Low Amplified Spontaneous Emission Threshold from Solution Processable Excited‐State Intramolecular Proton Transfer Chromophores

Abstract Chromophores undergoing excited‐state intramolecular proton transfer (ESIPT) feature a distinct four‐level photocycle and significant Stokes shift, which make them ideal for achieving population inversion upon photoexcitation, a fundamental prerequisite for lasing, by mitigating the concerns of self‐absorption owing to spectral overlap. This study focuses on functionalizing the hydroxyphenyl‐benzothiazole (HBT) ESIPT laser dye through the introduction of fluorene and phenyl‐carbazole moieties at the para‐position to the hydroxyl group, to demonstrate amplified spontaneous emission (ASE) activities. The synthesized HBT derivatives not only exhibit high thermal and photostability, but also high photoluminescence quantum yields, exceeding 40%, which is noteworthy for ESIPT materials emitting in the yellow–orange to red color ranges. Computational analyses show that these new HBT materials possess substantially lower ESIPT energy barriers, compared to their parent HBT, facilitating their efficient ESIPT processes. Moreover, these derivatives demonstrate a high radiative rate of 1.47 × 10⁸ s⁻¹. Thin films of a fluorene‐substituted HBT (HBT‐pFl) show a low solid‐state amplified spontaneous emission threshold value of 3.8 µJ cm⁻² at 582 nm, representing the lowest reported solid‐state ASE threshold for laser dyes in the orange emission region (575–600 nm) to date.