Molecular crystal structures and nonlinear optical properties in the series of dicyanovinylbenzene and its derivatives

X-ray single-crystal structures, molecular mechanics (MM) calculations of the optimal molecular dimers, and calculations of the static second-order polarizabilities (beta) were performed for a large series of methoxy-and dimethylamino-substituted derivatives of dicyanovinylbenzene and some of its analogues having large values of the molecular nonlinear optical susceptibilities. X-ray structural analysis has been performed for 3,4-dimethoxy- and 3,4,5-trimethoxy-dicyanovinylbenzenes (I, II), p-(dimethylamino)-dicyanovinylbenzene (III), 1,1-dicyano-2-phenyl-2-(2-methoxyphenyl) (2-MeO-C6H4-C(C6H5)=C(CN)(2)) (IV), and 4-(4-methoxyphenyl)-1,1-dicyano-1,3-butadiene (4-MeO-C6P4-CH=CH-CH=C(CN)(2)) (V). Crystal packing analysis and energetic MM calculations revealed the factors responsible for the formation of the centrosymmetric crystals. Compounds III and V were found to form acentric crystal structures (space groups P2(1) and Pc, respectively) and therefore are capable to the second-harmonic generation (SHG) in solid state. Qualitative data have demonstrated that compound V is rather active in SHG in the powder state (using Nd:YAG laser with lambda = 1064 nm) that may be important for its application. On the contrary, the powder of III is not active in SHG despite the "optimal" crystal packing that might be related to the strong absorption of the second harmonic light at lambda = 532 nm, but this compound gives strong SHG signal using the laser light with lambda = 1907 nm. Analysis of the influence of the different substituents in the aromatic ring on the calculated beta values in the series of the compounds studied was made.