Nonradiative Deactivation of Lanthanoid Excited States by Inner-Sphere Carboxylates

The vibrational deactivation of metal-centered
excited states is one of the fundamental processes that governs the
luminescence of inorganic luminophores. In molecular lanthanoid luminescence,
the most reliable way to modulate and systematically investigate these
processes is deuteration of X–H stretching modes (X = O, N,
C). Apart from the effect of these high-energy vibrational motifs,
very little is known about the impact of other oscillator fragments
present in lanthanoid complexes. We have developed a synthetic protocol
to efficiently and selectively label the popular chelator motif “pyridine-2-carboxylic
acid” with stable 13C/18O isotope at
the carboxylate group. The corresponding isotopologic lanthanoid complexes
(Ln = Sm, Eu, Ho) show a decrease of the local-mode carbonyl stretching
frequency of up to 5% after isotopic substitution. While this does
not seems to have any effect on the luminescence of lanthanoids with
medium- to high-energy gaps (Sm and Eu), we have found the first example
of a quantifiable luminescence isotope effect for one of the near-IR
transitions of holmium (3K8 → 5I5) that only involves the isotopic editing of the vibrational
environment at the four carbonyl oscillators.