Second-Harmonic Generation in Langmuir-Blodgett Monolayer of a Two-Dimensional Charge-Transfer Molecule: N,N′-Dioctadecyl-4,6-Dinitro-1,3-Diaminobenzene

A new amphiphilic dye, N,N′-dioctadecyl-4,6-dinitro-1,3-diaminobenzene (DIODD), which is capable of forming Langmuir-Blodgett (LB) films, has been synthesized and represents a model for a two-dimensional charge transfer (2-D CT) molecule. The stable Langmuir-Blodgett monolayers of a 1:1 mixture of dye and arachidic acid were deposited onto a glass substrate. The Langmuir-Blodgett monolayers exhibited a well-defined fringing pattern resulting from second-harmonic generation. These experimental results have been analyzed in connection with theoretical calculations for the model compounds. Our theoretical calculations demonstrate the superiority of two-dimensional charge transfer molecules over one-dimensional charge transfer molecules in the case in which the LB molecules incline from the normal to the substrate. The molecular orientation of the DIODD molecule is close to the optimum orientation applicable for maximizing χ111 (2). Second-order nonlinear optical susceptibilities χ111 (2) and χ133 (2) determined by analyzing the data were 2.2×10-8 and 7.7×10-9 esu, respectively at a tilt angle of 60°. This work presents the first example of a two-dimensional charge transfer molecule displaying second-harmonic generation from a Langmuir-Blodgett monolayer.