One pot oxygen mediated syntheses of stable radicals

Abstract

Air- and thermally-stable emissive carbon-centered radicals (CCRs) were synthesized at room temperature in open air by varying either the base concentration or the dilution factor of the reaction mixture. The carbon radical centers in the synthesized DCP˙, DCPC˙ and CP˙ have two adjacent C(sp²) in the chromenopyridine moiety and are further connected to C(sp³), C(sp²) or H atom, respectively. In situ generated DCPH, which contains contiguous C(sp³)–H bonds in chromenopyridine and dicyanomethyl moieties, is responsible for the base- and oxygen-mediated synthesis of these CCRs. Among these radicals, DCPC˙ having a π-tetramer in its crystal structure shows temperature-dependent electron paramagnetic resonance (EPR) in the solid state. CP˙ behaves differently in the NMR solvents DMSO-d₆ and D₂O producing nonaromatic and aromatic species, respectively. In water, CP˙ generates kinetically controlled green-red dual emissive cationic species CP(KC)⁺, which showed pH-dependent reversible absorbance and fluorescence ON–OFF patterns through neutral CP–H.