A Microscopic Model of Singlet Fission

Abstract

Singlet fission, where an electronically excited singlet on one chromophore converts into a doubly excited state on two, has gone from a curiosity in organic photophysics to a potential pathway for increasing solar energy conversion efficiencies. Focusing on the role of solvent-induced energy level fluctuations that would be present in a dye-sensitized solar cell, we present a microscopic model for singlet fission. Starting from an electronic model Hamiltonian, we construct diabatic states in a manifold of single and double excitations with total singlet multiplicity and then develop a multilevel non-Markovian theory of dynamics for electronic populations in the presence of energy level fluctuations. Depending on the energy scales, energy gap fluctuations can either facilitate or hinder interconversion steps that lead to singlet fission. We critically assess the Markovian approximation that leads to golden rule rates and study the role of intramolecular solvation dynamics and electron transfer.

Microscopic theory of singlet exciton fission. I. General formulation

“…9 The first is a mediated mechanism, whereby a charge transfer state acts as an intermediate in the transition from S 1 to T T ; theoretical studies of this mechanism in coupled molecular dimers have considered the static electronic parameters, 26 as well as the real-time dynamics in the limit of fast coherent transfer 27 and in the presence of a low-frequency solvent bath. 28 Alternatively, a direct mechanism has also been implicated, whereby the Coulomb potential yields a direct interaction between S 1 and T T , avoiding any intermediates. Some authors have invoked such a proposal to explain fission in crystalline tetracene and pentacene, based both on experiment 19 and quantum chemistry calculations of clusters.…”

Vibronic exciton theory of singlet fission. I. Linear absorption and the anatomy of the correlated triplet pair state

“…A viable approach to bridge the gap between these calculations and measurements of singlet fission is through microscopic modeling. Such arXiv:1703.01173v2 [cond-mat.mtrl-sci] 11 May 2017 has been proven successful in several studies on fission materials, [24][25][26][27][28][29][30] however, applications did not address the fission dynamics, 24,26,30 or were limited to a perturbative treatment of electronic couplings 25 or the vibrational degrees of freedom. [27][28][29] The present article forms the first entry in a series of studies in which we present a microscopic vibronic exciton model aimed at unraveling the mechanistic principles underlying singlet fission, with a particular emphasis placed on the role of vibronic coupling.…”

Understanding Electronically Non-Adiabatic Relaxation Dynamics in Singlet Fission

“…Here, E j is the energy of state j, V jk is the (effective) electronic coupling between state j and k, and 2λ is the reorganization energy for the energy transfer case (note the λ = π −1 ∫ dωJ(ω)/ ω is the reorganization energy for electron transfer processes). 19 c. Model System. We take the three-state model 17 for pentacene as an example to study the nonadiabatic relaxation dynamics in SF, that is,…”