Conjugated　polymers　with　strong　absorption　in　the　second　near-infrared　(NIR-II)　window　have　multiple　applications.　However,　the　development　of　new　type　of　NIR-II　conjugated　polymers　via　facile　and　green　methods　remains　challenging.　Herein,　this　work　reports　a　mild　and　convenient　transition-metal-free　method　to　synthesize　near-infrared　absorbing　quinoidal　conjugated　polymers　containing　para-azaquinodimethane　(AQM)　moieties.　The　AQM　quinoidal　conjugated　polymers　with　unique　molecular　structures　and　tunable　optoelectronic　properties　can　be　synthesized　by　combining　the　Knoevenagel　polycondensation　of　aromatic　dialdehyde　monomers　with　commercially　available　1,4-diacetyl-2,5-piperazinedione　and　the　following　alkylation　reaction.　The　resultant　polymer　PQ-DPP　shows　remarkable　NIR-II　absorption　with　a　narrow　band　gap　of　about　1.08 eV.　PQ-DPP　nanoparticles　exhibit　high　photothermal　conversion　efficiency　of　up　to　48%　under　1064 nm　laser　irradiation　(1 W　cm−2)　endowing　this　polymer　with　potential　in　bio-related　applications.　©　2024　Wiley-VCH　GmbH.