Novel distyryl BODIPY-fullerene dyads: Preparation, characterization and photosensitized singlet oxygen generation efficiency

Article abstract of DOI:10.1039/c7nj02010d

Novel types of mono- and bis-distyryl-BODIPY-fullerene dyads, 6 and 7, were prepared by Bingel cyclopropanation. Distyryl-BODIPY derivative (3) was reacted with methyl malonyl chloride and malonyl dichloride respectively for the synthesis of compounds 4 and 5 as light-harvesting antennae that contain one and two BODIPY units. Through the Bingel cyclopropanation of these BODIPYs (4 and 5) with fullerene-C₆₀ in the presence of CBr₄ and DBU, distyryl-BODIPY-fullerene dyads 6 and 7 were obtained. All newly synthesized compounds were characterized by MALDI-MS, ¹H and ¹³C NMR and elemental analysis. Moreover, the photophysical and photochemical properties of these heavy atom free dyads were investigated for the determination of their photosensitizer and fluorescence abilities in the near IR region to generate singlet oxygen.

Full text of DOI:10.1039/c7nj02010d

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Novel Distyryl BODIPYꢀFullerene Dyads: Preparation,
Characterization and photosensitized singlet oxygen generation
efficiency
a
a*
Hasan Ünlü , Elif Okutan
a
Department of Chemistry, Faculty of Science, Gebze Technical University, Gebze, Kocaeli,
Turkey
*
Author for correspondence:
Dr. Elif OKUTAN, Department of Chemistry, Gebze Technical University, P.O.Box: 141, Gebze
41400, Kocaeli, Turkey
Tel:
00 90 262 6053091
00 90 262 6053105
eokutan@gtu.edu.tr
Fax:
eꢀmail:

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Abstract
A novel type monoꢀ and bisꢀ distyrylꢀ BODIPYꢀ fullerene dyads 6 and 7 were prepared by
Bingel cyclopropanation. Distyrylꢀ BODIPY derivative (3) was reacted with methyl malonyl
chloride and malonyl dichloride respectively for the synthesis of compounds 4 and 5 as lightꢀ
harvesting antennas that contain one and two BODIPY units. Via the Bingel cyclopropanation of
these BODIPYs (4 and 5) with fullereneꢀ C60 in the presence of CBr and DBU, distyrylꢀ
4
BODIPYꢀ fullerene dyads 6 and 7 were obtained. All newly synthesized compounds were
1
13
characterized by MALDIꢀ MS, H C NMR and elemental analysis. Moreover, the photophysical
and photochemical properties of these heavy atom free dyads were investigated for determination
of their photosensitizer and fluorescence abilities in near IR region to generate singlet oxygen.
Keywords: Borondipyrromethenes, fullerenes, Singlet Oxygen, photochemistry, sensitizers,
UV/Vis spectroscopy

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1. Introduction
The covalent functionalization of the fullerenes to construct versatile adducts has developed
rapidly in the last few years and fullereneꢀ C60 is by far the most studied fullerene, can be
1
,2
considered a versatile building block in organic chemistry. Many fullereneꢀ C ꢀ based, donor–
6
0
acceptor systems, in which fullereneꢀ C is covalently linked to active groups, were synthesized,
6
0
3
ꢀ8
to investigate their photophyisical properties based on electron and energy transfer. The diverse
addition patterns of fullerene based conjugates make them attractive to generate widespread
9
application. For this purpose, several methodologies were developed to covalent linkage of
6
fullereneꢀ C with other molecules. An important application is the designing fullerene adducts
6
0
1
0
for material sciences. Some of these materials could be used in artificial light harvesting
1
1ꢀ13
systems owing to the high capacity of the fullereneꢀ C as an electron acceptor.
Furthermore,
6
0
these incorporations are important headway to understand and mimic the photosynthetic
materials. Amongst the organic dyes, derivatives of BF ꢀ chelated dipyrromethene (BODIPY),
2
“porphyrin’s little sister”, are extensively used in constituting these artificial photosynthetic
1
4
models thanks to their unique properties. BODIPYs generally act as excellent antennae with
favorable photophyisical properties like large molar absorptivities, high fluorescence quantum
yields, and relatively long singlet excited state lifetimes, that can be tuned easily via chemical
1
5, 16
modification of the BODIPY core.
One of the drawbacks of these chromophores is their
relatively short absorption and emission wavelength, which can be extended by introducing two
styryl groups at the 3ꢀ and 5ꢀ positions of the BODIPY core. With more extended πꢀ conjugated
1
7
system, distyryl BODIPYs, can absorb and emit at longer wavelengths (ca. 650 nm). BODIPY
1
8
dyes as photosensitizer are relatively new members of the photosensitizer community.
Alteration of the excited state dynamics via deliberate modifications that promote ISC and
collateral singlet oxygen generation is possible. One general method to enhance ISC involves
substitution of heavy atoms (eg., bromine and iodine) onto the BODIPY core owing to their
1
9,20
heavy atom effect.
However, BODIPYꢀ based photosensitizers that undergo efficient ISC
without requiring the spinꢀ orbital coupling of heavy atom substituents on the core structure
2
1
remained very rare. Thus it is still difficult to attract the triplet state of BODIPYs without heavy
atoms also it is not always convenient to prepare brominated/ iodated organic chromophore, since
2
2
the ISC property of the chromophore cannot be guaranteed. One common strategy to design

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new organic triplet photosensitizers is the functionalization of a known fluorophore, however it is
not applicable to design new organic chromophores as photosensitizer since there is no
2
3ꢀ27
constituted relationship between the ISC and molecular structures.
The idea of preparing BODIPYꢀ fullereneꢀ C dyads originated from the fact that BODIPYs are
6
0
attractive molecules due to their aforementioned unique properties. Moreover, BODIPY
2
8, 29
derivatives are frequently encountered in sophisticated photophysical systems.
Recently, we
have reported the synthesis, characterization, photophyisical/ photochemical properties of new
BODIPYꢀ fullerene and monostyryl BODIPYꢀ fullerene dyads, that showed efficient singlet
2
3
oxygen generation capacities.
To investigate the singlet oxygen capacities of dyads that have chromophore with extended
conjugation herein, we devised visible lightꢀ harvesting distyrylꢀ BODIPYꢀ fullerene (6, 7)
systems as Nearꢀ IR heavy atom free organic triplet photosensitizers (Scheme 1). Distyrylꢀ
BODIPYs were used as the light harvesting antennae, where fullereneꢀ C units act as the spin
6
0
convertor. The dyads (6 and 7) showed strong absorption of visible light with very small
fluorescence quantum yields. The synthesized dyads (6, 7) were used as oxidizer of 1,3ꢀ
1
diphenylisobenzofuran (DPBF) which was driven by photosensitization of singlet oxygen ( O ).
2
First, we synthesized light harvesting antennae distyrylꢀ BODIPY (3) that obtained in high yield.
Distyrylꢀ BODIPY (3) was reacted with methyl malonyl chloride to give BODIPYs 4 and
malonyl dichloride to obtain BODIPYs 5. Bingel cyclopropanation between BODIPYs (4 and 5),
fullereneꢀ C , and carbon tetrabromide gave novel BODIPYꢀ fullerene dyads (6ꢀ 7). The identity
6
0
of newly synthesized BODIPYs (4 and 5) and BODIPYꢀ fullerene dyads (6ꢀ 7) were confirmed
1
13
by H NMR, C NMR, matrixꢀassisted laser desorption/ ionization timeꢀ ofꢀ flight mass
spectrometry and elemental analysis techniques. The new dyads contain one or two different
lightꢀ harvesting antennae associated with long absorption wavelength, resulting in 642 nm
absorption bands.

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Scheme 1. Synthesis of the Distyrylꢀ BODIPYꢀ C Dyads
6
0
2
. Experimental
.1. General
2
Materials
The deuterated solvent (CDCl ) for NMR spectroscopy, silica gel, dichloromethane, piperidine,
3
trifluoroacetic acid, pꢀchloranil, triethylamine, Boron trifluoride diethyl etherate, pyridine and 4ꢀ
metoxybenzaldehyde were provided from Merck. Following chemicals were obtained from
Sigma Aldrich; fullerene, toluene, tetrahydrofuran, glacial acetic acid was purchased from Sigma
Aldrich. 4ꢀmetoxybenzaldehyde, potassium carbonate, 2,4ꢀdimethylpyrrole, methyl malonyl
chloride, malonyl chloride, carbon tetrabromide were purchased from Acros Organics. 3ꢀchloroꢀ

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1ꢀpropanol was provided from Alfa Aeasar. Benzene, acetonitrile were purchased from VWR.
DBU was provided from ABCR. All other chemicals used for the synthesis were reagent grade
unless otherwise specified.
Equipment
Electronic absorption spectra were recorded with a Shimadzu 2101 UV spectrophotometer in the
UVꢀvisible region. Fluorescence excitation and emission spectra were recorded on a Varian
Eclipse spectrofluorometer using 1 cm pathlength cuvettes at room temperature. The fluorescence
lifetimes were obtained using Horibaꢀ Jobinꢀ Yvonꢀ SPEX Fluorolog 3ꢀ2iHR instrument with
Fluoro HubꢀB Single Photon Counting Controller at an excitation wavelength of 620 nm and 630
nm. Signal acquisition was performed using a TCSPC module. Elemental analyses were obtained
using a Thermo Finnigan Flash 1112 Instrument. Mass spectra were acquired in linear modes
with average of 50 shots on a Bruker Daltonics Microflex mass spectrometer (Bremen, Germany)
1
13
equipped with a nitrogen UVꢀ Laser operating at 337 nm. H and C NMR spectra were recorded
in CDCl solutions on a Varian 500 MHz spectrometer. Analytical thin layer chromatography
3
(TLC) was performed on silica gel plates (Merck, Kieselgel 60 Å, 0.25 mm thickness) with F254
indicator. Column chromatography was performed on silica gel (Merck, Kieselgel 60 Å, 230ꢀ400
mesh). Suction column chromatography was performed on silica gel (Merck, Kieselgel 60 Å, 70ꢀ
230 mesh).
2. 2. Synthesis
2
3
Compounds 1 and 2 were prepared according to procedure described previously (Scheme 1).
2.2.1. Synthesis of compound 3
Compound 2 (100 mg, 0.251 mmol) and 4ꢀ(metoxy) benzaldehyde (73 mg, 0.54 mmol) dissolved
in benzene (40 mL). Piperidine (0.3 mL) and glacial acetic acid (0.3 mL) were added. The
solution was refluxed using Deanꢀ Stark apparatus. When the solution was concentrated, reaction
was followed by TLC until dark blueꢀcolored product became the major product. Compound 3

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was purified by silica gel column chromatography using Ethylacetate – Hexane (1:1) (230ꢀ400
mesh) (yield: 36%).
Spectral data of 3: Elemental analyses: Calc. (%) for C H BF N O : C, 71.93; H, 5.88; N, 4.41;
3
8
37
2
2
4
1
found C, 71.87; H, 5.82; N, 4.32. HꢀNMR (500 MHz, CDCl , 298 K, δ ppm): 7.62 (d, J = 16.4
3
Hz, 2H, transꢀ CH), 7.58 (d, J = 8.68 Hz, 4H, ArꢀCH), 7.21 (d, J = 8.71 Hz, 2H, ArꢀCH), 7.20 (d,
J = 16.28 Hz, 2H, transꢀCH), 7.02 (d, J = 8.7 Hz, ArꢀCH, 2H), 6.93 (d, J = 8.81 Hz, 4H, ArꢀCH),
6.61 (s, 2H, ꢀCH), 4.19 (t, J = 5.88 Hz, 2H, ꢀOCH ), 3.92 (t, J = 5.90 Hz, 2H, ꢀCH O), 3.85 (s,
2 2
1
3
6
H, ꢀCH ), 2.10 (m, J = 5.96 Hz, J = 5.94 Hz, 2H, ꢀCH ꢀ), 1.49 (s, 6H, ꢀCH ). CꢀNMR (126
3
1
2
2
3
MHz, CDCl , 298 K, δ ppm): 160.36, 159.33, 152.60, 141.80, 138.17, 135.61, 133.55, 129.78,
3
129.61, 129.01, 127.54, 117.40, 117.32, 114.94, 114.26, 65.69, 60.32, 55.38, 32.04, 14.86. MS
(
(
MALDIꢀTOF) (DIT) m/z (%): Calc.: 633.28 (22.48), 634.28 (100.00), 635.28 (40.53), 636.29
+
+
9.02), 637.29 (1.42), 638.29 (0.18); found: 635.44 [M+H] ; 614.94 [MꢀF] .
2.2.2. Synthesis of compound 4
Compound 3 (35 mg, 0.07 mmol) were dissolved in 15 mL of DCM. Pyridine (11 mg, 0.08
mmol) was added under Ar atmosphere. The mixture cooled on an ice bath and monomethyl
malonyl chloride (10 mg, 0.07 mmol) in 5 mL DCM was added dropwise. The mixture was
stirred for 12 h. Reaction mixture filtered. Solvent was removed. Compound 4 has been isolated
from column chromatography with silica gel (DCM) (230ꢀ400 mesh) (yield: 60%).
Spectral data of 4: Elemental analyses: Calc. (%) for C H BF N O : C, 68.67; H, 5.63; N, 3.81;
4
2
41
2
2
7
1
found C, 68.64; H, 5.61; N, 3.74. HꢀNMR (500 MHz, CDCl , 298 K, δ ppm): 7.62 (d, J = 16.65
3
Hz, 2H, transꢀ CH), 7.58 (d, J = 8.72 Hz, 4H, ArꢀCH), 7.21 (d, J = 8.56 Hz, 4H, ArꢀCH), 7.20 (d,
J = 16.65 Hz, 2H, transꢀCH), 7.01 (d, J = 8.67 Hz, 2H, ArꢀH), 6.93 (d, J = 8.79 Hz, 2H, ArꢀCH),
6
.61(s, 2H, ꢀCH), 4.41 (t, J = 6.3 Hz, 2H, ꢀOCH ), 4.11 (t, J = 6.1 Hz, 2H, ꢀCH O), 3.86 (s, 6H, ꢀ
2
2
OCH ), 3.75 (s, 3H, ꢀOCH ), 3.43 (s, 2H, ꢀCH ꢀ), 2.20 (m, J = 6.22 Hz, J = 6.2 Hz, 2H, ꢀCH ꢀ),
3
3
2
1
2
2
1
3
1
1
1
.49 (s, 6H, ꢀCH ). CꢀNMR (126 MHz, CDCl , 298 K, δ ppm): 166.91, 166.43, 160.35, 159.23,
3
3
52.59, 141.75, 138.11, 135.60, 133.54, 129.79, 129.59, 129.00, 127.58, 117.40, 117.30, 114.91,
14.25, 64.25, 62.39, 55.38, 52.55, 41.32, 28.53, 14.86. MS (MALDIꢀTOF) (No Matrix) m/z
(%): Calc.: 733.30 (22.25), 734.30 (100.00), 735.30 (44.72), 736.30 (11.43), 737.31 (2.12),
+
+
7
38.31 (0.31); found: 734.59 [M] ; 715.88 [MꢀF] .

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2.2.3. Synthesis of compound 5
Compound 2 (50 mg, 0.08 mmol) were dissolved in 15 mL of DCM. Pyridine (7 mg, 0.09 mmol)
was added under Ar atmosphere. The mixture cooled on an ice bath and malonyl chloride (14 mg,
0.10 mmol) in 5 mL DCM was added dropwise. The mixture was stirred for 12 h. Reaction
mixture filtered. Solvent was removed. Compound 5 has been isolated from column
chromatography with silica gel (DCM) (230ꢀ400 mesh) (yield: 56%).
Spectral data of 5: Elemental analyses: Calc. (%) for C H B F N O : C, 70.97; H, 5.58; N,
7
9
74
2
4
4
10
1
4
1
7
4
.19; found C, 70.94; H, 5.54; N, 4.14. HꢀNMR (500 MHz, CDCl , 298 K, δ ppm): 7.88 (d, J =
3
6.44 Hz, 2H, transꢀCH), 7.84 (d, J = 8.57 Hz, 4H, Arꢀ H), 7.49 (d, J = 8.56 Hz, 2H, Arꢀ CH),
.46 (d, J = 16.44 Hz, 4H, transꢀ CH), 7.29 (d, J = 8.61 Hz, 2H, Arꢀ CH), 7.20 (d, J = 8.59 Hz,
H, Arꢀ CH), 6.87(s, 2H, ꢀCH), 4.69 (t, J = 6.26 Hz, 2H, ꢀOCH ), 4.40 (t, J = 5.92 Hz, 2H, ꢀ
2
CH O), 4.12 (s, 6H, ꢀOCH ), 3.75 (s, 1H, ꢀCH ꢀ), 2.49 (m, J = 6.2 Hz, J = 6.1 Hz, 2H, ꢀCH ꢀ),
2
3
2
1
2
2
1
3
1
1
6
1
.76 (s, 6H, ꢀCH ). C NMR (126 MHz, CDCl , 298 K, δ ppm): 166.46, 160.37, 151.52, 141.70,
3
3
35.78, 135.68, 129.84, 129.58, 129.03, 128.25, 127.68, 117.48, 117.44, 117.28, 114.92, 114.26,
2.48, 55.37, 34.24, 29.71, 29.44, 21.19, 14.87. MS (MALDIꢀTOF) (DIT) m/z (%): Calc.:
334.56 (4.22), 1335.56 (37.68), 1336.55 (100.00), 1337.56 (74.53), 1338.56 (32.13), 1339.56
+
+
(
9.86), 1340.56 (2.37), 1341.57 (0.47); found: 1337.86 [M] ; 1317.27 [MꢀF] .
.2.4. Synthesis of compound 6
Compound 4 (20 mg, 0.027 mmol) were dissolved in 15 mL of toluene. C (27 mg, 0.038 mmol)
2
6
0
and CBr (9 mg, 0.027 mmol) was added to the solution. The mixture stirred for 30 min. 1,8
4
diazabicyclo [5.4.0]undecꢀ7ꢀene (DBU, 8.2 mg, 0.054 mmol) in toluene (10 ml) was added
dropwise. The mixture was stirred over night at room temperature. Compound 6 has been isolated
by flash column chromatography with DCM (230ꢀ400 mesh) (yield: 25%).
Spectral data of 6: Elemental analyses: Calc. (%) for C H BF N O : C, 84.30; H, 2.71; N,
1
02 39
2
2
7
1
1
.93; found C, 84.16; H, 2.69; N, 1.83. H NMR (500 MHz, CDCl , 298 K, δ ppm): 7.61 (d, J
3
=
18.04 Hz, 2H, transꢀCH), 7.58 (d, J=8.79 Hz, 4H, ArꢀCH), 7.23 (d, J =7.86 Hz, 2H, ArꢀCH),
.20 (d, J=15.77 Hz, 2H, transꢀCH), 7.04 (d, J =8.44 Hz, 2H, ArꢀCH), 6.93 (d, J =8.34 Hz, 4H,
ArꢀCH), 6.61 (s, 2H, ꢀCH), 4.77 (t, J=5.74 Hz, 2H, ꢀOCH ), 4.21 (t, J = 5.34 Hz, 2H, ꢀCH O),
7
2
2

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4
6
1
1
.08 (s, 3H, ꢀOCH ), 3.86 (s, 6H, ꢀOCH ), 2.41 (m, J = 6.01 Hz, J =6.0 Hz, 4H, ꢀCH ꢀ), 1.49 (s,
3 3 1 2 2
1
3
H, ꢀCH ). C NMR (126 MHz, CDCl , 298 K, δ ppm): 171.10, 160.34, 145.28, 145.22, 145.20,
3
3
44.91, 144.65, 143.89, 143.85, 143.05, 143.03, 142.93, 142.18, 141.91, 141.85, 141.54, 140.99,
38.78, 137.84, 136.57, 129.00, 128.19, 125.26, 60.36, 53.41, 41.34, 36.07, 21.02, 14.17, 11.40.
MS (MALDIꢀTOF) (DHB) (m/z (%): Calc.: 1451.28 (19.44), 1452.28 (100.00), 1453.28 (99.79),
1454.29 (54.26), 1455.29 (20.32), 1456.29 (5.83), 1457.0 (1.36); 1458.30 (0.27) found: 1453.35
+
[
M] .
2.2.5. Synthesis of compound 7
Compound 5 (25 mg, 0.02 mmol) were dissolved in 15 mL of toluene. C (18.7 mg, 0.026
60
mmol) and CBr (6.2 mg, 0.02 mmol) was added to the solution. The mixture stirred for 30 min.
4
1,8 diazabicyclo [5.4.0]undecꢀ7ꢀene (DBU, 5.7 mg, 0.04 mmol) in toluene (10 mL) was added
dropwise. The mixture was stirred over night at room temperature. Compound 7 has been isolated
by flash column chromatography with DCM (230ꢀ400 mesh) (yield: 21%).
Spectral data of 7: Elemental analyses: Calc. (%) for C H B F N O : C, 81.21; H, 3.53; N,
1
39 72
2
4
4
10
1
2
.73; found C, 81.15; H, 3.50; N, 2.66. H NMR (500 MHz, CDCl , 298 K, δ ppm): 7.59 (d, J
3
=
16.95 Hz, 4H, transꢀCH), 7.55 (d, J = 8.40 Hz, 8H, ArꢀCH), 7.22 (d, J =8.45 Hz, 4H, ArꢀCH),
.17 (d, J =16.95 Hz, 4H, transꢀCH), 7.04 (d, J =8.27 Hz, 4H, ArꢀCH), 6.91 (d, J =8.41 Hz, 8H,
ArꢀCH), 6.58 (s, 4H, ꢀCH), 4.76 (t, J = 6.17 Hz, 2H, ꢀOCH ), 4.22 (t, J = 5.85 Hz, 2H, ꢀCH O),
7
2
2
13
3
.84 (s, 12H, ꢀOCH ), 2.40 (m, J = 6.35 Hz, J = 5.86 Hz, 2H, ꢀCH ꢀ), 1.47 (s, 12H, ꢀCH ). C
3 1 2 2 3
NMR (126 MHz, CDCl , 298 K, δ ppm): 192.49, 192.46, 192.45, 180.30, 160.36, 144.74, 143.88,
3
1
1
5
2
43.86, 143.64, 143.19, 143.14, 143.13, 143.09, 143.05, 143.01, 142.80, 142.19, 141.86, 129.92,
29.57, 129.55, 129.50, 129.03, 129.03, 129.01, 128.96, 128.51, 114.25, 114.23, 114.22, 114.20,
3.42, 29.71, 22.66, 22.62, 14.12. MS (MALDIꢀTOF) (DIT) m/z (%): Calc.: 2052.54 (2.86),
053.54 (27.42), 2054.54 (85.03), 2055.54 (100.00), 2056.54 (69.77) 2057.55 (34.40), 2058.55
+
(
13.12), 2059.55 (4.08), 2060.56 (1.08), 2061.56 (0.25); found: 2055.19 [M] .
2.3. The parameters for fluorescence quantum yields
The fluorescence quantum yields (ΦF) of compounds 4ꢀ 7 were determined by the comparative
3
0
method (Eq. (1)).

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(1)
where F and FStd are the areas under the fluorescence emission curves of the compounds
(4ꢀ 7) and the standard, respectively. A and AStd are the respective absorbances of compounds (4ꢀ
11) and the standard at the excitation wavelengths, respectively. The refractive indices (n) of the
solvents were employed in calculating the fluorescence quantum yields in different solvents. Zinc
3
1
(
II) phthalocyanine (Ф = 0.2/ DMSO) was employed as the standard.
F
2.4. The parameters for singlet oxygen quantum yields
Singlet oxygen generating capability of compounds 6 and 7 were done using singlet oxygen trap
molecule 1,3ꢀ diphenylisobenzofuran (DPBF) and methylene blue (MB) as reference. Singlet
oxygen can be monitored using photobleaching and subsequent decrease in absorbance of DPBF.
2
6
30 nm (4.0 mW/cm ) led bulb was used as light source. 630 nm light was exposed from 5 cm
cell distance and absorbances were taken intervals for each solution (Eq. (2)).
(2)
where dyad and ref designate the “BODIPYꢀ fullerene photosensitizer” and “MB” respectively. k
is the slope of difference in change in absorbance of DPBF (414 nm) with the irradiation time. F
−
OD
is the absorption correction factor, which is given by F= 1−10
(OD at the irradiation
2
wavelength), and PF is light intensity (energy flux, mW/ cm ).
3. Results and discussion
3.1. Synthesis and structural characterization
2
3
The 4ꢀ(3ꢀhydroxypropoxy) benzaldehyde (1) and related BODIPY (2) were readily available
and it was quite obvious that this derivative can be converted to distyrylꢀ BODIPY (3) via double
Knoevenagel condensation. Compound 3 was transformed to malonyl moiety functionalized

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BODIPYs (4 and 5) by substitution reaction with methyl malonyl chloride and malonyl
dichloride in DCM in the presence of pyridine at 0 °C to obtain the compounds 4 and 5
respectively. Compounds 4 and 5 were capable of forming methanofullerenes with C . After
6
0
reacting in toluene with fullereneꢀ C60 in the presence of carbontetrabromide and DBU as
described by the literature procedure, removal of side products and unreacted fullereneꢀ C by
6
0
23, 32
column chromatograpy resulted in the isolation of a blue solids (6 and 7).
The isolated solids
1
13
were vacuum dried and identified as the desired products by H, C NMR, MALDIꢀ MS and
elemental analysis and the results were consistent with the assigned formulations (See ESI).
The mass spectra of all compounds (1ꢀ 7) were obtained by MALDIꢀTOF MS and the spectra
1
revealed the peak groups representing the protonated molecular ions (Fig. 1). Wellꢀ resolved H
NMR spectra of distyryl BODIPYꢀ fullerene dyads (6 and 7) showed sets of signals of mesoꢀ
aromatic and distyryl benzene protons for ~7ꢀ 8 ppm region. The observed 16 Hz protonꢀ proton
coupling constants at ~7.61 and ~7.20 ppm for compound 6 and ~7.59 and ~7.17 ppm for
compound 7 proved an E conformation of the double bonds as expected. The pyrrole rings ꢀCH
protons for compounds 6 and 7 appeared as sharp singlets ~6.6 ppm. The ꢀCH were appeared
2
~
4.2ꢀ 4.8 ppm as triplet and ~2.4 ppm as multiplet. The –OCH protons of malonyl moieties in
3
compound 6 showed ~4.1 ppm and styryl –OCH emerged for compounds (6, 7) ~3.8 ppm. The ꢀ
3
CH protons on the BODIPY cores were observed at ~1.4 ppm as sharp singlets (Fig. 2).
3
Fig. 1. MALDIꢀ MS spectra of (A) compound 6, (B) compound 7

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1
Fig. 2. Hꢀ NMR spectra of (A) compound 6, (B) compound 7 in CDCl3
3.2. Photoꢀphysical properties
3.2.1. Electronic absorption spectra
The UV/ Vis absorption spectra of the BODIPYs (4, 5) and dyads 6 and 7 were recorded in
dichloromethane and are shown in Fig. 3A. BODIPYs 4 and 5 showed major characteristic
absorption band at 642 nm that exhibited similar features of distyryl BODIPY compounds,
1
5
corresponding to the πꢀ π* transition of the BODIPY antennae. The molar extinction
4
4
ꢀ1
ꢀ1
coefficients (ε) of BODIPY dyes (4 and 5) at 642 nm were 8.9 x 10 and 18.3 x 10 M cm
respectively. Similarly, the maximum absorption bands of monoꢀ and bisꢀ distyrylꢀ BODIPYꢀ C60
antenna dyads 6 and 7 were observed at 643 nm as broader bands and the molar extinction

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4
4
ꢀ1
ꢀ1
coefficients were 8.1 x 10 and 1.6 x 10 M cm (Table 1). To collect the excitation energy
more efficiently, a photosensitizer should show strong absorption of visible or UV light where the
2
0
molar absorption coefficient of dyad 6 was higher than dyad 7.
The photostability is preferred for many fluorescent photosensitizers. The photostabilities of
BODIPYs (4 and 5) and dyads (6 and 7) were investigated for 60 min in dichloromethane
solutions (Fig. S10). The fluorescence intensities of BODIPYs 4 and 5 and dyads 6 and 7 were
remained the same as initial value. Consequently, it was found that all synthesized compounds 4ꢀ
7
showed excellent photostabilities.
Table 1. Photophyisical and Photochemical features of compounds 4ꢀ7
Absorption
Wavelength
λab (nm)
Emission
Wavelength
λem (nm)
*
**
***
****
ꢀ1
Compound
τF (ns)
ε
,
4
ꢀ1)
1
0 (M cm
4
5
6
7
0.58
0.31
0.03
3.97
8.9
18.3
1.6
642
642
643
644
653
655
655
656
0.04
0.89
0.42
2.49
0.015
0.005
ꢀ
ꢀ
8.1
*
Fluorescence quantum yield ** Singlet oxygen quantum yield
*
** Fluorescence lifetime
**** Molar extinction coefficients
†
below nanosecond level
3.2.2. Fluorescence spectra
The fluorescence emission spectra of compounds were investigated 6.7 µM dichloromethane
solution at room temperature. Upon excitation at the BODIPY units at 620 nm for BODIPYs 4
and 5 and 630 nm for dyads 6 and 7, BODIPYs 4 and 5 gave strong fluorescence bands at 653 nm
where emissions of dyads 6 and 7 effectively quenched (Fig. 3B). Fluorescence emission peaks
of BODIPY (4 and 5) exhibit characteristic emissions of distyryl BODIPYs such as narrow
bandwidth, with high quantum yield. When comparing to these two BODIPYs (4, 5) with the
monoꢀ and bisꢀ distyrylꢀ BODIPYꢀ C dyads (6 and 7) showed lover fluorescence. The presence
6
0

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of the C moiety in BODIPYꢀ fullerene dyads (6 and 7) effectively, leading to virtually invisible
6
0
fluorescence emission (Figure 3B).
Fig. 3. Electronic (A) absorption (B) emission spectra of compounds 4ꢀ 7 in dichloromethane.
3.2.3. Singlet oxygen measurements
Singlet oxygen can be generated via energy transfer from a triplet state of the photosensitizer to
ground state molecular oxygen. Since triplet photosensitizers are not only used for triplet energy
transfer, they find application area in the field of photocatalytic organic reactions, photodynamic
1
33
therapy (PDT), and photooxidation via sensitizing O . A good photosensitizer should absorb
2
the excitation light strongly preferably in the visible light region and has high yield of ISC to
efficiently outcome the triplet state. In general, most transition metal complexes based
photosensitizers, exhibit effective ISC, but weak absorption of visible. Since there is no
established relationship between ISC yield and molecular structure, the complexity of valuation
of the ISC quantum yield of an organic chromophore without heavy atoms generally hobble the
24, 25
developing a new triplet photosensitizer.
1
Herein nearꢀ IR lightꢀ harvesting fullereneꢀ C60 dyads were used as an O sensitizer for
2
photooxidation of DPBF (Fig. 4). The ease of tuning photophyisical properties of BODIPYs dyes

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empowered us to build up dyad systems that allowed working on the nearꢀ IR range. BODIPY
dyes without heavy atom essentially comes with their bright fluorescence and low intersystem
3
4
crossing yield that cause low singlet oxygen quantum yields. Therefore, first we studied
fluorescence quantum yields of compounds 4ꢀ 7. The fluorescence quantum yields (Φ ) of
F
compounds 4ꢀ 7 were measured in dichloromethane and unsubstituted ZnPc in DMSO solution
(λem = 677 nm) was employed as the standard. The fluorescence quantum yields of BODIPYs 4
and 5 and dyads 6 and 7 were found to be 0.58, 0.31, 0.015 and 0.005 respectively (Table 1). As
expected, the fluorescence quantum yields of BODIPYs 4 and 5 were sorely diminished after the
20
espousing with the fullereneꢀ C . Also, the fluorescence lifetimes (τ ) of the BODIPYs 4 and 5
6
0
F
were determined using the time correlated single photon counting (TCSPC) technique in
dichloromethane. The related lifetime spectra obtained using this technique were supplied in Fig.
S5 and were found to be 3.97 ns for BODIPY 4 and 2.49 ns for BODIPY 5. The lifetimes of
prepared dyads 6 and 7 were too low to be able to acquire required data in a nanosecond
instrument. Because emission in these dyads were much lower caused the lower emission
intensity, the lifetimes of dyads 6 and 7 are probably in picosecond level.
Thus, herein dyads 6 and 7 were used as heavy atom free organic triplet photosensitizers for the
formation of endoperoxide from DPBF to produce 1,2ꢀ dibenzoylbenzene. The singlet oxygen
generation capacities of the conjugates were determined by monitoring the dyeꢀsensitized
3
5
photooxidation of 1,3ꢀdiphenylisobenzofuran (DPBF). For this, a solution of the dyad 6 and 7
2
(
6.7 µM) and DPBF (absorbance around 1.1) was irradiated using red LED (4.0 mW/cm ) and the
decrease in the absorption band of DPBF at 414 nm was monitored. In order to facilitate
correlation to previously reported sensitizers, the activity was studied in dichloromethane.
Initially, to annihilate any contributions to the absorbance signals of DPBF from obscure
reactions without light, all solutions were kept for 20 min in dark. As expected, absorbance peaks
of DPBF for all solutions did not lead any chance. Upon irradiation of BODIPYꢀ fullerene dyads
3
1
(
6 and 7) (6.7 ꢀM) in aerated dichloromethane solutions, O was sensitized to O by the dyads,
2
2
1
DPBF was then decomposed by the reactive O , thus the characteristic absorption of DPBF at
2
414 nm decreased drastically almost 70% and 55% in 60 second (Fig. 4). The presence of singlet
oxygen was monitored by consistent decrease of the selective singlet oxygen trap molecule.
Singlet oxygen caused remarkable degradation of DPBF after each irradiation for 10 s of dyads 6

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21
and 7 under the light from 630 nm red led light. Under the same experimental conditions,
singlet oxygen measurements of the dyad molecules (6 and 7) and methylene blue as the
reference molecule were performed. Each solutions of dyads (6ꢀ 7) without DPBF were also
triggered with light for 15 min and no changes were observed in the absorbance intensities that
confirmed these compounds were not degraded under the conditions of singlet oxygen
measurement. Singlet oxygen quantum yields (Φ ) of compounds (6 and 7) were calculated
∆
2
3
according to the literature. Methylene blue used as the reference compound had a singlet
oxygen quantum yield of 0.57 under the conditions of the study in dichloromethane (Fig. S13).
Singlet oxygen quantum yields (Φ ) of dyads (6 and 7) were found 0.89, and 0.42 in
∆
dichloromethane, respectively. Singlet oxygen generation capacities of heavy atom free
BODIPYs 4 and 5 were also checked to assure the fullerenes ISC effect and be able to
comprehend with the capacities of dyads (Fig. S14 and Fig. S15). As expected the singlet oxygen
capacities of heavy atom free BODIPYs were too low and singlet oxygen quantum yields were
calculated as 3% and 4% respectively. After the cyclopropanation of fullerene to the BODIPYs 4
and 5 the Φ values were drastically decreased ~98% indicating enhanced ISC in dyads 6 and 7
F
2
5
(
Φ + Φ = 1; nonꢀradiative channel is neglected). Φ of dyad 6 was calculated to be 1.5ꢀ fold
F ISC ꢁ
3
6
that of the methlyne blue (Φ = 0.57) and 2.0ꢀ fold that of rose Bengal (Φ = 0.43). The ISC
ꢁ
ꢁ
yield was found to be much less efficient in dyad 7, which was substituted with bis BODIPY
antennae’s but showed lower molar extinction coefficient. Φ value of dyad 7 was calculated as
ꢁ
0.42 that is close to the rose Bengal. Since, light harvesting antennae of the dyads (6 and 7) were
designed to contain one and two BODIPY moiety, one may expect to see regularity in the singlet
oxygen generation abilities. However, the outcome was quite contrary to the expectation. This is
2
3ꢀ27
because there is no established relationship between the ISC and molecular structures.
Even a
slight derivatization, may diminish the ISC. The work reported in this and our previous paper
reminded us to use caution when designing other potential systems as photosensitizer.

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Fig. 4. Decrease in absorbance spectrum of DPBF in the presence of (A) compound 6 (6.7 ꢀM),
B) compound 7 (6.7 ꢀM) in dichloromethane.
(
Conclusion
In conclusion, novel monoꢀ and bisꢀ distryl BODIPYꢀ fullerene dyads were prepared and
1
13
characterized via elemental analysis, MALDIꢀ MS, H and C NMR spectra. The dyads showed
1
strong absorption of red light. Upon photoexcitation, the dyads were used as singlet oxygen ( O )
2
photosensitizers and the photosensitizing efficiency of the dyads compared to the known
methylene blue and rose Bengal triplet photosensitizers. We propose the BODIPYꢀ fullerene
dyads can be used as a structure motif for heavyꢀatomꢀfree organic triplet photosensitizers, for
which the absorption can be readily tuned by using different antennas to be able to work on
convenient range of visible light.
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Novel Distyryl BODIPY-Fullerene Dyads: Preparation,
Characterization and photosensitized singlet oxygen generation
efficiency
a
a*
Hasan Ünlü , Elif Okutan
a
Department of Chemistry, Faculty of Science, Gebze Technical University, Gebze, Kocaeli,
Turkey