Controlled 2D-Confinement of Phosphorescent Pt(II) Complexes on

Article pubs.acs.org/JPCC

Controlled 2D-Confinement of Phosphorescent Pt(II) Complexes on Quartz and 6H-SiC(0001) Surfaces Deb Kumar Bhowmick, Linda Stegemann, Manfred Bartsch, Naveen Kumar Allampally, Cristian A. Strassert,* and Helmut Zacharias* Physics Institute and Center for Nanotechnology (CeNTech), Westfälische Wilhelms-Universität Münster, Heisenbergstr. 11, 48149 Münster, Germany ABSTRACT: The photophysical properties of Pt(II)-based triplet emitters can be controlled by tuning the Pt(II)...Pt(II) distance. Herein we show that the functionalization and characterization of insulating and semiconducting substrates with triplet emitters can be achieved by a convenient wetchemical grafting of a pyridine-terminated silane linker on amorphous quartz and on silicon carbide single crystal surfaces. Coordination of the monodentate pyridine unit with phosphorescent Pt(II) complexes bearing tridentate luminophores forces the triplet emitters to aggregate by surface constraints, even if bulky substituents such as adamantyl moieties are present. We found that X-ray photoemission spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR FT-IR) constitute adequate tools to ascertain the degree of functionalization; in particular, high resolution XPS spectra revealed the electronic states at the surfaces, which were correlated with the frontier orbitals of the tridentate luminophores and the degree of aggregation of the complexes. Moreover, photoluminescence spectroscopy allowed us to assess the full extension of aggregation caused by the 2D-confinement, whereas fluorescence microscopy was used to evaluate the homogeneity of the phosphorescent layer.

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INTRODUCTION In the past decades, the functionalization of amorphous quartz1−6 and semiconducting surfaces7−10 with different linkers has gained major attention, based on their potential applications in the field of molecular electronic devices. Due to a very strong Si−C covalent network, SiC constitutes a versatile substrate possessing a large optically transparent band gap, high stability, and bioinertness. Diverse SiC polytypes have different band gaps,11 which can be tuned further by the judicious choice of adequate dopants. These characteristics of SiC have led to diverse applications in optoelectronic devices that can be used in relatively harsh conditions such as biosensors and transistors. Different approaches have been developed to produce welldefined monolayers on SiC surfaces:12−16 photochemical attachment,17,18 thermal grafting,19−21 radical attachment,22 and wet chemical functionalizations.23−26 The most common approach is the wet chemical functionalization, in which organosilanes are used to produce thin films under mild conditions.23−25,27 Applications of metal−organic assemblies have been widely discussed in the past years, ranging from catalysis to energy storage.28−35 While molecules in solution are randomly distributed, formation of metal−organic assemblies depends on many variables such as ligand structure, metal ions, solvents, and temperature,36−40 whereas deposition on solid surfaces can provide ordered molecular arrays, which can in turn lead to organized and directed intermolecular aggregation.41−44 In this © 2015 American Chemical Society

sense, the coordination chemistry of metals can be utilized to construct monomolecular films of organometallic complexes on solid substrates by using the right surface functionalization, allowing a controlled and defined deposition of supra-molecules to achieve an ordered array.28,41,45−47 Pt(II) complexes are known as triplet emitters48−51 which have potential applications in the field of organic light-emitting diodes52 (OLEDs) and light-emitting electrochemical cells53−59 (LEECs). Due to their d8 electronic configuration, they mainly form square planar complexes which have a tendency to stack into intermolecular aggregates. The variable degree of aggregation of these species can modulate their photoluminescence wavelength, intensity, lifetime, and quantum yields.56,60,61 Such aggregated species can be used for sensing applications, as the intermolecular distance and the resulting chromophoric Pt(II)...Pt(II) interactions strongly depend on the intercalation of small molecular species. In particular, Pt(II) can readily form stable complexes with terpyridine ligands48−51,62−65 and their N∧N∧N, N∧C∧N, and N∧N∧C analogues, some of them showing bright luminescence. Moreover, we have shown that aggregates of tridentate Pt(II) complexes can lead to gelating phosphorescent fibers, which can be also employed for the construction of Received: January 13, 2015 Revised: February 10, 2015 Published: March 3, 2015 5551

DOI: 10.1021/acs.jpcc.5b00377 J. Phys. Chem. C 2015, 119, 5551−5561

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The Journal of Physical Chemistry C solution-processed electroluminescent devices.60 Adamantyl- and tolyl-substituted tridentate Pt(II) complexes can be utilized as monomeric triplet emitters for vapor-deposited devices, as we have recently demonstrated.66 However, in both cases, the emitting dopants and the resulting transition dipole moments are randomly oriented within the electroluminescent layer. Brütting and co-workers have shown that a defined orientation of the emission dipoles could enhance the light outcoupling in running electroluminescent devices.67−72 Therefore, the organized deposition of supramolecules with a defined orientation on solid surfaces could provide a fundamental understanding leading to electroluminescent layers with enhanced efficiencies. Moreover, if aggregated species are used (see Strassert et al., ref 60), the enhancement of intermolecular interactions by 2D-confinement could improve the efficiency of such devices. This would also facilitate the design of optoelectronic architectures with sensing abilities, as the excited state properties depend on the Pt(II)...Pt(II) distance as a function of the environment. Finally, the defined orientation could modulate the extent of hybridization between the conductive surface and the doubly occupied dz2 orbitals of the metal center protruding out of the coordination plane, a phenomenon which we have recently demonstrated with the aid of scanning tunneling microscopy and scanning tunneling spectroscopy. The gained understanding allowed us to find submolecular electronic set-screws, and enabled the design and realization of deep-blue phosphorescent complexes. Interestingly, the directional interactions of the metal center with the substrate lead to a localized orbital hybridization and charge injection from the complexes into the metal. If this feature could be combined with a defined orientation of emissive aggregates, simplified electroluminescent devices consisting of an emissive layer of aggregates that interact via hybridization with the electrode could be achieved.1,73,74 In the present study, we introduce a new strategy for the functionalization and precise characterization of insulating and semiconducting substrates with triplet emitters based ot Pt(II) complexes with pincer ligands such as those previously described by Strassert and co-workers (vide supra).60,66 In particular, we report on the wet-chemical grafting of a pyridineterminated silane linker on amorphous quartz and on 6H-SiC(0001) single crystal surfaces, followed by coordination of the monodentate pyridine unit with phosphorescent Pt(II) complexes bearing dianionic tridentate luminophores. The complexes are forced to aggregate due to surface constraints, even if bulky adamantyl moieties are found on the tridentate ligand, as well as if no substituents are present. The resulting photoactive surfaces were characterized by X-ray photoemission spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR FT-IR) to ascertain the degree of functionalization. High resolution XPS spectra revealed the electronic states at the surfaces, which were correlated with the frontier orbitals of the tridentate luminophores and the degree of aggregation of the complexes. Optical emission and excitation spectra as well as the excited state lifetimes indicated that the Pt(II) complexes are completely aggregated due to the 2D-confinement on the surface. Fluorescence microscopy images enabled us to visualize the homogeneity of the phosphorescent layer.

Scheme 1. Schematic Representation for the Preparation of Pyridine-Terminated Quartz and SiC Surfacesa

a

A pretreatment generates OH-terminated surfaces, which are then functionalized with the 4-PETES linker unit.

chloride (99%) were bought from ABCR. Dimethyl sulfoxide (DMSO; 99.6%), H2SO4 (95%), NH3 (25%), triethylamine (99%), and methanol (99.5%) were purchased from VWR, and toluene anhydrous (99.8%), ethanol (99.8%), dichloromethane (DCM; 99.9%), H2O2 (35%), 2,6-pyridinedicarbonitrile (97%), and acetone (99.5%) from Sigma-Aldrich. 1-Adamantanecarboxylic acid chloride (97%), dry N,N-dimethylformamide (N,N-DMF; 99.8%), and ethylene glycol (99%) were bought from Acros Organic. All chemicals were used without further purifications. The n-type 6H-SiC (0001) wafers doped with nitrogen with a thickness of 330 ± 25 μm from PAM-XIAMEN company were used. Pt(DMSO)2Cl2. This precursor was prepared as described in ref 75. K2PtCl4 (4.82 g, 11.6 mmol) was added to a mixture of DMSO (2.46 mL, 35 mmol) and water (50 mL) at room temperature. The reaction mixture was stirred for 3 h at room temperature. The product precipitated out and was collected by filtration, and washed with water followed by diethyl ether. Tridentate Ligands. 2,6-bis(1H-tetrazol-5-yl)pyridine60,76 (L1), 2,6-bis(3-(p-tolyl)-1H-1,2,4-triazol-5-yl)-pyridine66,77 (L2), and 2,6-bis(3-(adamantan-1-yl)-1H-1,2,4-triazol-5-yl)-pyridine66,77 (L3) were synthesized and purified according to literature procedures. Substrate Functionalization. The quartz and 6HSiC(0001) samples were cleaned by sonication with distilled water, methanol, ethanol, and finally with acetone. The wafers were dried under a stream of N2. The quartz wafers were treated with H2SO4/H2O2 3:1, whereas the SiC substrates were oxidized in an oxygen plasma at 1100 °C and treated with 2.5% HF acid for 5 min. After that, the substrates were thoroughly washed with high purity water (Milli-Q) and dried under a stream of Ar similar to the procedure described in ref 27. Immediately thereafter, the wafers were dipped for 4 h into a solution of 3% 2-(4-pyridylethyl)triethoxysilane and 0.5% triethylamine (as a catalyst) in 15 mL of anhydrous toluene at 90 °C. The physisorbed molecules were removed by sonicating the substrates twice in toluene and twice in methanol and acetone. The samples were dried under a stream of Ar. The substrates were then used for initial characterization and further functionalization. The pyridine-terminated quartz and 6H-SiC substrates were dipped into a solution of the tridentate ligand (1.1 eq., 0.027 mmol), Pt(DMSO)2Cl2 (1 eq., 0.025 mmol), and DIPEA (4 eq., 17 μL) in 15 mL of MeOH at 65 °C for 24 h. Next, the samples were boiled in dichloromethane (DCM) for 1 h at 60 °C and then sonicated with DCM and MeOH for 5 min with each solvent.

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EXPERIMENTAL DETAILS Materials and Chemicals. 2-(4-Pyridylethyl)triethoxysilane (4-PETES; 95%), potassium tetrachloroplatinate (K2PtCl4; 99.9%), N,N-diisopropylethylamine (DIPEA; 99%), hydrazine monohydrate (99%), sodium carbonate (99.99%), and p-toluoyl 5552


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The Journal of Physical Chemistry C Scheme 2. Schematic Representation of the in Situ Preparation and Immobilization of Pt(II) Complexes on the Linker-Functionalized Quartz and SiC Surfaces

Surface Characterization Methods. X-ray Photoemission Spectroscopy (XPS). The samples were analyzed at pressures of about 10−9 mbar by using an Al Kα (hυ = 1486 eV) radiation source with monochromator and a hemispherical analyzer (Kratos Analytical Axis Ultra) at normal emission. Spectra were collected with a pass energy of 40 eV. For the X-ray source, a current emission of 10 mA and 12 kV anode voltage was used. Spectra were fitted with CASAXPS 2316 software. Shirley background corrections were used and were fitted with 70:30 Gaussian−Lorentzian functions. All signals were referenced to the signature of the C 1s peak of carbon at Eb = 284.6 eV. Attenuated Total Reflection Infrared (ATR-IR) Spectroscopy Measurements. The ATR-IR spectra were measured with a Bruker Vertex 70v Fourier transform IR spectrometer. Transmittance spectra were measured using a spectral resolution of Δυ̃ = 2 cm−1, using 512 scans for each measurement and referenced to background spectra of pre-treated quartz

(for quartz samples) and HF treated SiC wafers (for SiC samples). Spectra were taken at an incident angle of 68° of the source light. Photoluminescence Spectroscopy. Fluorescence microscopy images were recorded on an Olympus IX71 microscope equipped with a XC10 color CCD camera and a xenon lamp as excitation source. Steady-state 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 duration < 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 duration < 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 λ = 500 nm with 2.7 nm/mm dispersion and 1200 grooves/mm, or blazed at λ = 1250 nm 5553


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The Journal of Physical Chemistry C

Figure 1. ATR FT-IR spectra of pyridine linkers and Pt(II) complexes bound to 6H-SiC (a) and quartz (b) surfaces after subtraction of plain SiC (1.2 cm × 0.8 cm) or quartz (1.8 cm × 1.8 cm) spectra, respectively.

Scheme 2 schematically illustrates the immobilization of different Pt(II) complexes onto these substrates. The Pt(II) complexes were coordinated to the pyridine-terminated surfaces by immersing the functionalized substrates into a solution of the tridentate luminophore, Pt(DMSO)2Cl2, and DIPEA in MeOH. In this processes, the terminal pyridine groups of the silane linkers and the tridentate chromophores coordinatively bind to Pt(II), yielding a self-assembled monolayer of planar Pt(II) complexes on the surfaces. Each functionalization step involving the pyridine linker and the coordination to three different ligands was followed by IR and X-ray photoelectron spectroscopies, as classical NMR, MS, or elemental analysis techniques cannot be employed to evidence the formation of the complexes on the surfaces. In order to investigate the effect of the substitution pattern, three different classes of tridentate luminophores were chosen. They bear either no side group at all (L1, yielding cx1) or distinctly bulky substituents, namely, flat tolyl units (L2, leading to cx2) or adamantyl moieties (L3, yielding cx3). ATR-IR spectra (Figure 1) of all functionalized substrates were collected in a spectral range from 500 to 4000 cm−1 and compared with the spectra of the corresponding hydroxylterminated SiC and quartz surfaces. The spectral bands at 2852 and 2924 cm−1 are the most prominent features for most of the functionalized surfaces, corresponding to the symmetric and antisymmetric stretching modes, respectively, of the aliphatic CH2 moieties from the 2-(4-pyridylethyl)triethoxysilane linker. Due to the presence of adamantyl groups, the SiC and quartz surfaces functionalized with cx3 showed more intense signals at 2852 and 2907 cm−1, as compared to the pyridine linker alone and to surfaces functionalized with cx1 and cx2. At 2897 and 2963 cm−1, weak spectral features corresponding to symmetric and antisymmetric stretching of CH3, respectively, can be observed. These features exist on all surfaces and are indicative of the presence of unreacted ethoxy groups from the silane. Due to the tolyl-substituent, however, these bands appear

with 5.4 nm/mm dispersion and 600 grooves/mm), GlanThompson 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 < 120 ps (fwhm), 300−720 nm) and a cooled (−80 °C) NIR photomultiplier tube (Hamamatsu R5509-42) (transit time spread 1.5 ns (fwhm), 300−1400 nm). Steady-state and fluorescence lifetimes were recorded in TCSPC mode by using a PicoHarp 300 (minimum base resolution 4 ps). Emission and excitation spectra were corrected for source intensity (lamp and grating) by standard correction curves. Phosphorescence lifetimes were recorded by using a NanoHarp 250 (minimum base resolution 32 ns) in MCS mode. 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 by visual inspection of the weighted residuals and their autocorrelation. Confocal fluorescence measurements were carried out to study the photoluminescence images of surface-bound platinum complexes on quartz substrates and on the Si face of 6H-SiC(0001) substrates.

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RESULTS AND DISCUSSION The successive functionalization steps are schematically depicted in Schemes 1 and 2. Scheme 1 shows the hydroxylation of both amorphous quartz and single crystalline 6H-SiC(0001) surfaces, followed by functionalization with 2-(4-pyridylethyl)triethoxysilane (4-PETES). Upon treatment of quartz surfaces with an acid/peroxide mixture, and of SiC surfaces with oxygen plasma at 1100 °C followed by 2.5% HF for 5 min, hydroxylterminated quartz and SiC surfaces are obtained.78 When the hydroxylated surfaces are dipped into a solution of 4-PETES in toluene, the trisiloxane group of 4-PETES binds to the surface hydroxyl groups and further cross-polymerizes with neighboring 4-PETES molecules. Thus, pyridine-terminated quartz and 6H-SiC substrates are obtained. 5554


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The Journal of Physical Chemistry C stronger for surfaces functionalized with cx2, if compared with the other two complexes. Due to the lack of substituents, cx1 displays a spectral profile that is comparable to the free pyridine linkers when attached to the substrates. The functionalization was further investigated by XPS. The full spectra of differently functionalized SiC(0001) and quartz surfaces are shown in Figure 2. The XPS spectra of pyridinefunctionalized SiC (py@SiC) and quartz (py@Q) confirm the presence of the expected elements: C 1s, N 1s, O 1s, Si 2s, and Si 2p bands are observed at binding energies of 284.8, 398.6, 533, 153, and 102.2 eV, respectively. The N 1s and O 1s peaks appear at lower binding energies than those for the elemental molecular species, whereas the Si 2s and Si 2p peaks appear at higher binding energies than those for elemental Si. This indicates that both N (bound to C) and O atoms (bound to Si and C) sense more electropositive environments than in their elemental state. As a result of the oxidized Si moieties, the Si binding energy shifts to higher energies. After complexation of Pt(II), additional signals from Pt 4p at 520.7 eV, 4d3/2 at 333.2 eV, 4d5/2 at 315.9 eV, 4f5/2 at 76.3 eV, and 4f7/2 at 73.1 eV are observed, appearing at higher binding energies than for the metallic form. Upon complexation with the pyridine-terminated surface and the tridentate ligands (Scheme 2), the metal centers are surrounded by four electronegative nitrogen atoms, which causes the Pt(II) levels to shift to higher binding energies, as compared to the metallic form. Upon modification of the surfaces with cx2 (cx2@SiC and cx2@Q, blue traces in Figure 2) and cx3 (cx3@SiC and cx3@Q, magenta traces in Figure 2), residual chlorine traces can be detected as Cl 2s and 2p signals at 269.5 and 198.3 eV, respectively. However, the XPS spectra showed no traces of S, indicating the absence of physisorbed Pt(II)(DMSO)2Cl2. Different abundances of Cl are observed for each complex, depending on the size of the tridentate ligands. For cx1, the Cl signal is negligible. With increasing ligand size, however, the Cl abundance increases by about 19% and 35% for cx2 and cx3, respectively, with respect to cx1.79,80 The high resolution XPS spectra of C 1s and N 1s provide information about the surface bonding states, as shown in Figures 3 and 4, respectively. The spectra are analyzed after subtracting the Shirley background. For the py@SiC and py@ Q surfaces, the C 1s signal shows four different chemical states. The feature at 283.7 eV binding energy (red line) is due to the Si−C−C structural element, which is part of the aliphatic pyridine linker for all substrates (Schemes 1 and 2). The peak at Eb = 285 eV (green line) is caused by −C−C−C− and −C− CC− aliphatic and aromatic pyridine groups, respectively. The signal at 286 eV (dark blue line) originates from the −C− C−N structural features of the pyridine group, in which the N-bound carbon signal appears at higher binding energies than for the other components, due to the polar C−N bond. The peak at 283.3 eV binding energy (violet line in Figure 3a) is due to the Si−C−Si structural feature of SiC substrate, and is absent for the quartz substrate. The high resolution XPS spectra of N 1s (see Figure 4) give rise to a single chemical state, C−N−C, at 398.8 eV binding energy for the py@SiC and py@Q surfaces. For the surfaces functionalized with the Pt-complexes, the high resolution C 1s spectra show five different chemical states. For all three complexes, the peaks due to the −C−C−N structural feature from the linker arise always at the same binding energy of 286 eV, as previously also observed for the py@SiC and py@Q surfaces. However, the peak due to the −C−C−C− and −C−CC− structural features appears at 284.8 eV, which

Figure 2. X-ray photoelectron spectra of pyridine linkers and Pt(II) complexes bound to 6H-SiC (a) and quartz (b) where the black, red, blue, and magenta traces represent py, cx1, cx2, and cx3 functionalized surfaces, respectively.

is shifted by 0.2 eV to lower binding energies as compared to the py@SiC and py@Q surfaces. This is probably due to the π-accepting character of the pyridine units from the tridentate luminophore and the monodentate linker, which accept electron density from the metallic center. For the complexfunctionalized surfaces, one extra peak at Eb = 286.8 eV (light blue line) is originated by the −N−C−N− structural feature from the five-membered rings of the ligands. For the cx3functionalized surfaces, the relatively stronger contribution to the C 1s line from the Si−C substrate signal at Eb = 283.3 eV indicates a lower concentration of adamantyl-substituted Pt(II) complexes as compared to the other functionalizations, which was confirmed by comparison of the relative Pt/Si intensities (see below). For each functionalized surface, diverse N 1s chemical states are identified by high resolution XPS spectra. For cx2@SiC and cx3@SiC surfaces (and also for the corresponding quartz substrates), the signals corresponding to the C−N−N (blue line) of the five-membered rings and to the N−N−Pt and C−N−Pt structural features from the coordinating units (magenta line) appear at 399.4 and 400.4 eV, respectively. As compared to the free pyridine linkers, the C−N−C signals are shifted to higher binding energies due to coordination to the cationic center. 5555


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Figure 3. High-resolution C 1s XPS spectra of (a) SiC and (b) quartz surfaces with covalently attached pyridine and subsequently functionalized with complexes cx1, cx2, and cx3.

Figure 4. High-resolution N 1s XPS spectra of (a) SiC and (b) quartz surfaces with covalently attached pyridine and subsequently functionalized with complexes cx1, cx2, and cx3.

Table 1. Area Ratios of XPS Signals for Pt 4d and Si 2s after Subtracting a Shirley Background surface

Si 2s

Pt 4d(5/2)

Pt 4d(3/2)

ratio (Pt 4d(5/2)/Si 2s)

ratio (Pt 4d(3/2)/Si 2s)

rel density

cx1@SiC cx2@SiC cx3@SiC cx1@Q cx2@Q cx3@Q

500 1014 2213 623 725 1467

6242 3852 3470 9853 7879 7351

4157 2560 2440 6670 5254 4990

12.5 3.8 1.6 15.8 10.8 5.0

8.3 2.5 1.1 10.7 7.2 3.4

7.7 2.3 1 3.2 2.1 1

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Figure 5. High-resolution valence band XPS spectra of different complexes (cx1@SiC, cx2@SiC, and cx3@SiC).

Figure 7. Schematic orbital (top) and state (bottom) energy level diagrams of monomeric and aggregated Pt(II) complexes. LC, ligandcentered excitation; d−d, metal-centered excitation; MLCT, metal-toligand charge transfer excitations; MMLCT, metal−metal-to-ligand charge-transfer excitation. The energies of the states (bottom) are average values for monomeric and aggregated species, as discussed in the introduction; solid lines represent radiative transitions, whereas the dotted lines stand for radiationless processes.

On the cx1@SiC and cx1@Q surfaces, the structural features of C−N−N, C−N−Pt, and N−N−Pt are also present. However, a new strong tetrazole-related N−N−N peak at 400 eV (green line) is observable for cx1. Moreover, for the cx2@SiC and cx3@SiC surfaces (and also for the corresponding quartz substrates), the triazole-related C−N−C features at 398.6 eV (red line) display

lower binding energies than the pyridine-related signals from uncoordinated six-membered rings (py@SiC and py@Q). This can be attributed to the electron rich character of the fivemembered rings, and are obviously not present in cx1. As the Si signal originates from the substrate and linker molecules, and the Pt signal arises from the complex only, it is

Figure 6. Excitation spectra (black lines) monitoring the emission of the aggregates at 550 nm and emission spectra (red lines) of different complexes linked to SiC (a) and quartz (b) surfaces. All emission spectra were obtained by excitation at 375 nm. The sharp peaks between 400 and 500 nm are due to Raman scattering (black asterisk). 5557


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the frontier orbital energies. The energetic positions of these levels are calibrated with respect to the Ag 4d state, which was added to the samples. The HOMO energies differ for the three complexes: In the cases of cx1@SiC, cx2@SiC, and cx3@SiC, they are observed at 4.1, 3.2, and 2.9 eV below the Fermi level, respectively. Due to the tetrazole moiety in cx1@SiC, its HOMO energy level at Eb = 4.1 eV resides deeper under the Fermi level as compared to cx2@SiC and cx3@SiC surfaces, which possess triazole moieties instead. The peaks between 4.5 to 10 eV are due to the different frontier orbitals of the complexes, and tentatively assigned to HOMO-1 and HOMO-2. We have recently shown that neutral Pt(II) complexes bearing dianionic tridentate N∧N∧N ligands can be used as triplet emitters, particularly in electroluminescent devices.77 Depending on the substitution pattern, they can be employed either as monomeric or as aggregated species (schematic representations of orbital and state diagrams for monomers and aggregates are shown in Figure 7). For the monomeric species, the vibrationally structured emission occurs from metalperturbed ligand-centered triplet states (3MP-LC, an admixture of predominantly LC excitations and MLCT contributions), whereas the aggregates emit with a broad band from metal− metal-to-ligand charge-transfer states (3MMLCT). On both SiC and quartz, the emission spectra display a broad, unstructured luminescence profile which clearly originates from aggregated species as can be seen in Figure 6 (red lines). The absence of monomeric emission from 3MP-LC states (and their characteristic vibrational profile) is indicative of a dense packing of the complexes that favors the formation of aggregated species, even in the presence of bulky adamantyl substituents. The emission maxima of cx2 appear red-shifted as compared to cx1, which can be attributed to the higher-lying HOMO level of the triazole units compared to the tetrazole units. The extended conjugation due to the presence of the tolyl moieties on the triazole rings might also contribute to this behavior, as previously observed for the corresponding monomeric species.66 The fact that the luminescence of the adamantyltriazole-substituted cx3 appears blue-shifted as compared to cx1 indicates that for cx3 the average Pt(II)...Pt(II) distance is larger than that for cx1 and cx2. This is in accordance with the lower packing density observed in XPS (Table 1). The fact is reasonable in view of the bulky adamantyl substitution, as the bulkiness increasing the Pt(II)...Pt(II) distance evidently compensates the higher-lying HOMO of the triazole unit as compared to

Table 2. Excited State Lifetimes of the Photoluminescence Decay for the Complexes at SiC and Quartz Surfacesa substrate

component

fractional intensity [%]

fractional amplitude [%]

lifetime τ [μs]

cx1@SiC

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

19 52 29 21 50 29 61 38 1 29 58 13 26 54 20 22 55 23

5 36 59 61 34 5 65 32 3 63 35 2 61 36 3 56 40 4

0.1 0.5 2.3 0.1 0.4 1.6 0.1 0.36 2.3 0.1 0.36 1.4 0.25 0.9 3.5 0.2 0.6 2.3

cx1@Q

cx2@SiC

cx2@Q

cx3@SiC

cx3@Q

a

The excitation was carried out with picosecond pulses at λ = 375 nm.

possible to derive the relative coverage of the three systems by comparing the area of the Pt signal with respect to the Si signal. For all systems, the Pt 4d states are found at Eb = 333.2 eV (4d3/2) and 315.9 eV (4d5/2), whereas the Si 2s state is observed at Eb = 152.9 eV. The area ratio of the different systems between Pt 4d and Si 2s are shown in Table 1, suggesting that the cx1@SiC surface has an approximately 8-fold higher Pt density than the cx3@SiC surfaces, and an approximately 3.3 times higher Pt density than the cx2@SiC substrate. This trend correlates with the ligands’ structures, as cx3 bears two bulky adamantyl groups, cx2 possesses two flat tolyl substitutents, and cx1 features unsubstituted tetrazole rings, causing different sterical hindrances. As a result, the lowest saturation coverage is expected for the complex with the bulkiest substituent (adamantyl) and the highest coverage for the complex no substituent, as shown in Table 1. A similar trend is also observed for the functionalized quartz surfaces. In Figure 5, high resolution XPS spectra for low binding energies (Eb = 11 to 0 eV) of the functionalized surfaces show

Figure 8. Fluorescence microscopy images of cx1 (a), cx2 (b), and cx3 (c) on SiC (I) and quartz (II) surfaces, excited at 375 nm. All images are selected with an area of 160 × 160 μm2 of the surface. 5558


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optoelectronic architectures with sensing abilities, as the excited state properties could be switched by varying the Pt(II)...Pt(II) distance.

the tetrazole unit of cx1. The excitation spectra (black lines in Figure 6) also show the corresponding 1MMLCT absorption band at 440 nm (SiC) and 430 nm (quartz) along with ligandcentered transitions below 400 nm. The relative intensity of the aggregate-related 1MMLCT excitation band nicely correlates with the bulkiness of the tridentate luminophore: It is maximized for the tetrazole-substituted luminophores (cx1), and minimized with adamantyl-triazole (cx3) units. The excited state lifetimes listed in Table 2 are also indicative of the formation of aggregated species, and range from submicroseconds to a few microseconds. On 6H-SiC(0001), they are not significantly shortened by energy or electron transfer to the conduction band, as becomes evident by a comparison with the lifetimes on the quartz substrates. In the latter case, the electronically excited states of the complexes are energetically clearly located in the band gap of the substrate. We assign the multiexponential lifetimes to a variable functionalization density of the substrates with the 4-PETES linker. Cross-linking during the functionalization process leads to variable local densities of the Pt(II) aggregates (Table 2). The variable local functionalization density also explains the brighter spots visible in the fluorescence microscopy (Figure 8), in particular for cx2 and cx3 functionalized surfaces.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.A.S.). *E-mail: [email protected] (H.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is financially supported by the Deutsche Forschungsgemeinschaft via projects B9 and C7 of the Transregional Research Center TRR 61 “Multilevel Molecular Assemblies: Structure, Dynamics and Function”. D.K.B. acknowledges support from the NRW International Graduate School of Chemistry in Münster.

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REFERENCES

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CONCLUSION AND PERSPECTIVES We have shown that a defined functionalization and precise characterization of insulating and semiconducting substrates with triplet emitters can be achieved by a convenient wetchemical grafting of a pyridine-terminated silane on amorphous quartz and on 6H-SiC(0001) single crystal surfaces. Coordination of the pyridine unit with phosphorescent Pt(II) complexes forces the triplet emitters to aggregate by surface constraints, even if bulky substituents such as adamantane are present. XPS and ATR FT-IR constitute adequate tools to ascertain the degree of functionalization; in particular, high resolution XPS spectra revealed the electronic states at the surfaces, which were correlated with the frontier orbitals of the tridentate luminophores and the degree of aggregation of the complexes. Moreover, photoluminescence spectroscopy allowed us to assess the full extension of aggregation caused by the 2Dconfinement, whereas fluorescence microscopy was used to evaluate the homogeneity of the phosphorescent layer. The phosphorescent coating constitutes a new class of architecture in which triplet emitters are covalently linked to (semi)conductive or insulating substrates, resulting in controlled monolayers with defined constitution and suitable linkers. This opens new perspectives for the construction of printable electroluminescent devices in which the charge injection into the luminescent layer can be favored by the use of conjugated anchoring units. If electroluminescent aggregates are used, the enhancement of intermolecular interactions by 2D-confinement could improve the efficiency of such arrays. The organized deposition of (supra)molecules with a defined orientation of the molecular transition dipole moments also provides new insights which in running devices may lead to an enhanced efficiency of light generation. Moreover, the directional interaction of the metal centers with the substrate can lead to a localized orbital hybridization and charge injection from the complexes into the metal, as we have recently shown. Thus, simplified electroluminescent devices consisting of emissive layers of oriented aggregates that interact via Pt-electrode hybridization can be envisaged. Finally, the surface functionalization presented herein would also assist the design of 5559


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Controlled 2D-Confinement of Phosphorescent Pt(II) Complexes on

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