Two-Coordinate Coinage Metal Complexes as Solar Photosensitizers

Abstract

Generating sustainable fuel from sunlight plays an important role in meeting the energy demands of the modern age. Here we report the synthesis of new two-coordinate, molecular Cu(I) and Au(I) complexes that were designed to absorb visible photons (vis > 103 M-1cm-1), maintain long excited state lifetimes (~1-0.1s), and perform stable photo-induced charge transfer to a target substrate with remarkably potent photoreducing capabilities (E+/* up to 2.33 V vs. Fc+/0). The photoredox performance was evaluated in a variety of solvents, and we were able to understand the influence of ligand design and metal center on the photophysical properties. Interestingly, we found that the Cu(I) systems have competitive figures of merit with widely used scarce metal photosensitizers such as Ru(bpy)32+ and Ir(ppy)3. This work illuminates two-coordinate coinage metal complexes as promising, abundant metal, solar fuels photosensitizers that offer exceptional tunability and photoredox properties.

Intra- and Inter-Molecular Charge Transfer Dynamics of Carbene-Metal-Amide Photosensitizers

“…𝑀𝐴𝐶 experiencing a ~1.2 increase compared to the other gold compounds. And our analysis suggests that all the gold containing complexes have somewhat higher Δ𝐸 𝑆𝑇 than the copper complexes, consistent with theoretical calculations for the gap by Muniz, et al 23. …”

“…As the oxidative non-reversibility in 𝐴𝑢 𝐶𝑧 𝑀𝐴𝐶 is attributed to polymerization of the unsubstituted 3-, and 6-positions in the carbazole ligand, consistent with irreversible oxidation of bare Cz, 9 substitutions at these locations with tert-butyl groups in 𝐴𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 should impart reversibility, which is observed in its CV. 23,25 Still some degradation is observed upon adding MePI to the solution. We believe this instability is due again to excitation of a ligand field transition, this time of the cation, upon irradiation with 355 nm light.…”

“…We now determine 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑜 ~ 10 7 − 10 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑜 varies by over an order of magnitude in the compounds studied, reflects that there is a relatively small variation in ∆𝐸 𝑆𝑇 , all about 80-90 meV for Cu compounds, a value consistent with temperature dependent studies reported earlier. 8,23 Of course, for a given compound, the same spin-orbit coupling constant mediates both forward and reverse processes. On the other hand, we see a smaller but now solvent-dependent variations in the TADF, all close to 1 s (except Ag cMa compounds), 8 suggesting radiative and non-radiative relaxation of the singlet are sensitive, as expected, to different factors than the ISC processes.…”

“…The quenching rate constant, kq, of 𝐴𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 with MePI of (1.4 ± 0.1) • 10 10 M -1 s -1 is in agreement with that obtained previously. 23 Unlike electron transfer to MePI, using (𝐴𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )* to transfer a hole to BIH in THF (i.e.…”

“…Independently, a kq of (2.6 ± 0.3) • 10 8 M -1 s -1 has been established from a Stern-Volmer analysis using TCSPC (Figure S60), using the same methodology as Muniz et al. 23 Since BIH is a sacrificial donor, it promptly degrades after oxidation, preventing back electron transfer from (𝐴𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 ) -. Because of this, the 15 ms lifetime measured for (𝐴𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )corresponds to the lifetime before the ion either decomposes or more likely migrates out of the beam path (Figure 42b).…”