Polyimides with Excited‐State Intramolecular Proton Transfer and Room‐Temperature Phosphorescence Properties by End‐capping

Organic polymers possessing excited-state intramolecular proton transfer (ESIPT) significantly impact their electronic and optical properties through stimulus response, leading to unique photoluminescence behaviors such as large Stokes shifts, dual emission, and environmental sensitivity. In this work, polyimides (PIs) exhibiting both ESIPT and room-temperature phosphorescence (RTP) properties were designed and synthesized by end-capping with 5-amino-2-(2-hydroxyphenyl) benzothiazole (HBTA) and 5-amino-2-(2-hydroxyphenyl) benzimidazole (HBIA), respectively. HBTA-PI and HBIA-PI exhibit bright yellow-green and sky-blue fluorescence at 510 nm and 470 nm, with a relatively high photoluminescence quantum yields of about 27.2% and 25.1%, respectively. Importantly, HBTA-PI and HBIA-PI show excellent RTP emissions at 520 nm and 485 nm, with decay lifetimes of 76.5 ms and 53.2 ms. The theoretical results indicate that the effective and large spin-orbit coupling (SOC) constants from their keto forms in the excited state can enhance the intersystem crossing (ISC) process to produce triplet excitons. The rigid polyimide networks further inhibit the nonradiative decay of triplet excitons and enhance phosphorescence emission. Additionally, the excellent ESIPT properties of HBTA-PI and HBIA-PI enable the detection of HCl vapor, which induces multiple fluorescent and phosphorescent color changes. This work provides good examples of polyimides for organic polymers with ESIPT properties and multi-stimulus response emissions.