Positive exciton coupling

Exciton coupling is also observed in methacrylate derivatives of allylic alcohols151 and in a benzoate derivative of a homoallylic alcohol152. For benzoates of propargylic alcohols of S configuration 11 in which the O-Bz bond is assumed anti to the C-R1 bond, a Dc Voe calculation predicts a positive benzoate Cotton effect, in accordance with experimental153. 

Accordingly, this rotamer provides a positive rotatory contribution due to exciton coupling, although the amplitude of the exciton Cotton effect is smaller than that in the cyclic system, due to conformational averaging . 

FIGURE 15. Typical bisignate exciton coupling CD spectra (upper) and bell-shaped UV (lower) spectra for chromophores with electronic transitions near 300 nm. The shape of the observed CD curve is due to overlapping, oppositely-signed positive and negative CD transitions from electronic excitation into (two) exciton states... 

Exciton-coupled CD8, The systems have two or more chromophores, which are placed in chiral positions to each other. Each chromophore exhibits intense tt—tt transition generating a large electric transition moment , and at the same time, these moments make a large magnetic transition moment... 

The qualitative definition of exciton chirality is very simple (1) if two transition moments constitute a clockwise screw sense, CD shows positive first and negative second Cotton effects. On the other hand, (2) if they describe a counterclockwise screw sense, negative first and positive second Cotton effects are observed. In most cases, intense exciton-coupled CD Cotton effects are observed at the long-axis-polarized transition, and therefore the above results are rephrased as follows ... 

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