Organic electronic devices such as photovoltaics, light-emitting diodes (OLED’s), and field-effect transistors (OFET’s) are currently vigorously pursued as possibly inexpensive, processable replacements for the equivalent silicon-based devices. 1-3 One of the greatest obstacles for successful implementation of organic electronics is efficient electron transfer over long distances, i.e. high charge mobility. An additional challenge for photovoltaics in particular, is the requirement that photons over the entire solar spectrum must be efficiently absorbed. 2 Rylene imides have been both popular and successful constituents in organic molecular photovoltaic devices 4-7 because of their high absorption coefficients in the visible region, outstanding n-type conduction, 8 ease of synthetic modification, and excellent stability. The efficiency of electron or hole transport along monoreduced or oxidized chains of rylene imides remains an important, yet unexplored question.
My research focuses on intramolecular charge transfer among monoreduced/oxidized covalently bound linear (Figure 1) or cofacial perylene diimide assemblies, as well as self-assembled π-stacked systems. To facilitate these studies electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR), two of the most powerful tools used to explore charge delocalization or hopping among neighboring molecules, are employed. The isotropic hyperfine coupling constant (hfcc) depends upon the interaction of the electron and nuclear magnetic moments and is directly proportional to the spin density at the nucleus. In general, the odd electron density in aromatic radicals is delocalized over the π molecular orbitals, which are formed from the overlap of atomic carbon 2p z orbitals. The hfcc’s of a proton, a H, arise from π,σ-spin polarization, which can be directly related to the π-electron spin density on the carbon atom, ρ π, by McConnell’s relation, 9 a H = Qρ π, where Q is a proportionality constant.
Intramolecular electron hopping faster than 10 MHz is found to occur over two and three chromophores in monoreduced, perpendicularly bound, N-N linked perylene diimide systems. As seen in Figure 2, the ENDOR spectrum of the dimer shows a two-fold reduction of the hfcc’s compared to those of the monomer. This indicates that the unpaired electron is rapidly hopping between the two chromophores on the ENDOR timescale (10 MHz). Further insights into electron transfer processes within the oligomers are gained by exploring the temperature dependence in addition to counterion and solvent effects.
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(7) Shin, W. S. J., H.; Kim, Mi-Kyoung.; Jin, S.; Kim, Mi-Ra; Lee, J.; Lee, J. W.; Gal, Y. Journal of Materials Chemistry 2005, 16, 384.
(8) Jones, B. A. A., M. J.; Yoon, M.; Facchetti, A. Marks, T. J.; Wasielewski, M. R. Angewandte Chemie-International Edition 2004, 43, 6363.
(9) McConnell, H. M. Journal of Chemical Physics 1956, 24, 632.
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