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

ABSTRACT: 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 spincrossover (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···Euterminal = 17.66 Å) induces unprecedented optical reading of a [Fe(II)(Npyridine
∩Nbenzimidazole)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.