homepage    mcep   e-portfolio   chem501
  Home            MCEP home     E-portfolio    CHEM507
                                                                                                                   Molecular Spectroscopy Project:

FLUORESCENT WHITENING AGENTS
IN LAUNDRY DETERGENT & PAPER

1: History        2:Importance and Usage        3: Spectroscopy        4: Current Studies        5: Future Expts.

6: HS Classroom Lesson:        A: Lesson Plan        B: Worksheets        C: Instructor Guide
3. Chemical Structure & Spectroscopy
        Fluorescent whitening agents (FWAs) are effective brighteners because they fluoresce in a particular manner.  All optical brighteners absorb light in the near ultraviolet (UV) region (300-400 nm) without absorbing light in the visible region (otherwise, they would lessen the overall brightness) and then emit light in the visible region (400-780 nm) (1), increasing the amount of visible light given off, which results in brighter fabrics. 

Figure 1. Tonic water contains quinine that fluoresces
when exposed to UV light (i.e. a blacklight) (2).
fluorescing tonic water    Those optical brighteners that are particularly effective whitening agents, or FWAs, work in a manner similar to bluing to increase the perception of whiteness; they are colorless (do not absorb visible light) and absorb ultraviolet light, "converting" it to emit blue light (1).   Figure 1 to the left shows an example of a common substance that exhibits these properties: quinine in tonic water.
Figure 2. Diagram showing how yellow
and blue light mix to produce white light (3).
blue + yellow = white    This emission of blue light "whitens" fabrics that may naturally have a more neutral or yellowish white tint.  See Figure 2 to the right to recall how yellow and blue light mix to create white light (remember this from Part 1?).


What is fluorescence?

Figure 3. Electronic and vibrational energy level diagram for a substance that fluoresces.
Taken from http://www.resonancepub.com/spectrofluor.htm (an excellent site!) (4)
fluorescence    FWAs exhibit fluorescence as a result of specific electronic and vibrational transitions, shown in Figure 3 to the left:

1. Absorption of UV light.
    The FWA absorbs UV light as it is excited from the ground electronic state to the excited electronic state (i.e. the two curved Morse energy level surfaces in the diagram to the left).  The vibrational states (i.e. the smaller energy levels within each electronic energy level in the diagram to the left) also usually change because the internuclear distance must remain the same during this electronic transition (according to the Frank-Condon principle) (5,6).

2. Radiationless energy transfer.
    The FWA then relaxes to a lower vibrational state within the excited electronic state, often mediated by a collision or vibrational/ rotational motion within the molecule.  No electromagnetic radiation is emitted as energy is given off via some kinetic motion (5,6).

3. Emission of visible light.
Figure 4. Shows how the absorbed and emitted wavelength differ. 
Taken from http://www.resonancepub.com/spectrofluor.htm (4).

wavelength shift
    The FWA then relaxes from the excited electronic state back down to the ground excited state.  Because of the radiationless energy transfer, the energy of the light emission is less energetic than the energy of the initial light absorption.  The FWA initially had absorbed UV light, but emits visible light (5,6).  See Figure 4 to the right.


What compounds fluoresce in a way to produce FWA?
Table 1.  Structural formulas of some of the compounds FWAs are derived from (4).
structures    Most FWAs are "derivatives of stilbene. . . biphenyl and five membered heterocyclics, such as triazoles, oxoazoles or imidazoles. . . . six-membered heterocyclics, such as coumarins, naphthalimide, pyrazine, or triazine (7)."  The extensive pi-systems of these often heterocyclic aromatic compounds are associated with the closely spaced electronic energy levels that allow for energy transitions within the visible range (e.g. n-->pi transitions).  Table 1 to the right shows some of base structures that FWAs are derived from.

    Companies invest a lot of money to research, develop, and patent better FWAs.  Some of the ones sold commercially are Blankophor, Calcofluor, Fluolite, Leucophor, Photine, Pontamine White, Tinopal, and Uvitex (7).

    Various substituents are added to produce the correct optical properties, solubility, resistance to degradation, and fastness.  For example, the FWA's fastness or adherence to the particular fabric being used (via bonding or some strong intermolecular attraction) can be modified by  adding substituents that are more or less polar.The detailed structure of FWAs can be deduced by typical  methods of organic structure determination (i.e. analysis of Nuclear Magnetic Resonance (NMR) spectra, Mass spectra (MS), and Infrared (IR) absorption spectra). 



Methodology & Equipment

Figure 5. Schematic of a Fluorimeter (8).
fluorimeter     To quantitatively characterize the absorption and emission properties, it is most     To quantitatively characterize the absorption and emission properties, it is most useful to use a fluorimeter to measure the UV light absorption associated with the first electronic transition and the visible light emission associated with the second electronic transition.  A fluorimeter is similar to a regular spectrometer except it must be outfitted so that the light emitted by the sample cell must be carefulled filtered (perhaps with slits or a diffraction grating) so that photodetector can accurately assess absorption and emission of various wavelengths (9).  Thus, fluorimeters are high-end, sensitive spectrometers.  Figure 5 to the left shows how a fluorimeter operates and Figure 6 below shows an actual fluorimeter.
Figure 6. A fluorimeter (10).
fluorimeter    By calibrating the wavelength of the initial light used for excitation useful to use a fluorimeter to measure the UV light absorption associated with the first electronic transition and the visible light emission associated with the second electronic transition.  A fluorimeter is similar to a regular spectrometer except it must be outfitted so that the light emitted by the sample cell must be carefulled filtered (perhaps with slits or a diffraction grating) so that photodetector can accurately assess absorption and emission of various wavelengths (9).  Thus, fluorimeters are high-end, sensitive spectrometers.

    By calibrating the wavelength of the initial light used for excitation and the wavelength detected for the emitted light, it is possible to get absorption and emission spectra.


Spectra (11)
   
    The following are two examples of the spectra for the compounds FWAs are derived from.  They show absorption in the near UV (300-400 nm) and emission in the blue visible range (410-490 nm).  Click on the name to open up a link to the site where the spectra were found (it opens in a new window).
Click here to go to the index page for more spectra--the other coumarins are not as well suited for use as FWAs because they absorb in the blue visible range, which would actually cause some yellowing, rather than whitening and brightening.



    The next section explores current research concerning FWAs, including the design of novel FWAs and the investigation of possible toxicity.  Click on the button to the right to continue.



WORKS CITED:
  1. Maseka, K. (2005). The emission and absorption of radiation by white and dyed cotton fabrics laundered with fluorescent brightening agents. AATCC Review, 5(10), 35-38.
  2. Image of fluorescing tonic water from Wikipedia Commons. http://en.wikipedia.org/wiki/File:Tonic_water_uv.jpg (April 2, 2009).
  3. Image of additive color mixing. http://www.d.umn.edu/~mharvey/th1501color.html (January 7, 2009).
  4. Image of electronic and vibrational energy level diagram related to fluorescence.  http://www.resonancepub.com/spectrofluor.htm (April 2, 2009).
  5. Spectrofluorimetry. http://www.resonancepub.com/spectrofluor.htm (April 2, 2009).
  6. Moog, R.S., Spencer, J.N. & Farrell, J.J. (2004). ChemActivity 22: Electronic spectra of atoms and molecules. Physical Chemistry: A Guided Inquiry: Atoms, Molecules, and Spectroscopy, Houghton MIfflin Company, New York, p. 209, 212, 213.
  7. Science Tech Entrepeneur, July 2006 (2006).
    http://www.techno-preneur.net/information-desk/sciencetech-magazine/2006/july06/Fluorescent_brighteners.pdf  (January 7, 2009).
  8. Image of spectrophotometer (fluorimeter) from Wikipedia Commons. http://en.wikipedia.org/wiki/File:Spectrophotomer.JPG (April 2, 2009).
  9. Molecular fluorescence spectroscopy.  http://elchem.kaist.ac.kr/vt/chem-ed/spec/molec/mol-fluo.htm (April 3, 2009).
  10. Image of Jenway 62series fluorimeter. http://www.spectronic.co.uk/analytical-instruments/62seriesfluoro.htm (April 3, 2009).
  11. PhotochemCAD Spectra by Category.  http://omlc.ogi.edu/spectra/PhotochemCAD/html/index.html (April 3, 2009).