Intrinsically chiral chromophore

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. 

The origin of optical activity in molecules often reduces to the question of how the molecule acquires the electronic properties expected of a chiral object when it is formed from an achiral object. Most often an achiral molecule becomes chiral by chemical substitution. In coordination compounds, chirality commonly arises by the assembly of achiral units. So it is natural to develop ideas on the origins of chiral spectroscopic properties from the interactions of chirally disposed, but intrinsically achiral, units. Where this approach, an example of the independent systems model, can be used, it has obvious economic benefits. Exceptions will occur with strongly interacting subunits, e.g., twisted metal-metal-bonded systems, and in these cases the system must be treated as a whole—as an intrinsically chiral chromophore. ... 

In a recent review on predetermined chirality at metal centers, Khof and von Zelewsky described the chiral quadruply bonded species A and A-[Mo2Cl4(5,5-dppb)2] (where dppb = 2,3-bis(diphenylphosphino)butane) as M-configurational double helices. Such twisted multiply bonded complexes are paradigm examples of intrinsically chiral chromophores where the chiral center is not located at the metal atom but embraces the whole twisted M02X4P4 unit. The CD spectra of a range of chiral quadruply bonded compounds, with a variety of bidentate and monodentate chiral ligands (phosphines and amines) have been reported and can be explained by a simple metal localized theory. ... 

For alkyl-substituted A-nitrosazetidines, it was shown that the CD spectra can be interpreted on the basis of conformational diastereoisomerism, taking into account the nonplanarity of the nitrosazetidine chromophore137. For a particular configuration, the CE sign of the n - tt transition in the 350-400 nm region is determined by the intrinsic chirality of the chromophore and obeys a spiral rule, the same as that for nonplanar nitrosaziridines where the absence of the local plane of symmetry in the nitrosamine chromophore does not permit the use of the planar sector rule 148. 

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. 

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... 

This helicity rule was criticized in 1968 by Kuriyama (36). He demonstrated that for small torsion angles in enones the chirality contribution can be neglected by an electronic interaction of the type, observed in intrinsically symmetric chromophores, which are asymmetrically perturbed. This reasoning is particularly valid when a heteroatom is homo-conjugated with the unsaturated chromophore. 

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. 

The rotational strength calculated for I is as large as that of a butadiene twisted by 20°. In II, with an out-of-plane methyl, R increases by a factor of about 2. This shows that the contributions to R of dissymmetric substituents of chiral cisoid dienes may be comparable to and even outweigh the contributions arising from the intrinsic dissymmetry of the chromophore. 

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). 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

The power of circular dichroism (CD) spectroscopy is unfortunately limited to the study of chiral molecules having accessible chromophores, and cannot be applied to the study of achiral compounds. However, a great many compounds of interest are not intrinsically dissymmetric, so the induction of chirality into such species is a fundamental requirement for the performance of CD studies. In the present work, investigations into the CD induced in achiral molecules by the action of chiral agents is reviewed. 

The measurement of the time dependence of gium may be used to probe various chiral aspects of excited state energetics, molecular dynamics, and reaction kinetics. Although there are some time-dependent circular polarization effects due to molecular reorientations that parallel time-dependent linear polarization measurements, the most interesting studies are those that involve the time-dependence of intrinsic molecular chirality. For a sample containing one chiral luminescent lanthanide chromophore, it might be the case that there are processes that affect chirality occurring on the same time scale as emission that could be probed by time-dependent CPL. To date, however, there have been no reports of such studies, and all of the time-dependent CPL measurements have involved racemic mixtures. 

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