Elsevier

Inorganic Chemistry Communications

Volume 61, November 2015, Pages 165-168
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

https://doi.org/10.1016/j.inoche.2015.09.017Get rights and content

Highlights

  • Conformational polymorphism of the HMQ-CBS crystals was firstly identified.

  • Two crystalline phases with distinct anionic orientations and crystal packings were obtained.

  • Crystal in phase-I is a promising THz material, showing SHG efficiency of 0.5 times that of DAST.

  • Relationship between polymorphism and bulk NLO properties was discussed.

Abstract

Polymorphism is an important phenomenon to investigate the relationship between the chemical structures 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 molecular conformational changes and bulk NLO properties, and sheds light on the design of organic THz materials.

Graphical abstract

The existence of conformational polymorphism in the terahertz (THz) compounds of 2-[(E)-2-(4-hydroxy-3-methoxystyrl)-1-methyl]-quinolinium 4-chlorobenzenesulfonate (HMQ-CBS) was identified. 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.

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Introduction

Crystalline polymorphism has been considered as a fascinating research area because polymorphs exhibit different mechanical, physical and chemical properties, which makes it great importance in pharmaceutical 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 molecular conformations and the packing arrangements. Therefore, organic molecular compounds have great possibilities to exhibit the polymorphism because of their flexibility and weak intermolecular interactions in the solid state [3], [4], [5], [6], [7].

Styrylquinolinium derivatives with methylquinolinium electron acceptors 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]. Quinolinium-based crystals also exhibit large microscopic and macroscopic NLO susceptibilites [8], [9], [10]. Moreover, such quinolinium 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 macroscopic second-order NLO responses. In detail, the effective hyperpolarizability tensor component βijkeff of α-phase crystals is the diagonal component βijkeff = 178 × 10 30 esu, while the corresponding value for β-phase crystals is the off-diagonal component βijkeff = 60 × 10 30 esu [13]. However, the investigation on polymorphism of quinolinium derivatives 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) determined 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 that of DAST, indicating that it 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 molecular conformations and bulk NLO properties, which will shed light on the further studies of organic THz materials.

Section snippets

Experimental section

Metathesization of 2-[(E)-2-(4-hydroxy-3-methoxystyrl)-1-methyl]-quinolinium iodide with the silver-4-chlorobenzensulfonate in hot methanol afforded the raw materials of HMQ-CBS [14], [15]. The synthesis of iodide precursor was described in the literature including condensation of dimethylquinolinium iodide with vanillin in the presence of piperidine. Subsequently, the synthesized HMQ-CBS materials were recrystallized for multiple times from methanol solutions.

In order to investigate the

Results and discussion

Single-crystal X-ray structure determination reveals that both phases of HMQ-CBS crystals crystallize in monoclinic crystal system. However, phase-I belongs to the non-centrosymmetric space group of Pc and phase-II has the centrosymmetric space group of P21/n. Crystal structure of phase I has been recorded in the Cambridge Structural Database [14]. Here, we cited the reported structure from the reference for a consistent comparison.

Firstly, it is noteworthy that molecular conformation of the

Conclusion

Conformational polymorphism of the NLO crystals HMQ-CBS has been investigated, and a new polymorph was found with different anionic orientation and molecular arrangement. Both phases of HMQ-CBS crystals belong to the monoclinic system, but show different space groups (i.e. Pc and P21/n). The remarkable diversity between the two polymorphs of HMQ-CBS has been related to the molecular conformational changes, which influences the anionic orientation, crystal structure and the bulk NLO properties.

Acknowledgments

This work was supported by NSFC (21222102, 21373220, 51402296, 21171166, 21301172 and 21450110415), the 973 Key Program of MOST (2011CB935904), the NSF for Distinguished Young Scholars of Fujian Province (2014J06015), the “Chunmiao Project” of Haixi Institute of Chinese Academy of Sciences (CMZX-2013-002), the Youth Innovation Promotion Association CAS (2014262), and the CAS President’s International Fellowship Initiative.

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