Síntesis y evaluación de las propiedades fotofísicas de pirazoles e isoxazoles derivados de tetralona

Abstract

In this work, N-heterocyclic compounds derived from tetralone of structural interest were synthesized, and their photophysical properties were studied. Initially, β-enaminones derived from 1-tetralone and 6-methoxy-1-tetralone were obtained through microwave-assisted reactions. Subsequently, these β-enaminones were used as precursors for the synthesis of pyrazole derivatives 3a–d and isoxazole derivatives 3e,f through cyclocondensation reactions. The products were then subjected to a dehydrogenation process using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), aimed at forming the corresponding aromatic derivatives 4a–f. Finally, two of the aromatized pyrazole derivatives 4a,b were functionalized through an N-acetylation reaction, affording compounds 5a,b. The structural characterization of the compounds was performed by ¹H and ¹³C NMR spectroscopy, DEPT-135, and two-dimensional experiments such as COSY, HSQC, HMBC, and NOESY, complemented by HSMS-ESI. The products 3a–f were obtained in yields ranging from 59 to 86%, their aromatized analogues 4a–f in yields of 35–88%, and the acetylated derivatives 5a,b in yields of 60–77%. The photophysical study was carried out by absorption and fluorescence spectroscopy in solvents with different polarities, such as toluene, dichloromethane, and acetonitrile. The dihydro derivatives 3c–f exhibited absorption bands between 269 and 290 nm, whereas in the heteroarenes 4c–f, the absorption bands shifted to 316–322 nm, which is associated with an increase in conjugation in these compounds. The acetylated derivatives 5a,b showed bands around 350 nm due to intramolecular charge-transfer processes. Regarding emission, the isoxazole dihydro derivatives exhibited low fluorescence quantum yields (Φꜰ ≤ 5.02%) due to non-radiative processes, which improved in the heteroarenes 4c,d (Φꜰ ≤ 21%) but was higher in the acetyl derivatives 5a,b (Φꜰ = 34–58%). In particular, 5b exhibited the highest fluorescence quantum yield among all the compounds studied (58%). DFT calculations of the optimized geometry were performed to gain insight into the electronic behavior of 5b, where its HOMO and LUMO frontier orbitals exhibit a differentiated electron distribution across the donor and acceptor fragments, consistent with a donor–acceptor “push–pull” molecular system with intramolecular charge transfer. Overall, the results allowed the structure–property relationship of the studied azoles to be explored, in which structural modifications, such as the incorporation of electron-donating groups and changes in the electronic state, can modify their photophysical behavior and provide elements for the design of heterocyclic molecules with tunable or modular optoelectronic properties.