Optically operated second order optical effects in some substituted 4-(5-nitro-1,3-benzoxazol-2-yl)aniline chromophores

Article abstract of DOI:10.1016/j.dyepig.2017.02.009

In this work, we report on second order nonlinear optical (NLO) susceptibilities of some substituted 4-(5-nitro-1,3-benzoxazol-2-yl)aniline chromophores that were embedded into photopolymer matrices. Depending on the photo-induced nitrogen laser (waveleng

Full text of DOI:10.1016/j.dyepig.2017.02.009

Accepted Manuscript
Optically operated second order optical effects in some substituted 4-(5-nitro-1,3-
benzoxazol-2-yl)aniline chromophores
P. Szlachcic, A.A. Fedorchuk, A. Danel, B. Jarosz, A.M. El Naggar, A.A. Albassam, A.
Wojciechowski, E. Gondek, T. Uchacz, K. Stadnicka, G. Lakshminarayana, I.V. Kityk
PII:
S0143-7208(16)30843-9
10.1016/j.dyepig.2017.02.009
DYPI 5786
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 September 2016
Revised Date: 6 February 2017
Accepted Date: 7 February 2017
Please cite this article as: Szlachcic P, Fedorchuk AA, Danel A, Jarosz B, El Naggar AM, Albassam AA,
Wojciechowski A, Gondek E, Uchacz T, Stadnicka K, Lakshminarayana G, Kityk IV, Optically operated
second order optical effects in some substituted 4-(5-nitro-1,3-benzoxazol-2-yl)aniline chromophores,
Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.02.009.
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Graphical abstract

ACCEPTED MANUSCRIPT
Optically operated second order optical effects in some substituted
4
-(5-nitro-1,3-benzoxazol-2-yl)aniline chromophores
1
,*
2
1
1
3,4
3
P. Szlachcic , A.A. Fedorchuk , A. Danel , B. Jarosz , A.M. El Naggar , A.A. Albassam ,
5
6
7
7
8,*
A.Wojciechowski , E. Gondek , T. Uchacz , K. Stadnicka , G. Lakshminarayana , I.V.
5
Kityk
1
Institute of Chemistry, Faculty of Food Technology, Agricultural University,
ul. Balicka 122, 31-149 Kraków, Poland
Ivan Franko National University of Lviv, Faculty of Chemistry,
2
Kyryla I Mefodiya, 6, Lviv, 79005, Ukraine
Research Chair of Exploitation of Renewable Energy Applications in Saudi Arabia, Physics
and Astronomy Department, College of Science,
3
King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
4
Physics Department, Faculty of Science, Ain Sham University,
Abbasia, Cairo 11566, Saudi Arabia
Faculty of Electrical Engineering, Częstochowa University Technology,
5
ul. Armii Krajowej 17, PL-42-201 Częstochowa, Poland
6
Institute of Physics, Technical University of Kraków,
ul. Podchorążych 1, 30-084 Kraków, Poland
Faculty of Chemistry, Jagiellonian University,
7
ul. Ingardena 3, 30-060 Kraków, Poland
Wireless and Photonic Networks Research Centre, Faculty of Engineering, Universiti Putra
Malaysia,43400 Serdang, Selangor, Malaysia
8
1

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Abstract
In this work, we report on second order nonlinear optical (NLO) susceptibilities of some
substituted 4-(5-nitro-1,3-benzoxazol-2-yl)aniline chromophores that were embedded into
photopolymer matrices. Depending on the photo-induced nitrogen laser (wavelength 371 nm)
power density of photo-solidification we found an optimal photo-solidification parameter to
achieve maximum second harmonic generation (SHG). The influence of the π-conjugated
bonds on the output second order susceptibility and HOMO-LUMO level positions is
analysed. The performed experiments are compared with the theoretically calculated
microscopic hyper-polarizabilities obtained within the B3LYP DFT approach. The possible
operation by the output SHG efficiency versus the photoinduced laser power density is
presented. The role of the intra-molecular and surrounding polymers in the observed nonlinear
optical behaviour is explored.
Keywords:
second order optical chromophore;
photo-solidification;
benzoxazole;
DFT simulations;
X-ray diffraction
*
Corresponding authors: G. Lakshminarayana (E-mail: glnphysics@gmail.com)
Tel: +60-89466438
P. Szlachcic (E-mail: p.szlachcic@ur.krakow.pl)
Tel: +48-126624054
2

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1
. Introduction
Recently, one can observe an enhanced interest in the search of organic chromophores,
which possess second-order optical effects for their application in second-order nonlinear
optics (NLO) [1‒11] and in laser modulators, deflectors, telecommunications and optical
information processes [12‒14]. Such materials are developed in different directions:
modification of π-conjugated bonds, addition of highly polarized backside groups, and
formation of d-π metal–ligand charge transfer complexes [15‒18]. General theory on the
phenomenological and microscopic levels was developed in Refs. [19‒26]. Main efforts were
devoted for achieving the higher order hyper-polarizabilities [27, 28]. A flexible approach
describing the possible ways for enhancement of the second order susceptibilities near the
resonance and off-resonance cases was proposed recently [29, 30]. For this approach two
principal parameters are proposed to utilize – the energy of the transition from the ground to
the first excited states, and the second one – the number of effective conjugated electrons.
This article presents further development of this approach by incorporation of the
small size organic NLO chromophores into the photopolymer matrices. By varying the power
of photo-solidifying UV laser, it was shown a possibility of operation by second-order
nonlinear optical constants using external photo-solidifying laser field with assistance of dc-
electric aligned field. As a consequence, macroscopic susceptibility will be field oriented. All
the earlier works usually neglected this influence.
As a chromophore for the NLO effects, we have applied novel nitro-benzoxazole
derivatives presenting the non-symmetric chromophores which can be applied for nonlinear
optics of chromophores with relatively small size. Benzoxazole-derivatives have been already
studied for their NLO effects, but most of these researches were done on the polymers
containing benzoxazole moiety in the side chain [31‒37]. Non-polymeric derivatives have
been also studied, but till now only the molecules with different substituent architecture, when
compared to our research, were used [38‒46]. Small and simple donor-acceptor molecules
investigated in this work have not been studied yet, and the observed results could be used as
a reference for longer, differently substituted benzoxazole molecules. Benzoxazole-
derivatives proposed in the present work had been preliminarily studied for their NLO effects
in powder form, however the obtained efficiency was low (about 0.8 pm/V for λ=1064 nm).
Their interactions with the polymer matrix will be more efficient due to possible superposition
of the external fields. During the UV photo-solidification between the chromophore and
surrounding polymer matrix there remains some small liquid (non-solidified sheets on the
3

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borders of photopolymer-chromophore) which allows orienting spatially particular
chromophore by external fields as described previously [47,48]. We also performed the DFT
simulations of molecular hyper-polarizabilities of the synthesized compounds, in order to
compare with the measured second harmonic generation (SHG). The origin of the nonlinear
optical effects is discussed with respect to the space distribution of the particular HOMO and
LUMO levels as well as energy magnitudes of their levels.
4

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2
. Experimental
2
.1. General considerations
All the solvents used for the reactions and purifications were purchased from Dorchem
Polish supplier), and were of analytical grade. Dichloromethane used for spectroscopic and
(
electrochemical studies was of spectrophotometric grade (Alpha Aesar). Silica Gel 60 and
Aluminium Oxide 90 for a column chromatography were purchased from Merck.
Polyphosphoric acid, 4-nitro-2-aminophenol, 2-aminophenol, benzoic acid, 4-bromobenzoic
acid, N,N-dimethyl-4-aminobenzoic acid, diphenylamine, N-methylaniline, 1,2,3,4-
tetrahydroquinoline, (1-naphtyl)phenylamine, sodium tert-butylate, (2-biphenyl)di-tert-
butylphosphine, tetrabutylammonium hexafluorophosphate, ferrocene and Pd dba were
2
3
purchased from Alfa Aesar or Aldrich and used without further purification. CDCl with TMS
3
was purchased from Aldrich. Melting points were measured on an MEL-TEMP II apparatus
1
13
and they are uncorrected. H NMR and C NMR were recorded at 600 MHz and 150 MHz,
respectively, with a Bruker Avance III 600 spectrometer using CDCl as a solvent. The
3
chemical shifts were given in ppm (δ) from TMS as an internal standard. Elemental analyses
were conducted at Elementar Vario MICRO cube. Diffraction data were collected for single
crystals either with an Agilent Technologies SuperNova™ diffractometer, (low-temperature
Cryo-Jet device, Atlas CCD detector) [49], using Mo Kα radiation (except of D_RT which
was measured using Cu Kα radiation) or with the Nonius KappaCCD diffractometer using
graphite monochromated Mo Kα radiation (data collection: COLLECT [50], cell refinement:
HKL SCALEPACK [51], data reduction: HKL DENZO and SCALEPACK [51]). Cyclic
voltammetry (CV) measurements were carried out on a PalmSens3 potentiostat. One platinum
wire (ø = 0.5 mm) and platinum coil (ø = 1.6 mm) were used as a counter and working
+
electrodes, respectively. Non aqueous electrode (Ag/Ag ) designed and provided by ALS was
used as a quasi-reference electrode. The potential of quasi-reference electrode was calibrated
using the ferrocene as an internal standard. The solutions of compounds under the study
-4
-1
+
–
(
∼ 5⋅10 M) were prepared in suitable electrolyte (∼ 1⋅10 M solution of Bu N PF in
4 6
dichloromethane), the measurements were conducted at room temperature with the scan rate
of 100 mV/s. Prior to the measurements, the solutions were purged with argon to remove
residual oxygen. The dyes concentration used for spectroscopic measurements amounted to
5

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−
6
ca. 5⋅10 M. UV-Vis. electronic absorption spectra in dichloromethane were recorded using a
Shimadzu UV-2101 PC spectrometer.
2
.2. Synthetic procedures and NMR spectroscopy
2
.2.1. 2-(4-Bromophenyl)-5-nitro-1,3-benzoxazole: 4-nitro-2-aminophenol (50 mmol) and 4-
bromobenzoic acid (50 mmol) were mixed with polyphosphoric acid (100 g). The reaction mixture
was magnetically stirred for 24 hours at 110 °C (the flask was stopped with anhydrous calcium
chloride tube) applying known procedure [52]. Then the reaction mixture was poured into ice-water
and formed a waxy precipitate. After being neutralized with Na CO , the precipitate formed a light-
2
3
brown powder, which was filtered off, washed with water and dried in air. Crude product was
dissolved in CHCl and flashed through aluminium oxide. The obtained yellowish powder was pure
3
enough for the next steps, 8.59 g (54%). Small sample was recrystallized from toluene, light-yellow
1
needles, m.p. 248-249°C. H NMR (600 MHz, CDCl ): δ = 8.66 (d, J = 2.2 Hz, 1H), 8.34 (dd, J = 8.9,
3
1
3
2
.3 Hz, 1H), 8.13-8.15 (m, 2H), 7.71-7.73 (m, 2H), 7.69 (d, J = 8.9 Hz, 1H). C NMR (150 MHz,
CDCl ): δ = 165.1, 154.2, 145.6, 142.5, 132.5, 129.4, 127.6, 124.9, 121.4, 116.4, 110.8. Anal. Calcd.
3
for C H BrN O : C 48.93, H 2.21, N 8.78. Found: C 48.60, H 2.19, N 9.01.
1
3
7
2
3
2
.2.2. N,N-Dimethyl-4-(5-nitro-1,3-benzoxazol-2-yl)aniline (DMA): 4-nitro-2-aminophenol (10 mmol)
and N,N-dimethyl-4-aminobenzoic acid (11 mmol) were mixed with polyphosphoric acid (20 g). The
reaction mixture was magnetically stirred for 24 hours at 110 °C (the flask was stopped with
anhydrous calcium chloride tube). Then the reaction mixture was poured into ice-water. A precipitate
was filtered off, washed with water and dried in air. Crude product was crystallised twice from DMF.
1
Yellow-golden plates, 1.35 g (48%), m.p. 247-248 °C. H NMR (600 MHz, CDCl ): δ = 8.53 (d, J =
3
2
2
1
6
.2 Hz, 1H), 8.22 (dd, J = 8.8, 2.3 Hz, 1H), 8.08-8.11 (m, 2H), 7.57 (d, J = 8.8 Hz, 1H), 6.76-6.78 (m,
1
3
H), 3.10 (s, 6H). C NMR (150 MHz, CDCl ): δ = 167.2, 154.3, 153.0, 145.2, 143.3, 129.6, 120.0,
3
15.0, 112.6, 111.5, 109.9, 40.1. Anal. Calcd. for C H N O : C 63.60, H 4.63, N 14.83. Found: C
1
5
13
3
3
3.82, H 4.52, N 14.78.
6

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2
.2.3. General procedure for the synthesis of N-methyl-4-(5-nitro-1,3-benzoxazol-2-yl)-N-
phenylaniline (MPA), 4-(5-nitro-1,3-benzoxazol-2-yl)-N,N-diphenylaniline (DPA), N-(1-naphtyl)-4-
5-nitro-1,3-benzoxazol-2-yl)-N-phenylaniline (NPA) and 1-[4-(5-nitro-1,3-benzoxazol-2-yl)phenyl]-
(
1
,2,3,4-tetrahydroquinoline (THQ):
6
38 mg (2 mmol) of 2-(4-bromophenyl)-5-nitro-1,3-benzoxazole was dissolved in 15 ml of toluene
(
dried over molecular sieves 4A). Toluene was deoxygenated before use by purging with dry argon.
Then 269 mg (2.8 mmol) of sodium tert-butylate was added, followed by 37 mg (2 mol %) of
Pd (dba) and 30 mg (5 mol%) of (2-biphenyl)di-tert-butylphosphine, applying known procedure [53].
2
3
As the last, 2.4 mmol of corresponding amine was added; the flask was stopped with septa and the
reaction mixture was stirred at ambient temperature under argon atmosphere for 48 h. After that, the
reaction mixture was filtered; a precipitate was washed with toluene. The solvent was removed under
reduced pressure; the residue was dissolved in CHCl and flashed over Aluminium Oxide. The
3
solution was then concentrated and the product was adsorbed on Silica Gel 60 and the solvent was
removed. A dry adsorbent was put on the Silica Gel 60 column and eluted with toluene / petroleum
ether (1:1) mixture. Resulted yellow solid was crystallized from toluene.
2
.2.3.1. N-Methyl-4-(5-nitro-1,3-benzoxazol-2-yl)-N-phenylaniline (MPA): N-methylaniline was used
as an amine. In this case, the reaction was quenched with water, and the product was extracted with
ethyl acetate. The extract was dried over anhydrous MgSO and flashed through aluminium oxide.
4
Crude product was purified according to the general procedure to give 453 mg (66 %) of an yellow
1
powder, m.p. 185-188 °C. H NMR (600 MHz, CDCl ): δ = 8.54 (d, J = 2.2 Hz, 1H), 8.24 (dd, J = 8.8,
3
2
6
1
6
.2 Hz, 1H), 8.04-8.06 (m, 2H), 7.59 (d, J = 8.8 Hz, 1H), 7.42-7.45 (m, 2H), 7.25-7.27 (m, 3H), 6.86-
1
3
.88 (m, 2H), 3.42 (s, 3H). C NMR (150 MHz, CDCl ): δ = 166.8, 154.3, 152.3, 147.1, 145.2, 143.1,
3
29.9, 129.4, 126.2, 125.8, 120.2, 115.2, 114.5, 114.3, 110.1, 40.3. Anal. Calcd. for C H N O : C
2
0
15
3
3
9.56, H 4.38, N 12.17. Found: C 69.40, H 4.25, N 12.20.
2
.2.3.2. 4-(5-Nitro-1,3-benzoxazol-2-yl)-N,N-diphenylaniline (DPA): diphenylamine was used as an
1
amine following the general procedure to give 314 mg (39 %) of yellow plates, m.p. 199-201 °C. H
NMR (600 MHz, CDCl ): δ = 8.57 (d, J = 2.2 Hz, 1H), 8.26 (dd, J = 8.9, 2.3 Hz, 1H), 8.04-8.06 (m,
3
1
3
2
H), 7.61 (d, J = 8.9 Hz, 1H), 7.33-7.36 (m, 4H), 7.15-7.20 (m, 6H), 7.09-7.11 (m, 2H). C NMR
(
150 MHz, CDCl ): δ = 166.2, 154.3, 151.8, 146.4, 145.3, 143.0, 129.7, 129.2, 126.0, 124.8, 120.5,
3
1
4
20.4, 117.5, 115.6, 110.3. Anal. Calcd. for C H N O : C 73.70, H 4.21, N 10.31. Found: C 73.56, H
25 17 3 3
.06, N 10.28.
2
.2.3.3.
N-(Napht-1-yl)-4-(5-nitro-1,3-benzoxazol-2-yl)-N-phenylaniline
(NPA):
(1-
naphtyl)phenylamine was used as an amine following the general procedure to give 202 mg (22%) of
1
an yellow powder, m.p. 206-208°C. H NMR (600 MHz, CDCl ): δ = 8.55 (d, J = 2.2 Hz, 1H), 8.23
3
7

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(
dd, J = 8.9, 2.3 Hz, 1H), 7.99-8.02 (m, 2H), 7.92 (d, J = 8.3 Hz, 1H), 7.90 (d, J = 8.6 Hz, 1H), 7.85 (d,
J = 8.3 Hz, 1H), 7.57 (d, J = 8.9 Hz, 1H), 7.48-7.53 (m, 2H), 7.39-7.41 (m, 2H), 7.29-7.32 (m, 2H),
1
3
7
.23-7.25 (m, 2H), 7.09-7.12 (m, 1H), 6.96-6.98 (m, 2H). C NMR (150 MHz, CDCl ): δ = 166.3,
3
1
54.3, 152.3, 146.5, 145.3, 143.0, 142.0, 135.3, 130.9, 129.6, 129.3, 128.6, 127.6, 127.5, 126.9, 126.5,
1
26.4, 124.4, 124.3, 123.7, 120.4, 118.6, 116.8, 115.5, 110.2. Anal. Calcd. for C H N O : C 76.14,
2
9
19
3
3
H 4.19, N 9.19. Found: C 76.20, H 4.15, N 9.10.
2
.2.3.4. 1-[4-(5-nitro-1,3-benzoxazol-2-yl)phenyl]-1,2,3,4-tetrahydroquinoline (THQ): 1,2,3,4-
tetrahydroquinoline was used as an amine following the general procedure to give 155 mg (21%) of
1
light-orange plates, m.p. 189-190 (dec.)°C. H NMR (600 MHz, CDCl ): δ = 8.58 (d, J = 2.2 Hz, 1H),
3
8
.27 (d, J = 8.8, 2.3 Hz, 1H), 8.12-8.14 (m, 2H), 7.62 (d, J = 8.9 Hz, 1H), 7.31-7.33 (m, 2H), 7.20 (d, J
8.1 Hz, 1H), 7.14 (br d, J = 7.5 Hz, 1H), 7.06-7.09 (m, 1 H), 6.91 (td, J = 7.4, 1.2 Hz, 1H), 3.73 (t, J
=
=
=
1
3
6.1 Hz, 2H), 2.80 (t, J = 6.4 Hz, 2H), 2.05 (quintet, J = 6.2 Hz, 2H). C NMR (150 MHz, CDCl ): δ
3
166.3, 154.3, 151.8, 145.3, 143.0, 141.6, 129.29, 129.26, 129.22, 126.4, 121.6, 120.5, 119.8, 119.4,
1
17.5, 115.6, 110.3, 49.1, 27.4, 23.7. Anal. Calcd. for C H N O : C 71.15, H 4.61, N 11.31. Found:
22 17 3 3
C 71.01, H 4.52, N 11.35.
2
.2.4. N,N-Dimethyl-4-(1,3-benzoxazol-2-yl)aniline (D): 2-aminophenol (25 mmol) and N,N-dimethyl-
-aminobenzoic acid (25 mmol) were mixed with polyphosphoric acid (50 g). The reaction mixture
4
was magnetically stirred for 24 hours at 110 °C (the flask was stopped with anhydrous calcium
chloride tube). Then the reaction mixture was poured into ice-water. Precipitate was filtered off,
washed with Na CO solution and water and dried in air. Crude product was crystallized ten times
2
3
1
from toluene. Colourless plates, 3.50 g (59%), m.p. 185-187 °C (Lit. 182.3-183.7 °C [54]). H NMR
13
and C NMR match those found earlier [54]. Anal. Calcd. for C H N O: C 75.61, H 5.92, N 11.76.
1
5
14
2
Found: C 75.70, H 5.90, N 11.78.
2
.2.5. 2-Phenyl-5-nitro-1,3-benzoxazole (A): That compound was prepared according to a known
procedure from 10 mmol of 4-nitro-2-aminophenol and benzoic acid [55]. Colourless plates, 1.34 g
1
13
(
56%), m.p. 174-175 °C (Lit. 177 °C [56]). H NMR and C NMR match those found earlier [56].
Anal. Calcd. for C H N O : C 65.00, H 3.36, N 11.66. Found: C 65.28, H 3.35, N 11.50.
1
3
8
2
3
8

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Fig. 1. General molecular formula of the studied chromophores
9

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. Results and discussion
3
3
.1. X-ray crystallography
Crystal structure studies were performed for DMA (Fig. 2 (1)), DPA (Fig. 2 (2)) and THQ
(Fig. 2 (3)) derivatives and for reference compounds: 2-phenyl-5-nitro-1,3-benzoxazole
(acceptor-group-only) A (Fig. 2 (4) and N,N-imethyl-4-(1,3-benzoxazol-2-yl)aniline (donor-
group-only) D (Fig. 2 (5)). All studied crystals were obtained by slow cooling of hot and
saturated toluene solutions.
1
0

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Fig. 2. The shape of molecules for DMA_LT (1), DPA_LT (2), THQ_LT (3), A_LT (4) and D_LT
(
5) with the atom numbering scheme. Displacement ellipsoids are drawn at 50% probability level.
Crystal data, intensity measurement conditions and structure refinement details of the
compounds are given in Table S1 (see Supplementary Data). The programs used for the
crystal structures analyses were working under WinGX environment [57]. SIR92 was used for
solving the structures with direct methods [58]. SHELXL2013 was used for the refinement of
1
1

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the structures [59]. ORTEP3 was used in the preparation of molecular graphics [60]. The
supplementary materials could be found at CCDC [61].
Selected bond lengths (Å), valence angles (º), torsion angles (º) and dihedral angles (º) for
studied molecules correlated with the mean of the appropriate values found in the comparable
molecules taken from CCDC are listed in Table S2 (see Supplementary Data). The bond
lengths observed in the benzoxazole moiety are very similar in all studied compounds and
agree well with those deposited so far in CCDC data base (see references bellow Table S2 in
Supplementary Data). The external C(14)-N(20) bond length is significantly influenced by the
type of substituents on amine nitrogen atom N(20). The effect depends on conjugation
between the lone pair of amine group and π-electron system of phenyl ring, and was also
observed for 6-amino-1H-pyrazolo[3,4-b]quinoline derivatives [62]. The internal valence
angles in the oxazole ring match the values from CCDC data base, however those in the
benzene ring of benzoxazole moiety are clearly influenced by the nitro group at C(5) [63]. It
is worth to emphasize that the dihedral angle between the benzoxazole moiety and phenyl
substituent at C(2) is close to 0° in all studied structures, suggesting strong conjugation of
those π-systems. The packing of molecules is shown in Fig. S1‒S5 (see Supplementary Data).
As can be seen from Table S3 (see Supplementary Data) there is no strong or moderate
intermolecular interactions in the crystal structures. The geometry found for the molecules in
the crystalline state should be also preserved for the molecules in the polymer matrix and thus
could be considered as responsible for particular physical properties of the studied non-linear
optical materials.
3
.2. Non-linear optical studies
Such prepared microcrystalline chromophores were embedded into the liquid
oligoetheracrylate (OEA) photopolymers using a method similar to the described earlier [64].
The solidification was performed by 371 nm pulsed nitrogen laser with power density varied
2
up to 400 MW/cm by Glan polarizer. The additional dc-electric field was applied in order to
align the chromophores.
Second-order non-linear optical susceptibilities were measured by the Kurtz powder test
where a polymer supported the crystalline powder. The Nd: YAG laser operating at 1064 nm
and possessing pulses of 12 ns, maximal power 0.95 MW, pulse repetition frequency 13 Hz
1
2

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was applied as the fundamental ones. The exploration of photoinduced changes was
performed by a spectrophotometer with 2 nm spectral resolution connected to the CCD
camera. This detector registered the 532 nm doubled frequency signal and cut the
fluorescence scattering noise. The reference samples were BiBO single crystals.
3
As a consequence, we have determined the effective susceptibility of the polymer/powder
compounds. Taking into account the second-order non-linear optical susceptibilities we
obtained hyperpolarizability magnitudes. The optimized concentration for the titled molecules
was equal to 12 %. The increase of this content favoured aggregation which limit the
efficiency of the SHG output.
The microscopic view of such a poled polymer is presented in Fig. 3.
Fig. 3. Microscopic image of the photopolymerized dc-field aligned chromophore (the scale bar size is
equal to 100 µm).
The presented composite (Fig. 3) show an existence of the aligned crystallite chromophore.
Moreover during the solidification between the chromophore and the polymer there occurs
some voids which are responsible for the ability of the chromophore to be oriented in the
external dc-electric field.
The performed time kinetics SHG measurements (not presented here) have shown that
the most effective SHG output was achieved for the samples NPA.
The huge temporary shift of the SHG maxima (29.2 ns) indicates on the occurrence of the
trapping levels favouring additional light scattering. The output SHG is very sensitive to the
photo solidifying laser beam power density (see Fig. 4) with laser beam diameter about 2.2
mm.
1
3

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3.5
3.4
3.3
3.2
3.1
3.0
2.9
2.8
0
100
200
300
400
2
I [MW/cm ]
Fig. 4. Dependence of the effective second order susceptibilities versus the applied photoinduced
power density.
2
From Figure 4, one can see that the optimal photo-solidified power density is 180 MW/cm .
The further decrease is caused by the some cross-linking between the chromophore and the
OEA photopolymers, sufficiently suppressing the output SHG.
3
.3. Quantum chemical simulations
The electronic parameters for the titled compounds were investigated using Gaussian W09
package [65] at the DFT level [66, 67]. Initial geometries of molecules were estimated by
PM6 method and these models were subsequently optimized by DFT B3LYP/6-31 method
[
68, 69]. The principal parameters of the electronic structure for the titled compounds and the
hyper-polarizabilities are presented in the Tables 1, and 2, respectively.
1
4

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It is clearly seen that maximum second order susceptibility is observed for the NPA, and
minimum – for DMA. For convenience the experimentally determined second order
susceptibilities are given in the last row of the Table 1. There is observed an obvious
correlation between the calculated second-order hyper-polarizabilities and experimentally
measured effective second-order hyper-polarizabilities. Moreover, there is also observed a
correlation with the ground state dipole moments.
Table 1. Principal parameters of electronic structure for the titled compounds
E HOMO,
eV
E LUMO,
eV
dE,
eV
Dipole
moment
β (esu)
-30
-30
-30
-30
-30
THQ
DPA
NPA
MPA
DMA
-5.308
-5.416
-5.429
-5.520
-5.521
-2.530
-2.748
-2.736
-2.677
-2.653
2.778 9.1588
2.668 9.5178
2.693 9.6621
6.6715·10
6.4198·10
7.5531·10
2.843 10.5492 6.2090·10
2.868 10.7574 5.3118·10
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5

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Table 2. Values of the hyperpolarizabilities calculated theoretically (in esu) and compared
with the experimentally measured second order susceptibilities in pm/V at wavelength 1064
nm (in brackets).
DMA
DPA
MPA
NPA
THQ
βxxx
βxyy
βxzz
βyyy
βxxy
βyzz
βzzz
βxxz
βyyz
βxyz
βx
-609.3187
-23.9171
22.1679
15.5131
55.3835
-2.8579
0.0031
-725.8475
-46.8246
14.833
-697.4138
-24.9234
8.6417
-780.3962
-62.8461
-10.7969
25.7757
164.9506
-11.2767
-4.5096
54.3306
2.7321
744.5081
16.178
-28.3037
3.8524
-8.1277
-124.3877
-1.3703
1.9806
-0.8136
-70.1013
-2.9697
3.0718
55.8662
4.3444
6.5477
-0.0033
0.0021
-58.1248
-7.8128
1.0092
-46.1329
1.8660
103.0043
-1.4015
-9.8352
732.3824
64.063
0.0011
11.3957
-713.6955
-73.8846
-41.1951
6.2090·10
-1.5537
-854.0392
179.4496
52.5531
-611.0679
68.0387
0.0019
-757.8391
-133.8857
-63.957
6.6715·10
βy
βz
108.1505
-30
-30
-30
β(esu) 5.3118·10
-30
-30
7
.5531·10
6.4198·10
deff
deff
(1064
(0.56 pm/V)
(1.67 pm/V)
(1.06 pm/V)
(2.1 pm/V)
(1.49 pm/V)
nm)
The principal space distributions of the HOMO and LUMO molecular orbital levels are
presented in Fig. 5.
1
6

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DMA
HOMO
DMA
LUMO
1
7

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DPA
HOMO
DPA
LUMO
1
8

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MPA
HOMO
MPA
LUMO
1
9

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NPA
HOMO
NPA
LUMO
2
0

ACCEPTED MANUSCRIPT
THQ
HOMO
THQ
LUMO
Fig. 5. Space distribution of the HOMO and LUMO molecular orbitals.
This figure unambiguously confirms an existence of charge transfer between the donor-
acceptor groups. The space separation between these groups may be a consequence of the
different polarizabilities of the donor and acceptor groups determining the charge transfer.
The π-conjugated bonds play here crucial role, however the energy positions of the LUMO
also play substantial role.
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1

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Following the approach described previously [6,7,70,71], the fundamental limit for resonance
−
28
effective second-order optical hyperpolarizability β is equal to 57 × 10 esu. This is obtained
by taking into account of the influence of polymer. However, the effective hyper-
polarizabilities evaluated from the experimental SHG time kinetics curves were varied from
−
30
−30
7
,31 × 10
to 8,7 × 10
esu. This fact principally agreed with the intrinsic hyper-
polarizabilities determined by the ratio between the measured and the fundamental limits of
the mentioned model equal to about 0.013, which is comparable with the other organic
molecules [4]. Without the dc-field, one can observe decrease of the susceptibility up to 40–
5
0 % with respect to that maximally achieved.
This fact shows more efficient properties of the titled chromophores with respect to
other organic NLO chromophores including those possessing larger π-conjugated chains and
the d-localized states [71‒75].
3
.4. Electrochemical studies and UV-Vis absorption spectra – experimental HOMO-LUMO
energy levels
In order to determine the experimental HOMO energy levels, the electrochemical oxidation
potentials were examined by means of cyclic voltammetry (CV) and confirmed by
independent measurements of differential pulse voltammetry (DPV). The oxidation part of
cyclic voltammograms of studied benzoxazoles: DMA, MPA, DPA, NPA and THQ, together
with ferrocene, are depicted in Figure S6 (see Supplementary Data). The investigated
benzoxazoles exhibit reversible oxidation peaks (excluding THQ, for which reversal peak is
very weak) at the platinum electrode with reference to non-aqueous reference electrode
+
(
Ag/Ag , calibrated with ferrocene/ferrocenium redox couple).
According to the Koopmans’ theorem the energy of the HOMO is a good approximation to
the negative experimental ionization potential (−IP). Ionization potentials were calculated as
follows: − IP = EHOMO = − (1.4 ± 0.1) × (qV ) − (4.6 ± 0.08) eV where: q – electron charge in
CV
ē, VCV = E − the first oxidation potential obtained from CV measurements in V) [76].
ox
LUMO energy levels were determined from the following equation: LUMO = − (IP + E ),
0
0
where E − optical energy gap evaluated from the onset of the first absorption band of the
0
0
compounds in dichloromethane solution (the spectra are depicted in Figure 6).
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2

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Figure 6. Absorption spectra for studied molecules (DMA, blue; MPA, red; DPA, green, NPA, orange
and THQ, pink), recorded in dichloromethane solutions.
The values of oxidation potentials, optical energy gaps and HOMO/LUMO energy levels are
given in Table 3.
Table 3. Oxidation potentials, energy gaps, HOMO and LUMO energy levels for studied
molecules.
Eox/ferrocene [eV]
0.612
HOMO [eV]
-5.457
Egap [eV]
3.069
3.019
2.921
2.964
2.897
LUMO [eV]
-2.388
DMA
MPA
DPA
NPA
THQ
0.610
-5.454
-2.435
0.656
-5.518
-2.597
0.650
-5.510
-2.546
0.470
-5.258
-2.361
The HOMO energy levels agree quite well with those predicted by DFT calculations (see
Table 1). Calculated Egap and, in consequence, LUMO levels, seem to be underestimated.
However, one has to remember that calculated HOMO and LUMO energy levels could vary
depending on the chosen functional [77]. In addition, DFT methodology leads to problems of
accurate approximation of energy gap in push-pull molecules with possible rotations of donor
2
3

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group [78]. Finally there exists some influence of surrounding polymer which changes the
output susceptibilities [79‒81].
2
4

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5
. Conclusions
In summary, we have shown a possibility to use the substituted 4-(5-nitro-1,3-benzoxazol-2-
yl)aniline) chromophores embedded into the photopolymer matrices for the optically operated
second harmonic generation. It is shown that varying the effective external fields’ one can
operated by the output NLO parameters. The appropriate variation of the backside donor
group allows effectively operated by second order susceptibilities. More importantly, varying
the UV-induced power at 371 nm, one can operate by the photo-solidified power for operation
by the effective second order susceptibilities. The DFT simulations confirm the principal role
of the donor–acceptor intra-chromophore charge transfer in the observed output second-order
efficiencies. However, some role is played by external fields. The effects are crucially
dependent on the photoillumination time.
ACKNOWLEDGMENTS
The authors would like to thank, for purchasing part of the equipment used in the research, to
European Regional Development Fund within the Polish Innovation Economy Operational
Program (contract no. POIG.02.01.00-12-023/08). This research was also partly financed by
the Ministry of Science and Higher Education of the Republic of Poland basic funding for
University of Agriculture (DS 3133/2016). The project was also financially supported
by King Saud University, Vice Deanship of research chairs.
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