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

Abstract

The optical activity in porphyrins can easily be induced by a chiral environment, but it is difficult to determine the underlying mechanisms purely on an experimental basis. Therefore, in this study, magnitudes of the perturbational, dipolar, and direct covalent contributions to the electronic circular dichroism (CD) are evaluated with the aid of quantum chemical computations. Electronic properties of model porphyrin chromophores are analyzed. Time-dependent density functional theory (TD DFT), particularly with the hybrid B3LYP functional, appeared suitable for estimation of the electronic excitation energies and spectral intensities. The transition dipole coupling (TDC) between chirally stacked porphyrins was determined as the most important mechanism contributing to their optical activity. This is in agreement with previous experimental observations, where chiral matrices often induce the stacking and large CD signals. About a 10 times smaller signal could be achieved by a chiral orientation of the phenyl or similar residues covalently attached to the porphyrin core. Also, this prediction is in agreement with known experiments. Perturbation models realized by a chirally arranged porphyrin and a point charge, or by a porphyrin and the methane molecule, provided the smallest CD signals. The electrically neutral methane induced similar CD magnitudes as those of the charge, but spectral shapes were different. For a complex of porphyrin and the alanine cation, a significant influence of the solvent on the resultant CD spectral shape was observed, while for the charge and methane perturbations, a negligible solvent effect was found. Detailed dependence of the induced optical activity on variations of geometrical parameters is discussed. The simulations of the induced porphyrin activity can thus bring important information about the structure and intermolecular interactions in chiral complexes.

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

“…1, 4–6 Transition energies and intensities were computed for isolated indole rings with different basis sets, and also for indoles with methyl substituents which represent a connection to the peptide backbone, the β methylene carbon of Trp. The hybrid functional, B3LYP, was chosen because of previous successes using it for these excited states 16. To model the effect of solvation, we used the polarized continuum model (PCM),17 which considers the solute to be in a dielectric cavity whose electrostatic potential is derived from the dielectric of the solvent, in our case water (ϵ r = 78.39).…”

Molecular dynamics with restrictions derived from optical spectra

“…However, for many systems such a quasi-classical approach has been well-justified within the instantaneous mode approximations and other computational schemes. [18][19][20] Typically, the vibrational and electronic excitations observable in the spectra occur in a much faster time scale than common conformation motions and the coupling is minimized.…”

Optical and magnetic properties of antiaromatic porphyrinoids

“…[43][44][45][46][47][48] Due to their high applicability in various fields, it is desirable to un-derstand the electronic structure of porphyrinoids in detail, which may open up routes to a tailored design of new porphyrinoid based molecules with desired properties. 49,50 Common spectroscopic techniques that have been used for investigating porphyrinoids are nuclear magnetic resonance (NMR), 51 ultraviolet (UV) absorption, 1 magnetic circular dichroism (MCD), 52,53 electronic circular dichroism (ECD), 54,55 photoelectron (PE), 56 and two-photon absorption (TPA) [57][58][59] spectroscopies to mention the most common ones.…”