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

Abstract

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.