Unraveling Triplet Formation Mechanisms in Acenothiophene Chromophores

Abstract

The evolution of molecular platforms for singlet fission (SF) chromophores has fueled the quest for new compounds capable of generating triplets quantitatively at fast time scales. As the exploration of molecular motifs for SF has diversified, a key challenge has emerged in identifying when the criteria for SF have been satisfied. Here, we show how covalently bound molecular dimers uniquely provide a set of characteristic optical markers that can be used to distinguish triplet pair formation from processes that generate an individual triplet. These markers are contained within (i) triplet charge-transfer excited state absorption features, (ii) kinetic signatures of triplet−triplet annihilation processes, and (iii) the modulation of triplet formation rates using bridging moieties between chromophores. Our assignments are verified by time-resolved electron paramagnetic resonance (EPR) measurements, which directly identify triplet pairs by their electron spin and polarization patterns. We apply these diagnostic criteria to dimers of acenothiophene derivatives in solution that were recently reported to undergo efficient intermolecular SF in condensed media. While the electronic structure of these heteroatom-containing chromophores can be broadly tuned, the effect of their enhanced spin−orbit coupling and low-energy nonbonding orbitals on their SF dynamics has not been fully determined. We find that SF is fast and efficient in tetracenothiophene but that anthradithiophene exhibits fast intersystem crossing due to modifications of the singlet and triplet excited state energies upon functionalization of the heterocycle. We conclude that it is not sufficient to assign SF based on comparisons of the triplet formation kinetics between monomer and multichromophore systems.

Revealing the Singlet Fission Mechanism for a Silane-Bridged Thienotetracene Dimer

“…We will show longer-time TA data that support this. A recent literature report on a related tetracenethiophene derivative also reports no significant ISC . The ∼100 ps timescale for 1 TT formation corresponds to the fast dynamics observed using TCSPC as discussed above.…”

“…While this simple, scalable, 1-step reaction sequence starting from an easily prepared thienotetracene produces the desired dimer in modest (∼20%) yield, the main byproduct is the TIPSTT starting material, which could be recovered and reused. Further, the overall yield is comparable to the multistep yield of similar reported acenothiophene dimers, 29 and the absence of any reaction step requiring transition metal catalysts eliminates any concern of contamination with heavy elements. To obtain the highest levels of material purity, chromatography of the dimer was performed both on traditional silica gel and size-exclusion stationary phases.…”

“…A recent literature report on a related tetracenethiophene derivative also reports no significant ISC. 29 The ∼100 ps timescale for 1 TT formation corresponds to the fast dynamics observed using TCSPC as discussed above. Although overlapping ESA and bleach spectra prohibit deriving a quantitative TT yield from TA alone, we can reasonably take the 90% emission quenching as an estimate (with 100% being complete conversion).…”

Singlet fission in TIPS-anthracene thin films

“…45–47 Sfeir and coworkers have reported ISC via T 2 taking place in anthracenothiophene dimers. 48 These results were confirmed by the polarization patterns from electron-paramagnetic resonance spectroscopy and not resolvable in their TA measurements due to the rapid internal conversion of T 2 → T 1 . ISC and its effects on the MPL are further investigated here with the model (Fig.…”