Fine tuning of the photophysical and electroluminescent properties of DCM-type dyes by changing the structure of the electron-donating group

Abstract

Using 2H-indene-1,3-dione as an electron-withdrawing group and aromatic amines of different structures as electron-donating groups, four new DCM-type dyes, IN-1, IN-2, IN-3, and IN-4, were designed and synthesized for application in organic light-emitting diodes (OLEDs) as red light-emitting materials. IN-2 exhibits a red shift in fluorescence but a blue shift in absorption with respect to IN-1. Similarly, IN-4 exhibits a red shift in fluorescence but a blue shift in absorption with respect to IN-3. These unique photophysical properties were utilized to optimize the electroluminescence color purity and efficiency of these red emitters in OLEDs. In EL devices with the configuration of ITO/NPB (60 nm)/Alq₃ (or Gaq₃) : red dopant (2.0 wt%) (7 nm)/BCP (12 nm)/Alq₃ (45 nm)/LiF (0.3 nm)/Al (300 nm), IN-3 in Alq₃ and IN-1 in Gaq₃ show CIE coordinates of (0.64, 0.36) and (0.65, 0.34), and current efficiencies of 3.24 cd A⁻¹ and 3.02 cd A⁻¹, respectively, much better than the other guest/host combinations. This can be ascribed to the good spectrum overlap either between IN-3 and Alq₃ or between IN-1 and Gaq₃, indicating that the absorption spectrum has the same importance as the fluorescence spectrum in the improvement of color purity of red emitters.