Green hydrothermal synthesis yields perylenebisimide–SiO2 hybrid materials with solution-like fluorescence and photoredox activity

In organic–inorganic hybrid materials' (HMs) synthesis, it is intrinsically challenging to, at the same time, achieve (i) the concomitant synthesis of the components, (ii) nanoscopic interpenetration of the components, and (iii) covalent linking of the components. We here report the one-pot hydrothermal synthesis (HTS) of inorganic–organic HMs consisting of perylene bisimide (PBI) dyes and silica, using nothing but water as the medium and directly from the corresponding bisanhydrides, n-alkyl amines, and alkoxysilane precursors. First, in the absence of a functionalized alkoxysilane for linking, a mixture of the products, PBI and SiO2, is obtained. This evinces that the two products can be synthesized in parallel in the same vessel. Except for minor micromorphological changes, the concomitant synthesis does not affect each component's physicochemical properties. The PBI/SiO2 mixtures do not show synergistic properties. Second, through adding the linker aminopropyltriethoxysilane (APTS), covalently-linked class II hybrids are obtained. These PBI@SiO2 class II hybrids show synergistic materials properties: increased thermal stability is obtained in combination with nanoscopic homogeneity. The PBI moieties are dissolved in the solid SiO2 matrix, while being covalently linked to the matrix. This leads to solution-like fluorescence with vibronic fine-structure of the dyes. Moreover, through tuning the SiO2 amount, the band gaps of the class II hybrid materials can be systematically shifted. We exploit these optoelectronic properties by using the PBI@SiO2 hybrids as heterogeneous and reusable photoredox catalysts for the reduction of aryl halides. Finally, we present a detailed small-angle X-ray scattering and powder X-ray diffraction study of PBI@SiO2 synthesized at various reaction times, revealing the existence of an ordered PBI-oligomeric silesquioxane-type intermediate, which subsequently further condenses to the final nanoscopically homogeneous PBI@SiO2 material. These ordered intermediates point at HTS′ propensity to favor crystallinity (to date known for organic and inorganic compounds, respectively) to also apply to hybrid structures, and shed additional light on the long-standing question of structure formation in the early stages of sol–gel processes: they corroborate Brown's hypothesis (1965) that trifunctional hydroxysilanes form surprisingly well controlled oligomers in the early stages of polycondensation.


State-of-the-art on PBI@SiO 2 hybrid materials
In Table S1 we reviewed perylene bisimides@SiO2-based hybrid materials found in literature, as well as the current state of their synthesis methods, and their features regarding solid-state fluorescence and possible applications. The different synthetic strategies towards organic-inorganic hybrid materials consist of: (A) Strategies in which both organic and inorganic components are pre-made (e.g. dye loading into the matrix by gas/liquid sorption; mechanical mixing; ion exchange).
(B) Strategies in which one of the components is pre-made and the second component is formed subsequently (1: the dye is synthesized inside the matrix or 2: the matrix is made around the pre-made dye).
(C) Strategies in which the organic component needs to be modified to be incorporated in the premade inorganic component (1: the pre-made dye is modified with an alkoxy-group; then 2: sol-gel hybrid from hydrolysis of the modified-dye; 3: grafting of the modified-dye into the pre-made matrix). 1 PBI@SiO2 films non-covalent not reported --S1   APTS-Br-PBI@SiO2 (100 TEOS) 1 eq. Br-PBA -2 eq. 100 eq. 8.11 wt% dye (0.129 mmol dye/g HM) 13 APTS-Cl-PBI@SiO2 1 eq. Cl-PBA -2 eq. -a composition of the hybrid materials was determined by TGA.
The materials C5-PBI/SiO2, C8-PBI/SiO2 and C14-PBI/SiO2 consist of a mere mixture of dye and silica particles, and no further analysis was performed. The GC-MS spectra were collected in a ThermoScientific equipment (model Trace 1300 GC) with BGB5 column, and coupled to a single quadrupole mass spectrometer (ThermoScientific ISQ LT).

Quantum yield (f)
The f of the hybrid APTS-Br-PBI@SiO2 (100TEOS) was determined by direct method, using an integrating sphere. First, to measure the scattering of the incident light, a spectrum of an empty quartz cuvette was collected (S0, blue curve in Fig. S16). The sample was then placed into the cuvette in the integrating sphere, and a spectrum was collected (S1, pink curve in Fig. S16). In both measurements, the excitation wavelength was set to 430 nm, and the spectra were collected from 420-440 nm with step size of 0.1 nm, and integration time of 0.5 s. The emission spectrum of the sample was then collected from 445 nm to 700 nm, with step size of 0.1 nm, and integration time of 0.5 s, and it is depicted in green in Fig. S16. The QY is defined by the sum of all emitted photons (S2), divided by the sum of all absorbed photos (S0-S1): The corresponding areas of the peaks depicted in Fig S16 were calculated in Origin, by integrating the area below the curves, and were found as S0= 5.99x10 7 , S1= 3.56x10 7 and S2= 1.72x10 5 . Thus, the f value was determined as 0.7%:

Band gap calculation
The energy band gap of the hybrids and their individual components -all made by HTS -were estimated from the solid state UV-Vis spectroscopy, using Tauc's relationship (ahn) 1/n = A (hn − Eg) where A is a proportionality constant, 'hn' is the photon energy, 'a' the absorption coefficient and 'Eg' is the energy band gap of the material. 'n' is a constant whose value depends upon the type of transition, n = 1/2 for direct allowed transition and 2 for indirect allowed transition. 53 Figure S15    For comparison purposes, the procedure was also performed using (i) the homogeneous catalyst C3-Br-PBI instead of the HM; (ii) pure SiO2; (iii) with HM photocatalyst but without light; and (iv) without any catalyst.
In order to investigate the recyclability of the photocatalyst, the above-described procedure was repeated twice using the recovered catalyst. After 8h of reaction, the catalyst was recovered by centrifugation, dried under vacuum, and characterized by PXR, ATR-FTIR, TGA and SEM (Fig. S17).    Figure S19. GC-MS analysis of the filtrate from the second cycle of photoreduction reaction with APTS-Br-PBI@SiO2 (100 TEOS). On top, the gas chromatogram is shown and the main peaks highlighted: the retention time of iodobenzaldehyde was found at ∼8.1 min and benzaldehyde at ∼5.8 min. In the middle and bottom parts, are the MS spectra of iodobenzaldehyde and benzaldehyde, respectively.  Figure S20. GC-MS analysis of the filtrate from the third cycle of photoreduction reaction with APTS-Br-PBI@SiO2 (100 TEOS). On top, the gas chromatogram is shown and the main peaks highlighted: the retention time of iodobenzaldehyde was found at ∼8.1 min and benzaldehyde at ∼5.8 min. In the middle and bottom parts, are the MS spectra of iodobenzaldehyde and benzaldehyde, respectively.  Figure S21. GC-MS analysis of the filtrate from the photoreduction reaction with C3-Br-PBI (homogeneous catalyst). On top, the gas chromatogram is shown and the main peaks highlighted: the retention time of iodobenzaldehyde was found at ∼8.1 min and benzaldehyde at ∼5.8 min. In the middle and bottom parts, are shown the MS spectra of benzaldehyde and iodobenzaldehyde, respectively.  Figure S22. GC-MS analysis of the filtrate from the photoreduction reaction with APTS-Br-PBI@SiO2 (100 TEOS) in the absence of light. On top, the gas chromatogram is shown and the main peak from iodobenzaldehyde is observed at 8.069 min. No peak assigned to benzaldehyde was found. The asterisks indicate impurities from solvent.
5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7.0 7.  Figure S23. GC-MS analysis of the filtrate from the photoreduction reaction with SiO2. On top, the gas chromatogram is shown and the main peak from iodobenzaldehyde is observed at 8.069 min. No peak assigned to benzaldehyde was found. The asterisks indicate impurities from solvent.
5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7.0 7.  Figure S24. GC-MS analysis of the filtrate from the photoreduction reaction without catalyst. On top, the gas chromatogram is shown and the main peak from iodobenzaldehyde is observed at 8.069 min. No peak assigned to benzaldehyde was found. The asterisks indicate impurities from solvent.