Evidence of σ- and π-Dimerization in a Series of Phenalenyls

Abstract

Phenalenyl and a wide variety of its derivatives form stable radicals, which often associate in various aggregates with interesting properties that include magnetism and high electrical conductivity. The two main modes of aggregation involve π-stacking pancake multicenter bond formation and σ-bond formation. We explore the energetics of the various σ- and π-dimers for six phenalenyl derivatives with both computational and experimental methods. A modern density functional theory (M05-2X) is used to survey the potential energy surface revealing the mechanism of the aggregation. In order to enrich experimental data, the triphenyl and trimethyl derivatives are newly prepared and their aggregation behaviors are investigated by various analytical methods including ESR, (1)H NMR, UV-vis, and single-crystal X-ray diffraction. The agreement between computations and experiments are very good forming the basis of describing trends in this series. We find that π-dimer formation can proceed via an asynchronous concerted path from the monomers or in a stepwise process via σ-dimers. The strength of the π-stacking pancake interaction depends strongly on substituents and covers a wide range both in terms of binding energies and contact distances. The spin densities in the π-stacking dimers reflect these trends and display a wide range of diradicaloid characters. Many σ-dimer configurations compete some of which are separated by small barriers leading to fluxional structures between σ-bonded configurations or σ- and π-bonded configurations.

Recent Advances in the Chemistry of Phenalenyl

“…The dehydrogenation of the radical precursor (7) with p chloranil (Scheme 2) afforded deep blue crystals, contrary to our expectation. 29 The X ray crystallographic analysis of the blue crystal showed that 6 forms a discreet face to face π dimer (6 2 ) with a C(α) C(α) separation of 3.067 Å at 300 K (Figure 7), much shorter than that of 4 2 . The smaller interplanar separation leads to a stronger covalent bonding interaction of two unpaired electrons because of the larger overlap between the two SOMOs of adjacent phenalenyl units.…”

“…29 The dehydrogenation reaction of 9 with p chloranil produced a pink violet solution (Scheme 3), indicating the generation of 8 or a π dimer (8 2 π). However, the concentration of the solution yielded colorless platelet crystals.…”

Effects of Halo-Substitution on 2′-Chloro-5′-halo-phenyl-1,2,3,5-dithiadiazolyl Radicals: A Crystallographic, Magnetic, and Electron Paramagnetic Resonance Case Study

“…For radical 4, in the temperature range 120 -160 K, hyperfine structure appears to be additionally superimposed on the fine structure ( The axial ZFS parameter |D| of radicals 1 -4 (Table 3) appeared to be similar to those observed in other organic π radical dimers. [59][60][61][62] For most triplet state organic radicals, the 22 contribution of spin-orbit coupling to the ZFS is small, and quiet often the g anisotropy is small.…”

Near-infrared absorption of π-stacking columns composed of trioxotriangulene neutral radicals

“…Phenalenyl (PNL) is a carbon-centered neutral π-radical with a delocalized spin structure. [12][13][14][15][16][17][18][19] Several PNL derivatives form π-dimers, and exhibit absorption bands around 600 nm. [15][16][17] In a few cases, 1D π-stacking columns are formed with large intermolecular separations of~3.78 (ref.…”