Near‐Infrared‐III‐Absorbing and ‐Emitting Dyes: Energy‐Gap Engineering of Expanded Porphyrinoids via Metallation.

The synthesis of organometallic complexes of modified 26π‐conjugated hexaphyrins with absorption and emission capabilities in the third near‐infrared region (NIR‐III) is described. Symmetry alteration of the frontier molecular orbitals (MOs) of bis‐PdII and bis‐PtII complexes of hexaphyrin via N‐confusion modification led to substantial metal dπ–pπ interactions. This MO mixing, in turn, resulted in a significantly narrower HOMO–LUMO energy gap. A remarkable long‐wavelength shift of the lowest S0→S1 absorption beyond 1700 nm was achieved with the bis‐PtII complex, t‐Pt2‐3. The emergence of photoacoustic (PA) signals maximized at 1700 nm makes t‐Pt2‐3 potentially useful as a NIR‐III PA contrast agent. The rigid bis‐PdII complexes, t‐Pd2‐3 and c‐Pd2‐3, are rare examples of NIR emitters beyond 1500 nm. The current study provides new insight into the design of stable, expanded porphyrinic dyes possessing NIR‐III‐emissive and photoacoustic‐response capabilities. [ABSTRACT FROM AUTHOR]