Nanocage-Confined Synthesis of Fluorescent Polycyclic Aromatic Hydrocarbons in Zeolite

Polycyclic
aromatic hydrocarbons (PAHs) attract much attention
for applications to organic light-emitting diodes, field-effect transistors,
and photovoltaic cells. The current synthetic approaches to PAHs involve
high-temperature flash pyrolysis or complicated step-by-step organic
reactions, which lead to low yields of PAHs. Herein, we report a facile
and scalable synthesis of PAHs, which is carried out simply by flowing
acetylene gas into zeolite under mild heating, typically at 400 °C
and generates the products of 0.30 g g–1 zeolite.
PAHs are synthesized via acetylene polymerization inside Ca2+-ion-exchanged Linde type A (LTA) zeolite, of which the α-cage
puts a limit on the product molecular size as a confined-space nanoreactor.
The resultant product after the removal of the zeolite framework exhibits
brilliant white fluorescence emission in N-methylpyrrolidone
solution. The product is separated into four different color emitters
(violet, blue, green, and orange) by column chromatography. Detailed
characterizations of the products by means of various spectroscopic
methods and mainly mass spectrometric analyses indicate that coronene
(C24H12) is the main component of the blue emitter,
while the green emitter is a mixture of planar and curved PAHs. The
orange can be attributed to curved PAHs larger than ovalene, and the
violet to smaller molecules than coronene. The PAH growth mechanism
inside Ca2+-exchanged LTA zeolite is proposed on the basis
of mass spectral analyses and density functional theory calculations.