Synthesis, optical and electrochemical properties of a series of push-pull dyes based on the 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile (TCF) acceptor

Article abstract of DOI:10.1016/j.dyepig.2020.108807

A series of chromophores was designed and synthesized using 2-(3-cyano-4,5,5-trimethylfuran-2 (5H) -ylidene) malononitrile (TCF) as the electron acceptor and differing from each other by the use of thirteen different electron donors. The different dyes were characterized for their optical and electrochemical properties and theoretical calculations were also carried out to support the experimental results. By changing the electron donor in the thirteen dyes, chromophores absorbing between 430 nm and 700 nm could be synthesized. Solvatochromism of the different dyes was analyzed in 23 solvents of different polarity and a positive solvatochromism was determined for all chromophores using the semi-empirical solvent polarity scales based on the Kamlet-Taft parameters (π *) or the Catalan parameters (SdP and SPP).

Full text of DOI:10.1016/j.dyepig.2020.108807

Dyes and Pigments 184 (2021) 108807
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Synthesis, optical and electrochemical properties of a series of push-pull
dyes based on the 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)
malononitrile (TCF) acceptor
,
**
a
b
b
a
Guillaume Noirbenta , Corentin Pigot , Thanh-Tuan Bui , Sebastien Peralta , Malek Nechab ,
ˆ
´
´
a
a,*
´ ´
Didier Gigmes , Frederic Dumur
a Aix Marseille Univ, CNRS, ICR UMR7273, F-13397, Marseille, France
b
´
CY Cergy Paris Universite, LPPI, F-95000, Cergy, France
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of chromophores was designed and synthesized using 2-(3-cyano-4,5,5-trimethylfuran-2 (5H) -ylidene)
malononitrile (TCF) as the electron acceptor and differing from each other by the use of thirteen different
electron donors. The different dyes were characterized for their optical and electrochemical properties and
theoretical calculations were also carried out to support the experimental results. By changing the electron donor
in the thirteen dyes, chromophores absorbing between 430 nm and 700 nm could be synthesized. Sol-
vatochromism of the different dyes was analyzed in 23 solvents of different polarity and a positive sol-
vatochromism was determined for all chromophores using the semi-empirical solvent polarity scales based on the
Push-pull dyes
Solvatochromism
Claisen-schmidt condensation
Visible absorption
Near infrared absorption
TCF
Kamlet-Taft parameters (
π
*) or the Catalan parameters (SdP and SPP).
1. Introduction
molecular orbital (LUMO). Reducing the difference between the HOMO
and LUMO orbitals results in a bathochromic offset of the maximum
absorption. Even if the progress achieved in the design of push-pull dyes
is remarkable, there are still incentives for developing new materials
with more extended absorptions, thus providing panchromatic dyes
[17–19]. These structures are notably extensively studied in solar cells.
In this specific research field, low bandgap materials are actively
researched as these compounds can also be easily oxidized and transfer
an electron from the electron-donating material towards the
electron-accepting one upon sunlight irradiation [20–23]. Low bandgap
materials are also now extensively studied in photopolymerization due
to the improved light penetration in the photocurable resins at long
wavelength [24–28]. Thus, if the light penetration in the resin is limited
to a few hundreds of micrometers at 400 nm, this latter can reach 5 cm at
800 nm, justifying the search for new structures [29]. Therefore, the
access to thick samples is rendered possible by the use of
long-wavelength photoinitiators [30]. Parallel to this, for applications
such as visible light photopolymerization, dyes perfectly fitting the
emission of the irradiation sources are required. Therefore, a perfect
control of the absorption maxima of the dye that will be used as the
In recent decades, push-pull dyes have attracted a lot of attention due
to their numerous applications ranging from non-linear optics (NLO) [1,
2] to organic photovoltaics (OPV) [3,4], organic field effect transistors
(OFET) [5], organic light emitting diodes (OLED) [6] and polymeriza-
tion photoinitiators [7–12].
Typically, push-pull dyes are molecules comprising an electron
donor and an electron acceptor connected by mean of a
π -conjugated
and planar system [13,14]. This configuration results in a significant
electron delocalization from the donor to the acceptor facilitated by the
π
-conjugated spacer. By improving the electron-donating and the
electron-withdrawing ability of the two moieties, compounds with low
bandgaps can be obtained. These compounds also exhibit a character-
istic absorption band in the visible region corresponding to the intra-
molecular charge transfer (ICT) band whose position depends on the
nature of both the donor and acceptor groups as well as the number of
double bonds involved in the electronic delocalization [15,16]. This
transition typically corresponds to the transfer of an electron from the
highest occupied molecular orbital (HOMO) to the lowest unoccupied
* Corresponding author.
** Corresponding author.
E-mail addresses: guillaume.noirbent@univ-amu.fr (G. Noirbent), frederic.dumur@univ-amu.fr (F. Dumur).
https://doi.org/10.1016/j.dyepig.2020.108807
Received 16 July 2020; Received in revised form 25 August 2020; Accepted 25 August 2020
Available online 29 August 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

G. Noirbent et al.
Dyes and Pigments 184 (2021) 108807
photosensitizer is a prerequisite to efficiently initiate the polymerization
process and generate reactive species such as radicals or cations
[31–33]. In these different research fields, many electron acceptors have
been studied over the years as exemplified by malononitrile 1 [34],
substituted tricyanopropenes 2 [35], 1,1,3-tricyano-2-substituted pro-
penes 3 [36], dicyanovinyl-thiophen-5-ylidenes 4 [37], tetranitro-
fluorene 5 [38], pyran derivatives 6 and 7 [39,40], dicyanoimidazoles 8
[41], pyrazines 9 [42], hydantions and rhodanines 10 [43], (thio)bar-
bituric derivatives 11 [44], isoxazolones 12 [45], Meldrum derivatives
13 [46], indanedione derivatives 14 [47], 15 [48,49], benzo [d]thia-
zoliums 16 [50], benzo [d]imidazoliums 17 [51] and pyridinium 18
[52].
is concerned, synthetic approaches that do not require the use of tran-
sition metal catalysts, drastic reaction conditions (inert atmosphere,
anhydrous solvents, low temperature reactions) and the use of expensive
reagents are also highly appealing. In this field, one of the simplest way
to produce push-pull dyes is the Claisen-Schmidt condensation, con-
sisting in the coupling an aldehyde with an acceptor possessing an
activated methyl group under basic conditions. This approach clearly
competes with the traditional Knoevenagel reaction which is extensively
used when the electron acceptors possess an activated methylene group
such as in the case of (thio)barbituric or indane-1,3-dione derivatives
[64,65]. If TCF is an acceptor of interest for the design of dyes, such
push-pull structures have never been tested in photopolymerization for
example, providing a possible and new application for these dyes. This is
certainly attributable to the fact that photopolymerists prefer to test
dyes for which all the photophysical properties of photoinitiators have
already been examined prior being examined for polymerization tests.
To end, TCF-based dyes are also characterized by a remarkable thermal
stability, as evidenced in the literature and constituting another major
advantage of these dyes [66–69].
Among all electron acceptors, 2-(3-cyano-4,5,5-trimethylfuran-2
(5H)-ylidene)malononitrile (TCF) has been extensively studied due to
the ability to design push-pull dyes with ICT bands extending from the
visible range to the near infrared region [53–55]. Numerous derivatives
notably found applications in NLO and energy conversion [56–59].
Besides, a systematic study devoted to examine the influence of the
electron-donating group on the optical properties of TCF-based push--
pull dyes has never been carried out. Lower interest for TCF as the
electron acceptor compared to the aforementioned acceptors is also
justified by the fact that contrarily to indane-1,3-dione, barbituric and
thiobarbituric, this electron acceptor is not commercially available.
Indeed, the most popular electron acceptors are the benchmark ones.
Interest for TCF as the electron acceptor is also supported by the easiness
of synthesis of the dyes, but also by the easy access to TCF itself. Indeed,
TCF can be prepared in one step, by reaction of malononitrile and
3-hydroxy-3-methylbutan-2-one i.e. from cheap starting materials and
in high reaction yields (85–90%) [60–63]. As far as the synthesis of dyes
In this article, TCF has been used to synthesize a series of thirteen
push-pull dyes (see chemical structures of TCF1-TCF13 in Fig. 1).
Photophysical and electrochemical properties of the thirteen dyes, as
well as their solvatochromic properties were also studied. To support the
experimental results, theoretical calculations were made.
Fig. 1. Chemical structures of the thirteen dyes TCF1-TCF13 prepared with TCF as the electron acceptor and examined in this study.
2

G. Noirbent et al.
Dyes and Pigments 184 (2021) 108807
2. Results and discussion
for TCF3 (the dye exhibiting the most redshifted absorption for the
chromophores with a short spacer) to 679 nm for TCF6 (See Fig. 3a). It
has to be noticed that TCF6 possesses the most red-shifted absorption of
the series with a non-negligible contribution in the near-infrared region.
Such an absorption at long wavelength is relatively unusual for push-
pull dyes comprising TCF as the electron acceptor and an absorption
in this region was only reported for polymethine dyes [70–72] or
indolizine-based dyes in the literature [73]. Based on their chemical
structures, indolizine-based dyes are the most similar dyes compared to
the TCF-based dyes examined in this work due to their
donor-spacer-acceptor structures and the absorption maximum reported
for one of the indolizine-based dyes at 758 nm (AH25) is directly related
to the use of an extended electron donor, as in the case of TCF6. To the
best of our knowledge, TCF6 is the second push-pull dye exhibiting the
most red-shifted absorption after AH25 for a TCF-based dye. Besides,
due to the fact that the Michler’s aldehyde used in TCF6 only comprises
N,N-dimethylaniline groups as donors whereas AH25 comprises a
remarkable electron donor (i.e. an indolizine moiety associated with a
thiophene spacer), the absorption maximum of TCF6 is consequently
blue-shifted compared to that of AH25. It has to be noticed that if the
absorption maximum of TCF6 is greatly redshifted compared to most of
the TCF-based push-pull dyes reported in the literature, it remains
however blueshifted compared to other Michler’s aldehyde-based dyes
comprising stronger electron acceptors such as 3-(dicyanomethylidene)
indan-1-one (λmax = 680 nm in CH2Cl2) or 1,3-bis(dicyanomethylidene)
2.1. Synthesis of the dyes TCF1-TCF13
All dyes TCF1-TCF13 presented in this work have been synthesized
by a simple, green and straightforward two-component Claisen-Schmidt
condensation. This protocol consisted in reacting the electron acceptor
TCF containing an activated methyl group in its structure with a series of
13 aldehydes D1-D13 using piperidine as the catalyst and ethanol as the
solvent (See Fig. 2).
TCF1-TCF13 were obtained, after overnight reaction in ethanol
under reflux, with reaction yields ranging from 79% yield for TCF8 to
92% for TCF3 (see Table 1). All the compounds were obtained as solids
after evaporation of ethanol and their purification could be limited to a
simple precipitation in an ether:pentane mixture. For all compounds
TCF1-TCF13, their chemical structures could be confirmed by 1H, 13C
NMR, and HRMS analyses (see Supporting Information).
2.2. Optical properties
The thirteen chromophores examined in these works are “push-pull”
chromophores. All the compounds were characterized by UV–visible
spectroscopy to determine their absorption spectra in dichloromethane
(DCM). As shown in Fig. 4, this series of “push-pull” dyes exhibits an
intense absorption band with absorption maxima ranging from 437 nm
for TCF10 bearing the weakest electron donor of this series to 592 nm
for TCF3 possessing the strongest electron-donor. As specificity, all these
different dyes possess a short spacer in their backbones. Upon elongation
indane (λ
= 707 nm in CH2Cl2) [74].
max
Interestingly, in the case of TCF13, despites the presence of the
elongated spacer in its scaffold, an absorption maximum drastically
blue-shifted compared to that of TCF6 could be detected, peaking at 515
nm (vs. 679 nm for TCF6) so that position of its ICT band was similar to
of the
π-conjugated system, extension of the conjugation resulted in a
significant redshift of the absorption maximum, shifting from 592 nm
Fig. 2. Synthetic pathways to TCF1-TCF13 and the thirteen aldehydes used in this study.
3

G. Noirbent et al.
Dyes and Pigments 184 (2021) 108807
Table 1
Reaction yields obtained for the synthesis of TCF1-TCF13.
Compounds
TCF1
TCF2
TCF3
TCF4
TCF5
TCF6
TCF7
85
Reaction yields (%)
Compounds
84
89
92
88
83
86
TCF8
79
TCF9
85
TCF10
90
TCF11
83
TCF12
81
TCF13
86
Reaction yields (%)
Fig. 3. a) UV–visible absorption spectra of TCF1, TCF3-TCF5, TCF7-TCF12 in DCM. b) UV–visible absorption spectra of TCF2, TCF6, TCF13 in DCM.
than in TCF2. Indeed, in the case of TCF3, the electron donating ability
of the donor is reinforced by the presence of the butoxy group in ortho-
position of the
π-conjugated system, contributing to the electronic
delocalization while jointly improving the solubility of the dye. Finally,
the highest molar coefficients were found for the two dyes exhibiting an
extended π-conjugated spacer (i.e. TCF12 and TCF13), consistent with
the results reported in the literature [75]. Indeed, these two dyes exhibit
the largest oscillator strengths, resulting from the increase of the back-
bone length.
2.3. Solvatochromism
TCF1-TCF13 showed a sufficient solubility in common organic sol-
vents, so that their solvatochromism could be examined in 23 solvents of
different polarities. Besides, among the different solvents investigated,
ethanol and butane-1-ol were discarded for the solvatochromic study
due to absorption maxima in complete disagreement with those ob-
tained in the other solvents. This behavior may be assigned to the low
solubility of dyes in alcohols, but also to the formation of aggregates or
nanoparticles that drastically modify the positions of the absorption
maximum [76–79]. Similarly, irregular solvatochromic behaviors are
often reported in halogenated solvents (dichloromethane, chloroform)
so that these two solvents are often discarded for solvatochromic studies
[34,48,52]. Indeed, during the synthesis of TCF-based dyes, all com-
pounds precipitated in ethanol which was the solvent used for the re-
action. Even if absorption spectra of TCF1-TCF13 were nonetheless
recorded in alcohols, the coexistence of free molecules and aggregates in
solution adversely affected the position of the absorption maxima. A
summary of the absorption maxima for the thirteen dyes is provided in
Tables 2 and 3. As shown in Tables 2 and 3, a clear modification of the
absorption maxima with the solvent polarity could be evidenced, cor-
responding well to a solvatochromic behavior.
Fig. 4. UV–visible absorption spectra of TCF1-TCF13 in chloroform.
that of TCF1 (See Fig. 3b). This significant blue-shift can be assigned to
the weakness of the electron donating groups used in TCF13 i.e. the
para-methoxyphenyl groups. Due to their weak electron-releasing abil-
ity of the para-methoxyphenyl groups, elongation of the π-conjugated
spacer in TCF13 could not compensate the poor electron donating
ability of the donor so that the absorption maximum of TCF13 was blue-
shifted compared to that of TCF6.
On the basis of the position of the ICT bands, this series of 11 chro-
mophores bearing a short π-conjugated spacer can be divided into two
distinct groups: the first one comprises dyes bearing a weak electron
donor such as in the case of compounds TCF4, TCF5, TCF7, TCF9 and
TCF10 and a second group, those bearing a strong electron donor such as
in the case of compounds TCF2, TCF3, TCF8, TCF11 and TCF12.
Considering that all dyes have been prepared with the same electron
acceptor, a scale of electron-donating ability can be established for the
different donors. Examination of their molar extinction coefficients (see
Fig. 4) revealed that TCF2, TCF3, TCF6 and TCF12 had the highest
molar extinction coefficients of the series, reaching 103,600, 72,940,
58,290 and 44,250 Mꢀ 1 cmꢀ 1 respectively. The high molar extinction
coefficient of TCF3 can be assigned to the strong electronic delocaliza-
tion existing in this molecule, benefiting from a stronger electron donor
Analysis of the solvatochromism revealed the intramolecular nature
of the charge transfer, what could be verified by performing successive
dilutions of the solutions. Considering that the intensity of the charge
transfer band decreased linearly with the concentrations of dyes, it could
be concluded that the charge transfer observed in solution was of
intramolecular nature. These results were confirmed by the analysis of
the solvatochromism in solvents of different polarities, supporting the
4

G. Noirbent et al.
Dyes and Pigments 184 (2021) 108807
determined as being the Catalan’s, Reichardt’s and Kamlet-Taft’s scales.
Table 2
Optical properties of TCF1-TCF6 in 23 solvents.
More precisely, in the case of the Kamlet-Taft empirical scale, better
compounds
TCF1a
TCF2a
TCF3a
TCF4a
TCF5a
TCF6a
linear correlations were obtained while plotting ΔE = f (π*) instead
using a multiparameter approach (See Tables S3 and S6). While using
the Catalan empirical scale, linear correlations could be obtained with
two different parameters, namely the solvent dipolarity (SdP) and the
solvent polarity/polarizability (SPP) parameters. The different plots are
presented in SI. For most of the dyes, remarkable linear correlations with
large values for the square of the correlation coefficient (R2) could be
determined using the polarity/polarizability parameter for the Catalan
empirical scale (see Tables in SI). Conversely, a lower adequation was
found while using the solvent dipolarity parameter. Contrarily to the
Kamlet-Taft model in which all non-specific solvent effects are included
acetone
496
501
492
509
513
513
489
509
512
487
494
504
488
506
509
513
509
486
524
496
499
490
497
564
568
546
570
577
576
ndb
578
577
530
534
570
530
582
580
589
577
ndb
594
556
533
520
529
588
590
572
587
596
591
533
593
592
558
560
584
552
599
600
607
596
ndb
608
580
563
536
560
439
441
434
444
452
460
432
449
457
431
434
439
432
450
451
439
443
430
464
438
439
434
438
463
465
463
473
475
479
443
475
474
454
456
470
446
472
472
491
469
441
488
465
459
448
453
642
644
614
653
690
673
583
684
679
605
602
652
605
687
689
707
697
ndb
706
627
609
587
605
acetonitrile
AcOEt
anisole
butanol
chloroform
cyclohexane
1,2-dichloroethane
dichloromethane
diethyl carbonate
diethyl ether
diglyme
in the π* parameter, the advantage of the Catalan solvatochromic model
1,4-dioxane
DMA
is the ability to separate non-specific solvent effects such as the polarity
and the polarizability. With regards to the results obtained while using
the solvent dipolarity and the solvent polarity/polarizability parameters
(See Table S4), the solvent polarizability has been identified as the main
factor governing the spectral shifts of all dyes. Interestingly, in some
specific cases, as shown in Fig. 5, good correlations could be obtained
with three different polarity scales (Catalan’s, Reichardt’s and Kamlet-
Taft’s scales), as exemplified with TCF3. In fact, the best correlations
were obtained for the three dyes exhibiting the strongest electron do-
nors, namely TCF2, TCF3 and TCF6, squares of the correlation co-
efficients (R2) higher than 0.7 being obtained with the different polarity
scales. Noticeably, TCF5 that exhibits a weaker electron donor than
TCF2, TCF3 and TCF6 could also give a good correlation using the SPP
parameter (with R2 = 0.841). However, linear regression using the SdP
parameter furnished a lower square of the correlation coefficient of
0.692. More interestingly, its analogue TCF7 in which allyl groups have
been replaced by butoxy groups showed lower R2 coefficients, with R2 of
0.778 with the SPP parameter and 0.483 for the SdP parameter. Even
alkyl chains used as substituents should not drastically influence the
optical properties of TCF5 and TCF7, ability of the electron-rich allylic
group in TCF5 to form interactions different from that of the aliphatic
chains was clearly evidenced. Noticeable differences of their squares of
correlation coefficients are notably evidenced with the Catalan polarity
scale.
DMF
DMSO
ethanol
heptane
nitrobenzene
THF
toluene
triethylamine
p-xylene
a
Position of the ICT bands are given in nm.
nd: not determined.
b
presence of the intramolecular charge transfer by the similitude of the
absorption bands.
Various empirical polarity scales have been developed over the years
to interpret the solvent-solute interaction and the Kamlet-Taft’s [80],
Dimroth-Reichardt’s [79], Lippert-Mataga’s [81], Catalan’s [82],
Kawski-Chamma Viallet’s [83], McRae Suppan’s [84] and Bakhshiev’s
[85] scales are the most widely used. If polarity scales such as the
Kamlet-Taft’s or the Dimroth-Reichardt’s scales are adapted to examine
the solvatochromism in absorption, other polarity scales such as
Kawski-Chamma Viallet’s, McRae Suppan’s and Bakhshiev’s scales
examine the variation of the Stokes shifts vs. the solvent polarity.
Considering the number of molecules and the number of solvent
examined, only the solvatochromism in absorption was examined.
Among the different empirical polarity scales used to rationalize the
variation of the ICT bands in solution, the most suitable ones were
Considering that the
α factor of the Kamlet-Taft polarity scale is
indicative of the ability of the solvents to donate a hydrogen to form a
hydrogen bond with the solute [86], it can be thus concluded that due to
Table 3
Optical properties of TCF7-TCF13 in 23 solvents.
compounds
TCF7a
TCF8a
TCF9a
TCF10a
TCF11a
TCF12a
TCF13a
acetone
470
471
467
481
478
485
448
481
482
462
484
474
460
476
477
493
476
446
486
470
470
462
470
539
541
530
564
559
573
535
565
564
527
535
543
530
547
551
554
553
523
575
539
536
528
536
463
462
460
476
470
476
447
473
473
458
460
469
460
468
468
470
466
447
481
463
468
451
469
439
441
434
444
452
460
432
449
457
431
433
439
432
450
451
439
443
430
464
438
439
434
438
524
528
515
540
541
552
513
547
547
512
518
530
511
534
533
536
533
509
558
523
523
515
522
537
536
530
549
559
573
528
562
564
526
534
549
527
550
551
554
553
522
571
436
541
530
537
499
499
494
504
511
517
486
517
515
487
493
503
491
505
511
513
514
482
528
502
496
487
496
acetonitrile
AcOEt
anisole
butanol
chloroform
cyclohexane
1,2-dichloroethane
dichloromethane
diethyl carbonate
diethyl ether
diglyme
1,4-dioxane
DMA
DMF
DMSO
ethanol
heptane
nitrobenzene
THF
toluene
triethylamine
p-xylene
a
Position of the ICT bands are given in nm.
5

G. Noirbent et al.
Dyes and Pigments 184 (2021) 108807
Fig. 5. Linear correlations obtained while using three different polarity scales for TCF3 (Catalan a) and b) Kamet-Taft c), and Reichardt d) empirical scales).
the presence of heteroatoms in the TCF moiety as well as in the
electron-donating part, TCF1-TCF13 are thus good candidates to form
hydrogen bonds with the solvent. This behavior is reinforced in the case
of TCF2, TCF3 and TCF6 bearing accessible nitrogen atoms. For all dyes,
electron acceptors for which a shift as high as 85 nm was observed [52].
For comparisons, a solvatochromic shift of ca. 60 nm was observed for
3-(dicyanomethylidene)indan-1-one-based dyes and 50 nm for 1,3-bis
(dicyanomethylidene)indane-based dyes, also comprising Michler’s
aldehyde as the donor [87]. Therefore, it can be concluded that in this
study, the greatest solvatochromic shift ever reported with Michler’s
aldehyde-based dyes has been obtained with TCF6.
a
positive solvatochromism was found, with a decrease of the
HOMO-LUMO gap with the solvent polarity, suggesting a stabilization of
the electronic excited states relative to the ground state upon increase of
the solvent polarity.
Examination of their luminescence properties also revealed TCF1-
TCF13 to be weakly emissive. Indeed, photoluminescence of the TCF-
based dyes could only be measured for TCF1-TCF3, the other dyes
TCF4-TCF13 being not emissive (See Fig. 7). The largest Stokes shift was
found for TCF1, with a value of 70 nm. Conversely, small Stokes shifts
were determined for the two other dyes TCF2 and TCF3, being respec-
tively of 35 and 25 nm. These Stokes shifts are relatively small, and
comparable to that observed for other TCF-based dyes reported in the
literature [88,89].
Examination of the slopes obtained by plotting the HOMO-LUMO
gaps vs. the Taft parameters is indicative of the electronic redistribu-
tion occurring within the dyes upon excitation. The same holds true if
the Catalan or the Reichardt’s plots are considered. As shown in Fig. 6a,
all dyes proved to be sensitive to the solvent polarity. However, three
different groups could be identified (See Fig. 6b). Notably, TCF2, TCF3
and TCF6 that possess dialkylaniline groups in their electron-donating
parts are the most sensitive ones. However, this sensitivity is not
observed for the triphenylamine-based dyes (TCF8, TCF11 and TCF12)
which are quite close in structure to the dialkylaniline-based dyes TCF2,
TCF3 and TCF6. It can be assigned to the higher electronic delocaliza-
tion of the electron-lone pair of the nitrogen atom in the case of the
triphenylamine-based dyes, rendering the nitrogen atom less sensitive to
its environment. As a final group are all the dyes possessing aromatic
rings substituted with alkoxy groups (TCF4, TCF5, TCF9 and TCF10).
Considering that the alkoxy groups are the less donating groups, this is
also the dyes exhibiting the widest bandgaps. Due to the weak electron-
donating ability of their respective electron donors, only a small charge
redistribution upon excitation occurs. Among all dyes examined in this
work, the greatest solvatochromic shift was observed for TCF6,
approaching 120 nm. Indeed, if a maximum absorption at 583 nm was
found for TCF6 in cyclohexane, a redshift of the ICT band at 707 nm was
found in DMSO so that a shift as high as 120 nm could be determined
between the apolar and the highly polar solvent. It has to be noticed that
a strong solvatochromic behavior was previously reported in the liter-
ature for Michler’s aldehyde-based dyes comprising terpyridine as
2.4. Theoretical investigations
Optical properties of the thirteen dyes were also examined theoret-
ically by performing density functional theory (DFT) calculations at the
B3LYP/6-311G (d, p) level using the Gaussian09 program. For the
different calculations, dichloromethane was selected as the solvent and a
polarizable continuum model (PCM) was used to simulate the solvent.
Results of the theoretical investigations are summarized in Table 4
where the HOMO and LUMO energy levels, and the main transitions
involved in the ICT bands are reported. Simulated absorption spectra of
TCF1-TCF13 are given in Fig. 8.
As anticipated and due to the fact that the same electron acceptor has
been used to elaborate the thirteen dyes, only small variations of the
LUMO energy levels could be observed, ranging from ꢀ 3.59 eV for TCF9
to ꢀ 3.06 eV for TCF3. Conversely, a severe stabilization of the LUMO
energy level was found for TCF8 comprising two TCF groups, the LUMO
energy level decreasing to ꢀ 3.91 eV. On the contrary, values of the
6

G. Noirbent et al.
Dyes and Pigments 184 (2021) 108807
Fig. 6. Top: Variation of the HOMO-LUMO gaps of TCF1-TCF13 with the Taft parameters (
π
*). Bottom: The three sets of dyes (in red, blue and black) exhibiting the
same sensitivity to the solvent polarities. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
HOMO energy levels varied drastically from a dye to another one due to
the differences of electron donating groups. Thus, HOMO energy levels
ranging from ꢀ 5.59 eV for TCF6 to ꢀ 6.58 eV for TCF9 could be deter-
mined. Interestingly, comparison of the theoretical and experimental
optical bandgaps revealed a good adequation between the two values. A
severe mismatch was found for TCF8, for which a difference as high as
80 nm was determined between the experimental (564 nm) and the
theoretical (644 nm) values. This mismatch can be confidently assigned
to the fact that TCF8 is the only acceptor-donor-acceptor A-D-A struc-
ture of the series, rendering its UV–visible absorption spectrum more
difficult to model.
level on the TCF moiety. Interestingly, an asymmetric distribution of the
LUMO energy level of TCF8 could be found whereas the two TCF moi-
eties were expected to equally contribute to the LUMO level. On the
basis of the electronic distribution of the LUMO energy level of TCF8, it
can therefore support the severe mismatch found between the experi-
mental and theoretical position of the absorption maximum.
2.5. Electrochemical properties
All compounds examined in this study have been analyzed by cyclic
voltammetry (CV) to determine their electrochemical properties in a
dilute solution of dichloromethane. All CV curves are given in the ESI
and a set of curves is provided in Fig. 10. Redox potentials of all dyes
against the half-wave oxidation potential of the ferrocene/ferrocenium
cation pair are given in Table 5.
Optimized geometries and the electronic distributions of the HOMO-
LUMO orbitals of all dyes are provided in the supporting information. As
shown in Fig. 9, examination of the contour plots of TCF1-TCF13
revealed the electronic distribution to be consistent with that typically
observed for donor-acceptor D-A structures, with a HOMO energy level
mostly located onto the electron-donating part and the LUMO energy
In this study, all the compounds possess the same electron accepting
group but differ by the nature of their electron-donating groups. So, as
7

G. Noirbent et al.
Dyes and Pigments 184 (2021) 108807
Fig. 7. UV–visible and photoluminescence spectra of TCF1-TCF3 recorded in chloroform.
Table 4
Summary of simulated absorption characteristics in dilute dichloromethane of synthetized compounds. Data were obtained in dichloromethane solution.
HOMO -1 (eV)
HOMO (eV)
LUMO (eV)
LUMO +1 (eV)
λmax(theo)
(nm)
ΔEtheo (eV)
λmax(exp)
(nm)
ΔEexp (eV)
TCF1
TCF2
TCF3
TCF4
TCF5
TCF6
TCF7
TCF8
TCF9
TCF10
TCF11
TCF12
TCF13
ꢀ 6.39
ꢀ 6.80
ꢀ 6.61
ꢀ 7.16
ꢀ 7.00
ꢀ 6.06
ꢀ 6.95
ꢀ 6.86
ꢀ 7.26
ꢀ 7.17
ꢀ 6.81
ꢀ 6.71
ꢀ 6.88
ꢀ 6.32
ꢀ 6.01
ꢀ 5.76
ꢀ 6.43
ꢀ 6.26
ꢀ 5.59
ꢀ 6.19
ꢀ 6.33
ꢀ 6.58
ꢀ 6.51
ꢀ 6.12
ꢀ 5.95
ꢀ 6.15
ꢀ 3.57
ꢀ 3.29
ꢀ 3.06
ꢀ 3.48
ꢀ 3.35
ꢀ 3.19
ꢀ 3.30
ꢀ 3.91
ꢀ 3.59
ꢀ 3.53
ꢀ 3.57
ꢀ 3.41
ꢀ 3.49
ꢀ 1.85
ꢀ 1.35
ꢀ 1.17
ꢀ 1.48
ꢀ 1.41
ꢀ 1.38
ꢀ 1.35
ꢀ 3.66
ꢀ 1.55
ꢀ 1.59
ꢀ 1.92
ꢀ 1.63
ꢀ 1.84
498
515
519
461
481
604
484
644
453
469
546
566
550
ꢀ 2.49
ꢀ 2.41
ꢀ 2.39
ꢀ 2.69
ꢀ 2.58
ꢀ 2.05
ꢀ 2.56
ꢀ 1.92
ꢀ 2.74
ꢀ 2.64
ꢀ 2.27
ꢀ 2.19
ꢀ 2.25
512
577
592
457
474
679
482
564
473
457
547
564
515
ꢀ 2.42
ꢀ 2.15
ꢀ 2.09
ꢀ 2.71
ꢀ 2.62
ꢀ 1.83
ꢀ 2.57
ꢀ 2.20
ꢀ 2.62
ꢀ 2.71
ꢀ 2.27
ꢀ 2.20
ꢀ 2.41
expected, it leads to the detection of similar reduction potentials for all
dyes, proving that the reduction is located on the cyano group of the TCF
moiety (See Table 5).
potentials is also identified (TCF8, TCF11 and TCF12), proving an
oxydation process located on the nitrogen atom of the TPA moiety. As
they are better donor groups, they exhibit lower oxydation values, what
is consistent with the optical data.
Oppositely, the different donor parts lead to diverse oxidative po-
tentials. TCF4, TCF5, TCF7, TCF9 and TCF10 are all similar since
electrons of these dyes comprise an “-OR “ group on the phenyl ring.
Nevertheless, we may notice some disparities among this group of
compounds. Indeed, TCF10 has the highest oxydation potential of the
series probaly due to the long alkyl chain in ortho position causing a
torsion in the molecule. Moreover, in accordance with the bathochromic
effects observed with the UV spectra, only the para-substitution seems to
have an effect on these group of compounds since TCF5, TCF 7 and TCF
9 have very close oxydation potentials and this phenomenon is also
observed between TCF10 and TCF4, which are ortho and para sub-
stitued dyes (See Fig. 10).
Finally, the lowest oxydation potentials are all observed for dyes
with an amine group in para position of the phenyl ring but also with a
π
-conjugated spacer between the donor and the acceptor. Among this
group, we can detect the oxidation of the amine around 0.43 V for TCF2
and TCF3 which is not observed in the case of TCF6, which exhibit an
even better electron-donating moiety and for which an oxidation is
detected at 0.29 V. This oxidation process is certainly centered on the
double bond of the π-conjugated system separating the donor from the
acceptor. Formation of the radical cation on the vinyl spacer of push-pull
dyes is in agreement with results previously reported in the literature for
dyes based on Michler’s aldehyde D6 but also with the theoretical re-
sults obtained by DFT calculations and the experiment results
A second group composed of triphenylamine with close oxydation
8

G. Noirbent et al.
Dyes and Pigments 184 (2021) 108807
determined by UV–visible absorption spectroscopy [39]. It has to be
noticed that more electrochemical processes can be detected in the cy-
clic voltammogram of TCF6 compared to all the other dyes. Indeed, due
to the presence of the elongated π-conjugated vinylic spacer, dimeriza-
tion of the cation radical formed by oxidation of the vinylic spacer can
occur, complexifying the voltammogram, in oxidation and in reduction
[90]. To illustrate this, oxidation peaks at 0.37, 0.56 and 0.66 V were
detected at anodic potentials. Especially, dimerization is facilitated by
the presence of the strong electron donors, namely dimethylanilines. A
similarity of the anodic part of the cyclic voltammogram of TCF6 with
that previously reported in the literature for Michler’s aldehyde-based
dyes can be clearly evidenced, supporting the dimerization of the
cation radical [52]. Conversely, in TCF13, dimerization of the cation
radicals is not observed due to the presence of the weak methox-
ybenzene electron donor, adversely affecting radicals recombination.
Set apart these three main groups, TCF1 and TCF13 differs from the
other dyes by their electron donors. Notably, TCF1 comprises a carba-
zole acting as a remarkable electron donor whereas TCF13 comprises an
extended donor D13 of weaker electron donating ability than D6. While
comparing TCF1 and TCF13 with the other dyes, their oxydation po-
tentials are still lower than that of all the OMe-based compounds but
Fig. 8. Simulated absorption spectra of TCF1-TCF13 in dilute
dichloromethane.
Fig. 9. Contour plots of the HOMO and LUMO energy levels of TCF1 and TCF8.
Fig. 10. Comparisons between the cyclic voltammograms of TCF13/TCF6 (left) and TCF10/TCF4 (right) measured in dichloromethane scan rate 100 mV sꢀ 1, with
tetrabutylammonium perchlorate (TBAP) (0.1 M) as the supporting electrolyte.
9

G. Noirbent et al.
Dyes and Pigments 184 (2021) 108807
Table 5
Electrochemical characteristics of studied compounds TCF1-TCF13.
Ered
Ered onset
V/Fc
EOx
Eox onset
V/Fc
EOx
EOx
EHOMO
eV
ELUMO
eV
ΔEET
ΔEopt
V/Fc
V/Fc
V/Fc
V/Fc
eV
eV
TCF10
TCF4
TCF7
TCF9
TCF5
TCF13
TCF1
TCF11
TCF12
TCF8
TCF2
TCF3
TCF6
ꢀ 1.05
ꢀ 1.11
ꢀ 1.15
ꢀ 1.11
ꢀ 1.13
ꢀ 1.04
ꢀ 1.03
ꢀ 1.06
ꢀ 1.09
ꢀ 1.13
ꢀ 1.16
ꢀ 1.27
ꢀ 1.13
ꢀ 0.98
ꢀ 1.03
ꢀ 1.06
ꢀ 1.02
ꢀ 1.06
ꢀ 0.96
ꢀ 1.33
ꢀ 0.99
ꢀ 1.02
ꢀ 1.04
ꢀ 1.10
ꢀ 1.19
ꢀ 1.05
1.39
1.24
1.04
1.02
1.06
1.10
0.91
0.73
0.66
0.64
0.56
0.52
0.37
1.27
1.15
0.94
0.92
0.87
0.84
0.81
0.64
0.58
0.55
0.43
0.43
0.29
–
–
ꢀ 6.07
ꢀ 5.95
ꢀ 5.74
ꢀ 5.72
ꢀ 5.67
ꢀ 5.64
ꢀ 5.61
ꢀ 5.44
ꢀ 5.38
ꢀ 5.35
ꢀ 5.23
ꢀ 5.23
ꢀ 5.09
ꢀ 3.81
ꢀ 3.77
ꢀ 3.74
ꢀ 3.78
ꢀ 3.74
ꢀ 3.84
ꢀ 3.47
ꢀ 3.81
ꢀ 3.78
ꢀ 3.76
ꢀ 3.70
ꢀ 3.61
ꢀ 3.74
2.26
2.18
2.00
1.94
1.93
1.80
2.14
1.63
1.60
1.60
1.53
1.61
1.35
2.71
2.71
2.57
2.62
2.62
2.41
2.42
2.27
2.20
2.20
2.15
2.09
1.83
–
–
–
–
–
–
–
–
–
–
0.99
1.15
–
–
–
–
–
–
–
–
–
–
0.56
0.66
All potentials recorded in 0.1 M TBAP/dichloromethane. EHOMO (eV) = - 4.8 – Eox onset and ELUMO (eV) = ꢀ 4.8 – Ered onset.
remains greater than that determined for the TPA-based dyes and the
dyes based on the dimethylaniline donors.
Furthermore, the HOMO and LUMO energy levels of all the mole-
cules in this study were estimated from redox behaviors using the value
of the ferrocene ionization potential as the standard (4.8 eV vs vacuum).
This correction factor is based on calculations obtained by Pommerehne
et al. [91] These values are summarized in Table 5 and a comparison
between the values obtained experimentally and theoretically is pre-
sented in Fig. 11. A good agreement between the experimental and the
theoretical results are obtained. Especially, a between adequation be-
tween the theoretical and the experimental values of the LUMO level
that for the HOMO level was found for all dyes.
3. Conclusions
In this study, a series of thirteen dyes comprising the TCF group were
synthesized and their photophysical properties were analyzed. The
change of the electron donor fragment allows a shift of the maximum
absorption towards the near infrared by using Michler’s aldehyde as
electron donor compared to a donor comprising alkoxy chains on
phenyl. The solvatochromism of all the dyes was found to be linear and
Fig. 11. Comparison of frontier orbitals’ energy levels obtained from cyclic
positive, inducing a redistribution of the large charges during excitation.
It has been determined that the experimental and theoretical HOMO-
LUMO gaps showed a good correlation. Among the most interesting
findings of this work, TCF6 is the second TCF-based dye exhibiting the
most redshifted absorption after AH25 for push-pull dyes exhibiting a
donor-spacer-acceptor structure. Parallel to this, ICT band of TCF6 also
showed an exceptional solvatochromic shift as high as 120 nm from
cyclohexane to DMSO, making TCF6, the most solvatochromic dye ever
prepared with the Michler’s aldehyde.
voltammetry and DFT calculation.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
Author statement
The authors thank Aix Marseille University and The Centre National
de la Recherche (CNRS) for financial supports. 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) and Guillaume Noirbent (ANR-17-CE08-0054 VISICAT project).
ˆ
´
Guillaume Noirbent, Corentin Pigot, Thanh-Tuan Bui, Sebastien
´
´ ´
Peralta, Malek Nechab, Didier Gigmes, Frederic Dumur: Conceptuali-
zation, Methodology, Software.
ˆ
´
Guillaume Noirbent, Corentin Pigot, Thanh-Tuan Bui, Sebastien
´
´ ´
Peralta, Malek Nechab, Didier Gigmes, Frederic Dumur: Data curation,
Appendix A. Supplementary data
Writing- Original draft preparation.
ˆ
´
Guillaume Noirbent, Corentin Pigot, Thanh-Tuan Bui, Sebastien
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108807.
´
´ ´
Peralta, Malek Nechab, Didier Gigmes, Frederic Dumur: Visualization,
Investigation.
´ ´
Frederic Dumur Supervision.
ˆ
´
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´ ´
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