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 (ester derivatized O-PE hexamer)

chloroform

acetonitrile

Fluorescence Spectrum

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

Acknowledgements

• Prof. Gregory Tew, Ticora Jones and the Tew research group at UMass Amherst for O-PE work

• Prof. Jeffrey Moore at UIUC for providing M-PEs

• Prof. O’ Hara for everything

• Amherst College Chemistry department