Schiff bases represent
an essential class in organic chemistry
with antitumor, antiviral, antifungal, and antibacterial activities.
The synthesis of Schiff bases requires the presence of an organic
base as a catalyst such as piperidine. Base-free synthesis of organic
compounds using a heterogeneous catalyst has recently attracted more
interest due to the facile procedure, high yield, and reusability
of the used catalyst. Herein, we present a comparative study to synthesize
new Schiff bases containing indole moieties using piperidine as an
organic base catalyst and Au@TiO
2
as a heterogeneous catalyst.
In both methods, the products were isolated in high yields and fully
characterized using different spectral analysis techniques. The catalyst
was reusable four times, and the activity was slightly decreased.
The presence of Au increases the number of acidic sites of TiO
2
, resulting in C=O polarization. Yields of the prepared
Schiff bases in the presence of Au@TiO
2
and piperidine
were comparable. However, Au@TiO
2
is an easily separable
and recyclable catalyst, which would facilitate the synthesis of organic
compounds without applying any hazardous materials. Furthermore, the
luminescence behavior of the synthesized Schiff bases exhibited spectral
shape dependence on the substituent group. Interestingly, the compounds
also displayed deep-blue fluorescence with Commission Internationale
de l’Éclairage (CIE) coordinates of
y
< 0.1. Thus, these materials may contribute to decreasing the
energy consumption of the emitting devices.
Biocompatible luminogens
with aggregation-induced emission (AIE) have several applications
in the biology field, such as in detecting biomacromolecules bioprobes
and in bio-imaging. Due to their bioactivities and light-emitting
properties, many heterocyclic compounds are good candidates for such
applications. However, heterocyclic π-conjugated systems with
AIE behavior remain rare as strong intermolecular π–π
interactions usually quench their emission. In this work, new thienopyrimidine
heterocyclic compounds were synthesized and their structures were
verified by elemental analysis and Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), and 13C NMR
spectra. The photophysical properties of some compounds were investigated
in the solution and solid states. Density functional theory calculations
were also performed to confirm the observed photophysical properties
of the compounds. The studied dyes displayed AIE properties with spectral
shapes related to the aggregate structure and a quantum yield up to
10.8%. The emission efficiency of the powder is attributed to the
incorporation of multiply rotatable and twisted aryl groups to the
fused heterocyclic moieties. The dyes also showed high thermal stability
and potent antimicrobial activities against numerous bacterial and
fungal strains. Additionally, the cytotoxicity of the new compounds
was evaluated against the Caco-2 cell line, and molecular docking
was used to investigate the binding conformation of the most effective
compound with the MNK2 enzyme. Therefore, the presented structures
may potentially be used for bioapplications.
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