| Abstract
| - The introduction of the urea function as structure directing agent of diacetylene organogels (DA-OGs) has been achieved. Despite the urea function being one of the most frequently used structure directingagents for the formation of organogels, it has never been exploited in the fabrication and photopolymerizationof DA-OGs. The self-association of ureas involving two hydrogen bonds is much stronger than that ofurethanes or amides, and the resulting supramolecular assemblies are completely insoluble. In this context,1,1‘-(hexa-2,4-diyne-1,6-diyl)bis(3-(10-(triethoxysilyl)decyl)urea) 2 was synthesized. Compound 2 was solubleowing to the triethoxysilane function that we recently used in the fabrication of a silylated bis-urea-stilbeneorganogel. It formed an organogel, and its photopolymerization was studied in cyclohexane. The loss ofthe gel state and the formation of a red solution resulting from the polymerization were found to be theresult of the constraints introduced by the urea function in close vicinity to the polymerizable function. Toobtain an ureido substituted diacetylenic organogelator affording a blue highly conjugated polydiacetylene(PDA) without a sol−gel transition, a propylene spacer was introduced to move the urea function awayfrom the polymerizable function (derivative 3). The thermochromism exhibited by the latter in the solidstate was studied. Using the same setup and the same sample, UV−vis and FTIR spectra weresimultaneously recorded as a function of the temperature to highlight a relation between color changesand urea association mode changes. The data showed that the reversible thermochromic transition mustbe associated with a reversible supramolecular modification and, conversely, that irreversible chromictransitions are the result of irreversible structural modifications. The chromic effects of the acidic hydrolysis−polycondensation of the trialkoxysilyl groups to form a siloxane network were studied on a thin film of 3. Inthe same way, solvent effects on the color of the organogels of 3 were also investigated. Correlationscould be established between the different stimuli. These results provide a deeper understanding of theprecise molecular mechanism of the blue to red transition and of the reversibility of the purple to red transitiongenerally encountered in PDA thermochromism.
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