Spin-crossover tuning in synergistic luminescent Fe(ii) coordination compounds via topochemical [4 + 4] photocycloaddition of an anthracene-based ligand

Abstract

Coordination polymers (CPs) designed to couple spin crossover (SCO) and photoluminescence (PL) properties hold significant potential for sensor applications, enabling optical readout of spin states. We report the synthesis of the novel ligand L1 (1-(anthracen-9-ylmethyl)-1H-tetrazole), which undergoes a reversible [4 + 4] photocycloaddition to form the head-to-tail dimer L2 – a ditopic tetrazole ligand, key to the design of CPs. Utilizing these ligands, we synthesized a family of Fe(II) anthracene based SCO-PL coordination compounds with varying dimensionalities: 0D ([FeL1₄(CH₃CN)₂](ClO₄)₂·2CH₃CN, 1), 1D ([FeL1₂L2(CH₃CN)₂](ClO₄)₂·2CH₃CN, 2) and 2D ([FeL2₂(CH₃CN)₂](ClO₄)₂·2CH₃CN, 3). Notably, L1 retains its photoreactivity after coordination in 1, and the subsequent polymerization of 1 to 2 represents a rare example of a topochemical reaction. Magnetic studies confirmed thermally induced SCO behavior in all three coordination compounds (1–3), with increasing cooperativity correlating with greater structural rigidity. Variable-temperature PL spectra revealed an interplay between the spin transition and PL in 2 and 3, showing PL quenching in the low-spin state, which diminishes as the high-spin state population increases. Our findings provide a platform to further develop synergistic SCO-PL CPs with tunable magneto-optical properties, paving the way for future applications.