The resonance Raman spectrum of the simple alkyne bridge in 4,4'-dinitrotolane radical anion shows two distinct bands, providing proof of the solvent-dependent coexistence of charge-localized and -delocalized species. The Raman spectra of normal modes primarily involving the charge-bearing -PhNO(2) units also support the coexistence of two solvent-dependent electronic species. The temperature dependence of the spectra of the bridging unit shows an inverse relationship between the solvent reorganization energy (lambda(s)) and the temperature.

An electrically driven molecular rotor device comprised of a monolayer of redox-active ligated copper compounds sandwiched between a gold electrode and a highly doped P + Si substrate was fabricated. Current-voltage spectroscopy revealed a temperature-dependent negative differential resistance ͑NDR͒ associated with the device. Time-dependent density functional theory suggests the source of the observed NDR to be redox-induced ligand rotation around the copper metal center, an explanation consistent with the proposed energy diagram of the device. Beyond the traditional electronic devices, a number of nanoscale molecular devices have been studied and reported to exhibit NDR characteristics.3-7 Although various mechanisms have been proposed, including charging and discharging processes of electrons, chemical reaction, redox reaction, and the association-dissociation processes of molecules, their specific underlying physics remain unclear. These functional molecular units acting as state variables provide an attractive alternative to overcome the limits of conventional metaloxide-semiconductor field-effect transistor technology due to their potential scalability, low cost, low variability, highly integrateable characteristics, and the capability to exploit self-assembly processes.6-10 Thus the elucidation of switching mechanisms and the development of different operational approaches have drawn a lot of attention. In this letter, we demonstrate an electrically driven sandwich-type monolayer molecular rotor switch with NDR. The observed NDR behavior is attributed to rotational motion on solid support. Both calculations of time-dependent density functional theory and an observed temperature dependence of the NDR behavior support this hypothesis.Synthesized using a self-assembly approach, the molecular switch device shown in Fig. 1 is comprised of a heteroleptic copper compound covalently bonded to a highly doped silicon substrate. Each complex contains three subunits: a bifunctional stator ͓a bidentate ligand bonded to both a solid support ͑P + Si͒ and a Cu axle͔, the metal axle ͑Cu͒, and a diimine rigid rotator ͑2,9-dimethyl-1,10-phenanthroline͒. Preparation of the heteroleptic copper system was carried out through covalent grafting of a stator monolayer onto the hydroxylated surface of a P + Si substrate using silanol bonds. 11The stators were then used to chelate a copper metal axle that subsequently bonds to the rotator subunit. The device was completed by deposition of a Ti/gold film through a shadow mask on top of the molecular layer to form the top electrode. The number of molecules per unit area and per unit devices tested is approximately 1.1ϫ 10 14 cm −2 and 8 ϫ 10 10 , respectively. This copper compound exhibits two discrete, redox-dependent conformational states, Cu͑I͒ and Cu͑II͒. The Cu͑I͒ form has tetrahedral geometry while the Cu͑II͒ form is square planar.12 The compounds undergo a one electron redox-induced rotational conformational change depending on the oxidation state of the copper metal. Interconv...

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The charge transfer excited state of a mechanically interlocked [2]rotaxane (R(4+)) with a donor 1,5-dioxynaphthalene (DNP) unit in the rod and the acceptor cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring component, along with the analogous non-interlocked [2]pseudorotaxane (P(4+)), is studied by resonance Raman spectroscopy and electronic absorption spectroscopy. Resonance Raman excitation profiles are obtained, calculated quantitatively using time-dependent theoretical methods, and interpreted with the assistance of DFT calculations. The active vibrational modes are consistent with an electron transfer from the HOMO centered on the DNP unit to the LUMO on the CBPQT(4+) ring. Displacement vectors of highly distorted modes agree with the bonding changes predicted from the MO nodal pattern. Subtle changes in the frequency of some modes in the free components compared with those in R(4+) are observed. The largest distortions are found for modes involving ring breathing in the DNP unit of the rod and the paraquat units of the CBPQT(4+) ring. The individual mode contributions to the vibrational reorganization energy, as well as the total vibrational reorganization energy, are calculated. Very similar values of λ(v) for R(4+) and P(4+) are calculated (∼2910 cm(-1)), indicating that the mechanical stoppers in the interlocked system do not significantly affect the excited state properties of R(4+) compared with P(4+).