Ab Initio Modeling of the Electronic Circular Dichroism Induced in Porphyrin Chromophores

Induced photoelectron circular dichroism onto an achiral chromophore

An achiral chromophore can acquire a chiral spectroscopic signature when interacting with a chiral environment. This so-called induced chirality is documented in electronic or vibrational circular dichroism, which arises from the coupling between electric and magnetic transition dipoles. Here, we demonstrate that a chiroptical response is also induced within the electric dipole approximation by observing the asymmetric scattering of a photoelectron ejected from an achiral chromophore in interaction with a chiral host. In a phenol-methyloxirane complex, removing an electron from an achiral aromatic π orbital localised on the phenol moiety results in an intense and opposite photoelectron circular dichroism (PECD) for the two enantiomeric complexes with (R) and (S) methyloxirane, evidencing the long-range effect (~5 Å) of the scattering chiral potential. This induced chirality has important structural and analytical implications, discussed here in the context of growing interest in laser-based PECD, for in situ, real time enantiomer determination.

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Benchmarking computational methods and influence of guest conformation on chirogenesis in zinc porphyrin complexes

Different computational methods and influence of the guest conformation and solvent effect to analyze chirogenesis in zinc porphyrins by several chiral compounds have been investigated.

Optical and magnetic properties of antiaromatic porphyrinoids

Magnetic and spectroscopic properties of a number of formally antiaromatic carbaporphyrins, carbathiaporphyrins and isophlorins with 4n π electrons have been investigated at density functional theory and ab initio levels of theory.

Crystallographic and Spectroscopic Studies of a Host-Guest Complex Consisting of a Novel Zinc Trisporphyrinate and a Chiral Monoamine

We have designed and synthesized a novel zinc trisporphyrinate with a benzene tricarboxamide as the linker. In the presence of a large excess of 1-phenylethylamine, single crystals of the corresponding 1:3 host-guest complex were obtained, which provide the crystallographic structure of a host-guest complex consisting of an achiral porphyrin and a chiral monoamine. The structure reveals the 1-phenylethylamines adopt the "inside" binding mode that is stabilized by intramolecular hydrogen bonds. The NH2 of the 1-phenylethylamine is involved in both coordination and hydrogen bonding interactions. Circular dichroism (CD) and ultraviolet-visible spectra revealed that the 1:3 host-guest complex is dominant in the presence of a large excess of 1-phenylethylamine. The crystal structure shows there are two diastereomers of the 1:3 host-guest complexes. Density functional theory and TDDFT calculations suggest that one of the diastereomers is more energetically favorable, which dominates the CD signals.

Multi-scale modeling of electronic spectra of three aromatic amino acids: importance of conformational averaging and explicit solute-solvent interactions

Electronic transitions in the ultraviolet and visible spectral range can reveal a wealth of information about biomolecular geometry and interactions, such as those involved in protein folding. However, the modeling that provides the necessary link between spectral shapes and the structure is often difficult even for seemingly simple systems. To understand as to how conformational equilibria and solute-solvent interaction influence spectral intensities, we collected absorption (UV-vis), electronic circular dichroism (ECD), and magnetic circular dichroism (MCD) spectra of phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp) zwitterions in aqueous solutions, and compared them with quantum-chemical simulations. These aromatic amino acids provide a relatively strong signal in the accessible wavelength range. At the same time, they allow for a relatively accurate modeling. Energies and intensities of spectral bands were reproduced by the time-dependent density functional theory (TD DFT). The solvent was approximated by a continuum as well as clusters containing solvent molecules from the first hydration sphere. The ECD signal was found to be strongly dependent on molecular conformation, and the dependence was much weaker in UV-vis and MCD spectra. All spectral intensities, however, were significantly affected by the solvent approximation; especially for ECD and MCD the usual polarizable continuum solvent model did not yield satisfactory spectral shapes. On the other hand, averaging of the clusters obtained from molecular dynamics simulations provided an unprecedented agreement with the experiment. Proper modeling of the interactions with the environment thus makes the information about the molecular structure, as obtained from the electronic spectra, more accurate and reliable.

Porphyrins functionalized by quaternary pyridinium units

In this review, free-base and metalloporphyrins, functionalized on meso-positions by quaternary pyridinium units, also referred to as cationic porphyrins, are presented. The article consists of five parts. In the first part free-base porphyrins are described, especially taking account on generation of singlet oxygen; next parts concern metalloporphyrins. The second and third parts deal with zinc and manganese porphyrins, respectively; in the fourth part copper, palladium, and platinum porphyrins are presented. In the fifth part, describing porphyrins with various metal ions an attention is paid to porphyrin metal-organic frameworks (MOFs) and metal-organic materials (MOMs) in which metalloporphyrins are immobilized; syntheses and characterization of obtained products are shown.

TD-DFT modeling of the circular dichroism for a tryptophan zipper peptide with coupled aromatic residues

In this work, time dependent density functional theory (TD-DFT) is used to provide a reliable basis for interpretation of the electronic spectra of coupled tryptophan (Trp) residues, particularly those in a model Trpzip beta-hairpin peptide. Pairs of isolated indoles form chiral coupled chromophores whose computed electronic ultraviolet circular dichroism (CD) is in excellent agreement with observed transition wavelengths and intensities. The calculations were compared to experimental data for pairwise coupling in mutant Trpzip peptides that are recently available. A study of variation of the basis set, geometry optimization, and the solvent environment on the spectra showed limited impact on bandshapes. An alternative simplified computational scheme, dependent on the transition dipole coupling (TDC) mechanism, is shown to give a representation of qualitative aspects of the intense CD for the (1)B bands at 228 and 213 nm. The results confirm the origin of the Trpzip diagnostic CD as primarily a dipolar interaction between Trp sidechains, and show that quantum computations of electronic CD can provide a reliable basis for interpretation of these chirally coupled aromatic spectral phenomena.

Excitation and circular dichroism spectra of (+)-(S,S)-bis(2-methylbutyl)chalcogenides

Theoretical electronic spectra and natural circular dichroism (CD) spectra of (+)-(S,S)-bis(2-methylbutyl)chalcogenides, Ch[CH₂CH(CH₃)C₂H₅]₂ (Ch = S, Se, and Te), were calculated by the symmetry adapted cluster (SAC) and SAC-configuration interaction (SAC-CI) methods. Whereas the calculated CD spectrum for each stable conformation itself did not reproduce the corresponding experimental one, their Boltzmann-averaged spectra showed good agreement with the experimental results. We provided the assignment for each spectral band according to our calculation results. For the telluride compound, temperature dependence of the CD spectra was experimentally observed due to variation in the Boltzmann factor, and our calculations reproduced it qualitatively. The spectral features that we could not reproduce can be attributed to triplet transitions through the spin-orbit interaction effects as well as accuracy incompleteness on the calculation conditions.

Role of environmental factors on the structure and spectroscopic response of 5'-DNA-porphyrin conjugates caused by changes in the porphyrin-porphyrin interactions

We have explored the utility, strength, and limitation of through-space exciton-coupled circular dichroism in determination of the secondary structure of optically active chromophoric nanoarrays using the example of end-capped porphyrin- and metalloporphyrin-oligodeoxynucleotide conjugates. We put special emphasis on the explanation of the origin and significance of the distinctive multiple bands in the CD spectra (trisignate and tetrasignate CD bands). Such CD profiles are often observed in chiral aggregates or multichromophoric arrays but have never before been studied in detail. We found that variation of temperature and ionic strength has a profound effect on the geometry of the porphyrin-DNA conjugates and thus the nature of electronic interactions. At lower temperatures and in the absence of NaCl all three 5'-DNA-porphyrin conjugates display negative bisignate CD exciton couplets of variable intensity in the Soret region resulting from through-space interaction between the electric transition dipole moments of the two end-capped porphyrins. As the temperature is raised these exciton couplets are transformed into single positive bands originating from the porphyrin-single-strand DNA interactions. At higher ionic strengths and low temperatures, multisignate CD bands are observed in the porphyrin Soret region. These CD signature bands originate from a combination of intermolecular, end-to-end porphyrin-porphyrin stacking between duplexes and porphyrin-DNA interactions. The intermolecular aggregation was confirmed by fluorescence and absorption spectroscopy and resonance light scattering. DeVoe theoretical CD calculations, in conjunction with molecular dynamics simulations and Monte Carlo conformational searches, were used to mimic the observed bisignate exciton-coupled CD spectra as well as multiple CD bands. Calculations correctly predicted the sign and shape of the experimentally observed CD spectra. These studies reveal that the exciton-coupled circular dichroism is a very useful technique for the determination of the structure of optically active arrays.