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Article

(1E)-1,2-Diaryldiazene Derivatives Containing a Donor–π-Acceptor-Type Tolane Skeleton as Smectic Liquid–Crystalline Dyes

1
Faculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
2
Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
3
Faculty of Engineering and Design, Kagawa University, 2217-20, Hayashi-cho, Takamatsu 761-0396, Japan
*
Author to whom correspondence should be addressed.
Compounds 2024, 4(2), 288-300; https://doi.org/10.3390/compounds4020015
Submission received: 13 March 2024 / Revised: 5 April 2024 / Accepted: 12 April 2024 / Published: 17 April 2024
(This article belongs to the Special Issue Feature Papers in Compounds (2024))

Abstract

:
Considerable attention has been paid to (1E)-1,2-diaryldiazenes (azo dyes) possessing liquid–crystalline (LC) and optical properties because they can switch color through thermal phase transitions and photoisomerizations. Although multifunctional molecules with both LC and fluorescent properties based on a donor–π-acceptor (D-π-A)-type tolane skeleton have been developed, functional molecules possessing LC and dye properties have not yet been developed. Therefore, this study proposes to develop LC dyes consisting of (1E)-1,2-diaryldiazenes with a D–π-A-type tolane skeleton as the aryl moiety. The (1E)-1,2-diaryldiazene derivatives exhibited a smectic phase, regardless of the flexible-chain structure, whereas the melting temperature was significantly increased by introducing fluoroalkyl moieties into the flexible chain. Evaluation of the optical properties revealed that compounds with decyloxy chains exhibited an orange color, whereas compounds with semifluoroalkoxy chains absorbed at a slightly blue-shifted wavelength, which resulted in a pale orange color. The thermal phase transition caused a slight color change accompanied by a change in the absorption properties, photoisomerization-induced shrinkage, and partial disappearance of the LC domain. These results indicate that (1E)-1,2-diaryldiazenes with a D–π-A-type tolane skeleton can function as thermo- or photoresponsive dyes and are applicable to smart windows and in photolithography.

Graphical Abstract

1. Introduction

Materials whose dye properties change in response to external stimuli, such as heat or electric fields, are called functional dyes and are used in a wide range of applications, such as display devices, energy conversion materials, and recording materials [1,2,3,4,5,6]. Among the several dyes developed so far, 1,2-diaryldiazenes [7,8,9], which are well known as azo dyes, primarily exhibit a yellow-to-orange color, and their color changes due to transcis isomerization when irradiated with ultraviolet light. In addition, irradiation with visible light or heating causes cistrans isomerization; therefore, 1,2-diarydiazenes may have potential applications as optical switching materials using reversible photoisomerization [10,11].
Recently, significant attention has been paid to (1E)-1,2-diaryldiazene-based functional molecules that undergo photoisomerization and changes in the aggregate structure in response to temperature, i.e., liquid–crystalline (LC) dyes using a (1E)-1,2-diaryldiazene scaffold as a mesogen. It was reported that polymers with (1E)-1,2-diaryldiazene units introduced into their side chains undergo a phase transition accompanied by photoisomerization from the LC phase to the isotropic (Iso) phase upon light irradiation [12,13,14]. In addition, LC polymers containing (1E)-1,2-diaryldiazene units with one aromatic ring such as the tolane skeleton have been developed, and changes in their birefringence due to photoisomerization were reported [15,16].
Recently, our group explored molecules that have both LC and photoluminescence properties and successfully developed donor–π-acceptor (D–π-A)-type tolane derivatives that have an electron-donating alkoxy group and an electron-withdrawing cyano group at their molecular ends [17,18,19]. Among our achievements, the D–π-A-type tolane bearing a decyloxy (C10H21O) chain formed a nematic (N) LC phase as a mesophase (Figure 1a) [19]. However, the formation of a mesophase of smectic A (SmA) was observed when the flexible chain was changed from a decyloxy chain to a semifluoroalkoxy chain, viz., C4F9-C6H12O or C6F13-C4H8O, in which a fluoroalkyl unit was introduced into the flexible-chain end (Figure 1b) [20]. In addition to their LC properties, these tolanes were found to possess fluorescent properties with a quantum yield (ΦPL) of up to 0.21, even in the crystalline state.
Our next focus was on developing LC dyes with LC and dye properties. These are promising functional materials that can produce a wide variety of colors in response to various aggregated structural changes owing to their liquid crystallinity. Because (1E)-1,2-diaryldiazene derivatives with a tolane skeleton as a mesogen have hardly been reported, except for the polymer materials mentioned above, we designed D–π-A-type tolane derivatives with 4-cyanophenylazo units and various flexible chains introduced at both molecular ends, viz., 1ac, (Figure 1c). This paper describes the details of their synthesis and characterization and discusses the effects of the changes in the flexible-chain structure on their properties.

2. Materials and Methods

2.1. General

1H nuclear magnetic resonance (NMR) (400 MHz) and 13C NMR (100 MHz) spectra were acquired using an AVANCE III 400 NMR spectrometer (Bruker, Rheinstetten, Germany) in chloroform-d (CDCl3) solution, and chemical shifts are reported in parts per million (ppm) based on the residual protons or carbon in the NMR solvent. 19F NMR (376 MHz) spectra were acquired using an AVANCE III 400 NMR spectrometer (Bruker, Rheinstetten, Germany) in CDCl3 solution with C6F6 (δF = −163 ppm) as an internal standard. Infrared (IR) spectra were recorded using the KBr method on a FTIR-4100 type A spectrometer (JASCO, Tokyo, Japan). All IR spectra are reported in wavenumber (cm−1) units. High-resolution mass spectra (HRMS) were recorded on a JMS700MS spectrometer (JEOL, Tokyo, Japan) using the fast-atom bombardment (FAB) method. Before use, all chemicals of reagent grade were purified using standard methods. The melting temperature (Tm) was measured using a DSC-60 differential scanning calorimeter (SHIMADZU, Kyoto, Japan) under a nitrogen atmosphere at a scan rate of 5 °C min−1. The reaction progress was monitored by thin-layer chromatography (TLC) using silica gel TLC plates (Merck, Silica Gel, 60F254; Rahway, NJ, USA). Column chromatography was performed using silica gel (FUJIFILM Wako Pure Chemical Corporation, Wako-gel® 60 N, 38 μm to 100 μm; Osaka, Japan).

2.2. Materials

The (1E)-1-(4-cyanophenyl)-2-[4-[2-(4-semifluoroalkoxyphenyl)ethynyl]phenyl]diazenes 1ac were synthesized in two steps involving the Pd-catalyzed Sonogashira cross-coupling reaction between 4-(semifluoroalkoxy)-1-iodobenzene and 4-ethynylaniline (2ac), followed by oxidative dimerization with 4-aminobenzonitrile using in situ generated tert-butyl hypoiodite (t-BuOI) [21,22], according to the reaction sequence shown in Scheme 1. The synthetic procedure for the azo coupling process and the compound characterization data for 1ac are as described below. The synthetic details and characterization data for 3ac are described in the Supplementary Materials. The 1H, 13C, and 19F NMR spectra of 1ac and 3ac are shown in Figures S1–S16 in the Supplementary Materials.

2.3. Typical Synthetic Procedure for (1E)-1-(4-Cyanophenyl)-2-[4-[2-(4-decyloxyphenyl)ethynyl]phenyl]diazene (1a)

A mixture of freshly prepared 4-amino-4′-decyloxytolane (3a, 1.15 g, 3.30 mmol), 4-aminobenzonitrile (0.39 g, 3.30 mmol), and NaI (0.96 g, 13.2 mmol) in THF (40 mL) was placed in a 100 mL two-necked round-bottomed flask equipped with a magnetic stirrer bar. tert-Butyl hypochloride (tert-BuOCl, 1.56 mL, 13.2 mmol) was added to the solution under an argon atmosphere and stirred at room temperature. After stirring for 18 h, the resultant mixture was poured into an aqueous solution of sodium thiosulfate (Na2S2O3, 50 mL), and the solution was extracted thrice with CH2Cl2 (20 mL). The combined organic extracts were dried over anhydrous sodium sulfate (Na2SO4) and concentrated under vacuum to provide the crude product, which was purified by silica gel column chromatography using a mixed solution of hexane/CH2Cl2 (v/v = 1/1) as the eluent. The title compound was obtained as an orange solid in 20% yield (0.30 g, 0.65 mmol). To evaluate the phase transition and optical properties, the obtained compound was subjected to recrystallization using a 1/1 (v/v) mixed solvent system with CHCl3 as the good solvent and hexane as the poor solvent. Using various spectroscopic techniques, the molecular structure was identified correctly, and purity was proven sufficient for evaluating phase transitions and optical properties.

2.3.1. (1E)-1-(4-Cyanophenyl)-2-[4-[2-(4-decyloxyphenyl)ethynyl]phenyl]diazene (1a)

Yield: 20% (orange solid); Tm: 107 °C on the second heating process; 1H NMR (CDCl3): δ 0.88 (t, J = 6.8 Hz, 3H), 1.22–1.32 (m, 10H), 1.41–1.50 (m, 2H), 1.79 (quint, J = 6.8 Hz, 4H), 3.98 (t, J = 6.8 Hz, 2H), 6.89 (d, J = 8.8 Hz, 2H), 7.49 (d, J = 8.8 Hz, 2H), 7.66 (d, J = 8.8 Hz, 2H), 7.82 (d, J = 8.8 Hz, 2H), 7.94 (d, J = 8.8 Hz, 2H), 7.99 (d, J = 8.8 Hz, 2H); 13C NMR (CDCl3): δ 14.1, 22.7, 26.0, 26.1, 29.2, 29.3, 29.4, 29.6, 31.9, 68.1, 87.8, 93.3, 114.6, 118.4, 123.38, 123.40, 123.6, 127.9, 129.7, 130.0, 132.3, 133.2, 151.2, 154.5, 159.7; IR (KBr): ν 3435, 3069, 2956, 2879, 2312, 1577, 1438, 1367, 1070, 957, 790 cm−1; HRMS (FAB) Calcd for (M+) C31H33N3O: 463.2624, Found: 463.2629.

2.3.2. (1E)-1-(4-Cyanophenyl)-2-[4-[2-(4-(7,7,8,8,9,9,10,10,10-nonafluorodecyloxy)phenyl)ethynyl]phenyl]diazene (1b)

Yield: 13% (orange solid); Tm: 136 °C on the second heating process; 1H NMR (CDCl3): δ 1.42–1.60 (m, 4H), 1.66 (quint, J = 8.0 Hz, 2H), 1.83 (quint, J = 6.4 Hz, 2H), 2.00–2.17 (m, 2H), 3.99 (t, J = 6.4 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H), 7.49 (d, J = 8.8 Hz, 2H), 7.64 (d, J = 8.8 Hz, 2H), 7.66 (d, J = 8.8 Hz, 2H), 7.87 (d, J = 8.8 Hz, 2H), 7.90 (d, J = 8.8 Hz, 2H); 13C NMR (CDCl3): δ 20.1, 25.8, 28.8, 28.9, 30.7 (t, J = 22.7 Hz), 67.8, 87.9, 92.5, 97.8, 114.6, 114.9, 123.1, 124.5, 126.8, 132.2, 133.2, 138.4, 151.4, 152.0, 159.4. The four signals that should be assigned to C4F9 were split by the F atoms, and their intensities were too low for accurate assignment; 19F NMR (CDCl3, C6F6): δ −82.34 (t, J = 9.4 Hz, 3F), −115.80 to −116.10 (m, 2F), −125.71 to −125.88 (m, 2F), −127.28 to −127.46 (m, 2F); IR (KBr): ν 3067, 2938, 2864, 2212, 1592, 1474, 1392, 1287, 1107, 1045, 973, 879 cm−1; MS (FAB) m/z 176 ([C6H4-C≡C-C6H4]+, 100), 130 ([N=N-C6H4-CN]+, 6), 104 ([C6H4-N=N]+, 21), 102 ([PhCN]+, 20).

2.3.3. (1E)-1-(4-Cyanophenyl)-2-[4-[2-(4-(5,5,6,6,7,7,8,8,9,9,10,10,10-tridecafluorodecyloxy)phenyl)ethynyl]phenyl]diazene (1c)

Yield: 23% (orange solid); Tm: 126 °C on the second heating process; 1H NMR (CDCl3): δ 1.78–1.97 (m, 4H), 2.09–2.28 (m, 2H), 4.06 (t, J = 6.0 Hz, 2H), 6.95 (d, J = 8.8 Hz, 2H), 7.50 (d, J = 8.8 Hz, 2H), 7.61 (d, J = 8.8 Hz, 2H), 7.67 (d, J = 8.8 Hz, 2H), 7.88 (d, J = 8.4 Hz, 2H), 7.94 (d, J = 8.4 Hz, 2H); 13C NMR (CDCl3): δ 17.3, 28.7, 30.7 (t, J = 22.0 Hz), 67.3, 91.8, 98.0, 114.1, 123.0, 124.5, 130.0, 130.7, 131.3, 133.7, 138.40, 138.41, 145.6, 151.8, 151.9, 159.4. The six signals that should be assigned to C6F13 were split by the F atoms, and their intensities were too low for accurate assignment; 19F NMR (CDCl3): δ −82.07 (t, J = 9.4 Hz, 3F), −115.66 to −115.94 (m, 2F), −123.14 to −123.40 (m, 2F), −124.10 to 124.34 (m, 2F), −124.80 to −124.98 (m, 2F), −127.37 to −127.54 (m, 2F); IR (KBr): ν 3631, 3092, 2959, 2846, 2203, 1590, 1512, 1249, 1182, 1042, 980, 856 cm−1; MS (FAB) m/z 176 ([C6H4–C≡C–C6H4]+, [C6H12O–C6H4]+, 100), 130 ([N=N–C6H4–CN]+, 4), 104 ([C6H4–N=N]+, 12), 102 ([PhCN]+, 11)

2.4. Phase Transition Properties

The phase transition properties were evaluated by polarizing optical microscopy (POM) using a BX53 microscope (Olympus, Tokyo, Japan) equipped with heating and cooling stages (10.002 L, Linkam Scientific Instruments, Redhill, UK). The phase sequences, transition temperatures, and transition enthalpies were determined using differential scanning calorimetry (DSC, Shimadzu DSC-60 Plus) at a heating and cooling rate of 5 °C min−1 under a N2 atmosphere. Variable-temperature powder X-ray diffraction (VT-PXRD) measurements were performed using an FR-E X-ray diffractometer equipped with a two-dimensional R-axis IV detector (Rigaku, Tokyo, Japan) and a Cu Kα radiation source (λ = 0.154 nm).

2.5. Photophysical Properties

Ultraviolet–visible (UV–vis) light absorption spectroscopy was performed using a V-750 absorption spectrometer (JASCO, Tokyo, Japan). The transmission method was used to measure the solution samples, and the samples were prepared by dissolving the compounds in CH2Cl2 and adjusting their concentration to 1.0 × 10−5 mol L−1. The diffuse reflectance method was used to measure the powder samples, and pristine crystalline powders obtained by recrystallization were used.

2.6. Theoretical Assessment

Density functional theory (DFT) calculations were performed using the Gaussian 16 (Rev. B.01) suite of programs (Gaussian, Wallingford, CT, USA) [23], and geometry optimizations were performed at the CAM-B3LYP/6-311+G(d,p)//CAM-B3LYP/6-31+G(d) level of theory [24,25] using an implicit solvation model, the conductor-like polarizable continuum model [26], for CH2Cl2. The vertical electronic transitions were calculated using time-dependent DFT (TD-DFT) at the same level of theory.

3. Results and Discussion

Using (1E)-1-(4-cyanophenyl)-2-[4-[2-(4-decyloxyphenyl)ethynyl]phenyl]diazene (1a), (1E)-1-(4-cyanophenyl)-2-[4-[2-(4-(7,7,8,8,9,9,10,10,10-nonafluorodecyloxy)phenyl) ethynyl]phenyl]diazene (1b), and (1E)-1-(4-cyanophenyl)-2-[4-[2-(4-(5,5,6,6,7,7,8,8, 9,9,10,10,10-tridecafluorodecyloxy)phenyl)ethynyl]phenyl]diazene (1c) prepared according to Scheme 1, we initially assessed their phase transition properties by DSC and POM. Figure 2 shows the DSC thermograms obtained during the second heating and cooling processes of 1ac and the POM textures of 1ac in the mesophase (Figures S17–S19). Table 1 summarizes the thermophysical data, including the phase sequences, phase transition temperatures, and phase transition enthalpies of 1ac. The detailed phase sequence and transition enthalpies are collected in Tables S1–S3.
Compound 1a, with a decyloxy chain, was found to form a mesophase during heating and cooling; 1a is an enantiotropic LC molecule. During the heating process, endothermic peaks of 1a appeared at 107 °C and 160 °C, which revealed two different mesophases based on POM observation. Upon further heating, the POM image changed to a dark-field texture at 228 °C, indicating a phase transition to an Iso phase. However, at 228 °C, 1a also decomposed, and no clear endothermic peak was detected by DSC. POM observation revealed that, during the cooling process, a bright-field optical texture appeared at 225 °C, indicating a phase transition to the mesophase. The fact that the POM texture changed to a different form after the exothermic peak at 155 °C suggests that a mesophase-to-mesophase transition occurred. After further cooling, it was found that loss of fluidity occurred owing to energy release at 62 °C, resulting in a phase transition from the mesophase to the crystalline (Cr) phase. During cooling, the POM texture observed in the mesophase in the high-temperature region was a focal-conic fan-shaped texture with small domains, whereas a fan-shaped texture with large domains was observed in the mesophase in the low-temperature region. The POM image observed in the mesophase was a typical texture image of the smectic (Sm) phase, having orientational and positional order; the mesophase formed in 1a was considered an Sm phase. Compound 1b, having a short fluoroalkyl moiety at the flexible-chain end, also formed a mesophase exhibiting a fluid bright-field POM texture during both heating and cooling processes and exhibited enantiotropic LC properties. In the POM image of the mesophase, a fan-shaped texture was observed, similar to that of 1a, and it was concluded that the formed mesophase was the Sm phase (Figure 2b). Compound 1c, having a longer fluoroalkyl moiety at its flexible-chain end, also exhibited enantiotropic LC properties (Figure 2c). Unlike the other two compounds (1a and 1b), a single mesophase appeared during heating and cooling. The mesophase that appeared could be estimated to be any Sm phase, based on the focal-conic fan-shaped texture observed in the POM image.
The VT-PXRD measurements of 1a revealed that a diffraction peak corresponding to the (hkl) = (001) plane appeared at 2θ = 2.17° in the high-temperature region (180 °C) during the cooling process (Figure 3a, Figure S20, Table 2, Table S4).
Using Bragg’s equation, the interlayer distance (d-spacing) corresponding to the diffraction angle was determined as 4.07 nm. Considering that the molecular length calculated by DFT is 3.26 nm, it is presumed to be an SmA phase with a dimer in an antiparallel arrangement as a mesogen [27]. In the low-temperature region (100 °C), diffraction peaks corresponding to the (001) and (002) planes at 2θ = 2.34° (d = 3.77 nm) and 4.68° (d = 1.89 nm), respectively, and diffraction peaks corresponding to the (110), (200), and (210) planes in a range of 20–30° were observed. The wide-angle diffraction pattern was a typical SmE phase diffraction pattern; therefore, it can be concluded that the mesophase observed at 100 °C was the SmE. The interlayer distance of the SmE phase formed at 100 °C was 3.77 nm, i.e., smaller than that of the SmA phase formed at 180 °C. This is because, in the SmE phase, the cyanophenyl moiety at the molecular end deeply overlaps with the alkoxy chain at the other end, causing dense packing [27]. The VT-PXRD measurements of compound 1b revealed that diffraction peaks corresponding to the (001) and (002) planes were observed at 2θ = 2.22° (d = 3.97 Å) and 4.38° (d = 2.01 Å), respectively, in the high-temperature region (250 °C) (Figure 3b). Because the molecular length of 1b is 3.31 nm according to DFT calculations, it was determined to be an SmA phase that formed a dimer in an antiparallel arrangement. In the low-temperature region (150 °C), in addition to the diffraction peaks at 2θ = 2.18° (d = 4.05 nm) and 4.42° (d = 2.00 nm), diffraction peaks at 20.14° (d = 0.44 nm), 22.00° (d = 0.40 nm), and 27.83° (d = 0.32 nm) were observed. The diffraction peaks that appeared in the wide-angle region corresponded to the surface index (hkl) = (110), (200), and (210) diffractions, respectively, and the mesophase formed by these diffractions was concluded to be the SmE phase. When the VT-PXRD measurements were used to determine the mesophase of compound 1c, thermal decomposition of 1c occurred along with a phase transition to the Iso phase; therefore, no XRD pattern could be observed in the mesophase.
Changes in the flexible chains of 1ac affected the melting temperature (Tm), defined as the phase transition temperature from the Cr phase to the mesophase (Tm = 107 °C for 1a, 132 °C for 1b, and 176 °C for 1c). Comparing the Tm of 1ac, the alteration from a hydrocarbon-based flexible chain to a semifluoroalkoxy-based flexible chain (1a vs. 1b and 1c) resulted in a significant increase in Tm because the fluoroalkyl moiety of the flexible chain stabilized the Cr phase. Additionally, the length of the fluoroalkyl segment also affected the Tm. As the number of fluoroalkyl segments increased (1b vs. 1c), the aggregation effects between the fluoroalkyl moieties, such as fluorophilic interactions, became stronger and stabilized the Cr state, resulting in a significant increase in the Tm of 1c.
Next, we investigated the dye properties, that is, the UV–vis light absorption properties of 1ac. Dilute-solution samples were prepared by dissolving 1ac in CH2Cl2 and adjusting the concentration to 1.0 × 10−5 mol L−1. Pristine samples obtained via recrystallization from CH2Cl2/MeOH were used as solid samples. Figure 4 shows the absorption spectra obtained from the UV–vis absorption spectrometry measurements of 1ac in dilute solution and solid state (Figure S21), and the theoretical electronic transitions and related molecular orbitals, namely, the highest occupied molecular orbital (HOMO−1), the next highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and the next lowest unoccupied molecular orbital (LUMO+1), calculated by TD-DFT using the molecular structure of 1a as a representative (Figures S24–S26 for other compounds). The obtained optical data and the calculation results are summarized in Table 3 and Tables S5–S8.
A CH2Cl2 solution of 1a bearing a decyloxy group as a flexible chain provided an absorption band with a maximum absorption wavelength (λabs) of approximately 384 nm and an absorption band with a λabs of approximately 294 nm (Figure 4a). Compound 1b having a nonafluorodecyloxy group as a semifluoroalkoxy-type flexible chain had a λabs of approximately 377 nm, whereas compound 1c with a tridecafluorodecyloxy group showed two absorption bands with λabs of approximately 351 nm and 293 nm. To gain further information about the electronic transitions observed in 1ac, we performed TD-DFT calculations at the CAM-B3LYP/6-311+G(d,p)//CAM-B3LYP/6-31+G(d) level of theory [22,23,24,25]. The theoretical absorption wavelengths (λcalcd) with a large oscillator strength (f) were calculated to be 384 nm for 1a and 1b and 383 nm for 1c. Each allowed transition included three interorbital transitions; HOMO → LUMO (78%) was calculated as the primary electronic transition, in addition to HOMO−1 → LUMO (14%) and HOMO → LUMO+1 (4%) (Figure 4b). Focusing on the orbital diagrams of the HOMO and LUMO, thought to contribute the most to electronic transitions, it was found that the HOMO lobe was localized in the tolane skeleton, and the LUMO lobe locally covered the azobenzene unit. These results obtained theoretically showed that the absorption band in the long-wavelength region was primarily an intramolecular charge transfer (ICT) transition accompanied by the HOMO → LUMO transition. The energy gap between HOMO and LUMO (ΔEH-L) was 5.13 eV for 1a, 5.14 eV for 1b, and 5.16 eV for 1c, the order of which agreed with that of λabs, which was 1a (384 nm) > 1b (377 nm) > 1c (351 nm). Further investigation of the UV–vis absorption spectral changes upon UV irradiation (λex = 370 nm) revealed a decrease in the molar absorption coefficient (ε) of the absorption band assigned to the ICT π-π* transition (λabs = 384 nm) after the isosbestic point at 318 nm (Figure 4c for 1a and Figure S22 for 1b and 1c). On the other hand, the ε of the absorption band assigned to the n-π* transition of the 1,2-diaryldiazene compounds slightly increased after the isosbestic point at 430 nm (Figure S22). These results strongly suggest the photoisomerization of the 1,2-diaryldiazene compounds from the trans- to the cis-isomer upon UV irradiation [9,28,29].
The absorption properties of 1ac in the pristine solid state were evaluated by diffuse reflection. In the UV–vis diffuse reflection spectrum of 1a, two broad absorption bands centered at approximately 346 nm and 465 nm were observed, and the color of the pristine solid of 1a was orange (Figure 4d). Conversely, 1b and 1c with semifluoroalkoxy-type flexible chains had absorption spectra that were slightly blue-shifted compared to 1a, along with a decrease in the absorption intensity on the longer-wavelength side. As a result of the spectral change, the colors of the pristine solids of 1b and 1c were also pale orange. The spectral change of 1ac in the pristine solid state is attributable to the alteration in the aggregated structures (Figure S20). Compared to the absorption spectra in the CH2Cl2 solution, which acted as a molecular dispersion system, the absorption spectra of 1ac were observed to shift to longer-wavelength regions; all 1ac were inferred to form J-aggregate-like structures in the pristine solid state [30,31] owing to the high thermodynamic stabilization caused by the long flexible chains or long fluoroalkyl segments.
Generally, the color of a substance is expected to change due to the phase transition from the Cr to the LC phase because the LC phase has different aggregated structures compared to the pristine solid state. Therefore, we were interested in the absorption properties of compounds 1ac in the mesophase after the thermal phase transition (Figure 5, Figure S21). The measurement sample was prepared by heating the pristine solid to induce a phase transition to the mesophase and then freezing the aggregated structure by dipping it in liquid nitrogen.
When the absorption spectra were measured by the diffuse reflection method using the measurement samples prepared as described above, it was found that the absorption intensity on the long-wavelength side increased, whereas the absorption intensity on the short-wavelength side was maintained (Figure 5a–c). The color changed to deep red or deep orange after the thermal phase transition. Furthermore, we were interested in the changes in the mesophase due to photoisomerization (Figure 5d–f). To confirm the photoisomerization behavior in the mesophase, each sample was placed on a heating stage in the POM setup to induce a thermal phase transition and subsequently irradiated with UV light (λex = 370 nm) to investigate the changes in the POM texture. All samples had fan-shaped structures with large domains before UV irradiation; however, upon UV irradiation, the LC domains gradually shrank and partially disappeared (Figure 5d–f, Figure S23). Moreover, for 1c, which changed to the Iso phase through photoisomerization, it was found that the POM texture in the Sm phase reappeared immediately after stopping UV irradiation (Figure 5f). This phenomenon is similar to the photoresponse behavior of polymers with 1,2-diaryldiazene units in the side chains [14,15], and it was demonstrated that the present molecular system also exhibits the photoresponsiveness associated with transcis photoisomerization.

4. Conclusions

The study designed (1E)-1,2-diaryldiazenes, consisting of a D–π-A-type tolane skeleton and a 4-cyanophenylazo unit, as LC dyes with both LC and dye-like properties and synthesized three types of (1E)-1-(4-cyanophenyl)-2-[4-[2-arylethynyl]phenyl]diazene derivatives bearing decyloxy, nonafluorodecyloxy, and tridecafluorodecyloxy chains. Using readily available 4-decyloxy- or 4-semifluoroalkoxy-substituted phenylacetylene derivatives as a starting substrate, the target compound was successfully synthesized using two reaction protocols: Sonogashira cross-coupling and oxidative azo-coupling reactions. All three compounds used in this study exhibited enantiotropic LC properties and formed mesophases during both the heating and the cooling processes. POM and VT-PXRD measurements revealed that the compounds with a decyloxy or nonafluorodecyloxy chain showed an SmA phase in the high-temperature region and an SmE phase in the low-temperature region. An analog with a tridecafluorodecyloxy chain, which has a longer fluoroalkyl moiety in the flexible unit, also exhibited enantiotropic LC properties, in which the observed mesophase was the Sm phase, although the detailed mesophase structures were not identified due to thermal decomposition. In addition, the introduction of fluoroalkyl segments into the flexible chain led to a significant increase in Tm, whereas based on the stratified-dipole array theory, increasing the proportion of fluoroalkyl segments within the flexible chains stabilized the Cr phase and resulted in a significant increase in Tm. Optical property evaluations showed that all compounds exhibited an orange color in dilute solution, and their absorption wavelengths shifted slightly with modulations in the flexible-chain structure; the introduction of a fluoroalkyl moiety into the flexible chain slightly blue-shifted the absorption wavelength. It was also found that derivatives bearing a fluoroalkyl segment in the flexible chain caused a blue shift in the absorption band to the longer-wavelength side, which gave them a pale orange color even in the solid state, compared to the derivative with a decyloxy chain. The phase transition from the Cr phase to the mesophase changed the aggregated structures, resulting in a darker color tone with an increase in the absorption intensity on the long-wavelength side. Furthermore, when the compounds in the mesophase were irradiated with 365 nm UV light, a reduction in or the disappearance of the LC domain of the POM texture was observed, revealing that the compounds used in this study possess photoresponsiveness. The (1E)-1-(4-cyanophenyl)-2-[4-[2-arylethynyl]phenyl]diazene derivatives were found to be multifunctional molecules that exhibit thermoresponsiveness and photoresponsiveness and can be applied to smart windows that change color depending on the temperature and in photolithography, which uses changes in the state of materials due to light stimulation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/compounds4020015/s1, Figures S1–S16: NMR spectra of 1ac; Figures S17–S19: DSC thermograms of 1ac and POM texture images of 1ac in the mesophase; Figure S20: PXRD patterns of 1a and 1b at various temperatures; Figure S21: UV–vis absorption (in CH2Cl2 solution) and diffuse reflection spectra (in the pristine solid and aggregated states in the SmA phase) of 1ac and photographs of the pristine solid and aggregated states in the SmA phase of 1ac; Figure S22: UV–vis absorption spectral changes upon UV irradiation of 1b and 1c; Figure S23: POM image of the change of the SmA phase due to UV irradiation of 1ac; Figures S24–S26: Molecular orbitals diagrams of 1ac; Tables S1–S3: Phase transition data of 1ac; Table S4: X-ray diffraction data of 1a and 1b measured in the mesophase; Table S5: Theoretical electronic transitions of 1ac calculated by the TD-DFT method; Tables S6–S8: Cartesian coordinates of the optimized geometries of 1ac.

Author Contributions

Conceptualization, S.Y. and K.Y.; methodology, S.Y., K.Y. and M.H.; validation, S.Y. and K.Y.; investigation, S.Y., K.Y., Y.E. and M.H.; resources, S.Y., M.Y. and T.K.; data curation, S.Y., K.Y. and M.H.; writing—original draft preparation, S.Y., K.Y., M.Y. and T.K.; writing—review and editing, S.Y., K.Y., Y.E., M.H., M.Y. and T.K.; visualization, S.Y. and K.Y.; supervision, S.Y.; project administration, S.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Acknowledgments

We would like to express our sincere gratitude to Tsuneaki Sakurai (Kyoto Institute of Technology) for his valuable suggestions and cooperation in the measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Chemical structure of a D-π-A tolane containing (a) hydrocarbon chains and (b) semifluoroalkoxy chains developed in previous work. (c) Chemical structures of the target compounds in this work.
Figure 1. Chemical structure of a D-π-A tolane containing (a) hydrocarbon chains and (b) semifluoroalkoxy chains developed in previous work. (c) Chemical structures of the target compounds in this work.
Compounds 04 00015 g001
Scheme 1. Synthetic route to the target compounds 1ac.
Scheme 1. Synthetic route to the target compounds 1ac.
Compounds 04 00015 sch001
Figure 2. DSC thermograms of (a) 1a, (b) 1b, and (c) 1c during the second heating and cooling processes. The values shown in the thermograms indicate the phase transition temperatures. Abbreviations: Cr: crystalline; SmE: smectic E; SmA: smectic A; Sm: unidentified smectic; and Iso: isotropic phases. Inset: POM image in the mesophase.
Figure 2. DSC thermograms of (a) 1a, (b) 1b, and (c) 1c during the second heating and cooling processes. The values shown in the thermograms indicate the phase transition temperatures. Abbreviations: Cr: crystalline; SmE: smectic E; SmA: smectic A; Sm: unidentified smectic; and Iso: isotropic phases. Inset: POM image in the mesophase.
Compounds 04 00015 g002
Figure 3. VT-PXRD patterns of (a) compound 1a (100 °C and 180 °C) and (b) compound 1b (150 °C and 250 °C).
Figure 3. VT-PXRD patterns of (a) compound 1a (100 °C and 180 °C) and (b) compound 1b (150 °C and 250 °C).
Compounds 04 00015 g003
Figure 4. (a) UV–vis absorption spectra of 1ac in CH2Cl2 solution (concentration: 1.0 × 10−5 mol L−1). (b) Theoretical electronic transition of 1a calculated by TD-DFT. (c) UV–vis absorption spectral changes upon UV irradiation (λex = 370 nm). (d) UV–vis diffuse reflection spectra of 1ac in the pristine solid state.
Figure 4. (a) UV–vis absorption spectra of 1ac in CH2Cl2 solution (concentration: 1.0 × 10−5 mol L−1). (b) Theoretical electronic transition of 1a calculated by TD-DFT. (c) UV–vis absorption spectral changes upon UV irradiation (λex = 370 nm). (d) UV–vis diffuse reflection spectra of 1ac in the pristine solid state.
Compounds 04 00015 g004
Figure 5. UV–vis diffuse reflection spectra of (a) 1a, (b) 1b, and (c) 1c in the pristine solid state and aggregated states of (d) 1a, (e) 1b, and (f) 1c in the mesophase quenched by liquid N2.
Figure 5. UV–vis diffuse reflection spectra of (a) 1a, (b) 1b, and (c) 1c in the pristine solid state and aggregated states of (d) 1a, (e) 1b, and (f) 1c in the mesophase quenched by liquid N2.
Compounds 04 00015 g005
Table 1. Thermophysical data of 1ac 1.
Table 1. Thermophysical data of 1ac 1.
CompoundPhase Sequence and Phase Transition Temperature [°C]
(Enthalpy [kJ mol−1])
1a[H]Cr 71 (−9.5) Cr 97 (9.1) Cr 107 (8.1) SmE 160 (4.9) SmA 228 (−) 2 Iso
[C]Cr 23 (−0.64) Cr 62 (−4.2) SmE 155 (−6.2) SmA 225 (−) 2 Iso
1b[H]Cr 132 (2.2) SmE 205 (6.6) SmA 285 (7.2) Iso
[C]Cr 124 (−1.9) SmE 201 (−5.3) SmA 284 (−5.0) Iso
1c[H]Cr 57 (−2.2) Cr 107 (8.6) Cr 126 (1.6) Cr 176 (10.9) Sm 225 (−) 2 Iso
[C]G 81 Cr 138 (−10.3) Sm 223 (−) 2 Iso
1 Determined by DSC under a N2 atmosphere (scan rate: 5.0 °C min−1). [H] and [C] represent the second heating and cooling processes, respectively. Abbreviations: Cr, crystalline; SmE, smectic E; SmA, smectic A; Sm, unidentified smectic; Iso, isotropic; G, glassy phase. 2 Determined by POM because the endothermic and exothermic peaks of DSC could not be observed because of thermal decomposition.
Table 2. PXRD data of 1a, measured at 180 °C and 100 °C, and 1b, measured at 250 °C and 150 °C 1.
Table 2. PXRD data of 1a, measured at 180 °C and 100 °C, and 1b, measured at 250 °C and 150 °C 1.
1a 1b
Label2θ [°]/d Spacing [nm]hklLabel2θ [°]/d Spacing [nm]hkl
(180 °C) (250 °C)
12.17/4.0700112.22/3.97001
(100 °C) 24.38/2.01002
22.34/3.77001(150 °C)
34.68/1.8900232.18/4.05001
47.04/1.2500344.42/2.00002
520.44/0.43110520.14/0.44110
622.28/0.40200622.00/0.40200
728.23/0.32210727.83/0.32210
1 During the cooling process.
Table 3. Optical data of 1ac measured in CH2Cl2 solution and theoretical electronic transitions.
Table 3. Optical data of 1ac measured in CH2Cl2 solution and theoretical electronic transitions.
Compoundλ [nm] 1
(ε [103, L mol−1 cm−1])
λcalcd [nm] 2
(Oscillator Strength)
HOMO/LUMO [eV] 2
(∆EH-L)
Theoretical Transition 2
(Contribution)
1a294 (18.6), 384 (24.3)384 (f = 1.98)−7.22 eV/−2.08 eV
(5.13 eV)
HOMO → LUMO (78%)
HOMO−1 → LUMO (14%)
HOMO → LUMO+1 (4%)
1b377 (18.5)384 (f = 1.98)−7.23 eV/−2.09 eV
(5.14 eV)
HOMO → LUMO (78%)
HOMO−1 → LUMO (14%)
HOMO → LUMO+1 (4%)
1c293 (20.7), 351 (18.2)383 (f = 1.98)−7.25 eV/−2.09 eV
(5.16 eV)
HOMO → LUMO (79%)
HOMO−1 → LUMO (14%)
HOMO → LUMO+1 (4%)
1 In CH2Cl2 solution (concentration: 1.0 × 10−5 mol L−1). 2 Calculated at the CAM-B3LYP/6-311+G(d,p)//CAM-B3LYP/6-31+G(d) level of theory.
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Yamada, S.; Yoshida, K.; Eguchi, Y.; Hara, M.; Yasui, M.; Konno, T. (1E)-1,2-Diaryldiazene Derivatives Containing a Donor–π-Acceptor-Type Tolane Skeleton as Smectic Liquid–Crystalline Dyes. Compounds 2024, 4, 288-300. https://doi.org/10.3390/compounds4020015

AMA Style

Yamada S, Yoshida K, Eguchi Y, Hara M, Yasui M, Konno T. (1E)-1,2-Diaryldiazene Derivatives Containing a Donor–π-Acceptor-Type Tolane Skeleton as Smectic Liquid–Crystalline Dyes. Compounds. 2024; 4(2):288-300. https://doi.org/10.3390/compounds4020015

Chicago/Turabian Style

Yamada, Shigeyuki, Keigo Yoshida, Yuto Eguchi, Mitsuo Hara, Motohiro Yasui, and Tsutomu Konno. 2024. "(1E)-1,2-Diaryldiazene Derivatives Containing a Donor–π-Acceptor-Type Tolane Skeleton as Smectic Liquid–Crystalline Dyes" Compounds 4, no. 2: 288-300. https://doi.org/10.3390/compounds4020015

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