A Small Cationic Organo–Copper Cluster as Thermally Robust Highly Photo- and Electroluminescent Material

Abstract

Organic light emitting diods (OLEDs) are revolutionizing display applications. In this aspect luminescent complexes of precious metals such as iridium, platinum, or ruthenium play a significant role. Emissive compounds of earth-abundant copper with equivalent performance are desired for practical, large-scale applications such as solid-state lighting and displays. Copper(I)-based emitters are well-known to suffer from weak spin-orbit coupling and a high reorganization energy upon photoexcitation. Here we report a cationic organo-copper cluster [Cu 4 (PCP) 3 ] + (PCP = 2,6-(PPh 2) 2 C 6 H 3) that features suppressed non-radiative decays, giving rise to a robust narrow-band green luminophore with a photoluminescent (PL) efficiency up to 93%. PL decay kinetics corroborated by DFT calculations reveal a complex emission mechanism involving contributions of both thermally activated delayed fluorescence (TADF) and phosphorescence. This robust compound was solution-processed into a thinfilm in prototype OLEDs with external quantum efficiency up to 11% and a narrow emission bandwidth (65 nm FWHM).

Taking a snapshot of the triplet excited state of an OLED organometallic luminophore using X-rays

“…Combining the data obtained with all three X-ray methods, the following picture emerges: for the triplet excited state the charge shifts from the orbital delocalized on both P-coordinated and Ccoordinated Cu atoms and involving also the Cu-bound phosphorus atoms. This suggests that the charge moves to the phenyl rings, which is in agreement with DFT calculations which predicted more negative charges on the C atom of the bridging phenyl ring bound to Cu1 and Cu4 in the triplet state 31 . The electron transfer is accompanied by a structural re-arrangement involving an increase of the distance between C-coordinated Cu atoms by 0.05 Å and a decrease of the average distance between C-and P-coordinated Cu atoms by 0.04 Å. .…”

“…4, red line). We have calculated the scattering signals for the DFT-based models of the ground and excited triplet states reported 31 . A linear combination of such theoretical data and solvent signal (Fig.…”

“…As [Cu 4 (PCP) 3 ] + is a well-defined tetranuclear organo-copper cluster, in contrast to the vast majority of the copper complexes investigated for OLEDs which are either single or dinuclear coordination entities 11,16,21,32 . According to its ground state structure obtained with X-ray diffraction 31 (Fig. 1c), the four Cu centers are arranged in a concave kite-like rhombic structure with the angle between the two intramolecular planes defined by (Cu1-Cu2-Cu4) and (Cu1-Cu3-Cu4) of 29.3°.…”

“…Such conclusions were supported by theory and summarized in recent reviews 11,16 . For [Cu 4 (PCP) 3 ] + only basic experimental data are available 31 and the theory describing the mechanism of luminescence requires experimental verification. The organo-multicopper cluster motif does not have wellstudied analogues and it is not obvious how to transfer knowledge about the mechanism of luminescence of mononuclear Cu complexes to the multinuclear case.…”

The Pincer Platform Beyond Classical Coordination Patterns

“…Several multi‐core complexes were prepared in which PCP hosts tetra‐nuclear Au 4 ‐ strings (e.g. 61 in Figure 9‐IV) and route‐like structures, [ 142 ] as well as a Cu 4 distorted rhombic arrangement, [ 143 ] all showing close M–M interactions.…”