UNIQUE UV-VIS ABSORPTION AND CIRCULAR DICHROIC EXCITON-SPLIT SPECTRA OF A CHIRAL BISCYANINE DYE - ORIGIN AND NATURE

The exciton coupling between two cyanine dyes, characterized by intense and sharp absorption bands in the visible region, has been studied by UV-vis and CD spectroscopy because it provides a unique system of theoretical and applicational interests. Biscyanine dye 4b prepared from (1S,2S)-(+)-trans-1,2-cyclohexanediamine (1) shows two well-separated and strong electronic absorption bands at lambda(max) 550 (epsilon 182 000) and 480 nm (epsilon 191 000). These visible bands are accompanied by strong exciton split CD Cotton effects, lambda(ext) 546 (DELTAepsilon -232) and 475 nm (DELTAepsilon +231); however, the signs of the split Cotton effects are opposite those of the bis p-methoxycinnamide) 2 which exhibits positive first and negative second Cotton effects. The stable conformation UV-VIS and CD spectra of the model biscyanine (1S,2S)-5 as calculated by molecular mechanics and the pi-electron SCF-CI-DV MO method yielded typical exciton-split CD curves with nearly equal negative and positive band areas (''regional sum rule''): visible, lambda(max) 550 (epsilon 157 900) and 474 nm (epsilon 255 200); CD, lambda(ext) 550 (DELTAepsilon -140) and 474 nm (DELTAepsilon +136). The calculation indicated that the sign reversal in the bisignate CD of 4b in comparison with that of the bis(p-methoxycinnamide) 2 was due to a unique conformation of the two cyanine dye side chains; furthermore, it indicated that the large separation of the two bands in the visible and CD spectra was a result of strong exciton coupling between two cyanine dye electronic transitions characterized by their narrow bandwidths and location at long wavelength region. This is the first example in which exciton-split electronic absorption and CD bands appear as widely separated distinct peaks as large as 70 nm in solution.