Significant Effect of Guest Ions on the Fluorescence Behaviors of Palmatine-Based Supramolecular Systems

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 pi-pi 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.