13042
J. Phys. Chem. 1993,97, 13042-13045
ARTICLES Long-Distance Electron Transfer through Rodlike Molecules with Cubyl Spacers Basil Paulson,t**Kakumanu hamod,* Philip Eaton,* Gerhard Clow,**fiand John R. Miller'vt Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, and Department of Chemistry. University of Chicago, Chicago, Illinois 60637 Received: July 6, 1993: In Final Form: October 4, 1993' Rates of intramolecular electron transfer have been measured through cubanes. Pulse radiolysis was used to study the reactions B-CunN BCunN-3 where B = (4-biphenylyl), N (2-naphthyl), and Cu, is the spacer composed of n = 1-3 1,Ccubyl oligomers. Electronic couplings, V,for these reactions were found to decrease exponentially with increasing distance, V(R) = V(R0)exp[-0.5 B-R],with @ = 0.90 A-I. While the electrontransfer rates in these cubanes are a factor of 10 larger than in similar compounds with spacers based on cyclohexane, the distance dependence is almost identical to that for the cyclohexyl-type molecules, @ = 0.94 A-l. The unusual properties of the cubanes enhanced the electron-transfer rates but did not lead to a large difference in distance dependence.
Introduction In the last decade, numerous compounds have been studied to determine the distance dependence for rates of intramolecular electron-transfer (ET) reactions.'-z1 Generally, the rate was found to decrease approximately exponentially with donor-acceptor (DA) separation, but this dependence on distance, due primarily to through-bond interactions, varies markedly with the system studied. Previous work from these laboratories' studied the electrontransfer (ET) reaction B-SN + BSN(1) where B = (Cbiphenylyl), N = (Znaphthyl), and S represents spacer groups which varied from 1,3-~yclohexane(4-bonds) to 3,16-androstane (10-bonds). In this paper we report measurements of intramolecular ET rate as a function of distance for a seriesofcubaneoligomers,S = (1,4-cubyl), wheren = 1-3 (Chart I). The results from this study are compared with those for the cyclohexyl-typespacers1 to elucidate the important factors in the distance dependence of ET. These cubanes are rigid, rodlike molecules which accurately fix the donor-acceptor distance and are remarkably stable despite their highly-strained,p-rich C - C bonds.22923 We are interested in studying the cubane spacers because their properties of high connectivity, strong bond-to-bond couplings, and low ionization potential are featuresexpected to enhancelong-distanceelectronic couplings.2c29 The results presented here indicate that, despite the special properties of cubanes, enhancement of the interaction is substantial but not outstanding.
Experimental Section Preparation of the cubane compounds used in this study will be described elsewhere.*' The di-n-hexyl derivative of BCu3N was prepared because precursors to the unsubstituted tercubyl compound were insoluble. The anions were generated by irradiating solutions of the BSN with 30-ps pulses of 20-MeV electrons from the Argonne linac. Irradiation generates solvated electrons (
