Using inelastic electron tunneling spectroscopy (IETS) to measure the vibronic structure of nonequilibrium molecular transport, aided by a quantitative interpretation scheme based on Green's functiondensity functional theory methods, we are able to characterize the actual pathways that the electrons traverse when moving through a molecule in a molecular transport junction. We show that the IETS observations directly index electron tunneling pathways along the given normal coordinates of the molecule. One can then interpret the maxima in the IETS spectrum in terms of the specific paths that the electrons follow as they traverse the molecular junction. Therefore, IETS measurements not only prove (by the appearance of molecular vibrational frequencies in the spectrum) that the tunneling charges, in fact, pass through the molecule, but also can be used to determine the transport pathways and how they change with the geometry and placement of molecules in junctions. molecular electronics ͉ molecular junctions ͉ molecular transport T he electron-transfer process is crucial in chemistry, materials science, condensed matter physics, and electrical engineering. Although it is always modeled either explicitly or implicitly by pathways (how electrons actually move within the molecule), there is as yet no direct measurement or observation of such pathways. The pathways idea has been present in physical organic chemistry for years in connection with reaction mechanisms and has been widely used in the interpretation of electron tunneling pathways in proteins (1), but no distinct observations have been made. The absence of direct measurement of pathways is because the measurements are usually made starting with an equilibrium structure, exciting quickly (optical spectroscopy), and then observing the new perturbed structure. Although it is instructive to observe these initial and final states, they are static snapshots and cannot capture the dynamics of the electrontransport process. In molecular transport junctions, where current is moving continuously through the molecule, the nonequilibrium inelastic electron tunneling spectroscopy (IETS) probe permits direct observation of how different modes modulate the transport and, therefore, can be used to deduce actual pathways.It is well established that tunneling electrons can lose energy through excitation of a molecular vibrational level contained within the tunnel junction (2-5). The threshold for such excitation is eV ϭ -h where V is the bias voltage and -h is the energy of the molecular vibration. Peaks in d 2 I/dV 2 versus V, or more commonly the normalized quantity (d 2 I/dV 2 )/(dI/dV) versus V, correspond to molecular vibrations. IETS has become quite popular in the field of molecular electronics over the last 3 years (6-9) and has distinguished itself as a unique spectroscopic probe of molecular junctions. Because an IET spectrum is acquired directly from the measured transport characteristics (Fig. 1), the only added experimental requirement is the ability to cool the ju...
The synthesis, structure, and physical properties of a Heisenberg exchange-coupled cluster containing naphthalene groups are described. [Fe2(O)(O2CCH2C10H7)2(TACN-Me3)2]2+ (3) crystallizes in space group P1 with unit cell parameters a = 12.94(2) A, b = 14.84(2) A, c = 15.23(2) A, alpha = 101.12(7) degrees, beta = 90.8(1) degrees, gamma = 114.14(7) degrees, V = 2605(6) A3, and Z = 2 with R = 0.0425 and wR2 = 0.1182. Variable-temperature magnetic susceptibility data indicate that the two high-spin FeIII centers are antiferromagnetically coupled with J = -105 cm-1 (H = -2 JS1.S2), which is typical for this class of compounds. The room-temperature static emission spectrum of the compound in deoxygenated CH3CN solution is centered near 335 nm and has features reminiscent of both methyl-2-naphthylacetate (1) and [Zn2(OH)(O2CCH2C10H7)2(TACN-Me3)2]+ (2) with the following two caveats: (1) the overall emission intensity is roughly a factor of 10 less than that of the free ester (1, phi = 0.13) or the ZnII analogue (2, phi = 0.14), and (2) there is significant broadening of the low-energy shoulder of the emission envelope. Time-correlated single photon counting data revealed biphasic emission for 3 with tau 1 = 4.6 +/- 1 ns and tau 2 = 47 +/- 1 ns. The latter compares favorably with that found for 2 (tau = 47 +/- 1 ns) and is assigned as the S0-S1 fluorescence of naphthalene. Emission anisotropy, time-gated emission spectra, and nanosecond time-resolved absorption measurements all support the assignment of the 4.6 ns component as being due to a singlet excimer that forms between the two naphthylacetate groups of 3, a process that is likely mediated by the structural constraints of the oxo-bis-carboxylato diiron core. No direct evidence for intramolecular electron and/or energy transfer from the photoexcited naphthyl group to the iron-oxo core was obtained, suggesting that the short-lived excimer may contribute to circumventing such pathways in this type of system.
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