DOI: https://doi.org/10.21826/viiiseedmol202006 Photophysical characterization of 3-acyl-4-quinolones Karine N. de Andrade 1 , Amanda R. P. Costa 1 , Rodolfo I. Teixeira 2 , Micaeli L. da S. Moreira 2 , José Walkimar M. Carneiro 1 , Nanci C. L. Garden 2 , Fernanda da C. S. Boechat 1 , Maria Cecília B. V. de Souza 1 , Pedro N. Batalha 1* , Rodolfo G. Fiorot 1* 1 Universidade Federal Fluminense, Programa de Pós Graduação em Química, Niterói, RJ, Brazil. 2 Universidade Federal do Rio de Janeiro, Programa de Pós Graduação em Química, Rio de Janeiro, RJ, Brazil *pedrobatalha@id.uff.br, rodolfofiorot@id.uff.br Keywords: fluorescence, absorption, TD-DFT, quinolones Abstract 4-quinolones derivatives can present fluorescent properties, depending on their substituents and on the chemical environment (e.g., acidic medium), allowing their application as ion sensors. We theoretically evaluated the photophysical properties of previously synthesized 3- acyl-4-quinolones to verify how different substituents (R1=H, NH2 and R2 = OEt, OH, NHPh) affect their absorption profiles and the emission profile of a reference compound, PB3. All DFT and TD-DFT calculations were performed at B3LYP-D3/6-311++G(d,p) level and continuum polarization model for simulated acetonitrile as solvent. For PB2 (R1 = H, R2 = OEt), we observed hypsochromic shift compared to PB3 due to the increase of the gap between HOMO/LUMO (absence of electron-donating group), in accordance with experimental data. For R1=NH2 and R2=OH, NHPh (PB6 and PB10, respectively), the gap between HOMO/LUMO increases, resulting in a soft bathochromic shift for the simulated absorption spectra. In addition, we evaluated the effect of acid addition on the absorption and emission profile of PB3 and the results were compared with experimental data. Our thermodynamic results suggest that protonation occurs on the endocyclic carbonyl of the quinolone moiety, probably due to an increased aromatic character, as suggested by our NICS calculations. Finally, we associate the increase of the fluorescence in the acidic medium to the establishment of an intramolecular hydrogen bond and, thus, increased rigidity. Introduction 4-quinolones derivatives are recognized by their biological activities. As studies also show fluorescent properties in some compounds, as norfloxacin and 6-methoxy- 4-quinolone. 1,2 Therefore, 4-quinolones can be useful as ion sensors. 2 Despite these promising results, photophysical studies of these molecules are limited in the literature. 2 Herein, we simulated the photophysical properties for 4-quinolone derivatives (Figure 1) 3 . We computed all absorption wavelengths (and their main electronic transitions), evaluated the substituent effects, and confronted these results with experimental data. We also calculated the emission spectra of neutral and protonated PB3 to rationalize why this molecule presents a remarkable fluorescence increase in the presence of trifluoroacetic acid (TFA). Our focus on PB3 is due to their synthetic importance as a synthetic precursor (including for the other 4-quinolones). Figure 1: Target 4-quinolones studied. Methodology The effect of the substituent on the absorption profile of the 4-quinolones was evaluated by calculating the vertical excitation of PB3, PB2, PB6 and PB10. Then, we studied the fluorescence of PB3 in neutral and acidic media. All calculations were confronted with the experimental