Through the polymerization of a diphenylacetylene derivative and subsequent post-polymerization modification of the poly(diphenylacetylene), polymers with ionic liquid unit of imidazolium as side chain and bromide anion (P2) bis(trifluoromethylsulfonyl)imide (Tf₂N^-) anion (P3) as counterions were obtained. P3 exhibited an ultra-high sensitivity and an extremely low limit of detection (LOD) for picric acid (PA) detection in solution state, the Stern-Volmer constant (K_sv) was 108017 M^-1 and the LOD was 0.6 nM. Meanwhile, P3 exhibited good selectivity to PA over other 8 nitro-aromarics. Mechanistic studies confirmed the coexistence of static quenching and dynamic quenching mechanism, and photo-induced electron transfer (PET) and resonance energy transfer (FRET) processes may also occur during quenching, with electrostatic interactions playing a crucial role. DFT calculation results confirmed that after the exchange of bromide ions with Tf₂N^- anions, the electron-accepting role of the ionic liquid units is significantly weakened, leading to significantly higher sensitivity of P3 compared to P2. And contact-mode detection of PA was conducted using readily available filter paper strips and spin-coating films to better simulate practical application.