Competition between Singlet Fission and Charge Separation in Solution-Processed Blend Films of 6,13-Bis(triisopropylsilylethynyl)pentacene with Sterically-Encumbered Perylene-3,4:9,10-bis(dicarboximide)s

The photophysics and morphology of thin films of <i>N</i>,<i>N</i>-bis­(2,6-diisopropylphenyl)­perylene-3,4:9,10-bis­(dicarboximide) (<b>1</b>) and the 1,7-diphenyl (<b>2</b>) and 1,7-bis­(3,5-di-<i>tert</i>-butylphenyl) (<b>3</b>) derivatives blended with 6,13-bis­(triisopropylsilylethynyl)­pentacene (TIPS-Pn) were studied for their potential use as photoactive layers in organic photovoltaic (OPV) devices. Increasing the steric bulk of the 1,7-substituents of the perylene-3,4:9,10-bis­(dicarboximide) (PDI) impedes aggregation in the solid state. Film characterization data using both atomic force microscopy and X-ray diffraction showed that decreasing the PDI aggregation by increasing the steric bulk in the order <b>1</b> < <b>2</b> < <b>3</b> correlates with a decrease in the density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ∼100 nm solution-processed TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the TIPS-Pn results in competition between ultrafast singlet fission (^1*TIPS-Pn + TIPS-Pn → 2 ^3*TIPS-Pn) and charge transfer from ^1*TIPS-Pn to PDIs <b>1</b>–<b>3</b>. As the blend films become more homogeneous across the series TIPS-Pn:PDI <b>1</b> → <b>2</b> → <b>3</b>, charge separation becomes competitive with singlet fission. Ultrafast charge separation forms the geminate radical ion pair state ¹(TIPS-Pn^+•–PDI^–•) that undergoes radical pair intersystem crossing to form ³(TIPS-Pn^+•–PDI^–•), which then undergoes charge recombination to yield either ^3*PDI or ^3*TIPS-Pn. Energy transfer from ^3*PDI to TIPS-Pn also yields ^3*TIPS-Pn. These results show that multiple pathways produce the ^3*TIPS-Pn state, so that OPV design strategies based on this system must utilize this triplet state for charge separation