Systematic tuning of the emission colors and redox potential of Eu(ii)-containing cryptates by changing the N/O ratio of cryptands

Abstract

Eu(II) complexes have been widely investigated in organic light-emitting diodes (OLEDs), magnetic resonance imaging and photoredox catalysis due to their unique photophysical and electrochemical properties. To favor their further applications, it is of necessity to find a simple way to systematically regulate their emission colors and redox potentials. Herein, we have designed and synthesized a series of Eu(II)-containing cryptates EuX₂-N₂O₆ (N₂O₆ = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, X = Br or I, similarly hereinafter), EuX₂-N₄O₄ (N₄O₄ = 4,7,13,16-tetraoxa-1,10,21,24-tetraazabicyclo[8.8.8]hexacosane), and EuX₂-N₆O₂ (N₆O₂ = 4,7-dioxa-1,10,13,16,21,24-hexaazabicyclo[8.8.8]hexacosane), and studied their structures and properties systematically. These Eu(II) complexes exhibit tunable emission colors with maximum emission wavelengths in the range of 415–549 nm and photoluminescence quantum yields (PLQYs) in the range of 18–73%. On changing the N/O ratio of the cryptands, the emission colors, excited-state lifetimes, and redox potential of the corresponding Eu(II) complexes changed linearly. The results provide a clear basis for the design of tunable, luminescent Eu(II) complexes.