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Synthesis, optical and electrochemical properties of aseries of push-pull dyes based on the 2-(3-cyano-4,5,5-
trimethylfuran-2(5H)-ylidene)malononitrile (TCF)acceptor
Guillaume Noirbent, Corentin Pigot, Thanh-Tuân Bui, Sébastien Péralta,Malek Nechab, Didier Gigmes, Frédéric Dumur
To cite this version:Guillaume Noirbent, Corentin Pigot, Thanh-Tuân Bui, Sébastien Péralta, Malek Nechab, et al.. Syn-thesis, 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. Dyes and Pigments, Elsevier, 2021, 184,pp.108807. �10.1016/j.dyepig.2020.108807�. �hal-02936031�
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
Guillaume Noirbent 1,*, Corentin Pigot 1, Thanh-Tuân Bui 2, Sébastien Péralta 2, Malek Nechab 1,
Didier Gigmes 1 and Frédéric Dumur 1,*
1 Aix Marseille Univ, CNRS, ICR UMR7273, F-13397 Marseille France 2 CY Cergy Paris Université, LPPI, F-95000 Cergy, France
* Correspondence: [email protected], [email protected]
Abstract:
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).
Keywords: Push-pull dyes; solvatochromism; Claisen-Schmidt condensation; visible absorption; near
infrared absorption; TCF.
1. Introduction
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 polymerization
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 characteristic
absorption band in the visible region corresponding to the intramolecular 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 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 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 propenes 3[36], dicyanovinyl-thiophen-5-ylidenes 4[37], tetranitrofluorene 5[38], pyran
derivatives 6 and 7 [39,40], dicyanoimidazoles 8 [41], pyrazines 9 [42], hydantions and rhodanines 10
[43], (thio)barbituric derivatives 11 [44], isoxazolones 12 [45], Meldrum derivatives 13 [46],
indanedione derivatives 14 [47], 15 [48,49], benzo[d]thiazoliums 16[50], benzo[d]imidazoliums 17 [51]
and pyridinium 18. [52]
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 is concerned, synthetic approaches that
do not require the use of transition 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, consisting 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]
In this article, TCF has been used to synthesize a series of thirteen push-pull dyes (see chemical
structures of TCF1-TCF13 in Figure 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.
Figure 1. Chemical structures of the thirteen dyes TCF1-TCF13 prepared with TCF as the electron
acceptor and examined in this study.
2. Results and Discussion
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 Figure 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).
Figure 2. Synthetic pathways to TCF1-TCF13 and the thirteen aldehydes used in this study.
Table 1. Reaction yields obtained for the synthesis of TCF1-TCF13.
Compounds TCF1 TCF2 TCF3 TCF4 TCF5 TCF6 TCF7
Reaction yields (%) 84 89 92 88 83 86 85
Compounds TCF8 TCF9 TCF10 TCF11 TCF12 TCF13
Reaction yields (%) 79 85 90 83 81 86
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 Figure 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 of the π-conjugated system,
extension of the conjugation resulted in a significant redshift of the absorption maximum, shifting from
592 nm for TCF3 (the dye exhibiting the most redshifted absorption for the chromophores with a short
spacer) to 679 nm for TCF6 (See Figure 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)indane (λmax = 707 nm in CH2Cl2).[74]
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 that of TCF1 (See Figure
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 ability 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.
400 500 600 700
0.0
0.5
1.0
No
rma
lize
d a
bs
orp
tio
n in
ten
sit
y (
a.u
.)
Wavelength (nm)
TCF1
TCF3
TCF4
TCF5
TCF7
TCF8
TCF9
TCF10
TCF11
TCF12
a)
400 500 600 700 800 900
0.0
0.5
1.0
No
rmali
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ab
so
rpti
on
in
ten
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a.u
.)
Wavelength (nm)
TCF2
TCF6
TCF13
b)
Figure 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
On the basis of the position of the ICT bands, this series of 11 chromophores 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 Figure 4) revealed that TCF2, TCF3, TCF6 and TCF12 had the highest molar
extinction coefficients of the series, reaching 103600, 72940, 58290 and 44250 M-1.cm-1 respectively. The
high molar extinction coefficient of TCF3 can be assigned to the strong electronic delocalization existing
in this molecule, benefiting from a stronger electron donor 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 backbone length.
400 500 600 700 800
0
20000
40000
60000
80000
100000
(L
.mo
l-1.c
m-1)
Wavelength (nm)
TCF13
TCF12
TCF11
TCF10
TCF9
TCF8
TCF7
TCF6
TCF5
TCF4
TCF3
TCF2
TCF1
Figure 4. UV-visible absorption spectra of TCF1-TCF13 in chloroform.
2.3. Solvatochromism
TCF1-TCF13 showed a sufficient solubility in common organic solvents, 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 obtained 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 compounds precipitated in ethanol which was the solvent
used for the reaction. 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, corresponding well to a solvatochromic behavior.
Table 2. Optical properties of TCF1-TCF6 in 23 solvents.
compounds TCF11 TCF21 TCF31 TCF41 TCF51 TCF61
acetone 496 564 588 439 463 642
acetonitrile 501 568 590 441 465 644
AcOEt 492 546 572 434 463 614
anisole 509 570 587 444 473 653
butanol 513 577 596 452 475 690
chloroform 513 576 591 460 479 673
cyclohexane 489 nd² 533 432 443 583
1,2-dichloroethane 509 578 593 449 475 684
dichloromethane 512 577 592 457 474 679
diethyl carbonate 487 530 558 431 454 605
diethyl ether 494 534 560 434 456 602
diglyme 504 570 584 439 470 652
1,4-dioxane 488 530 552 432 446 605
DMA 506 582 599 450 472 687
DMF 509 580 600 451 472 689
DMSO 513 589 607 439 491 707
ethanol 509 577 596 443 469 697
heptane 486 nd² nd² 430 441 nd²
nitrobenzene 524 594 608 464 488 706
THF 496 556 580 438 465 627
toluene 499 533 563 439 459 609
triethylamine 490 520 536 434 448 587
p-xylene 497 529 560 438 453 605 1 Position of the ICT bands are given in nm 2 nd : not determined
Table 3. Optical properties of TCF7-TCF13 in 23 solvents.
compounds TCF71 TCF81 TCF91 TCF101 TCF111 TCF121 TCF131
acetone 470 539 463 439 524 537 499
acetonitrile 471 541 462 441 528 536 499
AcOEt 467 530 460 434 515 530 494
anisole 481 564 476 444 540 549 504
butanol 478 559 470 452 541 559 511
chloroform 485 573 476 460 552 573 517
cyclohexane 448 535 447 432 513 528 486
1,2-dichloroethane 481 565 473 449 547 562 517
dichloromethane 482 564 473 457 547 564 515
diethyl carbonate 462 527 458 431 512 526 487
diethyl ether 484 535 460 433 518 534 493
diglyme 474 543 469 439 530 549 503
1,4-dioxane 460 530 460 432 511 527 491
DMA 476 547 468 450 534 550 505
DMF 477 551 468 451 533 551 511
DMSO 493 554 470 439 536 554 513
ethanol 476 553 466 443 533 553 514
heptane 446 523 447 430 509 522 482
nitrobenzene 486 575 481 464 558 571 528
THF 470 539 463 438 523 436 502
toluene 470 536 468 439 523 541 496
triethylamine 462 528 451 434 515 530 487
p-xylene 470 536 469 438 522 537 496 1 Position of the ICT bands are given in nm
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 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 determined as being the Catalan’s, Reichardt’s and Kamlet-
Taft’s scales. More precisely, in the case of the Kamlet-Taft empirical scale, better 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 in the π* parameter, the advantage of the
Catalan solvatochromic model 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 Figure 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 donors, namely TCF2, TCF3 and TCF6,
squares of the correlation coefficients (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.
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 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, 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.
Figure 5. Linear correlations obtained while using three different polarity scales for TCF3 (Catalan a)
and b) Kamet-Taft c), and Reichardt d) empirical scales).
Examination of the slopes obtained by plotting the HOMO-LUMO gaps vs. the Taft parameters
is indicative of the electronic redistribution occurring within the dyes upon excitation. The same holds
true if the Catalan or the Reichardt’s plots are considered. As shown in Figure 6a, all dyes proved to be
sensitive to the solvent polarity. However, three different groups could be identified (See Figure 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 delocalization 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 literature for Michler’s aldehyde-based dyes comprising
terpyridine as 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.
0.0 0.2 0.4 0.6 0.8 1.0
1.8
2.0
2.2
2.4
2.6
2.8
3.0
E
(eV
)
Taft parameter (*)
TCF1
TCF2
TCF3
TCF4
TCF5
TCF6
TCF7
TCF8
TCF9
TCF10
TCF11
TCF12
TCF13
0.0 0.2 0.4 0.6 0.8 1.0
1.8
2.0
2.2
2.4
2.6
2.8
3.0
E
(e
V)
Taft parameter (*)
TCF2
TCF3
TCF4
TCF5
TCF6
TCF7
TCF8
TCF9
TCF10
TCF11
TCF12
Figure 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.
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 Figure 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 respectively 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]
400 450 500 550 600 650 7000.0
0.2
0.4
0.6
0.8
1.0
TCF3
TCF2F
luo
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nc
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nte
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(a
.u.)
(nm)
TCF1
400 450 500 550 600 650 7000.0
0.2
0.4
0.6
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Flu
ore
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a.u
.)
(nm)
400 450 500 550 600 650 7000.0
0.2
0.4
0.6
0.8
1.0
Flu
ore
sc
en
ce
in
ten
sit
y (
a.u
.)
(nm)
Figure 7. UV-visible and photoluminescence spectra of TCF1-TCF3 recorded in chloroform.
2.4. Theoretical investigations
Optical properties of the thirteen dyes were also examined theoretically 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 the 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 Figure 8.
200 300 400 500 600 700 8000
20000
40000
60000
80000
100000
120000
140000
160000
180000
Wavelength (nm)
TCF1
TCF2
TCF3
TCF4
TCF5
TCF6
TCF7
TCF8
TCF9
TCF10
TCF11
TCF12
TCF13
Figure 8. Simulated absorption spectra of TCF1-TCF13 in dilute dichloromethane.
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 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 determined. 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 structure of the series, rendering its UV-visible absorption
spectrum more difficult to model.
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 -6.39 -6.32 -3.57 -1.85 498 -2.49 512 -2.42
TCF2 -6.80 -6.01 -3.29 -1.35 515 -2.41 577 -2.15
TCF3 -6.61 -5.76 -3.06 -1.17 519 -2.39 592 -2.09
TCF4 -7.16 -6.43 -3.48 -1.48 461 -2.69 457 -2.71
TCF5 -7.00 -6.26 -3.35 -1.41 481 -2.58 474 -2.62
TCF6 -6.06 -5.59 -3.19 -1.38 604 -2.05 679 -1.83
TCF7 -6.95 -6.19 -3.30 -1.35 484 -2.56 482 -2.57
TCF8 -6.86 -6.33 -3.91 -3.66 644 -1.92 564 -2.20
TCF9 -7.26 -6.58 -3.59 -1.55 453 -2.74 473 -2.62
TCF10 -7.17 -6.51 -3.53 -1.59 469 -2.64 457 -2.71
TCF11 -6.81 -6.12 -3.57 -1.92 546 -2.27 547 -2.27
TCF12 -6.71 -5.95 -3.41 -1.63 566 -2.19 564 -2.20
TCF13 -6.88 -6.15 -3.49 -1.84 550 -2.25 515 -2.41
Optimized geometries and the electronic distributions of the HOMO-LUMO orbitals of all dyes
are provided in the supporting information. As shown in the Figure 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 level on the TCF moiety. Interestingly, an asymmetric distribution of the
LUMO energy level of TCF8 could be found whereas the two TCF moieties 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 experimental and theoretical
position of the absorption maximum.
Figure 9. Contour plots of the HOMO and LUMO energy levels of TCF1 and TCF8.
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 the Figure 10. Redox potentials of all dyes against the
half-wave oxidation potential of the ferrocene/ferrocenium cation pair are given in the Table 5.
In this study, all the compounds possess the same electron accepting group but differ by the
nature of their electron-donating groups. So,as 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).
Oppositely, the different donor parts lead to diverse oxidative potentials. 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
HOMO LUMO
TCF1
HOMO LUMO
TCF8
compounds since TCF5, TCF 7 and TCF 9 have very close oxydation potentials and this phenomenom
is also observed between TCF10 and TCF4, which are ortho and para substitued dyes (See Figure 10).
-1 0 1 2
-1
0
1 TCF6
I (µ
A)
Ewe/V (vs Ag wire)
-1 0 1 2
-4
-2
0
2
4
TCF13
I (µ
A)
Ewe/V (vs Ag wire)
-1 0 1 2
-2
-1
0
1
2
3
TCF4
I (µ
A)
Ewe/V (vs Ag wire)
-1 0 1 2
-6
0
6 TCF10
I (µ
A)
Ewe/V (vs Ag wire)
Figure 10. Comparaisons between the cyclic voltammograms of TCF13/TCF6 (left) and TCF10/TCF4
(right) measured in dichloromethane scan rate 100 mV·s-1, with tetrabutylammonium perchlorate
(TBAP) (0.1 M) as the supporting electrolyte.
Table 5. Electrochemical characteristics of studied compounds TCF1-TCF13.
Ered
Ered
onset EOx
Eox
onset EOx EOx EHOMO ELUMO ΔEET ΔEopt
V/Fc V/Fc V/Fc V/Fc V/Fc V/Fc eV eV eV eV
TCF10 -1.05 -0.98 1.39 1.27 - - -6.07 -3.81 2.26 2.71
TCF4 -1.11 -1.03 1.24 1.15 - - -5.95 -3.77 2.18 2.71
TCF7 -1.15 -1.06 1.04 0.94 - - -5.74 -3.74 2.00 2.57
TCF9 -1.11 -1.02 1.02 0.92 - - -5.72 -3.78 1.94 2.62
TCF5 -1.13 -1.06 1.06 0.87 - - -5.67 -3.74 1.93 2.62
TCF13 -1.04 -0.96 1.10 0.84 - - -5.64 -3.84 1.80 2.41
TCF1 -1.03 -1.33 0.91 0.81 0.99 1.15 -5.61 -3.47 2.14 2.42
TCF11 -1.06 -0.99 0.73 0.64 - - -5.44 -3.81 1.63 2.27
TCF12 -1.09 -1.02 0.66 0.58 - - -5.38 -3.78 1.60 2.20
TCF8 -1.13 -1.04 0.64 0.55 - - -5.35 -3.76 1.60 2.20
TCF2 -1.16 -1.10 0.56 0.43 - - -5.23 -3.70 1.53 2.15
TCF3 -1.27 -1.19 0.52 0.43 - - -5.23 -3.61 1.61 2.09
TCF6 -1.13 -1.05 0.37 0.29 0.56 0.66 -5.09 -3.74 1.35 1.83
All potentials recorded in 0.1M TBAP/dichloromethane. EHOMO (eV) = - 4.8 – Eox onset and ELUMO (eV) =
-4.8 – Ered onset
A second group composed of triphenylamine with close oxydation 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.
Finaly, 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 0.43V 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 results obtained by DFT calculations and
the experiment results determined by UV-visible absorption spectroscopy.[39] It has to be noticed that
more electrochemical processes can be detected in the cyclic voltammogram of TCF6 compared to all
the other dyes. Indeed, due to the presence of the elongated π-conjugated vinylic spacer, dimerization
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
0.66V 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
methoxybenzene 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 carbazole 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 potentials are still lower than that of all the OMe-based
compounds but remains greater than that determined for the TPA-based dyes and the dyes based on
the dimethylaniline donors.
1 2 3 4 5 6 7 8 9 10 11 12 13-8
-7
-6
-5
-4
-3
-2
-1
En
erg
y L
ev
els
(e
V)
HOMO-DFT
LUMO-DFT
HOMO-CV
LUMO-CV
Compounds TCFx (x=1-13)
Figure 11. Comparison of frontier orbitals’ energy levels obtained from cyclic voltammetry and DFT
calculation.
Furthermore, the HOMO and LUMO energy levels of all the molecules 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 presented in Figure 11. A good agreement between the experimental and the
theoretical results are obtained. Especially, a between adequation between the theoretical and the
experimental values of the LUMO level that for the HOMO level was found for all dyes.
4. 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 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.
Acknowledgements
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).
Conflicts of Interest
The authors declare no conflict of interest.
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