Surface plasmon resonance and nonlinear optical studies of Langmuir-Blodgett films of a betaine dye

Langmuir-Blodgett (LB) monolayers of 4-(N-hexadecylpyridinium-4-ylmethylide ne-amino)-2,6-dichlorophenolate have been investigated by surface plasmon r esonance (SPR) and second-harmonic generation (SHG) techniques. The thickne ss, the real and imaginary components of the dielectric permittivity and th e effective second-order susceptibility are l = 2.4 nm layer(-1), epsilon(r ) = 1.75 and epsilon(i) = 0.08 at 632.8 nm, chi(eff)((2)) = 20 pm V-1 at 1. 064 mu m respectively. The films protonate in air with loss of SHG and the colour changes from purple (lambda(max) = 510 nm) to yellow (lambda(max) = 375 nm). The process is reversible upon exposure to NH3 and, thus, the glas s\Ag\LB structures for SPR may be utilised as sensors by monitoring changes in the reflectance at a fixed angle of incidence. (C) 1999 Elsevier Scienc e B.V. All rights reserved.