Spectroscopic Signatures of Molecular Clusters and Molecular Cluster Aggregates from First Principles

Abstract

The second harmonic generation (SHG) response of (hetero)adamantane-type clusters and organotetrel molecules with the general formula [(RT)4E6] and [TR4] (T = group 14, R = organic substituents, E = C, S, CH_2), structures and substructures derived therefrom are investigated from first principles. The SHG response is modeled within density functional theory (DFT) applying a real time approach for the calculation of the nonlinear optical response. The structures and electronic properties used to model the optical response are determined within DFT. The need to approach the investigation using HSE is also explored. The investigated molecular clusters have been reported to efficiently convert infrared radiation into white light and are therefore extremely attractive functional materials for a multitude of applications. The investigations are part of the DFG funded FOR 2824 research group project, which aims to answer fundamental questions regarding the prerequisites for the nonlinear optical response which include a magnitude of factors. The relationships between optical properties and cluster symmetry, stoichiometry, substituent field, core composition, and further structural characteristics are investigated. In the case of heterogeneous dimers, the optical response of isolated clusters and cluster dimers strongly resembles that of a dominant cluster. Similarly, upon formation of cluster crystals, the compound inherits the optical characteristics of the parent molecules. It is predicted that the characteristic of the optical response is more strongly defined by the substituents and modifications of them, than modifications of the core. Core modifications have a higher impact if the resulting structure is more distorted from the ideal and symmetric adamantane configuration. Clear trends for modifications are identified to help model tailored responses. On the basis of our results, we are able to gain insight into the optical properties of larger structures by investigation of substructures due to the determined effect of inheritance of the optical characteristics of the response. Besides unraveling the relationships between structure/composition and optical response, our investigations allow the interpretation of the experimental investigations and inspire the synthesis of new compounds to tailor the optical properties. Having the knowledge of optical spectrum beyond the current usage helps to establish the correlation between excitation wavelength and the optical response. Moreover, we can help to answer the question whether other light sources beyond IR radiation could be employed to drive the material in a desired way.