Investigating ESIPT and donor-acceptor substituent effects on the photophysical and electrochemical properties of fluorescent 3,5-diaryl-substituted 1-phenyl-2-pyrazolines

This paper describes the synthesis, structural study, and evaluation of electrochemical and photophysical properties by UV-Vis absorption and fluorescence emission analysis (solution and solid-state) of a series of eight 3,5-aryl-substituted 1-phenyl-2-pyrazolines (5), where 3-aryl = 2-OH-C6H4 (5a-g) or Ph (5h), and 5-aryl = Ph (a, h), 1-naphthyl (b), 4-Br-C6H4 (c), 4-F-C6H4 (d), 4-OCH3-C6H4(e), 4-NO2-C6H4 (f), 4-(N(CH3)2)-C6H4(g). The UV-Vis absorption properties of 2-pyrazolines were evaluated in DCM, MeCN, AcOEt, EtOH, and DMSO as the solvent and showed a fluorescence shift for the polar aprotic solvents. The steady-state fluorescence emission exhibited a band in the blue region when excited at the least energetic transition of each compound, although the excited-state intramolecular proton (ESIPT) effect was not detected. In the solid state, compounds presented similar behavior regarding absorption and emission properties compared to the solution assays. With the electrochemical analyses performed for the synthesized 2-pyrazolines, it was possible to conclude that the redox potentials were influenced by the electronic and steric effects of the substituents on the aryl rings and, according to the electronic nature of the substituents, which electron-donating groups were favored. Finally, the TD-DFT analyses revealed that all compounds had delocalized electron density throughout the 2-pyrazolines unit and were not influenced by the substituent bonded at C-5. Nonetheless, LUMO orbital analysis showed that only derivatives 5b and 5f have this localized density over the substituents.