Chiral chromophores

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. 

K. Le Barbu Debus, N. Seurre, F. Lahmani, and A. Zehnacker Rentien, Formation of hydrogen bonded bridges in jet cooled complexes of a chiral chromophore as studied by IR/UV double resonance spectroscopy. 2 Naphthyl 1 ethanol/(methanol)n j 2 complexes. Phys. Chem. Chem. Phys. 4, 4866 4876 (2002). 

The isolated enantiomers S (M ) and R (Mr) of a chiral molecule M exhibit the same spectral features since their physical properties are identical. However, their aggregation with a chiral chromophore of defined configuration (Cr/s) leads to the formation of two diastereomeric complexes with different spectral properties, i.e., and [C /yM ]. The lcR2PI spectroscopy is able to discriminate between Mj and by measuring the spectral shift of the diastereomeric [C /yM ] and [Cj5/5-Mj ] complexes with respect to that of the bare chromophore Cr/s- It is convenient to define the diastereomeric clusters as homochiral when the chromophore and the solvent have the same configuration, and heterochiral in the opposite case. 

The mass resolved lcR2PI spectrum of the bare chiral chromophore Cr = (R)-(+)-l-phenyl-1-propanol, shows three intense signals at 37577 (A), 37618 (B), and 37624 cm (C) in the electronic Sj — Sq band origin region. A similar triplet falls at 38106, 38148, and 38155 cm . This pattern is common to substituted arenes and is interpreted as due to three stable conformers. Quantum chemical calculations at the RF1F/3-21G and B3LYP/6-31G levels of theory confirm this hypothesis. 

The photoisomerization of several copolymers was studied, in order to determine the effects of the structure and switching of the chiral side chain on the helicity of the main chain. A delicate balance of parameters was found, including separation and nature of the stereocenters, solvent, and concentration of azobenzene moieties J77 Stereoselectivity was often greatly enhanced if the chiral moieties were closer to each other. Accordingly, it was found that the incorporation of the stereocenter into a short, two-carbon spacer resulted in much more pronounced helical preference, as well as CD effects at lower chiral chromophore concentrations. The greater helical twist and improved thermal stability of the cis form (half-life 40h at RT) are notable features. 771 It was also found that the relationship between the trans-cis... 

In making decisions about which analytes are CD active it is not always a simple matter to inspect a molecular formula and to know for certain that the chiral center and the chromophore are mutually located in a way that will produce observable activity. And furthermore, although the molecular structure might suggest that a chiral chromophore is present, the substance might only be available as a racemic mixture in which case it is not optically active and therefore undetectable by CD. Trial and error is often the only proven criterion that works. 

One of the advantages of supramolecular chemistry over the covalent molecular approach resides in the possibility of inducing and/or transferring properties from singular molecules to self-organized multi-molecular assemblies. Chirality is one of these properties it stems from the electronic level (the helical nature of the electron rearrangement during a transition in chiral chromophores) and spreads up to the three-dimensional spatial disposition of atoms, or molecular components in molecular or supramolecular entities, respectively. 

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 . 

Chiral chromophores are discernible in samples with multiple overlapping absorbance bands. 

Samples must contain a chiral chromophore that absorbs in the near ultraviolet to infrared region. 

As (-H )-carvone (143) has the same absolute arrangement of the carbonyl and cyclopropyl moieties as 141, but opposite (n, tc ) CE, XXVIII has been ruled out as a conformational representative of 143 . For 143 with the gem-dimethyl moiety steric interactions will be severe in the conformations XXIX and (especially) XXX. XXVII is assumed to be the most favorable representation of 143. In XXVII the carbonyl n and cyclopropane p orbitals are almost orthogonal. Hence, there will be no contributions from an inherently chiral chromophore and the observed (n, n ) CE can be rationalized by the sum of the normal octant-like contributions of the C(6)methyl and gem-dimethyl group In XXVII all components of the gem-dimethyl group lie nominally in a (+) back... 

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... 

Fig. 2b is an idealized illustration of a single, uncomplicated Cotton effect. In reality, the occurrence of a complete curve in the electronic spectrum is rare. Complete dispersions are more likely to be observed in the vibrational spectral range because of the increased spectral resolution. However, even there, dispersions are too often complicated by extensive band overlap. The same is true for electronic spectra where hidden absorption bands coupled vibronic excitations and interferences from bands associated with other chiral chromophores contribute to producing anomalous ORD curves that are so complex they have little utility in quantitative analytical applications (Fig. 3). 

Those with chiral chromophores that absorb in the near-UV. 

Inherently chiral chromophores Chromophore itself is twisted and chiral and, therefore, the 7t—7t and/or n—it transitions become CD active, because both electric transition moment and magnetic transition moment < M 0> take nonzero values. The CD intensity of this group is stronger than that of group (a). 

The helicenes are unique with respect to their very high rotation values, which are a consequence of the inherent chiral chromophor enclosing the whole molecular skeleton. This also attracts interest with respect to the use of helices as chiral inductors for asymmetric syntheses. 

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