Unravelling Kinetics of Intramolecular Nd III → Fe II Energy Transfer in Spin Crossover Single Molecules: Dotting the i 's and Crossing the t 's.

Compared with the ripple of visible Eu III -based emission intensity induced by appended [Fe II N 6 ] spin crossover (SCO) units, as detected in the triple-stranded [EuFe( L1 ) 3 ] 5+ helicate, the lanthanide-based luminescent detection of Fe II spin-state equilibria could be improved significantly if the luminophore emission is shifted toward the near-infrared (NIR) domain. Replacing Eu III with Nd III in [NdFe( L1 ) 3 ] 5+ (i) maintains the favorable SCO properties in acetonitrile [critical temperature T 1/2 = 322(2) K], (ii) saturates nonradiative vibrational relaxation processes in the 233-333 K range, and (iii) boosts the crucial intramolecular Nd III → Fe II energy transfer rate processes, which are sensitive to the spin state of the Fe II metallic center. Consequently, the steady-state NIR Nd( 4 F 3/2 → 4 I J ) emission of the luminophore is amplified and linearly correlated with the low-spin-[Fe II N 6 ] and high-spin-[Fe II N 6 ] mole fractions controlled by the SCO equilibrium. This paves the way for a straightforward and direct NIR luminescent reading/sensing of the Fe II spin state in single molecules.