Mechano-Stimulus and Environment-Dependent Circularly Polarized TADF in Chiral Copper(I) Complexes and Their Application in OLEDs

Abstract

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exciton decay are highly attractive for electroluminescent devices (OLEDs) or next-generation photonic applications, such as spintronics, quantum computing, cryptography, or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure {Cu­(CbzR)­[( S/R )-BINAP]} [R = H (1), 3,6-tBu (2)] are efficient thermally activated delayed fluorescence (TADF) emitters with high radiative rate constants of k TADF up to 3.1 × 105 s–1 from 1/3LLCT states according to our temperature-dependent time-resolved luminescence studies. The efficiency of the TADF process and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupted by grinding of the crystalline materials. The origin of this pronounced mechano-stimulus photophysical behavior is a thermal equilibrium between the 1/3LLCT states and a 3LC state of the BINAP ligand, which depends on the relative energetic order of the excited states and is prone to inter-ligand C–H···π interactions. The copper­(I) complexes are also efficient CPL emitters displaying exceptional dissymmetry values g lum of up to ±0.6 × 10–2 in THF solution and ±2.1 × 10–2 in the solid state. Importantly for application in electroluminescence devices, the C–H···π interactions can also be disrupted by employing sterically bulky matrices. Accordingly, we have investigated various matrix materials for successful implementation of the chiral copper­(I) TADF emitters in proof-of-concept CP-OLEDs.

Towards Fast Circularly Polarized Luminescence in 2‐Coordinate Chiral Mechanochromic Copper(I) Carbene Complexes

“…The potential of molecular transition metal CP‐TADF emitters has recently been demonstrated by our study on trigonally coordinated copper(I) carbazolate complexes bearing chiral (S/R)‐BINAP ligands, which show efficient TADF with k r of 3 ⋅ 10 5 s −1 combined with high g lum of ±7 ⋅ 10 −3 in solution and up to ±2.1 ⋅ 10 −2 in the solid state [25] . The efficiency of the TADF process depends on the environment, but careful choice of the matrix materials allowed for successful implementation in CP‐OLEDs.…”

“…The rationale behind these structural peculiarities is not clear and certainly involves electronic and steric effects, such as C-(carbene)-MÀ N π conjugation and rehybridization of the ipso-N, but also packing effects in the solid state. [25] However, as demonstrated by the group of Marian, [14,23] the dihedral angle γ is the most important structural feature determining the efficiency of TADF in such D-M-A compounds.…”

“…The potential of molecular transition metal CP-TADF emitters has recently been demonstrated by our study on trigonally coordinated copper(I) carbazolate complexes bearing chiral (S/R)-BINAP ligands, which show efficient TADF with k r of 3 • 10 5 s À 1 combined with high g lum of � 7 • 10 À 3 in solution and up to � 2.1 • 10 À 2 in the solid state. [25] The efficiency of the TADF process depends on the environment, but careful choice of the matrix materials allowed for successful implementation in CP-OLEDs. Given our long-standing interest in d 10 coinage metal emitters, [26,27] in particular copper(I) carbene TADF complexes, [17,28] we decided to investigate the chiroptical properties of a series of linearly coordinated [Cu(amide)(cAAC)] complexes (cAAC = cyclic (alkyl)(amino)carbene)) featuring a D-M-A structure with chiral moieties either at the electrophilic carbene ligand or at the amide donor.…”

Thermally Activated Delayed Fluorescence Coinage Metal Cluster Scintillator

“…As shown in Figure e, at low temperature, 1 exhibits relatively long-lasting luminescence, mainly caused by the radiative transition from the excited triplet state to the ground state. With continuous heating, the thermally activated RISC process is turned on, and the luminescence lifetime of 1 gradually decreases along with the occurrence of the TADF process, and the lifetime variation can be described by the Boltzmann equation modified through excited-state dynamics. , …”

“…With continuous heating, the thermally activated RISC process is turned on, and the luminescence lifetime of 1 gradually decreases along with the occurrence of the TADF process, and the lifetime variation can be described by the Boltzmann equation modified through excited-state dynamics. 41,42 The photophysical parameters of 1 were investigated in detail. The photoluminescence rate (k PL ) was calculated with eq 1…”