47. Found: C, 74.85; H, 6.71; N, 10.36.
Synthesis of N,N-Dibutyl-2,5-dimethoxy-4-(4-((E)-2-(4′-methyl-
2,2′-bipyridin-4-yl)vinyl)styryl)aniline (19). Synthesis of this com-
pound follows the same procedure as described for 17. Aldehyde 11b
(0.19 g, 1 mmol) was used instead of aldehyde 5b. The crude product
was purified through column chromatography using silica gel (100−
200 mesh) and methanol/dichloromethane 5:95 v/v as mobile phase
to obtain compound 19 as a thick dark red gum. Yield: 0.22 g (80.%).
IR spectrum (ν/cm−1): 2953, 2925, 2853, 1721, 1587, 1501, 1458,
1373, 1205, 1042, 960, 824. 1H NMR (400 MHz, CDCl3): δ 8.62 (d, J
= 5 Hz, 1H), 8.57 (d, J = 5 Hz, 1H), 8.51 (s, 1H), 8.26 (s, 1H), 7.53
(s, 4H), 7.79 (d, J = 16 Hz, 1H), 7.43 (d, J = 16 Hz, 1H), 7.36 (d, J = 5
Hz, 1H), 7.14 (d, J = 5 Hz, 1H), 7.12 (t, 2H), 6.99 (d, J = 16 Hz, 1H),
6.51 (s, 1H), 3.87 (s, 3H), 3.86 (s, 3H), 3.16 (t, J = 7 Hz, 4H), 2.44 (s,
3H), 1.50 (quin, J = 7 Hz, 4H), 1.33−1.28 (m, 4H), 0.90 (t, J = 7 Hz,
6H). 13C NMR (100 MHz, CDCl3): δ 156.6, 155.9, 151.8, 149.4,
148.9, 148.2, 147.4, 145.8,141.3, 138.9, 134.7, 133.0, 127.3, 126.6,
125.6, 125.3, 1254.8, 123.9, 122.0, 120.9, 118.7, 118.1, 110.3, 105.2,
56.5, 56.3, 52.3 (2C), 29.3 (2C), 21.2, 20.5 (2C), 14.0 (2C). HRMS
(ESI/TOF-Q) m/z: (M + H)+, calcd for C37H44N3O2 562.3434, found
562.3432. Anal. Calcd for C37H43N3O2: C, 79.11; H, 7.72; N, 7.48.
Found: C, 79.03; H, 7.82; N, 7.36.
Synthesis of N,N-dibutyl-4-(2,5-dibutoxy-4-((E)-2-(4′-methyl-2,2′-
bipyridin-4-yl)vinyl)styryl)aniline (21). Synthesis of this compound
follows same procedure as described for 17. Aldehyde 13b (0.24 g, 1
mol) was used instead of aldehyde 5b. The dark brown crude was
purified on a silica gel (100−200 mesh) column using methanol/
dichloromethane 5:95 v/v as the eluent to obtain the compound 21 as
dark brown solid. Yield: 0.33 g (56%). IR spectrum (ν/cm−1): 2956,
2924, 2855, 1721, 1589, 1519, 1462, 1368, 1201, 1187, 1067, 968, 821.
1H NMR (400 MHz, CDCl3): δ 8.55 (d, J = 5 Hz, 1H), 8.49 (d, J = 4
Hz, 1H), 8.39 (s, 1H), 8.17 (s, 1H), 7.69 (d, J = 16 Hz, 1H), 7.35 (d, J
= 6 Hz, 2H), 7.32 (s, 1H), 7.19 (s, 1H), 7.13 (d, J = 16 Hz, 1H), 7.08−
6.98 (m, 4H), 6.56 (d, J = 8 Hz, 2H), 4.02 (t, J = 6 Hz,2H), 3.97 (t, J =
6 Hz, 2H), 3.22 (t, J = 7 Hz, 4H), 2.38 (s, 3H), 1.80 (sextet, J = 7 Hz,
4H), 1.52 (quin, J = 7 Hz, 8H), 1.29 (sextet, J = 7 Hz, 4H), 0.99−0.94
(m, 6H), 0.89 (t, J = 7 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ
156.6, 156.0, 151.7, 150.5, 149.3, 148.9, 148.1, 147.8, 146.6, 129.7,
129.4, 128.4, 127.9, 125.6, 125.0, 124.7, 124.3, 122.0, 120.2, 118.9,
118.1, 111.6, 111.2, 109.9, 69.2, 69.1, 50.8 (2C), 31.6, 31.5, 29.5, 21.2,
20.3 (2C), 19.5, 19.4, 14.0, 13.9 (2C). HRMS (ESI/TOF-Q) m/z: (M
+ H)+, calcd for C43H56N3O2 645.4373, found 646.4377. Anal. Calcd
for C43H55N3O2: C, 79.96; H, 8.58; N, 6.51. Found: C, 79.85; H, 8.62;
N, 6.45.
Synthesis of N,N-Dibutyl-4-(2,5-dibutoxy-4-((E)-2-(4′-methyl-2,2′-
bipyridin-4-yl)vinyl)styryl)-2,5-dimethoxyaniline (22). This com-
pound was synthesized by same procedure as described for 16.
Aldehyde 14b (0.53 g, 1 mmol) was used instead of aldehyde 5b. The
resultant dark red colored crude product was purified by a silica gel
column (100−200 mesh) eluting with methanol/dichloromethane in
5:95 v/v as mobile phase. The compound 22 was isolated as a dark red
gum. Yield: 0.48 g (69%). IR spectrum (ν/cm−1): 2955, 2924, 2854,
1
1734, 1588, 1505, 1463, 1377, 1202, 1042, 968, 853. H NMR (400
MHz, CDCl3): δ 8.62 (d, J = 5 Hz, 1H), 8.55 (d, J = 5 Hz, 1H), 8.46
(s, 1H), 8.23 (s, 1H), 7.77 (d, J = 16 Hz, 1H), 7.48 (d, J = 16 Hz, 1H),
7.41 (t, 1H), 7.36 (d, J = 16 Hz, 1H), 7.16 (t, J = 13 Hz, 2H), 7.12 (d,
J = 4 Hz, 3H), 6.55 (s, 1H), 4.08 (t, J = 6 Hz, 2H), 4.05 (t,, J = 6 Hz,
2H), 3.85 (d, J = 5 Hz, 6H), 3.17 (t, J = 8 Hz 4H), 2.42 (s, 3H), 1.89−
1.82 (m, 4H), 1.60 (quin,, J = 7 Hz, 4H), 1.53−1.45(m, 4H), 1.34−
1.28 (m, 4H), 1.03 (m, 6H), 0.91(t, J = 7 Hz, 6H). 13C NMR (100
MHz, CDCl3): δ 156.5, 155.9, 151.76, 151.71, 150.7, 149.2, 148.9
(2C), 148.2, 147.5, 146.6, 129.3, 128.4, 125.8, 124.7 (2C), 123.9,
122.12C), 121.2, 120.3, 118.9, 111.2, 110.4, 110.3, 105.5, 69.1 (2C),
56.5, 56.2, 52.5 (2C), 31.6, 31.5, 29.9 (2C), 21.1, 20.5, 19.5 (2C), 14.0
(3C). HRMS (ESI/TOF-Q) m/z: (M + H)+, calcd for C45H60N3O4
706.4584, found 706.4585. Anal. Calcd for C45H59N3O4: C, 76.56; H,
8.42; N, 5.95. Found: C, 76.65; H, 8.36; N, 5.85.
Synthesis of 4-(2,5-Dibutoxy-4-(2,5-dimethoxy-4-(pyrrolidin-1-
yl)styryl)styryl)-4′-methyl-2,2′-bipyridine (23). This compound was
synthesized following the same procedure for 17. Aldehyde 15b (0.24
g, 1 mmol) was used instead of aldehyde 5b. The crude product was
purified through column chromatography using silica gel (100−200
mesh) and methanol/dichloromethane 5:95 v/v as mobile phase to
obtain compound 23 as a dark red microcrystalline solid. Yield: 0.22 g
(71%). IR spectrum (ν/cm−1): 2958, 2928, 1737, 1712, 1587, 1452,
Synthesis of 4-(4-(2,5-Dimethoxy-4-(pyrrolidin-1-yl)styryl)styryl)-
4′-methyl-2,2′-bipyridine (20). Synthesis of this compound follows
the same procedure as described for 17. Aldehyde 12b (0.37 g, 1
I
DOI: 10.1021/acs.joc.5b02345
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The Journal of Organic Chemistry
Article
1
1357, 1217, 1117, 1040, 970, 821. H NMR (400 MHz, CDCl3): δ
Chem. Commun. 2008, 2903−2905. (c) Ajayaghosh, A.; Carol, P.;
Sreejith, S. J. Am. Chem. Soc. 2005, 127, 14962−14963. (d) Divya, K.
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8.64 (d, J = 5 Hz, 1H), 8.58 (d, J = 5 Hz, 1H), 8.50 (s, 1H), 8.27
(s,1H), 7.65 (d, J = 17 Hz, 1H), 7.49 (d, J = 16 Hz, 1H), 7.43 (d, J = 5
Hz, 1H), 7.39 (d, J = 16 Hz, 1H), 7.19 (t, 3H), 7.13 (d,J = 5 Hz, 2H),
6.30 (s,1H), 4.11 (t, J = 6 Hz, 2H), 4.07 (t, J = 6 Hz, 2H), 3.88 (s,
3H), 3.86 (s, 3H), 3.42 (t, J = 7 Hz, 4H), 2.46 (s, 3H), 1.97
(unresolved, 4H), 1.96−1.87 (m, 4H), 1.65−1.59 (m, 4H), 1.06 (q, J =
7 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 156.6, 156.0, 152.5, 151.7,
150.6, 149.3, 148.9, 148.1, 146.6, 144.1, 140.8, 129.7, 128.4, 125.6,
124.7, 124.4, 124.1, 122.0, 120.2, 119.5, 118.9, 116.5, 111.3, 110.8,
110.1, 99.8, 69.3, 69.2, 56.8, 56.5, 50.4 (2C), 31.67, 31.5, 25.09 (2C),
21.2, 19.5 (2C), 14.0 (2C). HRMS (ESI/TOF-Q) m/z: (M + H)+,
calcd for C41H50N3O4 648.3801, found 648.3807. Anal. Calcd for
C41H49N3O4: C, 76.01; H, 7.62; N, 6.49. Found: C, 76.12; H, 7.71; N,
6.58.
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ASSOCIATED CONTENT
* Supporting Information
Nazeeruddin, Md. K.; Gratzel, M. Tetrahedron Lett. 2010, 51, 6161−
̈
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6165. (d) Aubert, V.; Ishow, E.; Ibersiene, F.; Boucekkine, A.;
S
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Williams, J. A. G.; Toupet, L.; Metivier, R.; Nakatani, K.; Guerchais, V.;
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ACS Publications website at DOI: 10.1021/acs.joc.5b02345.
Le Bozec, H. New J. Chem. 2009, 33, 1320−1323. (e) Araya, J. C.;
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21−24.
X-ray crystallographic data for 18 (CIF)
Synthesis schemes, NMR and optical spectra, HRMS and
CHNS analysis plots, and computational details (PDF)
(10) Sandroni, M.; Favereau, L.; Planchat, A.; Akdas-Kilig, H.;
Szuwarski, N.; Pellegrin, Y.; Blart, E.; Le Bozec, H.; Boujtita, M.;
Odobel, F. J. Mater. Chem. A 2014, 2, 9944−9947.
AUTHOR INFORMATION
(11) Dragonetti, C.; Colombo, A.; Marinotto, D.; Righetto, S.;
Roberto, D.; Valore, A.; Escadeillas, M.; Guerchais, V.; Le Bozec, H.;
Boucekkine, A. J. Organomet. Chem. 2014, 751, 568−572.
(12) Le Bozec, H.; Guerchais, V. C. R. Chim. 2013, 16, 1172−1182.
(13) Nair, M. N.; Hobeika, N.; Calard, F.; Malval, J.-P.; Aloıse, S.;
Spangenberg, A.; Simon, L.; Cranney, M.; Vonau, F.; Aubel, D.;
Serein-Spirau, F.; Lere-Porte, J.-P.; Lacour, M.-G.; Jarrosson, T. Phys.
Chem. Chem. Phys. 2014, 16, 12826−12837.
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Corresponding Author
*E-mail: skdas@uohyd.ac.in; samar439@gmail.com. Phone:
+91 40 2301 1007. Fax: +91 40 2301 2460.
Notes
The authors declare no competing financial interest.
(14) Xu, D.; Yu, Z.; Yang, M.; Zheng, Z.; Zhu, L.; Zhang, X.; Ye, L.;
Wu, J.; Tian, Y.; Zhou, H. Dyes Pigm. 2014, 100, 142−149.
(15) Sarma, M.; Chatterjee, T.; Ghanta, S.; Das, S. K. J. Org. Chem.
2012, 77, 432−444.
ACKNOWLEDGMENTS
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We thank Science & Engineering Research Board (a statutory
body under the Department of Science and Technology),
Government of India, for financial support (Project No. SB/SI/
IC/034/2013). The 400 MHz NMR facilities at University of
Hyderabad by DST, Government of India, is gratefully
acknowledged. We acknowledge high performance computa-
tional facility at the Centre for Modeling, Simulation and
Design (CMSD), University of Hyderabad Campus. We thank
Mrs. Asia Perwez for helping us to record the mass HRMS
spectra of compounds 16−23. R.B. thanks UGC, India,
respectively for his fellowship. Single crystal X-ray diffraction
facility at University of Hyderabad by DST, Government of
India, is acknowledged. Our special thanks to Mr. Krishnachari
for helping us with quantum yield calculations. We are grateful
to Prof. S. Mahapatra, School of Chemistry, University of
Hyderabad, for giving us helpful suggestions in connection with
DFT calculations.
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