engleski

Semiquinone radicals as a base for novel organic (semi)conductors: how π-stacking determines magnetism and electrical conductivity

Semiquinones are a class of stable organic radicals with a great potential for design of novel organic electronics and multifunctional materials. Since the radicals are planar, they typically form π-stacks, which stabilise crystal packing and determine magnetic and electrical properties of the crystals. Stronger π interactions thus imply stronger exchange interactions and lower energy barrier for electron transfer. Two types of stacks π-have been described: 1) Peierls-distorted with alternating short (< 3.1 Å) and long (> 3.35 Å) [1] interplanar distances (i.e. comprising dimers of radicals with paired spins), which are diamagnetic and isolators and 2) stacks of equidistant radicals which are 1D antiferromagnetic and semiconductors. Despite its importance, π-stacking of planar organic radicals has been little studied from the fundamental point of view. Here we present a detailed study of different factors which influence (semi)conductivity of semiquinone systems in the solid state: 1) type of stack (Peierls-distorted or equidistant), 2) induction effect of electron-withdrawing substituents, 3) interplanar distance and 4) orientation of the stacked rings. As suitable systems we chose series of salts of three semiquinones (5, 6-dichloro-2, 3-dicyanosemiquinone, tetrachloro- and tetrabromosemiquinone [1]) with planar organic cations derived from N-methylpyridinium. Since only antiferromagnetic compounds with equidistant radicals are good semiconductors, we focused our attention to this type of stacks. Electronegativity (i.e. electron-withdrawing) of the substituents plays a double role: i) it stabilises the radical by enhancing delocalisation of π electrons and ii) lowers the energy of the HOMO orbital, thus increasing band gap related to electronic transport. Therefore, the most stable radicals (5, 6-dichloro-2, 3-dicyanosemiquinone, DDQ) are the least conductive, while less stable ones (tetrabromosemiquinone) are the best conductors. This effect is far more pronounced than increasing of the band gap by increasing interplanar separation, while orientation of the rings in the stacks apparently does not influence electrical conductivity.

semiquinone, pi-interactions, crystal engineering, semiconductors, magnetic properties

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