Conformational polymorphism on styrl quinolinium salts of 2-[(E)-2-(4-hydroxy-3-methoxystyr1)-1-methyl]-quinolinium 4-chlorobenzenesulfonate (HMQ-CBS): Potential nonlinear optical crystals for terahertz application

摘要

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-methoxystyr1)-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 ofP2i for phase-II. These two different crystalline phases exhibit the distinct anionic orientations and crystal packings, leading to significantly different nonlinear optical (NW) properties. Second harmonic generation (SHG) measurements reveal that crystals in phase4 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 phase4 might be a potential NW crystal forTHz 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 NW properties, and sheds light on the design of organic THz materials. (C) 2015 Elsevier B.V. All rights reserved.