1H NMR spectroscopy as a tool to determine accurate photoisomerization quantum yields of stilbene-like ligands coordinated to rhenium(I) polypyridyl complexes

In this work, the use of proton nuclear magnetic resonance, 1H NMR, was fully described as a powerful tool to follow a photoreaction and to determine accurate quantum yields, so called true quantum yields (Φtrue), when a reactant and photoproduct absorption overlap. For this, Φtrue for the trans–cis photoisomerization process were determined for rhenium(I) polypyridyl complexes, fac-[Re(CO)3(NN)(trans-L)]+ (NN = 1,10-phenanthroline, phen, or 4,7-diphenyl-1,10-phenanthroline, ph2phen, and L = 1,2-bis(4-pyridyl)ethylene, bpe, or 4-styrylpyridine, stpy). The true values determined at 365 nm irradiation (e.g. ΦNMR = 0.80 for fac-[Re(CO)3(phen)(trans-bpe)]+) were much higher than those determined by absorption spectral changes (ΦUV–Vis = 0.39 for fac-[Re(CO)3(phen)(trans-bpe)]+). ΦNMR are more accurate in these cases due to the distinct proton signals of trans and cis-isomers, which allow the actual determination of each component concentration under given irradiation time. Nevertheless when the photoproduct or reactant contribution at the probe wavelength is negligible, one can determine Φtrue by regular absorption spectral changes. For instance, Φ313 nm for free ligand photoisomerization determined both by absorption and 1H NMR variation are equal within the experimental error (bpe: ΦUV–Vis = 0.27, ΦNMR = 0.26; stpy: ΦUV–Vis = 0.49, ΦNMR = 0.49). Moreover, 1H NMR data combined with electronic spectra allowed molar absorptivity determination of difficult to isolate cis-complexes.