PT Unknown
AU Lutz, F
TI Supramolecular strategies for bimetallic photoredox catalysis
PD 10
PY 2020
DI 10.17185/duepublico/72842
LA en
AB 1 DNA-based photoredox systems To improve the efficiency of bicentric photocatalysts, it is necessary for both reaction partners to be in close proximity of each other. Yet, the interactions between the photosensitizer and the active catalyst are often unpredictable. To gain further insight into the structure-activity relationship of these systems, a library containing different photocatalytically active components allows fast screening of a multitude of different combinations. For this, single stranded oligonucleotides were to be functionalized with organic and metal-organic photosensitizers and water reducing catalysts. Within this project, several different heterogeneous and homogeneous protocols were investigated for the conjugation of amino-functionalized oligonucleotides with carboxylated small molecules. As this did not lead to the desired products when using the metalated compounds 2 - 5, the approach was changed. Using dibenzocyclooctyne-functionalized oligonucleotides, a new synthetic pathway for the conjugation via strain-promoted azide-alkyne cycloaddition was chosen. The synthesis of the appropriate azido-functionalized compounds 8-12 and 14 was successful in no more than three steps in medium to high yields. Again, different protocols using differing stoichiometries and solvents were investigated to conjugate oligonucleotides with several organic and organometallic compounds. While three DNA-conjugates were successfully generated (using 8, 12, and 14), some metal complexes showed either no conjugation or degradation of the oligonucleotides or their functionalized side chain. Further optimization of the synthetic protocols might lead to a more extensive library of DNA-based photocatalysts. Upon application of the resulting photocatalytic hybrids in irradiation experiments, no hydrogen could be detected after 16 hours of irradiation. This might either be attributed to a further degradation of the catalytic system and its oligonucleotide backbone or to a non-efficient hybridization of the single strands. Further experiments with longer or less rigid linkers between the oligonucleotide and the catalysts could increase stability by increasing the distance between the strands and the metal centres. Furthermore, the use of iron- or cobalt-based water reducing catalysts and metal-free or copper-based photosensitizers could increase the stability of the conjugated products. 2 CB8-based photoredox systems As the conjugation between oligonucleotides and a photocatalytic system did not lead to the generation of hydrogen, a new supramolecular strategy was tested. This was based on cucurbit[8]uril, as it can form 1:1:1 heteroternary complexes with naphthol- and viologen-based guest molecules. Again, short synthetic routes were employed to allow the generation of multiple guest molecules in short time. For this, based on nucleophilic substitution and etherification the naphthol- and viologen-functionalized metal complexes 20 – 24, 26 as well as the metal-free eosin-based photosensitizer 28 were synthesized. These complexes were shown to form 1:1:1 heteroternary inclusion complexes upon the addition of CB8 by optical and 1H-NMR spectroscopy. The combination of these photosensitizers showed almost no catalytic activity when using the palladium complex 23, which was then excluded from further experiments. The platinum complex 24 showed a varying amount of catalytic activity, based on the photosensitizer it was used in combination with. Using eosin-based photosensitizer 28, only little catalytic activity of around one turnover could be detected. The addition of CB8 had no influence on the amount of hydrogen generated, probably due to the low solubility of the photosensitizer. Adding a freely dissolved platinum salt (platinum(II) chloride) also showed no generated hydrogen. Using the ruthenium-based photosensitizer 20 showed comparable results about one turnover. When using the water reducing catalyst in excess (4 equivalents), hydrogen generation increased almost tenfold (9.37±0.75 / 11.8±0.45 turnovers in absence / presence of CB8). Increasing the amount of 20 or CB8 showed no improvement. Substituting water reducing catalyst 24 with ethylviologen led to no hydrogen generation, confirming the electron pathway from the photosensitizer via the water reducing catalyst to the water molecules. Iridium-based photosensitizer 21 showed higher activity in combination with 24 and CB8. Using the heteroternary system demonstrated a drastic increase in generated hydrogen of up to 147% when compared to the system without CB8 (1.26±0.51 / 3.11±0.01 turnovers in absence / presence of CB8). Adding an excess of photosensitizer 21 (4 equivalents) showed a further increase in hydrogen generation (11.1±0.11 turnovers, 356% in comparison to the 1:1:1 system), hinting at a relatively low stability or the ineffective rereduction of the photosensitizer, stopping the reaction after only a few turnovers. Control experiments, substituting platinum complex 24 by an unfunctionalized ethylviologen showed a drastic loss in activity (0.72±0.06 / 3.11±0.01 turnovers for 21 + EV + CB8 / 21 + 24 + CB8). Adding a freely dissolved platinum salt (platinum(II) chloride) showed no change in generated hydrogen. Furthermore, the “mercury-test” was applied to investigate the presence of active platinum nanoparticles. This showed an unaltered catalytic activity, ruling out the formation of catalytically active nanoparticles. As the functional groups of the photosensitizer and water reducing catalyst can enter CB8 either in a syn- or an anti-orientation, the bis-metalated platinum complex 26 was synthesized. Due to the forced syn-orientation, another drastic increase in catalytic activity was detected, especially for the optimized 1:4 stoichiometry of 21:26 (20-fold increase in comparison to the initial 1:1 case using the mono-Pt complex 24). In summary, these results show the viability of our supramolecular concept. The increased hydrogen-production can indeed by attributed to the close proximity of the photosensitizer and the water reducing catalyst, brought about by the formation of the heteroternary complex with the CB8 host. Considering the effect of CB8 on the photocatalytic systems being demonstrated successfully, increasing the variety of photosensitizers and water reducing catalysts is the next step to take for this project. Also, as the generated hydrogen was measured only once after 11 hours, further time-resolved experiments could show whether the catalytic activity follows a linear trend over time or a limited growth, hinting at either concentration or stability of the components as the limiting factor. Furthermore, as this reaction only represents one half-cell reaction of the complete conversion of water to hydrogen and oxygen, further experiments to implement the other half-cell reaction and to combine them within one reaction vessel are necessary to fully exploit the potential of photocatalytic water-splitting.
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