Synthesis of Near-Infrared-Absorbing Anionic Heptamethine Cyanine Dyes with Trifluoromethyl Groups

A novel anionic heptamethine cyanine (HMC) dye with two trifluoromethyl groups that selectively absorb near-infrared light is synthesized. When contrasted with previously studied anionic HMC dyes with substituents such as methyl, phenyl, and pentafluorophenyl groups, the trifluoromethylated dye displays a red-shifted maximum absorption wavelength (for instance, 948 nm in CH2Cl2) along with enhanced photostability. Furthermore, HMC dyes with broad absorption in the near-infrared region are synthesized by combining a trifluoromethylated anionic HMC dye with a cationic HMC dye as a counterion.

We previously reported that introducing perfluorophenyl groups into an anionic HMC dye resulted in more red-shifted λ max and higher photostability of the dye than those with methyl or phenyl groups [37]. Since the mechanism of photolysis of HMC dyes is based on the addition of an electrophilic singlet oxygen to a double bond, the introduction of electron-withdrawing properties into the HMC dye backbone should be the most powerful means of improving its photostability. Among them, the introduction of the CF 3 group, one of the most electron-withdrawing substituents, can be expected to be an effective means of improving the photostability of the dye. Herein, we report the synthesis and optical properties of novel anionic HMC-dye-bearing trifluoromethyl groups. Specifically, the trifluoromethylated dye achieved a more red-shifted λ max , lower highest occupied molecular orbital (HOMO) level, and higher photostability than the previously reported anionic HMC dyes.
On the other hand, the synthesis of cyanine-cyanine mixed dyes consisting of a cationic HMC skeleton and an anionic HMC skeleton to broaden the narrow absorption range of HMC dyes is an excellent method, but the optical properties and applications of On the other hand, the synthesis of cyanine-cyanine mixed dyes consisting of a cationic HMC skeleton and an anionic HMC skeleton to broaden the narrow absorption range of HMC dyes is an excellent method, but the optical properties and applications of cyanine-cyanine mixed dyes composed of cationic and anionic HMC skeletons to photoelectric conversion devices have been scarcely reported recently [38][39][40]. Additionally, their photostability has not been investigated so far. Therefore, a novel mixture of trifluoromethylated anionic HMC dye and cationic HMC dye was also synthesized and its absorption properties and photostability were investigated.

Synthesis of the Anionic Dye with Trifluoromethyl Groups 5a and 8a
Anionic HMC dyes with trifluoromethyl groups were prepared as follows. First, tricyanofurans bearing trifluoromethyl groups were synthesized (Scheme 1). 2-Hydroxy-2-(trifluoromethyl)propionitrile and methylmagnesium bromide were treated in dehydrated diethyl ether (Et2O) at −20 °C. The temperature was gradually increased to 0 °C and stirred for 30 min. The reaction was quenched with 10% HCl and stirred at 25 °C for 1 h, resulting in the production of 4,4,4-trifluoro-3-hydroxy-3-methylbutan-2-one (1a), which returned a 38% yield as determined by 19 F nuclear magnetic resonance (NMR) analysis [41]. A couple of reasons for the low yield of 1a may be due to the reaction not progressing sufficiently and the high volatility of 1a when the extraction solvent was removed. The ketone 1a was used in the following reaction without further purification. A mixture of 1a and 2 equiv. of malononitrile and a catalytic amount of lithium ethoxide in dehydrated THF was stirred under reflux for 8 h to obtain 2-(2-cyano-3,4-dimethyl-4-(trifluoromethyl)cyclopent-2-en-1-ylidene)malononitrile 2a at 25% yield [42]. In the synthesis of various tricyanofurans, it has been reported that only low yields can be obtained when hydroxyketones with electron-withdrawing substituents are used [43]. In the present study, the extremely strong electron-withdrawing CF3 group seems to have a significant influence on the low yield of tricyanofuran 2a.

Synthesis of the Anionic Dye with Trifluoromethyl Groups 5a and 8a
Anionic HMC dyes with trifluoromethyl groups were prepared as follows. First, tricyanofurans bearing trifluoromethyl groups were synthesized (Scheme 1). 2-Hydroxy-2-(trifluoromethyl)propionitrile and methylmagnesium bromide were treated in dehydrated diethyl ether (Et 2 O) at −20 • C. The temperature was gradually increased to 0 • C and stirred for 30 min. The reaction was quenched with 10% HCl and stirred at 25 • C for 1 h, resulting in the production of 4,4,4-trifluoro-3-hydroxy-3-methylbutan-2-one (1a), which returned a 38% yield as determined by 19 F nuclear magnetic resonance (NMR) analysis [41]. A couple of reasons for the low yield of 1a may be due to the reaction not progressing sufficiently and the high volatility of 1a when the extraction solvent was removed. The ketone 1a was used in the following reaction without further purification. A mixture of 1a and 2 equiv. of malononitrile and a catalytic amount of lithium ethoxide in dehydrated THF was stirred under reflux for 8 h to obtain 2-(2-cyano-3,4-dimethyl-4-(trifluoromethyl)cyclopent-2-en-1-ylidene)malononitrile 2a at 25% yield [42]. In the synthesis of various tricyanofurans, it has been reported that only low yields can be obtained when hydroxyketones with electron-withdrawing substituents are used [43]. In the present study, the extremely strong electron-withdrawing CF 3 group seems to have a significant influence on the low yield of tricyanofuran 2a.
The sodium salt of anionic HMC dye 4a was obtained via the reaction of dialdehyde 3 [44,45] with two equiv. of trifluoromethylated tricyanofuran 2a in sodium acetate in anhydrous acetic acid at 25 • C overnight (Scheme 2). The crude sodium salt of the HMC dye 4a was used in subsequent reactions without further purification. A mixture of crude 4a and tetrabutylammonium iodide (Bu 4 N + I − ) was stirred at 25 • C to afford the trifluoromethylated anionic HMC dye 5a with tetrabutylammonium cation. The overall yield of tricyanofuran 2a to trifluoromethylated anionic HMC dye 5a was 13% from 3. The low yield may be due to the reaction to synthesize the sodium salt 4a not progressing sufficiently.
The tetrabutylammonium salt of the anionic HMC dye 5a was stirred with 1.2 equiv. of the cationic HMC dye 7 [46], which was prepared via the reaction of indolenium salt 6 [47] with dialdehyde 3 in acetone overnight at 25 • C to afford the cyanine-cyanine mixed dye 8a with a yield of 92% (Scheme 3). The sodium salt of anionic HMC dye 4a was obtained via the reaction of dialdehyde 3 [44,45] with two equiv. of trifluoromethylated tricyanofuran 2a in sodium acetate in anhydrous acetic acid at 25 °C overnight (Scheme 2). The crude sodium salt of the HMC dye 4a was used in subsequent reactions without further purification. A mixture of crude 4a and tetrabutylammonium iodide (Bu4N + I − ) was stirred at 25 °C to afford the trifluoromethylated anionic HMC dye 5a with tetrabutylammonium cation. The overall yield of tricyanofuran 2a to trifluoromethylated anionic HMC dye 5a was 13% from 3. The low yield may be due to the reaction to synthesize the sodium salt 4a not progressing sufficiently. The tetrabutylammonium salt of the anionic HMC dye 5a was stirred with 1.2 equiv. of the cationic HMC dye 7 [46], which was prepared via the reaction of indolenium salt 6 [47] with dialdehyde 3 in acetone overnight at 25 °C to afford the cyanine-cyanine mixed dye 8a with a yield of 92% (Scheme 3). The sodium salt of anionic HMC dye 4a was obtained via the reaction of dialdehyde 3 [44,45] with two equiv. of trifluoromethylated tricyanofuran 2a in sodium acetate in anhydrous acetic acid at 25 °C overnight (Scheme 2). The crude sodium salt of the HMC dye 4a was used in subsequent reactions without further purification. A mixture of crude 4a and tetrabutylammonium iodide (Bu4N + I − ) was stirred at 25 °C to afford the trifluoromethylated anionic HMC dye 5a with tetrabutylammonium cation. The overall yield of tricyanofuran 2a to trifluoromethylated anionic HMC dye 5a was 13% from 3. The low yield may be due to the reaction to synthesize the sodium salt 4a not progressing sufficiently. The tetrabutylammonium salt of the anionic HMC dye 5a was stirred with 1.2 equiv. of the cationic HMC dye 7 [46], which was prepared via the reaction of indolenium salt 6 [47] with dialdehyde 3 in acetone overnight at 25 °C to afford the cyanine-cyanine mixed dye 8a with a yield of 92% (Scheme 3). The ultraviolet-visible-NIR (UV-vis-NIR) absorption spectra of the prepared trifluoromethylated anionic HMC dye 5a with Bu4N + cations and cyanine-cyanine mixed dye 8a in a dichloromethane (CH2Cl2) solution are shown in Figure 2a for the prepared

UV-vis-NIR Spectra and CV Measurements of Anionic HMC Dye 5a and Cyanine-Cyanine Mixed Dye 8a
The ultraviolet-visible-NIR (UV-vis-NIR) absorption spectra of the prepared trifluoromethylated anionic HMC dye 5a with Bu 4 N + cations and cyanine-cyanine mixed dye 8a in a dichloromethane (CH 2 Cl 2 ) solution are shown in Figure 2a for the prepared trifluoromethylated anionic HMC dye 5a and cationic HMC dye. Figure 2b shows the cyanine-cyanine mixed dye 8a. Table 1 summarizes λ max , molar absorption coefficient (ε), oxidation potential (E ox ), HOMO, and lowest unoccupied molecular orbital (LUMO) levels of the previously synthesized anionic HMC dyes.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 13 Table 1. UV-vis-NIR absorption spectra of anionic HMC dyes 5a-d, cationic HMC dye 7, cyaninecyanine mixed dye 8a in CH2Cl2, and electrochemical properties of each anionic HMC dye 5a-d in acetonitrile. As a result, the λmax of the dye 5a was observed at 948 nm, with negligible absorption in the visible region. However, cyanine-cyanine mixed dye 8a showed absorption from the cationic and anionic HMC skeletons at 785 and 949 nm, respectively, giving a broader absorption range than that of anionic HMC dye 5a. Compared with the various anionic HMC dyes synthesized, 5a showed a significant red shift in λmax, stabilization of each energy level, and decreased HOMO−LUMO energy gap. These results can be attributed As a result, the λ max of the dye 5a was observed at 948 nm, with negligible absorption in the visible region. However, cyanine-cyanine mixed dye 8a showed absorption from the cationic and anionic HMC skeletons at 785 and 949 nm, respectively, giving a broader absorption range than that of anionic HMC dye 5a. Compared with the various anionic HMC dyes synthesized, 5a showed a significant red shift in λ max , stabilization of each energy level, and decreased HOMO−LUMO energy gap. These results can be attributed to the trifluoromethyl groups being stronger electron-withdrawing substituents than the substituents of other anionic HMC dyes.

Dye
2.3. The Photostabilities of Anionic HMC Dye 5a and Cyanine−Cyanine Mixed Dye 8a The photostabilities of the anionic HMC dye 5a and cyanine-cyanine mixed dye 8a were evaluated by irradiating them with a white LED light (8.5 W, emitting blue LED + yellow phosphor, peak wavelength: 440 nm) in a CH 2 Cl 2 solution (1.0 × 10 −6 M) at 25 • C in a constant temperature chamber. The residual rates of dyes 5a and 8a, calculated from the change in absorbance at λ max in the UV-vis-NIR spectra, are illustrated in Figure 3a-d, respectively, and are compared with those of previous HMC dyes ( Table 2).   Ph 62 -5d Me 24 -7 --0 The residual rate of anionic HMC dye 5a after 10 days of light irradiation was 80%. This dye showed the best photostability compared to the anionic and cationic HMC dyes we synthesized previously. The observed results can be ascribed to the enhanced electronwithdrawing properties of the trifluoromethyl groups. These groups significantly suppress the electrophilic addition of singlet oxygen to the methine chain [37]. Furthermore, when photostability tests were carried out on the cyanine-cyanine mixed dye 8a under identical conditions, it was observed that the absorption peaks originating from the anionic HMC structure diminished more rapidly compared to those from the cationic HMC structure. This is because the methine chain of the anionic HMC skeleton, which is more electron-rich than the cationic skeleton, is more readily affected by the electrophilic addition to singlet oxygen, which matches the reported photodegradation mechanism. Interestingly, the photostability of the anionic dye in the cyanine-cyanine mixed dye 8a was much lower than that of anionic HMC 5a alone. In contrast, the photostability of the cationic dye in cyanine-cyanine mixed dye 8a was significantly improved compared to that of cationic HMC 7 alone.

Measurements
The 1 H NMR spectra of the compounds were obtained at 392 or 400 MHz in CDCl 3 , hexadeuteroacetone ((CD 3 ) 2 CO), or hexadeuterodimethyl sulfoxide ((CD 3 ) 2 SO) solutions using the residual solvent as the internal standard and a JEOL ECS-400 or ECX-400P Fourier transform NMR (FT-NMR) spectrometer. The 13 C NMR spectra of the compounds were obtained at 99 or 101 MHz in CDCl 3 , (CD 3 ) 2 CO, or (CD 3 ) 2 SO solutions, using the residual solvent as the internal standard, and a JEOL ECS-400 or ECX-400P FT-NMR spectrometer. The 19 F NMR spectra of the compounds were obtained at 369 or 376 MHz in CDCl 3 or (CD 3 ) 2 CO solutions, respectively, using CFCl 3 as the external standard and a JEOL ECS-400 or ECX-400P FT-NMR spectrometer. The data were reported as follows: (s = singlet, t = triplet, q = quartet, m = multiplet, br s = broad singlet, coupling constant(s), and integration). The melting points of the compounds were obtained using a Yanagimoto MP-S3 micro melting point apparatus and are uncorrected. The compounds' infrared (IR) spectra were recorded on a Shimadzu IR Affinity-1 instrument. Electrospray ionizationmass spectroscopy (ESI-MS) and HRMS measurements were performed using a Waters Xevo quadrupole time-of-flight (QTOF) mass spectrometer. The UV-vis-NIR absorption spectra of the dyes in solution were recorded using a Hitachi U-4100 instrument. The CV profiles were obtained using an HSV-110 automatic polarization system. TG-DTA experiments were performed using an SII EXSTAR 6000 thermogravimetry differential thermal analysis (TG/DTA) 6300 apparatus under a nitrogen atmosphere after heating to 80 • C under vacuum for 18 h; the measured values were uncorrected.

Synthesis of 2-(2-Cyano-3,4-dimethyl-4-(trifluoromethyl)cyclopent-2-en-1ylidene)malononitrile (2a) [42]
First, a mixture of 1a (0.601 g, 3.847 mmol) and malononitrile (0.510 g, 7.724 mmol) was dissolved in an anhydrous THF (4 mL) under an argon atmosphere. Subsequently, 0.269 mL of lithium ethoxide (1.0 M in EtOH, 0.269 mmol) was added dropwise to the solution. The reaction mixture was stirred under reflux for 8 h and then concentrated using a rotary evaporator. The residue was extracted with CH 2 Cl 2 (30 mL × 3), washed with brine, and the combined organic layers were dried over anhydrous Na 2 SO 4 . After the organic solvent was concentrated, the crude product was purified via column chromatography on silica gel using CH 2 Cl 2 as the solvent, followed by washing with methanol to produce 2-

Electrochemical Measurements of the Dyes
Electrochemical measurements of the dyes were performed in MeCN solutions (1.0 × 10 −3 M) containing Bu 4 NClO 4 (0.1 M). The E ox values were measured using three small electrodes. A silver quasi-reference electrode, a platinum wire, and a carbon electrode were used as the reference, counter, and working electrodes, respectively. All the electrode potentials were calibrated concerning the Fc/ferrocenium redox couple. Electrochemical measurements were performed at a scan rate of 200 mV s −1 . The E ox value of Fc vs. SCE was 0.380 V [48]. The E ox values vs. SCE were determined using the observed E ox (V vs. Ag) values of the dyes in MeCN solutions as follows: The energy of the HOMO (eV) was obtained using the E ox (V vs. SCE) values as follows [48]: HOMO (eV) = −(E ox (V vs. SCE) + 4.4) Molecules 2023, 28, 4650 10 of 12 The band gap (E 0-0 ) and energy of the LUMO (eV) were calculated using the λ onset abs value as follows: E 0-0 (eV) = 1240/λ onset abs (nm) LUMO (eV) = HOMO (eV) − E 0-0 (eV) 3.12. Methods for Evaluating Photostability CH 2 Cl 2 solutions of the dyes were maintained in an incubator at 25 • C and irradiated with white LED light (8.5 W).

Conclusions
We synthesized a novel anionic HMC dye 5a with trifluoromethyl groups in the dye skeleton and compared its properties with those of our previously synthesized anionic HMC dyes. The new anionic HMC dye 5a showed a more red-shifted absorption wavelength and improved photostability than our previously synthesized anionic HMC dyes. These properties are attributed to the electron-withdrawing characteristics of the trifluoromethyl groups of 5a, which are more potent than the substituents of the previously synthesized dyes.
We proceeded to synthesize a cyanine-cyanine mixed dye, 8a, composed of an anionic HMC skeleton bearing trifluoromethyl groups and a cationic HMC skeleton. Subsequent investigations into its absorption properties and photostability were conducted. Our findings revealed that the unique properties of both the anionic and cationic HMC skeletons were distinctly represented in this compound. In the cyanine-cyanine mixed dye 8a, the photostability of the cationic HMC skeleton was enhanced by the photodegradation of the anionic HMC skeleton. The enhanced properties of compounds 5a and 8a indicated their potential suitability for photovoltaic devices, thus presenting a significant advantage.
We are currently investigating organic solar cells that utilize only near-infrared light using HMC dyes with CF 3 groups.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.