A ring-fluorinated heptamethine cyanine dye: synthesis, photophysical properties, and vapochromic properties in response to ammonia

Abstract

Heptamethine cyanine dyes (HMCDs) have attracted considerable attention in biological and energy applications owing to their unique near-infrared (NIR) photophysical properties. Therefore, the development of molecules that change both visible and fluorescent colours in a stimulus-responsive manner by exploiting the NIR optical properties of HMCDs has been a subject of increasing interest. Most research results are based on a highly nucleophilic anion addition or reversible intramolecular addition reaction of a weakly nucleophilic neutral nucleophile with the C=N bond of the terminal indol-1-ium moiety. Examples of intermolecular addition of weakly neutral nucleophiles and the use of solid or polymer materials are not available. Here, we report the synthesis of a NIR-absorbing ring-perfluorinated HMCD. The HMCD's unique properties in various solvents and rapid and reversible vapochromic response to various amines, including NH₃, based on the noteworthy structural modification induced by fluorine atoms on the aromatic ring are also presented. The ring-fluorinated HMCD adsorbed on neutral filter paper responds quickly to even low-nucleophilicity NH₃ vapour. Repeatability tests on filter paper adsorbed with the ring-fluorinated HMCD and NH₃ and HCl vapours show excellent reproducibility in 13 blue-green and yellow colour transitions. These results are the first examples of intermolecular addition of weakly neutral nucleophiles into HMCDs and stimulus responsiveness not in solutions.

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Introduction

Polymethine cyanine dyes having azaheterocycles at both ends of the polymethine backbone offer advantages such as narrow absorption bands, high absorption coefficients, and readily tunable maximum absorption wavelengths (λ_abs) and maximum fluorescence wavelengths (λ_em) within the visible and near-infrared (NIR) regions.¹ In particular, heptamethine cyanine dyes (HMCDs), which exhibit absorption and fluorescence emissions in the NIR region, are attracting considerable attention as one of the most promising molecules for photo-science and -technology using NIR light, such as imaging,² therapy,³ and organic solar cells.^4,5

The development of molecules that change both the colours of the solutions or films and fluorescent colours in a stimulus-responsive manner by exploiting the NIR optical properties of HMCDs has been a subject of increasing interest. Changing both the colours of the solutions and fluorescent colours of the HMCD reported thus far is mostly based on (1) highly nucleophilic anion addition, such as cyanide anion (CN⁻), to the C=N bond of the terminal indol-1-ium moiety (Fig. 1(a))⁶ or (2) a reversible intramolecular addition reaction of weakly nucleophilic neutral nucleophiles, such as nitrogen, sulfur, and oxygen atoms, with the C=N bond of the indol-1-ium moiety or the C=C double bond at the meso position, depending on the pH of the solution (Fig. 1(b)).⁷ However, intermolecular addition of weakly neutral nucleophiles is not reported. In addition, all HMCDs were in solution, and no examples of the use of solid or polymer materials are available.

Fig. 1 Previous methods for changing the colour or fluorescence of HMCDs in solutions by intermolecular addition of cyanide anion (a) or intramolecular addition of neutral nucleophiles (b).

Ammonia (NH₃) is currently attracting attention as a fuel that can replace oil, coal, and natural gas for power generation because it does not emit carbon dioxide when burned. Ammonia is expected to be an energy carrier for the transport and storage of hydrogen.⁸ Although many studies on the vapochromism of organic dyes or metal complexes towards NH₃ have been reported,⁹ to the best of our knowledge, only one study has been conducted on the reversible vapochromic responsiveness of HMCDs adsorbed on silica gels to various amines, as shown in Fig. 2.¹⁰ The results revealed that the vapochromism due to the intermolecular addition of weakly nucleophilic neutral amines to the HMCD can be attributed to the adsorption of the dye on weakly acidic silica gel. However, the NH₃ vapour of the HMCD adsorbed on the weakly acidic silica gel was insufficient because NH₃ is not sufficiently nucleophilic.

Fig. 2 Previous method for changing the colour of HMCD towards vapour of amines.

The unique functions of organic dyes can be incorporated by introducing heteroatoms such as sulfur,¹¹ phosphorous,¹² and silicon¹³ into the electron-conjugated system or on the aromatic rings of the functional dyes. The functional dyes carrying partially ring-fluorinated aromatics or heteroaromatics have garnered attention as fluorescent dyes,¹⁴ organic field-effect transistors,¹⁵ organic photovoltaic cells,¹⁶ and amine-response dyes¹⁷ owing to their excellent properties, including their atomic size that is almost as small as that of a hydrogen atom, their highest electronegativity, their strong carbon–fluorine bond energy, and some interactions with other elements.¹⁸ However, only limited examples of stimulus-responsive NIR-absorbing molecules or materials carrying ring-fluorinated aromatics or heteroaromatics are available.¹⁹ Although only a few patents on the preparation of ring-fluorinated HMCDs exist, the structure and optical and other properties of such HMCDs have not yet been reported.²⁰ Here, we report (1) the synthesis of a NIR-absorbing ring-perfluorinated HMCD and its (2) crystal structure, (3) unique properties in various solvents, and (4) rapid and reversible vapochromic response to various amines including NH₃, based on the noteworthy structural modification induced by fluorine atoms on the aromatic ring. The results reveal the first examples of intermolecular addition of weakly neutral nucleophiles to HMCD and stimulus responsiveness not in solutions.

Results and discussion

Synthesis

As shown in Scheme 1 (eqn (1), successive treatment of a commercially available 2,3,4,5-tetrafluoroaniline with sodium nitrite in water at −10 °C and tin chloride(II) hydrate at room temperature in the presence of concentrated hydrochloric acid yields 2,3,4,5-tetrafluorophenyl hydrazine hydrochloride. The obtained crude hydrazine hydrochloride reacts with 3-methyl-2-butanone in acetic acid at 120 °C in the presence of sodium acetate and results in the formation of 4,5,6,7-tetrafluoro-2,3,3-trimethyl-3H-indole (1) in 22% yield from 2,3,4,5-tetrafluoroaniline.

Scheme 1 Preparation of the ring-fluorinated and non-fluorinated HMCDs (4a and 4b, respectively).

The reaction between 1 and an excess amount of the most reactive haloalkane, iodomethane, does not proceed because the nucleophilicity of 1 is significantly reduced owing to the strong electron-withdrawing property of the fluorine atom. Treatment of 1 with 2 equiv. of the Meerwein reagent (trimethyloxonium tetrafluoroborate) in chloroform for 24 h under reflux results in the formation of 4,5,6,7-tetrafluoro-1,2,3,3-tetramethyl-3H-indol-1-ium tetrafluoroborate (2a) in 84% yield. The tetrafluoroindolium salt (2a) smoothly reacts with dialdehyde (3) in dimethylformamide (DMF) even at 23 °C, which is a very low temperature compared with a typical reaction temperature (120 °C), for 18 h and results in the formation of 2-((E)-2-((E)-2-chloro-3-(2-((E)-4,5,6,7-tetrafluoro-1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-en-1-yl)vinyl)-4,5,6,7-tetrafluoro-1,3,3-trimethyl-3H-indol-1-ium tetrafluoroborate (4a) in 84% yield (Scheme 1, eqn (2)).²¹ The reaction of a fluorine-free indolium salt (2b) with 3 in DMF under the same reaction conditions (at 23 °C for 18 h) results in the formation of 2-((E)-2-((E)-2-chloro-3-(2-((E)-1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-en-1-yl)vinyl)-1,3,3-trimethyl-3H-indol-1-ium tetrafluoroborate (4b) with only 9% yield, together with a 20% yield of (E)-2-chloro-3-(2-((E)-1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-ene-1-carbaldehyde (5b) (Scheme 1, eqn (3)). The high-temperature (120 °C) conditions allow the reaction of 2b with 3 in DMF to proceed smoothly for 3 h, giving a satisfactory yield (76%) of nonfluorinated HMCD (4b). These results may be attributed to an increase in the acidity of the hydrogen atom on the methyl group at the 2-position of 2a owing to the strong electron-withdrawing property of the four fluorine atoms on the aromatic ring. Consequently, the formation of the enamine from 2a and successive condensation with 3 occur smoothly, even at room temperature to result in the formation of the corresponding ring-fluorinated HMCD (4a) in good yield.

Single-crystal structural analysis

Single crystals of 4a and 4b were prepared using the vapour diffusion method with dichloromethane (DCM) and hexane. Single-crystal X-ray analysis results of the obtained HMCDs (4a: CCDC No. 1987380, monoclinic, P2₁/c space group; 4b: CCDC No. 1987381, orthorhombic, Pca2₁ space group) are shown in Fig. 3 and 4.

Fig. 3 X-ray diffraction structures of HMCD 4a: twist and fold angles (a), intermolecular short contacts (b), and intramolecular short contacts (c).

Fig. 4 X-ray diffraction structures of the ring-fluorinated HMCD (4b): twist and fold angles (a), intermolecular short contacts (b), and intramolecular short contacts (c).

The twist and fold angles of the two indolium planes of HMCD 4b²¹ or the two ring-fluorinated indolium planes of HMCD 4a were measured by using the Olex2 software²² and are shown in Fig. 3(a) and 4(a). The twist angle between the two ring-fluorinated indolium planes of HMCD 4a is 45.25°, which is considerably larger than that (17.99°) between the two non-fluorinated indolium planes of HMCD 4b. These results are consistent with those of the ring-fluorinated trimethine cyanine dye.²³ The fold angle between the two ring-fluorinated indolium planes of HCMD 4a is 0.38°, which is smaller than that (4.82°) between the non-fluorinated indolium planes of HMCD 4b. These results are not consistent with those of the ring-fluorinated trimethine cyanine dye.²³ As shown in Fig. 4(b) and (c), compared with the ring-fluorinated HMCD 4a, in the structure of HMCD 4b, less intramolecular and intermolecular interactions occur between the fluorine atom and other atoms. By contrast, ring-fluorinated HMCD 4a exhibits six types of intramolecular interactions between fluorine atoms and other atoms, such as hydrogen, chlorine, and carbon atoms (Fig. 3(b)), and four types of intermolecular interactions between fluorine atoms and hydrogen, carbon, and nitrogen atoms (Fig. 3(c)). The packing of the molecules observed in the single-crystal X-ray analysis of HMCDs 4a and 4b is shown in Fig. S1 and S2 (ESI†). Molecular orientations are indicated with four colors: red, blue, magenta, and green, in both HMCDs. As shown in Fig. S1(a) (ESI†), in the case of ring-perfluorinated HMCD 4a, both the blue and magenta molecules and the red and green molecules are arranged in the same direction. In addition, the blue and magenta molecular arrangements and the red and green molecular arrangements are opposite. As shown in Fig. S1(g), (h), (i), and (j) (ESI†), the distances between the indolium ring and the methine double bond are 4.988, 5.091, and 6.439 Å, and the distance between the two indolium rings is ∼5.5 Å. No π–π stacking is observed. As shown in Fig. S2 (ESI†), in the case of fluorine-free HMCD 4b, the blue and magenta molecules and the red and green molecules are arranged in a herringbone shape. The distances between the indolium ring and the methine double bond are 4.274 and 5.918 Å, as depicted in Fig. S2(b) and (f) (ESI†). No π–π stacking is observed.

Ultraviolet-visible (UV-vis)-NIR absorption and fluorescence spectra and behaviour in various solvents

The UV-vis-NIR absorption (solid line) and fluorescence (dotted line) spectra of HMCDs 4a and 4b in DCM (1 × 10⁻⁶ M) are shown in Fig. 5.

Fig. 5 Normalised UV-vis-NIR absorption (solid line) and fluorescence (dotted line) spectra of HMCDs 4a and 4b in DCM (1 × 10⁻⁶ M).

The λ_abs values of 4a and 4b in DCM are within the NIR region (778 and 785 nm, respectively). The λ_abs value of ring-fluorinated HMCD 4a is 7 nm blue-shifted compared with that of non-fluorinated HMCD 4b. The molar absorption coefficients (ε) of 4a and 4b are 318 000 and 331 000, respectively. Similar to λ_abs, λ_em of ring-fluorinated HMCD 4a is 5 nm blue-shifted compared with that of non-fluorinated HMCD 4b. The Stokes shifts (SS) of HMCDs 4a and 4b are small (322 and 301 cm⁻¹, respectively). The fluorescence quantum yields (Φ_f) of both HMCDs are low, and the τ_f of ring-fluorinated HMCD 4a is slightly smaller than that of non-fluorinated HMCD 4b. The fluorescence lifetime (τ_s) of ring-fluorinated HMCD 4a is slightly shorter than that of non-fluorinated HMCD 4b. These results may be attributed to ring-fluorinated HMCD 4a having a higher non-radiative rate constant than non-fluorinated HMCD 4b (Table 1).

Table 1 Absorption maximum (λ_abs), molar absorption coefficient (ε), and fluorescence properties of HMCDs 4a and 4b in DCM

Dye	λ abs (nm)	ε (M^−1 cm^−1)	λ em (nm)	SS (cm^−1)	Φ f	τ s (ns)	k f (10^9 s^−1)	k nr (10^9 s^−1)
a Measured in DCM (1 × 10^−6 M). b Measured using an integrating sphere method. c Measured using a single-photon-counting method. d Radiative rate constant (kf = Φf/τs). e Non-radiative rate constant (knr = (1 − Φf)τs).
4a	778	318 000	798	322	0.11	0.9	0.13	1.01
4b	785	331 000	804	301	0.14	1.3	0.13	0.77

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The UV-vis-NIR absorption and fluorescence spectra of non-fluorinated HMCD 4b in various solvents, such as methanol (MeOH), ethanol (EtOH), 2-propanol (i-PrOH), acetonitrile (MeCN), and acetone, were recorded after preparing the solution and leaving it at room temperature for 3 h. The spectra and images are shown in Fig. 6 and summarised in Table 2. Similar to previous results with other anions,²²λ_abs (774–785 nm) and λ_em (795–804 nm) are in the NIR region, and negative solvatochromism is observed in the UV-vis-NIR and fluorescence spectra.

Fig. 6 UV-vis-NIR absorption spectra (a) and fluorescence spectra (b) of non-fluorinated HMCD 4b in various solvents (1 × 10⁻⁶ M).

Table 2 Absorption maximum and molar absorption coefficients of HMCDs 4a and 4b in various solvents (1 × 10⁻⁶ M)

Dye	Solvent	λ abs (nm)	ε (M^−1 cm^−1)
a Measured in solvents (1 × 10^−6 M).
4a	MeOH	400	49 000
	EtOH	402	46 000
	i-PrOH	399	39 000
	MeCN	404, 770	32 000, 103 000
	Acetone	406, 773	46 000, 15 000
	DCM	778	318 000
4b	MeOH	774	235 000
	EtOH	778	239 000
	i-PrOH	779	216 000
	MeCN	774	222 000
	Acetone	774	218 000
	DCM	785	331 000

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By contrast, ring-fluorinated HMCD 4a exhibits unique properties in various solvents. The spectra and photographs in various solvents are shown in Fig. 7 and summarised in Table 2. The peaks at approximately 774–779 nm disappear in the case of MeOH and EtOH, and a new peak appears at approximately 400 nm (Fig. 7(a)). As displayed in Fig. 7(b), the peaks around 774–779 nm are small in the case of i-PrOH, MeCN, and acetone. Furthermore, as shown in Fig. 7(d), fluorescence spectra of ring-fluorinated HMCD 4a excited at λ_abs (770–778 nm) in DCM, MeCN, and acetone are observed at 790–798 nm. However, note that no fluorescence spectrum exists at λ_abs (399–406 nm) in the case of MeOH, EtOH, i-PrOH, MeCN, and acetone.

Fig. 7 UV-vis-NIR absorption spectra in MeOH and EtOH (a), in i-PrOH, MeCN, and acetone (b), and in DCM (c). Fluorescence spectra (d) of ring-fluorinated HMCD 4a in various solvents (1 × 10⁻⁶ M).

Electrochemical and thermal properties

Thermogravimetry-differential thermal analysis (TG-DTA) was performed to determine the decomposition temperatures (T_dt) of HMCDs 4a and 4b. The results of T_dt are shown in Fig. S3 (ESI†) and listed in Table 3. Ring-perfluorinated HMCD 4a shows a lower T_dt (238.0 °C) than non-fluorinated HMCD 4b (244.9 °C).

Table 3 T _dt and electrochemical properties of HMCDs 4a and 4b

Dye	T dt (°C)	E ox (V vs. SCE)	HOMO^c (eV)	λ onset (nm)	HOMO–LUMO gap^e (eV)	LUMO^f (eV)
a Decomposition temperatures (Tdt) were determined by conducting TG-DTA. b Measured in MeCN containing the dyes (1 × 10^−3 M) and Bu4NClO4 (0.1 M) and recalculated as Eox (V vs. SCE). c HOMO (eV) = − (Eox (V vs. SCE) + 4.4). d Measured in CH2Cl2 (1 × 10^−6 M). e HOMO–LUMO gap (eV) = 1240/λonset^abs (nm). f LUMO (eV) = HOMO − (HOMO–LUMO gap).
4a	238.0	0.693	−5.09	875	1.42	−3.67
4b	244.9	0.478	−4.88	883	1.41	−3.47

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HMCDs 4a and 4b undergo oxidation at 0.693 and 0.478 V (versus the saturated calomel electrode (SCE)) in the cyclic voltammograms, respectively, as shown in Fig. S6 (ESI†). The E_HOMO values for HMCDs 4a and 4b were calculated using the values of E_ox in the cyclic voltammograms, and the E_LUMO values for HMCDs 4a and 4b were obtained by performing calculations using the E_HOMO values and the bandgap values, which can be obtained from the onset of the absorption spectra. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of aromatic ring-perfluorinated HMCD 4a were much lower than those of non-fluorinated HMCD 4b because of the high electronegativity of the fluorine atom.

Density functional theory (DFT) calculations

DFT calculations in the gas phase were performed at the B3LYP/6-31G(d,p) level with Gaussian 16 for HMCDs 4a and 4b performed to examine the electronic states of the HOMO and LUMO levels and bandgaps, and the results are illustrated in Fig. 8. Although a significant difference in the electron distribution between the HOMO and LUMO in HMCDs 4a and 4b is unlikely, the introduction of fluorine atoms into the aromatic ring of the dye lowers the levels of the HOMO and LUMO, especially the HOMO. Therefore, the bandgap of ring-fluorinated HMCD 4a is wider than that of non-fluorinated HMCD 4b. These results support that the λ_abs (778 nm) of ring-fluorinated HMCD 4a is blue-shifted compared with that of non-fluorinated HMCD 4b (785 nm).

Fig. 8 Energy diagram and molecular orbitals of the cationic parts of HMCDs 4a and 4b calculated using DFT at the B3LYP/6-31G(d,p).

Photostability

As presented in Fig. 9(a)–(c), the photostabilities of ring-fluorinated HMCD 4a and non-fluorinated HMCD 4b in an incubator at 25 °C under white-light-emitting diode (LED) irradiation (8.5 W) are measured in CH₂Cl₂ solutions (1.0 × 10⁻⁶ M). The results for the condition when the HMCDs are left in the dark without LED irradiation in CH₂Cl₂ are depicted in Fig. 9(d). CH₂Cl₂ is used as the solvent because the lifetime of singlet oxygen is longer in CH₂Cl₂ than in other solvents, such as alcohols, benzene, and alkanes. The residual rates of HMCD 2a and 2b are calculated from the changes at λ_abs in the UV-vis-NIR absorption spectra.

Fig. 9 Photostabilities of HMCDs 4a and 4b under white LED irradiation (8.5 W) in an incubator at 25 °C in CH₂Cl₂ solutions (1 × 10⁻⁶ M).

After 6 days of light irradiation, the residual rates of HMCDs 4a and 4b in CH₂Cl₂ are 92% and 78%, respectively (Fig. 9(c)). The photostability of ring-fluorinated HMCD 4a is higher than that of 2b. Non-fluorinated HMCD 4b in the CH₂Cl₂ solution kept in the dark does not degrade at all, and its residual rate is 99%. These results indicate that the introduction of a strong electron-withdrawing fluorine atom into the indolenine portion of HMCD stabilises the HOMO and the LUMO and reduces the electron density of the double bond, thus improving the photostability of the dye.²³

Stimulus response to amines

Based on the interesting absorption properties of ring-fluorinated HMCD 4a in various solvents, the stimulus responsiveness of ring-fluorinated HMCD 4a to other neutral nucleophiles such as triethylamine (TEA) in solutions was evaluated, as shown in Fig. 10.

Fig. 10 UV-vis-NIR absorption spectra (a) and photographs (c) of ring-fluorinated HMCD 4a (5 × 10⁻⁶ M) in the presence of various amounts of TEA in DCM and UV-vis-NIR absorption spectra after the solution is left at room temperature for 4 h (b). Photographs (d) of HMCD 4b (5 × 10⁻⁶ M) in the presence of various amounts of TEA in DCM. Standard deviation obtained from five measurements of the UV-vis-NIR spectra of HMCD 4a in the presence of 300 equiv. of TEA in DCM (1 × 10⁻⁵ M) after the solution is left at room temperature for 4 h (e). Linear relationship between the absorption logarithm (A₄₀₉/A₇₇₈) of HMCD 4a (5 × 10⁻⁶ M) with the amount of TEA in DCM (f).

Various amounts (10, 20, 30, 40, 50, 100, 200, and 300 equiv.) of TEA were added to a highly concentrated DCM solution (5 × 10⁻⁶ M) of HMCDs 4a and 4b at room temperature, and UV-vis-NIR spectra were recorded after the solution was prepared and left at room temperature for 4 h, as presented in Fig. 10 and Table 4.

Table 4 Absorption maximum of HMCDs 4a and 4b in the presence of various amounts of TEA in DCM (5 × 10⁻⁶ M) after the solution was prepared and left at room temperature for 4 h

Dye	Equiv. of TEA	λ abs (nm)
a Measured in CH2Cl2 (5 × 10^−6 M).
4a	0	778
	10	778, 416
	20	778, 413
	30	778, 412
	40	778, 412
	50	778, 412
	100	778, 409
	200	782, 409
	300	409
4b	0	785
	10	784
	20	784
	30	784
	40	784
	50	784
	100	784
	200	784
	300	784

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The results of the UV-vis-NIR spectra and the photographs obtained under white LED irradiation are shown in Fig. 10. Although the values (784–785 nm) of λ_abs in the DCM solution of non-fluorinated HMCD 4b in the presence of various amounts of TEA are almost unchanged in the UV-vis-NIR spectra, the λ_abs of ring-fluorinated HMCD 4a shifts from 778–782 nm to 409–416 nm, and the colour of the DCM solution of the HMCD 4a changes from green to yellow (Fig. 10(c)). An excellent linear relationship between the absorption log (A₄₀₉/A₇₇₈) and the equiv. of TEA is observed, as shown in Fig. 10(f). Based on this linearity, the limit of detection (LOD) for 4a is determined as LOD = 3s/k, where s is the standard deviation (Fig. 10(e)) obtained from five measurements, and k is the slope of the line. The results indicate that the LOD of ring-fluorinated HMCD 4a is 2.97 × 10⁻⁶ M.

¹H, ¹³C, and ¹⁹F nuclear magnetic resonance (NMR) spectra of ring-fluorinated HMCD 4a in deuterochloroform (CDCl₃) without or with an excess amount of TEA were recorded to assess the structural changes in ring-fluorinated HMCD 4a in a DCM solution. The results of not only ¹H, ¹⁹F, and ¹³C NMR spectroscopy of ring-fluorinated HMCD 4a without or with an excess amount of TEA but also high-resolution mass spectrometry (HRMS) are shown in Fig. 11. Two types of vinyl protons (H_a and H_b) in ¹H NMR of 4a, four types of aromatic fluorine atoms in ¹⁹F NMR, and four types of alkyl carbons and an imine carbon atom (173 ppm) (magenta-filled circles) in ¹³C NMR are observed,^17b because ring-fluorinated HMCD 4a has a resonance structure, that is, a particular electronic structure arising from a symmetric, positively charged, amino-terminated, 7-numbered polymethine chain.

Fig. 11 ¹H NMR spectra (a), ¹⁹F NMR spectra (b), and ¹³C NMR spectra (c) of ring-fluorinated HMCD 4a without or with an excess amount of TEA in CDCl₃ and HRMS results (d).

Because of the collapse of the symmetric resonance structure caused by the addition of TEA to aromatic ring-fluorinated HMCD 4a, four types of vinyl protons, H_a, H_a′, H_b, and H_b′, appear in the ¹H NMR spectra, similar to our previously reported ¹H NMR spectra of ring-fluorinated trimethine cyanine dye with n-hexylamine added to the C=N bond.^17b Eight types of aromatic fluorine atoms in the ¹⁹F NMR spectra and nine types of alkyl carbons (orange-filled circles) in the ¹³C NMR spectra are observed. The peak at 173.0 ppm corresponding to the iminium carbon disappears and shifts to 156.9 ppm in the ¹³C NMR spectra. These results suggest that the addition of TEA to ring-fluorinated HMCD 4a proceeds smoothly with CDCl₃ at room temperature to yield the corresponding TEA adduct with a molecular weight of 728.3011, as confirmed by performing HRMS with high-performance liquid chromatography (Fig. 11(d)).

Finally, the reversible vapochromic responses of the filter papers adsorbing ring-fluorinated HMCD 4a and non-fluorinated HMCD 4b to amine vapour were investigated. The filter papers adsorbing ring-fluorinated HMCD 4a and non-fluorinated HMCD 4b were prepared by soaking white filter papers in a DCM solution (5 × 10⁻⁴ M) of HMCDs 4a and 4b overnight and drying at room temperature overnight. The green colour of the filter paper adsorbing non-fluorinated HMCD 4b did not change in the presence of TEA vapour (Fig. 12(b)). However, notably, when the blueish-green filter paper adsorbing ring-fluorinated HMCD 4a was exposed to TEA or low-nucleophilicity ammonia vapour, the green filter paper responded within 1 s and showed a rapid and visible colour change from blueish-green to pale yellow (Fig. 12(a)). Non-fluorinated HMCD 4b adsorbed on weakly acidic silica gel changed only slightly in response to NH₃ vapor,¹⁰ whereas ring-fluorinated HMCD 4a adsorbed on the neutral filter paper reacted quickly, even with low-nucleophilicity NH₃ vapour.

Fig. 12 Photographs of ring-fluorinated HMCD 4a adsorbed on the filter paper with TEA vapour (a), HMCD 4b adsorbed on the filter paper with TEA vapour (b), ring-fluorinated HMCD 4a adsorbed on the filter paper with NH₃ vapour (c), and HMCD 4b adsorbed on the filter paper with NH₃ vapour (d).

Exposure of ring-fluorinated HMCD 4a adsorbed on the filter paper to the vapour of other primary amines, such as ethylamine, n-propylamine, and n-butylamine, and secondary amines, such as diethylamine, pyrrolidine, and piperidine, instantly causes the same vapochromic-responsive behaviour, along with a colour change from blue to green to pale yellow (Fig. 13(a) and (b)). No response is observed when ring-fluorinated HMCD 4a adsorbed on the filter paper is exposed to an aromatic amine, such as aniline (Fig. 13(d)).

Fig. 13 Vapochromic discolouration and colouration test of ring-fluorinated HMCD 4a adsorbed on the filter paper with various primary amines (a), secondary amines (b), and aniline (c).

However, the colour of the filter paper adsorbing ring-fluorinated HMCD 4a does not revert from pale yellow to blueish-green when left in air. However, ring-fluorinated HMCD 4a adsorbed on the filter paper is reversibly converted to the original HMCD when exposed to HCl vapour, as shown in Fig. 13(a)–(c). Fig. 14 presents the results of the repeatability tests with NH₃ and HCl vapours on ring-fluorinated HMCD 4a adsorbed on the filter paper. The filter paper adsorbing ring-fluorinated HMCD 4a exhibits excellent repeatability in terms of colour switching, which occurs 13 times.

Fig. 14 Vapochromic discolouration and colouration repeatability test of ring-fluorinated HMCD 4a adsorbed on the filter paper with NH₃ and HCl vapours.

The unique vapochromic-responsive phenomenon, along with the colour change towards various primary and secondary amines including ammonia, can be ascribed to the nucleophile addition into the iminium double bond of the aromatic ring-fluorinated HMCD (4a) on the filter paper, similar to that in the DCM or CDCl₃ solution.

Conclusions

The introduction of fluorine atoms into the aromatic ring of HMCDs reduced the HOMO and LUMO levels of the HMCDs. Consequently, various primary alcohols, despite being weakly neutral nucleophiles, were added to ring-fluorinated HMCD 4a in an alcohol solvent, and the colour of the solution changed from green to yellow. This phenomenon was also observed when TEA was added to a DCM solution of ring-fluorinated HMCD 4a. The addition of TEA to the iminium double bond of HMCD 4a was confirmed by performing ¹H, ¹³C, and ¹⁹F NMR and HRMS on the solution. Based on the unique properties of ring-fluorinated HMCD 4a, for the first time, we developed an HMCD adsorption filter paper, a near-infrared absorbent exhibiting reversible vapochromic response to weakly nucleophilic ammonia. These results are the first examples of intermolecular addition of weakly neutral nucleophiles into HMCDs and stimulus responsiveness not in solutions.

Experimental

Measurements

¹H NMR spectra were recorded at 392 or 400 MHz in CDCl₃, hexadeuteroacetone ((CD₃)₂CO), and hexadeuterodimethyl sulfoxide ((CD₃)₂SO) solutions, with tetramethylsilane (Me₄Si) as an internal standard, using a JEOL ECS-400 or ECX-400P FT-NMR spectrometer. ¹³C NMR spectra were obtained at 99 or 101 MHz in CDCl₃, (CD₃)₂CO, and (CD₃)₂SO solutions, with Me₄Si as an internal standard, using the JEOL ECS-400 or ECX-400P FT-NMR spectrometer. ¹⁹F NMR spectra were recorded at 369 or 376 MHz in CDCl₃, (CD₃)₂CO, and (CD₃)₂SO solutions, with CFCl₃ as an external standard, using the JEOL ECS-400 or ECX-400P FT-NMR spectrometer. The data were reported as s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet, m = multiplet, br s = broad singlet, coupling constant(s), and integration. The melting points were determined using Yanagimoto MP-S3 micro-melting-point apparatus and were uncorrected. Electrospray ionisation mass spectroscopy in MeOH was performed using a JEOL JMS-T100LP instrument (Accu TOF LC-plus). UV-vis absorption spectra were recorded on Hitachi U-4100 and PerkinElmer Lamda950 instruments. Fluorescence spectra were recorded using a Jasco FP-8600 spectrofluorometer. The absolute fluorescence quantum yields were obtained using a HAMAMATSU Quantaurus-QY C11347-01 instrument. The fluorescence lifetimes were measured using a HAMAMATSU Quantaurus-Tau compact fluorescence lifetime spectrometer (C11367-01). Cyclic voltammetry was performed by employing an automatic polarisation system (HSV-110). TG-DTA was performed on an SII EXSTAR 6000 TG/DTA 6300 system under nitrogen after applying heat treatment at 100 °C under vacuum for 12 h, and the measured values were uncorrected. The X-ray crystal structure was evaluated using a Rigaku AFC 10 (CCD: Saturn 724 +) + VariMax Mo Optic system. 4,5,6,7-Tetrafluoro-2,3,3-trimethyl-3H-indole (1),²¹ 1,2,3,3-tetramethyl-3H-indol-1-ium tetrafluoroborate (2b),²⁴ and (E)-2-chloro-3-(hydroxymethylene)cyclohex-1-enecarbaldehyde (3)²⁵ were prepared according to previously reported methods.

Synthesis

4,5,6,7-Tetrafluoro-1,2,3,3-tetramethyl-3H-indol-1-ium tetrafluoroborate (2a). A super-dehydrated chloroform (7.0 mL) solution of 4,5,6,7-tetrafluoro-2,3,3-trimethyl-3H-indol (0.324 g, 1.4 mmol) and trimethyloxonium tetrafluoroborate (0.419 g, 2.8 mmol) was stirred at 80 °C for 1 d. The reaction mixture was quenched with diethyl ether (100 mL × 3). The precipitate was purified via suction filtration to obtain 4,5,6,7-tetrafluoro-1,2,3,3-tetramethyl-3H-indol-1-ium tetrafluoroborate (0.392 g, 84%).

Yield 84%; m.p. 151–155 °C; R_f 0.65 (hexane/dichloromethane = 2/1); IR (KBr) 1523 (C=N) cm⁻¹; HRMS (ESI) found: m/z 246.0892; calcd for C₁₂H₁₂NF₄: M-BF₄, 246.0906; ¹H NMR (acetone-d₆) δ 1.70 (s, 6H, CH₃ × 2), 2.82 (s, 3H, CH₃), 4.13 (s, 3H, NCH₃); ¹³C NMR (acetone-d₆) δ 14.2 (s), 20.7 (s), 38.7 (s), 57.2 (s), 125.6 (d, J = 18.7 Hz), 127.2 (s), 138.7 (dm, J = 256.9 Hz), 142.1 (dm, J = 255.4 Hz), 143.8 (dd, J = 250.2, 12.5 Hz), 200.6 (s); ¹⁹F NMR (acetone-d₆) δ −155.7 (dd, J = 19.2, 3.0 Hz, 1F), −155.5 (td, J = 19.2, 3.0 Hz, 1F), −152.3 (tm, J = 18.1 Hz, 1F), −151.9 (s, 4F), −147.1 (tm, J = 18.1 Hz, 1F).

2-((E)-2-((E)-2-Chloro-3-(2-((E)-4,5,6,7-tetrafluoro-1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-en-1-yl)vinyl)-4,5,6,7-tetrafluoro-1,3,3-trimethyl-3H-indol-1-ium tetrafluoroborate (4a). A DMF (1.5 mL) solution of 4,5,6,7-tetrafluoro-1,2,3,3-tetramethyl-3H-indol-1-ium tetrafluoroborate (0.094 g, 0.3 mmol) and (E)-2-chloro-3-(hydroxymethylene)cyclohex-1-enecarbaldehyde (0.025 g, 0.15 mmol) was stirred at 23 °C for 18 h. The reaction mixture was cooled to room temperature, quenched with ice water (150 mL), filtered to remove the liquid, and concentrated under vacuum. The precipitate was purified by employing silica gel chromatography (dichloromethane/methanol = 20/1) to obtain 2-((E)-2-((E)-2-chloro-3-(2-((E)-4,5,6,7-tetrafluoro-1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-en-1-yl)vinyl)-4,5,6,7-tetrafluoro-1,3,3-trimethyl-3H-indol-1-ium tetrafluoroborate 4a (0.089 g, 84%).

Yield 84%; T_dt 238.0 °C; R_f 0.43 (dichloromethane : methanol = 20 : 1); IR (KBr) 1558 (C=N) cm⁻¹; HRMS (ESI) found: m/z 627.1813; calcd for C₃₂H₂₈ClF₈N₂: M-BF₄, 627.1808; ¹H NMR (CDCl₃) δ 1.82 (s, 12H, CH₃ × 4), 1.89–1.92 (m, 2H, –CH₂CH₂CH₂–), 2.67 (t, J = 6.00 Hz, 4H, –CH₂CH₂CH₂–), 3.79 (s, 6H, NCH₃ × 2), 6.16 (d, J = 14.2 Hz, 2H, vinyl H), 8.30 (d, J = 14.2 Hz, 2H, vinyl H); ¹³C NMR (CDCl₃) δ 20.6 (s), 26.4 (s, 2C), 34.6 (d, J = 9.3 Hz), 50.8 (s), 102.6 (s), 122.6 (d, J = 15.9 Hz), 126.6 (s), 130.4 (s), 135.4 (dd, J = 249.4, 13.2 Hz), 138.0 (dt, J = 252.3, 14.9 Hz), 141.7 (dt, J = 252.3, 14.9 Hz), 143.3 (dd, J = 249.4, 13.2 Hz), 144.9 (s), 152.0 (s), 173.0 (s); ¹⁹F NMR (CDCl₃) δ −159.5 (t, J = 19.9 Hz, 2F), −157.5 (dd, J = 19.9, 14.2 Hz, 2F), −153.4 (tm, J = 19.9 Hz, 2F), −153.2 (s, 4F), −146.5 (dd, J = 19.9, 14.2 Hz, 2F).

2-((E)-2-((E)-2-Chloro-3-(2-((E)-1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-en-1-yl)vinyl)-1,3,3-trimethyl-3H-indol-1-ium tetrafluoroborate (4b). A DMF (3.0 mL) solution of 1,2,3,3-tetramethyl-3H-indol-1-ium tetrafluoroborate (0.2658 g, 1.0 mmol) and (E)-2-chloro-3-(hydroxymethylene)cyclohex-1-enecarbaldehyde (0.088 g, 0.5 mmol) was stirred at 120 °C for 3 h. The reaction mixture was cooled to room temperature, quenched with ice water (150 mL), filtered to remove the liquid, and concentrated under vacuum. The precipitate was purified by performing silica gel chromatography (dichloromethane/methanol = 20/1) to obtain 2-((E)-2-((E)-2-chloro-3-(2-((E)-1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-en-1-yl)vinyl)-1,3,3-trimethyl-3H-indol-1-ium tetrafluoroborate (0.2226 g, 76%).

Yield 76%; T_dt 244.9 °C; R_f 0.43 (dichloromethane : methanol = 20 : 1); IR (KBr) 1551 (C=N) cm⁻¹; HRMS (ESI) found: m/z 483.2547; calcd for C₃₂H₃₆ClN₂: M-BF₄, 483.2562; ¹H NMR (acetone-d₆) δ 1.77 (s, 12H, CH₃ × 4), 1.91–1.97 (m, 2H, –CH₂CH₂CH₂–), 2.77 (t, J = 6.20 Hz, 4H, –CH₂CH₂CH₂–), 3.79 (s, 6H, NCH₃ × 2), 6.41 (d, J = 14.2 Hz, 2H, vinyl H), 7.32 (td, J = 7.3, 1.4 Hz, 2H, aryl H), 7.41–7.49 (m, 4H, aryl H), 7.62 (d, J = 7.3 Hz, 2H, aryl H), 8.44 (d, J = 14.20 Hz, 2H, vinyl H); ¹³C NMR (acetone-d₆) δ 21.6 (s), 26.9 (s), 28.0 (s), 32.0 (s), 50.1 (s), 102.5 (s), 112.0 (s), 123.2 (s), 126.2 (s), 127.4 (s), 129.6 (s), 142.2 (s), 144.1 (s), 144.7 (s), 150.0 (s), 174.3 (s); ¹⁹F NMR (acetone-d₆) δ −151.7 (s, 4F).

(E)-2-Chloro-3-(2-((E)-1,3,3-trimethylindolin-2-ylidene)ethylidene)cyclohex-1-ene-1-carbaldehyde (5b):²⁷R_f 0.21 (hexane : dichloromethane = 1 : 2); IR (KBr) 1643 (C=O) cm⁻¹; ¹H NMR (chloroform-d) δ 1.66 (s, 6H, CH₃ ×2), 1.74–1.81 (m, 2H, -CH₂CH₂CH₂-), 2.48 (t, J = 5.84 Hz, 2H, -CH₂CH₂CH₂-), 2.58 (tm, J = 5.84 Hz, 2H, -CH₂CH₂CH₂-), 3.23 (s, 3H, -NCH₃), 5.46 (d, J = 12.8 Hz, 1H, vinyl H), 6.72 (d, J = 7.79 Hz, 1H, aryl H), 6.94 (tm, J = 7.79 Hz, 1H, aryl H), 7.19–7.24 (m, 2H, aryl H) 7.82 (d, J = 12.8 Hz, 1H, vinyl H), 10.3 (s, 1H, CHO); ¹³C NMR (chloroform-d) δ 21.0 (s), 24.7 (s), 26.8 (s), 28.4 (s), 29.5 (s), 46.5 (s), 93.0 (s), 106.8 (s), 121.0 (s), 121.8 (s), 123.4 (s), 128.0 (s), 128.7 (s), 131.2 (s), 139.3 (s), 144.5 (s), 148.8 (s), 163.0 (s), 191.1 (s).

Preparation of the dye-adsorbed filter paper

The filter paper was dipped in a DCM solution (5 × 10⁻⁴ M) of the 4a and 4b dyes and allowed to stand in a refrigerator overnight to allow each dye to be adsorbed on the filter paper. After being removed and air-dried, the filter paper samples were exposed to the vapours of various amines.

Author contributions

SA performed the experiments on the synthesis and photophysical properties, analysed the data, and wrote part of the manuscript. TA and YU examined the single-crystal X-ray structure and analysed and summarised the data. YH performed the experiments on optical properties. YK and TI contributed useful discussions and comments on the synthesis, photophysical properties, and single-crystal X-ray structural analyses. KF conceived and designed the study and revised the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by JSPS KAKENHI (JP20K05647 and 23H02003 to KF). We would also like to thank Editage (https://www.editage.com) for pre-submission English language editing.

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11. For recent examples, see. (a) S. Takahashi, M. Murai, Y. Hattori, S. Seki, T. Yanai and S. Yamaguchi, Sulfur-Bridged Cationic Diazulenomethenes: Formation of Charge-Segregated Assembly with High Charge-Carrier Mobility, J. Am. Chem. Soc., 2024, 146, 22642–22649; (b) W. Wu, K. Yan, Z. He, L. Zhang, Y. Dong, B. Wu, H. Liu, S. Wang and F. Zhang, J. Am. Chem. Soc., 2024, 146, 11570–11576; (c) M. Taki, K. Kajiwara, E. Yamaguchi, Y. Sato and S. Yamaguchi, Fused Thiophene-S,S-dioxide-Based Super-Photostable Fluorescent Marker for Lipid Droplets, ACS Mater. Lett., 2021, 3, 42–49; (d) S. Tian, H. Ma, X. Wang, A. Lv, H. Shi, Y. Geng, J. Li, F. Liang, Z.-M. Su, Z. An and W. Huang, Utilizing d–pπ Bonds for Ultralong Organic Phosphorescence, Angew. Chem., Int. Ed., 2019, 58, 6645–6649; (e) J. Yang, X. Zhen, B. Wang, X. Gao, Z. Ren, J. Wang, Y. Xie, J. Li, Q. Peng, K. Pu and Z. Li, The Influence of the Molecular Packing on the Room Temperature Phosphorescence of Purely Organic Luminogens, Nat. Commun., 2018, 9, 840.
12. For recent examples, see. (a) K. Andoh, M. Murai, P.-A. Bouit, M. Hissler and S. Yamaguchi, Dithieno[3,2-b; 2’,3’-f]phosphepinium-Based Near-Infrared Fluorophores: px–π* Conjugation Inherent to Seven-Membered Phosphacycles, Angew. Chem., Int. Ed., 2024, 136, e202410204; (b) N. Inai, S. Yamaguchi and T. Yanai, Theoretical Insight into the Effect of Phosphorus Oxygenation on Nonradiative Decays: Comparative Analysis of P-Bridged Stilbene Analogs, ACS Phys. Chem. Au, 2023, 3, 540–552; (c) N. König, Y. Godínez-Loyola, H. Weiske, S. Naumov, P. Lönnecke, R. Tonner-Zech and C. A. Strassert, Evamarie Hey-Hawkins, Access to Strong Thieno[3,2-b]phosphole-Based Solid-State Emitters via Manganese(III)-Mediated Oxidative Annulation, Chem. Mater., 2023, 35, 8218–8228; (d) H. Ogasawara, Y. Tanaka, M. Taki and S. Yamaguchi, Late-Stage Functionalisation of Alkyne-Modified Phospha-Xanthene Dyes: Lysosomal Imaging Using an Off-On-Off Type of pH Probe, Chem. Sci., 2021, 12, 7902–7907; (e) M. A. Gonzalez, A. S. Walker, K. J. Cao, J. R. Lazzari-Dean, N. S. Settineri, E. J. Kong, R. H. Kramer and E. W. Miller, Voltage Imaging with a NIR-Absorbing Phosphine Oxide Rhodamine Voltage Reporter, J. Am. Chem. Soc., 2021, 143, 2304–2314; (f) M. Grzybowski, M. Taki, K. Kajiwara and S. Yamaguchi, Effects of Amino Group Substitution on the Photophysical Properties and Stability of Near-Infrared Fluorescent P-Rhodamines, Chem. – Eur. J., 2020, 26, 7912–7917.
13. For recent examples, see. (a) M. Murai, M. Abe, S. Ogi and S. Yamaguchi, Diazulenylmethyl Cations with a Silicon Bridge: A π-Extended Cationic Motif to Form J-Aggregates with Near-Infrared Absorption and Emission, J. Am. Chem. Soc., 2022, 144, 20385–20393; (b) J. B. Grimm, L. Xie, J. C. Casler, R. Patel, A. N. Tkachuk, N. Falco, H. Choi, J. Lippincott-Schwartz, T. A. Brown, B. S. Glick, Z. Liu and L. D. Lavis, A General Method to Improve Fluorophores Using Deuterated Auxochromes, JACS Au, 2021, 1, 690–696; (c) T. Tsuda, S.-M. Choi and R. Shintani, Palladium-Catalyzed Synthesis of Dibenzosilepin Derivatives via 1,n-Palladium Migration Coupled with anti-Carbopalladation of Alkyne, J. Am. Chem. Soc., 2021, 143, 1641–1650; (d) N. B. Alexey, Modular Synthetic Approach to Silicon-Rhodamine Homologues and Analogues via Bis-aryllanthanum Reagents, Org. Lett., 2021, 23, 2604–2609; (e) A. S. Moses, O. R. Taratula, H. Lee, F. Luo, T. Grenz, T. Korzun, A. St. Lorenz, F. Y. Sabei, S. Bracha, A. W. G. Alani, O. D. Slayden and O. Taratula, Nanoparticle-Based Platform for Activatable Fluorescence Imaging and Photothermal Ablation of Endometriosis, Small, 2020, 16, 1906936; (f) M. Pengshung, P. Neal, T. L. Atallah, J. Kwon, J. R. Caram, S. A. Lopez and E. M. Sletten, Silicon Incorporation in Polymethine Dyes, Chem. Commun., 2020, 56, 6110–6113.
14. (a) J. B. Grimm, A. N. Tkachuk, R. Patel, S. T. Hennigan, A. Gutu, P. Dong, V. Gandin, A. M. Osowski, K. L. Holland, Z. J. Liu, T. A. Brown and L. D. Lavis, Optimized Red-Absorbing Dyes for Imaging and Sensing, J. Am. Chem. Soc., 2023, 145, 23000–23013; (b) S. O. Oloo, G. Zhang, P. Bobadova-Parvanova, S. Al Horani, M. Al Horani, F. R. Fronczek, K. M. Smith, M. da Graça and H. Vicente, Synthesis and Regioselective Functionalization of Tetrafluorobenzo-[α]-Fused BOPYPY Dyes, Inorg. Chem., 2024, 63, 9164–9174; (c) Y. Hayashi, N. Suzuki, T. Maeda, H. Fujiwara and S. Yagi, Photophysical Properties of 4-(5-Methylthiophen-2-yl)pyridiniumcyclic Enolate Betaine Dyes Tuned by Control of Twisted Intramolecular Transfer, New J. Chem., 2021, 42, 9770–9779; (d) T. D. Moseev, M. V. Varaksin, D. A. Gorlov, V. N. Charushin and O. N. Chupakhin, Transition-Metal-Free C-H/C-Li Coupling of Nonaromatic 2H-Imidazole 1-Oxides with Pentafluorophenyl Lithium in the Design of Novel Fluorophores with Intramolecular Charge Transfer Effect, J. Org. Chem., 2020, 85, 11124–11133; (e) K. Santra, I. Geraskin, M. Nilsen-Hamilton, G. A. Kraus and J. W. Petrich, Characterization of the Photophysical Behavior of DFHBI Derivatives: Fluorogenic Molecules that Illuminate the Spinach RNA Aptamer, J. Phys. Chem. B, 2019, 123, 2536–2545; (f) R. III Chavez, L. Diodati, D. L. Wheeler, J. Shaw, A. L. Tomlinson and M. Jeffries-El, Evaluating the Impact of Fluorination on the Electro-optical Properties of Cross-Conjugated Benzobisoxazoles, J. Phys. Chem. A, 2019, 123, 1343–1352.
15. For recent examples, see. (a) Y. Chang, Y.-S. Wu, S.-H. Tung, W.-C. Chen, C.-C. Chueh and C.-L. Liu, ACS Appl. Mater. Interfaces, 2023, 15, 15745–15757; (b) K. Iguchi, T. Mikie, M. Saito, K. Komeyama, T. Seo, Y. Ie and I. Osaka, N-type Semiconducting Polymers Based on Dicyano Naphthobisthiadiazole: High Electron Mobility with Unfavorable Backbone Twist, Chem. Mater., 2021, 33, 2218–2228; (c) K. Ortstein, S. Hutsch, A. Hinderhofer, J. Vahland, M. Schwarze, S. Schellhammer, M. Hodas, T. Geiger, H. Kleemann, H. F. Bettinger, F. Schreiber, F. Ortmann and K. Leo, Energy Level Engineering in Organic Thin Films by Tailored Halogenation, Adv. Funct. Mater., 2020, 30, 2002987; (d) H. Wei, Y. Liu, Z. Liu, J. Guo, P.-A. Chen, X. Qiu, G. Dai, Y. Li, J. Yuan, L. Liao and Y. Hu, Effect of Backbone Fluorine and Chlorine Substitution on Charge-Transport Properties of Naphthalenediimide-Based Polymer, Semiconductors, Adv. Electron. Mater., 2020, 6, 1901241; (e) T. Hodsden, K. J. Thorley, J. Panidi, A. Basu, A. V. Marsh, H. Dai, A. J. P. White, C. Wang, W. Mitchell, F. Glöcklhofer, T. D. Anthopoulos and M. Heeney, Core Fluorination Enhances Solubility and Ambient Stability of an IDT-Based n-Type Semiconductor in Transistor Devices, Adv. Funct. Mater., 2020, 30, 2000325; (f) A. Y. Sosorev, V. A. Trukhanov, D. R. Maslennikov, O. V. Borshchev, R. A. Polyakov, M. S. Skorotetcky, N. M. Surin, M. S. Kazantsev, D. I. Dominskiy, V. A. Tafeenko, S. A. Ponomarenko and D. Y. Paraschuk, Fluorinated Thiophene-Phenylene Co-Oligomers for Optoelectronic Devices, ACS Appl. Mater. Interfaces, 2020, 12, 9507–9519; (g) K. Yamamoto, S. Kato, H. Zajaczkowska, T. Marszalek, P. W. M. Blom and Y. Ie, Effects of Fluorine Substitution in Quinoidal Oligothiophenes for Use as Organic Semiconductors, J. Mater. Chem. C, 2020, 8, 3580–3588.
16. For recent examples, see. (a) Z. Jia, S. Qin, L. Meng, Q. Ma, I. Angunawela, J. Zhang, X. Li, Y. He, W. Lai, N. Li, H. Ade, C. J. Brabec and Y. Li, High performance tandem organic solar cells via a strongly infrared-absorbing narrow bandgap acceptor, Nat. Commun., 2021, 12, 178; (b) H. Ji, J. Li, M. Du, J. Yang, A. Tang, G. Li, Q. Guo and E. Zhou, Fluorination of the Quinoxaline-Based p-Type Polymer and n-Type Small Molecule for High VOC Organic Solar Cells, J. Phys. Chem. C, 2021, 125, 10876–10882; (c) G.-U. Kim, C. Sun, J. S. Park, H. G. Lee, D. Lee, J.-W. Lee, H. J. Kim, S. Cho, Y.-H. Kim, S.-K. Kwon and B. J. Kim, Importance of Terminal Group Pairing of Polymer Donor and Small-Molecule Acceptor in Optimizing Blend Morphology and Voltage Loss of High-Performance Solar Cells, Adv. Funct. Mater., 2021, 31, 2100870; (d) Y. Firdaus, V. M. Le Corre, S. Karuthedath, W. Liu, A. Markina, W. Huang, S. Chattopadhyay, M. M. Nahid, M. I. Nugraha, Y. Lin, A. Seitkhan, A. Basu, W. Zhang, I. McCulloch, H. Ade, J. Labram, F. Laquai, D. Andrienko, L. J. A. Koster and T. D. Anthopoulos, Long-Range Exciton Diffusion in Molecular Non-Fullerene Acceptors, Nat. Commun., 2020, 11, 5220; (e) J. Yao, B. Qiu, Z.-G. Zhang, L. Xue, R. Wang, C. Zhang, S. Chen, Q. Zhou, C. Sun, C. Yang, M. Xiao, L. Meng and Y. Li, Cathode Engineering with Perylene-Diimide Interlayer Enabling over 17% Efficiency Single-Junction Organic Solar Cells, Nat. Commun., 2020, 11, 2726; (f) A. D. Fenta, S.-F. Liao, S.-W. Li, C.-F. Lu and C.-T. Chen, Increasing the Fluorine Substituent of Thieno[3,4-c]pyrrole-4,6-dione Terthiophene Copolymers Progressively Narrows the Nanofibrils and Enhances the Efficiency of Fullerene-Based Polymer Photovoltaics, Macromolecules, 2020, 53, 7073–7083; (g) Q. Nie, A. Tang, P. Cong, L. Chen, Q. Zhang, H. Ji, G. Li, Q. Guo and E. Zhou, Wide Band Gap Photovoltaic Polymer Based on Pyrrolo[3,4-f]benzotriazole-5,7-dione (TzBI) with Ultrahigh VOC Beyond 1.25 V, J. Phys. Chem. C, 2020, 124, 19492–19498; (h) P. Lei, B. Zhang, Y. Chen, Y. Geng, Q. Zeng, A. Tang and E. Zhou, Gradual Fluorination on the Phenyl Side Chains for Benzodithiophene-Based Linear Polymers to Improve the Photovoltaic Performance, ACS Appl. Mater. Interfaces, 2020, 12, 38451–38459; (i) S. Wen, Y. Li, N. Zheng, I. O. Raji, C. Yang and X. Bao, High-efficiency organic solar cells enabled by halogenation of polymers based on 2D conjugated benzobis(thiazole), J. Mater. Chem. A, 2020, 8, 13671–13678; (j) H. Jiang, X. Li, H. Wang, G. Huang, W. Chen, R. Zhang and R. Yang, Appropriate Molecular Interaction Enabling Perfect Balance Between Induced Crystallinity and Phase Separation for Efficient Photovoltaic Blends, ACS Appl. Mater. Interfaces, 2020, 12, 26286–26292; (k) Y. Wang, H. Zhong, Y. Hong, T. Shan, K. Ding, L. Zhu, F. Liu, H. Wei, C. Yu and H. Zhong, Tailoring the Molecular Geometry of Polyfluoride Perylene Diimide Acceptors towards Efficient Organic Solar Cells, J. Mater. Chem. C, 2020, 8, 8224–8233; (l) T. Liu, Y. Zhang, Y. Shao, R. Ma, Z. Luo, Y. Xiao, T. Yang, X. Lu, Z. Yuan, H. Yan, Y. Chen and Y. Li, Asymmetric Acceptors with Fluorine and Chlorine Substitution for Organic Solar Cells toward 16.83% Efficiency, Adv. Funct. Mater., 2020, 30, 2000456.
17. For recent examples, see. (a) R. Kani, Y. Miwa, Y. Kubota, T. Inuzuka, S. Kutsumizu and K. Funabiki, Rapid and Dual Optical CO₂-Responsive Polydimethylsiloxane Elastomer with Fluorinated Cyanine Dye, Chem. – Asian J., 2023, 18, e20230170; (b) R. Kani, Y. Kubota, T. Inuzuka and K. Funabiki, Aromatic Fluorine Atom-Induced Highly Amine-Sensitive Trimethine Cyanine Dye Showing Colorimetric and Ratiometric Fluorescence Change, RSC Adv., 2022, 12, 25587–25592.
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Footnote

† Electronic supplementary information (ESI) available. CCDC 1987380 and 1987381. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ma00962b

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