Improved Photostability of a CuI Complex by Macrocyclization of the Phenanthroline Ligands

Abstract The development of molecular materials for conversion of solar energy into electricity and fuels is one of the most active research areas, in which the light absorber plays a key role. While copper(I)‐bis(diimine) complexes [CuI(L)2]+ are considered as potent substitutes for [RuII(bpy)3]2+, they exhibit limited structural integrity as ligand loss by substitution can occur. In this article, we present a new concept to stabilize copper bis(phenanthroline) complexes by macrocyclization of the ligands which are preorganized around the CuI ion. Using oxidative Hay acetylene homocoupling conditions, several CuI complexes with varying bridge length were prepared and analyzed. Absorption and emission properties are assessed; rewardingly, the envisioned approach was successful since the flexible 1,4‐butadiyl‐bridged complex does show enhanced MLCT absorption and emission, as well as improved photostability upon irradiation with a blue LED compared to a reference complex.


Crystal data for 11
Single crystals were grown by vapor diffusion technique using dichloromethane as solvent and diethyl ether as anti-solvent. Solid state structure in the manuscript are displayed with rotation ellipsoids at 50% probability. Hydrogen atoms, solvent moleclues and the PF 6counter ions were omitted for clarity. Color code: N: blue, Cu: yellow, C: gray for one and purple for the other ligand for clarity.

Photostability investigations
A very similar strategy as in our recent investigation on acridinium dyes was used to compare the photostabilities of 1 and 15. [1] We irradiated diluted and deoxygenated solutions of both complexes, and monitored their UV-Vis spectra over two hours of photoirradiation. The cuvettes were irradiated in the sample chamber of the spectrophotometer (see Figure SI4) with a 455 nm LED from Thorlabs (M455L3-C1, 500 mW optical output). Our cuvette holder permits LED irradiation of the whole detection volume. For recording the absorption spectra after the desired irradiation times, the LED was blocked for 2 minutes.
The absorptions of both irradiated solutions at the LED peak wavelength (455 nm) were standardized to 0.10, which ensures that almost the same amount of light is absorbed while sample heating is avoided. Concentrations of ~36 M for 1 and ~211 M for 15 were required for these standardized conditions.
The UV-Vis data displayed in figure 6 of the main paper show good photostability for 1 under our test conditions. A significant decrease of the MLCT absorption band is expected to occur upon photodecomposition, but all spectra are virtually identical; the maximum relative absorption variation is with less than 0.5% within the accuracy of the analysis.
The situation is completely different for 15. Our photostability assay revealed a constant decrease of the MLCT band, which amounts to 7% after 2 hours of photoirradiation (compare, lower part of figure 6a and figure 6b). Assuming that the decomposition products do not absorb at the detection wavelength allows us to set a lower limit for the concentration change caused by photodecomposition: 14.8 M (211 M x 0.07). To be able to make a relative stability statement, we assume a maximum absolute concentration change for 1 under identical irradiation conditions of 0.4 M, corresponding to an experimental error of up to 1% hiding the alteration of the observed MLCT band (36 M x 0.01). Taking the widely differing starting concentration of both complexes into account [2] the actual photodecomposition of 15 is thus significantly faster (compared to that of 1) than the absorption spectra in figure 6 suggest. Hence, the analysis presented in this section revealed the photodegradation of 1 (macrocycle) to be slower than that of 15 by a factor of at least ~37.
In order to identify the photodecomposition product, we irradiated complex 15 in an NMR tube. The peaks in acetonitrile-d 3 are rather broad, which is the reason why we performed this experiment in deuterated dichloromethane. In the NMR spectrum recorded after irradiation with a blue high-power LED at 440 nm (from Kessil), which was very recently purchased and has a much higher output than the 455 nm LED used for the investigations presented in figure 3 of the main paper, we observed noticeable photodecomposition after 3h of irradiation, but we could not detect the release of the free ligand ( Figure SI3). The photodecomposition products could not be identified. 1
A spectroelectrochemical cuvette from ALS Japan was used as electrochemical cell to scope with the very limited amount of available material. Two platinum wires served as working and auxiliary electrode respectively, and a silver/silver chloride electrode served as reference.
The measurements were conducted in HPLC grade acetonitrile (CH 3 CN) and tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) was used as supporting electrolyte at a concentration of 0.1 M. All recorded potentials are given relative to Ag/AgCl/3M KCl. In all the experiments, the scan rate was 100 mV/s for CV and the pulse frequency was 15 Hz for SWV.
For all complexes (1, 2, 3, 4, 5 and 15) no redox signals at negative potentials were recorded, indicating that the ligand-centered reduction steps occur outside of the potential window accessible with our set-up and skills (-1.5 to 1.5 V). And indeed, the reduction of copper(I) diimine complexes is reported in literature between -1.5 and -1.7 V vs. SCE, 3 corresponding to values between -1.532 and -1.732 V vs. Ag/AgCl/3M KCl.
The recorded voltagramms of the complexes 1, 2, 3, 4, 5 and 15 are displayed in figure SI5 and the extracted redox values are listed in table SI3. Due to the limited stability and/or isolation properties of the complexes 2-5, the redox studies were performed with the best available sample quality. However, the appearance of additional oxidation waves in 5 and even more pronounced in 2, is most likely rather due to impurities than due to intrinsic redox features of the parent complexes.