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

Neel Deorukhkar; Charlotte Egger; Arnulf Rosspeintner; Claude Piguet

doi:10.1021/jacs.4c05546

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

J Am Chem Soc. 2024 Jul 17;146(28):19386-19396. doi: 10.1021/jacs.4c05546. Epub 2024 Jul 2.

Authors

Neel Deorukhkar¹, Charlotte Egger¹, Arnulf Rosspeintner², Claude Piguet¹

Affiliations

¹ Department of Inorganic and Analytical Chemistry, University of Geneva, 30 Quai E. Ansermet, CH-1211 Geneva 4, Switzerland.
² Department of Physical Chemistry, University of Geneva, 30 Quai E. Ansermet, CH-1211 Geneva 4, Switzerland.

PMID: 38953864
DOI: 10.1021/jacs.4c05546

Abstract

Compared with the ripple of visible Eu^III-based emission intensity induced by appended [Fe^IIN₆] 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(⁴F_3/2 → ⁴I_J) emission of the luminophore is amplified and linearly correlated with the low-spin-[Fe^IIN₆] and high-spin-[Fe^IIN₆] 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.