2,4-Bis[4-(dihexadecylamino)-2-hydroxyphenyl]-squaraine: Second-Harmonic Generation from Langmuir - Blodgett Monolayers of a Centro-symmetric Dye in which the Chromophores are Isolated from the Substrate

2,4-Bis[4-(dihexadecylamino)-2-hydroxyphenyl]squaraine is vertically oriented in the Langmuir–Blodgett film and the chromophore layer is separated from the glass substrate by the hydrophobic dihexadecylamino groups. The Langmuir–Blodgett monolayer exhibits moderately strong second-harmonic generation (SHG) with an effective susceptibility of c. 10 pm V-1 at 1064 nm and an intensity, relative to the mean SHG from films of the extensively studied hemicyanine dye (E)-4-[2-(4-dimethylaminophenyl)ethenyl]-N-docosylpyridinium bromide, of c. 0·3. The SHG cannot arise from the polarization of the dye by the underlying substrate because the conjugated donor-(acceptor)-donor moiety is separated by a 2 nm thick hydrophobic barrier. Instead, the non-linear optical behaviour is attributed to a non-centrosymmetric aggregate structure.