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Article

N-Phenylphenothiazine Radical Cation with Extended π-Systems: Enhanced Heat Resistance of Triarylamine Radical Cations as Near-Infrared Absorbing Dyes

by
Masafumi Yano
1,*,
Minami Ueda
1,
Tatsuo Yajima
1,
Koichi Mitsudo
2 and
Yukiyasu Kashiwagi
3
1
Faculty of Chemistry, Material and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita 564-8680, Japan
2
Division of Applied Chemistry, Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan
3
Osaka Research Institute of Industrial Science and Technology, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan
*
Author to whom correspondence should be addressed.
Colorants 2024, 3(4), 350-359; https://doi.org/10.3390/colorants3040024
Submission received: 13 November 2024 / Revised: 7 December 2024 / Accepted: 10 December 2024 / Published: 11 December 2024
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Abstract

:
N-Phenylphenothiazine derivatives extended with various aryl groups were designed and synthesized. These derivatives have bent conformation in crystal and exhibit high solubility. Radical cations obtained by one-electron oxidation of these derivatives gave stable radical cations in solution and showed absorption in the near-infrared region. A radical cation was isolated as a stable salt, which exhibited heat resistance up to around 200 °C. A design strategy for radical cation-based near-infrared absorbing dyes, which are easily oxidized and stable not only as a solution but in solid form, is described.

1. Introduction

Near-infrared radiation is light in the 750–2500 nm wavelength range, which is invisible to the naked eye, but has attracted a lot of attention in recent years. Recently, dyes with absorption in the near-infrared region have attracted much attention [1]. These dyes are expected to find applications in dye-sensitized solar cells [2], bioimaging [3,4], telecommunication technologies [5,6], organic photodetectors [7], security inks [8] and photodynamic therapy [9]. Compared to near-infrared absorbing materials based on inorganic compounds [10,11], these are lighter, more flexible and soluble in organic solvents, so large-area devices can be expected to be fabricated using printing processes [12,13]. At present, dyes capable of absorbing near-infrared light in the longer wavelength region are desired. One example of the use of such molecules is dyes for dye-sensitized solar cells. Approximately half of the energy of the light that falls from the sun is in the near-infrared light region. Absorbing dyes for use in dye-sensitized solar cells are being investigated, though. Most of these are only capable of absorbing ultraviolet and visible light and are unable to utilize the energy of light in the near-infrared region. Dyes that absorb near-infrared rays are also expected to be developed into shielding filters, which can be used to reduce the temperature rise inside a room by blocking only near-infrared rays from sunlight entering through the window. Until now, several near-infrared absorbing dyes have been designed and synthesized, and their physical properties have been elucidated [14]. Typical examples of these dyes are molecules based on oligoacenes [1,15,16,17] polyenes [18], porphyrins [19,20,21,22] and phthalocyanine [23] skeletons. These dyes are closed-shell molecules and employ a strategy of narrowing the HOMO (Highest Occupied Molecular Orbital)-LUMO (Lowest Unoccupied Molecular Orbital) gap of closed-shell molecules by extending π-systems. This molecular design rule is a promising approach to obtain near-infrared absorbing dyes. However, when trying to construct a more extended π system network to have absorption in the longer wavelength direction, one faces (1) lower yields due to complex and multi-step synthetic routes, (2) instability due to increased HOMO energy levels and (3) lower solubility. To overcome this low solubility, attempts have been made to introduce solubility-enhancing substituents into the dye molecules, which would further complicate the synthetic route [1,14]. For these reasons, the development of near-infrared absorbing dyes is currently in a synthetic chemical blockage. Near-infrared absorbing organic dyes using closed-shell molecules currently reported are limited to an extreme absorption wavelength of about 1400 nm. However, the near-infrared region extends further to 2500 nm, and if dyes with absorption in longer wavelengths can be developed, they can be expected to be used in the fields mentioned above and in new fields. New approaches to solve these problems have been sought in the last few years. It is widely known that radical cations obtained by one-electron oxidation of triphenylamine derivatives have a strong absorption around 650 nm [24]. This absorption is due to the transition between HOMO and SOMO (Singly Occupied Molecular Orbital), which is characteristic of open-shell species. Recently, we have reported that stable radical cations obtained by one-electron oxidation of triphenylamine derivatives extended with aromatic rings are promising near-infrared absorbing dyes [25,26,27,28,29]. These precursors can be easily synthesized in a few steps from commercially available reagents and show good solubilities in common organic solvents. Upon one electron oxidation, these molecules give stable radical cations with intense absorption in the near-infrared region. Among them, we have previously designed triphenylamine derivatives 1–3 and reported that the corresponding radical cations 1•+–3•+ have strong absorption in the near-infrared region [26]. We also found that the absorption wavelength is affected by the electronic effect of the substituent R, and that introducing electron-donating groups effectively shifts the absorption to longer wavelengths. All of the extended π-based triphenylamine radical cationic near-infrared absorbing dyes we have reported so far are π-extended derivatives of the unmodified triphenylamine core, and there is no precedence of modification of the triphenylamine core itself. Several triphenylamine derivatives in which the two aromatic rings are intramolecularly bridged by various spacers, such as oxygen, sulfur, carbonyl and methylene units, have been reported [30,31]. These constrained triphenylamine derivatives exhibit interesting properties due to their planar-fixed conformations and effective orbital interaction. Among them, N-phenylphenothiazine is known to be readily oxidized to give stable radical cations, which are used in hole transport layers for solar cells [32], organic electroluminescence [33], organic photocatalysis [34,35] and catalysis for atom transfer radical polymerization [36]. Furthermore, this radical cation has a broad absorption in the near-infrared region below 1000 nm [35]. With these results in mind, we considered that N-phenylphenothiazine with an extended π system is easily oxidized, and the resulting radical cation would be a stable dye with strong absorption in the near-infrared region. Furthermore, due to the constrained conformational, these radical salts can also be expected to be highly heat tolerant. We report here the syntheses and electrochemical, spectroscopic and thermal properties of N-phenylphenothiazine derivatives with extended π system with various aryl groups 4–6 (Figure 1). As mentioned above, N-phenylphenothiazine is a promising unit that can be used in various fields, but no examples of its use as a near-infrared absorbing dye have been reported.

2. Results and Discussion

2.1. Syntheses and Crystal Structure

The synthetic route for 4–6 is depicted in Scheme 1. These compounds were readily synthesized in one step by modifying the previously reported method [26]. Various aryl groups were introduced into the N-phenylphenothiazine derivative with three bromine atoms under Suzuki coupling conditions. The corresponding aryl boronic acid was reacted with 3,7-dibromo-10-(4-bromophenyl)-10H-phenothiazine [37] under Pd-catalyzed Suzuki coupling reaction conditions to give the target compound in moderate yield. These compounds were easily purified by recrystallization. For details of the synthetic procedure, see the Supplementary Materials.
For compounds 4 and 6, single crystals suitable for crystal structure analysis were obtained by recrystallization from methanol/chloroform, respectively. Crystallographic data and ORTEP diagrams for compound 6 are shown in Table S1 and Figure 2, respectively. The crystal structure of compound 3 without the sulfur bridging structure [38] is also shown for comparison. The phenothiazine ring of 6 is bent 25 degrees between two benzene rings, and the other benzene ring attached to the N atom of the phenothiazine ring is inclined 78 degrees to the mean plane of the phenothiazine ring. In the crystal, the bent 6 molecules form columns due to intermolecular aromatic ring stacking interactions between the phenothiazine rings (C(3)…S(1) = 3.46 Å) and CH-π interactions between the benzene and phenothiazine rings (H(26)…C(13) = 2.75 Å). There are no notable intercolumnar interactions other than two CH-π interactions (H(20)…C(4) = 2.84 and H(3)…C(22) = 2.81 Å). In contrast, the compound 3 molecule has a planar shape due to the absence of bridging S atoms between the benzene rings of the TPA core and is packed in the crystal in an intricate layered structure with many intermolecular interactions. Due to this difference in intermolecular interactions in the crystal, 6 is expected to be more soluble in organic solvents than 3.

2.2. Solubility

Solubility is an essential property in the solution deposition process, which has attracted much attention in recent years due to the ease of fabricating large-area devices using a coating process [39]. Generally, a solubility of 0.1 wt% or more, ideally more than 1 wt%, is desirable for use in device fabrication by the coating process. In addition, high solubility in environmentally friendly solvents has been in demand in recent years. To a sample of 5.0 mg, 100 µL of solvent was added repeatedly. The resulting suspension was shaken and sonicated at room temperature (20–25 °C). The solution was sealed during the dissolution process to prevent volatilization of the solvent. The total amount of solvent was converted to solubility (wt%). Compounds 4–6 showed higher solubility compared to compounds 1–3 without a bridging structure. Compound 4 had a solubility of more than 1 wt% in common organic solvents such as dichloromethane, anisole and toluene. Compared to compound 1 without sulfur bridging, it showed about twice the solubility in these solvents. This would be due to the bent conformation caused by the sulfur bridging, as described above, and a significant dipole moment from the center of the molecule in the direction of the sulfur atom. Compounds 4–6 showed high solubility even in ethyl acetate, which is recommended for industrial-scale use due to its low environmental impact. See Table S2 for a comparison of the solubility of compounds 1–6 in common organic solvents.

2.3. Electrochemical Properties

The electrochemical behavior of 4–6 was studied by cyclic voltammetry (CV) in dichloromethane with 0.1 M tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) as a supporting electrolyte at ambient temperature. Compound 4 showed a reversible redox couple (E0 = 0.15 V vs. Fc/Fc+) (Figure 3). This result indicates that the corresponding radical cation 4•+ is stable in solution. Compared to the redox potential of 1 (E0 = 0.33 V vs. Fc/Fc+), the cathodic shift was about 0.18 V, indicating that sulfur bridging makes triarylamines more readily oxidized. No change in the voltammogram of 4 was observed after repeating anodic and cathodic sweeping 10 times at a 25 mV/s sweep rate (Figure S3). Upon further anodic sweep, a second electron transfer was observed; however, this process was electrochemically irreversible (Figure S6). See also the Supplementary Materials for cyclic voltammograms of compounds 5 and 6. The electrochemical properties of compounds 1–6 are shown in Table 1. As compounds 4–6 were found to give stable radical cations in solution, these absorption spectral measurements were examined.

2.4. Photophysical Properties

To examine the spectroscopic properties of 4–6, UV-vis absorption spectrum measurements were obtained in dichloromethane. Compound 4 exhibited an absorption maximum at around 283 nm with a large molar extinction coefficient (ε = 1.67 × 105 M−1 cm−1). This absorption is thought to be derived from the HOMO to the LUMO. Upon excitation at 283 nm, 4 exhibited strong blue fluorescence at 456 nm (Figure 4). The results of absorption and fluorescence emission measurements for compounds 1–6 are shown in Table 2. Absorption and emission spectra of compounds 5 and 6 are shown in the Supplementary Materials.
The UV-vis-NIR spectra of oxidized species 4•+–6•+ were examined in dichloromethane: when two equivalents of SbCl5 were added to a solution of 4, the color of the solution became reddish brown (Figure S11), indicating the formation of oxidized species 4•+. New absorption peaks appeared at 879 nm (Figure 5). This absorption is thought to be derived from the HOMO to the SOMO. The absorption intensity of this peak (log ε = 4.23) was smaller than that of the unbridged congener 1•+ (log ε = 4.65). When the solution of 4•+ was allowed to stand under a nitrogen atmosphere, no decrease in peak intensity was observed over 60 min (Figure 6) at room temperature. UV titration experiments for 4 showed that the solution, obtained by SbCl5 oxidation, contains only two chemical species, 4 and 4•+ (Figure S18). No new absorption was observed when an excess amount (10 eq.) of SbCl5 was added, indicating that no dication 42+ was formed under the condition. Spectroscopic data for the radical cation 1•+–6•+ are shown in Table 3.
Fluorescence emission spectra of 4•+–6•+ in dichloromethane are shown in Figures S22–S24. Upon the addition of 2.0 equivalents of SbCl5, the emission of 4–6 were quenched almost completely, respectively. The existence of radical cation species was also confirmed by ESR spectra (see Figures S27–S29). The ESR spectra of 4•+–6•+ were detected at g = 2.002, 2.003 and 2.003, respectively, indicating the localization of the unpaired electron at the nitrogen center.

2.5. Isolation of Radical Cation Salts

As radical cation 4•+ was found to be stable in solution and to have absorption in the near-infrared region, an attempt was made to isolate this radical cation salt. When one equivalent of tris(4-bromophenyl) ammoniumyl hexachloroantimonate was added to a dichloromethane solution of 4, the color of the solution changed to reddish brown. Upon the addition of a large excess of diethyl ether to the solution, 4•+·SbCl6− was obtained as a green powder (Figure 7a). Details of the isolation procedure are described in the Supplementary Materials. The absorption spectrum of the isolated 4•+·SbCl6− in solution was in perfect agreement with 4•+ generated in solution (Figure S25). The salt can be stored under ambient conditions for more than several weeks without any decomposition. To estimate the heat tolerance of the isolated salt, we evaluated the thermal stabilities of compounds 4•+·SbCl6− using thermogravimetry and differential thermal analysis (TG-DTA) in the solid state under a nitrogen flow. The TG-DTA charts of compounds 4•+·SbCl6− showed an endothermic peak with weight loss at 205 °C, as shown in Figure 7b. This change is thought to be due to the thermal decomposition of the radical cation salts. The temperature at which a 5% weight loss occurs (T95) in its initial state for 4•+·SbCl6− was estimated to be 195 °C. On the other hand, the decomposition temperature (Td) and T95 for 1•+·SbCl6− without sulfur bridge were 175 and 174 °C, respectively (Figure S26). This suggests that fixing the conformation is a practical approach to enhancing the thermal properties of triphenylamine radical cation-based near-infrared absorbing materials.

3. Conclusions

In summary, we synthesized three π-extended N-phenylphenothiazine derivatives 4–6 as near-infrared absorbing material precursors and examined their spectroscopic, electrochemical and thermal properties. These compounds were easily prepared in moderate isolated yield. X-ray crystallographic analysis showed that 4 and 6 took a bent conformation, respectively. In the crystal, these compounds had a folded structure. Compounds 4–6 gave stable radical cations upon one-electron oxidation. The sulfur-bridged 4–6 was found to be more readily oxidized than the non-sulfur-bridged derivatives 1–3. These radical cations 4•+–6•+ exhibited a peak with a maximum absorption of around 700–900 nm. Fixation of the conformation by sulfur bridging of the triphenylamine framework was found to (1) improve solubility, (2) thermodynamically stabilize the radical cation by a cathodic shift in the redox potential and (3) improve the thermotolerance of the radical cation. Furthermore, compound 4 can be easily isolated as stable radical cation salts 4•+·SbCl6−. This radical salt exhibited higher thermostability than the non-bridged derivative 1•+·SbCl6−. The approach to bridging the triphenylamine skeleton presented in this paper will be one of the promising molecular design rules for near-infrared absorbing dyes. Investigation of π-extended triphenylamines with various cross-linked units is currently underway in our laboratory.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/colorants3040024/s1, Table S1. Crystallographic data for 4 (CCDC 2393033) and 6 (CCDC 2393032); Table S2. Solubility (wt%) of compounds 1–6 in dichloromethane, anisole, toluene and ethyl acetate at room temperature; Figure S1. Cyclic voltammogram of 5 in dichloromethane (1 × 10−3 M) with Bu4NPF6 as a supporting electrolyte. The scan rate is 100 mV/s; Figure S2. Cyclic voltammogram of 6 in dichloromethane (1 × 10−3 M) with Bu4NPF6 as a supporting electrolyte. The scan rate is 100 mV/s; Figure S3. Cyclic voltammogram of 4 in dichloromethane (1 × 10−3 M) repeating sweep ten cycles. The scan rate is 25 mV/s. Bu4NPF6 (1 × 10−1 M) was employed as a supporting electrolyte; Figure S4. Cyclic voltammogram of 5 in dichloromethane (1 × 10−3 M) repeating sweep ten cycles. The scan rate is 25 mV/s. Bu4NPF6 (1 × 10−1 M) was employed as a supporting electrolyte; Figure S5. Cyclic voltammogram of 6 in dichloromethane (1 × 10−3 M) repeating sweep ten cycles. The scan rate is 25 mV/s. Bu4NPF6 (1 × 10−1 M) was employed as a supporting electrolyte; Figure S6. Cyclic voltammogram of 4 in dichloromethane (1 × 10−3 M) with Bu4NPF6 as a supporting electrolyte in the range of −0.15 and 1.06 V (vs. Fc/Fc+) The scan rate is 100 mV/s; Figure S7. Cyclic voltammogram of 5 in dichloromethane (1 × 10−3 M) with Bu4NPF6 as a supporting electrolyte in the range of −0.15 and 1.06 V (vs. Fc/Fc+) The scan rate is 100 mV/s; Figure S8. Cyclic voltammogram of 6 in dichloromethane (1 × 10−3 M) with Bu4NPF6 as a supporting electrolyte in the range of −0.15 and 1.15 V (vs. Fc/Fc+) The scan rate is 100 mV/s; Figure S9. UV-vis (solid line) and fluorescence emission (dotted line) spectra of 5. The concentration is 1 × 10−5 M for UV-vis and 1 × 10−6 M for fluorescence emission spectra; Figure S10. UV-vis (solid line) and fluorescence emission (dotted line) spectra of 6. The concentration is 1 × 10−5 M for UV-vis and 1 × 10−6 M for fluorescence emission spectra; Figure S11. Dichloromethane solution of 4 in the absence (left) and presence of 2.0 equivalents of SbCl5 (right). [4] = 1 × 10−5 M; Figure S12. Dichloromethane solution of 5 in the absence (left) and presence of 2.0 equivalents of SbCl5 (right). [5] = 1 × 10−5 M; Figure S13. Dichloromethane solution of 6 in the absence (left) and presence of 2.0 equivalents of SbCl5 (right). [6] = 1 × 10−5 M; Figure S14. Absorption spectra of 5 before (dotted line) and after oxidation with 2.0 equivalents of SbCl5 (solid line) in dichloromethane at room temperature. [5] = 1 × 10−5 M; Figure S15. Absorption spectra of 6 before (dotted line) and after oxidation with 2.0 equivalents of SbCl5 (solid line) in dichloromethane at room temperature. [6] = 1 × 10−5 M; Figure S16. Time-course of absorption spectra of 5 with 2.0 equivalents of SbCl5 in dichloromethane at room temperature recorded every 15 min. The initial concentration of 5 is 1 × 10−5 M. Inset: time-course of the peak intensity at 736 nm; Figure S17. Time-course of absorption spectra of 6 with 2.0 equivalents of SbCl5 in dichloromethane at room temperature recorded every 15 min. The initial concentration of 6 is 1 × 10−5 M. Inset: time-course of the peak intensity at 688 nm; Figure S18. Absorption spectra of 4 after oxidation with 0, 0.3, 0.6, 0.9, 1.2, 1.5 and 2.0 equivalents of SbCl5 in dichloromethane at room temperature. [4] = 1 × 10−5 M; Figure S19. Absorption spectra of 5 after oxidation with 0, 0.3, 0.6, 0.9, 1.2, 1.5 and 2.0 equivalents of SbCl5 in dichloromethane at room temperature. [5] = 1 × 10−5 M; Figure S20. Absorption spectra of 6 after oxidation with 0, 0.1, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8 and 2.0 equivalents of SbCl5 in dichloromethane at room temperature. [6] = 1 × 10−5 M; Figure S21. Absorption spectrum of SbCl5 in dichloromethane at room temperature. [SbCl5] = 1 × 10−5 M; Figure S22. Emission spectra of 4 before (solid line) and after oxidation with 2.0 equivalents of SbCl5 (dotted line) in dichloromethane at room temperature. Excitation at 346 nm. [4] = 1 × 10−6 M; Figure S23. Emission spectra of 5 before (solid line) and after oxidation with 2.0 equivalents of SbCl5 (dotted line) in dichloromethane at room temperature. Excitation at 348 nm. [5] = 1 × 10−6 M; Figure S24. Emission spectra of 6 before (solid line) and after oxidation with 2.0 equivalents of SbCl5 (dotted line) in dichloromethane at room temperature. Excitation at 348 nm. [6] = 1 × 10−6 M; Figure S25. Absorption spectra of isolated 4•+·SbCl6– (solid line) and 4 after oxidation with 2.0 equivalents of SbCl5 (dotted line) in dichloromethane at room temperature; Figure S26. TG-DTA charts for 1•+·SbCl6− under N2 purge. Solid and dotted lines represent TG and DTA, respectively; Figure S27. ESR spectrum of 4•+ in CH2Cl2; Figure S28. ESR spectrum of 5•+ in CH2Cl2; Figure S29. ESR spectrum of 6•+ in CH2Cl2; Figure S30. 1H NMR spectrum of 4 (400 MHz, CD2Cl2); Figure S31. 13C NMR spectrum of 4 (100 MHz, CD2Cl2); Figure S32. 1H NMR spectrum of 5 (400 MHz, CD2Cl2); Figure S33. 13C NMR spectrum of 5 (400 MHz, CD2Cl2). Figure S34. 1H NMR spectrum of 6 (400 MHz, CD2Cl2). Figure S35. 13C NMR spectrum of 6 (400 MHz, CD2Cl2).

Author Contributions

Idea and writing: M.Y.; organic synthesis and physical properties measurement: M.U.; idea and ESR measurement and writing: T.Y.; idea, X-ray measurements and DFT calculation: K.M.; idea, writing, and IR measurement: Y.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by JSPS KAKENHI Grant Number JP23K04711 (to M.Y.) and the ESPEC Foundation for Global Environment Research and Technology (Charitable Trust) (to M.Y.).

Data Availability Statement

See Supplementary Materials for detailed data.

Acknowledgments

The authors would like to thank Hitoshi Ishida, Kansai University, for the measurements of UV-vis-NIR and spectra.

Conflicts of Interest

The authors declare no competing financial interests.

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Colorants 03 00024 g001
Figure 1. Triphenylamines examined previously (1–3) and in this study (4–6).
Figure 1. Triphenylamines examined previously (1–3) and in this study (4–6).
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Scheme 1. Syntheses of 4–6.
Scheme 1. Syntheses of 4–6.
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Figure 2. Molecular structure of 3 and 6.
Figure 2. Molecular structure of 3 and 6.
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Figure 3. Cyclic voltammogram of 4 in dichloromethane (1 × 10−3 M) with Bu4NPF6 as a supporting electrolyte (1 × 10−1 M).
Figure 3. Cyclic voltammogram of 4 in dichloromethane (1 × 10−3 M) with Bu4NPF6 as a supporting electrolyte (1 × 10−1 M).
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Figure 4. UV-vis (solid line) and fluorescence emission (dotted line) spectra of 4 in CH2Cl2. The concentration is 1 × 10−5 M for UV-vis and 1 × 10−6 M for fluorescence emission spectra.
Figure 4. UV-vis (solid line) and fluorescence emission (dotted line) spectra of 4 in CH2Cl2. The concentration is 1 × 10−5 M for UV-vis and 1 × 10−6 M for fluorescence emission spectra.
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Figure 5. Absorption spectra of 4 before (dotted line) and after oxidation with 2 equivalents of SbCl5 (solid line) in dichloromethane at room temperature. [4] = 1 × 10−5 M.
Figure 5. Absorption spectra of 4 before (dotted line) and after oxidation with 2 equivalents of SbCl5 (solid line) in dichloromethane at room temperature. [4] = 1 × 10−5 M.
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Figure 6. Time course of absorption spectra of 4 with 2.0 equivalents of SbCl5 in dichloromethane at room temperature recorded every 15 min. The initial concentration of 4 is 1 × 10−5 M. Inset: time course of the peak intensity at 879 nm.
Figure 6. Time course of absorption spectra of 4 with 2.0 equivalents of SbCl5 in dichloromethane at room temperature recorded every 15 min. The initial concentration of 4 is 1 × 10−5 M. Inset: time course of the peak intensity at 879 nm.
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Figure 7. (a) Isolated 4•+·SbCl6− powder, (b) TG-DTA charts for 4•+·SbCl6− under N2 purge. Solid and dotted lines represent TG and DTA, respectively.
Figure 7. (a) Isolated 4•+·SbCl6− powder, (b) TG-DTA charts for 4•+·SbCl6− under N2 purge. Solid and dotted lines represent TG and DTA, respectively.
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Table 1. Electrochemical data for 1–6.
Table 1. Electrochemical data for 1–6.
CompoundE0 (V vs. Fc/Fc+)
10.33
20.37
30.42
40.15
50.20
60.23
Table 2. Spectroscopic data for 1–6.
Table 2. Spectroscopic data for 1–6.
CompoundAbsorption SpectraFluorescence Spectra
λmax (nm)log ελmax (nm)
13434.75412
23454.83411
33444.78416
42835.22456
52814.98459
62825.07461
Table 3. Spectroscopic data for 1•+–6•+.
Table 3. Spectroscopic data for 1•+–6•+.
CompoundAbsorption Spectra
λmax (nm)log ε
1•+10534.65
2•+9254.78
3•+8624.34
4•+8794.23
5•+7354.28
6•+6874.28
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MDPI and ACS Style

Yano, M.; Ueda, M.; Yajima, T.; Mitsudo, K.; Kashiwagi, Y. N-Phenylphenothiazine Radical Cation with Extended π-Systems: Enhanced Heat Resistance of Triarylamine Radical Cations as Near-Infrared Absorbing Dyes. Colorants 2024, 3, 350-359. https://doi.org/10.3390/colorants3040024

AMA Style

Yano M, Ueda M, Yajima T, Mitsudo K, Kashiwagi Y. N-Phenylphenothiazine Radical Cation with Extended π-Systems: Enhanced Heat Resistance of Triarylamine Radical Cations as Near-Infrared Absorbing Dyes. Colorants. 2024; 3(4):350-359. https://doi.org/10.3390/colorants3040024

Chicago/Turabian Style

Yano, Masafumi, Minami Ueda, Tatsuo Yajima, Koichi Mitsudo, and Yukiyasu Kashiwagi. 2024. "N-Phenylphenothiazine Radical Cation with Extended π-Systems: Enhanced Heat Resistance of Triarylamine Radical Cations as Near-Infrared Absorbing Dyes" Colorants 3, no. 4: 350-359. https://doi.org/10.3390/colorants3040024

APA Style

Yano, M., Ueda, M., Yajima, T., Mitsudo, K., & Kashiwagi, Y. (2024). N-Phenylphenothiazine Radical Cation with Extended π-Systems: Enhanced Heat Resistance of Triarylamine Radical Cations as Near-Infrared Absorbing Dyes. Colorants, 3(4), 350-359. https://doi.org/10.3390/colorants3040024

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