Effect of dimer formation on the growth cessation of polar organic crystals

Despite their excellent nonlinear optical, electrooptic, piezoelectric, pyroelectric, photorefractive, and triboluminescent properties, the growth of polar organic crystals of large size and high quality has been hindered by growth cessation phenomena that occur in solution growth. Investigation of the crystal growth behavior of a highly polar organic nonlinear optical material, 3-methyl-4-methoxy-4′-nitrostilbene (MMONS), reveals that there exists an upper limit for the solution concentration, above which the crystals stop growing. Electric field induced second harmonic generation measurements show that the solution consists of two species, monomers and dimers of MMONS molecules, at room temperature and that the dimer concentration increases significantly as the solution concentration increases, while the monomer concentration decreases. The corresponding equilibrium constant of a dimer, K, is 0.61 (mol/L)^-1. The analysis of UV-visible absorption spectra also confirms the EFISH results. Analysis of the crystal structure shows that dimers attached on the crystal surface may cause a gradual degradation of crystal transparency and a buildup of lattice strain as the solution temperature increases from ∼16-40°C and may cause the growth cessation at solution temperatures above ∼40°C.