The subtle role of heteroaromatics in the second-order susceptibility in a series of amphiphilic styryl dye Langmuir-Blodgett films

A series of amphiphilic styryl dyes: 2-(4-dihexadecylaminostyryl) benzothiazole methiodide (BTM), 2-(4-dihexadecylaminostyryl) benzoxazole methiodide (BOM), and 2-(4-dihexadecylaminostyryl) benzimidazole methiodide (BIM), were synthesized; their monolayers were successfully deposited on quartz slides by the Langmuir-Blodgett technique and formed H aggregates. Second-order susceptibilities χ(zzz)/((2)) of the monolayers were determined with a polarized laser beam (Nd: YAG, λ = 1.064 μm). The values χ(zzz)/((2)) are 215, 132, and 67 pm V^-1 for BTM, BOM and BIM, respectively. Charge distributions over the entire molecules for the dye models were calculated using the MINDO/3 method in the MOPAC 7.0 program. Experimental data and semiempirical quantum chemical calculation results show that electron deficiencies of the heteroaromatic rings play an important role in determining the second harmonic generation of organic dye LB films.