Origin of the Nonlinear\nOptical Response in Organotetrel\nMolecules, (Hetero)adamantane-Type Clusters with Organic Substituents,\nand Related Species

The nonlinear optical response of (hetero)adamantane-type\nclusters\nand organotetrel molecules with the general formula [(RT)<sub>4</sub>E<sub>6</sub>] and [TR<sub>4</sub>] (T = group 14, R = organic substituents,\nE = S, CH<sub>2</sub>) is investigated from first principles. These\nclusters have been reported to efficiently convert infrared radiation\ninto white light and are therefore extremely attractive functional\nmaterials for a multitude of applications. We demonstrate that the\noptical nonlinearities of the clusters in the range from 0 to 3 eV\nhave their origin in electronic transitions within the substituents.\nThe cluster core does not directly take part to the generation process;\nhowever, it strongly affects the intensity of the linear and nonlinear\nresponse. The relationships between optical properties and cluster\nsymmetry, stoichiometry, substituent field, core composition, and\nfurther structural characteristics are investigated by systematical\nvariation of R and T. This also demonstrates the possibility to finely\ntune the intensity as well as the frequency dependence of the optical\nresponse. Upon formation of cluster dimers, the intensity of the nonlinearities\ndepends on the overall dimer geometry. In the case of heterogeneous\ndimers, the optical response strongly resembles that of a dominant\ncluster. Similarly, upon formation of cluster crystals, the compound\ninherits the optical characteristics of the parent molecules.