Spin density encodes intramolecular singlet exciton fission in pentacene dimers.

TLDR: The significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers is presented and phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes are revealed.

Abstract: The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character. Singlet exciton fission – the separation of photoexcited singlet states into two triplet states – holds promise for enhancing photocurrents in photovoltaic technologies. Krishnapriya et al. characterize how electron delocalization over the bridges in a series of pentacene dimers controls this process.