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Conjugated Polymers for Light-Driven Hydrogen Evolution from Water

Periodic Reporting for period 1 - PolymersForSolarFuel (Conjugated Polymers for Light-Driven Hydrogen Evolution from Water)

Reporting period: 2018-03-01 to 2020-02-29

With a steadily increasing demand of the global energy consumption and reliance of geopolitically sensitive sources of energy, such as petroleum and coal, there has never been such an urgency to explore alternative clean, renewable energy supplies. Aside from the obvious limitations in availability, those raw materials and their combustion products are considered polluting and low-efficient. The clean, sustainable production of hydrogen is one promising strategy for future zero-emission energy supply. In this context, photocatalysis using heterogeneous semiconductors for water splitting has received much attention. Progress has been made in the application of both inorganic and organic semiconductors, the latter triggered by the studies on carbon nitride and later conjugated polymers. The modularity of these materials over a wide range of monomer building blocks allows the transfer of photocatalytically active subunits from one class of materials into another. This allows us, in principle, to build structure-property relationships where molecular effects are deconvoluted from solid state packing effects. However, the modularity of these materials taken together with a wide range of accessible monomer building blocks results in a very large possible chemical space, even for ‘simple’ (A-B)n-type co-polymers.

The aim of the project PolymersForSolarFuel was to address globally relevant challenges in the field of renewable energy generation and storage. Combination of established concepts from the fields of photovoltaics, photocatalysis, and polymer synthesis built the foundation of this project and enabled the development of novel sustainable materials for solar-driven evolution of hydrogen from water. The PolymersForSolarFuel project aims to: a) investigate organic materials and contribute to an overall database of photoactive compounds, b) select most promising candidates through property-related screening, c) cross-examine physical (two-component) and chemical (one-component) combinations of such materials and identify most promising final candidate(s) and d) develop scale-up protocols and assemble a prototype of a feasible size. By meeting these goals, a better structure-function relationship in photocatalytically active polymers that facilitates the search of new catalysts within the chemical space will be achieved.
To investigate the effects of polymer's interaction with water on its photocatalytic activity, two soluble phenylene co-polymers containing sodium 3,3’-(9H-fluorene-9,9-diyl)bis(propane-1-sulfonate) (I, monomer was synthesized) or 9,9-bis[3,3'-(N,N-dimethylamino)propyl]fluorene (II, monomer was commercially available) subunits were prepared. Further, co-polymer of dibenzo[b,d]thiophene dioxide and sodium 3,3’-(9H-fluorene-9,9-diyl)bis(propane-1-sulfonate) (III) as well as composites of polymer I and phenylene dibenzo[b,d]thiophene dioxide co-polymer (literature known P7) were prepared successfully. All compounds were analysed by NMR (monomers), IR, UV/vis, PL and TGA. All polymers were tested towards their photocatalytic activity in a reaction mixture of water/methanol/triethylamine (1:1:1). While charged and fully soluble polymer I showed no activity, polymer II was only mildly soluble in chloroform and its thin films on glass showed good activity (1.2 mmol h-1 g-1) under visible light irradiation (>420 nm). However, the films proved to act self-sacrificial during irradiation and therefore not suitable for catalysis. Also, co-polymer III and the composite P7/I showed only poor to no reactivity during irradiation in the reaction mixture.

To investigate the effects of polymer's conductivity on its photocatalytic activity, ladder polymers were prepared. As known from literature, among conjugated polymers dibenzo[b,d]thiophene dioxide co-polymers show excellent activities in hydrogen evolution from water. Thus, the monomer 1,4-dibromo-2,5-bis(methylsulfinyl)benzene and two ladder polymers (cLaP1 and cLaP2) as well as their parent conjugated polymer (cLiP1) were prepared successfully. All compounds were analysed by NMR (monomers), IR, UV/vis, PL and TGA. All polymers were tested towards their photocatalytic activity in a reaction mixture of water/methanol/triethylamine (1:1:1). Ladder polymer cLaP1 (1.3 mmol h-1 g-1) exceeded both - the conjugated parent polymer cLiP1 and its oxidised successor cLaP2. Encouraged by these result, two further pigment-based ladder polymers were synthesised: poly(1,6-dihydropyrazino[2,3- g]quinoxaline-2,3,8-triyl-7-(2H)-ylidene-7,8-dimethylidene) (IV) and oligo-perinone (V). However, both pigment-based ladder polymers did not show any photocatalytic activity. Further ladder polymers were not pursued due to the extended synthetic load that is unfeasible within the project’s timeframe.

To investigate the effects of reactive sites in a polymer on its photocatalytic activity, salophene-type polymers and their (non-precious) metal complexes were prepared. Two types of polymers were successfully synthesised: all-salophene ladder-type polymers (VI) and phenylene spaced conjugated salophene polymers (VII). For both, monomers were successfully prepared prior to polymerisation and/or metalation and analysed by NMR, IR and UV/vis. For metalation, appropriate zinc, nickel, cobalt, ruthenium and vanadyl precursor metal salts were chosen. All polymers and their complexes were analysed by IR, UV/vis, PL and TGA. Zn-containing ladder-type polymers showed minor photocatalytic activity while no other salophene-type polymer succeeded. These finding require further investigation, also by means of theoretical computations.

Overall, the result of the study suggest that the interplay of different polymer properties cannot be deconvoluted nor predicted to give a synthetic handbook for the search of photocatalytically active polymers. More importantly, several trade-offs within a specific series of polymers have to be considered. Thus, while polarity and increased interaction with water is advantageous in one class of polymers (e.g. pyridine containing polymers known from literature), an extremely improved interaction with water, i.e. total solubility, may be disadvantageous. In the same manner, the trade-off between the thermodynamic driving forces and optimised light absorption result in a specific optimum within a series of catalysts.
Findings of this study contributed to the publication of the review “Current understanding and challenges of solar-driven hydrogen generation using polymeric photocatalysts”. Further, the study on ladder polymers (cLiP1, cLaP1 and cLaP2) was published and presented at two conferences.
The project PolymersForSolarFuel had a great impact on the personal development of the fellow. Both scientific publications and presentations sparked interest and fruitful discussions within the scientific community. Further, the review will help a new generation of young scientist to get an educated overview of the field of photocatylalytic hydrogen generation via polymeric photocatalysts. Ladder polymers as well as redox-active salophene-type metalated polymers remained previously relatively unregarded and pose a step beyond the state-of-art in this project.