A subphthalocyanine-pyrene dyad: Electron transfer and singlet oxygen generation

Article abstract of DOI:10.1039/c7pp00166e

A light harvesting subphthalocyanine-pyrene dyad has been synthesized and characterized by linking pyrene (Py) with subphthalocyanine (SubPc) at its axial position with the B-O bond through the para position of the benzene group. Upon photoexcitation at the pyrene unit of the dyad, an efficient electron transfer from the singlet-excited state of Py to SubPc was observed. The electron transfer features were also observed by exciting the SubPc entity, but with slower rates (～10⁸ s^-1). From the electrochemical measurements, the negative driving forces for charge separation via both the singlet states of Py and SubPc in the polar solvents indicate that the electron transfer is thermodynamically feasible. Interestingly, the examined compounds showed relatively high efficiency for producing the singlet oxygen (Φ_Δ = ～0.70). The collected data suggested the usefulness of the examined subphthalocyanine-pyrene dyad as a model of light harvesting system, as well as a sensitizer for photodynamic therapy.

Full text of DOI:10.1039/c7pp00166e

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10.1039/C7PP00166E.
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DOI: 10.1039/C7PP00166E
Photochemical & Photobiological Science
PAPER
Subphthalocyanine–pyrene dyad: Electron transfer and
singlet oxygen generation
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Mohamed E. ElꢀKhouly,a,b* Ahmed ElꢀRefaey,a Wonwoo Nam,b* Shunichi
Fukuzumi,b,c* Özge Göktuğd and Mahmut Durmuꢁd*
www.rsc.org/
Light harvesting subphthalocyanineꢀpyrene dyad has been synthesized and characterized by linking
pyrene (Py) with subphthalocyanine (SubPc) at its axial position with the BꢀO bond through the
para position of the benzene group. Upon photoexcitation at the pyrene unit of the dyad, an
efficient electron transfer from the singletꢀexcited state of Py to SubPc was observed. The electron
transfer features were also observed by exciting the SubPc entity, but with slower rates (~ 108 s–1).
From the electrochemical measurements, the negative driving forces for charge separation via both
singlet states of Py and SubPc in the polar solvents indicate that the electron transfer is
thermodynamically feasible. Interestingly, the examined compounds showed relatively high
efficiency for producing the singlet oxygen (Φꢀ = ~ 0.70). The collected data suggested the
usefulness of the examined subphthalocyanineꢀpyrene dyad as a model of light harvesting system,
as well as a sensitizer for photodynamic therapy.
porphyrinoids, subphthalocyanines (SubPcs) have attracted
special attention in the recent years as photoactive building
blocks for the artificial photosynthetic systems,26 organic
Introduction
solar cells,27ꢀ29 and organic light emitting diodes OLED30,31
applications for their conically shape with three Nꢀfused units
arranged around a central boron atom and aromatic πꢀelectron,
strong absorption in the visible region (500–700 nm), high
energy of the singlet (2.16 eV) and triplet (1.40 eV) states,
and relatively low reorganization energies.32ꢀ47 In the reported
SubPcꢀbased molecular systems, SubPcs were linked with
other photoactive species via different routes involving
peripheral and/or axial approaches.32ꢀ47
Development of relatively simple donorꢀacceptor models
designed to mimic the events of photosynthetic reaction
center has been an important goal for light energy
conversions and development of molecular electronic
devices.1ꢀ25 In the past years, various conjugated macrocycles
such as porphyrins, phthalocyanines and naphthalocyanines
were widely employed as building blocks for the photoactive
and electroꢀactive assemblies.1ꢀ25 Among numerous
a
On the other hand, pyrene derivatives can utilize as
electron donors and electron acceptors in the light harvesting
systems.48 Due to its planar and rigid structure, incorporation
of pyrene unit into molecular materials would offer an
effective way to control the πꢀπ stacking interactions and
enhance charge carrier mobility.49 In addition, pyrene
Department of Chemistry, Faculty of Science, Kafrelsheikh University,
Kafrelsheikh 33516, Egypt
b Department of Chemistry and Nano Science, Ewha Womans
University, Seoul 03760, Korea
c Faculty of Science and Engineering, Meijo University, SENTAN, Japan
Science and Technology Agency (JST), Nagoya, Aichi 468ꢀ8502, Japan.
d Department of Chemistry, Faculty of Basic Sciences, Gebze Technical
University University, Gebze 41400, Turkey
chromophores
showed
unique
photophysical
and
photochemical properties, such as high fluorescence quantum
yields, long excitedꢀstate lifetimes, and good thermal
stabilities.50ꢀ53
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Taking these properties into consideration, we present here respectively. Measurements of fluorescence quantum yields
the photoinduced electron transfer of a dyad composed of were carried out on a Hamamatsu C9920−0X(PMAꢀ12)
subphthalocyanine (SubPc) covalently bonded with the U6039−05 fluorescence spectrofluorometer. Fluorescence
pyrene entity (Py) at the axial position through BꢀO bond (Fig. lifetimes and singlet oxygen measurements have been
recorded by utilizing a high performance fluorescence
lifetime spectrometer FluoTime 300 (PicoQuant, Germany).
Lifetimes were evaluated with software FluoFit attached to
the equipment. Nanosecond timeꢀresolved transient
absorption measurements were carried out using the laser
system provided by UNISOKU Co., Ltd. Measurements of
nanosecond transient absorption spectrum were performed
according to the following procedure. A mixture solution in a
quartz cell (1 cm × 1 cm) was excited by a Nd:YAG laser
(Continuum SLIIꢀ10, 4ꢀ6 ns fwhm). The photodynamic
measurements were monitored by continuous exposure to a
xenon lamp for the visible region and halogen lamp for the
nearꢀIR region as a probe light and a photomultiplier tube
(Hamamatsu 2949) as a detector. All measurements were
carried out in deaerated solvents.
1). Such combination between the curved πꢀconjugation
system of the SubPc and the planar πꢀconjugation of pyrene
via axial approach has yet to be reported. Advantage of the
axial approach is to preserve the electronic characteristics of
the SubPc macrocycles.32ꢀ47,54ꢀ60 This behavior is quite
different from the reported pyreneꢀfused subphthalocyanine
molecule via peripheral approach, which causes an alteration
of the spectroscopic properties of subphthalocyanine entity.61
The photochemical behavior of the subphthalocyanineꢀpyrene
(SubPcꢀPy) dyad has been explored by using the steadyꢀstate
absorption and fluorescence, cyclic voltammetry and timeꢀ
resolved emission techniques. In addition, the ability of the
examined subphthalocyanines to act as an efficient sensitizer
for photodynamic therapy has been reported in this study.
Fast and efficient electron transfer
Electrochemical measurements in tetrahydrofuran were
performed on a potentiostat / Galvan state / ZRAꢀ05087
electrochemical analyzer. The measurements were taken by
using carbon electrode as working electrode, silverꢀsilver
chloride (Ag/AgCl) as a reference electrode and platinum as a
counter electrode. All measurements were carried out in
h
ν
345 nm
tetrahydrofuran
containing
tetraꢀnꢀbutylammonium
hexafluorophosphate (TBAPF6, 0.1 M) as the supporting
electrolyte at the scan rate = 40 mV s–1.
!
hν
500 nm
Slow electron transfer
Compound 2 was synthesized and characterised according
to literature procedures (Scheme 1).63 Compound 3 axially
terminal iodophenol substituted boronsubphthalocyanine 2
(50 mg, 0.081 mmol) and pyreneboronic acid (40 mg, 0.16
mmol) were dissolved in 1,4ꢀdioxane (5 mL) and then
Pd(PPh3)4 (37.63 mg, 0.03 mmol) and Cs2CO3 (530.47 mg,
1.63 mmol) were added to this solution in argon atmosphere.
This reaction mixture was stirred at reflux temperature until
complete conversion of the starting material to the product.
After cooling to room temperature, this mixture was extracted
with CH2Cl2/water. The organic CH2Cl2 phase was dried by
anhydrous Na2SO4 and then filtered off. The residue was
purified by preparative thinꢀlayer chromatography with silica
gel using CH2Cl2 solution as an eluent. The target
subphthalocyanine 3 was obtained as a pink solid. Yield 36
Fig. 1 Molecular structure of the examined SubPcꢀpyrene (SubPcꢀPy) dyad.
Experimental
1,4ꢀDioxane and toluene were dried as described in Perrin
and Armarego before use.62 Chlorosubphthalocyanine (1), 4ꢀ
iodophenol,
pyreneboronic
acid,
Cs2CO3
and
tetrakis(triphenylphosphine)palladium(0) were purchased
from Aldrich. The chromatographic methods such as thinꢀ
layer chromatography (TLC) (Silica gel 60 HF254) and
preparative thinꢀlayer chromatography were performed for
separation and purification of the target compounds.
FTꢀIR spectra were recorded on a Perkin Elmer Spectrum
100 FTꢀIR spectrometer. Positive ion and linear mode
MALDIꢀMS of complexes were obtained in 2,5ꢀ
dihydroxybenzoic acid (DHB) as MALDI matrix using
nitrogen laser accumulating 50 laser shots using Bruker
Microflex LT MALDIꢀTOF mass spectrometer. 1HꢀNMR
spectra were recorded in CDCl3 solutions on a Varian 500
MHz spectrometer. Optical absorption and fluorescence
measurements were carried out on JASCO spectrophotometer
(Vꢀ780) and JASCO spectrofluorometer (model FPꢀ8300),
mg, (64%). FTꢀIR
ν
max/cmꢀ1: 3041 (AromaticꢀCH), 1607,
1517, 1455, 1382, 1285, 1252, 1132, 1004, 915, 842, 734,
1
696. HꢀNMR (CDCl3), (
δ:ppm): 8.91 (dd, J = 5.7, 3.0 Hz,
ArH, 6H), 8.14ꢀ8.08 (m, pyreneꢀH, 3H), 8.02 (s, pyreneꢀH,
2H), 7.96 (s, pyreneꢀH, 1H), 7.94 (dd, J = 5.8, 2.8 Hz, ArH,
6H), 7.90 (s, pyreneꢀH, 2H), 7.74 (d, J = 7.9 Hz, pyreneꢀH,
1H), 7.02 (d, J = 8.3 Hz, phenylꢀH, 2H), 5.57 (d, J = 8.3 Hz,
phenylꢀH, 2H) (Fig. S1). UVꢀVis (DCM): λmax 561. MALDIꢀ
2 | J. Name., 2012, 00, 1-3
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TOFꢀMS m/z: Calcd. for C46H25BN6O: 688.22, Found: examined solvents (TN, THF, MeCN and DMF). Such
688.618 [M]+ (Fig. S2).
fluorescence quenching of the singletꢀexcited state of pyrene
can be explained by the electron transfer from the singlet state
of pyrene to SubPc generating Py•+–SubPc•– and/or energy
transfer from the singletꢀexcited state of Py to SubPc. The
finding that the quenching process increases with increasing
the solvent polarity may suggest that the electron transfer is
dominant.
Scheme 1 Synthesis of pyreneꢀSubPc and its precursor, where (i)
toluene, (ii) pyreneboronic acid, 1,4ꢀdioxane, Pd(PPh3)4, Cs2CO3.
Upon exciting the SubPc with 500ꢀnm light excitation, the
fluorescence spectrum of SubPc 2 in toluene showed a
maximum at 576 nm with fluorescence quantum yield (Φf) of
Results and discussion
0.16. By the axial linkage of SubPc with Py, the emission
intensity of the singlet SubPc in toluene exhibited no
The absorption spectra of the intense magenta solutions of fluorescence quenching suggesting no electron transfer
SubPcꢀPy 3, as well as the control SubPc 2 and pyrene, in
acetonitrile are shown in Fig. 2. The absorption spectrum of
the SubPc 2 consists of a highꢀenergy Bꢀband (300 nm) and a
lower energy Qꢀband (558 nm) arisen from πꢀπ* transitions
associated with 14 πꢀelectron systems, analogous to those of
porphyrins and phthalocyanines. The absorption spectrum of
pyrene showed maxima at 335, 319 and 307 nm. In the case
of SubPcꢀPy dyad 3, the absorption spectrum showed four
absorption bands at 561, 508, 342, and 304 nm. Since these
absorption bands are identical to the sum absorption of both
SubPc and Py entities, one can suggest no ground state
interaction between the SubPc and pyrene entities. Similar
absorption bands were observed for SubPcꢀPy dyad in toluene
(TN) and tetrahydrofuran (THF) and dimethylformamide
(DMF), only there is a small red shift of the Qꢀband (~2 nm)
in DMF (Supporting Information’s, Fig. S3).
process from Py to the singletꢀexcited state of SubPc in
toluene. When changing the solvent to more polar solvents,
the emission intensity of the singlet SubPc was considerably
decreased with increasing solvent polarities, THF (Φf = 0.12),
MeCN (Φf = 0.09), and DMF (Φf = 0.08). This observation
suggests the occurrence of electron transfer from pyrene to
the singletꢀexcited state of SubPc in the polar solvents, but
not in the lessꢀpolar toluene.
Fig. 3 (Left) Fluorescence spectra of SubPcꢀPy 3 in the indicated solvents, as
well as the pyrene control; λex = 334 nm. (Right) Fluorescence spectra of
SubPcꢀPy 3 in the indicated solvents, as well as the SubPc control; λex = 500
nm.
The fluorescence lifetime measurements (Fig. 4) track the
above considerations in a more quantitative way, giving
kinetic data of the electronꢀtransfer process in the polar
solvents. In THF, the timeꢀprofile of the singletꢀexcited state
of SubPc control exhibited a single exponential decay with a
lifetime of 2.0 ns. In the case of SubPcꢀPy 3, the lifetimes of
the singlet SubPc entity were found to be 1.80 ns (THF), 1.53
ns (MeCN), and 1.35 ns (DMF), which is considerably shorter
than that of the SubPc control. In the less polar toluene, the
lifetime of the singlet SubPc entity of 3 is nearly the same as
that of SubPc control. These observations agree well with the
steadyꢀstate fluorescence measurements. Based on the
lifetimes of the singlet states of SubPc control and SubPcꢀPy
3, the rate constants of electron transfer (ket) were determined
to be 8.8 × 107 sꢀ1 (THF), 1.8 × 108 sꢀ1 (MeCN), and 3.3 × 108
sꢀ1 (DMF).
Fig. 2 Steadyꢀstate absorption spectra of SubPcꢀPy dyad, as well as SubPc
and Py control compounds in acentonitrile.
Photophysical behavior was firstly investigated by steadyꢀ
state fluorescence using 334ꢀnm light excitation, which is
selectively excited the pyrene moiety. As seen from Fig. 3
(left), the fluorescence spectrum of the control pyrene in
acetontrile showed strong emission bands at 371 and 391 nm.
In the case of SubPcꢀPy dyad in acetonitrile, the emission
bands of the singlet pyrene were heavily quenched in all
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The observed electronꢀtransfer process of SubPcꢀPy 3 from (Supporting Informations, Figs. S8 and S9). This observation
both the steady state and timeꢀresolved emission suggests that the singletꢀexcited state of pyrene donates its
measurements was supported by the electrochemical electron to the attached SubPc entity forming the chargeꢀ
measurements (Supporting Information, Fig. S4). The cyclic separated state (Py•+ꢀSubPc•–), which in turn decayed to
voltammogram of 3 in deaerated THF solution containing (nꢀ populate the low lying 3SubPc* (Fig. 6).
C4H9)4NClO4 as a supporting electrolyte exhibited the first
reduction potential (Eox) of the SubPc entity at ꢀ1.0 V vs.
Ag/AgCl, while the first oxidation potential (Eox) of the Py
entity was recorded at 1.0 V vs. Ag/AgCl. Based on the redox
values, the driving force for the chargeꢀrecombination (ꢁGCR
)
process in THF was determined to be 2.0 eV.64 From the
electrochemical data and energy of the singletꢀexcited state of
Py (3.12 eV), the driving forces of charge separation (ꢀꢁGCS
)
via the singlet Py were determined to be 0.69, 1.12, 1.32, and
1.42 eV in TN, THF, MeCN and DMF, respectively. The
negative ꢀGCS of 3 via the singlet excited state of pyrene in
both the polar and less polar solvents indicate the higher
exothermicity for electron transfer, as shown in Fig. 3.
Similarly, the driving forces of charge separation (ꢀꢁGCS) of 3
via the singlet SubPc were determined to be ꢀ0.45, 0.15, 0.35,
and 0.45 eV in TN, THF, MeCN, and DMF, respectively. The
estimated negative ꢁGCS via the singlet SubPc indicate that
the electron transfer is thermodynamically feasible only in the
polar solvents, but not in the less polar solvents.
Fig. 5 Nanosecond transient absorption spectra at the indicated time intervals
of the SubPcꢀPy dyad 3 in deaerated acetonitrile solution. Inset shows the
decay profiles of the triplet SubPc in argonꢀsaturated and oxygenꢀsaturated
solutions.
(a) In polar solvents
1Py*-SubPc
3.12 eV
fast kCS
Py-1SubPc*
Py•+ - SubPc•–
2.15 eV
Slow kCS
2.00 eV
kCR
Py-3SubPc*
1.56 eV
hν
500 nm
hν
345 nm
kT
Py-SubPc
(a) In toluene
Py-1SubPc*
2.15 eV
isc
3O2
1O2
Py-3SubPc*
1.56 eV
Fig. 4 Decay time profiles of the singlet SubPc emission at 570 nm of the
SubPcꢀPy 3 in the indicated solvents, as well as the SubPc 2 in THF; λex
=
k
C
S
470 nm.
hν
500 nm
kT
The complementary nanosecond transient measurements of
SubPc–Py 3 in the polar solvents with 570 nm laser excitation,
which selectively excited the SubPc moiety, exhibited an
absorption band in the visible region with a maximum at 450
nm (Figs. 5, S5 and S6) that assigned to the tripletꢀexcited
state of SubPc (3SubPc*).65 This assigment was confirmed by
recording the transient absorption spectrum of SubPc control
in acetonitrile (see supporting information, Fig. S7). From the
Py-SubPc
Fig. 6 (Upper figure) Energy level diagram depicting the photoinduced
intramolecular electron transfer events of SubPcꢀPy 3 via the singlet Py and
SubPc entities in the polar solvents. (Lower figure) The singlet oxygen
generation via the triplet SubPc of 3 in toluene.
3
As shown in Fig. 5, the decay of SubPc*ꢀPy and SubPc
control in the toluene, was accelerated on addition of oxygen
(k = 7.0 × 105 sꢀ1) indicating that the triplet SubPc was
significantly quenched because of the energyꢀtransfer process
yielding the singlet oxygen. By assuming that the
concentration of O2 = 0.2 mM, the rate constant of the
3
firstꢀorder kinetic, the decay of SubPc*ꢀPy was determined
to be 1.9 × 104 sꢀ1, which is comparable to that of the SubPc
control. Upon exciting the SubPcꢀPy 3 by using 355 nm light,
which is selectively excited the Py moiety, the nanosecond
transient absorption spectra of 3 showed similar features
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energyꢀtransfer process was determined to be 3.5 × 109 Mꢀ1 sꢀ1. Upon exciting the SubPc entity, similar electronꢀtransfer
In order to determine the efficiency of the examined SubPc 2 features were observed. Based on the timeꢀresolved emission
and SubPcꢀPy 3 to produce singlet oxygen, which is the key measurements, the electronꢀtransfer rate constant were
to the application of photodynamic cancer therapy property,66 determined to be 8.8 × 107 sꢀ1 (THF), 1.8 × 108 sꢀ1 (MeCN),
two methods are generally used. The first method is to and 3.3 × 108 sꢀ1 (DMF). The finding that the fluorescence
measure singlet oxygen generation such as photooxidation of quenching increase with increasing the solvent polarities
the chemical quenchers for singlet oxygen,67 while second support the electron transfer character. Interestingly, the
method is to measure directly photoluminescence of singlet examined SubPc showed higher ability to generate the singlet
oxygen at 1274 nm.68 The ability of SubPc control and oxygen, which is the key to the application of photodynamic
SubPcꢀPy dyad to generate the singlet oxygen was confirmed cancer therapy. These findings suggest the usefulness of the
here by directly observing the characteristic emission band of examined materials as light harvesting materials and
the generated singlet oxygen in the NIR region. As shown in sensitizers for the medical application such as photodynamic
Fig. 6, a strong photoluminescence for the singlet oxygen at therapy.
1274 nm was observed in toluene. By comparison with
fullerene C60 as a standard (quantum yield of singlet oxygen
in toluene = 0.95),69 the quantum yield of singlet oxygen
Acknowledgements
This work was supported financially by the Science and
Technology Development Fund (STDF), Egypt (Grant
Numbers 5537 and 12436) and JSPS KAKENHI (16H02268
to S.F.), Japan.
generation in toluene (Φꢀ) was determined to be ~ 0.70 (for
SubPc control) and ~ 0.68 (for SubPcꢀPy dyad). The finding
that the ability of SubPcꢀPy 3 to produce the singlet oxygen is
close to that of the SubPc 2 was rationalized by the absence of
electron transfer from pyrene to the attached SubPc in the less
polar toluene. Therefore, it is most likely that the 1SubPc*ꢀPy
is mainly decayed via the intersystem crossing to populate the
3SubPc*ꢀPy, which in turn, transfers the energy from the
triplet SubPc to the oxygen generating the singlet oxygen (Fig.
7). The decay profile of the singlet oxygen generation was
fitted with a single exponential decay, from which the lifetime
of the singlet oxygen was determined to be 29.4 ꢁs, which is
similar to the reported value.70
Notes and references
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Fig. 7 Emission spectrum of singlet oxygen produced by SubPcꢀPy 3 in
toluene; λex = 470 nm.
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CONCLUSIONS
We reported herein the synthesis, electrochemical,
photophysical and photochemical properties of the
subphthalocyanines dyad, as a model for the light harvesting
system. The steadyꢀstate fluorescence measurements revealed
a significant fluorescence quenching of the pyrene entity due
to the electronꢀtransfer process to the attached SubPc entity.
M. E. ElꢀKhouly, O. Ito, P. M. Smith and F. D’Souza,
Intermolecular and supramolecular photoinduced electron
transfer processes of fullerene–porphyrin/phthalocyanine
systems, J. Photochem. Photobiol. C, 2004, 5, 79ꢀ104.
This journal is © The Royal Society of Chemistry 20xx
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