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

A series of amphiphilic styryl dyes: 2-(4-dihexadecylaminostyryl) benzothia zole methiodide (BTM), 2-(4-dihexadecylaminostyryl) benzoxazole methiodide (BOM), and 2-(4-dihexadecylaminostyryl) benzimidazole methiodide (BIM), wer e synthesized; their monolayers were successfully deposited on quartz slide s by the Langmuir-Blodgett technique and formed H aggregates. Second-order susceptibilities chi(zzz)((2)) of the monolayers were determined with a pol arized laser beam (Nd : YAG, lambda = 1.064 mu m). The values of chi(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 se miempirical quantum chemical calculation results show that electron deficie ncies of the heteroaromatic rings play an important role in determining the second harmonic generation of organic dye LB films.