Synthesis, Spectral Studies and Quantum-Chemical Investigations on the Powerful Fluorophores: Imidazo[4,5-a]acridines

Synthesis, characterisation, optical properties and theoretical calculations of a new fluorescent heterocyclic system: 3H-benzo[a]pyrazolo[3,4-j]acridine

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.

Theoretical Investigation on the Kinetics and Mechanism of the Synthesis of Fluorescent 3,8-Disubstituted-3H-Imidazo [4,5-a] Acridine-11-Carbonitriles

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.