Triplet Emitting C^N^C Cyclometalated Dibenzo[c,h]Acridine Pt(II) Complexes

Abstract

In a series of Pt(II) complexes [Pt(dba)(L)] containing the very rigid, dianionic, bis-cyclometalating, tridentate C^N^C2‒ heterocyclic ligand dba2– (H2dba = dibenzo[c,h]acridine), the coligand (ancillary ligand) L = dmso, PPh3, CNtBu and Me2Imd (N,N’-dimethylimidazolydene) was varied in order to improve its luminescence properties. Beginning with the previously reported dmso complex, we synthesized the PPh3, CNtBu and Me2Imd derivatives and characterized them by elemental analysis, 1H (and 31P) NMR spectroscopy and MS. Cyclic voltammetry showed partially reversible reduction waves ranging between ‒1.89 and ‒2.10 V and increasing along the series Me2Imd < dmso ≈ PPh3 < CNtBu. With irreversible oxidation waves ranging between 0.55 (L = Me2Imd) and 1.00 V (dmso), the electrochemical gaps range between 2.65 and 2.91 eV while increasing along the series Me2Imd < CNtBu < PPh3 < dmso. All four complexes show in part vibrationally structured long-wavelength absorption bands peaking at around 530 nm. TD-DFT calculated spectra agree quite well with the experimental spectra, with only a slight redshift. The photoluminescence spectra of all four compounds are very similar. In fluid solution at 298 K, they show broad, only partially structured bands, with maxima at around 590 nm, while in frozen glassy matrices at 77 K, slightly blue-shifted (~580 nm) bands with clear vibronic progressions were found. The photoluminescence quantum yields ΦL ranged between 0.04 and 0.24, at 298 K, and between 0.80 and 0.90 at 77 K. The lifetimes τ at 298 K ranged between 60 and 14040 ns in Ar-purged solutions and increased from 17 to 43 µs at 77 K. The TD-DFT calculated emission spectra are in excellent agreement with the experimental findings. In terms of high ΦL and long τ, the dmso and PPh3 complexes outperform the CNtBu and Me2Imd derivatives. This is remarkable in view of the higher ligand strength of Me2Imd, compared with all other coligands, as concluded from the electrochemical data.

Enhanced luminescence properties through heavy ancillary ligands in [Pt(C^N^C)(L)] complexes, L = AsPh3 and SbPh3

“…3 (right) shows selected transition difference densities for the TD-DFT calculated transitions for [Pt(dba)(SbPh 3 )] as a representative example, while illustrating their mixed yet predominantly 1 LC and 1 MLCT character, in agreement with previously reported data for related compounds. 25,28–31,34,35,51,52 Across the series P > As > Sb, a slight red-shift of the UV-vis absorption energies is both observed experimentally and predicted theoretically, matching the electrochemical data and the electronic structure modelled by DFT.…”

“…The new complexes [Pt(C^N^C)(AsPh 3 )] and [Pt(C^N^C)(SbPh 3 )] were synthesised in the same way as the previously reported lighter congeners [Pt(C^N^C)(PPh 3 )] 50,51 via a facile ligand exchange reaction from the corresponding [Pt(C^N^C)(dmso)] precursors as shown in Scheme 1 in good yields ranging from 64% to 84% (more information in the ESI†). The new complexes were characterised by nuclear magnetic resonance (NMR) spectroscopy (see Fig.…”

“…Details on data collection, structure solution and refinement as well as selected structural parameters are listed in Tables S1–S3 (ESI†) alongside with the data for the previously reported [Pt(dpp)(PPh 3 )] and [Pt(dba)(PPh 3 )]. 50,51 The crystal structures feature long Pt⋯Pt distances ranging from about 7 to 10 Å (far beyond a significant interaction between the Pt atoms, typically below 3.5 Å); thus, no significant Pt⋯Pt interactions nor π-stacking interactions were found. 52,53 In contrast, the dba containing structures show pronounced π-stacking of the acridine moieties with centroid distances between 3.5 and 3.6 Å.…”

“…UV-bands in both systems have been previously assigned to transitions into LC (π–π*) states, whereas mixed LC/metal-to-ligand charge transfer (MLCT) states were discussed for increasingly long-wavelength bands. 31,51,69,70…”

“…Herein, we report two series of double cyclometalated Pt( ii ) complexes [Pt(C^N^C)(PnPh 3 )] (Pn = P, As, Sb) based on the two established C^N^C ligand systems dpp (H 2 dpp = 2,6-diphenyl-pyridine) 31,50 and dba (H 2 dba = dibenzo[ c , h ]acridine) 31,51 which showed an enhancement of the phosphorescence quantum yields for their heavier congeners. The role of radiative and radiationless deactivation rates was studied by combining extensive photoluminescence (PL) and transient absorption spectroscopy (TAS) with density functional theory (DFT) calculations.…”