Bis-Metal Complexes of Doubly N-Confused Dioxohexaphyrins as Potential Near-Infrared-II Photoacoustic Dyes
Keito Shimomura, Hiroto Kai, Yuma Nakamura, Yongseok Hong, Shigeki Mori, Koji Miki, Kouichi Ohe, Yusuke Notsuka, Yoshihisa Yamaoka, Masatoshi Ishida, Dongho Kim, Hiroyuki Furuta
Abstract
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-M2-2a) were formed as the 26π-conjugated structural isomer of the trans-dioxohexaphyrin species (i.e., t-M2-2a). 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-Zn2-2a. In contrast, the coplanar t-M2-2a 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.