Magneto‐Structural Correlations in a Mixed Porphyrin(Cu2+)/Trityl Spin System: Magnitude, Sign, and Distribution of the Exchange Coupling Constant

The combination of multi-frequency continuous-wave electron paramagnetic resonance and density functional theory calculations were used to determine the magnitude, sign, and distribution of the exchange coupling constant J in a biradical consisting of a trityl radical and a copper(II)tetraphenylporphyrin. The ferromagnetic exchange coupling and its distribution are related to the structure and dynamics of the biradical.

Abstract

Tetrathiatriarylmethyl radicals (TAM or trityl) are receiving increasing attention in various fields of magnetic resonance such as imaging, dynamic nuclear polarization, spin labeling, and, more recently, molecular magnetism and quantum information technology. Here, a trityl radical attached via a phenyl bridge to a copper(II)tetraphenylporphyrin was synthesized, and its magnetic properties studied by multi-frequency continuous-wave electron paramagnetic resonance (EPR) spectroscopy and magnetic measurements. EPR revealed that the electron spin-spin coupling constant J between the trityl and Cu²⁺ spin centers is ferromagnetic with a magnitude of −2.3 GHz (−0.077 cm⁻¹, convention) and a distribution width of 1.2 GHz (0.040 cm⁻¹). With the help of density functional theory (DFT) calculations, the obtained ferromagnetic exchange coupling, which is unusual for para-substituted phenyl-bridged biradicals, could be related to the almost perpendicular orientation of the phenyl linker with respect to the porphyrin and trityl ring planes in the energy minimum, while the J distribution was rationalized by the temperature weighted rotation of the phenyl bridge about the molecular axis connecting both spin centers. This study exemplifies the importance of molecular dynamics for the homogeneity (or heterogeneity) of the magnetic properties of trityl-based systems.

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