•Two hexaazatriphenylene based chemsensors (1 and 2) were synthesized.•Both 1 and 2 exhibit off/on fluorescent response for Cd2+.•The response of 1 and 2 to Cd2+ is immune to the interference of other cations.•The coordination modes of 1 and 2 with Cd2+ were analyzed with different methods.•Based on coordination modes, the mechanism of fluorescence response was discussed.Two hexaazatriphenylene (HAT) derivatives, 2,3,6,7-tetramethyl-10,11-di(pyridin-2-yl)dipyrazino[2,3-f:2',3'-h]quinoxaline (1) and 2,3-dimethyl-6,7,10,11-tetra(pyridin-2-yl)dipyrazino[2,3-f:2',3'-h]quinoxaline (2), were designed and synthesized. Both 1 and 2 exhibited high off–on fluorescent selectivity for Cd2+ over many other metal ions, and the detection limits were determined to be 0.6 and 5.0 μM, respectively. The stoichiometry and coordination mode of blue fluorescent 1-Cd2+ and cyan fluorescent 2-Cd2+ were determined with fluorescence titration fit, Job's plot analysis, 1H NMR titration and X-ray crystallography, and the fluorescence enhancement mechanism was analyzed with density functional theory calculation.Two HAT derivatives exhibit highly selective off–on fluorescent response for Cd2+ based on CHEF mechanism, which are further investigated by the X-ray crystallographic method and DFT calculation.