Spontaneous Formation of Fluorescent Nanofibers and Reticulated Solid from Berberine Palmitate: A New Example of Aggregation-Induced Emission Enhancement in Organic Ion Pairs

Abstract

The salt formed between the large aromatic berberine cation and the long-chain palmitate anion was synthesized and used to prepare aqueous suspensions of particles owing to a solvent-exchange method. Under these conditions, elongated particles were readily obtained. They were studied by transmission microscopy with polarized light, as well as by fluorescence and electron microscopy. They were shown to be probably crystallized nanofibers, which were stable in suspension. Unexpectedly, upon filtration and drying, these fibers evolved to give a reticulated solid. The fluorescence properties of the compound were analyzed in solution, in aqueous suspension and in the powder crystalline state. Interestingly, berberine palmitate is virtually not fluorescent in aqueous solution because of the quenching effect of water, but transition to the solid state was accompanied by a strong increase in fluorescence intensity. This phenomenon was explained by the original molecular arrangement in the solid state. Actually, in the crystal, the anions form a distinct layer, which limits parallel-stacking of the fluorescent cations. Moreover, the berberine cations are protected from the access of water molecules, and so no quenching effect can take place. This example confirms that the newly introduced concept of ion-pair aggregation-induced fluorescence enhancement can be extended to a variety of structures. It also shows the interest of ion pairs for preparing fluorescent nanofibers and reticulated solids using a solvent-exchange method that is particularly easy to implement.

Unusual Aggregation‐Induced Emission of a Coumarin Derivative as a Result of the Restriction of an Intramolecular Twisting Motion

“…Although this approach enables the straightforward preparation of AIE luminogens, in a sense it kills imagination and constrains the diversity of AIE luminogens. Hence, many π‐conjugated planar traditional chromophores are rarely considered to be AIE‐active, owing to the lack of aromatic rotors, and there are only limited reports on new hypotheses for unusual AIE systems, such as the restriction of intramolecular charge transfer for electron‐donor–acceptor molecules, and the formation of ion pairs for the berberine palmitate dye 28–30…”

Aggregation‐Induced Emission: The Whole Is More Brilliant than the Parts

“…The space limitation forces us to be highly selective in choosing examples for presentation and discussion. As a compensation to this pitiful loss, we herein provide a list of selected papers published recently by different research groups in the area in the Reference Section . Although much work has been done in the area, many intriguing possibilities remain to be explored.…”

Weak C—H...X(X= O, N) hydrogen bonds in the crystal structure of dihydroberberine

“…Surprisingly, while the structure of berberine crystallized with a variety of counter-anions has been reported (Moniot & Shamma, 1979; Abadi et al , 1984; Kariuki & Jones, 1995; Marek, Sečkářová et al , 2003; Tong et al , 2010; Chahine et al , 2011), as have the structures of an assortment of 8-substituted derivatives of dihydroberberine (Man et al , 2001; Marek, Hulová et al , 2003; Dostál et al , 2004; Maier et al , 2010; Man et al , 2011), a determination of the crystal structure of dihydroberberine itself has not been undertaken. Herein, we report its crystal structure from X-ray diffraction data at 150 K.…”