Visualization of nonemissive triplet species of Zn(ii) porphyrins through Cu(ii) porphyrin emission via the reservoir mechanism in a porphyrin macroring

Abstract

A macroring composed of three Cu(II) porphyrins (CuP) and three slipped-cofacial Zn(II) porphyrin (ZnP₂) dimers exhibited near-IR emission from the CuP part. The emission lifetime of the macroring (15 μs) was 500 times longer compared to that of a Cu porphyrin monomer (e.g., Cu(II)TPP; TPP = meso-tetraphenylporphyrin). The observed emission is ascribed to emission via the reservoir mechanism from the trip-doublet (²T₁) state in CuP thermally activated from the T₁ state of ZnP₂, which is located ca. 1030 cm⁻¹ below the ²T₁ state of CuP. The near-IR emission of the macroring was significantly quenched by O₂, whereas that of the Cu porphyrin monomer was hardly quenched, indicating that the quenching event mainly occurred on the T₁ state of the ZnP₂ parts. The nonemissive triplet state of a Zn porphyrin at room temperature was visualized through emission from a neighboring Cu porphyrin.