Red shifting due to nonplanarity in alkylporphyrins: solid-state polarized UV-vis spectra and ZINDO calculations of two nickel(II)octaethylporphyrins.

We present the first demonstration of red shifting upon nonplanarity in alkylporphyrins using two pure conformations having known structures with identical substituents. The traditional view about the relationship of spectral red shifting to nonplanar deformation in porphyrins has been that the deformation from planar to nonplanar forms is in itself the cause of the shifting, but recently this view has been challenged. Among the new arguments is that the substituents required to effect conformational change also bring about nuclear rearrangements in the porphyrin complex which is the actual cause of the red shifting. Octaethylporphyrinatonickel(II), however, exists in both planar and ruffled forms which are determined only by the crystal structure, thus making the issue of different substituents moot. Using a polarized specular reflectance UV-vis microspectrophotometer, we have obtained polarized spectra of pure, solid samples of both forms of NiOEP. We find Soret band red shifting in the solid state that is much larger than previous reports of solution spectra and also report Q-band red shifting. We performed ZINDO calculations on monomers and dimers of both forms of NiOEP, based upon reported structures, and have reproduced the reported solution transition energies and our solid-state spectra as well as the red shifts that we and others have found experimentally. We conclude that, at least in this system, red shifting does indeed result primarily from conformation changes in the porphyrin.