Thermochromism in polydiacetylene solutions

Abstract

A novel planar ↔ nonplanar visual thermochromic conformational transition of polydiacetylene molecules in poor solvents is reported. The conformational transition is associated with both a color change (blue or red ↔ yellow) and a change in the state; the yellow solution (liquid) transforms to a blue or red gel (solid). The color transition occurs within a narrow range of temperature and has a large associated hysteresis. The enthalpy of the conformational transition is 29 kJ/mole of repeat unit. Fourier‐transform infrared studies show that molecules acquire a planar conformation in red or blue gels by formation of intramolecular H bonds between the adjacent substituent groups. Virtually all H bonds break (a nonplanar conformation) when the gels turn into yellow solutions.

Solutions of polydiacetylenes in good and poor solvents : a light and neutron scattering study

“…For instance, when hexane is added to a solution of P-3BCMU in chloroform at room temperature, it turns blue [4] ; and when a solution of P-4BCMU in toluene is cooled from high temperature, it turns red [5]. These color transitions correspond to changes in the electronic structure of the carbon chain skeleton.…”

“…On the other hand, it is generally believed that the conjugated backbone is in the trans configuration of the double bonds, but it has been suggested that yellow solutions consist primarily of the cis configuration [9]. Since [10], is related to pure intramolecular coil to rod conformational changes [5,[11][12][13] [14,15]. The second model corresponds to an aggregation of semi-rigid chains, or parts of such chains, leading to clusters [6,16] : this is the fringed micelles model [17].…”

“…Indeed, no long range cooperativity among hydrogen bonds is expected, unlike in polypeptides. The simplest way is then to use the Guinier approxi-mation for CP (q), and equation (5) [5,6]. This color transition corresponds to changes in the electronic structure of the conjugated backbone of the macromolecules.…”

“…A more detailed study of the transition and of the gel structure is in progress [42]. 5. Search for red solutions of isolated rod molecules.…”

“…It has been suggested that the color transition is a single chain phenomenon corresponding to an intramolecular coil to rod change [5,11,13] ; aggregation as well as gelation then occur as a result of the rodlike conformation of the molecules, but are not the driving force for the transition [14]. Solutions of isolated rod molecules should therefore be observed for low enough polymer concentrations.…”