Exciton-chirality model

Finally, the optical activity of polymers that contain a very large number of inherently achiral chromophores in a chiral arrangement may be treated by methods developed in solid-state physics, like the other optical properties of polymers. The resulting theoretical description can be viewed as a generalization of the exciton-chirality model. 

In addition to the CD exciton chirality method, we have recently reported that the theoretical calculation of the CD spectra by the jt-electron SCF-CI-dipole velocity MO method8-l4 has become an important tool for determination of the absolute configuration of a variety of twisted and conjugated n-electron systems. In fact, we have recently determined the absolute stereochemistry of (8aS)-(+)-l,8a-dihydro-3,8-dimethylazulene 10, a labile biosynthetic intermediate for 1,4-dimethylazulene 11 isolated from a liverwort, by application of the MO method to the theoretical calculation of the CD spectra of the twisted tetraene system (8a/ )-12.15 In that case, we have also succeeded in the experimental verification of the absolute configuration theoretically determined, by comparison of the CD spectra of the natural product with those of synthetic chiral model compounds (8a5)-(+)-... 

The CD exciton chirality method has been successfully applied to a variety of natural products to determine their ACs. This method enables one to deduce the AC of a chiral compound without any reference compound, and therefore, it is established as a nonempirical method. The principles of the CD exciton chirality method are explained using the steroidal bis(p-dimethylaminobenzoate) shown below as a model compound, where the nonempirical nature of this method is easily proved.8 11 1... 

As described above, 3,6-bis(/ -Cl-benzoate) 11 was reported as the exception of the CD exciton chirality method. To resolve this problem, in 1978 the steroidal model compound 12 was synthesized.38 Compounds 12 and 11 have the same relative configurations at key positions as shown in 12a and 11a (Figure 13). The CD spectrum of 12 showed a positive couplet in agreement with the positive twist of conformation 12a, indicating that the conformation of the benzoate group is not twisted by an intramolecular H-bond. Since 3,6-bis(/>-Cl-benzoate) 11 and 12 showed CD couplets of the same sign, 11a should have the same AC as 12a as shown in Figure 13. From these results, the ACs of 11, 9, and 7 were reversed.38... 

A subsequent investigation of chirality in such systems allowed the isolation of the regiosymmetric tetramer 61 by means of a model approach involving oxidation of the main dimer, i.e. the 4,4 -biindolyl 52b (03XA1133). Xhe resolution of the enantiomers of 52, 53, 57 (R = COCH3) and all the diasteroisomers of 61 (R = C0CH3, R = H) was achieved and their absolute stereochemistry was deduced by the exciton chirality method. 

Theoretical calculation can also be used to determine the direction of electric dipole transition moments of a chromophore within a complex system (e.g., for the use of exciton chirality method). Although TDDFT calculations could be used for this purpose," the DeVoe polarizability model has also been applied for various compounds, including supramolecular systems. ... 

For polymer 127 with two chiral side chains per monomeric unit, a right-handed helical order of the aggregates has been proposed by interpreting their CD spectra on the basis of the exciton theory and model studies.216,217... 

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