Violenes

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

Figure 15, Plot of the logarithms of disproportionation constants against the coulomb repulsion integrals for a series of 1,3,3-trimethylindolenin violenes (174).

Electrochromism is observed in reversible redox systems, which exhibit significant color changes in different oxidation states. Violenes, whose general structure is represented in Figure 1, are typical examples that exhibit electrochromism (1). 

Violene is constructed by a conjugated chain with two redox active chromophores. It is classified as two types (see Figure 2). As noted from the position of the end groups, one type is separated by an aromatic core and the other is located inside a cyclic 7c-system. In both cases, the colored species in this system are the radical ionic states. Thus, in general, the violene system does not show high redox stability (2). 

Wurster Type Violenes Weitz Type Violenes... 

Stabilization of the redox cycle is relatively important in construction of potentially useful electrochromic materials, because the molecules needed for application require high redox-stability. Recently, S. Hiinig et al. proposed the concept of violene-cyanine hybrid to produce stabilized organic electrochromic materials (3). The hybrid is constructed by a violene-type redox system containing delocalized closed-shell polymethine dyes as end groups. The hybrid is expected to exhibit the color of a cyanine dye, by an overall two-electron... 

Figure 3. General structure of Hiinig s violene-cyanine hybrid. One or two of the moieties X=C-Y represent cyanines, oxonols, or merocyanines.

Figure 4. Wurster type violene-cyanine hybrid.

The dication 212+ composed of two methylium units connected to a p-phenylene spacer would be a candidate for new Wurster type violene-cyanine hybrid (Figure 12) (15). The reaction of four molar amounts of azulene 6b with terephthalaldehyde yielded the hydro precursor. Synthesis of the dication 212+ was accomplished by hydride abstraction with DDQ in almost quantitative yield. The dication 212+ was expected to show destabilization, but instead it exhibited high thermodynamic stability just like the corresponding monocation 3b+. 

Figure 12. Wurster type violene-cyanine hybrid composed of two methylium units connected via a p-phenylene spacer.

The combination of the di(l-azulenyl)methylium units produced dicationic species with high stabilities. However, transformation of the dication 212+ to the quinoid compound 21 as a Wurster type violene-cyanine hybrid was not achieved due to the instability of the reduced species. 

We have established the conversion between the two colored species by electrochemical reaction utilizing the concept of a Wurster type violene-cyanine hybrid. Dications 222+ and 232+ showed significant changes in their absorption spectra in different oxidation states. Therefore, dications 222+ and 232+ could function as new violene-cyanine hybrids, in which the four end groups (X and Y) in the general structure are azulenes (Figure 4). 

Two color changes could be induced by the violene-cyanine hybrid designed from the Wurster type violene. However, the multiple color changes... 

For the central violene rc-system, we chose benzene with acetylene spacers for decreasing steric hindrance between the large cationic centers. Therefore, the tetracation 254 salt was designed as a first example for a hybrid with a cyanine unit at the both termini (27). As illustrated in Figure 22, tetracation 254 exemplifies a new hybrid system, which produces another cyanine substructure 252 by two-electron transfer. In this case, overall, four electron transfer should afford a fully neutralized species such as a neutral diradical. 

Figure 23. The dication 26designed as a typical example for the violene-...

Synthesis of novel polyelectrochromic materials with the new structural motif utilizing stabilized carbocations has been demonstrated by several examples. The Wurster type violene-cyanine hybrid systems (22z+ and 232+) exhibit the presumed two color changes. The new cyanine-cyanine hybrid system (24 ) with a cyanine unit at the one terminus exhibits three color changes. 

The report will emphasize mainly type A with a radical cation SEM ( violene ) as the intermediate oxidation level. Structural types B and C will be dealt with essentially for the sake of completeness and for comparison. 

From Wurster s salts and the Weitz system i/it is well known that the intermediate oxidation level of the radical cations ( violenes ) is characterized by an e ecially long wavelength absorption. This general behaviour may be demonstrated with 32, n = 1 In Fig. 7 the similar absorption curves of RED and OX can be seen from... 

Already before systematical investigations were started, it was anticipated that the known vinylene shift of 90—120 nm of the cyanine type may also be found in the violene type This situation has subsequently been expressed in a general theory of polymethines ... 

In both cases the number of ir-electrons differs from that of the sp2-atoms by one. Accordingly a linear correlation is expected between the absorption maxima of cyanines and violenes, and the number of vinylene groups or 7r-electrons. 

Quantitative MO-LCAO calculations as well as treatments as a one dimensional electron gas have been advanced. Special parameters, however, have to be introduced to account for different end groups and branching of the rr-system. Empirically a linear correlation between and n is verified in all cases so far investigated. That is, violenes behave like cyanines. The vinylene shift amounts to 100—150 nm in contrast to that of the corresponding forms OX and RED with 20-40 mm ... 

Correlation of Xmax of Violenes with the Number of rr-Electrons... 

Equation (4), which corresponds to the well known correlation for cyanines describes Xmax of vinylogous violenes extremely well. 

The specific effect of the different end groups is expressed by variations of k (branching, heteroatom) and b (additional n-electrons) X As can be read from Table 9, the correlation coefficient r is close to 1.0 in most cases. In some cases for n = 0 there are deviations which obviously arise from steric hindrance . Thus these data are omitted from the correlation. With cyanines one finds k 65 which some of the violenes exhibit too. Figure 9 again demonstrates the discussed correlation. 

In the field of cyanine dyes aza-substitution has been studied extensively for nearly 100 years and even a theoretical treatment of the observed hypsochromic and batho-chromic shifts is possible In case of the violenes with their even number of methine groups between the two ends, symmetrical bis- or tetraaza-substitution is most suitable both for practical and theoretical (electrochemical) reasons. 

The electrochemical properties of the imidazo[4,5-,7 [l,3,2]diazaborolidines 56, especially the AgEM constants (10 ), are close to the boron-bridged violenes indeed, the values obtained for 56 are on a par with those of the acceptor part in organic metals derived from TCNQ or -dicyanochinodiimine (DCNQI) (AgEM 10 -10 ) <2004CC1860>. 

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