Bisignate cotton effect

Chiral aggregates of 40 were subjected to multiple cycles of heating (50°C) and cooling (-10°C) to investigate the thermal stability of the aggregate chirality.29 In the CD spectra of the aggregates at 60%/40% THF/methanol, the bisignate Cotton effect observed at 20°C completely disappears at 50°C... 

Figure 4.23 Chiral orientation of the preferred helicity of single molecules of the polysilane for the origin of bisignate Cotton effect.

Upon threading the oligosilane molecules into the asymmetric y-cyclodextrin cavity to form a pseudorotaxane-type aggregate,335 a bisignate Cotton effect became apparent at about 290 nm, indicating induced PSS helicity. The reason for the bisignate nature of the signal was not clear. 

Often the UV absorption tails into the visible due to light scattering by the microparticles, although the Amax is usually very close to that in solution. The aggregate CD spectrum, however, is markedly different from the solution-state spectrum, showing an intense bisigned Cotton effect coincident with the UV due the Davidov coupling. 

With the same (.y)-2-methylbutyl side chain in each case, the solution CD was, as expected, almost identical, with positive Cotton effects. However, on addition of MeOH to form aggregates, while 127 showed a negative bisigned Cotton effect, 128 showed a positive bisigned Cotton effect, as shown in Figure 53. 

Table XI. Conformational structures (a and b) and Newman projection diagrams (c and d) of (15,25) and (lR,2R)-cyclohexane diol (a) and their bis-p-dimethylaminobenzoate derivatives (b). CD data for the bisignate Cotton effects of the latter are shown below.

The CD spectra of mono-substituted [2.2]metacyclophanes (24) were also reported [37]. All the metacyclophanes exhibited very similar bisignate Cotton effects, except for the acetyl derivative, in which the additional Cotton effect induced by the carbonyl group was observed. 

To determine the ACs of acyclic 1,2-glycols, the CD exciton chirality method has been applied to their dibenzoates or bis(2-anthroates), which show typical bisignate Cotton effects (see Section 9.04.4) as exemplified in Figures 27 and 28.72,73 Acyclic dibenzoates or bis(2-anthroates) can rotate around the bond connecting two benzoate or 2-anthroate chromophores, and therefore the CD sign depends on the conformational equilibrium. From the data of many examples, general rules were derived as shown in Figures 27 and 28. 

The di-LCPAs in solution and in cast film showed monosignate and bisignate Cotton effects in their CD spectra, respectively. These results suggest the formation of intrachain and interchain helical structures, respectively. In spite of this, the di-LCPAs in solution and in cast film exhibited no CPL [21]. However, (/f)-/(S)-Poly-4 in an N -LC film showed bisignate CPL bands centered around the PL emission band (Fig. 11.12a). (/f)-Poly-4 exhibited positive and negative CPL bands at wavelengths shorter and longer than 530 nm, respectively. (S)-Poly-4 exhibited CPL bands with handedness opposite to those (/ )-Poly-4. The CPL spectra of (/ )-/(S)-Poly-4 in an N -LC film showed gem values of 1.8 x 10 and... 

Figure 3 An example of a VT-CD experiment. A dithienopyr-role polymer 2 shows a temperature-controlled random coil-helix transition. The bisignate Cotton effect observed at lower temperatures indicates the helical chiral ordered conformation, while the CD becomes almost silent at higher temperatures, which suggests the absence of the chiral orientation of chromophores. Schematic representations of a random coil and a helix conformation of 2 are also shown. (Reprinted with permission from Ref. 8. Copyright 2008 American Chemical Society.)...

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