D-π-A chromophores with a quinoxaline core in the π-bridge and bulky aryl groups in the acceptor: Synthesis, properties, and femtosecond nonlinear optical activity of the chromophore/PMMA guest-host materials

Article abstract of DOI:10.1016/j.dyepig.2020.108801

Novel D-π-A chromophores with quinoxaline/quinoxalinone core in the π-conjugated bridge and various bulky groups in the acceptor moiety have been synthesized and systematically investigated at molecular level by UV–Vis spectroscopy, DFT calculations, electrochemical and TGA-DSC methods as well as at materials level by the example of PMMA-based composite polymer materials doped with different chromophore contents using molecular modeling and SHG technique. Chromophores exhibit positive dioxane/chloroform solvatochromic shift of ca. 50 nm, high values of first hyperpolarizability and dipole moment, small energy gap and good thermal stability. Tolyl and cyclohexylphenyl substituents unlike phenyl can be treated as most effective isolating groups, preventing chromophore pronounced aggregation even at 30 wt% content. Femtosecond nonlinear optical (NLO) activity was studied for poled thin guest-host polymer films with various chromophore weight content. Film DBA-VQ_PhV-TCF_PhCy(25 wt%)/PMMA with bulky cyclohexylphenyl groups in chromophore acceptor shows maximal NLO coefficient, d₃₃, values among the studied materials (37 pm/V) as well as good long-term stability of NLO response together with excellent chromophore thermal stability (T_d = 256 °C). Composite materials doped with quinoxaline chromophores are photostable with respect to laser pulses with peak intensities up to 11 GW/cm².

Full text of DOI:10.1016/j.dyepig.2020.108801

Dyes and Pigments 184 (2021) 108801
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
D-π-A chromophores with a quinoxaline core in the π-bridge and bulky aryl
groups in the acceptor: Synthesis, properties, and femtosecond nonlinear
optical activity of the chromophore/PMMA guest-host materials
,
Alexey A. Kalinina *, Liliya N. Islamovaa, Artemiy G. Shmelevb, Guzel M. Fazleevaa,
Olga D. Fominykha, Yulia B. Dudkinaa, Tatyana A. Vakhoninaa, Alina I. Levitskayaa,
Anastasiya V. Sharipovaa, Anvar S. Mukhtarova, Ayrat R. Khamatgalimova, Irek R. Nizameeva,
Yulia H. Budnikovaa, Marina Yu Balakinaa
a Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center of RAS, Arbuzov Str. 8, 420088, Kazan, Russia
b Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, Sibirsky Tract 10/7, 420029, Kazan, Russia
A R T I C L E I N F O
A B S T R A C T
Novel D- -A chromophores with quinoxaline/quinoxalinone core in the π-conjugated bridge and various bulky
Keywords:
π
NLO chromophore
Divinylquinoxaline bridge
TD-DFT calculations
Voltammetry
groups in the acceptor moiety have been synthesized and systematically investigated at molecular level by
UV–Vis spectroscopy, DFT calculations, electrochemical and TGA-DSC methods as well as at materials level by
the example of PMMA-based composite polymer materials doped with different chromophore contents using
molecular modeling and SHG technique. Chromophores exhibit positive dioxane/chloroform solvatochromic
shift of ca. 50 nm, high values of first hyperpolarizability and dipole moment, small energy gap and good thermal
stability. Tolyl and cyclohexylphenyl substituents unlike phenyl can be treated as most effective isolating groups,
preventing chromophore pronounced aggregation even at 30 wt% content. Femtosecond nonlinear optical (NLO)
activity was studied for poled thin guest-host polymer films with various chromophore weight content. Film
DBA-VQPhV-TCFPhCy(25 wt%)/PMMA with bulky cyclohexylphenyl groups in chromophore acceptor shows
Molecular modeling
NLO coefficient
maximal NLO coefficient, d3 values among the studied materials (37 pm/V) as well as good long-term stability
3,
of NLO response together with excellent chromophore thermal stability (Td = 256 ◦C). Composite materials
doped with quinoxaline chromophores are photostable with respect to laser pulses with peak intensities up to 11
GW/cm2.
1. Introduction
technology [6–8]. The key constructing blocks for such materials are the
second-order NLO chromophores. To achieve large optical non-
Nonlinear optical (NLO) materials are presented as acentric inor-
ganic crystals, for example, lithium niobate [1], and promising for the
generation of intense coherent deep- UV light fluorooxoborates [2,3],
useful for THz generation organic crystals [4] and various poled thin
film chromohore-containig organic materials: composite polymer ma-
terials, molecular glasses, molecular composite glasses [5]. Organic
materials with quadratic NLO properties attract considerable research
interest due to their potential use in optoelectronic devices to promote
development of such areas as telecommunications and information
linearities (large molecular first hyperpolarizability) various “push-pull”
molecules have been designed and systematically investigated [9–11].
Molecular engineering of organic chromophores has evolved in various
directions. There was an increase in the length of the chromophore
π
-bridge due to the introduction of additional vinyl units [12,13],
including conformationally locked-in cyclopentene [14], cyclohexene
[14–18], pyrane [19] or tetrahydronaphtalene [20] ring frameworks.
The incorporation of heterocyclic moiety, mainly thiophene ring [18,
21–23], in the
π-bridge was widely used in the design of NLO
* Corresponding author.
E-mail addresses: kalesha007@mail.ru (A.A. Kalinin), nailisllil1989@mail.ru (L.N. Islamova), sgartjom@gmail.com (A.G. Shmelev), Fazleeva_GM@mail.ru
(G.M. Fazleeva), fod5@yandex.ru (O.D. Fominykh), dudkinayu@iopc.ru (Y.B. Dudkina), tanyavakhonina@yandex.ru (T.A. Vakhonina), april-90@mail.ru
(A.I. Levitskaya), A.V.Sharipova@yandex.ru (A.V. Sharipova), mukhtarovy@yandex.ru (A.S. Mukhtarov), ayrat_kh@iopc.ru (A.R. Khamatgalimov), irek.rash@
gmail.com (I.R. Nizameev), yulia@iopc.ru (Y.H. Budnikova), mbalakina@yandex.ru (M.Y. Balakina).
https://doi.org/10.1016/j.dyepig.2020.108801
Received 8 May 2020; Received in revised form 18 August 2020; Accepted 19 August 2020
Available online 23 August 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
chromophores. Other heterocyclic, as a rule
π
-excessive, moieties, such
2.2. Chromophores synthesis
as pyrrole [24], furan [25], which serve as auxiliary donor, were also
used. Variation of donor [14,15,17,25–32] and acceptor [13,15,18,
33–36] terminal fragments including the selection of the optimal ratio of
donor-acceptor strength was another important structural factor in the
design of NLO chromophores. NLO activity of more complex chromo-
phore systems has also been investigated; among them were Y-type
chromophores with two donor fragments [23,37–39], bichromophores
[40–42], macrocyclic chromophores [43], etc. Heteroaromatics played
an important role in the design of NLO chromophores, but mostly
Aldehydes 6, 7 and Me-TCFAr acceptors 8b,c were synthesized ac-
cording to the literature [52,53,58–60].
2.2.1. 2-(3-Cyano-4,5-dimethyl-5-phenylfuran-2(5H)-ylidene)
malononitrile (8a)
To stirred solution of 3-(4-cyclophenyl)-3-hydroxybutan-2-one (1.0
g, 61 mmol) in dry pyridine (5 mL) malononitrile (0.81 g, 12.3 mmol)
was added at 0 ◦C. The temperature of the reaction mixture was slowly
raised to rt and the reaction mixture was stirred for another 5 h. Then the
reaction mixture was poured into water and the precipitate was filtered
and washed with water. When a few drops of acetic acid were added to
the solution, a precipitate again was formed, which was filtered and
washed with water. Yield 0.75 g (54%). White-gray powder, mp 177 ◦C
(EtOH).1H NMR (400 MHz, CDCl3): 7.50–7.46 (m, 3H, m,р-Ph),
7.23–7.19 (m, 2H, о-Ph), 2.24 (s, 3Н, СН ), 2.02 (s, 3Н, СН ). 13C NMR
π
-excessive heterocycles were incorporated into the
mophores, thiazole being the only exception [44,45]. Two decades ago it
was shown theoretically that the introduction of a -deficient hetero-
cycle in the -bridge can lead to the enhancement of the first hyper-
π-bridge of chro-
π
π
polarizability values [46]. Some azine functionalized π-conjugated dyes
with NLO and photoluminescence properties have been recently devel-
oped [47–49]. According to our theoretical studies, the chromophores
3
3
with the quinoxaline core in the
π
-bridge exhibit significant NLO activity
(100 MHz, CDCl3): 182.2, 175.7, 133.9, 130.5, 129.6, 125.0, 110.8,
[50,51], and for some chromophores it was confirmed experimentally
[52,53]. Our present investigations focus on the novel type of NLO
110.3, 109.0, 104.8, 101.5, 58.9, 22.3, 14.5. IR (νmax, cmꢀ 1, KBr): 3067
–
–
–
–
–
(CH), 2997 (CH), 2230 (С≡N), 2219 (C N), 1620 (C C), 1597 (C C).
–
–
chromophores with incorporated π-deficient quinoxaline/quinoxalinone
core in the
π
-bridge, bulky aromatic substituent in acceptor and its effect
2.2.2. Synthesis of chromophores with phenylquinoxaline moiety
on optical, electrochemical and thermal properties of chromophores as
well as on femtosecond NLO activity of guest-host chromophor-
A solution of aldehyde 6 (115 mg, 0.25 mmol), Me-TCFAr (8a-c)
(0.22 mmol) and anhydrous ethanol (2 mL) was stirred for 7–14 h (7h in
case of 8a, 14 h in case of 8b, 10 h in case of 8c) at 70 ◦С, then cooled to
rt. After removal of the solvent by rotary evaporation, the residue was
purified by silica-gel column chromatography (eluent: petroleum ether/
ethyl acetate = 7:1→ petroleum ether/ethyl acetate = 3:1) to give a
product as black powder.
e-containing polymer materials. Composite materials doped with D-π-A
chromophores with bulky (isolating) group in donor moiety show
enhancement of macroscopic NLO activity due to a decrease in the
dipole-dipole
interaction
and
the
establishment
of
a
non-centrosymmetric organization of chromophores in the materials
[54–57].
2.2.2.1. 2-(3-Сyano-4-((E)-2-(6-((E)-4-(dibutylamino)styryl)-3-phenyl-
quinoxalin-2-yl)vinyl)-5-methyl-5-phenylfuran-2(5H)-ylidene)malononi-
trile (DBA-VQPhV-TCFPh). Yield 87 mg (56%). Rf = 0.47 (hexane/ethyl
2. Materials and techniques
acetate 2:1). 1H NMR (600 MHz, CDCl3): δ 8.06–8.00 (m, 3H, Н-7,8
2.1. Chromophores characterization
–
quinoxaline, 1H of –CH CHꢀ TCF), 7.94 (s, 1H, Н-5 quinoxaline), 7.54
–
The NMR, UV–Vis, IR and MALDI spectra were registered on the
equipment of Assigned Spectral-Analytical Center of FRC Kazan Scien-
tific Center of RAS. NMR experiments were performed with Bruker
AVANCE-600 and AVANCE-400 (600 and 400 MHz for 1H NMR, 150
and 100 MHz for 13C NMR) spectrometers. Chemical shifts (δ in ppm) are
referenced to the solvents. The mass spectra were obtained on Bruker
UltraFlex III MALDI TOF/TOF instrument with p-nitroaniline as a ma-
trix. UV–Vis spectra were recorded at room temperature on a Perki-
nElmer Lambda 35 spectrometer using 10 mm quartz cells. Spectra were
registered with a scan speed of 480 nm/min using a spectral width of 1
nm. All samples were prepared in solution with the concentrations of ca
~3.5 × 10ꢀ 5 mol/L. The thermal stabilities of chromophores were
investigated by simultaneous thermal analysis (thermogravimetry/dif-
ferential scanning calorimetry - TG/DSC) using NETZSCH (Selb, Ger-
many) STA449 F3 instrument. Approximately 2.6–3.7 mg samples were
placed in an Al crucible with a pre-hole on the lid and heated from 30 to
600 ◦C. The same empty crucible was used as the reference sample.
High-purity argon was used with a gas flow rate of 50 mL/min. TG/DSC
measurements were performed at the heating rates of 10K/min. The
reaction progress and the purity of the obtained compounds were
controlled by TLC on Sorbfil UV-254 plates with visualization under UV
light. Voltammograms were recorded with a BASi Epsilon potentiostat/
galvanostat (USA) at 25 ◦C in dichloromethane solutions under dry ni-
trogen atmosphere. All measurements were carried out with 0.2 M
Bu4NBF4 as the supporting electrolyte, platinum working electrode
(0.02 cm2), platinum auxiliary electrode, and Ag/AgNO3 reference
electrode. All potentials were referred against the ferrocenium/ferro-
cene redox couple.
(dd, J = 7.4, 7.4 Hz, 1H, p-Ph), 7.49–7.31 (m, 10H, Ph, m-Ph, 3,5-H
–
aniline, 1H of –CH CHꢀ DBA), 7.29 (d,
J
=
15.7, 1H,
–
–
–
–CH CHꢀ TCF), 7.10 (d, J = 8.0 Hz, 2H, o-Ph), 7.03 (d, J = 16.3 Hz, 1H,
–
–CH CHꢀ DBA), 6.66 (d, J = 8.6 Hz, 2H, 2,6-H aniline), 3.33 (t, J = 7.6
–
Hz, 4H, NСН ), 2.06 (s, 3H, Me), 1.65–1.57 (m, 4H, NCH2CH2),
2
1.43–1.35 (m, 4H, N(CH2)2CH2), 0.98 (t, J = 7.3 Hz, 6Н, СН ). 13C NMR
3
(150 MHz, CDCl3): 175.0, 172.5, 155.8, 148.9, 143.2, 143.1, 143.0,
142.80, 141.5, 137.2, 135.0, 133.9, 130.5, 129.8, 129.6, 129.4, 128.9,
128.8, 125.7, 124.7, 123.4, 121.7, 120.0, 111.7, 111.2, 110.6, 109.7,
103.3, 99.1, 59.2, 50.8, 29.5, 24.1, 20.3, 14.0. IR (νmax, cmꢀ 1, KBr):
– –
– –
–
–
3446 (CH), 2928 (CH), 2228 (CN), 1579 (C C, C N), 1511 (C C).
MALDI-TOF: 707 [M+H]+.
2.2.2.2. 2-(3-Сyano-4-((E)-2-(6-((E)-4-(dibutylamino)styryl)-3-phenyl-
quinoxalin-2-yl)vinyl)-5-methyl-5-(p-tolyl)furan-2(5H)-ylidene)malononi-
trile (DBA-VQPhV-TCFTol). Yield 97 mg (61%). Rf = 0.22 (hexane/ethyl
acetate 3:1). 1H NMR (600 MHz, CDCl3): δ 8.06–8.00 (m, 3H, Н-7,8
–
quinoxaline, 1H of –CH CHꢀ TCF), 7.96 (s, 1H, Н-5 quinoxaline), 7.54
–
(dd, J = 7.4, 7.4 Hz, 1H, p-Ph), 7.47–7.30 (m, 7H, o,m-Ph, 3,5-H aniline,
–
–
–
1H of –CH CHꢀ DBA), 7.30 (d, J = 15.7, 1H, –CH CHꢀ TCF), 7.12 (d,
–
–
–
J = 8.3 Hz, 2H, 3,5-H p-Tol), 7.04 (d, J = 16.3 Hz, 1H, –CH CHꢀ DBA),
6.98 (d, J = 8.3 Hz, 2H, 2,6-H p-Tol), 6.66 (d, J = 8.8 Hz, 2H, 2,6-H
aniline), 3.32 (t, J = 7.6 Hz, 4H, NСН ), 2.38 (s, 3H, MeC6H4), 2.03
2
(s, 3H, MeTCF), 1.65–1.55 (m, 4H, NCH2CH2), 1.43–1.34 (М, 4H, N
(CH2)2CH2), 0.97 (t, J = 7.3 Hz, 6Н, СН ). 13C NMR (150 MHz, CDCl3):
175.0, 172.8, 155.8, 148.9, 143.2, 1433.0, 142.9, 141.9, 140.8, 137.2,
133.9, 131.9, 130.3, 129.8, 129.4, 129.3, 128.8, 128.7, 125.7, 124.8,
123.4, 121.7, 120.2, 111.7, 111.3, 110.7, 109.7, 103.3, 99.3, 59.0, 50.8,
29.5, 24.1, 21.3, 20.3, 14.0. IR (νmax, cmꢀ 1, KBr): 3443 (CH), 2929 (CH),
+
– –
– –
–
–
2230 (CN), 1583 (C C, C N), 1514 (C C). MALDI-TOF: 721 [M+H] .
2

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
2.2.2.3. 2-(3-Сyano-5-(4-cyclohexylphenyl)-4-((E)-2-(6-((E)-4-(dibuty-
lamino)styryl)-3-phenylquinoxalin-2-yl)vinyl)-5-methylfuran-2(5H)-yli-
dene)malononitrile (DBA-VQPhV-TCFPhCy). Yield 85 mg (49%). Rf
where z-axis coincides with the charge transfer direction along the
molecule. Average linear polarizability αav is calculated as
=
ꢀ
)
1
0.37 (hexane/ethyl acetate 3:1). 1H NMR (400 MHz, CDCl3): δ 8.07–7.97
α
(av) = αxx
+
αyy
+
αzz
,
3
–
(m, 3H, Н-7.8 quinoxaline, 1H of –CH CHꢀ TCF), 7.93 (s, 1H, Н-5
–
quinoxaline), 7.57 (dd, J = 7.3, 7.3 Hz, 1H, p-Ph), 7.50–7.27 (m, 8H, o,
and βtot as
–
–
–
m-Ph, 3,5-H aniline, 1H of –CH CHꢀ DBA, 1H of –CH CHꢀ TCF), 7.14
–
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
∑
1
3
βtot
=
β2x + β2y + β2z ; βi = βiii
+
(βikk + βkik + βkki), i = x, y, z.
(d, J = 8.5 Hz, 2H, 3,5-H p-CyPh), 7.03 (d, J = 16.5 Hz, 1H,
–
i=∕k
–CH CHꢀ DBA), 6.99 (d, J = 8.5 Hz, 2H, 2,6-H p-CyPh), 6.66 (d, J =
–
8.7 Hz, 2H, 2,6-H aniline), 3.33 (t, J = 7.6 Hz, 4H, NСН ), 3.40 (t, J =
2
The calculations were performed by Jaguar program package [68,
7.6 Hz, 4H, NСН ), 2.55–2.46 (m, 1H, Cy), 2.03 (s, 3H, Me), 1.92–1.81
2
69].
(m, 4H, Cy), 1.80–1.73 (m, 1H, Cy), 1.67–1.56 (m, 4H, NCH2CH2),
1.48–1.35 (m, 8H, N(CH2)2CH2, 4H of Cy), 1.32–1.20 (m, 1H, Cy), 0.98
(t, J = 7.3 Hz, 6Н, СН ). 13C NMR (100 MHz, CDCl3): 175.1, 172.7,
2.4. Atomistic modeling
3
155.8, 150.7, 148.9, 143.2, 143.0, 142.9, 142.8, 141.5, 137.2, 133.8,
132.2, 129.8, 129.4, 129.3, 128.8, 128.7, 128.0, 125.7, 124.8, 123.4,
121.7, 120.1, 111.7, 111.3, 110.7, 109.7, 103.2, 99.3, 58.9, 50.8, 44.2,
34.2, 34.1, 29.4, 26.7, 26.0, 24.1, 20.3, 14.0. IR (νmax, cmꢀ 1, KBr): 3462
To estimate the isolating ability of substituents in acceptor fragment
of the chromophores, molecular modeling of composite materials with
PMMA as polymer matrix and chromophores-guests was carried out. The
chromophore content in considered composite polymer materials was
20, 25 and 30 wt%. Composite polymer materials were packed in the
amorphous cell under compressive protocol (NPT ensemble) (Fig. S1) in
the course of multistage simulation to get density close to that of real
polymer. Polymer matrix was modeled by 10 chains each containing 60
monomer units. Such oligomer size provides sufficient flexibility of
chains, it contains 10 Kuhn segments [70] Polymer chains and chro-
mophores in such a cell experience steric hindrances due to environ-
ment. Chromophore number in a cell was tuned according the
chromophore molecular weight and the desired weight content in
composite material; weight content is indicated in round parentheses
after composite system notation. Molecular modeling was performed
with Desmond program [71] using OPLS3e force field [72].
– –
– –
–
–
(CH), 2927 (CH), 2229 (CN), 1582 (C C, C N), 1512 (C C). MALDI-
TOF: 789 [M+H]+.
2.2.3. 2-(3-Cyano-5-(4-cyclohexylphenyl)-4-((E)-2-(6-((E)-4-
(dibutylamino)styryl)-3-oxo-4-propyl-3,4-dihydroquinoxalin-2-yl)vinyl)-5-
methylfuran-2(5H)-ylidene)malononitrile (DBA-VQonV-TCFPhCy
)
A mixture of aldehyde 7 (60 mg, 0.13 mmol), Me-TCFPhCy (8c) (46
mg, 0.13 mmol) and anhydrous ethanol (1 mL, A and B) or anhydrous
ethanol (2.2 mL) and methylene chloride (0.8 mL, C) was stirred for 6 h
(A), 40 h (B) and 144 h (C) at 70 ◦С (A), 50 ◦С (B) and rt (C), respec-
tively, then cooled to rt (A and B). After removal of the solvent by rotary
evaporation, the residue was purified by silica-gel column chromatog-
raphy (eluent: methylene chloride/ethyl acetate = 250:1) to give a
product as black powder. Yield 34 mg (33%, A), 58 mg (56%, B), 36 mg
(35%, C). Rf = 0.32 (hexane/ethyl acetate 2.5:1). 1H NMR (600 MHz,
2.5. Film fabrication, poling and NLO measurements
–
CDCl ): δ 7.99 (d, J = 16.2 Hz, 1H, –CH CH-TCF), 7.78 (d, J = 8.6 Hz,
–
3
–
–
1H, H-8 quinoxaline), 7.67 (d, J = 16.2 Hz, 1H, –CH CH-TCF), 7.56 (d,
The guest-host polymer materials were prepared with PMMA as
J = 8.6 Hz, 1H, H-7 quinoxaline), 7.43 (d, J = 8.5 Hz, 2H, 3,5-H aniline),
polymer matrix (Tg = 103 ◦C) and chromophores under study as guests.
7.29 (d, J = 8.4 Hz, 2H, C6H4Cy), 7.26 (d, J = 8.4 Hz, 2H, C6H4Cy), 7.24
Thin polymer films were cast onto glass substrates (cover glasses 100 μm
–
(d, J = 16.0 Hz, 1H, –CH CH-An), 7.15 (s, 1H, H-5 quinoxaline), 6.94
–
thick) from a 5–7% solution of the polymer in cyclohexanone via spin-
coating at 5000 rpm for 90 s. After casting, the samples were kept in a
vacuum drying oven at room temperature for 10–16 h to remove the
solvent residue. Films were poled at the corona-triode setup in the
corona discharge field, voltage 6.5 kV, poling time ~20 min, the dis-
tance from the tungsten needle electrode to the surface of the film being
1 cm; the field was applied to the films heated to 108–118 ◦C. Second
harmonic generation (SHG) was performed by the femtosecond ampli-
fied laser system which allowed measuring the SHG intensity emitted by
the sample without any micro-objective or another focusing system. The
principal scheme of the experimental setup is presented in the Supple-
mentary information and in [73]. The laser system produced pulses with
the following parameters: the wavelength is 1028 nm, the pulse repe-
–
(d, J = 16.0 Hz, 1H, –CH CH-An), 6.65 (d, J = 8.5 Hz, 2H, 2,6-H ani-
–
line), 4.26–4.19 (m, 1H, N–CH2), 4.19–4.12 (m, 1H, N–CH2), 3.33 (t, J
= 7.4 Hz, 4H, N–CH2), 2.60–2.49 (m, 1H, Cy), 2.21 (s, 3H, CH3),
1.90–1.79 (m, 6H, 4H of Cy, N(CO)CH2CH2), 1.78–1.72 (m, 1H, Cy),
1.67–1.64 (m, 4H, N–CH2CH2), 1.44–1.34 (m, 8H, N-(CH2)2CH2, 4H of
Cy), 1.32–1.23 (m, 1H, Cy), 1.09 (t, J = 7.4 Hz, 3H, CH3), 0.98 (t, J = 7.4
Hz, 6H, CH3). 13C NMR (100 MHz, CDCl3): 175.1, 172.7, 154.7, 150.7,
149.1, 146.36, 143.7, 140.6, 134.0, 133.9, 133.0, 132.6, 131.6, 128.9,
128.0, 125.9, 122.9, 122.0, 121.5, 121.1, 111.8, 111.4, 110.9, 110.7,
110.0, 103.2, 99.8, 58.6, 50.8, 44.2, 44.1, 34.2, 34.1, 29.5, 26.7, 26.0,
24.4, 20.6, 20.3, 14.0, 11.4. IR (νmax, cmꢀ 1, KBr): 3068 (C–H), 3030
–
–
N), 1657
–
+
–
(C–H), 2957 (C––H), 2–928 (C–H), 2855 (C–H), 2229 (C
(C O), 1582 (C N, C C). MALDI-TOF: 771 [M+H] .
– – –
tition rate is 3 kHz, pulse duration is 200 fs, a pulse energy is 164 μJ, and
mean power of the laser beam is 492 mW. The beam diameter of 3 mm
2.3. DFT calculations
resulted in the peak pulse intensity of about 11.6 GW/cm2. SHG in-
tensity was measured by using a z-cut α-quartz crystal as a source of a
Structure and NLO characteristics of chromophores under study were
calculated in the framework of Density Functional Theory (DFT).
Chromophores geometrical parameters were optimized in gas phase at
reference signal. We followed [74] to obtain the NLO coefficient of the
sample d33,s
:
√̅̅̅̅̅̅̅̅̅
/
B3LYP/6-31G(d) level and electric characteristics (dipole moments, μ,
d33,s
d11,q
l
ls
=
Is Iq c,q F,
components of linear polarizability tensor, αij
, and first hyper-
polarizability tensor, βijk) were calculated at the M06-2X/aug-cc-pVDZ
level; the use of M06-2X density functional [61,62] and Dunning basis
sets [63,64] are recognized as an adequate choice for this purpose
[65–67]; further on the computational level for electric characteristics
calculations is denoted as B3LYP/6-31G(d)//M06-2X/aug-cc-pVDZ.
These chromophores are systems with intramolecular charge transfer,
where d11,q is known quartz nonlinear coefficient (0.45 pm/V), Is and Iq
are SHG intensities produced by the sample and the quartz, respectively,
and measured in the same configuration, lc,q is quartz coherence length
related to 1028 nm (assumed as 13 μm), ls is sample thickness (of 0.3 μm
mean value), F is correction factor (1.2 when lc,q≫ls). It was assumed in
analyzing experimental data that d33/d13 ≈ 3.
the dominating component of first hyperpolarizability tensor is βzzz
,
3

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
3. Results and discussion
52–56 nm) is one and a half times greater than that for quinoxalinone-
based chromophore DBA-VQonV-TCFPhCy (Δλ
=
33 nm). Sol-
-A chro-
-bridge [55,75–77]. However
it should be noted that the solvatochromic behavior of the chromophore
3.1. Synthesis
vatochromic inversion is a characteristic phenomenon for D-
mophores with a heterocyclic core in the
π
π
The synthetic routes for chromophores with quinoxaline/quinox-
alinone core in the
π-bridge are shown in Scheme 1. Starting from
DBA-VQonV-TCFPhCy with the quinoxalinone core in the π-bridge,
amphimethylbromoquinoxalines 1 and 2, chromophore precursors 6 and
7 have been synthesized in the two step reactions: palladium-catalyzed
Heck reaction of compounds 1/2 and 3 and followed by Riley oxidation
of ethenes 4 and 5 with selenium dioxide. Finally, condensations of al-
dehydes 6 and 7 with the various Me-TCFAr acceptor 8a-c gave chro-
which shows the predominance of the positive solvatochromic shift over
the negative one, makes it similar to FTC-type chromophores [55] with
thiophene moiety for which this phenomenon is the same, unlike the
closely related chromophore DBA-VQPhV-TCFPhCy with a quinoxaline
core in the π-bridge, for which this phenomenon is opposite. The value of
mophores DBA-VQPhV-TCFPh, DBA-VQPhV-TCFTol
TCFPhCy and DBA-VQonV-TCFPhCy (Scheme 1, Fig. 1).
,
DBA-VQPhV-
the solvatochromic effect often directly correlates with the value of the
first hyperpolarizability for related systems [45,54,78]. Chromophores
with dialkyl aniline donor and tricyanofuranyl acceptor bonded by
–
–
–
vinylene-Z-vinylene (-CH CH-Z-CH CH-)
π-bridge, where Z are
–
–
–
3.2. Photophysical properties
vinylene (-CH CH-) [78], thiophene [45,54,55], thiazole [45], qui-
noxaline/quinoxalinone (Table 1) moieties, exhibit close dioxane/ch-
loroform solvatochromic shift (53 nm, 46–61 nm, 47 nm, 51–53 nm,
respectively).
The chromophores photophysical properties were studied in seven
solvents of different polarity: dioxane ( = 2.2), chloroform ( = 4.8),
1,2-dichloromethane ( = 8.9), dimethylformamide ( = 36.7), aceto-
nitrile ( = 37.5), dimethylsulfoxide (45.0) and ethylene carbonate (EC,
= 89.8). UV–Vis absorption spectra of all chromophores exhibit a
similar broad * intramolecular charge-transfer (CT) absorption band
ε
ε
ε
ε
ε
ε
3.3. DFT calculations of chromophores first hyperpolarizability
π-π
in the visible region in the range 597–662 nm (Fig. 2, Table 1).
Chromophores showed a positive solvatochromic shift from dioxane
to chloroform solution and then solvatochromism inversion was
observed giving negative solvatochromic shift from chloroform to polar
solvent (CH3CN, DMF, DMSO or EC). The values of positive sol-
vatochromic shift for all chromophores are close (Δλ = 51–53 nm),
while negative solvatochromic shift for chromophores with quinoxaline
The values of chromophores electric characteristics are presented in
Table 2 together with the data for pristine 7-DBA-VQV-TCF chromo-
phore [53] which does not contain any bulky group in the acceptor
fragment.
The comparison of βtot values for all five chromophores are also given
in Fig. S2. To study the mutual effect of the acceptor substituent and the
nature of the π-electron bridge we have compared the electric charac-
core in the
π-bridge when passing from chloroform to acetonitrile (Δλ =
teristics of two chromophores with TCFPhCy acceptor and quinoxaline/
Scheme 1. Synthesis of TCF-based chromophores with divinylquinoxaline π-bridge.
4

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
Fig. 1. Chemical structure of the synthesized chromophores.
Combination of redox-active moieties in a sole molecule of a D-π-A
qinoxalinone bridges: DBA-VQPhV-TCFPhCy and DBA-VQonV-TCFPhCy.
The data of Table 2 and Fig. S2 demonstrate the effect of bulky sub-
stituents in the acceptor fragment on the value of first hyper-
polarizability. The introduction of phenyl and tolyl groups
(chromophores DBA-VQPhV-TCFPh and DBA-VQPhV-TCFTol gives most
promising examples: the value of βtot differs insignificantly from the
value for 7-DBA-VQV-TCF, besides the presence of substituents results
in the flattening of the structure (Fig. S3) which promotes the increase of
the first hyperpolarizability. According to the visualization of the
dihedral angle between the plane of the quinoxaline bridge and the
plane of TCF fragment, the value of this angle does not exceed 20◦ for 7-
DBA-VQV-TCF, it becomes significantly smaller for DBA-VQPhV-TCFPh
and DBA-VQPhV-TCFTol (6◦ and 13◦, respectively) and is equal to 46◦
and 77◦ for DBA-VQPhV-TCFPhCy and DBA-VQonV-TCFPhCy, respec-
tively. The increase of the angle correlates with the decrease of the first
hyperpolarizability value. We have also analyzed the frontier orbitals of
the chromophores under study (Fig. 3); the corresponding energies
together with the value of the energy gap are given in Table 3. The
visualization of frontier orbitals (Fig. 3) confirms that all chromophores
are CT ones, in all of them HOMO occupies mainly donor fragment and
adjacent region of the bridge, while LUMO is concentrated at the other
part of the bridge and the acceptor fragment. Orbital energies have very
close values as well as ΔEg values for all chromophores (Table 3).
chromophore typically leads to significant changes in its redox proper-
ties in comparison with those of individual fragments. The chromophore
molecules proposed in this study include dialkylarylamine donor, which
is oxidized reversibly, quinoxaline core, which is easily and reversibly
reduced and oxidized [79–81], as well as TCFPh terminal electron
acceptor, also capable to reversible oxidation [82] and reduction [83].
The electrochemical study of chromophores assembled from these
redox-active units capable to reversible electron transfers (oxidations
and reductions) is of great interest as assessing and predicting the energy
gap values (optical and electrochemical) of chromophores, these esti-
mations providing an important tool for selection of structures with the
highest NLO activity.
The redox properties of the NLO chromophores were studied by cy-
clic and differential pulse voltammetries (CV and DPV) in dichloro-
methane containing tetrabutylammonium tetrafluoroborate (0.2 M) as
the supporting electrolyte. These results are presented in Table 4 and in
Fig. 4.
To estimate the HOMO-LUMO energy gaps according to electro-
chemistry data we used recently proposed approach [84], based on close
relation of the oxidation and reduction potentials with the energies of
the HOMO and LUMO levels of organic NLO chromophore. Due to
irreversibility or low reversibility of redox processes we used DPV po-
tentials values (as the ones closest usually to the formal oxidation and
reduction potentials) to calculate the frontier orbitals energy [27,35,76,
77,84,85].
3.4. Electrochemical study
The electrochemistry picture is very similar for all chromophores
under study: quasi-reversible oxidation and irreversible reduction are
observed; energy gaps for chromophores vary insignificantly what is in
accordance with the DFT estimations (Table 4). These rather close
values for the studied chromophores seem to be quite expected: in
conformity with the HOMO and LUMO visualization (Fig. 3) the
For structures with several redox-active centers linked by unsatu-
rated bridges it is especially interesting to clarify the relationship be-
tween redox properties and peculiarities of intramolecular electron
transfer as well as mutual influence of the centers on these properties.
Redox processes are the basis of numerous functions in biology, chem-
istry, including materials chemistry and particularly NLO activity.
5

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
Fig. 2. UV–Vis experimental spectra of studied chromophores in different solvents.
Table 1
Photophysical properties of the studied chromophores.
λmax, nm (
ε
, 103⋅Mꢀ 1 cmꢀ 1
)
Δλmaxa, nm
Δλmaxb, nm
1,4-dioxane
CHCl3
CH2Cl2
CH3CN
DMF
DMSO
EC
DBA-VQPhV-TCFPh
DBA-VQPhV-TCFTol
DBA-VQPhV-TCFPhCy
DBA-VQonV-TCFPhCy
599 (34.2)
596 (34.0)
597 (35.9)
609 (39.2)
652 (28.3)
646 (25.4)
648 (30.8)
662 (30.9)
636 (26.9)
632 (28.4)
633 (28.6)
657 (38.3)
596 (30.4)
592 (34.4)
596 (30.6)
629 (34.3)
604 (20.3)
604 (24.0)
608 (22.2)
639 (20.0)
614 (22.0)
612 (28.5)
617 (26.8)
647 (21.7)
608 (24.9)
607 (27.4)
611 (21.2)
643 (27.9)
53
50
51
53
56
54
52
33
a
–dioxane/CHCl3.
b
–CHCl3/CH3CN.
1240/λmax, λmax taken from Table 1), because chromophore DBA-
VQonV-TCFPhCy has the largest λ amongst the studied here com-
Table 2
max
Electrical characteristics of the studied chromophores calculated at the B3LYP/
6-31G(d)//M062X/aug-cc-pVDZ level.
pounds. The largest electrochemical gap is characteristic of chromo-
phore 7-DBA-VQV-TCF, the basic structure from which all other
derivatives with substituents were obtained. This observation also co-
incides with the results of the optical gap estimation, namely, the large
ΔEoptmax value for 7-DBA-VQV-TCF (λmax = 619 nm [53]. EHOMO almost
coincide for all chromophores, ELUMO change insignificantly, being
slightly lower only for DBA-VQonV-TCFPhCy. Relatively, the results of
the energy trends estimation by electrochemical calculation and optical
data (ΔEλomptax) are in good agreement (Table 5), although the absolute
Chromophores
μ
(D)
αav (10ꢀ 24
βzzz (10ꢀ 30
βtot (10ꢀ 30
esu)
esu)
esu)
7-DBA-VQV-TCF
[53]
19.5
118.8
789
875
DBA-VQPhV-TCFPh
DBA-VQPhV-TCFTol
DBA-VQPhV-TCFPhCy
DBA-VQonV-TCFPhCy
20.2
20.1
16.8
21.5
126.6
130.0
129.4
123.6
815
775
622
472
890
841
619
517
values of the optical gaps calculated on the basis of λ
are over-
max
estimated traditionally.
introduction of the substituents to the chromophores acceptor fragment
almost does not affect the frontier orbitals. However, quantitatively the
electrochemical estimations give notably smaller values than DFT and
3.5. Thermal properties
optical ones. The smallest gap ΔEel corresponds to DBA-VQonV-TCFPhCy
,
which coincides with the estimated optical gap ΔEopt
(ΔEopt
=
max
max
The thermal stabilities of chromophores were investigated by
6

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
Fig. 3. Frontier orbitals of 7-DBA-VQV-TCF (a), DBA-VQPhV-TCFPh (b), DBA-VQPhV-TCFTol (c), DBA-VQPhV-TCFPhCy (d) and DBA-VQonV-TCFPhCy (e).
simultaneous TG/DSC analysis. Fig. 5 shows the thermogravimetric
Table 3
curves of studied chromophores. The investigated quinoxaline-based
chromophores exhibit the similar character of weight loss and have
high thermal stability: the decomposition temperatures at which 5%
mass loss occurs at heating are above 267 ◦C. However, only for chro-
mophores DBA-VQPhV-TCFPhCy TGA and DSC decomposition tempera-
ture agrees well (values Tad and Tbd in Table 6). For chromophores DBA-
VQPhV-TCFPh and DBA-VQPhV-TCFTol TGA and DSC decomposition
temperatures are markedly different; similar behavior was found for the
Frontier orbital energies and ΔEDgFT values for the studied chromophores.
Chromophores
E(HOMO), eV
E(LUMO), eV
ΔEDgFT,eV
7-DBA-VQV-TCF
ꢀ 5.18
ꢀ 5.17
ꢀ 5.16
ꢀ 5.18
ꢀ 5.20
ꢀ 3.28
ꢀ 3.24
ꢀ 3.22
ꢀ 3.29
ꢀ 3.05
1.90
1.93
1.94
1.89
2.15
DBA-VQPhV-TCFPh
DBA-VQPhV-TCFTol
DBA-VQPhV-TCFPhCy
DBA-VQonV-TCFPhCy
7

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
Table 4
Electrochemical data (CV peak potentials, DPV potentials) for oxidation-reduction of chromophores and calculated frontier orbitals energy values (eV). EHOMO = ꢀ (E
[DPV,ox vs. Fc+/Fc] + 4.8)(eV), ELUMO = ꢀ (E[DPV,red vs. Fc+/Fc] + 4.8)(eV).
Chromophores
Cyclic votammetry (E, V)
DPV potentials (V)
Oxidation
EHOMO, eV
ELUMO, eV
ΔEel, eV
Oxidation
Reduction
Reduction
7-DBA-VQV-TCF
Efp = 0.25; Erp = 0.13;
Efp = ꢀ 0.92; irrev.
0.21
ꢀ 0.83
ꢀ 5.01
ꢀ 5.01
ꢀ 5.00
ꢀ 5.00
ꢀ 5.00
ꢀ 3.97
ꢀ 4.00
ꢀ 3.98
ꢀ 3.99
ꢀ 4.05
1.04
1.01
1.02
1.01
0.95
ΔE = quasi-rev.
DBA-VQPhV-TCFPh
DBA-VQPhV-TCFTol
DBA-VQPhV-TCFPhCy
DBA-VQonV-TCFPhCy
Efp = 0.24; Erp = 0.14;
ΔE = quasi-rev.
Efp = ꢀ 0.90; irrev.
Efp = ꢀ 0.91; irrev.
Efp = ꢀ 0.91; irrev.
Efp = ꢀ 0.83; irrev.
0.21
0.20
0.20
0.20
ꢀ 0.80
ꢀ 0.82
ꢀ 0.81
ꢀ 0.75
Efp = 0.23; Erp = 0.14;
ΔE = quasi-rev.
Efp = 0.25; Erp = 0.14;
ΔE = quasi-rev.
Efp = 0.24; Erp = 0.12;
ΔE = quasi-rev.
Conditions: CH2Cl2/0.2 M Bu4NBF4, Pt working electrode; AgNO3 reference electrode, recalculated to Fc+/Fc; substrate concentration 1–2 mM.
Fig. 5. TGA curves of DBA-VQPhV-TCFPh (a), DBA-VQPhV-TCFTol (b) and
DBA-VQonV-TCFPhCy (c).
Fig. 4. Cyclic voltammograms of investigated chromophores in CH2Cl2 – 0.2 M
Bu4NBF4, scan rate 100 mV/s.
Table 6
Thermal properties of the studied chromophores.
Chromophore
DBA-VQPhV-TCFPh
DBA-VQPhV-TCFTol
DBA-VQonV-TCFPhCy
Table 5
Energy gaps of the studied chromophores, evaluated by different methods.
a
Tdb
,
◦C
◦C
◦C
287
232
189
275
236
195
267
256
248
Chromophore
λmax, nma
ΔEopt,max eVa
ΔEDgFT
ΔEel, eVa
Td
,
Tm
,
7-DBA-VQV-TCF
619 [53]
636
2.00
1.95
1.96
1.96
1.89
1.90
1.93
1.94
1.89
2.15
1.04
1.01
1.02
1.01
0.95
a
TGA, temperature at which 5% mass loss occurs at heating.
DSC.
DBA-VQPhV-TCFPh
DBA-VQPhV-TCFTol
DBA-VQPhV-TCFPhCy
DBA-VQonV-TCFPhCy
b
632
633
657
chromophores-guests are noncovalently bound with each other by π-π
a
In CH2Cl2.
interactions of different structural moieties, the number of bound
chromophores depends on the chromophore concentration in the ma-
terial. Among stacked structures both parallel and antiparallel
arrangement of chromophore dipole moments is revealed for all the
chromophores. In composite material DBA-VQPhV-TCFPh(20)/PMMA
chromophore 7-DBA-VQV-TCF [53]. The studied compounds appeared
as highly crystalline ones with melting points (Tm) at 189 ◦C, 195 ◦C and
248 ◦C for chromophores with Ph, Tol and CyPh substituents in
acceptor, respectively (Fig. 6, Table 6). The presence of an endothermic
peak at 148 ◦C for DBA-VQPhV-TCFTol attracts attention, which is
apparently indicates at the recrystallization, since no mass loss is
observed at this temperature.
(Fig. 7(a)) 12 chromophores out of 21 are noncovalently bound by π-π
interactions, all pairwise except one trimer (Fig. 7(b), (c)). In two
chromophores intramolecular bonds between their rings are formed.
Phenyl substituent in acceptor (Ph(A)) also participates in the non-
covalent bonds formation, such bonds are formed between phenyl sub-
stituents in the bridge (Ph(B)) and in acceptor (Ph(B)-Ph(A), between
bridge (B) and Ph(A) (B-Ph(A)).
3.6. Atomistic modeling of PMMA-based composite materials
In the case of DBA-VQPhV-TCFPh(25)/PMMA 17 chromophores out
of 28 are bound noncovalently, all the moieties are involved in bonding
including Ph(A) in four cases.
All the considered polymer composite systems were packed in
amorphous cell with density, close to that of real polymer, ranging from
1.051 to 1.065 g/cm3; the results of simulations are summarized in
Table 7. In all the composite systems studied here some proportion of
At further increase of the chromophore DBA-VQPhV-TCFPh load up
8

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
distributed over the cell (Fig. S5(a)). At 30 wt% content (Fig. S5(b)) 15
chromophores of 36 are noncovalently bound in DBA-VQPhV-
TCFTol(30)/PMMA system, tolyl substituent participating in non-
covalent binding. In some chromophores intramolecular bonds are
also realized with participation of tolyl substituent. In the case of tolyl-
substituted acceptor maximum number of chromophores in aggregate is
only 3 independently on the chromophore content in material.
In DBA-VQPhV-TCFPhCy(20)/PMMA material the chromophores are
rather uniformly distributed (Fig. S6(a)), 4 out of 19 chromophores are
involved in π-π-interactions with each other, all pairwise. Phenyl frag-
ment of acceptor substituent does not participate in noncovalent
bonding. Dimers are bound via donor-donor and donor-bridge
interactions.
In DBA-VQPhV-TCFPhCy(25)/PMMA the chromophores are still
rather uniformly spread over the material, 10 chromophores out of 25
are noncovalently bound with each other, maximum number of chro-
mophores in aggregate reaches 4 units, all parallel to each other. Only
one antiparallel pair of stacked chromophores was detected. In DBA-
VQPhV-TCFPhCy(30)/PMMA (Fig. S6(b)) 16 chromophores of 33 are
Fig. 6. DSC curves of DBA-VQPhV-TCFPh (a), DBA-VQPhV-TCFTol (b) and DBA-
VQonV-TCFPhCy (c).
noncovalently bound with each other by π-π interactions of various
moieties. In some chromophores intramolecular bonds between Phe(B)
and phenyl ring of PhCy substituent in acceptor are revealed. PhCy(A)
participates in noncovalent binding with various moieties of other
chromophores (D, Ph(B), B), but in all the revealed cases furanyl ring
itself is not involved in noncovalent bonding of chromophores; similar
observation was made when considering composite materials with
chromophores-guests, containing furanyl acceptor [52].
Table 7
Composite systems with considered chromophores-guests: number of chromo-
phores in a cell (N), number of noncovalently bound chromophores (Nb),
portion of noncovalently bound chromophores (%), maximum number of
chromophores in aggregate (M), material density
ρ
(g/cm3).
%
Composite polymer materials with a NLO chromophore which is
representative of another class – quinoxalinone ones - were also
considered, chromophore-guest is DBA-VQonV-TCFPhCy with quinox-
Composite system
N
Nb
M
ρ
DBA-VQPhV-TCFPh(20)/PMMA
DBA-VQPhV-TCFPh(25)/PMMA
DBA-VQPhV-TCFPh(30)/PMMA
DBA-VQPhV-TCFTol(20)/PMMA
DBA-VQPhV-TCFTol(25)/PMMA
DBA-VQPhV-TCFTol(30)/PMMA
DBA-VQPhV-TCFPhCy(20)/PMMA
DBA-VQPhV-TCFPhCy(25)/PMMA
DBA-VQPhV-TCFPhCy(30)/PMMA
DBA-VQonV-TCFPhCy(20)/PMMA
DBA-VQonV-TCFPhCy(25)/PMMA
21
28
36
20
28
36
19
25
33
20
26
12
17
23
4
57
61
64
20
25
42
21
40
48
33
27
3
2
5
3
4
3
2
4
3
2
3
1.065
1.053
1.065
1.056
1.052
1.060
1.065
1.051
1.053
1.062
1.053
alinone fragment in
π-electron bridge. In DBA-VQonV-TCFPhCy(20)/
PMMA composite system (Fig. S7(a)) 6 chromophores out of 20 are
noncovalently bound (Table 7), all pairwise, and one stacking dimer is
formed due to interaction of phenyl moiety of PhCy substituent with one
of bridge rings. In all dimers chromophores are antiparallelly arranged.
In DBA-VQonV-TCFPhCy(25)/PMMA (Fig. S7(b)) composite system 7 out
7
15
4
10
16
6
of 26 chromophores are π-π bound, maximum size of an aggregate is
7
three, in one dimer Ph of PhCy substituent participates in bonding with
bridge of the neighbouring chromophore, this substituent does not
prevent chromophore parallel arrangement.
to 30 wt% 23 chromophores out of 36 are involved in noncovalent
bonding with each other, chromophores are bound most antiparallely,
Ph(A) forms noncovalent bonds with donor (D), B and Ph(B). In some
Summarizing consideration of composite materials with
chromophores-guests, one can see, that the proportion of noncovalently
bound chromophores increases with the chromophore content growth in
all composite materials, the size of the aggregate is maximal for DBA-
VQPhV-TCFPh(30)/PMMA, reaching 5 units. As for other quinoxaline-
based substituted chromophores DBA-VQPhV-TCFTol and DBA-VQPhV-
TCFPhCy, the size of the chromophore aggregate does not exceed 4 units
(Table 7) for all three considered concentrations. In composite materials
DBA-VQonV-TCFPhCy the chromophore content increase does not result
in growth of the chromophore aggregate size, and proportion of chro-
mophores involved in noncovalent binding is very close for two
considered concentrations.
chromophores intramolecular π-π-stacking is realized (Ph(B)-Ph(A)). At
30 wt% content the chromophores begin to aggregate with the forma-
tion of clusters up to 5 molecules (Table 7), i.e. aggregation becomes
pronounced. Chromophore distribution in material is rather nonuniform
– chromophores are concentrated in one place in a cell (Fig. S4). The
proportion of chromophores involved in noncovalent bonding with each
other is increased from 57 to 64% with the chromophore content growth
from 20 to 30%, but pronounced chromophore aggregation is revealed
only at 30 wt% content.
Only 4 chromophores out of 20 are noncovalently bound by π-π in-
Tolyl and cyclohexylphenyl substituents can be treated as most
effective ones, preventing chromophore pronounced aggregation even
at 30 wt% content. As for phenyl substituent, it can’t completely isolate
the chromophores-guests, the size of an aggregate reaches 5 units.
teractions in composite material DBA-VQPhV-TCFTol(20)/PMMA, all
pairwise except one trimer. Tolyl substituent in acceptor also partici-
pates in the intermolecular noncovalent bonds formation with Ph(B) (Ph
(A)-Ph(B)), with bridge (Ph(A)-B). Intramolecular π-π-stacking is also
revealed in three chromophores, such chromophores do not interact
with any other ones. The number of non-covalently bound chromo-
phores increases to 7 in DBA-VQPhV-TCFTol(25)/PMMA, i.e. 25% (7 of
3.7. NLO performance
The thin films of guest-host polymer materials were prepared with
PMMA as polymer matrix and studied chromophores as guests to study
macroscopic NLO activity. Polymer films were poled in a corona
discharge field. The quality of orientation was controlled by the ab-
sorption change in UV–Vis spectra detected before and after poling
28) of chromophores are involved in intermolecular π-π interactions,
phenyl ring of acceptor substituent participates in noncovalent bonding
with acceptor and bridge rings of other chromophores. It should be
mentioned, that at 25 wt% content no pronounced aggregation of
chromophores was revealed – maximum size of one aggregate is only 4
chromophores, although chromophores are not very uniformly
(Fig. 8), and characterized by order parameter,
η (Table 8), calculated by
the following formula: = 1-A/A0, where A and A0 are the absorptions
η
9

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
Fig. 7. Amorphous cell with DBA-VQPhV-TCFPh(20)/PMMA (a); examples of intramolecular (b) and intermolecular (c) noncovalent bonding.
Fig. 8. UV–Vis electron absorption spectra registered before and after poling for DBA-VQPhV-TCFPhCy/PMMA (a) and DBA-VQonV-TCFPhCy/PMMA (b) films with
chromophore load 25 wt%.
of the polymer films after and before poling [5,37,86–88].
% chromophores load results in most pronounced chromophores
aggregation.
NLO coefficients were obtained by the SHG technique using ampli-
fied femtosecond pulses at a wavelength of 1028 nm. For each case the
effective generation of the second harmonic is observed in a wide range
of pump beam incidence angles (20◦–80◦) with a maximum in the vi-
cinity of 60◦ (Fig. 9).
Composite materials DBA-VQPhV-TCFTol/PMMA exhibit similar
behavior to that for DBA-VQPhV-TCFPh/PMMA: NLO activity is maximum
at 20–25 wt% chromophore load, but d33 values are 10% higher than in the
case of DBA-VQPhV-TCFPh/PMMA at the same chromophore content. NLO
NLO activity for chromophore DBA-VQPhV-TCFPh with large value
of first hyperpolarizability (Table 2) was studied more thoroughly and
thin polymer films doped with this chromophore in various concentra-
tions (15 wt%, 20 wt%, 25 wt% and 30 wt%) were fabricated. The
composite materials DBA-VQPhV-TCFPh(20)/PMMA and DBA-VQPhV-
TCFPh(25)/PMMA) show close d33 values 29 pm/V, which are
maximum amongst considered here DBA-VQPhV-TCFPh/PMMA com-
posite materials (Table 8, Fig. 10). The increase of the chromophore load
up to 30 wt% leads to decrease of NLO activity of polymer material DBA-
VQPhV-TCFPh(30)/PMMA. This observation is in line with the results
for DBA-VQPhV-TCFPh/PMMA composite materials simulation – 30 wt
activity of DBA-VQPhV-TCFPhCy/PMMA unlike DBA-VQPhV-TCFPh/
PMMA and DBA-VQPhV-TCFTol/PMMA demonstrates a more pronounced
weight concentration dependence: starting from 23 pm/V at 20 wt% load,
d33 value arises to 37 pm/V at 25 wt% chromophore content. Increase in the
chromophore load up to 30 wt% leads to decrease of NLO coefficient to 27
pm/V for DBA-VQPhV-TCFPhCy(30)/PMMA. This value is by 17% larger
than d33 of DBA-VQPhV-TCFPhCy(20)/PMMA, whilethed33 valuesforDBA-
VQPhV-TCFPh(30)/PMMA and DBA-VQPhV-TCFTol(30)/PMMA, on the
contrary, are by 38% and 35% less than the values of DBA-VQPhV-
TCFPh(20)/PMMA and DBA-VQPhV-TCFTol(20)/PMMA, correspondingly.
The presence of a bulky substituent in the p-position of phenyl ring in TCF
10

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
decrease in NLO activity of DBA-VQonV-TCFPhCy/PMMA with an increase
in the chromophore content from 20 wt% to 25 wt% polymer films doped
with 10 wt% and 15 wt% of chromophore DBA-VQonV-TCFPhCy were also
fabricated. Maximum of d33 values (32 pm/V) arises at 15 wt% chromo-
phore load, wherein NLO activities of poled films DBA-VQonV-
TCFPhCy(20)/PMMA and DBA-VQonV-TCFPhCy(25)/PMMA are close to
Table 8
NLO coefficients (d33), order parameters (
thickness (h).
η), λmax of polymer film and film
Sample, chromophore(load, wt
%)/polymer
h, nm
λ, nm
η
d33, pm/V
DBA-VQPhV-TCFPh(15)/PMMA
DBA-VQPhV-TCFPh(20)/PMMA
DBA-VQPhV-TCFPh(25)/PMMA
DBA-VQPhV-TCFPh(30)/PMMA
DBA-VQPhV-TCFTol(20)/PMMA
DBA-VQPhV-TCFTol(25)/PMMA
DBA-VQPhV-TCFTol(30)/PMMA
DBA-VQPhV-TCFPhCy(20)/PMMA
DBA-VQPhV-TCFPhCy(25)/PMMA
DBA-VQPhV-TCFPhCy(30)/PMMA
DBA-VQonV-TCFPhCy(10)/PMMA
DBA-VQonV-TCFPhCy(15)/PMMA
DBA-VQonV-TCFPhCy(20)/PMMA
DBA-VQonV-TCFPhCy(25)/PMMA
350
450
400
330
260
410
140
330
530
260
342
220
405
400
619
618
621
624
619
621
625
627
626
630
647
646
648
649
0.28
0.27
0.20
0.21
0.27
0.26
0.33
0.27
0.25
0.30
0.37
0.35
0.30
0.25
22
29
29
16
31
35
23
23
37
27
21
32
28
27
each other and differ insignificantly form that for DBA-VQonV-TCFPhCy
/
PMMA(15). The molecular modeling of polymer composites DBA-VQonV-
TCFPhCy/PMMAdid not revealpronouncedchromophoreaggregationat 20
and 25 wt% chromophore load, thus demonstrating good isolating ability of
CyPh substituent. According to DFT calculations, chromophore DBA-
VQonV-TCFPhCy possesses a large dipole moment in comparison with
chromophore DBA-VQPhV-TCFPhCy (22 D and 17 D, correspondingly,
Table 2), and this fact may be the reason for the shift of the NLO activity
maximum towards the low chromophore content in the polymer matrix.
The decrease in d33 value of DBA-VQonV-TCFPhCy/PMMA (Table 8, Fig. 10)
in comparison with that for DBA-VQPhV-TCFPhCy/PMMA correlates with
the first hyperpolarizability of chromophores DBA-VQonV-TCFPhCy and
DBA-VQPhV-TCFPhCy (Table 2).
3.8. Temporal and photochemical and stability
Three materials with chromophores containing Ph, Tol, CyPh sub-
stituents in the acceptor fragment preserve more than 90% of d33 after
annealing at 50 ◦C for 100 h (92%, 93 and 91% for materials with Ph, Tol
and CyPh substituents, respectively). In contrast, only 52% of the initial
d33 value of the poled film of DBA-V-TCF(20)/PMMA, without qui-
noxaline core in the π-bridge, was kept after 100 h.
Сomposite materials doped with quinoxaline chromophores are
photostable with respect to laser pulses with intensities up to 11.6 GW/
cm2 and up to at least 700 J exposure dose.
4. Conclusion
Four chromophores - representatives of a novel class of NLO D-
chromophores with -deficient heterocyclic moiety in the conjugated
bridge were synthesized. Quinoxaline or quinoxalinone cores have been
incorporated into chromophore -bridge together with bulky group
π-A
π
π
Fig. 9. The experimental SHG intensity on the incidence angle of femtosecond
laser radiation dependence for DBA-VQPhV-TCFPhCy/PMMA with chromophore
load 25 wt%.
introduced in acceptor moiety to study the effect of these factors on
electrochemical, linear and nonlinear optical, and thermal properties of
these compounds as well as on chromophore aggregation in polymer
materials doped with title chromophores and on NLO activity of thin
poled polymer film of guest-host materials. Chromophores exhibit a
similar broad π-π* intramolecular charge-transfer (CT) absorption band
in the visible region in the range 597–662 nm in UV–Vis absorption
spectra, positive solvatochromic shift at changing solvent from dioxane
to chloroform; solvatochromism inversion was observed to give negative
solvatochromic shift at changing solvent from chloroform to polar sol-
vent. The introduction of a bulky cyclohexylphenyl substituent leads to
non-planar geometry of the chromophores, decreasing the value of the
first hyperpolarizability. The value of the energy gap decreases when
passing from quinoxaline-based to quinoxalinone-based chromophores.
The thermal stability of chromophores grows with increasing the size of
the substituent in acceptor. Tolyl and cyclohexylphenyl substituents in
acceptor moiety promote the decrease of pronounced aggregation even
at 30 wt% content. Femtosecond nonlinear optical activity of poled thin
PMMA films, doped with chromophores with various weight content,
have been investigated by SHG technique (λ = 1028 nm). The films are
photostable with respect to laser pulses with peak intensities up to 11
GW/cm2. DBA-VQPhV-TCFPhCy/PMMA, doped with 25 wt% of chro-
Fig. 10. The d33 values chromophore/PMMA materials.
acceptor leads to an increase in macroscopic NLO activity, even at higher
chromophore content in polymer film, despite a decrease in first hyper-
polarizability of chromophore DBA-VQPhV-TCFPhCy compared to that for
DBA-VQPhV-TCFPh according to DFT; thus, cyclohexylphenyl group ex-
hibits good isolating ability, what is supported by molecular modeling re-
sults. In order to clarify the NLO effect in more detail, chromophore DBA-
VQonV-TCFPhCy with quinoxalinone core instead of quinoxaline one in the
mophore with quinoxaline core in the
π-bridge and bulky cyclo-
hexylphenyl group in acceptor, exhibits d33 values maximal among
those for the studied materials; it demonstrates a good temporal stability
of NLO response and excellent chromophore thermal stability.
π
-bridge and TCFPhCy acceptor was synthesized (Scheme 1, Fig. 1). Due to a
11

A.A. Kalinin et al.
Dyes and Pigments 184 (2021) 108801
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Acknowledgements
The authors gratefully acknowledge the financial support of the
Russian Science Foundation (grant no. 16-13-10215) for the study of
NLO activity of chromophores with phenylquinoxaline moiety and
Russian Foundation for Basic Research (grant No. 19-29-08001) for the
study of chromophore electrochemical properties and comparative
assessment of energy gaps calculated by different methods. We are
grateful to Professor E.S. Nefediev, the Head of the Department of
Physics of Kazan National Research Technological University and
Spectroscopy, Microscopy and Thermal Analysis Laboratory (Center for
collective use “Nanomaterials and Nanotechnologies” KNRTU) where
microscopic investigations were carried out.
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Appendix A. Supplementary data
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