Multiple Enol–Keto Isomerization and Excited-State Unidirectional Intramolecular Proton Transfer Generate Intense, Narrowband Red OLEDs

Abstract

A novel series of excited-state intramolecular proton transfer (ESIPT) emitters, namely, <b>DPNA</b>, <b>DPNA-F</b>, and <b>DPNA-</b><i>^<b>t</b></i><b>Bu</b>, endowed with dual intramolecular hydrogen bonds, were designed and synthesized. In the condensed phase, <b>DPNAs</b> exhibit unmatched absorption and emission spectral features, where the minor 0-0 absorption peak becomes a major one in the emission. Detailed spectroscopic and dynamic approaches conclude fast ground-state equilibrium among enol-enol (EE), enol-keto (EK), and keto-keto (KK) isomers. The equilibrium ratio can be fine-tuned by varying the substitutions in <b>DPNA</b>s. Independent of isomers and excitation wavelength, ultrafast ESIPT takes place for all <b>DPNAs</b>, giving solely KK tautomer emission maximized at >650 nm. The spectral temporal evolution of ESIPT was resolved by a state-of-the-art technique, namely, the transient grating photoluminescence (TGPL), where the rate of EK* → KK* is measured to be (157 fs)^-1 for <b>DPNA-</b><i>^<b>t</b></i><b>Bu</b>, while a stepwise process is resolved for EE* → EK* → KK*, with a rate of EE* → EK* of (72 fs)^-1. For all <b>DPNAs</b>, the KK tautomer emission shows a narrowband emission with high photoluminescence quantum yields (PLQY, ∼62% for <b>DPNA</b> in toluene) in the red, offering advantages to fabricate deep-red organic light-emitting diodes (OLED). The resulting OLEDs give high external quantum efficiency with a spectral full width at half-maximum (FWHM) as narrow as ∼40 nm centered at 666-670 nm for <b>DPNAs</b>, fully satisfying the BT. 2020 standard. The unique ESIPT properties and highly intense tautomer emission with a small fwhm thus establish a benchmark for reaching red narrowband organic electroluminescence.