2D–π–A type pyran-based dye derivatives: Photophysical properties related to intramolecular charge transfer and their electroluminescence application

Five different types of 2D–π–A (vinylcyanoacetate)pyran derivatives, each having two donor (2D) and one acceptor (A) groups showed significantly large Stokes shifts. Their fluorescence bands shifted to longer wavelengths with an increase in solvent dielectric constant. The π-conjugated moiety, such as a carbazolyl group, as an electron donor did not impart red-emission saturation. A locked ring structure of the electron donor group led to narrowing of the full width at half maximum (fwhm) and to increased dipole moment in the ground state; in addition, bulky donor groups resulted in molecular stability and restrained electronic perturbation on the excited state. An EL device based on the dyes displayed bright and saturated red light with a high luminance of about 2000 cd/m2 at a current density of 160 mA/cm2 at 16 V.