Dipolar NLO Chromophores Bearing Diazine Rings as π-Conjugated Linkers

Article abstract of DOI:10.1021/acs.joc.7b01442

The synthesis of a series of push-pull derivatives bearing triphenylamine electron-donating group, cyclopenta[c]thiophen-4,6-dione electron acceptor and various π-linkers including (hetero)aromatic fragments is reported. All target chromophores with systematically varied π-linker structure were further investigated by electrochemistry, absorption measurements, and EFISH experiments in conjunction with DFT calculations. Based on electrochemical and photophysical measurements, when a polarizable 2,5-thienylene moiety is embedded into the chromophore π-backbone the highest intramolecular charge transfer (ICT) is observed. Benzene, pyrimidine, and pyridazine derivatives exhibit lower polarizability and extent of the ICT across these π-linkers. The elongation of the π-conjugated system via additional ethenylene linker results in a significant reduction of the HOMO-LUMO gap and an enhancement of the NLO response. Whereas it does not significantly influence electrochemical and linear optical properties, the orientation of the pyrimidine ring seems to be a key parameter on the μβ value due to significant variation of the dipolar moment (μ) value. In 2a and 2c, pyrimidine is oriented to behave as an acceptor and thus generate dipolar molecule with μ above 5 D, whereas in 2b and 2d ground state dipole moment is significantly reduced. This study seems to indicate a high aromaticity of pyrimidine and pyridazine derivatives, close to the benzene analogues and significantly higher than thiophene analogues.

Full text of DOI:10.1021/acs.joc.7b01442

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Article
Dipolar NLO chromophores bearing diazine rings as #-conjugated linkers
Milan Klikar, Pascal le Poul, Ales Ruzicka, Oldrich Pytela, Alberto Barsella, Kokou
Dodzi Dorkenoo, Francoise Robin-le Guen, Filip Bures, and Sylvain Achelle
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01442 • Publication Date (Web): 24 Aug 2017
Downloaded from http://pubs.acs.org on August 24, 2017
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The Journal of Organic Chemistry is published by the American Chemical Society.
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Dipolar NLO chromophores bearing diazine
rings as π-conjugated linkers
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†
, ‡
†
‡
‡
§
Milan Klikar, Pascal le Poul, Aleš Růžička, Oldřich Pytela, Alberto Barsella, Kokou D.
§
†
,‡
,†
Dorkenoo, Françoise Robin-le Guen, Filip Bureš,* and Sylvain Achelle*
†
Institut des Sciences Chimiques de Rennes UMR CNRS 6226, IUT de Lannion, Université
‡
de Rennes 1, rue Edouard Branly, BP 30219, F22302 Lannion Cedex, France. Institute of
Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice,
Studentská 573, Pardubice, 53210, Czech Republic. ‡ Department of General and Inorganic
Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice,
§
53210, Czech Republic
.
Département d’Optique ultra-rapide et Nanophotonique, IPCMS-
CNRS, 23 Rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France.
Corresponding authors: *E-mails: sylvain.achelle@univ-rennes1.fr; filip.bures@upce.cz
Synopsis TOC
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Abstract
The synthesis of a series of push-pull derivatives bearing triphenylamine electron-donating
group, cyclopenta[c]thiophen-4,6-dione electron acceptor and various π-linkers including
(
hetero)aromatic fragments is reported. All target chromophores with systematically varied
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π-linker structure were further investigated by electrochemistry, absorption measurements and
EFISH experiments in conjunction with DFT calculations. Based on electrochemical and
photophysical measurements, when a polarizable 2,5-thienylene moiety is embedded into the
chromophore π-backbone the highest intramolecular charge transfer (ICT) is observed.
Benzene, pyrimidine and pyridazine derivatives exhibit lower polarizability and extent of the
ICT across these π-linkers. The elongation of the π-conjugated system via additional
ethenylene linker results in a significant reduction of the HOMO-LUMO gap and an
enhancement of the NLO response. Whereas it does not significantly influence
electrochemical and linear optical properties, the orientation of the pyrimidine ring seems to
be a key parameter on the µβ value due to significant variation of the dipolar moment (µ)
value. In 2a and 2c is pyrimidine oriented to behave as an acceptor and thus generate dipolar
molecule with µ above 5 D, whereas in 2b and 2d is ground state dipole moment significantly
reduced. This study seems to indicate a high aromaticity of pyrimidine and pyridazine
derivatives, close to the benzene analogues and significantly higher than thiophene analogues.
Introduction
During the past two decades, there has been a great interest in the design of dipolar
push-pull chromophores bearing an electron donating (D) and an electron-withdrawing group
(A) separated by a π-conjugated spacer (D-π-A design). The push-pull structure leads to an
efficient intramolecular charge transfer (ICT) from the donor to the acceptor part and
generation of a molecular dipole. The ICT can easily be illustrated by two limiting resonance
1
forms (neutral and zwitterionic arrangement). Dipolar chromophores are known for their
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interesting electro-optic activities and this design has been efficiently used in the conception
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of fluorescent probes, chromophores for dyes sensitized solar cells (DSSC), two-photon
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absorption dyes and 2 order nonlinear optic (NLO) chromophores. Second-order NLO
materials have found applications in blue/green laser obtained from red sources through
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frequency doubling, such as second-harmonic generation or terahertz wave generation.
The properties of dipolar chromophores can easily be tuned by modification of the
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donor/acceptor couple, however this strategy inclines to reach its limit. The optimization of
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π-conjugated bridge is also a key parameter that affects the ICT. Experimental and
theoretical studies have confirmed that the aromaticity of the π-conjugated bridge is directed
towards the ICT. In this context, it has been shown that replacement of a benzene ring by a
less aromatic and easily polarizable heterocycles such as thiophene, pyrrole or furan imparts
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an enhanced ICT in the chromophore.
Diazines are six-membered heterocycles with two nitrogen atoms. Depending on the
position of the nitrogen atoms, pyridazine (1,2-diazine), pyrimidine (1,3-diazine) and pyrazine
(1,4-diazine) can be distinguished. The diazine rings have been extensively used as electron-
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withdrawing part of push-pull structures. Only few examples of incorporating diazine rings
as π-linker in push-pull systems have been described so far, all of them focused on pyrimidine
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systems for photovoltaic application. It has been shown that the replacement of
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,4-phenylene π-spacer by a pyrimidine ring leads to higher light harvesting efficiency.
It
is generally admitted that the aromaticity decreases with increasing number of embedded
nitrogen atoms within the ring. However, the available reports on the aromaticity and stability
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of diazines are contradictory with no consistent conclusions. Ortíz et al. have studied
experimentally and theoretically aminopyrimidine-based donor-acceptor compounds; they
have shown that a subtle trade-off between aromaticity and push-pull effect explains their
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electronic properties. Alonso and co-workers have calculated the aromaticity of pyrimidine
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12a
and their benzene analogues using various descriptors. They have shown that pyrimidine is
less aromatic than benzene and, in contradiction to benzene, the appended substituents
decrease its aromaticity. On the contrary, Wang et al. claimed that diazine aromaticity is
similar to benzene and the observed lower relative energy of pyridazine isomer is due to a
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weak N-N bond rather than its reduced aromaticity.
In order to explore diazines as π-conjugated linkers in push-pull molecules, we have
designed a series of seven dipolar push-pull chromophores 1−2 (Chart 1). These molecules
possess 2,5-disubstituted pyrimidine or 3,6-disubstituted pyridazine moieties as a part of the
π-system, diphenylamino donor, and cyclopenta[c]thiophen-4,6-dione electron acceptor
(
ThDione). As a structural analogue to indan-1,3-dione, ThDione has recently been prepared
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in our group and proved to be very powerful electron-withdrawing group. The properties of
diazine-based chromophores 1−2 will be critically compared with 1,4-phenylene and
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,5-thienylene analogues 3 (Chart 2). Additional chromophores 4a−b are model compounds
to study effect of the π-linker length (Chart 2). Hence, we report herein their synthesis,
thermal, electrochemical, and (non)linear optical properties completed with the DFT
calculations.
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Chart 1
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Chart 2
Results and discussion
Synthesis
The chromophore 4b was synthesized by direct Knoevenagel condensation between
commercial available 4-(N,N-diphenylamino)benzaldehyde 8b and ThDione 11 (Scheme 1) in
the yield of 71 %. Biphenyl aldehyde 8a was prepared from 4-bromotriphenylamine 9 and
4
-formylphenylboronic acid pinacol ester 10 via Suzuki-Miyaura cross-coupling reaction.
Subsequent Knoevenagel condensation with ThDione 11 gave target chromophore 4a
Scheme 1).
(
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Scheme 1. Synthesis of 4a−b.
The chromophores 1a, 2a−b, 3a and 3d were obtained from the corresponding dihalogeno
derivatives 13, 16, 19, and 21 via two successive Suzuki-Miyaura cross coupling reactions
followed by a Knoevenagel condensation reaction with ThDione 11 (Schemes 1−4). Due to a
π-electron deficient character of both chloro-diazines 13 and 16, the cross-coupling reactions
can smoothly be carried out without the use of special and expensive Pd-precatalysts and
15
ligands. Commercially available 3,6-dichoropyridazine 13 underwent mono cross-coupling
reaction with 4-formylphenyl-boronic acid 12 to intermediate 14 (Scheme 2). A second
Suzuki-Miyaura reaction with 4-diphenylaminophenylboronic acid 15 afforded aldehyde 5a
that upon treatment with 11 provided target chromophore 1a. The synthetic route towards
pyrimidine derivatives 2a−b starts from 5-bromo-2-chloropyrimidine 16, which underwent
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regioselective Suzuki-Miyaura reaction with 12 and 15 to monocoupled intermediates 17
and 18 (Scheme 3). Their final Knoevenagel condensation with 11 afforded chromophores
2a−b. Starting from 1-bromo-4-iodobenzene 19 and 2,5-dibromothiophene 21, a similar
reaction sequence was employed to obtain 3a and 3d, respectively (Schemes 4−5).
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Scheme 2. Synthesis of 1a.
Scheme 3. Synthesis of 2a−b.
Scheme 4. Synthesis of 3a.
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Scheme 5. Synthesis of 3d.
Starting from commercially available 3-chloro-6-methylpyridazine 23, a Suzuki-Miyaura
cross-coupling with 12 or 15 afforded the intermediates 24 and 26 with good yields (Scheme
6). Protection of the aldehyde function in 26 led to acetal 27. Aldehydes 5b−c were obtained
exclusively as E isomers by condensation reaction between 24 and 27, bearing active methyl
group, and aldehydes 25 and 28 using dimsyl potassium according to a reported
10d,17
procedure
and followed by acidic cleavage of the acetal protecting group. Target
chromophores 1b−c were obtained by Knoevenagel condensation of 5b−c with ThDione 11.
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Scheme 6. Synthesis of 1b−c.
Starting from 5-bromo-2-methylpyrimidine 29, target chromophores 2c−d were obtained in a
similar way (Scheme 7).
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Scheme 7. Synthesis of 2c−d.
Chromophores 3b−c were obtained in a four step synthesis starting from 33 (Scheme 8). The
first step is an Arbuzov reaction leading to phosphonate 34. The second step, affording the
intermediates 35 and 36 exclusively as E isomers, involved the Horner-Wadsworth-Emmons
reaction with the aldehydes 25 and 28 and subsequent cleavage of the acetal to aldehyde in
35. Suzuki-Miyaura cross-coupling reactions of 35 and 36 with boronic acids 15 and 12
afforded aldehydes 7b−c that underwent final Knoevenagel condensation with ThDione 11.
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Scheme 8. Synthesis of 3b−c.
All new chromophores 1−4 exhibited good solubility in organic solvents, especially in
1
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chlorinated ones. The IR, H and C NMR and HRMS data were consistent with the expected
3
structures. For molecules with a vinylene linker, the J(H,H) coupling constant of ∼16 Hz of
the vinylic protons clearly support the selective formation of E-configured double bonds.
X-Ray analysis
Only chromophore 3d provided crystals suitable for X-ray analysis by slow diffusion of
hexane into its dichloromethane solution. The remaining chromophores proved difficult to
crystalize as single crystals. The plot shown in Figure 1 confirms the proposed molecular
structure of 3d as well as its arrangement in the solid state. Its structure is composed of nearly
linear π-conjugated path between the triphenylamine donor and ThDione acceptor, which is
documented by the largest interplanar angle found within the molecule of 16.3(3)°. This linear
arrangement allows efficient overlap of π-electrons and facilitates ICT over the whole
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π-conjugated system. In contrast to coplanar arrangement of the acceptor, phenyl rings of the
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amino donor adopt propeller shaped structure, typical for triarylamines. Aromaticity of the
π-linker in 3d can be evaluated by calculating bond-length alternation (BLA) within the
phenylene and thienylene particular subunits. The BLA, expressed as quinoid character
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9,8a
20
(
δ
r)
In unsubstituted benzene,
indexes I and I of benzene and thiophene are 100 and 66, respectively. The calculated values
of the 1,4-phenylene moiety within the triphenylamino donor are r = 0.006 Å and I = 94.9,
whereas the 1,4-phenylene ring close to the ThDione has r = 0.027 and I = 87.6. Central
,5-thienylene moiety possess Bird index I equal to 63.8. These values imply that all three
or Bird index (I
6
/I
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), helps to assess the extent of the ICT over the given π-system.
δ
r equals 0, in a fully quinoid ring, δr would be 0.1 Å. The Bird
6
5
δ
6
δ
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2
5
moieties partially loss aromaticity and adopt quinoid character. Especially the phenylene ring
close to the ThDione is significantly polarized compared to unsubstituted benzene as well as
the second one. However, the thiophene can be polarized even further with I up to 58, for
5
2
1
instance in T-shaped chromophores. The supramolecular arrangement of 3d reveals pairs of
molecules interconnected by π-π stacking interactions separated by 3.208(3) Å.
Dichloromethane molecules are accommodated inside the cavities made between two
diphenylamine parts of neighboring molecules.
Figure 1. X-ray molecular representation of chromophore 3d (150 K, R = 0.05, CCDC 1549497).
Molecule of DCM omitted for clarity.
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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Thermal analysis
Thermal behaviour of compounds 1−4 was studied by differential scanning calorimetry
DSC). Figure 2 shows thermograms of representative compounds 1b, 2c and 3b, Table 1 lists
all measured melting points (T ) and temperatures of thermal decompositions (T ). The
(
m
d
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
measured melting points range from 206 to 302 °C. The temperature of decomposition was
estimated within the range of 270−330 °C.
Figure 2. Representative DSC curves of compounds extended 1b, 2c and 3b determined with a
2
scanning rate of 3 °C/min under N .
All compounds 1−4 exhibited relatively sharp peaks of melting and gradual decomposition
processes. In the case of 2a−b, 3a, 3d and 4a the dehydration/desorption of residual
water/solvent was recorded within the range of 80−125 °C. Compounds 2b (195−205 °C), 3a
(
160−170 °C), 3b (205−215 °C) and 4a (180−190 °C) underwent exothermic, irreversible
monotropic solid-solid transition of metastable crystals. A slow solid-solid transition was also
observed for 2a, which showed metastable form melting at about 200 °C and immediate
crystallization to a more stable form. Compound 3d showed two/three crystalline forms
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successively melting within the range of 237 to 246 °C. Slow monotropic solid-solid
transition was observed for 4a whose α´-form melted (208 °C) briefly before the melting of
α-form (215 °C). Pyridazine derivatives 1a−c and pyrimidine 2d are not stable in liquid
phase, their melting processes were immediately followed by distinctive decomposition. A
gradual decomposition in liquid phase was observed for pyrimidine 2a−c and
benzene/thiophene derivatives 3b/3d. On the contrary, compounds 3a/3c and 4a−4b were
stable in liquid phase for additional 70−100 °C above their melting points.
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11
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53
54
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58
59
60
In general, all target chromophores exhibited large thermal robustness. However the diazine
compounds 1−2 showed slightly higher thermal stability in respect to benzene/thiophene
analogues 3. Regardless on the π-backbone, a very narrow decomposition window of about
35 °C was revealed for all compounds 1−3. The highest thermal robustness in this series was
recorded for pyrimidine derivative 2d with T
with shorter π-benzene linkers falls within the whole series with T
°C, respectively. The latter showed the highest T of 330 °C.
Table 1. Summarized thermal and electrochemical properties of chromophores 1−4.
m
/T
d
above 300 °C. Thermal behaviour of 4a−b
m
and T above 200 and 300
d
d
T [
a]
T
d
[b]
E
[c]
E
[c]
[c]
E
[d]
E
[d]
[e]
3[e]
m
1/2(ox1)
(red1)
ꢀE
HOMO
LUMO
λmax
εmax·10
Compound
-1 -1
(°C)
270
291
260
232
253
275
302
206
279
230
237
215
230
(°C)
282
295
270
290
298
298
306
306
298
302
290
302
330
(V)
(V)
(V)
2.00
(eV)
(eV)
(nm/eV)
442/2.81
(M ·cm )
20.9
1a
0.60
0.59
0.44
0.59
0.59
0.58
0.45
0.51
0.49
0.43
0.44
0.58
0.61
−1.40
−1.39
−1.39
−1.44
−1.41
−1.37
−1.37
−1.45
−1.47
−1.43
−1.42
−1.42
−1.59
−5.40
−5.39
−5.24
−5.39
−5.39
−5.38
−5.25
−5.31
−5.29
−5.23
−5.24
−5.38
−5.40
−3.40
−3.41
−3.41
−3.36
−3.39
−3.43
−3.43
−3.35
−3.33
−3.37
−3.38
−3.38
−3.20
1
b
1.98
1.83
2.03
2.00
1.95
1.82
1.96
1.96
1.86
1.86
1.99
2.20
449/2.76
454/2.73
443/2.80
450/2.75
451/2.75
459/2.70
444/2.79
470/2.64
459/2.70
506/2.45
505/2.46
36.1
25.5
1c
2
a
29.8
2
b
25.8
2c
38.1
2d
3a
33.1
21.7
3
b
42.2
3c
41.3
3d
4a
4b
36.5
46.3
490/2.53
25.9
[
a]
[b]
T
m
= melting point (the point of intersection of a baseline and a tangent of thermal effect = oneset).
T
d
= thermal
[
c]
decomposition (pyrolysis in N
2
atmosphere).
E1/2(ox1) and E(red1) are potentials of the first oxidation and reduction (half-
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1
1
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1
1
1
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2
2
2
2
2
2
2
2
2
2
3
3
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4
4
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5
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5
5
5
5
5
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wave for oxidation), respectively, as measured by CV; all potentials are given vs ferrocene as internal reference (E = 0.51 V
[
d]
[e]
-5
vs. Ag electrode); ꢀE = E1/2(ox1) - E (red1)
.
3
–EHOMO/LUMO = E1/2(ox1/red1) + 4.8 (Ref.22). Measured in CHCl at c ≈ 10 M.
Electrochemistry
Electrochemical measurements of chromophores 1−4 were carried out in CH Cl containing
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
4 4
.1 M Bu NBF in a three electrode cell by cyclic voltammetry (CV) (see experimental
details). All potentials were measured vs. Ag electrode and are given vs. ferrocene as internal
standard. The acquired data are summarized in Table 1. The half-wave potential (E1/2(ox1)) of
the first oxidation and the reduction potential E(red1) were acquired within the range of 0.43 to
0
.61 and −1.59 to −1.37 V, respectively. The first oxidation and reduction are typical one-
electron processes, followed by subsequent oxidations and reductions. Whereas the first
oxidations of 1−4 were reversible or quasi-reversible, all first reductions were irreversible
2
3
processes. According to the literature, the first oxidation is most likely localized on the
triphenylamine donor and the adjacent part of the π-linker, the first reduction process mostly
involves ThDione acceptor. The half-wave potential of the first oxidation and reduction were
further recalculated to the energies of the HOMO and the LUMO and these values were
24
visualized for compounds 1−3 in the energy level diagram (Figure 3).
With the same electron donor and acceptor in chromophores 1−3, the observed variations in
E
1/2(ox1) and E(red1) are obviously dependent on the type and extent of the used π-linker (Table
). In the case of molecules 4a−4b, the variations of E1/2(ox1) and E(red1) are predominantly
function of the π-linker length.
1
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Energy level diagrams of final chromophores 1−3.
Whereas the LUMO energies become steady within the range from −3.33 to −3.43 eV for
chromophores 1−3, the principal changes are seen in the HOMO levels. For instance, the
LUMO levels of chromophores bearing heterocyclic π-linkers (1a−c, 2b−d, and 3d) possess
almost identical ELUMO values of about −3.40 eV. The chromophores 3a−c with 1,4-phenylene
moieties showed only slightly increased values to about −3.35 eV. The electrochemical
HOMO-LUMO gap (ꢀE), which represents the most straightforward way for evaluating the
π-system variation, is mostly dictated by the HOMO levels. Hence, with the HOMO energies
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1
1
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1
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2
2
2
2
2
2
2
2
2
3
3
3
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3
3
3
3
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4
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4
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5
5
5
5
5
6
ranging from −5.23 to −5.40 eV, the ꢀE values are affected by the following structural
features:
•
The placement of the olefinic subunit. Whereas ethenylene subunit inserted closely to
the acceptor (e.g. 1b, 2c, and 3b; ꢀE = 1.98, 1.95, and 1.96 eV; Figure 3b) has only
diminished effect compared to 1a, 2a, and 3a (ꢀE = 2.00, 2.03, and 1.96 eV; Figure
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
a), its placement on the donor side (e.g. 1c, 2d, and 3c; ꢀE = 1.83, 1.82, and 1.86 eV;
Figure 3c) reduced the HOMO-LUMO gap by 0.1–0.2 eV.
•
•
Orientation of the heteroaromatic ring (e.g. pyrimidine in 2a and 2b) affected the ꢀE
only negligibly (2.03 and 2.00 eV; Figure 3a).
The type of six-membered (hetero)aromatic π-linker embedded in the chromophore
π-backbone
pyridazine→pyrimidine→benzene; ꢀE = 2.00→2.03/2.00→1.96 eV; Figure 3a).
• The lowest HOMO-LUMO gaps were electrochemically measured for chromophores
c, 2d, and 3c with pyridazine, pyrimidine, and benzene units and the π-system
plays
only
minor
role
(e.g.
1a→2a/2b→3a;
1
extended closely to the triphenylamine donor (Figure 3b).
•
Polarizable 2,5-thienylene moiety embedded in the chromophore π-backbone as in 3d
showed similar ICT transparency with ꢀE equal to 1.86 eV without additional
extension of the π-system (Figure 3a).
In summary, the electrochemical investigations of chromophores 1–3 revealed only minor
influence of the benzene vs. pyridazine/pyrimidine π-linkers. Based on these measurements,
we can assume similar polarizability and extent of the ICT across these π-linkers.
Figure 4 depicts the influence of π-linker length (number of 1,4-phenylene units) on the
electronic behaviour of chromophores 3a and 4a−b. As expected, the elongation of the
π-conjugated system linker narrowed the electrochemical gap, especially when going from 4b
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to 4a (ꢀE = 2.20 vs. 1.99 eV). Insertion of the third 1,4-phenylene unit as in 3a influenced the
HOMO/LUMO levels only negligibly. This saturation is in accordance with our previous
25
observation that ICT transmittance across three or more phenylene units is diminished.
10
11
12
13
14
15
16
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23
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29
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31
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33
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35
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37
38
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Energy level diagram of final chromophores 3a and 4a−4b.
UV-vis spectroscopy
The UV-Vis electronic spectra of target chromophores 1–4 were recorded in chloroform
(Figure 5), the fundamental linear optical properties are summarized in Table 1. TD-DFT
calculated UV-Vis spectra (a representative spectrum for 3a is provided in Fig. S39) revealed
HOMO to LUMO transition appearing at around 590 nm. However, experimental spectra
showed the longest wavelength absorption maxima (λmax) within the range of 442-506 nm.
These correspond to a HOMO-1 to LUMO and HOMO to LUMO+1 transitions. As indicated
by the TD-DFT calculations, the oscillator strength of the pure HOMO to LUMO transition is
significantly affected by rotation of the particular 1,4-phenylene (or diazine) units and
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1
1
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1
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1
1
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1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
therefore are predicted to be not visible (dihedral angle of about 40°). The observed high
energy bands are most likely populated by the most stable rotamers, which, in addition, would
also indicate low or no emissive character of target chromophores. Experimental optical gaps
(
1240/λmax) for molecules 1−4 were visualized in Figure 6, TD-DFT obtained values are
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
correlated in the SI (Fig. S40). As can be seen, the general trends deduced from the
electrochemical measurements are also obeyed in absorption behavior of 1–3. Namely,
pyridazine, pyrimidine, and benzene derivatives 1a, 2a/b, and 3a exhibited optical gaps
within a narrow range of 2.75−2.81 eV. Pyrimidine analogues 2a and 2b showed almost
identical optical gaps of 2.80 and 2.75 eV resembling the eletrochemicaly measured ones
(
2.03 and 2.00 eV). Hence, the orientation of the electron donor and the acceptor does not
affect the optical properties significantly. Insertion and placement of an additional olefinic
subunit reduced the optical gaps most significantly. In the benzene series 3a–c, this effect is
more pronounced when the ethenylene subunits separates the acceptor part (compare 3b vs.
3
a) than the triphenylamine donor (3c vs. 3a). This is in a slight contradiction to the
electrochemical data. However, the optical behavior of chromophores 1 and 2 mimics exactly
the trends seen by the aforementioned electrochemical measurements. The most
bathochromically shifted CT-band (λmax = 506 nm) and the lowest optical gap (2.45 eV) has
been measured for thiophene derivative 3d.
Surprisingly, absorption CT-bands of shorter π-benzene analogues 4a−4b are significantly
red-shifted compared to other chromophores 1−3, especially with benzene derivative 3a (Fig.
5
c and 6). This optical behavior is inconsistent with the electrochemical conclusions
(
decreased ꢀE value when the number of 1,4-phenylene units is increased). The observed red
shift for 4a and 4b, relative to 1-3, may be due to their shorter pi-linker, that may lead to the
stronger DA interaction in the excited states.
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
-5
Figure 5. UV-Vis spectra of chromophores 1−4 measured in chloroform (c ~ 10 M).
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3
4
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6
7
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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4
4
4
4
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5
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5
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0
1
2
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4
5
6
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 6. Comparison of energy gaps of chromophores 1−4 in chloroform.
The absorption data of target chromophores 1–4 in three selected solvents of different polarity
toluene, chloroform, and methanol) are shown in the SI (Fig. S6−S7). When going from less
polar toluene to more polar chloroform, the CT-bands have shifted only negligibly (ꢀ
0–9 nm). This undistinguished solvatochromic behavior is a common feature of similar
(
λ
max
=
10
diazine push-pull chromophores. However, in contrast to benzene/thiophene chromophores
−4, the absorption spectra of 1–2 measured in protic methanol showed significantly
suppressed and hypsochromically shifted CT-bands as a result of nonspecific hydrogen-
3
2
6
bonding of the solvent to the central diazine moiety.
Emission spectroscopy
In contrast to final chromophores 1–3 that are non-emissive, aldehyde precursors 5a, 6a−b,
7a, and 7d are highly fluorescent probably due to the weaker electron-withdrawing property
of the formyl fragment compared with ThDione. As these molecules also represent push-pull
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2
3
4
5
6
7
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9
molecules, we have investigated their UV-Vis and photoluminescence (PL) spectroscopic
properties in chloroform. Table 2 summarizes the gathered data, representative spectra are
shown in the SI (Fig. S9−S10). All these compounds display yellow emission in chloroform
with high fluorescence quantum yield (0.34−0.78) and large Stokes shifts, which is a clear
indication of the high polarizability of their π-conjugated systems due to the ICT. Push-pull
luminophores generally exhibit intense emission solvatochromism. More precisely, broad
structureless red-shifted emission is observed on increasing the solvent polarity along with a
successive decrease of the fluorescence intensity due to the stabilization of highly polar
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
10e-h,27
emitting state by polar solvents.
The emission behavior of selected aldehydes was
studied in a variety of different aprotic solvents and the results are reported in Table 3. All
compounds show a distinct positive emission solvatochromism. When increasing solvent
28
T
polarity (according to the Dimroth-Reichardt polarity parameter E (30)), a bathochromic
shift of the emission band has clearly been observed. As an example, the color changes under
UV irradiation for compound 6b in various solvents are shown in SI (Fig. S12). The emission
maxima plotted versus E (30) showed a good correlation and linearity for all five aldehydes
T
(Fig. S11). Surprisingly, the weakest solvatochromic range, proportional to the slope of the
regression line, is observed for the thiophene derivative 7d, whereas the highest is observed
for the pyridazine aldehyde 5a. Although this fluoro-solvatochromic study does not quantify
the ICT, it may serve as a clear indicator of its occurrence in 5a, 6a−b, 7a, and 7d.
3
Table 2: UV/Vis and PL of selected aldehydes in CHCl .
abs [a]
max
PL [a]
max
λ
λ
[b] Stokes shift
–1
Compound
ΦF
(
cm )
(
nm/eV) (nm/eV)
5
a
a
395/3.14 557/2.23 0.34
391/3.17 558/2.22 0.63
398/3.12 550/2.25 0.78
366/3.39 563/2.20 0.45
411/3.02 566/2.19 0.75
7363
7654
6944
9650
6663
6
6
b
7a
7
d
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
[
a]
–5
–5 [b]
All spectra were recorded at room temperature at c = 1.0 × 10 M to 3.0 × 10 . Fluorescence quantum yield
(
±10%) determined relative to 9,10-bis(phenylethynyl)anthracene in cyclohexane (
F
Φ = 1.00).
Table 3: Emission solvatochromism of selected aldehydes in various aprotic solvents.
Heptane
PL
Toluene
PL
THF
PL
CH
2
PL
Cl
2
Acetone
PL
CH CN
DMSO
PL
3
PL
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Compound
λ
λ
λ
λ
λ
λ
λ
max
max
max
max
max
max
max
(
nm)
(nm)
(nm)
37.4
(nm)
(nm)
(nm)
(nm)
[a]
[a]
[a]
[a]
[a]
[a]
[a]
30.9
33.9
433
478
482
467
500
40.7
592
607
578
583
578
42.2
45.6
45.1
[
b]
[b]
[b]
5
a
a
-
552
558
546
532
541
640
632
609
603
590
-
-
[b]
[b]
6
440
449
438
480
-
-
6b
626
648
646
622
[
b]
7a
-
7d
616
[
a]
–1 [b]
Dimroth–Reichardt polarity parameter, kcal⋅mol . No signal detected.
EFISH experiments
The second-order NLO responses of chromophores 1−3 were measured by the EFISH
technique in CHCl solution with a non resonant incident wavelength of 1907 nm. The second
3
harmonic at λ = 953 nm stay well clear of the absorption bands of the chromophores.
29
Experimental details on EFISH measurements are given elsewhere. EFISH measurements
provide information about the scalar product µ
hyperpolarisability tensor and the dipole moment vector. This product is derived
according to equation 1 and considering (-2ω,ω,ω,0), the third-order term, as negligible for
β (2ω) of the vector component of the first
3
0
β
γ
0
the push-pull compounds under consideration. This approximation is usually used for push-
pull organic and organometallic molecules.
γ
EFISH = µ
β
/5kT +
γ
0
(-2ω,ω,ω,0)
Eq. 1
Table 4. µ
β values for compounds 1−3.
1a
1b 1c 2a 2b 2c 2d 3a 3b 3c 3d
-
48
[a]
µ
β
[10 esu] 220 300 290 220 70 410 50 150 330 330 530
[
a]
-3
µβ (2ω) at 1907 nm in CHCl . Molecular concentrations used for the measurements were in the range 10 to
3
-
2
1
0 M. µβ ± 10%.
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5
1
2
3
4
5
6
7
8
9
All the µ
β values for compounds 1−3 are positive indicating of excited states, which are more
polarized than the ground state and that both ground and excited state are polarized in the
-48
same direction. The µ
β values listed in Table 4 are comprised between 50 and 530×10 esu.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The following tendencies may be observed:
•
•
As expected, extension of the π-conjugated core by addition of an ethenylene linker
results in a significant enhancement of the NLO response (1a vs. 1b−c, 2a vs. 2c and
3a vs. 3b−c) except in case of 2b and 2d that exhibit the lowest NLO responses.
Orientation of the pyrimidine ring seems to be a key parameter: the NLO response of
2a and 2c being dramatically higher than that of 2b and 2d respectively. This trend is
in contradiction with electrochemical and absorption data.
• The type of six-membered (hetero)aromatic π-linker in the chromophore scaffold
significantly tunes the NLO response. The pyrimidine ring with electron-donating
fragment in C5 position (2a and 2c) seems to be the best configuration. When
compared with their 1,4-phenylene analogues 3a−c, pyridazine derivatives 1 gave
slightly increased or similar NLO values.
•
Chromophore 3d with 2,5-thienylene moiety in the π-conjugated core exhibits the
-48
highest NLO response (530×10 esu) in accordance with the most red-shifted CT
absorption band observed for this compound.
Quantum chemical calculations
Spatial and electronic properties of all target chromophores 1−4 were investigated using
3
1
Gaussian W09 package at the DFT level. Initial geometries of molecules 1−4 were estimated
3
2
by PM3 method implemented in ArgusLab and these were subsequently optimized by DFT
B3LYP/6-311++G(2d,f,p) method. Energies of the HOMO and LUMO, their differences,
ground state dipole moments µ and first hyperpolarizabilities β at 1907 nm were also
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6
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
calculated on the DFT B3LYP/2P-311++G(2d,f,p) level in vacuum. All calculated data are
summarized in Table 5.
Table 5. Calculated values of energies
E
HOMO/LUMO (ꢀE), dipole moments
(ꢁ), first
hyperpolarizabilities and ꢁβ coefficients for compounds 1−4.
β
E
[
a]
[a]
LUMO
[a]
-30[a]
-48[b]
ꢀ
E
HOMO
E
ꢁ
β
·10
(esu)
ꢁβ ·10
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Compound
(
eV)
(eV)
(eV)
(D)
4.14
(esu ·cm)
3113
1
a
2.34
2.25
2.27
2.46
2.22
2.33
2.19
2.32
2.26
2.22
2.30
2.46
2.85
−5.46
−5.42
−5.40
−5.54
−5.39
−5.49
−5.35
−5.35
−5.34
−5.25
−5.33
−5.44
−5.64
−3.12
−3.17
−3.13
−3.08
−3.17
−3.16
−3.16
−3.03
−3.08
−3.03
−3.03
−2.98
−2.79
752
1188
1201
587
1b
1c
4.40
5.33
5.34
1.34
5.17
2.41
2.86
3.97
3.73
4.27
3.28
4.19
5227
6401
3135
1349
4534
3391
2031
4839
4890
4394
1889
746
2a
2b
1007
877
2
c
2d
1407
710
3a
3b
3c
1219
1311
1029
576
3d
4a
4b
178
[
a]
[b]
-18
Calculated at the DFT B3LYP/2P-311++G(d,f,p) level; 1D = 1×10 esu·cm.
As can be deduced from the energy level diagram shown in Figure 7, the DFT calculation
slightly overestimates the electrochemical ELUMO and fits relatively well the EHOMO. However,
both these quantities as well as their differences showed reasonable correlations (Fig.
S41−S43) and, therefore the used DFT method can be considered as a suitable tool for the
description of electronic properties of 1−4. Similar conclusions as from electrochemical data
can also be concluded from the DFT calculations. However, the following points should be
highlighted:
• Insertion of the olefinic subunit slightly reduces the calculated gap ꢀE by about
0.05−0.15 eV (e.g. 1a vs. 1c/1d).
•
Orientation of the pyrimidine ring affects the calculated ꢀE more significantly for both
doublet of chromophores 2a/2b and 2c/2d than what has been observed by
electrochemical measurements (Table 1).
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• Similarly to electrochemical data, type of (hetero)cycle embedded in the chromophore
π-backbone plays only minor role (e.g. 1b→2c→3b;
pyridazine→pyrimidine→benzene; ꢀE = 2.25→2.33→2.26 eV).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
• Polarizable 2,5-thienylene moiety embedded in the chromophore π-backbone as in 3d
showed similar calculated ꢀE of 2.30 eV as diazine/benzene analogous 1a (2.34 eV),
2
a/2b (2.46/2.22 eV) and 3a (2.32 eV). This is in contrast to electrochemical data.
• Extension of the π-linker obviously narrowed the calculated gap ꢀE (e.g. 4b→4a→3a;
.85→2.46→2.32 eV).
2
•
The lowest HOMO-LUMO gaps were calculated for chromophores 1b, 2d, and 3c
with extended π-system based on pyridazine (1b; 2.25 eV), pyrimidine (2d; 2.19 eV),
and benzene (3c; 2.22 eV) units.
Figure 7. Energy level diagram of calculated energies
EHOMO/LUMO at the DFT
B3LYP/2P-311++G(d,f,p) level (red) and analogical electrochemical EHOMO/LUMO (black) for
chromophores 1−4.
The HOMO and LUMO localizations in representative chromophores 1c, 2c and 3c are shown
in Figure 8. For complete listing see the SI (Fig. S26−S38). In general, the HOMO as well the
HOMO-1 are localized on the triphenylamine donor in all target chromophores 1−4. The
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1
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7
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
LUMO generally occupies ThDione acceptor and the adjacent olefinic unit. The LUMO+1 is
spread either on the fused thiophene ring of the ThDione acceptor in benzene/thiophene
compounds 3−4 or on the central diazine π-backbone of pyridazine/pyrimidine chromophores
1
−2.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 8. HOMO/HOMO-1 (red) and LUMO/LUMO+1 (blue) localizations and HOMO/LUMO mix
in chromophores 1c, 2c and 3c
.
The calculated ground state dipole moments range from 1.34 to 5.34 D (Table 5). Their values
in series 2 are dictated by the orientation of the pyrimidine ring. In 2a and 2c is pyrimidine
oriented to behave as an acceptor and thus generate dipolar molecule with
ꢁ
above 5 D,
whereas in 2b and 2d is ground state dipole moment significantly reduced.
-30
-30
The calculated values of the first hyperpolarizabilities β range from 180×10 to 1400×10
esu for target chromophores 1−4 (Table 5). As expected, the lowest
β
values were calculated
-30
for less extended molecules 4a/4b (580/180×10 esu) as well as for compounds 1a, 2a and
-30
3a without embedded olefinic π-units (750, 590, 710×10 esu). On the other hand, the
-
highest
β values were gained for 1c, 2d and 3c with extended π-linker (1200, 1400, 1310×10
3
0
esu). In general, the calculated first hyperpolarizabilities do not depend on the type of
central (hetero)aromatic π-backbone, but mostly on the π-linker extension as well as by
orientation of the pyrimidine unit in series 2. The latter property The calculated ꢁβ products
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obey the trends of the experimental values obtained by EFISH experiment as can be deduced
from their correlation (Fig S44 in SI).
Conclusions
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In conclusion we have successfully synthesized a series of thirteen D-π-A chromophores
bearing diphenylamino and ThDione fragments as electron-donor and acceptor respectively
and various (hetero)aromatic ring as π-linkers. Whereas the 2,5-thienylene linker significantly
enhances the polarizability of the chromophores when compared with 1,4-phenylene analogue
as show by the red-shifted CT absorption band and narrowed electrochemical gap. Pyrimidine
and pyridazine chromophores and their benzene analogues exhibit similar ICT.
-48
2,5-Thienylene derivatives exhibits also the best NLO response (µβ = 530×10 esu).
Pyridazine chromophores and their benzene analogues exhibits similar NLO response
whereas the orientation of the pyrimidine plays a significant role on the µβ value due to
significantly higher µ values when the ThDione electron withdrawing group is localized on
the C2-position of the pyrimidine ring. Taking into account that the aromaticity reduces the
ICT, the position of CT band, electrochemical gap and NLO response seem to indicate a high
aromaticity of pyrimidine and pyridazine derivatives, close to the benzene analogues and
significantly higher than thiophene analogues.
Experimental Section
General Conditions. 5-Bromo-2-methylpyrimidine 29 was purchased from Interchim. The
3
3
compound 34 was obtained according to reported procedure. In air- and moisture-sensitive
reactions, all glassware was flame-dried and cooled under nitrogen. Thermal behavior of the
target compounds were measured in open aluminous crucibles under N inert atmosphere.
2
DSC curves were determined with a scanning rate of 3 °C/min within the range 25–400 °C.
NMR spectra were acquired at room temperature. Chemical shifts are given in parts per
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