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.

The local structure of four amorphous organotin sulfide compounds is investigated by X-ray absorption spectroscopy. These compounds exhibit a nonlinear optical (NLO) response upon irradiation with a continuous-wave near-infrared laser. Their basic structural motif is a hetero-adamantane cluster with different organic substituent, but depending on the morphology of the compound and the choice of the organic substituent, the nature of the NLO response changes to either a second harmonic generation or the generation of a supercontinuum, potentially appearing as white light. The structural results provide an experimental evidence that the nature of the NLO properties is tied to distortions occurring at the cluster core, with almost ideal clusters in compounds that show second harmonic generation, and strongly distorted clusters in the case of compounds that generate a supercontinuum. These distortions may enable a closer proximity of the cluster cores, altering the intermolecular order, and thereby influencing the NLO response.

The microscopic structure of a new infrared-driven amorphous white-light-generating material was explored by X-ray diffraction, EXAFS and Reverse Monte Carlo simulation. In this material, structural disorder appears to be prerequisite for this nonlinear optical effect. The results are consistent with quantum chemical predictions, but it is also found that the molecular cores are distorted, which is identified as a crystallization inhibitor. Sulfur atoms thereby form a uniform vibrational network, which may be responsible for the high capability of the material to absorb infrared radiation.

The Front Cover illustrates the generation of a transmission electron microscopy diffraction pattern from an amorphous adamantane-type cluster compound and the conversion of infrared laser light into directed white light. Cover design by Elisa Monte. More information can be found in the Research Article by Kerstin Volz and co-workers.

The microscopic structure of two new infrared-driven amorphous white light generators, namely [(PhSn)(4)S-6] and [(CpSn)(4)S-6] and a related amorphous material [(NpSn)(4)S-6], showing 2nd harmonics generation instead, were explored by X-ray and neutron diffraction, EXAFS and Reverse Monte Carlo simulations to explore relations between their extreme nonlinear optical behavior and microscopic structural properties. The current state of research is reported and experimental and simulation results are discussed. The prominent observation is that the molecular units are distorted in the WLG materials which seems not to be the case for the 2nd harmonics generator. Associated is the formation of a net of similarly spaced intra- and intermolecular sulfur atoms which is interpreted as a vibrational network that could explain the high receptivity of the material for infrared radiation. It is also found that the molecules arrange in chains with staggered configuration regarding the arrangement of the organic ligands.

Amorphous materials are an integral part of today's technology-they commonly are performant and versatile in integration. Consequently, future applications increasingly aim to harvest the potential of the amorphous state. Establishing its structure-property relationship, however, is inherently challenging using diffraction-based techniques yet is extremely desirable for developing advanced functionalities. In this article, we introduce a set of transmission electron microscopy-based techniques to locally quantify the structure of a material. This unique approach allows to clearly identify the spatial distribution of amorphous and crystalline regions and to quantify atomic arrangements of amorphous regions of a representative model system. We study an ensemble of well-defined, functionalized adamantane-type cluster molecules exhibiting exceptionally promising nonlinear optical properties of unclear origin. The nanoscopic structure for three model compounds ([(PhSn)(4)S-6], [(NpSn)(4)S-6], [(CpSn)(4)S-6]) correlates with their characteristic optical responses. These results highlight the advantageous properties of amorphous molecular materials when understanding the microscopic origin.

The local structure of four amorphous organotin sulfide compounds exhibiting nonlinear optical (NLO) properties is investigated by low‐energy X‐ray absorption spectroscopy (XANES and EXAFS). The basic structural motif of a heteroadamantane cluster with different organic substituents is confirmed by the experiments and by comparison with computer simulations of the near‐edge structure. Essential information is obtained on a special role of the nonaromatic but electron‐rich cyclopentadienyl substituents, which are able to affect the structure of the heteroadamantane cluster core. The EXAFS fits also indicate a structural distinction between two groups within these compounds, which exhibit fundamentally different NLO responses: compounds that show a second harmonic generation can be described well using the single‐molecule approach, whereas the compounds exhibiting the generation of a supercontinuum manifest additional structural contributions.