Chiral exciton

Tryptycenes can be chiral, but how can one determine the absolute configuration The CD of (+)-dimethyl 5,12-dihydro-5,12[1, 2 ]benzonaphthacene-l, 15-dicarboxylateshows an intense (+) chirality exciton couplet (Ac +151, Aeffo -178) for the intense (emax -84,000) 233 nm transition. As with the pyranosides, the binary couplets must be... 

Simonyi M, Bikadi Z, Zsila F, and Deli J. 2003. Supramolecular exciton chirality of carotenoid aggregates. Chirality 15(8) 680-698. 

The configuration of the 4R,5R-dihydrodiol was established by application of the exciton chirality method (6). To minimize undesired interactions between the electric transition dipoles of the two j>-N,N-dimethylaminobenzoate chromophores and the dihydrodiol chromo-phore, a 4,5-dihydrodiol enantiomer was first reduced to 1,2,3,3a,4,5,7,8,9,10-decahydro and 4,5,7,8,9,10,11,12-octahydro derivatives (6). We found that it is not necessary to reduce the chrysene chromophore of a BaP 4,5-dihydrodiol enantiomer (Figure 2). Similarly, the absolute configurations of the K-region dihydrodiol enantiomers of BA (7), 7-bromo-BA (8), 7-fluoro-BA (9), 7-methyl-BA (10). and 7,12-dime thy 1-BA (DMBA) (7 ) can also be determined by the exciton chirality method without further reduction. 

The absolute configuration of the 7,8-dihydrodiol metabolite was also established to be 7R,8R by the exciton chirality method (11.12), Our result (Figure 2) is in agreement with those reported earlier (11.12). 

The absolute configuration of the 9,10-dihydrodiol metabolite was established to be 9R,10R both by nuclear magnetic resonance spectroscopy and by the structures of the hydrolysis products formed from the svn and anti 9,10-dihydrodio 1-7,8-epoxides which were synthesized from the same 9,10-dihydrodiol enantiomer (13). The absolute configuration of a BaP trans-9.10-dihvdrodiol enantiomer, after conversion to a tetrahydro product, can also be determined by the exciton chirality method (Figure 2) (19.20). 

Figure 2. The exciton chirality CD spectra of bis-p-N,N-dimethyl-aminobenzoyl derivatives of BaP 7.8.9.lO-tetrahvdro-trans-7.8-diol (1.0 A fc/ml, derived from BaP trans-7.8-dihvdrodiol metabolite),...

We have tried to express the results of Weigang s treatment in pictorial form (Scheme 6), applying the language of the exciton chirality rulesld to the coupling of the chromophore transition dipole moments with those induced in the nearby bonds. These are regarded... 

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. 

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 1-17. Exciton chirality of acyclic allylic benzoates and the sign of the predicted benzoate Cotton effects. The thick line denotes the electric transition moment of the benzoate group. Reprinted with permission by Am. Chem. Soc., Ref. 61. 

Harada N, Nakanishi K, Exciton chirality method and its application to configurational and conformational studies of natural products, Accounts Chem Res 5 257-263, 1972. 

The Cotton effects may be classified into three types168 those arising from chirally perturbed local achiral chromophores (ketones, /i.y-unsaturated ketones, double bonds, benzoates, aromatic compounds) those arising from inherently achiral chromophores, such as conjugated dienes or a,/3-unsaturated ketones those arising from interaction of the various electric transition moments when two or more chromophores which are chirally disposed are positioned nearby in space, intra- or intermolecularly (exciton chirality method)169. 

A useful method for the establishment of the absolute configurations of chiral primary amines involves formation of their salicylidene derivatives and application the salicylidcnamino chirality rule 172. Other procedures for primary amines are based on, either the CD of their V-(2,4-diniirophenylsulf enyl)- V-(4-methylphenylsulfonyl) derivatives as described in Section 4.3.4.I.5.251, or on the exciton chirality method313. 

Recently, a method was developed which allows assignment of the configuration around a secondary carbinol center combining kinetic resolution and the exciton chirality CD method241 (see Section 4.3.4.1.4., p 458). 

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