Tetraphenyl Tetrel Molecules and Molecular Crystals: From Structural Properties to Nonlinear Optics

The efficient light–matter interaction of molecular materials renders them prime candidates for (electro-)­optical devices or as nonlinear optical media. In particular, white-light generation is highly desirable for applications ranging from illumination to metrology. In this respect, cluster compounds have gained significant attention as they can show highly brilliant white-light emission. The actual microscopic origin of the optical nonlinearity, however, remains unclear and requires in-depth investigations. Here, we select the family of group 14 tetraphenyl tetrels with chemical formula X­(C₆H₅)₄ and X = C, Si, Ge, Sn, and Pb as the model system, and we study the properties of single molecules and molecular crystals. Calculations in the framework of the density functional theory yield the structural, vibrational, and electronic properties, electronic excitations, linear optical absorption, as well as second- and third-order optical susceptibilities. All well agree with the experimentally determined structural and vibrational properties, as well as the linear and nonlinear optical responses of specifically grown crystalline [X­(C₆H₅)₄] samples with X = Si, Ge, Sn, and Pb. This thorough characterization of the compounds yields deep insight into this material class on the path toward understanding the origin of the characteristic white-light emission.