Crystal growth and investigation of efficient non-linear optical materials in the mixed (2,-4-dinitrophenyl)-L-alanine (DPA) and 2-methyl-4-nitroaniline (MNA) system

Abstract

When a 1∶1 weight mixture of (2,-4-dinitrophenyl)-L-alanine (DPA) and 2-methyl-4-nitroaniline (MNA) is dissolved in a 50∶50 volume mixture of ethyl acetate and hexane, two types of crystals were observed on slow evaporation. In the first stage of crystallization, light-yellow plates of MNA measuring up to 25 mm×5 mm×1 mm were formed, which belong to the triclinic space group P1 with lattice parameters a=0.7621 (3) nm, b=0.795 2(5) nm, c=0.8200 (2) nm, α=111.84(2)°, β= 93.76(2)°, γ=116.84(3)°, Z=2. The volume of the unit cell containing two molecules is 361.9(1)×10⁻³ nm³. The powder SHG efficiency of the triclinic polymorph (MNA(T)) is about the same as that of the monoclinic MNA(M). The Raman spectrum of the new crystal is similar to that of MNA(M) crystallized from the vapour phase or solution, except for the change in the intensity of a few bands. The DSC data show minor differences in the melting points and specific heats for the two phases. The second stage of crystallization resulted in the formation of orange-yellow coloured crystals of the DPA:MNA complex, in the monoclinic space group P2₁ with lattice parameters a =0.6876(1) nm, b=0.7621(3) nm, c=1.7626(4) nm, β=96.62(2)°, V=917.5 ×10⁻³ nm³, Z=2 for C₁₆H₁₇N₅O₈. The Raman spectrum of DPA:MNA consists of bands from both DPA and MNA. The complex melts at 138(1)°C with a specific heat of 25.3(4) cal g⁻¹ according to the DSC measurements. The powder SHG efficiency is nearly the same as that of methyl-(2,4-dinitriphenyl)-aminoproanoate (MAP).