Electron-rich oligofluoranthene has been successfully synthesized by a one-step direct chemical oxidative oligomerization of fluoranthene. Key advantages include easy synthesis, high synthetic yield and low cost when compared with electropolymerization. Oligofluoranthene in solution is a very strong cyan fluorescence emitter with 12.2 times higher intensity than the fluoranthene monomer. The strong fluorescence can be effectively quenched by specific electron-deficient species, enabling the fabrication of low-cost, high-performance chemosensors for the selective detection of Fe(III) ions and the explosive 2,4,6-trinitrophenol (picric acid). A concentration range of >9 orders of magnitude with exceedingly low detection limits down to 10−12 M is possible. No sample enrichment is needed likely due to the synergistic effects of well-distributed π-conjugated electrons with a conical stereo configuration that may enhance the detection ability. Common interferents appear to have little effect as Fe(III) can be selectively detected in both tap water and seawater containing many other metal ions and picric acid can be detected at low concentrations even in the presence of inorganic acids.