Noncentrosymmetric organic solids with very strong harmonic generation response

Abstract

The molten reaction of 2-naphthol, 4-(aminomethyl)pyridine, and 4-pyridinecarboxaldehyde at about 180°C yields trans-2,3-dihydro-2,3- di(4′-pyridyl)benzo[e]indole (1) which possesses two chiral centers, rather than an expected Betti-type reaction product with only one chiral carbon center. The same reactions, using 3-pyridinecarboxaldehyde, 4-cyanobenzaldehyde, or 3- cyanobenzaldehyde instead of 4-pyridinecarboxaldehyde produce the related compounds trans-2,3-dihydro-2-(4′-pyridyl)-3-(3″-pyridyl)benzo-[e] indole (2), trans-2,3-dihydro-2-(4′-pyridyl)-3-(4″-cyanophenyl) benzo[e]indole (3), and trans-2,3-dihydro-2-(4′-pyridyl)-3-(3″- cyanophenyl)benzo[e]indole (4), respectively. This reaction proceeds with a high degree of stereoselectivity with a trans/cis ratio of about 98:2 at elevated temperature. Compounds 1, 2, and 4 crystallize in a noncentrosymmetric space group (Pca21, Pca21, and Cc), while compound 3 has a chiral space group (P21). These successfully acentric packing arrangements are probably due to the molecule bearing both two chiral centers and potential hydrogen-bonding groups. Furthermore, the reaction of racemic 6-hydroxy-2′-methyl-2-naphthaleneacetic acid with ethyl-2-cyano-1- (4′-pyridyl)acrylic acetate in the presence of piperidine gives 1-pyridyl-2-ethoxycarbonyl-3-amino-1H-naphtho-[2,1-b]pyran-2′- methylacetic acid (5), which likewise crystallizes in a chiral space group. All of compounds are second harmonic generation (SHG) active, and have a very strong SHG response approximately about 8.0, 5.0, 12.0, 6.0, and 1.4 (for 1-5 compounds) times that of urea. Ferroelectric property measurements indicate that compounds 1, 2, 4, and 5 may display ferroelectric behavior.