Polymorphism, crystal growth and characterization of an organic nonlinear optical material: DAPSH

Abstract

We have investigated the crystal growth, structural and physical properties of the organic nonlinear optical salt trans-4′-(dimethylamino)-N-phenyl-4-stilbazolium hexafluorophosphate (DAPSH). The chromophore in this recently created material shows a first molecular hyperpolarizability considerably larger than that of the related cation in the well-studied salt DAST (trans-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate). Through recrystallization of DAPSH using various solvents and conditions we have obtained three different crystalline phases. Phase a of DAPSH has a large second-order nonlinear optical susceptibility χ2. It crystallizes from acetone solutions and grows as purple needles, which are non-centrosymmetric with the space group Cc being suitable for electro-optic applications. Phase b with the triclinic non-polar space group P appears as reddish-brown thin plates when grown from DMSO solutions. It shows large third-order nonlinear optical susceptibility χ3 and can be easily grown as thin films, making it a good candidate for all-optical switching applications. Phase c with the non-polar space group P21/m grows as bright orange plates when crystallized from (50 : 50 vol%) ethanol–acetone solutions. This material shows no significant bulk nonlinear optical activity.