Five adducts consisting of carboxylic acid-substituted indoles with
nitro-substituted benzoic acids have been synthesized and tested for
second-order non-linear optical properties. These were indole-2-carboxylic
acid with 2,4-dinitrobenzoic acid (1), 3,5-dinitrobenzoic acid (2), and
2,4,6-trinitrobenzoic acid (3), and indole-3-acetic acid with
3,5-dinitrobenzoic acid (4), and 2,4,6-trinitrobenzoic acid (5). Compound (2)
produced clear, yellow crystals (space group P
-1 with a 6·8400(7),
b 15·150(2), c
16·097(2) Å, α 84·911(9), β 87·088(10),
γ 77·865(9)°, Z 4) which allowed the
structure to be determined by X-ray diffraction. Of the five adducts,
compounds (3) and (5) gave second harmonic intensities of 0·15 and
0·16 times respectively that of a urea standard.