The Quantum Coherent Mechanism for Singlet Fission: Experiment and Theory

Abstract

The absorption of one photon by a semiconductor material usually creates one electron-hole pair. However, this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron-hole pairs. This process, where a singlet exciton transforms to two triplet excitons, can have quantum yields as high as 200%. Singlet fission may be useful to solar cell technologies to increase the power conversion efficiency beyond the so-called Shockley-Queisser limit. Through time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene, our lab has recently provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state (also called a correlated triplet pair). More importantly, we found that population of the multiexciton state rises at the same time as the singlet state on the ultrafast time scale upon photoexcitation. This observation does not fit with the traditional view of singlet fission involving the incoherent conversion of a singlet to a triplet pair. However, it provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately after optical excitation. In this Account, we review key experimental findings from TR-2PPE experiments and present a theoretical analysis of the quantum coherent mechanism based on electronic structural and density matrix calculations for crystalline tetracene lattices. Using multistate density functional theory, we find that the direct electronic coupling between singlet and multiexciton states is too weak to explain the experimental observation. Instead, indirect coupling via charge transfer intermediate states is two orders of magnitude stronger, and dominates the dynamics for ultrafast multiexciton formation. Density matrix calculation for the crystalline tetracene lattice satisfactorily accounts for the experimental observations. It also reveals the critical roles of the charge transfer states and the high dephasing rates in ensuring the ultrafast formation of multiexciton states. In addition, we address the origins of microscopic relaxation and dephasing rates, and adopt these rates in a quantum master equation description. We show the need to take the theoretical effort one step further in the near future by combining high-level electronic structure calculations with accurate quantum relaxation dynamics for large systems.

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

“…2 to include two-electron couplings is straightforward (and essentially invokes a direct coupling between the singlet excited states and triplet pairs, H s1−t1t1 ). Nevertheless, their magnitudes are known to be an order of magnitude smaller than the one-electron integrals discussed above, 4,17,22,28,36 and a significant contribution of such couplings to fission dynamics is therefore unlikely, at least in systems like pentacene. 28 We have performed additional calculations including typical values for the two-electron couplings, and do not find noteworthy differences for the results reported in this work.…”

Singlet fission in linear chains of molecules

“…This should justify the ultrafast dynamics, in conjunction with high density of ME states and dephasing produced by environmental effects. 32 Casanova studied tetracene and two of its derivatives with ab initio quantum chemical calculations, excluding in all the cases the role of CT states as possible intermediates of the SF mechanism, but confirming their role for second order coupling. 37 Recently, Van Voorhis group calculated the energy levels and the electronic coupling between S1 and a TT state for dimers belonging to the crystallographic structures of different pentacene and tetracene derivatives, using constrained DFT (CDFT) and CDFT based configuration interaction (CDFT-CI).…”

“…[32][33][34][35] Zimmermann et al employed post-HF methods to study a pentacene dimer model. The ME state was characterized as the intermediate state leading to SF.…”

“…46 The results for this kind of approach suggest that SF occurs via a "super-exchange" mediated mechanism, through CT states, even if these states possess very high energies. [32][33][34] Very recently, Reichman's group extended the application of this theoretical framework to the study of crystalline pentacene. 47 This was achieved with a combination of ab initio calculations and semi-empirical modeling.…”

Coherent Dynamics of Mixed Frenkel and Charge-Transfer Excitons in Dinaphtho[2,3-b:2′3′-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization

“…CT states can act as precursors for interfacial CT states [33][34][35]; mixing of CT states with Frenkel excitons would facilitate charge separation in photovoltaic materials. They have also gained recent attention due to their relevance to singlet fission [36][37][38], in which singlet to triplet conversion can proceed via sequential CT steps [39][40][41].Here we focus on dinaphtho[2,3-b:, a p-type OSC originally introduced by Takimiya and co-workers [42]. DNTT and its derivatives [5,[43][44][45][46][47][48][49] have gained attention due to their high hole mobility values and air stability.…”