Nanosecond Time-Resolved Fluorescence Spectroscopy in the Physical Chemistry Laboratory: Formation of the Pyrene Excimer in Solution

Laser-based spectroscopic techniques have become firmly established in modern physical chemistry research because they are powerful tools for the study of atomic and molecular structure and dynamics. For example, a 1992 survey of experimental articles in issues of the Journal of Physical Chemistry and the Journal of Chemical Physics found that some 40% used lasers in some way (1). Until recently, however, very few undergraduate physical chemistry laboratories have incorporated laser spectroscopy experiments, so that few undergraduate students were exposed to this exciting and very important area of chemistry. This is reflected by the dearth of laser experiments in the chemical education literature prior to the present decade. (For a list of the experiments published before 1989, see the review by Steehler [2]). Progress in resolving this problem has appeared in the last few years with the publication of a number of new laser experiments for the physical chemistry laboratory, including no fewer than eight articles in a text devoted to modernizing the physical chemistry curriculum (3). A growing number of experiments are appearing in this Journal (4–12); other experiments were published in a book by Zare et al. (13) or are available from laser manufacturers (14). Many of these make use of moderately priced nitrogen/dye laser systems. These versatile devices have many potential applications in an undergraduate teaching laboratory, including moderately high-resolution spectroscopy via the use of associated dye lasers, and experiments employing nanosecond time resolution. The experiment that we will describe here combines a nitrogen laser with a /4-meter monochromator, a digital oscilloscope, and a computer-interfaced boxcar-gated integrator for nanosecond time-resolved fluorescence spectroscopy. The effective time resolution is <15 ns. Admittedly, more rigorous techniques such as time-correlated single-photon counting offer better time resolution (0.02–0.5 ns) and superior dynamic range, and thus are better suited for examining subtle changes in fluorescence time profiles. The lower precision of the nitrogen-laser techniques described here does limit the interpretation of the data to an extent. From a pedagogical standpoint, however, these laser experiments have the important advantage that the same instrumentation is used both to obtain emission spectra and to examine their time evolution, so that students can readily appreciate the complementary aspects of the spectral and kinetic data. The experiment studies the excited-state behavior of pyrene (Fig. 1). The pyrene molecule offers two advantages: it absorbs strongly at the wavelength of the nitrogen laser, and it has a fairly long excited-state lifetime (e.g., 382 ns in deoxygenated cyclohexane [15]). In this experiment, several aspects of the fluorescence behavior of pyrene are studied, as follows.