AGGREGATION-INDUCED 2ND-HARMONIC GENERATION

The molecular structure of -bis[6-(N-butyl-2,3,4-trihydroquinoloyl)]sq uaraine is centric, but its Langmuir-Blodgett films exhibit second-har monic generation (SHG) with an effective monolayer susceptibility of 1 20 pm V-1 at 1.064 mu m. This is not an inherent property of the molec ule, and the intensity is too strong to be associated with the glass-m onolayer interface. Instead, the SHG is attributed to a noncentrosymme tric aggregate structure and is enhanced by intermolecular charge tran sfer between the central acceptor and terminal donor of adjacent molec ules. Electrospray ionization mass spectrometry has provided evidence of aggregation, even in very dilute solution, with the spectrum showin g mass peaks that correspond to the monomer [mass/charge (m/z) = 457, [M + H](+)] and dimeric aggregate (m/z = 913, [2M + H](+)). These peak s persist in solid solution, and spin-coated films of the dye in poly( vinyl acetate) show weak SHG. (C) 1998 Optical Society of America.