Theoretical Modeling of Singlet Fission

Abstract

Singlet fission is a photophysical reaction in which a singlet excited electronic state splits into two spin-triplet states. Singlet fission was discovered more than 50 years ago, but the interest in this process has gained a lot of momentum in the past decade due to its potential as a way to boost solar cell efficiencies. This review presents and discusses the most recent advances with respect to the theoretical and computational studies on the singlet fission phenomenon. The work revisits important aspects regarding electronic states involved in the process, the evaluation of fission rates and interstate couplings, the study of the excited state dynamics in singlet fission, and the advances in the design and characterization of singlet fission compounds and materials such as molecular dimers, polymers, or extended structures. Finally, the review tries to pinpoint some aspects that need further improvement and proposes future lines of research for theoretical and computational chemists and physicists in order to further push the understanding and applicability of singlet fission.

Quantum Master Equation Approach to Singlet Fission Dynamics in Pentacene Linear Aggregate Models: Size Dependences of Excitonic Coupling Effects

“…Thus, there has been so far a great deal of theoretical and experimental studies on revealing the mechanism of SF, searching the key factors for controlling the SF efficiency, and constructing molecular and material design guidelines for efficient SF systems. [5][6][7][8][9][10] The SF study is composed of three steps concerning (i) energy level matching at the single molecular level, (ii) electronic coupling at the molecular aggregate level, and (iii) exciton dynamics including vibronic couplings. [9] In step (i), Smith and Michl presented requirements for efficient SF molecules based on thermodynamic and kinetic conditions, that is, the energy level matching conditions for the monomer: (a) 2E (T 1 )~E(S 1 ) or 2E(T 1 ) < E(S 1 ) and (b) 2E(T 1 ) < E(T 2 ), [5,6] where S 1 and T 1 indicate the lowest singlet and triplet exciton states, respectively, and T 2 the second triplet exciton state.…”

“…[11][12][13] It states that molecules with relatively small diradical character (~0.1 < y 0 <~0.5) as well as with much smaller tetraradical character (y 1 /y 0 <~0.2) tend to satisfy these two conditions. [10] Step (ii) focuses on the electronic couplings between monomers in aggregates, [5,6] which are related to the transition probability between the exciton states, that is, singlet Frenkel exciton (FE) state (e.g., S 1 S 0 ), charge transfer (CT) exciton state (e.g., CA, where C and A indicate cation and anion monomers, respectively) and double-triplet (correlated triplet pair) exciton state 1 (T 1 T 1 ), which is referred to as TT hereafter. Namely, in addition to the energy level matching conditions in step (i), design of intermolecular configuration (crystal packing) is found to be indispensable for describing the SF rate, which is proportional to transition probability from FE to TT state.…”

Application of Intramolecular Singlet Fission in Photovoltaics: Control over Multiexciton Generation and Triplet–Triplet Annihilation

“…Inspired by these factors, many researchers seek an approach to enable direct charge carrier extraction from the 1 (TT) state of iSF chromophores. Recent advances in understanding this complex quantum state have helped to realize the viability of 1 (TT) state as a multiple electron donor . The first reported success at harvesting multiple electrons from the 1 (TT) state of an iSF chromophore was by Kim et al .…”

“…Recent advances in understanding this complex quantum state have helped to realize the viability of 1 (TT) state as a multiple electron donor. 5 The first reported success at harvesting multiple electrons from the 1 (TT) state of an iSF chromophore was by Kim et al based on a theoretically designed model system. 6 The model system consisted of quinoidal bithiophene with β,β'-solubilizing groups (QOT2), found to undergo iSF, and the two-electron acceptor anthraquinone (AQ).…”

Potential Use of Squarates and Croconates as Singlet Fission Sensitizers

“…One alternative may be the downconversion of short wavelength photons. In organic materials this phenomenon is called singlet fission (SF) [57,58,59] and consists in a (spin-allowed) photophysical process in which one singlet excited state splits into two triplet states to generate two low-energy excitons per absorbed photon. In addition to the general requirements for solar cell sensitizers, singlet fission chromophores must fulfill two additional energy conditions: [57] (i) the energy of the first singlet excited state (E[S 1 ]) should be slightly higher (or equal) to twice the energy of the first triplet state (E[T 1 ]), in such a way that the process is exoergic (or isoergic).…”