Turning-On of Coumarin Phosphorescence in Acetylacetonato Platinum Complexes of Cyclometalated Pyridyl-Substituted Coumarins

Two pyridine-functionalized coumarins differing with respect to the site of pyridine attachment to the coumarin dye (3 in L1 or 7 in L2) and with respect to the presence (L1) or absence (L2) of a peripheral NMe2 donor were prepared and used as cyclometalating ligands towards the Pt(acac) fragment. X-ray crystal structures of complexes 1 and 2 show strong intermolecular interactions by π-stacking and short Pt∙∙∙Pt or C-H∙∙∙O hydrogen bonding that result in the formation of sheetlike packing patterns. The NMe2 donor substituent has a profound influence on the absorption and emission properties of the free coumarin dyes; L1 emits strongly while L2 is only weakly emissive. On binding to Pt(acac) the strong fluorescence of L1 is partially quenched while coumarin phosphorescence is observed from cyclometalated L1 and L2. The ligand-centered nature of the LUMO was confirmed by IR spectroelectrochemistry while the assignment of the phosphorescence emission as ligand-based rests on the vibrational structuring, the negligible solvatochromism, the small temperature-induced Stokes shifts on cooling to 77 K, the emission lifetimes, and strong oxygen quenching. (TD-)DFT calculations confirm our experimental results and provide an assignment of the electronic transitions and the spin density distributions in the T1 state. OPEN ACCESS Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-0-303768 Erschienen in: Inorganics ; 3 (2015), 2. S. 55-81 http://dx.doi.org/10.3390/inorganics3020055


Introduction
Platinum-based emitters with cyclometalating arylimine ligands derived from arylpyridines, arylthiazoles or -benzothiazoles or arylpyrazoles combine several attractive features such as an easy synthesis from well accessible precursors, good general stabilities, rather high quantum yields, and reasonably long lifetimes of their emission.Furthermore, their emission color is easily tuned by varying the substituents on the arene or imine constituents or the other coligand(s) .The Pt coordination center with its high spin-orbit coupling constant serves to promote efficient intersystem crossing (ISC), hence populating a lower-lying excited triplet sate from excited singlet state(s), while, at the same time, promoting a relatively rapid decay of the T1 state to the S0 ground state by the same mechanism [19,23].Cyclometalated Pt(II) complexes thus find applications as emitter materials for OLEDs, as sensitizers for electroluminescent devices or dye-based solar cells [5,7,15,16,22,27,28], as oxygen sensors [17,29,30], or for triplet-triplet-annihilation-based upconversion [9,30].
Particularly important are representatives where the other coligand is a β-diketonate, either acetylacetonate (acac) or its pivaloyl or bis(trifluoromethyl) (hfac) analogues, the latter providing better solubilities and diminished π-stacking.The success of those derivatives arises from the rigid, planar structure imposed by the two chelate ligands with bond angles rather close to the ideal values, their low propensity to tetrahedral distortion or twisting of one chelate plane with respect to the other in the excited state (the so-called D2d distortion), and the strong ligand field imposed by the C^N cyclometalating ligand.All these factors prevent rapid radiationless decay from the emissive triplet state through thermal equilibration with higher-lying non-emissive d-states by introducing large ligand-field splitting [19,20].
A major drawback of such complexes is, however, their usually rather low absorption in the visible (Vis) with extinction coefficients rarely approaching 10 4 .A possible way to circumvent that problem is to resort to a donor-acceptor-type architecture of the cyclometalating ligand which, by adding a strong intraligand charge-transfer component, red-shifts and intensifies the lowest-energy band [1,5,15,16,27,29] or the covalent attachment of a dye to the ligand.If the dye-based 1 π→π* state is energetically above the emissive triplet state it may well act as an antenna.Hence, the emissive triplet state, which mostly assumes a metal-to-ligand charge transfer (MLCT) character, is also populated by energy transfer from the excited 1 π→π* state of the dye [31,32].Alternatively, the Pt atom can promote the otherwise forbidden ISC of the attached dye and thus populate an emissive ligand-based triplet state ( 3 IL), which then emits with much lower luminescence lifetimes when compared to the 3 IL ( 3 π*) state of the parent dye [1,9,17,30,[33][34][35][36].To these ends, coumarins, and, in particular the benzothiazole-functionalized coumarin 6, have been repeatedly used as cyclometalating ligands and have been found to endow their metal complexes with interesting photophysical properties [1,30,[32][33][34][35]37].
We here report on two Pt(acac) complexes with pyridyl-functionalized cyclometalating coumarins that differ with respect to the position of the pyridine-coumarin linkage.In these two complexes, the Pt atom is either attached to the lactonyl or the phenyl ring of the coumarin entity.We demonstrate that the point of attachment and the substitution pattern influence the absorption and emission properties and either result in a relatively long-lived 3 IL or dual ligand-based fluorescence and phosphorescence emission.The electronic transitions and the nature of the emissive states are probed by quantum chemical calculations paired with IR spectroelectrochemical measurements.

Synthesis, NMR Spectroscopy, and Structural Characterization
Complexes 1 and 2 of this study are shown in Chart 1. Ligand L1 is literature-known [38], while ligand L2 was prepared by Negishi coupling from 4-methylcoumarin-7-trifluoromethanesulfonate (4-MCOTf) [39] and 2-pyridyl zinc chloride according to the method of Manabe [40] using diisopropyl(2naphthyl)phosphine/Pd(OAc)2 as the (pre)catalyst (Scheme 1) [41].The synthesis of the cyclometalated platinum complexes 1 and 2 follows the well-established sequence of first reacting the corresponding arylpyridine with K2[PtCl4] or PtCl2(dmso)2 in water/2-methoxyethanol and then treating the resulting chloro-bridged dimers {(L1)PtCl}2 or {(L2)PtCl}2 with acetylacetone in the presence of Na2CO3 base in 2-methoxyethanol as the solvent (Scheme 2) [1,42,43].Complexes 1 and 2 were obtained in overall yields of 17% and 63%, respectively, after purification by column chromatography (1) or recrystallization (2).The rather low yield of 1 is a consequence of its propensity to competing decomposition with release of L 1 under the reaction conditions and the tedious chromatographic purification.Successful cyclometalation was evidenced by the loss of the resonance signal of the respective proton of the free ligand, appreciable downfield shifts of the resonance signal(s) of the proton(s) adjacent to the site of Pt attachment, and the additional splitting of these signals by 3 JPt-H satellites of ca.15-20 Hz.We also note strong coordination shifts of 10.2, 11.9, and 6.3 ppm to lower field for the Pt-bonded carbon atom C6 and the quaternary carbon atoms that connect the two aromatic rings when comparing complex 2 and L2 (see Experimental Section; NMR spectra are displayed as Figures S1-S7 of the Electronic Supporting Information (ESI)).
Single crystals for X-ray diffraction studies were obtained by layering saturated solutions of the complexes in dichloromethane (DCM) with petroleum ether.The molecular structures are displayed in Figure 1 along with the atomic numbering scheme, while Table 1 summarizes the most important bond lengths, bond angles, and dihedral angles.Details of the data collection, structure refinement, and full lists of metric parameters can be found as Tables S1-S3 of the ESI.Angle sums of 359.9 (1) or 360.0° (2) at the coordination center indicate that the Pt(II) atom attains its typical square planar coordination geometry.Small distortions from the ideal geometry are imposed by the planar five-membered N^C chelate, which contracts the N-Pt-C bond angle to 81.3(2) (complex 1) or 82.4(4)° (complex 2) and concomitantly opens the other cis-angles.In complex 1, only the adjacent angle C(7)-Pt-O(3) is affected and becomes obtuse at 99.0(2)°.In 2, however, that effect is almost evenly distributed over all three remaining cis-angles, which are in the range of 91.2(4) to 93.5(3)°.Thus, in 1 the acac ligand bends over to the pyridyl ring of the cyclometalated C^N ligand while it is on-center in 2. The small dihedral angles formed by the conjoint rings of the cyclometalated ligand or the aryl or pyridyl constituents of that chelate and the adjacent donor function of the acac ligand (Table 1) as well as the small interplanar angles between the normals to the best planes of the five-membered Pt(C^N) and the six-membered Pt(O^O) chelates of 5.70 (1) or 1.08° (2) attest to a high degree of coplanarity with only a minute twist for complex 1.This is an important prerequisite for good luminescence properties.At ca. 1.97 (N), 1.98 (C7), and 2.01 (O3) or 2.08 Å (O4) the Pt-N, Pt-C, and Pt-O bond lengths offer no peculiarities with respect to similar structures [1,3,5,6,29,44,45].As usual, the Pt-O bond opposite the aryl C atom is considerably longer than that opposite the pyridine N donor as a consequence of the larger trans influence of the σ-bonded aryl ligand [44].

Bond parameter
In the crystal, complexes 1 and 2 show a variety of intermolecular interactions that lead to interesting packing motifs.Individual molecules of 1 arrange into staggered head-to-tail dimers that are held together by a short Pt•••Pt contact and by π-stacking interactions between the electron-rich acac and the electron-poor pyridyl and coumarin lactonyl rings.Figure 2 provides two different views of these "dimers."The Pt•••Pt distance of 3.164(1) Å is on the short end of such contacts [5,46,47] and well within a range that is usually only observed for dinuclear complexes, where such close contacts are enforced by small bridging ligands such as dppm [48][49][50][51] or in linear chain structures [52,53].The planes of the Pt(O^O) and the Pt(C^N) chelate rings of the two associated molecules are only slightly tilted by 5.6° with a stacking distance of 3.33 Å as defined by the distance between the centroids of the Pt(acac) and the Pt,N(2),C(5),C(6),C(7) planes.

Absorption and Emission Properties
Ligands L1 and L2 differ strongly with respect to their optical properties.In CH2Cl2 L2 shows a broad, structured absorption with individual peak maxima at 306 and 330 nm and an additional shoulder near 350 nm; extinction coefficients are of the order of ca.1.6 × 10 4 M −1 cm −1 (see Figure 4 and Table 2).In contrast, L1 has a red-shifted and appreciably more intense absorption at λmax = 414 nm (ε = 3.5 × 10 4 M −1 cm −1 ), suggesting that the underlying transition comprises some charge-transfer components from the NMe2 donor to the electron-accepting lactonyl and pyridyl constituents of L1 [38].Similar findings have also been reported for dialkyl-or diarylamino-substituted coumarins [5,29,30,[33][34][35]54].Clear differences are also seen in the emission spectra: While L1 emits intensely at 470 nm with a quantum yield of 83%, L2 is only weakly emissive (ΦF = 3.6%) at r. t. in solution (Table 3).The differences in the absorption spectra of L1 and L2 also prevail in their complexes 1 and 2. Thus, the π→π* absorptions of the parent coumarin dye are fully preserved in complex 2 and their positions are only very slightly shifted with respect to the free dye.Additional, weaker bands appear at lower energies, suggesting some contribution of the Pt(acac) moiety to these transitions.The latter notion is further substantiated by the moderate yet observable negative solvatochromism of these bands [55], which also indicates that the underlying excitations involve some limited degree of charge transfer (Table 2).For complex 1, the main absorption experiences a distinct red shift of 2650 cm −1 and now appears as a vibrationally structured band.In contrast to the L2/2 couple of compounds, the absorptivity is somewhat reduced on metal coordination.A second distinct peak that has no equivalent in free L1 is seen in the near UV at 371 nm.With a view to related complexes, that peak is assigned to an MLCT transition [1,3,54].In agreement with that notion, the respective band is clearly solvatochromic (see Figure 5 and Table 2).Additional, weaker features show up deeper in the UV with increased intensities when compared to the free ligand L1 (see Figures 4,5 and Table 2).All these bands are nearly invariant to the solvent, indicating only small changes in the polarities of the involved states (Figure 5).Insight into the nature of the relevant transitions and involved states was sought by (TD-)DFT calculations on the fully optimized structures of complexes 1 and 2. We note that the PBE0-calculated structures reproduce the experimental parameters better than the B3LYP ones (see Tables S9 and S10 of the ESI), while the latter functional gives a better match between calculated and experimental spectra (see Figure S18 of the ESI).The B3LYP-calculated spectra reproduce the general absorption features very well but overestimate transition energies by about 1700 cm −1 , which is not uncommon for such complexes.Only one strong visible absorption feature at 431 nm is calculated for complex 1, which further argues for a vibrational origin of the experimentally observed band structuring.That main absorption originates from the HOMO→LUMO transition and is best described as a π→π* transition within the cyclometalated coumarin dye with some degree of charge-transfer (CT) from the electron-rich dimethylaminophenyl ring to the electron accepting lactonyl and pyridyl constituents of the cyclometalating ligand.The additional, weaker excitations in the (near) UV at calculated wavelengths of 345 and 324 nm are mainly HOMO-1→LUMO and HOMO→LUMO+2 in character.They involve transitions from a state with strong Pt(acac) contributions of 72% to the same acceptor orbital (λcalc = 345 nm) and a π→π* transition with a minor coumarin→acac (L→L') component (λcalc = 324 nm).These transitions most likely account for the bands at 370 nm and the lower energy part of the composite band near 320 nm.They are followed by a set of two likewise closely spaced transitions mainly of a HOMO-2→LUMO+1 (λcalc = 287 nm) and HOMO-1→LUMO+3 (λcalc = 244 nm) character.Graphical representations of the relevant molecular orbitals are provided in Figure 6.A full list of calculated electronic transitions, fragment contributions to the relevant MOs in the frontier orbital (FMO) region and a more complete set of graphical MO representations can be found as Tables S6 and S8  Selected FMOs of complex 2 that are relevant for the observed electronic transitions are displayed in Figure 7; listings of fragment contributions and TD-DFT calculated transitions and a more complete set of calculated MOs may be found as Tables S7 and S8 and Figure S20 of the ESI.In contrast to complex 1, the donor orbitals HOMO and HOMO-1 are delocalized over the cyclometalating coumarin-pyridine hybrid ligand and the Pt(acac) moieties with a dominant contribution of the negatively charged coumarinyl part of L2 to the HOMO.As for complex 1, the LUMO is essentially delocalized over the dye ligand with rather similar contributions from the coumarin and pyridyl subunits.The absence of a strong donor substituent and the shift of the site of pyridyl attachment from the lactonyl to the phenyl part of the coumarin thus have the combined effect of decreasing the polarization of the MOs having strong contributions of the cyclometalating ligand when compared to complex 1.The calculated HOMOLUMO transition underlying the band at λcalc = 422 nm is thus best described as a blend of intraligand CT from the phenyl part of the cyclometalated ligand to the appended pyridyl and lactonyl acceptor units and CT from the Pt(acac) moiety to the cyclometalating ligand.That transition most likely accounts for the lowest energy band of complex 2 at 432 nm.It is followed by a second band of likewise moderate absorptivity, which is assigned as the excitation from HOMO-1 to the same acceptor orbital with some larger degree of CT from the Pt(acac) fragment.The most intense band of the electronic spectrum at λcalc = 320 nm arises mainly from the HOMO-3→LUMO excitation.Like for complex 1, it is essentially a π→π* type transition within the cyclometalated ligand, with some minor degree of electron donation from the Pt(acac) fragment.This transition most likely accounts for the low-energy part of the strong, broad feature in the experimental absorption spectrum.Other components of that composite band have a strongly mixed character, with involvement of several different MOs down to HOMO-9 and up to LUMO+5 (see Table S7 of the ESI).
Electrochemistry is a potent tool to probe for the energies of the immediate frontier MOs.In cyclic voltammetry (DCM, 0.1 M NBu4PF6, r. t.), complex 1 displays an irreversible oxidation at a peak potential Ep of 0.53 V. On extending the anodic scan to more positive values a second, likewise chemically irreversible process is observed at Ep = 0.67 V.The associated peak current is considerably smaller than that of the initial process and does not vary appreciably with sweep rate, which does not allow us to assign it to either the second oxidation of 1 or to a product generated during the decomposition of its oxidized form.The cathodic scan (THF, 0.1 M NBu4PF6, r. t.) reveals a chemically reversible (ip,a/ip,c = 1.0; ip,a and ip,c denote the anodic and cathodic peak currents of the wave) reduction at a half-wave potential E1/2 of -2.27 V. Representative voltammograms of complex 1 are displayed as Figure S15 of the ESI.
As complex 2 lacks the NMe2 donor substituent of complex 1 its FMO energies are expected to be lower.Indeed, calculated energies are −5.76 eV for the HOMO and −2.24 eV for the LUMO as compared to −5.22 eV and −1.93 eV for complex 1.As a consequence the oxidation and reduction potentials are shifted anodically with respect to 1. Thus, the peak of the irreversible oxidation appears at 0.65 V (here one should, however, bear in mind that the peak potentials of chemically irreversible electrode processes provide no stringent thermodynamic information but are also influenced by the rate and equilibrium constant of the chemical follow-up process).Notwithstanding, the E1/2 of the first, chemically reversible reduction of complex 2 is −2.09V and hence by 180 mV more positive than that of complex 1.In further contrast to 1, complex 2 can be reduced by one further electron within the potential window of the THF/0.1 M NBu4PF6 supporting electrolyte.The second reduction gives rise to a chemically partially reversible process (ip,a/ip,c ≈ 0.7) at E1/2 = −2.40V (see Figure S16 of the ESI).
This provided us with the opportunity to directly probe for the identity of the primary reduction site by means of IR spectroelectrochemistry. Considering that the LUMO of complex 2 is entirely based on the coumarin/pyridyl-derived ligand L2, reduction should induce major changes of the ligand-based stretches and, in particular, those associated with the lactonyl moiety.
In their IR spectrum the free ligand L2 and complex 2 both feature a weak band near 1770 cm −1 and a prominent absorption at 1715 cm −1 that can be assigned to the lactonyl moiety of the coumarin (see Figures S12 and S14 of the ESI).Additional rather intense bands of complex 2 are observed in the region above 1500 cm −1 with prominent peaks at 1576, 1563, and 1514 cm −1 in the solid or at 1577, 1564, and 1522 cm −1 in THF solution.IR spectroscopic changes on reduction were monitored under in situ conditions in an optically transparent thin-layer electrolysis (OTTLE) cell based on the design of Hartl et al. [56].On stepwise reduction to first the radical anion 2 − and then to dianion 2 2− the original carbonyl bands bleach out while intense bands at 1673 and 1644 cm −1 (2 − ) and then at 1604 cm −1 (2 2− ) appear (Figure 8), thus indicating a stepwise shift of the lactonyl band to lower energies on negative charging.Both reductions are notably accompanied by distinct sets of isosbestic points, attesting to clean conversions, even for the second reduction step.These observations support our notion that the LUMO of 2 is centered on the cyclometalating ligand.
Table 3 summarizes the photophysical data for the free ligands L1, L2 and their associated Pt(acac) complexes 1 and 2. While uncoordinated L2 is only weakly emissive (ΦF = 3.6%), complex 2, on excitation into the low-energy absorption band, shows a rather intense, vibrationally structured emission with a quantum yield Φ of 21.3%.Very similar results were obtained in 2-MeTHF with only small differences with respect to emission wavelength and quantum yield.The main peak at 550 nm is the most blue-shifted one; additional, less intense peaks are found at 594 and 640 nm (see Figure 9).The large Stokes shift of 4970 cm −1 and the strong quenching by atmospheric oxygen indicate phosphorescence emission from the T1 state.Vibrational structuring is a characteristic asset of a 3 IL-based emission arising from a ligand-centered excited state where the heavy metal atom has only small contributions but efficiently triggers the otherwise forbidden S1→T1 intersystem crossing (ISC) of a coordinated ligand owing to its large spin-orbit coupling constant [19].Typical examples of such behavior are Pt alkynyl complexes [57][58][59][60][61][62][63][64][65][66][67].The latter notion is further supported by the computed spin density surface for the T1 state of complex 2 as it is depicted in Figure 10a.Thus, the unpaired spins are evenly distributed over the cyclometalated coumarin-pyridyl ligand, with only small contributions from the Pt atom.At r. t. the lifetime of the emission is 1.6 µs.While such a lifetime would be extremely short for a purely organic phosphor, it is rather typical of the triplet emission of a complex, where higher lying and thermally accessible states with high metal contribution provide a pathway for radiationless decay [19].In keeping with that hypothesis, the emission lifetime increases to 15.3 µs in a frozen 2-MeTHF matrix.Under these conditions further sharpening and resolution of vibrational sublevels is observed along with only a slight solvent-and matrix-induced blue shift of the main peak to 539 nm (Figure 9c).We note here that weak solvatochromism and small thermally induced Stokes shifts are well-established tokens of ligand-based emissions [8,33,68].
Excitation spectra recorded at λ = 550 nm retrace the features of the absorption spectrum, yet with reduced intensities of the multiple UV transitions (see Figure 9(b)).This probably indicates that only some components of the highly mixed transitions in the UV (vide supra) populate the lower-lying emissive ligand-centered T1 state.On excitation at λ = 420 nm, complex 1 displays a similarly structured emission, which, on account of the Stokes shift of 3840 cm −1 and the almost quantitative quenching under atmospheric oxygen, is likewise assigned as phosphorescence from the T1 state.The quantum yield of 21.0% is essentially identical to that of complex 2 whereas the emission lifetime of 12.4 µs at r. t. is appreciably longer.Again, the behavior in 2-MeTHF was very similar.The calculated spin density map closely resembles that of complex 2 and reasserts that the phosphorescence emission is also ligand-based.
In striking contrast to complex 2, however, complex 1 also fluoresces with a main peak at λ = 486 nm, a pronounced shoulder at 457 nm, and a quantum yield of 6.4% at r. t.The latter emission is invariant to atmospheric oxygen (see Figure 11) and its lifetime is 2.6 ns.In a rigid glassy 2-MeTHF matrix at T = 77 K all emission peaks are seen to sharpen.As a consequence the composite fluorescence emission is resolved into two separate peaks at essentially identical wavelengths as in a fluid solution.As for complex 2, the fine structuring of the phosphorescence emission is enhanced along with a slight blue-shift of the main emission peak to 557 nm (Table 3) while the lifetime of the phosphorescence emission increases to a remarkable value of 103 µs.Excitation spectra recorded at the peak of the phosphorescence and well within the envelope of the fluorescence emission retrace important features of the absorption spectrum.Given the high fluorescence quantum yield of L1 of 83% and the fact that the high energy portion of the fluorescence excitation spectrum closely resembles features in the absorption spectrum of L1, it is likely that the fluorescence peak at the higher energy originates from trace impurities of free L1 that are neither observed in NMR spectra nor show up in the absorption spectrum of 1.In that respect we note that small quantities of free L1 may be released during the observed slow decomposition of complex 1 in solution, particularly under UV irradiation.It should be emphasized, though, that the fluorescence excitation spectrum also features a prominent peak at 465 nm, which is specific to 1 and completely absent in L1.We thus conclude that complex 1 indeed shows dual ligand-based fluorescence and phosphorescence emission.Whilst not unprecedented, dual emission from the S1 and the T1 states from a metal-bonded ligand in a platinum or palladium complex is still rare and has been observed on only a few occasions [36,54,[69][70][71].Key to the observation of dual emission is that the Pt coordination center has only limited contribution to the relevant frontier orbitals, as is the case here (see Figures 6 and 10).Another prerequisite is that the ISC rate resembles that of the fluorescence emission.We note that complexes exhibiting dual fluorescence and phosphorescence emissions have already been used as luminescence sensors for the quantitative detection of singlet-oxygen [70].Such dual emitters have some advantages over two-component sensor systems in that there is no need to account for different bleaching rates of the phosphorescent sensor and the fluorescent standard that is needed for quantifying phosphorescence quenching by oxygen.
To conclude, binding of the pyridyl-functionalized coumarins L1 and L2 as cyclometalating ligands to the Pt(acac) fragment has the effect of turning on room temperature phosphorescence emission with a quantum yield of ca.21% and partial quenching of the strong fluorescence emission of L1.Thus, while the site of pyridyl attachment to the coumarin dye and the presence of the NMe2 donor have profound impact on the absorption and emission properties of the free ligands (L1 absorbing strongly in the visible and being a very strong emitter whereas L2 absorbs exclusively in the UV and is only weakly emissive), both ligands perform equally well in terms of quantum efficiency when cyclometalated to Pt(acac) with similar emission wavelengths of 566 and 550 nm, respectively.Experimental and quantum chemical studies show that both complexes emit from a metal-perturbed ligand-centered triplet state.Hence, the role of the heavy metal ion is to trigger the intersystem crossing of the ligand and to render the triplet state emissive with a comparatively short phosphorescence lifetime.Complex 1, featuring that ligand, is a further example of the still rare class of complexes showing dual fluorescence and phosphorescence emissions.

Materials and General Methods
All reactions were performed under an atmosphere of nitrogen using standard Schlenk techniques.THF and MeOH were dried by standard procedures.All solvents used for reactions or analytical purposes were degassed by saturation with nitrogen or by three freeze-pump-thaw cycles prior to use.All reagents were commercially available and used without further purification.Ligand L1 [38], 4-methylcoumarin-7-trifluoromethanesulfonate (4-MCOTf) [39], and PtCl2(dmso)2 [72] were prepared according to literature procedures.
Infrared spectra were measured on a Perkin Elmer Spectrum 100 Series ATR-FT-IR spectrometer.NMR spectra were recorded on a Bruker Avance III 400 Automation or a Bruker Avance III 600 Cryoplatform as CDCl3 or as CD2Cl2 solutions at 298 K. 1 H NMR spectra were referenced to the signal of the residual protonated solvent, 13 C NMR spectra to the solvent signal, and 31 P NMR spectra to an external standard (85% H3PO4).The assignment of 1 H and 13 C signals was supported by 1 H, 1 H-COSY, 1 H, 1 H-TOCSY, 1 H, 13 C-gHSQC, and 1 H, 13 C-gHMBC measurements.Elemental analyses (C, H, N) were performed at in-house facilities.Mass spectra were measured on a FAB-MS modified Finnigan MAT 312 spectrometer.
X-Ray diffraction analysis was performed on a STOE IPDS-II diffractometer equipped with a graphite-monochromated radiation source (λ = 0.71073 Å) and an image plate detection system at 100 K.The structures were solved by direct methods (SHELXS-97), completed with difference Fourier syntheses, and refined with full-matrix least-squares using SHELXL-97 [73] minimizing ω(F0 2 -Fc 2 ) 2 .Molecular structures in this work are plotted with Mercury 3.1.
UV-Vis electronic absorption spectra in acetonitrile, dichloromethane (DCM), and THF were obtained at ambient conditions in HELLMA quartz cuvettes with 0.1 cm optical path lengths on a TIDAS Diode-Array PGS NIR und MCS UV/NIR spectrometer from j&m Analytik AG.Emission and excitation spectra in degassed dichloromethane (DCM) at room temperature were recorded on a Perkin Elmer Luminescence spectrometer LS 50 in quartz cuvettes from HELLMA that were modified by glass joints, allowing spectra to be recorded under rigorous exclusion of air.The absorbance of all samples amounted to 0.04 to 0.06.Steady-state luminescence spectra at 77 K were recorded on a Fluorolog-3 instrument (FL322) from Horiba Jobin Yvon.Time-resolved luminescence spectra were measured on a LP920-KS instrument from Edinburgh Instruments equipped with an R928 photomultiplier and an iCCD camera from Andor.Luminescence was excited with the frequency-doubled output from a Quantel Brilliant b laser and the pulse length of approximately 10 ns or at a luminescence spectrometer picoQuant FT-300 with time-resolved single photon counting.Modified quartz cuvettes and tubes were employed for these optical spectroscopic experiments.The concentration of samples was ca.1.5 ×10 −5 M. Electrochemical investigations in degassed THF were performed in a vacuum tight one-compartment cell on a BAS CV50 potentiostat using Pt or glassy carbon disk electrodes from BAS as the working electrode, a Pt counter electrode, and a Ag pseudoreference electrode.Potentials were measured by adding decamethylferrocene (Cp*2Fe) and were referenced to the ferrocene/ferrocenium couple (under our conditions the half-wave potential of the Cp*2Fe 0/+ couple is −550 mV in CH2Cl2 or −455 mV in THF with respect to the Cp2Fe 0/+ ).The design of the spectroelectrochemical cell follows that of Hartl et al. [56].

Quantum Chemical Calculations
The ground state electronic structures were calculated by density functional theory (DFT) methods using the Gaussian 09 [74] program packages.Quantum chemical studies were performed without any symmetry constraints.Open shell systems were calculated by the unrestricted Kohn-Sham approach (UKS) [75].Geometry optimization followed by vibrational analysis was made either in vacuum or in solvent media.The quasirelativistic Wood-Boring small-core pseudopotentials (MWB) [76,77] and the corresponding optimized set of basis functions [78] for Pt and 6-31G(d) polarized double-ζ basis sets [79] for the remaining atoms were employed together with the Perdew, Burke, Ernzerhof (PBE0) [80,81], or the Becke exchange and correlation functional (B3LYP) [82,83].Solvent effects were described by the polarizable conductor continuum model (PCM) [84][85][86][87] with standard parameters for dichloroethane.Absorption spectra and orbital energies were calculated using time-dependent DFT (TD-DFT) [88] and the same functional/basis set combinations mentioned above.For easier comparison with the experiment, the obtained absorptions were converted into wavelengths and broadened by a Gaussian distribution (full width at half maximum = 3000 cm −1 ) using the program GaussSum [89].Molecular orbitals were visualized with the GaussView program [90].(830 mg, 2 mmol) were suspended in 32 mL of a water/2-methoxyethanol mixture (1:3).The reaction mixture was heated to reflux for 48 h at 80 °C.The obtained dark green solution was cooled to 0 °C and 70 mL of purified water were added.A brown precipitate was filtered off, washed with ethanol, and dried in vacuo.Yield: 650 mg, 0.72 mmol, 72%., acetylacetone (0.22 mL, 2.16 mmol) and Na2CO3 (760 mg, 7.2 mmol) were suspended in 18 mL of 2-methoxyethanol and refluxed for 16 h at 100 °C.The precipitated crude product was filtered off and the residue was repeatedly extracted with 20 mL portions of dichloromethane.The combined organic phases were concentrated to a total volume of 20 mL.The product was purified by column chromatography on aluminum oxide (dichloromethane) to provide an orange solid.Yield: 150 mg, 0.25 mmol, 17%.Diisopropyl(1-naphthyl)phosphine [91]: A solution of 1-bromonaphthalene (0.87 mL, 6.3 mmol) in THF (20 mL) was cooled to −78 °C.n-BuLi (4.1 mL, 1.6 M in n-hexane) was added dropwise.Thereafter, the mixture was allowed to reach r. t. for a short time and was then recooled to −78 °C.Diisopropylchlorophosphine (1 mL, 6.3 mmol) was added over 5 min.The reaction was stirred for 48 h at ambient conditions and the mixture was washed with nitrogen-saturated aqueous potassium carbonate solution for 10 min.The aqueous layer was extracted with diethylether (2 × 20 mL).The organic phases were combined, dried over potassium hydroxide, and the solvent removed under reduced pressure.The crude product was suspended in n-hexane.The liquid phase containing the product was separated from the precipitate by syringe.The solvent was removed in vacuum and product was recrystallized from 2 mL of n-hexane to give the product as a colorless solid.Yield: 165 mg, 0.68 mmol, 11%.

7-(Pyridin-2-yl)-4-methylcoumarin (L2):
A mixture of 2-bromopyridine (0.6 mL, 6.08 mmol) in 20 mL of THF was cooled to −78 °C.t-BuLi (6.6 mL, 1.9 M in n-pentane) was added dropwise.The reaction mixture was stirred for 0.5 h and a solution of ZnCl2 (0.83 g, 6.08 mmol) in THF (10 mL) was slowly added.The resulting mixture was stirred for 0.5 h at -78 °C and then allowed to reach r. t.After 0.5 h, the solvent was removed in vacuo.A solution of palladium(II)acetate (22.7 mg, 0.101 mmol) and diisopropyl(1-naphthyl)phosphine (29.8 mg, 0.122 mmol) in THF (5 mL) and a solution of 4-MCOTf (1.25 g, 4.05 mmol) in THF (20 mL) were added.The reaction was stirred for 21 h at 60 °C.The formation of a dark green suspension and a black precipitate of Pd(0) were observed.The solvent was removed and the remaining solid was suspended in DCM.This suspension was filtered over silica gel (3 cm).The filtrate was concentrated to 70 mL, washed with 50 mL of aqueous NH4Cl solution, and dried over MgSO4.The solvent was removed under reduced pressure.Chromatographic purification on silica gel (ethyl acetate/petroleum ether 4:5) and recrystallization from dichloromethane layered with petroleum ether provided the product as a colorless solid.Yield: 210 mg, 0.89 mmol, 22%. 3 mmol) were suspended in 6.6 mL of a water/2-methoxyethanol-mixture (1:3) and refluxed for 16 h at 80-90 °C.The obtained yellow suspension was allowed to reach r. t.Fourty milliliters of purified water were added and the mixture was cooled to 0 °C.A precipitate was filtered off, washed with ethanol (3 × 10 mL), and dried in vacuo to yield a yellow solid.Yield: 140 mg, 0.15 mmol, 99%.

Figure 1 .
Figure 1.Structures of complexes 1 (left) and 2 (right).Hydrogen atoms are omitted for reasons of clarity; ellipsoids are drawn at a 90% probability level.

Figure 2 .
Figure 2. Side and top view on two molecules of 1 that associate through a short Pt•••Pt contact of 3.164(1) Å and π-stacking of 3.33 Å.

Figure 3 .
Figure 3. Packing of individual molecules of complex 2 in the crystal, displaying the different interactions of the molecule in the middle with four partners belonging to the sheets above and below.Close contacts are indicated by dark blue dotted lines.

Figure 4 .
Figure 4. Comparison of the electronic absorption spectra of (a) L1 and 1; (b) L2 and 2 in CH2Cl2 solution.

Figure 6 .
Figure 6.Selected molecular orbitals of complex 1 in the FMO region.

Figure 7 .
Figure 7. Selected molecular orbitals of complex 2 in the FMO region.

Figure 8 .
Figure 8. Spectroscopic changes in the IR during electrochemical reduction of complex 2 to 2 − (a) and of 2 − to 2 2− (b) in a 0.2 M NBu4PF6/THF solution at r. t.

Figure 9 .
Figure 9. Emission spectra of aerated and deoxygenated solutions of 2 (a) in DCM recorded at λexc = 412 nm; (b) comparison of the absorption, emission, and excitation spectra (λ = 550 nm) of 2 in DCM at r. t.; (c) comparison of emission spectra of 2 at r. t. and at T = 77 K in a frozen 2-MeTHF matrix.

Table 2 .
Electronic absorption data for complexes 1 and 2 in different solvents.