Long-Range Conjugation-Sensitized Erbium(III) Shortwave-Infrared Emission in Porphyrin Molecular Wires

Abstract Shortwave infrared (SWIR; 1.0–2.0 μm) molecular emitters are increasingly attractive for bioimaging, optical sensing, and photonic technologies because photons in this spectral region experience reduced scattering and background interference. Yet the SWIR window is not optically uniform. Wavelength-dependent scattering and vibrational absorption create distinct optical subwindows, making spectral programmability essential for optimizing photon transport. Here we report Lanwire, an erbium(III)-coordinated ethynylene-conjugated porphyrin molecular wire platform that combines programmable excitation with invariant single-band emission in the low-loss >1.5 μm subwindow. Incorporation of a single Er3+ center into molecular wires containing two to five linearly conjugated porphyrins progressively red-shifts Q-band absorption from 705 to 850 nm, while preserving the same 1535 nm emission output. Efficient ligand triplet formation (∼82–92%) and near-unity triplet-to-Er3+ energy transfer are observed for the shorter 2–3-mer Lanwires, whereas the triplet yield decreases to ∼62–65% for the longer 4–5-mer series. The lower triplet yields in the longer Lanwires arise from a marked decrease in the intersystem crossing (ISC) rate, by up to ∼5-fold, which remains fast (∼109 s–1) but loses its kinetic advantage over competing internal conversion. Calculations using the corresponding closed-shell Lu3+ analogues reveal progressive redistribution of triplet spin density away from the lanthanide-coordinated porphyrin toward adjacent conjugated units, suggesting that changes in singlet–triplet spatial organization may contribute to regulating ISC. By integrating excitation programmability with fixed SWIR output, Lanwire enables high-resolution in vivo imaging and excitation-encoded multispectral fluorescence imaging. Our work establishes long-range delocalized conjugation-mediated sensitization as a design principle for programmable lanthanide emitters that couple extended π-electronic systems with the unique emissive properties of 4f centers.