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Beilstein J. Org. Chem. 2020, 16, 2282–2296.
Introduction
Push-pull organic molecules are a class of organic dyes com- chemosensors for sensing of specific ions. In addition, some de-
prising of electron-donating and accepting groups in a rivatives also show NLO activity [25,34-37].
donor–π–acceptor (D–π–A) system. The dyes exhibit strong
absorption and emission properties in solution and in the solid Herein, we report on the synthesis and full photophysical char-
state [1-3]. One important feature of these molecules is an acterization of a series of new styryl-based organic chro-
exceptional polarizability which is a crucial criterion for NLO mophores containing a free amino group and the corresponding
materials. Nowadays, organic NLO materials bearing strong Schiff base derivatives. The photophysical, pH sensitivity, NLO
donor–acceptor groups with a π-bridge have shown extensive properties, and thermal stabilities of all synthesized dyes were
usages in signal processing, optical storage, and telecommuni- investigated. Density Functional Theory (DFT) calculations
cation devices [4-10]. The flexibility to change electron-donor were also employed to gain insight into the experimental data.
and acceptor groups in the molecules allows to tune intramolec-
Results and Discussion
Synthesis
As depicted in Scheme 1, dye 2 was obtained in good yield by
ular charge transfer (ICT) intensity and it can increase NLO
Some ions such as proton (H+) and hydroxide (OH−), in reacting malononitrile and 4-aminoacetophenone (compound 1)
aqueous environments, play significant roles in industrial, bio- according to our previous published procedure [31,32].
logical, and environmental processes [15-17]. Their presence in
aqueous media directly determines the pH level that affects The styryl-based organic chromophores 3–7 having a free
organisms living in the corresponding area [18-22]. Therefore, amino and strong donor-acceptor groups and a pyridin-2-yl
pH-sensitive dyes play a critical role in various sensor applica- moiety were synthesized by the base-catalyzed condensation of
tions for easy determination of such ions. These dyes show dif- 2 with different aldehyde derivatives in ethanol, as depicted in
ferent spectral properties upon protonation/deprotonation pro- Scheme 1. Further reactions of the dyes 3–7 with salicylalde-
cesses [23-25]. Fluorescent dyes as chemosensors offer unique hyde under basic conditions resulted in the formation of an azo-
merits such as low energy consumption, ease of handling, high methine bridge on each amine end yielding dyes 8–12 as the
selectivity, and notable sensitivity [26,27]. Moreover, some final products. All dyes were obtained in moderate to excellent
fluorescent dyes demonstrate remarkable ICT characteristics, yields (58–96%) and generally without the need for chromato-
and could therefore serve as NLO dyes [11-14].
graphic purification. The structures of all the new synthesized
dyes were confirmed by FTIR, 1H, 13C NMR, and HRMS
Among dyestuffs classes, the push-pull fluorescent dyes are analyses (Figures S1–S40 in Supporting Information File 1).
renowned to own such special behaviors. The push-pull dyes Moreover, the stereochemistry of the double bonds was deter-
generate higher charge delocalization upon excitation, thus en- mined on the basis of the coupling constants of the vinylic
hance both polarizability and fluorescence emission [12-14,18]. hydrogens in the 1H NMR spectra (J ≈ 15–16 Hz) and revealed
The charge delocalization upon excitation leads to a red-shifted that the dyes are stable as E-stereoisomers [31].
emission which is viable for the detection of various substrates
in biological tissues and samples [28-30]. Recently, there was Optimized geometries
an increasingly growing interest in studies regarding push-pull The optimized geometries of dyes 3–7 and 8-12 were obtained
organic molecules comprising of the dicyanomethylene group by performing DFT calculations at the B3LYP/6-31+G(d,p)
as a strong electron-accepting group coupled with various level of theory. The structures are illustrated in Figure 1 and
donors connected via a π-conjugation bridge [31-33]. Such dyes Figures S41 and S42 (Supporting Information File 1). As shown
offer good NLO characteristics when compared to Disperse Red in Figure 1, the different substituents were planar to the
1 as well as remarkable thermal stabilities with dissociation dicyanomethylene group. A twist was observed between the
temperatures up to 300 °C [31,32].
dicyanomethylene and aminophenyl groups (salicylidenyl
moiety for 8–12) with an angle of about 50°. In addition, the
Schiff bases containing an azomethine group are one of the dyes 8–12 exhibited strong intramolecular O–H···N hydrogen
most widely used organic dyes because of their easy and cheap bonds with lengths in the range of 2.63–2.64 Å (Figure 1,
synthetic accessibility through various methodologies and suit- Figures S41 and S42 in Supporting Information File 1).
able photophysical properties. In addition, they exhibit a broad
range of biological activities such as antimicrobial, antifungal, Absorption and emission properties
antiviral, and anticancer activity, to name a few. Moreover, To assess the effect of the solvent on the absorption and emis-
Schiff bases bearing a salicylidene moiety are also used as sion spectra of 3–7 and 8–12 (c = 10 μM for absorption, and
2283
?akmaz, Deniz
