Theoretical Analysis of the Porphyrin−Porphyrin Exciton Interaction in Circular Dichroism Spectra of Dimeric Tetraarylporphyrins

Abstract

Chiral bis-porphyrins are currently the subject of intense interest as chiral receptors and as probes in the determination of structure and stereochemistry. To provide an improved framework for interpreting the circular dichroism (CD) spectra of bis-porphyrins, we have calculated the CD spectra of chiral bis-porphyrins from three classes: I, where porphyrins can adopt a relatively wide range of orientations relative to each other; II, porphyrins have a fixed relative orientation; III, porphyrins undergo pi-stacking. The calculations primarily utilized the classical polarizability theory of DeVoe, but were supplemented by the quantum mechanical matrix method. Class I was represented by three isomers of the diester of 5alpha-cholestane-3,17-diol with 5-(4'-carboxyphenyl)-10,15,20-triphenylporphin (2-alphabeta, 2-betaalpha, 2-betabeta). Careful analysis of the torsional degrees of freedom led to two to four minimum-energy conformers for each isomer, in each of which the phenyl-porphyrin bonds had torsional angles near 90 degrees. Libration about these bonds is relatively unrestricted over a range of +/-45 degrees. CD spectra in the Soret region were calculated as Boltzmann-weighted averages over the low-energy conformers for each isomer. Three models were used: the effective transition moment model, in which only one of the degenerate Soret components is considered, along the 5-15 direction; the circular oscillator model, in which both Soret components are given equal weight; and the hybrid model, in which the 10-20 oscillator is given half the weight of the 5-15 oscillator, to mimic the effect of extensive librational averaging about the 5-15 direction. All three models predict Soret exciton couplets with signs in agreement with experiment. Quantitatively, the best results are given by the hybrid and circular oscillator models. These results validate the widely used effective transition moment model for qualitative assignments of bis-porphyrin chirality and thus permit application of the exciton chirality model. However, for quantitative studies, the circular oscillator or hybrid models should be used. The simplified effective transition moment and hybrid models are justified by the librational averaging in the class I bis-porphyrins and should only be used with such systems. Two class II bis-porphyrins were also studied by DeVoe method calculations in the circular oscillator model, which yielded good agreement with experiment. Class III bis-porphyrins were represented by 2-alphaalpha, for which the calculations gave qualitative agreement. However, limitations in the conformational analysis with the close contacts and dynamic effects in these pi-stacked systems preclude quantitative results.

Bottom-Up Hierarchical Self-Assembly of Chiral Porphyrins through Coordination and Hydrogen Bonds

“…The development of the equilibria in the self-assembly of each metalloporphyrin was studied at different temperatures by UV-visible absorption and CD spectroscopies, which gave information about the orientation that the chromophores adopt with respect to one another as the intensity of the optical signals is directly proportional to the interactions between electronic transitions. 62 Solutions of compounds Zn-(1-4) in methylcyclohexane/3% CHCl 3 were prepared (at a concentration 5 μM), and CD spectra were recorded in the range of temperatures from 338 K to 263 K after a heating and cooling cycle to ensure that equilibrium conditions applied.…”

Structural determinations by circular dichroism spectra analysis using coupled oscillator methods: An update of the applications of the DeVoe polarizability model

“…Another very recent, interesting application of DeVoe calculations was reported by Fleischhauer and co-workers, 61 who carried out a theoretical analysis of the porphyrin -porphyrin exciton interaction in the CD spectra of some dimeric tetraarylporphyrins. The porphyrin derivatives studied were divided into three classes.…”

A Cationic Zinc Porphyrin as a Chiroptical Probe for Z‐DNA

“…They have been covalently linked to specific chiral sequences, and their CD spectral properties have been used to resolve complicated stereochemical issues [12][13][14][15] as well as monitor polynucleotide dynamics. [16] Also, the noncovalent interactions of cationic or anionic water-soluble achiral porphyrins with chiral templates have been exploited to build supramolecular species to detect and/ or amplify the handedness of the matrix.…”