Singlet Fission among two Single Molecules

Singlet fission (SF) is a photophysical process where a singlet excitation generates two triplet excited states, enhancing exciton multiplication potentially useful for solar energy conversion. Since SF typically outcompetes radiative decay, single molecule studies of SF have remained elusive. Here, we present single molecule spectroscopy of a terrylenediimide (TDI) dimer at room and cryogenic temperatures. By analysing the stream of photons emitted by single dimers, the rates of formation and decay of SF-born triplet states were determined. We report considerable static and dynamic heterogeneities of the SF process which are reflected in broad rate distributions as well as the occasional occurrence of delayed fluorescence and rate fluctuations during spin evolution. Cryogenic experiments point to the formation of a coherent multiexciton superposition state which decays into the singlet exciton from which a correlated triplet pair evolves. Our results establish single molecule spectroscopy as a new avenue into mechanistic details of the SF process which often are drowned by ensemble av-eraging.

💡 Research Summary

This paper presents the first single‑molecule investigation of singlet fission (SF) using a terrylene‑diimide (TDI) dimer as a model system. The authors first characterize the dimer in bulk solution and in a solid zeonex matrix, showing that the absorption and fluorescence spectra are red‑shifted relative to the monomer and that the 0‑0/0‑1 vibronic intensity ratio increases, indicating exciton delocalization and electronic coupling (estimated dipole‑dipole coupling V ≈ 240 cm⁻¹). Time‑resolved fluorescence of the dimer reveals a biexponential decay: a dominant short component (≈2.2 ns, 95 %) and a minor long component (≈80 ns, 5 %) attributed to delayed fluorescence, whereas the monomer shows a single exponential decay (≈3.4 ns).

To probe SF at the single‑molecule level, the authors record photon streams from individual TDI molecules under continuous‑wave excitation and analyze the intensity autocorrelation function g²(τ). For monomers, a three‑level kinetic model (bright S₁, dark triplet, ground) yields an intersystem crossing rate k_ISC ≈ 1.2 × 10⁶ s⁻¹ and a triplet decay rate k_T ≈ 2.5 × 10⁴ s⁻¹, consistent with literature values.

For dimers, the g²(τ) curves display a much higher contrast and a much shorter bunching time, reflecting rapid transitions between a bright state and a dark SF‑generated state. Fitting the data with an effective three‑level model (bright S₁, SF‑born dark state, triplet) gives an overall SF formation rate k_SF ≈ 3 × 10⁸ s⁻¹—about three orders of magnitude faster than the monomer ISC rate—while the triplet decay rate remains essentially unchanged. Importantly, the extracted k_SF and the delayed‑fluorescence rate (k_delayed) vary widely from molecule to molecule, revealing substantial static heterogeneity due to variations in inter‑chromophore distance and orientation. Moreover, some molecules exhibit correlated temporal fluctuations of k_SF and k_delayed, indicating dynamic heterogeneity arising from slow spin‑evolution processes (mixing among ¹(TT), ³(TT), and ¹(T₁T₁) states).

Low‑temperature (1.4 K) high‑resolution absorption spectra of individual dimers show broadened features, which the authors interpret as evidence for the formation of a coherent superposition of the delocalized singlet ¹(S₁S₀) and the correlated triplet pair ¹(T₁T₁) on a sub‑100 fs timescale. This superposition rapidly decoheres into a pure singlet state, which then evolves through spin‑mixing to produce the observable triplet pair and ultimately non‑interacting triplets (T₁ + T₁).

Overall, the study demonstrates that single‑molecule fluorescence autocorrelation analysis can not only quantify the average SF rate but also uncover the distribution of rates and spin dynamics that are hidden in ensemble measurements. The findings provide valuable mechanistic insight for designing molecular dimers with optimized SF for solar‑energy conversion and for exploiting SF‑derived spin coherence in quantum information applications.