LAUSR

Meeting the challenges of realizing organic electrically pumped laser, low threshold gain materials with minimum optical loss are highly demanded, especially those with insignificant nonradiative processes during lasing. Herein, the low optical loss amplified spontaneous emission (ASE) and high Q‐factor (2,700) lasing are discovered in a solution‐processed benzothiadiazole‐cored multifluorene derivative. The excellent lasing properties of this molecule inspire to focus on the origin of its intrinsic optical loss. The ultrafast time‐resolved spectroscopy studies reveal that the low intrinsic optical loss originates from a relatively “simple” excited state interaction including just singlet–singlet absorption and stimulated emission, excluding additional loss processes such as intermolecular charge‐separation and triplet–triplet absorption that are detrimental to achieve high quality lasing. Quantitative descriptions on ASE process are also discussed based on both transient absorption spectra and transient photoluminescence spectra. This study shows new options and deeper understanding for designing low threshold gain materials.