Intersystem Crossing Outcompetes Triplet-Pair Separation from ¹(TT) below 270 K in Anthradithiophene Films

Singlet fission (SF) and triplet-triplet annihilation (TTA) are processes which may be exploited to boost the efficiency of solar energy technology. Despite being studied since the late 1960s, the mechanism of singlet fission is still not fully understood. This is partly because the main technique used to study singlet fission, optical or visible/near-IR transient absorption spectroscopy, cannot distinguish between the strongly coupled triplet-pair state ¹(TT), weakly interacting triplet pairs (T..T), and independent triplet states T₁ + T₁. To solve this problem, we combine transient optical spectroscopy performed as a function of magnetic field and transient electron spin resonance (ESR) spectroscopy to probe the different steps involved in the singlet fission mechanism. By using transient photoluminescence spectroscopy performed as a function of magnetic field to selectively probe the second step of singlet fission: ¹(TT) ⇌ (T..T), we show that in a well-studied model system, anthradithiophene (diF-TES-ADT), this step is highly temperature-dependent, even though the first step, ¹S → ¹(TT), is not. Transient ESR measurements confirm the absence of singlet fission at temperatures between 40 and 250 K for this system, with clear signatures of triplets generated by intersystem crossing and evidence for decay by triplet-triplet annihilation, further supported by magnetic field effect measurements. We conclude that in polycrystalline diF-TES-ADT, intersystem crossing outcompetes triplet hopping at temperatures below 270 K, enabling direct intersystem crossing from the bound triplet pair ¹(TT) to an independent triplet state T₁ localized on a single chromophore. The generated triplets can re-encounter and decay through triplet-triplet annihilation.