Exciton chirality splitting

For a system containing two chromophores i and j, the exciton chirality (positive or negative) governing the sign and amplitude of the split Cotton effect can be theoretically defined as below59 ... 

Figure 13. Definition of exciton chirality. Summation of the two Cotton effects (broken lines) separated by Davydov splitting A). gives the curves shown in solid lines. Adapted from N. Harada, K. Nakanishi. Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry, University Science Books. Mill Valley, California. 1983...

The (4R)-absolute configuration of a new chromophore of native visual pigment (159) (negative Cotton effect at 375 nm, negative Cotton effect at 254 nm) was established by the CD exciton chirality method applied to the 4-(dimethylamino)cinnamate (160). The split negative (381 nm) and positive (338 nm) exciton effects of 160 show a counterclockwise helicity between pentaenal and a-4-(dimethylamino)cinnamate chromophores355. 

Although exciton coupling leads to shifted and broadened, if not split, UV-visible spectra of the composite molecule, when the chromophores are held in a chiral orientation, exciton coupling can be detected even more clearly in the CD spectrum as two oppositely-signed CE s typically corresponding to the relevant UV-visible absorption band(s). The signed order of the CD transitions correlates with the relative orientation of the relevant electric dipole transition moments, one from each chromophore, and hence the absolute configuration of the composite molecule (Exciton Chirality Rule.)[2] For the bis-anthra-... 

A helical arrangement within columns was also found for other metal 3-diketonate complexes provided with chiral side chains (32) by Serrano and co-workers.35,36 These compounds form rectangular columnar mesophases with helical order within the columns. A spin-coated sample of 32 showed a positive exciton-splitted signal in the CD spectra, which was interpreted as a left-handed (M) helix. Annealing of the film resulted in much higher optical activities and a shift of the absorption maxima. The observed optical changes clearly point to a chiral organization of the columns in the mesophase. 

The signs of the exciton-split Cotton effect reflect the absolute configuration of the molecule if the direction of the transition moment in the chromophore is established. For example, chiral 9,9 -spirobifluorene derivative 1 has R configuration as indicated by a strong positive CD couplet (A = +1111.7) due to the LBb band of the anthracene chromophores, the electric transition moment of which is polarized along the long axis of the chromophore (Figure 14)100... 

The amplitude (A) of the exciton Cotton effect is inversely proportional to the square of the interchromophoric distance. Thus, weak exciton split Cotton effects are expected for remote dibenzoates. Nevertheless, exciton Cotton effects were used for the assignment of the configuration of dibenzoates in a steroidal skeleton separated by as many as seven or eight C—C bonds158. In one application, the absolute configuration of Wieland-Miescher ketone (—)-2 was established by the use of the dibenzoate chirality rule for the 4-bromobenzoylated derivatives of the epimeric 1,5-diols 3 and 4, obtained by reduction of (-)-2159. 

The CD spectrum of the 1,2-dibenzoate 6 in 9 1 MeOH/dioxane showed a pair of typical exciton-split Cotton effects with opposite signs centred upon the UV absorption (227 nm) of the benzoate chromophore AE235 5 -15.9 and A 221 5 +6.66. The negative longer wavelength Cotton effect clearly defines the negative chirality between the two electric transition dipoles of the benzoate chromophores... 

Exciton-coupled circular dichroism (ECCD) spectroscopy was performed on the Aib-Stb-D peptide to fiirther substantiate the interaction between the methyl stilbene side-chains, which was observed via PL experiments. The interaction between the excited state of chromophores in a chiral environment causes split Cotton effects upon absorption of circularly polarized light by the chromophores (29, 30). It can be observed from Figure 7 that Aib-Stb-D exhibits a split CD Cotton effect presumably because of the chiral presentation of the methyl stilbene molecules on the same side of the a-helical peptide the asymmetric nature of the split observed may be due to some other electronic transitions or of additional background ellipticity, as has been observed in other systems (29, 31). The ECCD results confirm the close proximity of the methyl stilbene side chains mediated by the peptide backbone, which permits interaction between the side-chains in the excited state. 

Nakanishi s method is based on the splitting of CD waves when two chromopho-res on a chiral molecule are close in space (the dibenzoate rule). Such is the case with dibromobenzoates of sugars which have a 1,2 or 1,3 relationship. In the first version of the method, saponins were permethylated, methanolyzed with acid and the liberated OH positions were p-bromobenzoylated (44). As terminal sugars are fully methylated and UV transparent, they need not be considered. Branched sugars (two substitutions at least) 3deld di-or tri-benzoates with exciton-split CD curves. The difference in Ae values of the two extrema of split CD curves is directly related to the respective positions of the benzoates (1, 2 eq-eq = 1, 2 eq-ax = 62 1, 2 ax-ax = 6 1, 3 eq-eq = 0 1, 3 eq-ax = 16). The sensitivity of circular dichroism makes the method suitable for microassays. Typical analyses are performed on a 100 pg scale (nanomolar levels). 

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