AUTHOR=Hamze Rasha , Idris Muazzam , Muthiah Ravinson Daniel Sylvinson , Jung Moon Chul , Haiges Ralf , Djurovich Peter I. , Thompson Mark E. 

TITLE=Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes Bearing Bulky NHC Benzimidazolyl Carbene

JOURNAL=Frontiers in Chemistry

VOLUME=8

YEAR=2020

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2020.00401

DOI=10.3389/fchem.2020.00401

ISSN=2296-2646

ABSTRACT=<p>The structural, photophysical and electrochemical properties of three luminescent 2-coordinate coinage metal (i.e., M = Cu, Ag, Au) complexes bearing a sterically bulky benzimidazolyl carbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene (<bold>BZI</bold>), and carbazolide (Cz) as the anionic ligand were investigated. All the complexes emit in the deep blue region (~430 nm) with relatively narrow spectra (full width at half maximum = 44 nm, 2,300 cm<sup>−1</sup>) characterized by vibronic fine structure in nonpolar media (methylcyclohexane at room temperature), and with high photoluminescence quantum yields (Φ<sub>PL</sub> &gt; 80%) and radiative rate constants (<italic>k</italic><sub><italic>r</italic></sub> ~ 7.8 × 10<sup>5</sup> s<sup>−1</sup>). The luminescence is solvatochromic, undergoing a red-shift in a polar solvent (CH<sub>2</sub>Cl<sub>2</sub>) at room temperature that are accompanied by a decrease in quantum yields (Φ<sub>PL</sub> &lt; 23%) and radiative rate constants (<italic>k</italic><sub><italic>r</italic></sub> &lt; 4.0 × 10<sup>4</sup> s<sup>−1</sup>), whereas the non-radiative rate constants remain nearly constant (<italic>k</italic><sub><italic>nr</italic></sub> ~ 1.0 × 10<sup>5</sup> s<sup>−1</sup>). The radiative rate is controlled via thermally assisted delayed fluorescence (TADF) and temperature-dependent luminescence studies of the gold complex (<bold>Au</bold><sup>BZI</sup>) in methylcyclohexane solution reveal an energy difference between the lowest singlet and triplet excited states of 920 cm<sup>−1</sup>. An organic light-emitting diode (OLED) fabricated using <bold>Au</bold><sup>BZI</sup> as a luminescent dopant has an external quantum efficiency of 12% and narrow, deep-blue emission (CIE = 0.16, 0.06).</p>