In the Laboratory
Laser-Induced Fluorescence of Lightsticks Carl Salter, Kevin Range, and Gail Salter Chemistry Department, Moravian College, Bethlehem, PA 18018
For nearly twenty years commercial lightsticks have been the key ingredient in many interesting chemical demonstrations and laboratory experiments (1–3). Most of these experiments employ the traditional 6-inch Cyalume lightstick1 and they usually focus on the reaction rate and temperature dependence of the chemiluminescence. Recently we began investigating the relatively new 11/2-inch Cyalume lightsticks in an attempt to develop “microscale” investigations of the mechanism of chemiluminescence. These mini lightsticks (38 mm; o.d. 4.5 mm) are available in red, yellow, green, and blue, are less expensive than their larger counterparts, and generate less waste. Omniglow Corporation lists the duration of the chemiluminescence as “4–6 hours”; our experiments indicate that the light intensity decays to 1% of initial intensity in about 15–20 hours, and the mini lightsticks are often glowing faintly a day after activation. We wanted to develop a physical chemistry experiment in which the chemiluminescence spectra of the lightsticks are compared with the laser-induced fluorescence spectra of the material removed from the used lightstick. Unfortunately, we had great difficulty obtaining fluorescence from solutions made from the contents of a spent lightstick, probably because we were diluting the material too much. Quite by accident, we discovered that the used lightsticks themselves will fluoresce when inserted in a laser beam, making it very easy to demonstrate that the same color is produced in both fluorescence and chemiluminescence. As discussed in reference 1, the chemiluminescent reaction in a lightstick involves the oxidation of diphenyl oxalate ester by hydrogen peroxide: O C O
O
+ H 2 O2
C
2
OH + 2 CO2
O
During the course of the oxidation reaction an intermediate is produced which transfers energy to a dye molecule. This intermediate is thought to be 1,2-dioxetanedione, which has a highly strained four-membered ring:
O
O C
C
O
O
Dye*
Lightstick chemiluminescence
C CO2
Dye Dye*
Laser 337 nm
Laser-induced fluorescence Dye
Scheme I. Mechanisms of lightstick chemiluminescence and laser-induced fluorescence. Dye* represents an electronically excited state of the dye molecule.
chemiluminescence had ceased. Note the strong similarity of the two spectra: both show peaks at 630 and 480 nm with shoulders at 670 and 420 nm, and the relative sizes of these features are roughly the same. (The sharp peak in the LIF spectrum at 674 nm is the laser line in 2nd order.) The LIF spectrum of a fresh, unused red lightstick is identical. Lightsticks of other colors yield similar CL and LIF spectra, but the agreement is most striking for the red lightstick because the features of its spectrum are well separated. (CL and LIF spectra for lightsticks of other colors can be viewed at our Web site: www.cs.moravian.edu/chemistry/lightstick/ l_st_spec.html). The LIF spectrum was obtained using a Laser Scientific VSL-337-NDS nitrogen laser (337 nm, 3 ns pulse width) operating at 20 Hz. The lightstick was irradiated endon. Both spectra were recorded by suspending a lightstick directly in front of the entrance slit of a CVI Digikrom 480 monochromater equipped with a Hamamatsu R928 photomultiplier tube operating at 1100 V; slits at 2 mm. The CL spectrum is 100 times more intense than the LIF spectrum owing to the small duty cycle of the laser.
0.5
O C
C
O
O
As shown in Scheme I, the decomposition of the dioxetane is catalyzed by a dye molecule, producing an electronically excited dye molecule, which returns to its ground state by emitting a photon. This mechanism suggests that if the dye molecule can be excited by some other mechanism (such as laser irradiation), the same visible light should result. The different colors available in lightsticks are due to the presence of different dye compounds. The spectra presented in Figure 1 bear out this mechanism. Here the chemiluminescence (CL) spectrum obtained from a red lightstick is compared with the laser-induced fluorescence (LIF) spectrum of the same lightstick taken after the 84
PMT Signal / Volts
O
0.4
CL 0.3
LIF (x 100)
0.2
0.1
0.0 350
400
450
500
550
600
650
700
750
Wavelength / nm Figure 1. Luminescence intensity (PMT signal voltage) vs wavelength for a red mini lightstick. CL: chemiluminescence; LIF: laser-induced fluorescence.
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
In the Laboratory
As a demonstration, the LIF of a lightstick is straightforward and dramatic. Because the effect can be produced directly from the lightstick without any intervening wet chemistry or material manipulation, it will make sense to a general audience or a general chemistry class. As a physical chemistry experiment, it makes an excellent introduction to the technique of laser-induced fluorescence and to topics in spectroscopy and photochemistry.
1. CYALUME is a trademark of American Cyanamide Company; production of the lightsticks is licensed to Omniglow Corporation. 11/2inch lightsticks are available from Edmund Scientific.
Acknowledgments
Literature Cited
We acknowledge the support of NSF DUE-9551365, which provided the instrumentation used in this study. Kevin Range received support from the Council of Undergraduate Research Academic–Industrial Undergraduate Research Partnership (AIURP) Fellowship sponsored by Hewlett-Packard.
We thank Richard Rosa of Omniglow Corporation for helpful discussions and Edmund Scientific for making red mini lightsticks available. Note
1. Schreiner, R.; Testen, M. E.; Shakhashiri, B. Z.; Dirreen, G. E.; Williams, L. G. In Chemical Demonstrations; Shakhashiri, B. Z., Ed.; University of Wisconsin Press: Madison, 1983; Vol. 1, pp 146–152. 2. Estell, J. K. J. Chem. Educ. 1991, 68, 225. 3. Bindel, T. H. J. Chem. Educ. 1996, 73, 356–358.
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