Spectroscopic Characterization of Solvent-Sensitive Foldamers

1. Spectroscopic Characterization of Solvent-Sensitive Foldamers Debanti Sengupta Advisor: Professor O’ Hara

2. Background • S0 to S2 transition: Absorption • S1 to S0 transition: Fluorescence Diagram from: http://www.shsu.edu/~chemistry/chemiluminescence/JABLONSKI.html

3. 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

4. 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.

5. 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

6. M-PE Results Absorption spectrum • UV studies – ratio of one peak to another decreases chloroform acetonitrile

7. Cisoid vs. Transoid Transoid Cisoid Transoid to cisoid shift responsible for absorbance blue shift in acetonitrile from chloroform

8. 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 M-PE results chloroform acetonitrile

9. 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

10. Ortho-substituted Phenylene Ethynelene Foldamers • 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 R: R = R =

11. O-PE Foldamers • Folding dependent on: • Chain length • Solvent • Sidegroups (ester/ether/mixed) • Temperature

12. Es6 (ester derivatized O-PE hexamer) Fluorescence Spectrum chloroform acetonitrile

13. Es6 Fluorescence Solvent Titration – Acetonitrile to Chloroform Two peaks correspond to two conformers: 420 nm conformer favoured in less polar solvent like chloroform

14. Es6 Fluorescence Solvent Titration – Acetonitrile to Water 360 nm conformer favoured in more polar solvent like water

15. 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

16. 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