Fluorescence Properties of Perylene and Pyrene Dyes Covalently

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Fluorescence Properties of Perylene and Pyrene Dyes Covalently Linked to 6H-SiC(0001) and Silicate Surfaces Deb Kumar Bhowmick, Linda Stegemann, Manfred Bartsch, Cristian Alejandro Strassert, and Helmut Zacharias J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09900 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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The Journal of Physical Chemistry

Fluorescence Properties of Perylene and Pyrene Dyes Covalently Linked to 6H-SiC(0001) and Silicate Surfaces Deb Kumar Bhowmick, Linda Stegemann, Manfred Bartsch, Cristian A. Strassert*, and Helmut Zacharias* Physics Institute, Center for Nanotechnology (CeNTech) and Center for Soft Nanoscience, University of Münster, Heisenbergstr. 11, 48149 Münster, Germany Abstract:

The functionalization of wide bandgap semiconductors and insulators with self-assembled layers of polycyclic aromatic hydrocarbons (PAHs) has gained major attention due to various potential applications such as sensors, in molecular electronics, or in solar cells and organic light emitting diodes. Their fluorescence properties depend among others on the spatial arrangement and aggregation of the molecules on the surface. In the present study, silane linkers of different length and conjugation were employed to functionalize silicon carbide and silicate surfaces with two PAHs. High-resolution X-ray photoemission spectroscopy (XPS) ascertained the degree of functionalization, supported by attenuated total reflection Fourier transform infrared spectroscopy and contact angle measurements. The homogeneity of the functionalized layers was evaluated by fluorescence microscopy. Using ultraviolet excitation, both a blue and a broad band fluorescence emission extending from 500 nm to beyond 600 nm were observed. Lifetime measurements revealed generally longer decays for the longer aliphatic AUDTES linker in comparison with the shorter aliphatic and aromatic spacers. Even though silicon carbide favors aggregation and shows a higher functionalization efficacy than the silicate substrates, the lifetimes of the fluorophores are not extensively influenced by the nature of the substrate but rather by the spatial arrangement of the luminescent species. Introduction: Organic-inorganic interfaces play an important role in modern optoelectronics. By choosing appropriate materials, the electronic structure of such interfaces may be adapted to various applications, in particular in the fields of molecular electronics,1-2 DNA and protein biosensors,31 ACS Paragon Plus Environment


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solar cells,5-6 long range electron7 and proton8 transfer reactions. The stability of the interfaces

is, however, always an important issue.9 Due to the strong Si-C network, silicon carbide (SiC) is one of the best substrates to obtain stable interface systems, similar to silicon10-11 and silicate.12-13 Further, different polytypes of SiC provide an opportunity to adapt the band gap,14 which opens the possibility to develop electronics used mainly under harsh conditions.15 The bio-inertness of SiC enables its use as bio-sensor and for bio-medical applications.16-19 For these reasons, the functionalization of SiC surfaces has become an important focus of research.20-22 Polycyclic aromatic hydrocarbons (PAHs) are very important for interface systems because of their stability, high fluorescence quantum yields and interesting optical and electronic properties.23-24 For highly conjugated polycyclic aromatic hydrocarbons, like perylenes and pyrenes, the spectral fluorescence properties and lifetimes strongly depend on the chemical environment and on their different aggregated forms.25-26 In surface-bound states, the photophysical properties of such systems can change dramatically.27 Perylenes and pyrenes have been studied in the past because of the very distinct optical behaviour from their monomeric and aggregated forms.28-29 Their HOMO-LUMO energies suggest that pyrene can act as a good electron donor, while perylene and in particular benzoperylene act as electron acceptors.30 Different strategies have been developed to obtain self-assembled monolayers on inorganic substrates, like wet chemical functionalization,31 thermal grafting,30 photochemical22,31 and radical attachment.32 Wet-chemical functionalization is the most convenient method to form selfassembled layers. To obtain an organic layer on a semiconductor surface, one needs linker molecules acting as connectors between the inorganic and the organic systems. A variety of linkers has been used for wet-chemical functionalization of different substrates, e.g. thiolate mainly for different metal surfaces,33-35 phosphonate36-37 and silane38-39 linkers for oxide and hydroxylated surfaces. The choice of linkers can strongly influence the adsorbate distribution on the surfaces, and can also have a strong influence on its optoelectronic properties. Only a few studies have been reported on the influence of different linkers between an adsorbate and a substrate, especially on semiconductor surfaces.40 Further, when comparing contact angles and fluorescence quantum yields of umbelliferone on silicate, it was found that for longer aliphatic silanes with a higher packing density, self-quenching of fluorescence41 occurred. Earlier, we reported on the functionalization of 6H-SiC(0001) and quartz with benzo[ghi]perylene and 2 ACS Paragon Plus Environment

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aminopropyl triethoxysilane (APTES) linker molecules,20 and triplet-emitting Pt(II) complexes in combination with 2-(4-pyridylethyl)triethoxysilane.42 In the present study, we discuss the fluorescence properties of two different PAH-based chromophores, namely benzo[ghi]perylene1,2-dicarboxylic anhydride and 1-pyrenecarboxaldehyde reacted with amino groups on insulating silicate and semiconducting 6H-SiC(0001) substrates. We also study the influence of linkers with different chain lengths and structures (aliphatic vs. aromatic) on these interface systems. The successful functionalization was assessed by static water contact angle, ATR FT-IR and XPS measurements. Time-resolved confocal fluorescence microscopy was employed to investigate the functionalization. Depending on the nature of the linkers, the distribution and the molecular orientation of the dyes with respect to the substrate change and directly affect the fluorescence lifetimes of the fluorophores.

Experimental Section Chemicals: The chemicals which were used during the course of experiment included MeOH (VWR), EtOH (VWR), CH2Cl2 (VWR), dry toluene (Acros), benzo[ghi]perylene-1,2dicarboxylic anhydride (Sigma Aldrich), 1-pyrenecarboxaldehyde (ABCR), 3-aminopropyl triethoxysilane (APTES; from Acros), 11-aminoundecyl-triethoxysilane (AUDTES; from ABCR), p-aminophenyltrimethoxysilane (p-APTMS; from ABCR), dimethyl formamide (DMF; from VWR), Zn(OAc)2 (Sigma Aldrich), H2SO4 (95% in water) (VWR), H2O2 (35% in water) (AppiChem), HF (Sigma Aldrich), acetone (VWR), dichloroethylene (VWR), triethylamine (VWR), imidazole (ABCR). These chemicals were used without further purification. Substrates: Double side polished 6H-SiC (0001) wafers (n-type doping, thicknesses: 330 ± 25 µm, resistivity: 0.02-0.1 Ωcm) from PAM-XIAMEN were used. Standard microscope slides (sodium-calcium-silicate glass) were used as silicate substrates, showing an optical bandgap of about 3.85 eV. Surface modification: Silicate and 6H-SiC wafers were cut into 8 mm × 10 mm pieces. These were sonicated in distilled water, followed by sonication with acetone, dichloroethylene, MeOH, and EtOH. Finally, the samples were again sonicated with deionized water. Then, these samples were treated with an acid-peroxide solution (H2SO4 : H2O2 in a 3:1 mixture) for 4 hours at 80˚C 3 ACS Paragon Plus Environment


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followed by three times washing with deionized water. (Caution: Acid-peroxide solution can explode on contact with organic material which can cause serious injury. All proper safety and precautions should be taken when using this reagent). Thereafter, the silicate slides were used for linker immobilization. 6H-SiC wafers were heated at 1000˚C under O2 atmosphere at 1000 mbar pressure for 4 hours and then treated with 48% HF acid for 3 min. These were thoroughly washed with deionized water and dried under a flow of N2. These samples were used for immobilization of linker molecules. Linker immobilization: The wafers were treated under Ar atmosphere with three different linker molecules, namely APTES, AUDTES, and p-APTMS as 4% solutions, in dry toluene at 100˚C for 4 hours with catalytic amounts of triethylamine. After completion of the reaction, the wafers were sonicated with toluene, ethanol, and acetone, each for 15 min. Then, the wafers were dried under nitrogen flow and used for characterization and further modification. Benzo[ghi]perylene-1,2-dicarboxylic anhydride immobilization: Linker-modified silicate and 6H-SiC wafers were allowed to react with a 1.5 mmolar solution of benzo[ghi]perylene-1,2dicarboxylic anhydride, imidazole, and copper acetate anhydride in dry toluene at 130˚C for 6 hours. Then, these functionalized substrates were sonicated with toluene followed by ethanol and acetone, respectively, for 10 min each. The wafers were dried under a flow of N2. 1-Pyrenecarboxaldehyde immobilization: Linker-modified silicate and SiC wafers were treated with a 10 mmolar solution of 1-pyrenecarboxaldehyde in dry ethanol at 45°C for 2 hour. After that step, the surfaces were cleaned by sonication in dry ethanol and dry acetone, for 5 min each. Finally the wafers were dried under nitrogen flow. 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 and a hemispherical analyzer (Kratos Analytical Axis Ultra) at normal emission. All signals were referenced to the signature of the carbidic C 1s peak of SiC at Eb = 282.8 eV. Contact Angle Measurements: Contact angle measurement is an effective technique to investigate the surface hydrophobicity and morphology, and was performed on all surfaces using 4 ACS Paragon Plus Environment

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an Erma Contact Angle Meter G-1. The humidity during the contact angle measurements was determined by ambient air, and amounted to about 50%. 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 ∆ῦ = 2 cm-1, using 512 scans for each measurement. 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. Images were taken in epi-fluorescence mode with a 60x water immersion objective (NA 1.20) and a 40x objective (NA 0.95) through borosilicate coverslips (0.13-0.16 mm, n=1.52) with samples on top. The excitation light was filtered by a 360-370 nm filter, and the emission was measured through a 400 nm dichroic and a 420 nm longpass filter. Time-, space-, and spectrally resolved confocal microscopy measurements were carried out with a PicoQuant MicroTime 200 featuring two single-photon avalanche diode detectors (from MPD and Perkin-Elmer). Pulsed diode lasers operating at 375 nm and 440 nm and pulse durations of τL < 40 ps and < 70 ps, respectively, were used as excitation sources. Time-correlated photon counting data were acquired through a PicoHarp 300 with a temporal resolution of 4 ps, and the data analysis was carried out with the SymPhoTime software package from PicoQuant (version 5.0). Single-crystal spectra were measured with a Peltier-cooled Andor Newton back-illuminated EMCCD connected to a Shamrock SR-163 (Andor) spectrograph. Time-, space-, and spectrally resolved confocal fluorescence measurements were carried out to study the lifetime of surfacebound benzo[ghi]perylene and 1-pyrene derivatives on silicate substrates and on the Si face of 6H-SiC (0001) substrates.

Results and Discussion The chemical functionalization of organic layers on solid surfaces strongly depends on the nature of the linkers. It is expected that a shorter chain length will lead to a more random distribution, 5 ACS Paragon Plus Environment


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whereas a longer chain length will lead to a more uniform surface coating, due to the stronger van der Waals interactions between adjacent hydrocarbon chains of the linker backbones. This leads to a higher availability of the reactive group for subsequent connection to dyes, resulting in a different distribution of the fluorophores on the surfaces. Figure 1 schematically depicts the reaction steps on amorphous silicate and single crystalline 6H-SiC(0001) surfaces with organic dyes by wet-chemical processing. The first step involves the hydroxylation of the wafers, followed by the attachment of reactive silane linkers (p-APTMS, APTES or AUDTES). The immobilization through silanization of the hydroxyl groups at the surfaces with methoxy- or ethoxysilanes is achieved by treatment in dry toluene, yielding amino-terminated surfaces. Finally, benzo[ghi]-perylene-1,2-dicarboxylic anhydride was allowed to bind to these active surfaces to yield an imide bond, while pyrene dyes give an imine bond (Figures 1c,d). Directly after the activation of silicate and 6H-SiC, the static water contact angles are α < 20° and α < 15°, respectively, which confirms the hydrophilicity of the surfaces due to the insertion of hydroxyl groups. The contact angles after the silanization and the dye attachment are shown in Table 1. The results indicate an increase of the contact angles, confirming the higher hydrophobicity upon functionalization with organic moieties. For the linker-functionalized surfaces, the contact angles grow as follows: AUDTES < p-APTMS < APTES. The trend suggests that the hydrophilic amino groups pointing outwards from the surface are more exposed for AUDTES and less available for APTES. For the dye-functionalized surfaces, p-APTMS linkers show the smallest contact angles, whereas aliphatic linkers display the highest ones. Attenuated total reflection (ATR) FT-IR spectroscopy is an important tool to assign different moieties on surfaces by their characteristic vibrational features. The ATR FT-IR spectra (Figs. S1 and S2, Supporting Information) show the CH2 symmetric and anti-symmetric stretching frequencies at ῦ = 2852 and 2923 cm-1 for AUDTES and APTES, as well as for the dyefunctionalized surfaces due to the aliphatic CH2 backbones from the linkers. A very broad signal from 3100 to 3700 cm-1 appears for all three silanized surfaces due to NH symmetric stretching and OH stretching from adsorbed water and unreacted surface hydroxyl groups. This confirms the functionalization of the hydroxyl-terminated surfaces. Since the p-APTMS-functionalized surfaces do not have any aliphatic CH2 moieties, the characteristic symmetric and asymmetric vibrational signals are absent (Figs. S1b and S2b, Supporting Information).

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The differently functionalized surfaces were further characterized by X-ray photoelectron spectroscopy (XPS) to investigate their chemical composition. The full XPS spectra of hydroxylated 6H-SiC(0001) (black) and silicate (red) are shown in Fig. 2. These surfaces show the different element signals of O 1s at 532 eV, Si 2s at 154 eV and Si 2p at 103 eV for silicate, and one extra elemental signal of C 1s at 282.8 eV for SiC. The full XPS spectra for linker- and dye-functionalized SiC surfaces are shown in Fig. 3, and in Fig. S3 for functionalized silicate surfaces (Supporting Information). The spectra show the presence of different elements, namely C 1s at 285.2 eV, N 1s at 400.5 eV, O 1s at 532.2 eV, as well as Si 2s at 154 eV and Si 2p at 103 eV for all linkers and the dye molecules on SiC (Fig. 3) and silicate surfaces (Fig. S3). There is a clear increase of the C 1s signal in comparison with the hydroxyl-terminated SiC and silicate surfaces (Fig. 2). Also, a new feature due to N 1s arises for the functionalized surfaces, which is absent for the hydroxyl-terminated SiC and silicate surfaces. The O 1s signal present at all surfaces is caused by the hydroxyl groups, by the silane linkers, and by the perylene dyes. For functionalized silicate surfaces, part of the O 1s signal arises from the substrate itself. The Sirelated signals originate from the substrate itself and the silane linkers, which are observed for all functionalized surfaces. High resolution C 1s (Figs. 4 and S4, Supporting Information) and N 1s (Figs 5 and S5, Supporting Information) XPS spectra show different signals due to the presence of distinct chemical features. C 1s signals of dye-functionalized surfaces show five different components for SiC and four components for silicate surfaces. The extra component at 282.8 eV for SiC surfaces is due to the Si-C-Si structural feature arising from the substrate43. The other four spectral components are common for both silicate and SiC surfaces. The component at 284.3 eV corresponds to Si-C-O and Si-C-C structural features. The most intense component at 285.2 eV is due to –C-C- and –C=C- bonds for AUDTES- and APTES-linked dyes, while it appears at 284.7 eV for p-APTMS (Figs. 4b and S4b, Supporting Information). The spectral feature at 286.4 eV arises due to C-N bonds. These components appear for both perylene- and pyrenefunctionalized surfaces. The highest energy component for the perylene-functionalized surfaces appears at 288.5 eV due to the –N-C=O structural feature of the imide and the amide bonds, whereas for pyrene-functionalized surfaces the –C=N- spectral feature from the imine bond appears at a lower binding energy (287.9 eV). This confirms different binding modes of the amino groups with these two different dyes. 7 ACS Paragon Plus Environment


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For functionalized SiC surfaces, the C 1s intensity of the substrate (Si-C-Si) with respect to other components is different for each linker. The relative intensity is lowest for p-APTMS and highest for AUDTES. During the functionalization of the hydroxylated surfaces, these silanes not only polymerize along the plane of the substrates, but also in an out-of-plane fashion, leading to the growth of a thicker organic layer at the substrate. The out-of-plane growth of the linkers increases with shorter chain lengths and decreasing van der Waals interactions between the linker chains. Even though p-APTMS contains more carbon atoms than APTES, it appears that the stiffer aromatic ring favors a random out-of-plane growth of the layers. We approximately quantified the linker functionalization on the SiC surface by comparing the N 1s signal with respect to the carbidic C 1s signal of different linker functionalized SiC surfaces. We arrive at approximately a monolayer coverage for the AUDTES linker, a few (five) monolayers for APTES, and multi-monolayers (> 20 ML) for p-APTMS using standard quantification procedures described in the literature.44-47 The high-resolution N 1s XPS signal (Figs. 5 and S5, Supporting Information) gives rise to four components for the perylene- and three for the pyrene-functionalized 6H-SiC(0001) and silicate surfaces. For both substrates, the four different chemical environments of the N 1s in perylenefunctionalized surfaces are due to unreacted amino groups at 399.2 eV, to protonated amines at 402.2 eV and due to the amide bonds at 400.1 eV. The 401.2 eV feature corresponds to the imide group of the perylene. The perylene dye binds to the linkers in two different ways, namely by a single amide bond or by a cyclic imide. Pyrene binds to the linkers through an imine double bond, which therefore gives rise to only three different chemical environments for the N 1s signal. Among them, two components are due to unreacted amine and protonated amine groups at 399.2 eV and 402.2 eV, respectively. The third component at 400.4 eV corresponds to the – C=N-C- structural feature arising from the reaction between the aldehyde group of the dye and the amine moiety of the silanized surface. On both substrates, this peak corresponding to the imine-N 1s appears at the same binding energies. One can compare the relative areas in the high-resolution N 1s spectra for the peaks corresponding to the reacted (–N(C=O)2, –NHC=O, –N=C) and the unreacted (–NH2 and –NH3+) amino groups to get an indication of the efficacy of the surface functionalization. In Table 2, the integrated N 1s areas are shown for the three linkers and for both dyes on both substrates. This 8 ACS Paragon Plus Environment

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analysis yields 15% to 35% of unreacted linkers. The efficacies of both dyes are higher on the smoother 6H-SiC (0001) than on amorphous silicate, irrespective of the silane. For AUDTES, the influence of the substrates is less pronounced. Due to its long and flexible aliphatic chain, the orientation of the reactive amine group with respect to the surface does not depend on the orientation of the silane group on the substrate. For the more rigid backbones of the shorter linkers (APTES, p-APTMS), the orientation of the amine groups change when the binding of the silane group to the substrate vary due to the surfaces roughness. In Figs. 6 and 7, the fluorescence spectra are shown for benzo[ghi]perylene-1,2-dicarboxylic anhydride upon reaction with AUDTES (Fig. 6) and p-APTMS (Fig. 7) at 6H-SiC(0001) (Figs. 6a and 7a) and silicate (Figs. 6b and 7b). The magenta spectra were obtained after excitation at λ = 375 nm and using a longpass filter at λ = 400 nm for observation. The blue spectra were obtained after excitation at λ = 440 nm and observation through a longpass filter with an onset at λ = 460 nm. Upon excitation at λ = 375 nm, the emission spectra show three bands48 centered at λ = 445 nm, 520 nm and 590 nm. Excitation at λ = 440 nm shows one broad band peaking at λ = 520 nm. Excitation at λ = 375 nm causes solely transitions between X 1A1(S0) and 1B2(S1) states, which causes a structured emission around λ = 445 nm from 1B2(S1) to X 1A1(S0) states, as described by Salama and coworkers49 and by Kelly and coworkers.29 The unstructured broad emission near λ = 590 nm is due to aggregated benzo[ghi]perylene, as recently also has been reported.48 As we have previously described for benzo[ghi]perylene bound to an APTES linker, irradiation at λ = 375 nm excited both the monomeric dye, which emits at 439 nm, as well as the aggregated moieties, which emit at 480 nm and 615 nm. On the other hand, irradiation at λ = 440 nm excited only aggregates. The emission bands observed in the present study are broader than previously reported,20 and the band at 480 nm appears shifted to a longer wavelength of 520 nm with a very broad emission profile. However, the behavior of these emission bands is similar to the ones we observed previously for different excitation wavelengths.20 This observation leads to the conclusion that the 440 nm excitation preferentially addresses π-stacked complexes, as compared to the 375 nm excitation for all benzo[ghi]perylene-functionalized SiC or silicate surfaces. Figure 8 shows the fluorescence spectra of 1-pyrene bound to APTES (black), AUDTES (red) and p-APTMS (blue) linkers on 6H-SiC(0001) (Fig. 8a) and silicate (Fig. 8b) surfaces upon 9 ACS Paragon Plus Environment


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excitation at λ = 375 nm. A longpass emission filter with an onset at λ = 400 nm was used. Pyrene-functionalized SiC surfaces show bands at λ = 440 nm and at 520 nm with a broad extension to the red. The emission band at λ = 440 nm has a low intensity as compared to the other band. These emission bands are similar to those observed for unsubstituted pyrene,50 but shifted to longer wavelengths and broadened.51 The excitation at λ = 375 nm causes transitions only from S0 to S1 states, as for other transitions higher excitation energies would be needed. The band at λ = 440 nm is thus caused by a relaxation from the S1 to the S0 ground state, while the broad band emissions at 520 nm is caused by the formation of multiple aggregates of pyrene molecules.51 On SiC surfaces, the broad extension of the emission to the red suggests the formation of additional aggregated pyrene. Pyrene-functionalized silicate surfaces (Figure 8b) show less intense monomeric bands at λ = 440 nm, and a more intense aggregate emission at 520 nm. For silicate substrates, the extension of the emission to the red is weaker than for SiC substrates, which makes the resulting fluorescence profile narrower. These observations suggest that the pyrene dyes are more prone to form aggregates on 6H-SiC(0001) substrates than on silicate substrates. The fluorescence images and spatial lifetime distributions of the functionalized surfaces are shown in Figures 9 to 13. Figures 9 and 10 show two-dimensional spatial distributions of benzo[ghi]perylene- and 1-pyrene-functionalized surfaces, respectively, upon excitation at λ=360 -370 nm and detection through a 400 nm dichroic and a longpass filter at λ=420 nm. The fluorescence distribution of the perylene-functionalized surfaces is different for AUDTES and pAPTMS. AUDTES-linked perylene has a significantly higher tendency to form bright islands than perylene bound to p-APTMS linkers. This tendency to form micrometer-sized islands is higher on SiC than on silicate, probably due to the smoother nature of the SiC surface. The pyrene distribution also varies when the dye is bound to different linkers (Figure 10). In this case, the tendency to the formation of highly emissive islands increases as the stiffness of the linker’s backbone decreases. More islands are observed for AUDTES than for p-APTMS, for which only a few islands appear. Again, island formation is more pronounced on SiC than on silicate. This phenomenon is therefore attributed to the nature of linkers and the substrate surfaces. Due to the stiff aromatic backbone of p-APTMS, the distribution of the amino groups on the substrate is random within the less organized organic layer. As a result, weakly emissive 10 ACS Paragon Plus Environment

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dye clusters are formed on the substrate. On the other hand, AUDTES bears the longest and most flexible aliphatic chain with highest van der Waals interactions, leading in part to a well-directed distribution of the amine groups. This causes stacking arrangement of the dyes upon binding to this linker. Thus, a high tendency to the formation of highly emissive clusters is observed on the substrate. Steady state fluorescence lifetimes were obtained by two-dimensional time-resolved confocal microscopy measurements (figs. 11-13). The area selected for lifetime averaging is 25 µm × 25 µm, which gave a good signal-to-noise ratio. For perylene-functionalized surfaces, two excitation wavelengths, namely λ = 375 nm and λ = 440 nm were used, and for pyrenefunctionalized surfaces only one excitation wavelength at λ = 375 nm was employed. Figures 11 and 12 show typical intensity and average lifetime images of perylene-functionalized SiC and silicate surfaces, obtained for two excitation wavelengths at λ = 375 nm and λ = 440 nm. In Fig. 13, the corresponding intensity and average lifetime images of pyrene-functionalized SiC and silicate surfaces are shown. For perylene- and pyrene-functionalized surfaces, the observed fluorescence decay curves are displayed in Figs S6-S8, Supporting Information, respectively. There, the logarithm of the fluorescence intensity is plotted vs. the decay time. In nearly all cases it was necessary to fit the data with three exponential decay components according to ௧

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భ

మ

య

݂ሺ‫ݐ‬ሻ = ‫ܣ‬଴ exp ቀ− ఛ ቁ + ‫ܣ‬ଵ ݁‫ ݌ݔ‬ቀ− ఛ ቁ + ‫ܣ‬ଶ ݁‫ ݌ݔ‬ቀ− ఛ ቁ. This requirement shows that different molecular environments are responsible for the luminescence. For benzo[ghi]perylene, two short lifetimes, one with about τ1 ~ 0.5 ns the other with τ2 ~ 1.5 ns, dominate the fluorescence decay. The third lifetime observed is in the range of τ3 ~ 4 to 7 ns with a relative intensity of about 10% of the total decay curves. The lifetimes observed for the aromatic p-APTMS linker are significantly shorter than those for the aliphatic linkers. Further, the less-ordered silicate surface causes the very short lifetime τ1 to remain undetected (or too close to the instrumental limit) for the p-APTMS and AUDTES linker. The results observed for benzo[ghi]perylene are summarized in Table 3.



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A similar behavior is found for pyrene attached to these linkers, and is summarized in Table 4. Also in this case, three lifetimes are necessary to fit the experimental data, with the two shorter ones dominating the decay again. These two fast decays show lifetimes in the range of about τ1 ~ 0.3 ns and τ2 ~ 0.6 to 1.8 ns, while the third decay, again with about 10% of the intensity, appears at about τ3 ~ 3 to 8 ns. Also for the pyrene dye, the p-APTMS linker results in the shortest lifetimes for all components, whereas the AUDTES linker results in the longest lifetimes. Moreover, the silicate surfaces yields shorter lifetimes than the single crystal SiC surface. For the two longer decay components, a general increase of the fluorescence lifetimes is observed from τ(p-APTMS) < τ(APTES) < τ(AUDTES). This holds for both dyes and for both substrates. For the perylene-functionalized surfaces, the fluorescence lifetimes observed upon excitation at 440 nm are longer than upon excitation at 375 nm. This implies that the stacking moieties (band at 520 nm) of surface-bound benzo[ghi]perylene have a longer fluorescence lifetime. Therefore, the main reason for the increase of the lifetime of benzo[ghi]perylene is the predominance of the aggregate emission. For both perylene- and pyrene-functionalized surfaces, the lifetimes do not differ much for the two different substrates, but they are slightly longer on SiC than on silicate. This implies that the emitters are well-isolated from the substrates by these linkers. Moreover, due to a smoother SiC single crystal surface than that of the amorphous silicates, more stacking of the perylene occurs, leading to longer lifetimes. The increased fluorescence lifetime for AUDTES-functionalized surfaces compared to the other two linkers is again due to the predominance of stacked dyes. For the pyrene-functionalized surfaces, the fluorescence spectra (Figure 8) suggest that on single crystal SiC substrates, the tendency to form aggregates is higher than on silicate. Also the lifetimes (Table 4) of pyrene on SiC substrates are slightly longer than for silicate substrates. This suggests that pyrene also shows a slightly longer lifetime for the stacked moieties than for the monomeric species, as we already observed on perylene-functionalized surfaces.

Conclusion 6H-SiC(0001) and silicate substrates were successfully reacted with benzo[ghi]perylene-1,2dicarboxylic anhydride and 1-pyrenecarboxaldehyde by imide and imine bonds, respectively, 12 ACS Paragon Plus Environment

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with the aid of three different amino-silane linkers (p-APTMS, APTES, AUDTES). The successful functionalization was confirmed by contact angle measurements, ATR FT-IR, and XPS, indicating that stiffer and shorter linkers cause thicker linker coverage. The reaction efficacies of the dyes with the amino-functionalized surfaces range from 65% to 85%. The stiffer (p-APTMS) linker shows a higher efficacy as the rigid backbone causes most of the active amine group to point outwards. Longer, flexible linkers (AUDTES) show higher reaction efficacies than shorter moieties (APTES) due to higher van der Waals interaction with adjacent linkers and a subsequently higher availability of reactive amino groups. The reaction efficacy for the shorter linkers shows a higher dependence on the nature of the substrate. On single crystal SiC substrates, it is higher than on amorphous silicate substrates. Fluorescence spectra show both monomeric and aggregated species of these dyes. The fluorescence images indicate an inhomogeneous distribution of the fluorophores and the formation of highly emissive islands for both dyes. The island formation is more prominent for longer and flexible linkers than for shorter and stiffer linkers, a phenomenon that is more pronounced on 6H-SiC(0001) than on silicate. Confocal microscopy measurements yield three fluorescence lifetimes for each dyefunctionalized surface. They are slightly longer on SiC substrates than on silicate, which implies that the aggregation is more efficient on the semiconductor. There is also an increased fluorescence lifetime for the different linker systems: τ(p-APTMS) < τ(APTES) < τ(AUDTES), due to an enhancement of the aggregation.

Acknowledgements: The authors thank Prof. Dr. Lifeng Chi (Physikalisches Institut and Center for Nanotechnology, University of Münster) for providing the contact angle instrumentation. Deb Kumar Bhowmick acknowledges support via the NRW International Graduate School of Chemistry, Münster, which is funded by the Government of the state Nordrhein-Westfalen and the University of Münster. The project is financially supported by the Deutsche Forschungsgemeinschaft via projects B09 and C07 of the Transregional Research Center TRR 61 “Multilevel Molecular Assemblies: Structure, Dynamics and Function”.



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Supporting Information Additional information regarding ATR-IR spectroscopy, XPS measurements and fluorescence decay curves can be found in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org

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Table 1. Contact angle measurements.

Surface

Silicate

6H-SiC(0001)

Static water contact angle after dye functionalization [°] benzo[ghi] pyrene perylene 73 76

Linker

Static water contact angle [°]

p-APTMS

64

APTES20

68

85

79

AUDTES

58

78

79

p-APTMS

65

74

75

APTES20

64

78

78

AUDTES

60

79

76

Table 2. Functionalization efficacy.

Surface

Dye

Linker

Total area of reacted and unreacted N 1s

Area of unreacted N 1s

Percentage of unreacted linker [%]

p-APTMS

4198

714

17

APTES20

1678

464

28

6H-SiC

AUDTES

12275

2897

23

(0001)

p-APTMS

4445

625

14

APTES

930

181

19

AUDTES

812

208

25

p-APTMS

1074

273

25

APTES20

364

125

34

AUDTES

4024

974

24

p-APTMS

886

253

28

APTES

3483

1185

34

AUDTES

1399

325

23

benzo[ghi]perylene

pyrene

benzo[ghi]perylene Silicate pyrene



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Table 3. Excited state lifetimes for the fluorescence decay of benzo[ghi]perylene-1,2functionalized 6H-SiC and silicate surfaces. Substrate

Excitation wavelength [nm]

Linker

375 p-APTMS 440

375 6H-SiC(0001)

APTES20 440

375 AUDTES 440 375 p-APTMS 440 375 Silicate

APTES20 440 375 AUDTES 440

Component 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 1 2 1 2 3 1 2 3 1 2 1 2

Rel. amplitude [%] 42 51 7 69 26 5 36 47 17 27 54 19 48 44 8 49 42 9 83 17 76 24 48 42 10 41 46 13 68 32 68 32

Lifetime τ [ns] 0.1±0.1 0.3±0.1 1.3±0.1 0.2±0.1 0.5±0.1 2.4±0.1 0.5±0.1 1.4±0.1 4.3±0.1 0.5±0.1 1.8±0.1 4.7±0.1 0.6±0.1 2.2±0.1 7.5±0.1 0.6±0.1 2.3±0.1 6.9±0.1 0.5±0.1 2.7±0.1 0.8±0.1 3.5±0.1 0.5±0.1 1.2±0.1 3.6±0.1 0.5±0.1 1.5±0.1 4.2±0.1 1.3±0.1 5.6±0.1 1.5±0.1 6.3±0.1


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Table 4. Excited state lifetimes for the fluorescence decay of 1-pyrene-functionalized 6H-SiC and silicate surfaces. Substrate

6H-SiC(0001)

Linker

Excitation wavelength [nm]

p-APTMS

375

APTES

AUDTES

p-APTMS

Silicate

APTES

AUDTES

375

375

375

375

375

1

Rel. amplitude [%] 65

2

35

4.3±0.1

1

58

0.3±0.1

2

34

1.2±0.1

3

8

5.3±0.1

1

51

0.4±0.1

2

38

1.8±0.1

3

11

8.9±0.1

1

57

0.2±0.1

2

35

0.6±0.1

3

8

2.8±0.1

1

52

0.2±0.1

2

39

0.8±0.1

3

9

2.9±0.1

1

40

0.6±0.1

2

45

2.4±0.1

3

15

7.9±0.1

Component

Lifetime τ [ns] 0.7±0.1



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(a)

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(b) H2SO4:H2O2 :: 3:1 60°C, 4 h

NH2 NH2 NH2

Silicate

OH OH

OH p-APTMS

Silicate/SiC

Si Si Si O O O OO O

SiC I. Sacrificial oxidation II. 48% HF, 5 min

(I)

Silicate/SiC

H2N H2N H2N

OH OH

OH

Silicate/SiC

Toluene (dry), triethylamine (cat.)

Si Si Si O O O OO O (II)

APTES

Silicate/SiC

90°C, 4 h H2N H2N H2N

AUDTES

Si Si Si O O O OO O

(III)

Silicate/SiC

(d)

NH2

NH2 NH2

Surface I/II/III

O

N

EtOH (dry) 45oC, 2 h

Surface I/II/III

Figure 1: Schematic representation of the hydroxylation of 6H-SiC(0001) and silicate surfaces (a) and silanization with APTES, p-APTMS, AUDTES of the hydroxylated surfaces (b). Schematic representation of the immobilization of benzo[ghi]perylene-1,2-dicarboxylic anhydride (c) and 1-pyrene carboxaldehyde (d) on amino-functionalized surfaces. Surface I, II, III denote the different linkerfunctionalized substrates.


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Figure 2: X-ray photoelectron spectra of hydroxylated 6H-SiC(0001) (black) and hydroxylated silicate (red) surfaces20. The line at Eb = 494 eV belongs to a spurious Na impurity.



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Figure 3: XPS of (a) AUDTES (black), pyrene- (blue) and benzo[ghi]perylene- (red) functionalized 6H-SiC(0001) surfaces; (b) of p-APTMS (black), pyrene- (blue) and benzo[ghi]perylene-1,2 (red) functionalized 6H-SiC(0001) surfaces; (c) pyrene-functionalized 6H-SiC(0001) surface with APTES linker (black).


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Figure 4: High-resolution C 1s XPS data of a benzo[ghi]perylene- and pyrene- functionalized 6H-SiC(0001) surfaces with AUDTES (a), p-APTMS (b) linkers and pyrene- functionalized 6HSiC(0001) surfaces with APTES (c). Dotted line: experimental data; solid lines: fitted data.



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Figure 5: High-resolution N 1s XPS data of a benzo[ghi]perylene- and pyrene- functionalized 6H-SiC(0001) surfaces with AUDTES (a), p-APTMS (b) linkers and pyrene-functionalized 6HSiC(0001) surfaces with APTES (c).


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Figure 6: Fluorescence spectra of a benzo[ghi]perylene-functionalized (a) 6H-SiC(0001) and (b) silicate surfaces with the AUDTES linker. Magenta: excitation at 375 nm and observation through a long pass filter at λ=400 nm; blue: excitation at 440 nm and observation through a long pass filter at λ=460 nm.



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Figure 7: Fluorescence spectra of a benzo[ghi]perylene-functionalized (a) 6H-SiC(0001) and (b) silicate surface with a p-APTMS linker. Magenta: excitation at 375 nm and observation through a long pass filter at λ=400 nm; blue: excitation at 440 nm and observation through a long pass filter at λ=460 nm.


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Figure 8: (a) Fluorescence spectra of a pyrene-functionalized (a) on 6H-SiC(0001) and (b) on silicate surfaces with APTES (black), AUDTES (red), p-APTMS (blue) linkers. Excitation at 375 nm and observation through a long pass filter at λ=400 nm.



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Figure 9: Fluorescence microscopy images of (a) perylene-p-APTMS-SiC (b) perylene-pAPTMS-silicate (c) perylene-AUDTES-SiC (d) perylene-AUDTES-silicate. Excitation at λ=360 -370 nm, observation via 400 nm dichroic and a long pass filter at λ=420 nm.


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Figure 10: Fluorescence microscopy images of (a) pyrene-p-APTMS-SiC (b) pyrene-p-APTMSsilicate (c) pyrene-APTES-SiC (d) pyrene-APTES-silicate (e) pyrene-AUDTES-SiC (f) pyreneAUDTES-silicate surfaces. Excitation at λ=360 - 370 nm, observation via a 400 nm dichroic and a long pass filter at λ=420 nm.



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Figure 11: Average lifetime and intensity images for (a) perylene-p-APTMS-SiC excited at 375 nm using a long pass filter at λ=400 nm, (b) perylene-p-APTMS-SiC excited at 440 nm using a long pass filter at λ=460 nm; (c), (d) as (a), (b) for a silicate substrate. All images are selected at different positions of the surface with an area of 25 µm × 25 µm.


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Figure 12: Average lifetime and intensity images of (a) perylene-AUDTES-SiC excited at 375 nm using a long pass filter at λ=400 nm, (b) perylene-AUDTES-SiC excited at 440 nm using a long pass filter at λ=460 nm, (c), (d) as (a), (b) for a silicate substrate. All images are selected at different positions of the surface with an area of 25 µm × 25 µm.



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Figure 13: Average lifetime and intensity images of (a) pyrene-p-APTMS-SiC, (b) pyrene-pAPTMS-silicate, (c) pyrene-APTES-SiC, (d) pyrene-APTES-silicate, (e) pyrene-AUDTES-SiC, (f) pyrene-AUDTES-silicate surfaces. Excitation wavelength of λ=375 nm and detection through a long pass filter at λ=400 nm. All images are selected at different positions of the surface with an area of 25 µm × 25 µm.


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Graphical TOC entry


Fluorescence Properties of Perylene and Pyrene Dyes Covalently

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