ISSN:
0021-8383
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Non-Kelulé polymethines are polymethine dyes that cannot be written by single Kekulé-type formulae. As demonstrated with 3,3′-dimethyl-3,3′-pyridocyanines (1) the electron distribution and term level ordering of them differ greatly from those of the related simple open-chain polymethines (streptopolymethines), considering the latter ones as constitutive elements of these dyes according to Dähne's polymethine concept. Non-Kekulé dyes are, however, closely related to isoconjugated classical polymethine dyes. The peculiar features of the non-Kekulé polymethines are well understood in terms of simple MO theory. Orbital tracing assisted by MOA (molecular orbital analysis) reveals crossing between the frontier orbitals of these dyes and those of the related open-chain polymethine substructures. Additional insight into peculiarities of 1 is gained at the semiempirical level by analyzing the π-electronic system in terms of the heterocyclic end groups and the interconnecting odd-alternant hydrocarbon (carbopolymethine) chain. Numerical predictions are made based on MNDO optimum geometries. According to CNDO/S and PPP calculations the non-Kekulé dyes considered absorb at longer wavelengths than the isoconjugate classical dyes. In the case of the first members of series 1 the lowest-energy transition is less determined by the characteristic polymethinin charge redistribution inherent to polymethine dyes than by notable heterocycle-to-chain charge transfer (CT). The interfragmental CT upon excitation reduces the transition probability of this electronic transition. Some calculations performed beyond the monoexcitation approximation indicate a certain birdicaloid character of the non-Kekuléan dye structures. The exceptional position of these dyes is also indicated by the higher heats of formation of 1 relative to the isomeric classical dyes.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/prac.19893310414
