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The fluorescence mechanism of dual-emission carbon quantum dots (DCQDs) is investigated by the improved intercept method, of which the DCQDs with high quantum yield are synthesized by hydrothermal method by using the precursor of sulfadiazine. The research of the morphology, chemical properties and fluorescence properties on DCQDs, shows that DCQDs have graphene-like structure and well-resolved lattice fringes, and that DCQDs fluorescence emission as well intensity has reversibility between acid and alkaline. Based on the ultraviolet absorption spectrum (UV–vis) of the DCQDs, the band gap of DCQDs is estimated by the improved intercept method. Then, the change law of DCQDs emission wavelength at different excitation wavelengths is studied by using the estimated band gap. It is found that the improved intercept method is well consisted with the emission change law of DCQDs at different excitation wavelengths. In addition, the influence of different concentration of Fe3+ on the estimated band gap of DCQDs shows that the Fe3+ has big influence on the band gap of 3.99 eV and 3.06 eV but almost no effect on band gap of 4.93 eV and 3.67 eV. It indicates that the quenching of Fe3+ to DCQDs may be due to the band gap caused by surface defect is changed by Fe3+. Also, DCQDs are used as probe to detect Fe3+ and used as spray ink. Thereby, the improved intercept method may provide a new direction for researching the fluorescence mechanism of carbon quantum dots.

Three donor‐acceptor (D‐A) polymers P1–P3 are designed and synthesized, with 4,7‐dibromo‐5,6‐difluorobenzo[c][1,2,5]thiadiazole (FBT) as A and thiophene derivatives as D. All polymers show good thermal stability. The UV–vis absorption spectra show that P1 has too strong aggregation in solution, and it has no obvious temperature‐dependent aggregation (TDA) performance, which is also revealed in the variable temperature UV–vis absorption spectra. P2 film shows the same aggregation as in solution, P3 film shows stronger aggregation than that in solution. Polymers P1 and P2 both have strong TDA properties. Organic field effect transistors (OFETs) are prepared based on polymers P1, P2, and P3. OFETs devices prepared with high molecular weight polymers have better performance, and the polymer with 2‐nonyltridecyl side chain leads to the highest hole mobility around 0.252 cm2 V−1 s−1. The results show that the application of high molecular weight polymers and the selection of appropriate side chains are very important for the charge transport performance of OFETs.