Electron Spin-Lattice Relaxation in Triplet-State Oligoacenes: A First-Principles-Based Approach

Organic chromophores in photo-excited triplet states can transiently exhibit uneven population distribution among spin sublevels, which is called spin polarization. Spin polarization can be realized even at room temperature and low magnetic fields, and it has recently attracted much attention due to its potential application in quantum technologies such as quantum sensing and quantum computing. Molecular spin polarizing agents offer tunability of their physical properties through molecular modification, and from the perspective of molecular design, it is desirable to understand the structure-property relationships. One of the key properties of the polarizing agents is the spin-lattice relaxation time, which determines the lifetime of spin polarization. This process is governed by the complicated coupling between electron spins and phonons in the surrounding host environment, and many aspects remain poorly understood. Conventional theoretical studies have usually used modeled phonon spectral densities, however, these approaches overlook the detailed specificities of molecular environments, making it challenging to analyze molecular structures or packing effects on relaxation dynamics. Recently first-principles calculations have been increasingly applied to study spin relaxation for the system of metal complexes and solid-state paramagnetic defects. However, as far as we know, no reports have been published on the relaxation of excited organic molecules in triplet states. Here, in this study, we performed first-principles calculations of the electron spin-lattice relaxation of organic chromophores in triplet state using Redfield relaxation model and quantum chemical calculations. By examining the contribution of each molecular vibrational mode to the relaxation, it was found that librational motions make a dominant contribution to the relaxation because they directly fluctuate the principal axes of zero-field splitting. Additionally, it was discovered that in pentacene, the out-of-plane molecular vibrational mode has a non-negligible contribution to the relaxation, which may explain the anisotropy of the spin-lattice relaxation observed in the experiment, where transitions between \T{Y} and \T{Z} states occur relatively rapidly. First-principles calculation of spin dynamics will likely be helpful in elucidating the molecular mechanisms of spin relaxation.