Syntheses, Crystal Growths, and Nonlinear Optical Properties for 2-Carboxylic acid-4-nitropyridine-1-oxide Crystals with Two Different Arrangements of Chromophores

A novel chromophore 2-carboxylic acid-4-nitropyridine-1-oxide (POA) has been synthesized, and the selective control of its crystal growth leads to two new second-order nonlinear optical (NLO) crystal materials named POA-I and POA-II accordingly. POA-I and POA-II are both orthorhombic, with space group P212121. Their cutoff ultraviolet–visible absorptions are located at about 380 nm. The Kurtz powder measurement shows that their second harmonic generation (SHG) efficiencies are about 4.6 and 9.8 times as large as that of KH2PO4 (KDP), respectively. The calculations of the SHG NLO susceptibilities are performed by the time-dependent density-functional theory (TDDFT) method coupled with the sum-over-state formula. It is found that a larger susceptibility originates from larger charge transfers and a stronger interaction among chromophores in the POA-II crystal.