Metal-free pyridinium salts with strong room-temperature phosphorescence and microsecond radiative lifetime

Easily processed metal-free phosphorescent luminophores with a fast rate of phosphorescence are emerging as promising materials for advanced optoelectronics. Alkylation of a modified vitamin B6 vitamer (pyridoxine) affords a family of pyridinium-derived ionic pairs 1-7 exhibiting variable anion-π interactions in the solid state. Such a noncovalent cation-anion network promotes tunable room-temperature phosphorescence (RTP, λ _em = 510-565 nm) in crystalline materials stemming from anion(I^-)-π(pyridinium⁺) charge transfer. Systematic X-ray structural and computational studies manifest the key role of the anion(I^-)-π(pyridinium⁺) distance in the spin-orbit coupling, hence the observed RTP. For the studied pyridinium salts with RTP, the radiative rate constants (k _r) reach up to 0.9-1.3 × 10⁵ s^-1 which are competitive with those of many noble metal emitters. Ion pair 2 reached an RTP with a quantum yield of 93% and was successfully demonstrated as an excellent X-ray scintillating dye in neat films. The demonstrated strategy of attaining intense RTP in small metal-free accessible molecules, i.e., atom-photon economy, represents a new twist in designing efficient and sustainable photofunctional molecular materials.