Two-dimensional charge-transfer molecules based on N,N＇-dialkyl-2,4-dinitro-1,5-diaminobenzene have been synthesized. H-1 and C-13-NMR spectroscopy, elemental analysis and mass spectroscopy were used to elucidate their chemical structures. The physical and non-linear optical properties of this new family of dyes containing alkyl chain; C3H7, C6H13, C8H17, C10H21, C11H23 and C18H37 are discussed, taking into account the possible role of alkyl chain length. These two-dimensional molecules have a significantly large off-diagonal beta component in contrast to one-dimensional molecules. These compounds showed no powder second-harmonic generation (SHG) at 1.064 mu m being centrosymmetric, however, their poled guest-host systems with poly(methyl methacrylate) and co-crystals with p-nitroaniline were SHG active. Powder SHG as high as 37 times that of urea was observed from N,N＇-dihexyl-2,4-dinitro-1,5-diaminobenzene with its mixture with p-nitroaniline. Second-harmonic generation of N,N＇-dioctadecyl-2,4-dinitro-1,5-diaminobenzene (DIODD) was studied as Langmuir-Boldgett monolayers. The Langmiur-Blodgett monolayer of a 1:1 mixture of DIODD and arachidic acid showed second-order non-linear optical coefficients d(11) and d(13) of 11 x 10(-9) and 3.85 x 10(-9) esu, respectively, at a tilt angle of 60 degrees. For the first time, a relationship between the microscopic polarizabilities and the molecular orientation of two-dimensional charge-transfer molecules has been established. In the light of the present experimental and theoretical data analysis, the potential of two-dimensional charge-transfer molecules for second-order non-linear optics is discussed.