2,4-BIS[4-(N,N-DIBUTYLAMINO)PHENYL]SQUARAINE - X-RAY CRYSTAL-STRUCTURE OF A CENTROSYMMETRIC DYE AND THE 2ND-ORDER NONLINEAR-OPTICAL PROPERTIES OF ITS NONCENTROSYMMETRIC LANGMUIR-BLODGETT-FILMS

Crystals of the title compound exist as a green monoclinic phase [spac e group P2(1)/c with a = 9.046(1), b - 19.615(2), c =9.055(1) Angstrom , beta=116.107(5)degrees, Z=2] and a purple triclinic phase. The chrom ophore is both planar and centrosymmetric and its dimensions indicate a tendency towards a quinoidal structure with extensive delocalisation . The Langmuir-Blodgett (LB) films show two types of aggregation with absorption maxima at 655 nm and 540 nm. These monolayers also exhibit strong second harmonic generation (SHG) comparable to the intensity fr om films of hemicyanine dyes. The anomalous non-linear optical propert ies are attributed to a serendipitous non-centrosymmetric packing arra ngement within the films and to an intermolecular charge transfer cont ribution to the bulk second-order susceptibility.