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 byappended [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 improvedsignificantly if the luminophore emission is shifted toward the near-infrared (NIR) domain. ReplacingEuIII with NdIII in [NdFe(L1)3]5+ (i) maintains the favorable SCO properties in acetonitrile (criticaltemperature 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 processeswhich 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 withthe LS-[FeIIN6] and HS-[FeIIN6] mole fraction controlled by the SCO equilibrium. This paves the wayfor a straightforward and direct NIR luminescent reading/sensing of FeII spin state in singlemolecules.