On the Optical Properties of Diamino-Pillared Graphene Architectures

Pillared graphenes are stacked graphene layers separated by spacers, or "pillars", of various types and have attracted significant attention in material science. In graphene networks, the pillars are used to increase interlayer distances and control their electronic conductivity, mechanical strength, and chemical reactivity. Such materials are currently investigated for a wide range of potential applications, including optoelectronics, flexible electronics, energy storage, catalysis, and sensing. In this work, we deal with the optical behavior of a special class of these promising materials, in which graphene sheets, piled one on top of the other, are covalently connected by diamino organic molecules via carbon-nitrogen single bonds. In particular, we studied elemental molecular quantities strongly related to molecules and materials' linear and nonlinear optical profiles. Properties such as optical gaps, UV-vis absorption spectra, excited states, and the first dipole hyperpolarizabilities have been computed and analyzed within the density functional theory framework. The obtained results suggest that both the linear and the nonlinear optical profiles of graphene architectures interconnected by conventional organic diamino molecules are dominated by the optical responses of the graphene layers. On the other hand, the diamino pillars are indirectly involved, in the optical behavior of these species, through local structural modifications on the framework of the graphene sheets as a result of the cross-linking process. Specifically, the performed computations, conducted in large finite graphene flakes of specific aromaticity patterns, exposed that local structural patterns may trigger surprisingly strong variations with respect to their optical absorption profiles, excited states, and nonlinear optical behavior. Such local structural patterns involve intercyclic carbon- carbon single bonds formed between sp3-hybridized carbon atoms belonging to two neighboring aromatic sextets. As far as the nonlinear optical properties are concerned, Kohn-Sham coupled perturbed computations on systems of various cross-linking patterns showed that NLO-inactive graphene sections, in terms of quadratic nonlinear optical responses, could be converted to materials of important nonlinearities. Finally, no evident dependence of the optical absorption profiles was observed with respect to the type (aromatic, conjugated, or fully saturated) of the pillar used.