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

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posted on 22.03.2007, 00:00 by Takeshi Yamamura, Takaharu Mori, Yoshiki Tsuda, Tomotaka Taguchi, Norikazu Josha
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 Å 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.