A physico-chemical investigation of fluorine-enriched quinolines

Article abstract of DOI:10.1039/c8nj00916c

Quinoline derivatives bearing fluoroalkyl groups at both 2 and 4 positions are scarcely described in the literature. Nevertheless, the addition of fluorine onto the quinoline core might bring about new interesting physico-chemical properties. To confirm t

Full text of DOI:10.1039/c8nj00916c

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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A Physico-chemical Investigation on Fluorine-Enriched Quinolines
Fallia Aribi,a,b Armen Panossian,a,b Denis Jacquemin,c Jean-Pierre Vors,b,d Sergii Pazenok,b,e Frédéric
R. Leroux*a,b and Mourad Elhabiri*a
Received 00th January 20xx,
Accepted 00th January 20xx
Quinoline derivatives bearing fluoroalkyl groups at both 2 and 4 positions are scarcely described in the literature.
Nevertheless, the addition of fluorine onto the quinoline core might bring new interesting physico-chemical properties. To
confirm this hypothesis a homogenous series of 2,4-bis(fluoroalkyl)-substituted quinolines was synthesized under mild
reaction conditions and their physico-chemical properties were thoroughly investigated by various techniques to illustrate
their potential usefulness as new building blocks for applications ranging from organic chemistry to material science.
DOI: 10.1039/x0xx00000x
www.rsc.org/
introduction of fluorinated moieties onto quinoline derivatives
has become an increasingly attractive target in several
Introduction
Since their discovery in 1834 by F. Runge, quinoline cores
have enjoyed a huge success in several fields, notably,
medicinal chemistry[1] like antimalarial (Primaquine®) or anti-
tumoral (Lenvima®) drugs, local anaesthetics (Nupercainal®),
agrochemistry[2] as fungicides (Fortress®) and herbicides
(Pestanal®) as well as in materials science[3] (Figure 1). For
more than 150 years, the synthesis of quinoline derivatives has
been explored and many methods are now available to access
them. The oldest reaction is the so-called Skraup synthesis
developed in 1880,[4] but, several alternative methods have
been developed, e.g., the Doebner-Von Miller reaction,[5] the
Conrad-Limpach synthesis,[6] the Friedländer reaction,[7] the
Pfitzinger reaction,[8] the Combes reaction,[9] and the Meth-
Cohn synthesis.[10] Most of these methods involve simple to
complex aryl-amines or related substrates as starting
materials.[11]
chemical areas.
Other metal-free reactions, combining high yields and short
reaction times, were also proposed.[12] Later, metals like
rhodium, platinum, copper, palladium, iron and others allowed
the formation of quinoline derivatives using catalytic amounts
of metal.[13] While quinolines are valuable scaffolds, fluorine is
an important heteroatom in life science, since it can improve
the physico-chemical properties of a bioactive molecule, e.g.,
by altering the acido-basic properties of proximate groups, by
increasing the lipophilicity of the molecule, or by improving its
stability toward oxidative degradation.[14] Therefore, the
Figure 1. Commercially available quinolines as agrochemicals and
pharmaceuticals.
However, reaching this goal constitutes
a significant
challenge. The introduction of fluorine onto heteroarenes is
usually performed thanks to the Balz-Schiemann reaction, the
Halex process, or by using fluorinating agents,[15] whereas
perfluoroalkyl groups can be introduced by classical
nucleophilic fluorination following a Swarts-type reaction in
presence of HF or SbF5 or using nucleophilic (Ruppert-Prakash
reagent) or electrophilic (Umemoto or Togni reagents)
techniques.[16] However, most of these methods allow the
insertion of only one fluorinated substituent onto quinolines.
In the literature, only a limited number of works reported the
synthesis of quinoline derivatives bearing several fluorinated
substituents. In 2002, Sloop et al.[17] and then in 2012, Baran et
al.[18] performed the synthesis of quinolines bearing fluoroalkyl
substituents at both C2 and C4 positions. Nevertheless, such
compounds are generally obtained in low yields or with a poor
a.Université de Strasbourg, Université de Haute-Alsace, CNRS (ECPM), UMR 7042-
LIMA, 25 Rue Becquerel, 67000 Strasbourg, France. E-mail:
frederic.leroux@unistra.fr and elhabiri@unistra.fr
b.Joint laboratory Unistra-CNRS-Bayer (Chemistry of Organofluorine Compounds),
France.
c. Laboratoire CEISAM - UMR CNRS 6230, CNRS – Université de Nantes, 2 rue de la
Houssinière, 44322 Nantes, Cedex 3, France
d.Bayer S.A.S, 14 Impasse Pierre Baizet, BP99163, 69263 Lyon Cedex 09, France.
e. Bayer AG, Alfred-Nobel-Strass 50, 40789 Monheim, Germany
Electronic Supplementary Information (ESI) available: [Absorption data; emission
data; cyclic voltammetry data; DFT calculations]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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CHF2
N
CHF2
regioselectivity due to the use of unsymmetrical reactants.
Recently, we developed a new, efficient, mild and completely
regioselective method allowing to access, from readily
available starting materials, to scarcely described
polyfunctionalized fluoroalkylated quinolines.[19] The strategy
is based on the use of FARs (Fluoroalkyl Amino Reagents)
(Figure 2).[20]
F
F3CO
Rf1
N
Rf1
2aꢀe
3aꢀe
Rf2
Rf1
CHF2
N
O
NH2
N
Rf1
Rf1
MeO
N
R1
R2
Rf1
DCM, r.t., t, desiccant
R1
R2
1aꢀe
4aꢀf
29ꢀ98%
Rf2
R: Me, Et
Rf2
Rf1
CHF2
N
R1: H, Me, CH2Br, CN, CO2
H
NR2
R2: H, F, OMe, Cl, NMe2, OCF3
Rf1: CHF2, CF3
F
N
BF4
Me2N
R1
R2
Rf1
Rf2: CHF2, CHFCl, CHFCF3
MeCN, 50 °C,
19 h
N
Rf1
R1
R2
5aꢀc
14ꢀ88%
CHF2
N
R1
6aꢀc
CHF2
F
Rf1
Cl
N
Rf1
R1
7aꢀe
Scheme 1. Synthetic routes to fluorinated quinolines and chemical
structures and labelling of the investigated compounds.
In a previous contribution,[19] we reported on the reactivity
of
these
quinoline
derivatives
towards
further
functionalization. In fact, they can be hardly oxidized and
require harsh reaction conditions presumably due to their low
electron-density. To confirm this hypothesis, we performed
electrochemical studies on a homogenous series of quinoline
derivatives to determine their physico-chemical properties,
and we concomitantly carried out spectrophotometric
(absorption and emission) analyses. Indeed, some of these
compounds, and more specifically those substituted by
electron-donating groups, were found to be brightly
fluorescent in solution. Density Functional Theory (DFT) and
Time-Dependent DFT (TD-DFT) calculations allowed assessing
and rationalizing the experimental data. To the best of our
knowledge, similar complementary analyses were not
performed on this type of compounds so far.
Figure 2. Commercially available Fluoroalkyl Amino Reagents (FARs) [Top];
Activation of FARs by Lewis acids converting them into their corresponding
iminium salts [Bottom].
This synthesis consists in a two-step reaction; the first
corresponds to the condensation of anilines onto fluorinated
ketones to provide fluorinated N-aryl acetimines. The latter
intermediate is an analogue of the one formed during the
Meth-Cohn synthesis, which uses the Vilsmeier reagent.[21] In
the second step, the vinamidinium intermediate further
cyclises into quinolines after electrophilic aromatic
Results and Discussion
Spectroscopic properties. In small aromatic molecules
substitution and rearomatizaton following
cyclization process (Figure 3, Scheme 1).
a Combes-like
containing one or two six-membered cycles, three sets of
absorption bands, corresponding to
π-π* transitions, are
usually observed in the near UV region. According to Platt's
theory and notation,[22] these absorptions can be ascribed to
transitions from the 1Ag ground state to 1La, 1Lb and 1Bb
electronic excited states. These
π-π* absorption bands are
generally labelled III (1Ag→1Bb lowest excited state of B3u
symmetry), II (1Ag→1La, lowest excited state of B2u symmetry)
and I (1A→1Lb, lowest excited-state of B3u symmetry), the latter
is forbidden by symmetry and associated with weak
1
absorptivities.[23] The La excited state typically corresponds to
a HOMO→LUMO electronic promoꢀon with large polarizaꢀon
and energy along the short molecular y axis, while the 1Lb
excited state is characterized by a nearly equal admixture of
HOMO-1→LUMO and HOMO→LUMO+1 characters with a
total polarization along the long x axis (Figure 4, Figure 5 and
Figure S1, ESI).[23b, 24] It is noteworthy that the configuration
interaction between HOMO-1→LUMO and HOMO→LUMO+1
is large enough to bring the energy of the 1Lb below
Figure 3. Analogy with the Meth-Cohn and the Combes syntheses.
2 | J. Name., 2012, 00, 1-3
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HOMO→LUMO (1La) transition. Therefore, in naphthalene, the
lowest singlet excited state corresponds to a HOMO-1→LUMO
40
30
20
10
0
III
and HOMO→LUMO+1 combination rather than
HOMO→LUMO transiꢀon (Figure 4 and Figure 5).
a
X4
N
III
III
II
X4
II
I
1La
280 nm
I
CHF2
X4
II
N
CF3
I
310 nm
1Lb
250
300
λ (nm)
350
400
1Ag
Figure 5. Comparison between the electronic absorption spectra of
naphthalene (cyclohexane)[26], quinoline (1,2-dichloroethane) and 1a (1,2-
dichloroethane) measured at 25 °C.
Figure 4. Electronic energy diagrams of naphthalene molecular orbitals and
polarization diagrams for the two lowest absorption bands I and II.
A significant increase of the absorptivity of band III, that is
red shifted, is observed as well. Additionally, no significant
influence of the nature of substitution was observed (CHF2
versus CHFCl versus CF3 versus CHFCF3, Figure S3 and Table S1,
ESI). The fact that the shape of the electronic absorption
spectra did not vary with respect to the fluorine substitution
pattern on either position 2 or 4 suggests that adding a limited
number of this strong -I atom (σm = +0.34),[27] is sufficient to
obtain the largest possible effect.
Quinolines are heteroaromatic nitrogen heterocycles that
are structurally equivalent and π-isoelectronic to naphthalene.
While the absorption spectrum of isoquinoline is similar to
that of naphthalene, that of quinoline is markedly different.
This can be ascribed to the inductive perturbation of the
electronegative α-nitrogen atom (Figure S1, ESI) as well as the
significant change in the relative positions of MO energy
levels.[23a] Quinoline exhibits two strong absorption bands (III
and IV) instead of the 1Bb band of naphthalene, whose maxima
are centered at ca. 226 nm (in C2H4Cl2, Figure 5) and 203 nm,
respectively.[23a] In addition, the first (1Lb) and second (1La)
absorption bands, that are more easily accessible in the
UV−visible window of the spectrum, are poorly separated from
We next examined the effect of fluorine-group substitution
on the benzene-type unit of derivatives 1a and 1d. More in
details, fluorine and trifluoromethoxy groups were introduced
either at the C6 (2a
for F and 3b 3d for OCF3) or C8 (2c for F and 3e for OCF3)
positions, while the pyridyl substitution pattern was conserved
with either CHF2/CF3 1a series) or CHF2/CHF2 1d series).
Regardless of the considered system 1a or 1d), the
, 2d for F and 3a, 3c for OCF3), C7 (2b, 2e
,
1
each other. Transition from the ground state to the La state
usually gives rise to a broad absorption, while transition to the
1Lb state results in a sharp and fine structured absorption
showing a hallmark vibronic progression, Figure S2, ESI).
(
(
(
introduction of a fluorine or trifluoromethoxy unit on either C6
or C7 positions led to a significant hypochromic shift of band III
with a gradual hypsochromic shift (Tables S2-S5 and Figures
Lastly, the absorptivity of the quinoline 1Lb absorption is
significantly increased with respect to that of naphthalene as
the transition moment of 1Lb band is almost parallel to the long
S4-S7, ESI) compared to the unsubstituted
(1a or 1d)
1
axis of the molecule and also due to a contribution of the Bb
structures. In contrast, substitution at the C6 position has only
a weak impact on the position and shapes of bands I (1Lb) and
II (1La), but a more noticeable effect on their absorptivities. In
other words, fluorine-based groups substituted at the C6
state.[25] Finally, the n→π* transitions that can be anticipated
for heteroaromatic nitrogen heterocycles like quinoline are
“forbidden” by symmetry (ε ≤ 100 M-1 cm-1) and are therefore
1
1
usually buried under the intense π-π* transitions.
position weakly influences the two low La and Lb electronic
transitions of the 1a and 1d quinoline chromophore.
Interestingly, substitution at the C7 position has a larger effect
on both the 1La (hypsochromic shift) and 1Lb (bathochromic
shift) transitions contributing to split these two sets of
Substitution of positions C2 and C4 of the quinoline (pyridyl
unit) by fluorinated alkyl groups (e.g. CHF2, CHFCl, CF3 or
CHFCF3) alters the electronic absorption spectrum (Figure S1,
ESI). Indeed, both the 1Lb (band I) and 1La (band II)
π-π*
transitions experience bathochromic displacements of ca. 7
and 15 nm (Figure 5) with respective hypo- and hyperchromic
shifts (Figures S1 and S3, ESI). These effects contribute to
blend even more these two sets of transitions that appeared
closer from one another (Figure 5).
absorptions. These effects can be ascribed to the π-electron
donor (+M) character of F which stabilizes the 1Lb state (i.e.,
bond-centered excess charge density character) but
destabilizes the 1La state (i.e., atom-centered excess charge
density). Substitution of a fluorine atom at C8 stands in an
interesting contrast with a blending of the 1Lb and 1La
transitions suggesting that inductive effects (-I) of the
substituent govern the response in that case.
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For the OCF3 group (σm = +0.38 and σp = +0.35, strong Furthermore, a spectroscopic study of 5-substituted quinolines
electron withdrawer –I and weak
hypsochromic effect of the 1La band can be observed after magnified and controlled by tuning the relative energies of the
π-electron donor +M), a in aqueous solutions showed that their photobasicity can be
substitution at the C6 or C7 positions of either 1a or 1d 1La states of the acid and base forms.[30] The 1La transitions
1
quinolines series, while the Lb absorption remains unaffected. shifted from ca. 370 nm to ca. 457 nm when going from the
Strikingly, substitution at the C8 position has a weak or trifling neutral to the protonated 5-aminoquinoline leading to the
effect.
To study the impact of the nature of the substitution at C6,
highest ꢀpKa separation (pKa*-pKa = 10.6).
Based on these reported investigations, we anticipated that
we selected compound 1a (Scheme 1) as the reference the absorption lying at 406 nm (ε406 = 7.4 x 103 M-1 cm-1) for 5a
scaffold. The electronic absorption spectra of the F (2a), OCF3 (i.e., under deprotonated form in pure 1,2-dichloroethane)
(
3a), OCH3 (4a) and NMe2 (5a) derivatives of 1a are depicted in corresponds to the 1La transition (with an ICT character), while
Figure 6.
those centered at 324 nm (ε324 = 7.24 x 103 M-1 cm-1) and 270
nm (ε270 = 2.8 x 104 M-1 cm-1) correspond to bands I (1Lb) and
III, respectively.
Regardless of the considered solvent (methanol[29] or 1,2-
dichloroethane), the presence of the fluorinated substituents
at C2 and C4 positions induces a further bathochromic shift of
ca. 20 nm of the main absorption bands for 5a compared to
the reference 6-dimethyl-amino-quinoline (vide supra).[29]
Furthermore, despite the absence of mesomerism between N1
and NMe2 in 5a, a large (moderate) red shift of the 1La (1Lb)
absorption is observed, suggesting a ground state electronic
perturbation likely confined to the benzene ring as suggested
by previous 13C NMR for 6-substituted quinolines.[28] To get
further insights into these electronic effects, 13C NMR chemical
4
3
2
1
0
1a (H)
2a (F)
3a (OCF3)
CHF2
N
R
CF3
4a (OCH3)
5a (N(CH3)2)
250 300 350 400 450 500
(nm)
shifts of 1a and its C6-substituted analogues (2a, 3a, 4a and
λ
5a) were recorded in CDCl3 (Table S7 and Figures S30-S34, ESI).
Figure 7 illustrates the variations of the chemical shifts of the
quinoline carbons as a function of the nature of substitution at
the C6 position for the 1a and the quinoline[28] series,
respectively. As a consequence of the sizeable -I effect of the F
Figure 6. Influence on the absorption spectra of the substitution on the C6
position for quinoline derivatives bearing a CF3 in C2 and a CHF2 in C4
(derivative 1a as a scaffold reference). Solvent: 1,2-dichlorethane, T =
25.0(2) °C.
and OCF3 substituents, the 13
C δ of the C6 carbon is deshielded
As discussed above, substitution by fluorinated groups such
as F (2a) or OCF3 (3a) induces minor changes of the absorption
bands of the quinoline (Figure 6 and Table S6, ESI). By contrast,
the OCH3 (4a) and NMe2 (5a) substituents, that display strong
mesomeric effects (σp = -0.27 and σm = 0.12 for OCH3 and σp ~ -
0.83 and σm = -0.15 for NMe2), lead to marked alterations of
the absorption properties of the quinoline. Band III
experienced strong bathochromic and hypochromic shifts,
concomitantly to significant alterations of both the shape and
position of bands I (1Lb) and II (1La). In particular, for the
dimethylamino derivative, a broad and intense absorption
emerges in the UV-visible region, which is typical of a change
from a local excited state to an intramolecular charge-transfer
(ICT) excited state (see TD-DFT calculations, vide infra).
(lower electron density), while the neighbouring carbons (C5
and C7) are concomitantly shielded. The 13C chemical shifts of
the other carbons belonging either to the pyridine subunit or
far from the C6 position, e.g., C8, are unaffected. We note that
fluorinated substitution induces marked effects compared to
substitution with other halogens such as Br (σp = 0.23 and σm
=
0.39) or Cl (σp = 0.23 and σm = 0.37) (Figure 7B).[28] This feature
agrees with our absorption data and suggests the absence of
ICT in the whole aromatic system for these substitutions.
40
2a (F)
3a (OCF3)
30
CHF2
5
8
4
4a (OCH3)
5a (NMe2)
6
R
3
20
10
For 6-substituted quinolines with strong
π electron-donating
7
substituents, 13C NMR studies[28] revealed marked upfield
shifts of the ortho-carbons and minor upfield shifts of the
para-carbon (i.e., with respect to the C6 position) for the
phenyl ring, whereas the changes were negligible for the
pyridyl moiety. On the other hand, the absorption spectrum of
6-dimethylamino-quinoline recorded in methanol,[29] depicted
absorptions at ca. 250, 295, and 380 nm, whereas the
acidification of a methanolic solution (0.01 N HCl), leading to
the protonation of the N-quinoline unit, shifted the main
absorption bands to 270, 300 and 440 nm, respectively.
2
N
CF3
Deshielding
0
ꢀ10
ꢀ20
ꢀ30
Shielding
C2 C3 C4 C5 C6 C7 C8
Carbon position
(A)
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resonance effects (+M) are the key for NMe2. For OCH3,
inductive and mesomeric (+M, σp = -0.27) effects can be
suggested. Consequently, bands I, II and III are significantly
40
30
20
10
0
Br
OCH3
5
8
4
6
R
3
modified with the two
π electron-donating substituents (OCH3
NMe2
Cl
Deshielding
7
2
and NMe2, ICT character), but only slightly tuned by the
fluorinated groups (F and OCF3).
N
By considering the influence of the substitution pattern at
the C7 position (Table S11, Figure S11, ESI) in the 1d series (F:
2e, OCF3: 3d, OCH3: 4e, NMe2: 5c), one observes an obvious
effect of the F and OCF3 substitution with hypsochromic and
ꢀ10
ꢀ20
ꢀ30
Shielding
1
1
bathochromic shifts of the La and Lb transitions, respectively
(vide supra). For NMe2, the 1La band is bathochromically
shifted to 405 nm (ε405 = 3.03 x 103 M-1 cm-1), with also a
significant red shift of band III. By contrast with the 6-
substituted analogue 4d, no absorption in the visible region is
observed for the OCH3 substitution at the C7 position 4e. The
common feature for 4d and 4e derivatives is the strong
bathochromic shifts of their respective 1Lb transitions. 13C NMR
studies (Table S9, Figure S9, Figure S35 and Figures S40-S43,
ESI) revealed the expected strong deshielding of the C7 signal
with concomitant shielding of the C6 and C8 signals, the
shielding of the C6 signal being larger for the NMe2
substitution than for its OCH3 counterpart. For the electron-
donating NMe2 substituent, a significant ICT character can be
suggested by the shielding of the C3 signal borne by the pyridyl
ring.
C2 C3 C4 C5 C6 C7 C8
Carbon position
(B)
Figure 7. Variation of the 13C NMR shifts of the quinoline carbon atoms as a
function of the nature of the substituent on position C6. (A) 1a series, this
work; (B) quinoline; ref. [28]. Solvent: CDCl3; T = 298 K.
For the OCH3 (4a) and NMe2 (5a) derivatives, the same
trends are found with, however, apparent shielding effects of
the 13C signals of the C2 and C4 carbons, suggesting resonance
process along the whole aromatic system and therefore an ICT
character. This is in line with the large bathochromic shifts
1
evidenced for the La transitions of 4a and 5a as well as with
TD-DFT calculations (vide infra). For the C2- and C4-
unsubstituted quinolines (Figure 7B),[28] substitution by a NMe2
group at the C6 position also induce a shielding of the C2
position likely as a result of this resonance effect. The C2 and
C4 substitutions with electron withdrawing fluorinated
functions exacerbate this electronic effect with respect to
quinoline (Figure 7A/B).
Emission properties. To get further insights into the
auxochromic effects, we recorded the emission spectra of the
compounds considered in this work. More precisely, upon
exc
excitation on the
π
-π
* transitions (
λ
exc = 314 – 322 nm, Aλ
≤
0.1), the emission spectra of the C2- and C4-substituted
quinolines were measured (Figure S12, ESI). These compounds
are characterized by very weak emission signals with maxima
ranging from 360 to 425 nm close to that of the parent
quinoline.[31] As already observed above, the introduction of
fluorinated units on the pyridyl ring has a little impact on the
emission properties of the quinoline unit. Furthermore,
The 1d series (CHF2 at C2 and C4 positions) was also
considered and the absorption electronic spectra of the 6-
substituted derivatives with F (2d), OCF3 (3c), OCH3 (4d) and
NMe2 (5b) groups can be found in Table S10 and Figure S10.
Overall, the auxochromic effects discussed for the 1a series
hold in the 1d series. The substitution pattern (CF3 versus
CHF2) on the pyridyl ring (1d versus 1a) does not modify the
spectral variations induced by the substituents at the C6
substitution by fluorine
(1a series, Figure S13, ESI) or
trifluoromethoxy (1d series, Figure S14, ESI) on the phenyl ring
at the C6 and C7 positions has also a weak influence on the
position, shape and intensity of the quinoline-centered
emission. By contrast, substitution at the C8 position by
fluorine drastically increases the intensity of the emission of
the quinoline moiety, and simultaneously shifts it by ca. 20 nm
(Figure S13, ESI). The corresponding compound 2c strongly
fluoresces compared to the "best" quinolines of our series
(Table 1). This indicates that the observed quinoline-centered
emission originates from the S1→S0 radiative deactivation and
is related to the 1La absorption, that is, respects the Kasha's
rule.[32] Substitution at the C8 position with other groups (e.g.,
CH3), has also a clear impact on the quinoline emission profile
(Figures S15 and S16, ESI).
position. The F (2d) and OCF3 (3c) substitutions at the C6
position of the 1d backbone induce only minor alterations on
the absorptions I, II and III (vide infra). Furthermore, the 1La
transitions are bathochromically shifted to 446 nm (ε446 = 934
M-1 cm-1) and 400 nm (ε400 = 6.93 x 103 M-1 cm-1) for 4d (6-
methoxy) and 5b (6-dimethylamino), respectively. For all
series, the C6-OCH3 substitution yields weaker absorptivity for
the absorption lying at low energies compared to the NMe2
substitution. This suggests a process controlled by both
inductive (-I, σm = +0.12) and mesomeric (+M, σp = -0.27)
effects for OCH3, while for NMe2 σm = -0.15, σp = -0.83), the
mesomer donating effect dominates. For the OCH3
substitution, this is further shown by the large red shift of the
1Lb transitions.
The 13C NMR investigation (Table S8, Figure S8 and Figures
S35-S39, ESI) for the 1d series substituted at the C6 position
led to the same conclusions as for 1a (vide supra). Inductive
We next turned our attention to the effect of methoxy and
dimethylamino groups on the emission of the fluorinated
quinolines 1a and 1d. Substitution by OCH3 group (-I and +M
directing group) at the phenyl ring (C6 and C7, 1a and 1d
electronic (-I) effects dominate for
F and OCF3, while
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series) markedly increases the intensity of the emission band Table 1) or monofluorination (e.g., 2c). Figure 8 summarizes in
of the quinoline unit without affecting its position (Table 1, relative terms the effects of the substitution pattern at the C6-
Figures S17 and S18, ESI). By contrast, substitution of the C6 position (Figure S21, ESI for 1a series, C7-position and Figures
and C7 position by NMe2 drastically alters both the intensity S22-S23, ESI for 1d series, C6- and C7-positions).
(significant increase with respect to either 1a or 1d parent
quinolines, Figures S19 and S20, ESI) as well as the position
1.0
(bathochromic shifts of ca. 100 nm with respect to the
1a (H)
2a (F)
methoxy substitution), (Table 1).
0.8
0.6
0.4
0.2
0.0
4a (OCH3)
Table 1. Emission properties in 1,2-dichloroethane of the substituted
5a (N(CH3)2)
quinoline derivatives.a
CHF2
λem (nm)
361
Qrel = Ax/Ar
0.008
nd
Compound
R
Rf1 = CF3
1a
1d
2a
2b
2c
3c
3d
3e
6b
7b
6a
Rf1 = CHF2
360
N
CF3
Rf1 = CF3, 6-F
355
0.010
0.014
0.399
0.013
0.006
0.024
0.032
0.010
0.025
x 10
Rf1 = CF3, 7-F
361
x 10
Rf1 = CF3, 8-F
381
Rf1 = CHF2, 6-OCF3
Rf1 = CHF2, 7-OCF3
Rf1 = CHF2, 8-OCF3
Rf1 = CHF2, 7-F, 8-CH3
Rf1 = CHF2, 7-Cl, 8-CH3
Rf1 = CF3, 7-F, 8-CH3
Rf1 = CF3, 7-Cl, 8-CH3
Rf1 = CF3, 6-OCH3
350 400 450 500 550 600 650 700
(nm)
383
λ
364
381
Figure 8. Influence on the emission spectra of the substitution on the C6
position for quinoline derivatives bearing a CF3 in C2 and a CHF2 in C4
(derivative 1a as a scaffold reference). Solvent: 1,2-dichlorethane, T =
25.0(2) °C. Emission and excitation band widths = 15 and 20 nm; band-pass
filter at 290 nm; 1% attenuator. The emission intensities have been
normalized with respect to the absorbances of the quinoline solutions and
therefore reflect the relative quantum yields. The absorbances at excitation
wavelength are always below 0.1 to prevent inner filter effects.
375
373
381
7a
4a
4b
4c
4d
4e
405
383
390
439
381
385
nd
0.264
0.099
1.000
0.215
0.104
Rf1 = CF3, 7-OCH3
Rf1 = CF3, 8-OCH3
Rf1 = CHF2, 6-OCH3
Rf1 = CHF2, 7-OCH3
Rf1 = CHF2, 8-OCH3
Electrochemical properties. We next describe in this section
the electrochemical properties (Table 2) of the fluorinated
quinoline derivatives (Scheme 1) focusing mainly on the 1d
series to assess the influence of the substitution pattern at the
benzene- and/or pyridyl-type rings. All compounds exhibit two
well-defined cathodic peaks in the potential range from -1.4 V
to -2.4 V with, in most cases, no evidence for anodic peaks
(Figure 9A, Figures S24-S29, ESI). The repetitive scans were
found to be fairly reproducible suggesting the absence of
adsorption processes or degradation products under these
experimental conditions. In aprotic medium, these two well-
defined reduction steps likely correspond to the successive
formation of the monoradical-anion (1st step) and the dianion
(2nd step), respectively. This is in line with literature data based
on related quinolines investigated in aprotic solvents such as
DMF.[33] The marked difference with data in DMF is based on
the fact that the 1st step is a reversible one-electron transfer,
while the 2nd is an irreversible one-electron process. Under our
experimental conditions, the two cathodic signals were
assigned to irreversible one-electron transfers as evidenced by
the absence of a current signal in the reverse scan as well as
the linear dependence of the cathodic potentials as a function
of the scan rate (Figure 9B). Furthermore, the linear
dependence of the peak current (ipc) of the two cathodic
signals of 1d on the square root of the sweep rate (v0.5) (Figure
4f
435
0.926
Rf1 = CF3, 6-NMe2
Rf1 = CHF2, 6-NMe2
Rf1 = CHF2, 7-NMe2
5a
5b
5c
478
474
492
0.620
0.562
0.265
aSolvent: 1,2-dichlorethane, T = 25.0(2) °C. See Scheme 1 for the chemical
structures of the defined quinolines. Qrel stands for relative fluorescent
quantum yield with respect to the most emissive derivative, namely 4c. For
all the compounds: emission and excitation band widths = 15 and 20 nm,
respectively; band-pass filter at 290 nm; 1% attenuator and λexc = 300 nm.
This is consistent with the UV-vis. absorption results:
substituents with strong mesomeric character drastically tune
both the absorption and the emission properties in rather
similar fashions. By contrast, strong inductive electron-
withdrawing groups (F, OCF3) have much less impact on the
spectroscopic properties of the quinolines. The OCH3 occupies
an intermediate spot due to its mesomer (+M) and inductive (-
I) electronic effects. The C6-substitution by OCH3
systematically led to compounds that fluoresce more strongly
with respect to the C7-substituted analogues (Table 1).
Similarly to substitution at the C8 position by F or OCF3, a
significant increase of the emission signal concomitant with a
bathochromic shift of 50-60 nm was observed upon C8-
substitution with a methoxy group. The red shifts that were
measured are much larger than with fluorine or
trifluoromethoxy moieties. As a consequence, the S1→S0
emission of 4c and 4f is shifted to ca. 430-440 nm (Table 1).
We noted, that steric interactions are probably a dominating
factor for the C8-substitution as shown by the marked
alterations of the quinoline emission (i.e., red shifts and
9C) indicates
a diffusion controlled electron process.
Therefore, the substituted quinolines based on 1d participate
to electrode reaction as diffusional species. In aprotic solvents
such as 1,2-dichloroethane, the fluorinated quinolines
reduction thus occurs via two successive one-electron
enhancement) upon methyl substitution (e.g., 6a, 7a, 6b, 7b,
6 | J. Name., 2012, 00, 1-3
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reference electrode = KCl (3 M)/Ag/AgCl; working electrode = glassy carbon
disk of 0.07 cm2 area.
transfers. The monoradical-anion is formed in the Epc1
reduction step, and is then reduced to its related
dihydroquinoline dianion in a second Epc2 step (Figure 9A). The
quinolines considered in this work are not subject to any inter-
or intramolecular proton transfers and therefore the
electrochemical properties are strictly associated to the
successive formation of the two anionic reduced species
(Figure 9). In addition, an important property of the
fluorinated quinolines of the 1d series is a broad potential
range of redox inertness. Indeed, no redox reaction occurs for
potential values ranging from +2.5 V to ~-1.3 V.
A clear relationship (Figure 10) can be observed between the
reduction potentials of the first (Epc1) and second (Epc2
)
electrochemical processes. This suggests that the substituents
borne by the quinoline 1d induce the same electronic effects
on the parent unit and its 1-electron reduced counterpart.
Comparison of cyclic voltammograms of the 1a and 1d
quinolines (Figure S24, ESI) shows that the two-successive
irreversible one-electron transfers are preserved irrespective
of the pyridyl-ring substitution pattern. A significant anodic
shift of the first reduction peak of ca. 120 mV can be observed
on substitution of the C2-CHF2 group by a CF3, while both the
position and shape of the second reduction peak are seemingly
unaffected. This agrees with the stronger inductive electron-
0
50 mV sꢀ1
Epc1
100 mV sꢀ1
150 mV sꢀ1
ꢀ50
200 mV sꢀ1
Epc2
withdrawing effect of CF3 (σm = +0.43, σp = +0.54) compared to
CHF2 ( m = +0.29, p = +0.32) and suggests that the separation
250 mV sꢀ1
CHF2
300 mV sꢀ1
σ
between the two reduction waves are intrinsically dependent
σ
ꢀ100
N
CHF2
on the pyridyl-ring substitution.
CHF2
N
ꢀ150
ꢀ200
ꢀ1.8
ꢀ1.9
1d
CHF2
ꢀ2.0
ꢀ1.5
ꢀ1.0
ꢀ0.5
0.0
Potential (V) vs. KCl(3 M)/Ag/AgCl
(A)
CHF2
ꢀ2.0
ꢀ1.6
ꢀ1.7
ꢀ1.8
ꢀ1.9
ꢀ2.0
ꢀ2.1
R
N
CHF2
Epc1
ꢀ2.1
1d series
ꢀ1.7
ꢀ1.6
ꢀ1.5
Epc1 (V) vs. KCl(3 M)/Ag/AgCl
Epc2
Figure 10. Variation of Epc2 (V, second cathodic step) as a function of Epc1 (V,
first cathodic step). Solvent: 1,2-dichlorethane; T = 25.0(2) °C; I = 0.1 M
(NBu4BF4); v = 200 mV s-1; reference electrode = KCl (3 M)/Ag/AgCl; working
electrode = glassy carbon disk of 0.07 cm2 area; auxiliary electrode = Pt wire.
The dashed line is only provided as a guide for the eyes.
0.0
0.1
0.2
v (V sꢀ1)
0.3
(B)
Table 2. Electrochemical data (1st and 2nd reduction processes) measured
using cyclic voltammetry (CV) for all the substituted quinolines of the 1d
series.a
ꢀ60
ꢀ80
CHF2
N
1d
CHF2
Compound
Rf1 = CF3
Rf1 = CHF2
Epc1ꢀEpc2 (V)
ꢀ100
ꢀ120
ꢀ140
ꢀ160
Epc1 (V)
ꢀ1.52
ꢀ1.64
ꢀ1.56
ꢀ1.52
ꢀ1.48
ꢀ1.45
ꢀ1.67
ꢀ1.66
ꢀ1.74
ꢀ1.68
ꢀ1.60
ꢀ1.54
Epc2 (V)
ꢀ2.00
ꢀ2.01
ꢀ1.92
ꢀ1.87
ꢀ1.83
ꢀ1.79
ꢀ2.00
ꢀ2.03
ꢀ2.11
ꢀ2.00
ꢀ1.94
ꢀ1.88
1a
1d
2d
2e
3c
3d
4d
4e
5b
5c
6b
7b
0.48
0.37
0.36
0.35
0.35
0.34
0.33
0.37
0.37
0.32
0.34
0.34
ipc1
Rf1 = CHF2, 6ꢀF
Rf1 = CHF2, 7ꢀF
Rf1 = CHF2, 6ꢀOCF3
Rf1 = CHF2, 7ꢀOCF3
Rf1 = CHF2, 6ꢀOCH3
Rf1 = CHF2, 7ꢀOCH3
Rf1 = CHF2, 6ꢀNMe2
Rf1 = CHF2, 7ꢀNMe2
Rf1 = CHF2, 7ꢀF, 8ꢀCH3
ipc2
0.2
0.3
0.4
0.5
v
0.5 (V0.5 sꢀ0.5)
(C)
Figure 9. (A) Cyclic voltammograms of 1d (~ mM) as a function of the sweep
rate. (B) Variation of the cathodic potentials as a function of the sweep rate
demonstrating the irreversible character of the two successive one-electron
transfers. (C) Variation of the cathodic current intensities as a function of
the square root of the scan rate indicating diffusion controlled redox
processes. Solvent: 1,2-dichlorethane; T = 25.0(2) °C; I = 0.1 M (NBu4BF4);
Rf1 = CHF2, 7ꢀCl, 8ꢀCH3
a
Solvent: 1,2-dichlorethane; T = 25.0(2) °C; I = 0.1 M (NBu4BF4); v = 200 mV s-1
reference electrode = KCl (3 M)/Ag/AgCl; working electrode = glassy carbon disk
of 0.07 cm2 area; auxiliary electrode = Pt wire. See Scheme 1 for the chemical
;
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be rationalized by the Hammett[27a] σ constants (Figure 11 and
the above discussions).
structures of the defined quinolines.
Let us now discuss the first one-electron process associated
to the Epc1 values. By considering the presence of the fluorine
substitution on the benzene-type ring (Figure S25, ESI), one
clearly observes that the first (and second) reduction peak is
anodically shifted of 80 mV (2d, fluorine at the C6-position)
and 120 mV (2e, fluorine at the C7 position). This indicates that
the fluorine-based quinolines are more prone to reduction
than the parent quinoline 1d. Therefore, the F-substituent
more oxidant
0.2
0.1
0.0
CHF2
(H3C)2
N
with electron-withdrawing capacities (σm = +0.34 and σp
=
N
CHF2
ꢀ0.1
ꢀ0.2
+0.06) decreases the electronic density of the electroactive
quinoline 1d unit and facilitates the reduction of the
corresponding compounds. This effect is even more marked
for F-substitution at the C7 position than at the C6 position as
evidenced by the further ca. 40 mV anodic shift of the first
cathodic signal. These observations are in agreement with the
absorption and emission data and confirm that F substitution
has mainly an electron-withdrawing inductive effect in the
present series. The introduction of the methyl substituent at
the C8-position (6b and 7b) confers to the corresponding
systems less oxidant properties due to the inductive donor
properties of the CH3 group.
less oxidant
ꢀ1.2 ꢀ0.9 ꢀ0.6 ꢀ0.3 0.0 0.3 0.6 0.9
Σ (σm σp
+
)
Figure 11. Plot of ꢀEpc1 (Epc1(sample)- Epc1(1d)) vs. Σ(σm + σp) for the substituted
fluorinated quinolines of the 1d series (black circles: C6-subsituted; blue
circles: C7-susbtituted). Solvent: 1,2-dichloroethane; I = 0.1 M (NBu4BF4); T =
25°C; v = 200 mV s-1; reference electrode = KCl(3 M)/Ag/AgCl; working
electrode = glassy carbon disk of 0.07 cm2 area. σm stands for a meta-
substituted group with respect to the reactive centre and σp designates a
para-substituted function. In this approach, we used the sum (σm + σp) to
describe the electronic effect of a substituent on the electroactive quinoline
core.
On the other hand, it has been reported that the
electrochemical reduction process of aryl halogenides may
lead to the cleavage of the C-halogen bond along the
formation of the monoradical-anion intermediate.[34] For
fluoro-substituted azaaromatics such as quinoline, the stability
of the monoradical-anions was found to be higher than that of
Figure 11 illustrates the variation of
irreversible electron transfer. clear linear relationship
between Epc1 and the nature of the electronic substituent
σm p)) can be observed. This suggests that the quinoline
ꢀEpc1 for the first
A
ꢀ
(
Σ
(
+ σ
comparable
defluorination pathways.[33b,
fluoro-substituted
34]
arenes
preventing
unit is solely influenced by inductive or mesomeric electronic
effects irrespective of the position of the substitution (C6, C7
or C8) on the benzene-type ring. Electron-withdrawing
substituents decrease the electronic density of the quinoline
unit and render these compounds “more oxidant”, while
electron-donating substituents increases the electronic density
and thereby decreases the propensity of the corresponding
redox compounds to be reduced. With the exception of the
dimethylamino substitution, the intercept at the origin is close
to zero (Figure 11) indicating that the electron transfers are
not complicated by coupled chemical reactions such as proton
For 2d
,
2e, 6b or 7b (Figure
S29, ESI), we underline that no dehalogenation was observed.
For the OCF3 substitution, the larger electron-withdrawing
properties (σm = +0.38 and σp = +0.35) compared to F lead to
the most oxidant compounds of the 1d series (Table 2).
Additionally, the CV voltammograms of both 3c and 3d exhibit
a third irreversible reduction peak at ca. -2.2V as well as an
irreversible oxidation peak at +1.32 V and +1.36 V, respectively
(Figure S26, ESI). We tentatively associate those to the
dissociation of the ●CF3 radical leading to a phenolic unit that
can give rise to an oxidation signal. No such behaviour was
observed for OCH3 substitution (Figure S27, ESI).
transfer
or
complexes
formation
(dimerization,
oligomerization…). Deviation from such a linear relationship,
however, occurs for the dimethylamino function substituted at
the C6 or C7 positions. This might result in a limited
conjugation of the dimethylamino group with the quinoline
unit, therefore limiting the cathodic shifts of the reduction
potentials. These data agree with those derived from the
spectrophotometric absorption and 13C studies.
As suggested by the analysis of the optical spectra, the OCH3
substituent can act either as an electron-withdrawing unit or a
mesomer electron-donating group. As a consequence, the
substitution by OCH3 either at the C6 or C7 positions has a
weak effect on the electrochemical properties.
The NMe2 substitution at the C6 or C7 position of 1d led to
the largest cathodic shift of the two successive reduction
peaks (Figure S28, ESI). This agrees very well with the electron-
donating capacities of this group. A cathodic shift of ca. 100
mV (+40 mV) was observed for the C6 (C7) substitution.
TD-DFT calculations. We have finally performed TD-DFT
calculation to obtain complementary insights into the low-lying
excited-states of the synthesized molecules. Experimentally,
very similar optical spectra were obtained in the 1a-1e series
The reactivity of the electroactive monoradical anion
intermediates mainly depends on the quinoline electronic
properties, i.e., the potentials of the first reduction wave can
be related to the electron affinities of the molecule, which can
(Figure S3 in the ESI). For 1a, 1b, 1c, 1d and 1e we compute a
vertical transition wavelength of 322 nm, 328 nm, 324 nm, 312
nm and 318 nm, all values that are close from one another as
well as to experiment (Table S1 in the ESI). These similitudes in
8 | J. Name., 2012, 00, 1-3
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the 1a-1e series are consistent with alike ICT characters.
Indeed, we computed very similar ICT distances (1.83 ± 0.04 Å)
in that series, values that can be viewed as relatively mild. In
Figure 12, we show density difference plots for 1a and 1c. The
excited-state topologies are similar, with an ICT from the
phenyl (mostly in blue) to the pyridyl (mostly in red) rings, the
two-carbon bearing the CF3 and/or CHF2 substituents acting as
acceptors, as expected. Substitution at C6 has a minor impact
on the first band experimentally (e.g., see Table S2 in the ESI),
and we indeed computed for 2a, a vertical excitation at 315
nm, very close to the one of 1a. As can be seen in Figure 12,
Conclusions
Based on the advantageous use of FARs, a homogeneous
series of unreported polyfunctionalized fluoroalkylated
quinolines has been prepared. Their physico-chemical
properties have been investigated by absorption and emission
spectrophotometries, 13C NMR spectroscopy, cyclic
voltammetry and TD-DFT calculations. Regarding their
absorption properties, substitution of positions C2 and C4 of
the quinoline core by fluoroalkylated groups (CHF2, CHFCl, CF3
or CHFCF3) moderately affects both the 1Lb (band I) and 1La
the additional fluorine atom acts as a trifling secondary
π-
donor for the first band, but does not significantly perturb the
topology of the excited-state, confirming that the main impact
of F is -I. At C7 (2b), the fluorine atom induces a small
bathochromic shift (324 nm) compared to 1a, which is
qualitatively in line with experiment, but this does not change
the shape of the excited-state significantly (Figure 12). In any
case, these effects are rather small and have a negligible
impact on the computed ICT distance that remains similar to
the one obtained for 1a. Much larger changes are found when
adding strong donor groups at C6, as TD-DFT returns the
lowest band at 359 nm in 4a and 427 nm in 5a. These large
bathochromic shifts are in line with experiment with maxima
at 340 and 406 nm, respectively (see Figure 6). As can be seen
in Figure 12, when adding a methoxy group, the ICT character
of the state increases, the oxygen playing a significant role; the
ICT distance attaining 2.17 Å according to our calculations. The
use of a dimethylamino donor group has an even stronger
impact on the topology of the excited-state and further
increases the ICT distance up to 2.32 Å.
(band II) π-π* transitions. The excited state topologies of the
lowest state predicted by TD-DFT calculations were also found
to be fairly similar. Due to the presence of fluoroalkylated
substituents, ICT (ICT distances of ca. 1.83 Å) was evidenced
from the phenyl to the pyridyl rings, the C2 and C4 carbons
acting as acceptors Substitution by fluorinated (F or OCF3)
groups at the C6, C7 or C8 positions induced minor changes of
the absorption band of the quinoline reflecting alike ICT
character. The fluorine atom at the C7 was, however, shown to
act as a very weak secondary π-electron donor, but without
significant alteration of the excited state topology. The OCH3
and NMe2 substituents, that display strong mesomeric effects,
stood in interesting contrast and led to marked alterations of
the absorption properties of the quinoline. Band III
experienced strong bathochromic and hypochromic shifts
concomitant to alterations of the shape and position of bands I
and II. For the NMe2 derivative,
a broad and intense
absorption emerges in the visible region. These features are
consistent with an increase of the ICT character of the low
lying excited states (computed ICT distances of 2.17 and 2.32 Å
for OCH3 and NMe2, respectively). 13C NMR data further
support these findings. Indeed, F and OCF3 substitutions
mainly influence the neighbouring carbons, whereas for
derivatives bearing OCH3 and NMe2 substitutions, the shielding
of the C2 and C4 13C signals confirm a resonance process along
the whole aromatic system and thus excited states with
increased ICT character.
The substituents electronic effects were also probed by
spectrofluorimetry. Introduction of fluorinated groups has a
little influence on the emission profile of the quinoline unit
(i.e., very weak emission signals with close maxima). Steric
interaction induced by C8 substitution (F, OCF3 or CH3),
however, drastically increases the emission intensity. As a
consequence, compound 2c with C8-F substitution fluoresces
more strongly than the "best" quinolines of our series.
Substitution by OCH3 also markedly increases the intensity of
the emission band of the quinoline unit, though without
affecting its position, while substitution by NMe2 drastically
alters both the intensity and the position. Substituents with
strong mesomeric character drastically tune both the
absorption and the emission properties in similar fashions.
Groups having predominantly inductive electron-withdrawing
properties (F, OCF3) have much less impact on the
Figure 12. Density difference plots between the lowest excited-state and the
ground-state for selected compounds. The blue (red) regions indicate
decrease (increase) of electronic density upon excitation. Contour threshold:
3 x 10-3 au.
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spectroscopic properties of the quinolines. The OCH3 stands
Emission studies. The fluorescence emission spectra were
occupying an intermediate spot due to its mesomer (+M) and recorded from 300 to 800 nm on a Perkin-Elmer LS-50B
inductive (-I) electronic effects. maintained at 25.0(2) °C by the flow of a Haake FJ thermostat.
With respect to their electrochemical properties, all the The light source was a pulsed xenon flash lamp with a pulse
compounds exhibit two well-defined cathodic responses in the width at half-peak height <10 μs and power equivalent to 20
-1.4 V / -2.4 V range with no evidence for anodic peaks. kW. The slit width was adjusted for both excitation and
Importantly, these fluorinated quinolines were found to be emission to reach the maximum of intensity. The
characterized by redox inertness over a broad potential range spectrofluorimetric signal of the emission were measured at
(from +2.5 V to ca. -1.3 V) in line with the harshness of absorbances <0.1 to minimize reabsorption processes.
reaction conditions used to further chemically functionalize
Electrochemistry. The redox potentials of the fluorinated
quinoline derivatives were measured by cyclic voltammetry
(CV) at 25 °C in 1,2-dichloroethane solvent. Cyclic voltammetry
of the quinolines (~ mM solutions) was carried out using a
Voltalab 50 potentiostat/galvanostat (Radiometer Analytical
MDE15 polarographic stand, PST050 analytical voltammetry
and CTV101 speed control unit) controlled by the Voltamaster
4 electrochemical software. A conventional three-electrode
cell (10 mL) was employed in our experiments with a glassy
carbon disk (GC, s = 0.07 cm2) set into a Teflon rotating tube as
a working electrode, a Pt wire as a counter electrode, and KCl
(3 M)/Ag/AgCl reference electrode (+210 mV vs NHE).[22] Prior
to each measurement, the surface of the GC electrode was
carefully polished with 0.3 µm aluminium oxide suspension
(Escil) on a silicon carbide abrasive sheet of grit 800/2400.
Thereafter, the GC electrode was copiously washed with water
and dried with paper towel and argon. The electrode was
installed into the voltammetry cell along with a platinum wire
counter electrode and the reference. The solutions containing
ca. 10-3 M of the quinoline derivatives were vigorously stirred
and purged with O2-free (Sigma Oxiclear cartridge) argon for
15 minutes before the voltammetry experiment was initiated,
and maintained under an argon atmosphere during the
measurement procedure. The voltammograms (cyclic
voltammograms - CV) were recorded at room temperature
(23(1)°C) in 1,2-dichloroethane with 100 mM tetra-n-
butylammonium tetrafluoroborate (Alfa Aesar, +99%) as
supporting and inert electrolyte.[23a, 35] The voltage sweep rate
was varied from 50 to 300 mV s-1 and several cyclic
voltammograms were recorded from +2.5 V to -2.5 V. Peak
potentials were measured at a scan rate of 200 mV s-1 unless
otherwise indicated. Redox potentials were determined from
oxidation and reduction potentials.
these compounds. The clear relationship found between the
reduction potentials of the first (Epc1) and second (Epc2
)
electrochemical processes suggests that the phenyl
substitution pattern induces the same electronic effects on the
parent unit and its 1-electron reduced counterpart. The
potential Epc1 - Epc2 separation was, however, found to be
intrinsically dependent on the pyridyl-ring substitution pattern
as evidenced by the anodic shift (ca. 120 mV) of only the first
reduction peak when going from a C2-CHF2 to a C2-CF3
substitution. Further substitution with electron-withdrawing
fluorine-containing groups decreases the electronic density of
the electroactive quinoline and facilitates the reduction of the
corresponding compounds. The most oxidant system is indeed
reached with OCF3 substitution. By contrast, NMe2 substitution
leads to the largest cathodic shift of the two successive
reduction peaks in line with the electron-donating capacities of
this group.
The present extensive physico-chemical study allowed getting
deeper insights into the electronic properties of these
polyfunctionalized fluoroalkylated quinolines. By fine-tuning
the substitution of the quinoline pattern (easily feasible by the
developed synthetic methodologies), valuable properties (i.e.,
electrochemical, spectroscopic, inertness/robustness) can be
conferred to this compound, which opens the way to targeted
design in several areas such as fluorescent and robust
quinolines for medicinal chemistry and bio-imaging.
Experimental Section
Solvents and Materials. The fluorine-enriched derivatives 1-7
were synthesized according to literature procedures.[19] The
electrochemical and spectrophotometric (absorption and
emission) properties of the substituted quinoline derivatives
were examined in spectroscopic grade 1,2-dichloroethane
(Carlo Erba, 99.8% for spectroscopy). All the stock solutions
were prepared by weighting solid products using a Mettler
Toledo XA105 Dual Range (0.01/0.1 mg - 41/120 g). The
complete dissolution of the ligands was obtained with the help
of ultrasonic bath (Bandelin Sonorex RK102). All the physico-
chemical measurements were carried out at 25.0(2) °C.
DFT calculations. All calculations have been made with the
Gaussian16 program,[36] with the B3LYP exchange-correlation
functional using the 6-311++G(d,p) atomic basis set for the
ground-state properties and the 6-311++G(2d,p) atomic basis
set for the excited-state properties. We have applied the
ultrafine integration grid for all calculations and used default
convergence thresholds but for a tightened SCF convergence
(at least 10-8 au) and geometry optimization (10-5 au on rms
forces) thresholds. Our calculations have been made in three
steps: i) geometry optimizations of the ground-state structure
without imposing symmetry constraints; ii) frequency
calculation to confirm that the structures are true minima of
the potential energy surface; and iii) TD-DFT calculations of the
Absorption studies. The UV-visible absorption spectra were
recorded from 220 to 800 nm with an Agilent Cary 5000
spectrophotometer maintained at 25.0(2) °C by the flow of a
Cary Varian Dual Cell Peltier accessory. Quartz optical cells
with pathlength of 1 cm were used for these measurements.
10 | J. Name., 2012, 00, 1-3
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ARTICLE
1797-1803; c) Y.-C. Wu, L. Liu, H.-J. Li, D. Wang and Y.-J.
Chen, J. Org. Chem., 2006, 71, 6592-6595; d) S. E. Denmark
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electronic excited-states. For all steps, we used the Polarizable
Continuum Model (PCM) approach to model solvation effects
(here 1,2-dichloroethane) using the corrected linear-response
approach[37] in its non-equilibrium limit for the TD-DFT part.
Intramolecular charge-transfer parameters (distances) have
been computed using Le Bahers's method.[38]
6
M. Conrad and L. Limpach, Ber. Dtsch. Chem. Ges., 1887, 20,
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7
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Conflicts of interest
9
A. Combes, Bull. Soc. Chim. Fr., 1888, 49, 89-92.
10 a) O. Meth-Cohn, B. Narine and B. Tarnowski, J. Chem. Soc.,
Perkin Trans. 1, 1981, 1520-1530; b) O. Meth-Cohn,
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There are no conflicts to declare
11 a) K. T. Finley, in Kirk-Othmer Encyclopedia of Chemical
Technology, John Wiley & Sons, Inc., 2000, p. 32; b) A. L.
Zografos, in Heterocyclic Chemistry in Drug Discovery (Ed.: J.
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Acknowledgements
We thank the Centre National de la Recherche Scientifique
(CNRS) and the University of Strasbourg (UMR 7042-LIMA) and
are very much grateful to Bayer S.A.S. for a grant to F.A.. The
French Fluorine Network (GIS Fluor) is also acknowledged. The
authors are grateful to Matthieu Chessé for fluorescence data
acquisition. This work used computational resources of the
CCIPL Waves cluster installed in Nantes thanks to the support
of the Région des Pays de la Loire.
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DOI: 10.1039/C8NJ00916C
New Journal of Chemistry
Graphical Abstract
A Physico-chemical Investigation on Fluorine-Enriched Quinolines
F. Aribi, A. Panossian, D.
Jacquemin, J.-P. Vors, S.
Pazenok, F. Leroux, M.
Elhabiri
A
homogenous series of 2,4-
bis(fluoroalkyl)-substituted
quinolines was synthesized under
mild reaction conditions and their
physico-chemical (absorption and
emission, electrochemistry, TD-
DFT) properties were thoroughly
investigated.