Stable near‐infrared‐IIb dyes based on bis‐Pd(II) complexes of bis(TIPS‐ethynyl)‐substituted doubly N‐confused hexaphyrins

We herein report the design, synthesis, and characterization of novel dye molecules based on bis‐Pd(II) complexes of bis(TIPS‐ethynyl)‐substituted doubly N‐confused hexaphyrins ( t ‐Pd 2 ‐3 and c ‐Pd 2 ‐3 ). These complexes exhibit distinct absorption and emission properties in a sub‐channel of the second near‐infrared (NIR‐IIb) region. The anisotropic π‐extended derivative, t ‐Pd 2 ‐3 , which features two silyl‐protected ethynyl groups at the 5,20‐positions, has a rigid, rectangular hexapyrrolic core that provides distinct NNNC coordination environments. The resulting bis‐Pd(II) complex, t ‐Pd 2 ‐3 , exhibits characteristic molecular orbital mixing in the S 0 → S 1 transition—specifically in the Q‐state band—with reduced configuration interaction between the HOMO and LUMO, which leads to enhanced NIR‐IIb absorption and emission beyond 1500 nm. This pronounced Q‐band absorption contributes to the generation of NIR‐IIb photoacoustic (PA) signals, rendering it a potential PA contrast agent. The current study offers new insights into the design of stable, expanded porphyrinic dyes for applications in photon‐to‐thermal therapy and imaging.