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.
