Triarylmethane dye ethynologue with a fused julolidine motif as a compact dye in the near infrared range

Abstract

Near-infrared (NIR) dyes have garnered significant attention for applications in bioimaging and optoelectronics due to their low phototoxicity and excellent tissue penetration. However, many conventional NIR dyes rely on extended π-conjugated systems and require complex multi-step syntheses. In this study, we report the development of compact NIR-absorbing dyes based on triarylmethane dye ethynologues. A series of compounds was synthesized by incorporating a julolidine unit as a strong electron-donating group and modifying either electron-donating or electron-withdrawing substituents on the aromatic rings adjacent to the carbocation center. The julolidine moiety effectively red-shifted the absorption into the NIR region, while electron-donating groups enhanced photostability and resistance under acidic conditions. Nevertheless, the presence of alkyne linkages makes the dyes susceptible to degradation under basic conditions, as well as to oxidation and thermal decomposition, limiting their long-term durability. Ongoing efforts focus on overcoming these drawbacks to advance the practical applications of these dyes in photocatalysis, photonic devices, and solar energy conversion systems. This work offers a promising approach toward the sustainable design of compact, efficient, and synthetically accessible NIR dyes.

---

1. Introduction

Organic dyes, such as coumarin,¹ azo,² rhodamine,³ cyanine,^4,5 polymethin,⁶ and squarylium dyes,^7,8 are not only used as colourants but also as functional materials in a wide range of applications, including fluorescent probes for bio-imaging, optical switches based on structural isomerization, non-linear optical devices, and dye-sensitized solar cells. The absorption wavelengths (λ_abs) of these compounds can be varied by tuning the electronic properties of the entire molecular structure. For example, replacing the dialkylamino moiety in the electron-donor structure with a fused julolidine skeleton suppresses rotation around the C–N bond, leading to red-shifted absorption as a result of increased molecular planarity and enhanced electron-donating ability (Fig. 1).^9–14

Fig. 1 Chemical structures of previously reported dyes containing julolidine skeletons.

Among various organic dyes, near-infrared (NIR) dyes that absorb light in the NIR region have attracted increasing interest in recent years owing to their low phototoxicity and excellent tissue permeability. Many new NIR absorbers have been developed to date, such as carbon nanotubes,^15,16 quantum dots,^17,18 and rare-earth-containing nanoparticles.^19,20 In addition, NIR dyes made from pure organic compounds, such as squarylium, polymethine, cyanine, rhodamine^21,22 and borondipyrromethene derivatives^23–25 have also been discovered. However, these dyes consist of extended π-conjugated structures and require multi-step synthesis, resulting in the generation of substantial chemical waste. Therefore, there is an urgent demand for the development of facile synthetic processes to produce NIR dyes with compact π-conjugated structures.

Crystal violet and malachite green are typical triarylmethane dyes, exhibiting violet and blue-green coloration, respectively (Fig. 2a).²⁶ They possess a unique and relatively compact molecular structure with an electron-donating terminal group and a central carbocation-type electron-deficient site. Moreover, triarylmethane dye ethynologues, with a carbon–carbon triple bond (C≡C) between the aromatic ring and the carbocation site, can further red-shift the absorption wavelength (λ_abs) to up to 688 nm due to the elongation of the π-conjugation (Fig. 2b).^27,28 However, since their first report by Nakatsuji et al. in 1988, no further research on the triarylmethane dye ethynologues has been conducted.

Fig. 2 Chemical structures of (a) triarylmethane dyes, and (b) their ethynologues. (c) Molecular design of the compact NIR organic dye candidate proposed in this study.

Aiming to develop NIR dyes with compact molecular structures, our research group investigated the synthesis and photophysical properties of various triarylmethane dye ethynologue derivatives 1a–d. These compounds contain an electron-accepting unit (a diarylmethyl cation with electronic properties controlled by substituents R¹ and R² on the aromatic ring) and an electron-donating julolidine skeleton. In this study, we present a detailed discussion of the molecular design based on quantum chemical calculations, as well as their synthesis and optical properties.

2. Results and discussion

2.1. Molecular design and synthesis

First, the molecular geometries of compounds 1a–d were optimized based on density functional theory (DFT) calculations, and the vertical transition process was investigated using the time-dependent (TD)-DFT method. Two electronic transitions with relatively large oscillator strength (f) were obtained for each compound (Fig. 3 and Fig. S19–S22 in ESI†). The one with a larger f was an electronic transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), while the other was an electronic transition from HOMO−1 to LUMO, except for 1a (which was a transition from HOMO−2 to LUMO). Both HOMO and LUMO lobes were localized throughout the molecule, which means that the HOMO → LUMO transition is a result of π–π* transition. At the same time, the electronic transition from an orbital lower than the HOMO, viz., HOMO−1 or HOMO−2, to the LUMO is also attributed to the π–π* transition involving two aromatic rings attached to the carbocation. The absorption wavelengths corresponding to the HOMO → LUMO transition, as determined from the TD-DFT calculation, were 586 nm for 1aA, 616 nm for 1bA, 626 nm for 1cA, and 618 nm for 1dA, indicating that these absorptions are close to the NIR wavelengths. As a control for 1cA, TD-DFT calculations were performed for a molecular structure with an N,N-dimethylaniline skeleton instead of the julolidine moiety. The theoretical absorption wavelength was 601 nm (Fig. S26, ESI†). The initial quantum chemical calculations showed that the triarylmethane dye ethynologue, which has a julolidine donor unit, could be a suitable candidate for the NIR dye.

Fig. 3 Theoretical electronic transition process for 1a (left) and 1c (right), as determined using quantum chemical calculations at the level of M06-2X/6-31+G(d)//M06-2X/6-311++G(d,p) basis set (CPCM, solvent = CH₂Cl₂).

Therefore, we synthesized 1a–d to elucidate their efficacy as NIR dyes (Scheme 1a). 9-Ethynyljulolidine, prepared in three steps from julolidine, was treated with n-butyllithium in tetrahydrofuran (THF) at −78 °C for 0.5 h, followed by the addition of benzophenone. The resulting solution was stirred overnight at room temperature to obtain the corresponding ethynylcarbinol 2a in 58% yield as a brown solid. Various substituted benzophenones were used as substrates under the same reaction conditions, and the corresponding ethynylcarbinols 2b–d were obtained in 34–83% yield as a brown solid. To the resulting ether solution of 2a, an aqueous solution of HClO₄ (1.1 equiv.) was slowly added at −78 °C and stirred for 5 min at room temperature. The resulting precipitate was separated by fast filtration to obtain the corresponding triarylmethane dye ethynologue 1a in 52% yield as a dark blue solid. Using a similar protocol, the other derivatives 1b–d were obtained in 44–68% yield, also as dark blue solids. As a control compound, triarylmethane dye A, in which the C≡C moiety is removed from ethynologue 1a with a julolidine skeleton, was also synthesized in two steps starting from 9-bromojulolidine (Scheme 1b). For all derivatives, the formation of carbocations before and after the dehydration process was confirmed by NMR analysis (Fig. S12–S17, ESI†). As an example, Fig. 4 shows the ¹H NMR spectra of precursor 2a before dehydration and ethynologue 1a after dehydration. After the dehydration, the proton signal “h” derived from the hydroxy group of precursor 2a disappeared. In contrast, all proton signals associated with the julolidine skeleton and other aromatic rings demonstrated a slight shift to the higher magnetic field due to the resonance effect. This result confirms the successful formation of carbocations after the dehydration reaction.

Scheme 1 Synthesis procedure for (a) novel triarylmethane dye ethynologues 1a–d and (b) triarylmethane dye A containing a julolidine skeleton.

Fig. 4 NMR analysis for 2a and 1a to confirm the formation of carbocation in the dehydration process.

2.2. Photophysical property

Next, the absorption behaviors of precursor carbinol 2a–d and the corresponding carbocation 1a–d (Fig. 5 and Table 1) were investigated in CH₂Cl₂ solutions (1.0 × 10⁻⁵ mol L⁻¹).

Fig. 5 Absorption spectra in the CH₂Cl₂ solution of (a) 1a/2a, (b) 1b/2b, (c) 1c/2c, and (d) 1d/2d, and (e) A (concentration: 1.0 × 10⁻⁵ mol L⁻¹). Inset: Photographs of the solution under daylight.

Table 1 Photophysical data of 1a–d and 2a–d in CH₂Cl₂ solution

Compound	λ abs [nm] (ε [10^3, L mol^−1 cm^−1])	Compound	λ abs [nm] (ε [10^3, L mol^−1 cm^−1])
a Concentration: 1.0 × 10^−5 mol L^−1.
2a	316 (25.9)	1a	298 (10.8), 383 (8.8), 411 (7.4), 624 (31.8), 689 (26.3)
2b	315 (28.4)	1b	299 (14.2), 392 (17.1), 438 (21.9), 669 (58.6), 740 (66.4)
2c	313 (26.6)	1c	320 (7.2), 412 (8.2), 453 (18.4), 688 (31.6), 756 (52.5)
2d	319 (25.0)	1d	333 (6.4), 418 (15.8), 440 (16.4), 662 (12.7), 726 (11.8)
		A	271 (17.8), 347 (12.3), 473 (56.8)

---

The absorption spectrum of the control compound A was also measured under the same conditions. The CH₂Cl₂ solution of ethynylcarbinol 2a was transparent with a maximum absorption wavelength (λ_abs) of approximately 316 nm (Fig. 5a, dashed line). The triarylmethane dye ethynologue 1a in CH₂Cl₂ was dark blue. In addition, it exhibited several absorption bands between 298 and 411 nm, with the main absorption bands at approximately 624 and 689 nm (Fig. 5a, solid line). Considering the TD-DFT calculation results described above, the absorption band on the long-wavelength side can be attributed to π–π* transition involving the HOMO → LUMO transition, while that on the short-wavelength side is attributed to π–π* transition involving the HOMO−2 → LUMO transition.

The CH₂Cl₂ solutions of 2b (with one methoxy group at the 4-position of the aromatic ring attached to the sp³-hybridized carbon of carbinol), 2c (with two methoxy groups at the 4,4′-position), and 2d (with one methoxy group and one trifluoromethyl group at the 4,4′-position) were transparent with λ_abs in the UV region (313–319 nm) (Fig. 5b and c, dashed line). Contrary to the λ_abs of 2a, the introduction of an electron-donating or electron-withdrawing group on the benzene ring had little effect on its λ_abs; all 2b–d showed similar λ_abs. The absorption studies of ethynologues 1b–d in CH₂Cl₂ solution demonstrated two absorption bands: one on the long-wavelength side involving the HOMO → LUMO transition and the other on the short-wavelength side involving the HOMO−1 → LUMO transition, similar to 1a. All absorption bands of 1b–d caused a redshift of λ_abs compared to that of 1a. In particular, all λ_abs in the longer-wavelength side were in the NIR region of above 700 nm (approximately 740 nm for 1b, 756 nm for 1c, and 726 nm for 1d). Based on the TD-DFT calculations, the introduction of a methoxy group destabilizes the HOMO energy level through antibonding orbital interactions, resulting in a redshift of λ_abs. As shown in Fig. 5e, a CH₂Cl₂ solution of triarylmethane dye A containing a julolidine skeleton exhibited a single absorption band with λ_abs at approximately 473 nm and appeared yellow. This experimental result shows that the C≡C moiety in the ethynologue structure is effective in inducing a significant red shift in the absorption wavelength.

Furthermore, the solvent effect on the absorption behavior of 1c was explored, in which two 4-methoxyphenyl moieties are bonded to a carbocation. In addition to the aforementioned medium-polarity CH₂Cl₂ (E_T30 = 40.7)²⁹ and low-polarity toluene (E_T30 = 40.7), acetonitrile (E_T30 = 45.6) was used as a high-polarity solvent in this study.

As shown in Fig. 6, the λ_abs of the absorption band attributed to the π–π* transition (HOMO → LUMO) was observed at 758 nm in low-polarity toluene, 756 nm in medium-polarity CH₂Cl₂, and 745 nm in high-polarity acetonitrile. As the solvent polarity increases, λ_abs exhibits a hypsochromic shift, indicating that the triarylmethane dye ethynologue shows a negative solvatochromic effect. This behavior suggests that increased solvent polarity preferentially stabilizes the ground state, which is consistent with the ionic nature of the ethynologue molecule.

Fig. 6 Absorption spectra of 1c in different solvents (toluene: blue, CH₂Cl₂: green, and MeCN: red line). Inset: The normalized absorption spectra.

Next, the photoluminescence (PL) properties of the triarylmethane dye ethynologues 1a–d in CH₂Cl₂ solution were evaluated. Upon excitation at their respective absorption maxima (λ_abs), 624 nm for 1a, 669 nm for 1b, 688 nm for 1c, and 662 nm for 1d, PL bands were observed at 732 and 857 nm for 1a, 730 and 854 nm for 1b, 718 and 856 nm for 1c, and 728 and 852 nm for 1d (Fig. S18, ESI†). However, the PL intensities were very weak, and the absolute quantum yield (Φ_PL) was below 0.01 in all cases. This is likely due to self-absorption arising from the narrow Stokes shift. Another possible explanation is non-radiative deactivation via twisted intramolecular charge transfer (TICT) excited states, as these ethynologues possess D–π–A character and exhibit large dipole moments up to 15.2 D.^30–32

2.3. Stability evaluation

To assess photostability, the CH₂Cl₂ solutions of ethynologues 1a–d were irradiated with red LED light (λ = 640 nm, 20 W) for various durations (0.5, 1.0, 2.0, and 16 hours), and their absorption spectra were recorded.

Upon 0.5 hour irradiation, the absorbance of compound 1a at approximately 624 nm decreased slightly, and prolonged exposure led to further decline. Simultaneously, a new absorption band emerged at approximately 493 nm, and an isosbestic point appeared at approximately 540 nm, indicating a gradual structural change under red-light irradiation (Fig. 7a). Compounds 1b and 1c, which bear one and two methoxy groups, respectively, showed negligible changes in absorption after short-term irradiation, though a slight decrease was observed after 16 hours (Fig. 7b and c). In contrast, compound 1d, containing both electron-donating and electron-withdrawing substituents, exhibited a marked decrease in absorbance after 16 hours (Fig. 7d). Furthermore, compound A, which lacks the C≡C moiety present in ethynologue 1a, showed no change in absorption even after 16 hours of irradiation with green LED light (λ = 525 nm, 20 W), demonstrating high photostability. These results suggest that although the C≡C unit reduces photostability, introducing electron-donating groups into the aromatic rings attached to the carbocation center can mitigate the instability.

Fig. 7 UV-vis absorption spectra of CH₂Cl₂ solutions (1.0 × 10⁻⁵ mol L⁻¹ concentration) of (a) 1a, (b) 1b, (c) 1c, (d) 1d, and (e) A at each LED light irradiation time (0 h, 0.5 h, 1.0 h, 2.0 h, and 16 h).

In addition, the absorption characteristics of compound 1c were studied under acidic and basic conditions (Fig. 8). When an Et₃N/CH₂Cl₂ solution at approximately 10 times the concentration of the original 1c solution was tested, no significant change in the absorption spectrum was observed, despite a slight decrease in absorbance. Increasing the Et₃N concentration up to 100-fold led to a pronounced decrease in the absorbance in long-wavelength region. In contrast, the addition of trifluoroacetic acid (TFA)/CH₂Cl₂ at concentrations up to 10 000 times higher resulted in only a slight decrease in absorbance. These results suggest that the ethynologue derivatives are relatively stable under acidic conditions but degrade under basic conditions. One possible mechanism involves nucleophilic attack on the C≡C unit, facilitated by the electron-withdrawing effect of the adjacent carbocation; however, this mechanism remains speculative at present.

Fig. 8 UV-vis absorption spectra of CH₂Cl₂ solution of ethynologue 1c (concentration 1.0 × 10⁻⁵ mol L⁻¹) under (a) basic conditions using Et₃N or (b) acidic conditions using CF₃COOH (TFA).

Subsequently, the thermal stability of the triarylmethane dye ethynologues 1a–d was evaluated by thermogravimetric analysis (Fig. 9).

Fig. 9 Thermogravimetric analysis thermograms of 1a–d and reference compound A. Heating rate: 10 °C min⁻¹.

The 5% weight loss temperature was used as the decomposition temperature (T^5%_d). The T^5%_d values of 1a–d were 127, 113, 138, and 107 °C, respectively, indicating that all the derivatives have poor thermal stability. To clarify the relationship between the molecular structure and thermal stability, the T^5%_d values were evaluated for derivative A, which does not have an alkyne structure, and derivative B, in which a 4-methoxyphenyl group was introduced instead of a julolidine skeleton. The T^5%_d of A was 223 °C, which is approximately 1.8 times higher than that of 1a, indicating that A is more stable than 1a. In the synthesis of B using 4-methoxyphenyl as the electron-donating group, precursor B–OH was successfully obtained as a white solid (Scheme 2). However, the treatment of B–OH with an aqueous solution of HClO₄ did not result in the formation of B. This is due to the weak electron-donating ability of the methoxy group in the corresponding ethynologue B, which is insufficient to stabilize the carbocation. These results suggest that the alkyne unit in the ethynologues can reduce thermal stability, likely due to its susceptibility to oxidation or hydrolysis.²⁴

Scheme 2 Chemical structures of control compounds A and B, and the synthetic pathway for compound B.

3. Conclusions

In this study, triarylmethane dye ethynologues incorporating a julolidine moiety as an electron-donating group were synthesized, and their various properties were evaluated. The introduction of the julolidine unit resulted in a red shift in absorption. Furthermore, the incorporation of one or more electron-donating groups into the aromatic ring attached to the carbocation yielded NIR dyes with relatively compact molecular structures. These electron-donating substituents also enhanced photostability compared to the unsubstituted derivative. pH stability tests revealed that the ethynologue derivatives were stable under acidic conditions but underwent rapid fading under basic conditions, likely due to decomposition. In addition, the alkyne moiety was found to be susceptible to oxidation and hydrolysis, contributing to reduced thermal stability. Therefore, enhancing photostability, thermal stability, and pH stability is essential for the practical applications of these compounds in photocatalysis, photonic devices, and solar cells. Our group is currently pursuing the development of compact NIR dyes that address these stability challenges.

4. Experimental

4.1. General method

¹H and ¹³C NMR spectra were obtained with a Bruker AVANCE III 400 NMR spectrometer (¹H: 400 MHz and ¹³C: 100 MHz) in chloroform-d (CDCl₃) or acetone-d₆ solution, and the chemical shifts are reported in parts per million (ppm) using the residual proton in the NMR solvent (CHCl₃: 7.26 ppm for ¹H and 77 ppm for ¹³C NMR or acetone: 2.04 ppm for ¹H and 205.87 ppm for ¹³C NMR). ¹⁹F NMR (376 MHz) spectra were obtained with a Bruker AVANCE III 400 NMR spectrometer in CDCl₃ or acetone-d₆ with C₆F₆ (δ_F = −163 ppm) as an internal standard. Infrared spectra (IR) were recorded in a KBr method with a JASCO FT/IR-4100 type A spectrometer; all spectra were reported in wavenumber (cm⁻¹). High resolution mass spectra (HRMS) were recorded on a JEOL JMS-700MS spectrometer using fast atom bombardment (FAB) methods. Elemental analysis of 1a–d was performed using a CHN Corder MT-5 device (Yanaco).

All reactions were conducted using dried glassware with magnetic stirrer bars in an Ar atmosphere. Column chromatography was performed using silica gel (Wakogel® 60 N, 38–100 μm), and thin-layer chromatography (TLC) was conducted using silica gel TLC plates (silica gel 60F₂₅₄, Merck, Darmstadt, Germany). Detailed synthetic procedures for the main products 1a–d were described below, and those for other compounds were provided in ESI.†

4.2. Synthesis of 1,1-diphenyl-3-(julolidin-9-yl)prop-2-yn-1-ium perchlorate (1a)

1,1-Diphenyl-3-(julolidin-9-yl)prop-2-yn-1-ol (2a, 0.382 g, 1.01 mmol) and Et₂O (10 mL) were added to a two-necked round-bottomed flask with a Teflon®-coated stirring rod and cooled to −78 °C. Perchloric acid dissolved in Et₂O (10 mL) was slowly added to the solution at −78 °C and stirred at room temperature for 5 min. After 5 min, precipitate formed during the reaction was separated by suction filtration to obtain the desired 1,1-diphenyl-3-(julolidin-9-yl)prop-2-yn-1-ium perchlorate 1a in 52% yield (0.242 g, 0.524 mmol) as a purple solid.

4.3. 1,1-Diphenyl-3-(julolin-9-yl)prop-2-yn-1-ium perchlorate (1a)

Yield: 52% (purple solid); M.p. = 148–155 °C; ¹H NMR (acetone-d₆): δ 2.14 (quin, J = 6.0 Hz, 4H), 2.92 (t, J = 6.0 Hz, 4H), 3.99 (t, J = 5.6 Hz, 4H), 7.53 (s, 2H), 7.57–7.70 (m, 6H), 7.80–7.84 (m, 4H) ppm; IR (KBr); ν 3501, 3025, 2947, 2853, 2049, 1618, 1560, 1450, 1239, 1127, 923, 878 cm⁻¹; HRMS (FAB) m/z [M⁺] calcd for C₂₇H₂₄N: 362.1903, found: 362.1903; anal. calcd for C₂₇H₂₄ClNO₄: C, 70.20; H, 5.24; N, 3.03. Found: C, 67.80; H, 5.44, N, 2.95. Due to the poor solubility of this compound in solvents, no clear ¹³C NMR signal was observed.

4.4. 1-(4-Methoxyphenyl)-1-phenyl-3-(julolidin-9-yl)prop-2-yn-1-ium perchlorate (1b)

Yield: 68% (blue solid); M.p. = 173–175 °C; ¹H NMR (acetone-d₆): δ 2.12 (quin, J = 6.0 Hz, 4H), 2.90 (m, 4H), 3.92 (t, J = 5.6 Hz, 4H), 3.98 (s, 3H), 7.18 (d, J = 8.8 Hz, 2H), 7.59–7.72 (m, 3H), 7.78–7.83 (m, 2H), 7.89 (d, J = 8.8 Hz, 2H) ppm; IR (KBr); ν 3494, 3052, 2956, 2844, 2054, 1594, 1430, 1246, 1123, 1017, 954, 920, 838 cm⁻¹; HRMS (FAB) m/z [M⁺] calcd for C₂₈H₂₆NO: 392.2009, found: 392.2011; anal. calcd for C₂₈H₂₆ClNO₅: C, 68.36; H, 5.33; N, 2.85. Found: C, 65.23; H, 5.50, N, 2.68. Due to the poor solubility of this compound in solvents, no clear ¹³C NMR signal was observed.

4.5. 1,4-Bis(4-methoxyphenyl)-3-(julolidin-9-yl)ylprop-2-yn-1-ium perchlorate (1c)

Yield: 60% (blue solid); M.p. = 179–182 °C; ¹H NMR (acetone-d₆): δ 2.09 (quin, J = 6.0 Hz, 4H), 2.88 (t, J = 6.0 Hz, 4H), 3.85 (t, J = 5.6 Hz, 4H), 7.19 (d, J = 8.8 Hz, 4H), 7.48 (s, 2H), 7.88 (d, J = 8.8 Hz, 4H) ppm; IR (KBr); ν 33 461, 3011, 2941, 2841, 2070, 1587, 1508, 1311, 1239, 1158, 960, 901, 839 cm⁻¹; HRMS (FAB) m/z [M⁺] calcd for C₂₉H₂₈NO₂: 422.2115, found: 422.2118; anal. calcd for C₂₉H₂₈ClNO₆: C, 66.73; H, 5.41; N, 2.68. Found: C, 61.78; H, 5.17, N, 2.60. Due to the poor solubility of this compound in solvents, no clear ¹³C NMR signal was observed.

4.6. 1-(4-Methoxyphenyl)-1-(4-(trifluoromethyl)phenyl)-3-(julolidin-9-yl)prop-2-yn-1-ium perchlorate (1d)

Yield: 44% (blue solid); M.p. = 179–182 °C; ¹H NMR (acetone-d₆): δ 2.11–2.18 (m, 4H), 2.89–2.95 (m, 4H), 3.95–4.00 (m, 7H), 7.17 (d, J = 8.8 Hz, 2H), 7.47 (s, 1H), 7.59 (s, 1H), 7.88 (d, J = 8.8 Hz, 2H), 7.92 (d, J = 8.8 Hz, 2H), 7.99 (d, J = 8.8 Hz, 2H) ppm; ¹⁹F NMR (acetone-d₆): δ −61.76 (s, 3F) ppm; IR (KBr); ν 3471, 3031, 3004, 2956, 2843, 2054, 1590, 1510, 1328, 1248, 1107, 921, 842 cm⁻¹; HRMS (FAB) m/z [M⁺] calcd for C₂₉H₂₅F₃NO: 460.1883, found: 460.1879; anal. calcd for C₂₉H₂₅ClF₃NO₅: C, 62.20; H, 4.50; N, 2.50. Found: C, 58.52; H, 5.05, N, 3.22. Due to the poor solubility of this compound in solvents, no clear ¹³C NMR signal was observed.

4.7. Julolidin-9-yldiphenylmethylium perchlorate (A)

Yield: 39% (red solid); ¹H NMR (acetone-d₆): δ 2.12 (quin, J = 6.0 Hz, 4H), 2.85 (t, J = 6.8 Hz, 4H), 3.98 (t, J = 5.6 Hz, 4H), 7.30–7.35 (m, 5H), 7.54–7.60 (m, 3H), 7.64–7.69 (m, 2H) ppm; IR (KBr); ν 3058, 3021, 2950, 2830, 1607, 1580, 1492, 1446, 1301, 1203, 1145, 1029, 981, 936, 867 cm⁻¹; HRMS (FAB) m/z [M⁺] calcd for C₂₅H₂₄N: 338.1903, found: 338.1912. Due to the poor solubility of this compound in solvents, no clear ¹³C NMR signal was observed.

4.8. Absorption and photoluminescence measurements

Ultraviolet-visible (UV-vis) absorption spectroscopy was conducted using a V-750 absorption spectrometer (JASCO). Absorption spectra were obtained by the transmission method using a solution with a concentration of 1.0 × 10⁻⁵ mol L⁻¹. PL spectra in the solution were acquired using an RF-6000 spectrofluorophotometer (Shimadzu, Kyoto, Japan). The absolute quantum yields of the solution were measured using a Quantaurus-QY C11347-01 absolute PL quantum yield spectrometer (Hamamatsu Photonics, Hamamatsu, Japan).

4.9. Photostability assessment

The UV-vis absorption spectrum was measured after irradiating a dye sample (1.0 × 10⁻⁵ mol L⁻¹) in a CH₂Cl₂ solution with red LED light (λ = 620 nm, 20 W, Kessil LED) or green LED light (λ = 525 nm, 20 W, Kessil LED) for a specified time (0.5 h, 1.0 h, 2.0 h, and 16 h).

4.10. pH stability

Dye sample 1c was mixed with Et₃N or TFA solutions of various concentrations to prepare a 1.0 × 10⁻⁵ mol L⁻¹ CH₂Cl₂ solution, and the UV-vis absorption spectrum was measured.

4.11. Thermal stability assessment

Thermal decomposition temperature (T^5%_d), defined as the temperature at which the weight decreases by 5%, was measured by thermogravimetric analysis (TGA) using TGA-50 instrument (Shimadzu, Kyoto, Japan).

4.12. Quantum chemical calculation

Density functional theory (DFT) calculations were performed using the Gaussian 16 (Rev. B.01) program set.³³ Geometry optimizations were conducted at the M06-2X/6-31+G(d) level of theory,³⁴ with the CH₂Cl₂ solvation effects considered using the conductor-like polarizable continuum model (CPCM).³⁵ Time-dependent DFT (TD-DFT) calculations were performed using the same level of theory to evaluate vertical electronic transitions.³⁶

Author contributions

Conceptualization: S. Y.; investigation: H. K., K. K., and S. Y.; methodology: H. K., K. K., and S. Y.; data curation: H. K., and S. Y.; project administration: S. Y.; software: K. K.; resources: M. Y., T. K., and S. Y.; supervision: S. Y.; validation: S. Y.; visualization: S. Y.; funding: S. Y.; writing – original draft: H. K., K. K., and S. Y.; writing – review and editing: H. K., K. K., M. Y., T. K., and S. Y.

Conflicts of interest

There are no conflicts to declare.

Data availability

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

Acknowledgements

This research was partially funded by the Iketani Science and Technology Foundation.

References

1. J. Zhang, M. Chen, C. Bai, R. Qiao, B. Wei, L. Zhang, R. Li and C. Qu, Front. Chem., 2020, 8, 800.
2. S. Oh, J. Baek, S. Kim and T. Yoon, RSC Adv., 2017, 7, 19497–19501.
3. S. Pandey, D. L. Arachchige, R. J. Schwandt, S. K. Dwivedi, I. Kathuria, H. Liu and R. L. Luck, J. Mater. Chem. B, 2025, 13, 7865–7881.
4. H. Huang, M. Li, J. Gu, S. Roy, J. Jin, T. Kuang, Y. Zhang, G. Hu and B. Guo, J. Nanobiotechnol., 2024, 22, 788.
5. N. Sellet, J. Frey, M. Cormier and J. P. Goddard, Chem. Sci., 2024, 15, 8639–8650.
6. J. M. Hales, J. Matichak, S. Barlow, S. Ohira, K. Yesudas, J. Brédas, J. W. Perry and S. R. Marder, Science, 2010, 327, 1485.
7. S. A. Al-horaibi, A. Al-Odayni, A. Alezzy, M. ALSaeedy, A. Al-Adhreai, W. S. Saeed and A. Hasan, J. Mol. Struct., 2023, 1290, 135943.
8. T. Maeda, Y. Hamamura, K. Miyanaga, N. Shima, S. Yagi and H. Nakazumi, Org. Lett., 2011, 13, 5994–5997.
9. P. Zhang, W. Liu, G. Niu, H. Xiao, M. Wang, J. Ge, J. Wu, H. Zhang, Y. Li and P. Wang, J. Org. Chem., 2017, 82, 3456–3462.
10. S. Yamada, J. Bessho, H. Nakasato and O. Tsutsumi, Dyes Pigm., 2018, 150, 89–96.
11. J. B. Grimm, A. N. Tkachuk, R. Patel, S. T. Hennigan, A. Gutu, P. Dong, V. Gandin, A. M. Osowki, K. L. Holland, Z. J. Liu, T. A. Brown and L. D. Lavis, J. Am. Chem. Soc., 2023, 145, 23000–23013.
12. B. H. Kim, S. W. Park, D. Lee, I. K. Kwon and J. P. Kim, Bull. Korean Chem. Soc., 2014, 35, 2453–2459.
13. F. A. Mikhailenko and L. V. Balina, Chem. Heterocycl. Compd., 1982, 18, 334–336.
14. K. A. Bello and J. O. Ajayi, Dyes Pigm., 1996, 31, 79–87.
15. M. Yoon, Y. Lee, S. Lee, Y. Cho, D. Koh, S. Shin, C. Tian, Y. Song, J. Kang and S. Y. Cho, Sens. Diagn., 2024, 3, 203–217.
16. B. Oruc and H. Unal, ACS Omega, 2019, 4, 5556–5564.
17. H. M. Gil, T. W. Price, K. Chelani, J. G. Bouillard, S. D. J. Calaminus and G. J. Stasiuk, iScience, 2021, 24, 102189.
18. Y. Ma, Y. Zhang and W. W. Yu, J. Mater. Chem. C, 2019, 7, 13662–13679.
19. A. Skripka, D. Mendez-Gonzalez, R. Marin, E. Ximendes, B. del Rosal, D. Jaque and P. Rodríguez-Sevilla, Nanoscale Adv., 2021, 3, 6310–6329.
20. J. Meng, Y. Cui and Y. Wang, ACS Appl. Nano Mater., 2024, 7, 11008–11018.
21. M. Dai, Y. J. Yang, S. Sarkar and K. H. Ahn, Chem. Soc. Rev., 2023, 52, 6344–6358.
22. Y. Sun, J. Liu, X. Lv, Y. Liu, Y. Zhao and W. Guo, Angew. Chem., Int. Ed., 2012, 51, 76354.
23. M. Nakamura, H. Tahara, K. Takahashi, T. Nagata, H. Uoyama, D. Kazuhara, S. Mori, T. Okujima, H. Yamada and H. Uno, Org. Biomol. Chem., 2012, 10, 6840–6849.
24. Y. Kage, H. Karasaki, S. Mori, H. Furuta and S. Shimizu, Chem. – Eur. J., 2019, 84, 1648–1652.
25. T. Rappitsch and S. M. Borisov, Chem. – Eur. J., 2021, 27, 10685–10692.
26. X. Zhang, M. Liu, Y. Hu, X. Wang, R. Wei, C. Yao, C. Shi, Y. Qium, T. Yang, X. Luo, J. Chen, W. Sun, H. Chen, X. Qian and Y. Yang, Adv. Mater., 2025, 37, 2411515.
27. S. Akiyama, S. Nakatsuji, K. Nakashima, M. Watanabe and H. Nakazumi, J. Chem. Soc. Perkin Trans. 1, 1988, 3155–3161.
28. S. G. R. Guinot, J. D. Hepworth and M. Wainwright, Dyes Pigm., 2000, 47, 129–142.
29. C. Reichardt, Chem. Rev., 1994, 94, 2319–2358.
30. K. Kobayashi, S. Yamada, M. Morita, T. Sakurai, M. Yasui and T. Konno, J. Mater. Chem. C, 2025, 14, 1369–1377.
31. C. Wang, W. Chi, Q. Qiao, D. Tan, Z. Xu and X. Liu, Chem. Soc. Rev., 2021, 50, 12656–12678.
32. S. Sasaki, G. P. C. Drummen and G. Konishi, J. Mater. Chem. C, 2016, 4, 2731–2743.
33. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson and H. Nakatsuji, et al.Gaussian16, Revision B.01, Gaussian Incorporated, Wallingford, CT, USA, 2016.
34. M. Walker, A. J. A. Harvey, A. Sen and C. E. H. Dessent, J. Phys. Chem. A, 2013, 117, 12590–12600.
35. H. Li and J. H. Jensen, J. Comput. Chem., 2004, 25, 1449–1462.
36. R. B. Gerber, V. Buch and M. A. Ratner, J. Chem. Phys., 1982, 77, 3022–3030.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details, NMR spectra for 1a–d, DFT calculation details, Cartesian coordinates, and optional data for absorption behaviour. See DOI: https://doi.org/10.1039/d5nj01519g

---