Spectroscopic Characterization of Solvent Sensitive Foldamers Debanti Sengupta

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

Background • S 0 to S 2 transition: Absorption • S 1 to S 0 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 Absorption spectrum chloroform acetonitrile • UV studies – ratio of one peak to another decreases

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

M-PE results Fluorescence Spectrum • 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 redshifted peak appears to be like UV spectrum of foldamer in pure acetonitrile chloroform acetonitrile

M-PE Anisotropies • For 10 mer, 18 mer, 24 mer: anisotropies at 350 nm are ~10 m. A 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

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= =

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

Es 6 (ester derivatized O-PE hexamer) Fluorescence Spectrum chloroform acetonitrile

Es 6 Fluorescence Solvent Titration – Acetonitrile to Chloroform Two peaks correspond to two conformers: 420 nm conformer favoured in less polar solvent like chloroform

Es 6 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 MPEs • Prof. O’ Hara for everything • Amherst College Chemistry department