Triplet Pair States in Singlet Fission

Abstract

This account aims at providing an understanding of singlet fission, i.e., the photophysical process of a singlet state ( S1 ) splitting into two triplet states (2 × T1 ) in molecular chromophores. Since its discovery 50 years ago, the field of singlet fission has enjoyed rapid expansion in the past 8 years. However, there have been lingering confusion and debates on the nature of the all-important triplet pair intermediate states and the definition of singlet fission rates. Here we clarify the confusion from both theoretical and experimental perspectives. We distinguish the triplet pair state that maintains electronic coherence between the two constituent triplets, 1(TT) , from one which does not, 1(T···T) . Only the rate of formation of 1(T···T) is defined as that of singlet fission. We present distinct experimental evidence for 1(TT) , whose formation may occur via incoherent and/or vibronic coherent mechanisms. We discuss the challenges in treating singlet fission beyond the dimer approximation, in understanding the often neglected roles of delocalization on singlet fission rates, and in realizing the much lauded goal of increasing solar energy conversion efficiencies with singlet fission chromophores.

Modulating the Electronic and Solid‐State Structure of Organic Semiconductors by Site‐Specific Substitution: The Case of Tetrafluoropentacenes

“…We believe that the details of the SF process could provide a possible explanation for the presence of a third species. During a SF process two neighboring, spin‐entangled triplet excitons are formed, which can be electronically coupled resulting in a multiexciton state 1 (TT) . This electronic coupling can be overcome by spatial separation of the triplets into T 1 +T 1 via a spin‐entangled, but electronically decoupled state 1 (T…T), as suggested recently .…”

“…This photophysical process converts an excited singlet state into two triplet states on neighboring molecules and has received increasing attention due to its potential to boost solar cell efficiencies above the Shockley–Queisser limit . It proceeds via a triplet pair state 1 (TT) in which the two molecules involved are already in their triplet state, but the overall character of the state is still a singlet . Most importantly, the triplets are electronically coupled and spin‐entangled .…”

“…[58,59] This electronic coupling can be overcome by spatial separation of the triplets into T 1 + +T 1 via as pin-entangled, but electronically decoupled state 1 (T…T), as suggested recently. [58] Thus, the difference between the two triplet species observed in the TA data of 12 might be the electronic coupling. In this case, species 2c an be assigned to 1 (TT) and species3 to 1 (T…T) or T 1 + + T 1 ,g iven that these latter two cannotb ed istinguished in TA.T he fact that 1 (TT) is observed as comparably long-lived speciesi n12 might be due to the lower long-range order in the thin film compared with 11,r educing the delocalization of the 1 (TT) and increasing the dephasing time.…”

“…[57] It proceeds via at riplet pair state 1 (TT) in which the two molecules involved are already in their triplet state, but the overall character of the state is still as inglet. [58] Most importantly,t he triplets are electronically coupled and spin-entangled. [59] Ap romising approach to optimize SF efficiencies for applicationsi s chemicalmodification,which in turn often affects molecular arrangement.…”

Ab InitioLinear and Pump–Probe Spectroscopy of Excitons in Molecular Crystals

“…39 Like other acene crystals, naphthalene exhibits a low-lying multi-exciton state -an overall singlet state that is qualitatively composed of two triplet excitons on neighboring molecules. [40][41][42] We shall refer to this state as S ME throughout. Multi-exciton states are of particular interest as precursors to singlet fission, since in principle they allow a single photon to generate two free charges.…”