Quintet multiexciton dynamics in singlet fission

Abstract

Singlet fission (SF), in which two triplet excitons are generated from a single absorbed photon, is a key third generation solar cell concept. Conservation of angular momentum requires that SF populates correlated multiexciton states, which can subsequently dissociate to generate free triplets. However, little is known about electronic and spin correlations in these systems since, due to its typically short lifetime, the multiexciton state is challenging to isolate and study. Here, we use bridged pentacene dimers, which undergo intramolecular SF while isolated in solution and in solid matrices, as a unimolecular model system that can trap long-lived multiexciton states. We combine transient absorption and time-resolved electron spin resonance (ESR) spectroscopy to show that spin correlations in the multiexciton state persist for 100s of nanoseconds. Furthermore, we confirm longstanding predictions that SF produces triplet pair states (TT) of quintet character -the first direct observation of quintet states in closed shell organic molecules. We compare two different pentacene-bridge-pentacene chromophores, systematically tuning the coupling between the pentacenes to understand how differences in molecular structure affect the population and dissociation of multiexciton quintet states.

Unified model for singlet fission within a non-conjugated covalent pentacene dimer

“…The pentacenes in NC are coupled just strongly enough to allow efficient SF before any other excited state processes occur, but weakly enough that the 1 (T 1 T 1 ) state formed by SF can undergo spin-mixing to form the 5 (T 1 T 1 ) on a timescale sufficiently slow to allow time-resolved electron paramagnetic resonance (TREPR) spectroscopy to observe both its formation and decay into independent triplet states (T 1 +T 1 ). This contrasts with recent observations of quintet states formed by SF in tetracene films and conjugated pentacene dimers1516, and allows us to use complementary time-resolved optical and TREPR spectroscopy to observe the intermediates and kinetics for the entire 1 (S 1 S 0 )   1 (T 1 T 1 )   5 (T 1 T 1 )→(T 1 +T 1 ) sequence. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.…”

“…It is also possible that the 1 (T 1 T 1 ) state can undergo spin evolution to yield the 5 (T 1 T 1 ) quintet state, which upon spin decoherence can produce two non-interacting triplet states (T 1 +T 1 )1516. In addition, it has recently been suggested that the two separated T 1 states can be populated directly from the 1 (S 1 S 0 ) state without participation of any intermediate electronic states12.…”

Fluctuating exchange interactions enable quintet multiexciton formation in singlet fission

“…The spectra at the end of the time evolutions, ′ (| |, τ , 0 ), are generated using the resfields function in EasySpin 5.2.25 31,32 and a methodology that we have previously described. 8 We use the probability distribution function (as shown in figure 2),…”

Using Structurally Well-Defined Norbornyl-Bridged Acene Dimers to Map a Mechanistic Landscape for Correlated Triplet Formation in Singlet Fission

“…A full assignment will require spin-sensitive measurements such as time-resolved EPR. 15,26,36,37 Discussion of a Common Model. As these TIPS-BP1' studies will help us to understand data in the full series of molecules ( Fig.…”