Bis-Metal Complexes of Doubly N‑Confused Dioxohexaphyrins as Potential Near-Infrared-II Photoacoustic Dyes

We investigated the detailed photophysical properties of a series of bis-metal (Zn and Cu) dioxohexaphyrin complexes as potential second near-infrared (NIR-II)-light responsive dyes. A cisoid-configured 28π-electron-conjugated dioxohexaphyrin analogue ( c -3a) containing two peculiar “confused pyrrole” moieties in the framework is identified as a reduced isomer derivative of a transoid 26π-dioxohexaphyrin ( t -2a). The symmetry-altered structure of c -3a affords a heteroleptic inner environment within the NNNN/NNOO donor core, which imparts its highly flexible electronic features and nonplanar geometry. The macrocycle c -3a can be transformed into the corresponding 26π-electron congener ( c -2a) having a coplanar rectangular structure by unique solvent-mediated redox reactivity. Furthermore, upon metal complexation, saddle-distorted bis-metal complexes ( c- M 2 -2a) were formed as the 26π-conjugated structural isomer of the trans-dioxohexaphyrin species (i.e., t -M 2 -2a). These isoelectronic dioxohexaphyrins demonstrate precise geometry-dependent photophysical properties. Broad tailing NIR-II absorption, weak emissive character, and rapid-decay of the S1 state are observed for c -Zn 2 -2a. In contrast, the coplanar t -M 2 -2a exhibits efficient photoacoustic response upon laser excitation with NIR-II light (λ > 1000 nm). To the best of our knowledge, this is the first example of an expanded porphyrin-based photoacoustic contrast agent responsive to NIR-II light.