Structure determination in a new type of amorphous molecular solids with different nonlinear optical properties: a...

The microscopic structures of two amorphous molecular solids with extremely nonlinear optical properties have been studied. They consist of organotetrel chalcogenide clusters with the chemical formula [(RSn)₄S₆]. The basic molecular building blocks are adamantane-like {Sn₄S₆} cores with organic ligands R attached to the Sn atoms. While the material equipped with R=naphthyl generates frequency doubling upon irradiation with a simple infrared laser diode, the material decorated with R=phenyl responds by emitting brilliant white light. The structural differences were investigated using x-ray scattering and extended x-ray absorption fine structure combined with molecular Reverse Monte Carlo. Transmission electron microscopy and scanning precession electron diffraction were used to examine structural differences from mesoscopic down to microscopic scales. Characteristic differences were found on all scales. While close core-to-core distances between {Sn₄S₆} cluster cores and molecular distortions are found in the white light emitting material, undistorted molecules and significantly larger core distances characterize the material showing frequency doubling. Here however, results of scanning precession electron diffraction reveal the formation of nanocrystalline structures in the amorphous matrix, which we identify as cause for the suppression of white light emission.