Noncentrosymmetric organic solids with very strong harmonic generation response

The molten reaction of 2-naphthol, 4-(aminomethyl)pyridine, and 4-pyridinecarboxaldehyde at about 180 degrees C yields trans-2,3-dihydro-2,3-di(4'-pyridyl)benzoe]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)benzoe]indole (2), trans-2,3-dihydro-2-(4'-pyridyl)-3-(4"-cyanophenyl)benzoe]indole (3), and trans-2,3-dihydro-2-(4'-pyridyl)-3-(3"-cyanophenyl)benzoe]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 (Pca2(1), Pca2(1), and Cc), while compound 3 has a chiral space group (P2(1)). 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-naphtho2,1-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.