Optical dephasing in solution: A line shape and resonance light scattering study of azulene in isopentane and cyclohexane

Study on the modulation of spectral properties of the formylperylene-methanol clusters by excited-state hydrogen bonding strengthening

In the present work, the charge transfer (CT) process within the formylperylene (FPe)-methanol (MeOH) systems facilitated by intermolecular hydrogen bonding interactions is theoretically studied in both the ground state S0 and the first singlet excited state S1. The geometric structures, electronic spectra and the infrared spectra of the FPe monomer as well as the various hydrogen-bonded FPe-MeOH complexes in both states were calculated with the density functional theory (DFT) method and time-dependent density functional theory (TD-DFT) methods, respectively. It is demonstrated that the total effect of the intermolecular hydrogen bonding between FPe and the MeOH molecules becomes strengthened in the ground state as the number of the MeOH molecules hydrogen-bonded to the FPe molecule increases from zero to three, which induces large increases in the dipole moment as well as systemic redshifts of the absorption spectra of FPe. Furthermore, upon photoexcitation of the FPe molecule, the intermolecular hydrogen bonds formed in the various hydrogen-bonded FPe-MeOH complexes are further strengthened which leads to even larger dipole moments as well as obvious redshifts of the fluorescence spectra. The calculated electronic spectra of the various hydrogen-bonded FPe-MeOH complexes are in agreement with the steady-state absorption and fluorescence spectra of FPe observed in the binary mixed solvents with different MeOH concentration. The intermolecular hydrogen bonding strengthening in both the ground and excited states are further confirmed by the infrared spectra shifts. Moreover, the vitally important role played by the intermolecular hydrogen bonding interaction and its strengthening upon electronic excitation in the CT process is discussed.

Characterization of the surface enhanced raman scattering (SERS) of bacteria

The surface enhanced Raman scattering (SERS) of a number of species and strains of bacteria obtained on novel gold nanoparticle (approximately 80 nm) covered SiO(2) substrates excited at 785 nm is reported. Raman cross-section enhancements of >10(4) per bacterium are found for both Gram-positive and Gram-negative bacteria on these SERS active substrates. The SERS spectra of bacteria are spectrally less congested and exhibit greater species differentiation than their corresponding non-SERS (bulk) Raman spectra at this excitation wavelength. Fluorescence observed in the bulk Raman emission of Bacillus species is not apparent in the corresponding SERS spectra. Despite the field enhancement effects arising from the nanostructured metal surface, this fluorescence component appears "quenched" due to an energy transfer process which does not diminish the Raman emission. The surface enhancement effect allows the observation of Raman spectra of single bacterial cells excited at low incident powers and short data acquisition times. SERS spectra of B. anthracis Sterne illustrate this single cell level capability. Comparison with previous SERS studies reveals how the SERS vibrational signatures are strongly dependent on the morphology and nature of the SERS active substrates. The potential of SERS for detection and identification of bacterial pathogens with species and strain specificity on these gold particle covered glassy substrates is demonstrated by these results.

Interaction of cetylpyridine bromide with nucleic acids and determination of nucleic acids at nanogram levels based on the enhancement of resonance Rayleigh light scattering

Resonance Rayleigh light scattering (RRLS) spectra of cetylpyridine bromide (CPB)-nucleic acid system and their analytical application have been first studied. The effective factors and optimum conditions of the reaction have been investigated. After CPB and nucleic acid are mixed together, a new absorption peak located at 300 nm appeared, which is due to the formation of new ion associate of CPB-nucleic acid. The new associate can result in two apparent RRLS peaks at 310-400 and 460-480 nm. The RRLS peak of the corrected spectra located at 290-350 nm, which indicate that the RRLS is originated from the absorption of CPB-nucleic acid associate. The peak at 460-480 nm disappears in the corrected RRLS spectra, which indicated that this peak is originated from the strong line emission of the Xe lamp. Under the optimum conditions, the enhanced intensity of RRLS is proportional to the concentration of nucleic acid in the range of 5.0 x 10(-9)-5.0 x 10(-5) g ml(-1) for calf thymus DNA (ctDNA), 1.0 x 10(-8)-4.0 x 10(-5) g ml(-1) for fish sperm DNA (fsDNA) and 1.0 x 10(-8)-5.0 x 10(-5) g ml(-1) for yeast RNA (yRNA). The detection limits (S/N = 3) are 4.3, 8.7 and 7.4 ng ml(-1), respectively. Synthetic samples were determined satisfactorily.

MOLECULAR ELECTRONIC SPECTRAL BROADENING IN LIQUIDS AND GLASSES

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.