Phenothiazine and pyridine-N-oxide-based AIE-active triazoles: synthesis, morphology and photophysical properties

Abstract

Aggregation-induced emission (AIE) low molecular weight compounds based on triazoles, phenothiazine and pyridine-N-oxide units bonded by short flexible chains have been obtained by a “click” chemistry reaction. The photophysical properties were explored by UV-vis and photoluminescence spectroscopy in solution, water suspension, and amorphous and crystalline films. The UV-vis absorption spectra indicated a typical behavior for nanoparticle formation. An emission intensity enhancement of 233-fold higher was registered for the crystalline films compared to solutions, clearly indicating an aggregation-induced emission behavior. The morphology study, in suspension and film, monitored by dynamic light scattering, scanning electron microscopy and polarized light microscopy methods, indicated that nano- and micro-crystals of rose-like shapes and fibers were formed.

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1. Introduction

Aggregation-induced emission (AIE) is an intriguing optical phenomenon for which aggregation works constructively in improving light emission. Evidenced by Tang and co-workers in 2001, it consists of an appearance of emission for aggregated compounds that lack light emission in solution.¹ The challenging AIE phenomenon is considerably opposite to the notorious aggregation-caused quenching (ACQ) of light emission in the condensed phase and is in good agreement with the requirements of optoelectronic devices, which need solid-state emissive substrates. The signature contribution of Tang's group on AIE compounds is the propeller-like design, in which peripheral rigid aromatic units are linked by rotatable single bonds to a conjugated stator. This structural approach proved to facilitate the AIE effect because of the restricted-intramolecular rotation (RIR),¹ and a large number of AIE compounds have been prepared based on this design, mainly using silole-, tetraphenylethene-, and biphenyl-based triazoles.² In addition, applying the RIR design, other groups started to work in this challenging domain, continuously increasing the number and variety of AIE fluorogens.² Tetraphenylethylene,^3–5 carbazole,⁶ dicarbazolyl,⁷ maleimide,⁸ persulfurated benzene core,⁹ distyrylanthracene,^10–12 binaphthyl,¹³ pyrrole, indole,¹⁴ pyridine,¹⁵ imine,¹⁶ metal complexes,^17–19 and PEG chains²⁰ have been used as the building blocks for numerous AIE active compounds, with practical applications in biomedical field.^21–24 There are few results regarding the AIE properties of phenothiazine-containing compounds. Phenothiazine-hydrazone²⁵ or phenothiazine linked with anthracene groups^26,27 or triarylamine²⁸ exhibited AIEE (aggregation-induced enhanced emission) or AIE effect.

Phenothiazine-based compounds have been intensely studied in the past decades due to their applications in biological field, e.g., in cancer therapy as photosensitizers for selective photodamage of cancer cells (photodynamic therapy),²⁹ or farnesyltransferase inhibitors,³⁰ drugs for Alzheimer's disease.³¹ Moreover, these compounds have applications in electronic and optical devices as organic light-emitting diodes,³² solar cells,³³ and chemical sensors.³⁴ Phenothiazine use in the active substrate of optoelectronic devices is based on its strong electron-donating characteristic, which promotes a good conjugation and therefore low band gap, which results in improved charge carrier mobility and shifting in the visible domain of light emission.³⁵ Moreover, due to the butterfly-like shape of phenothiazine-fused ring, its derivatives considerably preserve the fluorescence quantum yield in solid state, compared to their solution.³⁶ However, as a majority, the luminescence of phenothiazine compounds is partially quenched in the solid state, compared to solution state, an effect attributed to aggregation (ACQ).

The present study describes the synthesis, self-assembling and optical properties of phenothiazine and pyridine-N-oxide-based triazole compounds, which exhibit an AIE effect. All the structural blocks are biologically friendly units. Compared to the previously reported AIE compounds, their design distinguishes itself by the fact that small rigid units are linked by flexible spacers, which assure a high mobility in solution and work towards self-assembling in ordered architectures.³⁷

2. Results and discussions

2.1. Synthesis and structural characterization

Three phenothiazine and pyridine-N-oxide-based triazoles have been synthesised through the “click” chemistry of pyridine-N-oxide carrying triple-bond functionality with azide-containing phenothiazine. The intermediate pyridine-N-oxide carrying a triple bond was prepared by the cycloaddition reaction of propargyl bromide with sodium 2-sulfidopyridine-1-oxide, whereas the intermediate azide was prepared from corresponding 10-chloroacetyl-10H-phenothiazine derivative and sodium azide using tetrabutylammonium bromide as a phase transfer catalyst. The synthetic route to the target compounds (noted 5a, 5b and 5c) is illustrated in Scheme 1. The molecular structure of the intermediates and final triazole compounds was confirmed by elemental analysis, ¹H- and ¹³C-nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopic analysis.

Scheme 1 Synthetic pathway to phenothiazine and pyridine-N-oxide-based triazoles.

In the FTIR spectra of the studied triazoles, the stretching band characteristic to the H–C≡ units (3168 cm⁻¹ and 2107 cm⁻¹) disappears, indicating the complete consumption of the triple bond during the cycloaddition reaction, whereas triazole units are formed. All the other characteristic absorption bands of the synthesised compounds are also present, confirming the accurate structure.

The ¹H- and ¹³C-NMR spectra of the obtained compounds further confirmed the target compounds by the presence of all chemical shifts characteristic to protons and carbon atoms. The proton bands have accurate coupling constants and integral ratio. The proton signal at 3.25 ppm attributed to the proton linked directly to the triple-bond carbon is missing, indicating its disappearance during the reaction, whereas the signal attributed to the CH₂ protons (3.93 ppm) adjacent to sulfur is shifted to a higher frequency (4.33 ppm), reflecting the deshielding effect of the electron-withdrawing newly formed unit of triazole.

All three samples (5a, 5b, 5c) were obtained in crystal form, as demonstrated by polarized optical microscopy (POM) observation. The 5a sample exhibits birefringent grains, whereas 5b and 5c samples show birefringent needles (Fig. 1). The crystalline samples melt at high temperatures and freeze in an amorphous state during cooling. It can be inferred that the amorphous glass is stable for longer duration because no crystallization was observed even after six months.

Fig. 1 POM images of studied compounds: (a) 5a, (b) 5b, and (c) 5c at a magnification of 40×, 10× and 2×, respectively.

The samples (5a, 5b, and 5c) show good solubility in dimethylsulfoxide and poor solubility in other common organic polar solvents. They are completely insoluble in water and non-polar solvents.

2.2. Photophysical properties

UV-vis absorption. The photophysical properties of the studied triazoles were investigated in solution, suspension, and amorphous and crystalline films. DMSO, a well-known biocompatible dispersant,³⁸ was used as a solvent, whereas water, the principal component of living organisms, was used as a non-solvent such that premises for any possible biologic application were guaranteed. Due to their insolubility in water, the triazole molecules must aggregate in aqueous mixtures with high water fractions.

The UV-vis absorption spectra of the triazole-based compounds in THF or DMSO solution (0.66 × 10⁻⁵ g mL⁻¹) show an identical trace profile, consisting of three overlapping absorption maxima, corresponding to the π–π* transition in the three isolated chromophore units of the dye molecules, with an absorption edge of around 344 nm (Fig. 2a, Table 1). The wavelength of absorption light is relatively low because the flexible chains within the molecular structure hinder the extent of conjugation. Different concentrations of DMSO solutions show variations in absorption intensity but no significant change in absorption maxima, suggesting the absence of significant ground state intermolecular interactions (Fig. 2b).

Fig. 2 UV-vis spectra of (a) triazole compounds in THF; (b) compound 5a at different DMSO concentrations (inset: the concentration in g mL⁻¹).

Table 1 UV-vis absorption maxima and absorption edge in solution, water suspension, and amorphous and crystalline films

Code	λmax (λedge)/nm DMSO	λmax (λedge)/nm DMSO–H2O	λmax (λedge)/nm amorphous film	λmax (λedge)/nm crystalline film
5a	259; 276; 325 (344)	225; 258; 301 (453)	379 (577)	251 (501)
5b	254; 277; 325 (345)	221; 260; 302 (467)	380 (567)	250 (497)
5c	248; 379; 324 (344)	224; 257; 301 (449)	379 (575)	251 (487)

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The UV-vis spectra recorded for the DMSO–water solutions (0.66 × 10⁻⁵ g mL⁻¹) in the same conditions showed drastic changes in the shape: the spectral intensity increases and their absorption edge is strong red shifted around 100 nm, whereas the absorption band maxima are blue-shifted around 30 nm (Table 1) giving rise to a right-tailed curve (Fig. 3). The level-off tails at longer wavelengths are attributed to the scattering effect of the nanoparticles obtained by the aggregation of chromophoric units in the water.³⁹ The 30 nm blue shifting of the absorption maximum is an unusual behaviour, suggesting randomly twisted conformations with poor conjugation.^37,40 The comparison of the UV-vis spectra of solution and water suspension (Fig. 3b) suggests that dipolar aprotic DMSO solvent promotes the planarization of the chromophore units by strong physical forces. By adding a large amount of water, the hydrophobic dye molecules are constrained to segregate by self-assembling as nano/micro-aggregates. Within the aggregates, the chromophoric units adopt more twisted conformations because of the absence of a strong polar solvent. The clarity of DMSO–water solutions showing no precipitates clearly indicates macroscopically homogeneous solutions; thus, sustaining the hypothesis of nano-/micro-sized aggregates.

Fig. 3 UV-vis spectra (a) of the samples in water DMSO-solutions and (b) sample 5b in DMSO and DMSO–water.

The UV-vis spectra of the triazole compounds in solid state were registered for the films obtained in amorphous and crystalline states. The amorphous film spectra decreased in intensity, and their absorption maxima are strongly red-shifted (around 55 nm, compared to DMSO solutions); this behaviour reflects the planarization of chromophoric units due to reconformation by aggregation (Fig. 4a).⁴¹ This is consistent with the method of film preparation – casting from DMSO solutions during a heating stage – which creates conditions for maintaining the planarity of the chromophore π-conjugated units as obtained in highly polar DMSO. It also favours a tighter packing of the molecules, and thus π–π* staking. In the absence of an extended conjugation (interrupted by the flexible chains), the strong red-shift can be associated with intermolecular charge-transfer transitions possibly originating from the excessive charge density of donor-acceptor pyridine-N-oxide chromophoric units.⁴² Moreover, the presence of the triazole ring in the dye molecules provides additional possibilities for intermolecular charge transfer by H-bonding, π–π* stacking or donor–acceptor interactions.⁴³

Fig. 4 UV-vis spectra of (a) amorphous and (b) crystalline film samples.

On the contrary, the crystalline films casted from the DMSO–water solution on a glass substrate show a strong blue-shifting of the absorption maxima (Fig. 4b), suggesting twisted conformations that hinder the intermolecular charge transfer.

Comparing the absorption edge of the samples in all four different states (Table 1), a strong red shifting can be observed for the samples in suspension, as well as amorphous and crystalline films, compared to that in solutions, suggesting the formation of micro-/nano-sized aggregates.

Photoluminescence behaviour. The photoluminescence behaviour of the studied compounds, in solution, water suspension, as well as amorphous and crystalline films, has been explored by exciting the samples at their absorption maxima wavelengths. The fluorescence intensity was calibrated using the Raman scattering peak of water.⁴⁴

The highly diluted solutions of DMSO emit a UV light of very weak intensity, in the 350–375 nm wavelength domain, which is almost superposed with the baseline (Fig. 5a and b). There is no influence of the exciting light wavelength on the emission profile, but the emission intensity increases when the exciting light has a lower wavelength. The Stokes shift has low value, indicating no significant geometrical rearrangements in the excited state.⁴⁵ Because the sample solution was highly diluted (0.66 × 10⁻⁵ g mL⁻¹), the emission effect could be attributed to the isolated species with no disturbance effect from the chromophoric interactions, but due to the aggregate formation. Thus, the weak luminescence appears to be the result of dynamic intramolecular rotations, which effectively consume the exciton energy of the chromophore units, and thus rendering the molecules non-radiative.

Fig. 5 PL spectra of the triazoles: (a) DMSO solutions and water-suspensions (the inset indicates the sample code and the solvent) and (b) water suspensions of 5c sample (0.66 × 10⁻⁵ g mL⁻¹) with different water fractions (the inset indicates the water fraction).

The situation completely changed in the case of water solutions; all three samples emitted in a wide wavelength domain between 320 and 520 nm (Fig. 5a and b). The prominent broad emission curve could be related to an increased number of molecular conformations appearing because of the free intramolecular rotation, allowed by the σ-bonds between chromophoric rigid moieties. Mainly, the emission curve has three emission maxima, the most intense being in the UV domain. When illuminated with a UV lamp (360 nm), the solutions appear to have a weak violet-bluish luminescence. Compared to the DMSO solutions, the emission intensity in a water suspension is 2-fold higher for sample 5a, 48-fold higher for sample 5c and 263-fold higher for sample 5b (Table 2). The emission strengthening when water is added into the DMSO solutions is attributed to the segregation of aggregates, accompanied by the physical constraint of the intramolecular rotations. As a consequence, the non-radiative channels are blocked, whereas the radiative channels are opened up. Segregation is possible because of the immiscibility of triazoles in water, which results in local increases of luminophore concentration. During the segregation process, the formation of aggregates with random structures is more probable owing to the large molecular conformation number, which appears in the DMSO solution. The random aggregates mitigate the π–π stacking interactions of the chromophoric units, and thus the possibility of excimer formation. To better understand the influence of aggregation on the emission properties, the PL spectra were obtained for aqueous mixtures with various water contents. Fig. 5b shows the water fraction-dependent emission spectra of the sample 5c, as an example. The emission profile did not show any changes until the water fraction (f_w) reached 95%, whereas the emission intensity swiftly increased when the water content increased and reached the maximum for f_w of 99.2%. This clearly indicates the aggregation-induced emission. A similar behaviour was observed for the other samples.

Table 2 The light emission intensity of the studied triazoles^a

Code	DMSO solution	DMSO–water suspension	Amorphous films	Crystalline films
a Calculated by integrating the calibrated emission curves.^38
5a	28	57.8	2483	6527
5b	35.5	9353	1544	6414
5c	34.5	1659	1301	5745

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For the amorphous film samples, the emission intensity increased 88-, 43-, and 37-fold higher in comparison to the DMSO solutions (Table 2). The emission curves cover a large UV and visible domain, showing five overlapping emission bands with the maximum around 420 nm, corresponding to the violet-bluish light emission (Fig. 6a). The multiple superposed emission bands confirm various motifs of self-assembling of the aggregates in various amounts, suggesting the various planarity degrees of the molecules. It is expected that the molecules, as a whole, adopt twisted conformations, in which planar chromophoric units inside the molecules are tilted to one another at various angles. These random conformations promote different self-assembling motifs during slow DMSO evaporation, with longer or shorter intermolecular distances between fluorophoric units. The longer distances among fluorophoric units hinder the excimer and exciplex formation and cancel the non-radiative channels responsible for π–π stacking, favoring the increases of emission intensity.

Fig. 6 PL spectra of (a) amorphous and (b) crystalline films; (c) crystalline films of 5a, 5b, and 5c under a UV illuminating lamp and 5b under polarized light.

Compared to amorphous films, the crystalline films casted from DMSO–water solutions emit even more intense light, with the most intense emission maxima at 420 and 480 nm, both in the blue domain (Fig. 6b). The emission intensity is 233-, 180- and 166-fold higher compared to the DMSO solution, and 2.6-, 4.1- and 4.4-fold higher compared to the amorphous films (Table 2). The films show a strong birefringence under polarized light and violet-bluish fluorescence when illuminated with a UV lamp (Fig. 6c). The significant increases of the luminescence in the crystalline state can be attributed to the increase in the aggregate amount with longer intermolecular distances, prompted by the water non-solvent, which forces the fast segregation, and thus does not allow a close packing.

2.3. Morphology

To better understand the solvent influence on optical behavior, especially to observe the nano-/micro-aggregate formation, dynamic light scattering (DLS) on both types of solution samples (DMSO and DMSO–water) was performed. For the DMSO solution samples, no nanoparticles were detected, which indicates that in this strong polar aprotic solvent, the molecules are dissolved as isolated species. The data completely changed for the DMSO–water solutions. The sample 5a reveals typical spherical particles with a hydrodynamic radius of around 56.8 nm and PDI = 0.59, indicating nanoparticle formation. The samples 5b and 5c exhibit a high aspect ratio, corresponding to the presence of nano- and micro-fibers (Table 3).⁴⁶ DLS measurements allow us to make the supposition that in a DMSO–water mixture, the studied compounds form fine suspensions despite their clear appearance.

Table 3 Diffusion light scattering on the water suspension samples

Code	Dimension (nm)	DPI
5a	56.8 ± 14.5	0.59
5b	8733 ± 1673.4; 418.9 ± 39; 4395.2 ± 1150.3	2.434
5c	34.4 ± 10.1; 1148.5 ± 261.5; 18 973.9 ± 5396.5	1.059

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For a visual confirmation of the nano- and micro-particle formations and their shapes, scanning electron microscopy (SEM) of the samples obtained by casting DMSO–H₂O suspension on a glass support has been performed. As can be seen in Fig. 7, sample 5a exhibits rose-like crystals, and 5b and 5c samples show entangled rod-like nano- and micro-fibers that form networks.

Fig. 7 Scanning electron microphotographs of the studied samples.

2.4. Mechanism of aggregation-induced emission

Based on all the photophysical and morphological data collected, a scenario of aggregation-induced emission of these flexible triazoles can be obtained.

In a diluted solution, the flexible chains allow for active rotations of planar fluorophoric units (phenothiazine, pyridine-N-oxide, and triazole). The intramolecular rotations serve as non-radiative channels for the excited states to decay, and thus promote rich conformation variety. As a consequence, the aggregation occurs as a result of two antagonistic features – structural flexibility and fluorophore planarity – which promote random aggregate formation with a variety of intermolecular distances, depending on the environment, physical constraints and segregation time. Due to the large number of random conformers, the aggregation is driven by the self-assembly of flexible units, whereas the rigid units are packed in different conformations with intermolecular distances in various amounts. The short intermolecular distances within the fluorophore layers facilitate the π–π interactions, and thus favor the formation of excimers and exciplexis, which quench the luminescence. However, the longer intermolecular distances are able to restrict the formation of detrimental excimers and exciplexis, and thus are able to block the non-radiative channels and to open up the radiative ones, leading to the observed AIE effect. Consequently, the emission intensity depends on the amount of larger intermolecular distances into aggregates. The amorphous films obtained from solutions favor a larger amount of shorter distances between the more planar fluorphoric units, whereas the crystalline films obtained from suspensions facilitate a larger amount of longer distances between fluorophoric units. This explains why crystalline films obtained from DMSO–water mixtures with poor solvating power have better luminescence compared to amorphous films obtained from polar DMSO with high solvating power. In addition, it is expected that the formation of nano- and micro-sized aggregates will enlarge emission surfaces, compared to the continuous amorphous glassy films, and thus further promote the enhancement of light emission intensity.

3. Experimental

3.1. Reagents

Propargyl bromide, sodium-2-sulfidopyridine-1-oxide, and phenothiazine were purchased from Aldrich and used without further purification. Solvents (CH₃CN, t-BuOH, THF, and DMSO) were of high purities and used as purchased.

3.2. Equipment

IR spectra were recorded on a Bruker Tensor 27 in reflectance mode, using a Gemini sampling accessory to collect the horizontally attenuated total reflectance (ATR) spectra using a ZnSe crystal. IR spectra were recorded by the accumulation of at least 64 scans, with a resolution of 2 cm⁻¹.

¹H and ¹³C NMR spectra were recorded on a BRUKER Avance DRX 400 MHz spectrometer, equipped with a 5 mm direct detection QNP probe with z-gradients. The chemical shifts are reported as δ (ppm) relative to the residual peak of the DMSO solvent.

The particle size and size distribution were analyzed on a dynamic light scattering equipment Delsa Nano C, Beckman Coulter. The samples were prepared by the dilution of the triazole compounds in pure DMSO or DMSO–H₂O mixture (0.66 × 10⁻⁵ g mL⁻¹). The scattered light was measured at a fixed angle of 160°. The temperature was set at 25° ± 0.1 °C.

Scanning electron microscopy (SEM) images were acquired using a scanning electron microscope SEM EDAX-Quanta 200, at an accelerated electron energy of 10 or 15 keV. The samples were prepared by drop casting water suspension solution on microscope glass. The solvent was evaporated in air at room temperature.

The thermotropic behaviour of the triazole compounds was studied by observing the textures using an Olympus BH-2 polarized light microscope under cross polarizers with a THMS 600 hot stage and LINKAM TP92 temperature control system.

UV-vis absorption and photoluminescence spectra were recorded on a Carl Zeiss Jena SPECORD M42 spectrophotometer and a Perkin-Elmer LS 55 spectrophotometer, respectively, in solution and film, using 10 mm quartz cells and glass plates, respectively. The fluorescence intensity was calibrated using the Raman scattering peak of water.³⁵ The luminescence improvement was calculated by integrating the emission bands.

The optical study in solution was performed using stock solutions in DMSO. To confirm the absorption maxima, the UV-vis spectra were recorded in a wider wavelength domain, using solutions in THF, which has lower UV absorption than DMSO.

The determination of carbon, hydrogen, nitrogen and sulfur content of the compounds was performed on a 2400 Series II CHNS Perkin-Elmer elemental analyzer.

3.3. Synthesis

Three phenothiazine and pyridine-N-oxide-based triazoles were synthesised through the “click” chemistry of pyridine-N-oxide carrying a triple-bond functionality and azide-containing a phenothiazine (Scheme 1). Their synthesis is presented below.

Phenothiazine azides, namely, 2-azido-1-(10H-phenothiazin-10-yl)ethanone,⁴⁷ 2-azido-1-(2-chloro-10H-phenothiazin-10-yl)ethanone,⁴⁸ and 2-azido-1-(2-(trifluoromethyl)-10H-phenothiazin-10-yl)ethanone,⁴⁹ were synthesized in our laboratories according to published procedures as follows:

2-(Prop-2-yn-1-ylthio)pyridine-N-oxide. Propargyl bromide (12 mmol) was added to a suspension of sodium-2-sulfidopyridine-1-oxide (10 mmol) in 15 mL CH₃CN. The reaction mixture was stirred at reflux for 48 h. The resulting solid was collected by filtration, dried and recrystallized from ethanol to obtain pure 2-(prop-2-yn-1-ylthio)pyridine-N-oxide as an ochre solid with a 68% yield. mp 134–136 °C.

IR ν cm⁻¹: 3168 (νH–C≡), 2107 (νC≡C), 1588, 1553, 1472 (νC=C in aromatic rings), 1235 (νN → O), 1149 (ν pyridine), 836 (ν aromatic ring).

¹H NMR (DMSO-d₆, 400 MHz), δ (ppm): 8.33 (d, J = 6.4 Hz, 1H, ArH); 7.41–7.49 (m, 2H, ArH); 7.26 (dt, J = 8.6, 2.0 Hz, 1H, ArH); 3.93 (d, J = 2.4 Hz, 2H, CH₂); 3.25 (t, J = 2.4 Hz, 1H, H–C≡).

¹³C NMR (DMSO-d₆, 100 MHz), δ (ppm): 149.7 (C), 138.0 (CH), 125.4 (CH), 122.2 (CH), 121.4 (CH), 79.2 (C), 74.1 (CH), 17.7 (CH₂).

General procedure for the preparation of triazole derivatives. In a round bottom flask, a mixture of the azide (3 mmol) and 2-(prop-2-yn-1-ylthio) pyridine-N-oxide (3 mmol) was dissolved in a solvent mixture of t-BuOH and CH₃CN in a volume ratio of 5 : 3. When a homogeneous mixture was obtained, a 10% solution of sodium ascorbate (0.6 mmol) and copper sulfate (0.3 mmol) in water was added. The reaction mixture was subjected to magnetic stirring at 50 °C for 3–24 h, as monitored by thin-layer chromatography; subsequently, it was allowed to cool to room temperature. To remove the inorganic salts, cold water and ammonium hydroxide were added into the reaction flask. The resulting suspension was vigorously stirred at room temperature to complete the triazole precipitation, and crude product was separated by filtration and finally washed with ethyl acetate to obtain pure 1,2,3-triazole derivatives.

2-[4-(1-Oxy-pyridin-2-ylsulfanylmethyl)-[1,2,3]triazol-1-yl]-1-phenothiazin-10-yl-ethanone. White solid, mp 242–245 °C, 78% yield.

IR ν cm⁻¹: 1698 (νN–C=O), 1579, 1552, 1472 (νC=C in aromatic rings), 1260 (νN → O), 1147 (ν pyridine), 759 (ν aromatic ring).

¹H NMR (DMSO-d₆, 400 MHz) δ (ppm): 8.30 (d, J = 6.0 Hz, 1H, ArH), 8.02 (s, 1H, ArH), 7.75 (broad signal, 1H, ArH), 7.60 (d, J = 7.6 Hz, 3H, ArH), 7.44 (t, J = 7.6 Hz, 2H, ArH), 7.33–7.38 (m, 3H, ArH), 7.21 (t, J = 6.4 Hz, 1H, ArH), 5.52 (broad signal, 2H, CH₂CO), 4.33 (s, 2H, CH₂S).

¹³C NMR (DMSO-d₆, 100 MHz) δ (ppm): 164.7 (C), 150.3 (C), 142.2 (C), 138.0 (CH), 137.1 (2C), 132.2 (2C), 128.1 (CH), 127.6 (2CH), 127.0 (CH), 125.3 (2CH), 125.2 (2CH), 122.1 (2CH), 121.2 (2CH), 51.4 (CH₂), 24.3 (CH₂).

Elemental analysis calc. for C₂₂H₁₉N₅O₂S₂ (449.5): C 58.78; H 4.26; N 15.58; O 7.12; S 14.26. Found: C 58.69; H 4.31; N 15.72; S 14.32.

1-(2-Chloro-phenothiazin-10-yl)-2-[4-(1-oxy-pyridin-2-ylsulfanylmethyl)-[1,2,3]triazol-1-yl]-ethanone. White solid, mp 237–240 °C, 76% yield.

IR ν cm⁻¹: 1704 (νN–C=O), 1575, 1554, 1471 (νC=C in aromatic rings), 1247 (νN → O), 1151 (ν pyridine), 754 (ν aromatic ring).

¹H NMR (DMSO-d₆, 400 MHz) δ (ppm): 8.30 (d, J = 6.0 Hz, 1H, ArH) 8.02 (s, 1H, ArH), 7.84 (broad signal, 1H, ArH), 7.76 (broad signal, 1H, ArH), 7.60–7.62 (m, 3H, ArH), 7.33–7.48 (m, 4H, ArH), 7.21 (t, J = 6.4 Hz, 1H, ArH), 5.61 (broad signal, 1H, CH₂), 5.53 (broad signal, 1H, CH₂CO), 4.33 (s, 2H, CH₂S).

¹³C NMR (DMSO-d₆, 100 MHz) δ (ppm): 174.4 (C), 164.8 (C), 150.4 (C), 142.3 (C), 138.3 (C), 138.1 (CH), 136.5 (C), 131.8 (C), 131.4 (C), 129.2 (CH), 128.2 (CH), 127.8 (CH), 127.5 (CH), 127.0 (CH), 126.9 (CH), 125.3 (CH), 125.2 (CH), 122.1 (2CH), 121.2 (CH), 51.4 (CH₂), 24.3 (CH₂).

Elemental analysis calc. for C₂₂H₁₈ClN₅O₂S₂ (484): C 54.6; H 3.75; Cl 7.3; N 14.4; O 6.6; S 13.2. Found: C 54.35; H 3.91; N 14.7; S 13.28.

2-[4-(1-Oxy-pyridin-2-ylsulfanylmethyl)-[1,2,3]triazol-1-yl]-1-(2-trifluoromethyl-phenothiazin-10-yl)-ethanone. White solid, mp 230–232 °C, 70% yield.

IR ν cm⁻¹: 1702 (νN–C=O), 1551, 1470 (νC=C in aromatic rings), 1245 (νN → O), 1151 (ν pyridine), 763 (ν aromatic ring).

¹H NMR (DMSO-d₆, 400 MHz) δ (ppm): 8.30 (d, J = 6.4 Hz, 1H, ArH), 8.07 (s, 1H, ArH), 8.02 (s, 1H, ArH), 7.83 (d, J = 8.0 Hz, 1H, ArH), 7.70 (d, J = 8.4 Hz, 1H, ArH), 7.64 (d, J = 7.6 Hz, 1H, ArH), 7.60 (d, J = 8.4 Hz, 2H, ArH), 7.50 (t, J = 7.6 Hz, 1H, ArH), 7.41 (t, J = 7.6 Hz, 1H, ArH), 7.35 (t, J = 7.6 Hz, 1H, ArH), 7.20 (dt, J = 7.6, 1.6 Hz, 1H, ArH), 5.69 (broad signal, 1H, CH₂CO), 5.46 (broad signal, 1H, CH₂CO), 4.33 (s, 2H, CH₂S).

¹³C NMR (DMSO-d₆, 100 MHz) δ (ppm): 165.0 (C), 150.4 (C), 142.3 (C), 138.1 (CH), 137.5 (C), 136.3 (C), 131.4 (C), 129.0 (2CH), 128.4 (CH), 128.1 (C), 128.0 (CH), 127.0 (CH), 125.3 (CH), 125.2 (CH), 125.1 (C), 124.1 (CH), 123.9 (CH), 122.4 (C), 122.1 (CH), 121.2 (CH), 51.3 (CH₂), 24.3 (CH₂).

Elemental analysis calc. for C₂₃H₁₈F₃N₅O₂S₂ (517.5): C 53.38; H 3.51; F 11.01; N 13.53; O 6.18; S 12.39. Found: C 53.19; H 3.56; N 13.60; S 12.43.

Sample preparation for AIE measurements. Stock solutions with a concentration of 1% w/v were prepared by dissolving 10 mg triazole in 1 mL DMSO. Parts of the stock solution were transferred into 10 mL vials and diluted with appropriate amounts of DMSO or water to obtain 0.66 × 10⁻⁵ g mL⁻¹ solutions.

Film preparation by drop casting. Amorphous films have been prepared by drop casting 1% compound solution in DMSO on a glass support heated at 100 °C.

Ordered films, in form of micro- and nano-crystals have been prepared by drop casting a solution obtained by diluting 20 μL of 1% DMSO solution in 3 mL water.

4. Conclusions

Three triazoles based on phenothiazine and pyridine-N-oxide units were prepared by the “click” chemistry cycloaddition of pyridine-N-oxide carrying triple-bond functionality and azide-containing phenothiazine. The small rigid structural blocks are linked together by short flexible units and were chosen to be biologically friendly. All three compounds exhibit aggregation-induced emission phenomena, with a 233-fold higher emission in the crystalline state compared to that of the solution, and with around 4-fold higher emission compared to the amorphous state. The colour of the emitted light is red-shifted from UV light – in solution to bluish light – in water suspension and films. In solution and solid state, these compounds form nano- and micro-aggregates.

The increase in emission is in fact a consequence of restricted intramolecular rotations in the solid state, which prompts the random self-assembling of luminophore units with large intermolecular distances that mitigate stacking interactions and consequently open up the radiative channels. The self-assembling of these units in nano- and micro-particles further increases the emission intensity of crystalline films versus those of amorphous.

The flexible triazoles studied in this paper attract attention because of a new structural design in which small biologically friendly luminophore units are linked together by small flexible chains. This design enlarges the variety of the AIE luminogens based on the RIR mechanism to the flexible molecules, guiding further efforts in the development of new AIE structures for appropriate applications; the biological ones being especially envisaged. The ability to emit light by aggregation in aqueous solutions can be used for designing contrast agents, which can be further used for the in vivo luminescence imaging of deep tissues in medical optical imaging.

Acknowledgements

The research leading to these results has received funding from the Romanian National Authority for Scientific Research, CNCS–UEFISCDI, project number PN-II-PT-PCCA-2013-4-1861 (contract number 272/2014). One of the authors (Carmen Dumea) gratefully acknowledges the financial support of the European Social Fund within the Sectorial Operational Program Human Resources Development 2007–2013, through the Grant POSDRU/159/1.5/S/137750, “Project Doctoral and Postdoctoral programs support for increased competitiveness in Exact Sciences research”. This paper is dedicated to the 65th anniversary of Petru Poni Institute of Macromolecular Chemistry of Romanian Academy, Iasi, Romania.

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