Cholesteric liquid crystal induced circular dichroism (LCICD). V. Mechanistic aspects of LCICD

Abstract

7656 Figure 4. Energy surface (one-electron contributions) of the butadiene-cyclobutene ground configuration as a function of independent rotational angles $ and $'. Dotted lines: $ = -$' conrotation, @ = +$' disrotation. (Note: @' = +90° is electronically equivalent to $' = -90" and the surface repeats periodically for angles outside the given range.) path in this molecular state lies on a smooth energy slope, with no intervening energy maximum between the two paths. (This is true on either side of the peak value!) This in effect represents a highly unstable situation with strong restoring forces (negative derivatives of the energy with respect to the coordinates) directed toward 4' = -4. Within this simplified model a synchronized disrotational motion appears therefore extremely improbable, not only on the grounds of a higher activation energy.The energy difference between con-and disrotation is primarily controlled by the energy of the new u bond. We expect therefore the same type of qualitative behavior in all reacting states with an appreciable value of the bond order: the energy minimum path is close to the predicted preferred rotation; the opposite rotation is energy wise unstable. The model supports the view of the concerted electrocyclic reaction as simultaneous rotations of the two reacting centers; along the minimum energy path, the rotations are, however, not in perfect synchronization. A twofold axis or a symmetry is then not retained during the reaction.This observation detracts in no way from any previous conclusions as con-and disrotation refer to convenient, but not necessarily physically realized reference paths. It points, however, again to the fact that the bond order criterion is by its very nature a qualitative criterion for the prediction of rotations in electronically controlled electrocyclic reactions. Abstract: The influence of cholesteric liquid crystalline mesophase pitch (degree of helicity), temperature, texture, solute concentration, and cholesteric matrix on the intensity and sign of the cholesteric liquid crystal induced circular dichroism (LCICD) in pyrene was investigated. The spectroscopic information (polarizations of electronic transitions, detection of hidden transitions) provided by the LCICD spectra is also discussed. The LCICD intensity is dependent on pitch of the cholesteric mesophase, temperature, and texture. The sign of the LCICD is dependent on the position of Xa of the cholesteric pitch band relative to the wavelength of absorption, cholesteric matrix properties, as well as the chirality of the mesophase. The molecular ellipticity, [e] (deg cm2/dmol), for pyrene in a single cholesteric matrix is independent of solute concentration over a range in which the concentration was altered by two orders of magnitude. The concentration independence of [e] indicates the lack of a mechanism change over the concentration range studied and that solute-solute interactions do not play an important role in the origin of LCICD for dilute solutions of solutes in cholesteric mesophas...

Phase Behavior and Effect of Enantiomerism on Potassium N -Dodecanoyl Alaninate/Water/Decanol Systems

“…7) indicates that the phase (L a D ) has also helical arrangements like the Ch phase, but the degree of helical twist in the L a D phase is significantly lower (i.e., longer pitch) than in the Ch phase (35). If the L a D phase would be a twophase region consisting of Ch-and L a phases, it would have shown an LCICD band of intensity comparable to that observed in the Ch phase, because the LCICD intensity is almost independent of the concentration of the pyrene dissolved in the mesophase as it was put forward by Saeva et al (35).…”

Induced Circular Dichroism of Anionic Porphyrin TPPS Aggregates in DNA Solutions

“…Therefore, the cholesteric liquid crystal structure is believed to be a good chiral template, which can precisely control the molecular orientation through intermolecular interactions. Many achiral dye molecules can be arranged helically in the cholesteric liquid crystals and show induced CD signals, [8][9][10][11][12][13][14] that is liquid crystal induced CD.DNA is a chiral molecule, which is used as a chiral template to induce many achiral dyes to show induced CD signals. [15][16][17][18][19][20][21] DNA aqueous solution can form cholesteric liquid crystal structure when the concentration is higher than the critical value 22 and can induce the chirality of achiral dye molecules, such as Ethidium bromide, Bisbenzimide, etc.…”

“…Therefore, the cholesteric liquid crystal structure is believed to be a good chiral template, which can precisely control the molecular orientation through intermolecular interactions. Many achiral dye molecules can be arranged helically in the cholesteric liquid crystals and show induced CD signals, [8][9][10][11][12][13][14] that is liquid crystal induced CD.…”

Phthalocyanine, Porphyrin, Cyclodextrin, and Polymer Systems Suitable for Studying by Circular Dichroism, Magnetic Circular Dichroism, and/or Electrochemistry

“…F) Pyrene£-CDx: 86 The band of pyrene at the longest wavelength is polarized along the long axis (Figure 45g). 84 Associated with this band, pyrene in £-CDx (2:1 complex, confirmed by Job's continuous variation experiments) shows a minus-to-plus CD pattern in ascending energy, so that it was confirmed that two pyrene molecules take the form of a left-handed conformer in the cavity of £-CD (Figure 45j). G) Charge-transfer complex formation in CDxs: 87 If we dissolve electron-deficient and -rich compounds in solution, they may form CT complexes.…”

“…D) Pyrenetetrasulfonate: 83 The polarization direction of the absorption bands of pyrene had been well established as short as ca. 240 nm 84 and by us up to ca. 200 nm using the ¢-CDxpyrene system (Figure 45g).…”