Estimation of Dye Configuration from Conventional Chiroptical Spectra of Porphyrin Integrates:  Combination of Exciton Theory with Monte Carlo Molecular Structural Simulation

Abstract

Dye integrates (arrays and aggregates) are the subject of current interest in photochemical devices. However, they are in general not suitable for X-ray crystallography because of their poor crystallinity. Here, we improved a simple method of estimating dye configurations in porphyrin integrates from their visible absorption (AB) and circular dichroism (CD) spectra. For this purpose, we calculate the dipolar and optical rotatory strengths of an integrate on the basis of the exciton theory for a given porphyrin configuration, generate the theoretical AB and CD spectra of the dye integrate using a phenomenological line shape function, a Voigt function with an adjustable line width, and optimize the configuration by minimizing the square sum (S) of the difference between the observed and calculated spectra. We adopted two optimization methods to achieve a least-squares fit between the calculated and observed spectra: the Metropolis Monte Carlo (MC) method, which incorporates S into the molecular force-field energy as a constraint, and the quasi-Newton (QN) method, which numerically minimizes S and uses no molecular force field. To check the feasibility of these methods, we simulated the AB and CD spectra of Tröger's base and meso-meso-linked porphyrins using the QN program, then compared the dye configurations with their X-ray structures. The calculated dye configuration of Tröger's base porphyrin is sufficiently in agreement with that of the X-ray structure (RMSD=0.21 A for the ZnS4 center), whereas that of meso-meso-linked porphyrin was not. These results were discussed in terms of charge transfer between two porphyrins. Finally, we applied the QN and MC methods to the structural estimation of a newly prepared peptide-linked bis(porphyrin) Boc-(PorZn,S)2-OBut. The best configurations estimated by these two methods were sufficiently in agreement with each other.

Axial, Helical, and Planar Chirality in Directly Linked Basket-Handle Porphyrin Arrays

“…Meso - meso , β-β doubly linked dimers are helically chiral due to the steric hindrance of the remaining opposing β-protons (Figure D) . For all chiral examples from Figure , the enantiomers were resolved, and their ECD spectra were reported. ,, Although several meso-meso linked porphyrin dimers were described in the literature, their axial chirality has rarely been recognized − but rather often overlooked, and to the best of our knowledge, until now, no absolute configuration of such chiral systems has been elucidated. In the case of β-meso and β-β dimers, the absolute stereostructures were assigned by comparison of their experimental ECD spectra with the calculated ones. ,, The absolute configuration of the doubly linked dimer was initially assigned by a wrong application of the exciton chirality method and has later been revised by using quantum-chemical ECD calculations.…”

“…The intensively studied meso - meso coupled dimers are achiral as long as the meso substituents orthogonal to the axis are identical. ,,,,− Nonidentical substituents lead to the loss of all mirror planes within the dimer and therefore to an unsymmetric and thus axially chiral structure (Figure A). We have discovered several examples of such meso-meso linked systems in the literature, where, however, the axial chirality was not recognized at all, and only one case where enantiomers had been resolved but absolute configurations were not assigned. − The same chirality criteria apply for β-meso linked bisporphyrins: If the meso -coupled building block comprises two identical meso substituents orthogonal to the axis, the resulting bisporphyrin is achiral . In all cases with nonidentical meso substituents, the resulting structure is chiral (Figure B).…”

Porphyrin Arrays Responsive to Additives. Fluorescence Tuning

“…Irrespective of the absence or presence of en, it was impossible to deconvolute the CD spectra of 6 ′ using ideal positive and negative pairs of Cotton-effect that exhibit zero-cross points at the absorption maxima . This suggests that several energy bands that differ in wavelength, intensity, and line width contribute to the AB and CD spectra.…”