Influence of Donor-Substituents on Triphenylamine Chromophores Bearing Pyridine Fragments

Article abstract of DOI:10.1002/ejoc.201900026

Efficient synthetic routes that combine different palladium-catalyzed cross-coupling reactions have been developed for the preparation of a new family of push-pull derivatives in which pyridine was used as the acceptor group and different para-substituted diphenylamines as the donor groups. All compounds showed absorption in the UV/Vis region and blue-green emission with high quantum yields. Significant red shifts were observed in the absorption and fluorescence emission maxima on increasing the electron-donating ability of the substituents or on incorporating a π-conjugated linker. This finding can be explained on the basis of enhanced intramolecular charge transfer (ICT). Strong emission solvatochromism confirmed the formation of an intramolecular charge-separated emitting state. The HOMO-LUMO energy gaps have been estimated by experimental electrochemical measurements and the results were interpreted with the aid of DFT calculations. The thermal behavior of all materials has also been studied by differential scanning calorimetry.

Full text of DOI:10.1002/ejoc.201900026

A Journal of
Accepted Article
Title: Influence of donor-substituents on triphenylamine chromophores
bearing pyridine fragments
Authors: Jiri Tydlitat, Michaela Feckova, Pascal le Poul, Oldrich Pytela,
Milan Klikar, Julian Rodriguez-Lopez, Françoise Robin-le
Guen, and Sylvain Achelle
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900026
Link to VoR: http://dx.doi.org/10.1002/ejoc.201900026
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
FULL PAPER
A series of push-pull
Push-pull chromophores
chromophores with a pyridine
electron-withdrawing fragment
and methoxy-, 9H-carbazol-9-yl-
and 5-methoxythiophen-2-yl-
substituted diphenylamino
groups as electron-donating units
was designed.
Jiří Tydlitát,* Michaela Fecková,
Pascal le Poul, Oldřich Pytela,
Milan Klikar, Julián Rodríguez-
López,* Françoise Robin-le Guen
and Sylvain Achelle*,
Page No. – Page No.
Influence of donor-substituents
on triphenylamine
chromophores bearing pyridine
fragments
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
Influence of donor-substituents on triphenylamine
chromophores bearing pyridine fragments
Jiří Tydlitát,*,a Michaela Fecková,a,b Pascal le Poul,b Oldřich Pytela,a Milan Klikar,a Julián
Rodríguez-López,*,c Françoise Robin-le Guenb and Sylvain Achelle*,b
a
Dr. Jiří Tydlitát, Michaela Fecková, Prof. Oldřich Pytela, Dr Milan Klikar Institute of
Organic Chemistry and Technology, Faculty of Chemical Technology, University of
Pardubice, Studentská 573, Pardubice 53210, Czech Republic. jiri.tydlitat@upce.cz
http://bures.upce.cz/tydlitat.html
b Michaela Fecková, Dr. Pascal le Poul, Prof. Françoise Robin-le Guen, Dr. Sylvain Achelle
Université de Rennes, CNRS, Institut des Sciences Chimiques de Rennes - UMR6226,
F35000 Rennes, France. sylvain.achelle@univ-rennes1.fr https://iscr.univ-rennes1.fr/omc/dr-
sylvain-achelle
c Prof. Julián Rodríguez-López Facultad de Ciencias y Tecnologías Químicas, Universidad de
Castilla-La Mancha, 13071 Ciudad Real, Spain. julian.rodriguez@uclm.es
Abstract
Efficient synthetic routes that combine different palladium-catalyzed cross-coupling reactions
have been developed for the preparation of a new family of push-pull derivatives in which
pyridine was used as the acceptor group and different para-substituted diphenylamines as the
donor groups. All compounds showed absorption in the UV-Vis region and blue-green
emission with high quantum yields. Significant red shifts were observed in the absorption and
fluorescence emission maxima on increasing the electron-donating ability of the substituents
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
or on incorporating a π-conjugated linker. This finding can be explained on the basis of
enhanced intramolecular charge transfer (ICT). Strong emission solvatochromism confirmed
the formation of an intramolecular charge-separated emitting state. The HOMO-LUMO
energy gaps have been estimated by experimental electrochemical measurements and the
results were interpreted with the aid of DFT calculations. The thermal behavior of all
materials has also been studied by differential scanning calorimetry.
Keywords: Luminescence; Nitrogen heterocycles; Charge transfer; Triphenylamine;
Solvatochromism
1. Introduction
During the past two decades there has been great interest in the design of push-pull structures
based on a π-conjugated core substituted with electron-donating (D) and electron-attracting
(A) groups due to their applications in emissive materials for sensors1 and organic light-
emitting diodes (OLEDs),2 bulk heterojunction3 and dye-sensitized solar cells (DSSCs),4 field
effect transistors,5 and nonlinear optics (NLO).6 The photophysical and electronic properties
of push-pull chromophores, which are closely linked to intramolecular charge transfer (ICT)
occurring in the molecule, can be easily tuned by tailoring the D/A couple,7,8 the π-conjugated
linker,7,9 or by designing advanced quadrupolar or tripodal arrangements.10
The diphenylamino group has been extensively used as an electron-donating group in push-
pull structures used as emissive materials,9b,11 NLO chromophores11d,12 and dyes for DSSCs.13
The electron-donating strength of the diphenylamino group is lower than that of dialkylamino
analogs14 but the diphenylamino derivatives generally have higher fluorescence quantum
yields,9b,11d, 15 thus making them more interesting for applications related to emission. A
significant branching effect that enhances the two-photon absorption (TPA) has been
observed on the triphenylamine core,16 but the polysubstitution of the triphenylamine core by
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
two of three electron-withdrawing arms generally leads to a blue shift of the emission.16b,17
Reinforcing the electron-donating strength of the diphenylamino group would allow red-
shifted emission to be obtained while maintaining a high fluorescence quantum yield. The 4-
[bis(4-methoxyphenyl)amino]phenyl group has been described by Marder and co-workers as
a promising combination of a reasonably high π-donor strength with the high stability
previously reported for 4-(diarylamino)phenyl donors.14 Some examples of methoxy-, 18
diphenylamino-19 and carbazolyl-19a, 20 substituted diphenylamino emissive materials have
been described in the literature. Nevertheless, to the best of our knowledge, systematic
comparisons have not been performed with regards to their unsubstituted analogs.
The pyridine fragment is a moderately strong electron-withdrawing group that can be used in
push-pull structures.21 The presence of a nitrogen atom with a lone electron pair enables the
photophysical properties of pyridine derivatives to be tuned upon protonation, complexation
and hydrogen-bond formation. In this context, the incorporation of this heterocycle as an
electron-withdrawing fragment in push-pull structures appears to be interesting for the design
of fluorescent sensors.22 We recently described acid-responsive triphenylamine derivatives
bearing pyridine fragments. 23 White photoluminescence was obtained in solution by
controlled protonation of the blue-emitting pyridine derivatives, which resulted in yellow-
orange emissive acidified forms.
In the work described here we designed a series of selected methoxy-, 9H-carbazol-9-yl- and
5-methoxythiophen-2-yl-substituted triphenylamines 1–6 linked to pyridine fragments and
assessed the structure-property relationships. The photophysical and electrochemical
properties were compared to those of the recently described23 unsubstituted triphenylamine
analogs A–C (Figure 1).
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European Journal of Organic Chemistry
Figure 1. Structures of target compounds 1–6 and their unsubstituted analogs A–C.
2. Results and Discussion
Synthesis. The methoxy-substituted triphenylamine derivatives 1–3 were obtained from
commercially available 4-bromo-N,N-bis(4-methoxyphenyl)aniline (Scheme 1). The Suzuki–
Miyaura cross-coupling reaction with pyridin-4-ylboronic acid gave chromophore 1 in good
yield, whereas compound 2 was synthesized in moderate yield by Heck cross-coupling with
4-vinylpyridine according to a reported procedure.24 The synthesis of the acetylenic derivative
3 required a four-step route: a halogen-metal exchange with iodine as electrophile to give
iodo-derivative 7, a Sonogoshira cross-coupling reaction with triphenylsilylacetylene
followed by deprotection of the trimethylsilyl group to obtain alkyne 9 and, finally, a second
Sonogashira cross-coupling reaction with 4-iodopyridine to afford the target chromophore in
good yield.
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European Journal of Organic Chemistry
Scheme 1. (i) pyridin-4-ylboronic acid, PdCl2(PPh3)2, Na2CO3, dioxane/H2O 4:1, ∆, 18 h (ii)
4-vinylpyridine, Pd(OAc)2, P(o-tol)3, diethylamine, DMF, 90 °C, 24 h (iii) n-Buli, I2, THF, –
78 °C, 3.5 h (iv) trimethylsilylacetylene, PdCl2(PPh3)2, CuI, iPr2NH, 60 °C, 1 h (v) K2CO3,
MeOH, rt, 18 h (vi) 4-iodopyridine, PdCl2(PPh3)2, CuI, dioxane/Et3N 4:1, 90 °C, 18 h.
The grafting of two carbazole fragments onto the triphenylamine core by a copper-catalyzed
reaction with either 4-bromo-4',4''-diiodotriphenylamine or 4,4',4"-triiodotriphenylamine led
to the bromo- and iodo-derivatives 10 and 11, respectively. These compounds were readily
converted into chromophores 4 and 5 in moderate yield by Suzuki–Miyaura cross-coupling
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
with pyridin-4-ylboronic acid or Heck cross-coupling with 4-vinylpyridine, respectively
(Scheme 2).
Scheme 2. (i) carbazole, 1,10-phenanthroline, CuI, Cs2CO3, DMF, 18 h, 200 °C (ii) pyridin-4-
ylboronic acid, PdCl2(PPh3)2, Na2CO3, dioxane/H2O 4:1, ∆, 18 h (iii) 4-vinylpyridine,
Pd(OAc)2, P(o-tol)3, diethylamine, DMF, 90 °C, 24 h.
Finally, compound 6 was prepared in two steps from 4-bromo-4',4''-diiodotriphenylamine. An
initial double Suzuki–Miyaura cross-coupling reaction with 2-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-5-methoxythiophene gave intermediate 12, which after a second Suzuki–
Miyaura reaction with pyridin-4-ylboronic acid led to the desired compound 6 in good yield
(Scheme 3).
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European Journal of Organic Chemistry
Scheme
3.
(i)
2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-5-methoxythiophene,
PdCl2(PPh3)2, Na2CO3, 1,4-dioxane/H2O 4:1, ∆, 18 h (ii) pyridin-4-ylboronic acid,
PdCl2(PPh3)2, Na2CO3, dioxane/H2O 4:1, ∆, 18 h.
Photophysical properties. The UV/Vis and photoluminescence (PL) spectroscopic data for
compounds 1–6 measured in CHCl3 at room temperature are presented in Table 1. The
analyses were carried out using low concentrations of chromophores (0.5–1.5 × 10–5 M). To
facilitate comparison of the photophysical properties, the results for unsubstituted
triphenylamine analogs A–C are also included in Table 1. As representative examples, the
spectra of compounds 1, 4 and 6 are provided in Figure 2. Compounds 1–6 exhibit blue-green
emission with quantum yields between 0.31 and 0.80. In all cases, a significant red shift in the
emission maximum was observed in comparison to the unsubstituted analogs A–C. It can be
seen from Figure 2 that the absorption maxima of 1, 4 and 6 depend on the electron-donating
substituent and the value increases in the following order: 9H-carbazol-9-yl (4) < methoxy (1)
< 5-methoxythiophen-2-yl (6). As far as the emission is concerned, the 9H-carbazol-9-yl
derivative 4 is also the most strongly blue-shifted whereas the methoxy- and 5-
methoxythiophen-2-yl- derivatives 1 and 6 exhibit similar emission maxima at λmax = 511 nm.
As observed for compounds A–C and other families of chromophores,9b,25 absorption and
emission increase when an acetylenic linker is placed between the triphenylamine fragment
and the pyridine moiety and an even more remarkable increase is observed when a vinylic
linker is incorporated [in the methoxy series λmax (1) < λmax (3) < λmax (2) and in the carbazole
series λmax (4) < λmax (5)]. Compounds 1–6 exhibit red-shifted absorption in the presence of
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
camphorsulfonic acid due to protonation of the pyridine ring, which enhances the ICT.
Nevertheless, in contrast to unsubstituted triphenylamine derivatives A–C, the protonated
forms of 1–6 are not emissive.
Table 1. UV/Vis and PL data in CHCl3.
UV/vis λmax, nm UV/vis λmax, nmb
PL
Stokes shift
Compda
c
ΦF
cm–1
(ε
, mM–1·cm–1)
352 (26.6)
(ε
, mM–1·cm–1)
430 (39.0)
λmax, nm
511
533
521
473
510
511
444
485
456
0.60
0.48
0.52
0.44
0.80
0.31
0.50
0.45
0.78
8840
6301
7760
8184
6231
7604
6130
5288
5097
1
2
399 (38.1)
371 (36.5)
341 (50.1)
387 (36.9)
368 (68.5)
349 (25.6)
386 (28.0)
370 ((31.0)
485 (41.5)
460 (37.9)
429 (32.4)
468 (41.9)
445 (35.9)
416 (28.9)
467 (27.5)
449 (31.0)
3
4
5
6
A23
B23
C23
a All spectra were recorded at room temperature at c = 0.5 × 10–5 to 1.5 × 10–5 M. b Data
c
in the presence of 10–2 M camphorsulfonic acid. Fluorescence quantum yield (±10%)
determined relative to 9,10-bis-phenylethynylanthracene in cyclohexane (ΦF = 1.00).26
Figure 2. Normalized absorption (dashed lines) and emission spectra (solid lines) of
compounds 1, 4 and 6 in CHCl3 solution.
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Emission solvatochromic studies were performed in an effort to gain an insight into the
photophysical processes of these new push-pull molecules. Thus, the emission spectra of
compounds 1–6 were recorded in a series of aprotic solvents of different polarity (Table 2,
27
Figure 3). As expected for emissive push-pull derivatives,9b,
strong positive
solvatochromism was observed on increasing the polarity, as estimated by the Dimroth–
Reichardt scale. 28 For each compound the emission maximum was plotted versus the
Dimroth–Reichardt polarity parameter (Figure S40, see Supporting Information) and in all
cases good linearity was obtained. The slope of the corresponding regression line (SP) allows
an evaluation of the ICT into the chromophores. As expected, the highest SP values were
obtained for compounds 2 and 3, which exhibit the most red-shifted absorption and emission
in CHCl3. Compounds 1–3, 5 and 6 have higher SP values than their unsubstituted analogs.
Surprisingly, compound 4 has a lower SP value than A, thus confirming that the carbazole
fragments on the triphenylamino core have a significantly weaker influence on the ICT than
the two other substituents evaluated (methoxy and 5-methoxythiophen-2-yl).
Table 2. Emission solvatochromism in various aprotic solvents.
Heptane Toluene 1,4-Dioxane THF
30.9a 33.9a 36.0a 37.4a
λmax, nm λmax, nm λmax, nm λmax, nm λmax, nm λmax, nm λmax, nm
CH2Cl2 Acetone MeCN
40.7a 42.2a 45.6a
SPb
Compd
7.22
9.94
9.91
5.37
7.19
7.08
5.60
6.38
7.11
451
468
460
426
455
454
389
424
398
475
500
480
443
471
469
412
448
421
477
511
484
444
478
473
-
508
556
522
466
509
496
433
475
450
523
574
540
476
524
519
451
498
473
536
590
560
487
536
532
458
500
481
554
607
605
504
555
552
472
515
497
1
2
3
4
5
6
A23
B23
C23
-
-
ET(30), Dimroth–Reichardt polarity parameter in kcal mol–1. Slope of the regression line
a
b
λmax (nm) versus ET(30) in nm mol kcal–1.
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Figure 3. Fluorescence color changes for 5 in various solvents (from left to right: n-heptane,
toluene, 1,4-dioxane, CHCl3, CH2Cl2 and MeCN). Photographs were taken in the dark upon
irradiation with a hand-held UV lamp (λem = 366 nm).
Electrochemical properties. The redox properties of the push-pull molecules 1–6 were
studied by cyclic voltammetry (CV) in DMF, using NBu4PF6 (0.1M) as the supporting
electrolyte in a three-electrode cell (working electrode: glassy carbon; counter electrode: Pt;
reference: Ag wire). All measurements were obtained using Fc+/Fc as the internal reference.
The potential values are given vs. saturated calomel electrode (SCE)29 for comparison with
unsubstituted triphenylamine molecules A–C23 (Table 3). The cyclic voltammograms are
presented in Figure 4.
Table 3. Electrochemical data,a EHOMO and ELUMO values.b
Compd
Eox (V)
0.79c
0.75c
0.79c
0.99d
0.94e
0.77e
1.07
Ered (V)
–2.26c
–2.02d
–2.08d
–2.34e
–1.92d
–2.31c
–2.15
EHOMO (eV)
–5.14
ELUMO (eV)
–2.09
∆E (eV)
3.04
2.76
2.88
3.33
2.87
3.08
3.22
2.87
3.05
1
2
3
4
5
–5.10
–5.14
–5.34
–5.29
–5.12
–5.42
–5.41
–5.44
–2.33
–2.27
–2.00
–2.42
–2.04
–2.20
–2.54
–2.40
6
A23
B23
C23
1.06
1.09
–1.82
–1.96
a Potentials are given vs. SCE, glassy carbon working electrode, Pt counter electrode, 20 °C in
DMF, 0.1M NBu4PF6 as supporting electrolyte, 100 mV s–1 scan rate. Ferrocene was used as
b
internal reference (E1/2 = +0.44 V vs. SCE). EHOMO, ELUMO were determined empirically
from the oxidation and reduction potentials according to the empirical relationship EHOMO = –
(Eox + 4.35) eV; ELUMO = –(Ered + 4.35) eV. c E1/2. d Quasi-reversible. e Irreversible.
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European Journal of Organic Chemistry
Figure 4. Cyclic voltammograms of compounds 1–6 with NBu4PF6 as the supporting
electrolyte, measured in DMF (working electrode: glassy carbon; counter electrode: Pt;
reference: Ag wire), 100 mV s–1 scan rate.
Compounds 1–6 all exhibited one or several oxidation processes. Molecules 1–3, which bear
4-methoxy substituents on two phenyl groups of the triphenylamine moiety, showed a first
reversible redox process (E1/2 = +0.75/+0.79 V) that became quasi-reversible for carbazole
derivatives 4–5 and irreversible for compound 6, which contains methoxythiophene branches,
at a scan rate of 100 mV s–1. The potential values for the first oxidation waves of 4 (E1/2ox = +
ox
0.99 V) and 5 (Ep = +0.94 V), which bear carbazole substituents, are higher than those for
compounds 1–3 and 6 but approach the values obtained for A, B and C.23 The lower oxidation
potentials obtained for 1–3 can be explained by the increase in the electron-donating effect of
the triphenylamine containing methoxy groups. Incorporation of a thienyl ring in compound 6
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
did not affect the potential value (when compared to 1). Finally, the incorporation of
carbazole units only has a weak influence on the electron-donating character of the substituted
triphenylamine.
Compounds 1 and 6 exhibited a first reversible reduction process on scanning from 0 to –2.5
V, whereas the process was quasi-reversible for compounds 2, 3 and 5. A loss of reversibility
was observed for 4. Potential values (E1/2red or Epred) were in the range from –1.92 V to –2.34
V vs. SCE. As reported previously for molecules A–C, compounds bearing an acetylenic or
vinylic π-linker are reduced at more positive potentials. The same trend was observed for the
carbazole derivatives 4 and 5. These results show that the reduction of the pyridine moiety is
easier after the incorporation of either an acetylenic or vinylic π-linker.
The values of the EHOMO, ELUMO and the energy band gap (∆E) were estimated for all
molecules from the experimental Eox and Ered potentials by the empirical relationship EHOMO
=
–(Eox + 4.35) eV; ELUMO = –(Ered + 4.35) eV (Table 3). The results indicate that the 4-
methoxytriphenylamine derivatives 1–3 have a lower energy gap in comparison to molecules
A–C. Furthermore, the energy gap remained almost unchanged on incorporating a thienyl
group (in 6) between the methoxy substituent and the phenyl groups of the triphenylamine.
Also, the HOMO-LUMO overlap values obtained for chromophores 4 and 5 in comparison
with molecules A and C confirm the weak effect of donor modification by grafting a
carbazole moiety. The trends outlined above are consistent with the variations in λmax of the
charge transfer bands observed in the electronic spectra of compounds 1–6. Contrary to most
cases, the estimated electrochemical gap of 4 (3.33 eV) is greater than that for A (3.22 eV).
However, this is also consistent with the λmax measurements obtained from the electronic
spectra: the maximum wavelength for 4 is lower (341 nm) than that for A (349 nm).
Thermal properties. The thermal behavior of compounds 1–6 was studied by differential
scanning calorimetry (DSC). The thermographs are shown in Figures S33–S38 of the
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European Journal of Organic Chemistry
Supporting Information, while the melting points (Tm) and thermal decomposition
temperatures (Td) are listed in Table 4.
Table 4. Thermal properties of compounds 1–6.
Compd
Tm (°C)
Td (°C)
1
155
330
2
-
285
3
-
285
4
-
300
5
249
260
6
-
200
Only chromophores 1 and 5 exhibited an endothermic peak due to melting. All of the other
compounds decomposed directly without melting. Carbazole derivative 5, with an embedded
ethenylene spacer, was not stable in the liquid state (Tm = 249 °C). This compound gradually
decomposed upon continued heating above its melting point. Loss of residual water was also
observed for 5 up to 120 °C. This finding is in contrast with the methoxy derivative 1, which
contains an unextended π-linker, which was stable in the liquid phase for a further 175 °C
above its melting point. Structural analogs 2 and 3, which differ only in the π-spacer (double
or triple bond), exhibited almost identical thermal behavior, with a decomposition temperature
estimated at Td = 285 °C. Similar DSC curves were also recorded for molecules 4 and 6, for
which a broad endothermic peak due to the enantiotropic solid-solid transition was detected at
150 °C and 90 °C, respectively. This transition was immediately followed by a second
undefined endothermic phenomenon and subsequent exothermic decomposition for both
chromophores.
The gradual elongation of the π-spacer in the methoxy analogs 1/2/3 led to a reduction in the
thermal stability (none/ethenylene/acetylene spacer → Td = 330/285/285 °C). An analogous
effect was also observed for the carbazole derivatives 4/5 (none/ethenylene spacer → Td =
300/260 °C). The lowest Td value was determined for compound 6, which contained
methoxythiophene donors. In general, the enlargement of the π-linker seemed to decrease the
thermal robustness of the compounds under investigation. Regarding the variation of the
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
donors, this structural modification increased the thermal stability in the order 5-
methoxythien-2-yl → N-carbazole → methoxy group.
DFT calculations. The Gaussian W09 package30 was employed to investigate the spatial and
electronic properties of chromophores 1–6 at the DFT level. The initial geometries of the
molecules 1–6 were estimated and optimized using the DFT B3LYP/6-31G(d) and DFT
B3LYP/6-311G(d,p) methods. The HOMO and LUMO energies, their differences, and the
ground state dipole moments were calculated at the DFT B3LYP/6-311++G(d,f,p) level and
the results are summarized in Table 5.
Table 5. DFT calculated properties of chromophores
Compd Symmetry group EHOMO (eV) ELUMO (eV) ∆E (eV) µ (D)
C2
C1
C2
C2
C1
C2
C2
C1
C2
–5.13a
–5.07a
–5.16a
–5.54a
–5.46a
–5.03a
–5.44b
–5.36b
–5.45b
–1.41a
–1.91a
–1.80a
–1.83a
–2.25a
–1.60a
–1.57b
–2.05b
–1.94b
3.72
3.16
3.36
3.71
3.21
3.43
3.88
3.30
3.51
4.1
5.3
5.5
1.6
2.1
2.8
3.8
4.8
5.0
1
2
3
4
5
6
A23
B23
C23
a Calculated at the DFT B3LYP/6-311++G(d,f,p) level. b Calculated at the DFT
B3LYP/6-311++G(2d,p) level.
The calculated energies of the HOMO and LUMO of pyridine derivatives 1–6 ranged from –
5.54 to –5.03 and from –2.25 to –1.41 eV, respectively. It is clear that the calculated
EHOMO/LUMO values reflect structural differences between the individual chromophores and
show a close correlation with the electrochemical data (Figure 5). The electron-donating
ability of methoxy, carbazole and methoxythiophene moieties, the nature of the π-linker, and
the electron-withdrawing character of the pyridine ring all have a significant effect on the
calculated HOMO-LUMO gaps (∆E). The calculated gaps, which are within the range ∆E =
3.72–3.16 eV, correlate well with the electrochemical data, with differences not exceeding
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0.68 eV. The calculated HOMO-LUMO gaps for the linear chromophores 1–3 resemble those
obtained for unsubstituted triphenylamine derivatives A–C.23 The largest ∆E values (3.72 and
3.88 eV) were calculated for compounds 1 and A (without a spacer), whereas the narrowest
gaps of 3.16 and 3.30 eV were computed for compounds 2 and B, which contain a vinyl
spacer. It can also be seen that the introduction of two electron donors on the central
triphenylamine core reduced the HOMO-LUMO gaps when compared to the parent
derivatives A–C (Figure 5). The calculated ∆E values are almost identical for the methoxy
and carbazole derivatives. On the other hand, for methoxythienyl derivative 6 the ∆E value is
significantly lower. The HOMO/LUMO localizations are represented in Figure 6. The HOMO
is located either on the central amino donor atom (1–5), the carbazole (4 and 5) or the
phenylene units bearing the methoxythiophene substituents (6). On the other hand, the LUMO
occupies the pyridine acceptor and the adjacent π-system. Hence, all molecules 1–6 can be
considered as charge-transfer chromophores. The calculated ground state dipole moments
reflect the molecular structures and symmetry. Chromophores 4–6 bear bulky carbazole and
methoxythiophene groups and these showed low dipole moments of 1.6–2.8 D, whereas
methoxy derivatives 1–3 have dipole moments in the range 4.1 to 5.5 D.
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
Figure 5. Energy level diagram for linear chromophores 1–3 and A–B.
Figure 6. Representative HOMO (red) and LUMO (blue) localizations in 1–6.
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
3. Conclusions
A series of push-pull derivatives has been designed with a pyridine electron-withdrawing
fragment and methoxy-, 9H-carbazol-9-yl- and 5-methoxythiophen-2-yl-substituted
diphenylamino groups as electron-donating units. The reinforcement of the electron-donating
character of the diphenylamino group leads to enhanced ICT into the push-pull structure with
respect to their unsubstituted diphenylamino derivatives, as demonstrated by spectroscopic
and electrochemical measurements as well as by DFT calculations. This enhanced ICT also
explains the emission quenching observed for compounds 1–6 upon protonation in contrast to
the results observed for A–C.23
4. Experimental section
General: All reactions were carried out in vacuum-dried Schlenk flasks under Ar. All
compounds and solvents were purchased from Aldrich or TCI and were used without further
purification, except for tetrahydrofuran, which was freshly distilled from Na/benzophenone
ketyl. 4-Iodopyridine31 and 4-bromo-N,N-bis(4-iodophenyl)aniline19b were obtained according
to reported procedures. Column chromatography was carried out using silica gel 60 (particle
size 0.040–0.063 mm, 230–400 mesh; Merck) and commercially available solvents. Thin-
layer chromatography (TLC) was conducted on aluminium sheets coated with silica gel 60
1
13
F254 and compounds were visualized with a UV lamp (254 or 360 nm). H and C NMR
spectra were recorded in CDCl3 at 500 MHz and 125 MHz, respectively, at 25 °C on a Bruker
AscendTM instrument, using the appropriate solvent signal (CDCl3 7.25 and 77.23 ppm) as
internal reference. The apparent resonance multiplicities are described as s (singlet), d
(doublet), t (triplet) and m (multiplet). High-resolution MALDI mass spectra were measured
on an LTQ Orbitrap XL MALDI mass spectrometer (Thermo Fisher Scientific, Bremen,
Germany) equipped with a nitrogen UV laser (337 nm, 60 Hz). The LTQ Orbitrap instrument
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
was operated in positive-ion mode over a normal mass range (m/z 50–2000) with a resolution
of 100,000 at m/z = 400. 2,5-Dihydroxybenzoic acid (DHB) was used as a matrix. Thermal
properties were measured by differential scanning calorimetry DSC on a Mettler-Toledo
STARe System DSC 2/700 equipped with an FRS 6 ceramic sensor and HUBER TC100-MT
RC 23 cooling system in open aluminum crucibles under an N2 atmosphere. DSC curves were
obtained at a scanning rate of 3 °C min–1.
DFT calculations: HOMO/LUMO localizations in 1–6 were derived from the calculations
using the Gaussian W09 package at the DFT level. Geometries for visualizations were
optimized using the DFT B3LYP/311G method and are visualized as red-HOMO localization
and blue-LUMO localization. The visualizations were generated in the program OPchem.32
4-Iodo-N,N-bis(4-methoxyphenyl)aniline (7): In a Schlenk tube under an inert atmosphere
of argon, 4'-bromo-N,N-bis(4-methoxyphenyl)aniline (1.31 g, 3.42 mmol) was dissolved in
anhydrous THF (30 mL). The resulting solution was cooled to –78 °C and n-BuLi (1.51 mL,
2.5 M solution in hexanes, 3.764 mmol) was added dropwise. The reaction mixture was
stirred at –78 °C for 30 min and iodine (0.869 g, 3.42 mmol) in anhydrous THF (5 mL) was
added dropwise. The resulting mixture was slowly warmed and stirred for 3 h. Solvents were
removed under reduced pressure and the mixture was diluted with CH2Cl2 (50 mL) and
saturated aqueous solution of NH4Cl (70 mL). The aqueous layer was washed with CH2Cl2 (2
× 30 mL). The combined organic layers were washed with brine (50 mL) and dried with
Na2SO4. After removal of the solvent under reduced pressure, the residue was purified by
flash chromatography (SiO2, CH2Cl2/hexane 1:2) and subsequent recrystallization from
boiling ethanol to give compound 7 as a colorless solid (0.864 g, 2.00 mmol, 59%). 1H NMR
(400 MHz, CDCl3): δ = 3.79 (s, 6H), 6.66–6.83 (m, 2H), 6.80–6.84 (m, 4H), 7.01–7.03 (m,
4H), 7.39–7.41 (m, 2H). HRMS (MALDI) m/z calcd. for C20H18INO2: 595.0131 [M+], found
595.0144. The spectroscopic data are consistent with those reported in the literature.33
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4-Methoxy-N-(4-methoxyphenyl)-N-{4-[(trimethylsilyl)ethynyl]phenyl}aniline (8): In a
Schlenk tube under an inert atmosphere of argon, N,N-bis(4-methoxyphenyl)-4'-iodoaniline
(394 mg, 0.914 mmol), trimethylsilylacetylene (152 µL, 1.10 mmol), CuI (4 mg, 0.021 mmol)
and PdCl2(PPh3)2 (32 mg, 0.046 mmol) were dissolved in anhydrous diisopropylamine (25
mL). The resulting solution was stirred at 60 °C for 1 h. The mixture was cooled and
diisopropylamine was removed under reduced pressure. The mixture was diluted with diethyl
ether (50 mL) and water (70 mL). The aqueous layer was washed with diethyl ether (2 × 30
mL). The combined organic layers were washed with brine (50 mL) and dried with Na2SO4.
After removal of the solvent under reduced pressure, the residue was purified by column
chromatography (SiO2, CH2Cl2/hexane 1:2) to give compound 8 as a yellow oil (350 mg,
1
0.872 mmol, 95% yield). H NMR (400 MHz, CDCl3): δ = 0.24 (s, 9H), 3.78 (s, 6H), 6.77–
6.84 (m, 6H), 7.05–7.08 (m, 4H), 7.25–7.23 (m, 2H). EI-MS (70 eV) m/z (rel. int.): 401 (M+,
100), 386 (40), 193 (10). The spectroscopic data are consistent with those reported in the
literature.34
4-Ethynyl-N,N-bis(4-methoxyphenyl)aniline (9): In a round-bottomed flask N,N-bis(4-
methoxyphenyl)-4'-(trimethylsilylethynyl)aniline (350 mg, 0.872 mol) and K2CO3 (311 mg,
1.31 mol) were dissolved in methanol (50 mL). The reaction mixture was stirred for 18 h at
room temperature. The methanol was removed under reduced pressure and the mixture was
diluted with CH2Cl2 (50 mL) and water (100 mL). The aqueous layer was washed with
CH2Cl2 (2 × 30 mL), brine (50 mL) and dried with Na2SO4. After removal of the solvent
under reduced pressure, the residue was purified by column chromatography (SiO2,
CH2Cl2/hexane 1:2) to give compound 9 as a yellow oil (232 g, 0.704 mmol, 60%). 1H NMR
(400 MHz, CDCl3): δ = 3.01 (s, 1H), 3.76 (s, 6H), 6.77–6.84 (m, 6H), 7.01–7.05 (m, 4H),
7.23–7.25 (m, 2H). EI-MS (70 eV) m/z (rel. int.): 329 (M+, 100), 314 (70). The spectroscopic
data are consistent with those reported in the literature.31
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N,N-Bis[4-(9H-carbazol-9-yl)phenyl]-4-bromoaniline (10): In a glass autoclave 4'-bromo-
N,N-bis(4-iodophenyl)aniline (210 mg, 0.365 mmol), carbazole (152 mg, 0.909 mmol), 1,10-
phenanthroline (120 mg, 0.666 mmol), CuI (60 mg, 0.315 mmol) and Cs2CO3 (490 mg, 1.50
mmol) were dissolved in dry DMF and argon was bubbled through for 5 min. The reaction
mixture was stirred at 200 °C for 18 h. The resulting mixture was cooled and the DMF was
removed under reduced pressure. Saturated aqueous NaHCO3 (70 mL) was added and the
mixture was extracted with CH2Cl2 (3 × 50 mL). The organic layer was washed with brine (70
mL) and dried with Na2SO4. After removal of the solvent under reduced pressure, the residue
was purified by column chromatography (SiO2, CH2Cl2/hexane 1:2) to give compound 10 as a
1
colorless solid (86 mg, 0.131 mmol, 36%). H NMR (400 MHz, CDCl3): δ = 7.18–7.20 (m,
2H), 7.28–7.31 (m, 4H), 7.36–7.38 (m, 4H), 7.41–7.50 (m, 14H), 8.15 (d, J = 7.5 Hz, 4H).
HRMS (MALDI) m/z calcd. for C42H28BrN3: 653.1461 [M+], found 653.1473. The
spectroscopic data are consistent with those reported in the literature.20a
N,N-Bis[4-(9H-carbazol-9-yl)phenyl]-4-iodoaniline (11): This compound was obtained
from tris(4-iodophenyl)amine (623 mg, 1 mmol) and carbazole (335 mg, 2 mmol) by the same
procedure as described for 10. The crude product was purified by column chromatography
1
(SiO2; petroleum ether:CH2Cl2, 4:1). Colorless solid (217 mg, 31%). H NMR (300 MHz,
CDCl3):
δ
= 7.07–7.10 (m, 2H), 7.28–7.53 (m, 20H), 7.67–7.70 (m, 2H), 8.15–8.18 (m, 4H)
= 86.8, 109.9, 120.1, 120.5, 123.5, 125.3, 126.1, 126.6,
13
ppm. C NMR (75 MHz, CDCl3):
δ
128.3, 132.9, 138.8, 141.1, 146.3, 147.3 ppm. HRMS (MALDI) m/z calcd. for C42H28IN3:
701.1322 [M+], found 701.1319.
4-Bromo-N,N-bis[4-(5-methoxythiophen-2-yl)phenyl]aniline (12): In a Schlenk tube under
an inert atmosphere of argon, 4'-bromo-N,N-bis(4-iodophenyl)aniline (218 mg, 0.379 mmol),
2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-5-methoxythiophene (200 mg, 0.833 mmol),
and Na2CO3 (80 mg, 0.755 mmol) were dissolved in dioxane/water 4:1 (25 mL) and argon
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was bubbled through for 5 min, followed by the addition of PdCl2(PPh3)2 (11 mg, 0.016
mmol). The reaction mixture was stirred at 90 °C for 18 h. The mixture was then cooled and
the solvents were removed under reduced pressure. The residue was diluted with CH2Cl2 (50
mL) and water (70 mL). The aqueous layer was extracted with CH2Cl2 (2 × 30 mL). The
combined organic layers were washed with brine (50 mL) and dried with Na2SO4. After
removal of the solvent under reduced pressure, the residue was purified by column
chromatography (SiO2, CH2Cl2/hexane 1:2) to give compound 12 as an orange solid (148 mg,
1
0.270 mmol, 71%). H NMR (500 MHz, CDCl3): δ = 3.90 (s, 6H), 6.16 (d, J = 3.9 Hz, 2H),
6.86 (d, J = 3.9 Hz, 2H), 6.96–6.98 (m, 2H), 7.02–7.04 (m, 4H), 7.32–7.36 (m, 6H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 60.4, 104.9, 115.3, 120.2, 124.7, 125.4, 126.0, 130.0, 130.1,
132.5, 145.8, 146.7, 165.8 ppm. HRMS (MALDI) m/z calcd. for C28H22BrNO2S2: 547.0270
[M+], found 547.0278.
General Procedure for the Suzuki–Miyaura cross-coupling: In a Schlenk tube under an
inert atmosphere of argon, the appropriate TPA bromide (1.0 equiv.), pyridine-4-boronic acid
(1.2 equiv.) and Na2CO3 (1.0 equiv.) were dissolved in dioxane/water 4:1 (25 mL) and argon
was bubbled through for 5 min, followed by the addition of PdCl2(PPh3)2 (0.05 equiv.). The
resulting solution was stirred at 90 °C for 18 h. The mixture was then cooled and the solvents
were removed under reduced pressure. The residue was diluted with CH2Cl2 (50 mL) and
water (70 mL). The aqueous layer was washed with CH2Cl2 (2 × 30 mL). The combined
organic layers were washed with brine (50 mL) and dried with Na2SO4. After removal of the
solvent under reduced pressure, the residue was purified by column chromatography (SiO2,
CHCl3/ethyl acetate 1:1).
General Procedure for the Heck cross-coupling: Palladium acetate (10%) and tris-o-
tolylphosphine (20%) were introduced into a dry and degassed NEt3/DMF (9 mL, 2:1, v/v)
mixture, which was stirred for 15 min. The mixture was degassed again by gentle bubbling of
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
nitrogen. The appropriate halogen derivative (1.0 equiv.) and vinylpyridine (1.3 eq.) were
added. The mixture was then stirred at 85–90 °C under nitrogen for 3 h. After cooling, the
NEt3 was removed under vacuum. The crude mixture was diluted with CH2Cl2 and washed
with water and saturated NaHCO3. After removal of the solvent under reduced pressure, the
residue was purified by column chromatography (SiO2).
4-Methoxy-N-(4-methoxyphenyl)-N-[4-(pyridin-4-yl)phenyl]aniline (1): This compound
was prepared according to the general procedure for the Suzuki–Miyaura cross-coupling
starting from 4-bromo-N,N-bis(4-methoxyphenyl)aniline (200 mg, 0.521 mmol). Purification
by column chromatography afforded the pure chromophore 1 as a yellow solid (156 mg,
1
0.406 mmol, 78%). H NMR (500 MHz, CDCl3): δ = 3.80 (s, 6H), 6.82–6.86 (m, 4H), 6.96–
6.97 (m, 2H), 7.08–7.10 (m, 4H), 7.43–7.46 (m, 4H), 8.56–8.57 (m, 2H) ppm. 13C NMR (125
MHz, CDCl3): δ = 55.7, 115.0, 119. 9, 120.8, 127.3, 127.6, 129.0, 140.4, 147.94, 150.0,
150.3, 156.5 ppm. HRMS (MALDI) m/z calcd. for C25H22N2O2: 382.1676 [M+], found
382.1681. The spectroscopic data are consistent with those reported in the literature.35
(E)-4-Methoxy-N-(4-methoxyphenyl)-N-{4-[2-(pyridin-4-yl)vinyl]phenyl}aniline (2): This
compound was prepared according to general procedure for the Heck cross-coupling starting
from 4-bromo-N,N-bis(4-methoxyphenyl)aniline (300 mg, 0.78 mmol, 1 equiv.) and 4-
vinylpyridine (107 mg, 1.02 mmol, 1.3 equiv). The crude product was purified by column
1
chromatography (SiO2; ethyl acetate/petroleum ether 1:1). Yellow solid (188 mg, 59%). H
NMR (300 MHz, CDCl3):
δ
= 3.80 (s, 6H), 6.79–7.90 (m, 7H), 7.08 (d, J = 6.9 Hz, 4H), 7.22
13
(d, J = 16.5 Hz, 1H), 7.34–7.30 (m, 4H) 8.52 (d, J = 6.0 Hz, 2H) ppm. C NMR (75 MHz,
CDCl3):
δ = 55.5, 114.8, 119.7, 120.5, 122.8, 127.0, 127.9 (2C), 132.9, 140.3, 145.2, 149.4,
150.1, 156.3 ppm. HRMS (MALDI) m/z calcd. for C27H25N2O2: 409.1910 [M + H+], found
409.1910.
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4-Methoxy-N-(4-methoxyphenyl)-N-[4-(pyridin-4-ylethynyl)phenyl]aniline (3): In
a
Schlenk tube under an inert atmosphere of argon, N,N-bis(4-methoxyphenyl)-4'-
(ethynyl)aniline (232 mg, 0.704 mmol), 4-iodopyridine (159 mg, 0.776 mmol) and CuI (3 mg,
0.016 mmol) were dissolved in anhydrous dioxane/trimethylamine 4:1 (25 mL) and argon was
bubbled through for 5 min, followed by the addition of Pd(PPh3)4 (16 mg, 0.014 mmol). The
resulting mixture was stirred at 90 °C for 18 h. After cooling, the solvents were removed
under vacuum. The residue was diluted with CH2Cl2 (50 mL) and saturated aqueous NH4Cl
(70 mL). The aqueous layer was washed with CH2Cl2 (2 × 30 mL). The combined organic
layers were washed with brine (50 mL) and dried with Na2SO4. After removal of the solvent
under reduced pressure, the residue was purified by column chromatography (SiO2,
CHCl3/ethyl acetate 1:1) to give the chromophore 3 as a yellow oil (252 mg, 0.620 mmol,
88%). 1H NMR (500 MHz, CDCl3): δ = 3.79 (s, 6H), 6.82–6.86 (m, 6H), 7.06–7.08 (m, 4H),
13
7.30–7.32 (m, 4H), 8.53–8.55 (m, 2H) ppm. C NMR (125 MHz, CDCl3): δ = 55.7, 85.9,
95.5, 112.4, 115.1, 118.8, 125.5, 127.6, 132.2, 133.0, 139.9, 149.77, 149.82, 156.7 ppm.
HRMS (MALDI) m/z calcd. for C27H22N2O2: 406.1676 [M+], found 406.1680. The
spectroscopic data are consistent with those reported in the literature.36
N,N-Bis[4-(9H-carbazol-9-yl)phenyl]-4-(pyridin-4-yl)aniline (4): This compound was
prepared according to the general procedure for Suzuki–Miyaura cross-coupling starting from
compound 10 (200 mg, 0.306 mmol). Purification by column chromatography afforded the
1
pure chromophore 4 as a yellow solid (112 mg, 0.171 mmol, 56%). H NMR (500 MHz,
CDCl3): δ = 7.30–7.33 (m, 4H), 7.39–7.41 (m, 2H), 7.44–7.47 (m, 8H), 7.50–7.56 (m, 10H),
7.66–7.68 (m, 2H), 8.16–8.18 (m, 4H), 8.66–8.67 (m, 2H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 110.0, 120.2, 120.6, 121.2, 123.5, 124.5, 125.7, 126.1, 128.4, 128.4, 132.74,
133.14, 141.1, 146.3, 147.6, 148.5, 150.5 ppm. HRMS (MALDI) m/z calcd. for C47H32N4:
652.2621 [M+], found 652.2638.
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European Journal of Organic Chemistry
(E)-N,N-Bis[4-(9H-carbazol-9-yl)phenyl]-4-[2-(pyridin-4-yl)vinyl]aniline
(5):
This
compound was prepared according to the general procedure for Heck cross-coupling starting
from compound 11 (200 mg, 0.29 mmol, 1 equiv.) and 4-vinylpyridine (39 mg, 0.37 mmol,
1.3 equiv). The crude product was purified by column chromatography (SiO2; ethyl acetate).
1
Yellow solid (80 mg, 41%); Rf = 0.6 (SiO2; ethyl acetate). H NMR (300 MHz, CDCl3):
δ =
3
6.98 (d, J = 16.5 Hz, 1H), 7.29–7.58 (m, 27H), 8.16–8.18 (m, 4H), 8.57–8.59 (m, 2H) ppm.
13C NMR (75 MHz, CDCl3):
δ = 109.9, 120.1, 120.5, 120.9, 123.5, 124.3, 125.0, 125.6,
126.1, 128.3, 128.5, 131.4, 132.5, 133.0, 141.1, 144.9, 146.3, 147.9, 150.3 ppm. HRMS
(MALDI) m/z calcd. for C49H34N4: 678.2778 [M+], found 678.2774.
4-(5-Methoxythiophen-2-yl)-N-[4-(5-methoxythiophen-2-yl)phenyl]-N-[4-(pyridin-4-
yl)phenyl]aniline (6): This compound was prepared according to the general procedure for
the Suzuki–Miyaura cross-coupling starting from compound 12 (200 mg, 0.365 mmol).
Purification by column chromatography afforded the pure chromophore 6 as a yellow-orange
solid (166 mg, 0.303 mmol, 83%). 1H NMR (500 MHz, CDCl3): δ = 3.90 (s, 6H), 6.16 (d, J =
4.0 Hz, 2H), 6.88 (d, J = 4.0 Hz, 2H), 7.09–7.10 (m, 4H), 7.16–7.17 (m, 2H), 7.37–7.39 (m,
13
4H), 7.46–7.47 (m, 2H), 7.51–7.53 (m, 2H), 8.60–8.61 (m, 2H) ppm. C NMR (125 MHz,
CDCl3): δ = 60.4, 104.9, 120.3, 121.1, 123.4, 125.2, 126.0, 127.9, 130.0, 130.3, 131.5, 145. 7,
147.7, 148.6, 150.4, 165.8 ppm. HRMS (MALDI) m/z calcd. for C33H26N2O2S2: 546.1430
[M+], found 546.1444.
Acknowledgements
This work was supported by the European Regional Development Fund-Project "High
sensitive sensors and low density materials based on polymeric nanocomposites -
NANOMAT (No. CZ.02.1.01/0.0/0.0/17_048/0007376)". J.R.-L. thanks the Ministerio de
Economía y Competitividad/Agencia Estatal de Investigación/FEDER for financial support
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10.1002/ejoc.201900026
European Journal of Organic Chemistry
(project CTQ2017-84561-P). Funding from the Junta de Comunidades de Castilla-La
Mancha/FEDER is also gratefully acknowledged by J.R.-L. and S.A. (project
SBPLY/17/180501/000214). M. F. thanks the Région Bretagne, France, for funding.
Keywords: Push-pull; Intramolecular charge transfer; Nitrogen heterocycles;
Triphenylamine; Fluorescence
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European Journal of Organic Chemistry
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