A Cyclic Alkyl(amino)carbene as Two‐Atom π‐Chromophore Leading to the First Phosphorescent Linear Cu^I Complexes

Abstract

The members of a series of linear and trigonal copper(I) complexes bearing a cyclic alkyl(amino)carbene (CAAC) ligand show surprising photophysical properties compared to those of the corresponding N-heterocyclic carbene (NHC) complexes. Whereas the linear NHC complexes [CuX(NHC)] are almost non-emissive, [CuX(CAAC)] (X=Cl, Br, I) and [Cu(CAAC) ]PF show very bright emissions from their triplet excited states in the blue to green region, displaying quantum yields of up to 65 % in the solid state, even though the π-acceptor comprises only the carbene C and N atoms with no other π conjugation. [Cu(CAAC) ]PF is the fastest Cu -based triplet state emitter characterized to date, not displaying thermally activated delayed fluorescence (TADF), with an intrinsic lifetime of only 10.6 μs, that is, k =9.4×10 s , competitive with many Pt - and Ir -based emitters. In order to test the stability of such linear copper CAAC complexes in devices, some of our compounds have been applied in proof-of-principle organic light-emitting diodes (OLEDs). This case study thus demonstrates for the first time the use of CAACs as suitable π-chromophores for Cu -based phosphorescent emitters, and their implementation in OLEDs underlines the general applicability of this class of ligands in materials science.

Copper and Gold Cyclic (Alkyl)(amino)carbene Complexes with Sub‐Microsecond Photoemissions: Structure and Substituent Effects on Redox and Luminescent Properties

“…The structures of the Cu I thiocyanates with Me2 La nd Et2 Ll igands differ significantly.T he first indication that all complexes possess an N-bound SCN ligand was provided by the nearly identical 13 CNMR chemical shifts of ( Me2 L)CuNCS (d = 134.6), ( Et2 L)CuNCS, (d = 134.7) and ( Ad L)CuNCS (d = 135.7), which agreedw ith previously reported N-bound thiocyanate complexes. [14] The IR stretching frequencies of ( Me2 L)CuNCS (2085 cm À1 )a nd ( Ad L)CuNCS (2081 cm À1 )a re consistentw ith Nbound terminal NCS ligands, whereas the higher value observed for ( Et2 L)CuNCS (2111 cm À1 )i ndicates bridging character.…”

“…As we have shown before, [1] this points to emissions from solvent exciplexes, with >90% of the emission being due to slow components with lifetimes of 300-800 ns, which increases with increasing steric protection of the metal centre ( Me2 L)Cu < Shortly before the submission of this manuscript a publication appeared which claimed that ( Me2 L)CuX and [( Me2 L) 2 Cu] + complexes show only phosphorescence, with emission lifetimes of tens of microseconds. [13] Here we show that more than one emission mechanism operates, i.e. prompt emission in the nanosecond range, together with a minor component with microsecond life time.…”

“…Av ery recentp aper claimed that the 2:1c ompounds [( Me2 L) 2 Cu]X are unstable for X = Cl, Br or Iand could not be isolated. [13] In our hands, [( Me2 L) 2 Cu]I proved stable to air and Scheme1.Preparation of copper CAAChalide and pseudo-halide complexes.…”

“…However,h igh yields of the bromo and iodo complexes can be obtained by metathesis of ( Me2 L)CuClw ith NaBr or NaI, respectively.A n excess of Me2 La ffords the 2:1c omplex [( Me2 L) 2 Cu]I.T he halide complexes show good solubility in polar non-protic solvents like dichloromethane, THF, MeCN, DMF or acetonea nd are moderately soluble in toluene or ethanol but insoluble in hexane.Av ery recentp aper claimed that the 2:1c ompounds [( Me2 L) 2 Cu]X are unstable for X = Cl, Br or Iand could not be isolated. [13] In our hands, [( Me2 L) 2 Cu]I proved stable to air and Scheme1.Preparation of copper CAAChalide and pseudo-halide complexes.…”

Copper(I) Phosphinooxazoline Complexes: Impact of the Ligand Substitution and Steric Demand on the Electrochemical and Photophysical Properties

“…Hence, only in C5 and C7 , where the substituents at adjacent ligands are pointing towards each other, the flattening distortion is hampered (for S 0 and S 1 structures see Figure S10). Another promising alternative to prevent unwanted flattening upon photoexcitation is the design of linear Cu I complexes based on for example, cyclic alkyl(amino)carbenes, N ‐heterocyclic carbenes and different pyridine or amide ligands . The coplanar arrangement of the ligands can suppress non‐radiative decay and reduce structural reorganization resulting in highly efficient Cu I emitters.…”