In Search of Organic Compounds Presenting a Double Exchange Phenomenon

Abstract

The objective of this paper is to design a consistent series of organic molecules that may present a double exchange mechanism and study their low energy spectrum using spin unrestricted Density Functional Theory. For this purpose, organic tetra-methylene methane units having an S = 1 spin ground state and diamagnetic organic bridges are taken as building blocks for constructing molecules having two or more magnetic units. When biunit systems are ionized, the ground state of the resulting molecular ions may be either a quartet, if the spectrum is ruled by a double exchange mechanism, or a doublet, if it obeys the logic of a monoelectronic picture. A strategy based on the physical analysis of the leading interactions is followed in order to energetically favor a high-spin ground state. It is shown that the most promising compounds involve bridges that have both a large gap between the highest occupied and the lowest unoccupied molecular orbitals and small coefficients on the atoms to which the magnetic units are connected. While the followed strategy enables one to conceive organic compounds exhibiting a double exchange phenomenon, it is shown that the electronic mechanism ruling the spectrum of such organic double exchange compounds is different from that of their inorganic homologues. A new method to reconstruct the spectrum of low energy from various spin unrestricted DFT solutions is proposed and applied. Finally monodimensional and bidimensional periodic lattices based on the most promising organic architecture are suggested.

When a single hole aligns several spins: Double exchange in organic systems

“…A previous paper has succeeded to show, from density functional theory (DFT) calculations, that one might conceive branched polycyclic aromatic systems which have a singlet ground state when neutral, and which have a quartet ground state after ionization. 8 This work has confirmed that in organic chemistry too a single hole may align three spins. 9,10 The suggested systems, based on Chichibabin type branched poly-aromatic chains, slightly deviate from the double-exchange canonical models since in their neutral ground state they only involve two unpaired electrons.…”

“…Let us recall, for instance, that it is responsible for the so-called colossal magnetoresistivity in manganites. 6 Up to now the systems exhibiting the phenomenon essentially involve magnetic transitionmetal ions, bearing several unpaired electrons in a high-spin ground state, such as Ni 2+ (d 8 ), or Mn (d 7 ). They can be periodic metal-oxide lattices or coordination complexes, for which experiments confirm the high spin multiplicity of the ground state.…”

“…In such a case, there is competition and mixing between states with three unpaired electrons and states with one unpaired electron only, while the low-energy states of the typical double exchange systems only involve states with three unpaired electrons. 8 In this context, we found it useful to look for molecular organic systems which would remain closer to the orthodox double-exchange situation, starting from magnetic units which are doubtlessly of high-spin multiplicity. We have selected fused polycyclic molecules since we have recently shown that it is possible to conceive systems with several delocalized unpaired electrons of parallel spins.…”

Why 1,2-quinone derivatives are more stable than their 2,3-analogues?

“…The good performance of the UB3LYP method is confirmed by recent papers [25][26][27][28][29][30]. On the basis of previous results [31], spin contamination corrections were not included.…”

“…It should be noted that Clar structure shown in Scheme 5b is reinforced by all applied aromaticity indices (see Table S1 in Supporting Information). Summarizing, in 2,3-quinones for n > 2, the diradical singlet situation is favored, as in the case of the acenes [25,59,61,63] and in other polycyclic aromatic hydrocarbons [26][27][28][29][30][64][65][66][67][68][69] and graphene nanoflakes [70,71]. Therefore, for 2,3-isomers, only the results of the ground states (singlet closed shell for n ≤ 2 and singlet open shell for n ≥ 3) are presented below.…”