PSORALEN-OLEFIN PHOTOPRODUCI'S: FIRST OBSERVATION OF A PHOTO-ENE REACTION

Abstract Methyl-substituted psoralens (4'-(hydroxymethyl)-4,5',8-trimethylpsoralen and 4,5',8-trimethylpsoralen) are found to yield an ene product as well as the expected [2+2] cycloaddition product from photochemical reaction with simple olefins. As determined by absorbance, liquid chromatography-mass spectrometry and nuclear magnetic resonance, both products are formed at the pyrone side of the respective psoralen. The product distribution is dependent on olefin concentration as well as the nature of the olefin. In deoxygenated solutions, cyclic olefins form as much as 50% ene product, while unsubstituted straight-chain olefins form as little as 3%. In oxygenated solutions, the product distribution is strongly affected by singlet oxygen.     

On-column refractive index detector for flash chromatography

A universal detector for on-column analysis in flash chromatography is reported. The detection scheme takes advantage of refractive index changes as analytes move through an illuminated region of the column. The column packing material is a diffuse scattering medium when the refractive index of the solvent is significantly different than that of the packing material. The magnitude of the 'signal' depends on the degree to which the refractive index mismatch is changed. An empirical model that qualitatively accounts for the observed trends is presented. This detection scheme provides a simple, inexpensive means to monitor the end of a flash chromatography column in order to determine the exit time of the species of interest, thus greatly reducing the post-column analysis time. Additionally, the detector is movable along the length of the column, offering the potential to monitor separations as they occur.     

2D correlation analysis of the continuum in single molecule surface enhanced Raman spectroscopy

The role and the nature of the continuum in Surface Enhanced Raman Spectroscopy (SERS) are unclear. Here, two-dimensional (2D) covariance and correlation analysis is applied to single molecule SERS spectra on silver colloids with and without rhodamine 6G (native colloid). The resulting 2D covariance and correlation maps show that the sharp molecular Raman peaks from rhodamine 6G and the molecule responsible for the SERS peaks from the native colloid are correlated to different continua even though both continua are present in each data set. This suggests that two distinct active sites on the silver colloids produce the two different continua, and that each site has some molecular specificity.     

Optical characterization of pristine poly(3‐hexyl thiophene) films

We describe a comprehensive model for the optical properties of pristine films of poly‐(3‐hexylthiophene) (P3HT). The presented model is anisotropic with the optical axis normal to the substrate plane, which is consistent with previous x‐ray diffraction studies that show preferential edge‐on packing of the polymer chains on the substrate. Peak locations and spacings are defined using a Huang‐Rhys vibronic progression consistent with known phonon energies. We demonstrate that the model fits variable‐angle spectroscopic ellipsometry and normal‐incidence transmission data well, and accurately predicts angle‐ and polarization‐dependent transmission and reflection data. The spectral features of the optical constants used in the model are in excellent agreement with published spectroscopic data on P3HT. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

The role of O2 in SERS-active thin metal film photodynamics

Optical emission from thin Ag films excited by visible light under a non-oxygen atmosphere was analyzed as a function of time and excitation power. The emission behavior under anaerobic conditions was strikingly different from that obtained under an oxygen-containing atmosphere. Specifically, emission intensity increased as a function of time with no photoinduced signal decay. A simple mechanistic model was developed and demonstrated to accurately predict the photodynamic trends observed under both conditions. The model involves oxygen-mediated cyclic production and destruction of photoactive silver clusters on the thin film. The evidence presented here strongly supports the hypothesis that oxygen plays a key role in the "blinking" phenomenon observed on SERS-active Ag films.     

Molecular orientation of a model liquid crystal alignment layer

An analytical methodology, involving the use of a combination of second harmonic generation (SHG) and linear dichroism, was utilized to probe the molecular orientation and angular distribution of a model liquid crystal (LC) alignment layer. In order to determine which film structure would be best suited for use as an alignment layer, the azo dye o-methyl red (MR) was covalently bound to a glass substrate using both monofunctional and trifunctional silane chemistry. The influence of solvent on the orientation and angular distribution of both thin films was also investigated. For the monofunctional silane film under water, the mean orientation angle of the MR molecular long axis was 67±4° and the width of an assumed Gaussian distribution was 32±2°. Under hexanes, the mean orientation angle was the same within error (63±1°) but the distribution width narrowed considerably to 22±1°. Molecular orientation within the trifunctional silane film exhibited little dependence on solvent. Under water, the mean orientation angle and angular distribution width were 76±3° and 30±1°, respectively. With hexanes as the solvent, the mean orientation angle and angular distribution width were 79±1° and 30±1°, respectively. Orientation insensitive SHG measurements indicated that surface coverage in the tri-functional silane film was twice that in the mono-functional silane film. The observed orientational differences were attributed to differences in the forces that dictate molecular orientation for the two systems. Based on the higher orientation angle, higher surface coverage and the lack of solvent dependence, MR-tri exhibits more desirable characteristics for use as an LC alignment layer.