Role of spin density and interchromophoric coupling in singlet fission

Abstract

Singlet exciton fission (SEF) is the transformation of a photoexcited singlet exciton into two triplets localized on two chromophores coupled together. This process therefore produces two triplet excitons at the expense of one photon and hence can potentially improve the solar cell efficiency. An investigation into the excited state dynamics of three potential SEF candidates, pentacene dimers linked via diketopyrrolopyrrole derivative bridges, elucidates the significance of electronic spin density distribution in facilitating efficient intramolecular SEF (iSEF) in π-bridged pentacene dimers. Based on transient absorption spectroscopy measurements, efficient iSEF was only observed for the phenyl-derivative as only the 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. An extraordinary correlation between the energy of excitation and SEF dynamics has been observed. The triplet absorption feature evolving as well as decaying at slower rate with decreasing energy of excitation was studied using transient absorption and photoluminescence measurements. This color sensitivity and origin of longer lived triplet species generated upon higher energy photoexcitation are analyzed to form a photophysical model. This contribution discourses the possibility multiple mechanistic pathways possible for an intramolecular dimer system to undergo SEF. In an attempt to experimentally analyze the practical challenges of increasing biradicaloid character of small molecules, a series of dicyanomethylated diketopyrrolopyrrole derivatives and their exciton dynamics was explored. For the most conjugated molecule SEF like features are observed in the fs TA of thin film which needs to be further investigated.