Phenyl-triggered photophysical switching between normal fluorescence and delayed fluorescence in phthalonitrile-based luminophores

Abstract

Herein, a facile strategy for switching luminescent properties between normal fluorescence and thermally activated delayed fluorescence (TADF) is presented. Two luminophoric molecules, VPN-Ph and VPN-H, combining phthalonitrile as an electron-accepting core and triphenylamines as electron-donating peripheries with and without two phenyl groups, are newly developed. A comparative study on their structural and photophysical properties is conducted. While non-phenyl VPN-H does not exhibit TADF but normal fluorescence, phenyl-introduced VPN-Ph exhibits TADF with a high photoluminescence quantum yield as a consequence of the phenyl-triggered steric congestion. By virtue of the TADF feature, an organic light-emitting diode (OLED) incorporating VPN-Ph as an emitter achieves a maximum external electroluminescence (EL) quantum efficiency as high as 28.0%, which is five times higher than that of the VPN-H-based OLED. Thus, phenyl-triggered geometric modulation has a drastic impact on the resulting photophysical and EL properties, leading to TADF on/off switching.