Site-Selection Fluorescence Spectrometry of Polycyclic Aromatic Hydrocarbons in Vapor-Deposited Argon Matrices

Abstract

The use of tunable laser excitation to effect significant sharpening in the inhomogeneously-broadened fluorescence spectra of polycyclic aromatic hydrocarbons (PAHs) isolated in argon matrices at 15 K is reported. The broad-band-excited fluorescence spectra of most PAHs in solid argon exhibit complex multiplet structures assumed due to the location of individual PAH molecules in different types of discrete “sites” in the polycrystalline matrix. Under laser excitation, the fluorescence spectra of PAHs are strongly dependent on excitation wavelength. For most PAHs, there exists an “optimum” excitation wavelength, at which a “single-site” fluorescence spectrum comprising one intense feature having a bandwidth of SO cm−1 or less is obtained. The excitation and emission wavelengths for site-selection fluorescence of eleven PAHs matrix isolated in argon are tabulated, and the trade-offs implicit in choice of optimum excitation wavelengths for site-selection fluorometric detection of PAHs in multicomponent samples are addressed. Quantitative results for five constituents of a sixteen-component standard reference material exhibit excellent agreement with certified values.

Absorption and emission spectroscopy of perylene (C20H12) isolated in Ne, Ar, and N2 matrices

“…Also, the increasing shift when going from site a to site b indicates that the perylene molecule is more perturbed in the latter site. These observations seem to agree with the conclusions of MacDonald et al 16 that substitutional sites for PAHs in inert matrices are formed by displacement of six or more inert gas atoms.…”

“…This temperature-dependent effect is often observed in matrices when a large number of different sites is formed. 16 Amongst the complex structure observed in Ar and N 2 , different sites in the solid matrix named a to e in Ar and a to d in N 2 have been identified. Other substructures have been assigned to low frequency spacings whose possible origin is discussed in Sec.…”

Derivatization of benz[a]anthracene metabolites for detection by laser excited Shpol'skii spectrometry

“…For example, the ultraviolet excitation at 63 K of 1-hydroxypyrene and 1-aminopyrene yields fluorescence line widths far broader than those observed for their nonpolar parent pyrene (36). Two apparent shortcomings of Shpol'skii spectrometry are the inability to find Shpol'skii solvents for some important fluorescent polar analytes and the rapid photodegradation of such compounds under laser illumination in Shpol'skii matrices (18,37). To overcome these limitations either a suitable crystalline host must be found to accommodate the analyte, or the analyte has to be modified (derivatized) to fit the host.…”

Laser-induced phosphorescence spectrometry of halogenated naphthalene derivatives prepared in vapor-deposited parent molecule matrices

“…However, until the introduction of various procedures for improving spectral resolution, the applications of fluorometry in multicomponent analysis have been limited. The usage of laser excitation coupled with improved sampling methodologies (e.g., laserexcited Shpol'skii (11,12), fluorescence line narrowing (13,14), or site-selection (15,16) spectrometry) has resulted in high spectral resolution, low detection limits, and capabilities for the direct analysis of complex samples, such as oil shales and coal tars. In view of the spectacular results that have been achieved with laser-induced fluorescence (LIF) spectrometry, it would be highly desirable to extend the methodology to compounds that phosphoresce, rather than fluoresce.…”

“…With this method a monochromatic excitation source (i.e., a laser) is only those molecules in resonance with the excitation wavelength will absorb radiation. The resulting fluorescence is characterized by narrow emission line widths (13)(14)(15)(16). In vitreous solids the singlet states of the energy-selected molecules are energetically equivalent, even though they are positioned in different sites in the low-temperature solid, and the triplet states will often be energetically nonequivalent.…”