2,4-bis [4-(N-methyl-N-alkylamino)phenyl]squaraines: Structure-property relationship and dependence of the type of aggregate upon the hydrophobicity of the alkyl group

The molecular packing in Langmuir-Blodgett (LB) films of the title compound, 2,4-bis[4-(N-methyl-N-alkylamino)phenyl]squaraine, is influenced by chromophore-dominated interactions when the alkyl chains are short and by van der Waals interactions when they are sufficiently hydrophobic. The butyl to dodecyl analogues pack parallel to the substrate with interlayer spacings of 8.2-8.9 Angstrom from grazing incidence X-ray synchrotron diffraction(GIXD). In contrast, the tetradecyl to docosyl analogues adopt a "U-shaped" configuration, with the chromophore parallel to the substrate, and the d spacing increases from 19.8 to 24.1 Angstrom. The transition is manifested by an abrupt change in the molecular area in contact with the substrate and by subtle differences in the Linear and second-order nonlinear optical properties. Freshly deposited monolayers have a broad absorption maximum at 660-695 nm and exhibit second-harmonic generation, but with time, films of the higher alkyl analogues, tetradecyl to docosyl, alter to an H-aggregate phase (lambda(max) = 530 nm) with loss of SHG. Long-term stability has been observed for films of 2 4-bis[4-(N-methyl-N-hexylamino)phenyl]squaraine: the LB monolayer has an optimum susceptibility of chi(zzz)((2)) approximate to 710 pm V-1 at 1.064 mu m, a thickness from atomic force microscopy (AFM) and surface plasmon resonance (SPR) of 5.0 +/- 0.5 Angstrom (cf. 8.5 +/- 1.5 Angstrom layer(-1) in the bulk film), and real and imaginary components of the dielectric permittivity of epsilon(r) = 3.0 +/- 0.1 and epsilon(i) = 0.7 +/- 0.1 respectively at 532 nm.