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. 2023 Nov 29;28(23):7834.
doi: 10.3390/molecules28237834.

The Effect of Monodentate Co-Ligands on the Properties of Pt(II) Complexes Bearing a Tridentate C^N*N-Luminophore

Stefan Buss  1   2 , Leon Geerkens  1   2 , María Victoria Cappellari  1   2 , Alexander Hepp  1 , Jutta Kösters  1 , Cristian A Strassert  1   2
Affiliations

The Effect of Monodentate Co-Ligands on the Properties of Pt(II) Complexes Bearing a Tridentate C^N*N-Luminophore

Stefan Buss et al. Molecules. .

Abstract

In this study, the insertion of different monodentate co-ligands on Pt(II) complexes bearing a monoanionic C^N*N luminophore as a tridentate chelator was achieved beyond the previously reported chlorido- ([PtCl(L)]) and cyanido-decorated ([PtCN(L)]) analogues. To investigate the impact of the auxiliary ligand on the photophysical properties, we introduced a neutral carbonyl-ligand and observed a lower photoluminescence quantum yield (ΦL) than with a cyanido moiety. However, the direct substitution of the chlorido co-ligand by a NO-related derivative was not successful. Interestingly, the attempted reduction of the successfully inserted nitrito-N-ligand in [PtNO2(L)] resulted in the oxidation of the Pt(II)-center to Pt(IV), as demonstrated by X-ray diffractometry. For comparison, the trifluoroacetato Pt(II) and chlorido Pt(IV) complexes ([PtTFA(L)] and [PtCl3(L)], respectively) were also synthesized. The photophysical characterization revealed similar photoluminescence profiles for all complexes, indicating a weak effect of the co-ligand on the excited state; in fact, all complexes display emission from metal-perturbed ligand-centered states (even the Pt(IV) species). Nonetheless, longer excited state lifetimes (τav) suggest a reduced thermally-activated radiationless deactivation via metal-centered states upon exchange of the chlorido units for other monodentate entities, yet without significantly improving the overall ΦL at room temperature. The irreversible oxidation waves (measured via cyclic voltammetry) mostly stem from the Pt(II)-center; the co-ligand-related drop of these potentials correlates with the increasing σ-donating capacities of the ancillary ligand. In summary, an enhanced π-acceptor capacity does not necessarily improve the ΦL and can even impair radiative rates by compromising the perturbative participation of the metal center on the emissive triplet state; in addition, strong σ-donor abilities improve the phosphorescence efficiencies by hampering the thermal population of dissociative electronic configurations related to the participation of antibonding d*-orbitals at the metal center.

Keywords: co-ligand exchange; photophysics; soluble triplet emitters; structural characterization; synthesis of Pt(II) complexes; time-resolved photoluminescence spectroscopy.

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Conflict of interest statement

The authors have no conflict of interest to declare.

Figures

Figure 1
Figure 1
Selected N*N^C-chelated Pt(II) complexes from previous studies. Complexes with a phenylacetylido co-ligand (left) [55] as well as the chlorido-[PtCl(L)] (center) and cyanido-containing analogues [PtCN(L)] (right) are shown as well [42,56].
Figure 2
Figure 2
Schematic representation of the co-ligand exchange reactions and the oxidation towards Pt(IV) species.
Figure 3
Figure 3
Molecular structure in the crystalline phase of [PtNO2(L)] (left; CCDC-Nr.: 2298207) and [Pt(TFA)2NO2(L)] (right; CCDC-Nr.: 2298209). Hydrogen atoms are omitted for clarity (left). Displacement ellipsoids are shown at 50% probability.
Figure 4
Figure 4
Molecular structure in the crystalline phase of [PtTFA(L)] (left; CCDC-Nr.: 2298208) and [PtCl3(L)] (right; CCDC-Nr.: 2298210). Displacement ellipsoids are shown at 50% probability.
Figure 5
Figure 5
Steady-state photoluminescence spectra at 298 K in liquid DCM at RT (left) for [PtCN(L)] (black), [PtCO(L)] (red), [PtNO2(L)] (blue), and [PtTFA(L)] (orange), as well as at 77 K in frozen glassy matrices of DCM/MeOH (v:v = 1:1) (right) of [PtCl(L)] (black) and [PtCl3(L)] (red). All solutions used for the emission spectra were optically diluted (A < 0.1), and the spectra were normalized to the highest intensity.
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