Preparation of Pyridinium Iodide-CdI₂ Hybrids for Nonlinear Optical Materials

In order to obtain a new type of organic-inorganic hybrid material for nonlinear optics, two pyridinium-CdI₂ crystals, i.e., bis(4-cyano-1-methylpyridinium) tetraiodocadmate (CNP/CdI₄) and bis(4-dimethylamino-1-methylpyridinium) tetraiodocadmeate (DAP/CdI₄) were prepared. From X-ray crystal structure analyses, the inorganic parts of both of the crystals are found to be the tetrahedral [CdI₄]^2- structure. Although the CNP/CdI₄ crystals have a centrosymmetric layered structure, the DAP/ CdI₄ crystals do not have layered structures resulting in non-centrosymmetric space group Cc and second-order nonlinear optical activity. Thus, the second-harmonic generation (SHG) coefficients (d) of the DAP/CdI₄ crystal were estimated using the oriented gas model. In spite of the shorter cutoff wavelength of DAP/CdI₄ compared with that of the well-known 2-methyl-4-nitroaniline (MNA) crystal, the maximum d value was expected to reach one-third of that of the MNA in the diagonal component.