SilsesquioxaneâTerpyridine Nano Building Blocks for the Design of

Article pubs.acs.org/IC

Silsesquioxane−Terpyridine Nano Building Blocks for the Design of Three-Dimensional Polymeric Networks Esther Carbonell,* Lucia A. Bivona, Luca Fusaro, and Carmela Aprile* Unit of Nanomaterial Chemistry (CNano), Department of Chemistry, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium S Supporting Information *

ABSTRACT: Novel polyhedral oligomeric silsesquioxanes (POSS) decorated with eight terpyridine moieties were synthesized in a one-pot procedure via Heck coupling reaction with the aim of investigating the possible formation of three-dimensional extended supramolecular organizations. The monosubstituted analogue was also prepared and used as a model compound. Both POSS-based nanostructures were extensively characterized via 1H, 13C, and 29 Si nuclear magnetic resonance (NMR), ultraviolet−visible spectroscopies and combustion chemical analysis. The assembly of these nanocaged bricks and two different metal ions (Zn2+ and Fe2+) was investigated via 1H NMR as well as absorption and emission spectroscopy. Both mono- and octa-terpyridine-functionalized POSS (O-POSS) displayed interesting photophysical properties. Moreover, under selected conditions, the O-POSS forms stable gels at room temperature and can easily be shaped in the form of a film with potential applications in nanotechnology.

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INTRODUCTION Natural and synthetic organic polymers play a crucial role in diverse fields in our daily life. Traditional synthetic organic polymers are usually obtained via self- or co-condensation of one monomeric unit or different monomeric units, resulting in the repetition of motifs connected by covalent bonds. Their final properties are strictly dependent on the nature of the monomer as well as the configuration, tacticity, and molar mass of the final material. However, to tune their properties, a novel synthetic strategy should be often envisaged. One of the most promising and emerging areas is represented by the formation of polymers via ligand−metal interaction [metallopolymers (MP)].1−7 The main advantage of MP synthesized via selfassembly is the dynamic nature of their interactions. Because MP are governed by noncovalent interactions, they are often sensitive to external stimuli such as temperature, solvents, pH, concentration, light, ultrasound, etc., lending to the material an ability to alter its response.8 Moreover, their electrochemical, optical, and magnetic response can easily be tuned by modifying the metal ion and preserving the structure of the ligand.5,9 One of the most used chelating ligand for the design of MP is represented by the terpyridine moiety as reflected in the increasing number of articles published about this topic.10 This great deal of attention can mainly be ascribed to the extraordinary binding affinity of terpyridine ligands for most transition metal ions. Among the possible structures, the most employed terpyridine is represented by a Kröhnke-type motif bearing a phenyl ring at position 4′.11,12 A wide variety of Kröhnke-type terpyridines have been synthesized and connected through different spacers to finely tune their absorption spectra and photophysical properties.13 As a consequence, a broad range of metallo-supramolecular architectures based on © 2017 American Chemical Society

terpyridine−metal complexes have already been reported in the literature.13−15 Terpyridines have been extensively used to form complexes with divalent first-row transition metal ions. These ligands form stable complexes according to the Irving−Williams series (Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+), with the corresponding formation of octahedrally coordinated metal centers often connecting two terpyridine units. In some cases, the complexes can also be formed even at room temperature (e.g., Fe2+, Co2+, and Zn2+). The stability of the complexes as well as their final properties is dependent on the metal ion, temperature, pH, and solvent.16−18 Specific metal ions are selected as a function of the potential application19−22 (e.g., light-emitting devices, photovoltaic solar cells, magnetic materials, and chemosensors) of the final complexes. Because of their versatility, the interest of the scientific community rapidly moved from the investigation of isolated terpyridine-based complexes to the construction of one- and two-dimensional supramolecular architectures.23−28 Recently, the study was also extended to three-dimensional (3D) networks. For this purpose, highly symmetric structures such as polyhedral oligomeric silsesquioxanes could represent exceptional pivots for the development of 3D organizations. Because of their rigid cagelike inorganic core surrounded by organic peripheries, silsesquioxane nanostructures exhibit a nice combination of interesting properties, including thermal and mechanical stability as well as good solubility in a broad range of organic solvents.29,30 Moreover, the silsesquioxane corners can easily be functionalized, allowing a facile tuning of the features of POSS. The integration of the Received: February 21, 2017 Published: May 24, 2017 6393

DOI: 10.1021/acs.inorgchem.7b00471 Inorg. Chem. 2017, 56, 6393−6403

Article

Inorganic Chemistry Scheme 1. Synthetic Routes to M-POSS and O-POSS via the Heck Coupling Reaction

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POSS nanostructure within a polymeric matrix allows one to obtain improved performance in terms of thermal resistance, oxidation stability, and surface hardening.31−34 Moreover, in the past several years, photophysical studies of octa-functionalized POSS with a wide variety of chromophores31,35−40 have been performed to investigate the influence of the nanocage as well as the effect induced by the 3D organization of the chromophore. Some examples of functionalized POSS as building blocks for the formation of coordination polymeric networks have been reported. In one of them, POSS structures were octa-functionalized with carboxylic groups41 and used to form ordered polymers via coordination with Cu(II). Recently, POSS functionalized with terpyridine units42 were also reported. However, the terpyridines were connected to the silsesquioxane nanocages via flexible aliphatic chains, and no extended photophysical investigation was performed. In the work presented here, novel POSS bearing terpyridine moieties were prepared via Heck coupling reaction to study the photophysical properties of the 3D polymeric networks. The synthesis of the monotopic (M-POSS) and polytopic ligand (O-POSS) was successfully achieved, and the POSS-based nanostructures were extensively characterized via 1H, 13C, and 29 Si nuclear magnetic resonance (NMR) and ultraviolet−visible (UV−vis) spectroscopies as well as combustion chemical analysis. The self-assembly processes in the presence of Fe2+ and Zn2+ metal ions were investigated via 1H NMR as well as absorbance and emission spectroscopy. Both mono- and polyterpyridine-functionalized POSS displayed interesting photophysical properties. Moreover, under selected conditions, the O-POSS formed stable gels at room temperature and can easily be shaped in the form of a film.

EXPERIMENTAL SECTION

Materials and Methods. Monovinyl-isobutyl-substituted (MV) POSS, octavinyl-substituted (OV) POSS, anhydrous N,N-dimethylformamide (DMF), Et3N (99.5%), iron(II) triflate (≥85%), and zinc(II) triflate (98%) were purchased from Sigma-Aldrich. 4′-(4Bromophenyl)-2,2′:6′,2″-terpyridine, palladium acetate, and tris(2methylphenyl)phosphine were purchased from TCI chemicals. Methanol (MeOH) and dichloromethane (DCM) used for the spectrofluorometric measurements were of spectroscopic grade and were purchased from Carl Roth. Quantitative 1H NMR experiments were performed at 25 °C on a Varian VNMRS spectrometer operating at 9.4 T (400 MHz for 1H) equipped with a 5 mm broadband probe, using the following acquisition parameters: relaxation delay of 8.0 s, acquisition time of 2.0 s, excitation pulse of 90°, and 32 transients. Homonuclear and heteronuclear two-dimensional (2D) spectra were recorded at 25 °C on a Jeol ECZR spectrometer operating at 11.7 T (500 MHz for 1H and 125 MHz for 13C) equipped with a 5 mm inverse probe. The chemical shift scale was referenced to the residual signal of CDCl3 (δ = 7.26 ppm). Solid-state 13C and 29Si NMR spectra were recorded at room temperature on a Bruker Avance-500 spectrometer operating at 11.7 T (125 MHz for 13C and 99.3 MHz for 29Si) using a 4.0 mm probe and a spinning frequency of 8 kHz. Combustion chemical analysis (C, H, N, and S) was performed on a Thermo Finnigan Flash-45 EA 1112 apparatus. UV−vis measurements were performed on a Cary 5000 spectrophotometer (Varian) and fluorescent measurements on a Cary Eclipse instrument (Agilent Technologies). The measurements were taken using 10 mm Suprasil quartz cuvettes from Hellma Analytics. Mass spectra were recorded using an electrospray ionization instrument (ESI) for the M-POSS and with a model 5800 MALDI-TOF instrument (ABSciex) in reflector mode (1500 shots every position) in a mass range of m/z 900−4000 for the O-POSS (in DCTB matrix). Transmission electron microscopy images were recorded with a PHILIPS TECNAI 10 instrument at 80 eV. Synthesis of M-POSS. Monovinyl-isobutyl-substituted POSS (200 mg, 0.24 mmol), 4′-(4-bromophenyl)-2,2′:6′,2″-terpyridine (140 mg, 0.36 mmol), palladium acetate (5.4 mg, 0.024 mmol), and tris(2methylphenyl)phosphine (14.6 mg, 0.048 mmol) were charged in a 6394


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Figure 1. 1H NMR spectra of (a) MV, (b) BrPhterpy, and (c) M-POSS. dried flask and purged with N2. Anhydrous DMF (7 mL) and dry triethylamine (2 mL) were added. The reaction mixture was heated at 100 °C for 3 days under a N2 atmosphere. Then the mixture was filtered to remove the palladium metal, and the filtrate was cooled, precipitating the M-POSS: white powder; 55% yield; 1H NMR (400 MHz, CDCl3) δ 0.69−0.62 (m, 14H), 0.95−1.00 (m, 42H), 1.81−1.95 (m, 7H), 6.27−6.22 (d, 1H), 7.27−7.22 (d, 1H), 7.37−7.34 (t, 2H), 7.59−7.57 (d, 1H), 7.92−7.86 (m, 4H), 8.69−8.67 (d, 2H), 8.74−8.73 (d, 2H), 8.75 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 22.60, 23.96, 25.82, 118.77, 119.81, 121.47, 123.96, 127.43, 127.61, 136.99, 138.39, 138.65, 147.38, 149.24, 149.77, 156.06, 156.30; MAS 29Si NMR (99.3 MHz) δ −68.27, −80.17. Elemental analysis (%) of C51H79N3O12Si8. Calcd: C, 53.23; H, 6.92; N, 3.65. Found: C, 52.45; H, 6.79; N, 3.22. Synthesis of O-POSS. Octavinyl-substituted POSS (100 mg, 0.16 mmol), 4′-(4-bromophenyl)-2,2′:6′,2″-terpyridine (745 mg, 1.92 mmol), palladium acetate (14.4 mg, 0.064 mmol), and tris(2methylphenyl)phosphine (39 mg, 0.128 mmol) were charged in a dried flask and purged with N2. Anhydrous DMF (14 mL) and dry triethylamine (4 mL) were added. The reaction mixture was heated at 100 °C for 7 days under a N2 atmosphere. Then the mixture was filtered to remove the palladium metal and concentrated in vacuo. The residue was precipitated in acetonitrile using a centrifuge (20 °C, 4500 rpm, 10 min). The solid was washed five times with acetonitrile (5 × 25 mL) using the ultrasound bath for 10 min and centrifuged. The same procedure was repeated with methanol (5 × 25 mL) until the solution remained clean and once again with acetone: pale brown powder; 68% yield; MAS 29Si NMR (99.3 MHz) δ −80.29. Elemental analysis (%) of C51H79N3O12Si8. Calcd: C, 71.48; H, 4.17; N, 10.87. Found: C, 67.04; H, 3.99; N, 9.78.

In both cases, the covalent anchoring of the heterocyclic organic molecules to the nanocage was achieved by reacting the vinyl-functionalized POSS nanostructures with the commercially available 4′-(4-bromophenyl)-2,2′:6′,2″-terpyridine (BrPhterpy) via Pd-catalyzed Heck reaction (Scheme 1). Both mono- and octa-functionalized POSS were extensively characterized via different techniques, including NMR and UV−vis spectroscopies. Figure 1 shows quantitative 1H NMR spectra of the M-POSS and, for comparison, of the starting materials used for its preparation. The good performance of the Heck reaction can easily be followed via the shift of the vinyl signals of the silsesquioxane at higher frequencies accompanied by the modification of their multiplicity, as well as by the evident shift of the Hf and Hg signals of the terpyridine. The complete assignment of the 1H NMR signals of M-POSS was achieved by 1 H homo- and heteronuclear 2D NMR experiments (Figures S1−S3). The two doublets of olefinic protons Hi and Hh are located at ∼6.25 and ∼7.25 ppm, respectively, and their 3J coupling (19.1 Hz) suggests a trans conformation. Analogous quantitative experiments cannot be performed on the O-POSS because of the substantial difference between the NMR spectra of the two compounds. While M-POSS displays a well-resolved pattern of 1H signals, O-POSS presents broad bands in the aromatic regions (Figure S4). This behavior was previously observed for octa-functionalized POSS structures displaying aromatic substituents and could be attributed to the presence of aggregates due to π−π stacking interaction between the aromatic functionalities as well as to chemical exchange.31 Unfortunately, a better resolution of the 1H spectra was not obtained by dissolving the O-POSS in DMSO, recording spectra at higher temperatures (≤115 °C), or decreasing the OPOSS concentration. Combustion chemical analysis performed on both samples confirms the finding obtained via NMR spectroscopy for the M-POSS and indicates a degree of functionalization of the O-POSS corresponding to an average

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RESULTS AND DISCUSSION The synthesis of polyhedral oligomeric silsesquioxanes (POSS) bearing terpyridine moieties was successfully achieved in a onestep procedure. The M-POSS was obtained starting from the monovinyl-heptaisobutyl-substituted silsesquioxane (MV in Scheme 1). The octavinyl-substituted silsesquioxane (OV in Scheme 1) was selected for the preparation of the POSS decorated with eight terpyridines (O-POSS). 6395


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Figure 2. Solid-state 13C P-MAS (left) and 29Si MAS NMR (right) spectra of (a and e) MV, (b and f) M-POSS, (c and g) OV, and (d and h) OPOSS. Asterisks denote spinning side bands.

with that of the commercially available 4′-(4-methylphenyl)2,2′:6′,2″-terpyridine as a reference ligand. UV−vis spectra of M-POSS and O-POSS show absorption bands with maxima at 292 and 287 nm, respectively, similar to the UV−visible band of 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine in dichloromethane [λmax = 280 nm (see Figure S8a)]. This band is red-shifted by 12 nm for the M-POSS and 7 nm for the O-POSS with respect to the reference. As expected, for the octa-functionalized silsesquioxane, a relevant hyperchromic effect (almost 8 times larger than that for M-POSS) was observed. These absorption bands obey Beer’s law in the selected concentration range (see Experimental Section and Figures S9 and S10), indicating that there is no significant complex aggregation in dichloromethane in these concentration ranges. For both POSS derivatives, the emission bands are also clearly red-shifted (see Figure S8b). Moreover, the O-POSS shows a very broad emission band between 360 and 450 nm that can be attributed to the possible formation of excimers whose bands usually appear between 410 and 450 nm.45 Because this band is not concentration-dependent, it could be attributed to the formation of an internal excimer.46 Moreover, the O-POSS emission intensity is significantly quenched. This behavior could be due to the interaction of the terpyridine units (self-quenching) as a result of the proximity effect induced by the nanocage.38,44 Both M- and O-POSS display interesting features in light of the formation of novel supramolecular structures by metal interaction. It is known that terpyridine moieties display good chelating properties for a wide range of metal ions representing an important building block in metallosupramolecular chemistry. The formation of metallopolymers from O-POSS-based supramolecular organizations was, hence, investigated by selecting two different transition metal ions. Among the different possibilities, Zn(II) and Fe(II) were selected for this study. The formation of M-POSS dimers via complexation of one metal center with two terpyridine units could be considered as a model reaction for the formation of extended polymeric networks. M-POSS nanostructure was hence selected for an initial investigation. Moreover, because of the well-resolved 1H NMR spectrum, quantitative information can easily be obtained following the evolution of the 1H NMR signals of M-POSS during complexation. 1H NMR titration experiments of M-POSS were, hence, performed using both

of 7.2 terpyridine molecules per silsesquioxane core. This average value indicates the presence of a distribution of species with a predominance of hepta-functionalized structures. Because of the difficulties encountered with solution-state NMR characterization of O-POSS, 13C and 29Si NMR experiments were performed under solid-state conditions (ssNMR). To allow a meaningful comparison among all the samples, only the solid-state NMR spectra of both silsesquioxanes are herein presented; the liquid-state 13C NMR spectrum of the M-POSS is shown in Figure S5. The 13C CP-MAS spectrum of M-POSS is characterized by a pattern of signals similar to those observed in solution. As previously observed during solution-state 1H NMR experiments, a different pattern of signals, compared to that of the MV precursor, is observed. These additional signals can easily be attributed to the organic functionalities. The signals observed in the 13C CP-MAS ssNMR spectrum of the OPOSS are very similar to those observed in the M-POSS spectrum. As expected, a significant broadening was also observed. The stability of the silsesquioxane core under the selected synthesis condition was proven via 29Si NMR experiments (Figure 2, right). 29Si MAS ssNMR spectra of MV and M-POSS display two signals in the region of the T3 units with a relative abundance of 7:1, corresponding to the Si atoms linked to the isobutyl groups and to the vinyl functionality. As expected, only one signal typical of a closed T8R8 structure corresponding to completely condensed T3 silicon units was observed for the OPOSS. It is worth noting that this resonance line is dramatically broadened (see Figure 2h). It is known that the solid-state 29Si NMR spectra are strongly dependent on the T−O−T angle;43 the broadening of the signals can hence be ascribed to the distribution of conformations with the consequential distortion of the T−O−T angles. This behavior was previously observed in other POSS structures displaying a distribution of species.44 Mass spectrometry [ESI-TOF (Figure S6)] confirmed the structure of M-POSS, while in the case of O-POSS, the use of MALDITOF/TOF (Figure S7) was needed to obtain structural information. The optical features of mono- and octa-functionalized POSS derivatives were characterized as well via both UV−vis and fluorescence spectroscopy, and their behavior was compared 6396


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Figure 3. 1H NMR titration experiments with M-POSS in a 7:3 CD3Cl/MeOD mixture upon addition of (a) 0, 0.26, 0.5, and 1.3 equiv of Zn(OTf)2 and (b) 0, 0.26, 0.5, and 1.0 equiv of Fe(OTf)2. Changes in normalized integrated areas of selected signals upon addition of 0−1 equiv of (c) Zn(OTf)2 and (d) Fe(OTf)2.

Zn(OTf)2 and Fe(OTf)2. The samples were dissolved in a 7:3 CDCl3/MeOD mixture and the spectra recorded at 298 K. Upon addition of Zn(OTf)2 to a solution of M-POSS, a new pattern of signals corresponding to the formation of the terpyridine−Zn complex was observed (see Figure 3a). A ratio of approximately 1:1 between the free and complexed M-POSS was evidenced by a [Zn2+]/[M-POSS] ratio of ≈0.25, and the complete disappearance of the signals corresponding to the free ligand was achieved upon addition of ∼0.5 equiv of Zn2+. These results clearly indicate that the formed complex (Zn@2M-POSS) has a Zn2+:ligand ratio corresponding to 1:2 and is characterized by a large association constant. The broadening of the resonance lines observed in the first part of the experiment (between 0 and 0.5 equiv) could be attributed to chemical exchange between the free and complexed ligands or the presence of solvated forms. To understand if the solvent could play the role of a competitive ligand, an excess of Zn(OTf)2 was added to the NMR tube. As expected, a novel series of signals in slow exchange on the chemical shift time scale appeared in the presence of 1.3 equiv; the ratio between the Zn@2M-POSS complex and the new species was ∼1:1 (Figure 3c). This experiment suggests that the Zn@2M-POSS complex could dissociate forming a mixture in which a novel species corresponding to a complex with a 1:1 Zn2+:ligand ratio (Zn@ M-POSS) was generated. The free coordination sites of zinc are most probably occupied by solvent molecules as described in the literature.47 A schematic representation of the two possible complexes is shown in Figure 4. Analogous titration experiments were performed using iron(II) triflate. As in the previous case, the presence of Fe(OTf)2 in the M-POSS solution causes the appearance of a new pattern of signals in slow exchange on the chemical shift time scale (see Figure 3b). After addition of an amount of

Figure 4. Schematic representation of the mono- and bis-metal complexes.

Fe(OTf)2 corresponding to an [Fe2+]/[M-POSS] ratio of ≈0.25, the integrated areas of the complexed and free ligands were in a ratio of ∼1:1. The complete disappearance of the signals corresponding to the M-POSS was obtained for an [Fe2+]/[M-POSS] ratio of 0.5 (see Figure 3d). It is worth noting that in the presence of Fe2+ the possible broadening of the resonance lines due to chemical exchange between the free and complexed ligands was not observed. Moreover, no further evolution of the 1H NMR spectra was observed after addition of an excess of Fe(OTf)2. These results clearly indicate that the formed Fe@2M-POSS complex has an Fe2+:ligand stoichiometry of 1:2 and, in agreement with the literature, an increased stability compared to that of the analogous zinc-based dimers.48 The formation of both Zn@2M-POSS and Fe@2M-POSS was hence investigated via UV−vis and fluorescence spectroscopy. UV−visible titration of M-POSS with Zn(OTf)2 clearly displays an isosbestic point at 313 nm, suggesting that only a single equilibrium between the coordinated and free M-POSS ligand exists. No absorption peak was observed at wavelengths longer than 350 nm, demonstrating the absence of a metal to ligand charge transfer (MLCT) transition (see Figure 5a). A plateau was reached slightly after addition of 0.5 equiv of Zn2+, confirming the formation of a Zn@2M-POSS complex, in agreement with the finding obtained via nuclear magnetic 6397


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Figure 5. (a) UV−vis absorption spectra of M-POSS in CH2Cl2 (1.0 × 10−5 M) upon titration with Zn(OTf)2 in EtOH (1.3 × 10−3 M). The inset shows the normalized absorption changes at 340 nm (black squares) and the normalized emission intensity changes at 360 and 420 nm (red circles and blue triangles, respectively). (b) Emission spectra of M-POSS in CH2Cl2 upon titration with Zn(CF3SO3)2. λex = 315 nm, and OD = 0.24. Slits = 2.5 nm.

Figure 6. (a) UV−vis absorption spectra of M-POSS in CH2Cl2 (1.1 × 10−5 M) upon titration with Fe(OTf)2 in EtOH (1.32 × 10−3 M). The inset shows the normalized absorption changes at 573 nm (black squares) and the normalized emission intensity changes at 348 nm (red circles). (b) Emission spectra of M-POSS (1.1 × 10−5 M) in CH2Cl2 upon addition of small amounts of Fe(OTf)2 (1.32 × 10−3 M) in EtOH at λex = 310 nm (OD = 0.32). Slits = 2.5 nm.

isosbestic point, complete quenching of the emission band at 362 nm was observed (Figure 6b). The plots describing the variation of the absorption at 573 nm as well as the emission intensity at 348 nm (superimposed in the inset of Figure 6a) reach a plateau after addition of 0.5 equiv of Fe(OTf)2, indicating the formation of an Fe@2MPOSS dimer. This also supports the previous observation via NMR. Once the excellent chelating properties of the M-POSS ligand had been proven, the behavior of the O-POSS was investigated by 1H NMR titration experiments to study the formation of 3D metallopolymeric structures. However, in this case, substantially different results were obtained. As previously reported, the 1H NMR spectra of O-POSS were characterized by nonresolved resonance lines, the aromatic signals being broad and superimposed irrespective of the solvent or temperature used (see Figure 7a,b for 0 equiv of salt). When Fe(OTf)2 or Zn(OTf)2 was added, the progressive formation of a precipitate was observed. This behavior represents evidence of the interaction between the octa-functionalized silsesquioxane and both Fe2+ and Zn2+. However, because of the low solubility of the complexes formed in the concentration range

resonance spectroscopy. However, the UV−vis titration experiment did not provide clear evidence of the formation of the Zn@M-POSS species. In the presence of an excess of Zn2+, an only slight decrease in absorption at 340 nm was observed (see Figure 5a, inset). Considering the previously published NMR results, this decrease could be attributed to the formation of the monocomplex.17,49 The change in the fluorescence spectrum upon addition of increasing amount of Zn(II) triflate is evidenced in Figure 5b. The emission of the M-POSS at 363 nm was greatly enhanced (2-fold) and red-shifted to 420 nm after formation of the Zn@ 2M-POSS complex. This band can be assigned to the terpyridine−zinc complex, in agreement with the literature.13 Analogous experiments were performed using iron(II) triflate as the titrating agent. Upon UV−visible titration of M-POSS with Fe(OTf)2, evident variations in the absorption spectra were observed (Figure 6a). A red shift of the terpyridine absorption band at 293 nm with an isosbestic point at 310 nm and the concomitant appearance of a typical MLCT band centered at 573 nm were clearly visible (Figure 6a). The deep violet color of the final complex is attributed to the presence of the previously mentioned MLCT band. Upon excitation at the 6398


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Figure 7. 1H NMR titration experiments with O-POSS in a 7:3 CD3Cl/MeOD mixture upon addition of (a) 0, 1.3, and 6.2 equiv of Zn(OTf)2 and (b) 0, 1.0, and 4.9 equiv of Fe(OTf)2. Changes in normalized integrated area upon addition of 0−7 equiv of (c) Zn(OTf)2 and (d) Fe(OTf)2.

Figure 8. (a) UV−vis absorption spectra of O-POSS (1.6 × 10−6 M) in CH2Cl2 upon titration with a Zn(OTf)2 solution (1.3 × 10−3 M) in EtOH. The inset shows the normalized absorption changes at 343 nm (black squares) and the normalized emission intensity changes at 368 nm (red circles) and 481 nm (blue triangles). (b) Emission spectra of O-POSS in CH2Cl2 upon addition of small amounts of Zn(OTf)2 (1.3 × 10−3 M) in EtOH at λex = 310 nm (OD = 0.25). Slits = 5 nm.

intensity in the linear regime, a Zn2+:O-POSS stoichiometry of 4:1 was obtained (4Zn@O-POSS). Considering the presence of an almost octa-functionalized ligand, this result corresponds to 0.5 equiv of Zn2+ per terpyridine unit, suggesting that all the terpyridine units decorating the silsesquioxane cage are acting as the active ligand for the metal centers. As one can clearly observe in Figure 7d, in the presence of Fe2+ the intensity of the ligand signal decreases linearly over the entire concentration range and the titration was complete for an [Fe2+]/[O-POSS] ratio of ≈4. This observation indicates that in this case a 4Fe@O-POSS complex was formed. The UV−visible titration experiment with O-POSS (1.6 × 10−6 M in dichloromethane) with Zn(OTf)2 evidenced a

used for NMR experiments, it was not possible to characterize these species using NMR spectroscopy. The titration experiments were, hence, performed following the disappearance of the integrated areas of O-POSS 1H NMR signals (see Figure 7a,b). When the O-POSS was titrated with Zn(OTf)2, a linear decrease in the intensities of the NMR signals was observed up to a [Zn 2+ ]/[O-POSS] ratio of ≈3. At higher Zn 2+ concentrations, the variation follows a nonlinear trend and the complete disappearance of the signal was achieved for a [Zn2+]/[L] ratio of ≈6 (Figure 7c). This behavior is in agreement with the previous results obtained using M-POSS as the ligand and can be ascribed to the competitive coordination of the solvent molecules for the metal center. It is worth emphasizing that upon extrapolation of the variation in signal 6399


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Figure 9. (a) UV−vis absorption spectra of a 1.6 × 10−6 M solution of O-POSS in CH2Cl2 upon titration with an Fe(OTf)2 solution (1.3 × 10−3 M) in EtOH. The inset shows the normalized absorption changes at 574 nm (blue triangles) and the normalized emission intensity changes at 368 nm (black squares) and 495 nm (red circles). (b) Emission spectra of O-POSS (1.6 × 10−6 M) in CH2Cl2 upon addition of small amounts of Fe(OTf)2 (1.3 × 10−3 M) in EtOH at λex = 310 nm (OD = 0.3). Slits = 5 nm.

or 4 equiv of metal salt, respectively) in a 6:4 CH3Cl/MeOH mixture was prepared. After some days, a gel was obtained via slow solvent evaporation only in the presence of the O-POSS (4Zn@OPOSS gel and 4Fe@O-POSS gel). The gelation tests were performed using a test tube inversion method.53 The photographs of the 4Zn@O-POSS and 4Fe@O-POSS gels are shown in Figure 10. As expected, the 4Zn@O-POSS gel

decrease in the intensity of the band at 286 nm and a corresponding increase in the intensity of the band at 332 nm with a clear isosbestic point at 310 nm (Figure 8a) as previously mentioned for the M-POSS. Upon excitation at 310 nm, a new emission band at 480 nm was observed in the presence of Zn2+ (see Figure 8b). This band corresponds to the formation of the Zn complex and is notably red-shifted (60 nm) compared to the emission band of Zn@M-POSS (λmax = 420 nm) under the same conditions. Surprisingly, a very broad band was observed in the intermediate points of the titration (between 0.3 and 0.8 equiv), indicating the existence of multiple species in the excited state. To understand the remarkable shift in the intensity of the emission band, we performed the titration experiment under more diluted conditions (see Figure S11). In this case, we observed an emission band at 450 nm (red-shifted by 30 nm with respect to that of Zn@M-POSS). As reported in previous works,46,50−52 the progressive shift of this band (MPOSS at 420 nm, diluted O-POSS at 450 nm, and concentrated O-POSS at 480 nm) could be attributed to the formation of aggregates in the excited state induced by metal complexation. Analogous titration experiments were also performed in the presence of Fe(OTf)2 using two different concentrations of OPOSS. Similar changes in the absorption spectra were observed (Figure 9a and Figure S12a). The plots of changes in the intensity of absorption (574 nm) and emission (368 and 495 nm) reached a plateau after addition of ∼4 equiv of Fe2+ per OPOSS unit (Figure 9a, inset), confirming the formation of the 4Fe@O-POSS complex. Upon excitation at 310 nm, quenching of the emission band at 385 nm (λmax) was observed for both O-POSS concentrations (see Figure 9b and Figure S12b). However, the appearance of a new band around 500 nm was evidenced only in the case of the most concentrated solution. After addition of a sufficient amount of cation, the novel band promptly disappeared. This band could be attributed to the formation of intramolecular or intermolecular excimers, and its disappearance could be ascribed to the formation of the 3D network, hindering all the possible direct interactions between terpyridine units. To fully study the assembly process, a more concentrated solution (10−4 M M-POSS or O-POSS for the addition of 0.5

Figure 10. Photographs of the 4Zn@O-POSS and 4Fe@O-POSS gels without (left) and with (right) UV irradiation at 366 nm.

also displays an intense fluorescence emission under a 366 nm UV lamp and the 4Fe@O-POSS gel shows a characteristic purple color corresponding to the MLCT band. This behavior represents a further indication of the formation of an extended 3D cross-linked network in the presence of OPOSS ligands trapping solvent molecules within the porous irregular organization. Evidence of the previously mentioned irregular porous organization was obtained through transmission electron microscopy (Figure 11) of the lyophilized gels. Moreover, these materials can also easily be shaped in the form of a film. Figure 12 shows the film of 4Zn@O-POSS without (a) and with (b) ultraviolet irradiation at 366 nm. As shown in the figure, the formed film is transparent, homogeneous, and highly emissive under UV exposure. The fluorescence emission of 4Zn@O-POSS in the form of a film could have a potential application in nanotechnology, such as in electronics and photonics,54 or in organic light-emitting devices.55 6400


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Inorganic Chemistry

Figure 11. Transmission electron microscopy images of the 4Zn@O-POSS lyophilized gel. Panels a−c show different areas of the same sample.

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Figure 12. 4Zn@O-POSS film (a) without and (b) with UV irradiation at 366 nm.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

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CONCLUSIONS Novel 3D structures were obtained via assembly of silsesquioxane-based ligands bearing terpyridine units with two different metal ions (Fe2+ and Zn2+). The formation of analogous POSS-based dimers via complexation of one metal center with two terpyridine units was used as model reaction for the formation of extended polymeric networks. All the complexes were extensively investigated via NMR, UV−vis, and fluorescence spectroscopy. These studies allow highlight the high degree of coordination of the O-POSS ligand as well as the interesting photophysical properties of the formed 3D complexes. Additional transmission electron microscopy investigations further evidenced the tendency of 4metal@O-POSS to form a 3D organization with an irregular porous structure. The capability of the organic−inorganic hybrid network to trap solvent molecules confers on the 4metal@O-POSS polymers the characteristics of gels. The gels were stable under ambient conditions. Morover, the O-POSS can also easily be shaped in the form of a film with important implications for nanotechnology.2,54,55

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2D NMR spectra (Figures S1−S3) and 13C NMR spectra (Figure S5) of M-POSS, 1H NMR spectrum of O-POSS (Figure S4), ESI-TOF-MS and MALDI-TOF/TOF spectra of M-POSS and O-POSS (Figures S6 and S7), UV−vis and emission spectra of M-POSS and O-POSS (Figure S8), Beer−Lambert calibration curve of M-POSS and O-POSS in CH2Cl2 (Figures S9 and S10), and UV− vis and emission titration experiments of O-POSS under dilute conditions (Figures S11 and S12) (PDF)

ORCID

Carmela Aprile: 0000-0002-3193-3239 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.C. gratefully acknowledges the FNRS for the CR mandate (Appl ID 22367853). This research used resources of the nuclear magnetic resonance service of the University of Namur. This service is a member of the “Plateforme Technologique Physico-Chemical Characterisation” (PC2). The ESI-TOF and MALDI-TOF analyses were performed in the SCIE University of Valencia section of Mass Spectrometry and Proteomics Unit, a member of ISCIII ProteoRedProteomics Plataform.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00471. 6401


Article

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