Ligand-controlled and nanoconfinement-boosted luminescence employing Pt(ii) and Pd(ii) complexes: from color-tunable aggregation-enhanced dual emitters towards self-referenced oxygen reporters

Abstract

In this work, we describe the synthesis, structural and photophysical characterization of four novel Pd(ii) and Pt(ii) complexes bearing tetradentate luminophoric ligands with high photoluminescence quantum yields (Φ _L) and long excited state lifetimes (τ) at room temperature, where the results were interpreted by means of DFT calculations. Incorporation of fluorine atoms into the tetradentate ligand favors aggregation and thereby, a shortened average distance between the metal centers, which provides accessibility to metal-metal-to-ligand charge-transfer (³MMLCT) excimers acting as red-shifted energy traps if compared with the monomeric entities. This supramolecular approach provides an elegant way to enable room-temperature phosphorescence from Pd(ii) complexes, which are otherwise quenched by a thermal population of dissociative states due to a lower ligand field splitting. Encapsulation of these complexes in 100 nm-sized aminated polystyrene nanoparticles enables concentration-controlled aggregation-enhanced dual emission. This phenomenon facilitates the tunability of the absorption and emission colors while providing a rigidified environment supporting an enhanced Φ _L up to about 80% and extended τ exceeding 100 μs. Additionally, these nanoarrays constitute rare examples for self-referenced oxygen reporters, since the phosphorescence of the aggregates is insensitive to external influences, whereas the monomeric species drop in luminescence lifetime and intensity with increasing triplet molecular dioxygen concentrations (diffusion-controlled quenching).

Scheme 1. Synthesis of the tetradentate ligand precursors (LH and LF) and the corresponding metal complexes PtLH, PtLF, PdLH, and PdLF. All the precursors and complexes were characterized by high resolution mass spectrometry (HRMS) as well as ¹H-, ¹³C- and ¹⁹F-NMR spectroscopy, where all the signals were unambiguously assigned (see ESI, Fig. S1–S34 and Table S1†).

Fig. 1. X-ray diffractometric results regarding dimer formation in crystals of PtLH (top-left), PtLF(1) (top-medium), PtLF(2) (top-right), PdLH (bottom-left) and in PdLF (bottom-right) with the corresponding π⋯π and C–H⋯π interactions and indication of the resulting metal–metal distances.

Fig. 2. Absorption spectra (molar absorption coefficients as a function of wavelength) and normalized (to the highest intensity) emission spectra of PtLH (black), PtLF (red), PdLH (green) and PdLF (blue). In both graphs, absorption and emission spectra of the complexes are shown (solid lines for DCM at 298 K; dashed lines for 1 : 1 DCM/MeOH glassy matrices at 77 K; in all cases, λ_exc = 350 nm and c = 10⁻⁵ M).

Fig. 3. (a) Normalized absorption spectra calculated for the monomeric (solid lines) and dimeric complexes (light-colored lines) and the corresponding spectra (dashed lines) of all four complexes measured in DCM. (b) Normalized theoretical and experimental emission spectra of PtLH, PtLF, PdLH and PdLF (77 K) in DCM. The calculated absorption and emission spectra were obtained with the PBE0 functional and the SDD basis set.

Fig. 4. (Top) Monomers and dimers of the four complexes in the T₁ geometry. (Bottom) Decomposition of the T₁ state for the monomeric and dimeric complexes into contributions originating from MLCT, LMCT, LC, and LLCT configurations determined with the package TheoDORE. The dimers in the ground state of the complexes can be seen in Fig. S72.

Fig. 5. HAADF image and corresponding EDX maps of 100 nm-sized polystyrene nanoparticles loaded with PdLFi.e. PS(PdLF-0.5 mM).

Fig. 6. Φ _L vs. loading concentration of PS with the complexes PtLH/PtLF (left) and PdLH/PdLF (right).

Fig. 7. Emission spectra of PS (loaded with different concentrations of PtLH, PtLF, PdLH, and PdLF; 1 mg mL⁻¹ of nanoparticles, dispersed in air-equilibrated deionized water). (a) Emission spectra of the PS(PtLH-series) under 405 nm excitation; inset: photograph of the PS(PtLH-series) dispersed in deionized water with c_L ranging from 0.1 mM to 2.0 mM. (b) Emission spectra of the PS(PtLF-series) under 388 nm excitation; inset: photograph of the PS(PtLF-series) dispersed in deionized water with c_L ranging from 0.1 mM to 2.0 mM. (c) Emission spectra of the PS(PdLH-series) under 335 nm excitation; insets: normalized emission spectra of the PS(PdLH-series) (top) and photograph of the PS(PdLH-series) dispersed in deionized water with c_L ranging from 0.1 mM to 4.0 mM (bottom). (d) Emission spectra of the PS(PdLF-series) under 368 nm excitation; insets: normalized emission spectra of the PS(PdLF-series) (top) and photograph of the PS(PdLF-series) dispersed in deionized water with c_L ranging from 0.1 mM to 4.0 mM (bottom). The photographs were taken under 365 nm illumination with a UV lamp.

Fig. 8. Amplitudes of the photoluminescence decay components for (a) PS(PtLF-0.5 mM) and (b) PS(PdLF-0.5 mM) derived from biexponential and triexponential fits, plotted as a function of emission wavelength, together with the respective emission spectra (solid lines). (a) The amplitude B1 (red dotted line) corresponds to the aggregate lifetime of τ₁ = 4.8 μs and the amplitude B2 (green dotted line) to the monomer emission with a lifetime of τ₂ = 11.0 μs. (b) The amplitude B1 (red dotted line) corresponds to the fixed lifetime τ₁ = 5 μs of the aggregated complex and B2 (green dotted line) and B3 (blue dotted line) correspond to the lifetimes τ₂ = 28 μs and τ₃ = 240 μs, which are both assigned to the monomeric PdLF.

Fig. 9. (a) Emission intensity of PS(PtLF-0.5 mM) as a function of ³O₂ partial pressure; inset: ratiometric emission intensity-based Stern–Volmer plot. (b) Luminescence lifetime (monitored at 490 nm) as a function of ³O₂ partial pressure. (c) Lifetime (τ₄₉₀)-based Stern–Volmer plot and (d) luminescence lifetime detected at 635 nm as a function of ³O₂ partial pressure.