EFFECTS OF PI-PI INTERACTIONS ON MOLECULAR-STRUCTURE AND RESONANCE RAMAN-SPECTRA OF CRYSTALLINE COPPER(II) OCTAETHYLPORPHYRIN

Single-crystal resonance Raman measurements were performed on two crystalline phases of copper(II) octaethylporphyrin (CuOEP): triclinic B and triclinic B forms. These are compared to previously acquired resonance Raman data on the triclinic A and triclinic B phases of nickel(II) octaethylporphyrin (NiOEP). The difference in crystal packing between the triclinic A and B structures allows for more extensive pi-pi-interactions in the B form than in the A form. Differences in the single-crystal Raman spectra of the triclinic A and B forms of both Cu- and NiOEP are attributed to these pi-pi-interactions. Specifically, the Raman core-size marker lines, nu(3), nu(2), and nu(10), and the oxidation-state marker line nu(4), are affected by the different packing interactions in the two crystals, and the same Raman shifting patterns between the triclinic A and B phases are observed for both Cu- and NiOEP. The metal centers exert a slight influence over the packing-induced frequency shifts in the Raman modes. Spectral results on the CuOEP crystals are also compared to solution Raman data on pi-pi-aggregated and monomeric copper(II) uroporphyrin (CuUroP). Similar to the A and B forms of NiOEP, the CuOEP triclinic A and B crystalline phases mimic the monomer and salt-induced aggregate of CuUroP in solution, as evidenced by similar magnitude upshifts in the Raman modes upon aggregation. A comparison of the structure calculated using molecular mechanics and the structure obtained from X-ray diffraction gives some insight into the effect that pi-pi-interactions have on the structure of CuOEP in the triclinic B crystal.