Background and methods
Results and comments
Using spectroscopy to monitor anthracene
dimerisation
Don Praveen Amarasinghe
MOAC Doctoral Training Centre
University of Warwick
24th September 2013
Background and methods
Contents
1
Background and methods
2
Results and comments
Results and comments
Background and methods
Results and comments
Anthracene
Small molecule with non-trivial structure.
Used in calibration.
Wavelength shifts in calibration samples.
Could dimer formation
1
1
be the problem?
Chandross, E. A.; Ferguson, J.; McRae, E. G. J. Chem. Phys. 1966, 45
(10), 3546–3553.
Background and methods
Spectroscopic methods
Absorbance spectroscopy
Fluorescence spectroscopy
Raman (linear difference) spectroscopy
Results and comments
Background and methods
Absorbance spectroscopy
Results and comments
Background and methods
Results and comments
Absorbance spectroscopy
2
Measure intensity of light before, I0 , and after, I passing
through sample. Absorbance, A, is calculated using:
I0
A = log10
I
Beer-Lambert law — A ∝ Concentration.
2
Figure taken from http://faculty.sdmiramar.edu/fgarces/
Background and methods
Results and comments
Fluorescence spectroscopy
What about light emitted from the sample?
Measure intensity of light emitted from sample at right angles
to the path of excitation light.
Background and methods
Results and comments
Raman spectroscopy
Used to study the vibrational energy levels of a molecule.
Shine laser (UV-vis) light at sample. Record when the light
scattering is inelastic.
Virtual
Energy
States
Vibrational
Energy
States
Rayleigh
Scattering
Stokes
Raman
Scattering
Anti - Stokes
Raman
Scattering
Background and methods
Results and comments
Raman linear difference spectroscopy
Based on work by Kowalska et al. 3
Dry samples on polyethylene (PE) film and stretch to align
the molecules.
Shine polarised Raman laser light at sample — parallel and
perpendicular to stretch direction.
“Subtract perpendicular from parallel”.
3
Kowalska et al. Anal. Chem. 2012, 84, 1394–1401.
Background and methods
Results and comments
Solution preparation
Anthracene solutions prepared with methylcyclohexane solvent
at concentrations between 0.05 and 2 mg ml−1 .
Background and methods
Contents
1
Background and methods
2
Results and comments
Results and comments
Background and methods
Absorbance spectra
Spectra of anthracene solutions in cuvettes
Results and comments
Background and methods
Results and comments
Absorbance spectra
Spectra of anthracene solutions dried on taut, unstretched PE film.
Background and methods
Results and comments
Absorbance spectra
Spectra of anthracene solutions dried on stretched PE film.
Background and methods
Results and comments
Fluorescence spectra
Fluorescence emission spectra with excitation radiation at 252 nm.
Background and methods
Results and comments
Raman spectra
R L S - P o w d e r
R L S - 2 m g /m l A n th ra c e n e o n 1 .8 x S tre tc h e d P E
R L D - 0 .5 m g /m l A n th ra c e n e o n 1 .8 x S tre tc h e d P E
0
R L D - 2 m g /m l A n th ra c e n e o n 1 .8 x S tre tc h e d P E
5 0 0
1 0 0 0
1 5 0 0
2 0 0 0
W a v e n u m b e r (c m -1 )
2 5 0 0
3 0 0 0
3 5 0 0
Background and methods
Results and comments
Raman spectra
Summary of findings:
The Raman spectra obtained from powder samples and
solutions dried on film are consistent with previously published
Raman spectra of anthracene 4 .
Raman linear difference data indicate changes in orientation of
molecules between different concentrations.
1557
1480
1163
1008
cm−1
cm−1
cm−1
cm−1
(C-C in plane stretching)
(C-C in plane stretching)
(C-H in plane bending)
(C-H in plane bending)
However these changes appear in either stretched PE film
data or unstretched film data - not both.
4
Abasbegovic, N.; Vukotic, N.; Colombo, L. J. Chem. Phys. 1964, 41 (9),
2575–2577.
Background and methods
Results and comments
Remarks
Need to study concentrations between 1 mg/ml and 2 mg/ml.
Difficulties in maintaining solubility of more concentrated
solutions.
Prevalence of noise in Raman spectra of low concentrations of
anthracene.
Spectral changes might be due to higher order structures or
other effects!
Background and methods
Results and comments
Conclusions
Absorbance spectroscopy data appear to indicate the
formation of ground-state dimer with increasing concentration
of solution, but there are inconsistencies (Beer-Lambert law).
Fluorescence spectra also indicates ground-state dimer
formation.
Raman linear difference spectral data can be used to analyse
anthracene, however the data obtained cannot be used to
draw concrete conclusions.
More work required to investigate higher-order structures and
their spectral effects.
Background and methods
Results and comments
Acknowledgements
Kasra Razmkhah, Nikola Chmel & Alison Rodger.
All of the Biophysical Chemistry Lab on 6th Floor Chemistry.
Birmingham Science City scheme for the Raman instruments.
MOAC DTC staff and fellow students.
The University of Warwick and EPSRC.
Thank you for listening! :-) Any questions?