Symmetric chromophores

In the early eighties, Ciba-Geigy discovered a new type of heterocyclic pigments, based on a symmetric chromophor, the 1,4-diketopyrrolo(3,4c)pyrrole system (141) (R = alkyl, aryl, Cl, Br, CF3) ... 

The binding of a symmetric chromophore to a protein or nucleic acid often induces CD in that chromophore. For example, the bands of enzyme-bound pyridoxal and pyridoxamine phosphates shown in Fig. 14-9 are positively dichroic in CD, but the band of the quinonoid intermediate at 20,400 cm-1 (490 nm) displays negative CD. When "transimination" occurs to form a substrate Schiff base (Eq. 14-26), the CD is greatly diminished. While the coenzyme ring is known to change its orientation (Eq. 14-39 Fig. 14-10), it is not obvious how the change in environment is related to the change in CD. 

One-electron theory is typically applied to molecules in which the chromophore is only part of the whole molecule. This chromophoric part is locally symmetric, but the entire molecule is dissymmetric. It is the dissymmetric perturbation of the locally symmetric chromophore by the rest of the molecule that induces the optically active absorption bands. An example of such a molecule is (+)-3-methylcyclohexanone, in which the lowest energy transition is the n-it transition which is centered on the carbonyl group [31]. Locally, this carbonyl group is symmetric, and without the perturbing effects of the rest of the molecule, it would be optically inactive like formaldehyde. However, the coulombic field of the methyl group statically perturbs the carbonyl group and the subsequent mixing causes the transition to become... 

In Ref 105 the (n, n ) circular dichroism of the tricyclic ketones 152-155 was analyzed with the aim to see whether the systems must be viewed as inherently chiral chromophores or rather inherently symmetric chromophores, respectively. If they fall more nearly into the inherently symmetric classification the octant rule should be applicable. As a basis for the classification the order of magnitude of the rotational strengths was used together... 

Referring to the very similar rotational strengths of a-fenchocampherone (159) and spiro(cyclopropane-l,7 -norbornanone) (155) (Figure 13 Table 9) it was suggested that 155 has an inherently symmetric chromophore in which the normal octant perturbers dominate the contributions to the chirality of the (n, k ) transition. 

C. Symmetric Chromophores with Large Two-Photon Absorptivities... 

Symmetrical chromophores with the structure 94 bear as electron-rich aromatic bridge a thiophene moiety substituted at the end with electron-withdrawing benzothiazole groups. In this A-n-A chromophore, TPA significantly... 

The chromophore of 2,3-naphthalenedicarboximide exhibits an intense Bb transition around 260 nm, which is polarized along the long axis of the chromophore. This C2v-symmetrical chromophore is ideally suitable for the CD exciton chirality method because the long-axis-polarized transition moment is exactly parallel to the C—N bond of amine moiety. This is an advantage of the 2,3-naphthalenedicarboximide group and hence the use of this chromophore is highly recommended for primary amines. 

In a previous paper (9) the angular-overlap model, based upon the operator A of Eq. (8), apart from the term e(r), was applied to a series of symmetrical chromophores. In Table 8 the results are given for the same chromophores, based upon the barred operator A of Eq. (16b). 

The monocations and anions of porphyrins produce complicated electronic spectra [Falk (56), Salek (755)], which indicate a less symmetric chromophor than in the neutral base or the dications and dianions. One might therefore conclude that the "uneven proton is localized at one nitrogen atom and not sitting in the center of the porphyrin cavity like a metal ion in the metalloporphyrins. 

The examination of additive contributions (1, 3, 5, 6, ) such as configurational, conformational, and vicinal effects of optically active ligands has been useful in the correlation of stereochemical effects and CD spectra. The ligand-polarization model (20, 21, 22, 24) of optical activity depends upon the polarizability of the perturbing groups which constitute the dissymmetric environment around the symmetric chromophore. Phenyl subsitutents which have large anisotropic polarizability can make contributions with signs reversed from those expected (20). 

The comparison of optical rotatory dispersion (ORD) or circular dichroism (CD) curves gives more reliable correlations. If the molecule contains an asymmetrically perturbed symmetric chromophore, the sign of the Cotton effect reflects the stereochemistry of the environment of the chromophore. 

For many inherent symmetric chromophores sector rules have been derived. With an increasing... 

The ribose phosphate backbone of nudeic acids does not absorb significantly above 180 nm. Therefore when we meastire CD of nucleic adds, the most important contributors are the bases the purines and the pyrimidines. In themselves, both these kinds of bases arc symmetrical chromophores. However, they become optically active when attached to a ribose sugar means of an N-glycosidic bond. Their optical activity increases further when they assume helical structures. 

In light of the above discussion, the carbonyl group, or for that matter any symmetric chromophore whose associated optically active transitions are readily amenable to investigation, becomes an ideal probe with which to search out the structural subtleties of a particular molecular framework. >2 > In addition, and perhaps more important, the rotational strengths of a chromophore provide the theoretician with experimentally accessible quantities with which to test his hypotheses as to the molecular origin of optical activity. Unfortunately, their accessibility is not all one might desire. 

For the case of symmetric chromophores, what appears to be a fruitful theoretical approach is perhaps best illustrated by way of reference to a concrete example, and it will probably come as no surprise to the reader that the example we have chosen to use involves the carbonyl group of saturated ketones. From the point... 

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