Breaking the Limit - Synthesis and Properties of p-Amino-Triazatriangulenium, the Most Stable Carbenium Ion

Abstract

Highly stabilized tris(dialkylamino)-triazatriangulenium (A-TATA⁺) and tris(methoxy)-triazatriangulenium (MeO-TATA⁺) carbenium ions have been synthesized and characterized as fluorescent dyes. As a result of the extensive delocalization of positive charge in the planar triangulenium system and the presence of six strongly electron-donating groups, these new carbenium ions display extreme cation stability. A linear correlation between cation stability (pK_R+) and reduction potential (E_red) has been established for the trianguleniums, leading to an estimated pK_R+ value of 32 for A-TATA⁺, making A-TATA⁺ the most stable carbenium ion reported to date, surpassing the previous record by 7 pK_R+ units. Furthermore A-TATA⁺ displays unique dye properties in the blue spectral region, with intense absorption at 447 nm (ε = 84000 cm⁻¹ M⁻¹), and a high fluorescence quantum yield at 460 nm (ϕ_f = 45%).