Symmetry chromophores

A sector rule for the rotatory strength of low-symmetry chromophores such as lactones, lactams, and peptides (Fig. 17) has been described by Ong et al (1977) and Weigand (1979, and references therein) and compared with the quadrant rule (Schellman, 1968). One of the most important conclusions is that the longest-wavelength singlet absorption (based on the perturbation theory) of an ester group and that of an amide arise from basically different... 

Magnetic circular dicliroism (MCD) is independent of, and thus complementary to, the natural CD associated with chirality of nuclear stmcture or solvation. Closely related to the Zeeman effect, MCD is most often associated with orbital and spin degeneracies in cliromophores. Chemical applications are thus typically found in systems where a chromophore of high symmetry is present metal complexes, poriihyrins and other aromatics, and haem proteins are... 

The reduced symmetry of the chromophore, which still contains 187t-electrons and is therefore an aromatic system, influences the electronic spectrum which shows a bathochromic shift and a higher molar extinction coefficient of the long-wavelength absorption bands compared to the porphyrin, so that the photophysical properties of the chlorins resulting from this structural alteration render them naturally suitable as pigments for photosynthesis and also make them of interest in medical applications, e.g. photodynamic tumor therapy (PDT).2... 

Now look at octahedral complexes, or those with any other environment possessing a centre of symmetry e.g. square-planar). These present a further problem. The process of violating the parity rule is no longer available, for orbitals of different parity do not mix under a Hamiltonian for a centrosymmetric molecule. Here the nuclear arrangement requires the labelling of d functions as g and of p functions as m in centrosymmetric complexes, d orbitals do not mix with p orbitals. And yet d-d transitions are observed in octahedral chromophores. We must turn to another mechanism. Actually this mechanism is operative for all chromophores, whether centrosymmetric or not. As we shall see, however, it is less effective than that described above and so wasn t mentioned there. For centrosymmetric systems it s the only game in town. 

A distorted conjugated pair of double bonds is an intrinsically chiral chromophoric system, and its overall chiroptical properties depend on the reduced symmetry of the chromophore itself as well as on the perturbing action of a dissymmetric environment. 

In 4 and 5 the chromophore is planar and the optical activity arises from the lack of a vertical symmetry plane (i.e. that bisecting the diene moiety), owing to the presence of the Ci—CH3 bond, which has no counterpart in the other half of the molecule11. 

The diene chirality rule (hereafter referred to as DR) constitutes a simple tool for correlating the sign of the lowest energy tt —> n transition (] A —> 1B in C2 symmetry) of the distorted diene to the chirality (left or right-handed) of the chromophore. The validity of this rule is based on the assumption that the CD of the distorted chromophore is determined by its intrinsic helicity alone and that external dissymmetric perturbations have only minor effects on the optical activity. 

Mason and coworkers10 studied the chiral bicyclic derivatives 2, 3, 33 and 34, having known absolute configuration. These molecules possess a planar s-cis diene chromophore and formally their chirality is due to the presence of the D or CH3 substituents, which rule out all the symmetry planes. However, it is interesting to point out a peculiar structural... 

Only for 4-R-substituted 1,2,4-triazoles, isoxazoles and 1-alkyl-tetrazoles (Fig. 1), has the Fe(II)N6 spin crossover chromophore been found to consist of six chemically identical heterocyclic ligands. These spin transition materials are of particular interest. Since only a single N-donor ligand is involved in the synthetic procedure, the formation of mixed ligand species is avoided, and hence rather high yields are usually obtained. In addition, the choice of such relatively small heterocyclic ligands favours almost regular Oh symmetry about the Fe(II) ion. This is especially so for low-spin Fe(II). 

Trzaska and co-workers showed a similar propeller mechanism for the formation of helical columns from disclike metallomesogens (29-31).34 These metallomesogens also have C3 symmetry and 30 and 31 are provided with chiral side chains. In the hexagonal columnar mesophase these chiral side chains induce a Cotton effect in the chromophore of the helically arranged core. Heating the mesophase to the isotropic liquid results in the disappearance of the Cotton effect because of the loss of helical order. This effect illustrates the need for the molecules to be positionally ordered in order for the side-chain chirality to be transferred to the supramolecular column. 

The chromophore should have as high symmetry as possible so that polarization of the transition bands is established in the geometry of the chromophores. 

The crystal structure can be considered as a structure regularly stacked with bimolecular layers along the a-axis. Within the bimolecular layer, two molecules related by inversion symmetry face each other in the tail-to-tail fashion with their molecular axes inclined by about 26° to the bilayer surface. This inclination enables the head-to-tail arrangement of azobenzene chromophores as expected from the spectroscopic study. 

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