Experimental and theoretical study of the photochemical properties and acidochromism of nonconventional 2,4,6-triarylpyridine-based materials

Abstract

2,4,6-Triarylpyridines (TAPs) represent a versatile class of N-heterocycles with broad applications in many fields. This study presents an investigation on photochemical properties and acidochromism behavior of novel 2,6-bis(4-(dodecyloxy)phenyl)-4-(4-substituted-phenyl)pyridine-based derivatives (1a-1g). Photochemical study in dichloromethane (DCM) and acetone (ACT) solutions showed a small solvent effect for the TAP derivatives, except for the carbazole derivative 1a, which was characterized as the most emissive material in all conditions, but especially in DCM solution for blue emission (λEM = 406 nm, ϕf = 0.36). The presence of electron-donating groups at the 4-phenyl moiety contributes to more intense and redshifted emissions in both DCM and ACT solutions, as well as in the solid state. In aqueous ACT, while TAPs bearing EDGs such as 1a presented more intense emissions depending on the water fraction, an opposite effect was verified for fluorinated derivative 1f. The acidochromism of selected TAPs 1a and 1f (fluorinated derivative) in aqueous acetone was characterized by the decreasing of the emission intensity at 436 nm and 388 nm, respectively, in the presence of aqueous HCl for both TAPs, however, the protonated 1f showed strong emission in higher wavelength (λEM = 476 nm) in acidic medium. A 1H NMR study allowed a better understanding of the structural features associated to the acidochromism. The photochemical and acidochromism results obtained for the TAP derivatives are strongly supported by computational results, and particularly noteworthy is the favoring of the resonance effect as consequence of the decreasing in the torsion angle between the 4-alkoxyaryl substituents and the pyridine core after protonation of 1f. The emissive properties and acidochromism capacity of 1a and 1f allowed the potential application of TAP-adsorbed paper strips as disposable sensors for qualitative detection of gaseous HCl.