Chapter 3: Representative Systems Analysed by the Exciton Coupling Method

A wide range of examples of the use of exciton coupling theory in the conformational analysis of natural and synthetic dimers and polymers are described. Dimers of two identical interacting chromophores represent the most widely studied systems in this regard. The sign sequence observed in the CD spectra for the two bands that arise from a Davydov splitting of an excited state, often referred to as an exciton couplet, has been found to be related to the relative alignments of the interacting chromophores. Molecules in which there is a clockwise twist arrangement in the alignment of the electric dipole transition moments (edtms) of the first and second chromophores are defined as having positive excition chirality based on a plus-to-minus sequence in CD intensity in ascending energy terms, while those with an anticlockwise twist arrangement are defined as having a negative chirality based on a minus-to-plus sequence. A wide range of examples of CD spectra that are dominated by exciton couplets are provided, including 5α-cholestane-2β,3β- and 3β,6β-diol bis(π-dimethylaminobenzoate), (6R,15R)-(+)-6,15-dihydro-6,15-ethanonaphtho[2,3-c] pentaphene, sugar benzoates, haematoxylin (a natural catechol product), oligonaphthalenes, a bisporphyrin system coordinated by a shared chiral axial ligand, biscyanine dye and cyclotriveratrylenes. The analyses of the CD spectra of the helical structures of N-alkylated poly(p-benzamide)s are provided as examples of how exciton coupling theory is applied to synthetic polymer. The exciton coupling of non-identical chromophores is also examined with allylic alcohols and conjugated enones, esters and lactones used as examples.