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-Pd and bis-Pt complexes of hexaphyrin via N-confusion modification led to substantial metal d -p interactions. This MO mixing, in turn, resulted in a significantly narrower HOMO-LUMO energy gap. A remarkable long-wavelength shift of the lowest S →S absorption beyond 1700 nm was achieved with the bis-Pt complex, t-Pt -3. The emergence of photoacoustic (PA) signals maximized at 1700 nm makes t-Pt -3 potentially useful as a NIR-III PA contrast agent. The rigid bis-Pd complexes, t-Pd -3 and c-Pd -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.