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 (Q 1 ) 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 Q 1 and doublet states by calculating the zero field splitting and g value, respectively. The polarization pattern of the Q 1 ZnPc-ATEMPO is A / E, and is opposite to that ( E / A ) of the Q 1 MgPc-ATEMPO, where the A and E denote absorption and emission of the microwaves, respectively. The Q 1 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 |Q 1,±1/2‘〉 and to |Q 1,±3/2‘〉, respectively. The selective ISC to |Q 1,±1/2‘〉 is reasonably explained by the z component of SOC between the Q 1 and higher excited doublet states, while the ISC to |Q 1,±3/2‘〉 originates from the x and y components of SOC. These experiments have proved that the selectivity of ISC to the Q 1 state is dependent on the components of SOC.