Engineering of non-linear optical crystals displaying a quasi perfect polar alignment of chromophores

Abstract

Four non-linear optically active crystals based on organic salts obtained by bis-protonation of four diamines (trans-(±)-1,2-, trans-(1R,2R)-(–)-, cis-1,2-diaminocyclohexane or R-(–)-2-methylpiperazine) by 2-methoxy-4-nitrophenol have been prepared. Their crystal structures and non-linear optical efficiencies have been studied. The two adjacent proton acceptor sites in 1,2-diaminocyclohexanes play an important role in aligning the 2-methoxy-4-nitrophenolate chromophores in a quasi-perfect polar arrangement by anchoring through short hydrogen bonds. In the case of the methylpiperazinium derivative the symmetrical 1,4 location of proton acceptor sites directs the chromophores to near antiparallel alignment by forming short hydrogen bonds. The second harmonic generation efficiencies at 1.064 µm of Nd ³⁺ :YAG laser light are equivalent to that of POM (3-methyl-4-nitropyridine-1-oxide) for the three diammoniocyclohexane derivatives and equivalent to twice that of urea for the methylpiperazinium derivative.