Octatetraenes radical cations

One distinctive feature of polyene radical cations (especially of the long ones) is their great photosensitivity. For example, exposure of a sample of matrix isolated all-fra s octatetraene radical cation to diffuse daylight leads, within about an hour, to a new pho-tostationary equilibrium containing at least six rotamers. It is by virtue of this sensitivity, which is in part due to the very large absorptivities of the second EA bands, that a highly selective rotamer interconversion can be achieved, using a very narrow bandwidth . 

Although site effects are not as prevalent in UV-vis absorption as they are in IR spectra, they do exist and manifest themselves sometimes very clearly in band systems that comprise sharp peaks. An example is the radical cation of all-trans-octatetraene whose first absorption band consists of multiple peaks that can be selectively bleached by highly monochromatic light. The site stmcture can become more evident in laser-induced fluorescence, where excitation of individual sites is possible down to the level of single molecules in favorable cases, but a discussion of this fascinating phenomenon is beyond the scope of this chapter. 

In subsequent argon matrix isolation studies, similar bands were found when hexatriene or cyclohexadiene are ionized , and eventually, five of the six possible rotamers of hexatriene radical cation were identified by selective, wavelength-specific in ter conversions . Similar results were later obtained for octatetraene , where six of the twenty possible rotamers are formed on ionization in argon (Figure 31) which could be interconverted and identified by selective photolysis . Interestingly, in the case of the butadiene radical cation the s-cis rotamer could not be detected, even if the diene radical cation was formed from the cyclobutene radical cation . In contrast, in a recent resonance Raman study, some weak bands were detected and assigned to the s-cA-butadiene radical cation which might have escaped detection in the earlier ESR and EA experiments . ... 

Later, gaseous 1,3,5-cyclooctatiiene radical cations 12 were also studied by CID mass spectrometry, together with the ions generated from the acyclic isomo", 1,3,5,7-octatetraene... 

In the case of nonalternant systems, however, the parallel between neutral molecules and the corresponding radical cations breaks down. Consider, for example, heptafulvene (Fig. 7.9b). Heptafulvene can be derived by union of tropylium with CH3" (Fig. 7.9b). Likewise, the open-chain analog of heptafulvene, octatetraene (Fig. 7.9d) can be derived by union of the odd AH cation heptatrienylium with CH3 (Fig. 7.9c). Since both heptafulvene and octatetraene are classical conjugated polyenes, their total n energies should be the same. 

The semiempirical quantum chemical consideration led to the conclusion that the discrepancy can be a consequence of ion-pair formation (Zuilhof Lodder 1995). In the case of cyclo-octatetraene, the ion-pairing phenomenon deserves more detailed explanation. While the dianion of cyclo-octatetraene is completely planar and meets all the requirements of aromaticity, the anion radical of cyclo-octatetraene is a nonaromatic species and is not completely planar. In the equilibria just considered, both the anion radical and the dianion had alkali cations as counterparts. The dialkali salt of the dianion has two cations symmetrically located over and beneath the octagonal plane. Distortions from ion pairing between the dianion plane and these two alkali cations are reciprocally compensated. With the... 

Electro-optical modulators are other examples whose efficiency is enhanced in the presence of ion radicals. These devices are based on sandwich-type electrode structures containing organic layers as the electron-hole-injecting layers at the interface between the electrode and the emitter layer. The presence of ion radicals lowers the barrier heights for the electron or hole injection. Anion radicals (e.g., anion radicals from 4,7-diphenyl-l,10-phenanthroline—Kido Matsumoto 1998 from tetra-arylethynyl-cyclo-octatetraenes— Lu and co-workers 2000) or cation radicals (e.g., cation radicals from a-sexithienyl—Ku-rataetal. 1998 1,1 -diphenyl-2-[phenyl-4-Ar,ALdi(4 -methylphenyl] ethylene—Umedaand co-workers 1990, 2000), all of them used as electron or hole carriers. 

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