Riboflavin and Eosin Y Supported on Chromatographic Silica Gel as Heterogeneous Photocatalysts

Abstract

The application of photocatalysis for organic synthesis, both in the laboratory and on an industrial scale, will depend on the achieving of good yields and the ease with which it can be applied. Selective irradiation of the photocatalyst with LED light has made it possible to activate the reactions easily, without the need for UV or heat filters. However, a common problem is the need to separate the photocatalyst from the reaction products through extraction and chromatography isolation processes. These procedures make it difficult to recover and reuse the catalyst, which is not compatible with scale-up applications. Photocatalysts attached to heterogeneous supports resulted in an alternative, which facilitates their removal and reuse. In this study, we use chromatographic silica gel as a low-cost heterogeneous support to bind photosensitizers such as Riboflavin or Eosin Y. The modified silica gel was analyzed by FTIR-ATR and diffuse reflectance UV-visible spectroscopy, thermogravimetric analysis, and optical microscopy. These hybrid materials have a suitable size for easy separation by decantation and were found to be photoactive against two photooxidation reactions. These easy-to-handle materials open the door to effective applications for photoinduced organic synthesis methods at medium to large scale.

Scheme 1. Riboflavin and Eosin Y Structures, and Their AM1 Energetic Minimization 3D Images at Room Temperature (∼300 K)

Figure 1

Chromatographic silica gel modified with RF (up panels) or Eosin Y (bottom panels). From left to right, hybrid material re-suspended in water (A, E), after decanting (B, F) and the recovered solid (C, G). Finally, the hybrid material with RF is seen under the light of 395 nm (D), or for Eo, under Green LED (517 nm) in DMF.

Scheme 2. Reactions for Surface Modification of Silica Gel for Chromatography

(a) A succinic linker is attached to the silica gel in the first step. In the second step, SG–Succ reacts with riboflavin to give the hybrid material SG–Succ–RF. (b) The Eosin Y disodium salt reacts with APTES to form an amide using DCC. In a second step, the APTES–Eosin product was transferred to a second–round flask and reacted with the silica gel, pre–activated with pyridine in DMSO to obtain the SG–APTES–Eo.

Figure 2

Analysis for modified SG with RF and a Succinic linker. (A) FTIR-ATRs of the SG-Succ. The detailed analysis of the spectra subtraction present signals that suggest the presence of the Si-O-CO-CH₂-CH₂-COOH residue. (B) FTIR-ATRs of SG-Succ-RF and the spectra subtraction showing RF signals. (C) TGA analysis for both “dry” SG (black), SG-Succ (blue), SG-Succ-RF (dark yellow), and riboflavin (red). The light blue shadow shows the shape difference due to the presence of the organic linker. (D) Diffuse reflectance spectroscopy at 45° for SG (black), SG-Succ (blue), SG-Succ-RF (dark yellow), and riboflavin powder (red) or riboflavin solution in ethanol (orange).

Figure 3

Size distribution histogram of silica gel particles and microscopy images for (a) SG (b) SG-Succ), (c) SG-Succ-RF, and (d) the discolored SG-Succ-RF after incubation in water for 48 h. Note that 800–1000 crystals were counted for the histogram.

Figure 4

(1) DMA photooxidation with photogenerated singlet oxygen. (A) experimental procedure used in the experiment. (B) DMA photolysis followed by UV–Vis Spectroscopy. In insert B1, the decay of the 379 nm band is shown in detail. In insert B2, in the region close to 452 nm, the presence of soluble Riboflavin released from the hybrid material is not observed. (C) Comparative experiments of the decrease in absorbance (ΔAbs 379 nm) by photosensitization of DMA with SG-Succ-RF under Blue LED irradiation (orange) or in the dark (violet) and compared with RF in solution (Abs 0.3, in red; Abs 0.005 in pink). As an additional control, the photobleaching of DMA under blue light (black) or in the presence of SG alone is shown (blue).

Figure 5

(B) O₂-uptake at different irradiation times of aqueous suspensions of: (black line) SG-Succ-RF (0.03% w/v); (red line) SG-Succ-RF (0.03% w/v) + FFA (0.5 mM); and (blue line) SG-Succ-RF (0.03% w/v) + FFA (0.5 mM) + NaN₃ (2.3 mM). (C) Number of cycles for which SG-APTES-Eo can be reused at pH 6 plus visible irradiation. 0.5 mM FFA plus SG-APTES-Eo, cycle 1 (black), cycle 2 (red), cycle 3 (blue), cycle 4 (green), cycle 5 (orange). SG-APTES-Eo (magenta).

Figure 6

DMA consumption cycles. In each cycle, 5 μmol DMA in 3 mL DMF in the presence of 0.1 g of SG-Succ-RF or SG-APTES-Eo and irradiated for1 h with blue LED or Green LED, respectively.