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.
Highlights
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
The calculations reveal that the spectral differences of 2P-TDPP and 2P-SeDPP compared to 2PPDPP are caused by stronger delocalization of the frontier orbitals and smaller energy gaps
In a dimer such as 2P-PDPP the triplets generated by intramolecular singlet exciton fission (iSEF) will reside on the two pentacene units, separated by the PDPP bridge
Summary
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. We present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. 1234567890():,; Singlet exciton fission (SEF) is a spin-allowed process that was first discovered in anthracene crystals in 19651 It involves the absorption of one photon by a singlet ground state molecule, followed by separation of the photoexcited singlet excited state into two triplets residing on two different chromophores. Acene dimers have proven more fruitful, with several reports of nearly quantitative intramolecular singlet fission yields in a wide range of structures[11,15,16,17] These species are evidence for tunability of rate and yield of iSEF, and even changes in underlying mechanism[16,17] through the electronic nature of the bonding interaction. Theoretical analysis of the spin orbitals and S1 states reveal that spin localization and partial 1TT character of S1, explains the rapid singlet fission of the phenyl derivative
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