Visual conformational transitions of water soluble polydiacetylenes: Effects of pH and electrolyte on absorption and fluorescence spectra

Abstract

Visual conformational transitions (yellow↔red or purple) of some water soluble polydiacetylenes (RC–C≡C–CR)_x, where R=–(CH₂)_3,4OCONHCH₂COO⁻K⁺ and –(CH₂)_2,3COO⁻K⁺, which is analogous to the helix↔coil conformational transition of polypeptides are reported. The conformational transitions, nonplanar↔planar (or twisted) is introduced by varying the concentration of an electrolyte and the pH, and are characterized by the absorption and emission spectroscopy. The yellow solutions of polydiacetylenes turn red or purple upon increasing the concentration of an electrolyte such as KCl and upon lowering the pH, from‐12 to 2. The electrolyte concentration dependent absorption spectra display an isosbestic point at 471 nm. No isosbestic point is observed during pH titration. Explanations are forwarded for this discrepancy. The yellow solutions of all the polymers are found to fluoresce. Depending on the side groups used, the red or purple solutions either fluoresce with a substantial reduction in intensity or do not fluoresce at all. Fluorescence maxima show dependence on λ_exc when the side group R=Pxn(CH₂)_2,3COO⁻K⁺. Also the maxima of the corrected fluorescence excitation spectra of such polymers are found at different wavelengths than the maxima of the corresponding absorption spectra.(AIP)