Novel BODIPY-bridged cyclotriphosphazenes * Dedicated to Prof. Dr. Adem Kılıç on the occasion of his retirement.

Three new 2-component unsubstituted ( 4P ), diiodo- ( 5P ), and dibromo- ( 6P ) distyryl-BODIPY-bridged cyclotriphosphazene dimers were designed and synthesized. The newly synthesized BODIPY-cyclotriphosphazene systems were characterized by 1 H, 13 C, and 31 P NMR spectroscopy. The photophysical properties of the distryl-BODIPYs (4–6) and BODIPY-cyclotriphosphazene dyads ( 4P – 6P ) were studied by UV-Vis absorption and fluorescence emission spectroscopy. In these derivatives, the bino-type cyclotriphosphazene derivative bearing unsubstituted BODIPY unit 4P exhibited high fluorescence and no singlet oxygen generation due to the lack of spin converter. The attachment of heavy atoms (iodine and bromine) enabled the production of singlet oxygen. The bino-type BODIPY-cyclotriphosphazenes ( 5P and 6P ) were also used as triplet photosensitizers in the photooxidation of 1,3-diphenylisobenzofuran to endoperoxide via generation of the singlet oxygen in dichloromethane. The singlet oxygen production of these compounds was also investigated via a direct method and produced a singlet oxygen phosphorescence peak at 1270 nm.


Introduction
Phosphazenes are one of the most studied classes of inorganic heterocyclic molecules [1]. The significance of this class is due to their wide application areas, such as medicine, biology, flame-retardant additives, membranes, microlithography, and ionic crystals [2][3][4][5]. In these studies, cyclotriphosphazenes are widely used as a core platform to develop tailor-made systems, because cyclotriphosphazene can easily be tuned by nucleophilic substitution reactions [6]. The readily available reactive phosphorus-chloride bonds provide an immense range of different molecular systems for targeted applications [7][8][9]. Moreover, phosphazenes possess thermal and chemical stabilities under different conditions, which is very important for specific applications areas [10,11].
BODIPYs were proven to be versatile dyes with high molar absorption coefficients, large fluorescence quantum yields, and good solubility in various solvents [19]. A BODIPY core can be easily functionalized to tune the photophysical properties, such as the bathochromic shift of the UV-Vis absorbance via a Knoevenagel condensation reaction . [20]. However, the high fluorescent quantum yield of BODIPYs generally suggests trivial intersystem crossing, which hinders their role as triplet photosensitizers. To overcome this obstacle, brominated or iodinated BODIPY derivatives stand out by using the spin-orbital coupling effect of the heavy atom [19,21,22]. BODIPY-decorated cyclotriphosphazene derivatives have been readily reported as efficient singlet oxygen generators [23,24]. However, BODIPY-bridged cyclotriphosphazene derivatives, which can provide additional branching points, have never been reported. Herein, we reported the first examples of BODIPY-bridged cyclotriphosphazene dimers (4P-6P). Unsubstituted, iodinated, and brominated distyryl-BODIPY derivatives (4)(5)(6) bearing alkoxy functional groups were reacted with trimer and mono-BODIPY-bridged cyclotriphosphazene (4P-6P) derivatives were synthesized (Scheme). All of the prepared compounds were characterized by mass, 31 P, 1 H, and 13 C NMR spectroscopy. The photophysical properties of BODIPYs (4-7) and BODIPY-bridged cyclotriphosphazenes (4P-6P) were studied via UV-Vis and fluorescence emission techniques. Singlet oxygen generation abilities of BODIPY-bridged cyclotriphosphazenes (4P-6P) were investigated via both indirect and direct methods.

Equipment
Absorption spectra of the compounds were inscribed with a Shimadzu 2101 UV spectrophotometer (Kyoto, Japan) in the UV-Vis region. Fluorescence excitation and emission spectra were obtained with a Varian Eclipse spectrofluorometer (Palo Alto, CA, USA) (1-cm path-length cuvette, RT). Singlet oxygen phosphorescence around 1270 nm was investigated using a Horiba Jobin-Yvon fluorometer (Kyoto, Japan) with a Hamamatsu

Parameters for fluorescence quantum yields
The fluorescence quantum yields ( Φ F ) of compounds 4-6 and 4P-6P were calculated by the relative procedure in Eq. (1) [25]: where the areas under the fluorescence emission curves of the compounds (4-6 and 4P-6P) and the standard are F and F Std , respectively. A and A Std represent the absorbances of compounds 4-6 and 4P-6P, respectively.
η values, as the refractive indices of the solvents, were considered in the determination of the fluorescence quantum yields in different solvents. Cresyl violet ( Φ F = 0.54/methanol) [26] was used as the standard.

Parameters for singlet oxygen
Singlet oxygen ( Φ ∆ ) phosphorescence measurements were carried out using a Horiba Jobin-Yvon fluorometer with a Hamamatsu NIR PMT 5509. The intensity of singlet oxygen formation was obtained according to Eq. (2): Singlet oxygen quantum yields of compounds 4-6 and 4P-6P were determined by employing DPBF as a trap molecule and methylene blue (MB) as a reference. Singlet oxygen generations were monitored by the decrease in absorbance of DPBF. A 630-nm (4.0 mW/cm 2 ) LED bulb was used as a light source and exposed from a cuvette distance of 5 cm, and absorbances were recorded at intervals for each irradiation (Eq. (3)): .
Here, compound and ref designate the BODIPYs and BODIPY-cyclotriphosphazene (4-6 and 4P-6P) and MB, respectively. k is the slope of the change in maximum absorbance of DPBF (414 nm) with the irradiation time. F is the absorption correction factor (F = 1 −10 −OD , where OD is absorption at the irradiation wavelength), and PF is the light intensity (energy flux, mW/cm 2 ) .

Synthesis of compound 5
Compound 2 (100 mg, 0.173 mmol) and 4-hydroxybenzaldehyde (49 mg, 0.401 mmol) were dissolved in benzene (40 mL) under an argon atmosphere. Piperidine (0.3 mL) and glacial acetic acid (0.3 mL) were added to the solution and the reaction mixture was refluxed using a Dean-Stark apparatus until it was concentrated. The reaction was followed by TLC until the major product was a dark blue-colored compound. The reaction mixture was extracted from dichloromethane and water. Next, compound 5 was purified by silica gel column chromatography using dichloromethane and methanol (98:2) (yield: 73%).

Synthesis of compound 6
Compound 3 (100 mg, 0.207 mmol) and 4-hydroxybenzaldehyde derivative (64 mg, 0.518 mmol) were dissolved in benzene (40 mL) under an argon atmosphere. Piperidine (0.3 mL) and glacial acetic acid (0.3 mL) were added to the solution and the reaction mixture was refluxed using a Dean-Stark apparatus until it was concentrated. The reaction was followed by TLC until the major product was a dark blue-colored compound. The reaction mixture was extracted from dichloromethane and water. Compound 6 was purified by silica gel column chromatography using dichloromethane and methanol (97:3) (yield: 58%).

Synthesis of compound 4P
A 100-mL round-bottomed flask was charged with tetrahydrofuran (50.0 mL) and purged with Ar for 15 min.

Synthesis of compound 5P
A 100-mL round-bottomed flask was charged with tetrahydrofuran (50.0 mL) and purged with Ar for 15 min.

Synthesis of compound 6P
A 100-mL round-bottomed flask was charged with tetrahydrofuran (50.0 mL) and purged with Ar for 15 min.
Compound 6 (30 mg, 0.04 mmol) and cesium carbonate (34 mg, 0.1 mmol) were added to the reaction flask and stirred for 30 min. Trimer (30.2 mg, 0.08 mmol) was poured into the reaction mixture and stirred for 16 h at room temperature. The precipitated salt (CsCl) was filtered off and the solvent was removed at reduced pressure.
The resulting green solid was subjected to column chromatography using n-hexane and tetrahydrofuran (4:1) as the mobile phase (yield: 30%).

Synthesis and structural characterization
The synthetic pathways to prepare the BODIPYs (4-6) and BODIPY-bridged cyclotriphosphazenes (4P-6P) in this study are shown in the Scheme. Compounds 1-3 were synthesized according to methods in the literature [27][28][29]. The synthetic strategy to prepare distyryl-BODIPYs (4-6) bearing the -OH functional group was also based on methods in the literature and initially included the reactions of BODIPYs 1-3 with 4hydroxybenzaldehyde in benzene using a Dean-Stark apparatus via Knoevenagel condensation conditions [27].
BODIPYs (4)(5)(6) in tetrahydrofuran solutions were then reacted with an excess of hexachlorocyclotriphosphazene (trimer) in the presence of Cs 2 CO 3 to prepare BODIPY-bridged cyclotriphosphazenes (4P-6P) from nucleophilic displacement reactions. The progress of the reactions was followed by TLC and the products were purified by silica-gel column chromatography with proper eluent systems (see Section 2). Identifications of the BODIPYs (1-6) and BODIPY-cyclotriphosphazenes (4P-6P) were performed using 31 P, 1 H, and 13 C NMR spectroscopy and mass spectrometry. The results confirmed the established formulations, where the protondecoupled 31 P NMR spectra of BODIPY-bridged cyclotriphosphazenes 4P-6P displayed the expected AX 2 spin systems with 2 sets of signals corresponding to the P-OPhCl groups at ∼11.9-12.1 ppm and the PCl 2 groups at ∼22.5 ppm.

Photophysical properties
The electronic UV-Vis absorption spectra of BODIPY-cyclotriphosphazenes 4P-6P were recorded in different solvents ( Figure 1). The absorption profile of compound 4P was observed to be similar in all of the studied solvents (dichloromethane, acetone, ethanol, chloroform, acetonitrile, tetrahydrofuran, and dimethyl sulfoxide) with maximum absorptions at ∼622 nm. I 2 -BODIPY-cyclotriphosphazene 5P exhibited parallel absorption peaks with different intensities, where the maximum absorption intensity observed was in dichloromethane at 638 nm and the lowest was in dimethyl sulfoxide with a slight ( ∼8 nm) bathochromic shift. Compound 6P was influenced by the solvent and exhibited the most intense absorbances in chloroform, ethanol, and dichloromethane at 640 nm, whereas the lowest absorption was displayed in acetonitrile with an additional peak at ∼683, while the other selected solvent intensities were in the middle. Because all three compounds (4P-6P) displayed optimum absorption characteristics, dichloromethane was selected as the solvent to be studied.
The concentrations of the compounds were fixed at 2 µ M and measured at room temperature. Next, the molar extinction coefficients ( ε) of BODIPYs 4-6 and BODIPY-cyclotriphosphazene derivatives 4P-6P were determined for compounds 4-6 as 7.18, 5.22, and 6.44 M −1 × cm −1 and for compounds 4P-6P as 13.74,  Table. The electronic absorption characters of the distyryl-BODIPYs (4-6) and BODIPY-bridged cyclotriphosphazene derivatives (4P-6P) were studied in dichloromethane (Figure 2A). Maximum absorbance wavelengths of compounds 4-6 were determined as 638, 655, and 638 nm, which is characteristic for distyryl-BODIPY derivatives and caused by S 0 -S 1 transitions [19]. These characteristic transition bands were also observed for compounds 4P-6P at 625, 658, and 639 nm, respectively. The absorption bands of the BODIPYbridged cyclotriphosphazenes were observed to be blue-shifted when compared to the parent BODIPY derivatives (4)(5)(6). Fluorescence emission spectra of the BODIPYs (4-6) and BODIPY-cyclotriphosphazenes (4P-6P) upon excitation at 580 nm are presented in Figure 2B. BODIPY derivative 4 exhibited strong emission at 634 nm with an excitation wavelength at 580 nm in dichloromethane. The fluorescence emission of diiodo-and dibromo-BODIPY derivatives 5 and 6 was ∼678 nm, whereby the fluorescence intensity of dibromo-BODIPY 6 stayed between that of 4 and 5, as expected [30]. The maximum fluorescence emissions of the BODIPY-bridged cyclotriphosphazenes (4P-6P) were at 634, 655, and 653 nm with hypsochromic shifts of 17-25 nm compared to those of related BODIPY derivative fluorescence with a similar but slightly broader spectral shape. The fluorescence quantum yields were calculated via the relative method using cresyl violet as the reference ( Φ ref = 0.54/methanol) [26] and quantified as 0.32, 0.01, and 0.01 for compounds 4-6 and 0.25, 0.05, and 0.14 for 4P-6P, respectively (Table). Moreover, the fluorescence lifetimes were obtained using TCSPC and the results are depicted in Figure 3 and the Table.

Photochemical properties
Photosensitizers can be described as chemical tools that generate reactive oxygen species (ROS) via light illumination [31]. As one of the ROS, singlet oxygen is produced through energy delivery from a photosensitizer's triplet energy state to triplet oxygen (ground state molecular oxygen, 3 O 2 ) [32]. A fashionable strategy to enhance the efficiency of the triplet energy state for BODIPY chemistry is the covalent attachment of iodine or bromine to the 2 and 6 positions of the BODIPY core [30]. Herein, singlet oxygen production of the unsubstituted (4P), diiodo-(5P), and dibromo-(6P) substituted BODIPY-bridged cyclotriphosphazene systems was studied.   was terminated with red LED (630 nm) systematically. Reductions in the absorption bands of DPBF were monitored to enable calculation of singlet oxygen generation yields via the indirect method ( Figure 4). As expected, the absorbance of DPBF as a trap molecule (414 nm) did not display any significant alteration upon irradiation of compound 4P due to the lack of a heavy atom effect, as expected ( Figure 4A). In the case of compounds 5P and 6P, a heavy atom effect (iodine and bromine) was asserted and as soon as irradiation was initiated, about 80% and 14% of the DPBF absorptions vanished in 25 s (Figures 4B and 4C). Moreover, the photooxidation experiment of DPBF was performed with MB as the standard ( Φ ∆ = 0.52 in dichloromethane) under the same experimental conditions ( Figure 4D). Data for the compounds were plotted as the maximum absorption of the trap molecule versus the irradiation time to obtain the slopes of the graphics. The singlet oxygen quantum yields were determined via a relative method using MB as the standard, according to Eq. (3) (see Section 2). The singlet oxygen quantum yields of compounds 5P and 6P were determined as 0.89 and 0.24, respectively. The iodinated BODIPY-cyclotriphosphazene system 5P exhibited more singlet oxygen yield than MB and compound 6P. In addition, the photostabilities of the BODIPY-cyclotriphosphazenes (4P-6P) were studied since photostability upon excitation is an expected behavior [6]. Compounds 4P-6P were excited with the same LED used in the photooxidation studies of DPBF for 20 min and no significant alteration occurred in the absorptions ( Figure 5).
The singlet oxygen generation characters were then examined by measuring singlet oxygen signature phosphorescence at 1270 nm for dyads (4P-6P). Compounds 4P-6P and MB were excited at their excitation wavelengths with a xenon-arc source and detected with a near-IR sensitive detector. Equal absorptivities (0.2) for all of the compounds and MB in dichloromethane were excited and the BODIPY-bridged cyclotriphosphazene derivative bearing iodine substituents (5P) gave the strongest phosphorescence at 1270 nm, which was consistent with the photochemical singlet oxygen experiments. MB and compound 6P also displayed phosphorescence peaks with reduced intensity (Figure 6).

Conclusions
Three novel BODIPY-bridged cyclotriphosphazenes composed of 2 cyclotriphosphazene and unsubstituted-(4P), iodinated-(5P), and brominated-(6P) distyryl BODIPY subunits were designed and prepared via nucle-   to have shifted to blue relative to the parent BODIPY derivatives. The singlet oxygen generation efficiencies and photostabilities of compounds 4P-6P were investigated by both direct and indirect (chemical) methods by comparing them with MB as the standard. The dimer, containing unsubstituted BODIPY-cyclotriphosphazene 4P, exhibited no singlet oxygen generation, as expected, while the heavy atom-substituted iodine (5P) and bromine (6P) were found to have generated singlet oxygen with both methods. Iodination of BODIPY was found to be more effective than bromination to generate triplet photosensitization, and the singlet oxygen quantum yields were calculated with the indirect method and found as 0.89 and 0.24 for 5P and 6P, respectively. We suggest that BODIPY-bridged cyclotriphosphazenes exhibit excellent capacity as photosensitizers and can be used for the development of novel systems.