Intra- and Inter-Molecular Spin Coupling in Phenalenyl Dimeric Systems

Abstract

Phenalenyl is a triangular aromatic molecule made of three fused benzene rings, carrying an unpaired electron, and many of its derivatives show crystal structures with stacked radicals. Here, we investigate the inter-molecular binding in phenalenyl dimers by state-of-the-art computational methods and phenomenological models. Aside from being important for the supramolecular assembly of such radical molecules, the theoretical insight is relevant in methodological aspects, due to the interplay of long-range exchange coupling effects and van der Waals forces. We used comparative wave function-based and density functional theories. Drawing the potential energy surfaces as a function of inter-planar separation and mutual rotation of the monomer units, we found an interesting pattern which is not discovered in previous computational reports on the title systems. The dependence can be nicely interpreted by a transparent phenomenological model based on an orbital overlap paradigm of exchange coupling. We also brought forth a simplified phenomenological valence bond (VB) model of inter-molecular coupling, which is realized on the background of the VB spin model inside of the aromatic monomers and calibrated with the corresponding ab initio data. As the systems can be considered good candidates with potential applications in spintronics and organic magnetism, the theoretical rationalization opens up prospective ways to realize such promises.

The Density Functional Theory Account of Interplaying Long-Range Exchange and Dispersion Effects in Supramolecular Assemblies of Aromatic Hydrocarbons with Spin

“…Although the potential energy curves for phenalenyl dimers were considered in several instances [ 31 , 32 , 33 , 34 ], some details remained unobserved, and the following discussion pays the right attention to the implied issues, particularly to certain DFT specificities. In a recent study [ 25 ], we considered and rationalized, by magneto-structural orbital models, energy profiles and spin coupling as a function of the general rotation of phenalenyl units around a common C 3 axis. In the following, in order to debate DFT specific aspects, we are confining ourselves to the study of staggered conformation, with D 3d symmetry of the dimer, by varying the inter-planar distance between monomers.…”

“…The Morse-type fit is applied on the data labelled with LS in the previous discussion. The exchange coupling is fitted with a simple exponential, as we employed in other instances [ 25 , 59 ]: …”

“…In previous studies, we scrutinized spincoupling and resonance effects in monomeric [ 24 ] and dimeric [ 25 ] triangulenes, particularly relying on complete active space (CAS) and valence bond (VB) calculations. Recently [ 25 ], we analyzed the full landscape of singlet and triplet states together with related exchange coupling, as a function of separation and mutual rotation of two phenalenyl radicals sharing a common third-order rotation axis.…”

“…In previous studies, we scrutinized spincoupling and resonance effects in monomeric [ 24 ] and dimeric [ 25 ] triangulenes, particularly relying on complete active space (CAS) and valence bond (VB) calculations. Recently [ 25 ], we analyzed the full landscape of singlet and triplet states together with related exchange coupling, as a function of separation and mutual rotation of two phenalenyl radicals sharing a common third-order rotation axis. The calculations, made with Complete Active Space (CAS) methods followed by second order corrections (PT2) and, comparatively, with B3LYP density functional, were used as support of an interesting qualitative magneto-structural correlation model.…”

“…In the following sections, we focus on staggered conformation because, in experimental instances, it is more frequent than the eclipsed conformation [ 19 ]. Another reason for confining ourselves to staggered geometry is because it is expected to render results similar to the eclipsed one, especially in exchange coupling, as computational data from previous results [ 25 ] and outlined magneto-structural rationalization supports. Then, we avoid a certain redundancy, as the convened limitation serves the proposed aims well.…”