Tunable single and dual emission behavior of imidazole fluorophores based on D-π-A architecture

• A series of imidazole derivatives were synthesized and characterized.
• Influence of solvent polarity on spectral behavior was investigated.
• Kamlet–Taft and Catalán solvent parameters were used to analyze the solvatochromism.
• Interplay between LE and ICT state was analyzed.
• TD-DFT method was used to study the geometric and electronic structure of molecules.

A series of three D-π-A molecules containing 4,5-diphenyl-1H-imidazole moiety as the donor (D) with cyanoacetic acid, rhodanine-3-acetic acid and 4-nitrophenyl acetonitrile as the acceptor (A) connected by a biphenyl bridge have been synthesized and characterized. Crystallography of BIC shows C4-phenyl group to be twisted nearly perpendicular to the imidazole unit and the remaining groups are in plane. The absorption spectra of the compounds are dominated by intramolecular charge transitions that arise from the imidazole core to the acceptor groups. The compounds display distinct dual and single emission behavior in different solvent environments, namely locally excited (LE ∼ 430 nm) and intramolecular charge transfer (ICT ∼ 550 nm). The Lippert–Mataga analysis suggested that the excited state dipole moment of ICT band is higher than the LE band. The influence of general and specific solvent effects on photophysical properties of the fluorophores was discussed with Catalán four parameter and Kamlet–Taft three parameter solvent scales. These results suggest that the major influence from solvent dipolarity takes part in a prominent role on changing the emission behavior of the compounds. The quantum yield of the compounds ranged between 0.07 and 0.55 in different solvents and it was shown to depend on the substitution pattern most notably that based on acceptor groups. All the compounds exhibit two lifetimes in the singlet excited state corresponding to LE state (short lived) and ICT state (long lived). In addition, the triplet state absorption properties of the compounds are also investigated by transient absorption measurement. The geometric structure and electronic structure of the molecules in the ground state is studied with DFT methods, whereas the energies of the lowest singlet excited states are calculated by employing TD-DFT methods. The results show that most prominent bands in acetonitrile solvent with higher oscillator strength for BIC, BIN and BIR are due to LE.

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