The Solitonic Nature of the Electronic Structure of the Ions of Linear Conjugated Systems

Abstract

Features of the electron density distribution in the ground and excited states, the position of the energy levels, and the equilibrium molecular geometry of the ions of linear p-electron systems, including conjugated polymers and oligomers, cationic and anionic polymethine dyes, and the radical-ions of a,w-substituted polyenes, are reviewed. By using the soliton concept it was possible to explain many unusual properties of this type of organic compound: The quasimetallic conductivity, the marked change in the spectral characteristics of ionic dyes absorbing and emitting light in the near IR region of the spectrum, etc. It was shown on the basis of semiempirical and nonempirical calculations that the charges in collective p-electronic systems form soliton waves, the width of which does not depend on the length of the molecule. In the excited state the size of the solitons increases significantly. The introduction of donor and acceptor terminal groups can destroy the symmetry of the charge distribution and molecular geometry at a conjugation chain of critical length exceeding the dimensions of the soliton. It was established that the injection of electrons/holes not only leads to the appearance of a soliton level at the center of the energy gap but is also accompanied by significant displacement of the top of the valence band and the bottom of the conduction band.

Location of the charge and geometric components of soliton waves in cations of unsymmetrical conjugated systems

“…Thus, it was established that in a,w-dialkylamino-substituted polymethine cations or the radical-cations of polyenes, where the length of the conjugation chain exceeds the width of the soliton wave, the latter is displaced to one of the terminal groups, i.e., the symmetrical distribution of electron density in the symmetrical molecule is destroyed. At the same time with the introduction of a methoxyl group OCH 3 as terminal group the distribution of electron density and the molecular geometry remain symmetrical even with a comparatively large number of vinylene groups in the conjugation chain [2,[4][5][6][7]. The symmetry of the soliton waves is not destroyed by phenyl and other alternant carbocyclic groups in contrast to the nonalternant terminal groups, which displace the center of the soliton at a critical elongation of the polymethine chain [2,4,5].…”

“…[R 1 -(CH) m -R 2 ] z , z = 0, 1, 2 I such as polyenes, polymethine dyes, and donor-acceptor compounds continue to find wide application as highly polarized media, molecular switches or conductors in fields associated with the conversion of luminous energy [1] and also serve as suitable model compounds for the development of new theoretical concepts (e.g., see the review [2] and the references therein). Their principal characteristics are determined by the collective system of p electrons.…”

“…At the same time with the introduction of a methoxyl group OCH 3 as terminal group the distribution of electron density and the molecular geometry remain symmetrical even with a comparatively large number of vinylene groups in the conjugation chain [2,[4][5][6][7]. The symmetry of the soliton waves is not destroyed by phenyl and other alternant carbocyclic groups in contrast to the nonalternant terminal groups, which displace the center of the soliton at a critical elongation of the polymethine chain [2,4,5]. Accordingly, a significant difference in the effect of the terminal groups is observed experimentally.…”

“…The injection of charge leads to the appearance of impurity or soliton levels in the gap [3], and a soliton-like wave of partial alternating positive and negative charges is generated on the atoms. At the same time a wave of changes in the length of the carbon-carbon bonds, which also alternate -so-called geometric or topological solitons, appears [2,4]. In unsubstituted conjugated systems both the charge and the geometric solitons are located at the center of the molecule.…”

“…In unsubstituted conjugated systems both the charge and the geometric solitons are located at the center of the molecule. Earlier [2,5] it was shown that the form of the soliton waves hardly depends at all on the length of the conjugation chain, so that only the central part of the solution wave is observed in the case of short molecules. The introduction of terminal groups can lead to a significant change both in the form and in the location of the solitons.…”