In our attempt to prepare compound 4,5,9,10-tetraamino-2,7-bis(2-octyldodecyl)benzo[lmn] [3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (4NH2NDI) according to the reported procedure for constructing larger conjugated molecules, an unexpected by-product 2,8-bis(2-octyldodecyl)-5,5,11,11-tetraphenyl-5,6,11,12-tetrahydroimidazo[4′,5′:5,6]benzo[1,2,3,4-lmn]imidazo[4,5-f] [3,8]phenanthroline-1,3,7,9(2H,4H,8H,10H)-tetraone (4NHNDI) was obtained through the undesired hydrolysis of 4,5,9,10-tetrakis((diphenylmethylene)amino)-2,7-bis(2-octyldodecyl)benzo[lmn] [3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (4NNDI) when a drop of water was added in this reaction. The undesired hydrolysis was furthered confirmed by the direct hydrolysis of pure 4NNDI in organic solvent with a drop of aqueous HCl solution.4NHNDI was fully characterized by NMR and mass spectroscopy and was further confirmed through a single crystal analysis. The absorption and emission wavelengths of 4NHNDI were red-shifted ~ 20 and 30 nm with the increased polar of the solvents due to the dipole–dipole interactions between 4NHNDI and the polarity solvents. Interestingly, 4NHNDI can act as an efficient anion sensor for fluoride ion over a wide range of other anions (ClO4−, Br−, BF4−, I−, NO3−, Cl−, PF6−, AcO−, HSO4- and CN−) owing to the four NH fragments in the main backbone. Surprisingly, 4NHNDI didn't show any sensing behavior in aqueous solutions to all eleven anions including F- anion, which might be due to the steric effects from long alkyl chains in aqueous solution. However, this phenomenon could allow us to recover the quenched spectrum induced by F− anion through adding H2O in pure THF solution.