Self-Association and Nitroaromatic-Induced Deaggregation of Pyrene Substituted Pyridine Amides

The
self-assembly features of the bis-pyrene methyl amide functionalized
pyridine and benzene “tweezers” 1 and 2 were studied in organic solution and in the solid state.
These systems were found to display remarkably different self-association
features and optical properties, which was rationalized by control
experiments using compounds bearing pyrenemethyl esters, alkyl groups,
or a single pyrene substituent (3–6). As dilute solutions in chloroform, tweezers 1 displays
both pyrene monomer and excimer emission features reflecting intramolecular
contacts between the pyrene subunits. At higher concentrations in
chloroform, as well as in the solid state, tweezers 1 self-assembles to form a linear supramolecular polymer. In contrast,
tweezers 2 does not interact in an intermolecular fashion
and photoexcitation produces emission features characteristic of a
pyrene monomer. DFT (density functional theory) and TDDFT (time dependent
density functional theory) calculations revealed that the lowest vertical
transitions are forbidden and that S1 of 1 is an emissive state. In contrast to 1 and 2, both pyrene-free control systems 5 and 6 were found to form linearly self-assembled supramolecular arrays
in the solid state, albeit of differing structure. Upon exposure to
trinitrobenzene (TNB), the self-assembled structures formed from 1 undergo deaggregation to form TNB complexes. This change
is reflected in both an easily discernible color change and a quenching
of the fluorescence emission intensity. Changes in the optical features
were also seen in the case of 2. However, notable differences
between these two ostensibly similar systems were seen.