Singlet heterofission in tetracene-pentacene thin-film blends

Singlet fission (SF), the photophysical process where an excited singlet state is converted into two triplets on the ultrafast timescales, is nowadays of utmost interest in the scientific community due to the capability to overcome the Shockley-Queisser limit of single junction solar cells. Among the different approaches to control the key parameters of SF, the combination of two different chromophores is a promising way to tune the energy balance and the electronic coupling. In this work we investigate, by means of ultrafast transient absorption spectroscopy, thin film blends of two prototypical SF chromophores: pentacene (PEN) and tetracene (TET). In blends of 5% PEN we observe, for the first time, heterofission of a singlet from PEN to two triplets on one PEN and one TET, after direct excitation of PEN chromophores. For PEN concentrations exceeding 5% there is no evidence of this process, since homofission outcompetes heterofission thanks to its exothermicity. We therefore show that intermolecular heterofission is observable in weakly interacting systems bridging the gap between studies of doped single crystals and strongly coupled systems such as heterodimers.