Experimental Evidence for a Selection Rule of Intersystem Crossing to the Excited Quartet States: Metallophthalocyanines Coordinated by 4-Amino-TEMPO

Metallophthalocyanines (MPcs; M = Zn, Mg) coordinated by 4-amino-TEMPO (ATEMPO) in the excited quartet (Q₁) state, which are constituted by the excited triplet MPc and doublet ATEMPO, have been studied by a time-resolved electron paramagnetic resonance (TREPR) method. Two components were observed for both the Zn and Mg complexes, and are reasonably assigned to the Q₁ and doublet states by calculating the zero field splitting and g value, respectively. The polarization pattern of the Q₁ ZnPc-ATEMPO is A/E, and is opposite to that (E/A) of the Q₁ MgPc-ATEMPO, where the A and E denote absorption and emission of the microwaves, respectively. The Q₁ TREPR spectrum with the E/A polarization has been observed for the first time. The A/E and E/A polarization patterns are reproduced by selective intersystem crossing (ISC) to |Q₁,±1/2′〉 and to |Q₁,±3/2′〉, respectively. The selective ISC to |Q₁,±1/2′〉 is reasonably explained by the z component of SOC between the Q₁ and higher excited doublet states, while the ISC to |Q₁,±3/2′〉 originates from the x and y components of SOC. These experiments have proved that the selectivity of ISC to the Q₁ state is dependent on the components of SOC.