INFLUENCE OF SYMMETRY ON THE VIBRATIONAL-SPECTRA OF ZN(TPP), ZN(TPC), AND ZN(TPIBC)

To evaluate the effects of sequential pyrrole ring reduction, we have examined the vibrational spectra of tetracoordinate ZnII complexes of meso-tetraphenyiporphyrin [Zn(TPP)], mero-tetraphenylchlorin [Zn(TPC)], and meso-tetraphenylisobacteriochlorin [Zn(TPiBC)]. The phenyl substituents of these macrocycles fall into distinct symmetry classes: for Zn(TPP), all four phenyl groups are equivalent; for Zn(TPC), the phenyl groups are of two different types; and for Zn(TPiBC), there are three types of phenyl substituent. This variation leads to marked differences in the resonance Raman spectra of the three complexes, particularly for the phenyl vibrational modes. Thus, a single totally symmetric Cm-phenyl stretch [v(Cm-Ph)] is observed for Zn(TPP) at 1236 cm−1. In contrast, two v(Cm-Ph) modes are observed at 1247 and 1236 cm−1 for Zn(TPC). For Zn(TPiBC), three v(Cm-Ph) modes are observed at 1269, 1252, and 1235 cm−1. Other phenyl-related vibrational modes undergo similar splittings as the symmetry of the phenyl substituents varies from Zn(TPP) to Zn(TPC) to Zn(TPiBC). The vibrational patterns for these macrocycles are in harmony with 1H NMR results demonstrating one, two, and three type(s) of meso-phenyl substituents for H2TPP, H2TPC, and H2TPiBC, respectively. The vibrational spectra of Zn(TPC) and Zn(TPiBC) are more complex than the spectra of Zn(TPP), owing to their lowered symmetries. Nonetheless, the spectra of Zn(TPiBC) display features distinct from those of Zn(TPC). © 1990, American Chemical Society. All rights reserved.