Photoinduced Water Splitting Dynamics by Organic Chromophores – WSPLIT

The WSPLIT project joins the effort of three research groups with complementary expertise. The ICR group focuses on method development, software implementation, and applying theoretical chemistry methods for simulating molecular excited states. The team is responsible for the development of the Newton-X platform. The ISMO group has long experience studying molecular complexes isolated in the gas phase. The team is specialized in dynamical studies using pump-probe picosecond experiments. The PIIM team has been working for many decades the study of photoreactive systems isolated in molecular clusters and hydrogen-bonded systems isolated in rare gas matrices. The team has expertise on supersonic expansion combined with a time-of-flight mass spectrometry.

In WSPLIT, we have experimentally measured the yield of the WSPLIT reaction in several aromatic organic molecules. This mapping gave us a deep insight into how to select molecules to enhance the reaction. We learned, for instance, that reducing the flexibility for out of planevibrations, is one of the main factors impacting the WSPLIT efficiency, because as this type of motion induces heat loss to the environment. Additionally, we have developed and implemented new theoretical methods to help in the simulations of the processes of interest. All these methods are publically available.

The acquired knowledge about the molecular mechanisms that enhance or prevent the WSPLIT reaction opens new avenues for investigations. We can optimize the yield by selecting rigid molecules to minimize out-of-plane distortions. The molecular candidates in this class are polycyclic aromatic azo-carbons. Preliminary results with heptazine indicate that this may be a fruitful strategy. Other possibilities involve the investigation of charged anionic or protonated species.

The three WSPLIT partners have published 12 scientific papers directly related to the project, being two reviews in the prestigious Chemical Reviews. A final publication is submitted for publication and can be accessed as a preprint at the ChemRxiv. Several publications span phenomenological studies where experiments and theory are employed to explore the nature of the WSPLIT reaction. Others report the development and implementation of novel theoretical methodologies.

Photoinduced water splitting is the ultimate source of storable and clean energy in the form of H2. For over four decades, to optimize this process has been the grail of sustainable energy research.

The possibility of splitting water through photoinduced radical reactions enabled by a simple organic chromophore is a recent theoretical prediction, which already counts on preliminary experimental corroboration. These reactions, however, are not the ones commonly expected in water splitting processes, as they involve hydrogen radicals rather than protons. For this reason, they may represent a deep paradigm shift in the H2 photocatalysis from water, if a series of bottlenecks in terms of recombination rates can be eliminated.

At this point, not much is known about these radical reactions. The WSplit project has been set up exactly to apply state-of-the-art experimental and computational techniques to close this knowledge gap. The project joins the expertise of three research terms, two experimental and one computational, to disentangle the reactive process through a combination of a large battery of techniques, including theoretical simulations, time-resolved experiments, and laser spectroscopy coupled to mass spectrometry. This analysis will be applied to isolated microsolvated clusters. The goals of the project are:
• To develop and use advanced experimental and computational methods to characterize the radical dynamics in microsolvated pyridine-water clusters.
• To use this basic knowledge to search for ways to reduce the recombination rate in these photoreactions.
• To test the efficiency of this class of radical reactions when photoinduced by other organic chromophores.

The choice of systems to be investigated is based on the preliminary experimental and theoretical information available on these radical reactions. It is also set to allow clean spectral signatures from the experiments done in small cold cluster obtained in supersonic jet, allowing an optimal synergy with the theoretical simulations.

The theoretical work will be split in two axes: method developments and simulations. The method development axis will focus on the implementation of a mixed quantum-classical dynamics approach to simulate tunneling using rare-events sampling and machine learning algorithm. This methodology will be then used to investigate the excited-state nonadiabatic dynamics of (Py)k(H2O)n clusters (k = 1,2; n = 1-3).

The experimental work will also be split in two axes: time-resolved spectroscopy and photochemical measurements. The first axis will focus on measuring the picosecond dynamics of the photoinduced dissociation of water. The second axis will use laser spectroscopy coupled to mass spectrometry to find the role of the OH radical in the reaction dynamics, to determine what is controlling the NH dissociation of the pyridinyl radical, and to check whether other organic chromophores may be more productive than pyridine.

WSplit is a three years’ project focused on fundamental research. It is the first step of a larger research programme, whose goal is to build a water-splitting photocatalytic cell using an organic chromophore as a co-catalyst. The maturation of the project will allow us to orient the investigations towards more applied topics in the near future; to access European funds through “Future and Emerging Technologies” calls, as the processes proposed here perfectly fits in the “Key Enabling Technologies” concentration area of the Horizon 2020.