Unravelling Kinetics of Intramolecular NdIII→ FeIIEnergy 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 FeIIspin-state equilibria could be improved significantly if the luminophore emission is shifted toward the near-infrared (NIR) domain. Replacing EuIIIwith NdIIIin [NdFe(L1)3]5+(i) maintains the favorable SCO properties in acetonitrile [critical temperature T1/2= 322(2) K], (ii) saturates nonradiative vibrational relaxation processes in the 233–333 K range, and (iii) boosts the crucial intramolecular NdIII→ FeIIenergy transfer rate processes, which are sensitive to the spin state of the FeIImetallic center. Consequently, the steady-state NIR Nd(4F3/2→ 4IJ) emission of the luminophore is amplified and linearly correlated with the low-spin-[FeIIN6] and high-spin-[FeIIN6] mole fractions controlled by the SCO equilibrium. This paves the way for a straightforward and direct NIR luminescent reading/sensing of the FeIIspin state in single molecules.