Long-distance electron transfer through rodlike molecules with cubyl spacers

Abstract

Rates of intramolecular electron transfer have been measured through cubanes. Pulse radiolysis was used to study the reactions B-CunN (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.BCunN-3 where B = (4-biphenylyl), N IntroductionIn 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 (D-A) 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) reactionwhere 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-type spacers1 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 features expected to enhance long-distance electronic couplings.2c29 The results presented here indicate that, despite the special properties of cubanes, enhancement of the interaction is substantial but not outstanding. Experimental SectionPreparation 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 (<2 X l e 5 M) which attach to the donor and acceptor Abstract published in Advance ACS Absrrucrs, November 15, 1993. BCu 0 NCU groups to form an equal distribution of B-SN and BSN-. The intramolecular ET reaction is then observed photometrically as it proceeds to equilibrium. Experimental methods for sample preparation and acquisition of transient absorption data and fitting of the data have been desc...

Ab initio quantum chemical study of electron transfer in carboranes

“…We also note here that the ET coupling matrix element for the 10-vertex carborane is nearly the same as that for the smaller carbon-based cage structured bicyclo[1.1.1]pentane [13]. This suggests that it is possible to control the ET coupling strength between a pair of active reaction centers not only by structural modification but also by the chemical nature of the spacer element, as also noted by others [25][26][27][28][29].…”

“…The HOMO and HOMO-1 orbitals correspond to the symmetric and anti-symmetric combination of the p-orbitals of the terminal end groups, respectively. The KT approach has been successfully used to study ET properties of a wide range of organic structures [25][26][27][28] and generally yields V AB values comparable to more rigorous methods [29]. However, it was noted in a previous study [13] that the magnitude of V AB obtained in the KT approach is generally larger than that obtained using the MH two-state approach at a similar level of theory.…”

Electron Transfer through Organic Molecules

“…[1][2][3][6][7][8][9][10][11][12][13]15,16,33,44,56,57 A general feature of these results under nonresonant tunneling conditions is that the transconductance decreases exponentially with increasing length. Experimental measurements include intramolecular donor-acceptor systems, 7-9 biochemical systems, 3,6 electrochemistry through alkanethiolate SAMs, [10][11][12]15 metal-insulator-metal junctions 16 and those by atomic force microscopy combined with a conducting probe tip.…”

On the Coherent Description of Diffusion‐Influenced Fluorescence Quenching Experiments II: Early Events

“…For bimolecular reactions, the Brownian motion of freely diffusing species in liquid solution strongly affects the observed kinetics. [1] Therefore, freezing the relative motion of the reaction partners by chemical linking [2][3][4] or by studying the reaction in rigid media [5,6] have been common strategies to overcome the unmitigated electron-transfer events. Unfortunately, these approaches either modify the reactants or the reaction medium.…”