Pyrazolo[3,4-b]hydroquinoline-5-Ones: Optoelectronic Properties and DFT Computational Studies

This study explores the structure-property relationships in a series of pyrazolo[3,4-b]hydroquinoline-5-ones (PzHQs), a promising class of fluorophores with potential applications in bioimaging, sensors, and advanced materials. We synthesized PzHQs with diverse electron-donating and -withdrawing substitutions at the N1-, C3-, and C4-positions of the heterocyclic rings to systematically investigate their electronic, steric, and push-pull effects on their fluorescence emissions. The results demonstrated that, unlike the electron-withdrawing groups, electron-donating substituents enhance the fluorescence intensity. Furthermore, we identified and quantified several key environmental factors influencing the fluorescence emission, including the critical self-quenching concentration (CQC), the presence of molecular oxygen, and solvent polarity. A complementary DFT computational study at the B3LYP/6-31++G** level, using the CPCM solvent model, was performed, providing theoretical support that aligns with and rationalizes the experimental findings.