Effects of the Separation Distance between Two Triplet States Produced from Intramolecular Singlet Fission on the Two-Electron-Transfer Process

Abstract

To harvest two triplet excitons of singlet fission (SF) via a two-electron transfer efficiently, the revelation of the key factors that influence the two-electrontransfer process is necessary. Here, by using steady-state and transient absorption/ fluorescence spectroscopy, we investigated the two-electron-transfer process from the two triplet excitons of intramolecular SF (iSF) in a series of tetracene oligomers (dimer, trimer, and tetramer) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) as an electron acceptor in solution. Quantitative two-electron transfer could be conducted for the trimer and tetramer, and the rate for the tetramer is faster than that for the trimer. However, the maximum efficiency of the two-electron transfer in the dimer is relatively low (∼47%). The calculation result of the free energy change (ΔG) of the secondelectron transfer for these three compounds (−0.024, −0.061, and −0.074 eV for the dimer, trimer, and tetramer, respectively) is consistent with the experimental observation. The much closer ΔG value to zero for the dimer should be responsible for its low efficiency of the two-electron transfer. Different ΔG values for these three oligomers are attributed to the different Coulomb repulsive energies between the two positive charges generated after the two-electron transfer that is caused by their various intertriplet distances. This result reveals for the first time the important effect of the Coulomb repulsive energy, which depends on the intertriplet distance, on the two-electron transfer process from the two triplet excitons of iSF.

Employing Singlet Fission into Boosting the Generation of Singlet Oxygen and Superoxide Radicals for Photooxidation Reactions

“…This is coincident with the previous observation that two free triplets of SF could transfer electron quantitatively to the electron acceptor. 65,75 Notably, the TT state is also quenched slightly by O 2 , which may be caused by the TTET process from the triplet or quintet sublevels of the TT state to O 2 . This is consistent with the fact that the Φ Δ value in oxygen-saturated toluene/CHCl 3 is lower slightly than the corresponding T 1 yield.…”

Unraveling Triplet Formation Mechanisms in Acenothiophene Chromophores

“…Historically, this has involved the enhancement of intersystem crossing (ISC) rates through the design of molecules with singlet–triplet energy level degeneracies − or by incorporating heavy atoms to increase spin–orbit coupling . A related approach involves the incorporation of heteroatoms with low energy nonbonding orbitals, which facilitates fast triplet formation through a process in which a change in spin angular momentum can be offset by a change in orbital angular momentum (El-Sayed rule). , More recently, a strategy for triplet generation in organic molecules has emerged in the form of multichromophore systems capable of singlet fission (SF), which includes a growing list of small molecule chromophores, oligomers, and polymers. − The advantage of SF materials is that a photogenerated singlet exciton rapidly decays into a pair of triplet excitons, allowing for enhanced triplet yields with a quantum yield above 1 on ultrafast time scales. − In optoelectronic applications, the formation of the triplet pair facilitates the utilization of energy that would normally be lost as heat. , Furthermore, it has recently been proposed that photochemical applications may directly benefit from the distinct properties of the triplet pair compared to an individual triplet. , As such, a key challenge is distinguishing between processes that generate an individual triplet versus those that generate a triplet pair. ,, This is necessary to both properly characterize quantum yields and design harvesting interfaces that account for their differing lifetimes and recombination pathways.…”

“…27,28 Furthermore, it has recently been proposed that photochemical applications may directly benefit from the distinct properties of the triplet pair compared to an individual triplet. 29,30 As such, a key challenge is distinguishing between processes that generate an individual triplet versus those that generate a triplet pair. 29,31,32 This is necessary to both properly characterize quantum yields and design harvesting interfaces that account for their differing lifetimes and recombination pathways.…”

Intramolecular singlet fission and triplet exciton harvesting in tetracene oligomers for solar energy conversion

“…19). 184 In these compounds, the lifetimes of the triplet state are almost identical, but the separation distances between the two triplet states are different. This allowed us to reveal the effect of the separation distance between the two triplet excitons on the electron-transfer process independently.…”

“…(f) Plots of the pseudo-first-order rate constant (k obs ) vs. concentration of TCNQ. Reproduced with permission 184. Copyright 2022, the American Chemical Society.…”