Efeitos estruturais nas propriedades acidocrômicas de derivados de 2,4,6-Triarilpiridina

Abstract

Aromatic N-heterocyclic compounds have proven to be highly relevant for the design of emissive materials with diverse applications, such as in the fields of sensors and organic electronics. This study presents the synthesis and investigation of the acidochromic properties of derivatives of the 2,4,6-triarylpyridine (TAP) heterocyclic core. In total, ten TAP derivatives were designed: four containing 4-dodecyloxyphenyl substituents at the 2 and 6 positions of the pyridine core, as well as 4-carbazolylphenyl (1a), 4-piperidinophenyl (1b), 2-thienyl (1c), and 4-fluorophenyl (1d) substituents at the 4 position of the heterocyclic core; three derivatives lacking alkoxy groups, bearing 2- thienyl (1e), phenyl (1f), and 4-fluorophenyl (1g) substituents at the 4 position of the pyridine core; and three additional derivatives (1h, 1i, and 1j) containing 4-fluorophenyl substituents at the 2 and 6 positions of the pyridine core, along with 4- dodecyloxyphenyl, phenyl, and 4-fluorophenyl substituents at the 4-position of the heterocyclic core, respectively. All compounds were synthesized and characterized by IR, 1H and 13C NMR, except for compounds 1f, 1i and 1j, which were only characterized by IR. The TAP derivatives were obtained in moderate yields (25–60%) through multicomponent reactions involving two equivalents of acetophenones and one equivalent of appropriate aldehydes, in the presence of ammonium acetate and cobalt (II) chloride as a catalyst, under solvent-free conditions. The emissive properties of the TAP derivatives were evaluated in a 70:30 v/v acetone-water solution, and all compounds exhibited fluorescence, with maximum emission wavelengths ranging from 355 nm to 445 nm. Among these, the piperidinyl derivative 1b showed the highest emission intensity and the longest emission wavelength. All evaluated compounds displayed acidochromism upon the addition of HCl. Notably, while acid addition suppressed the emission of derivative 1a, the other TAP derivatives were characterized by the quenching of the original emission band, accompanied by the emergence of a new emission band between 390 nm and 486 nm. The presence of electron-donating dodecyloxy groups induced longer wavelength emissions after acid addition, with maximum emission values ranging from 461 nm to 486 nm for 1h-H+ and 1b-H+, respectively. Conversely, the absence of alkoxy groups resulted in smaller shifts in emission wavelengths before and after acid addition. 1H NMR analysis conducted for compound 1d revealed that the alkoxy groups actively participate in the stabilization of the pyridinium cation formed upon protonation of the TAPs. This stabilization likely contributes to the larger differences in emission wavelengths observed for these derivatives before and after acid addition. As a practical application, compound 1d adsorbed onto filter paper was evaluated as a disposable sensor, demonstrating its utility for the detection of gaseous HCl.