Fluorescence properties, aluminium ion selective emission changes and self-assemblies of positional isomers of 4-(hydroxyphenylthio)naphthalene-1,2-diones

•Three positional isomers of 4-(hydroxyphenylthio)naphthalene-1,2-dione are characterised.•4-(4-hydroxyphenylthio)naphthalene-1,2-dione shows dual-fluorescence emission.•Al3+ ions selectively in fluorescence emissions of the isomers are observed.•One isomer showed ON–OFF fluorescence with increments of Al3+ ions.•Hydrogen bonded motifs of positional isomers involved in proton transfer are analysed.

Three positional isomers namely 4-(hydroxyphenylthio)naphthalene-1,2-dione having the para-, ortho- or meta-hydroxyphenylthio units were synthesized and characterized. The isomer having para-hydroxyphenylthio unit showed dual-fluorescence in certain solvents such as dimethylacetamide, which could be tuned to single fluorescence emission by adding methanol. Fluorescence emission intensities of these isomers increased by 5–6-fold on addition of aluminium ions, with an exception being the isomer having the ortho-hydroxyphenylthio unit, which showed a two tier change of fluorescence intensity with an initial enhancement followed by a decrease. The fluorescence lifetime decay study in the presence and absence of aluminium ions suggests that at low concentration of aluminium ions exciplex formation took place. Crystal structures of these isomers as well as methanol and dimethylacetamide solvates of para-hydroxyphenylthio unit containing isomer were determined. Self-assembling patterns have provided information on the nature of hydrogen bonds contributing to proton transfer.

Graphical abstractCompound 1 shows dual-fluorescence emissions whereas compounds 2 and 3 show single emission in dimethylacetamide. Aluminium ions increase the fluorescence emission intensity of the compounds.Figure optionsDownload full-size imageDownload as PowerPoint slide