Abstract
Two probes, AIE-1 and AIE-2, were synthesized to investigate the effect of substitutional functional group on aggregation (aggregation-caused quenching (ACQ) or aggregation-induced emission (AIE)) and intramolecular charge transfer (ICT) behavior as well as on the cell imaging aspect. The yellow-color non-substituted probe AIE-1 showed weak charge-transfer absorption and an emission band at 377 nm and 432 nm, whereas the yellowish-orange color substituted probe AIE-2 showed a strong charge-transfer absorption and an emission band at 424 nm and 477 nm in THF solvent. The UV-Vis studies of AIE-1 and AIE-2 in THF and THF with different water fractions showed huge absorption changes in AIE-2 with high water fractions due to its strong aggregation behavior, but no such noticeable absorption changes were observed for AIE-1. Interestingly, the fluorescence intensity of AIE-1 at 432 nm gradually decreased with increasing water fractions and became almost non-emissive at 90% water. However, the monomer-type emission of AIE-2 at 477 nm was shifted to 584 nm with a 6-fold increase in fluorescence intensity in THF-H2O (1:9, v/v) solvent mixtures due to the restriction of intramolecular rotation on aggregation in high water fractions. This result indicates that the probe AIE-1 shows ACQ and probe AIE-2 shows AIE behaviors in THF-H2O solvent mixtures. Furthermore, the emission spectra of AIE-1 and AIE-2 were carried out in different solvent and with different concentrations to see the solvent- or concentration-dependent aggregation behavior. Scanning electron microscope (SEM) and dynamic light scattering (DLS) experiments were also conducted to assess the morphology and particle size of two probes before and after aggregation. Both of the probes, AIE-1 and AIE-2, showed less toxicity on HeLa cells and were suitable for cell imaging studies. Density functional theory (DFT) calculation was also carried out to confirm the ICT process from an electron-rich indole moiety to an electron-deficient cyano-phenyl ring of AIE-1 or AIE-2.
Highlights
Fluorescent materials with changeable optical properties have attracted considerable attention because of their valuable applications in chemistry, materials science, and biology [1].Molecules 2020, 25, 585; doi:10.3390/molecules25030585 www.mdpi.com/journal/moleculesthe development of suitable fluorescent materials is essential for high-technology applications, such as organic optoelectronic devices [2], organic photovoltaic devices (OPVs) [3], organic field effect transistors [4], data storage devices [5], and sensors [6]
The aggregation-induced emission enhancement (AIEE)/aggregation-caused quenching (ACQ) behaviors were explained by the twisted to planer conformational changes, including the restriction of intramolecular single bond rotation in their aggregation state
This AIEE or ACQ results mainly depended on the types of stacking of aggregation-induced emission (AIE)-1 or AIE-2 molecules in their aggregation state
Summary
Fluorescent materials with changeable optical properties have attracted considerable attention because of their valuable applications in chemistry, materials science, and biology [1]. Most fluorescent materials suffer for their non-emissive nature in their aggregate or condensed state (ACQ effect) due to strong intermolecular π–π interaction, which is harmful for their practical applications [11]. Cyano-functionalized fluorophores are well known for their intrinsic luminescent properties, highly photostability, mechanofluorochromism, and AIE phenomenon [31]. Keeping in mind all the reported examples of cyano-functionalized derivatives, we were interested in developing some new donor-acceptor-containing cyano-functionalized derivatives with an easy synthesis procedure and from low-cost materials because of their high photostability, biomedical applications, and intrinsic luminescent properties. We synthesized and characterized two cyano-functionalized fluorophores, AIE-1 and AIE-2, based on AIE/ACQ and ICT characteristics to investigate the structural-dependent aggregation behaviors, solvatochromism, cell cytotoxicity, and cell imaging applications (Figure 1). The electron-rich indole moiety transferred electrons to the electron-deficient cyano-functionalized phenyl ring through conjugated π-systems
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