Bodipy-based photosensitizers with long alkyl tails at the meso position: efficient singlet oxygen generation in Cremophor-EL micelles

Article abstract of DOI:10.1016/j.tetlet.2016.02.033

Bodipy dyes with n-decyloxyphenyl-(4, 5) and pentadecyl-(8) meso substituents can easily embed themselves into micellar structures formed from Cremophor-EL. In micelles of approximately 20 nm median size, heavy-atom substituted dyes show remarkable photos

Full text of DOI:10.1016/j.tetlet.2016.02.033

Tetrahedron Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Bodipy-based photosensitizers with long alkyl tails
at the meso position: efficient singlet oxygen generation
in Cremophor-EL micelles
a
b
c
c
a,b,
⇑
Bilal Kilic , Nisa Yesilgul , Veli Polat , Zuhal Gercek , Engin U. Akkaya
a
UNAM-National Nanotechnology Research Center, Bilkent University, 06800 Ankara, Turkey
Department of Chemistry, Bilkent University, 06800 Ankara, Turkey
Department of Chemistry, Bulent Ecevit University, 67100 Zonguldak, Turkey
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Bodipy dyes with n-decyloxyphenyl-(4, 5) and pentadecyl-(8) meso substituents can easily embed them-
selves into micellar structures formed from Cremophor-EL. In micelles of approximately 20 nm median
size, heavy-atom substituted dyes show remarkable photosensitization properties as evidenced by the
rate of reaction with an anthracene-based selective singlet oxygen trap in buffered aqueous solutions.
Considering the ease of Bodipy derivatization and the advantages of Cremophor-EL carried therapeutic
agents, these photosensitizing agents may offer novel targeting opportunities and enhanced chemical
and photophysical stability.
Received 28 December 2015
Revised 4 February 2016
Accepted 9 February 2016
Available online xxxx
Keywords:
Bodipy
Photosensitizers
Singlet oxygen
Organic chromophores
Organic photochemistry
Ó 2016 Elsevier Ltd. All rights reserved.
Introduction
One of the most effective methods of drug delivery is using lipo-
2
1,22
somal or micellar systems.
In principle it becomes straightfor-
Photodynamic therapy is a promising methodology for the
treatment of certain cancerous and non-cancerous diseases.
ward to include various targeting groups in addition to the active
agent itself into a micellar or liposomal construct. In this work,
we targeted Bodipy derivatives with heavy atom substituents
which would facilitate intersystem crossing. In addition, to ensure
that these compounds would prefer a micellar structure as
opposed to bulk aqueous solution, we incorporated long alkyl
chains at the meso positions (C-8) of the Bodipy dyes. The micelle
forming agent examined was Cremophor EL (Kolliphor EL), a syn-
thetic, non-ionic surfactant made by the reaction of Castor oil
(mostly triglyceride) with ethylene oxide, which provides a poly-
ethylene glycol chain.
1
–8
The treatment protocol involves bringing together three compo-
nents, namely light, molecular oxygen, and a photosensitizer. In
cases where the excited photosensitizer can efficiently undergo
intersystem-crossing to the triplet manifold, excitation energy in
turn, can be transferred to the ground state (triplet) molecular oxy-
gen generating singlet excited molecular oxygen. Singlet oxygen
produced in this way, is the primary cytotoxic agent in photody-
9
namic therapy.
Bodipy-based photosensitizers have received considerable
attention as alternative photosensitizers in recent years.8
,10–14
The
reasons for this attention are twofold: unlike other photostable
Results and discussion
1
5
16,17
dyes such as PDIs and squaraines,
the absorption peak of
these dyes are easily tunable to use the entire visible and even
0
The synthetic plan for 8-alkyl and 8-(4 -alkoxy)aryl derivatives
the near IR region of the spectrum,18 and there are multiple routes
differs to some extent (Scheme 1). p-Hydroxybenzaldehyde was
converted to the corresponding decyl ether by treating it with 1-
bromodecane in acetonitrile at reflux. 4-Decyloxybenzaldehyde
2) was then first treated with 2,4-dimethylpyrrole and TFA in
DCM, followed by oxidation with DDQ. Without isolation, the
dipyrrin intermediate was then treated with Et N and BF O to
1
9,20
to transform these dyes into efficient singlet oxygen generators.
With these considerations, and due to their strong absorptivity in
the visible region, it is clear that Bodipy dyes offer significant poten-
tial comparable to porphyrins and phthalocyanines.
(
3
ꢀEt
3 2
yield the green fluorescent Bodipy dye 3. The 2,6-positions of the
Bodipy core were then brominated and iodinated using different
⇑
Corresponding author. Tel.: +90 312 290 3570; fax: +90 312 266 4068.
E-mail address: eua@fen.bilkent.edu.tr (E.U. Akkaya).
http://dx.doi.org/10.1016/j.tetlet.2016.02.033
040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
0
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2
B. Kilic et al. / Tetrahedron Letters xxx (2016) xxx–xxx
Scheme 1. Synthesis of the targeted photosensitizers 4, 5, and 8.
protocols. The reaction with iodic acid and iodine in ethanol at
0 °C gave diiodo compound 4, while treatment of 3 with NBS at
6
room temperature resulted in compound 5.
In a different approach, palmitoyl chloride was reacted with
2
,4-dimethylpyrrole followed by installation of the difluoroboron
bridge to yield dye 7. Iodination was performed as for compound
, with iodic acid and iodine in ethanol at 60 °C, to give diiodo com-
4
1
13
pound 8. All new compounds were characterized by H, C NMR
spectroscopy and HRMS. Absorption spectra of the photosensitiz-
ers were acquired in DCM, and the major absorption bands in the
visible corresponding to S
30 nm (Fig. 1).
Micelles containing Bodipy compounds 4, 5, and 8 were pre-
0 1
–S transition were located around
5
pared following literature protocols.2
charge of the micelles were studied using dynamic light scattering
ESI). The micelles vary in size between 13 and 27 nm. Absorption
3–25
The size and the surface
(
and emission spectra for the micelle embedded compounds were
also acquired in buffered aqueous solutions and were not altered
to any significant extent.
The singlet oxygen generation capacity of the photosensitizers
Figure 1. Normalized absorption spectra of photosensitizers 4, 5, and 8 in DCM.
in the micelles were studied using a selective singlet oxygen trap
0
2
,2 -(anthracene-9,10-diyl)bis(methylene)dimalonic acid. The
absorbance of the trap molecule was adjusted to be approximately
.0 in an oxygen saturated solution. Micellar photosensitizers at a
concentration of 4.0 M in aqueous buffer solutions were excited
in the presence of the trap compound. Initially, a few data points
were acquired to eliminate any possibilities of a reaction occurring
in the dark. The cuvettes were then exposed to an LED array opti-
mized for 520 nm. The change in the absorption spectra of the
anthracene based trap molecule can be clearly seen in Figure 2. It
is important to note that when kept in the dark, no changes in
absorption took place, and also, in the absence of photosensitizers
under irradiation with the same source, no change in the absorp-
tion took place (ESI).
The data for each photosensitizer were plotted as the change in
absorption of the trap molecule at 382 nm versus irradiation time
(Fig. 3). It is clear that all three photosensitizers show significant
efficiencies for singlet oxygen generation.
1
l
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B. Kilic et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
confident that similarly functionalized Bodipy dyes will find prac-
tical applications within the context of photodynamic therapy. Our
work along that direction is currently in progress.
Acknowledgments
The authors are grateful to Department of Chemistry and
UNAM, Bilkent University; Department of Chemistry Bulent Ecevit
University.
Supplementary data
Supplementary data (all experimental methods and procedures,
additional spectroscopic and analytical data) associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1
016/j.tetlet.2016.02.033.
References and notes
1
2
3
4
5
.
.
.
.
.
Bonnett, R. Chem. Soc. Rev. 1995, 24, 19.
Dolmans, D. E. J. G. J.; Fukumura, D.; Jain, R. K. Nat. Rev. Cancer 2003, 3, 380.
Brown, S. B.; Brown, E. A.; Walker, I. Lancet Oncol. 2004, 5, 497.
Castano, A. P.; Mroz, P.; Hamblin, M. R. Nat. Rev. Cancer 2006, 6, 535.
Robertson, C. A.; Hawkins Evans, D.; Abrahamse, H. J. Photochem. Photobiol. B
2009, 96, 1.
0
Figure 2. Singlet oxygen mediated bleaching of the trap molecule (2,2 -(anthra-
cene-9,10-diyl)bis(methylene)dimalonic acid) in the presence of Bodipy 5 (4.0 M)
l
in water. The light source was an LED array with peak emission at 520 nm, fluence
2
rate 2.5 mW/cm .
6. Lovell, J. F.; Liu, T. W. B.; Chen, J.; Zheng, G. Chem. Rev. 2010, 110, 2839.
7
8
.
.
Zhao, J. Z.; Wu, W. H.; Sun, J. F.; Guo, S. Chem. Soc. Rev. 2013, 42, 5323.
Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew, L. V.; Chung, L. Y.; Burgess, K. Chem.
Soc. Rev. 2013, 42, 77.
9.
Dougherty, T. J.; Gomer, C. J.; Henderson, B. W.; Jori, G.; Kessel, D.; Korbelik, M.;
Moan, J.; Peng, Q. J. Nat. Cancer Inst. 1998, 90, 889.
1
1
0. Erbas-Cakmak, S.; Akkaya, E. U. Angew. Chem., Int. Ed. 2013, 52, 11364.
1. Kolemen, S.; Isik, M.; Kim, G. M.; Kim, D.; Geng, H.; Buyuktemiz, M.; Karatas, T.;
Zhang, X. F.; Dede, Y.; Yoon, J.; Akkaya, E. U. Angew. Chem., Int. Ed. 2015, 54,
5
340.
2. Erbas-Cakmak, S.; Bozdemir, O. A.; Cakmak, Y.; Akkaya, E. U. Chem. Sci. 2013, 4,
58.
3. Turan, I. S.; Cakmak, F. P.; Yildirim, D. C.; Cetin-Atalay, R.; Akkaya, E. U. Chem.
Eur. J. 2014, 20, 16088.
1
1
8
14. Topel, S. D.; Cin, G. T.; Akkaya, E. U. Chem. Commun. 2014, 8896.
15. Ozcan, O.; Yukruk, F.; Akkaya, E. U.; Uner, D. Top. Catal. 2007, 44, 523.
16. Oguz, U.; Akkaya, E. U. Tetrahedron Lett. 1998, 39, 5857.
17. Kukrer, B.; Akkaya, E. U. Tetrahedron Lett. 1999, 40, 9125.
18. Buyukcakir, O.; Bozdemir, O. A.; Kolemen, S.; Erbas, S.; Akkaya, E. U. Org. Lett.
2009, 11, 4644.
19. Cakmak, Y.; Kolemen, S.; Duman, S.; Dede, Y.; Dolen, Y.; Kilic, B.; Kostereli, Z.;
Yildirim, L. T.; Dogan, A. L.; Guc, D.; Akkaya, E. U. Angew. Chem., Int. Ed. 2011, 50,
11937.
2
2
0. Huang, L.; Yu, X. R.; Wu, W. H.; Zhao, J. Z. Org. Lett. 2012, 14, 2594.
1. Ballut, S.; Makky, A.; Loock, B.; Michel, J.-P.; Maillard, P.; Rosilio, V. Chem.
Commun. 2009, 224.
22. Temizel, E.; Sagir, T.; Ayan, E.; Isik, S.; Ozturk, R. Photodiag. Photodyn. Ther.
2
014, 11, 537.
3. Synthesis of photosensitizer 4: Compound 3 (90.0 mg, 0.189 mmol) and iodine
200 mg, 0.78 mmol) and ethanol (50 mL) were added to a 250 mL round
2
0
Figure 3. Singlet oxygen mediated bleaching of the trap molecule (2,2 -(anthra-
(
cene-9,10-diyl)bis(methylene)dimalonic acid) in the presence of photosensitizers 4,
lM) in PBS buffered water (pH 7.2). Absorbance at 382 nm was plotted
as a function of time. The light source was an LED array with peak emission at
bottomed flask, and to this solution was added iodic acid (111 mg, 0.63 mmol)
in water (2 mL). The reaction mixture was stirred at 60 °C and the progress was
5, and 8 (4.0
monitored by TLC (1:1 CHCl
Na (50 mL) was added and the product extracted with CHCl
The solvent was removed in vacuo and the residue purified by silica gel column
chromatography using 1:1 CHCl
/Hexanes as eluant. Red solid (80 mg, 70%). 1
NMR (400 MHz, CDCl , 300 K): d = 7.14 (d, J = 8.5 Hz, 2H), 7.05 (d, J = 8.5 Hz,
H), 4.04 (t, J = 6.5 Hz, 2H), 2.66 (s, 6H), 1.84 (m, 2H), 1.52 (s, 6H), 1.47 (m, 2H),
3
/Hexanes). Upon completion, saturated aqueous
2
5
20 nm, and a fluence rate of 2.5 mW/cm .
2
S
2
O
3
3
(3 ꢁ 50 mL).
3
H
3
H
Singlet oxygen quantum yields in Cremophor EL micelles in
2
1
aqueous solutions were calculated (ESI) and vary between 0.14
and 0.46. Interestingly, the bromo derivative, photosensitizer 5,
seems to be the most effective compound in micelle-embedded
form. A singlet oxygen quantum yield of almost 50% in a micellar
construct is remarkable and compares very well with well-known
13
3 C
.32 (m, 12H), 0.91 (t, J = 6.7 Hz, 3H). C NMR (100 MHz, CDCl ): d = 160.2,
156.5, 145.4, 141.8, 129.0, 126.4, 115.4, 85.6, 68.3, 32.0, 29.6, 29.5, 29.4, 29.3,
+
2
6.1, 22.7, 17.2, 16.0, 14.2. HRMS (ESI) calcd for C29
H
37BF I
2 2
2
N O (MꢂH)
730.10199, found 730.10249; = 0.68 ppm.
D
2
4. Synthesis of photosensitizer 5: Compound 3 (75 mg, 0.23 mmol) was dissolved
in DMF/DCM (1:1, 50 mL). Then, N-bromosuccinimide (370 mg, 2.05 mmol) in
DCM (25 mL) was added to the reaction mixture dropwise over 15 min. The
reaction mixture was stirred at room temperature for 2 h. The progress of the
reaction was monitored by TLC. The reaction mixture was extracted with water
2
6
photosensitizers.
Conclusion
and DCM and the organic phase dried with Na
in vacuo and the residue purified by silica gel column chromatography using
2
SO
/Hexanes as eluant. Yield: 165 mg (95%). 1H NMR (400 MHz, CDCl
,
3
4
. The solvent was removed
3
3
:1 CHCl
00 K): d
3
We have reported examples of Bodipy based photosensitizers
containing micelle embedding side-chains. We have demonstrated
that in aqueous solutions, stable micelles of such compounds can
be prepared which act as effective photosensitizers. We are
H
= 7.15 (d, J = 8.7 Hz, 2H), 7.03 (d, J = 8.7 Hz, 2H), 4.04 (t, J = 6.6 Hz,
2H), 2.62 (s, 6H), 1.85 (m, 2H), 1.53 (m, 2H), 1.45 (s, 6H), 1.32 (m, 12H), 0.91 (t,
J = 6.8 Hz, 3H). 13C NMR (100 MHz, CDCl
): d = 160.2, 153.7, 142.5, 140.7,
30.9, 129.0, 126.1, 115.4, 111.7, 68.3, 31.9, 29.6, 29.6, 29.4, 29.3, 29.2, 26.1,
3
C
1
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4
2
B. Kilic et al. / Tetrahedron Letters xxx (2016) xxx–xxx
+
2
2.7, 14.1, 13.9, 13.6. HRMS (ESI) calcd for C29
found 635.12963; = 8.23 ppm.
5. Synthesis of photosensitizer 8: Compound 5 (400 mg, 0.80 mmol) and iodine
500 mg, 1.78 mmol) were added to ethanol (50 mL) in a 250 mL round
H37BBr
2
F
2
N
2
O (MꢂH) 635.1244,
chromatography using 1:1 CHCl
H NMR (400 MHz, CDCl
3
–Hexanes as eluant. Red solid (500 mg, 80%).
, 300 K): d = 3.01 (t, J = 8.4 Hz, 2H), 2.63 (s, 6H), 2.49
(s, 6H), 1.63 (m, 2H), 1.52 (m, 2H), 1.29 (m, 22H), 0.91 (t, J = 6.8 Hz, 3H).
NMR (100 MHz, CDCl ): d = 155.2, 146.4, 142.2, 131.4, 86.3, 31.9, 31.7, 30.3,
29.7, 29.7, 29.6, 29.6, 29.5, 29.4, 29.4, 29.4, 22.7, 18.9, 16.1, 14.1. HRMS (ESI)
1
D
3
H
13
C
(
3
C
bottomed flask, and to this solution was added iodic acid (400 mg, 2.27 mmol)
+
in water (3 mL). The reaction mixture was stirred at 60 °C and was monitored
calcd for C28
H
43BF
I
2 2
N
2
(MꢂH) 709.16, found 709.16034;
D = 0.56 ppm.
by TLC (1:1 CHCl
50 mL) was added, and the product extracted with CHCl
solvent was removed in vacuo and the residue purified by silica gel column
3
/Hexanes). Upon completion, aqueous saturated Na
2
2
S O
3
26. For a compilation of quantum yields of various photosensitizers, please check
the following website: https://www3.nd.edu/~ndrlrcdc/Compilations/QY/
IntroQY.htm.
(
3
(3 ꢁ 50 mL). The
Please cite this article in press as: Kilic, B.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.02.033