A naphthalimide-based fluorescent gelator (N1) containing an alkenyl group has been designed and characterized. This material is able to gelate alcohols via a precipitate-to-gel transformation when triggered with ultrasound for less than 2 min (S-gel). The gelation process in n-propanol was studied by means of absorption, fluorescence, and IR spectra, scanning electron microscopy (SEM) images, and X-ray diffraction patterns. The fluorescence intensity of N1 decreased during the gelation process in a linear relationship with the sonication time. The S-gel of N1 could be used to sense aliphatic and aromatic amines by measuring the change in the signal output. For example, the addition of propylamine to the S-gel of N1 resulted in a dramatic enhancement of the fluorescence intensity, accompanied by a gel-to-sol transition. On the contrary, when the S-gel of N1 was treated with aromatic amines such as aniline, fluorescence was quenched and there was no gel collapse. The sensing mechanisms were studied by (1)H NMR, small-angle X-ray scattering, SEM and spectroscopic experiments. It is proposed that isomerization of the alkenyl group of N1 from the trans to cis form occurs when the S-gel is treated with propylamine, resulting in a gel-sol transition. However, the aromatic aniline molecules prefer to insert into the gel networks of N1 via hydrogen-bonding and charge-transfer interactions, maintaining the gel state. As potential applications, testing strips of N1 were prepared to detect aniline.
This report describes how the luminescence of naphthalimide could be tuned by various physical stimuli, including heat, sonication, and grinding. Herein, instant and switchable control of color and fluorescent emissions has been achieved by the sonication-triggered gelation of an organic liquid with naphthalimide-based organogelators (N3-N7). Green emissive suspensions of the gelators in organic liquids are transformed into orange emissive gels upon brief irradiation with ultrasound with an emission wavelength red-shift of approximately 60 nm and fluorescence intensity quenching by a factor of 20, which can subsequently be reversed by heating. When sonication-triggered S-gels are evaporated to S-xerogels, the solid state xerogels (N3, N4, N6, N7) exhibit mechanochromism, the color of which changes from red to yellow and the emission color of which changes from orange to green with enhanced intensity by grinding. This mechanochromic property can be reversed through a regelation process. The mechanochromic character of the S-xerogel of N3 is thus applied to quantitatively sense the mechanical pressure range from 2 to 40 MPa through fluorescence changes, reflecting a new type of application for gelation assembly. The physical stimuli triggered fluorescence changes of these compounds strongly depend on the molecular structure and solvent. The results demonstrate that the different aggregation modes and long-range order arrangement of the molecules regulated by the stimulus may affect the internal charge transfer (ICT) process of the naphthalimide groups, resulting in the tunability of the photophysical properties of the gelators. This report provides a new strategy for tunable and switchable control of luminescence through nonchemical stimuli in aggregation-based monocomponent systems.
In this work, two naphthalimide-based compounds, 1a and 1b, have been designed and synthesized. Both compounds can form stable two-component gels in n-propanol or n-butanol upon addition of α-cyclodextrin (α-CD) followed by sonication at room temperature. Interestingly, the 1a/α-CD gel is thixotropic and very sensitive to water. Addition of a small amount of water induces rapid gel collapse, allowing further development of the gel as a visual relative humidity sensor. Specificity of the sensor has been confirmed using several approaches, such as scanning electron microscopy and fluorescence, Fourier transform infrared, and H NMR spectroscopy experiments. The results show that α-CD acts as a junction for the assembly of 1a or 1b through hydrogen bonding between hydroxyl and amide groups. Upon addition of water, α-CD interacts with the adamantane group of 1a via an incomplete host-guest encapsulation, resulting in the dissociation of the hydrogen-bonding-assisted two-component assembly, accompanied by gel collapse.
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