Substituted 2,4-Di(pyridin-2-yl)pyrimidine-Based Ruthenium Photosensitizers for Hydrogen Photoevolution under Red Light

Abstract

The photocatalytic reduction of water to form hydrogen gas (H 2 ) is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris(bipyridine) and its many derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis(terpyridine) analogues, however, have only recently gained attention for this application because of their poor photophysical properties. Yet, by the introduction of electron-donating or -withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be considerably improved. In this study, we report a series of nonsymmetric 2,6-di(pyridin-2yl)pyrimidine ligands with peripheral pyridine substituents in different positions and their corresponding ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital, leading to a red-shifted emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than the previously reported bis(terpyridine) complexes used as PSs. Interestingly, the pyridine substituent in the 4-pyrimidine position has a greater impact on both the photophysical and electrochemical properties. This correlation between the substitution pattern and properties of the complexes is further investigated by using time-dependent density functional theory. In hydrogen evolution experiments under blue-and red-light irradiation, all investigated complexes exhibit much higher activity compared to the previously reported ruthenium(II) bis(terpyridine) complexes, but none of the complexes are as stable as the literature compounds, presumably because of an additional decomposition pathway of the reduced PS competing with electron transfer from the reduced PS to the catalyst.

Red Light-Based Dual Photoredox Strategy Resembling the Z-Scheme of Natural Photosynthesis

“…The combination of [Cu(dap) 2 ] + and DCA complements and expands the known photochemical applications of these two individual components when used separately. 68 , 77 , 83 , 89 , 117 , 159 − 167 Red light-driven applications play important roles in other important contexts, for example, hydrogen production, 47 , 48 , 168 , 169 medical applications, 158 , 169 − 173 and polymerizations. 174 − 178 Now, red light as well as multiphoton excitation-based mechanisms seem to become of increasing interest for synthetic organic photoredox chemistry, 50 , 126 , 179 − 181 and we hope the insights gained from our work will be useful in that greater context.…”

Exploring Multidimensional Chemical Spaces: Instrumentation and Chemical Systems for the Parallelization of Hydrogen Evolving Photocatalytic Reactions

“…Additionally, this method may interrupt the reaction as changing the condition in the headspace may affect the activity in solution. Rather than sampling by hand repeatedly throughout the hours long reaction, Hanan’s group developed a reaction sampling mechanism, which is operated by an Arduino computer and routes samples in real time to a GC for measurement. ,,,− A very similar system has also seen use by Beller and co-workers particularly for the exploration of cobalt based PSs. − These systems are described as using a constant flow of argon into the reaction vessel with an outflow directly into the GC. By this method, the headspace remains a consistent composition and does not substantially change in pressure.…”

“…Limitations in these systems, beyond operating with a single experiment at one time, also include the inability to gather kinetic information from the reaction due to singular sampling, and the need for high energy consumption through high wattage lamps which also require cutoff filters. The challenge of sampling has been partially addressed by the Hanan lab through the addition of automated gas sampling throughout the reaction process using Arduino single-board microcontrollers. − These setups allowed for initial exploration in photocatalytic water reduction; however, for the field to truly expand and explore the vast chemical space surrounding these reactions, innovation of more efficient and ingenious reactors is necessary.…”

Structurally modified T-metal complexes and organic dyes as photosensitizers and earth-abundant T-metal catalysts for photo-driven hydrogen evolution in artificial photosynthetic systems

“…The higher stability of PS3 and PS5 resulted in higher TONs than those of the slightly more active complexes PS4 and PS6. 71 Chen et al in 2017 prepared and characterized a linear pentadentate aminopyridine ligand 2,6-bis[(methyl(pyrid-2-ylmethyl)amino)-N-methyl]pyridine (Py Me pam). 73 The choice of the pentadentate amino pyridine ligand was made on the basis of its easy synthesis, versatility and redox properties.…”

“…58 However, the synthesis of the [Ru(tpy) 2 ] 2+ complexes where tpy ligands were judiciously substituted as reported by Hanan et al made it possible to use them for photocatalytic hydrogen production. 70,71 The use of the catalyst C25 with [Ru(bpy) 2 ] 2+ and H 2 A gave a TON of 2700. Furthermore, the stability of the catalyst before and aer photocatalysis paved a way for its incorporation into other multicomponent systems.…”