Confinement of Aggregation-Induced Emission Molecular Rotors in Ultrathin Two-Dimensional Porous Organic Nanosheets for Enhanced Molecular Recognition

Despite the rapid development of
molecular rotors over the past
decade, it still remains a huge challenge to understand their confined
behavior in ultrathin two-dimensional (2D) nanomaterials for molecular
recognition. Here, we report an all-carbon, 2D π-conjugated
aromatic polymer, named NUS-25, containing flexible tetraphenylethylene
(TPE) units as aggregation-induced emission (AIE) molecular rotors.
NUS-25 bulk powder can be easily exfoliated into micrometer-sized
lamellar freestanding nanosheets with a thickness of 2–5 nm.
The dynamic behavior of the TPE rotors is partially restricted through
noncovalent interactions in the ultrathin 2D nanosheets, which is
proved by comparative experimental studies including AIE characteristics,
size-selective molecular recognition, and theoretical calculations
of rotary energy barrier. Because of the partially restricted TPE
rotors, NUS-25 nanosheets are highly fluorescent. This property allows
NUS-25 nanosheets to be used as a chemical sensor for the specific
detection of acenaphthylene among a series of polycyclic aromatic
hydrocarbons (PAHs) via fluorescent quenching mechanism. Further investigations
show that NUS-25 nanosheets have much higher sensitivity and selectivity
than their stacked bulk powder and other similar polymers containing
dynamic TPE rotors. The highly efficient molecular recognition can
be attributed to the photoinduced electron transfer (PET) from NUS-25
nanosheets to acenaphthylene, which is investigated by time-resolved
photoluminescence measurements (TRPL), excitation and emission spectra,
and density functional theory (DFT) calculations. Our findings demonstrate
that confinement of AIE molecular rotors in 2D nanomaterials can enhance
the molecular recognition. We anticipate that the material design
strategy demonstrated in this study will inspire the development of
other ultrathin 2D nanomaterials equipped with smart molecular machines
for various applications.