Influence of the monodentate ancillary ligand on the photophysical properties of Pt(II) complexes bearing a symmetric dianionic tridentate luminophore

2.1 Synthesis and structural characterization

The tridentate luminophore was synthesized as reported [3], [12], [13], [14] and used for the complexation with Pt(II) in a one-pot reaction in combination with PPh₃ as the monodentate ancillary ligand to yield the previously described complex C1, according to literature procedures [1]. Alternatively, progressively smaller ancillary ligands, namely a PPh₂Me and a PPhMe₂, yielded complexes C2 and C3, respectively. For comparative purposes, a flat 4-amylpyridine was employed as reported in the past to yield complex C4 [1]. The structural formulae can be found in Scheme 1, and were confirmed in the case of C2 by X-ray diffractometry of single crystals grown by slow evaporation of the solvent (Fig. 1). A detailed description of the structural characterization of C2 and C3 (including NMR spectroscopy, X-ray diffractometry and mass spectrometry) can be found along with the synthetic and purification methods in the Experimental Section.

Scheme 1:

Molecular formulae of complexes C1 to C4.

Fig. 1:

Molecular structure of C2 as obtained by single-crystal X-ray diffractometry (displacement ellipsoids are shown at the 30% probability level, H atoms as spheres with arbitrary radii).

2.2 Photophysical characterization

The UV/Vis absorption spectra of C1–C4 in fluid solution at room temperature are shown in Fig. 2. It can be seen that the spectra do not differ significantly from one another regarding the wavelength and the molar absorption coefficients of the maxima down to 270 nm. The absorption band peaking at 300 nm can be assigned to a transition into a predominantly ligand-centered excited singlet state (¹LC, ππ* excitation) with main contributions of the tridentate luminophore and a slight contribution of the metal center, as previously indicated by TD-DFT calculations [1], [3]. The small, unstructured band in the range of 350–450 nm can be attributed to the transition into the lowest excited singlet state that can be mainly described as a singlet HOMO-LUMO (¹MLCT) excitation involving the π orbitals of the tridentate ligand mixed with the metal-centered d orbitals and the π* orbitals of the luminophore. The spectra show no influence of the ancillary ligand and no traceable aggregation in the ground state. Interestingly, the relative intensity of the band below 250 nm drops with a decreasing number of phenyl units, and can be therefore related to the ancillary ligand. This was confirmed by the observation that the introduction of the pyridine unit led to a distinct band at 250 nm.

Fig. 2:

UV/Vis absorption spectra of complexes C1 to C4 in dichloromethane at room temperature (5×10⁻⁶m). Spectra of complexes C1 and C4 were previously published by Sanning et al. [1] and are shown for comparison.

The excitation and emission spectra of C1 to C4 in fluid solution at room temperature are depicted in Fig. 3. The excitation spectra show the same behavior as the absorption spectra (vide supra): Strong bands are observed in the UV region (300 nm) and attributed to the abovementioned transitions into the ¹LC excited states, whereas the weak bands peaking at 400 nm can be assigned to the ¹MLCT (HOMO-LUMO) excitation. The slight overlap with the triplet emission indicates that ³MLCT states might also be optically accessible due to spin-orbit coupling. A significant influence of the ancillary ligand cannot be observed, neither for the excitation nor for the emission spectra. The latter show their main maximum around 460 nm, a second maximum around 490 nm and a shoulder around 510 nm. This clear vibrational progression of the emission spectra is indicative of emission from metal-perturbed ligand-centered triplet states (³MP-LC states). The lifetimes are relatively short and the photoluminescence quantum yields low in deaerated solutions (see Table 1), which indicates fast radiationless deactivation pathways, most likely via dark metal-centered states (³MC states). These dissociative dd* excitations, which are optically inactive due to the parity-related Laporte prohibition, are nonetheless thermally accessible as a consequence of the high energy of the emissive states, providing conical intersections back into the ground state. Interestingly, a reduction of the number of phenyl groups (C3) and the introduction of the more rigid pyridine ligand (C4) slightly enhance the quantum yields (Φ_L), enabling the quantification of the radiative (k_r) and radiationless (k_nr) deactivation rate constants, which are of the same order of magnitude for both complexes (Table 1).

Fig. 3:

Excitation and emission spectra of C1 to C4 in deaerated dichloromethane at room temperature (5×10⁻⁶m). The spectra of C1 and C2 were previously published by Sanning et al. [1], and are shown for comparison.

Considering the findings of previously published STS measurements and (TD)-DFT calculations, the highest density of probability of the HOMO and the LUMO is rather found on the tridentate luminophoric ligand than on the ancillary ligand, and thus the excitation and emission wavelengths are not expected to differ upon exchange of the latter [1], [4]. In solution at room temperature, these expectations are met (vide supra). In frozen glassy matrices at 77 K, however, the aggregation tendency of C4 is strong enough to lead to a broad, unstructured emission band around 600 nm. This band can be assigned to the emission from a ³MMLCT state as a consequence of the planarity of the complex bearing a flat ancillary 4-amylpyridine that favors stacking and metal-metal interactions (Fig. 4). The complexes with the bulky ancillary phosphane ligands do not show any aggregation, and the lifetime of the excited state of the monomeric species is found in the µs range, underlining the emission from ³MP-LC states, and coincident for C1 and C4 (carrying stronger π-accepting ancillary ligands), as well as for C2 and C3 (bearing weaker π-accepting ligands). It is thus demonstrated that aggregation can be avoided not only by the use of a very bulky ancillary ligand such as PPh₃ but also by a significantly smaller monodentate species such as PPhMe₂. This is additionally underlined by the excitation and emission spectra in the solid state (see Fig. 5). Only C4 shows the characteristic broad, unstructured, red-shifted emission associated with the aggregated species. C1–C3 show solely emission from monomeric ³MP-LC states in the solid state. For C1–C3, the lifetimes are in the µs range but not monoexponential, indicating ³MP-LC emission from different conformations or other weakly coupled species. The quantum yield in the solid state is significantly higher for C1 and C2 as compared to C3 and C4. The smaller volume and planarity of C3 and C4 enhance the possibility to interact with neighboring molecules (triplet-triplet annihilation, for instance), favoring radiationless deactivation pathways while the bulky ancillary ligands of C1 and C2 prevent such processes to a larger extent. While the k_r values in the solid state coincide with those in solution for the ³MP-LC emissive states of C1–C3, it is clear that coupling and collisional thermalization with the solvent favors the radiationless deactivation, and the k_nr rises in solution accordingly. For C4 in the solid state, on the other hand, the stronger participation of the metal in the multi-centered ³MMLCT states enhances spin-orbit coupling and thus the phosphorescence (larger k_r), whereas the higher charge-transfer character (with a distorted excited state), the red-shifted emission (energy gap law) and a high density of vibrational states rise the k_nr [15]. In the solid state, the different packing originating from the four ancillary ligands also affects the relative intensities of the vibrational peaks in the emission spectra (see Fig. 5).

Fig. 4:

Excitation and emission spectra of C1 to C4 (5×10⁻⁶m) in frozen glassy matrices of dichloromethane-methanol (1:1) at 77 K. Spectra for C1 and C4 were previously published by Sanning et al. [1] and are shown for comparison.

Fig. 5:

Excitation and emission spectra of C1 to C4 in the solid state (powder). Spectra for C1 and C4 were previously published by Sanning et al. [1] and are shown for comparison.

4.1 Synthesis and characterization

C2 and C3 were synthesized and purified as described for C1 by using the corresponding phosphane and by working under strictly inert atmosphere (in the absence of molecular oxygen) [1].

Characterization of complex C2. Yield: 60%. – ¹H NMR (600 MHz, [D₈]THF): δ=8.29 (t, J=7.9 Hz, 1H), 7.96 (d, J=7.9 Hz, 2H), 7.86 (dd, J=13.0, 7.6 Hz, 4H), 7.53–7.48 (m, 2H), 7.44 (m, 4H), 2.59 (d, J=12.0 Hz, 3H). – ³¹P NMR (243 MHz, [D₈]THF): δ=–4.43. – ¹⁹F NMR (282 MHz, [D₈]THF): δ=–65.43. – HRMS [(+)-ESI, MeOH]: m/z=743.08330 (calcd. 743.08350 for [M+H]⁺), 765.06502 (calcd. 765.06544 for C₇H₇N₅ONa, [M+Na]⁺), 1507.13957 (calcd. 1507.14112 for [2M+Na]⁺).

Characterization of complex C3. Yield: 47%. – ¹H NMR (600 MHz, [D₈]THF): δ=8.25 (t, J=7.9 Hz, 1H), 7.98 – 7.89 (m, 4H), 7.46 (dt, J=4.4, 2.4 Hz, 3H), 2.32 (d, J=12.3 Hz, 6H). – ¹⁹F NMR (564 MHz, [D₈]THF): δ=–65.33. – HRMS ((+)-ESI, MeOH): m/z=681.06773 (calcd. 681.06740 for [M+H]⁺), 703.04980 (calcd. 703.04935 for C₇H₇N₅ONa, [M+Na]⁺), 1383.11056 (calcd. 1383.10863 for [2M+Na]⁺).

4.2 Crystal structure determination of C2

Crystal structure data: Formula C₂₄H₁₆F₆N₇PPt, M_r=742.50, yellow crystal, 0.18×0.18×0.07 mm³, triclinic, space group P1̅ (No. 2), a=7.8921(1), b=10.6139(3), c=16.0496(4) Å, α=90.746(1), β=97.765(1), γ=108.217(1)°, V=1263.1(1) ų, Z=2, T=223(2) K, ρ_calcd. =1.95 g cm⁻³, μ(MoKα) =5.7 mm⁻¹, 11265 reflections collected (±h, ±k, ±l), 4925 independent (R_int=0.035) and 4815 observed reflections [I>2σ(I)], 381 refined parameters, R=0.026 [I>2σ(I)], wR2=0.068 (all data), max./min. residual electron density 0.64/–1.77 e Å⁻³.

The intensity data were collected on a rotating anode Nonius κ-CCD diffractometer with Montel mirror-monochromatized MoKα radiation (λ=0.71073 Å) by using an ω-φ scan method at 223(2) K. Absorption corrections were applied using multi-scan techniques (0.43≤T≤0.69). The structure was solved by Direct Methods and refined by full-matrix least-squares techniques. The non-hydrogen were refined anisotropically. The hydrogen atoms attached to carbon atoms were generated geometrically and refined as riding atoms. Exceptions and special features: One CF₃ group was found to be disordered over two positions. Several restraints (SADI, SAME, ISOR and SIMU) were used in order to improve the refinement stability. Programs used: Data collection, Collect [16]; data reduction, Denzo-SMN [17]; absorption correction, Denzo [18]; structure solution, Shelxs-97 [19]; structure refinement, Shelxl-97 [20] and graphics, XP [21].

CCDC 1485361 (compound C2) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.

4.3 Photophysical characterization

Absorption spectra were measured on a Varian Cary 5000 double-beam UV/Vis/NIR spectrometer and are baseline-corrected. Steady-state excitation and emission spectra were recorded on a FluoTime300 spectrometer from PicoQuant equipped with a 300 W ozone-free Xe lamp (250–900 nm), a 10 W Xe flash-lamp (250–900 nm, pulse width<10 µs) with repetition rates of 0.1–300 Hz, an excitation monochromator (Czerny-Turner 2.7 nm/mm dispersion, 1200 grooves/mm, blazed at 300 nm), diode lasers (pulse width<80 ps) operated by a computer-controlled laser driver PDL-820 (repetition rate up to 80 MHz, burst mode for slow and weak decays), two emission monochromators (Czerny-Turner, selectable gratings blazed at 500 nm with 2.7 nm/mm dispersion and 1200 grooves/mm, or blazed at 1250 nm with 5.4 nm/mm dispersion and 600 grooves/mm), Glan–Thompson polarizers for excitation (Xe lamps) and emission, a Peltier-thermostatized sample holder from Quantum Northwest (–40 to 105°C), and two detectors, namely a PMA Hybrid 40 (transit time spread FWHM<120 ps, 300–720 nm) and a R5509-42 NIR photomultiplier tube (transit time spread FWHM 1.5 ns, 300–1400 nm) with external cooling (−80°C) from Hamamatsu. Steady-state and fluorescence lifetimes were recorded in TCSPC mode by a PicoHarp 300 system (minimum base resolution 4 ps). Emission and excitation spectra were corrected for source intensity (lamp and grating) by standard correction curves. Lifetime analysis was performed using the commercial FluoFit software. The quality of the fit was assessed by minimizing the reduced chi squared function (χ²) and visual inspection of the weighted residuals and their autocorrelation. Luminescence quantum yields were measured with a Hamamatsu Photonics absolute PL quantum yield measurement system (C9920-02) equipped with a L9799-01 CW Xenon light source (150 W), monochromator, C7473 photonic multi-channel analyzer, integrating sphere and employing U6039-05 PLQY measurement software (Hamamatsu Photonics, Ltd., Shizuoka, Japan). All solvents used were of spectrometric grade.