6MNEP: A molecular cation with large hyperpolarizability and promise for nonlinear optical applications

Article abstract of DOI:10.1039/d0tc01829e

Molecular organic crystals are strategically designed for nonlinear optical applications using push-pull chromophores as the core feature. In this approach, electron-donating and accepting groups are connected through a π-conjugated bridge to obtain planar molecules with high hyperpolarizability. However, the non-centrosymmetric packing that is required for nonlinear optical (NLO) applications is a critical challenge that must be addressed to design useful materials. In this article, we present the new organic cation 6MNEP that shows a large hyperpolarizability and can be crystallized in ideal non-centrosymmetric structures for NLO applications, when paired with T and 4NBS anions. The 6MNEP cation was obtained by extending the conjugation length of already existing chromophores. We compare the 6MNEP crystals with other crystals that also have cations with extended conjugation lengths, but result in centrosymmetric crystal structures. Using the effective hyperpolarizability, we found 6MNEP-T and 6MNEP-4NBS to have 1.6 to 2.5 times larger macroscopic nonlinearities than benchmark NLO organic crystals. Additionally, the significantly lower absorption wavelength compared with other state-of-the-art crystals make 6MNEP-T and 6MNEP-4NBS promising materials for NLO applications like intense terahertz generation.

Full text of DOI:10.1039/d0tc01829e

View Article Online
View Journal
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: G. A. Valdivia, K.
C. Kenney, E. W. Jackson, J. Bloxham, A. X. Wayment, D. J. Brock, S. J. Smith, J. Johnson and D. J.
Michaelis, J. Mater. Chem. C, 2020, DOI: 10.1039/D0TC01829E.
Volume
Number
6
5
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
7
February 2018
Pages 911-1266
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
rsc.li/materials-c
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2050-7526
PAPER
Nosang V. Myung, Yong-Ho Choa et al.
A
noble gas sensor platform: linear dense assemblies of
a multi-layered
single-walled carbon nanotubes (LACNTs) in
ceramic/metal electrode system (MLES)
rsc.li/materials-c

Page 1 of 16
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/D0TC01829E
6
MNEP: A molecular cation with large hyperpolarizability and promising for
nonlinear optical applications
Gabriel A. Valdivia-Berroeta, Karissa C. Kenney, Erika W. Jackson, Joseph Bloxham, Adam
#
*
X. Wayment, Daniel J. Brock, Stacey J. Smith, Jeremy A. Johnson, David J. Michaelis
Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT
8
4602
#jjohnson@chem.byu.edu
dmichaelis@chem.byu.edu
*
Abstract
Molecular organic crystals are strategically designed for nonlinear optical applications using
push-pull chromophores as the core feature. In this approach, electron-donating and accepting
groups are connected through a π-conjugated bridge to obtain planar molecules with high
hyperpolarizability. However, the non-centrosymmetric packing that is required for nonlinear
optical (NLO) applications is a critical challenge that must be addressed to design useful
materials. In this article, we present the new organic cation 6MNEP that shows a large
hyperpolarizability and can be crystallized in ideal non-centrosymmetric structures for NLO
applications, when paired with T and 4NBS anions. The 6MNEP cation was obtained by
extending the conjugation length of already existing chromophores. We compare the 6MNEP
crystals with other crystals that also have cations with extended conjugation lengths, but result
in centrosymmetric crystal structures. Using the effective hyperpolarizability, we found
6
MNEP-T and 6MNEP-4NBS to have 1.6 to 2.5 times larger macroscopic nonlinearities than
benchmark NLO organic crystals. Additionally, the significantly lower absorption wavelength
compared with other state-of-the-art crystals make 6MNEP-T and 6MNEP-4NBS promising
materials for NLO applications like intense terahertz generation.

Journal of Materials Chemistry C
Page 2 of 16
View Article Online
DOI: 10.1039/D0TC01829E
1
. Introduction
Many non-centrosymmetric organic crystals have been studied for nonlinear optical (NLO)
1
2
applications such as second-harmonic (SHG), sum-frequency (SFG), and terahertz (THz)
3
4
generation. These types of NLO materials can feature ultrafast response times, large second-
order susceptibilities, and the possibility of incorporating different functional groups to
improve the NLO activity. In order to create efficient NLO crystals, highly conjugated and
5
6
polar organic molecules can be optimized to have large molecular hyperpolarizabilities. In
addition, organic molecules must exhibit non-centrosymmetric packing in the crystalline state,
which is required for NLO activity.7
Push-pull chromophores suitable for NLO applications are designed by connecting electron-
donating (ED) with electron-accepting (EA) groups using a π-conjugated aromatic bridge (Bd),
as seen in Figure 1. For instance, the DAS cation in the stilbazolium crystal DAST (4-
dimethylamino-N-methyl-4-stilbazolium tosylate) features a dimethyl amino ED group
8
connected through a styryl bridge (Bd) to a pyridinium EA moiety (Figure 1a). Hydrogen
9
10
bonding ED groups such as hydroxyl and methoxy have also been employed in fabricating
push-pull molecules (Figure 1b). On the other side of the bridge, EA groups such as
pyridinium, quinolinium, and indolium12 have found widespread application in NLO
8
11
material design.
Figure 1 a) Push-pull chromophore design using electron donating (ED), bridging (Bd), and
electron accepting (EA) groups, as seen in DAST. b) Common examples of ED, Bd, and EA
groups. c) 6MNEP-T and 6MNEP-4NBS molecular structures.

Page 3 of 16
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/D0TC01829E
An ideal NLO organic crystal combines a chromophore exhibiting large molecular
hyperpolarizability with non-centrosymmetric packing in the crystalline state. However,
different conformations can be observed for organic chromophores. The ideal scenario for NLO
applications is to have the molecules arranged in a non-centrosymmetric, head-to-tail
1
3
conformation. To quantify this, an order parameter has been defined that corresponds to the
cosine cubed of the angle between the hyperpolarizability vector and the polar axis of the
3
14
crystal (cos (θ )). For centrosymmetric crystals, this angle is 90°, and therefore the order
p
parameter and the effective hyperpolarizability are both 0. For non-centrosymmetric crystals,
the order parameter value will be closer to 1 if the chromophores are aligned in an ideal head-
to-tail conformation. Due to the difficulty of predicting solid-state arrangements for push-pull
chromophores, usually the molecular hyperpolarizability is optimized and then slight
modifications are introduced to induce non-centrosymmetric molecular packing.12
A common approach to increase hyperpolarizability (β), while keeping the ED and EA groups
unchanged (see Figure 1a), is to increase the π-conjugation length. Using this strategy, the bis-
alkene derivative of DAST, DACSC (4-((1E,3E)-4-(4-(dimethylamino)phenyl)buta-1,3-dien-
1
-yl)-1-methylpyridin-1-ium 4-chlorobenzene sulfonate),15 was recently designed and
characterized (Figure 2). Crystals incorporating the DASC cation show the required non-
centrosymmetric packing; however, they co-crystalize with water and have sub-optimal cation-
cation alignment.
Figure 2 DACSC molecular structure and extended conjugation molecules developed by
Ajito et al (1-3).
Figure 2 also shows chromophores designed by Tsuji et al. with methoxy ED groups and
extended conjugation lengths using either (E)-buta-1,3-dien-1-ylbenzyl (1, 3) or styryl (2)
1
6
bridges connected to pyridinium (1, 3) or 6-isoquinolinium (2) EA moieties. Improved β
values along with reduced maximum absorption wavelengths were found for the fused-ring
molecules (2). Despite the good characteristics of these π-extended systems, crystal structures

Journal of Materials Chemistry C
Page 4 of 16
View Article Online
were not reported and only qualitative second-harmonic generation tests were performed to
DOI: 10.1039/D0TC01829E
prove the NLO activity of the chromophores.16
To compare the macroscopic nonlinearities for several organic NLO compounds, we use the
effective hyperpolarizability (βiiieff) that corresponds to the product of the molecular β with the
order parameter. This parameter has been shown to correlate strongly with measured NLO
properties like SHG and THz generation.11, 17-19 Table 1 shows the reported order parameter
1
8
and effective hyperpolarizability values for benchmark organic NLO crystals DAST and
HMQ-TMS
trimethylbenzenesulfonate),19
2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 4‐methylbenzenesulfonate)
OHP-CBS (4‐(4‐hydroxystyryl)‐1‐methylpyridinium 4‐chlorobenzenesulfonate). Due to its
large molecular hyperpolarizability and optimal molecular packing, HMQ-TMS has the largest
effective hyperpolarizability of these benchmark crystals.
(2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium
2,4,6-
HMQ-T
as well as
1
1
(
and
1
7
Table 1 Order parameter (cos (θ )), DFT calculated effective hyperpolarizability (βiiieff), and
3
p
powder second harmonic measurements relative to DAST for organic nonlinear optical
crystals. βiiieff correlates strongly with NLO properties like second harmonic generation and
THz generation.11, 19
-30
Property/
Chromophore
DAST11, 18
HMQ-TMS19
HMQ-T11
OHP-CBS17
Order parameter βiiieff (10 esu)
Powder SHG
3
(cos (θ ))
p
0.83
1.0
0.91
1.0
0.97
0.97
161
185
155
118
297
292
1.0
-
0.77
0.36
-
6
MNEP-T
6
MNEP-4NBS
-
In this article, we introduce 6MNEP, a cationic push-pull chromophore. This cation features a
methoxy ED group connected to a pyridinium EA group through a 2-vinylnaphtyl bridge.
When this organic cation is combined with T and 4NBS anions, as shown in Figure 1c, it
produces crystals with large molecular hyperpolarizability and ideal non-centrosymmetric
packing in the crystalline state. We also report the synthesis and X-ray structure determination
of other four centrosymmetric molecular cations with π-extended conjugation length and
compare them with 6MNEP-T and 6MNEP-4NBS.
2
2
. Results and discussions
.1 Chromophores design
To design the π-extended chromophores, methoxy and dimethyl amino ED groups were
combined with (E)-buta-1,3-dien-1-ylbenzyl and 2-vinylnaphtyl π-extended bridges in
conjunction with pyridinium and 6-isoquinolinium EA groups to yield unique cations, as
shown in Figure 3. The methoxy derivatives 6MNEP, MBDM, and 6MEIQ along with

Page 5 of 16
Journal of Materials Chemistry C
View Article Online
dimethyl amino-functionalized chromophores 6DMIQ and DACS were paired with T (p-
DOI: 10.1039/D0TC01829E
toluenesulfonate) and 4NBS (4- nitrobenzenesulfonate) anions to form organic salts. In order
to compare the molecular hyperpolarizability of the designed π-extended chromophores, ab-
2
0
initio calculations in the gas phase were performed for all the compounds. The β value of
gas
state-of-the-art DAST cation (Figure 1a) was also computed and utilized as reference.
Geometry optimization and single-point hyperpolarizability calculations were performed, as
explained in the experimental section. βgas values were found to be 286 (6MNEP), 243
(
1
MBDM), 299 (6MEIQ), 394 (6DMIQ), and 282 (DACS) x10-30 esu. These values are from
.56 to 2.52 times higher compared to the β obtained for the DAS cation (155 x 10-30 esu).
These results confirm the positive impact of π-extended systems on the molecular
hyperpolarizability.
Figure 3 Molecular structures of π-extended chromophores. From top to bottom, 6MNEP,
6
MEIQ, MBDM, 6DMIQ, and DACS paired with T and 4NBS anions
Out all of the designed cations, 6MNEP was the only one that showed ideal head-to-tail
conformations for NLO applications when combined with T and 4NBS anions. As shown in
Table 1, combining the ideal molecular packing in the crystalline state with the large
hyperpolarizability values obtained from single-crystal structure coordinates, 6MNEP-T and
6
MNEP-4NBS show effective hyperpolarizability values from 1.6 to 2.5 times higher
compared with state-of-the-art organic crystals.

Journal of Materials Chemistry C
Page 6 of 16
View Article Online
DOI: 10.1039/D0TC01829E
2
.1 X-ray crystal structures analysis
To evaluate the molecular packing of the featured organic salts, the different cations were
synthesized with 4-methyl (T) and 4-nitro (4NBS) benzene sulfonate anions, and single crystals
were grown via slow evaporation techniques. Figure 4 shows the crystal structures obtained
via X-ray diffraction experiments for 6MNEP, 6MEIQ, and MBDM. Arrows from ED to EA
groups are drawn to indicate parallel and antiparallel arrangements. Lattice and refinement
parameters for the obtained crystal structures are detailed in the SupportiInformationion. As
observed in Figure 4, 6MNEP-T and 6MNEP-4NBS feature similar molecular packing and the
same triclinic P1 space group with nearly perfect alignment for NLO applications. Other
chromophores with known NLO activity, such as DSNS and OHP-CBS,17, 21 have crystallized
in the same space group. As shown in Figure S2, 6MNEP was also paired with 3NBS (3-
nitrobenzenesulfonate) and 4TFS (4-(trifluoromethyl)benzenesulfonate) anions, however the
3
NBS derivative is centrosymmetric and the 4TFS compound, while non-centrosymmetric,
features sub-optimal molecular packing.
Figure 4 Crystal structures of π-extended methoxy derivatives. Blue arrows point from ED to
EA groups and indicate parallel (6MNEP) and antiparallellel (MBDM, 6MEIQ) packing.
Similar to 6MNEP, 6MEIQ also features a fused ring system with extended conjugation.
However, solvent molecules were co-crystallized with 6MEIQ in the solid state. This could

Page 7 of 16
Journal of Materials Chemistry C
View Article Online
explain the centrosymmetric molecular packing obtained for 6MEIQ-T and 6MEIQ-4NBS (see
DOI: 10.1039/D0TC01829E
Figure 4). In order to eliminate these crystallized solvent molecules, the methyl attached to the
quaternary isoquinolinium nitrogen was replaced by an ethyl group. Figure S3 shows the
molecular packing of 6MEIQE-4NBS with no co-crystallized solvent molecules, but the
molecular packing maintains its centrosymmetric alignment. On the other hand, the bisalkene
chromophore MBDM crystallizes in the centrosymmetric monoclinic space group P2 /c when
1
combined with the T anion, while MBDM-4NBS features a P2 2 2 non-centrosymmetric
1
1 1
space group, as observed at the bottom of Figure 4. Despite the acentric nature of the
orthorhombic MBDM-4NBS crystal, the arrangement of the molecular cations is antiparallel,
and therefore we expect no NLO activity. Figure S2c shows the molecular packing of MBDM-
N2S (N2S = 2-naphthalenesulfonate), which also forms a centrosymmetric arrangement. In
order to induce head-to-tail relative conformations, the methyl moiety in the quaternary
nitrogen of MBDM was substituted for an ethyl group. As shown in Figure S4, similar
conformations with non-centrosymmetric space groups, but poor molecular alignment, were
obtained for the ethylated compounds when combined with 3NBS, 4NBS, N2S, and CBS (4-
chlorobenzenesulfonate).
Figure 5 Crystal structures of π-extended dimethylamino derivatives. Pink arrows go from ED
to EA groups and indicate parallel and antiparallel packing.
As shown in Figure 5, all the chromophores with dimethylamino ED groups (6DMIQ, DACS)
feature centrosymmetric space groups. Other organic crystals with dimethylamino groups
(
DAST and 4DEP) have been shown to form centrosymmetric packing when co-crystallized
2
2
water molecules are present. However, DACS-4NBS does not show any co-crystallized water

Journal of Materials Chemistry C
Page 8 of 16
View Article Online
molecules, and the observed molecular packing is still centrosymmetric. In order to eliminate
DOI: 10.1039/D0TC01829E
the water molecules from the 6DMIQ crystals, the cation was modified with an N-ethyl group
instead of a methyl group on the isoquinolinium nitrogen. The crystal structures for the
ethylated molecular cation paired with different anions are shown in Figure S5. Even though
co-crystallized water molecules are not present, the molecular packing is still centrosymmetric
and, therefore, NLO inactive.
Figure 6 Hirshfeld surfaces of a) 6MNEP-T and b) MBDM-T. c) Fingerprint plots for O-H
intermolecular interactions present in 6MNEP-T (pink) and MBDM-T (blue)
As we see above, 6MNEP shows optimal molecular packing, and therefore we examined the
crystal structure of 6MNEP-T in more detail and compared it with MBDM-T. As shown in
Figure 4, 6MNEP-T features parallel relative conformations in contrast to antiparallel packing
observed in MBDM-T. The drastic differences in molecular packing observed in methoxy
functionalized chromophores could be attributed to the presence of an extra aromatic ring in
the ED side of 6MNEP. The increased Van der Waals volume induced by the extra ring in

Page 9 of 16
Journal of Materials Chemistry C
View Article Online
6
MNEP could be responsible for the favorable interactions in the crystalline state that induce
DOI: 10.1039/D0TC01829E
the head-to-tail conformations. However, similar conformations were not observed in 6MEIQ,
which also features an extra aromatic ring compared to MBDM. This is mainly due to the
presence of co-crystallized solvent molecules, and therefore correlations between one extra
aromatic ring and non-centrosymmetric packing are impossible to establish.
To characterize specific intermolecular interactions in 6MNEP-T and MBDM-T that could help
explain the drastic differences in molecular packing, we calculated Hirshfeld surfaces, as
explained in the method sections. Figure 6a-b shows the Hirshfeld surfaces of 6MNEP-T and
MBDM-T, normalized to the atomic Van der Waals volumes, characterizing O-H
intermolecular interactions. As observed in Figure 6c, 6MNEP-T features overall shorter
interaction distances compared with MBDM-T. This difference is explained by strong cation-
cation interactions between the ED methoxy group and the methyl attached to the quaternary
nitrogen of the EA moiety. As seen in Figure 6b, this same interaction is not observed in
MBDM-T and appears to be responsible for the ideal alignment observed in 6MNEP
chromophores.
2
.1.1 Molecular alignment stabilities for 6MNEP-T
Figure 7 Intermolecular distances for 6MNEP-T crystal structures at 100 and 296 K.

Journal of Materials Chemistry C
Page 10 of 16
View Article Online
Organic NLO crystals are utilized in applications involving high-power lasers; therefore, the
DOI: 10.1039/D0TC01829E
stability of the parallel molecular alignments for 6MNEP chromophores in the crystalline state
must be evaluated. X-ray diffraction data shows the same crystal structure for 6MNEP-T at
1
00 and 296 K (see Supporting Information), with small differences due to thermal expansion.
In addition, intermolecular distances analysis reveals similar cation-cation and anion-anion
distances at both temperatures, as observed in Figure 7.
2
.2 Hyperpolarizabilty calculations and NLO properties estimation
To evaluate the applicability of 6MNEP-T and 6MNEP-4NBS as potential NLO crystals, the
effective hyperpolarizability, βiiieff = β cos (θ ), was calculated. This parameter is also known
3
4
tot
p
as the macroscopic optical nonlinearity, and it has been extensively used to compare between
different organic NLO crystals showing a high correlation with experimental SHG
measurements.11, 19 In this study, β was obtained from DFT calculations using the measured
tot
3
crystal atomic coordinates. The order parameter (cos (θ )) was calculated from the angle
p
between β and the ac-crystallographic plane, as depicted in Figure 8. In Table 1, we compare
the values of βiiieff for 6MNEP-T and 6MNEP-4NBS with state-of-the-art organic salt crystals,
1
9
11
18
17
eff
namely, HMQ-TMS, HMQ-T, DAST, , and OHP-CBS. Significantly improved β
iii
values are observed for 6MNEP-T and 6MNEP-4NBS, ranging from 1.6 to 2.5 times higher
when compared with HMQ-TMS and OHP-CBS, respectively. Powder SHG test results are
also included, where available, to show the positive correlation between βiiieff and the
experimental determinations. It is important to note that phase-matching conditions are not
considered in SHG experiments or βiiieff calculations. To the best of our knowledge, the
calculated βiiieff values for 6MNEP-T and 6MNEP-4NBS are the highest ones reported so far
in the literature.
Figure 8 Angle between the molecular hyperpolarizability (blue) and the polar axis (red) for
a) 6MNEP-T, and b) 6MNEP-4NBS.
To further confirm the second-order nonlinearity estimations obtained with βiiieff, we decided
to perform additional calculations, including two and four cations in the gas phase, and
frequency-dependent hyperpolarizabilities studies in methanol solution using the polarizable
continuum model (PCM). Conformations employed for two and four cations calculations are

Page 11 of 16
Journal of Materials Chemistry C
View Article Online
shown in Figure S7, and the obtained β results compared with other NLO organic crystals
tot
DOI: 10.1039/D0TC01829E
are shown in Table 2. 6MNEP chromophores exhibit values from 1.4 to 2.8 times higher than
DAST, HMQ-TMS, HMQ-T, and OHP-CBS for all the configurations analyzed. More
importantly, our calculations reflect the sub-optimal order parameter of DAST, which shows a
βtot of 0.89 times the expected value for a four cations arrangement. This reduced βtot
determination obtained for DAST strongly contrasts with the ideal values obtained for 6MNEP
salts, where no lessening of the hyperpolarizabilitywas observed for 6MNEP-T, and only a 4%
reduction was determined for the 6MNEP-4NBS four cations calculations.
Table 2 One, two, and four cation calculations for DAST, HMQ-TMS, HMQ-T, OHP-CBS,
6
MNEP-T, and 6MNEP-4NBS. Calculations were performed using the B3LYP/NLO-V
configuration. Molecular arrangements employed to calculate the β values are shown in
tot
Figure S7 and were obtained from X-ray coordinates as deposited in the CCDC database (See
methods for CCDC IDs).
Configuration/
Chromophore
DAST
HMQ-TMS
HMQ-T
One cation β
Two cations β
Four cations β
tot
tot
tot
-30
-30
-30
(10 esu)
188
(10 esu)
(10 esu)
412
470
367
239
673
643
673
838
752
485
1288
1194
209
183
115
318
OHP-CBS
6
MNEP-T
6
MNEP-4NBS
311
Table 3 Polarizable continuum model calculations in methanol for DAST, HMQ-TMS, HMQ-
T, OHP-CBS, 6MNEP-T, and 6MNEP-4NBS. Optimization and polar calculations were
performed in methanol from 800 to 1900 nm using the B3LYP/NLO-V method. Frequency-
-30
dependent hyperpolarizability values, β(-ω;ω:0), are reported in 10 esu units.
Chromophore/
Wavelength (nm)
DAST
HMQ-
TMS
474
HMQ-
T
467
OHP-
CBS
306
6MNEP- 6MNEP-
T
4NBS
668
672
676
681
687
693
699
707
715
723
732
743
8
00
521
524
527
531
535
540
544
550
556
562
568
576
668
672
676
681
687
693
699
707
715
724
734
743
9
00
477
481
485
489
494
499
505
512
519
526
534
470
473
477
482
486
491
497
503
510
517
525
307
309
310
312
314
317
319
322
325
328
331
1
1
1
1
1
1
1
1
1
1
000
100
200
300
400
500
600
700
800
900
Solution phase calculations in methanol were performed, as explained in the methods section.
The β(-ω;ω:0) values obtained for the studied organic NLO materials are shown in Table 3.

Journal of Materials Chemistry C
Page 12 of 16
View Article Online
For the anlyzed chromphores, a monotonic increase in the βtot values with respect to the
DOI: 10.1039/D0TC01829E
wavelength was observed. This is an indication of the abscence of important absorption effects
in the analyzed wavelength range. From 800 to 1900 nm, 6MNEP-T and 6MNEP-4NBS
outperform the other NLO organic materials. However, the improvement for 6MNEP
chromophores is lower compared with βiiieff and two or four cations calculations. The
enhancement factor for 6MNEP-T and 6MNEP-4NBS in frequency-dependent calculations
corresponds from 1.3 to 2.2 times compared to DAST and OHP-CBS, respectively. Despite
the lower improvement factors observed for 6MNEP materials in solution phase calculations
in methanol compared with other NLO organic crystals, 6MNEP-T and 6MNEP-4NBS still
display higher nonlinearities compared with DAST, HMQ-TMS, HMQ-T, and OHP-CBS.
2
.3 Crystal and optical properties
Table 4 Wavelength of maximum absorption (λ ) in methanol solution, transparency range
max
cut-off in the crystalline state and melting point for 6MNEP-T, 6MNEP-4NBS, HMQ-TMS,
,
HMQ-T, DAST, and OHP-CBS.
Property/
Chromophore
DAST18
HMQ-TMS19
HMQ-T11
OHP-CBS17
λmax (nm)
Transparency
cut-off (nm)
Melting point
(°C)
256
274
273
~270
224
234
475
439
439
390
399
399
595
680
595
490
475
-
6
MNEP-T
6
MNEP-4NBS
An important consideration in designing organic NLO crystals is to keep the wavelength of
maximum absorption (λ ) shifted to values < 410 nm to avoid two-photon absorption
max
processes that occur when the crystals are irradiated with high-power lasers. A common system
employed to pump these NLO crystals is the Ti:Sapphire laser with a central wavelength of
8
00 nm. As observed in Table 4, the λmax values for 6MNEP-T, 6MNEP-4NBS, HMQ-TMS,
HMQ-T, DAST, and OHP-CBS are 399, 399, 439, 439, 475, and 390 nm, respectively.
Therefore, the likelihood of getting two-photon absorption damage in HMQ-TMS, HMQ-T, or
DAST crystals when pumped with a Ti:Sapphire system is higher compared with 6MNEP-T
and 6MNEP-4NBS crystals. On the other hand, OHP-CBS features a lower λ value of only
max
9
nm less when compared with the 6MNEP salts. However, the βiiieff parameter is 2.5 times
greater for the newly designed chromophores. Similar trends, as obtained in solution, are
observed for transparency cut-off parameters with 6MNEP-T and OHP-CBS having lower
values with respect to the other featured crystals and below 500 nm. Another critical advantage
of OHP-CBS is related to improved phase-matching conditions in the THz region, that have
been attributed to the yellow color of OHP-CBS crystals, compared with orange or red crystals
such as HMQ-T and DAST. Both 6MNEP-T and 6MNEP-4NBS exhibit yellow-colored
crystals as it is confirmed by the λmax of 399 nm, and therefore good phase-matching conditions
in the THz region are expected. Finally, melting point data reveal similar stability for 6MNEP
salts compared with benchmark NLO organic crystals, as detailed in Table 4.

Page 13 of 16
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/D0TC01829E
Slow evaporation and slow-cooling methods with different organic solvents are currently
being tested to grow large crystals of 6MNEP-T and 6MNEP-4NBS. To date, only
polycrystalline solids have been obtained, as confirmed by XRD experiments. Similar
crystallization problems have been reported for the nonlinear crystal DSNS (4-dimethylamino-
N-methyl-4-stilbazolium naphthalene-2-sulfonate),21 however, a study from Guan et al.
showed the possibility of growing DSNS nanowires with detectable SHG activity.23 We
propose that 6MNEP compounds can be utilized for similar applications, and we are currently
pursuing these efforts.
3
. Conclusions
Chromophores with π-extended conjugation lengths were synthesized and crystal structures
reported for 6MNEP, MBDM, 6MEIQ, 6DMIQ, and DACS with T and 4NBS anions. Despite
the improved β values for these chromophores, only compounds containing 6MNEP cations
crystallized with nearly perfect alignment for NLO applications. The β eff parameters of
iii
6
MNEP-T and 6MNEP-4NBS were from 1.6 to 2.5 times higher compared with state-of-the-
art NLO organic crystals. Work is currently underway to produce large-high-quality crystals
optimal for NLO applications.
4
. Experimental Section
Synthesis and NMR characterization: 6MNEP, MBDM, and DACS organic salts were
synthesized by a condensation reaction at room temperature between the 1,4-dimethyl
pyridinium sulfonate salts and the corresponding aldehydes in the presence of piperidine as the
catalyst. 6DMIQ and 6MEIQ derivatives were obtained by Wittig reaction of the corresponding
benzaldehydes to obtain N,N-dimethyl- and 4-methoxy- 4-vinylaniline. The vinyl compound
was reacted with 6-bromoisoquinoline with Pd(OAc) as catalyst to obtain the 6DMIQ and
6
MEIQ cation precursors. Anions were added by alkylation reactions in toluene. Additional
details about the reaction procedures, yields, and NMR data are provided in the Supporting
Information.
Calculations: Density functional theory calculations were performed using the Gaussian 09
software. The B3LYP functional, along with the 6-311++G** basis set, were employed to
optimize molecular geometries and then find the β tensor components. This configuration was
selected due to the extensive validation presented in the literature with other NLO organic
materials. In addition, we performed a calculation optimization study with different DFT and
semi-empirical methods; the obtained results are presented in the Supporting Information (See
Table S1). The total magnitude of β was calculated using the following equations:
Eq. 1a
훽푖 =
∑푗 ( 푖푗푗
훽 + 훽 + 훽푗푗푖)
푗푖푗

Journal of Materials Chemistry C
Page 14 of 16
2
2
2
View Article Online
DOI: 10.1039/D0TC01829E
훽푡표푡 = 훽푥 + 훽푦 + 훽푧
Eq. 1b
Calculations with two and four cations were set up with the x-ray crystal coordinates for DAST
CCDC ID: 1175744), HMQ-TMS (CCDC ID: 931931), HMQ-T (CCDC ID: 824804), OHP-
CBS (CCDC ID: 1586163), 6MNEP-T (CCDC ID: 1867834), 6MNEP-4NBS (CCDC ID:
867835). Cations' conformations examples are provided in Figure S7. To facilitate the
(
1
convergence of the calculations, the B3LYP/NLO-V method was employed.
Frequency-dependent calculations were performed by optimizing DAST, HMQ-T, HMQ-
TMS, OHP-CBS, 6MNEP-T, and 6MNEP-4NBS in a methanol polarizable continuum model.
The same solvent model was employed to run polar calculations for wavelengths from 800 to
1
900 nm. Hyperpolarizability, β(-ω;ω:0), values were obtained directly from the Gaussian 09
output.
X-ray crystallography: Small single crystals for X-ray diffraction experiments were obtained
by slow evaporation of the compounds in methanol. The crystals were harvested in oil, mounted
in a loop, and centered using a video camera. Diffraction data were collected using a Cu (Kα,
1
.54178 Å) rotating anode as the X-ray source and a CCD camera as the detector. Diffraction
peaks were integrated, scaled, and corrected using the APEX3 software package. Chemical
2
4
structures were solved with the dual-space algorithm incorporated in SHELXT. Refinement
2
of the initial structural solutions was performed against F on all data using least squares full-
2
5
matrix strategy as it is included in SHELXL-2014. Unit cell, and refinement parameters are
reported in the Supporting Information.
Hirshfeld analysis: Hirshfeld surfaces were calculated using the Crystal Explorer software.
Surfaces were calculated considering the atomic Van der Waals radius of the atoms inside the
surface. Plots characterizing the inner atom – surface distances (d ) and the outer atom – surface
i
distances (d ) were also generated.
e
Optical Characterization: UV-Vis absorption measurements were performed in a Carry 5e
spectrometer using a 1-cm quartz cell and methanol as solvent. Data were collected from 300
to 500 nm, and the absorption intensity was normalized for comparison between chromophores.
Absorption traces are shown in Figure S6b.
Transparency cut-off value determinations: Measurements were performed using a JASCO V-
7
70 spectrometer using a 2nm data interval and 0.06 seconds response times. A large and highly
twinned crystal of 6MNEP-T was utilized in the measurements. The obtained trace is shown in
Figure S6a.
Differential Scanning Calorimetry: Melting points and heats of fusion were obtained with a
TA Instruments Q2000 modulated differential scanning calorimeter (MDSC). The MDSC
baseline was calibrated with sapphire disks and the thermocouples were calibrated using

Page 15 of 16
Journal of Materials Chemistry C
View Article Online
ASTM method E967-08. This method measures melting points or phase transitions of indium,
DOI: 10.1039/D0TC01829E
adamantane, water, tin, and lead to calibrate across the range 210 K to 600 K using a cubic
spline. Using this calibration, experimental uncertainty in temperature is estimated to be much
lower than 1%. A heating rate of 5°C/min was employed. DSC traces are shown in Figure S8.
5
. References
1
2
.
.
T. P. Radhakrishnan, Accounts of Chemical Research, 2008, 41, 367-376.
M. Kitazawa, R. Higuchi, M. Takahashi, T. Wada and H. Sasabe, Applied physics
letters, 1994, 64, 2477-2479.
3
.
.
Z. Yang, L. Mutter, M. Stillhart, B. Ruiz, S. Aravazhi, M. Jazbinsek, A. Schneider, V.
Gramlich and P. Günter, Advanced Functional Materials, 2007, 17, 2018-2023.
S.-H. Lee, M. Jazbinsek, C. P. Hauri and O. P. Kwon, CrystEngComm, 2016, 18, 7180-
4
7203.
5
6
.
.
R. W. Boyd, Nonlinear optics, Elsevier, 2003.
H. S. Nalwa and S. Miyata, Nonlinear optics of organic molecules and polymers, CRC
press, 1996.
7.
S. H. Lee, B. J. Kang, J. S. Kim, B. W. Yoo, J. H. Jeong, K. H. Lee, M. Jazbinsek, J.
W. Kim, H. Yun, J. Kim, Y. S. Lee, F. Rotermund and O.-P. Kwon, Advanced Optical
Materials, 2015, 3, 756-762.
8
9
.
.
P. Y. Han, M. Tani, F. Pan and X. C. Zhang, Optics Letters, 2000, 25, 675-677.
F. D. J. Brunner, O. P. Kwon, S.-J. Kwon, M. Jazbinšek, A. Schneider and P. Günter,
Optics express, 2008, 16, 16496-16508.
10.
11.
12.
F. D. J. Brunner, S.-H. Lee, O. P. Kwon and T. Feurer, Optical Materials Express, 2014,
4, 1586-1592.
P. J. Kim, J. H. Jeong, M. Jazbinsek, S. B. Choi, I. H. Baek, J. T. Kim, F. Rotermund,
H. Yun, Y. S. Lee and P. Günter, Advanced Functional Materials, 2012, 22, 200-209.
G. A. Valdivia‐Berroeta, E. W. Jackson, K. C. Kenney, A. X. Wayment, I. C. Tangen,
C. B. Bahr, S. J. Smith, D. J. Michaelis and J. A. Johnson, Advanced Functional
Materials, 2019, 1904786.
13.
14.
15.
M. Jazbinsek, U. Puc, A. Abina and A. Zidansek, Applied Sciences, 2019, 9, 882.
J. Zyss and J. L. Oudar, Physical Review A, 1982, 26, 2028.
S. J. Sundaram, J. V. Ramaclus, P. Antony, M. Jaccob and P. Sagayaraj, New Journal
of Chemistry, 2018, 42, 18865-18872.
1
6.
7.
K. Tsuji, N. Nishimura, X.-M. Duan, S. Okada, H. Oikawa, H. Matsuda and H.
Nakanishi, Bulletin of the Chemical Society of Japan, 2005, 78, 180-186.
C. U. Jeong, B. J. Kang, S. H. Lee, S. C. Lee, W. T. Kim, M. Jazbinsek, W. Yoon, H.
Yun, D. Kim, F. Rotermund and O.-P. Kwon, Advanced Functional Materials, 2018,
1
1801143.
1
8.
9.
P.-J. Kim, J.-H. Jeong, M. Jazbinsek, S.-J. Kwon, H. Yun, J.-T. Kim, Y. S. Lee, I.-H.
Baek, F. Rotermund and P. Günter, CrystEngComm, 2011, 13, 444-451.
J.-H. Jeong, B.-J. Kang, J.-S. Kim, M. Jazbinsek, S.-H. Lee, S.-C. Lee, I.-H. Baek, H.
Yun, J. Kim, Y. S. Lee, J.-H. Lee, J.-H. Kim, F. Rotermund and O.-P. Kwon, Scientific
reports, 2013, 3.
1
20.
21.
22.
S.-J. Kwon, O. P. Kwon, J.-I. Seo, M. Jazbinsek, L. Mutter, V. Gramlich, Y.-S. Lee, H.
Yun and P. Günter, The Journal of Physical Chemistry C, 2008, 112, 7846-7852.
B. Ruiz, Z. Yang, V. Gramlich, M. Jazbinsek and P. Günter, Journal of Materials
Chemistry, 2006, 16, 2839-2842.
G. A. Valdivia‐Berroeta, L. K. Heki, E. A. McMurray, L. A. Foote, S. H. Nazari, L. Y.
Serafin, S. J. Smith, D. J. Michaelis and J. A. Johnson, Advanced Optical Materials,
2
018, 1800383.
S. Guan, X. Wang, Y. Li, T. Tian, G. Xu, S. Yuan and B. Cai, Crystal Growth & Design,
019, 19, 780-786.
23.
2

Journal of Materials Chemistry C
Page 16 of 16
View Article Online
DOI: 10.1039/D0TC01829E
2
4.
5.
G. M. Sheldrick, Acta Crystallographica Section A: Foundations and Advances, 2015,
71, 3-8.
2
P. Müller, Crystallography Reviews, 2009, 15, 57-83.