Palmatine as a natural isoquinoline alkaloid possesses
biological
activity, low toxicity, biodegradability, fluorescence, and antibacterial
properties; however, the low fluorescent intensity and poor stability
hindered its practical applications, especially in bioimaging, detection
of probes, and functional dyes as the fluorophore. Serial aromatic
carboxylic acids as guest ions were introduced by replacing anions
of the host Palmatine to fabricate new crystalline Palmatine-based
supramolecular salts or cocrystals. The structures and fluorescent
properties of the prepared supramolecules were examined by single-crystal
X-ray diffraction, thermogravimetry, ultraviolet spectrophotometry,
fluorescence spectrometry, and infrared spectrometry. It was found
that the fluorescence performance could be tuned by introducing guest
molecules based on salicylic acid with different push–pull
electronic groups to affect the π–π conjugation
system. The introduction of –NO2 in salicylic acid
can result in a decrease in fluorescence on account of withdrawing
electrons from the benzene ring, thereby reducing the probability
of electron transition. Surprisingly, the fluorescence intensity of
Palmatine-aminosalicylic acid (–NH2) exhibited a
10-fold increase due to the decline of the energy gap between the
highest occupied molecular orbital and the lowest unoccupied molecular
orbital. These findings provided a theoretical basis for further regulating
the fluorescence performance of Palmatine and other natural dyes through
the introduction of guest molecules to form the supramolecular systems.