Based on the AM1 studies of the electronic structure of 1, 3-N, N'- bis-4-(4'-nitro benzenediazo) phenyl squaraine (BNBPS), the interacting model in which a fluorinion approaches vertically the planar four-member ring of BNBPS has been proposed and the stable geometry of [BNBPS-F]^- has been found. The chromogenic mechanism has been suggested in terms of the calculated total energies of BNBPS and [BNBPS-F]^- at ground and first excited states, respectively. The interactions between BNBPS and Cl^-, Br^-, I^- have also been calculated and the results are in agreement with the experimental phenomena.

Key words: AZO DYES, NITROBENZENE, FLUORINE ION, COLOUR RENDERING