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

Abstract

A quantum-chemical investigation into the location and form of solitons in the cations of unsymmetrical polymethine systems was undertaken. It was shown that the introduction of one terminal group (electron-donating or electron-accepting) with intrinsic level outside the energy gap of the main chromophore leads to displacement of the charge and geometric waves without substantial distortion of their form; the introduction of an amino group with a high level for the unshared electron pair is accompanied by displacement of the solitons to the end of the conjugation chain, so that half of the soliton wave projects onto the molecule. In unsymmetrical polymethine cations with two nitrous groups the geometric soliton is closer to the less basic residue; replacement of one terminal residue by a methoxy group moves the position of the soliton to the end of the chromophore. Linear polymethine systems[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. In the case of neutral molecules the compounds are insulators with a comparatively large energy gap and with a uniform distribution of electron density on the carbon atoms in the conjugation chain. 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. 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. 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 232 0040-5760/08/4404-0232

In‐hospital complications of autologous hematopoietic stem cell transplantation for lymphoid malignancies

“…As was established earlier, unsubstituted polymethines with the relatively long conjugated chain may demonstrate a symmetry breaking, resulting in a shift of a charge wave from the center to one of the chain ends. The substituted molecules often show this symmetry breaking at shorter chain lengths 10, 11, 16, 18, 23, 24, 35…”

Molecular design of near infrared polymethine dyes: A review

“…There are two definitive peculiarities of soliton: a) its dimension which does not depend on the extension of the linear conjugated system; b) its mobility, i.e. soliton can move along the π-system under influence of donor/acceptor strength of terminal groups [146][147][148][149], asymmetry of the dye molecule [150] or under external electric field [151]. It should be noticed that in ionic π-system with several charges, each of charges exists as a separate soliton [153].…”

Location of charge and geometric soliton waves in dications of α,ω-substituted polyenes

“…At the same time the value of the parameter Dq m at the center of the chain is significantly reduced while the amplitude of alternation of the bond lengths remains maximum. Thus, both solitons move toward the end of the chain like the localization of the soliton waves in the monoamino-substituted polymethine cations [13]. As known, calculations (both by the AM1 method and ab initio) give similar cardinal migration of the soliton waves for the symmetrical monocations of streptocyanines (VIII):…”

“…Earlier [13] it was proposed to evaluate this by interrupting the conjugation between the terminal groups and the chromophore by withdrawing the substituent from the plane of the main part of the molecule. It should be noted that the relative position of the UEP level must depend on the total charge of the p system.…”

“…It should be noted that the relative position of the UEP level must depend on the total charge of the p system. We Earlier [13] during investigation of the localization of the solitons in the cations of linear conjugated systems it was proposed to separate the terminal groups into two types depending whether their intrinsic levels fall outside or inside the energy gap. In the first case the terminal group only slightly displaces the soliton waves from the center of the chromophore in the monosubstituted polymethine cation, while in the second the soliton waves are localized at the terminal group.…”