Abstract
A series of heteroleptic [Cu(N^N)(P^P)][PF6] complexes is reported in which N^N is a di(methylsulfanyl)-1,10-phenanthroline (2,9-, 3,8- or 4,7-(MeS)2phen) or di(methoxy)-1,10-phenanthroline (2,9-, 3,8- or 4,7-(MeO)2phen) and P^P is bis(2-(diphenylphosphano)phenyl)ether (POP) or 4,5-bis(diphenylphosphano)-9,9-dimethylxanthene (xantphos). The effects of the different substituents are investigated through structural, electrochemical and photophysical studies and by using DFT and TD-DFT calculations. Introducing methylsulfanyl groups in the 2,9-, 3,8- or 4,7-positions of the phen domain alters the composition of the frontier molecular orbitals of the [Cu(N^N)(P^P)]+ complexes significantly, so that ligand-centred (LC) transitions become photophysically relevant with respect to metal-to-ligand charge transfer (MLCT). Within this series, [Cu(2,9-(MeS)2phen)(POP)][PF6] exhibits the highest photoluminescence quantum yield of 15% and the longest excited-state lifetime of 8.3 μs in solution. In the solid state and in frozen matrices at 77 K, the electronic effects of the methylsulfanyl or methoxy substituents are highlighted, thus resulting in luminescence lifetimes of 2 to 4.2 ms at 77 K with predominantly LC character for both the 3,8- and 4,7-(MeS)2phen containing complexes. The results of the investigation give new guidelines on how to influence the luminescence properties in [Cu(N^N)(P^P)]+ complexes which will aid in the development of new sustainable and efficient copper(i) emitters.
Highlights
We seek to expand the knowledge of such complexes by incorporating symmetrically substituted, isomeric di(methylsulfanyl)phen and di(methoxy)phen ligands (Scheme 1), and we investigate the influence of these electron-donating groups on the structural, electrochemical and photophysical properties of these compounds
This partitioning of orbital character means that functionalization of the N^N ligand with electron-withdrawing or electron-donating substituents can be used for the tuning of the highest-occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap
Taking [Cu(POP)(phen)]+ as a starting point, we were interested in assessing how the introduction of methoxy and methylsulfanyl substituents into various positions of symmetric phen ligands would influence the character of the molecular orbitals (MOs) in the HOMO and LUMO manifolds in these types of copper(I)
Summary
The foundation for a move to copper(I) was built by McMillin and co-workers around 1980, when they revealed the photoluminescent behaviour of copper(I) complexes containing phosphane or bis(phosphane) in combination with 2,20-bipyridine (bpy) or 1,10-phenanthroline (phen) ligands. These complexes commonly exhibit a metal-to-ligand charge transfer (MLCT) excited-state with a radiative emission to the electronic ground state in the visible spectral range. [Cu(POP)(2,9-(MeS)2phen)][PF6] was isolated as an orange solid (170 mg, 0.16 mmol, 88%). [Cu(xantphos)(4,7-(MeS)2phen)][PF6] was isolated as a yellow solid (68 mg, 0.06 mmol, 58%). 31P{1H} NMR (202 MHz, acetone-d6, 298 K) d/ppm À13.1 (xantphos), À144.3 (septet, JPF = 707 Hz, PF6À). [Cu(POP)(4,7(MeO)2phen)][PF6] was isolated as an orange solid (87 mg, 0.09 mmol, 57%). HR ESI-MS positive mode m/z 841.1809 [Cu(4,7-(MeO)2phen)(POP)]+ Found: C 61.11, H 4.59, N 2.62; C53H44CuF6N2O3P3 requires C 61.96, H 4.32, N 2.73
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