Bodipy Based Iridium and Ruthenium Complexes: Synthesis, Photophysical and Photochemical Properties

doi:10.21203/rs.3.rs-3977995/v1

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Bodipy Based Iridium and Ruthenium Complexes: Synthesis, Photophysical and Photochemical Properties

https://doi.org/10.21203/rs.3.rs-3977995/v1

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The design, synthesis, and characterization of novel BODPY based iridium and ruthenium complexes were reported. The structures of new compounds were fully characterized by FT-IR, MALDI-TOF Mass analysis, ¹H, and ¹⁹F NMR spectroscopies. Precursor BODIPY was characterized by single-crystal X-ray diffraction technique for the first time. Photophysical properties including absorption and emission profiles and fluorescence lifetimes were investigated via UV–vis absorption and fluorescence emission spectroscopy. Photochemical properties were investigated via direct method and the singlet oxygen production capabilities of new complexes were determined by NIR phosphorescence methods. BODIPY-based ruthenium complex more efficient at producing ¹O₂ than that of the BODIPY-based iridium complex. Besides, both complexes showed more remarkable photosensitization ability than some commonly used photosensitizer based on BODIPY derivatives. This study points out that novel complexes are effective ¹O₂ photosensitizers that might be used for different application areas like as photodynamic therapy.

BODIPY

Iridium

Ruthenium

Photophysical property

Photochemical property

Singlet Oxygen

PDT

Cancer is one of the deadliest diseases worldwide [1–3]. One of the main methods used in cancer treatment today is cytotoxic agents, which are drugs used to prevent or stop the growth and proliferation of cancer cells [4–6]. Cisplatin complexes are commonly used as a type of cytotoxic agent in cancer treatment [7]. However, the use of these complexes comes with some disadvantages, including high toxicity, resistance development, and side effects [8, 9]. Transition metal complexes have been considered as potential chemotherapeutic agents since the discovery of cisplatin [10–16]. Iridium, and ruthenium contained complexes have recently emerged as a new hope in cancer treatment. Especially the BODIPY (boron dipyrromethene) complexes derived from these elements have become a promising alternative for cancer treatment by targeting some biochemical differences specific to cancer cells [17, 18]. BODIPY is a boron-containing compound and is also used for biological imaging due to its fluorescent properties. BODIPY targets some enzymes that are highly abundant in cancer cells but present in low amounts in normal cells, causing the inhibition of these enzymes in cancer cells and thus leading to their death [19]. The advantages of iridium and ruthenium BODIPY complexes over other cancer treatment methods include lower toxicity, less side effects and resistance development risk, higher selectivity, higher photophysical activity, and better targeting abilities [20, 21]. These features suggest that these complexes could be a more effective and safer option for cancer treatment. Although the effects of these complexes on cancer treatment are not yet fully understood, studies conducted so far have shown positive results. With future studies, the potential of iridium and ruthenium BODIPY complexes in cancer treatment can be further revealed [22–24].

Photodynamic therapy (PDT) has emerged as an important alternative in cancer treatment in recent years [25]. This method is based on the generation of reactive oxygen species (ROS), particularly singlet oxygen (¹O₂), in cells by specialized compounds called photosensitizers when they are localized and stimulated in the targeted region. In this context, iridium, and ruthenium BODIPY complexes have become important candidates for PDT in recent years [26]. These complexes are obtained by incorporating iridium or ruthenium metal complexes into the structure of BODIPY molecules. The heavy metals in the structures of these complexes affect the photophysical properties of the molecules, increasing the formation of ¹O₂. The use of iridium and ruthenium BODIPY complexes in photodynamic therapy applications is advantageous due to their high efficiency in ROS production [27, 28]. Additionally, the absorption properties of the complexes at different wavelengths allow for the expansion of application areas. Therefore, iridium and ruthenium BODIPY complexes are promising photosensitizer candidates for important medical applications such as cancer treatment.

This article has focused on the advantages of using iridium and ruthenium BODIPY complexes as photosensitizers. The structure, synthesis, properties, and applications of these complexes as photosensitizer will be discussed. In this way, a solid theoretical basis will be provided for the use of iridium and ruthenium BODIPY complexes as potential photosensitizers for PDT, which is used in the alternative treatment of cancer.

2.1. Materials and Equipment

All chemical reagents and deuterated solvent CDCl₃ for NMR spectroscopy were purchased commercially. All the reactions were performed under an atmosphere of argon. All reactions followed by thin-layer chromatography (TLC) with using silica gel plates. Purification of all compounds were done by column chromatography with using powder silica gel. Infrared spectra were measured on a Perkin Elmer Spectrum 100 FT-IR spectrophotometer. Mass spectra were recorded on a Bruker Daltonics Microflex mass spectrometer. All NMR spectra were recorded in CDCl₃ solutions on a Varian 500 MHz spectrometer. Electronic absorption spectra were recorded with a Shimadzu2101 UV spectrophotometer in the UV–Visible region. Fluorescence emission spectra were recorded on a Varian Eclipse spectrofluorometer. The fluorescence lifetimes were measured by Horiba- Jobin-Yvon-SPEX Fluorolog 3-2iHR instrument with FluoroHub-B Single Photon Counting Controller at an excitation wavelength of 470 nm. The singlet oxygen phosphorescence of the compounds at 1270 nm were measured by Horiba Jobin-Yvon fluorimeter with Hamamatsu NIR PMT 5509 at − 80°C.

2.2. Synthesis

Starting iridium (1) [29], and ruthenium (2) [30] complexes were synthesized and purified in accordance with the literature and used after data control.

2.2.1. Synthesis of bipyridine-functionalized BODIPY (3)

[2,2'-bipyridine]-6-carbaldehyde (370 mg, 2.3 mmol) was dissolved in dry CH₂Cl₂ (200 mL). 2,4-Dimethylpyrrole (456 mg, 4.8 mmol) and one drop of trifluoroacetic acid were added under N₂ atmosphere. The mixture was stirred at room temperature in the dark for 3h. After oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) (1.01 g, 2.3 mmol), triethylamine (TEA) (4 mL) and BF₃.Et₂O (5 mL) were added dropwise to the mixture (Scheme 1). The reaction solution was stirred for another 3h, and the brown mixture was washed with water and brine, dried over anhydrous magnesium sulfate, and concentrated at reduced pressure. The crude product was purified by silica-gel column chromatography (petroleum/EtOAc, 10:1, v/v) to yield 3 as red solid 354 mg, yield 43% (Fig. 1). Spectral data of compound 3: FT-IR νmax (cm^− 1): 2924, 2852, 1458, 1365, 1307, 1155, 974, 761 (Fig. S1). MS (MALDI-TOF) m/z (%): Calc. for (C₂₃H₂₁BF₂N₄): 402.26; found 402.563 [M⁺] and 383.478 [M-F] (Fig. S2a). ¹H NMR, in CDCl₃, δ ppm: 8.64 (d), 8.49 (d), 8.35 (d), 7.90 (t), 7.74 (t), 7.37 (d), 7.29 (d), 7.26 (d), 5.93 (s), 2.51 (s), 1.30 (s) (Fig. S3). ¹⁹F NMR, in CDCl₃, δ ppm: -146.5 (m) (Fig. S6a).

2.2.2. Synthesis of Iridium-BODIPY complex (4)

Compound (1) (100 mg, 0.1 mmol) was reacted with BODIPY (3) (75 mg, 0.2 mmol) in ethylene glycol (5.0 mL) under reflux for 15 h (Scheme 2). Upon cooling to room temperature, the mixture was extracted with water (30 mL× 3) and washed with diethyl ether (50 mL×3). The aqueous layer was transferred to vessel and it was placed on ice then 10 mL of aqueous ammonium hexafluorophosphate solution (1.0 g in 10 mL of deionized water) was slowly added, yielding a colored suspension. The red-purple precipitate was collected by filtration. The crude product was purified by silica-gel column chromatography (DCM/EtOH, 10:1, v/v) to yield 4 as purple solid 82 mg, yield 47% (Fig. 1). Spectral data of compound 4: FT-IR νmax (cm^− 1): 3635, 2921, 2852, 1602, 1434, 1363, 1306, 1021, 841, 796 (Fig. S1). MS (MALDI-TOF) m/z (%): Calc. for (C₄₅H₃₇BF₂IrN₆): 902.84; found 902,866 [M-PF₆⁻] (Fig. S2b). ¹H NMR, in CDCl₃, δ ppm: 8.99 (d), 8.73 (d), 8.42 (t), 8.10 (t), 8.01–7.90 (m), 7.90-7.00 (m), 5.30 (s), 2.17 (s) (Fig. S4). ¹⁹F NMR, in CDCl₃, δ ppm: -146.3 (m) (Fig. S6b).

2.2.3. Synthesis of Ruthenium-BODIPY complex (5)

Compound (2) (100 mg, 0.2 mmol) was reacted with BODIPY (3) (166 mg, 0.4 mmol) in ethylene glycol (5.0 mL) under reflux for 15 h (Scheme 2). Upon cooling to room temperature, the mixture was extracted with water (30 mL× 3) and washed with hexane (50 mL×3). The aqueous layer was transferred to vessel and it was placed on ice then 10 mL of aqueous ammonium hexafluorophosphate solution (1.0 g in 10 mL of deionized water) was slowly added, yielding a coloured suspension. The precipitate was collected by filtration. The crude product was purified by silica-gel column chromatography (DCM/EtOH, 10:1, v/v) to yield 5, as yellow solid 97 mg, yield 37% (Fig. 1). Spectral data of compound 5: FT-IR νmax (cm^− 1): 3636, 2952, 2919, 1594, 1433, 1360, 1311, 1023, 841, 766 (Fig. S1). MS (MALDI-TOF) m/z (%): Calc. for (C₄₃H₃₇BF₂RuN₈): 815,69; found 815,091 [M-2PF₆⁻] and 796.089 (Fig. S2c). ¹H NMR, in CDCl₃, δ ppm: 9.04 (t), 8.02 (d), 8.00-7.90 (m), 7.90–7.10(m), 6.98 (s), 5.30 (s), 2.04 (s) (Fig. S5). ¹⁹F NMR, in CDCl₃, δ ppm: -146.3 (m) (Fig. S6c).

2.3. Photopyhsical and photochemical measurements

UV-Vis and fluorescence spectroscopy were used for investigation of photophysical properties. All measurements were done in THF solutions. The fluorescence lifetimes of the compounds were directly measured and monoexponentially calculations were used. The fluorescence quantum yields (Φ_F) of compounds 3, 4 and 5 were determined by the comparative method using a methylene blue standard (Φ_F = 0.03 in MeOH) according to Eq. (1) [31]

where Φ_F(Std) is the fluorescence quantum yield of reference molecule. The areas under the fluorescence emission curves for compounds (3–5) and the reference molecule, respectively, are labeled as F and F_Std. A and A_Std are the respective absorbance of the compounds and the reference at the excitation wavelengths. η² and η²_Std are the refractive indices of the solvents of compound and reference molecule respectively. Concentration of the solutions fixed to 1x10^-5M.

Photochemical measurements were done in THF solutions by the phosphorescence of ¹O₂ and compared with Rose Bengal (RB) as the reference molecule. The capability of ¹O₂ generation was obtained by Eq. (2),

where, I_Δ is the area under the curve of ¹O₂ signature peak at 1270 nm; A is the absorbance, which was fixed to 0.21 for compounds 4, 5 and reference molecule; and Φ_Δ (std) is the ¹O₂ quantum yield of the reference molecule (RB, 0.82 in THF) [32].

2.4. X-ray crystallography

Crystallographic data for compound 3 was recorded on a Bruker APEX II CCD diffractometer using monochromatized Mo-Kα (λ = 0.71073 Å) X-radiation. Absorption correction was performed by the multi-scan method implemented in SADABS [33] and space groups were determined using XPREP implemented in APEX2 [34]. The structure was solved by direct methods (SHELXS-97) [35] and refined by full-matrix least squares on F² using SHELXL [36] in the Olex2 Software Package [37]. All non-hydrogen atoms were refined with anisotropic displacement factors and C-H hydrogen atoms were placed in calculated positions and allowed to ride on the parent atom. The final geometrical calculations and the molecular drawings were carried out with PLATON [38], MERCURY [39], and DIAMOND (Version 3.1) [40] programs. Structure determination was deposited with the Cambridge Crystallographic Data Centre with reference CCDC-2183625 for 3.

3.1. Synthesis and Characterization

Scheme 1 and Scheme 2 demonstrate the synthesis pathway of BODIPY (3) and, complexes (4 and 5), respectively. The literature-known iridium (1) [29], and ruthenium (2) [30] compounds, were treated with BODIPY dye (3) in ethylene glycol, 15 h, reflux to give novel compounds (4 and 5). The purity of compounds (3-5) was confirmed by FT-IR, matrix-assisted MALDI-MS, ¹H, and¹⁹F NMR spectrometry. The spectral data of compounds are given in the SI part (Fig. S1-S6). In addition, the crystal structure of compound 3 was determined by single crystal X-ray diffraction (Fig. 2). FT-IR spectrum of 3 shows the characteristic BODIPY peaks at 1365 (B-F str) and 1307 (B-N str). In FT-IR spectra of 4 and 5, the peaks at 841 cm⁻¹ belong to the PF₆⁻ counter ions of the complexes (Fig. S1). Molecular ion peak and the lack of one fluorine peak were related to the calculated values of compound 3 (Fig. S2a). Molecular ion peak and the lack of one PF₆^-were related to the calculated values of compound 4 (Fig. S2b), while molecular ion peak and the lack of two PF₆^-were related to the calculated values of compound 5 (Fig. S2c). The ¹H NMR spectra of compounds (3-5) were given in (Figs. S3-S5), respectively. The ¹H NMR spectra of compound 3 exhibited singlet signals for methyl protons at 1.30 and 2.51 ppm. The proton peaks of pyrrole exhibited at 5.93 ppm. The aromatic protons of the compound exhibited at 7.20-8.70 ppm interval. The ¹H NMR spectra of compound 4 exhibited singlet for signals for methyl protons at 2.17 ppm. The proton peaks of pyrrole exhibited at 5.30 ppm. The other proton peaks of the compound exhibited at 7.19-9.00 ppm interval. The ¹H NMR spectra of compound 5 exhibited singlet for signals for methyl protons at 2.04 ppm. The proton peaks of pyrrole exhibited at 5.30 ppm. All the aromatic proton peaks of the compound exhibited at 7.11-9.00 ppm interval. The ¹⁹F NMR spectra of compounds (3-5) were given in SI (Fig. S6).

3.2. X-ray crystallography

The crystal structure of compound 3 was determined by single-crystal X-ray crystallography, the selected data collection and refinement details were presented in Table 1.

Table 1. X-ray crystallographic data and refinement parameters for compound 3.

Compound	3
Empirical formula	C₂₃H₂₁B₁F₂N₄
Formula weight	402.25
Temperature (K)	150 (2)
Crystal system	Monoclinic
Space group	P21/n
a (Å)	16.5067(7)
b (Å)	6.9833(3)
c (Å)	17.3670(7)
b (°)	103.8983(16)
Volume (Å³)	1943.31(14)
Z	4
Density (calc, Mg/m³)	1.375
Absorption coeff. (mm^-1)	0.096
F(000)	840
q_max (°)	25.24
Reflections collected	3963
Independent reflections	3102
R_int(merging R value)	0.0996
Parameters	275
R (F²>2sF²)	0.0864
wR (all data)	0.1646
Goodness-of-fit on F²	1.285

The compound 3 is a BODIPY derivative which is meso- substituted with dipyridyl- moiety as shown in Figure 2. All bond and conformational parameters are found in normal range for previous BODIPY and dipyridyl derivatives [41,42]. The 16-membered main BODIPY core of the structure having two five-membered and one six-membered rings is nearly planar, and two fluorine atoms bonded to boron in a tetrahedral geometry located in two sides of those plane. Two pyridine rings of the dipyridyl moiety are also planar even single C18-C19 bond could have been let their rotation, the nitrogen atoms of two pyridine rings are found in trans positions respect to each other. The angle between planes of BODIPY core and dipyridyl- moiety is 67.74° (Fig. S7).

3.3. Photophysical and photochemical properties

Photophysical properties of novel compounds (3-5) in THF solutions were examined via UV-vis absorption and emission spectroscopy. Normalized absorption spectra of compounds (3-5) were shown in Figure 3a. Maximum absorption band of 3 was observed at 504 nm (ε = 108000 M⁻¹ cm⁻¹). Maximum absorption band of 4 and 5 were observed at 533 nm (ε = 79000 M⁻¹ cm⁻¹) and 558 nm (ε = 39000 M⁻¹ cm⁻¹), respectively. These bands can be assigned to the S₀-S₁ transitions. The molar absorption coefficient (ε) of 4 was seen to be higher than that of 5 (Table 2). Stokes shifts of the compounds 4 and 5 were about 61 and 62 nm respectively (Table 2). Absorption spectra of compounds (3-5) in THF solutions were investigated at different concentrations (10^-6-10^-5Mfor 3 and 10^-5-10^-4M for 4 and 5) and any change at absorption wavelength was observed (Fig. S8a, S9a, S10a). This result demonstrates that the relationship between the maximum absorption wavelength and concentration of the compounds are convenient with Lambert-Beer law, and this means that compounds have no aggregation between given concentration intervals. Emission spectra of compounds (3-5) in THF solutions were investigated at different concentrations (10^-6-10^-7Mfor 3 and 10^-4-10^-5M for 4 and 5). Normalized emission spectra of compounds (3-5) were shown in Figure 3b. The maximum emission band of 3 was observed at 518 nm when excited at 450 nm. The maximum emission band of 4 and 5 were at 594 nm and 620 nm when excited at 450 nm, respectively (Fig. 3b). Fluorescence lifetimes (τ_F) of the compounds were directly measured (Fig. S11) and τ_F values were determined by monoexponentially calculations as 1.3±0.004 ns, 0.9±0.02 ns and 0.02±0.014 for 3, 4, and 5, respectively (Table 2). It was observed that maximum emission wavelengths of BODIPY linked iridium (4) and ruthenium (5) complexes shifted red region (~76 nm for 4 and ~102nm for 5) with a concomitant decrease of ε values. As a result of this bathochromic shift, Stoke’s shifts of these derivatives increased. On the other hand, the direct technique was used to determine ¹O₂generation. For compounds 4, 5 and Rose Bengal (RB) as a standard, the ¹O₂ phosphorescence peaks at 1270 nm were measured in THF solutions. Compounds (4, 5, and RB) were each given the same absorbance values (0.210), after which they were stimulated by a xenon arc light source at the corresponding absorption maxima and determined by a near-IR detector (Fig. S12). Since same absorbance values of the compounds were examined, the area below the phosphorescence peaks can be straight correlated. Compounds 4 and 5 were produced ¹O₂. Using Eq. (2), the ¹O₂ quantum yields were calculated to be 0.36 for compound 4 and 0.41 for compound 5. The outcomes demonstrated that compound 5 produced ¹O₂ more effectively than compound 4. The increase in φ_Δ can be explained by the accounts for the rise in spin-orbital coupling (SOC) and the yield of intersystem crossing (ISC) [43]. These two new compounds have remarkable photosensitizing activity like as many BODIPY derivatives in literature and more effective than some other PSs based on BODIPY based Ir and Ru complexes [26, 27, 44-46].

Table 2. Photophysical properties of compounds^a

Compound	λ_abs (nm)	λ_ems(nm)	ε^b, M^-1cm^-1	Δ_stokes(nm)	Ƭ_Fc(ns)^c	ɸ_F^d	φ_Δ^e
3	504	518	108000	14	1.3±0.004	0.14	N.D.^f
4	533	594	79000	61	0.9±0.2	0.06	0.36
5	558	620	39000	62	0.02±0.014	0.05	0.41

^[a] THF.

^[b] Molar extinction coefficients.

^[c] Lifetime.

^[d] Fluorescence quantum yield.

^[e] Singlet oxygen quantum yield.

^[f] No phosphorescence for this compound.

In summary, bipyridine-functionalized BODIPY (3) and BODIPY-based iridium and ruthenium complexes (4 and 5) were synthesized and characterized for the first time. BODIPY (3) was obtained as a single-crystal and its structure was determined by single crystal X-ray diffraction study for the first time. The photophysical properties including absorption, emission profiles, and fluorescence lifetime of the compounds were investigated by UV-vis and fluorescence spectroscopy instruments. The ¹O₂ generation abilities of the compounds were also determined by NIR phosphorescence methods. BODIPY-based iridium and ruthenium complexes (4 and 5) showed higher ¹O₂ production ability when compared with precursor BODIPY (3). BODIPY-based ruthenium complex (5) showed more ¹O₂ quantum yields than BODIPY-based iridium complex (4). This means that the addition of ruthenium to the main core drastically increases the photosensitizer behavior of the molecules, while iridium possesses lower potency. Additionally, compared to certain regularly used PSs based on BODIPY based iridium and ruthenium derivatives, both complexes (4 and 5) were demonstrated good photosensitization potential. This study suggests new insight into the design and development of PDT agents for further studies.

Acknowledgements: The authors acknowledge to Prof. Dr. Fatma Yüksel for her supervision on single-crystal X-ray crystallography data.

Author Contributions: Beyza Cabir and Seda Cetindere equally contributed to this manuscript, study conception, design, and synthesis. All authors read and approved the final manuscript.

Funding: No fund for this work.

Data Availability: No datasets were generated or analyzed during the current study.

Ethics Approval: Not applicable.

Competing Interests: The authors declare no competing interests.

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Scheme 1 and 2 are available in the Supplementary Files section.

No competing interests reported.

MSSI22.02.2024.doc
scheme1.png
Scheme 1: Synthetic pathway of the compound 3.
scheme2.png
Scheme 2: Synthetic pathway of the compounds 4 and 5.

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Bodipy Based Iridium and Ruthenium Complexes: Synthesis, Photophysical and Photochemical Properties

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Abstract

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1. Introduction

2. Experimental

2.1. Materials and Equipment

2.2. Synthesis

2.2.1. Synthesis of bipyridine-functionalized BODIPY (3)

2.2.2. Synthesis of Iridium-BODIPY complex (4)

2.2.3. Synthesis of Ruthenium-BODIPY complex (5)

2.3. Photopyhsical and photochemical measurements

2.4. X-ray crystallography

3. Results and Discussion

4. Conclusion

Declarations

References

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Additional Declarations

Supplementary Files

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