Synthesis, and the optical and electrochemical properties of a series of push-pull dyes based on the 4-(9-ethyl-9H-carbazol-3-yl)-4-phenylbuta-1,3-dienyl donor

Article abstract of DOI:10.1039/d1nj00275a

A series of twelve dyes based on the 4-(9-ethyl-9H-carbazol-3-yl)-4-phenylbuta-1,3-dienyl donor were prepared with electron acceptors varying in their structures but also in their electron-withdrawing ability. For specificity, a butadienyl spacer was introduced between the donor and the acceptor to both lower the bandgap and furnish dyes with high molar extinction coefficients. The different dyesA-Nwere characterized using various techniques including UV-visible absorption and fluorescence spectroscopy, and cyclic voltammetry. All dyes showed an intense intramolecular charge transfer band located in the visible range. To further investigate the optical properties of the twelve dyes, their solvatochromism was investigated in twenty-three solvents of different natures, enabling linear correlations to be obtained on different polarity scales such as the Taft, Reichardt and Catalan scales. To support the experimental results, the optical properties were compared with those theoretically determined.

Full text of DOI:10.1039/d1nj00275a

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Synthesis, and the optical and electrochemical
properties of a series of push–pull dyes based on
the 4-(9-ethyl-9H-carbazol-3-yl)-4-phenylbuta-
1,3-dienyl donor†
Cite this: New J. Chem., 2021,
45, 5808
a
b
Corentin Pigot, *a Guillaume Noirbent,
Thanh-Tuan Bui,
ˆ
b
c
a
a
Sebastien Peralta,
Sylvain Duval,
Didier Gigmes,
Malek Nechab
and
´
´
a
Frederic Dumur
´ ´
*
A series of twelve dyes based on the 4-(9-ethyl-9H-carbazol-3-yl)-4-phenylbuta-1,3-dienyl donor were
prepared with electron acceptors varying in their structures but also in their electron-withdrawing ability.
For specificity, a butadienyl spacer was introduced between the donor and the acceptor to both lower
the bandgap and furnish dyes with high molar extinction coefficients. The different dyes A–N were
characterized using various techniques including UV-visible absorption and fluorescence spectroscopy,
and cyclic voltammetry. All dyes showed an intense intramolecular charge transfer band located in the
visible range. To further investigate the optical properties of the twelve dyes, their solvatochromism was
investigated in twenty-three solvents of different natures, enabling linear correlations to be obtained on
different polarity scales such as the Taft, Reichardt and Catalan scales. To support the experimental
results, the optical properties were compared with those theoretically determined.
Received 17th January 2021,
Accepted 16th February 2021
DOI: 10.1039/d1nj00275a
rsc.li/njc
complexes.33–36 For the design of organic molecules strongly
absorbing in the visible range and displaying molar extinction
A Introduction
During the past decades, organic dyes have attracted a coefficients on par with those of the transition metal complexes,
great deal of interest from both the academic and industrial push–pull dyes incorporating an electron donor connected to an
communities due to the numerous applications in which these electron acceptor by means of a p-conjugate spacer are the most
structures can be used. Notably, organic dyes can be employed obvious candidates and the most straightforward route to get
as singlet or triplet emitters for organic light-emitting molecules with high molar extinction coefficients.37–39 Indeed,
diodes,1–3 as light-absorbing materials for organic photo- the position of the intramolecular charge transfer (ICT) band can
voltaics (OPVs),4–6 as chromophores for non-linear optical be easily tuned by modifying both the strength of the electron
applications,7–9 as dyes for various biological applications donating and the electron accepting abilities of the two groups
including cancer phototherapy or biological labelling,10,11 as attached at both ends of the p-conjugated spacer.40–43 This
photoinitiators of polymerization,12–20 and as photoredox catalysts strategy is notably extremely useful to red-shift the ICT bands
for organic transformations or hydrogen production.21–24 and thus reduce the HOMO–LUMO gap (where HOMO and
Recently, significant efforts have been devoted to replace LUMO respectively stands for highest occupied molecular orbital
organic dyes by push–pull dyes in photopolymerization because and lowest unoccupied molecular orbital).44–47 Additionally, the
the absorption spectra of push–pull dyes are easy to tune.25–32 molar extinction coefficient can be drastically increased by
For many of the abovementioned applications, the possibility elongating the p-conjugated spacer between the donor and the
to design metal-free dyes is especially attractive, addressing the acceptor as a result of an improvement of the oscillator
toxicity issue often raised by the use of transition metal strength.47–49 However, elongation of the p-conjugation in these
structures is not an easy task, as exemplified with the synthesis
a Aix Marseille Univ., CNRS, ICR UMR7273, F-13397 Marseille, France.
of 3-(9-alkyl-9H-carbazol-3-yl)acrylaldehydes. Indeed, for this
aldehyde, only three syntheses are reported in the
literature.50–54 In fact, carbazole is a remarkable electron donor
exhibiting a relatively low oxidation potential, and good thermal
stability and this polyaromatic structure can also be easily
chemically modified.53,54 Carbazole is also a cheap compound
E-mail: pigotcorentin2@gmail.com, frederic.dumur@univ-amu.fr
b
´
CY Cergy Paris Universite, LPPI, F-95000 Cergy, France
c
´
Universite de Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois,
´
UMR 8181 – UCCS – Unite de Catalyse et Chimie du Solide, F-59000 Lille, France
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00275a
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Scheme 1 Synthetic
route
to
3-(9-alkyl-9H-carbazol-3-
yl)acrylaldehydes Cbz-1–Cbz-3.
and so this structure was extensively used in Organic Electronics,
with applications ranging from fluorescence emitters to host
materials for OLEDs.55–58
While coming back to the synthesis of 3-(9-alkyl-9H-
carbazol-3-yl)acrylaldehydes, the three procedures reported in
the literature are based on metal-catalysed syntheses, namely
the palladium-catalysed formylation of an alkenylzinc inter-
mediate using S-(4-nitrophenyl)thioformate to introduce the
aldehyde group for the synthesis of Cbz-1,59 the hydrozirconation
homologation method of Maeta and Suzuki for the synthesis of
Cbz-260 or the Pd-catalysed coupling reaction of acrolein diethyl
acetal on a brominated carbazole for the synthesis of Cbz-361–63
(see Scheme 1). If large scale syntheses are required, these
reactions are not adapted, requiring the use of multistep syntheses
taking recourse to expensive catalysts, dry solvents and imposing
a controlled atmosphere to proceed. Simpler synthetic methods
are thus actively being researched. Recently, several studies
Fig. 1 Chemical structures of dyes A–N.
were devoted to elaborate acrolein derivatives starting from donor by preparing first a carbazole–benzophenone adduct
benzophenone derivatives. This strategy was notably successfully later converted as an acrylaldehyde is unprecedented. To the
applied to the synthesis of Michler’s aldehyde starting from best of our knowledge, no such 3,3-diphenylacrylaldehyde
Michler’s ketone.64,65 More recently, new benzophenone deriva- derivative has been previously reported in the literature. However,
tives were also converted to their aldehyde analogues,66–71 the conversion of a carbazole–benzophenone adduct as an alkene
furnishing dyes exhibiting a strong positive solvatochromism.72,73 has recently been reported in the literature.74 The different dyes
In light of these results, a hybrid carbazole-based benzophe- were seen by a strong absorption extending over the visible range.
none Cbz-BP in which the electron donating ability is reinforced These dyes were characterized using various techniques including
by the presence of the carbazole moiety has been converted to its UV-visible absorption and fluorescence spectroscopy and cyclic
aldehyde analogue Cbz-3. The approach used to access this voltammetry. Theoretical calculations were also carried out to get
extended electron donor is unprecedented for carbazole derivatives. a deeper insight into the electronic transitions involved in the
This aldehyde part allowed an affordable synthesis of 12 dyes A–N optical transitions. Finally, solvatochromism of the 12 dyes
differing by the electron acceptors and containing two isolated was examined on several solvent polarities scales and linear
isomers: D/I (see Fig. 1). It must be noticed that this strategy correlations could be obtained using the Kamlet–Taft, Reichardt
consisting in generating an extended carbazole-based electron and Catalan empirical models.
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B Results and discussion
B1. Synthesis of the dyes
The extended aldehyde Cbz-BP has been synthesized in three
steps starting from 9-ethyl-9H-carbazole. By performing a Friedel–
Crafts reaction with benzoyl chloride in the presence of aluminum
chloride, (9-ethyl-9H-carbazol-3-yl)(phenyl)methanone could be
prepared in 84% yield. Addition of methylmagnesium iodide
followed by
a dehydration reaction furnished 9-ethyl-3-(1-
phenylvinyl)-9H-carbazole in almost quantitative yield (see
Scheme 2). Formylation of the alkene by a Vilsmeier–Haack
reaction gave Cbz-BP as a mixture of s-cis/s-trans isomers that
could not be separated via column chromatography. All attempts
of iodine-catalyzed thermal cis/trans isomerization in toluene did
not allow the modification of the ratio between isomers.75 If no
structural determination was carried out to identify which isomer
was the main product, a 2 : 1 ratio could be found on the proton
NMR spectrum of Cbz-BP. Interestingly, modification of the
reaction time during the Vilsmeier–Haack reaction did not modify
the ratio between isomers. Consequently, this mixture of isomers
was subsequently used for the synthesis of the different dyes.
Besides, in order to investigate the potential influence of the
isomerization onto the optical properties, one of the two isomers
could be obtained in almost pure form but in small quantity by
crystallisation at ꢀ20 1C in a mixture of THF/pentane solvents.
Finally, eleven of the twelve dyes of the series were prepared
via a Knoevenagel reaction carried out under basic conditions.
Except for dyes B and E for which diisopropylamine (DIPA) was
used instead of piperidine, all dyes could be obtained with
reaction yields ranging from 78 to 92% (see Scheme 3). In the
case of dyes B and E, the choice of DIPA as the base was
motivated by recent results reported in the literature mentioning
a nucleophilic attack of secondary amines on the cyano groups
of EA5, inducing a cyclization reaction and producing azafluo-
renone derivatives.76–80 However, piperidine could be used for
the synthesis of dye M, no nucleophilic attack of amine being
reported at present in the literature for EA11. Finally, dye F was
prepared by using a specific procedure. Indeed, due to the
Scheme 3 Synthetic route to dyes A–N.
remarkable stability of the EA3 anion in basic conditions, no
condensation reaction can occur. To circumvent this problem,
the condensation of Cbz-BP and EA3 in acetic anhydride
furnished dye F in 85% yield.
In the case of symmetric electron acceptors (EA1, EA3, EA4,
EA6, EA7, EA8 and EA12), the corresponding dyes (A, C, F–J and
N) were obtained as a mixture of s-cis and s-trans isomers.
Conversely, a more complex situation was found for dyes
prepared with EA2, EA5, EA9, EA10 and EA11, since in the
complement of a s-cis/s-trans mixture, the different electron
acceptors could adopt two different orientations so that a
mixture of 4 isomers could be theoretically obtained in these
cases (see Fig. 2). Besides, the steric hindrance induced by the
use of asymmetric electron acceptors such as EA2 and EA5
certainly favour an orientation over the other. It has to be
noticed that among the twelve electron acceptors issued in this
study, EA2,81 EA3,82 EA4,83 EA5,76 EA1184 and EA1285 had to be
prepared.
Interestingly, two isomers (D and I) could be isolated in pure
form. Indeed, due to the marked insolubility of one of the
isomers of dyes I, these could be easily separated from its
mixture via precipitation. Conversely, in the case of dye D
(which is a pure isomer of C), this latter was obtained by
opposing the isolated isomer of Cbz-BP to EA1 under basic
conditions. Finally, due to the exceptional electron-
withdrawing ability of EA12, a spontaneous deprotonation
can occur in highly polar solvents such as DMF. Therefore, N
could be prepared at room temperature in DMF, without using
a base, and obtained as a precipitate upon addition of pentane
Scheme 2 Synthetic route to Cbz-BP.
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Fig. 2 The four possible isomers when asymmetric electron acceptors
are used.
to the reaction media. Dye N was isolated in the pure form by
filtration, in 87% yield.
B2. Optical properties
All dyes showed good solubility in chloroform so that a comparison
of their optical properties could be established. A summary of the
optical properties is provided in Table 1 and Fig. 3 and 4. As shown
in the Fig. 3, a severe variation of the absorption maxima with the
electron-withdrawing ability of dyes A–N could be detected. Thus,
absorption maxima ranging from 475 nm (K) to 587 nm (N) and
610 nm (B) were found. Position of the ICT bands undergo a
redshift following the order of the electron acceptor strength.
Notably, comparison of the absorption maxima between A and C
revealed the elongation of the indane-1,3-dione-based acceptor EA4
to red-shift the absorption of A by ca. 30 nm compared to that
determined for C comprising EA1 as the acceptor. Interestingly, a
superimposition of the absorption spectra of A and J could be
Fig. 3 Normalized UV-visible absorption spectra of dyes A–N in
chloroform.
clearly evidenced, demonstrating that the electron-withdrawing
abilities of 1H-cyclopenta[b]naphthalene-1,3(2H)-dione EA4 and
1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione EA8 were
Table 1 Optical characteristics of the different compounds in chloroform with the values theoretically determined in CH2Cl2. HOMO–LUMO gaps (DE)
are also shown in this table
Compounds
A
B
C
D
E
F
G
lexp (nm)
ltheo (nm)
525
495
610
559
495
475
495
—
575
542
583
554
479
470
eexp (Mꢀ1 cmꢀ1
)
42 600
81 300
57 750
62 600
36 400
66 000
35 900
—
40 300
63 300
37 700
93 000
35 400
46 700
etheo (Mꢀ1 cmꢀ1
)
DEexp (eV)
DEtheo (eV)
2.36
2.51
2.03
2.22
2.50
2.61
2.50
—
2.15
2.29
2.12
2.22
2.59
2.64
Compounds
H
I
J
K
L
M
N
lexp (nm)
ltheo (nm)
490
477
475
—
526
506
467
487
467
487
558
550
501, 587
517, 656
eexp (Mꢀ1 cmꢀ1
)
45 100
54 400
43 200
—
61 600
68 600
49 600
72 600
47 300
72 700
55 100
96 200
51 300, 56 000
70 800, 8900
etheo (Mꢀ1 cmꢀ1
)
DEexp (eV)
DEtheo (eV)
2.53
2.60
2.61
—
2.36
2.22
2.65
2.55
2.65
2.55
2.22
2.25
2.11
1.89
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comparable, because the two dyes possessed the same
electron donor.
Among the different electron acceptors, the most interesting
one is undoubtedly EA12, which enables the design of a dye (N)
exhibiting two ICT bands located at 510 and 587 nm respectively.
For comparison, EA5 which is the strongest electron acceptor of
the series could only furnish a dye (B) with an absorption
maximum at 610 nm, and therefore only shifted by about
20 nm compared to dye N. Based on previous reports devoted
for tetranitrofluorene-based dyes, the presence of the two intense
ICT bands detected in the UV-visible absorption spectrum of
N can be assigned to a significant intramolecular charge transfer
occurring in the ground state.86 In particular, due to the
presence of four electron withdrawing nitro groups onto TNF,
numerous mesomeric forms can be written, differing by the
length of the p-conjugated spacer between the nitro and the
amino groups (see Fig. 4).
Fig. 5 Comparison of the UV-visible absorption spectra of C/D and H/I in
chloroform.
As anticipated, the highest molar extinction coefficients
could be found for the four dyes bearing the strongest electron
acceptors (EA5, EA8, EA11 and EA12), namely dyes B, J, M and
N. By increasing the electron acceptor strength, the oscillator
strength is logically increased, enhancing the molar extinction
coefficients (see Fig. S1, ESI†).
logically observed. Considering that severe variations of the
absorption maxima were found by varying the electron-
withdrawing groups, the experimental behavior observed for
dyes A–N was rationalized by theoretical calculations.
B3. Theoretical calculations
A comparison with the simulated UV-visible absorption spectra
in dichloromethane revealed the calculations to give absorption
maxima to be in perfect agreement with the experimental one
(see Fig. S1 and S2, ESI†). Finally, considering that 2 isomers
(D and I) could be separated from their respective mixtures (C and
H), a comparison between the absorption spectra of the isolated
isomer and the mixture could be established (see Fig. 5).
Interestingly, if the absorption spectrum of C perfectly super-
imposed that of D, a completely different behavior was found for
H and I, the ICT band of I being blue-shifted by 14 nm (476 nm vs.
490 nm for H). Considering that the isomer I is less conjugated
than its analogue, on the basis of the UV-visible absorption spectra,
it can be confidently proposed that I is the s-cis isomer of H.
Additionally, in this configuration, the s-cis configuration
imposes an internal torsion inside the molecules due to the
steric hindrance generated by the donor and the acceptor so
that a reduction of the conjugation between the two partners is
The energy levels of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
were determined by theoretical calculations and a representative
set of the electronic distribution is provided in Fig. 6. As
evidenced in Fig. 6, a HOMO energy level specifically localized
on the carbazole group and a LUMO energy level comprising
both the acceptor and the lateral phenyl ring could be found for
all dyes. While examining the energy levels of both the HOMOs
and LUMOs of all dyes, almost no variation of the HOMO level
was found, consistent with a localization of the HOMO onto the
carbazole moiety (see Table 2). Conversely, major differences
could be found for the positions of the LUMO levels, resulting
from their localization onto the acceptor moieties. As a result of
this, the variation of the HOMO–LUMO gap only originates from
the variation of the positions of the LUMO levels. More precisely,
by improving the electron-accepting ability, the LUMO level is
stabilized, decreasing the HOMO–LUMO gap.
B4. Solvatochromism
Push–pull dyes are compounds that are highly sensitive to their
environment (polarity, and polarizability of the solvents) and
position of the ICT bands can drastically vary with the solvent
effects.87,88 Generally, for push–pull dyes, both HOMO and
LUMO energy levels are precisely localized onto two different
parts of the molecule. In the present case, a positive solvato-
chromism is typically observed, evidencing an important
charge redistribution upon photoexcitation and a dipole
moment larger in the excited state than in the ground state.
This behavior is typically found for push–pull dyes exhibiting a
relatively small dipole moment in the ground state, which is
achievable by separating the donor from the acceptor by an
elongated spacer.89–93 To investigate the solvatochromic
Fig. 4 Different mesomeric forms supporting the presence of two ICT
bands for the TNF-based dye N.
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Fig. 6 Optimized geometries and HOMO/LUMO electronic distributions of dyes A, C and G.
Table 2 Theoretical positions of the HOMO and LUMO energy levels
the charge transfer detected for all dyes was verified by realizing
successive dilutions and linear plots could be obtained by
measuring the absorbance vs. the dye concentration. This point
being verified, the resulting data were evaluated by means of
the different abovementioned empirical models. Interestingly,
remarkable correlations were obtained on the Kamlet–Taft and
the Catalan scales as well as, to a lesser extent, on the Reichardt
scale. In particular, for the Catalan, several parameters have
been developed such as the solvent polarizability (SP), the
solvent dipolarity (SdP) and the solvent polarity/polarizability
(SPP) parameters.103–105 In this work, the best correlations were
Compounds
A
B
C/D
E
F
G
HOMO (eV)
LUMO (eV)
ꢀ5.55
ꢀ2.72
ꢀ5.48
ꢀ3.17
ꢀ5.56
ꢀ2.64
ꢀ5.71
ꢀ3.06
ꢀ5.83
ꢀ3.27
ꢀ5.69
ꢀ2.78
Compounds
H/I
J
K
L
M
N
HOMO (eV)
LUMO (eV)
ꢀ5.64
ꢀ2.79
ꢀ5.71
ꢀ2.99
ꢀ5.66
ꢀ2.72
ꢀ5.67
ꢀ2.72
ꢀ5.95
ꢀ3.33
ꢀ5.90
ꢀ3.91
properties of dyes, numerous solvent polarity scales have been obtained with the Catalan SPP parameters taking into account
developed over the years and the Kamlet–Taft’s,94 Dimroth– the polarity/polarizability of the solvents. This is also this
Reichardt’s,95 Lippert–Mataga’s,96 Catalan’s,97,98 Kawski– interaction, which is considered by the two other solvatochromic
Chamma–Viallet’s,99 McRae’s,100 Suppan’s,101 and Bakhshiev’s102 scales, namely the Kamlet–Taft and the Reichardt models.
scales can be cited as the most popular ones. Investigations of Therefore, the solvent–solute interactions are mainly governed
the solvatochromic properties of dyes A–N were carried out in by the polarity/polarizability of the solvents, which was con-
23 solvents of different polarities and changes in the longest- firmed by three polarity scales based on these interactions.
wavelength absorption maxima lmax for the 14 dyes are Among all dyes, the best correlations were obtained for dyes B
summarized in Table 3. First, the intramolecular nature of or K, as shown in the Fig. 7.
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Table 3 Summary of the optical properties of compounds A–H in solvents of various natures
Compounds
Aa
Ba
Ca
Da
Ea
Fa
Ga
Ha
Ia
Ja
Ka
La
Ma
Na
Acetone
Acetonitrile
AcOEt
Anisole
Butanol
Chloroform
Cyclohexane
1,2-Dichloroethane
Dichloromethane
Diethyl carbonate
Diethyl ether
Diglyme
1,4-Dioxane
Dimethylacetamide
Dimethylformamide
DMSO
Ethanol
Heptane
Nitrobenzene
THF
Toluene
Triethylamine
p-Xylene
510
508
502
516
525
526
488
515
517
499
496
508
503
516
517
524
522
485
528
507
509
—
585
579
581
591
604
610
563
592
597
575
572
588
573
595
596
598
596
560
617
583
581
ndb
572
456
489
479
491
498
495
466
488
489
474
472
483
477
488
489
493
492
464
500
482
483
470
479
456
476
477
489
497
495
466
488
489
475
469
483
477
491
488
495
491
464
502
478
483
470
479
553
550
552
562
569
542
564
564
548
547
548
559
548
564
560
565
563
539
579
556
551
ndb
553
546
545
551
573
ndb
583
543
576
577
546
547
574
546
577
576
580
ndb
539
587
563
554
ndb
550
456
461
457
471
475
478
446
468
471
456
451
463
458
470
469
474
475
443
479
460
464
455
462
466
466
465
474
486
489
459
479
480
462
462
471
467
472
472
476
480
457
488
467
471
467
468
462
463
458
466
474
475
450
470
469
459
455
466
459
471
468
471
466
446
481
460
465
455
464
460
504
499
508
521
524
488
516
514
496
497
511
501
518
512
516
517
485
528
503
506
ndb
504
456
455
455
464
461
466
445
462
463
452
453
461
454
466
463
468
458
442
473
458
459
450
458
457
456
455
464
462
468
445
465
464
452
451
461
456
466
465
468
459
442
473
458
460
450
459
534
526
525
549
550
559
520
551
553
521
528
545
525
542
539
541
548
514
568
533
527
522
528
562
561
561
582
568
588
ndb
586
586
553
ndb
569
559
568
577
581
ndb
ndb
562
567
567
ndb
569
508
a
b
Position of the ICT bands are given in nm. nd: not determined.
For all dyes, negative slope lines were found for the linear and N differ from the others for which similar slopes were found
regressions, irrespective of the polarity scales, indicative of for both the Kamlet–Taft or the Catalan plots. Comparison
positive solvatochromism. Considering that the absolute value between H and I is particularly interesting since I is a pure
of the slopes can provide information of the sensitivity of the isomer isolated from H. Besides, a completely different behavior
dyes to the solvent polarity and thus of the charge redistribution with regard to the solvent polarity was found, H exhibiting a
upon excitation, a summary of the different negative slope lines lower sensitivity than I. These results confirm that I is certainly
is provided in Fig. 8. Interestingly, in the series of 12 dyes, three the s-cis isomer, and that the internal torsion induced by the
different behaviors could be detected. Thus, only dyes E, F, H steric hindrance between the donor and the acceptor are
Fig. 7 Scatterplots with overlaid linear regression plots for dye B with (a) the Kamlet–Taft graph (b) the Reichardt graph (c) the Catalan graph and for dye
K (d) the Catalan graph.
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Fig. 8 Variation of the positions of the charge transfer band with the Kamlet–Taft and Catalan empirical parameters for dyes A–N.
drastically modified by the polarity of the solvents. Conversely, C each dye, energy of the first singlet excited state (ES1) could be
and its pure isomer D show similar slopes in both plots. Based determined. All data are summarized in Table 4.
on the NMR experiments, similarity of behaviors between C and
Carbazoles are known to be fluorescent compounds which
D arises from the fact that D is mostly composed of C, resulting explains why the different dyes were emissive.106–108 Only dye N
in a minor contribution of the second isomer on the optical comprising a TNF moiety as the acceptor was not emissive.
properties. A lower sensitivity to the solvent polarity is also found The lack of photoluminescence of compound N can be
for dyes E and N what is counter-intuitive considering that these attributed to the presence of the nitro groups, well-known to
two dyes possess strong electron acceptors. Finally, the most efficiently quench the photoluminescence of luminescent
negative slopes were determined for dye F bearing EA3 as the compounds.109,110 For all dyes, Stokes shift ranging from
acceptor. This pronounced sensitivity to the solvent polarity can 50 nm for compound B to 97 nm for compound M could be
be assigned to the presence of the numerous cyano groups on determined. The largest Stokes shift was obtained for dye M
EA3, favoring a major redistribution of the electronic density comprising the 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-
upon excitation when highly polar solvents are used.
2,5-dihydrofuran (TCF) group as the acceptor.111,112 These large
Stokes shifts are indicative of a significant electronic redistribution
between the ground state and the excited state.113 Additionally, the
fluorescence of the indane compounds (comprising EA1–EA5 as
the electron acceptors) seems to be highly dependent of the
number of the cyano functions. Indeed, the Stokes shift of F was
only 61 nm while a Stokes shift of 68 nm and 78/77 nm could be
respectively determined for compounds E and C/D. In fact, a
reduction of the Stokes shift with a redshift of the absorption
maxima was observed, consistent with a decrease of the gap
between the occupied and unoccupied orbitals. The same trend
was observed while comparing the Stokes shifts between B and E.
Indeed, due to the improved electron-withdrawing ability of EA2
compared to EA5, a reduction of the Stokes shift was detected,
going from E (68 nm) to B (50 nm).
B5. Photoluminescence spectroscopy
The emission properties of all dyes were examined in chloroform
as the solvent. Since almost all compounds were fluorescent, and
were therefore not sensitive to oxygen, all experiments were
carried out in air. Most of the dyes were photoluminescent so
that a Stokes shift could be determined for most of the dyes.
In addition, as shown in Fig. 9, by determining the crossing
point between the UV-visible and the fluorescence spectra of
B6. Electrochemical properties
All push–pull dyes were investigated by cyclic voltammetry (CV) in
deaerated acetonitrile solutions. The selected set of voltammograms
is shown in Fig. 9 and all CV curves are given in the ESI.† The redox
potentials of all compounds are summarized in the Table 5 in
which the redox potentials are given against the half wave oxidation
potential of the ferrocene/ferrocenium cation couple.
All the synthesized compounds exhibited irreversible single-
electron oxidation and single-electron reduction processes.
Concerning the oxidation, all the push–pull molecules were
Fig. 9 Normalized absorption and emission intercept curves of dye D in
chloroform.
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Table 4 Fluorescence properties of the different compounds recorded in chloroform
A
B
C
D
E
F
G
lex (nm)
526
612
116 279
2.13
610
660
200 000
1.92
495
573
128 205
2.28
495
572
129 870
2.28
575
643
147 059
2.01
583
644
163 934
2.01
479
562
120 482
2.34
lem (nm)
Stokes shift (cmꢀ1
ES1 (eV)
)
H
I
J
K
L
M
N
lex (nm)
lem (nm)
490
570
475
570
526
600
467
555
467
550
558
655
587
—
Stokes shift (nm)
ES1 (eV)
125 000
2.30
105 263
2.31
135 135
2.18
113 636
2.41
120 482
2.43
103 093
1.99
—
designed from the same donor moiety, namely the Cbz-BP,
which explains that comparable oxidation potential are
detected for all dyes, the oxidation of the carbazole group being
detected around 750 mV per Fc per Fc+. An oxidation process
lowered by ca. 100 mV was also observed for the compounds
L and M and reduction of the oxidation potential can be
explained by an oxidation process occurring on the amino
group of the rhodanine acceptors.
Regarding the reduction process, most of the molecules are
affected by a monoelectronic reduction located on the central
double bound, slightly impacted by the acceptor moiety.
Indeed, the stronger the electron withdrawing group, the
stronger the cathodic shift. A relevant example of this can be
obtained by comparing the reduction potentials of M and I
(See Fig. 10 and Table 5). In this case, EA11 is more accepting
than EA7, explaining the 200 mV cathodic shift occurring
during the reduction of the double bound.
Fig. 10 Cyclic voltammograms of I (up) and M (down) in acetonitrile
solutions (10ꢀ3 M) using tetrabutylammonium perchlorate (0.1 M) as the
supporting electrolyte. Scan rate: 100 mV sꢀ1
.
nitro groups prior to the central double bond. Reduction of the
nitro groups is frequently observed in the case of
poly(nitrofluorene) structures, explaining the difference of
200–500 mV observed between the reduction peak of this
compound and that of the other dyes.114,115
Nonetheless, a reduction at a smaller cathodic potential was
observed for N, presumably resulting from the reduction of the
Finally, by using the ferrocene (Fc) ionization potential value
as a standard and the equation established by Pommerehne
et al., energy levels of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
could be estimated from the redox potentials (see Table 5).116
Table 5 Outline of the electrochemical properties of dyes A–N compared
with DFT calculations. Eox, correspond to all the oxidation potentials and Ered
the reduction potentials. DEel corresponds to the difference between the
HOMO and the LUMO levels, electrochemically obtained. Finally, DEth is
the difference between the HOMO and LUMO levels, theoretically obtained.
Fc/Fc+ was used as internal reference standard
a
b
Eox onset
Ered onset
EHOMO ELUMO DEel
(eV) (eV)
DEth
(eV)
Compounds (V per Fc) (V per Fc) (eV)
A
B
C
D
E
F
G
H
I
J
K
L
M
N
0.71
0.76
0.74
0.74
0.72
0.74
0.77
0.75
0.77
0.75
0.63
0.63
0.67
0.69
ꢀ1.13
ꢀ1.00
ꢀ1.06
ꢀ1.14
ꢀ1.12
ꢀ0.95
ꢀ1.18
ꢀ1.14
ꢀ1.18
ꢀ1.07
ꢀ1.27
ꢀ1.29
ꢀ0.94
ꢀ1.18
ꢀ5.51 ꢀ3.67 1.84
ꢀ5.56 ꢀ3.80 1.76
ꢀ5.54 ꢀ3.74 1.80
ꢀ5.54 ꢀ3.66 1.88
ꢀ5.52 ꢀ3.68 1.84
ꢀ5.54 ꢀ3.85 1.69
ꢀ5.57 ꢀ3.62 1.95
ꢀ5.55 ꢀ3.67 1.93
ꢀ5.57 ꢀ3.62 1.95
ꢀ5.55 ꢀ3.73 1.82
ꢀ5.44 ꢀ3.53 1.91
ꢀ5.44 ꢀ3.51 1.93
ꢀ5.47 ꢀ3.86 1.61
ꢀ5.49 ꢀ3.62 1.87
2.83
2.31
2.92
2.92
2.65
2.56
2.91
2.85
2.85
2.72
2.94
2.95
2.62
1.99
a
Onset of the oxidation and reduction potentials versus ferrocene
(E(onset) vs. Fc). EHOMO and ELUMO were determined from the formulae:
EHOMO = ꢀ4.8 ꢀ Eox vs. Fc and ELUMO = ꢀ4.8 ꢀ Ered vs. Fc.113 b EHOMO
and ELUMO are given in eV.
Fig. 11 Comparison between frontier orbital energy levels obtained from
cyclic voltammetry and DFT calculations.
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2 S. Redon, G. Eucat, M. Ipuy, E. Jeanneau, I. Gautier-
Thanks to the determination of the HOMO and LUMO energy
levels by electrochemistry, a comparison could be made
between the experimental and the theoretical values of the
two energy levels (see Fig. 11). If a clear difference can be
noticed between the LUMODFT and the LUMOel, conversely, a
good adequation between the theoretical and the experimental
energy level of the HOMO orbital was found, as shown in the
Fig. 11. Even if the position of the LUMO energy levels was
overestimated by theoretical calculations, similar variations
were found between the LUMODFT and the LUMOel. This
difference can be attributed to the fact that influence of the
solvent on the LUMO energy level is not fully considered in the
calculations.
`
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C Conclusions
To conclude, a series of 12 push–pull dyes differing by the
electron-acceptors have been designed and synthesized.
Interestingly, the carbazole-based electron donor reported in
this work has been synthesized for the first time according to a
multistep synthesis. All dyes prepared using this electron donor
proved to be soluble in most of the common organic solvents,
even if electron acceptors such as (thio)barbituric acids and
tetranitrofluorene derivatives well known to produce low soluble
dyes have been used. By modifying the electron acceptors, a set
of dyes absorbing over the whole visible range have been
obtained. For all dyes, a positive solvatochromism could be
determined, evidencing that the excited state of the dye is
more polar than the ground state. If promising results have
been obtained with this electron donor, the asymmetry of this
elongated aldehyde is at the origin of the presence of numerous
isomers. As a result of this, complicated NMR results were
obtained. Future work will consist in developing a more
symmetric structure bearing two carbazole moieties, to first
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
12 P. Xiao, F. Dumur, T.-T. Bui, X. Sallenave, F. Goubard,
B. Graff, F. Morlet-Savary, J.-P. Fouassier, D. Gigmes and
The authors thank Aix Marseille University and The Centre
National de la Recherche Scientifique (CNRS) for financial
support. The Agence Nationale de la Recherche (ANR agency)
is acknowledged for its financial support through the PhD
grants of Corentin Pigot (ANR-17-CE08-0010 DUALITY project)
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