Aggregation-induced second-harmonic generation

The molecular structure of 2,4-bis[6-(N-butyl-2,3,4-trihydroquinoloyl)]squaraine is centric, but its Langmuir-Blodgett films exhibit second-harmonic generation (SHG) with an effective monolayer susceptibility of 120 pm V-1 at 1.064 mu m. This is not an inherent property of the molecule, and the intensity is too strong to be associated with the glass-monolayer interface. Instead, the SHG is attributed to a noncentrosymmetric aggregate structure and is enhanced by intermolecular charge transfer between the central acceptor and terminal donor of adjacent molecules. Electrospray ionization mass spectrometry has provided evidence of aggregation, even in very dilute solution, with the spectrum showing mass peaks that correspond to the monomer [mass/charge (m/z) = 457, [M + H](+)] and dimeric aggregate (m/z = 913, [2M + H](+)). These peaks persist in solid solution, and spin-coated films of the dye in poly(vinyl acetate) show weak SHG. (C) 1998 Optical Society of America.