Modulation of Eu(III)-Centered Luminescence by Fe(II) Spin-Crossover in Extended Trinuclear Helicates

The efficiency of intermetallic d-f energy transfers in molecular-based Fe(II)-Ln(III) dyads can be modulated by (i) the spin state of the [Fe(II)N₆] unit, (ii) the intermetallic distance, and (iii) the choice of the trivalent lanthanide (Ln(III)). The programming of a monotonous and reliable change in Ln-based luminescence upon spin-crossover (SCO) processes is highly desired for easy and fast optical monitoring of Fe(II)-spin state, but Ln···Fe separations over the nanometer range are required to be compatible with detectable Ln-based light emission. Moving from a nonluminescent rigid macrobicyclic dinuclear [FeEu(L8)₃]⁵⁺ triple-stranded helicate (Fe···Eu = 9.19 Å) toward the extended macrotetracyclic trinuclear [FeEuEu(L11)₃]⁸⁺ analog (Fe···Eu_terminal = 17.66 Å) induces unprecedented optical reading of a [Fe(II)(N_pyridine^∩N_{benzimidazole})₃]²⁺ spin-crossover platform upon modulation of the red emission of the terminal [EuN₆O₃] chromophore. The close-to-linear Eu(III)-based optical response to the high-spin Fe(II) mole fraction in solution contrasts with the less adapted wavy luminescence response reported pioneeringly for the hydrolysis-sensitive 2-(pyrimidin-2-yl)-1H-benzo[d]imidazole platform in [FeEu(L9)₃]⁵⁺. The rational programming of variable optical detection modes (wavy response, linear increase, linear decrease) is discussed and can be deduced from the modeling of the coupled SCO-Fe(II)/Ln(III) centers developed in this work.