Spin-Crossover Tuning and Structural Characterisation of Luminescent Fe(II) Coordination Compounds with BODIPY-Tetrazole Ligands

Abstract

Multifunctional spin crossover (SCO) materials that incorporate additional functionalities, particularly photoluminescence (PL), show great promise for applications in sensors, optical switches, and thermometry. Coupling SCO with PL enables a sensitive, non-destructive optical readout of the spin state and modulation of the PL signal by external stimuli, such as light, temperature, or pressure. However, combining an SCO centre with fluorescent groups often induces chemical and/or structural changes that quench or alter one or bothfunctionalities. Although several molecular, synergistic SCO-PL systems exist, none to date combine an abrupt, hysteretic spin transition at or near ambient temperature with substantial, spin-state-dependent PL modulation.A mononuclear, heteroleptic Fe(II) SCO-PL platform [Fe(L)4(RCN)2](A)2·nRCN was designed to enable independent tuning of SCO and PL properties, while facilitating synergistic coupling. The ligand L consists of the fluorophore 4,4-difluoro-4-bora-3a,4a-diaza-sindacene (BODIPY), which is covalently linked to a 1H-tetrazole coordinating unit. Coordination of L to Fe(ClO4)2·6H2O in CH3CN yielded [Fe(L)4(CH3CN)2](ClO4)2·2CH3CN as heteroleptic precursor for SCO optimisation. Varying the non-coordinating anions (A = BF4−, ClO4−, PF6−, SbF6−) revealed an abrupt, complete spin transition at a temperature (T1/2) of 265 K for the smaller, tetrahedral anions (BF4−, ClO4−), whereas the larger anions (PF6−, SbF6−) resulted in a more gradual, incomplete SCO at lower T1/2. The independent modification of the axial RCN co-ligands (CH3CN, CH3CH2CN, CH2CHCN) shifted T1/2 between 220-270 K via electronic effects. Replacing RCN with PhCN produced an incomplete spin transition at T1/2 = 88 K, whereasCH3(CH2)2CN co-ligands further stabilised the high-spin state at all measured temperatures, attributed to an increased distortion of the Fe(II) octahedral coordination sphere. In contrast, a homoleptic [FeL6](ClO4)2·4ClCH2CN analogue remained trapped in low-spin state at 300 K. Variable-temperature PL spectroscopy revealed pronounced spin-state-dependent PL quenching for [Fe(L)4(CH3CH2CN)2](ClO4)2·2CH3CH2CN, which was attributed to energytransfer from the BODIPY excited state to the Fe(II) absorption bands in the low-spin state. Other RCN derivatives exhibited weaker PL changes across T1/2, confirming a truly synergistic SCO-PL interplay. Overall, a modular SCO-PL platform has been established, in which the SCO behaviour and the PL properties can be controlled independently. This platform paves the way for further systematic tuning of SCO and PL properties, and the development of SCO-PL materialstailored to specific applications.