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Intersystem Crossing Outcompetes Triplet-Pair Separation from 1(TT) below 270 K in Anthradithiophene Films
Eman M Bu Ali1,2, Arnau Bertran3, Gabriel Moise3
1School of Mathematical and Physical Sciences, The University of Sheffield, Sheffield S3 7RH, U.K.
Singlet fission (SF) is crucial for solar energy. New methods reveal SF in diF-TES-ADT is temperature-dependent below 270 K, with intersystem crossing dominating over triplet hopping.
Area of Science:
- Photochemistry
- Materials Science
- Spectroscopy
Background:
- Singlet fission (SF) and triplet-triplet annihilation (TTA) are key processes for enhancing solar energy conversion efficiency.
- The precise mechanism of SF remains incompletely understood, particularly distinguishing between coupled triplet pairs and independent triplets using transient absorption spectroscopy.
Purpose of the Study:
- To elucidate the singlet fission mechanism by differentiating between coupled triplet pairs and independent triplets.
- To investigate the temperature dependence of specific SF steps in the diF-TES-ADT model system.
Main Methods:
- Combined transient optical spectroscopy with magnetic field effects.
- Employed transient electron spin resonance (ESR) spectroscopy.
- Utilized temperature-dependent transient photoluminescence spectroscopy.
Main Results:
- The second step of SF (1(TT) ⇌ (T..T)) in diF-TES-ADT is highly temperature-dependent, unlike the first step (1S → 1(TT)).
- Transient ESR confirmed the absence of SF between 40 and 250 K, showing triplets from intersystem crossing and decay via TTA.
- Magnetic field effects supported these findings.
Conclusions:
- In polycrystalline diF-TES-ADT, intersystem crossing outcompetes triplet hopping below 270 K.
- This enables direct intersystem crossing from 1(TT) to localized T1 states.
- Generated triplets can undergo triplet-triplet annihilation.
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