Mechanical-Bond-Induced Exciplex Fluorescence in an Anthracene-Based Homo[2]catenane

Collisional
intermolecular interactions between excited states
form short-lived dimers and complexes that lead to the emergence of
excimer/exciplex emission of lower energy, a phenomenon which must
be differentiated from the photoluminescence (PL) arising from the
monomeric molecules. Although the utilization of noncovalent bonding
interactions, leading to the generation of excimer/exciplex PL, has
been investigated extensively, precise control of the aggregates and
their persistence at very low concentrations remains a rare phenomenon.
In the search for a fresh approach, we sought to obtain exciplex PL
from permanent structures by incorporating anthracene moieties into
pyridinium-containing mechanically interlocked molecules. Beyond the
optical properties of the anthracene moieties, their π-extended
nature enforces [π···π] stacking that can
overcome the Coulombic repulsion between the pyridinium units, affording
an efficient synthesis of an octacationic homo[2]­catenane. Notably,
upon increasing the ionic strength by adding tetrabutyl­ammonium
hexafluoro­phosphate, the catenane yield increases significantly
as a result of the decrease in Coulombic repulsions between the pyridinium
units. Although the ground-state photophysical properties of the free
cyclophane and the catenane are similar and show a charge-transfer
band at ∼455 nm, their PL characters are distinct, denoting
different excited states. The cyclophane emits at ∼562 nm (quantum
yield ϕF = 3.6%, emission lifetime τs = 3 ns in MeCN), which is characteristic of a disubstituted anthracene–pyridinium
linker. By contrast, the catenane displays an exciplex PL at low concentration
(10–8 M) with an emission band centered on 650 nm
(ϕF = 0.5%, τs = 14 ns) in MeCN
and at 675 nm in aqueous solution. Live-cell imaging performed in
MIAPaCa-2 prostate cancer cells confirmed that the catenane exciplex
emission can be detected at micromolar concentrations.