Studies of 2‐Azaazulenium Derivatives: Unsymmetrical Trimethine Cyanine Dyes Bearing a 2‐Azaazulenium Moiety as One of the Terminal Groups

Abstract

We report here the synthesis of a series of symmetrical and unsymmetrical trimethine cyanine dyes derived from 2-azaazulene, combined spectral and quantum-chemical investigations of their molecular geometry and electron structure, as well as the nature of the lowest electron transitions. Based on the analysis of both calculations and experimental data obtained from absorption and (13)C NMR spectra, it was concluded that the 2-azaazulene residue can be treated as a weakly basic terminal group; its donor properties are provided with the participation of the HOMO-1, in contrast to the typical Brooker's terminal residues with their donor HOMOs. The new classification of the terminal groups of cyanine dyes, and hence the classification of types of unsymmetrical cyanines, is proposed. It is shown that the nature of the higher electron transitions (delocalized or local) in the cyanine dyes depends on their type. In the unsymmetrical trimethine cyanine of the mixed type, negative deviations are observed in their absorption spectra.

New unsymmetrical squaraine dyes derived from imidazo[1,5-a]pyridine

“…are described mainly by single excited configurations [37][38][39][40][41][42][43]. The calculations confirmed this regularity for the unsymmetrical squaraines 6a-d.…”

“…When compared to the correspondent symmetrical analogues [34], compound 6a displays absorption at approximately the same wavelength, while that of 6b and 6c is shifted hypsochromically (38 and 36 nm, respectively), a characteristic behavior of unsymmetrical squaraine dyes [34,35] (Table 2). Table 2 It was established earlier that end groups possessing highly localized orbitals produce the main contributions to the highly occupied MOs of cyanine dyes with relatively short polymethine chains [37][38][39][40]. Among short chain cationic cyanine dyes and their squaraine analogues, the local M A N U S C R I P T…”

“…It was ascertained that the two highest occupied MOs can be considered to be generated mainly from the highest MOs of both end residues conjugated with the delocalized π-electron system of the polymethine chain and, therefore, these MOs can be treated as donor orbitals [37][38][39][40].…”

Molecular design of near infrared polymethine dyes: A review

“…In the non-planar conformation without any conjugation, the frontier and nearest π-MOs (see Figure 4b) are located in the separate molecular fragments (although some MOs can involve the σ-orbitals, It was established that the HOMOs in the typical Brooker's cyanines 6-8 are generated mainly by terminal groups and hence they should be treated as donor orbitals [133][134][135][136]. Such a nature of the frontier level points clearly the first electron transition to be connected mainly with all molecular fragments, not only with the polymethine chain.…”

“…So, the highest occupied MOs of the terminal residues in the cationic dyes can be treated as a donor orbital; at the same time, the lowest vacant MO of the TG in the anionic PMDs can be considered as an acceptor orbital [133][134][135][136]. Because of the different sensitivities of the donor/acceptor levels and levels produced by the polymethine chain to the total charge, the donor level could occur higher than the highest occupied levels of the chain in cationic dyes, while the acceptor level could be disposed below than the lowest vacant level produced by the chain.…”