Understanding the Quenching Effects of Aromatic C–H- and C–D-Oscillators in Near-IR Lanthanoid Luminescence

Several series of selectively deuterated 2,2′-bipyridine-based
cryptates with the near-IR emissive lanthanoids Pr, Nd, Er, and Yb
are reported. The structural and luminescence properties of these
complexes have been comprehensively investigated. A combination of
experimental techniques (X-ray crystallography, lanthanoid-induced
NMR shift analysis, luminescence, vibrational near-IR absorption)
and theoretical concepts has been applied with a focus on nonradiative
deactivation through multiphonon relaxation of lanthanoid excited
states by aromatic, high-energy C–(H/D) oscillators. It is
shown that the characteristics for the overtones of these vibrational
modes deviate substantially from harmonic oscillators and that anharmonicity
within a local-mode Morse model is an essential parameter for any
accurate description. The spectral overlap integrals (SOIs) of lanthanoid
electronic states with aromatic C–(H/D) overtones are evaluated
quantitatively for different lanthanoid/oscillator combinations and
the implications for luminescence enhancement through deuteration
is discussed. Simple Gaussian functions are proposed as appropriate
mathematical forms for the empirical approximation of SOIs.