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Magneto-chiral dichroism (MChD) is an interesting phenomenon in which the absorbance of a chiral molecule depends on the magnetic field direction. As the MChD of two enantiomers is opposite in nature, MChD has received a considerable attention not only in magneto-optical devices but also for new asymmetric synthetic methods and as an explanation for the origin of the homochirality of life. Recently, several experimental observations of MChD have been reported in aromatic π-conjugated systems. In this review, we introduce these MChD observations, and discuss the theoretical explanations of the π-electronic properties of aromatic π-conjugated systems, such as the orbital angular momentum and the exciton chirality. Furthermore, the possibility of using MChD for the photoresolution of aromatic compounds is discussed.
© 2014 Optical Society of America
1. Introduction
Magneto-chiral dichroism (MChD) is one of magneto-optical effects in which the absorption coefficient of a chiral molecule for an unpolarized beam of light changes depending on whether an externally applied magnetic field is parallel or antiparallel to the propagation direction of the beam of light [1,2]. This phenomenon is of great interest as the MChD of two enantiomers is identical magnitude, but opposite sign. MChD has been given attention in the fabrication of magneto-optical devices and for use in new asymmetric synthetic methods; MChD is also a possible explanation for the origin of the homochirality of life [3–11].
The study of MChD originated from theoretical research. Wagniére and Meier calculated the magnetically induced changes for the absorption and emission rates of chiral molecules [1]. Based on these results and molecular theory, Barron and Vbrancich showed that the absorption coefficient of a chiral system for an unpolarized light is different when an externally applied magnetic field is parallel or antiparallel to the propagation direction, and an opposite effect was predicted for two enantiomers [2]. This effect was named MChD. The matrix elements resulting in circular dichroism (CD) and magnetic circular dichroism (MCD) intensities are similar to those for the MChD intensity, which correlates with the transition electric dipole moment, the transition magnetic dipole moment, and the transition electric quadrupole moment, in addition to the magnetic dipole moment between excited states [4,12]. The CD intensity corresponds to an imaginary part of the cross product of the transition electric dipole moment and the transition magnetic dipole moment, and chiral molecules show CD. On the other hand, the MCD intensity correlates with the transition electric dipole moment and the magnetic dipole moment between excited states, and the MCD is a property of all matter. Therefore, MChD is expected to be experimentally observed for chiral compounds that show both strong CD and MCD. In fact, the magnitude of gMChD can be roughly estimated by the product of gCD and gMCD (g = ΔAbs/Abs, where ΔAbs and Abs denote the absorption difference and total absorption.) [4].
Following these theoretical predictions, MChD was first observed in the 5D0→7F1,2 luminescent transition of a europium(III) complex [3]. Subsequently, several MChD observations were reported for different metal compounds [4–7]. In metals, this effect mainly originates from the fact that MCD, which is one of the important origins of MChD, is intensified by the d (or f) orbital-based degeneracy and angular momentum of the metals. Recently, several experimental observations of MChD have been reported for aromatic compounds that are correlated well with living beings. These MChD observations focused on both the orbital angular momentum of an aromatic π-conjugated system and the exciton chirality [8–10].
In this review, we describe the experimental observation of MChD for several aromatic compounds and provide theoretical explanations for the π-electronic properties of the aromatic compounds, including the orbital angular momentum and the exciton chirality. In addition, we discuss the possibility of photoresolution based on the MChD of aromatic compounds.
2. Theoretical background
2-1. Orbital angular momentum of aromatic π-conjugated systems
The MCD intensity correlates with the intensity of MChD. Although MCD effects are similar to CD effects, the origins of MCD and CD, which are Zeeman effects and molecular chirality, respectively, are completely different. If we consider diamagnetic aromatic compounds, the MCD intensity is proportional to the orbital angular momentum between the excited states. The magnitude of the orbital angular momentum is correlated with the magnetic quantum number, ml. Similar to atomic orbitals, π-electronic orbitals of aromatic π-conjugated systems are quantized by ml (Fig. 1(a)).Although the maximum ml values of d-orbitals (ml = 0, ± 1, ± 2) and f-orbitals (ml = 0, ± 1, ± 2, ± 3) are 2 and 3, respectively, those of π-electronic orbitals increase as the size of the π-conjugated systems is increased [13,14]. For example, the ml values of the HOMOs and LUMOs of C16H162-, which is a simple model of a porphyrin analogue, are ± 4 and ± 5, respectively. In this case, the orbital angular momentum of the excited states corresponds to ML = ± 1 or ± 9 (Fig. 1(b)). Thus, strong MCD signals are observed for the absorption band which has ML = ± 9 (Figs. 1(c)-(d)). Therefore, large aromatic π-conjugated systems, such as porphyrin analogues, are suitable for the observation of strong MCD and MChD signals.
Fig. 1 Explanation for the intensification of MCD by the orbital angular momentum of π-conjugated systems. (a) The relationship between atomic orbitals and molecular orbitals. (b) The energy state diagram of absorption and MCD for π-conjugated systems. (c) UV-vis and (d) MCD spectra of the protonated form of meso-tetrakis(4-sulfonatophenyl)porphine (H4TPPS4). Adapted from [11].
2-2. Exciton chirality
The CD intensity is also correlated with the MChD intensity. The CD intensity is a function of an imaginary part of the cross product of the transition electric dipole moment and the transition magnetic dipole moment. In conventional systems, CD transitions are very weak because the electric dipole moment and the magnetic dipole moment arise from during linear and rotational electronic motions, respectively. When a dimer is formed by two chromophores (Fig. 2(a)), the low exciton and high exciton states are generated owing to the exciton interaction between the two chromophores (Fig. 2(b)). When the transition electric dipole moments of the chromophores are twisted in a chiral dimer, the transition magnetic dipole moment is approximated by the vector products of transition electric dipole moments and position vectors, i.e. m = -iπ(E1 R1 × μ1 + E2 R2 × μ2), where Ei, Ri and μi denote the transition energy, the position vector and the transition electric dipole moment for the unit i, respectively. Thus, strong CD intensities can be observed for a system, such as an aromatic compound whose transition electric dipole moment is very intense. This phenomenon is called exciton chirality [15,16]. Because the CD sign of the low and high exciton states is opposite, dispersion-type CD spectra are seen for conventional chiral dimers. Furthermore, the absolute configuration of chromophores in the dimer can be determined by analyses of the CD spectra. When aromatic compounds with large transition electric dipole moments, such as porphyrin analogues, are twisted, the CD and MChD intensities are strengthened.
Fig. 2 Explanation of the exciton chirality using 1,1ʹ-binaphthyl as a model system. (a) Molecular structures of a pair of enantiomers. (b) The energy state diagram of the CD transitions for one of the enantiomers.
3. Magneto-chiral dichroism of aromatic π-conjugated systems
3-1. Magneto-chiral dichroism of organic compounds
Ishii and associates attempted to observe the first MChD of organic compounds that are correlated with living beings. The chiral J-aggregates of water-soluble porphyrins were employed in order to obtain the large orbital angular momentum and exciton chirality [8]. The chiral J-aggregates of the protonated form of meso-tetrakis(4-sulfonatophenyl)porphine (H4TPPS4) were formed by adding chiral tartaric acid, which induces Coulomb interactions between the positively charged pyrrole protons and the negatively charged sulfonate groups (Figs. 3(a)-(b)) [8,17,18]. The formation of the J-aggregates causes a considerable red-shift of the Soret band (π–π*) to 490 nm owing to the exciton interaction between the H4TPPS4 constituents (Fig. 3(f)). A very intense CD signal is observed in the J-band region because of the exciton chirality; positive/negative and negative/positive CD spectral patterns are induced by the addition of L- and D-tartaric acids, respectively (Fig. 3(g)). Density functional theory (DFT) calculations (B3LYP/6-31G*) indicate that the porphyrin units can interact to form left-handed helical, planar, or right-handed helical structures (Figs. 3(c)-(e)). The right- or left-handed helical structure is formed preferably when the pyrrole protons in the upper and lower porphyrin, which interact with the sulfonic acid functional groups in the lower and upper porphyrin, respectively, are both located at the right- or left-hand side. This structure is preferred because the Coulomb interactions between the sulfonic acid groups and the far pyrrole protons are more favourable owing to steric hindrance. On the other hand, the planar structure is generated preferably when the pyrrole protons involved in the interaction are located on the right- and left-hand sides of the upper and lower porphyrin, respectively. For a chiral tetramer adopting the optimized helical and planar structures, the experimentally obtained dispersion-type CD spectral pattern can be reproduced by calculations based on the exciton chirality method. Because of the large orbital angular momentum of the porphyrin unit, an intense, integral-type MCD signal (Faraday B term) is observed at the J-band (490 nm) of the chiral J-aggregates of H4TPPS4, which indicates the non-degeneracy of the J-band (Fig. 3(h)). When the J-aggregates are prepared by adding L-tartaric acid, a sharp, positive MChD signal is observed at 490 nm; the MChD signal of the J-aggregates prepared using D-tartaric acid is observed at the same wavelength, but the sign is negative (Fig. 3(i)). The peak position (490 nm) of the MChD spectra is identical to that of the spectrum obtained from the product of the CD and MCD spectra. This finding was consistent with previous reports [1,2,12], and thus, the presence of MChD in organic compounds was demonstrated for the first time.
Fig. 3 MChD of the chiral J-aggregates of H4TPPS4. The molecular structures of (a) H4TPPS4, (b) the J-aggregates of H4TPPS4, and the optimized H4TPPS1 dimer ((c) left-handed helical, (d) parallel, and (e) right-handed helical structures.), as well as (f) UV-vis, (g) CD, (h) MCD, and (i) MChD spectra of the chiral J-aggregates of H4TPPS4. The chiral J-aggregates of H4TPPS4 were prepared by the addition of L-tartaric acid (red line) or D-tartaric acid (blue line). Adapted from [8].
3-2. Magneto-chiral dichroism of light-harvesting antenna
In addition to the MChD observed for chiral J-aggregates of water-soluble porphyrins, it is important to confirm the presence of MChD for the π–π* transitions by demonstrating its existence in other systems. The observation of MChD is important, not only for the clarification of the asymmetry in biological systems, but also for the development of novel functions, such as asymmetric synthetic methods and magneto-optical devices. Ishii and associates showed the second example of MChD for π−π* transitions using the chiral J-aggregates of zinc chlorins (ZnChls, Figs. 4(a)-(b)) [9]. This supramolecular system is a model compound for light-harvesting (LH) antennas in green photosynthetic bacteria, i.e. chlorosomes, which are formed through the self-assembly of a large number of bacteriochlorophyll molecules without the assistance of proteins. In this system, the Zn ion was employed as the central metal ion instead of a Mg ion, because Zn chlorins are more stable than Mg chlorins.
Fig. 4 MChD of light-harvesting antenna. The molecular structures of (a) ZnChl, (b) the J-aggregates of ZnChl, and (c) the optimized ZnChl dimer, as well as (d) UV-vis, (e) MCD, (f) CD, and (g) MChD spectra of the chiral J-aggregates of ZnChls. The red arrows indicate the transition electric dipole moments of the ZnChl constituents at the Qy band. Adapted from [9].
UV-vis, MCD, and CD spectra in the Q band region are shown in Figs. 4(d)-(f) for the chiral J-aggregates of ZnChls. In the UV-vis spectrum of the chiral J-aggregates of ZnChls, the Qy band is remarkably red-shifted to 729 nm owing to the exciton interaction between the ZnChl constituents, whereas the shift of the Qx band is very small. This result indicates that the long axis of the J-aggregates is nearly parallel to the transition electric dipole moment of the Qy band. Thus, the J-aggregation increases the energy separation between the Qy and Qx bands. The MCD intensity for the Qy band of the J-aggregates, which originates from the large orbital angular momentum, is one third weaker than that of the corresponding monomer because the Faraday B-term is inversely proportional to the energy separation. DFT calculations (B3LYP/6-31G*) for the ZnChl dimer indicate that the 31-methoxy group of one ZnChl coordinates to the Zn ion of another ZnChl. In addition, the transition electric dipole moments of the Qy band are calculated to be almost parallel to the long axis of the dimer, which is consistent with the observed J-band. The chiral J-aggregates of ZnChls show an intense, inverse-S shaped CD signal in the J-band region (Fig. 4(f)). The inverse-S shaped CD spectral pattern originates from the exciton chirality owing to the twisted configuration between the chromophores with a large transition electric dipole moment; this configuration is consistent with the optimized structure (Fig. 4(c)). A positive MChD signal is seen at 733 nm (Fig. 4(g)), and this peak position is identical to that of the spectrum obtained from the product of the CD and MCD spectra, similar to that observed for the chiral J-aggregates of H4TPPS4. This second example of MChD for the π−π* transition emphasizes that, on the basis of the π-electronic properties, such as the orbital angular momentum and exciton chirality, MChD should be observed in aggregates of conventional organic aromatic compounds. From the viewpoint of photosynthesis, this observation of MChD in the LH antenna model compound indicates the possibility of MChD occurring during the LH process, which is attractive as a novel magnetic field effect on photosynthesis [19–26].
3-3. Magneto-chiral dichroism of single-molecule magnets
Recently, Jiang and associates reported the third example of MChD for π−π* transitions using chiral rare earth triple-decker complexes comprised of one phthalocyanine and two porphyrin ligands, which show single-molecule magnet (SMM) behaviour [10]. Rare earth double-decker complexes comprised of two phthalocyanine ligands are known to show the SMM behaviour [27]. This study focused on Dy2[Pc(OBNP)4](TClPP)2 (Fig. 5(a)). The chirality in this complex is induced by the binaphthyl substituents in the phthalocyanine unit. In the UV-vis spectrum of this complex, Soret bands (374 and 420 nm) and Q bands (497, 612, and 1000 nm) were observed (Fig. 5(b)), which are characteristic of rare earth triple-decker complexes comprised of one phthalocyanine and two porphyrin ligands. Intense CD signals are observed below 500 nm (Fig. 5(c)), which indicates that the peripheral chiral binaphthyl units induce the CD signals of the porphyrin and phthalocyanine chromophores, owing to the exciton chirality. The MCD spectrum of the Dy2[Pc(OBNP)4](TClPP)2 complex shows dispersion-type Faraday A terms, which originate from the orbital angular momentum of the porphyrin and phthalocyanine units (Fig. 5(d)). MChD signals are observed at 380 and 426 nm (Fig. 5(e)); the signs of these signals for the two enantiomers are opposite. The peak positions (380, 426 nm) of these signals are identical to those of the spectrum obtained from the product of the CD and MCD spectra.
Fig. 5 MChD of a single-molecule magnet. The molecular structures of (a) Dy2[Pc(OBNP)4](TClPP)2, as well as (b) UV-vis, (c) CD, (d) MCD and (e) MChD spectra of Dy2[Pc(OBNP)4](TClPP)2. Redrawn from [10].
In this study, the alternating-current (ac) magnetic susceptibility was also measured in the range of 1.0-780 Hz. The Dy2[Pc(OBNP)4](TClPP)2 complex shows a frequency dependence of in-phase and out-phase signals under an external direct current (dc) magnetic field (0.2 T), although there is no frequency dependence without the dc magnetic field. Thus, this complex is not only the third example of MChD for the π−π* transition, but also a field-induced SMM.
4. Photoresolution based on MChD
MChD can induce asymmetric photochemical reactions under magnetic fields, as the MChD of two enantiomers is opposite. Therefore, MChD is a candidate for explaining the origin of the homochirality of life, in addition to the earth’s rotational motion (the Coriolis force) and circularly polarized light (CPL) induced asymmetric photochemical reactions [28]. An excess of L-amino acids in the Murchison meteorite has attracted considerable attention in terms of the homochirality of life [29], and requires a clarification of the origin of organic asymmetric reactions in the universe. Asymmetric reactions based on CPL or MChD can occur in the universe. The occurrence of these reactions seems more likely in regions relatively close to neutron stars, where there are both enormous magnetic fields (108-1012 T) and synchrotron radiation that includes CPL [30,31]. It is well established that CPL can induce enantioselective photochemical reactions (photoresolution) on the basis of CD, i.e. differences in absorption coefficients of CPL between two enantiomers [32–34]. The enantiomeric excess (e.e.) owing to the CPL induced photoresolution is given by gCD/2. The e.e. would be small even by pure CPL from the synchrotron radiation because the gCD values of organic compounds including amino acids are small. On the other hand, Rikken and Raupach studied MChD-based photochemistry that results in enantioselective reactions [35]. A thermally racemic mixture of Δ- and Λ-tris(oxalato) Cr(III) complexes was employed. Because one enantiomer selectively absorbs unpolarized light under a magnetic field that is parallel (or antiparallel) to the propagation direction of the light, an e.e. is obtained for the racemic mixture of Δ- and Λ-tris(oxalato) Cr(III) complexes owing to magneto-chiral anisotropy. Similar to the photoresolution with CPL, the e.e. owing to MChD is given by gMChD/2. The gMChD value is considered to be proportional to the strength of the magnetic field. Therefore, although the gCD (~gMChD/gMCD) values of organic compounds are very small, the e.e. owing to MChD should be high for organic aromatic compounds when subjected to the enormous magnetic fields (108-1012 T) near the neutron stars. Thus, for example, MChD-based photochemical reactions of aromatic amino acids may contribute to asymmetric reactions in the universe because aromatic amino acids exhibit both CD and MCD. In addition, polycyclic aromatic hydrocarbons (PAHs), the most abundant organic molecules in the universe (20% of total cosmic carbon), were also found in the Murchison meteorite [36–38]. The MChD of PAH aggregates may result in asymmetric photochemical reactions that introduce a small bias in chirality. Therefore, MChD-based photoresolution in the universe is a plausible candidate for explaining of the origin of the homochirality of life, along with the CPL-induced photochemical reactions.
5. Conclusion
In this review, the MChD of aromatic π-conjugated systems was described in terms of theoretical explanations, experimental observations, and photoresolution. This review of MChD is useful for understanding the homochirality of life. From the viewpoint of applications, the MChD of aromatic π-conjugated systems has significant advantages, such as tuneable wavelengths and high chemical reactivity, which are useful for asymmetric synthetic methods and magneto-optical devices.
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