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Four new fluorescent dyes derived from the 3 H-benzo[ a]pyrazolo[3,4- j]acridine system were synthesised and fully characterised by 1H NMR, 13C NMR, mass and analytical data. These new fluorophores were prepared from the reaction of 1-alkyl-5-nitro-1 H-indazoles with 1-naphthylacetonitrile via nucleophilic substitution of hydrogen, in high yields. The optical properties of the dyes were also investigated and the results revealed that, in some cases, they have higher quantum yields compared with well-known fluorescent dyes such as fluorescein. Solvent effects on the fluorescence characteristics of the four compounds indicated that the emission wavelength is red-shifted with increasing solvent polarity. Furthermore, density functional theory calculations using the B3LYP hybrid functional and the 6-311++G(d,p) basis set provided the optimised geometries and relevant frontier orbitals of the compounds. Calculated electronic absorption spectra were also obtained using the time-dependent density functional theory method.

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