Pd(II)-Templated End-to-End Cyclization of Doubly N-Confused Bilatrienes: A Stable Singlet Diradicaloid Dye with Efficient Absorption across the Solar Spectrum

We report a highly stable singlet diradicaloid (Pd1-dimer), formed through a Pd(II)-templated end-to-end cyclization of a linear N-confused bilatriene (1). This unique Pd-bridged double-loop complex exhibits an open-shell singlet diradicaloid ground state with an extremely narrow energy gap. Interestingly, further coordination of Pd(II) at the NNCC core generates a distinct doublet radicaloid species (Pd21-dimer). Despite their open-shell electronic nature, both complexes exhibit remarkable stability under ambient conditions and reversible redox interconversion between their corresponding charged states. The complexes display broad visible-to-second near-infrared (NIR-II) absorption profiles arising from extremely narrow energy gaps. Efficient nonradiative relaxation associated with the open-shell electronic structure enables strong photothermal conversion, and Pd1-dimer exhibits a photothermal conversion efficiency of 47.8% in aqueous nanoparticle dispersions under 940 nm laser irradiation. Moreover, when integrated into a carbon nanotube-modified photothermal conversion membrane, applying a small amount of Pd1-dimer (e.g., 1.0 mg per membrane) yields a solar-to-vapor conversion efficiency of 84.4% and an evaporation rate of 1.22 kg m-2 h-1 under 1.0-sun irradiation. This work establishes open-shell porphyrinoid diradicaloids as a promising molecular platform for efficient solar-thermal energy conversion and interfacial seawater desalination.