Luminescent Copper(I)‐Complexes with an Anionic NHC obtained via a Coordination Polymer as Versatile Precursor**

Abstract

The anionic diamido N‐heterocyclic carbene 1 is used to prepare a series of linear as well as trigonal, heteroleptic CuI complexes with small molecular ligands such as pyridine derivatives or triphenylphosphine. A key role lies in the versatile precursor for these complexes, a moisture‐ and air‐stable 1D coordination polymer [1 ⋅ Cu]n composed of only the NHC ligand and CuI, such that the copper is linearly coordinated by the carbene carbon atom and one oxygen atom of the backbone of the carbene. This polymer can easily be cleaved into monomeric complexes by addition of the desired ligand to dispersions of the polymer in dichloromethane. In solution, the complexes are in equilibrium with this highly insoluble polymer and free ligand. Thus, analytical and spectroscopical experiments with the compounds are limited to their crystalline state, characterized by single crystal X‐ray diffraction experiments. Some of the complexes exhibit visible luminescence in the solid state upon irradiation with ultraviolet light. The spectral features (emission wavelength, Stokes shift, width of the emission band, vibrational fine structure) significantly differ among the complexes. Quantum mechanical computations reveal a subtle interplay of several factors such as coordination number and charge transfer character of the emissive state.

How to tune luminescent Cu(i) complexes with strong donor carbenes towards TADF?

“…Among the four complexes, 3a exhibits the largest apparent Stokes shift of 7605 cm −1 . 43 Like in 2a , a vibronic progression (Δ ≈ 1200 cm −1 ) is observable in the emission spectrum of the powder at 77 K 43 (Fig. S3, ESI†), which indicates an LC state as the emitting state.…”

“…In the present case, trigonal complexes are formed as well when the sterically less demanding pyridine instead of 2,6-lutidine is added to the zwitterionic NHC Cu( i ) coordination polymer in excess. 43…”

“…The deviations are particularly pronounced around the excitation wavelength of 375 nm. A quantum chemical analysis of the vertical excitation energies and electronic structures of the low-lying singlet and excited states in the FC region 43 had shown that the LC(carbene) and MLCT(d z 2 ) dominated states of 3a are blue shifted compared to their 2a congeners whereas the states exhibiting mainly LLCT(NHC-to-pyridine) or MLCT(d xz ) character are stabilized by the coordination of a second pyridine ligand. The 1 LC state, which was primarily excited in 2a , is hardly accessible in 3a at the excitation wavelength ( λ ex = 375 nm) used to record the emission spectra.…”

“…12,25,26,41,42 To prevent the accidental formation of trigonal complexes, we introduced methyl groups in ortho -position of the pyridine ring which increase the steric demand of the ligand, yielding the linear lutidine and 4-formyllutidine complexes 2a and 2b , respectively. 43 An alternative approach to increase the “pull” effect is the introduction of a second acceptor ligand resulting in the bis(formylpyridine) complex 3b , for which lower energies of the LLCT and MLCT states in comparison to the LC states are calculated (Fig. 1).…”

“…Our synthesis started from the recently described moisture and air-stable one-dimensional coordination polymer [Cu(An6DAC)] n which, upon treatment with the appropriate pyridine ligand, provided the respective linear or trigonal complexes, dependent on the particular substitution pattern of the pyridine derivative (Chart 1). 43 In addition to structural characterization of the synthesized compounds, excitation, absorption and steady-state emission spectra of four luminescent complexes were presented in that work, but the electronic nature of the emitting states, and their time-resolved luminescence properties with the corresponding temperature dependence had not yet been determined. This work is organized in a way that each complex is investigated in detail with respect to its energetic and dynamic properties.…”