Optical detection and spectroscopy of single molecules and single nanoparticles have been achieved at room temperature with the use of surface-enhanced Raman scattering. Individual silver colloidal nanoparticles were screened from a large heterogeneous population for special size-dependent properties and were then used to amplify the spectroscopic signatures of adsorbed molecules. For single rhodamine 6G molecules adsorbed on the selected nanoparticles, the intrinsic Raman enhancement factors were on the order of 10 14 to 10 15 , much larger than the ensemble-averaged values derived from conventional measurements. This enormous enhancement leads to vibrational Raman signals that are more intense and more stable than single-molecule fluorescence.

http://science.sciencemag.org/content/sci/275/5303/1102.full.pdf

Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

This is an overview of some of the important, challenging, and problematic issues in contemporary electron transfer research. After a qualitative discussion of electron transfer, its time and distance scales, energy curves, and basic parabolic energy models are introduced to define the electron transfer process. Application of transition state theory leads to the standard Marcus formulation of electron transfer rate constants. Electron transfer in solution is coupled to solvent polarization effects, and relaxation processes can contribute to and even control electron transfer. The inverted region, in which electron transfer rate constants decrease with increasing exoergicity, is one of the most striking phenomena in electron transfer chemistry. It is predicted by both semiclassical and quantum mechanical models, with the latter appropriate if there are coupled high- or medium-frequency vibrations. The intramolecular reorganizational energy has different contributions from different vibrational modes, whic...

Contemporary Issues in Electron Transfer Research

Efficient collection and detection of fluorescence coupled with careful minimization of background from impurities and Raman scattering now enable routine optical microscopy and study of single molecules in complex condensed matter environments. This ultimate method for unraveling ensemble averages leads to the observation of new effects and to direct measurements of stochastic fluctuations. Experiments at cryogenic temperatures open new directions in molecular spectroscopy, quantum optics, and solid-state dynamics. Room-temperature investigations apply several techniques (polarization microscopy, single-molecule imaging, emission time dependence, energy transfer, lifetime studies, and the like) to a growing array of biophysical problems where new insight may be gained from direct observations of hidden static and dynamic inhomogeneity.

https://science.sciencemag.org/content/sci/283/5408/1670.full.pdf

Illuminating single molecules in condensed matter.

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.

Chemical approaches to artificial photosynthesis. 2.

Resonance Raman excitation profiles have been measured across the broad, intense charge-transfer absorption band of the hexamethylbenzene/tetracyanoethylene complex in CCl 4 solution. Nonresonant Raman spectra of the complex and its perdeuterated derivative in the solid state have also been obtained with excitation at 1064 nm. Absorption of light within the charge-transfer band results in electron transfer from hexamethylbenzene to tetracyanoethylene to form a radical-ion pair

Mode-specific vibrational reorganization energies accompanying photoinduced electron transfer in the hexamethylbenzene/tetracyanoethylene charge-transfer complex

Resonance Raman Intensity Analysis of Excited-State Dynamics

The magnitudes, time scales, and underlying mechanisms responsible for broadening the electronic spectra of molecules in liquid solutions and glasses are reviewed. The emphasis is on experimental results from hole-burning, single-molecule, photon echo, and resonance Raman and fluorescence studies. The influence of the time scale of the measurement in distinguishing between homogeneous broadening (electronic dephasing) and inhomogeneous broadening is discussed, and the role of coupling of solvent phonons to the solute's electronic transition is stressed.

Molecular electronic spectral broadening in liquids and glasses.

Absorption, Fluorescence, and Resonance Raman Spectroscopy of the Hexamethylbenzene/Tetracyanoethylene Charge-Transfer Complex: Toward a Self-Consistent Model

Resonance Raman spectra, including absolute cross sections, have been measured for the triiodide ion in ethanol, ethyl acetate, and acetonitrile solvents using eight excitation wavelengths that span the two lowest absorption bands. Preresonance Raman spectra have also been obtained with 488 nm excitation. The apparent vibrational line widths observed on resonance are approximately twice the preresonant line widths in all three solvents. By quantitatively modeling the triiodide/solvent system using two intramolecular modes (the symmetric and antisymmetric stretches) and one low-frequency intermolecular or solvent mode, we are able to reproduce the vibrational broadening on resonance, which, within the context of our model, is due to unresolved combination bands between the intramolecular degrees of freedom and the intermolecular mode. The Raman spectra in ethanol clearly show one-quantum transitions in the antisymmetric stretching mode (ν3) which should be symmetry forbidden in the linear D∞h geometry, ind...

Anne B. Myers

Papers

Mode-specific vibrational reorganization energies accompanying photoinduced electron transfer in the hexamethylbenzene/tetracyanoethylene charge-transfer complex

Resonance Raman Intensity Analysis of Excited-State Dynamics

Molecular electronic spectral broadening in liquids and glasses.

Absorption, Fluorescence, and Resonance Raman Spectroscopy of the Hexamethylbenzene/Tetracyanoethylene Charge-Transfer Complex: Toward a Self-Consistent Model

Solvent Effects in the Raman Spectra of the Triiodide Ion: Observation of Dynamic Symmetry Breaking and Solvent Degrees of Freedom