3,8-Disubstituted-3H-imidazo[4,5-a]acridine-11-carbonitriles show very interesting optical properties. In some cases, they have higher quantum yields compared to well-known fluorescent dyes such as fluorescein. Hitherto, no detailed theoretical study has been reported on the mechanism of the synthesis of the titled compounds, hence an accurate and detailed theoretical investigation on the synthesis of these dyes is desirable. In this paper, density functional theory (DFT) methods have been employed to investigate the most reasonable mechanism in formation of these fluorophores. All species and related transition states were optimised and the relative energies of all species and the activation energies for all proposed mechanisms were obtained. Comparing the Ea values of all pathways reveals that intramolecular electrophilic aromatic substitution is the most plausible mechanism for the cyclisation reaction in the preparation of these dyes. Also, the effects of substituents in positions 3 and 8 were investigated and the results show that electron-donor substituents can reduce the activation energy for formation of the σ complex in the electrophilic aromatic substitution.

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