Spectral Manifestations of d–π Exchange and Charge Transfer Interactions in Cu(II) Porphyrins.

New absorption bands due to spin-forbidden transitions from the ground state to excited triplet states are found at 295 K in the near-IR absorption spectra of a number of Cu(II) complexes of octaethylporphyrin (OEP) that differ in the nature, number, and position of their side substituents. The frequency distribution, number, and nature of these transitions are analyzed using computer decomposition of complex contours into Gaussian components and additional data from the phosphorescence and phosphorescence excitation spectra (295–77 K). The d-π exchange integrals, determining the energy splitting ΔE = E(2T1) – E(4T1) in the compounds under study, are calculated on the basis of the experimental data obtained. It is shown that, in addition to the formally spin-allowed 2S0→2T1 transition (λmax = 833 nm), the absorption spectrum of the nonplanar CuOETPP molecule at 295 K exhibits the low-intensity spin-forbidden 2S0→4T1 transition (λmax = 937 nm). For this compound at 77 K, phosphorescence from the 4T1 state is observed (λmax = 955 nm), with a quantum yield equal to φPh = 0.0015 and a decay time amounting to τPh = 190 ns. For the CuOEP–Ph(o-NO2) molecule, which contains the electron-acceptor nitro group, direct absorption from the ground state S0 to a charge transfer (CT) state (λmax = 845 nm) is observed at 295 K. The extinction coefficient of this absorption and the energy of the CT state are determined. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]