A practical strategy for construction and regulation of multi-functional triazepinium salts via highly efficient I2-catalyzed cyclization

Lang Liu a, Mengyao She ab, Jun Zhang a, Zhaohui Wang a, Hua Liu a, Mi Tang a, Ping Liu *a, Shengyong Zhang a and Jianli Li *a
aMinistry of Education Key Laboratory of Synthetic and Natural Functional Molecule Chemistry, College of Chemistry & Materials Science, Northwest University, Xi'an, Shaanxi 710127, P. R. China. E-mail: lijianli@nwu.edu.cn
bMinistry of Education Key Laboratory of Resource Biology and Modern Biotechnology in Western China, The College of Life Sciences, Faculty of Life and Health Science, Northwest University, Xi'an, Shaanxi Province 710069, P. R. China

Received 20th December 2019 , Accepted 22nd March 2020

First published on 23rd March 2020

Synthesis of new functional organic molecules is a critical path that may greatly accelerate the evolution of organic optoelectronic materials. We have achieved a facile synthesis strategy to produce unique saddle-shaped multi-functional triazepinium salts that exhibit excellent solid-state fluorescence. Their fluorescence performance could be easily regulated by adjusting the dihedral angle between the main skeleton and the substituted moiety, giving a large Stokes shift, non-aggregation quenching, long lifetime, and multilevel-redox characteristics.


With the rapid evolution of photoelectric materials, increasing attention has been given to exploring and designing organic materials with fluorescence characteristics in the past decades.1 These materials underpin the development of some optoelectronic devices, such as organic light-emitting diodes (OLEDs),2 organic solid-state lasers,3 smart materials,4 fluorescent bioimaging,5 data recording and storage,6 security printing,7 and fluorescent molecular sensors.8 Most of the organic luminophores, e.g., perylene, pyrene, anthracene, BODIPY, etc., exhibit strong aggregation-caused quenching (ACQ).9–12 That is, they fluoresce strongly when in the dissolved or molecularly dispersed state, but become weakly or not at all emissive in the solid state. Likewise, the disadvantages of small Stokes shifts,13 pH sensitivity,14 and photobleaching15 severely limit the scope of practical applications. Thus, it will be extremely challenging to design organic molecules that could eliminate these deficiencies and gain the excellent performance of organic solid-state emitters.16

To overcome the impediments, many novel strategies for constructing fluorescent molecules have been developed.17 Among these strategies, inhibiting the non-radiative transition of the single-bond-rotor, double-bond-rotor, and loose-bolt effect at the molecular level seems to be the most effective route to improve the luminous efficiency and restrain the ACQ of the designed molecules. One of the ways of circumventing concentration quenching depends on the construction of a molecular architecture exhibiting excited-state intramolecular proton transfer.18 However, such methodologies were often limited by non-ideal application properties.19 Another direct approach is the introduction of a large steric bulky group.16 It is difficult to achieve this goal directly through the regulation of the skeleton core. Many efforts have been devoted to the structure modification to reduce the ACQ effect during the past decade, but the results of most of these studies performed on classic organic molecules are still not as expected.20,21 Therefore, it is urgent to exploit a new organic skeleton that could exhibit excellent solid-state fluorescence performance.

As is known, nitrogen-containing macrocyclic compounds are rich in electronic and optical properties,11,22 and their non-planar, rigid and flexible structures could suppress the π–π stacking interaction of fluorophores themselves against the ACQ effect, which may lead to desirable solid-state fluorescence. Among many multi-nitrogen-containing macrocyclic compounds, triazepine, an important bioactive seven-membered heterocyclic compound, is widely used in many drugs as their core molecular skeleton.23–25 To date, only a few methods for synthesizing triazepine derivatives have been reported, and most of them require a tedious synthesis process and expensive substrates.26–28 Thus, it is crucial to develop a convenient and economical strategy that could construct the triazepine moiety and regulate its optical properties.

Inspired by the previous literature and our research on the development of nitrogen heterocyclic compounds and fluorescence sensors, we put forward an efficient and metal-free strategy to synthesize a unique triazepinium salt fluorescent platform from 3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-amine and acetyl (Scheme 1). A series of triazepinium salts with a large Stokes shift, long fluorescence lifetime, high environmental stability, non-ACQ effect, and no photobleaching could be easily obtained using this strategy. Moreover, the formation of a fused ring structure in these molecules could enhance the rigidity of the molecular skeleton and increase the fluorescence quantum yield. An excellent solid-state fluorescence property could be implemented by adjusting the acetyl due to the restricted intramolecular rotation. Furthermore, the well-designed triazepinium salts also reveal abundant electrochemical characteristics that might be further used to form photoelectric materials or devices.

image file: c9gc04328d-s1.tif
Scheme 1 Synthesis of triazepinium salts.

Results and discussion

Syntheses of triazepinium salts

3,5-Di(pyridin-2-yl)-4H-1,2,4-triazol-4-amine (1a) and acetone (1b) were employed as substrates to optimize the condition of the cyclization reaction under the catalysis of I2. The C(sp3)–H of acetyl was activated under the guidance of previous works.29 As shown in Table 1, the most common solvent of this type of reaction, chlorobenzene, was used as the first option, but the result shows that there is no expected target product generated (Table 1, entry 1). In the following solvent screening experiment, the participation of 1,4-dioxane, 1,2-dichloroethane, acetonitrile, and toluene could not lead to the formation of the desired triazepinium salt Q1 (Table 1, entries 2–5), and the use of DMSO has realized the best results over DMF, with a yield of 45% (Table 1, entries 6 and 7). Furthermore, temperature has a certain influence on the yield of this reaction. When the temperature increased from 60 °C to 80 °C, the yield was increased to 85% with a reduced reaction time of 10 h (Table 1, entries 7, 8, 11, and 12). However, the reaction could not proceed without the addition of I2, which strongly demonstrates that I2 has taken part in the cyclization process and played an indispensable role (Table 1, entry 13). There was no difference in the reaction results between argon and air (Table 1, entry 15). In addition, the six products have been successfully synthesized on a gram scale (entry 11, given in Schemes S4–S9).
Table 1 Effect of various parameters on the yield of Q1

image file: c9gc04328d-u1.tif

Entrya Solvent Temp/°C Time/h Yieldb/%
a 3,5-Di(pyridin-2-yl)-4H-1,2,4-triazol-4-amine (0.5 mmol, 1 equiv.), acetone (0.6 mmol, 1.2 equiv.), I2 (0.3 mmol, 0.6 equiv.) and solvent (1 mL) were in a round-bottom flask for 10 h open to air. b Isolated yields. c Without I2. d I2 (0.6 mmol, 1.2 equiv.). e Under an argon atmosphere (balloon).
1 Chlorobenzene 60 6 n.d
2 Toluene 60 6 n.d
3 1,2-Dichloroethane 60 6 n.d
4 Acetonitrile 60 6 n.d
5 1,4-Dioxane 60 6 n.d
6 DMF 60 6 25
7 DMSO 60 6 45
8 DMSO 60 8 60
9 DMSO 60 10 71
10 DMSO 60 12 70
11 DMSO 80 10 85
12 DMSO 100 10 84
13c DMSO 80 10 n.d
14d DMSO 80 10 84
15e DMSO 80 10 83

After the determination of the optimal reaction conditions, the generality and limitations of this reaction were investigated. As shown in Table 2, 3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-amine and a series of acetyl derivatives were employed in this reaction, leading to the production of a variety of triazepinium salts efficiently (2–37), indicating an excellent tolerance of different functional groups. Using aromatic acetyl, polycyclic aromatic acetyl, and heteroaromatic acetyl as substrates could form the desired products in good yields of 60%–90%. It is obvious that the yields of the electron-withdrawing group containing aromatic acetyls are higher than that of the electron-donating group containing aromatic acetyls. Furthermore, compared with the para and meta position substituted substrates, an obvious steric hindrance was observed when the ortho-position of acetyl was substituted. In addition, the aliphatic acetyls also showed good reaction activity and afforded target compounds in good yields of 66%–85%, and the substitution of alkynes, alkanes, ethers, alcohols, and nitriles have no significant influence on this reaction. These results corroborated the feasibility of this method for constructing highly functionalized triazepinium derivatives.

Table 2 Synthesis of various triazepinium salts
Acetyl (0.6 mmol, 1.2 equiv.), 3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-amine (0.5 mmol, 1 equiv.), and I2 (0.3 mmol, 0.6 equiv.); DMSO (1 mL); 80 °C; 10 h.
image file: c9gc04328d-u2.tif

Based on the above experimental results, a possible and concise mechanism of the reaction is illustrated in Scheme 2. First, the substrates I are treated with I2 to form α-iodoketone(II) and HI. Then, the dehydration condensation reaction occurs between α-iodoketone(II) and 3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-amine(III) under the promotion of HI that was generated in the previous step and then produces IV. Next, the nucleophilic pyridine ring attacks methylene and releases iodide ions through a Menschutkin reaction.

image file: c9gc04328d-s2.tif
Scheme 2 Proposed mechanism for the synthesis of triazepinium salts.

Fluorescence properties and structure–function relationship

It is critical to research the photophysical properties involving the excited states of triazepinium salts for the realization of specific application in luminescent organic materials. The fluorescence properties of different triazepinium salts were investigated in the solid state. As shown in Fig. 1, most of these molecules exhibit excellent emission at a long wavelength of approximately 530–640 nm [Fig. 1(a)], while some triazepinium salts are incapable of displaying any fluorescence performance at all. The 4-pyridine-substituted molecule (Q28) has the longest maximum emission wavelength at 640 nm, while the methyl-substituted molecule (Q3) shows the shortest at 530 nm [Fig. 1(c)]. It is obvious that, when the aromatic acetyls without an electron-donating effect were introduced into the skeleton, strong solid-state fluorescence will be released, which is quite the opposite effect compared with the electron-donating groups. Moreover, the fluorescence will be quenched if the ortho-position of the substituent is occupied due to the destructed co-planarity between the triazole ring and the R moiety. It is surprising that the fluorescence decay times of Q5 and Q18 could reach 1.0 μs, which is quite long for organic molecules, indicating good performance for organic, long-afterglow luminescent materials [Fig. 1(b)]. More importantly, the absorption and emission spectra were sufficiently separated with a large Stokes shift of 207 nm, which could fully suppress fluorescence resonance energy transfer to avoid luminescence quenching and further enhance the fluorescence intensity [Fig. 1(d)].
image file: c9gc04328d-f1.tif
Fig. 1 (a) Emission spectra of partial triazepinium salts in the solid-state; (b) fluorescence lifetimes of some triazepinium salts; (c) emission spectra of Q3, Q5, and Q28 in the solid-state; (d) absorbance and emission spectra (λex = 390 nm and λem = 597 nm).

As can be seen from the above results, these triazepinium salts not only exhibit broadly similar fluorescence properties but also reveal different characters in detail. To illuminate these differences, X-ray crystallographic studies were conducted given its crucial role and successful application in explaining many fluorescence phenomena and mechanisms. It is worth noting that the skeleton was distorted to form a non-planar, saddle-shaped, seven-membered structure, which might provide a coalition of rigidity and flexibility in one molecule. Moreover, three nitrogen atoms containing seven-member rings would enhance the push–pull electronic system and promote the free flow of electrons, resulting in the enhancement of fluorescence performance.

Among all these triazepinium salts, we successfully achieved the single-crystal structure of Q5, Q35, Q12, and Q21 from DMSO/H2O. As illustrated in Table 3, the change of the non-R moiety barely impacted the dihedral angles between these three planes, which means that the electronic and steric effects of the R moiety comprise the most likely driving force leading to the different characters of these molecules. As shown in Fig. 2(e), the triazepinium salts can be regarded as the combination of two parts, i.e., the “main skeleton” (triazole ring) and the “branches” (R moiety). The dihedral angle between the “main skeleton” and “branches” of Q5, Q35, Q12, and Q21 is measured to be 9.19°, 39.44°, 59.84°, and 83.67°, respectively (Fig. 2). Based on the crystals, it is shown that, when the R part exhibits electron-donating tendency, the ring softness of the seven-membered ring increases, which leads to an increase in the angle between the R part and the main skeleton part. Conversely, when the R part exhibits non-electron-donating tendency, it has a tightening effect on the entire skeleton of the seven-membered ring, reducing the angle between the R portion and the main body. The large dihedral angels (Q12, Q21, and Q35) destroy the planar structure to quench fluorescence, but the small dihedral angle enhances conjugation to induce the strong and longest solid-state fluorescence performance of Q5. These results portend that its molecules are modified by the R moiety and the dihedral angles of the two parts. (The detailed crystal and structure refinement data are given in Tables S1–S4.)

image file: c9gc04328d-f2.tif
Fig. 2 Crystal structures and the dihedral angle between the “main skeleton” and the “branch” of Q5, Q35, Q12, and Q21.
Table 3 Dihedral angle of four triazepinium salts in crystal

image file: c9gc04328d-u3.tif

Compounds Anglea Angleb
a Dihedral angle between Plane 2 and Plane 1. b Dihedral angle between Plane 3 and Plane 1.
Q5 28.301 28.133
Q12 30.474 27.587
Q21 25.973 28.373
Q35 20.121 23.977

Furthermore, density-functional theory30 (DFT) has been employed to clarify the structure–function relationship of these triazepinium salts using the Gaussian 09 program.31 The structures were optimized under a combination of the basis of the 6-31G basis set for H and C elements, and 6-31+G** with the B3LYP functional32 [Fig. 3(a); S21]. All the optimized structures were confirmed to be local minima due to the non-existence of an imaginary frequency. The small HOMO–LUMO energy gap indicates the possibility of red emission. The effect of temperature on the solid-state emission performance and electrochemical activities of Q5 was further investigated. As shown in Fig. 3(b), the fluorescence intensity at the maximum wavelength gradually decayed with increasing temperature from 77.4 to 299.3 K, which may be caused by the accelerated molecular movement. Furthermore, a 30 nm red-shift of the maximum fluorescence peak could be observed when the temperature reached 242 K from room temperature. Moreover, in the cyclic-voltammetry experiment, three pairs of redox peaks were observed at −1.64/−1.52, −1.33/−1.27, and −1.15/−1.00 V [Fig. 3(c)], indicating a stronger oxidation–reduction activity.

image file: c9gc04328d-f3.tif
Fig. 3 (a) HOMO–LUMO energy values and gaps and their distributions of Q5, Q35, Q12, and Q21; (b) emission spectra (λex = 390 nm) of Q5 with increasing temperature (77.4 to 299.3 K); (c) cyclic voltammograms of Q5.


In summary, a facile strategy was successfully developed for constructing a class of untapped multi-nitrogen, seven-membered solid-state fluorophores, triazepinium salts, without chromatographic purification. The well-designed fluorophores have a series of advantages, such as a large Stokes shift, environmental stability, non-aggregation quenching, non-photobleaching, long lifetime, and adjustable emission. The structure–activity analysis demonstrated that the dihedral angle between the triazole ring and the R moiety caused by the different substituents controls the fluorescence performance of these molecules, leading to an adjustable fluorescence wavelength ranging from 530 to 640 nm. Moreover, this type of fluorophore reveals an obvious fluorescence red-shift along with decreasing temperature and exhibits multilevel-redox characteristics. The development of this synthetic strategy will provide a practical approach for the construction of brand-new multi-functional cores and can be further applied in photoelectric materials and fluorescence sensors.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the National Natural Science Foundation of China (No. 21572177, 21673173, and 21807087), the Key Research and Development Program of Shaanxi (No. 2019KWZ-07), the Key Science and Technology Innovation Team of Shaanxi Province (No. 2017KCT-37), the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JQ3038), the Opening Foundation of Key Laboratory of Resource Biology and Biotechnology in Western China (Northwest University) Ministry of Education (No. ZSK2018003), and the Xi'an City Science and Technology Project (No. 2019218214GXRC018CG019-GXYD18.4). We thank LetPub for its linguistic assistance during the preparation of this manuscript.

Notes and references

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Electronic supplementary information (ESI) available. CCDC 1944954–1944957. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9gc04328d
These authors contributed equally as the first authors.

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