H 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 (Φ_true), when a reactant and photoproduct absorption overlap. For this, Φ_true 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. Φ_NMR = 0.80 for fac-[Re(CO)₃(phen)(trans-bpe)]⁺) were much higher than those determined by absorption spectral changes (Φ_UV-Vis = 0.39 for fac-[Re(CO)₃(phen)(trans-bpe)]⁺). Φ_NMR 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 Φ_true by regular absorption spectral changes. For instance, Φ_313 nm for free ligand photoisomerization determined both by absorption and ¹H NMR variation are equal within the experimental error (bpe: Φ_UV-Vis = 0.27, Φ_NMR = 0.26; stpy: Φ_UV-Vis = 0.49, Φ_NMR = 0.49). Moreover, ¹H NMR data combined with electronic spectra allowed molar absorptivity determination of difficult to isolate cis-complexes.