The cationic dye, E-N-alkyl-4-[2-(4-alkyloxynaphthyl) ethenyl] quinolinium bromide has a wide-bodied chromophore with alkyl groups at opposite ends, i. e. CmH2m+1-D-π-A+-CnH2n+1 X-, where D and A are electron-donor and electron-acceptor groups respectively, π is a conjugated bridge and X- is the bromide counterion. It forms non-centrosymmetric Langmuir-Blodgett (LB) structures in which the packing is dependent upon the combination and compatibility of the alkyl groups. Films of two isomeric analogues where m + n = 34, have a layer thickness of 4.3 nm when the groups are significantly different (m = 12 and n = 22) which decreases to 2.5 nm when they are similar (m = 18 and n = 16). The latter adopts an interdigitating layer arrangement at both the upper and lower interfaces and it is the first example of a Lego-type structure with non-centrosymmetric alignment of the optically nonlinear chromophores. This is evidenced by the second-harmonic intensity, which increases quadratically with the number of layers, i. e. as .