Ab-DMRG Perspective for the Fundamentals and Analysis of Singlet Fission Materials

Abstract

Organic singlet fission (SF) materials have emerged as a new promising mechanism to potentially break the Shockley-Queisser limit, thus raising the peak efficiency of solar power conversion for photonic devices. However, the details of SF molecular machinery are experimentally poorly understood, and computationally challenging as well, due to a variety of fundamental questions associated with the quantum mechanical many-body nature of transient excited states. Inherent electronic issues such as double excitation and strong correlation which jointly determine SF efficiency have plagued application of the popular time-dependent density functional theory to SF materials. In this work, we have employed high-level multireference ab-initio Density Matrix Renormalization Group (ab-DMRG) computation on the low-lying singlet and triplet electronic states of pentacene dimer, i.e., a commonly established model system in literature. We have implemented an extensive wavefunction analysis tool, based on computed ab-DMRG one- and two-particle state and transition density matrices for providing an adequate notion of the nature of SF charge transfer between fragments and excited states. Projected local spin values are computed for specific fragments in the dimer to represent a concise spin distribution of excited states, and they quantify the direct evidence for SF processes that was previously unattainable. Vibronic coupling density analysis has been implemented and carried out to reveal the coupling mechanism between electronic and vibrational degrees of freedom for driving SF charge transfer. Finally, molecular strategies for designing more efficient SF candidates will be discussed based on all computed results.