Tetracene is an archetypal material undergoing singlet fission—the generation of a pair of triplet excitons from one singlet exciton. Here, using time-resolved electron spin resonance, we show how the spin dynamics in tetracene crystals are influenced by temperature and morphology. Upon cooling from 300 to 200 K, we observe a switch between singlet fission and intersystem crossing generated triplets, manifesting as an inversion in transient spin polarization. We extract a spin dephasing time of approximately 40 ns for fission-generated triplets at room temperature, nearly 100 times shorter than the dephasing time that we measure for triplets localized on isolated tetracene molecules. These results highlight the importance of morphology and thermal activation in singlet fission systems. In addition, we present recent findings on single crystal tetracene studied using optically detected magnetic resonance, where additional features in the spectrum are observed.
We acknowledge support from DFG SPP-1601 (BI 464/10-2, BE 5126/1-2) and the Nanoscale project within the excellence initiative of the Freie Universität Berlin. The work at Nanjing University is supported by the National Natural Science Foundation of China (21873047). We thank L. R. Weiss for insightful discussions.