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[Display omitted] • Two isomers of boron difluoride naphthaloylacetonates were synthesized. • Interrelation between crystal structure and luminescent properties has been revealed. • The complexes exhibited fluorescence, delay fluorescence and phosphorescence. • The role of the inversion of S 1 and T 2 levels in enhancing delay fluorescence is revealed. A comparative study of the luminescence properties of solutions and crystals of two isomers: boron difluoride 1-(1′-naphthyl)butanedionate-1,3 (α-NAcBF 2) and 1-(2′-naphthyl)butanedionate-1,3 (β-NAcBF 2) has been performed. An interrelation between the molecular and crystal structure of the studied complexes and their luminescence properties has been revealed. In the α-NAcBF 2 molecule, the plane of the naphthyl group was turned by 34.26° relatively to the chelate cycle, while the β-NAcBF 2 molecule was planar. The difference in the luminescence properties of the crystals of α-NAcBF 2 (452 nm) and β-NAcBF 2 (537 nm) was related to different abilities to form excimers. In β-NAcBF 2 crystals, J-aggregates consisted of dimers of antiparallel molecules comprising excimer traps. For the crystals and solutions of α-NAcBF 2 at 77 K, in addition to phosphorescence, the delayed fluorescence was observed. In case of β-NAcBF 2 , the delayed fluorescence was detected only for crystals, whereas the phosphorescence – for both crystals and solutions. [ABSTRACT FROM AUTHOR]