Photo-generated Extreme Reductive Power with Copper(I) Based Photosensitizers – PERCO

The objective of this proposal is to develop original and cheap photosensitive systems capable of generating a very strong reductive power upon illumination, which could be further used in various photochemical reactions or reduction catalysts activation. Reductive quenching (RQ) is particularly relevant within this frame: when a photosensitizer PS is promoted to its excited state PS*, it can be reductively quenched by a sacrificial donor: PS* + SD ? PS- + SD+. If PS is properly chosen such that E(PS/PS-) is very negative, strongly reductive species can thus be photo-generated. Heavy metal based complexes are very efficient in this area, but they are expensive and toxic. On the other hand, copper(I)-diimine complexes as PS (hereafter named PSCu) are elegant alternatives because they feature surprisingly similar photophysical properties than e.g. ruthenium complexes and substantially more cathodic reduction potentials than any heavy metal based PS, at much lower price and toxicity. Nevertheless, copper(I)-diimine complexes have almost never been used for RQ, despite these advantages. The reason is PSCu are weak photo-oxidants in general: the initial photo-induced charge separation PS* + SD -> PS- + SD+ is not thermodynamically feasible with usual SD.
The aim of the PERCO project is to adjust the electronic properties of SD and PSCu for efficient RQ, via molecular engineering of both protagonists. The driving force for RQ is related to the equation E(PSCu/PSCu-) + E00 – E(SD+/SD), where E00 is the energy of the excited state of PSCu. Setting as an upper limit E(SD+/SD) = 0.3 V vs. SCE (criterion 1), and as a lower limit E(PSCu/PSCu-) + E00 = 0.6 V vs. SCE, a minimum driving force of 300 meV is insured for RQ. New SD will be obtained by modifying pre-existing well-known SD with electron donating groups. In parallel, new PSCu will be designed such that E(PSCu/PSCu-) + E00 is more positive (no less than 0.6 V vs. SCE). This will be achieved by modifying the pi-accepting properties of ligands coordinating copper(I) and tuning the steric bulk of the latter coordination cage, altering respectively E(PSCu/PSCu-) and E00. Very importantly, we will pay special attention not to annihilate the remarkably reductive power of PSCu- nor its light harvesting efficiency during chemical engineering, by imposing 3 additional criteria: 2) E(PSCu/PSCu-) must remain below -1.6 V vs. SCE, and 3) E00 must lie below 2.5 eV to insure visible light sensitivity. At last, the excited state lifetime of obtained PSCu must be long lived enough (ca. 70 ns) to insure the efficacy of bimolecular RQ reaction (criterion 4). With all gathered information, we will isolate homo- and heteroleptic copper(I) complexes able to perform RQ with pre-existing or newly synthesized SD. In particular, we will append anchors on the diimine fragment to allow chemisorption on a p type semi-conductor (e.g.NiO). We will study the photo-induced hole transfer from grafted PSCu into the valence band of NiO, en route towards photo-electrochemical cells. Finally, photo-active systems designed in this project will be put to the test in bulk photolysis experiments: we will monitor the photo-induced degradation of organic products featuring very negative reduction potential, in presence of PSCu and SD, or NiO|PSCu hybrid systems. All compounds will be characterized by electrochemistry and steady-state spectroscopies, in order to interpret and quantify the behaviour of our systems. Ultrafast spectroscopy (optical and X-ray) will be performed to appraise the intricate photo-induced processes at stake. The success of the PERCO project is relying on a collaboration between CEISAM (Nantes) and the Chen group (Northwestern University), who is specialised in the photophysical behaviour of copper based photosensitizers. The successful achievements of this proposal will pave the way towards cheap, fully renewable artificial photosystems for H2O or CO2 photocatalytic reduction.