Photoelectric generation and second-order nonlinear optical characters of the dichromophore dye molecules

Centrosymmetric molecules can exhibit strong second-harmonic generation (SHG) by Langmuir-Blodgett (LB) technique. The macroscopic nonlinearity chi((2)), of the monolayer film reaches 245 pm V-1 for 4,4'-di{1-[1-(N,N'-dioctadecaneamino)phenylethylpyridinium bromide]octyl}dipyridinium bromide (PMVs) and that for 4,4'-di{1-[1-(N,N'-dioctadecaneamino)phenylethylpyridinium bromide)]propyl)dipyridinium bromide (PMV3) is 184 pm V-1. Steady cathodic photocurrent is obtained upon excitation of the dyes incorporated into LB films deposited on an ITO electrode. The observed quantum yield of the photocurrent strongly depends on the applied electrode potential, concentration of the electron acceptor or donor, pH of the electrolyte, and the chemical structure of the dye congeners. The quantum yield reaches 4.73% for PMV8, while that for PMV3 is 3.94%, under favorable conditions.