Highly efficient thermally activated fluorescence of a new rigid Cu(i) complex [Cu(dmp)(phanephos)]+

Abstract

The rigid [Cu(dmp)( phanephos)] + complex displays a high luminescence quantum yield of 80% at ambient temperature. In contrast to the long-lived phosphorescence of 240 µs at T < 120 K with a radiative rate of k r = 3 × 10 3 s −1 , the ambient-temperature emission represents a thermally activated delayed fluorescence (DF) with a decay time of only 14 μs and a radiative rate of k r (DF) = 6 × 10 4 s −1 . Evidence for the involvement of the excited singlet state in the emission process is presented. This material has high potential to be applied in efficient OLEDs taking advantage of the singlet harvesting mechanism.In the past decade, substantial investigations were carried out to develop novel materials for organic light emitting diodes (OLEDs), 1 in particular, to increase the device efficiency. In this respect, luminescent materials and excitation mechanisms play a crucial role. A breakthrough was reached by applying organometallic complexes based on the 3rd-row transition metals iridium and platinum. 2-6 These substances frequently display high phosphorescence quantum yields approaching even 100% and short emission decay times of a few μs. 7 Of special importance is the fact that by an electroluminescent excitation all triplet and singlet excitons formed in the emissive layer of an OLED can be utilized for light generation, whereby the emission stems from the lowest triplet state. This mechanism, representing the triplet harvesting effect, 8,9 is based on the properties of the organometallic compounds. It allows one to achieve much higher OLED efficiencies than obtainable with typical organic fluorescent (singlet) emitters by which only 25% of the total number of excitons can be exploited. However, the triplet emitters are based on high-cost platinum group metals. Therefore, alternative materials, such as Cu(I) complexes, 5,6,10-14 came into the focus of research. At first sight, Cu(I) complexes seem to exhibit substantial problems with regard to OLED applications: (1) compared to Ir or Pt, Cu as a 1st row transition metal induces much weaker spin-orbit coupling.15 As a consequence, transitions between the excited triplet state and the singlet ground state are largely forbidden. Thus, long phosphorescence decay times of several 100 μs are found. 5,6 Therefore, in an OLED, strong saturation effects would result. (2) Cu(I) complexes with reducible ligands, i.e. with energetically low-lying π* orbitals, such as aromatic diimines, often display distinct low-energy metal-toligand charge-transfer (MLCT) transitions in the visible part of the spectrum. With this type of electronic excitation, a flattening distortion of the molecular structure takes place.13a, [16][17][18] Such structural rearrangements are usually connected with an increase of non-radiative deactivation or even quenching of the emission due to an increase of the Franck-Condon factors that couple the excited state and the ground state. 19,20 This is especially distinct in non-rigid environments. 21This contribution presents a new, adequately designe...

Effects of N-heteroaromatic ligands on highly luminescent mononuclear copper(I)–halide complexes

“…As shown in Figure 4, the lifetimes of complexes 1-X changed gradually in the low-temperature region. However, a sharp decrease in the lifetime was observed with increasing temperature above 170 K. Assuming a two-state model involving the lowest excited singlet state (S 1 ) and the lowest excited triplet state (T 1 ), the observed lifetime can be expressed as a Boltzmann average by using equation (2) [11,[34][35][36].…”

Synthesis, structure, characterization and luminescent properties of copper(I) complexes based on bis-diimine bridging ligands

“…All the complexes have intense absorption bands near ~225(ε ≈ 80000-90000 M -1 cm -1 ) and 260 nm (ε = 60000-70000 M -1 cm -1 ), which are attributed to the π-π* transition of intraligand [5b, 20] . A broad absorption peak near 370 nm (ε≈11000-16000 M -1 cm -1 ) can be ascribed to the metal to ligand charge transfer (MLCT) states originating from a dπ-π* transitions [21] . Accounting for this, the selected DFT calculation of complexes 1, 2 and 3 have recently been carried out to investigate the electronic properties in the Dmol 3 package.…”

Electric-field induced nonlinear optical materials based on a bipolar copper (I) complex embedded in polymer matrices

“…Especially appealing is the presence of low-energy metal-toligand charge-transfer (MLCT) excited states, in which an electron is promoted from an occupied metal d-orbital to an empty p*-orbital of one of the organic ligands [8][9][10][11]. Since the first hyperpolarisability b can be resonance enhanced via such intra-molecular charge-transfer excitations, [12] complexes displaying MLCT electronic transitions, are expected to be well suited for the second harmonic generation.…”