Cyclometalated Spirobifluorene Imidazolylidene Platinum(II) Complexes with Predominant ³LC Emissive Character and High Photoluminescence Quantum Yields

Abstract

Two novel bidentate C^C* spiro cyclometalated platinum(II) complexes comprising a spiro-conjugated bifluorene ligand and different β-diketonate auxiliary ligands are synthesized and characterized. Their preparation employs a robust and elaborate synthetic protocol commencing with an N-heterocyclic carbene precursor. Structural characterization by means of NMR techniques and solid-state structures validate the proposed and herein presented molecular scaffolds. Photophysical studies, including laser flash photolysis methods, reveal an almost exclusively ligand-centered triplet state, governed by the C^C* spiro −NHC ligand. The high triplet energies and the long triplet lifetimes in the order of 30 μs in solution make the complexes good candidates for light-emitting diode-driven photocatalysis, as initial energy transfer experiments reveal. In-depth time-dependent density functional theory investigations are in excellent accordance with our spectroscopic findings. The title compounds are highly emissive in the bluish-green color region with quantum yields of up to 87% in solid-state measurements.

Blue Light-Emitting Cyclometalated Spirofluorenexanthene Imidazolylidene Platinum(II) Complexes

“…The C2-Pt bond lengths are 1.943(2) Å for complex 4 and 1.974(1) Å for 6 . The Pt(1)-C(6) bond is 1.980(1) Å or 2.001(1) Å, which is comparable to previous C^C*-platinum­(II) complexes. ,, …”

“…Strikingly similar, two close-lying bands are observed at 300 and 310 nm. These two absorption bands were also observed in our previous work on the SBF–NHC ligand motif. These transitions are studied in more detail in the computational section (Figures S53–S58) of this work.…”

“…In the case of complex 5 , a strong electronic contribution of the LUMO to the emissive state was found . It is worth mentioning that the same auxiliary ligand did not affect the photophysical properties of the respective (SBF-NHC)­Pt­(II) complex, as reported in our previous work . The bis­(pyrazolyl)­borate derivative 6 has a band-structured emission spectrum similar to that of complex 4 .…”

“…Strikingly similar, two close-lying bands are observed at 300 and 310 nm. These two absorption bands were also observed in our previous work 58 SBF-NHC platinum(II) complexes appear at 345−375 nm, absorption onsets for complexes 4 and 6 are rather undefined and shifted toward lower wavelengths (350 nm for 4 and 6). It is worth mentioning that the absorption onset of the absorption spectrum of complex 5 is red-shifted by about 25 nm, compared to the absorption onset of 4 and 6.…”

“…These complexes demonstrated high quantum yields as well as high triplet energies (Figure 1). 58 The strong ligand-centered ( 3 LC) character of the green-blue emission, which is mainly affected by the SBF-NHC-ligand, intrigued us to explore further spiroconjugated NHC motifs and their platinum(II) complexes. In this context, N^N-chelating bisazolyl borate auxiliary ligands were not investigated.…”