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Issue 36, 2017
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Using molecular vibrations to probe exciton delocalization in films of perylene diimides with ultrafast mid-IR spectroscopy

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Abstract

Ultrafast vibrational spectroscopy in the mid-infrared was used to directly probe the delocalization of excitons in two different perylenediimide (PDI) derivatives that are predicted to preclude the formation of excimers, which can act as trap sites for excited state energy in organic semiconductors. We identified vibrational modes within the conjugated C–C stretch modes of PDI molecules whose frequencies reported the interactions of molecules within delocalized excitonic states. The vibrational linewidths of these modes, which we call intermolecular coordinate coupled (ICC) modes, provided a direct probe of the extent of exciton delocalization among the PDI molecules, which was confirmed using X-ray diffraction and electro-absorption spectroscopy. We show that a slip-stacked geometry among the PDI molecules in their crystals promotes delocalized charge-transfer (CT) excitons, while localized Frenkel excitons tend to form in crystals with helical, columnar stacking geometries. Because all molecules possess vibrational modes, the use of ultrafast mid-infrared spectroscopy to measure ICC vibrational modes offers a new approach to examine exciton delocalization in a variety of small molecule electron acceptors for optoelectronic and organic photovoltaic applications.

Graphical abstract: Using molecular vibrations to probe exciton delocalization in films of perylene diimides with ultrafast mid-IR spectroscopy

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Publication details

The article was received on 17 Jul 2017, accepted on 29 Aug 2017 and first published on 29 Aug 2017


Article type: Paper
DOI: 10.1039/C7CP04819J
Citation: Phys. Chem. Chem. Phys., 2017,19, 24829-24839
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    Using molecular vibrations to probe exciton delocalization in films of perylene diimides with ultrafast mid-IR spectroscopy

    E. R. Kennehan, C. Grieco, A. N. Brigeman, G. S. Doucette, A. Rimshaw, K. Bisgaier, N. C. Giebink and J. B. Asbury, Phys. Chem. Chem. Phys., 2017, 19, 24829
    DOI: 10.1039/C7CP04819J

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