Organic Upconversion Devices for NIR–II Biometric Sensing and Imaging Enabled by an Isomerically Engineered Narrow-Bandgap Quinoidal Polymer

The limited availability of organic donor materials absorbing beyond 1000 nm fundamentally limits the development of solution-processed photodetectors and upconversion devices (UCDs) in the second near-infrared (NIR-II) window. Herein, we report DPP-TbT, a narrow-bandgap alternating copolymer realized through isomeric engineering of the thiophene backbone. By employing a thieno[3,4-b]thiophene (TbT) quinoidal donor fused to a diketopyrrolopyrrole (DPP) acceptor, the absorption onset is extended to ∼1300 nm, enabling intrinsic NIR-II photon harvesting. When blended with the benchmark non-fullerene acceptor Y6 to form bulk-heterojunction photodetectors, the DPP-TbT:Y6 system achieves a specific detectivity exceeding 1011 Jones at 1064 nm with a linear dynamic range (LDR) of 68 dB. Monolithic integration of this photodetector with a quantum-dot light-emitting diode further yields a UCD that directly transduces 1064 nm excitation into visible emission with a turn-on voltage of ∼2 V, a luminance of 3741 cd m-2, and a luminance-LDR of 60 dB. We further demonstrate real-time heart-rate monitoring via photoplethysmography, high-resolution biological imaging (5080 ppi), and visualization of hidden information through a 1000 nm long-pass filter. This work establishes isomeric engineering as a rational strategy for accessing ultra-narrow bandgaps and advancing low-cost, wearable NIR-II visualization and biometric sensing systems.