The multi-exciton formation via singlet exciton fission in organic semiconductors can potentially enhance solar power conversion efficiencies beyond the maximum power conversion efficiency. Singlet exciton fission (SEF) is the process of evolution of a photoexcited singlet exciton into two individual triplet excitons located on different chromophores, thereby producing two excitons at the expense of one photon. In the present contribution we provide compelling evidence on the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentancene dimers. We synthetically modulated the spin density distribution in a novel series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Remarkably we find that efficient iSEF is only observed for the phenyl-derivative while absent in other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a novel spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character. Our new perspective based on spin density distribution of the frontier molecular orbitals offers important insight into ultrafast iSEF process, and reveals a critical aspect to rationally design new materials.
Transient absorption of 10 μM of 2P-PDPP in chlorobenzene. (a) Contour diagram of the obtained broadband transient absorption spectra in between 50 fs to 1 nanosecond subsequent to 670 nm excitation. (b) Single wavelength kinetics at 504, 630 and 900 nm. Symbols are raw data and lines are multi-exponential fits from single wavelength kinetics analysis. (c) α-HOSOs of 2P-PDPP and 2P-TDPP (the symmetry equivalent b- orbitals are omitted). In 2P-PDPP a- and b- electrons are spatially separated because the SOs are polarized towards opposite pentacenes. In 2P-TDPP the SOs of a-and b-electrons are less polarized and involve the bridge more strongly.
Krishnapriya, K. C. et al. Spin density encodes intramolecular singlet exciton fission in pentacene dimers. Nat. Commun.10, 33 (2019).