Triazolyl Conjugated (Oligo)Phenothiazines Building Blocks for Hybrid Materials—Synthesis and Electronic Properties

Abstract

The Cu-catalyzed alkyne-azide 1,3-dipolar cycloaddition variant provides a highly efficient entry to conjugated triazolyl-substituted (oligo)phenothiazine organosilicon derivatives with luminescence and reversible redox characteristics. Furthermore, by in-situ co-condensation synthesis several representative mesoporous MCM-41 type silica hybrid materials with embedded (oligo)phenothiazines are prepared and characterized with respect to their structural and electronic properties. The hybrid materials also can be oxidized to covalently bound embedded radical cations, which are identified by their UV/Vis absorption signature and EPR signals.

1. Introduction

The mechanistic details of 1,3-dipolar cycloaddition were elucidated by Huisgen and his group almost six decades ago [1]. Ever since, it has become a versatile reaction for the formation of five-membered heterocycles, even in complex natural pro-duct syntheses [2]. However, it took almost 40 years for 1,3-dipolar cycloadditions, among other highly atom economical (cyclo)additions, to lead to the formulation of the “click chemistry” concept, coined by Sharpless and coauthors [3]. Although the synthesis of 1,2,3-triazoles by 1,3-dipolar cycloaddition of organic azides to alkynes was established and mechanistically well described [4], the regioselectivity of the pericyclic process is often poor or even absent for terminal alkynes and organic azides. Changing the mechanism from the thermal process to a stepwise organometallic copper-catalyzed process, named Meldal-Sharpless reaction [5,6] or copper-catalyzed alkyne-azide cycloaddition (CuAAC) [7], gave rise to the highly selective formation of 1,4-disubstituted triazoles. Mechanistic elucidation [8], catalyst design [9], and implementation into multicomponent methodologies [10] mark basic expansions of CuAAC over the years.

Besides application in drug discovery [11] and lead discovery [12] in medicinal chemistry [13] and bioorganic chemistry [14,15] due to the operational simplicity and broad tolerance for functional groups, CuAAC found rapid use as a valuable ligation tool in polymer and materials sciences, either for polymer analog transformation or for polymerization, as emphasized in several reviews since the early days [16,17,18,19,20,21,22,23]. Furthermore, meso-porous silica hybrid materials have also been successfully functionalized by azides employing CuAAC as a postsynthetic grafting tool [24] for internal and external surface deco-ration in biosensing applications [25]. Recently, we used CuAAC for preparing triethoxysilylated substituted fluorophore-containing precursors for the synthesis of Nile Red-embedded mesoporous silica hybrids as solvent polarity sensors that also operate in aqueous media [26]. Furthermore, we expanded the CuAAC approach for the synthesis of RGB fluorophore precursors that were used for mesoporous silica hybrid materials for energy down-converting phosphors and application in monolith coated light emitting diodes [27]. Based upon our experience in covalently anchoring phenothiazine electrophores in silica hybrid materials [28], either by postsynthetic grafting with carbamate linkers [29] or in-situ synthesis of the hybrid materials using the more hydrolysis stable urea linkers [30], we reasoned that CuAAC of phenothiazinyl alkynes with azido functiona-lized triethoxysilanes would provide highly luminescent, reversible redox systems that can be employed in hybrid silica in-situ synthesis. Herein we report the synthesis by CuAAC and electronic properties of triazolyl conjugated oligophenothiazines as luminescent electrophore precursors.

2. Results and Discussion

2.1. Synthesis

Our synthesis of linear conjugated triazolylsubstituted (oligo)phenothiazine precursors for hybrid materials’ synthesis bearing triethoxylsilyl propyl groups commences from ethynylated (oligo)phenothiazines 1 [31,32,33,34]. Mono- and diethynylated (oligo)phenothiazines 1 undergo the CuAAC at room temp in DMF with (3-azidopropyl)tri-alkoxysilanes 2 [35] to give conjugated triazolylsubstituted (oligo)phenothiazines 3 in excellent yield as oils (Scheme 1). Both triethoxy- and trimethoxysilyl derivatives 3 are readily obtained as well as methyl- and n-hexyl-substituted phenothiazine derivatives. The structures of compounds 3 were unambiguously assigned by IR, ¹H- and ¹³C-NMR spectroscopy and mass spectrometry.

For the synthesis of the conjugated and nonconjugated diphenothiazine dumbbells 8 decorated with conjugated triazoles bearing triethoxylsilyl propyl groups, first the tetrakis(ethynyl) compounds 7 were synthesized by Sonogashira alkynylation of the tetra-bromo derivatives 4 with (trimethylsilyl)acetylene (5) followed by desilylation of the crude product 6 (Scheme 2). Similarly to the formation of the precursors 3 the conjugated and nonconjugated diphenothiazine dumbbells 8 bearing conjugated triazoles with tri-ethoxylsilyl propyl groups were synthesized by CuAAC with (3-azidopropyl)triethoxy-silane (2a) in moderate yield. The structures of compounds 8 were unambiguously assigned by IR, ¹H- and ¹³C-NMR spectroscopy and mass spectrometry.

2.2. Electronic Properties of the Triazolyl Conjugated (Oligo)Phenothiazines

The ground and excited state electronic properties of the triazolyl conjugated (oligo)-phenothiazines 3 and 8 was studied by cyclic voltammetry, absorption and emission spectroscopy (Table 1).

All (oligo)phenothiazines triazolyl-conjugated (oligo)phenothiazines 3 and 8 possess fully chemically reversible first oxidation potentials between 650 and 800 mV (Table 1, Figure 1). Upon direct comparison of the first oxidation potentials of N-methyl phenothiazine (E_1/2^0/+1 = 770 mV) or N-hexyl phenothiazine (E_1/2^0/+1 = 730 mV) [32] to mono phenothiazines 3a (E_1/2^0/+1 = 750 mV) and 3b (E_1/2^0/+1 = 720 mV), and 3d (E_1/2^0/+1 = 730 mV) and 3e (E_1/2^0/+1 = 700 mV) it becomes apparent that the triazole electronically exerts a weak electron-releasing effect [22]. The directly conjugated dyads 3c and 3f are more electron-rich, with lowest first oxidations at E_1/2^0/+1 = 650 mV, and reveal two consecutive oxidations se-parated by 140 to 170 mV arising from diphenothiazinyl coupling [36]. Likewise, the N-bridged conjugated dyad 8a displays a similiar behavior of a coupled redox system consisting of two electrophore termini with a separation of the two one-electron oxidations of 180 mV (Figure 1 left). In contrast, the deconjugated dyad 8b only shows a single reversible first oxidation, which occurs simultaneously at both electrophore moieties, i.e., without any electronic coupling (Figure 1 right).

Electron absorption and emission spectroscopy clearly show that all compounds in this study are luminescent with typical huge phenothiazine Stokes shifts [36]. More specifically for the series 3, the absorption in the UV region is accompanied by a blue to greenish blue fluorescence for all representatives. The emission maxima lie between 446 and 470 nm. The Stokes shifts in a range from $\Delta \tilde{\nu }$ = 5700 to 9200 cm⁻¹ are caused by significant geometrical changes upon excitation from a highly non-planar ground state to a largely planarized excited state [37]. Although showing a different mode of ligation the dyads 8 possess very similar emission maxima λ_em,max at 455 (8a) and 456 nm (8b), indicating that the major contribution of the luminescence stems from the bis(triazolyl)phenothiazine structural element (Figure 2). Essentially the same reasoning for huge Stokes shifts arising from planarization of the excited state upon UV excitation also holds true for this series.

2.3. Silica Hybrid Materials from Selected Triazolyl Conjugated (Oligo)Phenothiazines 3b-d

Encouraged by our successful synthesis of fluorophore-containing silica hybrid materials from triazolyl ligated precursors [26,27] we reasoned this type of precursor ligation could be employ for embedding electrophores in the sense of an in-situ synthesis of meso-porous hybrid materials. Exemplarily the three triazolyl conjugated phenothiazine deri-vatives 3b, 3c, and 3d were chosen as substrates for the material synthesis. The in situ synthesis of silica hybrid materials was performed by first stirring an aqueous mixture of the surfactant cetyltrimethylammonium bromide (CTAB), ethylamine, precursor 3b, methanol (and THF for precursors 3c and 3d), and tetraethylorthosilicate (TEOS) at room temp for 24 h followed by heating to 100 °C for another 24 h (Scheme 3). After extraction of CTAB the powdery materials 3b–d@MCM-41 were obtained and the loadings of the electrophores 3b–d were determined by combustion analysis.

2.4. Structural Characterization of Silica Hybrid Materials 3b–d@MCM-41

First encouraging information on the silica hybrid materials 3b–d@MCM-41 is provided by the phenothiazine content that was determined by combustion analysis (Table 2). Expectedly, the smaller the precursor 3b–d the higher is the phenothiazine weight percentage in the derived hybrid materials 3b–d@MCM-41. Interestingly, the employed precursor concentration also affects phenothiazine loading as seen by the phenothiazine content of the samples 3c-1@MCM-41 and 3c-2@MCM-41.

Powder X-ray diffraction (XRD) was employed as a routine method to characterize the ordering of the mesostructure (Table 2, for a detailed discussion of the powder XRD data, see Supplementary Materials). The XRD patterns of the hybrid materials 3b–d@MCM-41 show the typical reflections of the [100] plane of hexagonal symmetry at 2θ = 2.0 to 2.1° corresponding to d-values between 4.20 to 4.50 nm. Additionally, overlapping broad bands can be observed in the 2θ rage of 3.4 to 4.8°, which can be assigned to the diffraction of the 110 and 200 planes of the hexagonally symmetric lattice of the MCM-41 type materials [38]. Therefore, periodic arrangement of pores in the materials are present, although these diffraction peaks are neither very sharp nor intense. This decreased intensity and broadening of the diffraction peaks disturbs the contrast between the framework and the pore channels [39,40], which can be attributed to structural distortions caused by the stiffness of precursor materials 3 in cylindrical channels [41]. In comparison to the cell parameter a₀ = 4.96 nm of pure silica MCM-41 [30] the cell parameters a₀ = 4.85–5.20 nm fall into a similar range. While material 3b@MCM-41 possesses a slightly shrunk lattice, the lattices of 3c,d@MCM-41 formed from precursors 3c and 3d reveal a slightly expanded lattice. Caused by the size and geometry, the mesostructure of material 3d@MCM-41 is similar to a material with a wormhole-like pore system (or HMS silica phases) [42].

Nitrogen physisorption measurements provide information on the pore structure and surface parameter of the hybrid materials 3b-d@MCM-41. Textural parameters derived from isotherms and the pore distribution curves (see Supplementary Materials) are collected in Table 2. The isotherms and their pore distribution curves clearly support the mesoporous structure of the materials 3b-d@MCM-41. The data of the two samples of 3c@MCM-41 with varying phenothiazine content expectedly give different surface areas and pore volumes.

The estimated wall thickness of 3d@MCM-41 of 2.76 nm is rather high compared to pure silica MCM-41 obtained under similar conditions as well as to other phenothiazine containing hybrid materials anchored on the internal surface [28,30]. Due to the relatively large dimensions of precursor 3d (Figure 3 highlights the rigid region and the maximum intramolecular Si-Si distance of the MM2-calculated expanded conformer) the ligation distributes this precursor also inside the channel walls. With an average Si-O bond length of 1.6 Å and the size of precursor 3d, two putative scenarios can be envisioned: First, the molecules are embedded inside the walls of the material, or second, the molecules are placed outside of the wall stretching across the pore with both silicon atoms as anchoring sites.

Furthermore, transmission electron microscopy (TEM) supports the mesoporous structure of the hybrid materials 3b–d@MCM-41 (for details, see Supplementary Materials), additionally revealing distortions from ideal parallel orientation of the channels caused by the large magnitude of some precursors even leading to worm-hole like morphology for the material 3d@MCM-41 formed from the largest precursor 3d in this series (Figure 4).

Spectroscopic characterization of the hybrid materials 3b-d@MCM-41 by FT-IR, ¹³C CP-MAS, and ²⁹Si CP-MAS NMR spectroscopy additionally supports the molecular structural features of the materials (for details, see Supplementary Materials). In particular, ¹³C CP-MAS NMR spectra of the materials 3b-d@MCM-41 in comparison the solution spectra of the corresponding precursors 3b-d manifests the unambiguous incorporation of the organic moieties within the materials without disintegration during the hydrothermal synthesis and the acidic extraction process. Finally, ²⁹Si CP-MAS NMR spectroscopy outlines the presence of organic/inorganic moieties as basic structural units constituting the hybrid mesoporous materials.

2.5. Optical Properties and Chemical Oxidation of Hybrid Materials 3b–d@MCM-41

As already seen from the optical properties of the precursors 3b–d the effect of conjugated triazolyl moieties in the hybrid materials 3b–d@MCM-41 not only affects the mode of ligation. Interestingly, some absorption and emission characteristics, although obtained from solution data for the precursors and in reflectance mode for the materials, are very similar for precursors and materials, but some also distinctly differ from each other (Table 3, for spectra, see Supplementary Materials).

Hybrid material 3b@MCM-41, which is a yellow powder, and its precursor 3b, exhibit very similar absorption and emission characteristics: two absorption bands at around 260 and 320 nm and one emission band at around 451 nm. This accounts for the absence of any interaction of the chromophore with the framework of the material. Essentially, also the chromophores are equally distributed in the material without any interchromophore interactions. The remarkably large Stokes shift (~9000 cm⁻¹) demonstrating a significant geometrical change upon the excitation from a highly non-planar ground state to largely planarized excited state is typical for phenothiazine derivatives [37]. The chromophores contain triazole moieties that can be protonated by sufficiently strong acids. Upon protonation of hybrid material 3b@MCM-41-H⁺ with trifluoroacetic acid a redshift of the longest wavelength absorption band from 318 to 364 nm can be observed (for details, see Supplementary Materials). The triazole protonation causes an increase of the substituents electron withdrawing character to a −M substituent which plausibly diminishes the HOMO-LUMO gap and explains the observed halochromicity.

The color of hybrid material 3c@MCM-41 is dark yellow and the red-shifted color can be explained with the large π-system of the constituting biphenothiazine chromophore, as also supported by the bathochromic shift of the absorption bands in comparison to hybrid material 3b@MCM-41. This extended π-system also causes a red shift of the emission bands at 484 nm (3c-1@MCM-41) and 500 nm (3c-2@MCM-41). Interestingly, the higher chromophore loading of material 3c-2@MCM-41 not only causes a pronounced red shift but even a more intense blue emission upon eyesight.

The adsorption and emission spectra of material 3d@MCM-41 containing a phenothiazine with two ligation positions possess in addition to two absorption bands at 270 and 330 nm typical for phenothiazines and a small shoulder appears at around 360 nm. Interestingly, the emission spectrum of this hybrid material displays dual emission with maxima around 445 and 390 nm, which are absent in the solution spectrum of precursor 3d. Considering that the molecules are uniformly distributed inside the silica framework, this dual emission is likely caused by conformational biases due to restricted molecular rotation and relaxation.

Encouraged by the apparent stability of radical cations of the phenothiazine containing precursors 3 as shown by reversible electrochemical oxidation under the conditions of cyclic voltammetry we decided to chemically oxidize the silica hybrid materials with one electron oxidants, such as antimony pentachloride and NOBF₄, in dichloromethane suspension at room temp (Scheme 4).

The color of the materials 3b–d@MCM-41 immediately changes from yellow to dark green-blue after addition of the oxidant. In the absorption spectra of the suspensions, radical cation absorption signatures at low energy are discernable. However, the stronger oxidation potential of SbCl₅ not only causes differences in the absorption spectra due to counter ion effects, but also due to more complete oxidation more intense absorption bands of the resulting oxidized hybrid materials 3b–d⁺@MCM-41 X⁻ are found (for spectra and details, see Supplementary Materials). Interestingly, the suspension of cationic material 3b⁺@MCM-41 BF₄⁻ also shows a protochromic behavior as the pristine materials 3b@MCM-41 (for spectra, see Supplementary Materials). In the solid state, all oxidized hybrid materials are deeply colored, mostly dark green to black. Remarkably also for material 3d@MCM-41 with phenothiazines embedded in the silica walls, these electrophores are reached by the chemical oxidant, as also observed by others [43]. Another interesting feature is that the obtained oxidized materials are obviously quite stable under a nitrogen atmosphere at ambient temperature for several weeks due to absence of any bleaching of the radical cations.

Finally, electron paramagnetic resonance (EPR) spectra were recorded for the oxidized hybrid materials 3b⁺@MCM-41 SbCl₆⁻ and 3d⁺@MCM-41 SbCl₆⁻ to provide further evidence of the presence of stable phenothiazine radical cations in the hybrid materials (Figure 5). The g-values of these two samples are 2.0054 and 2.0050, respectively. Although, the EPR signals are not well resolved and hyperfine coupling constants and an-isotropic g-tensors cannot be determined, the high symmetry of the EPR spectra indicates that the phenothiazine radicals find themselves in a quite isotropic environment in the hybrid materials. This remarkable chemical stability of phenothiazinyl radical cations in mesoporous silica hybrid materials makes them suitable for studying radical processes in mesoporous environments [44].

3. Materials and Methods

3.1. General Procedure (GP1) for CuAAC Synthesis of 1-(3-(Triethoxysilyl)propyl)-1,2,3-Triazol-4-yl Substituted (Oligo)phenothiazine Precursors 3

To a well-stirred mixture phenothiazinyl alkyne 1a–f (1.0 equiv), placed in a Schlenk tube with a magnetic stir bar and dissolved in dry DMF, copper iodide (10 mol%) was added (For details, see Table 4). Then, 3-(triethoxysilyl)propylazide (2a) or 3-(trimethoxy-silyl)propylazide (2b) (1.0 equiv per CC triple bond) was added, the reaction tube was sealed and the suspension was stirred under nitrogen at room temp for 16 h. The reaction mixture was filtered through a short plug of Celite^® and washed with dichloromethane. The solvents were removed in vacuo and the product was dried in vacuo for 14 h.

3.1.1. 10-Methyl-3-(1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazole-4-yl)-10H-phenothiazine (3a)

According to GP1 the triazole 3a (193 mg, 95%) was isolated as a brown oil. ¹H-NMR (CD₂Cl₂, 500 MHz): δ 0.62 (t, J = 8.0 Hz, 2 H), 1.20 (t, J = 7.0 Hz, 9 H), 2.02 (m, 2 H), 3.39 (s, 3 H), 3.80 (q, J = 7.0 Hz, 6 H), 4.36 (t, J = 7.0 Hz, 2 H), 6.87 (m, 2 H), 6.94 (dt, J = 7.5 Hz, J = 1.0 Hz, 1 H), 7.14 (dd, J = 1.0 Hz, J = 7.5 Hz, 1 H), 7.19 (m, 1 H), 7.58 (d, J = 2.0 Hz, 1 H), 7.64 (dd, J = 2.0 Hz, J = 8.5 Hz, 1 H), 7.73 (s, 1 H). ¹³C-NMR (CD₂Cl₂, 125 MHz): δ 7.1 (CH₂), 17.8 (CH₃), 23.9 (CH₂), 34.9 (CH₃), 52.2 (CH₂), 58.1 (CH₂), 113.8 (CH), 113.8 (CH), 118.8 (CH), 122.1 (CH), 122.5 (C_quat), 123.4 (C_quat), 123.6 (CH), 124.3 (CH), 125.1 (C_quat), 126.7 (CH), 127.2 (CH), 128.2 (C_quat), 145.1 (C_quat), 145.2 (C_quat). UV/Vis (CH₂Cl₂) λ_max (ε): 266 (49200), 319 nm (9950). IR (film): $\tilde{\nu }$ = 2889, 2098, 1467, 1334, 1260, 1103, 954, 751, 698, 561, 534 cm⁻¹. MS (FAB⁺) m/z (%): 484.3 (100, M⁺), 439.2 (18, M⁺−C₂H₅O), 422.2 (17), 376.3 (42, M⁺− CH₂Si(OC₂H₅)₃).

3.1.2. 10-Hexyl-3-(1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazole-4-yl)-10H-phenothiazine (3b)

According to GP1 the triazole 3b (228 mg, 97%) was isolated as a brown oil. ¹H-NMR (CD₂Cl₂, 500 MHz): δ 0.62 (t, J = 8.0 Hz, 2 H), 0.88 (t, J = 7.0 Hz, 3 H), 1.20 (t, J = 7.0 Hz, 9 H), 1.31 (m, 4 H), 1.44 (m, 2 H), 1.80 (m, 2 H), 2.03 (m, 2 H), 3.80 (q, J = 7.0 Hz, 6 H), 3.86 (t, J = 7.5 Hz, 2 H), 4.36 (t, J = 7.5 Hz, 2 H), 7.16 (m, 2 H), 7.45 (m, 2 H), 7.55 (d, J = 2.0 Hz, 1 H), 7.62 (dd, J = 8.5 Hz, J = 2.0 Hz, 1 H), 7.72 (s, 1 H), 7.87 (m, 1 H). ¹³C-NMR (CD₂Cl₂, 125 MHz): δ 7.1 (CH₂), 13.4 (CH₃), 17.8 (CH₃), 22.3 (CH₂), 23.9 (CH₂), 26.2 (CH₂), 26.5 (CH₂), 31.1 (CH₂), 41.7 (CH₂), 52.2 (CH₂), 58.1 (CH₂), 115.1 (CH), 115.2 (CH), 118.8 (CH), 122.0 (C_quat), 123.8 (C_quat), 124.2 (CH), 127.0 (C_quat), 128.1 (CH), 128.2 (C_quat), 131.3 (CH), 131.6 (CH), 131.7 (CH), 144.6 (C_quat), 144.7 (C_quat). UV/Vis (CH₂Cl₂) λ_max (ε): 268 (55400), 318 nm (14250). IR (film): $\tilde{\nu }$ = 2973, 2927, 1719, 1655, 1579, 1544, 1459, 1390, 1340, 1290, 1252, 1166, 1102, 1080, 957, 793, 750, 726, 698, 560, 528 cm⁻¹. MS (FAB⁺) m/z (%): 554.3 (100, M⁺), 509.3 (12, M⁺− C₂H₅O), 483.2 (6, M⁺− C₅H₁₁), 422.2 (12), 376.3 (24, M⁺− CH₂Si(OC₂H₅)₃).

3.1.3. 10,10’-Dihexyl-7-(1-(3-(trimethoxysilyl)propyl)-1H-1,2,3-triazole-4-yl)-10H,10’H-3,3’-biphenothiazine (3c)

According to GP1 the triazole 3c (970 mg, 98%) was isolated as a brown oil. ¹H-NMR (CD₂Cl₂, 500 MHz): δ 0.65 (t, J = 8.0 Hz, 2 H), 0.89 (m, 6 H), 1.31 (m, 8 H), 1.44 (m, 4 H), 1.79 (m, 4 H), 2.04 (m, 2 H), 3.57 (s, 9 H), 3.84 (m, 4 H), 4.36 (t, J = 7.0 Hz, 2 H), 6.89 (m, 5 H), 7.15 (m, 2 H), 7.32 (m, 4 H), 7.60 (m, s, 1 H), 7.64 (m, 1 H), 7.74 (s, 1 H). ¹³C-NMR (CD₂Cl₂, 125 MHz): δ 5.8 (CH₂), 13.5 (CH₃), 22.3 (CH₂), 23.6 (CH₂), 26.2 (CH₂), 26.3 (CH₂), 26.4 (CH₂), 31.1 (CH₂), 47.0 (CH₂), 47.1 (CH₂), 50.1 (CH₃), 52.1 (CH₂), 115.0 (CH), 115.1 (CH), 118.8 (CH), 121.9 (CH), 123.8 (CH), 124.1 (C_quat), 124.2 (CH), 124.4 (CH), 124.6 (C_quat), 124.7 (CH), 124.8 (CH), 124.9 (C_quat), 126.9 (CH), 133.5 (C_quat), 133.7 (C_quat), 143.4 (C_quat), 143.8 (C_quat), 144.3 (C_quat), 144.7 (C_quat), 146.1 (C_quat). UV/Vis (CH₂Cl₂) λ_max (ε): 278 (43200), 328 (14600), 367 nm (12300). IR (film): $\tilde{\nu }$ = 2926, 2852, 1672, 1577, 1508, 1458, 1376, 1332, 1251, 1191, 1082, 874, 807, 764 cm⁻¹. MS (MALDI) calcd. for [C₄₄H₅₅N₅O₃S₂Si]⁺ m/z: 793.3; found: 793.3 (M⁺).

3.1.4. 10-Methyl-3,7-bis(1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazole-4-yl)-10H-phenothiazine (3d)

According to GP1 the bistriazole 3d (140 mg, 88%) was isolated as a brown oil. ¹H-NMR (CD₂Cl₂, 500 MHz): δ 0.62 (t, J = 8.0 Hz, 4 H), 1.20 (t, J = 7.0 Hz, 18 H), 2.03 (m, 4 H), 3.42 (s, 3 H), 3.80 (q, J = 7.0 Hz, 12 H), 4.37 (t, J = 7.5 Hz, 4 H), 6.90 (m, 1 H), 7.45 (m, 2 H), 7.60 (m, 2 H), 7.74 (m, 1 H), 7.87 (m, 2 H). ¹³C-NMR (CD₂Cl₂, 125 MHz): δ 7.1 (CH₂), 17.8 (CH₃), 23.8 (CH₂), 35.0 (CH₃), 52.2 (CH₂), 58.1 (CH₂), 114.0 (CH), 118.8 (CH), 123.0 (C_quat), 123.7 (CH), 124.4 (CH), 125.3 (C_quat), 128.2 (CH), 131.7 (CH), 144.8 (C_quat), 146.1 (C_quat). UV/Vis (CH₂Cl₂) λ_max (ε): 276 (37800), 334 nm (7200). IR (film): $\tilde{\nu }$ = 2974, 1457, 1392, 1337, 1263, 1079, 957, 787, 698, 560 cm⁻¹. MS (MALDI) calcd. for [C₃₅H₅₃N₇O₆SSi₂]⁺ m/z: 756.335, found: 756.297 (M⁺).

3.1.5. 10-Hexyl-3,7-bis(1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazole-4-yl)-10H-phenothiazine (3e)

According to GP1 the bistriazole 3e (625 mg, 93%) was isolated as a brown oil. ¹H-NMR (CD₂Cl₂, 500 MHz): δ 0.62 (t, J = 7.7 Hz, 4 H), 0.88 (t, J = 7.0 Hz, 3 H), 1.20 (t, J = 7.0 Hz, 18 H), 1.33 (m, 4 H), 1.46 (m, 2 H), 1.83 (m, 2 H), 2.03 (m, 4 H), 3.80 (q, J = 7.0 Hz, 12 H), 3.89 (t, J = 7.3 Hz, 2 H), 4.37 (t, J = 7.3 Hz, 4 H), 6.93 (d, J = 8.5 Hz, 2 H), 7.57 (d, J = 2.0 Hz, 2 H), 7.63 (dd, J = 8.5 Hz, J = 2.0 Hz, 2 H), 7.73 (s, 2 H). ¹³C-NMR (CD₂Cl₂, 125 MHz): δ 7.0 (CH₂), 13.4 (CH₃), 17.8 (CH₃), 22.3 (CH₂), 23.9 (CH₂), 26.2 (CH₂), 26.4 (CH₂), 31.1 (CH₂), 41.7 (CH₂), 52.2 (CH₂), 58.1 (CH₂), 115.2 (CH), 123.8 (CH), 124.2 (C_quat), 124.3 (CH), 125.1 (CH), 144.2 (C_quat), 148.1 (C_quat). UV/Vis (CH₂Cl₂) λ_max (ε): 276 (54500), 335 nm (10100). IR (film): $\tilde{\nu }$ = 2974, 2928, 1871, 1736, 1719, 1655, 1618, 1560, 1458, 1400, 1365, 1296, 1257, 1166, 1079, 956, 789 cm⁻¹. MS (MALDI) calcd. for [C₄₀H₆₃N₇O₆SSi₂]⁺ m/z: 825.4; found: 825.4 (M⁺).

3.1.6. 10,10’Dihexyl-7,7’-bis(1-(3-(trimethoxysilyl)propyl)-1H-1,2,3-triazole-4-yl)-10H,10’H-3,3’-biphenothiazine (3f)

According to GP1 the bistriazole 3f (90 mg, 92%) was isolated as a brown oil. ¹H-NMR (CD₂Cl₂, 500 MHz): δ 0.64 (t, J = 8.0 Hz, 4 H), 0.88 (t, J = 7.0 Hz, 6 H), 1.33 (m, 8 H), 1.46 (m, 4 H), 1.82 (m, 4 H), 2.02 (m, 4 H), 3.55 (s, 18 H), 3.88 (t, J = 7.0 Hz, 4 H), 4.36 (t, J = 7.0 Hz, 4 H), 6.93 (d, J = 3.5 Hz, 2 H), 7.32 (d, J = 2.0 Hz, 2 H), 7.35 (dd, J = 8.0 Hz, J = 2.0 Hz, 2 H), 7.56 (d, J = 2.0 Hz, 2 H), 7.62 (dd, J = 8.0 Hz, J = 2.0 Hz, 2 H), 7.72 (s, 2 H). ¹³C-NMR (CD₂Cl₂, 125 MHz): δ 5.8 (CH₂), 13.4 (CH₃), 22.3 (CH₂), 23.7 (CH₂), 26.2 (CH₂), 26.4 (CH₂), 31.1 (CH₃), 47.7 (CH₂), 52.1 (CH₂), 115.1 (CH), 115.2 (CH), 118.7 (CH), 122.6 (C_quat), 122.8 (C_quat), 123.2 (C_quat), 123.4 (C_quat), 123.9 (CH), 124.2 (CH), 124.3 (CH), 124.8 (CH), 131.6 (C_quat), 142.8 (C_quat), 144.1 (C_quat). UV/Vis (CH₂Cl₂) λ_max (ε): 278 (50700), 336 nm (16100). IR (film): $\tilde{\nu }$ = 2926, 1719, 1611, 1560, 1459, 1400, 1400, 1340, 1254, 1191, 1079, 874, 803 cm⁻¹. MS (MALDI) calcd. For [C₅₂H₇₀N₈O₆S₂Si₂]⁺ m/z: 1022.44; found: 1022.5 (M⁺).

3.2. 1,4-Di(10H-phenothiazin-10-yl)benzene

In an oven-dried Schlenk tube with a magnetic stir bar were placed 10H-phenothiazine (3.99 g, 20.0 mmol), 1,4-diiodobenzene (3.30 g, 10.0 mmol), bis(dibenzylideneacetone)palladium(0) (350 mg, 6.00 mol%), tri(tert-butyl)phosphonium tetrafluoroborate (290 mg, 10.0 mol%), sodium tert-butanolate (2.21 g, 23.0 mmol), and dry 1,4-dioxane (80.0 mL). The dark solution was deaerated by a constant stream of nitrogen via syringe for 10 min. Then, the reaction mixture was heated to reflux for 16 h. After cooling to room temp the solvents were removed in vacuo and the residue was absorbed on celite^® and purified by column chromatography on silica gel (n-hexane/ethyl acetate (9:1 → 1:1) to give 1,4-di(10H-phenothiazin-10-yl)benzene [45,46] (4.44 g, 94%) as a light beige powder, Mp 258–260 °C. R_f (n-hexane/acetone 9:1): 0.30. ¹H-NMR (300 MHz, di-methylsulfoxide-d₆): δ 6.55 (dd, ³J = 8.1 Hz, ⁴J = 1.1 Hz, 4 H), 6.97 (ddd, ³J = 7.5, 7.8 Hz, ⁴J = 1.1 Hz, 4 H), 7.09 (ddd, ³J = 8.1 Hz, 7.8 Hz, ⁴J = 1.5 Hz, 4 H), 7.19 (dd, ³J = 7.5 Hz, ⁴J = 1.5 Hz, 4 H), 7.53 (s, 4 H). ¹³C-NMR (500 MHz, dimethylsulfoxide-d₆): δ 118.5 (CH), 122.3 (C_quat), 123.8 (CH), 127.4 (CH), 127.9 (CH), 130.4 (CH), 140.3 (C_quat), 143.6 (C_quat). MS (EI) m/z: 472 (5, [M]⁺), 352 (93, C₂₃H₁₆N₂S]⁺). IR $\tilde{\nu }$ [cm⁻¹]: 3061 (w), 3032 (vw), 1884 (vw), 1591 (m), 1556 (m), 1501 (s), 1483 (w), 1460 (vs), 1443 (s), 1406 (vw), 1369 (vw), 1304 (s), 1279 (w), 1254 (s), 1236 (s), 1184 (w), 1165 (vw), 1155 (vw), 1126 (m), 1084 (m), 1042 (s), 1017 (m), 961 (w), 930 (s), 840 (m), 739 (vs), 723 (s), 712 (s), 700 (vw), 664 (m), 627 (vs). Anal. calcd. for C₃₀H₂₀N₂S₂ [472.6]: C 76.24, H 4.27, N 5.93, S 13.57; Found: C 76.41, H 4.19, N 5.76, S 13.50.

3.3. 1,4-Bis(3,7-dibromo-10H-phenothiazin-10-yl)benzene (4a)

In an oven-dried 250 mL two-necked round bottom flask with a magnetic stir bar were placed 1,4-di(10H-phenothiazin-10-yl)benzene (1.35 g, 2.86 mm) and glacial acetic acid (823 mL). A solution of elementary bromine (740 μL, 14.3 mmol) in acetic acid (114 mL) was added dropwise over an hour to the vigorously stirred solution at room temp. The dark brown solution was then stirred at room temp for 16 h. Then a saturated aqueous solution of sodium sulfite (720 mg, 5.71 mmol) was added and the solution was stirred at room temp for 3 h. The precipitate was collected by suction and was washed three times with methanol. Then the product was dried under vacuo to give compound 4a (2.10 g, 93%) as a light yellow powder, Mp 356–358 °C. R_f (n-hexane/dichloromethane 9:1): 0.15. ¹H-NMR (300 MHz, dimethylsulfoxide-d₆): δ 6.33 (d, ³J = 8.8 Hz, 4 H), 7.21 (dd, ³J = 8.8 Hz, ⁴J = 2.3 Hz, 4 H), 7.39 (d, ⁴J = 2.3 Hz, 4 H), 7.62 (s, 4 H). ¹³C-NMR (75 MHz, dimethylsulfoxide-d₆): δ 114.8 (C_quat), 119.0 (CH), 122.6 (C_quat), 128.8 (CH), 130.3 (CH), 131.6 (CH), 139.6 (C_quat), 142.2 (C_quat). MS (EI) m/z: 792 (15, [M⁸¹Br₄]⁺), 790 (47, [M⁸¹Br₃⁷⁹Br]⁺), 788 (68, [M⁸¹Br₂⁷⁹Br₂]⁺), 786 (44, [M⁸¹Br₃⁷⁹Br]⁺), 784 (6, [M⁷⁹Br₄]⁺), 711 (15, [C₃₀H₁₆⁸¹Br₃N₂S₂]⁺), 709 (22, [C₃₀H₁₆⁸¹Br₂⁷⁹BrN₂S₂]⁺), 707 (20, [C₃₀H₁₆⁸¹Br⁷⁹Br₂N₂S₂]⁺), 705 (6, [C₃₀H₁₆⁷⁹Br₃N₂S₂]⁺), 630 (9, [C₃₀H₁₆⁸¹Br₂N₂S₂]⁺), 628 (12, [C₃₀H₁₆⁸¹Br⁷⁹BrN₂S₂]⁺), 626 (5, [C₃₀H₁₆⁷⁹Br₂N₂S₂]⁺), 358 (32, [C₁₂H₆⁸¹Br₂NS]⁺), 356 (65, [C₁₂H₆⁸¹Br⁷⁹BrNS]⁺), 354 (39, [C₁₂H₆⁷⁹Br₂NS]⁺), 272 (43, [C₁₈H₁₀NS]⁺), 196 (100, [C₁₂H₆NS]⁺). IR $\tilde{\nu }$ [cm⁻¹]: 3917 (vw), 2959 (vw), 2857 (vw), 2332 (w), 2305 (vw), 1582 (m), 1504 (s), 1481 (m), 1452 (vs), 1406 (w), 1379 (m), 1346 (vw), 1308 (s), 1279 (w), 1265 (w), 1254 (m), 1242 (m), 1200 (vw), 1186 (vw), 1152 (vw), 1123 (vw), 1096 (m), 1078 (w), 1067 (vw), 1018 (w), 951 (vw), 920 (w), 866 (m), 856 (m), 839 (m), 820 (vw), 797 (vs), 754 (m), 735 (w), 716 (vw), 685 (w), 669 (w). Anal. calcd. for C₃₀H₁₆N₂S₂Br₄ [788.2]: C 45.72, H 2.05, N 3.55, S 8.13; Found: C 45.83, H 1.99, N 3.46, S 7.95.

3.4. 1,4-Bis((3,7-dibromo-10H-phenothiazin-10-yl)methyl)benzene (4b)

In an oven-dried Schlenk tube with a magnetic stir bar were placed 3,7-dibromo-10H-phenothiazin 1a (3.57 g, 10.0 mmol), powdered potassium hydroxide (700 mg, 12.5 mmol), and dry N,N-dimethylformamide (30.0 mL) under nitrogen. The red solution was stirred at 60 °C (oil bath) for 2 d. Then a solution of α,α′-dibromo-para-xylene (1.32 g, 5.00 mmol) in N,N-dimethylformamide (20.0 mL) was added dropwise to the solution. The reaction mixture was stirred at 80 °C for 16 h. After cooling to room temp the solvent was removed in vacuo. The crude product was triturated with water (2 × 200 mL) and ethanol (100 mL). After drying under vacuo compound 4b [47] (3.18 g, 78%) was obtained as a light yellow powder, Mp 288–290 °C. R_f (n-hexane/acetone 7:3): 0.28. ¹H-NMR (300 MHz, dimethylsulfoxide-d₆): δ 5.06 (s, 4 H), 6.67 (d, ³J = 8.7 Hz, 4 H), 7.21 (dd, ³J = 8.7 Hz, ⁴J = 2.3 Hz, 4 H), 7.24 (s, 4 H), 7.34 (d, ⁴J = 2.3 Hz, 4 H). ¹³C-NMR (500 MHz, dimethylsulfoxide-d₆): δ 50.7 (CH₂), 99.4 (C_quat), 114.1 (CH), 117.5 (C_quat), 124.5 (CH), 126.9 (CH), 128.6 (CH), 130.0 (C_quat), 143.2 (C_quat). MS (EI) m/z: 818 (1, [M⁸¹Br₃⁷⁹Br]⁺), 816 [M⁸¹Br₂⁷⁹Br₂]⁺, 814 (1, [M⁸¹Br⁷⁹Br₃]⁺), 360 (9, [C₁₂H₆⁸¹Br₂NS]⁺), 358 (100, [C₁₂H₆⁸¹Br⁷⁹BrNS]⁺), 356 (58, [C₁₂H₆⁷⁹Br₂NS]⁺), 278 (74, [C₁₂H₇⁸¹BrNS]⁺), 276 (82, [C₁₂H₇⁷⁹BrNS]⁺), 196 (88, [C₁₂H₆NS]⁺), 104 (32, [C₈H₈]⁺). IR $\tilde{\nu }$ [cm⁻¹]: 2384 (vw), 2349 (vw), 2253 (vw), 1586 (w), 1485 (w), 1456 (vs), 1435 (w), 1387 (w), 1348 (m), 1296 (vw), 1258 (s), 1223 (m), 1157 (vw), 1121 (w), 1107 (m), 1082 (vw), 1045 (vw), 1007 (m), 982 (vw), 966 (vw), 930 (w), 825 (s), 795 (vs), 741 (s), 652 (m). Anal. calcd. for C₃₂H₂₀Br₄N₂S₂ [816.3]: C 47.09, H 2.47, N 3.43, S 7.86; Found: C 46.96, H 2.60, N 3.47, S 7.62.

3.5. General Procedure (GP2) of Synthesis of Tetrakis(ethynyl)diphenothiazines 7

In an oven-dried screw-capped Schlenk vessel with a magnetic stir bar under nitrogen atmosphere were placed the tetrahalogenated dyad 4 (1.00 equiv), CuI (20.0 mol%), PdCl₂(PPh₃)₂ (20.0 mol%), dry tetrahydrofuran (26.4 mL/mmol) and piperidine (13.5 mL/mmol). The reaction mixture was deaerated by a constant stream of nitrogen by a syringe for 10 min. Then, (trimethylsilyl)acetylene (5) (7.40 equivs) was added to the reaction mixture, which was subsequently heated at 67 °C (oil bath) under nitrogen for 21 h. After cooling to room temp, the solvents were removed under reduced pressure and the residue 6 was triturated with diethylether. The precipitated salts were separated by filtration and the solvents of the filtrate were evaporated in vacuo. The residue was purified by chromatography on silica gel. The product was dissolved in tetrahydrofuran (10.5 mL/mmol) and a 1.0 m solution of TBAF (6.00 equivs) was added. After stirring at room temp for 1 h, the solvents were removed in vacuo and the product 7 was purified by suspension in water, filtration and drying of the solid under vacuo.

3.5.1. 1,4-Bis(3,7-diethynyl-10H-phenothiazin-10-yl)benzene (7a)

According to GP2 compound 4a (4.21 g, 5.34 mmol), PdCl₂(PPh₃)₂ (729 mg, 1.07 mmol), CuI (202 mg, 1.07 mmol), THF (140 mL), piperidine (72 mL), and (trimethylsilyl)-acetylene (5.30 mL, 39.5 mmol) were reacted to give compound 7a (1.61 g, 53%) as an orange powder after chromatography on silica gel (hexane/dichloromethane 9:1), Mp > 400 °C (dec.). R_f (hexane/dichloromethane 4:1): 0.45. ¹H-NMR (600 MHz, tetrahydrofuran-d₈): δ 3.50 (s, 4 H), 6.33 (d, ³J = 8.7 Hz, 4 H), 7.04 (dd, ³J = 8.7 Hz, ⁴J = 2.0 Hz, 4 H), 7.16 (d, ⁴J = 2.0 Hz, 4 H), 7.71 (s, 4 H). ¹³C-NMR (151 MHz, tetrahydrofuran-d₈): δ 81.0 (CH), 82.5 (C_quat), 116.4 (CH), 119.4 (C_quat), 125.3 (CH), 129.5 (CH), 131.3 (C_quat), 133.2 (CH), 139.6 (C_quat), 143.0 (C_quat). MS (MALDI-TOF) calcd. for [C₃₈H₂₀N₂S₂]⁺ m/z: 568.110; Found: 568.120. IR $\tilde{\nu }$ [cm⁻¹]: 3287 (m), 2104 (m), 1599 (m), 1582 (m), 1503 (w), 1495 (w), 1468 (vw), 1387 (s), 1312 (vw), 1256 (vw), 1190 (w), 1152 (w), 1096 (w), 1077 (w), 1065 (vw), 1045 (vw), 1020 (m), 937 (m), 880 (s), 866 (w), 806 (vw), 779 (vw), 745 (w), 729 (w), 710 (w), 667 (s), 648 (vw). Anal. calcd. for C₃₈H₂₀N₂S₂ (568.7): C 80.25, H 3.54, N 4.93, S 11.27; Found: C 80.01, H 3.60, N 4.71, S 10.99.

3.5.2. 1,4-Bis((3,7-diethynyl-10H-phenothiazin-10-yl)methyl)benzene (7b)

According to GP2 compound 4b (2.86 g, 3.50 mmol), PdCl₂(PPh₃)₂ (491 mg, 700 μmol), CuI (133 mg, 700 μmol), THF (92 mL), piperidine (47 mL), and (trimethylsilyl)acetylene (3.69 mL, 25.9 mmol) were reacted to give compound 7b (1.13 g, 54%) as an orange powder after chromatography on silica gel (hexane/dichloromethane 9:1), Mp >400 °C (dec.). R_f (hexane/dichloromethane 4:1): 0.29. ¹H-NMR (600 MHz, tetrahydrofuran-d₈): δ 3.47 (s, 4 H), 5.11 (s, 4 H), 6.66 (d, ³J = 8.5 Hz, 4 H), 7.08 (dd, ³J = 8.5 Hz, ⁴J = 1.8 Hz, 4 H), 7.16 (d, ⁴J = 1.8 Hz, 4 H), 7.27 (s, 4 H). ¹³C-NMR (151 MHz, tetrahydrofuran-d₈): δ 79.0 (CH), 83.5 (CH), 116.6 (CH), 118.1 (C_quat), 124.0 (C_quat), 128.1 (CH), 130.9 (CH), 132.2 (CH), 136.5 (C_quat), 145.5 (C_quat). MS (MALDI-TOF) calcd. for [C₄₀H₂₄N₂S₂]⁺ m/z: 596.770; Found: 597.160. IR $\tilde{\nu }$ [cm⁻¹]: 3298 (w), 3055 (vw), 2108 (vw), 1599 (w), 1464 (vs), 1456 (s), 1400 (m), 1354 (m), 1339 (w), 1300 (w), 1256 (m), 1223 (s), 1163 (w), 1123 (w), 1007 (w), 1065 (w), 1011 (w), 893 (m), 881 (m), 816 (s), 745 (w), 704 (w), 633 (s), 604 (m). Anal. calcd. for C₄₀H₂₄N₂S₂ [596.8] C 80.51, H 4.05, N 4.69, S 10.74; Found: C 79.65, H 4.25, N 4.37, S 10.53.

3.6. General Procedure (GP3) of the CuAAC Synthesis of Tetrakis[(triethoxysilylpropyl)triazole] Substituted Diphenothiazines 8

In an oven-dried screw-capped Schlenk vessel with a magnetic stir bar under nitrogen atmosphere were placed the tetrakis(alkynylated) dyad 7 (1.00 equiv), CuI (10.0 mol% per alkynyl unit) and dry DMF (22.4 mL/mmol). After addition of (3-azidopropyl)tri-ethoxysilane (2a) (1.00 equiv per alkynyl unit), the turbid reaction mixture was stirred at room temp for 16 h, while the reaction mixture turned red. The solvent was removed under reduced pressure and the residue was purified by chromatography on silica gel.

3.6.1. 1,4-Bis(3,7-bis(1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)-10H-phenothiazin-10-yl)benzol (8a)

According to GP 3 compound 7a (260 mg, 457 μmol), CuI (34.8 mg, 183 μmol), CuI (34.8 mg, 183 μmol), (3-azidopropyl)triethoxysilane (2a) (588 mg, 2.38 mmol), and DMF (27 mL) were reacted to give compound 8a (235 mg, 33%) as a yellow crystalline solid after chromatography on silica gel (hexane/acetone 6:4), Mp > 350 °C (dec.). R_f (n-hexane/acetone 6:4): 0.15. ¹H-NMR (300 MHz, tetrahdrofuran-d₈): δ 0.62 (m_c, 8 H), 1.19 (t, ³J = 7.0 Hz, 36 H), 2.02 (m_c, 8 H), 3.80 (q, ³J = 7.0 Hz, 24 H), 4.36 (t, ³J = 6.8 Hz, 8 H), 6.57 (d, ³J = 8.5 Hz, 4 H), 7.54 (dd, ³J = 8.5 Hz, ⁴J = 2.0 Hz, 4 H), 7.63 (d, ⁴J = 2.0 Hz, 4 H), 7.67 (s, 4 H), 8.09 (s, 4 H). ¹³C-NMR (75 MHz, tetrahdrofuran-d₈): δ 14.1 (CH₂), 18.7 (CH₃), 20.8 (CH₂), 53.1 (CH₂), 59.1 (CH₂), 118.7 (CH), 120.2 (CH), 123.4 (C_quat), 124.6 (CH), 125.0 (CH), 128.1 (C_quat), 132.1 (CH), 141.9 (C_quat), 143.8 (C_quat), 146.8 (C_quat). MS (MALDI-TOF) calcd. for [C₇₄H₁₀₄N₁₄O₁₂S₂Si₄]⁺ m/z: 1558.190; Found: 1556.650. IR $\tilde{\nu }$ [cm⁻¹]: 2953 (m), 2922 (m), 2853 (m), 1653 (vw), 1601 (vw), 1458 (w), 1437 (w), 1364 (w), 1310 (w), 1271 (m), 1248 (vs), 1233 (s), 1167 (m), 1119 (w), 1086 (m), 1034 (s), 945 (w), 930 (w), 912 (vw), 874 (w), 858 (vw), 800 (m), 768 (m), 743 (vw), 723 (vw), 700 (vw), 644 (s), 637 (s), 621 (m).

3.6.2. 1,4-Bis((3,7-bis(1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)-10H-phenothiazin-10-yl)methyl)benzene (8b)

According to GP 3 compound 7b (148 mg, 248 μmol), CuI (18.9 mg, 99.2 μmol), CuI (34.8 mg, 183 μmol), (3-azidopropyl)triethoxysilane (2a) (319 mg, 1.29 mmol), and DMF (15 mL) were reacted to give compound 8b (138 mg, 35%) as yellow crystals after chromatography on silica gel (hexane/acetone 6:4), Mp >320 °C (dec.). R_f (hexane/acetone 6:4): 0.17. ¹H-NMR (500 MHz, tetrahydrofuran-d₈): δ 0.65 (m_c, 8 H), 1.18 (t, ³J = 7.0 Hz, 36 H), 2.73 (m_c, 8 H), 3.82 (q, ³J = 7.0 Hz, 24 H), 4.44 (t, ³J = 7.1 Hz, 8 H), 4.46 (s, 4 H), 6.61 (d, ³J = 8.6 Hz, 4 H), 7.56 (dd, ³J = 8.6 Hz, ⁴J = 2.0 Hz, 4 H), 7.67 (d, ⁴J = 2.0 Hz, 4 H), 7.77 (s, 4 H), 8.29 (s, 4 H). ¹³C-NMR (125 MHz, tetrahdrofuran-d₈): δ 8.2 (CH₂), 18.7 (CH₃), 25.1 (CH₂),52.3 (CH₂), 53.1 (CH₂), 59.0 (CH₂), 118.6 (CH), 120.9 (CH), 122.6 (C_quat), 124.6 (CH), 125.3 (CH), 127.9 (C_quat), 132.8 (CH), 141.7 (C_quat), 143.9 (C_quat), 146.7 (C_quat). MS (MALDI-TOF) calcd. for [C₇₆H₁₀₈N₁₄O₁₂S₂Si₄]⁺ m/z: 1586.250; Found: 1586.000. IR $\tilde{\nu }$ [cm⁻¹]: 2818 (vw), 2365 (w), 1597 (vw), 1456 (vs), 1414 (w), 1331 (m), 1258 (m), 1242 (m), 1198 (w), 1155 (w), 1140 (m), 1107 (w), 1038 (w), 1009 (vw), 961 (vw), 910 (vw), 872 (w), 862 (w), 806 (s), 745 (s), 725 (m), 706 (m), 650 (w).

3.7. Synthesis of Hybrid Materials 3b–d@MCM-41

3.7.1. Synthesis of Hybrid Material 3b@MCM-41

An aqueous solution of cetyltrimethylammonium bromide (CTAB) was mixed with ethylamine under stirring. Then, a solution of precursor 3b in methanol and tetraethylorthosilicate (TEOS) was added dropwise. The composition of the mixture of TEOS:3b:CTAB:EtNH₂:methanol:H₂O in molar ratio was 1.0:0.085:0.14:2.4:2.0:100. The reaction mixture was stirred at room temp for 24 h to obtain a homogenous mixture of the organosilane with TEOS. Then the mixture was stirred at 100 °C for 24 h. The product was collected by filtration, washed thoroughly with deionized water to pH-Neutrality. After drying at 50 °C under vacuo a powdery material was obtained. For extraction of the surfactant (CTAB) 1.0 g of the powder was suspended twice in ethanol (80 mL) and aqueous HCl (36%, 1.0 mL) and heated to reflux for 8 h. After cooling to room temp, the solid was filtered, washed with ethanol and dichloromethane and finally dried at 50 °C under vacuo. The material 3b@MCM-41 was obtained as a yellow powder. Anal. calcd for 3b@MCM-41: C 20.63, H 3.34, N, 3.94; which corresponds to a loading of 0.70 mmol/g.

3.7.2. Synthesis of Hybrid Material 3c-1@MCM-41

An aqueous solution of cetyltrimethylammonium bromide (CTAB) was mixed with ethylamine under stirring. Then a solution of precursor 3c in methanol/THF and tetraethylorthosilicate (TEOS) was added dropwise. The composition of the mixture of TEOS:3c:CTAB:EtNH₂:methanol:THF:H₂O in molar ratio was 1.0:0.03:0.14:2.4:2.0:2.0:100. The reaction mixture was stirred at room temp for 24 h to obtain a homogenous mixture of the organosilane with TEOS. Then the mixture was stirred at 100 °C for 24 h. The product was collected by filtration, washed thoroughly with deionized water to pH-Neutrality. After drying at 50 °C under vacuo a powdery material was obtained. For extraction of the surfactant (CTAB), 1.0 g of the powder was suspended twice in ethanol (80 mL) and aqueous HCl (36%, 1.0 mL) and heated to reflux temp for 8 h. After cooling to room temp, the solid was filtered, washed with ethanol and dichloromethane and finally dried at 50 °C under vacuo. The material 3c-1@MCM-41 was obtained as a yellow powder. Anal. calcd for 3c-1@MCM-41: C 19.92, H 2.92, N, 1.93; which corresponds to a loading of 0.28 mmol/g.

3.7.3. Synthesis of Hybrid Material 3c-2@MCM-41

An aqueous solution of cetyltrimethylammonium bromide (CTAB) was mixed with ethylamine under stirring. Then, a solution of precursor 3c in methanol/THF and tetraethylorthosilicate (TEOS) was added dropwise. The composition of mixture TEOS:3c:CTAB:EtNH₂:methanol:THF:H₂O in molar ratio was 1.0:0.055:0.14:2.4:2.0:2.0:100. The reaction mixture was stirred at room temp for 24 h to obtain a homogenous mixture of the organosilane with TEOS. Then the mixture was stirred at 100 °C for 24 h. The product was collected by filtration, washed thoroughly with deionized water to pH-neutrality. After drying at 50 °C under vacuo a powderous material was obtained. For extraction of the surfactant (CTAB) 1.0 g of the powder was twice suspended in ethanol (80 mL) and aqueous 36% HCl (1.0 mL) and heated to reflux temp for 8 h. After cooling to room temp the solid was filtered, washed with ethanol and dichloromethane and finally dried at 50 °C under vacuo. The material 3c-2@MCM-41 was obtained as a yellow powder. Anal. calcd for 3c-2@MCM-41: C 27.79, H 2.97, N, 3.04; which corresponds to a loading of 0.43 mmol/g.

3.7.4. Synthesis of Hybrid Material 3d@MCM-41

An aqueous solution of cetyltrimethylammonium bromide (CTAB) was mixed with ethylamine under stirring. Then, a solution of precursor 3d in methanol/THF and tetraethylorthosilicate (TEOS) was added dropwise. The composition of the mixture of TEOS:3d:CTAB:EtNH₂:methanol:THF:H₂O in molar ratio was 1.0:0.04:0.14:2.4:2.0:2.0:100. The reaction mixture was stirred at room temp for 24 h to obtain a homogenous mixture of the organosilane with TEOS. Then the mixture was stirred at 100 °C for 24 h. The pro-duct was collected by filtration, washed thoroughly with deionized water to pH-Neutrality. After drying at 50 °C under vacuo a powdery material was obtained. For extraction of the surfactant (CTAB), 1.0 g of the powder was suspended twice in ethanol (80 mL) and aqueous HCl (36%, 1.0 mL) and heated to reflux temp for 8 h. After cooling to room temp, the solid was filtered, washed with ethanol and dichloromethane and finally dried at 50 °C under vacuo. The material 3d@MCM-41 was obtained as a yellow powder. Anal. calcd for 3d@MCM-41: C 12.67, H 2.82, N, 2.78; which corresponds to a loading of 0.28 mmol/g.

4. Conclusions

In summary, we have successfully employed the CuAAC, a copper catalyzed variation of (3+2)-cycloadditions, for synthesizing triazole conjugated phenothiazines bearing propyl trialkoxylsilyl moieties as precursors for the synthesis of mesoporous silica hybrid materials by the in-situ co-condensation strategy. The precursor materials are all intensively luminescent by triazole substitution and reversible redox systems according to cyclic voltammetry. These favorable electronic properties can be directly included in the mesoporous hybrid materials as well. Exemplarily, four different materials were synthesized and extensively characterized for XRD, nitrogen absorption-desorption isotherms, TEM, solid state CP-MAS NMR and IR spectroscopy to gain an insight into mesoporous structuring and molecular ligation in the silica framework. The mesoporous periodicity of the obtained materials largely depends on the molecular dimensions of the employed precursors in the templated in-situ condensation synthesis. The electronic properties of the hybrid materials are affected by the molecular properties of the constituents, however, more dense packing and embedding leads to further cooperativity, e.g., intense fluorescence. Also, the stability of chemically oxidized materials displaying the formation of radical cations sets the stage for devising chemically and electrochemically reversible electrophores embedded in stabilizing silica frameworks. Further studies will be directed to develop periodically mesoporous materials with switchable charge transport properties.

Supplementary Materials

The following are available online, Supporting Information: ¹H- and ¹³C-NMR spectra of compounds 3 and 8, textural parameters and powder XRD measurements, N₂ adsorption/desorption isotherms, electron microscopy (TEM, SEM), FT-IR spectra, ¹³C CP MAS NMR spectra, ²⁹Si CP MAS NMR spectra, absorption and emission spectra of hybrid materials 3b–d@MCM-41 and absorption spectra of oxidized and protonated hybrid materials.

Author Contributions

The work consists of parts of the Ph.D. theses of H.K. and A.W.F., which were supervised by T.J.J.M., and the Ph.D. thesis of Z.Z., which was supervised by W.R.T. The conceptualization was jointly outlined by T.J.J.M. and W.R.T. Synthetic studies, analytical assignments, electroanalytical and photophysical investigations with the precursors 3 and 8 were performed by A.W.F. (compounds 3) and H.K. (compounds 8), who compiled and interpreted the obtained data. The syntheses and textural and structural characterization of the hybrid materials 3b–d@MCM-41 and their chemical oxidation was performed by Z.Z., who compiled and interpreted the obtained data. Writing of the original draft was done by T.J.J.M., and review and editing by H.K., A.W.F., Z.Z., and W.R.T. Project administration and funding acquisition was done by T.J.J.M. and W.R.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Deutsche Forschungsgemeinschaft, Schwerpunktprogramm SPP 1181, MU 1088/9-1, MU 1088/13-1 and TH 550/20-1, and Fonds der Chemischen Industrie.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data of this work are included in the manuscript and the Supplementary Materials.

Acknowledgments

The authors thank Burak Kürsat-Menekse and Wladislaw Pisetsky for experimental assistance in the course of their B.Sc. theses.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Not available.

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Figures, Schemes and Tables

Scheme 1. CuAAC synthesis of linear triazolyl conjugated triethoxylsilylpropyltriazolyl (oligo)phenothiazines 3.

Scheme 2. CuAAC synthesis of triazolyl conjugated tetrakis(triethoxylsilylpropyltriazolyl) substituted diphenothiazine dumbbells 8.

Figure 1. Cyclic voltammograms of triazolyl conjugated diphenothiazines 8a (left) and 8b (right) (recorded in dichloromethane at T = 298 K, ν = 100 mVs−1, elektrolyte: [Bu4N][PF6], Pt working electrode, Pt counter electrode, Ag/AgCl reference electrode, referenced to E1/20/+1(Me10Fc/Me10Fc+) = −95 mV).

Figure 2. Absorption (solid lines) and emission (dashed lines) spectra of triazolyl conjugated diphenothiazines 8a (black) and 8b (red) (recorded in dichloromethane at T = 298 K with c(8) = 10−3 m).

Scheme 3. In situ synthesis of selected triazolyl conjugated (oligo)phenothiazine silica hybrid materials 3b–d@MCM-41.

Figure 3. MM2-optimized stretched conformer of precursor 3d highlighting the intramolecular Si-Si distance of 2.2 nm and the rigid core with an extension of 1.7 nm.

Figure 4. TEM images of 3d@MCM-41. (A) Along the channel direction. (B) Perpendicular to the channel direction.

Scheme 4. Oxidation of the silica hybrid materials 3b-d@MCM-41 with single electron oxidants.

Figure 5. Q-band EPR spectra of (A) 3b+@MCM-41 SbCl6− and (B) 3d+@MCM-41 SbCl6− (recorded at T = 297 K).

Table 1

Selected electronic properties (cyclic voltammetric oxidation potentials, absorption and emission data by UV/Vis and fluorescence) of the triazolyl conjugated (oligo)phenothiazines 3 and 8.

Compound	Oxidation Potentials 1		Absorption 2	Emission 3	Stokes Shift 4
	E1/20/+1	E1/2+1/+2	λ max,abs	λ max,abs	$\mathbf{\Delta }\stackrel{\mathbf{̃}}{\mathbf{\nu }}$
	[mV]	[mV]	[nm] (ε [m−1cm−1])	[nm]	[cm−1]
3a	750	-	266 (49200), 319 (9950)	446, 469sh	8900
3b	720	-	268 (55400), 318 (14250)	450	9200
3c	650	790	278 (43200), 328 (14600), 367 (12300)	470, 505sh	6000
3d	730	-	276 (37800), 334 (7200)	459	8100
3e	700	-	276 (54500), 335 (10100)	460, 484sh	8100
3f	650	820	278 (50700), 336 (16100)	469, 506sh	5700
8a 5	700	880	279 (183000), 343 (20000)	455, 479	7200
8b 5	800	-	274 (83000), 335 (10000)	456, 485	7900

¹ Recorded in dichloromethane at T = 298 K, ν = 100 mVs⁻¹, electrolyte: [Bu₄N][PF₆], Pt working electrode, Pt counter electrode, Ag/AgCl reference electrode, referenced to E_1/2^0/+1(Fc/Fc⁺) = 450 mV. ² Recorded in dichloromethane at T = 298 K, c = 10⁻³ m. ³ Recorded in dichloromethane at T = 298 K, c = 10⁻⁶ m (excitation at the longest wavelength absorption band). ⁴ $\Delta \tilde{\nu }$ = 1/λ_max,abs−1/λ_max,em [cm⁻¹] (the corresponding absorption and emission bands for determining the Stokes shifts are high-lighted in bold face). ⁵ Cyclic voltammograms referenced to E_1/2^0/+1(Me₁₀Fc/Me₁₀Fc⁺) = −95 mV.

Table 2

Textural parameters of MCM-41 [30] and 3b–d@MCM-41 derived from powder X-ray diffraction and nitrogen adsorption-desorption analyses and the phenothiazine loading in the corresponding hybrid materials.

Sample	Content of Phenothiazine 1		d 100 2	a 0 3	S BET 4	V p 5	Dp 6	wt 7
	[mmol g–1]	[wt%]	[nm]	[nm]	[m2g–1]	[cm3g–1]	[nm]	[nm]
MCM-41	−	−	4.30	4.96	1123	0.98	2.83	2.13
3b@MCM-41	0.70	30.4	4.20	4.85	536	0.36	2.36	2.49
3c-1@MCM-41	0.28	18.3	4.44	5.13	1086	0.96	2.82	2.31
3c-2@MCM-41	0.43	28.1	4.50	5.20	543	0.48	2.77	2.43
3d@MCM-41	0.28	14.9	4.48	5.17	539	0.41	2.41	2.76

¹ Calculated from the CHN combustion analysis according to the content of nitrogen. ² d₁₀₀ is the d(100) spacing. ³ a₀ is the cell parameter (a₀ = 2d₁₀₀/√3). ⁴ S_BET is the BET surface area. ⁵ V_p is the pore volume. ⁶ D_p is the pore diameter; ⁷ w_t is the wall thickness a₀-D_p.

Table 3

Selected optical properties (absorption and emission data by UV/Vis and fluorescence) of the triazolyl conjugated (oligo)phenothiazines 3b–d ¹ and hybrid materials 3b–d@MCM-41 ².

Compound	Absorption	Emission	Stokes Shift 3
	λ max,abs	λ max,abs	Δ$\tilde{\nu }$
	[nm]	[nm]	[cm−1]
3b	268, 318	450	9200
3b@MCM-41	260, 320	451	9100
3c	278, 328, 367	470, 505sh	6000
3c-1@MCM-41	276, 330, 370	484, 509sh	6400
3c-2@MCM-41	275, 327, 376	500	6700
3d	276, 334	459	8100
3d@MCM-41	270, 327, 360	390, 445	5300

¹ Recorded in dichloromethane at T = 298 K (emission spectra were recorded by excitation at the longest wavelength absorption band). ² UV/Vis spectra of diffuse reflectance. ³ $\Delta \tilde{\nu }$ = 1/λ_max,abs − 1/λ_max,em [cm⁻¹] (the corresponding absorption and emission bands for determining the Stokes shifts are highlighted in bold face).

Table 4

Experimental details of the CuAAC synthesis of precursors 3.

Entry	Alkyne 1	Azide 2	CuI	DMF	Product 3 Yield
	[mg] ([μmol])	[mg] ([μmol])	[mg] ([μmol])	[mL]	[mg] (%)
1	100 (422) of 1a	105 (422) of 2a	8.0 (42)	3 mL	193 (95) of 3a
2	129 (422) of 1b	105 (422) of 2a	8.0 (42)	3 mL	228 (97) of 3b
3	735 (1250) of 1c	256 (1250) of 2b	24.0 (125)	7 mL	970 (98) of 3c
4	55 (211) of 1d	105 (422) of 2a	4.0 (42)	3 mL	140 (88) of 3d
5	270 (815) of 1e	402 (1630) of 2a	16.0 (82)	3 mL	625 (93) of 3e
6	59 (96) of 1f	41 (200) of 2b	1.9 (10)	3 mL	90 (92) of 3f