Highly efficient one-pot synthesis of α-free pyrylium salts with tunable fluorescence emission via ring-expanding reaction of triarylcyclopentadienes

A series of α-free pyrylium perchlorates were prepared by one-pot oxidation of substituted cyclopentadiene derivatives in the presence of perchloric acid. A tunable fluorescence with green to red emission in dichloromethane and green to orange emission in the solid state can be achieved by changing the aryl groups on 2- or 4-position of pyrylium ring. Furthermore, single crystal structures and quantum chemistry calculation were conducted to understand the relationship between structures and optical properties.