David H. Waldeck

Breakdown of Kramers theory description of photochemical isomerization and the possible involvement of frequency dependent friction

We present new data on the ground state isomerization rate of DODCI and compare our results to previously obtained excited state data. We find that the one‐dimensional Kramers expression does not fit the data over the whole viscosity range when the friction is measured by either the overall rotation time or the solvent viscosity. The deviation is qualitatively similar to that previously observed in diphenyl butadiene when the viscosity was used as the friction measure. Deviations from the simple hydrodynamic theory appear to depend on the reduced frequency for isomerization. We show how our results are consistent with the one‐dimensional Kramers theory when a frequency dependent friction is used to describe the influence of the solvent.

Carbon nanotube-polymer nanocomposite infrared sensor.

The infrared photoresponse in the electrical conductivity of single-walled carbon nanotubes (SWNTs) is dramatically enhanced by embedding SWNTs in an electrically and thermally insulating polymer matrix. The conductivity change in a 5 wt % SWNT-polycarbonate nanocomposite is significant (4.26%) and sharp upon infrared illumination in the air at room temperature. While the thermal effect predominates in the infrared photoresponse of a pure SWNT film, the photoeffect predominates in the infrared photoresponse of SWNT-polycarbonate nanocomposites.

Fluctuations in Biological and Bioinspired Electron-Transfer Reactions

Central to theories of electron transfer (ET) is the idea that nuclear motion generates a transition state that enables electron flow to proceed, but nuclear motion also induces fluctuations in the donor-acceptor (DA) electronic coupling that is the rate-limiting parameter for nonadiabatic ET. The interplay between the DA energy gap and DA coupling fluctuations is particularly noteworthy in biological ET, where flexible protein and mobile water bridges take center stage. Here, we discuss the critical timescales at play for ET reactions in fluctuating media, highlighting issues of the Condon approximation, average medium versus fluctuation-controlled electron tunneling, gated and solvent relaxation controlled electron transfer, and the influence of inelastic tunneling on electronic coupling pathway interferences. Taken together, one may use this framework to establish principles to describe how macromolecular structure and structural fluctuations influence ET reactions. This framework deepens our understanding of ET chemistry in fluctuating media. Moreover, it provides a unifying perspective for biophysical charge-transfer processes and helps to frame new questions associated with energy harvesting and transduction in fluctuating media.

Nonradiative damping of molecular electronic excited states by metal surfaces

Abstract In this review we discuss the interaction of a molecular excited state with a smooth substrate. Both theoretical and experimental work is treated. This discussion will concentrate on the classical treatment of the interaction because of its astounding success in comparison with experiment. We do however discuss the shortcomings of the classical treatment and some recent approaches to correcting these limitations. The experimental work is considered in detail but we focus on the region close to the substrate, less than 500 A away because the longer distance regime has been well reviewed. At the end of this article we briefly point out areas where future work is needed.

Picosecond pulse induced transient molecular birefringence and dichroism

Excitation of dye molecules in fluid solution can lead to transient birefringence (caused by change in the polarizability anisotropy) and transient dichroism (caused by change in the absorption cross section). In the anisotropic absorption method of Shank and Ippen, both effects are observed, and in some cases the measured signal is dominated by the birefringence contribution. Very small amounts of external birefringence can lead to erroneous values for the rotational reorientation time. We present an analysis of these effects in a Jones matrix formalism, and use the results to show how to obtain correct reorientation times. The changes in polarizability anisotropy for the dyes oxazine‐725 and DODCI are evaluated as +75 and +4 A3, respectively, at 585 nm and we believe these to be the first direct measurements of this change between ground and first singlet states. The rotational reorientation times of four molecules in ethanol are presented and compared with hydrodynamic predictions.

Ambient stability of chemically passivated germanium interfaces

The stability of any semiconductor surface passivating layer is key to applications. Second harmonic generation (SHG) can be used to probe the chemical state of semiconductor interfaces, as well as investigate the mechanisms of chemical transformation. While the SHG rotational anisotropy changes upon sulfidation or alkylation of Ge surfaces, SHG appears far less sensitive to H and Cl passivation of germanium surfaces than to silicon surfaces. Investigation of the stability of chemically modified germanium surfaces using a number of additional techniques, including atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), reveals that H- and Cl-terminated Ge(1 1 1) rapidly re-oxidize in ambient. S- and alkyl-terminations are more robust, showing little sign of oxide formation after a month in ambient. 2003 Elsevier B.V. All rights reserved.

Chiral-Induced Spin Selectivity Effect

The chiral-induced spin selectivity (CISS) effect was recently established experimentally and theoretically. Here, we review some of the new findings and discuss applications that can result from special properties of this effect, like the reduction of the elastic backscattering in electron transfer through chiral molecules. The CISS effect opens the possibility of using chiral molecules in spintronics applications and for providing a deeper understanding of spin-selective processes in biology.

Time resolved polarization spectroscopy: Level kinetics and rotational diffusion

Time resolved polarization spectroscopy is a powerful and sensitive technique for the study of dynamics in the liquid phase. Using this technique, it is possible to obtain information about various relaxation processes in solution, including the motions of molecules in different electronic states, and the transitions between these states. Since the experiment typically measures both electronic relaxation and molecular reorientation, the observed signal can have a complicated form which represents a coupling of these two effects in a way that depends on the particular scheme of decay processes which are present in a given system. We present a general algorithm for deriving the form of the experimental signal for an arbitrary scheme for systems exhibiting these phenomena, assuming that the molecular motion is described by asymmetric rotational diffusion. Several examples are presented and experimental results interpreted using the derived formulas, including cases where (1) the excited state diffusion tenso...

The chiroptical signature of achiral metal clusters induced by dissymmetric adsorbates

Using a dissymmetrically-perturbed particle-in-a-box model, we demonstrate that the induced optical activity of chiral monolayer protected clusters, such as Whettens Au28(SG)16 glutathione-passivated gold nanoclusters (J. Phys. Chem. B, 2000, 104, 2630-2641), could arise from symmetric metal cores perturbed by a dissymmetric or chiral field originating from the adsorbates. This finding implies that the electronic states of the nanocluster core are chiral, yet the lattice geometries of these cores need not be geometrically distorted by the chiral adsorbates. Based on simple chiral monolayer protected cluster models, we rationalize how the adsorption pattern of the tethering sulfur atoms has a substantial effect on the induced CD in the NIR spectral region, and we show how the chiral image charge produced in the core provides a convenient means of visualizing dissymmetric perturbations to the achiral gold nanocluster core.

Spintronics and Chirality: Spin Selectivity in Electron Transport Through Chiral Molecules

Recent experiments have demonstrated that the electron transmission yield through chiral molecules depends on the electron spin orientation. This phenomenon has been termed the chiral-induced spin selectivity (CISS) effect, and it provides a challenge to theory and promise for organic molecule-based spintronic devices. This article reviews recent developments in our understanding of CISS. Different theoretical models have been used to describe the effect; however, they all presume an unusually large spin-orbit coupling in chiral molecules for the effect to display the magnitudes seen in experiments. A simplified model for an electrons transport through a chiral potential suggests that these large couplings can be manifested. Techniques for measuring spin-selective electron transport through molecules are overviewed, and some examples of recent experiments are described. Finally, we present results obtained by studying several systems, and we describe the possible application of the CISS effect for memory devices.