Exciton Correlations in Intramolecular Singlet Fission

Abstract

We have synthesized a series of asymmetric pentacene-tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet-triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases.

The entangled triplet pair state in acene and heteroacene materials

“…We find that non-radiative decay of 1 (TT) is a dominant loss mechanism limiting further exploitation. Similarly fast non-radiative 1 (TT) decay has been observed in covalent acene dimers2463, carotenoid aggregates6264 and polymers566566, and understanding the mechanism of this process and minimizing it could allow for efficient solar cells with direct charge transfer from 1 (TT). Our work also has critical implications for TTA upconversion efficiency models67, where the implicit assumption is that the singlet-character TTA encounter complex (that is, 1 (TT)) converts to S 1 with unity efficiency.…”

“…Beyond applications of singlet fission and TTA, a clearer understanding of the 1 (TT) state will shed light on the nature of the interaction between triplet excitons and may open the way to new physics involving entangled bosons1011. Despite significant theoretical effort1011121314151617181920, few experiments have succeeded in directly probing the 1 (TT) state in any system2122232425, particularly in solid films relevant to eventual singlet fission devices. We are aware of a recent study of the 1 (TT) state in solid films of 6,13-Bis(triisopropylsilylethynyl) (TIPS)-tetracene, which crystallizes into a structure unique among singlet fission materials26.…”

Triplet Transfer Mediates Triplet Pair Separation during Singlet Fission in 6,13‐Bis(triisopropylsilylethynyl)‐Pentacene

“…Any contribution to the signal measured at 520 nm from transient absorptions of the excited S 1 population is also known to decay on the sub‐100 fs time scale of initial CTP formation and does not affect the transient absorption kinetics measured on the picosecond and longer time scales. Therefore, the T 1 → T n transition probed at 520 nm provides a measure of the triplet formation dynamics . We fit the growth kinetics with a biexponential function to quantify the amplitudes and time constants describing the singlet fission dynamics .…”

Singlet Fission: Progress and Prospects in Solar Cells

“…Dimers serve as an excellent platform for studying this state, as the two triplets are trapped adjacent to each other and cannot diffuse apart. Very recently, Sanders et al studied the triplet pair dynamics in a series of asymmetric pentacene−tetracene (PTn, n = 0, 1, 2) heterodimers derived from the bridge motif. The presence of long‐lived, delayed fluorescence in these compounds reveals a lifetime for correlated triplet pair states with singlet character can persist at least 50 ns.…”