Star-shaped Triphenylene-triazine Multi-stimuli Responsive Discotic Liquid Crystals: Synthesis, Properties and Applications

doi:10.6023/A23010006

Abstract

The electronic and optical properties of π-conjugated materials are determined by their molecular structures and supramolecular architectures. Discotic liquid crystals consist of π-conjugated aromatic core and peripheral alkyl chains, they self-organize into two-dimensional discotic columnar mesophase, which can be applied as charge carrier transport channel in optical and electronic thin-film devices. Order and dynamics are two unique features of discotic liquid crystalline semiconductors. Three star-shaped 1,3,5-triazine-triphenylene discotic liquid crystals TPPTn (n=6, 8, 12) were synthesized by Suzuki-Miyaura cross-coupling reaction and characterized. Thermogravimetric analysis (TGA) shows that TPPTn have good thermal stability, and their thermal decomposition temperatures were higher than 350 ℃. The ultraviolet-visible (UV-Vis) absorption spectra and fluorescence spectra showed that they have outstanding fluorescence emissions in solution, thin film and even in solid state, and the length of alkoxy chain affects a little the fluorescence properties of the film or solid state by affecting the molecular aggregation. TPPTn have the aggregation-induced emission (AIE) effect: longer the alkoxy chain, more obvious the AIE phenomenon, this AIE effectively overcomes the disadvantage of aggregation caused quenching (ACQ) of π-extended discotic liquid crystalline molecules. Further, TPPT6 exhibits significant solvatochromism from blue to orange with the increasing of solvent polarity, due to the intramolecular charge transfer (ICT), which provides possibility for visual recognition of solvent polarity. The fluorescent emission absolute quantum yield is up to 43%. What is more, TPPT6 exhibits a temperature-responsive fluorescence enhancement with increasing temperature in solution, which is different from the temperature quenching of traditional molecules, and the fluorescence exhibits a reversible switching from green to blue with increasing/decreasing of temperature. TPPT6 also has obvious acid-base stimuli- responsive fluorescent response by CF₃COOH and Et₃N. ¹H NMR titration proves that the protonation of triazine core is the mechanism to acid discoloration. Based on the acid-base stimuli response phenomena, two kinds of fluorescent safety inks have been designed and demonstrated. Differential scanning calorimetry (DSC), polarizing microscope (POM) and X-ray diffraction (XRD) indicate that these star-shaped molecules TPPTn have liquid crystal properties, self-organizing into columnar rectangular (Col_rec) mesophase with mesophase temperature range as wide as 278 ℃. Trifluoroacetic acid (CF₃COOH) modulates successfully the mesomorphic behavior of TPPTn by ionizing the triazine core. In addition, TPPT6 also displays electroluminescent properties, which has been applied to a green organic light-emitting diode (OLED), and the luminous efficiency of OLED reaches 7.41 lm/W. The star-shaped triphenylene-triazine TPPTn have potential applications as liquid crystalline semiconductors, fluorescence sensing, information encryption and organic light-emitting diodes.

Key words: triphenylene, triazine, multi-stimuli response, discotic liquid crystals, aggregation-induced emission, Suzuki- Miyaura cross-coupling reaction

Cite this article

Chongyang Zeng, Ping Hu, Biqin Wang, Wenyan Fang, Keqing Zhao, Bertrand Donnio. Star-shaped Triphenylene-triazine Multi-stimuli Responsive Discotic Liquid Crystals: Synthesis, Properties and Applications[J]. Acta Chimica Sinica, 2023, 81(5): 469-479.

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Fig. & Tab. 14

Scheme 1. Star-shaped compounds triphenylene-phenyl-triazine TPPTn: molecular structure, nomenclature, synthesis and yield

Figure 1. UV-Vis absorption spectra of TPPT6/8/12 in THF (a); fluorescent emission spectra (b); fluorescence photos of TPPTn in various states (c); electron distribution on frontier molecular orbitals of TPPT1 (d)

Figure 2. UV-Vis absorption spectra (a); fluorescence emission spectra (b) and Lippert-Mataga curve (c) of TPPT6 in different solvents, and inserted image is fluorescence photos (polarity increases from left to right)

Compound	Solvent	λ_a	ε (×10⁴)	λ_e	QY/%	Δν
TPPT6	CHX	360	19.62	449	43.02	5506
	toluene	362	21.27	454	42.20	5597
	EA	364	9.41	478	22.02	6552
	THF	364	24.89	490	18.44	7064
	EtOH	367	21.31	519	9.59	7980
	DMF	368	22.16	548	0.96	8925

Table 1. Photophysical property parameters of TPPT6

Figure 3. The UV-Vis absorption spectra (a), PL emission spectra (b) and fluorescence intensity change map (c) of TPPT6 in THF/H2O with different water fraction. (d) for the water fraction between 0~10% of the fluorescence spectra, inserted picture for the fluorescence quenching fitting curve

Figure 4. UV-Vis absorption and fluorescence spectra of TPPT6 in lower and higher temperature in CH3COOEt (a); fluorescence wavelength variation with alternating heating-cooling (b); fluorescence spectra at 25~75 ℃ (c), insert the picture for the linear fitting of temperature and I/I0; (d) the TPPT molecular conformations at 300 K and 350 K

Figure 5. Photophysical property of TPPT6 by acid-base (CF3COOH/Et3N) stimulation. UV-Vis absorption spectra (a); fluorescence periodic cycle diagram (b); emission spectra (c); solid sample fluorescence spectra before and after acid-base stimulation (d); sunlight/fluorescence images of solid samples (e); fluorescence images of thin films (f)

Compd.	2nd heating [1st cooling]/℃ (∆H/(kJ•mol^-1))
TPPT6	Col_rec 278 (36.26) I [I 263 (38.85) Col_rec]
TPPT8	Col_rec 231 (27.90) I [I 221 (29.40) Col_rec]
TPPT12	Cr -7 Col_rec 149 (12.19) I [I 153 (14.94) Col_rec -13 Cr]
TPPT6-H	Col_rec 221 I [I 220 Col_rec]

Table 2. Phase transition temperature and enthalpy change of TPPT6/8/12 and TPPT6-H

Figure 6. POM photo micrographs of TPPT6/8/12 and TPPT6-H

Figure 7. Representative SAXS/WAXS patterns of TPPTn compounds at one temperature (left column) and with different temperatures (right column)

Figure 8. Two-dimensional supramolecular packing model of TPPTn rectangular columnar liquid crystal phase (Colrec), a rectangular superlattice containing two molecules. (a) parallel and (b) antiparallel arrangements of the trimers

TPPT6	TPPT8	TPPT12
Col_rec a=7.104 nm, b=3.321 nm A=23.5924 nm² V_mol=4.645 nm³ h_mol=0.394 nm, N_mol=2	Col_rec a=8.692 nm, b=3.478 nm A=30.2308 nm² V_mol=5.450 nm³ h_mol=0.360 nm, N_mol=2	Col_rec a=9.868 nm, b=3.885 nm A=38.3372 nm² V_mol=7.255 nm³ h_mol=0.378 nm, N_mol=2

Table 3. Mesophase’s parameters of the star-shaped triphenylene-triazine TPPTn

Figure 9. TPPT6 application as safety ink. ink 1: diluted solution of TPPT6 in CHCl3; ink 2: ink 1 and CF3COOH

Figure 10. Electroluminescence spectra of TPPT6 (a), CIE 1931 chromaticity diagram (b), forward current versus luminous efficiency at constant voltage, light intensity relationship diagram (c), photos of OLED devices with and without electricity (d)

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