Probing the molecular character of periodic mesoporous organosilicates via photoluminescence of Lewis acid – base adducts †

Combining inorganic and organic fragments allows for the introduction of molecular properties into a functional hybrid material. Such materials can be rationally designed due to the modularity of the organic scaffold embedded in a stable inorganic matrix, typically silica. One approach relies on a co-condensation of an organosilane such as R–Si(OAlk)3 with tetraethoxysilane in the presence of a structure-directing agent, yielding mesostructured materials (usually, MCM or SBA-type) that incorporate organic groups (R) in their pores with high regularity. A related strategy utilizes an organic linker with two trialkoxysilyl groups [(AlkO)3Si–R–Si(OAlk)3] that can be condensed to give the so-called periodic mesoporous organosilicas. The latter high surface area hybrid materials feature organic functionalities uniformly distributed within the pores and the walls of an inorganic silica matrix. PMOs derived from aromatic precursors usually feature a 2D hexagonal framework of mesopores resulting in a long-range crystal order detected by XRD. These properties make PMOs attractive platforms for the post-synthetic modification towards molecularly defined heterogeneous catalysts. Phenylpyridine units have been previously utilized to form a PMO material (ppy-PMO, Scheme 1) and to subsequently immobilize several transition metal complexes. In the case of the Cp*Ir-ppy-PMO system, dynamic nuclear polarization surface enhanced NMR spectroscopy was used to differentiate between outer and inner layers of phenylpyridine ligands. Photoluminescence (PL) is another powerful structure-sensitive technique that was recently used to investigate the homogeneity of the environment of surface species. Encouraged by this result, we sought to extend the molecular level characterization of PMO materials by using PL. As solid molecular adducts of B(C6F5)3 and various O/N Lewis bases were reported to exhibit photoluminescence, we prepared related B(C6F5)3 adducts with PMO materials and compared their PL properties to respective molecular systems. Here, we demonstrate that PL decay of sites formed by B(C6F5)3 coordination to pyridine units of PMO materials is similar to the molecular analogues, which highlights the homogeneous environment and the molecular character of the organic units in PMO’s. We followed literature procedures to prepare ppy-PMO, and also synthesized its previously unknown, sterically more bulky analogue ppy-PMO (Scheme 1). Material biph-PMO was prepared according to the literature as a nitrogen-free reference PMO (Scheme 1, see ESI† file for synthetic details). As B(C6F5)3 was anticipated to react with free surface silanols, 10 the residual RSiOH groups of all three PMO materials were passivated using TMSBr (PMOpas), as evidenced by the disappearance of the OH band at 3720 cm 1 (Fig. S15 and 16, ESI†). All materials described herein exhibit a Type II isotherm in the N2 adsorption/desorption measurement, high surface areas and microporosity (pores o 2.4 nm by BJH analysis, Table 1). Incipient wetness impregnation of the three PMOpas with a toluene solution of B(C6F5)3 (0.2 equiv. B(C6F5)3 per nitrogen site of the PMOs) gave the respective B-PMOpas materials (Scheme 1). The H magic angle spinning (MAS) NMR spectrum of ppy-PMOpas has signals at d 2.0 and 6.9 ppm (Fig. S12, ESI†) that are assigned to aliphatic and aromatic protons of the ppy moiety, respectively, and the C cross-polarization CP-MAS NMR of ppy-PMOpas features aliphatic and aromatic peaks as well (Fig. S13, ESI†). The Si CP-MAS spectrum of ppy-PMOpas shows expected signals of T2 and T3-sites at 69.3 and 78.6 ppm, respectively (Fig. S14, ESI†). B NMR spectra provided insight into the binding of B(C6F5)3 in B-PMOpas materials. Free, non-interacting B(C6F5)3 gives a broad signal Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1–5, 8093 Zürich, Switzerland. E-mail: ccoperet@ethz.ch † Electronic supplementary information (ESI) available: Synthetic procedures and physico-chemical characterization. See DOI: 10.1039/c6cp02176j Received 2nd April 2016, Accepted 21st April 2016

Combining inorganic and organic fragments allows for the introduction of molecular properties into a functional hybrid material. 1,2Such materials can be rationally designed due to the modularity of the organic scaffold embedded in a stable inorganic matrix, typically silica. 1,2One approach relies on a co-condensation of an organosilane such as R-Si(OAlk) 3 with tetraethoxysilane in the presence of a structure-directing agent, yielding mesostructured materials (usually, MCM or SBA-type) that incorporate organic groups (R) in their pores with high regularity. 2A related strategy utilizes an organic linker with two trialkoxysilyl groups [(AlkO) 3 Si-R-Si(OAlk) 3 ] that can be condensed to give the so-called periodic mesoporous organosilicas. 1 The latter high surface area hybrid materials feature organic functionalities uniformly distributed within the pores and the walls of an inorganic silica matrix. 1 PMOs derived from aromatic precursors usually feature a 2D hexagonal framework of mesopores resulting in a long-range crystal order detected by XRD.These properties make PMOs attractive platforms for the post-synthetic modification towards molecularly defined heterogeneous catalysts. 3,4henylpyridine units have been previously utilized to form a PMO material (ppy-PMO, Scheme 1) and to subsequently immobilize several transition metal complexes.3d,e,5,6 In the case of the Cp*Ir III -ppy-PMO system, dynamic nuclear polarization surface enhanced NMR spectroscopy was used to differentiate between outer and inner layers of phenylpyridine ligands. 5hotoluminescence (PL) is another powerful structure-sensitive technique that was recently used to investigate the homogeneity of the environment of surface species. 7Encouraged by this result, we sought to extend the molecular level characterization of PMO materials by using PL.As solid molecular adducts of B(C 6 F 5 ) 3 and various O/N Lewis bases were reported to exhibit photoluminescence, 8 we prepared related B(C 6 F 5 ) 3 adducts with PMO materials and compared their PL properties to respective molecular systems.Here, we demonstrate that PL decay of sites formed by B(C 6 F 5 ) 3 coordination to pyridine units of PMO materials is similar to the molecular analogues, which highlights the homogeneous environment and the molecular character of the organic units in PMO's.
We followed literature procedures to prepare ppy-PMO, 3d,5 and also synthesized its previously unknown, sterically more bulky analogue ppy Me -PMO (Scheme 1).Material biph-PMO was prepared according to the literature 9 as a nitrogen-free reference PMO (Scheme 1, see ESI † file for synthetic details).As B(C 6 F 5 ) 3 was anticipated to react with free surface silanols, 10 the residual RSiOH groups of all three PMO materials were passivated using TMSBr (PMO pas ), as evidenced by the disappearance of the OH band at 3720 cm À1 (Fig. S15 and 16, ESI †).All materials described herein exhibit a Type II isotherm in the N 2 adsorption/desorption measurement, high surface areas and microporosity (pores o 2.4 nm by BJH analysis, Table 1).Incipient wetness impregnation of the three PMO pas with a toluene solution of B(C 6 F 5 ) 3 (0.2 equiv.B(C 6 F 5 ) 3 per nitrogen site of the PMOs) gave the respective B-PMO pas materials (Scheme 1).
The 1 H magic angle spinning (MAS) NMR spectrum of ppy Me -PMO pas has signals at d 2.0 and 6.9 ppm (Fig. S12, ESI †) that are assigned to aliphatic and aromatic protons of the ppy Me moiety, respectively, and the 13 12 All impregnated B-PMO pas materials contain no free B(C 6 F 5 ) 3 .2D 11 B multiple quantum (MQ-MAS) experiments allow to distinguish between B-O and B-N interactions in B-PMO pas materials (Fig. 1).The signal at À2 ppm is ascribed to a B-O surface adduct as it is present in all three materials including nitrogen-free B-biph-PMO pas .This boron chemical shift is also close to that of molecular B(C 6 F 5 ) 3 -Oadducts in solution. 13Impregnated pyridine-containing materials feature an additional signal centred around À4 ppm.This signal is attributed to the B(C 6 F 5 ) 3 -Py adduct since the molecular B(C 6 F 5 ) 3lutidine adduct in solution has a chemical shift at À3.9 ppm. 14Solid state NMR data of the molecular adduct of B(C 6 F 5 ) 3 and phenylpyridine also corroborates this assignment (Fig. S19, ESI †).
While instructive in distinguishing between the N/O boron adducts, the 11 B NMR only reveals one, relatively broad signal for a B-N interaction, which is not conclusive in terms of the broadness of the B-N sites distribution.We therefore set about to investigate the photoluminescence properties of B-PMO pas materials.Previously, coordination of B(C 6 F 5 ) 3 to the pyridine unit of the light emitting polymer was reported to lead to a red shift in the emission spectra, longer excited state lifetimes as well as higher quantum yields.8b A recent study reported on the photoluminescence properties of solid adducts of B(C 6 F 5 ) 3 with various acetophenone derivatives as well.8a DRIFT UV-VIS spectra show a red shift upon impregnation of our PMO pas materials with B(C 6 F 5 ) 3 (Fig. S21, ESI †).The photoluminescence spectra of all PMO pas obtained after excitation at 350 nm display no emission.Interestingly, both molecular models of the B-PMO pas adducts with alkoxy silyl moieties on Scheme 1 Synthesis of B-ppy-PMO pas , B-ppy Me -PMO pas and B-biph-PMO pas materials and their molecular models.This journal is © the Owner Societies 2016 the aromatic linkers (ppy Si and ppy Si Me , Scheme 1) show no photoluminescence as well, in contrast to the simple B(C 6 F 5 ) 3 adducts with ppy and ppy Me .The siloxane moieties are likely causing different relaxation pathways and/or compete for boron coordination, thereby suppressing the photoluminescence from the Lewis B-N adduct.No luminescence was observed for the biph-derived materials, pointing towards the necessity of a B-N interaction for photoluminescence.In the case of ppy and ppy Me systems, the molecular boron adduct as well as the impregnated material show photoluminescence.For the molecular adducts the maximum of emission is at 355 nm and 363 nm, while for the impregnated materials a red shift to 418 and 420 nm, respectively, is observed (Fig. 2A).Photoluminescence decay of the emitting samples described above features a bi-exponential mode (Fig. 2B, C and Table 2).The differences in the fitting parameters between molecular adducts (entries 1, 2) and impregnated materials (entries 3, 4) are relatively small and most likely not attributed to a larger distribution of sites in B-PMO pas , but caused by the presence of a greater number of deactivation pathways in the solid state that result in the quenching of photoluminescence.Note that a bi-exponential mode was also observed for the PL decay of molecular adducts of B(C 6 F 5 ) 3 with aromatic ketones.8a

Conclusions
We reported that PMOs with phenylpyridine moieties impregnated with B(C 6 F 5 ) 3 feature distinct signals for B-N and B-O interactions in 11 B ssNMR MQ-MAS spectra.The B-N Lewis acid-base pair provides photoluminescence in the solid state while the B-O pair does not.The comparison of photoluminescence decay featured by molecular adducts in the solid state with respective impregnated materials demonstrates the molecular nature of highly homogeneous surface sites in PMOs.
C cross-polarization CP-MAS NMR of ppy Me -PMO pas features aliphatic and aromatic peaks as well (Fig. S13, ESI †).The 29 Si CP-MAS spectrum of ppy Me -PMO pas shows expected signals of T 2 and T 3 -sites at À69.3 and À78.6 ppm, respectively (Fig. S14, ESI †). 11B NMR spectra provided insight into the binding of B(C 6 F 5 ) 3 in B-PMO pas materials.Free, non-interacting B(C 6 F 5 ) 3 gives a broad signal with d iso = 58 ppm (Z CS = 0.03 and C q = 4.3 MHz) due to the quadrupolar boron nucleus and the high symmetry of B(C 6 F 5 ) 3 (Fig. S18, ESI †).A priori, B(C 6 F 5 ) 3 can form adducts with O sites of silica as well as N sites of the phenylpyridine moiety. 11Coordination of a Lewis base (O or N) to B(C 6 F 5 ) 3 results in an upfield shift of the boron signal in the 11 B NMR.

Table 1
Surface areas of obtained PMO materials