ABSTRACT
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 (BZI), 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-1) characterized by vibronic fine structure in nonpolar media (methylcyclohexane at room temperature), and with high photoluminescence quantum yields (ΦPL > 80%) and radiative rate constants (k r ~ 7.8 × 105 s-1). The luminescence is solvatochromic, undergoing a red-shift in a polar solvent (CH2Cl2) at room temperature that are accompanied by a decrease in quantum yields (ΦPL < 23%) and radiative rate constants (k r < 4.0 × 104 s-1), whereas the non-radiative rate constants remain nearly constant (k nr ~ 1.0 × 105 s-1). The radiative rate is controlled via thermally assisted delayed fluorescence (TADF) and temperature-dependent luminescence studies of the gold complex (Au BZI) in methylcyclohexane solution reveal an energy difference between the lowest singlet and triplet excited states of 920 cm-1. An organic light-emitting diode (OLED) fabricated using Au BZI as a luminescent dopant has an external quantum efficiency of 12% and narrow, deep-blue emission (CIE = 0.16, 0.06).
ABSTRACT
A systematic study is presented on the physical and photophysical properties of isoelectronic and isostructural Cu, Ag, and Au complexes with a common amide (N-carbazolyl) and two different carbene ligands (i.e., CAAC = (5 R,6 S)-2-(2,6-diisopropylphenyl)-6-isopropyl-3,3,9-trimethyl-2-azaspiro[4.5]decan-2-ylidene, MAC = 1,3-bis(2,6-diisopropylphenyl)-5,5-dimethyl-4-keto-tetrahydropyridylidene). The crystal structures of the (carbene)M(I)(N-carbazolyl) (MCAAC) and (MAC)M(I)(N-carbazolyl) (MMAC) complexes show coplanar carbene and carbzole ligands and C-M-N bond angles of â¼180°. The electrochemical properties and energies for charge transfer (CT) absorption and emission compounds are not significantly affected by the choice of metal ion. All six of the (carbene)M(Cz) complexes examined here display high photoluminescence quantum yields of 0.8-1.0. The compounds have short emission lifetimes (τ = 0.33-2.8 µs) that fall in the order Ag < Au < Cu, with the lifetimes of (carbene)Ag(Cz) roughly a factor of 10 shorter than for (carbene)Cu(Cz) complexes. Detailed temperature-dependent photophysical measurements (5-325 K) were carried out to determine the singlet and triplet emission lifetimes (τfl and τph, respectively) and the energy difference between the singlet and triplet excited state, Δ ES1-T1. The τfl values range between 20 and 85 ns, and the τph values are in the 50-200 µs regime. The emission at room temperature is due exclusively to E-type delayed fluorescence or TADF (i.e., T1âΔS1âS0+hν ). The emission rate at room temperature is fully governed by Δ ES1-T1, with the silver complexes giving Δ ES1-T1 values of 150-180 cm-1 (18-23 meV), whereas the gold and copper complexes give values of 570-590 cm-1 (70-73 meV).
ABSTRACT
Luminescent complexes of heavy metals such as iridium, platinum, and ruthenium play an important role in photocatalysis and energy conversion applications as well as organic light-emitting diodes (OLEDs). Achieving comparable performance from more-earth-abundant copper requires overcoming the weak spin-orbit coupling of the light metal as well as limiting the high reorganization energies typical in copper(I) [Cu(I)] complexes. Here we report that two-coordinate Cu(I) complexes with redox active ligands in coplanar conformation manifest suppressed nonradiative decay, reduced structural reorganization, and sufficient orbital overlap for efficient charge transfer. We achieve photoluminescence efficiencies >99% and microsecond lifetimes, which lead to an efficient blue-emitting OLED. Photophysical analysis and simulations reveal a temperature-dependent interplay between emissive singlet and triplet charge-transfer states and amide-localized triplet states.
ABSTRACT
The photophysical properties of several Cu(i) complexes coordinated with cyclic (alkyl)(amino)carbene (CAAC) ligands were examined. All the compounds were found to be phosphorescent, regardless of whether they are 2-, 3- or 4-coordinated. Aggregate and excimer emission were observed from 2-coordinate CAAC-CuCl derivatives in methylcyclohexane solution. Emission from the complex 4-coordinated with a trispyrazolylborate ligand is red-shifted with respect to both the chloro-derivative and an analogous complex with an NHC ligand.
