Photoluminescence and electroluminescence of squarylium cyanine dyes

A series of squarylium cyanine dyes (Sq1, Sq2 and Sq3) were synthesized to explore their applications in organic electroluminescence devices (ELDs) with the aim of achieving highly efficient red emission. The absorption and fluorescence spectra of squarylium cyanine dyes (Sqs) in organic solvents, as well as their interaction with 8-hydroxyquinoline aluminum (Alq) in solution and in film, were investigated to help understand the EL mechanism. The basic device structure consists of a holetransport layer (HTL) of N,N′-diphenyl-N,N′-bis (3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD) and an emission layer (EML) of Sq-doped Alq. By the doping method, the EL color can be readily tuned from green (EL of Alq) to red (EL of Sq) upon varying application of voltage. The EL mechanism is suggested to be associated with the energy transfer from the excited Alq to the dopant Sq.