Incorporation of directing amide groups has been shown
to facilitate
the topochemical polymerization of 1,3-butadiyne (diacetylene) groups
in noncrystalline phases such as gels, amorphous solids, and liquid
crystals. It remains challenging to polymerize 1,3-butadiyne-containing
alkylthiolate ligands within their self-assembled monolayers on gold
nanoparticles (AuNPs), which enhances their stability and adds new
optical and electronic properties. Especially smaller AuNPs of sizes
below 5 nm in diameter have been reported to display sluggish photopolymerization
and are susceptible to photodegradation under UV irradiation. To probe
the effectiveness of the amide-directed photopolymerization of 1,3-butadiyne
ligands, small AuNPs in the 2–4 nm range were synthesized that
contain alkylthiolate ligands with and without amide and 1,3-butadiyne
groups. Their photopolymerization and photostability were studied
by transmission electron microscopy (TEM), UV–vis spectroscopy,
and Raman spectroscopy. AuNP with amide-free 1,3-butadiyne ligands
templated the polymerization of the 1,3-butadiyne ligands but fused
to large and insoluble particles during the polymerization process.
AuNPs with ligands containing both 1,3-butadiyne and amide groups
polymerized significantly faster, which slowed down photodegradation.
A UV irradiation (254 nm and 176 W/m2) for 5–10
min was found to be optimal for the AuNPs with directing amide groups
studied here, although their average core sizes grew from 3.8 to 4.0
nm in diameter and about 20% of the attached 1,3-butadiyne ligands
remained unreacted after 10 minutes of irradiation. About 75% of the
attached 1,3-butadiyne ligands were already polymerized during the
first 5 min of UV irradiation. This decrease in reactivity is reasoned
with a fast polymerization of ligands attached to facet sites and
slower polymerization rates for ligands attached to edge and corner
sites. Unexpectedly, photopolymerization occurred only in the presence
of solvent, whereas no polydiacetylene was generated when dry powders
of any of the diacetylene-containing gold nanoparticles were irradiated.