Resonance Raman Studies of Macrocyclic Complexes. 2. Antiresonance and Selective Intensity Enhancement in Synthetic Metal(II) Porphyrin Analogues

The resonance Raman spectra are reported for the Mn(II), Cu(II), Ni(II), and Co(II) complexes of the dianonic N₄ macrocyclic ligand 5,7,12,14-tetramethyldibenzo[b,i] [1,4,8,11]tetraazacyclotetradecahexenate (L^2-). Raman intensity patterns as a function of excitation frequency covering the argon ion laser emission lines (and the Rhodamine 6G dye laser emission region for Ni^IIL) have been constructed. Mn^IIL and Cu^IIL show a simple one-term frequency dependence with the ligand π → π* transition near 380 nm. Ni^IIL shows selective Raman intensity enhancement of low-frequency modes in resonance with a charge transfer band at 585 nm. Incontrast, the resonance behavior near the π → π* transition shows characteristic enhancement of high-frequency ligand modes. In each region a one-term theoretical curve fits the data well. Co^IIL shows a strong antiresonance interference effect near 460 nm within a weak charge transfer band on the shoulder of the π → π* transition. This is the first observation of Raman antiresonance involving only allowed electronic transitions. The data are fit with a two-term theoretical expression which demonstrates the origin of the antiresonance behavior. The interpretation of the Raman intensity patterns leads us to assign the 460 nm band in Co^IIL and the 585 nm band in Ni^IIL to charge transfer transitions and indicates that the 460 nm band is coupled to the ligand π → π* transition while the 585 nm band is not.