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.
Bibliographical noteFunding Information:
The works at Kyushu University, the University of Tokyo and RIKEN were supported by JSPS KAKENHI grant numbers JP19K05439, JP19H04586, JP20H00406, JP17H06173, and JP20H02728. Funding from the Tokuyama Research Foundation is gratefully acknowledged. The work at Yonsei University was supported by Strategic Research (NRF2016R1E1A1A01943379) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science. We thank Prof. M. Watanabe (Kyushu Univ.) for providing access to NMR instrumentation.
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