Blue fluorescence emitter based on HLCT and AIE: acidochromism behavior and narrow-band emission in OLED applications

Abstract

Owing to their low cost, less toxicity, and high exciton utilization, fluorescent emitters based on hybridized local and charge transfer (HLCT), namely, “hot exciton” materials, as potential organic light-emitting diode (OLED) materials have attracted significant interest. However, because pure blue fluorescence emitters possess a wide energy gap and π–π molecular stacking effects, developing efficient blue fluorescence materials with narrow full width at half maximum (FWHM) is still a challenge. Herein, a blue fluorescence emitter (CF₃–PPI–TPE) was successfully designed and synthesized by incorporating an aggregation-induced emission (AIE) unit of tetraphenylethylene (TPE) and HLCT unit of phenanthroimidazole (PPI). The HLCT excited state and AIE properties of CF₃–PPI–TPE were proved by photophysical measurements and time-dependent density functional theory (TD-DFT) calculations. In addition, CF₃–PPI–TPE exhibited obvious acidochromism behaviors with color change from blue to green after fuming with trifluoroacetic acid (TFA), and the color change was reversible after treatment with NH₃ gas. Non-doped (device I) and doped (as host, device II; as guest, device III) OLED features based on CF₃–PPI–TPE were explored. The color CIE coordinates of the three devices are (0.126, 0.099), (0.135, 0.051) and (0.135, 0.061), and their FWHM in OLEDs are 22.2 nm, 17.3 nm and 22.9 nm, respectively. In addition, their Commission Internationale de l'Eclairage (CIE_y) values are less than 0.07 in OLEDs. These results provide a new strategy for developing narrow FWHM and stimuli-responsive materials based on AIE and HLCT effects for OLEDs.