Abstract
A series of simply structured diacetylene-diamide-based gelators (DAGs) with aromatic terminals were synthesized, and their gelation and subsequent photopolymerization abilities were analyzed. DAGs with an adequate spacer length (n) and tolyl terminals (DA-Tn) interacted with aromatic solvents, such as benzene and xylenes, at elevated temperatures. During the subsequent cooling process, the DAGs interacted with each other through CH-π interactions at their terminal positions. They also formed one-dimensional hydrogen bonding arrays through secondary amides, leading to stable organogels. These gels polymerized into π-conjugated polydiacetylenes (PDAs) under ultraviolet irradiation. In the p-xylene gels of DA-Tn, the spacer length exerted characteristic odd-even effects on the photopolymerization rates over a certain range (n = 3-6), which can be explained by periodic changes in the uniformity of the molecular packing modes. When the gelling solvent was changed to cyclohexane, the gelation and photopolymerization abilities were greatly improved because the DA-Tn gel networks became highly crystallized and transparent to ultraviolet light (254 nm). The ultimate conversion to PDA from DA-T8/cyclohexane gels was 45.2 wt %. Applying photolithographic techniques to the DAG with excellent photopolymerizability in the film state, we successfully fabricated microscale photopatterns of PDA. We also established a convenient removal process (development process) of DA monomers in unexposed areas. The resulting PDA patterns were quite stable to ambient light stimuli.
Highlights
When irradiated with ultraviolet light or γ-rays in their solid states, diacetylene (DA) derivatives undergo 1,4-addition polymerization into π-conjugated polydiacetylenes (PDAs)
The DA diamides used in this study were prepared by two synthetic routes (A and B shown in Scheme 1)
DA diamides with alkylene spacers of n = 3 and 8 (DA-T3, DA-T8, and DA−B3) were prepared in one step by amide condensation of the primary amines with commercially available DA dicarboxylic acids a(n)
Summary
When irradiated with ultraviolet light or γ-rays in their solid states, diacetylene (DA) derivatives undergo 1,4-addition polymerization into π-conjugated polydiacetylenes (PDAs). Such reactions (called topochemical reactions) should meet strict requirements on the DA monomer assemblages (i.e., a 5.0 Å stacking distance and 45° tilt angle) to minimize the 1,4distance among adjacent diacetylene cores, enabling progressive photopolymerization.[1,2] The photopolymerization of DAs has been vigorously investigated in organized systems such as crystals,[1,2] Langmuir−Blodgett (LB) films,[3−5] and micelles or vesicles.[6] The π-conjugated chains of PDA change their conformations under an external stimulus (e.g., heat,[7−9] mechanical stress,[10−14] light,[15,16] or chemical species[17−24]) Because this conformational change affects their apparent colors, PDAs have been widely studied as chemical and physical sensors.[25,26] When doped with an appropriate electron donor such as iodine, PDAs function as electroconductive materials.[27−29] PDAs are densely packed in crystal form and are poorly soluble in common organic solvents.[1,2]. Using the DAG with excellent photopolymerizability, we established a photolithographic process that fabricates microscale photopatterns of PDA with the desired shapes
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