Three new nonplanar barbituric derivatives, named as TTB, TTTB, and TOB, were synthesized. The D-
Conventional organic luminescent materials have little fluorescence due to aggregation (aggregation-caused quenching (ACQ) effect), which severely limits the applications in solid state [
In our former work, a series of thiophene derivatives with one, two, and four branches attaching on thiophene, respectively, were synthesized [
Based on the former work, we further design and synthesize several dipolar compounds constituted by thiophene and barbitural acid in the current submission. In this work, the monomer, dimer, and trimer of thiophenyl serve as electron donor. The emphasis of this work is put on the relationship between the degree of the oligomerization in the electron donor and the AIE effect. The research results show that the compound with the trimer of thiophenyl exhibits the strongest AIE effect. Additionally, these barbituric derivative nanoaggregates exhibited strong mechanofluorochromism (MFC) effects. The detailed results are reported below.
In this paper, three barbituric derivatives were synthesized and exhibited very good AIE property. Piezochromic luminescent materials are a type of force-stimulated responsive materials that change the luminescence color and intensity by varying the physical packing of the molecules [
All the reagents were obtained commercially and used without further purification: 1,3-dimethylbarbituric acid (Aladdin, 98%), 2,2
The compound of 1a was synthesized according to literature methods [
Synthetic routes towards TTB, TTTB, and TOB compounds.
A mixture of 1,3-dimethylbarbituric acid (0.960 g, 6.18 mmol) and 2,2
A mixture of 1,3-dimethylbarbituric acid (0.670 g, 4.35 mmol) and 2,2
A mixture of 1,3-dimethylbarbituric acid (1.05 g, 6.74 mmol) and 1a (1.00 g, 5.62 mmol) in ethanol (10 mL) and two drops of acetic acid was refluxed for 6 h. The reaction was cooled to room temperature and the solid particles were filtered. The product was then purified by recrystallization with acetic acid to give 0.58 g (61% yield): 1H NMR (600 MHz, CDCl3):
As shown in Figure
(a) Absorption and (b) emission spectra of three compounds in THF solution (concentration:
The photoluminescence (PL) behaviors of compounds TTB, TTTB, and TOB were investigated in THF/H2O mixed solvent (Figure
(a) FL spectra of TTB, TTTB, and TOB in THF/H2O with different water fractions (
PL spectra of TTB, TTTB, and TOB in different solvents with varying polarities (concentration:
Plots of the electrostatic potential for TTB (a), TTTB (b), and TOB (c) with B3LYP/6-31G (d) on the isodensity surface of 0.001. The color is coded as red for strong negative and blue for strong positive.
Photographs of TTB, TTTB, and TOB at
The AIE index
The morphology and size of the nanoaggregates in the THF/H2O with different
SEM images (a) and DLS sizes (b) of TTB, TTTB, and TOB in THF/H2O (nanoaggregate state) at a concentration of
As shown in Figure
Energy levels of HOMO and LUMO, energy gap, and electron cloud distribution of three molecules calculated by B3LYP/6-31G (d) program.
Based on the MFC characteristics of nonplanar AIE molecules, the MFC performances of TTB, TTTB, and TOB with helical structure were investigated. As shown in Figures
Photos of TTB, TTTB, and TOB and color changes under grinding and fuming stimuli.
Normalized fluorescent spectra of TTB, TTTB, and TOB in different solid states: original nanoaggregate, ground, and fumed (excitation wavelength, TTB: 410 nm, TTTB: 530 nm, and TOB: 440 nm).
After grinding, the three ground powders were exposed to CH2Cl2 vapor at room temperature. As can be seen from Figure
Powder X-ray diffraction (XRD) were used to further explore the MFC mechanism of these nanoaggregates. As shown in Figure
XRD patterns of TTB, TTTB, and TOB in different solid states: original, ground, and fumed.
In summary, we reported three compounds (TTB, TTTB, and TOB) with typical AIE characteristics and highly reversible piezochromic and vapochromic properties. Among them, TTTB displayed the largest MFC spectra shift (26 nm) with visible color changes. It was confirmed that the emission bands of these compounds and their intensity dramatically changed by switching the nanoaggregates between crystalline and amorphous. These nanomaterials can be used in color emission and security [
The data used to support the findings of this study are available from the corresponding authors upon request.
The authors declare that they have no conflicts of interest.
This work was financially supported by the Natural Science Foundation of Shandong Province (ZR2018MB025), the Key R&D Program Projects of Shandong Province (2017GGX202001), and the Funds for Research Leader in Jinan (2018GXRC028).
Figure S1: the 1H-NMR spectra of TTB. Figure S2: the 1H-NMR spectra of TTTB. Figure S3: the 1H-NMR spectra of TOB. Figure S4: the 13C-NMR spectra of TTB. Figure S5: the 13C-NMR spectra of TTTB. Figure S6: the 13C-NMR spectra of TOB. Figure S7: the mass spectra of TTB. Figure S8: the mass spectra of TTTB. Figure S9: the mass spectra of TOB.