Synthesis and characterization of new second-order NLO chromophores containing the isomeric indolizine moiety for electro-optical materials

Article abstract of DOI:10.1016/j.dyepig.2017.08.047

To further explore the potential application of the indolizine donor group in nonlinear optical (NLO) chromophores, we have designed and synthesized new push-pull chromophores (MPI-1)-V-TCF and (MPI-3)-V-TCF, having isomeric 3(1)-methyl-2-phenylindolizin-

Full text of DOI:10.1016/j.dyepig.2017.08.047

Dyes and Pigments 147 (2017) 444e454
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and characterization of new second-order NLO
chromophores containing the isomeric indolizine moiety for
electro-optical materials
Alexey A. Kalinin*, Maxim A. Smirnov, Liliya N. Islamova, Guzel M. Fazleeva,
Tatyana A. Vakhonina, Alina I. Levitskaya, Olga D. Fominykh, Nataliya V. Ivanova,
Ayrat R. Khamatgalimov, Irek R. Nizameev, Marina Yu. Balakina
A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre, Russian Academy of Sciences, Arbuzov Str. 8, 420088 Kazan, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
To further explore the potential application of the indolizine donor group in nonlinear optical (NLO)
chromophores, we have designed and synthesized new push-pull chromophores (MPI-1)-V-TCF and
(MPI-3)-V-TCF, having isomeric 3(1)-methyl-2-phenylindolizin-1(3)-yl (MPI-1(3)) electron donor and
TCF acceptor, and compared them with dimethyl aniline (DMA) analogues. The Uv-Vis (linear optical)
properties and solvatochromic behavior were also investigated; theoretical predictions of molecular NLO
characteristics were done on the basis of the DFT calculations. The indolizine-based chromophores have
high thermal stability: the decomposition temperature is 348 ꢀC for (MPI-1)-V-TCF and 343 ꢀC for the
(MPI-3)-V-TCF, which is by 10e15 ꢀC higher than that for the chromophore DMA-V-TCF. An important
role is played by the bulky phenyl group, which prevents the aggregation of chromophores in guest-host
doped polymer films. In NLO activities, the doped (MPI-1)-V-TCF/PMMA and (MPI-3)-V-TCF/PMMA films
display the NLO coefficient, d33, values of 40 and 42 pm/V, correspondingly, at the chromophore load of
20 wt%.
Received 2 March 2017
Received in revised form
25 August 2017
Accepted 25 August 2017
Available online 26 August 2017
Keywords:
NLO chromophore
Indolizine donor
Poled polymer
DFT
Nonlinear optical coefficient
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
following strong donor moieties are synthesized and studied both
at the molecular and material levels: dialkylaminophenyl- [3e8]
Polymer electro-optic (EO) materials have attracted much
attention because of their potential applications in optical data
storage, transmission and information processing [1,2]. One of the
major challenges in the development of organic nonlinear-optical
(NLO) materials is the rational design of NLO chromophores to
and diarylaminophenyl- [8,9], diarylaminopyrrole [8], and diary-
laminothiophene [8], 1,3,3-trimethylindolylidene-2-methylene
[10], carbazole [11], phenothiazine [12], phenoxazine [12], tetra-
hydroquinoline [13], and julolidine [13e16] ones.
Сhromophores with dialkylaminothiophene [17], ferrocene
[18,19], pyranylidene-4-methylene [20], dihydrobenzothiazolylidene
and dihydroquinolinylidene donors [21] were also studied at the
molecular level.
Recently substantial progress in the enhancement of NLO
response has been achieved for chromophores with julolidine unit,
which is more strong donor in comparison with dialkylaniline one,
the structure of the chromophores containing such strong donor
with bulky group allowed to reach ultrahigh values of electro-optic
coefficient, r33 [14,16]. The importance of isolating group in the
donor was shown by the example of dialkylaniline donor [5e7].
Surprisingly, one may notice the lack of the research devoted to
chromophores with indolizine donors. Indolizines are used in the
synthesis of biindolizines [22] and redox-active macrocyclic
optimize their molecular first hyperpolarizability (
late these ( ) values into bulk NLO activities together with
improvement of thermal and photochemical stability. Most chro-
mophores have a -electron conjugated structure coupled to
b) and to trans-
b
p
electron donor and electron acceptor units. Chromophores with the
* Corresponding author.
E-mail addresses: kalesha007@mail.ru (A.A. Kalinin), macs-ne@mail.ru
(M.A. Smirnov), nailisllil1989@mail.ru (L.N. Islamova), Fazleeva_GM@mail.ru
(G.M. Fazleeva), tanyavakhonina@yandex.ru (T.A. Vakhonina), april-90@mail.ru
(A.I. Levitskaya), fod5@yandex.ru (O.D. Fominykh), ivanovanataliya@yandex.ru
(N.V. Ivanova), ayrat_kh@iopc.ru (A.R. Khamatgalimov), irek.rash@gmail.com
(I.R. Nizameev), mbalakina@yandex.ru (M.Yu. Balakina).
http://dx.doi.org/10.1016/j.dyepig.2017.08.047
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

A.A. Kalinin et al. / Dyes and Pigments 147 (2017) 444e454
445
compounds [23e26], giving such heterocyclophanes the ability to
respond to the external influence and to provide redox-switchable
binding of certain cations [27e29]; besides indolizines are used in
the development of functional materials for various applications
[30,31].
III MALDI TOF/TOF instrument with p-nitroaniline as a matrix. Uv-
Vis spectra were recorded at room temperature on a Perkin-Elmer
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 concen-
trations of ca ~1,10ꢁ5 mol/L. The thermal stabilities of chromo-
phores were investigated by simultaneous thermal analysis
(thermogravimetry/differential scanning calorimetry - TG/DSC)
using NETZSCH (Selb, Germany) STA449 F3 instrument. Approxi-
mately 3e4 mg samples were placed in an Al crucible with a pre-
hole on the lid and heated from 30 to 500 ꢀ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 10 K/min.
The use of indolizine donor can open new perspectives in the
development of new NLO chromophores. The great
nodal Carbon atom (Fig. 1) together with the total
p
-charge at the
p
-excessiveness
of this heterocyclic system specifies the effectiveness of the indo-
lozine donor which can result in the high values of first hyper-
polarizability of indolizine-containing chromophores.
Recently, a new class of NLO chromophores with donors con-
taining indolizine moiety has been proposed [32]. First hyper-
polarizability values of the chromophores, which are analogues of
FTC, CLD and OLD, containing 1-methyl-2-phenylindolizin-3-yl
(MPI-3) or 3-methyl-2-phenylindolizin-1-yl (MPI-1) donors
instead of diethylaminophenyl, 3-cyano-2-dicyanomethylene-5,5-
dimethyl-2,5-dihydrofuran-4-yl (TCF) acceptor, and 2,5-
The thickness and quality of doped polymer films was deter-
mined by AFM technique (Scanning probe microscope MultiMode
V (Veeco)) in intermittent-contact mode.
divinylthiophene, octatetraene and 2,20-divinylbithiophene
p-
2.2. Synthesis methodology
electron bridge, have been calculated by DFT technique at the M06-
2X/aug-cc-pVDZ0 computational level. The change of dimethylani-
line donor for indolizine one resulted in the growth of the first
hyperpolarizability values (Fig. 2).
The combination of indolizine donor with divinylquinoxaline
bridge provides the same tendency [32,33]. Besides the possibility
to introduce bulk substituent at the site 2 of indolizine system may
play a positive role in preventing aggregation of chromophores in
the material.
Here we describe our experimental approach to the design and
synthesis of two new NLO chromophores (MPI-1)-V-TCF and (MPI-
3)-V-TCF, containing the same vinylene bridge and TCF acceptor,
and isomeric indolizine donors; the effect of donor units on
macroscopic NLO coefficient is investigated. For this purpose the
chromophore DMA-V-TCF with dimethylaniline donor was also
synthesized. Thermal stability, linear optical properties and mo-
lecular hyperpolarizabilities (DFT calculation results) of the chro-
mophores, as well as NLO properties of the chromophore-based
materials were systematically compared to prove the benefits of
the indolizine-based donor in application to NLO chromophores.
The methods used for synthesis of the chromophores are shown
in Schemes 1e3. 2-Cyanomethylene-3-cyano-4,5,5-trimethyl-2,5-
dihydrofuran (TCF) was obtained by condensation of 3-hydroxy-
3-methyl-2-butanone with malononitrile in pyridine [34]. The al-
dehydes (MPI-1)-CHO and (MPI-3)-CHO were prepared according
to literature [35].
The 1- and 3-methyl-2-phenylindolizines were synthesized in
three steps from acetophenone and propiophenone [36]. The sec-
ond step of the reaction was carried out by heating a solution of
0.1 mol of bromoacetophenone (bromopropiophenone) and
0.12 mol of 2-ethylpyridine (2-picoline) in acetone (30 mL) at
stirring at 65 ꢀC for 8 h (30 h), then reaction mixture was cooled to
room temperature; the product was filtered and washed by acetone
to give white solid with 97% (81%) yield. The products of each step
were further employed without any additional purification.
2.2.1. General procedure for the chromophores synthesis
a. A solution of aromatic aldehyde (1.3 mmol) and TCF acceptor
(1.0 mmol) in anhydrous ethanol (18 mL) was allowed to stir at
79 ꢀC for 14 h and then cooled to room temperature. The crystal
product was filtered and washed by ethanol and hot mixture
ethanol:water (1:2).
2. Experimental section
2.1. Materials and instrumentation
b. To solution of aromatic aldehyde (1.3 mmol) and TCF acceptor
(1.0 mmol) in anhydrous pyridine (18 mL) several drops of
CH3COOH were added. The reaction was allowed to stir at 50 ꢀC
for 14 h and then cooled to room temperature. The crystal
product was filtered and washed by ethanol and hot mixture
ethanol:water (1:2).
2-Picoline, 2-ethylpyridine, acetophenone, propiophenone, 3-
hydroxy-3-methyl-2-butanone and malononitrile were purchased
from Aldrich or Acros. Organic solvents used were purified and
dried according to standard methods. The melting points presented
in Section 2.2.1 were determined on a Boetius hot-stage apparatus
and are uncorrected. Infrared (IR) spectra were recorded on the
Bruker Vector-22 FT-IR spectrometer. All 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) spectrome-
2.2.1.1. (E)-2-(1-(3-methyl-2-phenylindolizinyl)-vinyl)-3-cyano-5,5-
dimethylfuran-2(5H)-ylidene)-malononitrile
((MPI-1)-V-TCF).
ters. Chemical shifts (
d
in ppm) are referenced to the solvents CDCl3
Yield 24% (a), 43% (b). Dark blue powder, m.p.: 295e297 ꢀC. MALDI-
TOF: 417 [MH]þ, 439 [MþNa]þ, 455 [MþK]þ. IR (nmax, cmꢁ1, KBr):
and DMSO-d6. The mass spectra were obtained on Bruker UltraFlex
Fig. 1. Diethylaniline, unsubstituted indolizine and isomeric indolizines e sources of new donor fragments for NLO chromophores (with p-charge at the atom p-C in diethylaniline
and C1 and C3 atoms in indolizine derivatives calculated at the B3LYP/6-31G(d) level) [32].

446
A.A. Kalinin et al. / Dyes and Pigments 147 (2017) 444e454
Fig. 2. First hyperpolarizability values of chromophores FTC, CLD, OLD and their indolizine analogues, calculated at the B3LYP/6-31G(d)//M06-2X/aug-cc-pVDZ0 level [32].
Scheme 1. Chemical structures and synthesis of chromophore (MPI-1)-V-TCF. Reagents and Conditions: (i) Br2, rt, CHCl3; (ii) picoline, acetone, 65 ꢀC, 30 h; (iii) NaHCO3, H2O, reflux
30 min; (iv) DMF, POCl3, 45 min 35e40 ꢀC; (v) TCF, EtOH, reflux 14 h or AcOH/Py, 50 ꢀC, 14 h.
Scheme 2. Chemical structures and synthesis of chromophore (MPI-3)-V-TCF. Reagents and Conditions: (i) Br2, rt, CHCl3; (ii) picoline, acetone, 65 ꢀC, 8 h; (iii) NaHCO3, H2O, reflux
30 min; (iv) DMF, POCl3, 45 min 35e40 ꢀC; (v) TCF, EtOH, reflux 14 h or AcOH/Py, 50 ꢀC, 14 h.
2.2.1.2. (E)-2-(3-(1-methyl-2-phenylindolizinyl)-vinyl)-3-cyano-5,5-
dimethylfuran-2(5H)-ylidene)-malononitrile
((MPI-3)-V-TCF).
Yield 12% (a), 85% (b). Green powder, m.p.: 284e286 ꢀC. MALDI-
TOF: 417 [MH]þ, 439 [MþNa]þ, 455 [MþK]þ. IR (nmax, cmꢁ1, KBr):
2217, 1585, 1557, 1516, 1463, 1447, 1425, 1387, 1353, 1286, 1269,
1230, 1197, 1153, 1134, 1062, 1024, 921, 843, 768, 755. 1H NMR
Scheme 3. Chemical structures and synthesis of chromophore DMA-V-TCF. Reagents
and Conditions: (i) TCF, EtOH, reflux 14 h.
(400 MHz, DMSO-d6)
d
9.00 (d, J ¼ 7.0 Hz, 1H), 8.06 (d, J ¼ 15.0 Hz,
1H), 7.88 (d, J ¼ 8.6 Hz, 1H), 7.63 (dd, J ¼ 8.6, 7.0 Hz, 1H), 7.56 (dd,
J ¼ 7.5, 7.0 Hz, 2H), 7.48e7.53 (m, 1H), 7.38 (dd, J ¼ 7.5, 1.4 Hz, 2H),
7.32 (ddd, J ¼ 7.0, 7.0, 1.1 Hz, 1H), 6.03 (d, J ¼ 15.0 Hz, 1H), 2.17 (s,
2129, 1594, 1557, 1522, 1474, 1441, 1362, 1293, 1268, 1237, 1205,
1154, 1105, 1081, 1072, 928, 823, 752, 705. 1H NMR (400 MHz,
3H), 1.58 (s, 6H). 1H NMR (600 MHz, CDCl3)
d 8.35e8.55 (br, 1H),
8.46 (d, J ¼ 6.9 Hz, 1H), 7.59 (d, J ¼ 8.6 Hz, 1H), 7.57 (dd, J ¼ 7.4,
7.0 Hz, 2H), 7.52 (dd, J ¼ 7.3, 7.3 Hz, 1H), 7.40 (dd, J ¼ 8.6, 7.3 Hz, 1H),
7.36 (d, J ¼ 7.3 Hz, 2H), 7.13 (dd, J ¼ 7.3, 7.0 Hz, 1H), 5.57e5.77 (br,
DMSO-d6)
d
8.56 (d, J ¼ 6.6 Hz, 1H), 8.26 (d, J ¼ 8.8 Hz, 1H), 8.20 (d,
J ¼ 15.1 Hz, 1H), 7.69 (dd, J ¼ 8.8, 7.6 Hz, 1H), 7.55 (dd, J ¼ 7.3, 7.1 Hz,
2H), 7.49 (dd, J ¼ 7.3 Hz, 1H), 7.39 (d, J ¼ 7.1 Hz, 2H), 7.35 (ddd,
J ¼ 7.6, 6.6 Hz, 1H), 6.15 (d, J ¼ 15.1 Hz, 1H), 2.39 (s, 3H), 1.56 (s, 6H).
1H), 2.23 (s, 3H), 1.29 (s, 6H), 13C NMR (150 MHz, CDCl3)
d 177.22,
173.00, 141.21, 138.23, 133.85, 130.15, 129.67, 129.19, 128.78, 127.44,
126.07, 122.27, 118.33, 116.25, 114.39, 113.78, 112.97, 103.80, 95.90,
51.33, 26.13, 8.98.
1H NMR (600 MHz, CDCl3)
d
8.46e8.26 (br, 1H), 8.01 (d, J ¼ 6.8 Hz,
1H), 7.91 (d, J ¼ 8.7 Hz, 1H), 7.53 (dd, J ¼ 7.3, 7.1 Hz, 2H), 7.49 (dd,
J ¼ 7.1, 7.1 Hz, 1H), 7.42 (dd, J ¼ 8.7, 7.3 Hz, 1H), 7.34 (d, J ¼ 7.3 Hz,
2H), 7.11 (dd, J ¼ 7.3, 6.8 Hz,1H), 6.08e5.88 (br,1H), 2.41 (s, 3H),1.35
(s, 6H), 13C NMR (150 MHz, CDCl3)
d 177.18, 173.99, 140.17, 138.05,
2.2.1.3. (E)-2-(4-(N,N-dimethylaminophenyl)-vinyl)-3-cyano-5,5-
dimethylfuran-2(5H)-ylidene)malononitrile (DMA-V-TCF). Yield 83%
(a). Dark blue powder, m.p.: 317e319 ꢀC (lit. 290e292 ꢀC [37]).
MALDI-TOF: 331 [MH]þ, 353 [MþNa]þ, 369 [MþK]þ.
133.59, 130.27, 129.75, 128.97, 128.48, 126.49, 125.25, 124.81, 118.29,
115.63, 113.57, 113.30, 112.70, 111.51, 106.58, 96.01, 52.03, 26.26,
9.82.

A.A. Kalinin et al. / Dyes and Pigments 147 (2017) 444e454
447
2.3. Composite materials preparation
chloroform (4:1) led only to 13% yield of the chromophore [37]. All
the chromophores were completely characterized by 1H NMR, 13
C
Polymethyl methacrylate (PMMA) was obtained in the toluene
by free radical polymerization. To study the NLO effectiveness of
the chromophores the guest-host polymer materials were pre-
pared, where PMMA was used as polymer matrix (Tg ¼ 105 ꢀC) and
isomeric chromophores (MPI-1)-V-TCF and (MPI-3)-V-TCF were
used as guests, the DMA-V-TCF chromophore was used for com-
parison (as a reference one). Thin polymer films containing 20 wt.%,
25 wt.% and 30 wt.% of the chromophores were prepared by spin-
coating from 7% solution of polymer in cyclohexanone; films
thickness and roughness were characterized by AFM technique.
NMR, MS, Uv-Vis spectroscopic methods, and the obtained data
were in full agreement with the assumed structure.
The MALDI mass spectra of the chromophores (MPI-1)-V-TCF
and (MPI-3)-V-TCF displayed ion peaks at appropriate MHþ values.
1H NMR spectra of these compounds recorded in DMSO contain the
signals of the protons of phenylindolizine aromatic unit resonating
in the region of 8.6e7.3 ppm, the singlet signals of the protons of
two methyl groups (2.4 and 1.6 ppm) and the doublet signals of the
protons of vinylene
p-bridges resonating in the regions of
8.2e8.0 ppm and of 6.2e6.0 ppm. 1H NMR spectra of all the pre-
pared chromophores indicated the presence of only one isomer.
The coupling constants observed for the vinyl protons of the con-
jugated chains are of the order of 16 Hz, confirming their trans-
configuration.
2.4. Poling and NLO measurements
Films were poled at the corona-triode setup in the corona
discharge field, voltage 6.5 kV, poling time ~20 min, the distance
from the tungsten needle electrode to the surface of the film being
1 cm; the field was applied to the films heated to temperatures
close to glass transition temperature, Tg. The quality of orientation
was controlled by the absorption change in Uv-Vis spectra detected
3.2. Optical properties
Uv-Vis absorption spectra were recorded in a few aprotic sol-
vents with different polarity, so that the solvatochromic behavior of
each chromophore could be investigated in a wide range of
dielectric environments. The spectral data are summarized in
Table 1 and Fig. 2. The ICT energies Emax of the chromophores with
before and after poling, and characterized by order parameter,
calculated by the following formula:
h,
h
¼ 1 ꢁ A
;
=
A0
the same (acceptor and p-bridges)/(donor and p-bridges) indicate
the donor/acceptor strength of these units, respectively [39,40].
Compared with chromophore DMA-V-TCF, the lmax of chromo-
phores (MPI-1)-V-TCF and (MPI-3)-V-TCF in methylene chloride
(DCM) were shifted to the longer wavelength of 597 nm and
636 nm, respectively, what may indicate a greater donor strength of
the indolizine (MPI-1 and MPI-3) units compared to that for
dimethylaniline one (in the order DMA < MPI-1<MPI-3). When
passing from (MPI-1)-V-TCF to (MPI-3)-V-TCF bathochromic shift
of absorption maximum by 30e40 nm occurs depending on the
polarity of the solvent. Similar picture is observed when passing
from (MPI-1)-V-TCF to (MPI-3)-V-TCF doped films (see Section 3.5).
This gives evidence of the larger donor strength of the MPI-3 donor
in comparison with that of MPI-1, what is in accordance with
charge distribution in the source compounds of these donor units
(Fig. 1).
Absorption bands of both chromophores (MPI-1)-V-TCF and
(MPI-3)-V-TCF showed bathochromic shifts of 13e14 nm when
solvent was changed from dioxane to methylene chloride, whereas
they are 4 nm and 13 nm when passing from methylene chloride to
acetonitrile, respectively. Solvatochromic shift for DMA-V-TCF is
moderate when solvent was changed from methylene chloride to
acetonitrile and significant when it was changed from dioxane to
methylene chloride. This is assumed to give evidence about lower
polarizability of indolizine chromophores. The bathochromic shift
of the absorption maxima and the change of its form for less broad
one when passing from dioxane to chloroform or DCM seem to
where A and A0 are the absorptions of the polymer films after and
before poling [38].
Polymer NLO coefficients, d33, were measured by the second
harmonic generation (SHG) technique; pulse Nd3þ:YAG laser radi-
ation (
l
¼ 1064 nm, pulse duration 15 ns, power density at the
-quartz (х-cut) plate served as a standard) was
sample 10 kW/cm2;
a
used. In the course of measurements it was assumed that d33
/
d13 z 3.
3. Results and discussion
3.1. Synthesis and characterization
Schemes 1 and 2 show the synthetic approach for the chro-
mophores (MPI-1)-V-TCF and (MPI-3)-V-TCF. The heterocyclic al-
dehydes (MPI-1)-CHO and (MPI-3)-CHO were prepared according
to literature [35,36], the second step of the reaction being the
exception: the synthesis was carried out by heating the reaction
mixture in acetone. At the final step the Knoevenagel condensation
with TCF acceptor was carried out to obtain the desired isomeric
chromophores (MPI-1)-V-TCF and (MPI-3)-V-TCF. The reaction was
carried out in pyridine providing the high product yield, while
heating in ethanol led to lower yield. Similarly, the TCF acceptor
was condensed with the DMA-CHO in ethanol to give the chro-
mophore DMA-V-TCF with 83% yield. This reaction in ethanol/
Table 1
Uv-Vis absorption and solvatochromic data for the chromophores under study.
Chromophore
lmaxa, nm
lmaxb, nm
lmaxc, nm
lmaxd, nm
Dle
nm
,
Dlf
,
nm
(ε, 103 Mꢁ1cmꢁ1
)
(ε, 103 Mꢁ1cmꢁ1
)
(ε, 103 Mꢁ1cmꢁ1
)
(ε, 103 Mꢁ1cmꢁ1
)
(MPI-1)-V-TCF
(MPI-3)-V-TCF
DMA-V-TCF
583 (76)
623 (67)
526 (56)
595 (108)
636 (104)
575 (79)
597 (116)
636 (100)
575 (91)
593 (99)
623 (96)
569 (65)
4
13
6
14
13
49
a
In dioxane.
In CHCl3.
b
c
In CH2Cl2.
In CH3CN.
d
e
The difference between c lmax and d lmax
.
f
c
The difference between a lmax and lmax.

448
A.A. Kalinin et al. / Dyes and Pigments 147 (2017) 444e454
indicate that the ground state of indolizine-based chromophores
changes from the neutral one to that corresponding to the cyanine
limit with minimal bond length alternation, BLA [41,42]. At the
same time in the case of indolizine-based chromophores the sec-
ond less intensive band is present in blue region of the spectrum, its
intensity being maximal in dioxane (Fig. 3). These spectral features
can be attributed to chromophore aggregation, which strongly in-
creases with the solvent polarity decrease [40]. To clarify whether
the second band is determined by the aggregation, the spectra were
registered in chloroform for the cases of greater and lower chro-
mophores concentrations. As almost no changes were seen for
(MPI-1)-V-TCF, this gives evidence about the absence of aggrega-
tion for this chromophore in solution, the band in the blue region
being the intrinsic one. Some changes were obtained in the spectra
of isomeric (MPI-3)-V-TCF; in this case the band in the blue region
seems to be the superposition of the intrinsic band and the ‘ag-
gregation’ band (Table S3). For DMA-V-TCF in chloroform highly
intensive shoulder band is observed, which is absent in dioxane
though in this case the absorption band is broad. All these facts
testify to the significantly lower aggregation of indolizine-based
chromophores in solution.
a b n r c e A b s o
3.3. DFT calculations
Over the recent years the popularity of DFT technique for the
calculation of first hyperpolarizability has grown significantly due
to the essential progress in the development of new approaches, in
particular, new density functionals (DF) with appropriate descrip-
tion of exchange-correlational terms. Conventional DFs were
known to be ill-suitable for the estimation of molecules electric
characteristics already in the early 2000th [43e47]. The analysis of
the results obtained with various DFs, developed in the framework
of LDA, GGA, hybrid-GGA, and their comparison with the data ob-
tained with more sophisticated techniques, such as MP2 and
CCSD(T) have shown that the use of hybrid GGA functionals with
incorporation of the sufficient amount of exact Hartree-Fock ex-
change allows achieving reliable estimation of first hyper-
polarizability values [48]. Besides, the use of so-called range-
separated DFs with separate description of short- and long-range
parts of electron-electron interaction, in particular,
uB97X, was
shown to provide rather accurate values of polarizability and first
hyperpolarizability, which are in good agreement with the exper-
imental data [44,45,48]. Among the high-exchange DFs M06-2X
functional is shown to perform well for a wide range of proper-
ties for the molecules consisting of first row atoms [49,50].
Thus, here to predict the effectiveness of indolizine donor
moieties, the calculations of first hyperpolarizability of the sug-
b a n c e A b s o r
gested chromophores are performed by TD-DFT technique: static
values were estimated at the B3LYP/aug-cc-pVDZ0 as well as M06-
2X/aug-cc-pVDZ0 and B97X/aug-cc-pVDZ0 levels, which were
shown to provide reliable estimations of NLO characteristics
[48,51]; dynamic (at 1064 nm) values were calculated at the M06-
b
u
b
2X/aug-cc-pVDZ0 and B3LYP/aug-cc-pVDZ0 levels. The geometry of
the chromophores under study was optimized at B3LYP/6-31G*
level, which has good reputation for structural estimations. The
chromophores are shown to be planar lying in the x,y-plane; the
phenyl isolating group of the donor is almost perpendicular to the
plane of the indolizine moiety, the potential energy surface being
flat in the range 30ꢀ. In order to compare the obtained values of
first hyperpolarizability with those published in literature for
similar systems differing by the donor moiety, in particular, with
DMA-V-TCF, the calculations are also repeated in the framework of
most often used B3LYP density functional, though it is recognized
that the application of this computational level results in the
overestimation of hyperpolarizability values in comparison with
c e a b n r b s A o

A.A. Kalinin et al. / Dyes and Pigments 147 (2017) 444e454
449
the data obtained by more exact correlated methods such as MP2
[52].
The structure of the suggested chromophores permits assuming
the dominating component of the first hyperpolarizability tensor,
D
D
E
E
b2YYY
DR ¼
;
b2XYY
byyy, however, as the values of other components, in particular, bxyy
,
where hb2YYY i and hbX2YY i are orientational averages of hyper-
polarizability tensor components, which are proportional to the
scattered signal intensities for vertically and horizontally polarized
incident signals, respectively [54]. The DR characterizes the chro-
mophore, giving grounds to the determination of appropriate
hyperpolarizability tensor components from the analysis of HRS
experiment.
The calculations of molecular geometrical parameters and
electric properties are performed with Jaguar program [55]. The
obtained values of the electrical parameters are presented in
Table 2.
may reach ~30% of byyy, we have calculated the btot value defined as:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
¼ x; y; z:
X
1
3
btot
¼
b2x þ b2y þ b2z
;
bi
¼
biii
þ
ð
bikk
þ
bkik
þ bkkiÞ; i
isk
For the sake of comparison with the available literature data we
also provide bHRS(0) values from the computed hyperpolarizability
tensor components according to the following expression [53]:
X
X
X
6
35
16
105
38
105
b2HRS
¼
b2iii
þ
biiibijj
þ
b2iij
According to Table 2, one may see that M06-2X and
uB97X
density functionals provide rather close results, the difference in
first hyperpolarizability values not exceeding 20%. The regiose-
i
isj
isj
X
16
20
35
þ
biijbjkk
þ
b2ijk
;
lectivity of the addition of the donor to the
almost does not affect the values of hyperpolarizabilities, the (MPI-
3)-V-TCF having somewhat higher values than (MPI-1)-V-TCF.
In order to visualize the regioselectivity of the addition of the
donor to the -electron bridge, the isodensity maps of the frontier
p-electron bridge
105
ijk
b
which may be simplified without significant loss of accuracy for the
case when there is a dominant first hyperpolarizability value for:
p
HOMO and LUMO orbitals were calculated and the HOMO-LUMO
energy gaps were estimated (Table 3).
One may see that both the HOMO and the LUMO orbitals cover
the whole molecule, some shift of the electron density towards the
acceptor end being observed in the LUMO. This effect is quite weak
rffiffiffiffiffiffi
6
35
bHRS
z
byyy:
Depolarization ratio (DR) is also calculated according to
Table 2
The static electric characteristics for the chromophores (MPI-1)-V-TCF, (MPI-3)-V-TCF and DMA-V-TCF calculated in aug-cc-pVDZ0 basis set.
(MPI-1)-V-TCF
(MPI-3)-V-TCF
DMA-V-TCF
B3LYP/M06-2X B3LYP/B3LYP B3LYP/
u
B97X B3LYP/M06-2X B3LYP/B3LYP B3LYP/
u
B97X B3LYP/M06-2X B3LYP/B3LYP B3LYP/uB97X
mtot (D)
18.2
65.3
18.6
68.2
17.3
68.4
6.9
66.4
66.8
28.3
18.3
64.4
32.2
96.3
25.9
101.8
105.0
39.9
16.9
67.0
25.9
91.9
18.5
92.9
94.7
38.0
17.3
69.8
9.7
57.8
1.7
51.0
51.0
23.9
17.0
66.2
33.1
107.6
27.5
112.0
115.3
44.6
18.7
56.4
28.8
168.7
27.1
168.0
170.1
69.8
19.4
59.9
19.2
147.9
16.1
143.1
144.0
61.2
18.6
54.7
27.5
155.5
25.9
154.8
157.0
64.4
aav (,10ꢁ24 esu)
bxyy (,10ꢁ30 esu) 28.8
byyy (,10ꢁ30 esu) 90.2
bx (,10ꢁ30 esu)
by (,10ꢁ30 esu)
btot (,10ꢁ30 esu)
21.7
94.2
96.6
bHRS (,10ꢁ30 esu) 37.4
Table 3
Frontier molecular orbitals (isodensity maps) and energy gap values for the suggested chromophores.

450
A.A. Kalinin et al. / Dyes and Pigments 147 (2017) 444e454
Table 4
due to the very short p-electron bridge. Two chromophores, (MPI-
The dipole moments of ground and CT excited state, transition dipole moments,
oscillator strength and energy gaps calculated for the chromophores (MPI-3)-V-TCF
and DMA-V-TCF at B3LYP/aug-cc-pVDZ0 level.
3)-V-TCF and (MPI-1)-V-TCF, have somewhat different values of the
HOMO-LUMO energy gap, this difference being consistent with
their bHRS values.
The comparison of the values of dipole moment and hyper-
polarizability, btot, for (MPI-3)-V-TCF and (MPI-1)-V-TCF chromo-
phores with those of DMA-V-TCF, having DMA donor, demonstrates
the unexpectedly high values for the latter (170,10ꢁ30 esu), thus
the difference in btot values is in favor of DMA-V-TCF. However,
theoretical estimations carried out for model chromophores with
(MPI-3)-V-TCF
DMA-V-TCF
mg (D)
me (D)
mge (D)
F
17.7
21.36
10.83
1.1
19.41
27.16
12.06
1.47
E
ge (eV)
2.49
2.66
the same end groups but longer p-electron bridge (up to 5 vinylene
groups; Fig. S11) demonstrate that this difference becomes less
pronounced with the lengthening of the bridge and the preference
changes for the opposite when the bridge contains 4 vinylene
groups: for the chromophores with four and five vinylene groups in
the chain the hyperpolarizability for MPI-3-based chromophore
exceeds that for DMA-based by approximately 6% of the value
(Fig. 4). The analogous comparison carried out in Ref. [32] for
chromophores with DMA donor having the structure similar to that
of recognized champions FTC and CLD, provides the same result.
performed at M06-2X/aug-cc-pVDZ0 level, confirms this statement
[32].
For chromophores with charge transfer (CT) transitions domi-
nating the NLO response, the first hyperpolarizability can be
approximated in the framework of the so-called two-state model
(TSM):
3e2,Dmge m2ge
,
b0
y
bTSM
¼
;
Eg2e
Our previous computational study of chromophores with
p-elec-
tron bridges of various natures, containing indolizine donor,
where Dmge is the difference between dipole moments of the
ground and CT excited states, mge is the transition dipole moment
for the two states and Ege- the energy gap between them [56]. The
analysis of the first hyperpolarizability of the studied chromo-
phores by TSM elucidates the difference of this characteristic for the
chromophores with indolizine and DMA donors (Table 4); the
corresponding calculations (vertical transitions) are performed
with B3LYP functional which is recognized to give reliable results
for the estimation of Uv-Vis spectra parameters, in particular, lmax
[48].
According to these estimations, the difference of dipole mo-
ments between the ground and CT excited states is essentially
higher for DMA-V-TCF, this factor seems to be the governing one in
the formation of the first hyperpolarizability: the smaller value of
the energy gap for (MPI-3)-V-TCF does not change the situation. As
1400
1200
1000
800
600
400
200
0
a result, according to TSM
b is almost twice higher for DMA-V-TCF
in comparison with (MPI-3)-V-TCF. For the results of the calcula-
tions given in Table 2 for these chromophores approximately the
same ratio of hyperpolarizability values holds.
0
1
2
3
4
5
6
The dynamic first hyperpolarizability values calculated for the
(MPI-3)-V-TCF and DMA-V-TCF chromophores are presented in
Table 5. According to the performed estimations, the values of
n
Fig. 4. The dependence of btot on the bridge length (n) calculated at the M06-2X/aug-
cc-pVDZ0 level for (MPI-3)-Vn-TCF (black circles) and DMA-Vn-TCF (white triangles)
chromophores.
btot(2u) for two chromophores become rather close, the value for
(MPI-3)-V-TCF being even somewhat higher (644.7,10ꢁ30 esu
Table 5
The dynamic electric characteristics for the chromophores (MPI-3)-V-TCF and DMA-V-TCF calculated at the M06-2X/aug-cc-pVDZ0 and B3LYP/aug-cc-pVDZ0 levels at 1064 nm.
(MPI-3)-V-TCF
DMA-V-TCF
B3LYP/M06-2X
B3LYP/B3LYP
B3LYP/M06-2X
B3LYP/B3LYP
aav
aav (2
(ꢁ
(
u
u
;
) (,10ꢁ24 esu)
) (,10ꢁ24 esu)
,0)
72.27
137.03
76.08
210.18
60.67
95.89
65.72
136.47
b
u
u
bxyy (,10ꢁ30 esu)
byyy (,10ꢁ30 esu)
bx (,10ꢁ30 esu)
by (,10ꢁ30 esu)
btot (,10ꢁ30 esu)
37.86
132.94
33.76
137.50
141.58
18.17
82.37
0.50
86.37
83.38
39.40
229.19
39.57
229.92
233.30
30.64
210.38
23.88
210.78
212.13
b
(ꢁ2
u; u,u)
bxyy (,10ꢁ30 esu)
byyy (,10ꢁ30 esu)
bx (,10ꢁ30 esu)
by (,10ꢁ30 esu)
btot (,10ꢁ30 esu)
bHRS (,10ꢁ30 esu)
258.37
546.33
209.28
606.99
642.06
226.2
231.60
492.68
180.75
530.86
541.89
204.0
153.01
605.30
125.73
620.44
633.05
250.6
180.29
764.80
121.77
773.03
782.56
316.7

A.A. Kalinin et al. / Dyes and Pigments 147 (2017) 444e454
451
Fig. 6. DSC curves for (MPI-1)-V-TCF, (MPI-3)-V-TCF, and DMA-V-TCF.
Fig. 5. TGA curves for (MPI-1)-V-TCF, (MPI-3)-V-TCF and DMA-V-TCF.
DMA-V-TCF, the difference comprises about 10% in favor of DMA-V-
TCF due to larger byyy(2 ) values in this case.
Along with bHRSð2 Þ we have estimated the depolarization ra-
Table 6
Thermal stability of the studied chromophores.
u
u
Chromophore
(MPI-1)-V-TCF
(MPI-3)-V-TCF
DMA-V-TCF
tios, which for DMA-V-TCF is equal to 5.1, this value is in accordance
with those for linear noncenrosymmetric molecule being ~5 [53];
as for (MPI-3)-V-TCF the DR is equal to 5.5, the enhanced value may
be caused by the expanded conjugated system in the indolizine
donor moiety. Such high values obtained for DR correlate with
major dipolar contribution to the HRS response and a dominating
Td, oCa
Td, oCb
348 (343)
350
343 (340)
343
331 (327)
328
a
TGA, temperature at which 10 (5%) mass loss occurs at heating.
DSC.
b
byyyð2 Þ component.
u
against 629.5,10ꢁ30 esu). The tendency to the increase of the
hyperpolarizability values with the bridge elongation, noted above
3.4. Thermal stability
by the example of static
b values, will evidently be more pro-
nounced in the case of dynamic hyperpolarizabilities. As for
bHRS(2u), the values are also rather close for (MPI-3)-V-TCF and
The thermal stabilities of chromophores were investigated by
simultaneous TG/DSC analysis. Fig. 5 shows the thermogravimetric
Fig. 7. AFM surface images for polymer film (MPI-1)-V-TCF/PMMA before (a) and after (b) poling.
Table 7
NLO coefficients (d33, pm/V), film thickness (h, nm) and order parameters (
h
) for the studied polymer films at different chromophores content.a
(MPI-3)-V-TCF/PMMA DMA-V-TCF/PMMA
Chromophores content, wt%
(MPI-1)-V-TCF/PMMA
h
h
d33
h
h
d33
h
h
d33
20
25
30
295
0.45
40
e
255
310
450
0.33
0.35
0.27
42
24
29
180
320
310
0.30
0.49b
0.27
43
24
23
310
0.39
39
a
Accuracy o d33 measurements is about 15%.
b
The enhand value seems to result from the method of estimation: the Uv-Vis spectroscopy does not distinguish between polar and axial chromophores ordering; the
chromophores aggregation also leads to the increase of
h.

452
A.A. Kalinin et al. / Dyes and Pigments 147 (2017) 444e454
curves of (MPI-1)-V-TCF, (MPI-3)-V-TCF, and DMA-V-TCF. The
indolizine-based chromophores have high thermal stability: the
decomposition temperature (Td) is 348 ꢀC for (MPI-1)-V-TCF and
343 ꢀC for the (MPI-3)-V-TCF, which is 10e15 ꢀC higher than that
for the chromophore DMA-V-TCF with DMA donor (Table 6).
The studied chromophores are highly crystalline compounds
with melting point at 299 ꢀC, 286 ꢀC and 320 ꢀC for (MPI-1)-V-TCF,
(MPI-3)-V-TCF, and DMA-V-TCF, respectively (Fig. 6). The endo-
thermic enthalpies of melting are estimated as 95 J/g, 88 J/g and
57 J/g for (MPI-1)-V-TCF, (MPI-3)-V-TCF, and DMA-V-TCF, corre-
spondingly. The second exothermic peaks (350 ꢀC, 343 ꢀC and
328 ꢀC for (MPI-1)-V-TCF, (MPI-3)-V-TCF, and DMA-V-TCF,
respectively) correspond to thermal decompositions and are
consistent with above mentioned decomposition temperatures.
(MPI-3)-V-TCF/PMMA (Table 7). The analysis of the Uv-Vis
spectra for the studied polymer films shows that the ratio of the
intensity of absorption band without aggregation and that with the
3.5. NLO performance
According to the data of Sections 3.2 and 3.3 one may conclude
that there are several factors which may determine the NLO char-
acteristics at the material level. On the one hand, indolizine-based
chromophores have somewhat lower values of first hyper-
polarizability compared with that for DMA-based one (the differ-
ence is more pronounced for the static values, and is rather small
for the dynamic ones). On the other hand, the following features are
in favor of indolizine-based chromophores: the bathochromic shift
of UV absorption maximum (Section 3.2), smaller energy gap value
(Section 3.3), and the structurаl factor e the presence of bulky
phenyl substituent serving as isolation group. To study the effec-
tiveness of novel chromophores at the macroscopic level, they were
introduced into the PMMA polymer to produce guest-host material.
Polymer films were spin cast on glass substrate and poled at the
corona-triode at temperature 10 ꢀC higher than the Tg of the
polymer. The AFM surface images of the (MPI-1)-V-TCF/PMMA film
obtained before and after poling are presented on Fig. 7. The NLO
activity (d33 coefficients) was measured by SHG technique. The d33
values for the films (MPI-1)-V-TCF/PMMA, (MPI-3)-V-TCF/PMMA
and DMA-V-TCF/PMMA were measured with various chromophore
contents, corresponding data are presented in Table 7.
As an example the Uv-Vis absorption spectra for poled films at a
loading density of 30 wt% are presented on Fig. 8. The Uv-Vis
spectra for polymer films with indolizine chromophores are
similar to the chromophores spectra in solution, however the ab-
sorption for (MPI-3)-V-TCF containing film is somewhat greater
what seem to be due to instrumental error. Very strong aggregation
is characteristic for DMA-V-TCF/PMMA (Fig. 7c), as a result the
absorption band at 575 nm observed in chloroform (Fig. 2c), which
has the polarity rather close to that of PMMA, is no longer the most
intensive. For composite materials (MPI-1)-V-TCF/PMMA and
(MPI-3)-V-TCF/PMMA the aggregation is assumed to be essentially
lower. However, it is observed, what is illustrated by the ratio of
absorption maxima for corresponding bands for the case without
aggregation to that when aggregation occurs.
These data are in accordance with the corresponding values of
NLO coefficients. Maximum values of d33 were achieved for the
chromophore content of 20 wt% for all the films studied here
(Table 7), and the corresponding values were rather close, being 40,
42 and 43 pm/V for (MPI-1)-V-TCF, (MPI-3)-V-TCF and DMA-V-TCF,
respectively. The values of d33 at 20% of chromophores load are
similar to all studied materials, i.e. the lower hyperpolarizability
value for indolizine-based chromophores is compensated by their
lower tendency to aggregate. At 30% load of the indolizine-based
chromophores the composite materials have higher values of d33
in comparison with DMA-V-TCF/PMMA; in this case the effect of
the isolating group seems to be crucial, this effect being more
pronounced for (MPI-1)-V-TCF/PMMA than for.
Fig. 8. Uv-Vis absorption spectra for the studied films: (MPI-1)-V-TCF/PMMA (a),
(MPI-3)-V-TCF/PMMA (b) and DMA-V-TCF/PMMA (c).

A.A. Kalinin et al. / Dyes and Pigments 147 (2017) 444e454
453
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contribution from aggregation, practically is not changed when
passing from the (MPI-1)-V-TCF/PMMA composite with 20 wt%
load to that with 30 wt% load, while in the case of (MPI-3)-V-TCF/
PMMA this ratio decreases (Table S3), this fact gives evidence to
more essential role of isolating group in the case of the (MPI-1)-V-
TCF/PMMA. It is worth stressing, that this conclusion also holds in
the case of chromophore solutions in different solvents, as well as
for solutions of (MPI-3)-V-TCF in various concentrations (Fig. 3,
Table S3).
The stability of the NLO response is very important character-
istic of the NLO material. Our measurements demonstrate that after
10 days the essential part of nonlinearity is preserved: for the (MPI-
3)-V-TCF/PMMA films with 20% of chromophores load the d33 value
decreases by 30% of the response, while those for (MPI-1)-V-TCF/
PMMA and DMA-V-TCF/PMMA decrease by 45% and 40%, respec-
tively. For the films with 30% of chromophores load the d33 for
(MPI-3)-V-TCF/PMMA and (MPI-1)-V-TCF/PMMA are quite similar,
after 10 days its value decreases by ca 20% in comparison with the
initial, while the for DMA-V-TCF/PMMA decrease of d33 is the same
40% as in the case of the material with 20% load.
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4. Conclusion
Two novel NLO push-pull chromophores (MPI-1)-V-TCF and
(MPI-3)-V-TCF with isomeric indolizine electron donor moieties
have been synthesized. This is for the first time that the indolizine
donor was used at the development of second-order NLO materials.
Chromophore DMA-V-TCF with dimethylaniline donor was also
prepared for comparison. The indolizine-based chromophores
showed excellent thermal stability: the decomposition tempera-
ture is 348 ꢀC for (MPI-1)-V-TCF and 343 ꢀC for the (MPI-3)-V-TCF,
which is 10e15 ꢀC higher than that for the DMA-V-TCF. As for the
second-order NLO activities, the doped films (MPI-1)-V-TCF/PMMA
and (MPI-3)-V-TCF/PMMA exhibit close d33 values, 40 and 42 pm/V,
which is achieved at the chromophore concentration of 20 wt%.
These d33 values are comparable with that for DMA-V-TCF/PMMA
(43 pm/V), in spite of the fact that at the molecular level the NLO
activity of DMA-V-TCF is somewhat higher. Theoretical predictions
carried out at the DFT computational level give grounds to expect
that the lengthening of
p-electron bridge will result in more sig-
nificant growth of NLO activity for indolizine-based chromophores
in comparison with the chromophore with DMA donor, NLO char-
acteristics of chromophores with indolizine moieties becoming
higher than that for corresponding DMA-Vn-TCF chromophore
starting from the octatetraene bridge. An important role of the
bulky phenyl group in the donor, which prevents the aggregation of
chromophores in guest-host doped polymer films, is revealed. Thus
the indolizine donor may be considered promising for the devel-
opment of novel NLO active materials.
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Financial support of Russian Science Foundation (grant no. 16-
13-10215) is gratefully acknowledged.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.08.047.
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