In 1978 it was discovered, largely through the work of Fleischmann, Van Duyne, Creighton, and their coworkers that molecules adsorbed on specially prepared silver surfaces produce a Raman spectrum that is at times a millionfold more intense than expected. This effect was dubbed surface-enhanced Raman scattering (SERS). Since then the effect has been demonstrated with many molecules and with a number of metals, including Cu, Ag, Au, Li, Na, K, In, Pt, and Rh. In addition, related phenomena such as surface-enhanced second-harmonic generation, four-wave mixing, absorption, and fluorescence have been observed. Although not all fine points of the enhancement mechanism have been clarified, the majority view is that the largest contributor to the intensity amplification results from the electric field enhancement that occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure. Small in this context is gauged in relation to the wavelength of light. The special preparations required to produce the effect, which include among other techniques electrochemical oxidation-reduction cycling, deposition of metal on very cold substrates, and the generation of metal-island films and colloids, is now understood to be necessary as a means of producing surfaces with appropriate electromagnetic resonances that may couple to electromagnetic fields either by generating rough films (as in the case of the former two examples) or by placing small metal particles in close proximity to one another (as in the case of the latter two). For molecules chemisorbed on SERS-active surface there exists a "chemical enhancement" in addition to the electromagnetic effect. Although difficult to measure accurately, the magnitude of this effect rarely exceeds a factor of 10 and is best thought to arise from the modification of the Raman polarizability tensor of the adsorbate resulting from the formation of a complex between the adsorbate and the metal. Rather than an enhancement mechanism, the chemical effect is more logically to be regarded as a change in the nature and identity of the adsorbate.

Surface-enhanced spectroscopy

The dependence of the optical properties of spherical gold nanoparticles on particle size and wavelength were analyzed theoretically using multipole scattering theory, where the complex refractive index of gold was corrected for the effect of a reduced mean free path of the conduction electrons in small particles. To compare these theoretical results to experimental data, gold nanoparticles in the size range of 5 to 100 nm were synthesized and characterized with TEM and UV−vis. Excellent agreement was found between theory and experiment. It is shown that the data produced here can be used to determine both size and concentration of gold nanoparticles directly from UV−vis spectra. Equations for this purpose are derived, and the precision of various methods is discussed. The major aim of this work is to provide a simple and fast method to determine size and concentration of nanoparticles.

https://www.ucl.ac.uk/~ucaptt0/Thanh%20Anal%20Chem%20on%20size%20of%20Au.pdf

Determination of size and concentration of gold nanoparticles from UV-vis spectra

6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

The Grotthuss mechanism

ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

The Nature of Simple Photodissociation Reactions in Liquids on Ultrafast Time Scales

We report the first investigation of the fast excited-state intramolecular proton transfer of 3-hydroxyflavone. Picosecond stimulated emission and transient absorption spectroscopy demonstrate that the tautomer formation can be monitored by 620-nm transient absorption. The excited-state proton transfer was studied at 620 nm with femtosecond time resolution, and determined to be 240±50 fs in a nonpolar solvent environment. The fast excited-state proton transfer in methanol solution, however, was faster than the 125-fs instrument response

Direct observation of fast proton transfer: femtosecond photophysics of 3-hydroxyflavone

Chemical reactions that break alkane carbon-hydrogen (C–H) bonds are normally carried out under conditions of high temperature and pressure because these bonds are extremely strong (∼100 kilocalories per mole), but certain metal complexes can activate C–H bonds in alkane solution under the mild conditions of room temperature and pressure. Time-resolved infrared experiments probing the initial femtosecond dynamics through the nano- and microsecond kinetics to the final stable products have been used to generate a detailed picture of the C–H activation reaction. Structures of all of the intermediates involved in the reaction of Tp*Rh(CO)2 (Tp* = HB–Pz3*, Pz* = 3,5-dimethylpyrazolyl) in alkane solution have been identified and assigned, and energy barriers for each reaction step from solvation to formation of the final alkyl hydride product have been estimated from transient lifetimes.

https://science.sciencemag.org/content/sci/278/5336/260.full.pdf

The Mechanism of a C-H Bond Activation Reaction in Room-Temperature Alkane Solution

The vibrational dephasing of polyatomic molecules in condensed phases by intermolecular vibrational energy exchange is treated theoretically In the exchange model, dephasing arises from random modulation of the vibrational frequency caused by intramolecular anharmonic coupling to low frequency modes which are undergoing intermolecular energy exchange with the bath The exchange rates are temperature dependent and as a consequence manifest themselves experimentally as a temperature dependent broadening and shift of the Raman spectral line shape Using a reduced density matrix technique within the constraints of a Markoff approximation, the theory allows the time dependence introduced by the exchange process to be properly accounted for, and allows explicit expressions for the vibrational correlation function and corresponding spectral line shape functions to be derived and related to molecular parameters Application of the theory can have important consequences experimentally, since an analysis of the te

/pdf/the-origin-of-vibrational-dephasing-of-polyatomic-molecules-41j3v609tt.pdf

The origin of vibrational dephasing of polyatomic molecules in condensed phases

The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

https://science.sciencemag.org/content/sci/279/5348/202.full.pdf

Charles B. Harris

Papers

The Nature of Simple Photodissociation Reactions in Liquids on Ultrafast Time Scales

Direct observation of fast proton transfer: femtosecond photophysics of 3-hydroxyflavone

The Mechanism of a C-H Bond Activation Reaction in Room-Temperature Alkane Solution

The origin of vibrational dephasing of polyatomic molecules in condensed phases

Femtosecond dynamics of electron localization at interfaces