Deep-blue thermally activated delayed fluorescence (TADF) emitters for light-emitting electrochemical cells (LEECs)

Two deep blue thermally activated delayed fluorescence (TADF) emitters (imCzDPS and imDPADPS) that contain charged imidazolium groups tethered to the central luminophore were designed and synthesized as small molecule organic emitters for light-emitting electrochemical cell (LEEC) electroluminescent devices. The emission profile in doped thin films (5 wt% in PMMA) is very blue and narrow (λ_PL: 414 nm and 409 nm; full width at half maximum (FHWM): 62 nm and 46 nm for imCzDPS and imDPADPS, respectively) with good photoluminescence quantum efficiencies (Ф_PL: 44% and 49% for imCzDPS and imDPADPS, respectively). In neat films, emission maxima occur at 440 nm and 428 nm for imCzDPS and imDPADPS, respectively with comparable Φ_PL values of 44 and 61%, respectively. Both emitters exhibit biexponential emission decay kinetics (nanosecond prompt and microsecond delayed fluorescence) in both MeCN solution and thin film, characteristic of TADF behaviour. While imDPADPS did not show any emission in the LEEC device, that of imCzDPS gave an electroluminescence (EL) maximum at 470 nm and CIE coordinates of (0.208, 0.250), which makes this device amongst the bluest reported to date. However, the maximum device luminance achieved was 2.5 cd m^-2 and this poor brightness was attributed to the electrochemical instability of the emitter in LEEC architecture, as evidenced by the additional peak around 550 nm observed in the EL spectrum.