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Spectroscopic Characterization of Solvent-
Sensitive Foldamers
Debanti Sengupta
Advisor: Professor O’ Hara
Background
• S0 to S2 transition: Absorption
• S1 to S0 transition: Fluorescence
Diagram from: http://www.shsu.edu/~chemistry/chemiluminescence/JABLONSKI.html
Anisotropy
ISS technical notes
•Molecule excited with polarized light
•Measurement of level of polarization of fluorescence – indication of how fast molecule rotates
•Low anisotropy – rotates fast; High anisotropy – rotates slowly
Foldamers
• Fold in one solvent, unfold in another
• Practical applications: can provide binding pockets for rodlike chain molecules; model compounds to study how structure and energetics affect folding into secondary structures
Prince, R.B., Saven, J.G., J. Am. Chem. Soc. 1999, Vol 121.
Meta-substituted phenylene ethynelenes
Nelson, J.C., Saven, J. G. Science, 1997 Vol 277.
• Meta-phenylene ethynelenes – extensively characterized foldamers
• Display changes to fluorescence and UV-Visible spectra upon folding in acetonitrile and unfolding in chloroform
M-PE Results
• UV studies – ratio of one peak to another decreases
chloroform
acetonitrile
Absorption spectrum
Cisoid vs. Transoid
Cisoid
TMS
N3Et2
R
R
R
Transoid
TMS
N3Et2
R
R
R
Transoid to cisoid shift responsible for absorbance blue shift in acetonitrile from chloroform
M-PE results
• Fluorescence results: adding foldamer to folding solvent leads to a red shift (due to excimer formation) and dramatic quenching of fluorescence intensity
• Solvent titrations: both peaks present
• Excitation spectrum of red-shifted peak appears to be like UV spectrum of foldamer in pure acetonitrile
Fluorescence Spectrum
chloroform
acetonitrile
M-PE Anisotropies
• For 10mer, 18mer, 24mer: anisotropies at 350 nm are ~10 mA higher than at 420 nm
• 420 nm peak – folded state; therefore, folded state rotates more rapidly than unfolded.
• Folded state may be more compact than unfolded – analogous to a ball/puck rolling better rather than a rope
• Fold at a much lower number of monomers – 4 as opposed to 8 for m-PE molecules
• Center of helix formed is smaller than m-PE foldamers
Ortho-substituted Phenylene Ethynelene Foldamers
TMS
N3Et2
R
R
R
R
R
O
O
O
OR:R =R =
O-PE Foldamers
• Folding dependent on:
– Chain length
– Solvent
– Sidegroups (ester/ether/mixed)
– Temperature
Es6 Fluorescence Solvent Titration – Acetonitrile to Chloroform
Two peaks correspond to two conformers: 420 nm conformer favoured in less polar solvent
like chloroform
Es6 Fluorescence Solvent Titration – Acetonitrile to Water
360 nm conformer favoured in more polar solvent like water
O-PE Anisotropies
• Preliminary studies indicate 360 nm peak has lower anisotropy than 420 nm peak
• From M-PE case, possible conclusion – 360 nm peak corresponds to folded state, 420 nm peak corresponds to unfolded state
• More anisotropy work needed to prove this hypothesis