Syntheses, X-ray structures and second harmonic generation efficiencies of fused ring heterocycles

Second harmonic generation efficiencies of a number of fused ring heterocyclic compounds bearing a nitro group were measured using the Kurtz powder method. Among them, 5-nitroindole has the highest SHG efficiency, 20 times as high as that of urea reference. Several 5-nitroindole derivatives were synthesized in the hope of improving the SHG efficiency. None of the derivatives, however, exhibited higher SHG efficiency than the mother compound. X-ray crystal structures of some of these compounds as well as that of 5-nitroindole itself were determined. In the crystal structure of 5-nitroindole the angle (θ) between the molecular charge transfer axis and the polar axis of the crystal is 52.72 °, which is close to the optimum value 54.74 ° predicted by theory. On the other hand, crystal structures of l,2-dimethyl-5-nitroindole and 1-ethyl-5-nitroindole revealed θ values far from the optimum one, which is consistent with their low SHG efficiencies.