Unravelling Kinetics of Intramolecular NdIII→FeII Energy Transfer in Spin-Crossover Single Molecules: Dotting the i's and Crossing the t's.

Compared with the ripple of visible EuIII-based emission intensity induced by
appended [FeIIN6] spin crossover (SCO) units, as detected in the triple-stranded [EuFe(L1)3]5+
helicate, the lanthanide-based luminescent detection of FeII spin-state equilibria could be improved
significantly if the luminophore emission is shifted toward the near-infrared (NIR) domain. Replacing
EuIII with NdIII in [NdFe(L1)3]5+ (i) maintains the favorable SCO properties in acetonitrile (critical
temperature T1/2 = 322(2) K), (ii) saturates non-radiative vibrational relaxation processes in the 233-
333 K range and (iii) boosts the crucial intramolecular NdIIIFeII energy transfer rate processes
which are sensitive to the spin state of the FeII metallic center. Consequently, the steady-state nearinfrared
(NIR) Nd(4F3/24IJ) emission of the luminophore is amplified and linearly correlated with
the LS-[FeIIN6] and HS-[FeIIN6] mole fraction controlled by the SCO equilibrium. This paves the way
for a straightforward and direct NIR luminescent reading/sensing of FeII spin state in single
molecules.