Synthesis and Spectral Study of a New Family of 2,5-Diaryltriazoles Having Restricted Rotation of the 5-Aryl Substituent

Efficient synthesis of 2,5-diaryl substituted 4-azido-1,2,3-triazoles by the reaction of sodium azide with dichlorosubstituted diazadienes was demonstrated. The optical properties of the prepared azidotriazoles were studied to reveal a luminescence maximum in the 360–420 nm region. To improve the luminescence quantum yields a family of 4-azido-1,2,3-triazoles bearing ortho-propargyloxy substituents in the 5 position was prepared. Subsequent intramolecular thermal cyclization permits to construct additional triazole fragment and obtain unique benzoxazocine derivatives condensed with two triazole rings. This new family of condensed heterocycles has a flattened heterocyclic system structure to provide more conjugation of the 5-aryl fragment with the triazole core. As a result, a new type of UV/“blue light-emitting” materials with better photophysical properties was obtained.

Nowadays, there is also significant interest in the development of novel selective approaches to 2-substituted 1,2,3-triazoles. These compounds are particularly attractive since they have excellent fluorophore properties [19][20][21][22][23][24][25]. A great number of fluorophores has been developed over the years, but effective UV/blue-light-emitting molecules are still rare due to their relatively high energy gap, which may cause poor photostability or low quantum efficiency. However, the booming research area, such as OLED display study, in photoactive compounds has generated increasing needs for effective UV/blue-light-emitting molecules. Recently Yan and co-workers [22] reported N-2-aryl-1,2,3-triazoles ( Figure 1) as new UV/blue-light-emitting compounds with tunable emission and adjustable Stokes shift through planar intramolecular charge transfer. 1,1-Dichlorodiazadienes are a valuable class of electrophiles. These compounds can be prepared using the reaction of carbon tetrachloride witH-N-substituted hydrazones of aldehydes in the presence of CuCl as a catalyst. [26,27] Recently we demonstrated that these compounds are also interesting diazodyes [28,29]. The reaction of 4,4-dichloro-1,2-diazabuta-1,3-dienes with sodium azide has been found to open straightforward access to extremely rare 1,1-bis-azides ( Figure 2). These highly unstable compounds are prone to eliminate a N 2 molecule to cyclize into 4-azido-1,2,3-triazoles 1 bearing two aryl (heteroaryl) groups at positions 2 and 5. The reaction was found to be very general for the highly efficient synthesis of various 4-azidotriazoles. It was demonstrated that these heterocycles are highly attractive building blocks for subsequent preparation of 1,2,3-triazole-derived compounds [30]. The prepared azidotriazoles 1 [30] contain the same structural pattern as the diaryltriazoles used as UV/blue-light-emitting compounds [22] (Figure 1). Therefore, we decided to study optical properties of 4-azido-1,2,3-triazoles 1. This study is also devoted to investigation of the synthesis of 4-azido-2,5-diaryl-1,2,3-azidotriazoles prepared from arylhydrazines and ortho-propargyloxy-benzaldehydes. The presence in the structure of these compounds of a triple bond and the azido group opens up the possibility of intramolecular cyclization of these compounds to obtain unique condensed heterocycles containing two triazole rings (Figure 3b). Moreover, such structural modification can significantly affect the photophysical properties of the intramolecular cyclization products. In this case free rotation of the aryl fragment in the 5 position is impossible. The main aim of this work was studying of optical properties of 2-aryl 1,2,3-trizoles 1 and synthesis of their rigidified analogues obtained via intramolecular acetylene-azide cyclization (Figure 3b).
The emission spectra of compounds 1a-i have two trends: spectra 1a-1d and 1g-1i exhibited similar emission bands in the 363-372 nm range. On the contrary, the emission spectra of pyridine analogues 1e and 1f demonstrated slightly different character having maxima at 419 nm and Stokes shifts up to 108 nm ( Figure 5, Table 1). The reasons of such behavior are not clear at the moment and demand subsequent study, however it is most probably connected with the presence of additional nitrogen in the structure. Unfortunately, small fluorescence quantum yields were observed for all these compounds 1a-1i (Table 1).

Synthesis and Characterization of Condensed Analogues
Next, we decided to prepare some analogues of compounds 1a-1i having a flattened structure and restricted rotation of the aryl fragment at the 5vposition of the triazole. We expected to enhance the fluorescence quantum yields by such a structural transformation. ortho-Propagyloxybenzaldehydes were used as starting materials for this aim. A set of such aldehydes was prepared by alkylation of some salic aldehydes with propargyl bromide (Scheme 1). Scheme 1. Synthesis of ortho-propargyloxybenzaldehydes [31].
All steps of the synthesis were performed in one pot to give the target dienes in respectable yields (up to 77%). It should be pointed out that the method is amenable to the variation of functional groups in the structure of the starting aldehydes. Electronically and sterically different o-propargyloxybenzaldehydes can be used for this aim. Moreover, the corresponding naphthalene derivative was prepared as well (Schemes 2 and 3) [26]. In a similar manner, a set of dienes 2h-o having different aryl substituents at the nitrogen was prepared from the parent o-propargyloxy-benzaldehyde and substituted aryl hydrazines. We tried to perform variation of this part of molecules keeping in mind the influence of both electron and steric factors. For example, the corresponding dienes having methyl-, methoxy-and cyano groups, and different halogens can be prepared in up to 89% yield. Moreover, diene 2m having a bulky 2,6-dimethylphenyl substituent was synthesized too.  Having in hand a family of precursors for the synthesis of model azidotriazoles, the reaction with excess of sodium azide was studied. It was found that the synthesis is very general to give the target products in up to 97% yield. A set of 15 triazoles 3a-o was thus prepared having different substituents in the position 2 and 5 (Scheme 4) [30,32,33]. Next, intramolecular cyclization to form second triazole ring by thermal [2+3] cycloaddition was studied. Prepared compounds 3 have in the structure both an azide group and an acetylene fragment. We observed that spontaneous cyclization takes place slowly, even at room temperature, during storage. Smooth cyclization can be performed by reflux in o-xylene during 12 h in an argon atmosphere. As a result, a family of condensed triazole derivatives 4 having restricted rotation of substituent at the 5 position was prepared. We observed atropoisomerism [34] for some of prepared products. Their NMR spectra contain doubled set of signals (Scheme 5). Scheme 5. Intramolecular cyclization of o-propargyloxy substituted 4-azido-2,5-diaryl-1,2,3-triazoles.

Photophysical Properties of Compounds 4a-4o
UV-vis absorption and fluorescence spectroscopic measurements were performed for the synthesized compounds 4 to establish the relationship between the structure and photophysical properties of the prepared flattened derivatives 4a-o [35]. All these spectral data were obtained in dichloromethane (c = 10 −6 M for all compounds) at room temperature and the results are summarized in Table 2.
All the investigated compounds exhibited similar absorptions in the 250-305 nm range ( Figure 6). The absorption spectra for compounds having a phenyl group at the N(2) position demonstrated absorption maxima at 284-288 nm. However, naphthalene derivative 4g has a maximum of absorption shifted to 301 nm. More pronounced influence for absorption spectra was found when varying the substituents at N(2). The presence of an electron-withdrawing cyano group resulted in a bathochromic shift to 305 nm. On the other hand, a hypsochromic shift was observed for ortho-substituted derivatives, for example 4k.  The emission spectra of solutions of 4a-4o ( Figure 7) were recorded at an excitation wavelength corresponding to the maximum in the absorption spectra. Typical emission maxima obtained upon irradiation of the solutions were located in the blue region. The Stokes shifts were shown in the range from 31 to 112 nm. Obviously, the spectral characteristics of compounds 4a-4o depend on their electronic properties, conjugation of the substituents at the C(5) position and at the N(2) of the triazole. To our delight, much higher quantum yields (Φ F up to 0.616) were observed for all derivatives 4a-4o in comparison to triazole derivatives 1a-h (Φ F up to 0.017) having free rotation of the C(5) substituents. These data confirmed the attractiveness of our idea to synthesize and to study the photophysical properties of flattened intramolecular cyclizatioproducts. Analysis of the emission spectra for compounds 4a-4o showed that the substitution at the N(2) position has more influence on fluorescence properties. A presence of a methoxy or cyano group at the aromatic ring para-position at the triazole N(2) leads to enhanced quantum yields ( Table 2, Figures 6 and 7), whereas, ortho-substituted derivatives exhibited lower fluorescenc efficiency. In contrast, the quantum yields were below 5% for any substituents at the C(5) of triazole ring (compounds 4a-f, Φ F up to 0.007-0.044). Due to the extra ring the naphthyl derivative 4g showed moderate photophysical properties (Φ F = 0.079).
The presence of a 4-MeO-group in the aryl at the N(2)-triazole resulted in the most effective conjugation, which resulted in a fluorescence enhancement (comp. 4l, Φ F up to 0.616). Most probably, better internal charge transfer (ICT) from the electron-donor OMe-group to the relatively electronically deficient part of molecule is achieved. On the contrary, the 2-methoxy derivative has the small quantum yield (comp. 4k, Φ F up to 0.013) but it gives the largest Stokes shift (112 nm), that makes it a pretty interesting fluorophore material. Other ortho-substituted at N(2)-triazole derivatives also demonstrated low quantum yields (4i, 4j, 4m and 4n) to confirm the influence of steric hindrance on the photophysical properties. Most probably, lower conjugation of the aryl ring at the N(2)-triazole due to a distorted conformation is a reason for the reduced quantum yields in these cases.

Solvatochromism of Compound 4l
We also carried out a study of solvatochromic properties for compound 4l which demonstrated the highest quantum yields. The absorption and emission spectra of 4l were taken at a standard concentration in different solvents of various polarities, including dioxane, benzene, EtOAc, THF, dichloromethane, EtOH, DMF, MeCN [36]. The UV-Vis spectrum of 4l in low polarity benzene has a slight bathochromic shift relative to the spectrum in dichloromethane ( Figure 8). Increasing solvent polarity resulted in more significant bathochromic shifts of the emission maxima, indicating an ICT behavior, which is better stabilized in polar solvents, for example in THF or EtOH ( Figure 9). The intensity of the emission of compound 4l is also highly dependent on the solvent polarity. In particular, the quantum yield of 4l rises with increasing polarity of the solvents (benzene (0.457) Table 3). The lowest fluorescence quantum yield was observed in the nonpolar, aprotic solvent benzene because of the charge transfer phenomena [37]. The highest fluorescence quantum yield was observed in a polar, protic solvent, EtOH.

Experimental Details
All required fine chemicals were of reagent grade and were used directly without purification unless otherwise noted. 1 H-and 13 C-NMR spectra were acquired at 400.1 and 100.6 MHz, respectively, on an AVANCE 400 MHz spectrometer (Bruker, Karlsruhe, Germany) in chloroform-d (unless otherwise stated). 1 H-NMR coupling constants (J) are reported in Hertz. Data are reported as follows: chemical shift, multiplicity (s -singlet, br s -broad singlet, d -doublet, t -triplet, q -quartet, m -multiplet, dddoublet of doublets, ddd -doublet of double doublets), coupling constants, integration, and assignment (optionally). HRMS (ESI-MS) spectra were measured on MicroTof (Bruker Daltonics, Bremen, Germany). All IR data was obtained on a Nicolet iS5 One FT-IR spectrometer (Thermo Scientific, Madison, WI USA) using consoles of internal reflection iS3 with a ZnSe ATR element, dip angle 45 • C. All UV data was obtained on a Cary 60 UV-Visible spectrophotometer (Agilent, Santa Clara, CA USA) within 250-800 nm spectral range. The UV-spectra were recorded at 1 cm cuvettes at room temperature, dichloromethane was used as a solvent. Emission spectra were registered with a F2700 spectrofluorometer (Hitachi, Tokyo, Japan) in 1 cm quartz cells. The concentration of the compound in the solutions was 10 −5 M and 10 −6 M for both measurements. The relative fluorescence quantum yields (ΦF) were measured using quinine sulfate in 0.1 M H2SO4 (Φ f = 0.55) and 2-aminopyridine 0.1 M H2SO4 (Φ f = 0.60) as a standards [36].

General Procedure for the Preparation of
o-Propargyloxy-substituted dichlorodiazabutadienes were synthesized based on previous work [26]. A 20 mL screw neck vial was charged with DMSO (10 mL), the corresponding o-propargyloxybenzaldehyde [31] (1 mmol, 1 eq.) and hydrazine (1 eq.). After 2 h stirring TMEDA (2.5 eq.), CuCl (0.01 eq.) were added and carbon tetrachloride (10 eq.) was put into the reaction mixture during 5 min under cooling with a water bath. The next reaction was carried out at room temperature during 3 h (until TLC analysis showed complete consumption of corresponding hydrazone). The reaction mixture was then poured into water (200 mL), and extracted with DCM (3 × 20 mL). The combined organic phase was washed with water (3 × 50 mL), brine (1 × 30 mL), dried over anhydrous sodium sulfate and concentrated in vacuo of the rotary evaporator. The residue was purified by column chromatography on silica gel using appropriate mixtures of hexane and DCM (3/1) as eluent.

Conclusions
The synthesis and photophysical properties of a series of new differently substituted 2-phenyl-2H,8H-benzo[g]bis([1,2,3]triazolo)[5,1-c:4',5'-e] [1,4]oxazocines were investigated. The corresponding dichlorodiazenes containing propargyloxy groups were used as a key starting materials for this aim. Their reaction with sodium azide leads directly to the corresponding 4-azido-1,2,3-triazoles in up to 97% yield. Subsequent thermal cyclization resulted in efficient synthesis of condensed heterocycles having an additional triazole ring in up to 86% yield. The prepared oxazocine derivatives demonstrated interesting photophysical properties and much higher fluorescence quantum yields in comparison to non-cyclized triazole derivatives.