Coexistence of luminescence and spin-crossover in 2D iron(ii) Hofmann clathrates modulated through guest encapsulation

Abstract

The search for advanced multifunctional materials displaying two or more simultaneous and correlated physicochemical properties represents a key step in developing further practical applications. In this context, here we report the synthesis and physical characterization of the unprecedented two-dimensional Hofmann-type Coordination Polymers (HCPs) formulated as {Fe^II(AnPy)₂[M^I(CN)₂]₂}·XBz (AnPy = 4-anthracenepyridine; M = Ag, Au; XBz = substituted benzenic guest). These materials combine two functions, spin crossover (SCO) and fluorescence which arise, respectively, from the octahedrally coordinated Fe^II ions and the anthracene based AnPy ligands. The thermal dependence of the magnetic and calorimetric properties performed on microcrystalline samples, as well as single crystal UV-Vis absorption spectroscopy and structural studies reflect a multi-stepped nature of the SCO. The temperature at which the SCO takes place is markedly modulated by the nature of the XBz guests (X = I, Br, Cl, NO₂, CH₃) which are trapped within the interstitial spaces originating from the stacking of bimetallic layers. Two different crystallographic phases, one orthorhombic and another triclinic have been identified. The triclinic phase is not stable in air and transforms into the orthorhombic form likely prompted by the partial release of guests. Both phases afford new examples of symmetry breaking and spin-state ordering associated with the SCO. All compounds exhibit strong blue fluorescence that gradually vanishes as the temperature increases but they also present a prominent excimer/exciplex green fluorescence band for X = I (M = Ag, Au) and Cl (M = Au), whose intensity increases in parallel with the population of high-spin Fe^II ions. Hence, the latter compounds represent new examples of synergy between SCO and luminescence.