Larissa Bergmann

Direct observation of intersystem crossing in a thermally activated delayed fluorescence copper complex in the solid state.

An intersystem crossing time of 27 ps is measured in a copper complex that shows thermally activated delayed fluorescence. Intersystem crossing in thermally activated delayed fluorescence (TADF) materials is an important process that controls the rate at which singlet states convert to triplets; however, measuring this directly in TADF materials is difficult. TADF is a significant emerging technology that enables the harvesting of triplets as well as singlet excited states for emission in organic light emitting diodes. We have observed the picosecond time-resolved photoluminescence of a highly luminescent, neutral copper(I) complex in the solid state that shows TADF. The time constant of intersystem crossing is measured to be 27 picoseconds. Subsequent overall reverse intersystem crossing is slow, leading to population equilibration and TADF with an average lifetime of 11.5 microseconds. These first measurements of intersystem crossing in the solid state in this class of mononuclear copper(I) complexes give a better understanding of the excited-state processes and mechanisms that ensure efficient TADF.

Metal–Organic and Organic TADF-Materials: Status, Challenges and Characterization

This section covers both metal–organic and organic materials that feature thermally activated delayed fluorescence (TADF). Such materials are especially useful for organic light-emitting diodes (OLEDs), a technology that was introduced in commercial displays only recently. We compare both material classes to show commonalities and differences, highlighting current issues and challenges. Advanced spectroscopic techniques as valuable tools to develop solutions to those issues are introduced. Finally, we provide an outlook over the field and highlight future trends.

Are copper(I) complexes tough enough to be processed from solution

Organic light-emitting diodes (OLEDs) suggest fascinating possibilities: they promise to solve existing problems such as large-area lighting and display manufacturing in an elegant, efficient way. In so doing, they could enable innovative products such as flexible, transparent devices and smart packaging. However, OLEDs also present two key challenges: how to substitute commonly used compounds containing very rare metals (iridium and platinum) with readily available materials; and how to simplify a laborious vacuum process using wet-processing techniques such as coating and printing. The use of luminescent copper (Cu) complexes1, 2 could address both aspects, but there is a catch. Not every copper complex is suitable for solution processing. Owing to their electronic properties, only complexes with copper having the oxidation number +1 are suitable as emitters in luminescent devices. Such complexes contain one or more positively charged copper(I) ions that are connected to several ligands (binding molecules). Depending on the charge of the ligands, the complexes may contain counterions to obtain an electronically neutral compound. Some of these compounds are not air stable and are vulnerable to oxygen and water: see Figure 1(a). Dealing with this aspect is tedious during synthesis, but it has already been resolved. Both water and air must be carefully avoided in any event to produce OLED devices. In fact, some of the reported best copper(I) OLEDs contain substances very sensitive to air.3 Similarly, some copper(I) complexes show intrinsic weaknesses. They dissociate in solution, potentially forming other compounds and can therefore not be used in OLED devices.4 The key to properly controlling the structure and properties of Figure 1. Copper(I) complexes in solution degrade (a) owing to reaction with oxygen, water, or coordinating solvent molecules, or (b) because the formation of other complexes is sometimes favored. Cu: Copper. Br: Bromide. P: Phosphorus. Ph: Phenyl. n: Repeating unit.

Novel oligonuclear copper complexes featuring exciting luminescent characteristics

A series of highly luminescent mono-, di-, and trinuclear copper(I) complexes has been synthesized using modular ligand systems of easily accessible N^N, P^P or P^N ligands in order to show the rich structural diversity of copper(I) compounds. Those systems allow for the design of various emitting materials with desired photophysical properties, such as emission colors and high efficiencies. The complexes were characterized with well-established methods such as X-ray crystallographic studies or elemental analysis and, in addition, due to their interesting photoluminescence characteristics, their emission properties were further investigated by means of spectroscopic methods as well as DFT-calculations. In detail, various cationic and neutral mononuclear complexes have been synthesized in order to investigate the photophysical properties of this these different types of emitting compounds. It has been found that neutral copper(I) complexes show superior emission properties (with PLQY up to 89%) compared to their cationic counterparts. Furthermore, a series of dinuclear and trinuclear copper(I) complexes has been synthesized featuring an easy tunable emission maximum from sky blue to deep red (481 nm to 713 nm) with extraordinary high photoluminescence quantum yields up to 99%. In addition, a new crosslinking-technique has been developed to open up the door for a new way to fully solution processed OLED using these promising emitting compounds: Alkyne-substituted emitting complexes crosslink automatically with azide-polymers in a copper-catalyzed alkyne-azide Click reaction.

CCDC 1047160: Experimental Crystal Structure Determination

Related Article: Larissa Bergmann, Carolin Braun, Martin Nieger, Stefan Brase|2018|Dalton Trans.|47|608|doi:10.1039/C7DT03682E

Data underpinning - Direct observation of intersystem crossing in a Thermally Activated Delayed Florescence copper complex in the solid state

We are grateful to CYNORA GmbH for financial support. This work was supported by the Engineering and Physical Sciences Research Council (grant number EP/J009016/1) and by the European Union Seventh Framework Programme under grant agreement 321305.