Synthesis of fluorescent [1,2,4]triazolo[1,5-a]pyrimidines and study of their photophysical behavior

Abstract

The synthesis and photophysics of a family of [1,2,4]triazolo[1,5-a]pyrimidines bearing strategically tuned substituents are reported. Products were obtained in 58–94% yield by reacting 5-amino-1,2,4-triazoles with β-enaminones under solvent-free microwave conditions; notably, precursor heteroamines were obtained in 79–82% yield via a simple and novel protocol. Substituent-dependent solvatofluorochromism showed that dyes with a dimethylamino group exhibited exceptional optical behavior, including hydrochromism and acidochromism in acetonitrile (ϕF = 98% in THF at ∼450 nm; turn-off fluorescence to 10% in water and with TFA; LOD of 9.0–12.6 µM), due to intramolecular charge transfer (ICT) within the 7-arylated heterocyclic core; this process is altered by a methyl substituent on the triazole moiety. DFT and TD-DFT calculations were used to correlate the spectroscopic properties of eight of the synthesized products.