Nonlinear optical activity and second harmonic generation

4 Nonlinear optical activity and second harmonic generation 

The principal structural requirement for second order nonlinear effects in assemblies of molecules is the lack of a centre of symmetry, and considerable efforts have been expended in trying to induce potentially useful molecular entities to crystallize in non-centrosymmetric or polar crystals (Curtin and Paul 1981 Liter et al. 1991). As demonstrated below, this is a necessary, but not sufficient condition for obtaining nonlinear effects. True to form, the variety of crystallization experiments has led to a number of polymorphic structures, and to information about the relationship between the properties of these materials and their structures, as well as useful guidelines for attempting to obtain the desired non-centrosymmetric crystal structures. 

Perhaps the seminal work in this regard is that by Hall et al. (1986). This study summarizes work on three widely studied molecules whose crystals have been shown to have nonlinear optically active molecules 6-XX-6-XXII, acronymically designated 

In 6-XXII (MNP) the four different forms may be obtained from different solvents under different rates of cooling and evaporation. The space groups for the two forms for which crystal structures have been reported (Forms 2 and 3) are again both non-centrosymmetric monoclinic P2. The needles of Form 2 obtained by rapid cooling 

With the dilemma not resolved, Pandarese et al. carefully reexamined many individual crystals from the benzene-grown batch, and discovered that approximately 20 per cent of them belonged to the non-centrosymmetric orthorhombic space group P2i2i2i, thus explaining the source of the nonlinear effects. A method for purifying m-nitrophenol and growing crystals was developed by Wojcik and Marqueton in 1989 and the structure of the non-centrosymmetric orthorhombic form was published by Hamzaoui et al. in 1996. 

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