Conditions for forming excited multiplet states: magnetic interactions between excited triplet (phthalocyaninato)zinc and doublet nitroxide radical

Abstract

Excited state phthalocyanine complexes comprising (tetra-tert-butylphthalocyaninato)zinc (ZnPc) coordinated by nitroxide radicals have been studied by time-resolved electron paramagnetic resonance (TREPR). Five ZnPc complexes coordinated by an axial ligand containing a nitroxide radical NRX (where X (= 4, 5, 6, 8 or 10) denotes the number of bonds from the zinc to the nitroxide nitrogen) were selected, and the magnetic interactions between the excited triplet ZnPc and respective NRX have been investigated in terms of the conditions for forming the excited quartet (Q₁) state. Optimum structures calculated using a PM3 Hamiltonian show that the bond number X is well correlated with the distance between zinc and nitroxide nitrogen atoms (Δr). TREPR spectra of ZnPc complexes, which are coordinated by NR8 or NR10, are almost the same as that of ZnPc coordinated by pyridine (ZnPc–py), indicating that the electron exchange interaction, J, between the excited triplet ZnPc and doublet nitroxide is much smaller than the zero field splitting parameter D value (D(T₁) = 0.720 GHz) of the excited triplet ZnPc–py. On the other hand, TREPR spectra of the NR4, NR5 and NR6 complexes are assigned to the Q₁ state constituted by the excited triplet ZnPc and doublet nitroxide radical. The D value of the Q₁ state (D(Q₁)) decreases in the order ZnPc–NR6 (0.205 GHz) > ZnPc–NR5 (0.190 GHz) > ZnPc–NR4 (0.165 GHz). This decrease is interpreted in terms of a magnetic dipole–dipole interaction between the triplet ZnPc and doublet nitroxide, which is opposite in sign to D(Q₁), and increases in the order ZnPc–NR6 < ZnPc–NR5 < ZnPc–NR4. Calculations of resonance magnetic fields indicate that the |J| values of the NR4, NR5 and NR6 complexes are larger at least than the D(T₁) value. It is found that the D and |J| values are well correlated with the bond number X and distance Δr. This EPR study is useful for understanding the photophysical and photochemical properties of chromophores.