Single-molecule interfacial electron transfer dynamics manipulated by external electric current

Interfacial electron transfer (IET) dynamics in 1,1'-dioctadecyl-3, 3, 3', 3'-tetramethylindodicarbocyanine (DiD) dye molecules / indium tin oxide (ITO) film system have been probed at the ensemble and single-molecule level by recording the change of fluorescence emission intensity. By comparing the difference of the external electric current (EEC) dependence of lifetime and intensity for enambles and single molecules, it is shown that the single-molecule probe can effcienly demonstrate the IET dynamics. The backward electron transfer and electron transfer of ground state induce the single molecules fluorescence quenching when an EEC is applied to ITO film.Comment: 6 pages, 6 figure

Ground and excited state electron transfer dynamics

The focus of this work is the investigation of the factors controlling electron transfer in molecular electronic systems, in particular those affecting electron transfer to and from electronically excited states. To achieve this, a number of mono- and trimetallic osmium and ruthenium complexes were synthesised and characterised. Monolayers of an osmium polypyridyl complex bound to a platinum microelectrode via a ¿rara-l,2-bis-(4-pyridyl)ethylene bridge were formed to probe ground state electron transfer dynamics. This is compared to the rate of photoinduced oxidative electron transfer quenching which occurs in a trimetallic osmium complex where the metal centres are linked by the same bridging ligand. The rate constant for this quenching is 1.3 xlO8 s '1, compared to 2 x 106 s '1 for the ground state process with the same driving force. These investigations show that the strength of coupling across the bpe ligand is higher when it links two metal centres as opposed to when it bridges a metal centre and an electrode. Extensive experiments were carried out to quantify the effect of laser pulses on an unmodified electrode surface. Laser activation improves the heterogeneous kinetics of a solution phase redox probe by removing polishing debris and other adsorbed impurities. Laser-induced current transients observed following a single laser pulse are due to a rapid (jas) restructuring of the double-layer followed by a slow (ms) thermal decay within the metal electrode. A mathematical model has yielded values of the thermal diffusion coefficient as a function of applied potential. To investigate excited state heterogeneous electron transfer, monolayers of a ruthenium polypyridyl complex containing the bridging ligand, 2,2':4,4":4',4"- Quarterpyridyl are used. Using Rehm-Weller calculations, the excited state redox potentials occur a t -0.71 and +1.05 V for oxidation and reduction respectively. Laser excitation of these monolayers in conjunction with high-speed cyclic voltammetry was utilised to attempt to directly measure the excited state redox potentials of this complex. This experiment has not been entirely successful and suggestions for improvements to the experiment are discussed

Long-Lived Ultracold Molecules with Electric and Magnetic Dipole Moments

We create fermionic dipolar $^{23}$Na$^6$Li molecules in their triplet ground state from an ultracold mixture of $^{23}$Na and $^6$Li. Using magneto-association across a narrow Feshbach resonance followed by a two-photon STIRAP transfer to the triplet ground state, we produce $3\,{\times}\,10^4$ ground state molecules in a spin-polarized state. We observe a lifetime of $4.6\,\text{s}$ in an isolated molecular sample, approaching the $p$-wave universal rate limit. Electron spin resonance spectroscopy of the triplet state was used to determine the hyperfine structure of this previously unobserved molecular state.Comment: 5 pages, 5 figure

Coherent storage of photoexcited triplet states using 29Si nuclear spins in silicon

Pulsed electron paramagnetic resonance spectroscopy of the photoexcited, metastable triplet state of the oxygen-vacancy center in silicon reveals that the lifetime of the ms = \pm1 sub-levels differ significantly from that of the ms =0 state. We exploit this significant difference in decay rates to the ground singlet state to achieve nearly ~100% electron spin polarization within the triplet. We further demonstrate the transfer of a coherent state of the triplet electron spin to, and from, a hyperfine-coupled, nearest-neighbor 29Si nuclear spin. We measure the coherence time of the 29 Si nuclear spin employed in this operation and find it to be unaffected by the presence of the triplet electron spin and equal to the bulk value measured by nuclear magnetic resonance.Comment: 5 pages, 4 figure

On the Existence of Biradical-Ionic States of Donor-Acceptor Cyclophanes. A Simple MO-Theoretical Study

The longest wavelength absorption in the UV-VIS spectra of donor-acceptor cyclophanes corresponds to an excitation from the donor D to the acceptor A with the transfer of one electron, i.e. hv DA--+ D+A-. The size of the electron transfer should become larger with increasing donor-acceptor strength already in the ground state. One expects a biradical-ionic ground state D+Q A-• with a large q, say q > 1/2, for certain donor-acceptor combinations with a small enough difference between the ionization energy I0 of the donor and the electron affinity EA of the acceptor. In this work we investigate the ground state and the lowest excited singlet and triplet states of donor-acceptor cyclophanes within a semi- empirical four-orbital configuration interaction model. The dependence of energies and size of the electron transfer in these states on molecular parameters of donor and acceptor and on their mutual geometrical arrangement is elucidated. Our model leads to the simple approximate condition v•" ;;;; O for the occurrence of a biradical-ionic ground state. verr is a measure of the effective donor-acceptor strength which does not only depend on the difference I0 - EA but also on the Coulombic repulsion between an electron in D and one in A and on the stabilizing effect of the solvent. The symmetries of the frontier orbitals, i. e. the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor, determine whether the biradical- ionic ground state will be a triplet or a singlet. According to our simple model a triplet biradical-ionic ground state should occur if the frontier orbitals belong to different representations of the pointgroup of the phane. The transition from an ordinary ground state without an appreciable electron transfer from D to A ·into the biradical-ionic ground state is discontinuous. This transition is, however, continuous if the frontier orbitals belong to the same representation, although the magnitude of the electron transfer q changes significantly only in a narrow range of v•rr whose width is determined by the size of the transanular and through-bond interaction between the frontier orbitals. Finally we discuss how to choose donor and acceptor in a cyclophane in order to fulfill the condition for a biradical-ionic ground state

Novel Density-Wave States of Two-Band Peierls-Hubbard Chains

Based on a symmetry argument we systematically reveal Hartree-Fock broken-symmetry solutions of the one-dimensional two-band extended Peierls-Hubbard model, which covers various materials of interest such as halogen-bridged metal complexes and mixed-stack charge-transfer salts. We find out all the regular-density-wave solutions with an ordering vector $q=0$ or $q=\pi$. Changing band filling as well as electron-electron and electron-phonon interactions, we numerically inquire further into the ground-state phase diagram and the physical property of each state. The possibility of novel density-wave states appearing is argued.Comment: 10 pages, 6 PS figures, to appear in Phys. Lett.

Compton scattering beyond the impulse approximation

We treat the non-relativistic Compton scattering process in which an incoming photon scatters from an N-electron many-body state to yield an outgoing photon and a recoil electron, without invoking the commonly used frameworks of either the impulse approximation (IA) or the independent particle model (IPM). An expression for the associated triple differential scattering cross section is obtained in terms of Dyson orbitals, which give the overlap amplitudes between the N-electron initial state and the (N-1) electron singly ionized quantum states of the target. We show how in the high energy transfer regime, one can recover from our general formalism the standard IA based formula for the cross section which involves the ground state electron momentum density (EMD) of the initial state. Our formalism will permit the analysis and interpretation of electronic transitions in correlated electron systems via inelastic x-ray scattering (IXS) spectroscopy beyond the constraints of the IA and the IPM.Comment: 7 pages, 1 figur