New perylenebisimide decorated cyclotriphosphazene heavy atom free conjugate as singlet oxygen generator

Article abstract of DOI:10.1016/j.saa.2019.117232

Perylenebisimide-cyclotriphosphazene based inorganic-organic system was synthesized by a multistep procedure. The substitution reaction of asymmetric perylenebisimide (PBI) derivative with the hexachloroyclotriphosphazene (trimer) resulted in the formation of fully PBI decorated cyclotriphosphazene (5). The identity of newly synthesized compound (5) was confirmed by using ³¹P, ¹H and ¹³C NMR spectroscopies and mass spectrometry. The photophysical (UV- Vis absorption, fluorescence emission, fluorescence lifetime and fluorescence quantum yield) and photochemical (the singlet oxygen generation, and photostability) properties of this conjugate were investigated as novel heavy atom free triplet photosensitizer. The singlet oxygen quantum yield of the PBI-cyclotriphosphazene (5) was calculated to be 0.86 which is good for a heavy atom free triplet photosensitizer. These results will add to the development of cyclotriphosphazene based heavy atom free singlet oxygen triplet photosensitizer systems for applications in organic oxygenation reactions.

Full text of DOI:10.1016/j.saa.2019.117232

Accepted Manuscript
New perylenebisimide decorated cyclotriphosphazene heavy atom
free conjugate as singlet oxygen generator
Elif Okutan, Hande Eserci, Elif Şenkuytu
PII:
S1386-1425(19)30622-5
DOI:
https://doi.org/10.1016/j.saa.2019.117232
Article
Number:
Reference:
117232
SAA 117232
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
6 February 2019
27 May 2019
31 May 2019
Please cite this article as: E. Okutan, H. Eserci and E. Şenkuytu, New perylenebisimide
decorated cyclotriphosphazene heavy atom free conjugate as singlet oxygen generator,
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/
10.1016/j.saa.2019.117232
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ACCEPTED MANUSCRIPT
New Perylenebisimide Decorated Cyclotriphosphazene Heavy Atom
Free Conjugate as Singlet Oxygen Generator
Elif OKUTANa*, Hande ESERCİa, Elif ŞENKUYTUa
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:
e-mail:
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Abstract
Perylenebisimide-cyclotriphosphazene based inorganic-organic system was synthesized by a
multistep procedure. The substitution reaction of asymmetric perylenebisimide (PBI) derivative
with the hexachloroyclotriphosphazene (trimer) resulted in the formation of fully PBI decorated
cyclotriphosphazene (5). The identity of newly synthesized compound (5) was confirmed by
31
13
using P, 1H and C NMR spectroscopies and mass spectrometry. The photophysical (UV- Vis
absorption, fluorescence emission, fluorescence lifetime and fluorescence quantum yield) and
photochemical (the singlet oxygen generation, and photostability) properties of this conjugate
were investigated as novel heavy atom free triplet photosensitizer. The singlet oxygen quantum
yield of the PBI-cyclotriphosphazene (5) was calculated to be 0.86 which is good for a heavy
atom free triplet photosensitizer. These results will add to the development of
cyclotriphosphazene based heavy atom free singlet oxygen triplet photosensitizer systems for
applications in organic oxygenation reactions.
Keywords: perylene, cyclotriphosphazene, photophysical, photochemistry, singlet oxygen,
photosensitizer
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1. Introduction
Chlorocyclotriphosphazenes are robust heterocyclic rings, which can be derivatized with
numerous significant nucleophiles owing to the six pendant chlorine groups on the phosphorous
atoms [1]. In the past few decades a great amount of scientific attention was devoted to the
substitution reactions of and determination of properties of the resulting phosphazene derivatives
[2,3]. The potential applications of the chloro and organocyclophosphazene systems find usage in
areas in technology and medicine such as component in the preparation of the nanocomposites
[4-7], flame retardant additives [8], elastomers [9], liquid crystals [10], fluorescence
chemosensors [11], electrolytes for lithium-ion batteries [12] and singlet oxygen generators [13-
15]. Lately numerous studies show the use of cyclic phosphazenes as templates for the
construction of new multimodular systems especially by using chromophore groups such as
pyrene, naphthalene, naphtalimide, BODIPY to investigate photophysical and photochemical
properties [13, 16-18]. Specifically, iodinated/brominated-BODIPY-cyclotriphosphazene dyads
bearing heavy atoms or BODIPY-cyclotriphosphazene-fullerene triads as heavy atom triplet
photosensitizers to produce singlet oxygen where fullerene act as spin converter to enhance
intersystem crossing were extensively studied including our group and others [13-15,17]. Singlet
oxygen (1O2), the lowest excited state of molecular dioxygen, is one of the well-known reactive
oxygen species (ROS). It has substantial function in a considerable number of reactions which
display significance in chemical, biological, and environmental processes [19,20]. Recently we
reported the synthesis of three new perylene-cyclotriphosphazene systems bearing glycol groups
for efficient exfoliation of SWCNTs [21]. PBI (perylenebisimide) derivatives were commonly
used because of their optical properties, as well as chemical and thermal stability due to their
highly π-conjugated rigid molecular structure. Also, their molecular structures can be controllably
modified.
Here, we report on the synthesis, characterization, photophysical and photochemical properties of
a hexa-PBI substituted cyclotriphosphazene derivative in which PBI is substituted at the N
positions by a branched amine group. The presence of branched aliphatic chains in asymmetric
PBIs significantly enhances their solubility and inhibits the intermolecular interaction
aggregation and enhance the reactivity in solution. We demonstrate the synthesis of a PBI-
clotriphosphazene derivative (5) bearing six PBI unit in one molecule on a stable inorganic core.
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Providing a molecule that served as an efficient heavy atom free singlet oxygen generator. The
second part of the project was to investigate the photophysical and photochemical activities of the
newly synthesized PBI-cyclotriphosphazene (5) molecule.
2. Experimental
2.1. Materials
The deuterated solvent (CDCl3), silica gel, hydrochloric acid, pH colorplast were provided from
Merck. Following chemicals were purchased from Sigma Aldrich; ethanol, dichloromethane,
methanol, dihexyl ketone and ammonium acetate. Natrium cyanoborhydride, perylene-3,4,9,10-
tetracarboxylic dianhydride, t-buthanol, N,N-dimethylacetamide and phosphonitrilic chloride
were obtained from Acros Organics. Potassium hydroxide, imidazole and zinc acetate anyhdrous
were provided from Alfa Aeasar. Methanol was obtained from VWR. 6-aminohexanol and
tridecan-7-amine were purchased from T.C.I. All other chemicals were reagent grade.
2.2. Equipment
Electronic absorption spectra were recorded with a Shimadzu 2101 UV spectrophotometer in the
UV-visible region. The 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 Horiba- Jobin-Yvon-SPEX Fluorolog 3-2iHR instrument with
Fluoro Hub-B Single Photon Counting Controller at an excitation wavelength of 500 nm and 570
nm. Signal acquisition was performed using a TCSPC module. The Mass spectra were acquired
in linear modes with average of 50 shots on a Bruker Daltonics Microflex mass spectrometer
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31
(Bremen, Germany) equipped with a nitrogen UV-Laser operating at 337 nm. H, C and P
NMR spectra were recorded in CDCl3 solutions on a Varian 500 MHz and Bruker 500 mHz
spectrometers. Analytical thin layer chromatography (TLC) were performed on silica gel plates
(Merck, Kieselgel 60 Å, 0.25 mm thickness) with F254 indicator. Column chromatography were
performed on silica gel (Merck, Kieselgel 60 Å, 230-400 mesh). Suction column chromatography
was performed on silica gel (Merck, Kieselgel 60 Å, 70-230 mesh).
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2.3. The parameters for fluorescence quantum yields
The fluorescence quantum yields (ΦF) of the compound (4,5) were determined by the
comparative method in dichloromethane by comparing with the fluorescence of Rhodamine 6G
(ΦF=0.94 in EtOH) [26] as a standard (Eq. (1)) [27].
(1)
ΦF(Std) is the fluorescence quantum yield of standard. F and FStd are the areas under the
fluorescence emission curves of samples (4,5) and the standard, respectively. The A and AStd
stands for the respective absorbance of the samples and standard at the same excitation
wavelength. ɳ2 and ɳ2 are the refractive indices of solvents used for the sample and standard,
Std
respectively. The concentration of the solutions at the excitation wavelength fixed at 0.5µM.
2.4. The parameters for singlet oxygen quantum yields
Singlet oxygen generating capacity of compounds (4,5) were investigated using singlet oxygen
trap molecule 1,3- diphenylisobenzofuran (DPBF) and methylene blue (MB) as reference [28].
Singlet oxygen can be monitored using photobleaching and subsequent decrease in absorbance of
DPBF. 516 nm (90.50 μmol m−2 s−1) and 630 nm (210.7 μmol m−2 s−1) LED bulb were used as
light source and exposed from 5 cm cell distance and absorbances were taken intervals for each
solution (Eq. (2)).
(2)
where 5 and ref designate the “PBI-cyclotriphosphazene photosensitizer” and “MB” respectively.
k is the slope of absorbances of DPBF (414 nm) with the difference in change in irradiation time.
F is the absorption correction factor, that is given by F= 1−10−OD (OD is absorption at the
irradiation wavelength), and PF is photon flux (μmol m−2 s−1).
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3. Synthesis
Compounds 1-4 were synthesized according to the literature (Scheme 1) [21].
3.1. Synthesis of compound 5
Compound 4 (658 mg, 0.94 mmol) was dissolved in tetrahydrofuran in a 25 mL round-bottomed,
two-necked flask under an argon atmosphere. NaH (38 mg, 0.94 mmol, 60%) was added to the
reaction mixture and heated at reflux temperature of THF for 2 hours. Trimer (50 mg, 0.14 mmol)
was added to the reaction mixture. The reaction was stirred under reflux with a magnetic stirrer at
66 °C for 4 days. The reaction mixture was separated from the sodium chloride salts by filtration
through G4 filter. The solvent (THF) was removed by partial vacuuming on the rotary
evaporator. Compound 5 was isolated from the reaction mixture (23 mg, 4%, red solid) in silica
gel (70- 230 mesh) using dichloromethane: methanol (97: 3) as solvent.
Spectral data of compound 5: MS (MALDI-TOF) (DIT) m/z (%): Calc. for (C258H282N15O30P3):
4166.09; found 4166.67 [M]+ (Fig.S13). 31P NMR- decoupled to (202 MHz, CDCl3, 298 K) δ= -
19.98 (s, 3P, P(OPery)2); Spin system: A3 (Fig.S14). 31P NMR- coupled to (202 MHz, CDCl3, 298
K) δ= -19.97 (br, s, 3P, P(OPery)2); Spin system: A3 (Fig.S15). 1H-NMR (500 MHz, CDCl3, 298
K) δ= 8.73-8.59 (br, m, 8H), 5.24 -5.17 (m, 1H), 4.23 (t, 2H, J = 7.78 Hz), 3.69 (q, J = 6.0 Hz,
2H), 2.32-2.23 (m, 2H), 1.93-1.86 (m, 2H), 1.83 -1.78 (m, 2H), 1.66-1.61 (m, 2H), 1.51 (t, J = 3.6
Hz, 4H), 1.38-1.30 (m, 6H), 1.29-1.14 (m, 10H), 0.85 (t, J = 7.0 Hz, 6H) (Fig.S16). 13C NMR
(126 MHz, CDCl3, 298 K): δ= 163.46, 134.77, 131.46, 123.15, 123.02, 62.81, 54.83, 40.44,
32.61, 32.39, 31.78, 29.23, 28.02, 26.95, 25.30, 22.60, 14.06 (Fig.S17).
4. Results and discussion
As known the perylene core has three different positions for functionalization: ortho- (α-), bay-
(β-) and imide [29]. A key problem in perylene chemistry is their reluctance to solubility, because
of their planar and rigid nonpolar structure [30] whereas the introduction of a long
chain/branched alkyl groups to the N position enhances their solubility [31]. Thus, we have
imported a branched long chain amine to the N position of perylene core for solubility and
prepared asymmetric PBI derivative 4 via three-step reactions. Linking of the asymmetric PBI
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derivative 4 to the cyclotriphosphazene (trimer) was accomplished conveniently through
nucleophilic substitution reaction by using NaH as base in tetrahydrofuran (Scheme 1). All
1
compounds were fully characterized by 31P, H, 13C NMR and mass spectra. The detailed
procedures with the characterization data are given in the Experimental Section and ESI (Fig.S1-
S17).
Scheme 1. Synthesis of the Perylene-Cyclotriphosphazene (5)
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The MALDI-TOF mass spectrum of the PBI-cyclotriphosphazene (5) clearly show the calculated
1
molecular ion peak at m/z of 4166.67 (Fig. S13). Comparison of the HNMR spectra of
compounds 4 and 5 showed same pattern where the peaks of the compound 5 were observed to be
broader and all integrals confirmed proposed structure (Fig S16). The proton decoupled 31P NMR
spectrum of the compound 5 exhibited a first order 31P NMR spectrum as a sharp singlet peak at -
19.98 ppm because of the chemical environment equivalence of all the phosphorus nuclei (Fig
S14). It was unexpected to observe the peak at such upfield region where even the geminal
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substituted partial PBI derivatives of cyclotriphosphazene P NMR spectra were given parallel
resonance manner.
The electronic absorption behavior of newly synthesized PBI-cyclotriphosphazene (5) was
studied in dichloromethane. Maximum absorbance wavelength of compound (5) were determined
to be 458, 488, 528 nm which is characteristic for PBI derivatives (Fig. 1A) [32]. The molar
extinction coefficient of the PBI derivative (4) and PBI-cyclotriphosphazene (5) were 10.2×104
M-1 cm-1 and 11.8×104 M-1 cm-1, respectively (Table 1) (Fig.S18 and S19). Fluorescence spectra
of the PBI derivative (4) and the PBI-cyclotriphosphazene (5) upon excitation at 490 nm are
presented in Fig. 1B. The fluorescence emission maxima were recorded at 535, 573 and 622 nm
for compound 5. As seen from the Fig. 1B, compound 5 was found to be more absorptive than
compound 4 whereas the calculated fluorescence quantum yield of the parent compound (4) was
higher than perylene-cyclotriphosphazene (5) in dichloromethane. Using rhodamine 6G as the
reference ΦF=0.94 in EtOH [26] the fluorescence quantum yields were calculated to be 0.56 and
0.23 for compounds 4 and 5, respectively. The fluorescence lifetimes (τF) of the compounds 4
and 5 were determined using the time correlated single photon counting (TCSPC) technique in
dichloromethane. The related lifetime spectra obtained using this technique are shown in Fig. S21
and τF values were found to be 3.92 and 3.89 ns for compounds 4 and 5, respectively. Usually,
the τF’s for the PBIs with groups connected at the N-positions are around 3-4 ns as reported in the
literature and are dependent on solvent polarity [33]. The τF’s of the compounds (4, 5) are
consistent with the previously published results. The photophysical data of perylene derivative
(4) and PBI-cyclotriphosphazene (5) are listed in Table 1.
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Fig. 1 Electronic (A) absorption (B) emission spectra of compounds 4 and 5 in dichloromethane.
Table 1. Photophysical features of compounds 2-5
Absorption
Wavelength
λab (nm)
Emission
Wavelength
λem (nm)
Stokes
Shift
‡ΦΔ
Compound
*ε,
**τF (ns)
†ΦF
(nm)
104 (M-1 cm-1)
10.2
11.9
3.92
CHISQ=1.438
3.89
0.56
0.23
-
4
5
456, 487, 521
458, 488, 528
530, 572, 623
535, 573, 622
9
7
0.86
CHISQ=1.588
* Molar extinction coefficients
Fluorescence quantum yield
** Fluorescence lifetime
‡ Singlet oxygen quantum yield
†
CHISQ: Chi-square
In our continued effort to synthesize new heavy atom free singlet oxygen generators, especially
those bearing more than one light harvesting moieties in a single molecule which does not
demand heavy atoms for photosensitization. An inorganic heterocycle, cyclotriphosphazenes can
be used as a primary building block to prepare multi modular scaffolds [13, 15, 21]. The
absorbance of DPBF and compound 5 were adjusted in cuvette and to evaluate any possible side
reactions in dark media, the solutions were kept in dark for 15 min. whereas the absorbance peaks
of DPBF and compound 5 did not indicate any change. Then the green LED light (516 nm) was
turned on to excite compound 5 (2.1 µM) which immediately started to produce singlet oxygen.
The formation of singlet oxygen was pursued via absorbance variation of the trap at 414 nm.
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Singlet oxygen caused significant and regular degradation after each irradiation (10 s) (Fig. 2).
As reference photosensitizer methylene blue in dichloromethane was also studied under the same
experimental conditions to calculate the singlet oxygen quantum yield (Fig S21). Singlet oxygen
quantum yield of compound (5) was calculated as 0.86 in dichloromethane. Also,
dichloromethane solution of PBI-cylotrihosphazene (5) without DPBF was triggered with green
LED light for 20 min and no substantial change was observed in the absorbance intensities which
exhibit the photostability of the compound 5 (Fig. S22). Previously we reported the BODIPY-
cyclotriphosphazene- fullerene triad bearing one and three fullerene units as heavy atom free
triplet-photosensitizers [13, 14]. In these molecules singlet oxygen generation experiments
indicated the positive effect of fullerene-C60 unit as spin converter. Furthermore, Eçik et. al.
previously reported the singlet oxygen production of heavy atom substituted BODIPY-
cyclotriphosphazene dendrimeric molecules which also needed the effect of a heavy atom to
produce singlet oxygen. One of the practical examples of heavy atom free triplet photosensitizers
are orthogonal bis-BODIPYs with ~0.4-0.5 singlet oxygen quantum yields [34]. On the other
hand, PBIs are highly versatile dyes and pigments, which utilized as sensitizing agent for singlet
oxygen [35-38]. The perylene moiety, derivatized via heavy atom free substituents or ligands
bearing metals have been examined and have proven to be comparable photosensitizer to those of
typical singlet oxygen generators [39]. First examples of heavy atom free PBI derivatives were
prepared by Akkaya group which demonstrate efficient singlet oxygen generation [38] and Wu
et. al. reported the singlet oxygen quantum efficiencies of di(perylenebisimide) derivatives in the
range between 0.60-0.70 [36]. Since the high fluorescence quantum yields (ΦF) of PBIs [40]
generally propound negligible intersystem crossing efficiency, the enhancement of triplet-state
population for PBI dyes generally achieved via heavy atom functionalization [41-43] or
preparation of frameworks which enable nonradiative processes via nonplanarity of the distorted
cores [36,38]. Herein we developed a full PBI substituted cyclotriphosphazene derivative which
does not demand the effect of heavy atom and still produce singlet oxygen effectively possibly
caused by distortion of π-conjugated structures after substitution to the cyclotriphosphazene core.
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Fig. 2 Decrease in absorbance spectrum of DPBF in the presence of compound 5 (2.1 M) in
dichloromethane.
Conclusion
Without any doubt, triplet photosensitizers play a fundamental role in many applications such as
photocatalytic reactions which is independent from whether they are applied in organic or
hydrolytic media. In this article we presented novel example of the PBI-cyclotriphosphazene
system as heavy atom free triplet photosensitizer to present the first example of
cyclotriphosphazene based PBI dendrimeric system which does not require the presence of a spin
converter with a high singlet oxygen quantum yield (0.86).
Acknowledgment
This work was supported by the TUBITAK project no: 215-Z- 250
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New Perylenebisimide Decorated Cyclotriphosphazene Conjugate as
Heavy Atom Free Singlet Oxygen Generator
Elif OKUTANa*, Hande ESERCİa, Elif ŞENKUYTUa
a
Department of Chemistry, Faculty of Science, Gebze Technical University, Gebze,
Kocaeli, Turkey
Graphical abstract
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ACCEPTED MANUSCRIPT
New Perylenebisimide Decorated Cyclotriphosphazene
Conjugate as Heavy Atom Free Singlet Oxygen Generator
Hande ESERCİa, Elif ŞENKUYTUa, Elif OKUTANa*
a
Department of Chemistry, Faculty of Science, Gebze Technical University, Gebze,
Kocaeli, Turkey
Highlights
. Synthesis of novel cyclotriphosphazene derivatives bearing perylenediimide side groups
. Investigation of photophyisical and photochemical properties
. Heavy atom free triplet photosensitizers
. Investigation of singlet oxygen generation efficiency
17