Applications of magnetic circular dichroism spectroscopy to porphyrins and phthalocyanines

Magnetic circular dichroism (MCD) spectroscopy has widely been applied to porphyrins and phthalocyanines since around 1970, in order to elucidate their electronic structures. In this mini-review, some representative MCD results from the author's laboratory over the past 30 years are introduced, together with recent results from other laboratories. MCD studies on the following monomeric species are included: D(4h) type, adjacent vs. opposite type diaromatic ring-fused, non-planar, and reduced and oxidized species, as well as species showing temperature-dependent MCD signals. In addition, one example illustrates the use of MCD as a probe for the distal histidine residue in myoglobin. Recent results on dimers and oligomers are also reported. In particular, it is confirmed that the spectra of cofacial eclipsed dimers do not reflect the molecular symmetry of the constituent monomers. The spectra of rare-earth sandwich dimers and trimers are definitively assigned. Using spectra of planar oligomers of porphyrins, it is reiterated that it is often dangerous to assign the absorption bands of chromophores based only on the results of molecular orbital calculations. Some examples show that MCD can give information on the relative size of the DeltaHOMO (energy difference between the HOMO and HOMO-1) and DeltaLUMO (energy difference between the LUMO and LUMO+1); for example, if DeltaHOMO > DeltaLUMO, the MCD signal changes from minus to plus in ascending energy.     

Detailed assignment of the magnetic circular dichroism and UV-vis spectra of five-coordinate high-spin ferric [Fe(TPP)(Cl)

High-spin (hs) ferric heme centers occur in the catalytic or redox cycles of many metalloproteins and exhibit very complicated magnetic circular dichroism (MCD) and UV-vis absorption spectra. Therefore, detailed assignments of the MCD spectra of these species are missing. In this study, the electronic spectra (MCD and UV-vis) of the five-coordinate hs ferric model complex [Fe(TPP)(Cl)] are analyzed and assigned for the first time. A correlated fit of the absorption and low-temperature MCD spectra of [Fe(TPP)(Cl)] lead to the identification of at least 20 different electronic transitions. The assignments of these spectra are based on the following: (a) variable temperature and variable field saturation data, (b) time-dependent density functional theory calculations, (c) MCD pseudo A-terms, and (d) correlation to resonance Raman (rRaman) data to validate the assignments. From these results, a number of puzzling questions about the electronic spectra of [Fe(TPP)(Cl)] are answered. The Soret band in [Fe(TPP)(Cl)] is split into three components because one of its components is mixed with the porphyrin A2u72-->Eg82/83 (pi-->pi*) transition. The broad, intense absorption feature at higher energy from the Soret band is due to one of the Soret components and a mixed sigma and pi chloro to iron CT transition. The high-temperature MCD data allow for the identification of the Q v band at 20 202 cm(-1), which corresponds to the C-term feature at 20 150 cm(-1). Q is not observed but can be localized by correlation to rRaman data published before. Finally, the low energy absorption band around 650 nm is assigned to two P-->Fe charge transfer transitions, one being the long sought after A1u(HOMO)-->d pi transition.     

Substituent effects in porphyrin dimer complexes studied by IR spectroscopy

A series of lanthanide porphyrin dimers have been synthesized and investigated with IR spectroscopic techniques. The spectra of the porphyrin dimers are compared not only with each other but also with those of their component monomer units. The experimental results exhibit that the IR spectra of the porphyrin dimers are closely related to those of their corresponding monomers. A detailed analysis of the IR spectra between the porphyrin dimers and monomers suggest that the dimer molecules can be treated as regular derivatives of metalloporphyrin monomers despite the symmetries of these two systems being different. The dimerization of the porphyrin rings only result in frequency shifts and intensity changes of the IR spectra. These shifts are attributed to the induced π–π interactions between these two macrocycles. The downshifts of the frequencies observed in Ce(OEP)₂ further indicate that the π–π interactions intrinsically decrease the bond strength of the entire molecule. Additionally, only the relative intensities instead of the frequencies of the ethyl vibrations in the region 2800–3000 cm⁻¹ are observed to be sensitive to the types and the positions of the substituent groups. These observations suggest that these ethyl vibrational modes of the OEP moiety can be used as characteristic bands to monitor subtle deformations of the porphyrin rings caused by the substituent groups in the dimer complexes.     

Water dimer equilibrium constant of saturated vapor

The value and temperature dependence of the dimerization constant for saturated water vapor are determined. A general expression that links the second virial coefficient and the dimerization constant is obtained. It is shown that the attraction between water monomers and dimers is fundamental, especially at T > 350 K. The range of application for the obtained results is determined.     

Matrix isolation and density functional theory study of bis(trifluoromethyl)dioxodiazine: a photodimer of trifluoronitrosomethane

Trifluoronitrosomethane (CF3NO) was trapped in rare gas matrixes and irradiated at 633 and 670 nm. The infrared spectra of the postirradiation samples exhibit features consistent with cis and trans conformers of bis(trifluoromethyl)dioxodiazine, a previously uncharacterized species. The concentration dependence of the formation of the dimer is consistent with a mechanism in which monomers trapped in adjacent sites undergo excitation and subsequent reaction. The dimers reversibly form the monomer when irradiated with ultraviolet light. Density functional theory was used to determine the structure of the dimers and predict their infrared and Raman spectra. The predicted vibrational frequencies are in agreement with those observed. A third (skewed) conformation was predicted to have a triplet ground state, but no evidence of this species was observed. All three dimers exhibit significant diradical character, as evidenced by comparatively low N-N and high N-O stretching frequencies. Transition-state calculations predict the dimerization barrier to range from 17.1 (cis) to 35.0 (trans) kJ mol(-1) for the singlet dimers and to be 62.1 kJ mol(-1) for the triplet dimer. This is an example of nitroso dimerization that requires electronic excitation to proceed.     

Self-association of disulfonated deuteroporphyrin and its esters in aqueous solution and photosensitized production of singlet oxygen by the dimers

Dimerization of free acid and ester forms of disulfonated deuteroporphyrin is investigated in aqueous solution by absorbance and fluorescence spectroscopies. The dimerization equilibrium constant increases with the extent of esterification. In phosphate buffer saline (pH 7.4, 20 degrees C), it ranges from 1.4 x 10(6) M(-1) to 7.8 x 10(7) M(-1) for the free acid and the diethyl ester forms, respectively. The dimer formation is favored by an increase of ionic strength, as predicted by the Debye-Hückel law. The dimers display a marked shift to the blue of their Soret band. In agreement with the exciton model, a cofacial stacking of the molecules with some offset is postulated. The sulfonate groups on each molecule are likely to stand on opposite directions to reduce repulsion. Both the analysis of porphyrin self-association and careful examination of the fluorescence excitation spectra show that the dimers of disulfonated deuteroporphyrins do not fluoresce at all. The quantum yield of formation of singlet oxygen by the disulfonated deuteroporphyrins in deuterated methanol is 0.71, a value typical of monomers. In deuterated water, the yield is 0.44 for all the compounds studied though they are dimerized. The fact that nonfluorescent dimers of porphyrins can be efficient photosensitizers is emphasized.     

MCD studies of biphenyls

The magnetic circular dichroism (MCD) spectra of biphenyl derivatives with electron donating substituents at the C₄ and/or C₄′ positions of biphenyl were measured at room temperature in the wavelength regions of 200 to 350 nm. The biphenyls studied exhibited Faraday B terms caused by a magnetic coupling of the non-degenerate electronic states. A careful inspection of the MCD spectra has enabled us to find hidden bands which were buried in the intense, so-called “conjugation band” of biphenyl, resulting in the preferable assignments of the bands.     

Halogénoacétates de cuivre(II): 2° Partie. Étude magnétique et spectroscopique

Electronic spectra of chloro- and bromoacetates of copper (II) (hydrated and as addition compounds with ethyl acetate) have been obtained in the solid state at 298 °K and 77 °K. The results are discussed and compared with magnetic susceptibility measurements. The band at about 27.5 kK. is present in the monomers and in the dimers and cannot be used to characterize the dimer species. It is assigned to the npπ → d_x²?y² symmetry forbidden transition.     

N-Hydroxyurea dimers: A matrix isolation and theoretical study

The dimerization of N-hydroxyurea (NH₂CONHOH) has been investigated by FTIR matrix isolation spectroscopy and DFT(B3LYP)/6-311++G(2d,2p) calculations. The analysis of NH₂CONHOH/Ar matrix spectra and comparison with theoretical ones reveal the formation of two types of dimers with a strong OH⋯O hydrogen bond. There is an additional weak interaction between the oxygen atom of the OH group of the proton donor molecule and the NH or NH₂ group of the proton acceptor in both dimers, respectively. The identified structures correspond to local minima on the PES. The formation of the less stable structures not the most stable ones indicates that the creation of N-hydroxyurea dimers is related to the dipole-dipole interaction at the initial stage of the dimerization process, which favours generation of polar dimers.     

A DFT study on the dimerization of C62, H2-C62, and F2-C62

On the basis of calculations using the density functional theory, we show that C(62), a recently synthesized nonclassical fullerene, will presumably undergo dimerization with various isomers at elevated temperatures. This is shown by calculating the dimerization energy and the activation barrier of the dimerization. Eight possible isomers of the dimer were identified, all of which are more stable than the two isolated monomers. The relative stability of various isomers depends upon the kind of C=C bonds within the four-membered carbon ring involved in the dimerization. In addition, similar calculations were performed for the monomers and dimers of H(2)-C(62) and F(2)-C(62). Six isomers were identified for each of the dimers. Although less pronounced than the case of the C(62) dimer, all isomers of the H(2)-C(62) dimer are appreciably more stable than the individual monomers. Although a large steric repulsion due to F atoms significantly reduces the stability of F(2)-C(62) dimer, its two isomers are still more stable than separate monomers.     

Porphyrins XXXV . Exciton coupling in μ-oxo Scandum dimers

The absorption and emission spectra at room temperature and at 77 K are reported for the monomers and μ-oxo dimers of (OEP)Sc(III) and (TPP)Sc(III). [Here (OEP) is octaethylporphin and (TPP) is tetraphenylporphin.] Exciton coupling effects are strongest in the B(Soret) band of [(OEP)Sc]₂O dimer: (i) The peak is blue shifted by 11 nm; (ii) the Soret band has a long red tail out to 480 mn; (iii) the fluorescence polarization shows a broad negativ band ≈ 440 nm. A vibronic exciton coupling model can roughly interpret the data if there is substantial and variable tilting of the ring planes. Exciton effects are weaker in the B(Soret) band of [(TPP)SC]₂O, presumably because there is less tilting. The effect of dimer formation on the Q band of [(OEP)Scl₂O is to red shift the band ≈ 420 cm^?1 and to nearly double the Q(0,0) halfwidth; there is no change in fluorescence yield with dimerization. Presumably for Q bands exciton coupling is weaker than inhomogeneous broadening. Both the phosphorescence yield and triplet lifetime at 77 K drop by $\text{case2}\text{3}$ in the dimer, showing faster radiationless decay.     

Photonics of dimers of cyanine dyes

The results of study on the properties of dimers of thiamonomethine-and thiatrimethinecyanines (thiacarbocyanines) in the ground and electronically excited states in aqueous solutions are presented. Dimers of cyanine dyes have the sandwiched structure with near-parallel alignment of the polymethine chains of the monomers in the dimer. The formation of dimers is manifested by two absorption bands of different intensities due to splitting of the S* level of the monomers upon their resonance interaction. Dimers of thiacarbocyanines are characterized by a low fluorescence quantum yield φ_fl as compared to monomers; however, φ_fl of dimers of thiamonomethinecyanines are markedly higher than that of monomers. Dimers of cyanine dyes are also characterized by a relatively high quantum yield of intersystem crossing to the triplet state. In the triplet-triplet absorption spectra, two bands of different intensities are revealed, which are due to the splitting of the higher triplet level of the monomers that form the dimer. In the presence of electron donors (ascorbic acid, hydroquinone) and/or acceptors (p-benzoquinone, p-nitroacetophenone, methylviologen), the triplet state of dimers is quenched as a result of electron transfer yielding radical products. Dimers in the triplet state can serve as photosensitizers of redox reactions.     

The electronic and magnetic circular dichroism spectra of matrix-isolated scandium atoms and the scandium dimer

The electronic and magnetic circular dichroism (MCD) spectra of scandium atoms isolated in argon, krypton and xenon matrices have been measured and the bands assigned. Some aspects of the assignment present problems and the resulting matrix shifts are rather irregular. Magnetization studies of the above systems are also reported and the data show that there are particularly strong guest—host interactions in the case of Sc/Xe. Furthermore, they suggest that there is significant guest—host interaction in the ground electronic state. Computer simulation of the magnetization curves and the MCD spectra, using a crystal field model, enables some tentative suggestions concerning the nature of the matrix sites to be made. All sites show an axial character. MCD bands of a scandium dimer have been observed. The form and magnetization properties of one band support a ⁵Σ ground state for the molecule.     

Intermolecular association of tetrahydrofuran-2-carboxylic acid in solution: a vibrational circular dichroism study

Carboxylic acids are known for their strong intermolecular associations. With chiral carboxylic acids, this behavior can be studied using vibrational circular dichroism (VCD). Tetrahydrofuran-2-carboxylic acid 1, a chiral building block for beta-lactam antibiotics, is studied by emphasizing the effect of the dimerization. Experimental results indicate that for solutions of 1 in CDCl3 and CS2, a complex equilibrium exists between the monomers and dimers. B3LYP/aug-cc-pVTZ calculations are performed on both monomer and dimer structures. To simulate IR and VCD spectra, populations for monomer and dimers were approximated using a semiquantitative model. A good agreement between experimental and simulated spectra is obtained by taking into account both the monomeric and the dimeric structures, weighted using the experimentally determined populations.     

The first TDDFT and MCD studies of free base triarylcorroles: a closer look into solvent-dependent UV-visible absorption

Absorption spectra of several free base triarylcorroles were investigated by MCD spectroscopy. The MCD spectra exhibit unusual sign-reverse (positive-to-negative intensities in ascending energy) features in the Soret- and Q-type band regions, suggesting a rare ΔHOMO < ΔLUMO relationship between π and π* MOs in the corrole core.     

Magnetic circular dichroism study of directly fused porphyrins.

The magnetic circular dichroism (MCD) spectra of doubly and triply linked fused bisporphyrins (2MD and 2MT, M = Ni, Zn, Cu, Pd, and H2) and triply linked higher oligomers (3ZnT and 4ZnT) have been measured, and their Q-bands assigned based on the results of INDO/s calculations. In contrast to the Faraday A term observed for the Q(0,0) band of Ni(II) tetraphenylporphyrin, a single positive Faraday B term was observed for the lowest energy transition of the fused systems. The calculations indicated that the molecular orbitals (MOs) of the directly fused porphyrins consist of linear combinations of the constituent monomeric MOs, and that the effect of lowering the symmetry is always larger on the lowest unoccupied molecular orbital (LUMO) than on the highest occupied molecular orbital (HOMO). On the basis of Michl's perimeter model, these features can be correlated with the observed positive MCD signs in the near infrared region. A weak absorption band at 600-700 nm for the fused dimers can be assigned to a short-axis polarized Q transition.     

N,N'-Linked biazoles : Part 7. Oxidative dimerization of n-unsubstituted tetrahydroindazolones

The structure of the products obtained by oxidative dimerization of tetrahydroindazolones was established as being N(1)-C(3a') dimers. In one case a C(3a)-C(3a') dimer was also isolated, but N-N'-Linked dimers were never found, contrary to a previous report in the literature. A complete 13-carbon nmr study of the monomers and their fixed methylated derivatives was performed to determine the most abundant tautomer in each case, since the tautomerism study of tetrahydroindazolone and of its 4-ethoxycarbonyl derivative was needed to fully determine the structure of the dimers.     

A study of the electronic spectra of 9-fluorenone

The absorption and MCD spectra of 9-fluorenone are measured. The CD spectra of the β-cyclodextrin complex with 9-fluorenone are also measured. The assignment for the first (≈26.0 × 10³ cm^?1) and fourth (≈48.0 × ³ cm^?1) absorption band has been determined from the sign of the CD spectra by reference to the observed MCD spectra.     

Synthesis of Novel Symmetric Porphyrin Schiff Base Dimers by Solid–Liquid Reaction Methodology

A new convenient solid–liquid condensation reaction procedure for the synthesis of novel asymmetric and symmetric meso‐tetraarylporphyrin and metalloporphyrin Schiff bases is reported. The condensation reaction between β‐formyl porphyrin or metalloporphyrins and aromatic amines was carried out at solid–liquid interface by using neutral alumina powder as a solid support for β‐formyl porphyrin or metalloporphyrins and absolute ethanol as the carrier solvent for aromatic amines. Six different asymmetric porphyrin/metalloporphyrin Schiff bases were synthesized via solid–liquid interface reaction methodology. The same solid–liquid synthetic methodology was applied for the synthesis of six novel symmetric Schiff base porphyrin/metalloporphyrin dimers. The comparison of UV–visible spectra of porphyrin Schiff base monomers and dimers revealed that some degree of electronic perturbation has occurred upon dimerization as the Soret bands of the monomers underwent peak broadening along with red shifts. Column chromatography and crystallization were used to purify the compounds. Fourier transform infrared, UV–visible, elemental analysis, ¹H NMR, and mass spectrometry were used to characterize the newly synthesized compounds.     

The ν(OH/OD) band shape of strong hydrogen bonded dimers of phosphinic acids. Phenomenology and formation models

The infrared spectra of phosphinic acid R₂POOH dimers (R=CH₃, CH₂Cl, C₆H₅) have been studied in CCl₄ and CH₂Cl₂ solutions (T=300 K). The infrared spectra of deuterated R₂POOD dimers (R=CH₃, CH₂Cl) were also studied in the gas phase (T=400–550 K) and solid state (T=100–300 K). They are compared with previously studied spectra of the light (non-deuterated) dimers in the gas phase, in the solid state and in low-temperature argon matrices (T=12–30 K) in the 4000–400 cm⁻¹ spectral region. It is found that the strong and broad ν(OH) dimer bands have similar shapes, nearly equal values of bandwidth and low-frequency shift, and possess the Hadzi ABC structure irrespective of the type of acid, significant differences of dimerization enthalpies, influence of solvent, the type of H-bonded complexes (cyclic dimers in the gas phase, in solutions, and in inert matrices, and infinite chains in the solid state), and temperature in the range 12–600 K. Isotopic ratio of the first moments of light and deuterated acid bands has been measured. Analysis of the ν(OH/OD) band of hydrogen bonded dimers of phosphinic acids shows that the interaction between the two intermolecular bonds O–HOP in a cyclic complex plays virtually no role in the mechanism of the ν(OH/OD) band formation; the shape of ν(OH/OD) band is controlled mainly by the POOH(D)O fragment; and the band shape of strong hydrogen bonded complexes is formed by a number of vibrational transitions from the ground state to different combination levels in the region 3500–1500 cm⁻¹.