BODIPY decorated dendrimeric cyclotriphosphazene photosensitizers: synthesis and efficient singlet oxygen generators

Abstract

In the present work, syntheses of BODIPY decorated dendrimeric cyclotriphosphazenes (8–10) are described. The newly synthesized dendrimeric cyclotriphosphazenes have been characterized by ¹H, ¹³C, ³¹P NMR spectroscopies, and UV-Vis electronic absorption spectra. These derivatives show absorption in the NIR region with good molar extinction coefficients. Singlet oxygen generation capacities of novel compounds (8–10) are measured using the trap molecule 1,3-diphenylisobenzofuran. BODIPY decorated dendrimeric cyclotriphosphazenes (8–10) demonstrate high singlet oxygen quantum yields. Also, each of them shows chemical and photostability under the conditions of singlet oxygen measurement. The dendrimeric cyclotriphosphazenes are proposed as potential photosensitizers that can be used as efficient singlet oxygen generators.

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Introduction

Singlet oxygen is of great importance for various applications such as wastewater treatment, blood sterilization and photodynamic therapy (PDT).¹ Photodynamic therapy (PDT) is a very effective and novel technique for the treatment of malignant tumors.^2–5 Three main components are required for photodynamic activity: a photosensitizer (PS), light and singlet oxygen. During the photodynamic reaction, the PS is excited with red or near infrared light, generating reactive oxygen species such as singlet oxygen, and thus irreversibly damaging tumor cells.⁶ Photosensitizers are generally highly conjugated aromatic molecules and can be monitored by various optical imaging techniques.^7–9 According to their chemical structures, PSs can be classified into three groups:^4,6 (i) porphyrin-based photosensitizers (e.g., Photofrin), (ii) chlorophyll-based photosensitizers (e.g., chlorins) and (iii) dyes (e.g., phthalocyanine, and BODIPY). The main disadvantage of the porphyrin based PSs include their low absorptivity region in tissues.⁶ Compared with porphyrins, chlorophyll-based PSs have a deeper penetration in tissues, but their fast photobleaching rate reduces the PDT efficiencies of these molecules which is a major issue.¹⁰ In recent years, BODIPY dyes have attracted great attention as photosensitizers due to their many ideal characteristics such as environment insensitivity, high photostability and the high extinction coefficients.^3,11–17

Cyclotriphosphazene, is an important member of heterocyclic ring systems, has six very active phosphorous–chlorine bonds.^18,19 Its six-membered ring is resistant to the various reaction conditions, so it is commonly used as a platform to prepare of new molecules.^20–28 The chemical and physical properties of cyclotriphosphazenes can be changed via the substituted side groups on the phosphorus. This leads to the emergence of the new applications of cyclotriphosphazenes such as light emitting diodes,²⁵ fluorescence probes,^22,29,30 anticancer and antimicrobial agents.^31–33 Although many researchers have indicated the several other useful applications of them, none has focused on cyclotriphosphazene as a photosensitizer.

The purpose of our work was to prepare the first example of the dendrimeric cyclotriphosphazene as a photosensitizer. For this aim, near IR absorbing BODIPY decorated dendrimeric cyclotriphosphazenes (8–10) were synthesized (Fig. 1). Singlet oxygen generation capabilities and the appropriate photo degradation by light irradiation of these compounds were investigated.

Fig. 1 BODIPY decorated dendrimeric cyclotriphosphazenes (8–10).

Experimental

General methods

The used materials, equipment, photophysical and photochemical formulas and parameters were provided as ESI.†

Synthesis

Compound 1 was synthesized according to literature.³⁴

Synthesis of compound 2. CH₂Cl₂ (300 mL) was purged with Ar for 30 min. 2,4-Dimethyl pyrrole (0.46 g, 4.8 mmol) and compound 1 (0.48 g, 2.2 mmol) were added. The color of the solution turned into red after the addition of 3 drops of trifluoroacetic acid. The reaction mixture was stirred at room temperature for 12 h. Then, p-chloranil (0.54 g, 2.2 mmol) was added and the reaction mixture was stirred at room temperature for 30 min. Then triethyl amine (5 mL) and boron trifluoride diethyl etherate (5 mL) were added sequentially (Scheme S1†). After stirring at room temperature for 3 h, it was extracted with water. Organic layer was dried with Na₂SO₄ and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane–CH₂Cl₂ (1 : 2) as mobile phase. Fraction containing compound 2 was collected then the solvent was removed under reduced pressure (0.71 mmol, 310 mg, 33%, mp 97 °C). Maldi TOF (m/z) calc. 437.29, found: 437.99 [M⁺] (Fig. S1†). ¹H NMR (500 MHz, CDCl₃) δ_H 7.2 (d, J = 8.4 Hz, 2H), (Ar-H), 7.0 (d, J = 8.4 Hz, 2H), (Ar-H), 6.0 (s, 2H), (Ar-H), 4.1 (m, 2H), (OCH₂), 3.4 (m, 2H), (NCH₂), 2.6 (s, 6H), (CH₃), 1.9 (m, 2H), (CH₂), 1.8 (m, 2H), (CH₂), 1.4 (s, 6H), (CH₃), ppm (Fig. S2†). ¹³C NMR (126 MHz, CDCl₃) δ_C 159.4, 155.3, 143.1, 141.8, 131.8, 129.2, 127.1, 121.1, 115.0, 67.3, 51.2, 26.5, 25.8, 14.6 ppm (Fig. S3†).

Synthesis of compound 3. Compound 2 (305 mg, 0.70 mmol) and I₂ (443 mg, 1.74 mmol) were dissolved in ethanol (110 mL). Iodic acid, HIO₃ (271 mg, 1.54 mmol) was dissolved in a few drops of water and added into previous solution. The reaction mixture was stirred at 40 °C for a few hours until all reactant was consumed. Then, saturated sodium thiosulfate solution was added (50 mL) and it was stirred at room temperature for additional 30 min (Scheme S2†). Then, it was extracted with CH₂Cl₂ and water. Organic layer was dried with Na₂SO₄ and evaporated under reduced pressure (415 mg, quantitative). The crude product was purified by silica gel column chromatography using n-hexane–CH₂Cl₂ (1 : 1) as mobile phase. Fraction containing compound 3 was collected then the solvent was removed under reduced pressure (0.4 mmol, 274 mg, 53%, mp 155 °C). Maldi TOF (m/z) calc. 689.01, found: 689.09 [M⁺] (Fig. S4†). ¹H NMR (500 MHz, CDCl₃) δ_H 7.1 (d, J = 7.6 Hz, 2H), (Ar-H), 7.0 (d, J = 7.6 Hz, 2H), (Ar-H), 4.1 (dd, J = 6.0 Hz, 2H), (OCH₂), 3.4 (dd, J = 6.3 Hz, 2H), (NCH₂), 2.6 (s, 6H), (CH₃), 2.0 (p, 6.0 Hz, 2H), (CH₂), 1.9 (m, 2H), (CH₂), 1.4 (s, 6H), (CH₃) ppm (Fig. S5†). ¹³C NMR (126 MHz, CDCl₃) δ_C 159.8, 156.6, 145.3, 141.5, 131.7, 129.1, 126.8, 115.3, 67.4, 51.2, 26.5, 25.7, 17.2, 15.9 ppm (Fig. S6†).

Synthesis of compound 4. Compound 3 (150 mg, 0.22 mmol) and 4-methoxybenzaldehyde (74 mg, 0.54 mmol) were dissolved in benzene (30 mL). Piperidine (300 μL) and acetic acid (300 μL) were added. The reaction mixture was reflux using Dean–Stark apparatus until all aldehyde was consumed (Scheme S3†). Then, crude product was extracted with CH₂Cl₂ and water. Organic layer was dried with Na₂SO₄ and evaporated in vacuo. The crude product was purified by silica gel column chromatography using n-hexane–CH₂Cl₂ (1 : 2) as mobile phase. Fraction containing compound 4 was collected then the solvent was removed under reduced pressure (0.11 mmol, 100 mg, 35%, mp 198 °C). Maldi TOF (m/z) calc. 925.35, found: 926.38 [M⁺ + H] (Fig. S7†). ¹H NMR (500 MHz, CDCl₃) δ_H 8.2 (d, J = 16.6 Hz, 2H), (trans-CH), 7.6 (d, J = 7.2 Hz, 4H), (Ar-CH), 7.7 (d, J = 16.4 Hz, 2H), (trans-CH), 7.2 (d, J = 7.3 Hz, 2H), (Ar-CH), 7.0 (d, J = 7.4 Hz, 2H), (Ar-CH), 6.9 (d, J = 7.6 Hz, 4H), (Ar-CH), 4.1 (m, 2H), (OCH₂), 3.9 (s, 6H), (OCH₃), 3.4 (m, 2H), (NCH₂), 2.0 (m, 2H), (CH₂), 1.9 (m, 2H), (CH₂), 1.5 (s, 6H), (CH₃) ppm (Fig. S8†). ¹³C NMR (126 MHz, CDCl₃) δ_C 160.7, 159.8, 150.4, 145.7, 139.0, 138.6, 133.2, 130.9, 129.7, 129.3, 128.8, 127.4, 116.8, 115.8, 114.3, 67.4, 55.4, 51.2, 26.5, 25.8, 17.8 ppm (Fig. S9†).

Synthesis of compound 5. Compound 3 (150 mg, 0.22 mmol) and 4-bromo benzaldehyde (101.7 mg, 0.55 mmol) were dissolved in benzene (30 mL). Piperidine (300 μL) and acetic acid (300 μL) were added. The reaction mixture was reflux using Dean–Stark apparatus until all aldehyde was consumed (Scheme S4†). Then, crude product was extracted with CH₂Cl₂ and water. Organic layer was dried with Na₂SO₄ and evaporated in vacuo. The crude product was purified by silica gel column chromatography using n-hexane–CH₂Cl₂ (1 : 2) as mobile phase. Fraction containing compound 5 was collected then the solvent was removed under reduced pressure (0.09 mmol, 90 mg, 40%, mp > 300 °C). Maldi TOF (m/z) calc. 1023.09, found: 1024.48 [M⁺ + H] (Fig. S10†). ¹H NMR (500 MHz, CDCl₃) δ_H 8.1 (d, J = 16.7 Hz, 2H), (trans-CH), δ_H 7.7 (d, J = 16.7 Hz, 2H), (Ar-CH), (trans-CH), 7.6–7.5 (m, 8H), (Ar-CH), 7.2 (d, J = 7.1 Hz, 2H), (Ar-CH), 7.1 (d, J = 7.0 Hz, 2H), (Ar-CH), 4.1 (t, J = 5.3 Hz, 2H), (OCH₂), 3.4 (t, J = 6.5 Hz, 2H), (NCH₂), 2.0 (m, 2H), (CH₂), 1.9 (m, 2H), (CH₂), 1.5 (s, 6H), (CH₃) ppm (Fig. S11†). ¹³C NMR (126 MHz, CDCl₃) δ_C 160.0, 159.9, 150.2, 146.4, 140.1, 138.1, 135.5, 133.6, 132.0, 129.5, 129.0, 126.9, 123.4, 119.4, 115.5, 67.4, 51.2, 26.5, 25.8, 17.8 ppm (Fig. S12†).

Synthesis of compound 6. Compound 3 (150 mg, 0.22 mmol) and benzaldehyde (57.3 mg, 0.54 mmol) were dissolved in benzene (30 mL). Piperidine (300 μL) and acetic acid (300 μL) were added. The reaction mixture was reflux using Dean–Stark apparatus until all aldehyde was consumed (Scheme S5†). Then, crude product was extracted with CH₂Cl₂ and water. Organic layer was dried with Na₂SO₄ and evaporated in vacuo. The crude product was purified by silica gel column chromatography using n-hexane–CH₂Cl₂ (1 : 2) as mobile phase. Fraction containing compound 6 was collected then the solvent was removed under reduced pressure (0.12 mmol, 100 mg, 53%, mp 199 °C). Maldi TOF (m/z) calc. 865.30, found: 866 [M⁺ + H] (Fig. S13†). ¹H NMR (500 MHz, CDCl₃) δ_H 8.2 (d, J = 16.7 Hz, 2H), (trans-CH), 7.7 (d, J = 16.9 Hz, 2H), (trans-CH), 7.7–7.6 (M, 4H), (Ar-CH), 7.5–7.4 (m, 4H), (Ar-CH), 7.4–7.3 (m, 2H), (Ar-CH), 7.2 (d, J = 8.5 Hz, 2H), (Ar-CH), 7.1 (d, J = 8.0 Hz, 2H), (Ar-CH), 4.1 (t, J = 5.7 Hz, 2H), (OCH₂), 3.4 (t, J = 4.5 Hz, 2H), (NCH₂), 2.0–1.9 (m, 2H), (CH₂), 1.8 (m, 2H), (CH₂), 1.5 (s, 6H), (CH₃), ppm (Fig. S14†). ¹³C NMR (126 MHz, CDCl₃) δ_C 159.9, 150.5, 146.1, 139.7, 139.6, 136.7, 129.6, 129.2, 128.8, 127.8, 127.7, 127.5, 118.8, 115.3, 115.2, 67.4, 51.2, 26.5, 25.8, 17.7 ppm (Fig. S15†).

Synthesis of compound 7. Propargyl alcohol (3.04 mL, 52.60 mmol) were dissolved in 20 mL of dry THF under an argon atmosphere in a 250 mL three-necked round-bottomed flask. The reaction mixture was cooled in an ice-bath and NaH (60% oil suspension, 2.1 g, 52.6 mmol) in 30 mL of dry THF was quickly added to a stirred solution. Solution of cyclotriphosphazene (3.0 g, 8.62 mmol) in THF (30 mL) was added dropwise (Scheme S6†). The reaction was stirred for 3 h at room temperature and followed by TLC on silica gel plates. The reaction mixture was filtered to remove the formed sodium chloride and the solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography using n-hexane–CH₂Cl₂ (5 : 2) as mobile phase. Fraction containing compound 7 was collected then the solvent was removed under reduced pressure (6.02 mmol, 2.8 g, 70%, oily). Maldi TOF (m/z) calc. 465.27, found: 466.22 [M⁺ + H] (Fig. S16†). ³¹P NMR (proton decoupled) (202 MHz, CDCl₃) δ_P = 17.23 (s, 3P), ppm (Fig. S17a†). ¹H NMR (phosphorus coupled) (500 MHz, CDCl₃) δ_H = 4.61 (broad, 12H), (OCH₂) ve 2.54 (broad, 6H), (CH) ppm (Fig. S18†). ¹³C NMR (126 MHz, CDCl₃) δ_C 75.64, 60.33 ve 54.10 ppm (Fig. S19†).

Synthesis of compound 8. Compound 4 (150 mg, 0.162 mmol) was dissolved in CH₂Cl₂ (4 mL), CH₃OH (1 mL) and H₂O (1 mL). Compound 7 (12.56 mg, 0.027 mmol), sodium ascorbate (7.5 mg, 0.03 mmol), CuSO₄·5H₂O (6 mg, 0.03 mmol), and 2 drop of Et₃N were added and the mixture was stirred at room temperature for 2 days (Scheme S7†). The solvent was evaporated and the crude product was purified by silica gel column chromatography using MeOH–CH₂Cl₂ (5 : 100) as mobile phase. Fraction containing compound 8 was collected then the solvent was removed under reduced pressure (0.006 mmol, 35 mg, 21%, mp 237 °C). ³¹P NMR (proton decoupled) (202 MHz, CDCl₃) δ_P 17.43 (s, 3P), ppm (Fig. S20†). ¹H NMR (phosphorus coupled) (500 MHz, CDCl₃) δ_H 8.2 (d, J = 16.6 Hz, 12H), (trans-CH), 7.64–7.61 (m, 24H + 6H), (Ar-CH, trans-CH, NCH), 7.6 (m, 12H), (Ar-CH), 7.2 (d, J = 8.2 Hz, 12H), (Ar-CH), 7.0 (d, J = 8.2 Hz, 12H), (Ar-CH), 6.9 (d, J = 8.4 Hz, 24H), 4.6 (s, 12H), (POCH₂), 4.5 (t, J = 7.0 Hz, 12H), (NCH₂), 4.1 (t, J = 5.8 Hz, 12H), (OCH₂), 3.9 (s, 36H), (OCH₃), 2.2 (m, 12H), (CH₂), 1.9 (m, 12H), (CH₂), 1.5 (s, 36H), (CH₃) ppm (Fig. S21†). ¹³C NMR (126 MHz, CDCl₃) δ_C 160.7, 159.7, 150.4, 145.7, 145.3, 145.2, 139.0, 138.5, 133.2, 129.7, 129.6, 129.3, 127.5, 122.2, 116.8, 115.2, 114.3, 67.1, 66.1, 58.4, 55.4, 27.3, 26.2, 17.7 ppm (Fig. S22†).

Synthesis of compound 9. Compound 5 (150 mg, 0.147 mmol) was dissolved in CH₂Cl₂ (4 mL), CH₃OH (1 mL) and H₂O (1 mL). Compound 7 (11.35 mg, 0.024 mmol), sodium ascorbate (7.5 mg, 0.03 mmol), CuSO₄·5H₂O (6 mg, 0.03 mmol), and 2 drop of Et₃N were added and the mixture was stirred at room temperature for 2 days (Scheme S8†). The solvent was evaporated and the crude product was purified by silica gel column chromatography using MeOH–CH₂Cl₂ (5 : 100) as mobile phase. Fraction containing compound 9 was collected then the solvent was removed under reduced pressure (0.006 mmol, 38 mg, 24.05%, mp 254 °C). ³¹P NMR (proton decoupled) (202 MHz, CDCl₃) δ_P 17.39 (s, 3P), ppm (Fig. S24†). ¹H NMR (phosphorus coupled) (500 MHz, CDCl₃) δ_H 8.1 (d, J = 16.7 Hz, 12H), (trans-CH), 7.7 (d, J = 16.7 Hz, 12H), (trans-CH), 7.6 (s, 6H), (NCH), 7.57–7.49 (m, 48H), (Ar-CH), 7.2 (d, J = 8.2 Hz, 12H), (Ar-CH), 7.0 (d, J = 8.2 Hz, 12H), (Ar-CH), 4.6 (s, 12H), (POCH₂), 4.5 (t, J = 6.0 Hz, 12H), (NCH₂), 4.1 (m, 12H), (OCH₂), 2.24–2.16 (m, 12H), (CH₂), 1.94–1.86 (m, 12H), (CH₂), 1.5 (s, 36H), (CH₃) ppm (Fig. S25†). ¹³C NMR (126 MHz, CDCl₃) δ_C 159.8, 150.2, 146.4, 146.3, 145.3, 139.9, 138.1, 135.5, 133.5, 132.0, 129.5, 129.0, 127.1, 123.4, 122.2, 119.4, 115.3, 67.1, 66.1, 58.4, 27.5, 26.2, 17.8 ppm (Fig. S26†).

Synthesis of compound 10. Compound 6 (150 mg, 0.163 mmol) was dissolved in CH₂Cl₂ (4 mL), CH₃OH (1 mL) and H₂O (1 mL). Compound 7 (12.64 mg, 0.027 mmol), sodium ascorbate (7.5 mg, 0.03 mmol), CuSO₄·5H₂O (6 mg, 0.03 mmol), and 2 drop of Et₃N were added and the mixture was stirred at room temperature for 2 days (Scheme S9†). The solvent was evaporated and the crude product was purified by silica gel column chromatography using MeOH–CH₂Cl₂ (5 : 100) as mobile phase. Fraction containing compound 10 was collected then the solvent was removed under reduced pressure (0.007 mmol, 40 mg, 26.14%, mp 217 °C). ³¹P NMR (proton decoupled) (202 MHz, CDCl₃) δ_P 17.41 (s, 3P), ppm (Fig. S27†). ¹H NMR (phosphorus coupled) (500 MHz, CDCl₃) δ_H 8.2 (d, J = 16.7 Hz, 12H), (trans-CH), 7.7 (d, J = 16.7 Hz, 12H), (trans-CH), 7.7 (d, J = 7.2 Hz, 24H), (Ar-CH), 7.6 (s, 6H), (NCH), 7.4 (t, J = 7.4 Hz, 24H), (Ar-CH), 7.3 (d, J = 7.4 Hz, 12H), (Ar-CH), 7.2 (d, J = 8.7 Hz, 12H), (Ar-CH), 7.0 (d, J = 8.7 Hz, 12H), (Ar-CH), 4.6 (s, 12H), (POCH₂), 4.5 (t, J = 7.1 Hz, 12H), (NCH₂), 4.1 (t, J = 5.9 Hz, 12H), (OCH₂), 2.22–2.15 (m, 12H), 1.91–1.85 (m, 12H), 1.5 (s, 36H) ppm (Fig. S28†). ¹³C NMR (126 MHz, CDCl₃) δ_C 159.7, 150.5, 146.1, 145.4, 145.3, 139.6, 139.5, 136.6, 133.4, 129.6, 129.3, 128.8, 127.7, 127.5, 122.2, 118.8, 115.3, 67.1, 66.1, 58.4, 27.3, 26.2, 17.8 ppm (Fig. S29†).

Results and discussion

Cyclotriphosphazene core is known to be a functional and robust ring. In our design, we wanted to use cyclotriphosphazene ring due to its excellent core for synthesis of dendrimeric molecules. BODIPY molecules were chosen as a fluorophore because of their intrinsic characteristics. This framework, 4-(4-azidobutoxy)benzaldehyde derivative (1) and BODIPY dye (2) were synthesized according to the literature.³⁴ 2,6-Position of compound 2 was decorated with iodine to increase in the intersystem crossing for more effective singlet oxygen generation.³ Methyl groups in 3,5 positions of BODIPY were acidic enough to participate in Knoevenagel condensation reaction.³⁵ Compounds 4–6 were obtained by applying a series of Knoevenagel reactions including aromatic aldehydes (4-methoxybenzaldehyde, benzaldehyde, 4-bromo benzaldehyde) and the compound 3 (Scheme 1). This synthetic procedure was used to extend the degree of π-electron conjugation and to yield near-IR absorbing dyes. In addition to that electron-rich aromatic aldehydes which are different bearing terminal groups were preferably used to compare the effects on the singlet oxygen generation.

Scheme 1 Synthesis of compounds 3–6.

Nucleophilic substitution reactions of cyclotriphosphazene with the sodium salts of propargyl alcohol were carried out in order to prepare full propargyl alkoxide substituted cyclotriphosphazene (7) (Scheme 2).

Scheme 2 Click reactions of the BODIPY molecules (4–6) with compound 7.

Click reactions of the BODIPY molecules (4–6) with cyclotriphosphazene core (7) were the key step to obtain the target molecules (8–10) (Scheme 2). All compounds were characterized by mass spectrometry (compounds 2–7), ¹H and ¹³C NMR (compounds 2–10), ³¹P NMR (compounds 7–10). Structure analysis data of each new isolated compound have been presented in the Synthesis section.

NMR characterization of dendrimeric cyclotriphosphazenes

The click reactions of the BODIPY molecules (4–6) with compound 7 were carried out under the same conditions. The target compounds (8–10) were characterized using spectroscopic techniques. Comparison of ¹H NMR spectra of compounds 4 and 8 showed disappearance of the –CH₂N₃ protons resonating at 3.4 ppm and appearance of new triplet peak belonging to the –NCH₂– protons at 4.5 ppm. Also, two new peaks belonging to the –POCH₂ protons at 4.6 ppm and the triazole ring protons at 7.59 ppm for compound 8 were observed (Fig. 2).

Fig. 2 (a) ¹H NMR spectrum of compound 4. (b) ¹H NMR spectrum of compound 8.

All integral values confirmed proposed structure for compound 8. On the other hand, comparison of ¹³C NMR spectra of compounds 4 and 8, POCH₂ carbon signal appeared at high field (66.1 ppm) and the triazole ring two carbons signals appeared in the range of 110–165 ppm. Also, the –CH₂N₃ carbon signal at 51.2 ppm shifted at 58.4 ppm as –NCH₂ (Fig. S23†). The proton decoupled ³¹P NMR spectrum of the compound 8 was observed a sharp singlet peak because of the chemical environment equivalence of all the phosphorus nuclei (Fig. S20†). Structure analysis data of the other dendrimeric molecules (9, 10) have been presented in the Synthesis section.

Photophysical measurements

The electronic absorption behavior of newly synthesized dendrimeric cyclotriphosphazenes (8–10) were studied in dichloromethane. Maximum absorbance wavelength of compounds 8, 9 and 10 were determined to be 660, 642 and 638 nm, respectively (Fig. 3). Dendrimeric cyclotriphosphazenes showed molar extinction coefficients in the range (5–7) × 10⁴ M⁻¹ cm⁻¹ (Table 1).

Fig. 3 UV-Vis absorption spectra of compounds 8–10 (0.5 μM) in dichloromethane.

Table 1 Photophysical properties of dendrimeric cyclotriphosphazenes^a

Compound	λab, nm	ε^b, 10^4 M^−1 cm^−1	λem, nm	φΔ^c
a Dichloromethane.b Molar extinction coefficients.c Singlet oxygen quantum yields.
8	660	5.4	685	0.72
9	642	4.9	660	0.80
10	638	6.8	650	0.70

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The fluorescence emission maxima were recorded at 685 nm for 8, 660 nm for 9, 650 nm for 10. The compound 8 led to a larger shift towards red both in absorption and the emission spectra than any other compound (9 and 10) tested (Fig. S30†). The photophysical data of compound 8–10 were listed in Table 1.

Singlet oxygen measurements

In this work, singlet oxygen generation capacities of novel compounds (8–10) were performed using the trap molecule 1,3-diphenylisobenzofuran (DPBF) in dichloromethane. The absorbance of DPBF and compound 8 was adjusted around 1.1 and 0.3 respectively in cuvette (Fig. 4). In order to eliminate any contribution to the absorbance change of DPBF from dark reactions, this solution was kept for 20 min in dark. As expected, absorbance peak of DPBF did not cause any change. When the light was turned on, compound 8 (0.5 μM) in dichloromethane started singlet oxygen production. The presence of singlet oxygen was monitored by decrease of the selective singlet oxygen trap molecule, DPBF, absorbance at 414 nm. Singlet oxygen causes a remarkable degradation of DPBF.¹⁴ After each irradiation for 5 s under the light from 300 W quartz lamp, filtered to remove light with λ < 600 nm, absorbance was measured several times (Fig. 4). Under the same experimental conditions, singlet oxygen measurements of the other dendrimeric molecules (9, 10) and methylene blue as the reference molecule were performed. Decrease in absorbance spectra of DPBF were given in ESI (Fig. S31–S33†). Also, each solution of dendrimeric compounds without DPBF was triggered with light for 20 min and no changes were observed in the absorbance intensities of them (Fig. S34–S36†). This situation confirmed that these compounds were not degraded under the conditions of singlet oxygen measurement. Singlet oxygen quantum yields (φ_Δ) of compounds (8–10) were calculated according to the literature.^14,36 Methylene blue used as the reference compound had a singlet oxygen quantum yield of 0.57 under the conditions of the study in dichloromethane. Singlet oxygen quantum yields (φ_Δ) of compounds (8–10) were found 0.72, 0.80, 0.70 in dichloromethane, respectively. These were evidences that dendrimeric cyclotriphosphazenes can also be potential photosensitizers to be used for the singlet oxygen generation.

Fig. 4 (A) Decrease in absorbance spectrum of DPBF in the presence of compound 8 (0.5 μM) in dichloromethane. (B) Absorbance decrease of DPBF at 414 nm with time in dichloromethane in the presence of compound 8.

Conclusions

In this study, BODIPY decorated dendrimeric cyclotriphosphazenes (8–10) are synthesized and characterized by various techniques such as ¹H, ¹³C, ³¹P NMR, UV-Vis. These compounds show strong near IR absorption in the 630–660 nm region with good molar extinction coefficients and reveal respectable singlet oxygen quantum yields when compared with methylene blue under similar conditions. Dendrimeric cyclotriphosphazenes (8–10) produce singlet oxygen only under light excitation. Also, each of them shows chemical and photo stability under the conditions of singlet oxygen measurement. We propose that dendrimeric cyclotriphosphazenes are efficient generators of singlet oxygen and potential photosensitizers for photodynamic therapy when excited in the therapeutic window of the electromagnetic spectrum. The studies will continue in our laboratories to further explore their derivatives.

Acknowledgements

The authors thank the Scientific and Technical Research Council of Turkey for financial support TUBITAK (Grant 114Z445).

References

1. M. C. DeRosa and R. J. Crutchley, Coord. Chem. Rev., 2002, 351–371.
2. P. Mroz, A. Yaroslavsky, G. B. Kharkwal and M. R. Hamblin, Cancers, 2011, 3, 2516–2539.
3. A. Kamkaew, S. H. Lim, H. B. Lee, L. V. Kiew, L. Y. Chung and K. Burgess, Chem. Soc. Rev., 2013, 42, 77–88.
4. Z. Huang, Technol. Cancer Res. Treat., 2005, 4(3), 283–293.
5. J. P. Celli, B. Q. Spring, I. Rizvi, C. L. Evans, K. S. Samkoe, S. Verma, B. W. Pogue and T. Hasan, Chem. Rev., 2010, 110, 2795–2838.
6. A. P. Castano, T. N. Demidovaa and M. R. Hamblin, Photodiagn. Photodyn. Ther., 2004, 1, 279–293.
7. R. Bonnett, Chem. Soc. Rev., 1995, 19–33.
8. V. Çakır, M. Göksel, M. Durmus and Z. Biyiklioglu, Dyes Pigm., 2016, 125, 414–425.
9. J. Killoran, L. Allen, J. F. Gallagher, W. M. Gallagher and D. F. O'Shea, Chem. Commun., 2002, 1862–1863.
10. Y. Hongying, W. Fuyuan and Z. Zhiyi, Dyes Pigm., 1999, 43, 109–117.
11. S. Atilgan, Z. Ekmekci, A. L. Dogan, D. Guc and E. U. Akkaya, Chem. Commun., 2006, 4398–4400.
12. T. Yogo, Y. Urano, Y. Ishitsuka, F. Maniwa and T. Nagano, J. Am. Chem. Soc., 2005, 127, 12162–12163.
13. S. G. Awuahab and Y. You, RSC Adv., 2012, 2, 11169–11183.
14. Y. Cakmak, S. Kolemen, S. Duman, Y. Dede, Y. Dolen, B. Kilic, Z. Kostereli, L. Tatar Yildirim, A. L. Dogan, D. Guc and E. U. Akkaya, Angew. Chem., Int. Ed., 2011, 50, 11937–11941.
15. L. Yao, S. Xiao and F. Dan, J. Chem., 2013, 1–10.
16. J. Zhao, W. Wu, J. Sun and S. Guo, Chem. Soc. Rev., 2013, 42, 5323–5351.
17. J. Zhao, K. Xu, W. Yang, Z. Wang and F. Zhong, Chem. Soc. Rev., 2015, 44, 8904–8939.
18. C. W. Allen, Chem. Rev., 1991, 91, 119–135.
19. V. Chandrasekhar and S. Nagendran, Chem. Soc. Rev., 2001, 30, 193–203.
20. A.-M. Caminade, A. Hameaua and J.-P. Majorala, Dalton Trans., 2016, 1810–1822.
21. M. R. Rao, G. Gayatri, A. Kumar, G. N. Sastry and M. Ravikanth, Chem.–Eur. J., 2009, 15, 3488–3496.
22. G. Yenilmez Çiftçi, E. Şenkuytu, M. Durmuş, F. Yuksel and A. Kılıç, Dalton Trans., 2013, 14916–14926.
23. E. W. Ainscough, A. M. Brodie, P. J. B. Edwards, G. B. Jameson, C. A. Otter and S. Kirk, Inorg. Chem., 2012, 51, 10884–10892.
24. G. Franc, S. Mazères, C.-O. Turrin, L. Vendier, C. Duhayon, A.-M. Caminade and J.-P. Majoral, J. Org. Chem., 2007, 72, 8707–8715.
25. M. R. Rao, R. Bolligarla, R. J. Butcher and M. Ravikanth, Inorg. Chem., 2010, 49, 10606–10616.
26. G. Yenilmez Çiftçi, E. T. Eçik, M. Bulut, F. Yuksel, A. Kılıc and M. Durmus, Inorg. Chim. Acta, 2013, 398, 106–112.
27. S. J. Coles, D. B. Davies, R. J. Eaton, M. B. Hursthouse, A. Kılıç, R. A. Shaw and A. Uslu, Dalton Trans., 2006, 1302–1312.
28. B. Cosut, Dyes Pigm., 2014, 100, 11–16.
29. E. Şenkuytu, E. Tanrıverdi Eçik, M. Durmuş and G. Yenilmez Çiftçi, Polyhedron, 2015, 101, 223–229.
30. R. Kagit, M. Yildirim, O. Ozay, S. Yesilot and H. Ozay, Inorg. Chem., 2014, 53(4), 2144.
31. T. Yıldırım, K. Bilgin, G. Yenilmez Çiftçi, E. Tanrıverdi Eçik, E. Şenkuytu, Y. Uludağ, L. Tomak and A. Kılıç, Eur. J. Med. Chem., 2012, 52, 213–220.
32. M. Siwy, D. Seük, B. Kaczmarczyk, I. Jaroszewicz, A. Nasulewicz, M. Pelczynska, D. Nevozhay and A. Opolski, J. Med. Chem., 2006, 49, 806–810.
33. N. Asmafiliz, Z. Kılıc, A. Öztürk, T. Hökelek, Y. Koç, L. Açık, Ö. Kısa, A. Albay, Z. Üstündağ and A. O. Solak, Inorg. Chem., 2009, 48, 10102–10116.
34. S. Atılgan, T. Ozdemir and E. U. Akkaya, Org. Lett., 2010, 12(21), 4792.
35. A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891–4932.
36. N. Adarsh, R. R. Avirah and D. Ramaiah, Org. Lett., 2010, 12(20), 5720–5723.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07171f

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