Insights into the Spin-Lattice Dynamics of Organic Radicals Beyond Molecular Tumbling: A Combined Molecular Dynamics and
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Applied Magnetic Resonance
ORIGINAL PAPER
Insights into the Spin‑Lattice Dynamics of Organic Radicals Beyond Molecular Tumbling: A Combined Molecular Dynamics and Machine‑Learning Approach Alessandro Lunghi1 Received: 25 June 2020 / Revised: 7 August 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract The prediction of spin-lattice dynamics is a challenging computational and theoretical problem due to the complex interplay among atomic motion and spin interactions. Here, we employ machine-learning paradigms to generate a first-principlesaccurate and computationally inexpensive model that predicts both conformational energy and spin Hamiltonian parameters of an organic radical as function of its molecular distortions. This model is applied to the study of the g tensor correlation function during molecular dynamics in solution and it is shown that it is possible to access the time dependence of the spin Hamiltonian parameters without making assumptions on the type of molecular motion or spin-lattice dynamics’ time-scales involved. We further use these approach to disentangle inter- and intra-molecular contributions to the g tensor correlation function and provide new insights into the nature of vibronic-coupling in organic semiconductors.
1 Introduction The study of electronic spin-lattice dynamics provides a unique fingerprint of spin and its own chemical environment. Several Electronic Paramagnetic Resonance (EPR) applications are specifically designed to probe the spins’ environment, both in terms of molecular dynamics (MD) and electronic structure. For instance, traditional applications of EPR spectroscopy involve the use of organic radicals as spectroscopic probes for the study of bio-molecules dynamics [1], while EPR studies of transition metals compounds have long provided a fundamental mean to address the relation between electronic structure, coordination geometry and magnetism [2–4]. In recent years the study of spin-lattice dynamics has also found renewed interest in the context of molecular spintronics [5] and molecular quantum technologies [6].
* Alessandro Lunghi [email protected] 1
School of Physics, CRANN Institute and AMBER, Trinity College Dublin, Dublin 2, Ireland
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A. Lunghi
In these fields spin is used as a platform to store or process information. Long spin lifetimes and/or spin coherence times are required for practical applications of these technologies. The understanding of the processes leading to spin-lattice relaxation and how the resulting spectroscopic signals are affected is of central importance for all these applications. The study of spin phenomena is among the oldest efforts in quantum physics and the theory behind spin relaxation has been investigated in details. In a nutshell, the prediction of spin-lattice dynamics from first-principles requires the solution of the von Neumann equation for the spin density matrix under the effect of spin operators that change in time due to atomic thermal fluctuations. Although the theory of s
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