Color-Tunable Low-Threshold Amplified Spontaneous Emission from Yellow to Near-Infrared (NIR) Based on Donor–Spacer–Acceptor–Spacer–Donor Linear Dyes

Organic lasing dyes possessing small energy gaps commonly suffer from high thresholds, because of energy-gap law, which induces detrimental nonradiative relaxation. Herein, molecules based on alternating donor (D) and acceptor (A) moieties are described. Those linear molecules are built with a spacer between D and A, with a general formula of D–spacer–A–spacer–D. Such modification of the A core allows one to obtain color-tunable lasing properties from yellow to near-infrared (NIR). Into more details, triphenylamine (TPA) is selected as the D moiety, while benzothiadiazole (BTD) and benzo­[1,2-c:4,5c′]­bis­[1,2,5]­thiadiazole (BBTD) with strong electron-withdrawing capability, are chosen as A cores for yellow ((TPA-F)₂BTD) and NIR ((TPA-F)₂BBTD) lasing dyes, respectively. Both dyes exhibit high photoluminescence quantum yields (PLQYs) and fast fluorescence radiative rate. The corresponding amplified spontaneous emission (ASE) thresholds of yellow and NIR dyes are as low as 0.5 and 3.8 μJ cm^–2 in the solution-processing blending films, with wavelength peaks at 550 and 811 nm, respectively. In those dyes, the long alkyl chains appended on the fluorene conjugated spacer guarantee the high solubility and prevent aggregation. Consequently, for (TPA-F)₂BTD, the neat film allows one to reach PLQYs as high as 90%, with a low ASE threshold (1.9 μJ cm^–2) and good stability. Therefore, such promising molecular design allows the realization of color-tunable ASE or lasing dyes with very low thresholds.