Left-handed CPL

Helical undulators build on this principle by using two orthogonal magnetic field arrays [82, 83]. These permit transverse excursions in perpendicular x and y directions. If the arrays have a relative longitudinal shift, this introduces a phase to the induced perpendicular excursions and when the phase is 90° the electron trajectory can follow left- or right-handed corkscrew paths The emitted radiation is correspondingly right- or left-handed CPL. 

The P- and M-cis-syn-conformers give photoreactions, whose rates are proportional to el when left-handed CPL is used. Therefore, a high optical yield requires a high g-factor 69 After the excitation cyclization occurs leading to a DHP, which is readily oxidized to helicene. The precursor remains racemic as racemization in the ground state, is easy. Racemization in the excited state, however, may be slow, compared to cyclization. According to Kagan 69) this mechanism explains the asymmetric synthesis well. 

Indeed, direct irradiation of racemic autocatalyst 12 by left-handed CPL and the subsequent asymmetric autocatalysis produces highly enantioenriched (S)-alkanol 12 with > 99.5% ee (Scheme 13). On the other hand, irradiation with right-handed (r) CPL instead of /-CPL, formed (R)-12 with > 99.5% ee. The process provides direct correlation of the handedness of CPL with that of the organic compound with high enantiomeric excess [84],... 

This result supports the view that diverse ways exist to obtain chiral biomolecules via CPL or chiral inorganic or organic crystals combined with asymmetric autoctalysis. Kenso Soai and his team studied the effect of the structure of the substituents at position 2 of the pyrimidyl alkanol (Shibata et al. 1996). They found that using 2-alkynyl-pyrimidyl alkanol after three rounds of asymmetric autocatalysis, an astonishing amplification factor of 630,0000 was reached. In the reaction, either (+) or (—) crystals of Cytosine serve as initiators that were formed spontaneously by stirring. In the Soai reaction of chiral amplification, it is crucial that dimers of the O-Zinc diisopropyl intermediate are the active catalysts Racemic pyrimidine alcohols subjected to photolysis with either right- or left-handed CPL produced an ee of one isomer as shown in Fig. 3.4. 

Figure 2 shows the structures of four classes of so-called overcrowded alkenes (see Section 5.3.1), designed as molecular components for a chiroptical switch based on CPL irradiation.1191 Thanks to unfavorable steric interactions around the central ole-finic bond, the molecules are forced to adopt a helical shape. The chirality in these inherently dissymmetric alkenes - denoted M and P for left-handed and right-handed helices, respectively - therefore originates from distortion of the molecular... 

Scheme 4 Photochemical interconversion of P (right handed) and M (left handed) helicity in 12 by irradiation with l- or r-CPL [31]...

Figure 9 Schematic representation of the shift between right- and left-handed helices of a poly isocyanate with bicycloketone chromophore pendants subjected to irradiation with r- or l-CPL or noncircularly polarized light (non-CPL).

The measured quantities in CPL spectroscopy are the difference of intensity (A/) of the left handed (/l) and right handed (7r) circularly polarized components of the emitted light, and the total luminescence intensity (I) emitted by the sample ... 

The peculiar properties of lanthanide luminescence also make these ions attractive in the generation of CPL. In this technique, a pulse of light is used for excitation and circular polarization of the emitted light is measured using a photomodulator, allowing one to select alternatively left- and right-handed circularly polarized light. 

A study of the structures of molecules requires techniques that are sensitive to the three-dimensional shapes, and ideally the property of handedness. Handed molecules that have been separated, at least in part, from their antipodes, are said to be optically active in that they behave di rently toward the right and left components of circularly polarized light (CPL). All biological molecules of any complexity have this characteristic. The di rential interaction with CPL may manifest itself as a differential scattering, which is measured as a non-zero rotation of the plane of polarization of plane polarized light, measurable with a polarimeter at specific wave lengths, or as a function of wave length, X., as optical rotatory dispersion (ORD). 

The differential emission of left and right circularly polarized light from luminescent molecular systems is called circularly polarized luminescence (CPL), and is at the basis of the corresponding spectroscopic technique (CPL spectroscopy) [23-25]. CPL spectroscopy should not be confused with fluorescence detected circular dichroism (see Sect. 6.1.6) in the latter technique the differential absorption of the circularly polarized components is detected through fluorescence measurements, owing to the different extent of photoexcitation that left- and right-handed light can produce on a chiral molecule. 

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