Sandwich type dimers

The following conclusion of the theory (1 ) is extremely important. The radiative transition 2 > Sq in a sandwich dimer is forbidden. In case of a dimer of 04 symmetry, the transition 2 (4Eg) > Sg (A g) is forbidden because of parity. There is no principle difference in the splitting nature of 2 and states for sandwich type dimers with lesser than D4h symmetry and the 2 > Sq transition remains quasi forbidden. This makes it possible to explain low P2 values obtained in (1 ) by a decrease of the 2 > Sg transition radiative probability, i.e., by decreasing or 2 > Sq fluorescence quantum yield in dimeric TTA complexes. In the case of non-sandwich dimer structures with location of subunits in one plane, the So state also is split into two states (high 202y and low 2B3g). However, two radiative transitions S2(B2y)... 

Linear-dichroic spectra of SA monolayers prepared from mixtures of OTS and a cyanine-dye surfactant established the absence of dimerization and the orientation of the chromophore parallel to the substrate [183]. In contrast, the same cyanine dye underwent sandwich-type dimer formation in LB films and had its chromophore oriented perpendicular to the water surface [192]. These results highlight an important difference between LB and SA monolayers. Parameters which determine monolayer formation on an aqueous subphase are also responsible for the orientation and organization of the surfactants therein. Furthermore, the configuration of the surfactants is retained regardless of the structure of the substrate to which the floating monolayer was subsequently transferred to by the LB technique. Conversely, in SA monolayers, surfactant organization is primarily dependent upon the nature of the substrate [183]. 

E.p.r. studies459 indicate the formation of sandwich-type dimer complexes of V02+ and etioporphyrin (EP) in light petroleum at 77 K the e.p.r. data are consistent with an EP-EP plane distance of ca. 3.5 A. A planar ligand local environment for Vlv, possibly provided by a phthalocyanine- or porphyrin-type ligand, has been proposed460 to account for the axial microsymmetry of the V17 centre and the absence of zero-field splitting indicated by e.p.r. studies of vanadium in mineral oil. 

The aggregation of 29 and 30 was studied by absorption spectroscopy. By comparison with the absorption spectra of model squaraine aggregates, it was concluded that 29 forms sandwich type dimers, which is essentially controlled by C-0 dipole-dipole interactions. For 30, the aggregate band is red-shifted to that of the monomer, and this has been attributed to an aggregated form involving intermolecular charge-transfer interactions between the electron-... 

Tabie 13.14. Charge Transfer Efficiency in Mixed Sandwich Type Dimers... 

Anions of other buckybowls were also characterized by NMR spectroscopy. For example the dianions and tetranions of dibenzocorannulene 25 and dibenzocydo-pentacorannulene 60 were reported by Rabinowitz et al. [53], In these cases, however, the tetraanion stage of reduction could be achieved only with K metal. No sandwich type dimerization similar to tetralithio-1 was observed for these larger systems. 

The chiral penta-substituted corannulene, 1,3,5,7,9-penta-terfbutylcorannulene (37), which shows anomalous dynamic behavior [116], was reduced with Li and four reduction stages were observed [117]. The behavior of the anti-aromatic dianion and the aromatic tetraanion resemble that of the parent corannulene. In the final stage of the reduction, three distinct tetraanionic species could be detected. Two of the species are sandwich-type dimers, as in the case of corannulene, and are diastereomers, due to the chirality of 37 (RR/SS and meso - RS/SR). The third species was assigned as a tetraanionic monomer, which slowly disappears. 

Simple transition metal halides react cleanly with alkali metal boratabenzenes. In this way sandwich-type complexes 32 of V (27), Cr (64), Fe (58), Ru (61), and Os (61) have been made. The corresponding nickel complexes seem to be nonexistent, quite in contrast to NiCp2 in attempted preparations, mixtures of diamagnetic C—C linked dimers were obtained (29). In the manganese case, high sensitivity to air and water has precluded preparative success until now. Some organometallic halides have added further variations to the main theme. The complexes 33 of Rh and 34 of Pt were obtained from [(COD)RhCl]2 and [Me3PtI]4, respectively (61). 

C5Hs)2Ti]2, and the unexpected diamagnetism was confirmed. A further unusual feature, deduced from infrared spectral data, was that in one case the compound was not of the 7r-sandwich type, but instead the dimer apparently contained both v- and rr-bonded cyclopentadienyl groups (8). 

To improve the aqueous solubility of 5 we also synthesized a cyclopeptide 6 containing hydroxyproline subunits [24]. Although this compound is very water-soluble, and in solution adopts a similar conformation to 5, it only forms 1 1 complexes with anions. A reason for the inability of 7 to form 2 1 complexes could be that the hydroxyproline subunits in 6 are better solvated than prolines in aqueous solution, and the desolvation required for aggregation of two cyclopeptide molecules thus occurs less readily. Steric hindrance of hydroxyl groups from different cydopeptide moieties in the a dimeric complex of 6 could, moreover, also make aggregation difficult. Although peptide 6 cannot form sandwich-type complexes, it proved to be valuable for a quantitative determination of the anion affinity of 5, the results of which are summarized in Table 2.2.3 [24],... 

The reduction of 1,3,5,7,9-penta-tert-butylcorannulene (47) with lithium progresses in four steps, and affords a paratropic dianion and a diatropic tetraanion78. This is reminiscent of the reduction process of 43. In the final stage of the reduction, however, three distinct tetraanionic species could be detected two sandwich-type diastereomers, similar to 44, and another species assigned to a tetraanionic monomer of 47, that slowly disappears. This study showed that the peri derivatization of 43 with bulky alkyl groups has little effect on its dimerization. 

Tabata and coworkers31 found that, at low temperatures, there are two types of dimer cation—a non-bonded n-n sandwich type and a bonded linear type. 

The anthocyanin-ethyl-flavanol adducts thus formed are purple pigments ( nax 5 0 nm), and much more resistant towards acid-catalyzed cleavage than the equivalent colorless ethyl-linked flavanol dimers (29). This presumably results from the displacement of the hydration equilibrium toward the flavylium cation form, due to self association, since similar synthetic pigments have been shown to be greatly stabilized by sandwich-type stacking (28). 

Scheme 14 Di is identified as the low-energy sandwich-type excimer, Dj as the high-energy dimer, and (MM) as "relaxed monomer. Dj and (MM) are populated rapidly (< 10 ps) by an exciton diffusion mechanism from the initially excited monomer M, and Dj is formedfrom (MM) (but not from Dj ) with a rise time of about 2 ns...

The octakis(crown-ether) (78) does not present the same changes in absorption spectrum as described above with the tetrakis(crown-ether) (72) in the presence of metal ions (Li, Na, K ) there was no observable change in the spectrum. It was concluded that K" " forms an intramolecular sandwich-type complex and not an intermolecular dimer as does (72) <89CB291>. 

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