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-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.