(Received: February 28, 2005; Accepted for publication: July 25, 2005; Published on Web: September 12, 2005)
We developed a new program to estimate dye configurations in porphyrin arrays from their UV-vis. absorption and circular dichroism (CD) spectra. This program calculates 1) unit vectors of transition dipole moments from given molecular coordinates, 2) energies and wave functions for the array based on simple exciton theory, and 3) the UV-vis. and CD spectra from the obtained dipolar and optical rotatory strengths; then, 4) optimizes the porphyrin configuration so as to achieve least-squares fit between the observed and calculated spectra. The program generates a set of porphyrins in random configuration and minimizes the square sum (S) of the difference between observed and calculated spectra based on the quasi-Newton method.
The applicability of the exciton subroutine was evaluated by using two chirogenic porphyrin compounds: Troger's base porphyrin dimer (Crossley et al) and meso-meso linked bisporphyrin (Osuka et al). These porphyrins are the only two examples reported with UV-vis., CD spectra, and X-ray crystallographic data. Calculated dye-configuration of the former was in good agreement with crystal structure (RMSD = 0.21 A for planar ZnS4 center), whereas that of the latter was chemically meaningless. That is to say, the van der Waals surfaces of the two porphyrins partially collided with each other in the molecule, although the estimated structure was similar to the X-ray data (RMSD = 0.95 A for planar ZnS4 center). Consequently, our program is applicable to noncovalently-linked porhyrins, because the absorption spectra of directly-linked compounds are affected by charge transfer.
Keywords: Porphyrin, Exciton theory, Estimation of dye-configurations, UV-vis. absorption, Circular dichroism spectra
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