Tuning the Photophysical Properties of 2-Quinolinone-Based Donor-Acceptor Molecules through N- versus O-Alkylation: Insights from Experimental and Theoretical Investigations

Article abstract of DOI:10.1002/ejoc.201301085

A series of 2-quinolinone-based molecular systems with three different acceptor groups and N/O-alkylated quinolinone compounds have been synthesised in an attempt to understand their optical properties. All the compounds were characterised by ¹H NMR, ¹³C NMR and mass analysis. Absorption measurements revealed that charge-transfer transition was observed by introducing electron-withdrawing acceptor groups. Alkylation of 2-quinolinone at the O-position thwarts the charge-transfer transitions, whereas N-alkylation retains the charge-transfer property. The presence of resonance zwitterionic forms in the unalkylated and N-alkylated quinolinone compounds play an important role in charge-transfer transition. The effects of solvents on the absorption and emission properties of these compounds were probed through Lippert Mataga and E_T(30) correlation. The Stokes shifts of O-alkylated compounds were larger than those of the unalkylated and N-alkylated quinolinone compounds. The observed higher quantum yield and Stokes shift for these compounds will make them ideal fluorescent probes. Incorporation of acceptor groups and alkylation in the quinolinone moiety alters their energy levels. Good thermal stability was observed for both unalkylated and alkylated quinolinone compounds. The trends observed in the photophysical and electrochemical properties were supported by theoretical studies. The observed tunable optical properties, which were achieved through simple N- vs. O-alkylation, results in large Stokes shifts and thermal stability, which means that the 2-quinolinone-based molecular systems are expected to emerge as potential candidates for photovoltaic and biological applications.

Full text of DOI:10.1002/ejoc.201301085

FULL PAPER
DOI: 10.1002/ejoc.201301085
Tuning the Photophysical Properties of 2-Quinolinone-Based Donor–Acceptor
Molecules through N- versus O-Alkylation: Insights from Experimental and
Theoretical Investigations
Ganesan Paramaguru,[a] Rajadurai Vijay Solomon,[a] Sivanadanam Jagadeeswari,[a]
Ponnambalam Venuvanalingam,*[a] and Rajalingam Renganathan*[a]
Keywords: Donor-acceptor systems / Tautomerism / Fluorescence spectroscopy / Density functional calculations / Sensors
A series of 2-quinolinone-based molecular systems with
three different acceptor groups and N/O-alkylated quinol-
inone compounds have been synthesised in an attempt to
understand their optical properties. All the compounds were
shifts of O-alkylated compounds were larger than those of
the unalkylated and N-alkylated quinolinone compounds.
The observed higher quantum yield and Stokes shift for
these compounds will make them ideal fluorescent probes.
Incorporation of acceptor groups and alkylation in the quin-
olinone moiety alters their energy levels. Good thermal sta-
bility was observed for both unalkylated and alkylated quin-
olinone compounds. The trends observed in the photophysi-
cal and electrochemical properties were supported by theo-
retical studies. The observed tunable optical properties,
which were achieved through simple N- vs. O-alkylation, re-
sults in large Stokes shifts and thermal stability, which means
that the 2-quinolinone-based molecular systems are ex-
pected to emerge as potential candidates for photovoltaic
and biological applications.
1
characterised by H NMR, 13C NMR and mass analysis. Ab-
sorption measurements revealed that charge-transfer transi-
tion was observed by introducing electron-withdrawing ac-
ceptor groups. Alkylation of 2-quinolinone at the O-position
thwarts the charge-transfer transitions, whereas N-alkylation
retains the charge-transfer property. The presence of reso-
nance zwitterionic forms in the unalkylated and N-alkylated
quinolinone compounds play an important role in charge-
transfer transition. The effects of solvents on the absorption
and emission properties of these compounds were probed
through Lippert Mataga and ET(30) correlation. The Stokes
of the ICT property and molecular geometry relaxation
upon photoexcitation.[4] Several researchers have observed
that compounds that exist in keto/enol tautomerisation dis-
play large Stokes shift and that these materials can be
applied to various applications.[5]
Introduction
Donor–Acceptor molecular systems possess intramolec-
ular charge transfer (CT) properties, which is one of the key
parameters for their potential applications in various fields
such as organic photovoltaics, organic light-emitting diodes,
nonlinear optical (NLO) materials, and biological sensors.[1]
The CT properties of these systems can be easily tuned by
linking suitable donor and acceptor moieties. Over the
years, donor fragments such as pyrene, perylene, tri-
phenylamine, carbazole, coumarin, phenothiazine, and
indoline, and acceptor moieties such as cyano, nitrophenyl
acrylonitrile, carboxylate, and rhodanine substituents have
been successfully employed as donor–acceptor systems for
various applications.[2] Molecular systems that show large
Stokes shift are widely utilized as fluorescent probes for
biological applications, UV photostabilisers, laser dyes, and
photovoltaic materials.[3] Recent reports highlight the fact
that variation of Stokes shift can be related to the strength
2-Quinolinones, which are 2-pyridinone type molecular
systems, have interesting structural and spectral properties
due to their tendency to adopt tautomeric forms (keto and
enol) that are nearly equal in energy.[6] Nishiwaki et al. have
studied in detail the synthesis of quinolinone derivatives for
biological applications,[7] and Natarajan et al. have synthe-
sised several quinolinone-based metal complexes for anti-
cancer activities.[8] However, some molecules with a keto/
enol resonance unit suffer from poor solubility due to
strong intermolecular H-bonding, which constrains their
application in various fields.[9] Whereas introducing alkyl
chains in the lactam units is an important approach to im-
proving solubility,[10,9a,9b] introducing the substituents on
the N- or O-atom of the quinolinone moiety will increase
its solubility but may prevent the molecule from adopting
its characteristic tautomer forms, which may influence the
photophysical properties.
[a] School of Chemsitry, Bharathidasan University,
Tiruchirappalli 620024, Tamilnadu, India
E-mail: rrengas@gmail.com
http://www.bdu.ac.in/schools/chemistry/chemistry/
dr_r_renganathan.php
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301085.
Despite intense research into the biological activities of
quinolinone derivatives, to the best of our knowledge, no
studies have focused on the quinolinone framework in
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donor–acceptor systems by precluding specific tautomer electrochemical and thermal behaviour of the synthesised
forms and analysing the effect on photophysical properties. quinolinone compounds was analysed. Experimentally ob-
In this context, we have chosen two types of quinolinone served electronic properties of the quinolinone compounds
framework with three different acceptor groups. These two were further examined by Density Functional Theory
types of donor fragments are based on a common quinol- (DFT) and Time-Dependent DTF (TDDFT) calculations.
inone moiety (Q) to which is attached a methyl (MQ) or a The results gave deeper insights into factors affecting the
benzene moiety (BQ). Cyano acrylic acid (CA), nitrophenyl structural and photophysical properties of quinolinone de-
acetonitrile (NPAN), and Rhodanine acetic acid (RA) were rivatives for both photovoltaic and biological applications.
introduced into the above quinolinone framework as ac-
ceptor moieties and the magnitude of acceptor parts in ICT
Results and Discussion
properties were analysed. N- and O-alkylated quinolinone
derivatives were synthesised to assess the impact of the tau-
tomer on their optical properties. All the newly synthesised
quinolinone compounds (Scheme 1) were characterised by
1H NMR, 13C NMR and mass analysis. Upon alkylation,
striking differences were observed in the photophysical and
Stokes shift properties of the derivatives. Furthermore, the
Synthesis and Characterisation of Quinolinone Compounds
The synthetic routes used for the preparation of unalkyl-
ated and N- and O-alkylated MQ and BQ compounds with
various electron-withdrawing acceptor groups are depicted
in Schemes 2 and 3. Aldehydes MQA and BQA were em-
ployed as common starting precursors for the synthesis of
quinolinone-based donor acceptor compound. These start-
ing materials were synthesised by following reported meth-
ods.[11,8b] The final compounds MQX and BQX (X = C, N,
R) were conveniently prepared by the Knoevenagel conden-
sation of MQA and BQA, respectively, by using the corre-
sponding active methylene compounds as per literature pro-
cedures.[12] The characteristic –CH=C vinylic resonance
band appeared around 8.4–9.3 ppm in the 1H NMR spectra
for all the compounds, which is consistent with previous
reports.[12] The observed mass data further confirmed the
identity of the final products.
The alkylation of BQA and MQA (Scheme 2), which was
monitored by thin-layer chromatography (TLC), was car-
ried out with 1-bromooctane in the presence of base
(K2CO3) in N,N-dimethylformamide (DMF) and resulted
in the formation of a mixture of NMQA, OMQA, and
OBQA, respectively. TLC analysis of the reaction mixture
obtained from alkylation of MQA revealed two spots [Rf
(hexane/EtOAc, 1:0.1) = 0.6 (spot I), 0.2 (spot II)], whereas
only one spot was detected for BQA [Rf (hexane/EtOAc,
1:0.1) = 0.76]. 2-Quinolinone type compounds are known
Scheme 1. Structures of unalkylated and N- and O-alkylated quin-
olinone compounds with various acceptor groups.
Scheme 2. Synthetic route used for the preparation of NMQA, OMQA, and OBQA.
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A Study on 2-Quinolinone-Based Donor–Acceptor Molecules
Scheme 3. Synthetic route used for the preparation of the 2-quinolinone derivatives with various electron-withdrawing groups.
to exist in equilibrium between keto and enol form, as MQ and BQ series differed in the methyl and benzene sub-
shown in Scheme 4.[6] Hence, the alkylation of this type of stituent in the common quinolinone moiety and this sub-
compound involves an ambident anion that may result in stituent effect is one of the reasons for the formation of
mixtures of N- and O-alkylated products.[13] Based on this, preferred N-/O-alkylated products.
the formation of two spots in the alkylation of MQA may
correspond to the formation of both N- and O-alkylated
products, whereas either the N- or O-alkylated isomer was
1
formed in the alkylation of BQA. H NMR spectroscopic
analysis of the compounds forming spots I and II from the
alkylation reaction of MQA (see the Supporting Infor-
mation, SI2 and SI4, respectively) reveal only slight varia-
tions, which made identification of the corresponding prod-
uct difficult. However, the 13C NMR spectra of the com-
pounds making up spots I and II (see the Supporting Infor-
mation, SI5) were significantly different, with a major dif-
ference being observed in the region 40–70 ppm. Spot I ma-
terial gave rise to a signal at ca. 66 ppm, whereas this was
absent in the spectrum obtained from spot II, with the latter
having a signal at ca. 42 ppm. When comparing the N- and
O-alkylated MQ quinolinone compound (see the Support-
ing Information, SI5), the alkyl carbon (marked with an
asterisk) attached to the oxygen atom should appear more
downfield than when attached to a nitrogen atom, the elec-
tronegativity of which is less than oxygen. Hence, the mate-
rial generating a 13C NMR signal at ca. 66 ppm should
correspond to the O-alkylated product. Accordingly, spot II
Scheme 4. Possible equilibrium and resonance forms of unalkylated
and N/O-alkylated quinolinone compounds. “R” represents the
was assigned as the N-alkylated product. The product ob-
tained in the alkylation of BQA also possesses a signal at
various acceptor groups.
ca. 66 ppm and was accordingly assigned as O-alkylated
BQA. Recently, Torhan et al. reported the relationship be-
The assignment of spot I as the O-alkylated compounds
tween the ratio of N- and O-alkylation products and the by using NMR techniques was confirmed by single-crystal
nature of substituents in 2-pyridone type compounds.[13m] XRD structure analysis of OBQA.[14] A needle-like crystal
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was obtained from alkylated BQA in dichloromethane/ conjugation system in the benzene ring. All the compounds
methanol solvent. The single-crystal X-ray structure of this exhibit a high-energy absorption band around 300–400 nm,
compound (Figure 1, A; for details of crystal data see the which arises due to πǞπ* transitions[15] and a second band
Supporting Information, SI20) revealed that OBQA crys- in the visible region (400–500 nm). Fine tuning of the ab-
tallized in a triclinic crystal system in the P-1 space group. sorption characteristics of these compounds was observed
Due to the presence of alkyl chains, the possibility of π-π by introducing different acceptor groups. The visible ab-
stacking in the benzene-fused quinolinone is prevented, sorption of MQC and MQN appears at 417 and 418 nm,
which is clearly observed from the crystal packing (Fig- respectively, whereas the absorption of MQR falls at
ure 1, B). The obtained alkylated aldehydes were subjected 434 nm. Although, the conjugated system was larger in
to Knoevenagel condensation to give the final products MQN, MQC has a similar absorption region. This implies
(OMQX, NMQX, OBQX, X = C, N, R). All the alkylated that during the photoexcitation, the electron density around
1
products were characterised by H NMR, 13C NMR and the quinolinone moiety shifts towards the electron-with-
mass analysis.
drawing (EW) cyano group in MQC. MQR shows a further
bathochromic shift, reflecting the strong electron-with-
drawing capability of the rhodanine group. As a result, the
localisation of πǞπ* transition over the acceptor group
(charge-transfer transition) was enhanced by introducing
the electron-withdrawing groups. Hence, the band that ap-
pears in the visible region is due to charge-transfer transi-
tions, as observed in earlier reports.[16] Thus, the band
around 300–400 nm stems from πǞπ* transition localising
on the quinolinone segment, whereas the visible band origi-
nates from πǞπ* transition extended over the acceptor
segment.[17] Furthermore, the absence of a band in the vis-
ible region for the absorption of MQA (strong acceptor
group is absent) supports the origin of charge-transfer tran-
sitions being due to the presence of an acceptor group. In
the case of BQ derivatives, the BQR absorption appears at
473 nm, whereas those of BQC and BQN have maximum
Figure 1. (A) Single-crystal X-ray structure of OBQA. (B) Crystal
packing view of OBQA along the b-axis.
Electronic Properties
The absorption spectra in tetrahydrofuran (THF) of MQ
and BQ derivatives substituted with various electron-with-
drawing groups are presented in Figure 2, and the relevant
data are summarised in Table 1. Compared with methyl-
(MQ) and benzene- (BQ) substituted quinolinone deriva-
tives, the absorption of BQ derivatives appeared in the
Figure 2. Absorption spectra of unalkylated (A) MQX, and
(B) BQX compounds recorded in THF solvent [X = C, N, R
higher wavelength region due to the presence of the extra (1ϫ10–4 m)].
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A Study on 2-Quinolinone-Based Donor–Acceptor Molecules
Table 1. Photophysical data of unalkylated and N- and O-alkylated quinolinone compounds substituted with various acceptor groups
(THF solvent).
MQC MQN MQR BQC BQN BQR OMQC OMQN OMQR OBQC OBQN OBQR NMQC NMQN NMQR
λabs [nm]
417
3.22 1.64
418
431
3.56
444
0.17
435
1.67
473
2.32
332
0.81
358
1.68
397
2.79
336
0.61
346
0.68
419
1.74
402
0.64
416
1.2
430
2.8
Absorption
coefficient[a]
λemi [nm]
524
538
521
4007
0.35
522
3365
0.69
536
4331
0.67
529
2238
0.51
463
8522
0.51
493
7648
0.32
491
4822
0.14
490
4908
0.68
517
9559
0.66
499
3826
0.15
510
5267
0.57
535
5346
0.55
520
4025
0.19
Stokes shift[b] 4896 5336
Quantum
0.61 0.51
yield (φf)
[a] ε [104 m–1 cm–1]. [b] Δν [cm–1].
absorptions at 444 and 435 nm, respectively. These different completely absent and the peak corresponding to πǞπ*
acceptor groups have similar effects on the absorption trend transition localised on the quinolinone segment was more
for MQ and BQ derivatives. Comparatively, the RA moiety prominent. A similar disappearance of a CT band was ob-
possesses greater electron-withdrawing capability than the served for OBQN. For RA substituted compounds the band
other acceptor groups, whereas the CA moiety has compar- was shifted to a lower wavelength region. Similar behaviour
able electron-withdrawing character to that of the NPAN was observed for all OMQ compounds. The observed op-
acceptor. The observed absorption behaviour illustrates the tical properties of the O-alkylated MQX and BQX (X = C,
importance of incorporating acceptor groups in the quinol- N, R) compounds reveal that alkylation at the O-position
inone core structure to tune their optical properties.
prevents charge-transfer transition behaviour of the quinol-
The absorption behaviour of O-alkylated BQ derivatives inone derivatives. This assumption was further supported
in comparison to their unalkylated BQ derivatives is dis- by analysing the absorption behaviour of N-alkylated MQ
played in Figure 3 and the data is summarised in Table 1. compounds. The absorption spectra of the NMQ series in
Comparing the absorption characteristics of BQC and THF solvent (Figure 3, Table 1) clearly show that the ab-
OBQC, the CT band around 444 nm for the former was sorption behaviour of N-alkylated compounds retains the
Figure 3. Absorption spectra of alkylated OBQX, OMQX and NMQX (X = C, N, R) along with the absorption spectra of their respective
unalkylated compounds recorded in THF.
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CT transitions, as observed in unalkylated MQ compounds. for their use in photovoltaic applications. Among the vari-
This observation suggests that alkylation on the nitrogen ous acceptor groups, RA-substituted quinolinone had the
atom does not lead to any significant change in the CT highest ε, with the lowest ε values being found for CA sub-
absorption behaviour of the quinolinone compounds. These stituted compounds.
trends signify that the optical properties of 2-quinolinone
type derivatives can be easily tuned by varying the alkyl-
ation at the N- or O-position.
Table 2. B3LYP/6-31g(d) optimised C–N and C–O bond lengths
[Å].
Com-
C–N
C–O
Com-
C–N
C–O
The possible tautomer structures of unalkylated, N-alkyl-
ated, and O-alkylated quinolinones are shown in Scheme 4.
Nimlos et al. calculated the C–N bond length of the lactam
tautomer of 2-hydroxy quinoline by using semiempirical
methods and obtained a value (1.39 Å) that lies between
those of the neutral and zwitterionic form, indicating sig-
nificant contributions from both tautomer structures.[6a] To
unravel the structural properties of these molecules, DFT
calculations were carried out at the B3LYP/6-31g(d) level.
All these molecules have a rigid quinolinone unit and the
acceptor and alkyl groups are slightly twisted out of the
quinolinone plane. The important bond lengths, listed in
Table 2, show that the C–N bond lengths of unalkylated
and N-alkylated quinolinones fall between those of C–N
single (1.47 Å) and double (1.29 Å) bonds. This implies that
pound
pound
BQC
BQN
BQR
MQC
MQN
MQR
O-BQC
O-BQN
O-BQR
1.39849 1.22622
1.39575 1.22782
1.39615 1.22769
1.39256 1.22612
1.39012 1.22772
1.39073 1.22758
1.31232 1.34627
1.31149 1.34890
1.31109 1.34892
O-MQC
O-MQN
O-MQR
N-MQC
N-MQN
N-MQR
1.30719 1.34754
1.30654 1.34994
1.30610 1.35000
1.40659 1.22899
1.40265 1.23169
1.40326 1.23148
Excited State Properties
The emission spectra of all the quinolinone derivatives in
these structures have excellent delocalisation and that the THF are shown in Figure 4 and the related data are gath-
unalkylated and N-alkylated quinolinones favour zwitter- ered in Table 1. Based on the ground-state optical proper-
ionic forms, as previously reported.[6a] However, the C–N ties, it is more pertinent to analyse the excited properties in
bond lengths of O-alkylated quinolinone compounds were two different ways, such as influence of acceptor groups and
found to be closer to those of a double bond, thus favouring effect of alkylation. By varying the acceptor groups, a dif-
the neutral structure over the zwitterionic form. Based on ference in the excited behaviour of the quinolinone com-
this observation, unalkylated quinolinone apparently exists pounds was observed. The BQ series containing different
in equilibrium between the keto (lactam) and enol (lactim) acceptor groups showed emission trends as follows: BQN
form. Furthermore, the lactam form can also exist in a zwit- Ͼ BQR Ͼ BQC. In contrast in the observed absorption
terionic resonance structure.[6] N-Alkylated quinolinones trend, BQR shows more blueshifted emission behaviour
also have the possibility of existing in zwitterionic forms.[18] than BQN. Similarly, MQN shows lower energy emission
However, O-alkylated quinolinones can be represented in behaviour than MQR and MQC. In the alkylated quinol-
one form and there is no available zwitterionic resonance inone series, compounds having a NPAN segment have red-
structure. These different forms reveal that the common shifted emission behaviour. The observations from the ex-
possible structure among the unsubstituted and N-alkylated cited state behaviour points to the fact that the NPAN ac-
quinolinones is the zwitterionic form. The aromatic π den- ceptor segment has more extensive conjugation than other
sity is delocalised over the oxygen atom in the resonance EW groups, which is reflected in the emission properties
zwitterionic form, which increases the extent of the π-sys- of these quinolinone compounds, as discussed in previous
tem and creates a higher electron density around the pyrid- reports.[19] Comparing the unalkylated and N-alkylated MQ
inone moiety.[6] During photoexcitation, this higher electron compounds, similar emission behaviour is observed that is
density is attracted by the electron-withdrawing groups, and analogous to their ground state properties. As predicted
the localisation of the πǞπ* transition extends over the from the absorption behaviour, the emission maxima of the
acceptor groups, resulting in prominent CT bands in the O-alkylated quinolinone compounds appear at lower wave-
visible region, similar to previous reports.[6a] As a result of length regions compared with those of the N-alkylated and
the absence of a resonance zwitterionic form in O-alkylated unalkylated derivatives. The excitation spectrum of these
quinolinone compounds, no such extra electron density compounds was measured to gain further information on
around the oxygen atom exists, leading to the absence of a the nature of the emission behaviour and the spectra are
prominent CT transition. The molar extinction coefficient displayed in the Supporting Information (SI26). Unalkyl-
(ε [m–1 cm–1]) of these quinolinone compounds usually pro- ated and alkylated compounds have both πǞπ* conjuga-
vides information on their light-harvesting efficiency and tion and charger transfer bands in their excitation spectra,
their suitability for further application in photovoltaic ap- whereas only πǞπ* conjugation bands are observed for the
plications.[2q,2r] The ε values of all the compounds under O-alkylated series. The excitation spectra of all the com-
investigation were calculated at their absorption maxima (in pounds reflect their absorption behaviour. This implies that
THF) and are listed in Table 1 (see the Supporting Infor- the emission is not purely a result of charge transfer charac-
mation, SI25 for ε values in other solvents). The observed teristics and supports the contribution of “π” conjugation
ε values of all the compounds were found to be sufficient in the excited state, as stated for NPAN acceptor com-
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A Study on 2-Quinolinone-Based Donor–Acceptor Molecules
Figure 4. Emission spectra of unalkylated and N- and O-alkylated MQX and BQX compounds recorded in THF (X = C, N, R).
pounds. The fluorescence quantum yields of the quinol- pounds. Interestingly, the Stokes shifts for the O-alkylated
inone compounds were measured in THF solvent and the series is larger than for other compounds. For example, the
values are given in Table 1. These observed values reveal Stokes shift of BQN in DMF solvent is 2206 cm–1, whereas
that the quantum yield of these quinolinone compounds upon O-alkylation (OBQN), the Stokes shift increased dra-
depend on the EW acceptor part. For example, the quan- matically to 9684 cm–1. Similarly, a 4331 cm–1 Stokes shift
tum yield of MQC is 0.61 but it is reduced to 0.35 for of BQN in THF solvent is increased to 9559 cm–1 for
MQR. Similarly, the quantum yield of OBQC and OBQR OBQN. DFT studies reveal that the excited state of O-alkyl-
are 0.68 and 0.15, respectively. In general, the CA acceptor ated structures is perturbed more than the corresponding
substituted quinolinone compounds show higher quantum N-alkylated and unalkylated compounds. This leads to the
yield (φ = 0.69) whereas the quantum yield is very low with existence of nonradiative relaxation pathways such as struc-
RA acceptor compounds. Compounds with the NPAN ac- tural reorganisation in the excited state and results in a
ceptor have comparable quantum yields to those with the large Stokes shift.[3,17] The obtained large Stokes shift val-
CA acceptor. The observed difference in the quantum yield ues are analogous with the recently reported mole-
depending on the acceptor part is similar to those noted in cules,[3b,3d,3e] and such compounds can diminish the inner
our recent report.[2s] There was no significant difference in filter effect, which suggests the possible application of these
the quantum yield among alkylated and unalkylated quin- compounds as biological fluorescent probes.[3d,3e,3g]
olinone compounds.
The Stokes shift of these compounds in THF are summa-
rised in Table 1 and those of all compounds in various sol-
vents are given in the Supporting Information (SI25).
Solvatochromism
Among the different acceptor groups, NPAN substituted
The absorption and emission spectra in various solvents
compounds have relatively large Stokes shift in various sol- were measured to gain more information regarding the sol-
vents. The Stokes shifts of all compounds in THF solvent vent effect on the photophysical properties of the quinol-
followed the order: MQN Ͼ MQC Ͼ MQR, BQN Ͼ BQC inone compounds. The representative figures are shown in
Ͼ BQR, OMQN Ͼ OMQC Ͼ OMQR, NMQN Ͼ NMQC the Supporting Information (SI21–SI24) together with a
Ͼ NMQR, OBQN Ͼ OBQC Ͼ OBQR, which highlights summary of the relevant data (SI 25). A comparison of the
the small Stokes shift observed for RA substituted com- absorption and emission behaviour of all quinolinone com-
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pounds in various solvents (see the Supporting Infor- alkylated and unalkylated compounds suggests that the oxi-
mation, SI27) clearly reveals that the absorption of the RA dation process is mainly centred on the pyridinone moiety.
acceptor group substituted quinolinone compounds has The reduction behaviour of these compounds (Table 3 and
more redshifted absorption than other acceptor groups in the Supporting Information, SI30) show that among the
various solvents. Unalkylated and N-alkylated compounds different acceptor groups, the reduction potential of RA ac-
have more extensive changes in absorption characteristics in ceptor substituted unalkylated and N-alkylated compounds
various solvents than O-alkylated compounds. As discussed shifts towards more positive values, whereas the O-alkylated
above, NPAN acceptor group substituted compounds compounds with a CA acceptor part have more positive
shows redshifted emission behaviour compared with other reduction behaviour. The reduction potential for MQC (–
acceptor groups. Compared to their ground state, the ex- 0.95 V) remains almost the same as for its N-alkylated
cited state of these compounds is more sensitive to changes counterpart (NMQC, –0.92 V), whereas it was altered for
in solvent. A positive solvatochromism is expected if the OMQC (–0.85 V). Overall, the reduction behaviour of al-
emission behaviour is related to ICT,[20] however, the fact kylated and N-alkylated series behave in a similar manner.
that there is no clear trend observed on increasing the sol- These observations further suggest that the incorporation
vent polarity points to the involvement of other factors. A of acceptor groups and alkylation in the pyridinone moiety
better description of the solvent effect in both the ground plays an important role in tuning the energy levels of the
and electronic excited states of the compounds was at- quinolinone-based compounds.
tempted by using the Lippert–Mataga equation.[21] The
Lippert–Mataga plot of Stokes shift and orientation pol-
arisability of solvents for all the quinolinone derivatives (see
the Supporting Information, SI28) reveals a nonlinearity of
the Lippert–Mataga correlation, which shows the existence
of specific solvent effects.[22] Factors such as hydrogen-
bonding and polarisability are potential causes of the ob-
served solvent effect, as discussed in the literature.[23] The
use of the Reichardt–Dimroth polarity parameter ET(30)
is another important method with which to gain a better
understanding on the solvatochromic behaviour of the
quinolinone compounds.[24] The correlation of Stokes shift
with the ET(30) parameter for all the quinolinone com-
pounds (see the Supporting Information, SI29) reveals a de-
viation from linearity, indicating the involvement of solute–
solvent interactions other than dipole–dipole (charge trans-
fer) interactions in the excited state of the compounds.
Figure 5. Cyclic voltammograms of MQX (X = C, N, R) recorded
in THF (vs. AgCl/Ag electrode).
Electrochemical Properties
Table 3. Electro-optical data of the quinolinone compounds.
The electrochemical behaviour of the quinolinone com-
pounds was scrutinised by using cyclic voltammetry and
differential pulse voltammetry measurements; the voltam-
mograms of the quinolinone compounds are displayed in
Figure 5 for the MQ series (see the Supporting Information,
SI30 for other compounds) and the data are listed in
Table 3. The unalkylated quinolinone compounds exhibit
quasireversible oxidation processes, whereas irreversible
oxidation is observed for the alkylated compounds. Among
the various acceptor groups, the oxidation of the unsubsti-
tuted quinolinone with the RA acceptor group occurs at
higher potential, which signifies the strongest accepting be-
haviour by the rhodanine moiety. An anodic shift in the
oxidation of alkylated compounds was observed relative to
unalkylated compounds. The oxidation of N-alkylated com-
pounds was cathodically shifted compared with O-alkylated
derivatives. The introduction of alkyl chains onto the quin-
olinone compounds results in tuning the HOMO energy
level by 300 mV, as can be predicted from the potentials of
Eox
Es
Eox*
Ered
[V][a]
[eV][b]
[eV][c]
[V][a]
MQC
MQN
MQR
BQC
BQN
BQR
OMQC
OMQN
OMQR
OBQC
OBQN
OBQR
NMQC
NMQN
NMQR
0.78
0.84
1.08
0.84
0.83
1.05
1.19
1.24
1.21
1.34
1.33
1.23
1.1
2.97
2.64
2.59
2.71
2.64
2.50
3.08
3.05
2.90
3.02
2.85
2.74
2.92
2.61
2.62
–2.19
–1.8
0.95
0.87
0.85
0.87
0.80
0.85
0.85
0.86
0.99
0.78
0.93
0.97
0.92
0.85
0.83
–1.51
–1.87
–1.81
–1.45
–1.89
–1.81
–1.69
–1.68
–1.52
–1.51
–1.82
–1.41
–1.36
1.2
1.25
[a] Oxidation potential (Eox) and reduction potential (Ered) of the
quinolinone compounds (10–3 m) in DMF containing 0.1 m tetra-
butylammonium hexafluorophosphate (vs. AgCl/Ag electrode).
[b] Es was calculated from the intersection of absorption and emis-
all the compounds. This difference in the oxidation of the sion in DMF. [c] Eox* = Eox – Es.
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A Study on 2-Quinolinone-Based Donor–Acceptor Molecules
Thermal Properties
Thermogravimetric analysis (TGA) was performed to in-
vestigate the thermal stability of the quinolinone com-
pounds (see the Supporting Information, SI31). All the
compounds exhibit good thermal stability and temperatures
corresponding to 5% weight loss (Td) for the MQ and BQ
compounds with various acceptor groups were 250 (MQC),
320 (MQN), 290 (MQR), 233 (BQC), 310 (BQN), and
298 °C (BQR). Among the different acceptor groups, CA
exhibited lower Td, whereas quinolinones with the NPAN
group showed higher thermal stability relative to RA. This
is due to the presence of a –COOH group in CA and RA,
which leads to a decarboxylation process as reported pre-
viously[25] and lowers its Td compared with NPAN deriva-
tives. To examine the alkylation effect, the TGA of OMQR,
NMQR and OBQR was analysed and their Td values were
found to be 327, 324 and 322 °C, respectively, which were
higher than their parent unalkylated compounds. The sta-
bility of the alkylated compounds was comparable to quin-
olinone compounds with the NPAN group. The observed
thermal stability data suggests these quinolinone com-
pounds are suitable for use in various photovoltaic applica-
tions.
Theoretical Insights into the Optical and Electrochemical
Properties
Frontier molecular orbitals are often used to obtain
qualitative information about the optical and electrochemi-
cal properties of molecules. To understand the reactivity,
the frontier molecular orbitals and their corresponding en-
Figure 6. Frontier molecular orbitals (HOMO and LUMO) of
MQC, MQN, MQR, NMQC and OMQC.
ergy levels have been examined (see the Supporting Infor- MQC, a 14% contribution of the acceptor towards the
mation, SI32–SI34). The frontier molecular orbital HOMO is significantly increased to 46%. Furthermore, the
(HOMO and LUMO) pictures of MQ derivatives are de- quinolinone unit contributes 94 and 85% to HOMO in
picted in Figure 6 (see also the Supporting Information, OMQC and NMQC, whereas it reduced to 44 and 53%,
SI32 and SI33). From these calculations it is clear that the respectively, towards the LUMO. As evident from the mo-
HOMO is delocalised over the entire quinolinone unit and lecular orbital diagrams of the BQ derivative, the contri-
is extended slightly into the acceptor part. The LUMO is bution of the quinolinone unit predominates (60–90%) in
evenly delocalized on the acceptor group, and part of quin- the HOMO, whereas the contribution of the acceptor
olinone unit is also involved. The alkyl group takes part groups improved 30–40%. It is important to note that quin-
in neither HOMO nor LUMO orbitals. Furthermore, the olinone derivatives with a CA acceptor group have ca. 90%
frontier molecular orbitals show that both the HOMO and HOMO contribution from the quinolinone unit, whereas
LUMO levels of all these molecules have π character and the acceptor group contributes very little (Ͻ 12%); for the
are stabilized by the acceptor groups. Hence, the electronic LUMO, the contribution of the acceptor group increases to
transitions of these molecules arise from both intramolecu- 55%. The HOMO of RA substituted quinolinone gets
lar charge-transfer (CT) and πǞπ* transitions.
greater contributions from both the quinolinone unit as
To gain further insights from these frontier molecular or- well as the acceptor group. However, contributions from the
bitals, energetics of the HOMOs and LUMOs and their cor- alkyl group in N- and O-alkylated compounds towards
responding compositions were computed by using the HOMO and LUMO is up to 3 and 1%, respectively. This
QMForge program.[26] The compositions of the HOMO analysis provides further useful clues for the design of such
and LUMO orbitals were partitioned according to the con- molecules with various acceptor groups for use as improved
tribution from the quinolinone unit, the acceptor unit and candidates for optoelectronic applications.
the alkyl group, and the results are presented in Figure 7
Time-dependent DFT (TDDFT) calculations have been
(see also the Supporting Information, SI34). It is clear from carried out on the ground state geometries to understand
these calculations that the contribution of the quinolinone and explain the nature of transitions for the observed ab-
unit to the HOMO gradually decreased from 86 to 45% sorption spectra of these molecules. The details of the exci-
upon moving from MQC to MQR. At the same time, in tation energies, oscillator strength (f), and contributing con-
Eur. J. Org. Chem. 2014, 753–766
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761

P. Venuvanalingam, R. Renganathan et al.
FULL PAPER
terised by 1H NMR, 13C NMR and mass analysis. The sig-
nificance of these modifications on the photophysical prop-
erties of the quinolinone moiety was investigated by using
a range of experimental and theoretical techniques. Charge-
transfer transitions in the visible region were observed by
introducing electron-withdrawing acceptor groups to the
quinolinone framework. Among the different acceptor
groups, rhodanine acetic acid substituted quinolinone
showed better electron-withdrawing property as indicated
by the more redshifted absorption. A significant change in
the quinolinone photophysical properties was observed be-
tween N- and O-alkylated compounds. The observed pho-
tophysical properties among different compounds reveal
that the presence of zwitterionic resonance structures play
a crucial role in the charge-transfer behaviour. TDDFT
studies clearly predict the absorption trend and accounts
for the nature of the transitions. The Stokes shift of the O-
alkylated quinolinone was found to be larger among unal-
kylated and N-alkylated quinolinone compounds. The
higher quantum yield of these quinolinone-type compounds
facilitates their use in biological applications. The combina-
tion of acceptor groups and alkylation on the pyridinone
moiety plays an important role in tuning the energy levels
of the quinolinone-based compounds. Theoretical studies
suggest that the contribution of the quinolinone unit pre-
dominates in the HOMO, whereas the LUMO is delocalised
onto the acceptor fragments. The observed good thermal
stability for all the compounds makes them suitable candi-
dates for use in photovoltaic applications. Considering the
above results, such as altered photophysical properties by a
simple change in alkylation position, large Stokes shift, tun-
able electrochemical behaviour, and good thermal behav-
iour, signifies that quinolinone-based compounds should
emerge as potential candidates for photovoltaic and bio-
logical applications. Furthermore, the observed results de-
lineate the factors that determine the photophysical proper-
ties of quinolinone derivatives and should allow better mo-
lecular engineering to tune the structural and photophysical
properties of such compounds towards various applications.
Synthesis of a range of quinolinone-based donor and ac-
ceptor systems with potential use in photovoltaic and bio-
logical applications are in progress in our laboratory.
Figure 7. Percentage contribution from different segments towards
the HOMO and LUMO.
figurations for the most probable electronic transition of all
the molecules in THF are summarised in the Supporting
Information (SI35). In line with the experiments, two ab-
sorption bands were predicted in the range of 300–400 and
400–500 nm. The strongest absorption was observed at
372 and 482 nm for BQC, which arise from HOMO–
1 Ǟ LUMO (85%) and HOMO Ǟ LUMO (85%), respec-
tively. RA substituted quinolinone compounds were found
to display redshifted absorption compared with other ac-
ceptor groups, as expected from experimental observations.
As seen from the frontier molecular orbital diagrams, the
distribution of HOMO and LUMO in these molecules has
a significant overlap, which implies that the transitions are
from both the charge transfer as well as π-π* transitions.[17b]
MQN displays an intense absorption peak at 472 nm with
an excitation energy of 2.63 eV. This peak was mainly due
to HOMO Ǟ LUMO (85%) transition with oscillator
strength of 0.5652. Another interesting fact is that the
HOMO–1 Ǟ LUMO transitions are responsible for the
peak around 300–400 nm, whereas the peak obtained be-
tween 400–500 nm originates from HOMO Ǟ LUMO tran-
sitions. In line with experiments, O-MQC possesses the low-
est absorption (362 nm) among all, and was assigned to the
HOMOǞLUMO (98%) transition with an oscillator
strength of 0.6739. The absorption wavelength of the quin-
olinone compounds increases upon moving from CA,
NPAN to RA acceptor group, respectively, which is in good
agreement with experimentally observed absorption proper-
ties.
Experimental Section
Chemicals and Instrumentation: Phosphorous oxychloride, dimethyl
formamide, octyl bromide, rhodanine acetic acid, 4-nitrophenyl-
acetonitrile, and piperidine were purchased from Sigma–Aldrich.
4-Bromo aniline, acetyl chloride, cyanoacetic acid, ammonium
acetate, and acetic acid were purchased from LOBA chemicals. All
solvents were of Analar reagent grade and used as received. 1H
and 13C NMR spectra were obtained with a Bruker spectrometer
operating at 400 MHz and 100 MHz, respectively. Mass spectra
were obtained with a Bruker Daltonics, FT-ICR/APEX II, op-
erating in ESI mode. Absorption spectral measurements were re-
corded with
a JASCO V630 UV/Visible spectrophotometer.
Conclusions
Fluorescence and excitation measurements were carried out with a
JASCO FP-6500 spectrofluorimeter. Fluorescence quantum yield
A series of quinolinone core structures with different ac-
ceptor and alkylated groups were synthesised and charac- was calculated by using the equation: Φs = Φr [(Is/As)/(Ir/Ar)]
762
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A Study on 2-Quinolinone-Based Donor–Acceptor Molecules
(ηs/ηr)2, where Φr and Φs are the quantum yield of the reference
and sample. Perylene (Φ = 0.94) and diphenyl anthracene (Φ =
0.93) in cyclohexane was used as reference. Ir and Is are the inte-
grated photoluminescence area for the reference and sample,
respectively. Ar and As are the absorbance of the reference and
sample at the excitation wavelengths, and ηr and ηs are the refrac-
tive indexes of the solvents used for reference and sample, respec-
tively. Cyclic voltammetry and differential pulse voltammetry
(DPV) measurements were carried out with Princeton Applied Re-
search, Versastat II instruments in dimethyl formamide medium.
Tetrabutylammonium hexafluorophosphate (0.1 m) was used as
supporting electrolyte. The experimental setup consisted of a plati-
num working electrode, platinum wire counter electrode and a sil-
ver/silver chloride reference electrode. All samples were deaerated
by bubbling with pure nitrogen gas for ca. 5 min at room tempera-
ture. Thermogravimetric analyses (TGA) were performed at a heat-
ing rate of 10 °C/min. Thin-layer chromatography (TLC) was car-
ried out on aluminium sheets pre-coated with silica gel 60F254 (E.
Merck). Crystallographic data collection was performed with a
Bruker kappa Apex II CCD detector system and single-crystal X-
ray diffractometer equipped with a fine-focus sealed X-ray tube
using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å).
Structure solution and refinement were carried out by using the
SHELXL-97 software package. All calculations were performed by
using the WINGX software package and SHELX programme.
128.6, 128.2, 127.9, 126.9, 125.6, 124.9, 124.9, 124.5, 124.0, 123.2,
123.0, 114.9, 109.4, 104.1, 22.1 ppm. HRMS (ESI): Calcd. for [M
+ H]+ 291.0769; found 291.0804.
2-(4-Nitrophenyl)-3-(2-oxo-1,2-dihydrobenzo[h]quinolin-3-yl)acrylo-
1
nitrile (BQN): Yield 74%. H NMR (400 MHz, [D6]DMSO): δ =
9.06 (s, 1 H), 8.85 (s, 1 H), 8.22 (d, J = 8.8 Hz, 2 H), 8.04 (s, 1 H),
7.81–7.76 (m, 4 H), 7.65 (d, J = 9.2 Hz, 3 H) ppm. HRMS (ESI):
Calcd. for [M + H]+ 368.1035; found 368.1145.
2-{4-Oxo-5-[(2-oxo-1,2-dihydrobenzo[h]quinolin-3-yl)methylene]-2-
thioxothiazolidin-3-yl}acetic Acid (BQR): Yield 70 %. 1H NMR
(400 MHz, [D6]DMSO): δ = 12.67 (s, 1 H), 8.89 (d, J = 8 Hz, 1 H),
8.51 (s, 1 H), 7.99 (d, J = 8 Hz, 1 H), 7.80 (s, 1 H), 7.75–7.65 (m,
4 H), 4.72 (s, 2 H) ppm. 13C NMR (100 MHz, [D6]DMSO): δ =
167.3, 166.8, 161.2, 134.2, 130.2, 129.2, 128.6, 127.0, 125.5, 123.3,
123.2, 122.8, 115.8, 44.9 ppm. HRMS (ESI): Calcd. for [M + H]+
397.0316; found 397.0410.
General Procedure for the Preparation of OMQA, NMQA and
OBQA: MQA or BQA (1 equiv.) and K2CO3 (3 equiv.) were sus-
pended in DMF and heated to 120 °C. At 120 °C, 1-bromooctane
(1 equiv.) was added and the mixture was stirred at 120 °C until
the reactant spot was no longer visible by TLC. Upon completion,
the reaction mixture was cooled to room temperature and poured
into demineralised water and extracted with chloroform. The crude
product was purified by column chromatography. For BQA, one
spot was visible by TLC and column chromatography (hexane/ethyl
acetate, 1:0.03) gave the pure product. For MQA, hexane/ethyl
acetate (1:0.02) was used as eluent to obtain the first spot, and the
second spot was separated in hexane/ethyl acetate (1:0.06).
General Procedure for the Preparation of MQX and BQX (X = C,
N, R) Compounds: To a mixture of quinolinone aldehyde (MQA or
BQA) (1 equiv.) and the corresponding active methylene compound
(1 equiv.) in chloroform, piperidine (0.01 equiv.) was added and the
mixture was heated to reflux until TLC analysis showed the disap-
pearance of the aldehyde spot. After the completion of the reaction,
the precipitated solid was filtered and dried in vacuo to yield the
desired product.
6-Methyl-2-(octyloxy)quinoline-3-carbaldehyde (OMQA): Yield
1
27%. H NMR (400 MHz, [D6]DMSO): δ = 10.49 (s, 1 H), 8.5 (s,
1 H), 7.73 (d, J = 8.4 Hz, 1 H), 7.54–7.59 (m, 2 H), 4.56 (t, J =
6.8 Hz, 2 H), 2.49 (s, 3 H), 1.84–1.91 (m, 2 H), 1.47–1.54 (m, 3
H), 1.28–1.41 (m, 7 H), 0.89 (t, J = 6.8 Hz, 3 H) ppm. 13C NMR
(100 MHz, [D6]DMSO): δ = 189.6, 160.8, 147.5, 138.9, 134.6,
134.5, 128.6, 126.9, 124.2, 119.9, 66.5, 31.8, 29.7, 29.3, 29.2, 28.9,
26.2, 22.6, 21.2, 14.1 ppm. MS (ESI): Calcd. for [M + H]+ 300.2;
found 300.25.
2-Cyano-3-(6-methyl-2-oxo-1,2-dihydroquinolin-3-yl)acrylic
Acid
1
(MQC): Yield 76%. H NMR (400 MHz, [D6]DMSO): δ = 12.24
(s, 1 H), 8.77 (s, 1 H), 8.46 (d, J = 5.6 Hz, 1 H), 7.59 (s, 1 H), 7.49
(d, J = 6.8 Hz, 1 H), 7.27 (d, J = 8.4 Hz, 1 H), 2.36 (s, 3 H) ppm.
13C NMR (100 MHz, [D6]DMSO): δ = 162.9, 160.0, 147.7, 140.9,
138.1, 134.7, 131.9, 128.8, 123.4, 118.1, 115.7, 115.3, 104.7,
20.1 ppm. HRMS (ESI): calcd. for [M + H]+ 255.0769; found
255.0765.
2-(Octyloxy)benzo[h]quinoline-3-carbaldehyde (OBQA): Yield 87%.
1H NMR (400 MHz, [D6]DMSO): δ = 10.55 (s, 1 H), 9.12–9.14 (m,
1 H), 8.6 (s, 1 H), 7.87–7.89 (m, 1 H), 7.68–7.74 (m, 4 H), 4.74 (t,
J = 6.8 Hz, 2 H), 1.92–1.99 (m, 2 H), 1.31–1.46 (m, 10 H), 0.89 (t,
J = 6.8 Hz, 3 H) ppm. 13C NMR (100 MHz, [D6]DMSO): δ =
189.3, 161.5, 148.1, 138.5, 135.0, 130.2, 129.2, 127.8, 126.7, 125.7,
125.7, 125.3, 121.4, 119.0, 127.8, 126.7, 125.7, 125.7, 125.3, 121.4,
119.0,66.7, 31.8, 29.4, 29.3, 28.9, 26.2, 22.7, 14.1 ppm. MS (ESI):
Calcd. for [M + H]+ 336.2; found 336.17.
3-(6-Methyl-2-oxo-1,2-dihydroquinolin-3-yl)-2-(4-nitrophenyl)-
acrylonitrile (MQN): Yield 90 %. 1H NMR (400 MHz, [D6]-
DMSO): δ = 12.22 (s, 1 H), 8.68 (s, 1 H), 8.35 (d, J = 8.8 Hz, 2
H), 8.22 (s, 1 H), 8.02 (d, J = 8.8 Hz, 2 H), 7.59 (s, 1 H), 7.46 (d,
J = 8.0 Hz, 1 H), 7.27 (d, J = 8.4 Hz, 1 H), 2.37 (s, 3 H) ppm. 13C
NMR (100 MHz, [D6]DMSO): δ = 160.4, 140.5, 139.3, 137.4,
133.8, 131.8, 128.4, 127.0, 125.4, 124.4, 115.2, 20.2 ppm. HRMS
(ESI): Calcd. for [M + H]+ 332.1035, found 332.1067.
6-Methyl-1-octyl-2-oxo-1,2-dihydroquinoline-3-carbaldehyde
1
(NMQA): Yield 22%. H NMR (400 MHz, [D6]DMSO): δ = 10.48
(s, 1 H), 8.3 (s, 1 H), 7.51 (d, J = 6.8 Hz, 2 H), 7.29 (d, J = 9.6 Hz,
1 H), 4.29 (t, J = 8.0 Hz, 2 H), 2.43 (s, 3 H), 1.76 (m, 2 H), 1.44–
1.61 (m, 3 H), 1.28–1.39 (m, 7 H), 0.88 (t, J = 6.8 Hz, 3 H) ppm.
13C NMR (100 MHz, [D6]DMSO): δ = 190.5, 161.5, 140.7, 139.5,
135.2, 132.4, 131.5, 125.1, 119.5, 114.4, 42.4, 31.7, 29.3, 29.2, 27.5,
27.0, 22.6, 20.4, 14.0 ppm. MS (ESI): Calcd. for [M + H]+ 300.2;
found 300.25.
2-{5-[(6-Methyl-2-oxo-1,2-dihydroquinolin-3-yl)methylene]-4-oxo-2-
thioxothiazolidin-3-yl}acetic Acid (MQR): Yield 84 %. H NMR
1
(400 MHz, [D6]DMSO): δ = 12.20 (s, 1 H), 8.34 (s, 1 H), 7.76 (s, 1
H), 7.58 (s, 1 H), 7.45 (d, J = 8.0 Hz, 1 H), 7.25 (d, J = 8.4 Hz, 1
H), 4.72 (s, 2 H), 2.35 (s, 3 H) ppm. 13C NMR (100 MHz, [D6]-
DMSO): δ = 196.2, 167.3, 166.8, 160.3, 145.3, 137.5, 134.2, 131.9,
130.3, 128.5, 124.1, 123.4, 119.2, 115.2, 44.8, 20.2 ppm. HRMS
(ESI): Calcd. for [M + H]+ 361.0316; found 361.0370.
General Procedure for the Preparation of NMQX, OMQX and
OBQX (X = C, R) Compounds: To a mixture of alkylated quinol-
inone aldehyde (NMQA or OMQA or OBQA; 1 equiv.) and corre-
sponding active methylene compound (1 equiv.) in chloroform,
2-Cyano-3-(2-oxo-1,2-dihydrobenzo[h]quinolin-3-yl)acrylic Acid
(BQC): Yield 80%. 1H NMR (400 MHz, [D6]DMSO): δ = 12.71
(s, 1 H), 8.94 (s, 2 H), 8.52 (s, 1 H), 8.03–8.01 (d, J = 7.6 Hz, 1 H),
7.73 (s, 3 H), 7.69 (s, 1 H) ppm. 13C NMR (100 MHz, [D6]DMSO): piperidine (0.01 equiv.) was added and the mixture was heated to
δ = 162.2, 148.8, 148.3, 147.3, 146.4, 138.8, 134.2, 130, 129.1, 128.9, reflux until the aldehyde spot was no longer visible by TLC. Upon
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763

P. Venuvanalingam, R. Renganathan et al.
FULL PAPER
completion of the reaction, the precipitated solid was filtered and
dried in vacuo to give the desired product.
(400 MHz, CDCl3): δ = 8.14 (s, 1 H), 7.99 (s, 1 H), 7.7 (d, J =
8 Hz, 1 H), 7.53–7.5 (m, 2 H), 4.94 (s, 2 H), 4.53 (t, J = 6.4 Hz, 2
H), 2.49 (s, 3 H), 1.88 (t, J = 7.2 Hz, 2 H), 1.29–1.25 (m, 10 H),
0.88 (t, J = 6.4 Hz, 3 H) ppm. 13C NMR (100 MHz, [D6]DMSO):
δ = 193.3, 167.1, 166.2, 158.0, 144.6, 139.4, 134.3, 133.8, 127.8,
126.8, 126.2, 124.3, 124.1, 117.5,66.3, 45.1, 31.1, 28.6, 28.5, 25.5,
25.5, 22.0, 20.7, 13.8 ppm. HRMS (ESI): Calcd. for [M + H]+
473.1568; found 473.1874.
General Procedure for the Preparation of NMQN, OMQN and
OBQN Compounds: To a mixture of alkylated quinolinone alde-
hyde (NMQA or OMQA or OBQA; 1 equiv.) and corresponding
active methylene compound (1 equiv.) in acetonitrile, piperidine
(0.01 mol ratio) was added and the mixture was heated to reflux
until the aldehyde spot was no longer visible by TLC. Upon com-
pletion of the reaction, the precipitated solid was filtered and dried
in vacuo to yield the desired product.
2-Cyano-3-[2-(octyloxy)benzo[h]quinolin-3-yl]acrylic Acid (OBQC):
Yield 56%. 1H NMR (400 MHz, CDCl3): δ = 9.14 (s, 1 H), 9.08
(s, 1 H), 8.88 (s, 1 H), 7.88–7.82 (m, 1 H), 7.69 (s, 4 H), 4.69 (t, J
= 6 Hz, 2 H), 1.95 (t, J = 7.2 Hz, 2 H), 1.54–1.25 (m, 10 H), 0.88
(t, J = 6.4 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 162.8,
158.7, 146.4, 145.6, 138.2, 134.3, 129.2, 129.2, 127.9, 126.8, 125.6,
125.2, 124.2, 120.9, 116.0, 115.6, 66.5, 31.1, 28.6, 28.6, 28.1, 25.4,
22.0, 13.8 ppm. HRMS (ESI): Calcd. for [M + H]+ 403.2180; found
403.2021.
2-Cyano-3-(6-methyl-1-octyl-2-oxo-1,2-dihydroquinolin-3-yl)acrylic
1
Acid (NMQC): Yield 48%. H NMR (400 MHz, CDCl3): δ = 8.87
(d, J = 6.4 Hz, 2 H), 7.54 (s, 1 H), 7.51 (dd, J = 8.8, 1.6 Hz, 1 H),
7.28 (d, J = 4.4 Hz, 1 H), 4.3 (t, J = 8 Hz, 2 H), 2.43 (s, 3 H), 1.78–
1.73 (q, J = 7.6 Hz, 2 H), 1.46–1.25 (m, 10 H), 0.88 (t, J = 6.8 Hz,
3 H) ppm. 13C NMR (100 MHz, [D6]DMSO): δ = 162.8, 159.3,
148.3, 140.1, 138.0, 134.9, 132.0, 130.2, 122.6, 118.9, 115.6, 114.8,
105.3, 42.2, 31.1, 28.6, 28.5, 26.9, 26.1, 22.0, 19.8, 13.8 ppm.
HRMS (ESI): Calcd. for [M + H]+ 367.2021; found 367.2243.
2-(4-Nitrophenyl)-3-[2-(octyloxy)benzo[h]quinolin-3-yl]acrylonitrile
1
(OBQN): Yield 67%. H NMR (400 MHz, CDCl3): δ = 9.13–9.10
(m, 1 H), 9.03 (s, 1 H), 8.34 (d, J = 8.8 Hz, 2 H), 8.20 (s, 1 H),
7.91–7.89 (m, 3 H), 7.74 (s, 2 H), 7.71–7.69 (m, 2 H), 4.73 (t, J =
6.8 Hz, 2 H), 2.00–1.93 (m, 2 H), 1.46–1.30 (m, 10 H), 0.87 (t, J =
6.8 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 159.2, 147.8,
146.1, 140.2, 138.5, 137.1, 134.7, 130.2, 128.9, 127.8, 126.7, 126.5,
125.7, 125.3, 124.8, 124.3, 121.5, 117.1, 116.8, 110.2, 67.1, 31.8,
29.4, 29.3, 28.9, 26.3, 22.7, 14.1, 4.7 ppm. HRMS (ESI): Calcd. for
[M + H]+ 480.2286; found 480.2527.
3-(6-Methyl-1-octyl-2-oxo-1,2-dihydroquinolin-3-yl)-2-(4-nitrophen-
yl)acrylonitrile (NMQN): Yield 59%. 1H NMR (400 MHz, [D6]-
DMSO): δ = 8.77 (s, 1 H), 8.32–8.29 (m, 3 H), 7.9 (dd, J = 7.2,
1.6 Hz, 2 H), 7.54 (s, 1 H), 7.48 (dd, J = 8.8, 2 Hz, 1 H), 7.29 (d,
J = 7.29 Hz, 1 H), 4.31 (t, J = 7.6 Hz, 2 H), 2.45 (s, 3 H), 1.76 (t,
J = 8.0 Hz, 2 H), 1.57–1.27 (m, 10 H), 0.88 (t, J = 7.2 Hz, 3
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 160.7, 147.9, 140.1,
139.7, 138.4, 137.8, 134.1, 132.5, 130.4, 126.8, 124.5, 124.3, 120.0,
117.0, 114.2, 110.3, 43.3, 31.7, 29.3, 29.1, 27.5, 27.0, 22.6, 20.4,
14.0 ppm. HRMS (ESI): Calcd. for [M + H]+ 444.2287; found
444.2551.
2-(5-{[2-(Octyloxy)benzo[h]quinolin-3-yl]methylene}-4-oxo-2-thioxo-
thiazolidin-3-yl)acetic Acid (OBQR): Yield 67%. 1H NMR
(400 MHz, CDCl3): δ = 9.10 (t, J = 4.4 Hz, 1 H), 8.21 (s, 1 H),
8.12 (s, 1 H), 7.89–7.87 (m, 1 H), 7.73–7.67 (m, 4 H), 4.96 (s, 2 H),
4.73 (t, J = 6.8 Hz, 2 H), 2.01–1.94 (m, 2 H), 1.45–1.25 (m, 10 H),
0.88 (t, J = 6.8 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
193.1, 167.1, 166.1, 158.5, 144.5, 139.3, 134.2, 129.2, 128.9, 127.9,
126.7, 126.5, 125.4, 125.3, 124.1, 123.6, 121.5, 116.5, 66.5, 45.1,
31.1, 28.7, 28.6, 28.1, 25.5, 22.0, 13.8 ppm. HRMS (ESI): Calcd.
for [M + H]+ 509.1568; found 509.1844.
2-{5-[(6-Methyl-1-octyl-2-oxo-1,2-dihydroquinolin-3-yl)methylene]-
4-oxo-2-ioxothiazolidin-3-yl}acetic Acid (NMQR): Yield 69%. 1H
NMR (400 MHz, CDCl3): δ = 7.9 (s, 1 H), 7.82 (s, 1 H), 7.45 (d,
J = 5.2 Hz, 2 H), 7.24 (s, 1 H), 4.9 (s, 2 H), 4.27 (t, J = 8.0 Hz, 2
H), 2.43 (s, 3 H), 1.73 (t, J = 7.6 Hz, 2 H), 1.47–1.25 (m, 10 H),
0.88 (t, J = 6.8 Hz, 3 H) ppm. 13C NMR (100 MHz, [D6]DMSO):
δ = 196.0, 167.2, 166.7, 159.4, 144.1, 137.3, 134.4, 132.0, 130.3,
129.8, 123.5, 123.2, 120.0, 114.7, 44.9, 43.6, 42.0, 31.1, 28.6, 28.5,
26.9, 26.2, 22.1, 22.0, 19.9, 13.8 ppm. HRMS (ESI): Calcd. for [M
+ H]+ 473.1568; found 473.1874.
Theoretical Calculations: To understand the structure-property re-
lationships in the synthesised quinolinone compounds, DFT calcu-
lations were performed. Gas-phase optimisation of these molecules
were carried out at the B3LYP/6-31g(d) level. B3LYP[27] functional
is found to perform well for most organic molecules, and therefore
was adopted here.[28] All the optimised structures were confirmed
by the vibrational frequency calculations by obtaining no imagi-
nary frequencies. In an effort to rationalise the nature of electronic
transitions, the contributing configurations to the transitions and
charge transfer probability, the ten lowest singlet excited states were
calculated by means of TDDFT at the B3LYP/6-31g(d) level in
THF solvent by using the ground-state optimised geometries. The
Polarisable Continuum Model (PCM) was used to examine solvent
effects.[29] All the calculations were carried out with the
Gaussian 09 suite of program.[30]
(E)-2-Cyano-3-[6-methyl-2-(octyloxy)quinolin-3-yl]acrylic Acid
(OMQC): Yield 70%, 1H NMR (400 MHz, [D6]DMSO): δ = 12.67
(s, 1 H), 8.9–8.88 (d, J = 8 Hz, 1 H), 8.51 (s, 1 H), 8–7.98 (d, J =
8 Hz, 1 H), 7.80 (s, 1 H), 7.75–7.65 (m, 4 H), 4.72 (s, 2 H) ppm.
13C NMR (100 MHz, [D6]DMSO): δ = 162.7, 160.1, 158.1, 146.2,
145.4, 138.2, 137.8, 134.4, 134.0, 128.6, 128.0, 127.7, 127.5, 126.3,
124.1, 123.8, 118.2, 116.8,116.2, 66.2, 31.1, 30.3, 28.6, 28.5, 28.1,
25.4, 22.0, 20.8, 20.6, 20.1 ppm. HRMS (ESI): Calcd. for [M +
H]+ 367.2021; found 367.2171.
3-[6-Methyl-2-(octyloxy)quinolin-3-yl]-2-(4-nitrophenyl)acrylonitrile
1
(OMQN): Yield 65%. H NMR (400 MHz, CDCl3): δ = 8.86 (s, 1
H), 8.35–8.32 (m, 2 H), 8.13 (s, 1 H), 7.89–7.87 (m, 2 H), 7.72 (d,
J = 8.4 Hz, 1 H), 7.62 (s, 1 H), 7.53 (dd, J = 8.8, 2.0 Hz, 1 H), 4.53
(t, J = 6.8 Hz, 2 H), 2.51 (s, 3 H), 1.90–1.83 (m, 2 H), 1.51–1.25
(m, 10 H), 0.87 (t, J = 6.8 Hz, 3 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 158.7, 147.9, 145.9, 140.3, 139.3, 137.2, 134.6, 133.7,
128.9, 127.7, 126.8, 126.7, 124.3, 117.8, 117.0, 110.8, 66.8, 31.8,
29.3, 29.2, 28.8, 26.2, 22.6,21.2, 14.0 ppm. HRMS (ESI): Calcd. for
[M + H]+ 444.2286; found 444.2293.
Supporting Information (see footnote on the first page of this arti-
cle): 1H and 13C NMR spectra, solvatochromatic spectra and their
data, DFT optimized geometries, Frontier Molecular Orbitals and
results of percentage contribution towards FMOs.
Acknowledgments
2-(5-{[6-Methyl-2-(octyloxy)quinolin-3-yl]methylene}-4-oxo-2-thi-
R. R. thanks the Department of Science and Technology (DST)
(reference number SR/S1/PC-12/2011, DT: 20.09.2011) for the pro-
oxothiazolidin-3-yl)acetic Acid (OMQR): Yield 69 %. 1H NMR
764
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 753–766

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Published Online: December 3, 2013
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