Inorganic Chemistry Communications
Short communication
Conformational polymorphism on styrl quinolinium salts of
2-[(E)-2-(4-hydroxy-3-methoxystyrl)-1-methyl]-quinolinium
4-chlorobenzenesulfonate (HMQ-CBS): Potential nonlinear optical
crystals for terahertz application
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Maruthappan Manikandan, Tianliang Chen, Zhihua Sun , Shuquan Zhang, Junhua Luo
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 August 2015
Received in revised form 19 September 2015
Accepted 20 September 2015
Available online 25 September 2015
Polymorphism is an important phenomenon to investigate the relationship between the chemical struc-
tures and properties of functional materials. Here, we demonstrate the existence of conformational
polymorphism in the terahertz (THz) compounds of 2-[(E)-2-(4-hydroxy-3-methoxystyrl)-1-methyl]-
quinolinium 4-chlorobenzenesulfonate (abbreviated as HMQ-CBS). There are two different crystalline
phases (I and II, respectively). X-ray structural analyses reveal that HMQ-CBS crystallizes in the monoclinic
crystallographic system with the non-centrosymmetric space group Pc for phase-I, and centrosymmetric space
group of P21/n for phase-II. These two different crystalline phases exhibit the distinct anionic orientations and crystal
packings, leading to significantly different nonlinear optical (NLO) properties. Second harmonic generation (SHG)
measurements reveal that crystals in phase-I possess the SHG efficiency of about 0.5 times that of N,
N-dimethylamino-N-methylstilbazolium p-toluenesulphonate (DAST), which indicates that HMQ-CBS in phase-I
might be a potential NLO crystal for THz application. In contrast, crystals in the phase-II show no SHG responses.
Comparison of their crystal structures and NLO properties enables us to understand the correlation between molec-
ular conformational changes and bulk NLO properties, and sheds light on the design of organic THz materials.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Polymorphism
Nonlinear optics
THz crystal
Molecular packing
1. Introduction
Quinolinium-based crystals also exhibit large microscopic and
macroscopic NLO susceptibilites [8–10]. Moreover, such quinolinium
Crystalline polymorphism has been considered as a fascinating re-
search area because polymorphs exhibit different mechanical, physical
and chemical properties, which makes it great importance in pharma-
ceutical and material science fields [1,2]. Particularly, the engineering
of new materials with desirable functional properties requires the
knowledge of structure–property relationship, which could be achieved
by investigating the polymorphism systems. In organic polymorphisms,
the structure–property relationship is directly dependent on the molec-
ular conformations and the packing arrangements. Therefore, organic
molecular compounds have great possibilities to exhibit the polymor-
phism because of their flexibility and weak intermolecular interactions
in the solid state [3–7].
crystals show high terahertz (THz) wave generation efficiency at near
infrared and infrared wavelengths [11–12]. Interestingly, polymorphism
is quite common in the stilbazolium and quinolinium derivatives.
For instance, S.H. Lee et al. had reported the crystalline polymorphism
of 2-[(E)-2-(4-hydroxy-3-methoxystyrl)-1-methyl]-quinolinium 4-
methoxybenzenesulfonate, that is, two different crystalline states of
α-phase and β-phase. These polymorphs display the different macroscop-
ic second-order NLO responses. In detail, the effective hyperpolarizability
eff
ijk
tensor component β of α-phase crystals is the diagonal component
βeijfkf = 178 × 10−30 esu, while the corresponding value for β-phase
crystals is the off-diagonal component β = 60 × 10−30 esu [13].
eff
ijk
However, the investigation on polymorphism of quinolinium deriva-
Styrylquinolinium derivatives with methylquinolinium electron ac-
ceptors are recently found to be interesting materials for nonlinear optical
(NLO) applications, due to their higher electron accepting strength than
the methylpyridinium acceptor in the benchmark ionic NLO crystal of
N,N-dimethylamino-N-methylstilbazolium-4-methylbenzenesulfonate
(DAST, one of the most promising crystals for terahertz application) [8].
tives is rather rare, and the relationship between crystal structures
and NLO properties has not yet been fully investigated.
In the present work, we report the polymorphism of the NLO
molecular compound, 2-[(E)-2-(4-hydroxy-3-methoxystyrl)-1-
methyl]-quinolinium 4-chlorobenzenesulfonate (HMQ-CBS), which
exhibit two different crystalline states (phase I and phase II) deter-
mined by X-ray structural analyses and second harmonic generation
(SHG) measurements. The results of NLO properties display that
crystals in the phase-I possess the SHG efficiency of about 0.5 times
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Corresponding authors.
1387-7003/© 2015 Elsevier B.V. All rights reserved.