Fluorescence Thermochromism: Qualitative Analysis
Jacob Barrett, Michael J. Goldcamp, Alfred Conklin, Jr.
Wilmington College, Wilmington, Ohio, 45177
Introduction:
Materials:
When electrons in an excited state return to a lower energy level, they give
off photons in a phenomenon called luminescence. When the cause of a
molecule’s luminescence is another photon of higher wavelength (and thus
higher energy) than the emitted photon, the luminescence is further
classified into fluorescence. One method of such excitation involves
irradiating the compound with ultraviolet light, as shown in Figure 1. Once in
the excited state, any form of energy transfer, including collision with other
molecules, lessens or can even destroy a molecule’s fluorescence. A
common method of reducing thermal vibrations of molecules is by
immersing the substance in liquid nitrogen, which has a boiling point of 77
Kelvin (-196°C).
All materials were provided courtesy of Wilmington College, and purchased from SigmaAldrich, Fisher Scientific,
Ligand structures: ?Say something here!
Experimental:
Samples were prepared by combining excess amount of ligating compound with the
metal compound in vials, and dissolving in a solvent if necessary. Once all samples had
been made, filter paper was sectioned off into eighths and used as the carrier material
for each sample, as it (what?) showed minimal fluorescence. Each sample was pipetted
onto a section of filter paper, and any solvent or extra liquid was allowed to evaporate.
Qualitative data were recorded as the ligand-metal saturated paper was then irradiated
with ultraviolet light, followed by an immersion in liquid nitrogen and subsequent
irradiation with ultraviolet light. After these data were recorded, the paper was gently
heated to potentially remove any extra ligands. How do you know only “extra ”ligands
were removed? Once the heating was complete, the filter paper was allowed to return to
room temperature and the room and liquid nitrogen temperature irradiation data were
recorded.
Substance
Figure 1: mechanism of fluorescence; excitation of an electron by and emission of a photon.
While many compounds fluoresce when chilled with liquid nitrogen, there
are also many examples of compounds that fluoresce at room temperature
(295-300 Kelvin). Substances that are capable of fluorescing at room
temperature can have a different fluorescence at liquid nitrogen
temperature. One such compound is pyridine copper (I) iodide,
(C5H5N)nCuI, which shows intense yellow fluorescence at room
temperature. When chilled to liquid nitrogen temperature, however,
reduction of thermal vibrations allow for the emission of higher energy pinkpurple photons, which are otherwise quenched. This evidences? that it is
possible to utilize temperature to control the wavelength, and thus the color,
of a molecule’s fluorescence, and is the basis of the research done here.
A
C
B
D
Figure 2: A: luminescence of bis-pyridine copper (I) iodide at room temperature. B: luminescence of bis-pyridine
copper (I) iodide chilled in liquid nitrogen. C: luminescence of pyridine copper (I) iodide at room temperature. D:
luminescence of pyridine copper (I) iodide chilled in liquid nitrogen. Are these pictures you took? If so say so.
CuI + BenzilA
BenzilD
NiCl2 + 2-benzylpyridine
CuI + 2-benzylpyridine
NiCl2 + 2,2’-bipyridine (Bpy)N
NiCl2 + BpyA
CuI + BpyA
Ni(NO3)2 + BpyA
BpyA
NiCl2 + 2,6-diaminopyridineA
CuI + 2,6-diaminopyridineA
2,6-diaminopyridineA
CuI + dipicolylamine
CuI + ethylenediamine
NiCl2 + ethylenediamine
MnCl2 + ethylenediamine
MnCl2 + triphenylphosphine (PPh3)A
NiCl2 + PPh3A
CuI + PPh3D
PPh3A
CuI + tris(2-pyridylmethyl)amine (TPA)A
CuI + pyridine (pyr)
CuBr2 + pyr
Fe(NO3)3 + pyr
Co(NO3)2 + pyr
MnCl2 + pyr
CuCl2 + pyr
Cu(NO3)2 + pyr
AgNO3 + pyr
NiCl2 + pyr
Luminescence before
300K
green
green
none
none
none
none
weak
none
none
blue
blue
blue
weak
none
blue
none
weak
weak
green
weak
none
green
none
none
Absorbs
weak
none
none
none
blue
77K
green
green
none
blue-green
none
weak
weak
none
none
blue
blue
blue
weak
none
blue
blue
weak
none
green
weak
none
green
yellow
none
Absorbs
blue
peach
none
none
blue
and after heating
300K
none
none
none
none
none
none
weak
none
none
none
none
none
weak
none
none
none
weak
weak
weak
none
none
yellow
none
none
Absorbs
none
none
yellow
N/A
weak
77K
yellow
yellow
none
none
none
none
weak
none
none
none
none
none
weak
none
weak
blue
weak
none
red
none
none
pink-purple
yellow-orange
none
Absorbs
none
deep orange
yellow
N/A
weak
Table 1: Qualitative analysis of fluorescence of various ligand-metal complexes, ordered by ligating compound. Superscript letters indicate
solvent in the case of dry complexes (A for acetone, D for dichloromethane, and N for Acetonitrile. Furthermore, bolded luminescence data
indicate increased intensity but no change in wavelength, while italicized compounds indicate a molecule lacking a metal center. “Weak”
fluorescence is determined as barely observable, while “none” is characterized as having no discernible fluorescence.
pyridine
2,2’-bipyridine
triphenylphosphine
Ethylenediamine
2-Benzylpyridine
2,6-diaminopyridine
Benzil
Results, Conclusions, and Future Work:
Two compounds, benzil and diaminopyridine, were shown to have significant
fluorescence in absence of a metal complex. Generally, metal complexes with
nitrates showed little to no fluorescence, and one complex even absorbed ultraviolet
light. Compounds that coordinated with an odd number of aromatic rings (such as
pyridine or triphenylphosphine) generally fluoresced more reliably than other
complexes. Copper-halides complexes tend to fluoresce very well, especially when
pyridine is used as the ligating compound. For several complexes, however, there
are not enough data to support any generalized statement.
Future work would involve quantitative analysis of fluorescent compounds,
broadening the scope of ligating compounds investigated, as well as increasing the
number of metal-halide centers.
Acknowledgement:
Wilmington College provided funding for the materials for this work. Who was/were
you advisor(s)?
References:
1. Ford P. and Vogler A. Acc. Chem. Res. 1993, 26, 220-226
2. Tard et al. Chem. Mater. 2008, 20, 7010–7016
3. Knotter et al. Inorg. Chem. 1992, 31, 2196-2201
4. Perruchas et al. Inorg. Chem. 2011, 50, 10682–10692
5. Kyle et al. J. Am. Chem. Soc. 1991, 113, 2954-2965
6. Dias et al. J. Am. Chem. Soc. 2003, 125, 12072-12073