A kinetic model for photoswitching of magnetism in the high spin molecule [Mo(IV)(CN)₂(CN-Cu(II)(tren))₆](ClO₄)₈

Abstract

The heptanuclear complex [Mo(IV)(CN)₂(CN-CuL)₆]⁸⁺ switches from a paramagnetic dark state corresponding to six spin-½ Cu(II) ions to a predominantly high spin S=3 state, on prolonged irradiation with 406 nm laser radiation at low temperature. The system returns to a paramagnetic state on warming to room temperature. The temperature dependence of the χ_MT vs. T curve depends upon duration of irradiation. An earlier microscopic model showed that the excitation cross sections in different spin manifolds are similar in magnitude and that photomagnetism is not due to preferential excitation to the S=3 state. In this paper, we attribute photomagnetism to a long lived S=3 charge transfer excited state for which there appears to be sufficient experimental evidence. Based on this postulate, we model the photomagnetism by employing a kinetic model which includes internal conversions and intersystem crossings. The key feature of the model is the assumption of the existence of two kinds of S=3 states: one of which has no direct pathway for internal conversion and the other characterized by slow kinetics for internal conversion to the low-energy states. The trapped S=3 state can decay via a thermally activated barrier to the other S=3 state. The experimental χ_MT vs. T for two different irradiation times are fitted using Arrhenius dependence of the rate constants in the model.