Achiral chromophores

In particular most of the early studies on CPL were based on the incorporation of a luminescent achiral chromophore in a chiral nematic or cholesteric liquid crystal. Chiral nematic liquid crystals (CNLC) are intrinsically birefringent and exhibit a helical supramo-lecular architecture, which is characterized by the pitch length p (Figure 5.11). 

Photochemically Triggered Induced Circular Dichroism in Liposomes When an optically inactive chromophore is subject to the effect of optically active environment, optical activity may be induced at the absorption wavelength of the optically inactive chromophore. This phenomenon of induced circular dichroism(ICD) is often observed in polypeptides bearing various achiral chromophores on the side chain( ). The strong chiral environment caused by the peptide helix structure is responsible for this. Distance from, and orientation to, the chiral field decide the degree of ICD appearing on the achiral chromophore. 

Provided that an optically active molecular aggregate is photochemically perturbed to change the state of molecular alignment, the effect of a chiral environment on an achiral chromophore incorporated in the molecular aggregate will be also altered. It has been known that polypeptides bearing photochromic side groups change their optically active properties as a result of photochromic reaction(10-12). This phenomenon is likely to be related to non-linear photoresponsiveness. 

The Cotton effects may be classified into three types168 those arising from chirally perturbed local achiral chromophores (ketones, /i.y-unsaturated ketones, double bonds, benzoates, aromatic compounds) those arising from inherently achiral chromophores, such as conjugated dienes or a,/3-unsaturated ketones those arising from interaction of the various electric transition moments when two or more chromophores which are chirally disposed are positioned nearby in space, intra- or intermolecularly (exciton chirality method)169. 

For the optical activity of achiral chromophores with a dissymmetric environment, two types of theoretical treatments have been proposed coupled oscillator treatment and one-electron treatment. The charge distribution of the magnetic dipole transition correlates Coulombically with an electric dipole induced in the substituents, and the colinear component of the induced dipole provides, with the zero-th order magnetic moment, a non-vanishing rotational strength. 

Figure B3.5.3 The relation of ellipticity to the differential absorption of circularly polarized radiation. The oscillating radiation sine wave, 01, is proceeding out of the plane of the paper towards the viewer. (A) Plane-polarized radiation is made up of left- and right-handed circularly polarized components, OL and OR, respectively. Absorption by a chromophore in a nonchiral environment results in an equal reduction in intensity of each component, whose resultant is a vector oscillating only in the vertical plane—i.e., plane-polarized radiation. (B) Interaction of the radiation with achiral chromophore leads to unequal absorption, so that combination of the emerging vectors, OL and OR, leads to a resultant that describes an elliptical path as it progresses out of the plane of the paper. The ratio of the major and minor axes of the ellipse is expressed by tan 0, thus defining ellipticity. The major axis of the ellipse makes an angle (q) with the original plane, which defines the optical rotation. This figure thus demonstrates the close relation between optical rotation and circular dichroism.

A more serious explanation was offered recently72. The independent systems perturbation (ISP) approach was used to calculate the CD of the tt 3s and tt -> tt transitions. The ISP approach is interested in a particular transition belonging to an achiral chromophore A the other chromophores in the molecule interact with this transition giving it... 

The chromophores that are prevalent in the molecular structures of natural products are the aromatic ring and the carbonyl functional group. Symmetry elements associated with these functional groups means that they are achiral chromophores, so chirality has to be induced by their structural... 

Chiral molecules with formally achiral chromophore... 

The work just discussed deals with the situation in which the chromophore per se is chiral. This is one of two main classes of chiral molecules recognized by specialists in the field180,181) it is commonly called the case of the inherently chiral chromophore . The second class of chiral molecules are those which have an achiral chromophore in a chiral environment . 

When two identical achiral chromophors a and h are present in the same molecule and form a chiral arrangement, they can interact with each other and alleviate the degeneracy of the transition. In this way the VCD shows a so-called couplet a positive effect closely followed (going to longer wavelength) by a negative one (a positive couplet) or vice versa (a negative couplet). In the equation above i/ is the transition frequency. Tab the separation of the dipoles, and /z the dipole moment. 

For molecules with an achiral chromophore located in a chiral environment that is transparent in the region of the spectrum characteristic of the chromophore, it is advantageous to consider the effect of the chiral environment on the transition moments and Mo—r of the achiral chromophore... 

Finally, the optical activity of polymers that contain a very large number of inherently achiral chromophores in a chiral arrangement may be treated by methods developed in solid-state physics, like the other optical properties of polymers. The resulting theoretical description can be viewed as a generalization of the exciton-chirality model. 

The Induoibility of A. The (2 are functions purely of the achiral chromophore A. It is impossible to estimate these empirically by other techniques, so that the only approach would seem to be through simple model calculations. These are beyond the scope of this paper, so that the inducibility will be discussed in a general way, with a particular emphasis on its role in establishing CD/stereochemical correlations. 

C(5) The relative intensities of the CV of d-d transitions of A2, E symmetry is determined largely by the inducibilities, and thus the nature of the achiral chromophore. 

It may seem that the previous discussion leads to so many restrictions that one despairs at the use of d-d CD as a stereochemical probe at all. The source of this apparent dilemma is that the CD is sensitive not only to the absolute configuration of the chelate system, but also to any variations in the achiral chromophore. Such a negative conclusion is readily alleviated, however, if the restrictions are considered together with established empirical rules. In addition, the sensitivity to the achiral chromophore, though a partial hindrance to the establishment of a simple CD-stereochemical correlation to cover all complexes, is actually an appreciable advantage to the study of the effects of, for example, added anions on the metal-ligand bonding. We now illustrate these features with some examples. 

The anion effect thus effectively leads to a new achiral chromophore, because of the large effects on the charge transfer states. This has been exploited as a method of isolating the A band, and thus effecting an assignment of the absolute configuration. Such effects serve to illustrate how easily the d-d CD can be altered through variations in the achiral chromophore (C(s)). 

Additivity Rules. In addition to these direct CD-stereochemical correlation rules, there are a set of empirical additivity rules determined empirically by Douglas (see, e.g., reference (19) and collated references in Hawkins (2)). An exanple of such a rule is that for cases in which the chelate itself has an asymmetric carbon. The d-d CD contributions from the asymmetric carbon atom and that of the chelate system as a whole are found to be additive. The theoretical justification for this additivity follows directly from the model discussed in this paper, and has been discussed in some detail elsewhere (4). (It is worth noting that we implicitly used this additivity in considering the d-d CD as a sum of contributions from each of the chelates I, with the achiral chromophore adapted to D symmetry.)... 

A and A" is the same achiral chromophore and B-j and Bj have the same configurational relationship to A in the composite complex as in their respective substituent complexes, the circular dichroism of AB-jBj... is simply the summation of the CD s of A B., A"Bj, etc. The purpose of this effort is to exhibit an achiral 1 chromophore upon which exchange of chiral ligands induces negligible perturbation in the configurational relationship of the chelating ligands, and thus to allow the utility of the predicted additivity to be demonstrated experimentally. 

For intrinsically chiral species that are inert enough to be resolved conventionally, the measurement of natural optical activity in crystals has the same advantages as single crystal absorption measurements. In addition, however, it also affords the opportunity to determine rotational strengths of species which do not exhibit optical activity in solution. There are two classes of such materials 1) intrinsically achiral chromophores which crystallize in enantiomorphous space groups, and 2) intrinsically chiral but labile chromophores which spontaneously resolve on crystallization. 

There is a second type of chiral quadruple-bonded species in which the twist is zero or close to zero. " In these cases the M02 unit must be treated as an achiral chromophore and the CD (which is weak) can be explained by one electron static coupling. This theory has also been used to explain the CD of singly bonded Rh2(5 -mandelate)4 (EtOH)2. ... 

The optical activity in valence excitations of chiral metal complexes has been effectively treated using the model of an achiral chromophore (metal ion) in a chiral environment (ligands) and this model appears also appropriate for XAS in view of the core nature of the initial orbital state. The zero-order electric and magnetic transition moments arise from different transitions and must be mixed by some chiral environmental potential (V ). Considering the case of a lanthanide ion, and taking tihe electric dipole transitions for the Z.2,3 edge as Ip—Kj), a first-order perturbation expression for the rotational strength looks like ... 

CD may sometimes be used to deduce the strxicture of a system. This is only really viable when the system can be considered as a collection of chromo-phores (spectroscopically well-defined subunits of a molecule) each of which is only slightly perturbed by the rest of the system. In the rest of this section we shall consider the coupling of two intrinsically achiral chromophores that are chirally oriented with respect to one another. A range of applications of this theory is given in (4). We have to treat eda/mdf transitions separately from... 

When eda transitions occur in two non-identical achiral chromophores, as with the degenerate case, a linear motion of charge at a distance (in another chromo-phore) has a circular (magnetic) component locally. Thus if the two transitions couple, an induced CD signal (4) is found. The main differences between the non-degenerate and degenerate cases are that for the non-degenerate case ... 

The equation describing the octant rule dependence of the dynamic coupling induced CD of an transition is derived as follows (4, 7). One first divides the molecule into an achiral chromophore A composed of the C=0 bond and the two carbon atoms to which it is joined (Figure II). The rest of the molecule is divided into chromophores, C,. which are subunits of the molecule that do not exchange electrons with the rest of the molecule. For a molecule where (apart... 

---