T½ Tuning in a Synergistic BODIPY-Tetrazole Fe(II) Spin-Crossover-Photoluminescence System via Co-Ligand Variation.

Bifunctional materials combining spin crossover (SCO) and photoluminescence (PL), offering noncontact and highly sensitive spin-state detection, are promising candidates for application in photonic devices. However, independently tuning the SCO and PL properties to achieve synergistic SCO-PL behavior remains challenging. Herein, we present a series of heteroleptic Fe(II) coordination compounds with the general formula [Fe(L)4(RCN)2](ClO4)2•nRCN, bearing four BODIPY-derived fluorophores L (L = 4,4-difluoro-1,3,5,7-tetramethyl-8-[(1H-tetrazol-1-yl)methyl]-4-bora-3a,4a-diaza-s-indacene) in the equatorial positions and variable RCN co-ligands (CH3CH2CN, CH2CHCN, CH3(CH2)2CN, PhCN) in the apical positions. The systematic variation of the co-ligand modulates steric and/or electronic effects, thereby influencing the SCO behavior without perturbing the PL characteristics. Magnetic susceptibility measurements revealed T1/2 = 220 K and 270 K for R = CH3CH2 and R = CH2CH, respectively, indicating that electronic effects modulate T1/2. Conversely, for R = Ph and R = CH3(CH2)2 the SCO capability is progressively lost, with the latter showing no spin-switching at all. Temperature-dependent PL studies display an increase in emission upon the SCO, most pronounced for [Fe(L)4(CH3CH2CN)2](ClO4)2•2CH3CH2CN, which can be attributed to electronic coupling between the SCO center and the fluorophore. This work establishes a single-molecule SCO-PL platform, in which the SCO behavior can be tuned independently from the PL response, paving the way for tailored molecular sensors and optical switches.