Novel BODIPY-bridged cyclotriphosphazenes

Article abstract of DOI:10.3906/kim-1907-44

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,

Full text of DOI:10.3906/kim-1907-44

Turk J Chem
Turkish Journal of Chemistry
(2020) 44: 74 – 86
© TÜBİTAK
http://journals.tubitak.gov.tr/chem/
doi:10.3906/kim-1907-44
Research Article
Novel BODIPY-bridged cyclotriphosphazenes∗
Hande ESERCİ, Ezel ÖZTÜRK, Elif OKUTAN∗∗
Department of Chemistry, Faculty of Science, Gebze Technical University, Gebze, Kocaeli, Turkey

Received: 21.07.2019
•
Accepted/Published Online: 17.10.2019
•
Final Version: 11.02.2020
Abstract: Three new 2-component unsubstituted (4P), diiodo- (5P), and dibromo- (6P) distyryl-BODIPY-bridged cy-
clotriphosphazene dimers were designed and synthesized. The newly synthesized BODIPY-cyclotriphosphazene systems
1
13
31
were characterized by H,
C, and
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 4
P 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.
Key words: Boron-dipyrromethene, cyclotriphosphazene, photophysical, UV-Vis spectroscopy, photochemical
1. 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, mem-
branes, microlithography, and ionic crystals [2–5]. 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 [7–9]. Moreover, phosphazenes possess thermal and
chemical stabilities under different conditions, which is very important for specific applications areas [10,11].
Nucleophilic substitution reactions of phosphazenes with functional groups (alcohols, amines, thiols, etc.) have
been widely studied [12–15]. Recently, derivatization of a phosphazene core with organic chromophores has
attracted much attention. Specifically, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene BODIPY-cyclophosphazene
systems were synthesized and characterized by several groups to investigate their photophysical and photo-
chemical properties [16–18].
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
∗Dedicated to Prof. Dr. Adem Kılıç on the occasion of his retirement.
∗∗
Correspondence: eokutan@gtu.edu.tr
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This work is licensed under a Creative Commons Attribution 4.0 International License.

ESERCİ et al./Turk J Chem
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 cyclot-
riphosphazene dimers (4P–6P). Unsubstituted, iodinated, and brominated distyryl-BODIPY derivatives (4–6)
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
1
13
P, H, and 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.
2. Experimental
2.1. Materials
Chloroform-d (CDCl3), p-chloroanil, boron trifluoride diethyl etherate, silica gel, acetic acid, triethylamine, tri-
fluoroacetic acid, and piperidine were obtained from Merck (Darmstadt, Germany). Subsequent chemicals were
obtained from Sigma-Aldrich (St. Louis, MO, USA): ethanol, ethyl acetate, iodic acid, N-bromosuccinimide,
dichloromethane, tetrahydrofuran 2,4-dimethyl pyrrole, and glacial acetic acid. 4-Hydroxy-benzaldehyde, ben-
zene, and cesium carbonate were obtained from Alfa Aesar (Haverhill, MA, USA). The rest of the chemicals
used in the synthesis were reagent grade unless otherwise specified.
2.2. 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
NIR PMT 5509 (Hamamatsu, Japan) at –80 ◦ C. The fluorescence lifetime experiments were performed using a
Horiba Jobin-Yvon-SPEX Fluorolog 3-2iHR instrument at excitation wavelengths with time-correlated single-
photon counting (TCSPC) module for signal acquisition. Mass spectra were obtained in linear modes on a
Bruker Daltonics Microflex mass spectrometer (Billerica, MA, USA) equipped with a nitrogen UV-laser at 337
31
1
13
nm. The P, H, and C NMR spectra were provided in solutions (CDCl3) with a Varian spectrometer (500
MHz). Analytical thin-layer chromatographies (TLC) were carried out on silica gel plates (Merck, Kieselgel
60 Å, 0.25-mm thickness with F254 indicator). Suction column chromatographies were made with silica gel
(100–200 mesh).
2.3. Parameters for fluorescence quantum yields
The fluorescence quantum yields (ΦF ) of compounds 4–6 and 4P–6P were calculated by the relative procedure
in Eq. (1) [25]:
F×AStd×n2
ΦF = ΦF Std
,
(1)
75
FStd×A×n2Std

ESERCİ et al./Turk J Chem
where the areas under the fluorescence emission curves of the compounds (4–6 and 4P–6P) and the standard are
F and FStd , respectively. A and AStd 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 (ΦF = 0.54/methanol) [26] was used as the standard.
Scheme. Synthesis of BODIPY-bridged cyclotriphosphazenes (4P–6P).
2.4. 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.
76

ESERCİ et al./Turk J Chem
(2):
I∆ (compound) ×A(std)
Φ∆=Φ∆ (std)
(2)
I∆ (std) ×A(compound)
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
2
decrease in absorbance of DPBF. A 630-nm (4.0 mW/cm ) 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)):
[
] [
] [
]
k(compound)
k(ref)
F(ref)
PF(ref)
Φ∆(compound) =Φ∆(ref)
.
(3)
F(compound) PF(compound)
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
2
wavelength), and PF is the light intensity (energy flux, mW/cm ).
2.5. Synthesis
Compounds 1–4 were synthesized with respect to the literature (Scheme) [27–29].
2.5.1. 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%).
Spectral data of 5: MS (MALDI-TOF) (DIT) m/z (%): calc. for C33 H25 BF2 I2 N2 O2 : 784.19; found:
+
1
3
784.15 [M ]. H NMR (500 MHz, CDCl3 , 293 K, δ ppm): 8.00 (d, JH−H = 16.62 Hz, 2H, trans-CH), 7.41
3
3
(d, JH−H = 15.71, 2H, trans-CH), 7.40 (d, JH−H = 8.37, 6H, Ar-CH), 7.25 (br, 1H, Ar-CH), 7.17 (dd,
3JH−H = 6.29, JH−H = 6.30, 2H, Ar-CH) 6.75 (d, JH−H = 8.52 Hz, 4H, Ar-CH), 1.32 (s, 6H, -CH3).
3
3
13
C
NMR (126 MHz, CDCl3 , 293 K, δ ppm): 162.43, 154.48, 149.59, 143.46, 142.09, 139.18, 136.66, 133.28, 132.44,
132.29, 129.33, 115.75, 119.71, 29.53.
2.5.2. 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%).
77

ESERCİ et al./Turk J Chem
+
1
Spectral data of 6: MS (MALDI-TOF) (DHB) m/z (%): calc.: 690.19; found: 690.06 [M ]. H NMR
3
3
(500 MHz, CDCl3 , 293 K, δ ppm): 8.02 (d, JH−H = 16.6 Hz, 2H, trans-CH), 7.50 (d, JH−H = 16.9, 2H,
3
trans-CH), 7.46 (m, 7H, Ar-CH), 7.22 (m, 2H, Ar-CH), 6.80 (d, JH−H = 8.2 Hz, 4H, Ar-CH), 1.33 (s, 6H,
13
-CH3).
C NMR (126 MHz, CDCl3 , 293 K, δ ppm): 158.47, 148.41, 140.85, 139.19, 134.80, 131.79, 129.39,
129.33, 129.27, 128.66, 128.31, 115.73, 115.19, 29.55, 25.40, 13.50.
2.5.3. Synthesis of compound 4P
A 100-mL round-bottomed flask was charged with tetrahydrofuran (50.0 mL) and purged with Ar for 15 min.
Compound 4 (100 mg, 0.19 mmol) and cesium carbonate (159 mg, 0.49 mmol) were added to the reaction flask
and stirred for 30 min. Trimer (163.0 mg, 0.47 mmol) was poured into the reaction mixture and the reaction
mixture was stirred for 16 h at room temperature. The precipitated salt (CsCl) was filtered off and the solvent
was removed at reduced pressure. The resulting white solid was subjected to column chromatography using
n-hexane and ethyl acetate (3:2) as the mobile phase (yield: 55%).
+
31
Spectral data of 4P: MS (MALDI-TOF) (NOM) m/z (%): calc.: 1154.77; found: 1154.18 [M ].
P
2
2
NMR (202 MHz, CDCl3 , δ ppm): 22.47 (d, JP −P = 60.59 Hz, 2P, PCl2), 12.12 (t, JP −P = 60.59 Hz,
1
3
1P, PClOPh) spin system: A2 X. H NMR (500 MHz, CDCl3 , 293 K, δ ppm): 7.72 (d, JH−H =16.5 Hz,
2H, trans-CH), 7.68 (d, JH−H =7.8 Hz, 4H, Ar-CH), 7.53 (m, 3H, Ar-CH), 7.34 (m, 6H, Ar-CH), 7.24 (d,
3JH−H =16.9 H, 2H, trans-CH), 6.66 (s, 2H, -CH), 1.48 (s, 6H, -CH3). C NMR (126 MHz, CDCl3 , 293 K,
3
13
δ ppm): 152.27, 149.51, 149.41, 142.60, 139.72, 135.26, 134.92, 134.44, 133.59, 129.21, 128.94, 128.29, 121.78,
121.73, 120.12, 117.93, 25.62.
2.5.4. Synthesis of compound 5P
A 100-mL round-bottomed flask was charged with tetrahydrofuran (50.0 mL) and purged with Ar for 15 min.
Compound 5 (150 mg, 0.191 mmol) and cesium carbonate (149 mg, 0.459 mmol) were added to the reaction
flask and stirred for 30 min. Trimer (132.7 mg, 0.382 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 ethyl
acetate (3:1) as the mobile phase (yield: 40%).
Spectral data of 5P: MS (MALDI-TOF) (DIT) m/z (%): calc. for C33 H23 BCl10 F2 I2 N8 O2 P6 : 1406.56;
+
31
2
found: 1406.58 [M ].
P NMR (202 MHz, CDCl3 , δ ppm): 22.49 (d, JP −P = 61.08 Hz, 2P, PCl2), 11.93
2
1
(t, JP −P = 61.08 Hz, 1P, PClOPh) spin system: A2 X. H NMR (500 MHz, CDCl3 , 293 K, δ ppm): 8.06
(d, JH−H = 16.58 Hz, 2H, trans-CH), 7.52 (d, JH−H = 8.06 Hz, 2H, Ar-CH), 7.49 (d, JH−H = 16.57 Hz,
2H, trans-CH), 7.44 (br, 1H, Ar-CH), 7.40–7.37 (m, 4H, Ar-CH), 7.17–7.12 (m, 6H, Ar-CH) 1.43 (s, 6H, -CH3).
3
3
3
13
C NMR (126 MHz, CDCl3 , 293 K, δ ppm): 147.52, 141.84, 107.64, 104.96, 104.93, 66.93, 34.54, 31.99, 29.09,
25.38, 23.71, 23.17.
2.5.5. 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
78

ESERCİ et al./Turk J Chem
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%).
+
31
Spectral data of 6P: MS (MALDI-TOF) (DIT) m/z (%): calc.: 1312.56; found: 1312.091 [M ].
P
2
2
NMR (202 MHz, CDCl3 , δ ppm): 22.5 (d, JP −P = 60.88 Hz, 2P, PCl2), 11.96 (t, JP −P = 60.83 Hz, 1P,
1
3
PClOPh) spin system: A2 X. H NMR (500 MHz, CDCl3 , 293 K, δ ppm): 8.07 (d, JH−H = 17.1 Hz, 2H,
trans-CH), 7.67 (d, JH−H = 8.6, 4H, Ar-CH), 7.62 (d, JH−H = 17.2 Hz, 2H, trans-CH), 7.53 (m, 3H, Ar-CH),
7.30 (d, JH−H = 8.7 Hz, 6H, Ar-CH), 1.41 (s, 6H, -CH3).
3
3
3
13
C NMR (126 MHz, CDCl3 , 293 K, δ ppm):
129.51, 129.50, 129.15, 129.12, 128.06, 121.74, 121.70, 118.79, 110.56, 67.93, 53.36, 25.49, 13.69.
3. Results and discussion
3.1. 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 liter-
ature [27–29]. 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 4-
hydroxybenzaldehyde in benzene using a Dean-Stark apparatus via Knoevenagel condensation conditions [27].
BODIPYs (4–6) in tetrahydrofuran solutions were then reacted with an excess of hexachlorocyclotriphosp-
hazene (trimer) in the presence of Cs2 CO3 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
31
1
13
BODIPYs (1–6) and BODIPY-cyclotriphosphazenes (4P–6P) were performed using P, H, and C NMR
spectroscopy and mass spectrometry. The results confirmed the established formulations, where the proton-
31
decoupled P NMR spectra of BODIPY-bridged cyclotriphosphazenes 4P–6P displayed the expected AX2
spin systems with 2 sets of signals corresponding to the P-OPhCl groups at ∼11.9–12.1 ppm and the PCl2
groups at ∼22.5 ppm.
3.2. 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. I2 -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 µM 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,
79

ESERCİ et al./Turk J Chem
Figure 1. Absorption spectra of compounds (A) 4P, (B) 5P, and (C) 6P in various solvents.
7.87, and 10.40 M−1 × cm−1 , respectively, in dichloromethane. The photophysical data are summarized
in the 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 S0 –S1 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 BODIPY-
bridged cyclotriphosphazenes were observed to be blue-shifted when compared to the parent BODIPY derivatives
(4–6). 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 (Φref
= 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.
3.3. 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
3
triplet energy state to triplet oxygen (ground state molecular oxygen, O2) [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.
80

ESERCİ et al./Turk J Chem
Figure 2. (A) UV-Vis absorption (2 µM) and (B) fluorescence emission (1 µM, λex = 580 nm) features of compounds
4–6 and 4P–6P.
Figure 3. Fluorescence decay profiles of compounds (A) 4–6 and (B) 4P–6P using a laser excitation source of 670 nm.
The singlet oxygen formation abilities of the systems were first identified by chemical method via pursuing
the photooxidation of DPBF [6]. The 2 µM dichloromethane solutions of the BODIPY-cyclotriphosphazenes
(4P–6P) and DPBF (absorbance set to ∼2.3 ± 0.1) were first kept in the dark for 15 min, to eliminate
possible side reactions, and then the solutions were irradiated for 5 s until the DPBF (414 nm) absorption
81

ESERCİ et al./Turk J Chem
Table 1. Photophysical and photochemical features of compounds 4–6 and 4P–6P.
4
Compound Absorption Emission
wavelength
∆
∗ε, 10 (M−1 cm−1
)
∗∗τF (ns) ∗∗∗ΦF †Φ∆
(nSmto)kes
Êwavelength
λab (nm)
586, 638
601, 655
591, 638
576, 625
609, 658
591, 639
λem (nm)
651, 711
677
678
634, 693
655
4
5
6
4P
5P
6P
13
21
21
10
16
14
7.18
5.22
6.44
13.74
7.87
10.40
3.57
1.78
3.22
4.33
1.68
4.03
0.32
0.01
0.01
0.25
0.05
0.14
-
-
-
-
0.24
0.89
653, 712
*Molar extinction coefficients,
**fluorescence lifetime,
***fluorescence quantum yield,
†singlet oxygen quantum yield via chemical method.
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).
3.4. 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-
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ESERCİ et al./Turk J Chem
Figure 4. Absorbance spectra of DPBF after each irradiation in the presence of compounds (A) 4P, (B) 5P, (C) 6P,
and (D) MB in dichloromethane (2 µM).
Figure 5. The photostabilities of BODIPY-cyclotriphosphazene conjugates (4P–6P).
ophilic substitution reaction. Photophysical properties, such as UV-Vis absorption, fluorescence emission, fluo-
rescence quantum yield, and lifetime of the parent BODIPY derivatives (4–6) and BODIPY-cyclotriphosphazenes
(4P–6P) were studied. Absorption bands of the BODIPY-bridged cyclotriphosphazenes (4P–6P) were found
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Figure 6. Singlet oxygen phosphorescence with sensitization of compounds 4P–6P and MB in dichloromethane
(maximum absorptions were set to 0.2 for all compounds).
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 quan-
tum 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.
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