Photoinduced electron transfer (PET) within D4-A and D-A photosynthetic systems: Enhanced intramolecular PET achieved by increasing the number of donors

Article abstract of DOI:10.1016/j.dyepig.2010.03.010

Photoinduced electron transfer (PET) occurring within D₄-A was compared to that within D-A, in which A (electron acceptor) was a single zinc phthalocyanine moiety and D (electron donor) a phenothiazine unit, using UV-vis absorption, fluorescence emission and laser flash photolysis. D and A in both models were directly linked by covalent C-N bonds. The symmetrical D₄-A system, for which both the synthesis and purification were more straigthforward than for D-A, displayed a much faster PET rate constant; although the D-A system was more difficult to prepare, it exhibited less efficient PET. The charge separation state formed through PET showed long lifetimes on the μs scale in the cases of both the D₄-A and D-A molecules.

Full text of DOI:10.1016/j.dyepig.2010.03.010

Dyes and Pigments 87 (2010) 139e143
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photoinduced electron transfer (PET) within D4eA and DeA
photosynthetic systems: Enhanced intramolecular PET achieved by
increasing the number of donors
*
Xian-Fu Zhang , Hongwei Zheng, Shuangming Jin, Ruiyun Wang
Chemistry Department, Hebei Normal University of Science and technology, Qinhuangdao, Hebei Province, 066004 PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Photoinduced electron transfer (PET) occurring within D4eA was compared to that within DeA, in which
A (electron acceptor) was a single zinc phthalocyanine moiety and D (electron donor) a phenothiazine
unit, using UVevis absorption, fluorescence emission and laser flash photolysis. D and A in both models
were directly linked by covalent CeN bonds. The symmetrical D4eA system, for which both the synthesis
and purification were more straigthforward than for DeA, displayed a much faster PET rate constant;
although the DeA system was more difficult to prepare, it exhibited less efficient PET. The charge
Received 5 December 2009
Received in revised form
6 March 2010
Accepted 8 March 2010
Available online 17 March 2010
separation state formed through PET showed long lifetimes on the
and DeA molecules.
m
s scale in the cases of both the D4eA
Keywords:
Photoinduced electron transfer
Phthalocyanine
Ó 2010 Elsevier Ltd. All rights reserved.
Phenothiazine
Charge separation
Synthesis
Fluorescence quenching
1. Introduction
the rate and efficiency of PET within DneA can be increased or
decreased relative to that of the corresponding DeA. A comparative
investigation of PET within a DneA compared to that of its corre-
sponding DeA system is necessary to reveal such differences, but
this remains an unexplored aspect of intramolecular PET. This was
the purpose of this study insofar as a D4eA model (Fig. 1) was
compared to that of the corresponding DeA system. It is known
that PETcan occur in a D4eA system [12] but this was not compared
with that of the corresponding DeA system.
In a D4eA supramolecule there is only one entity, namely A, that
can accept electrons, but there are four equal donors which will
concurrently donate an electron to the same acceptor A during the
lifetime of the photo excited ZnPc. If the acceptor is allowed to hold
only one electron, competition occurs between the four donors. In
the case of intermolecular PET, it is well known that the greater the
number of donors, the faster is PET, as effective collisions between
donors and acceptors are increased. Intra-molecular PET, however,
proceeds through bonding or space that involves no collisions. In
general, a oneeone linking of DeA can make PET occur more
effectively; moreover, the direction of electron movement from any
D to A in the symmetrical D4eA system is against that on the
opposite side (Fig. 1 right), which can result in the canceling out or
slowing down of PET. In this work, however, it was found that PET
within the D4eA was significantly faster than that in the DeA.
Photoinduced electron transfer (PET) in molecular systems, that
contain both electron donors (D’s) and acceptors (A’s) tethered
together using various types of bridge, has been extensively
explored both experimentally and theoretically [1e9]. The domi-
nant models reported have been linearly linked DeA1eA2e/eAn
(n ꢀ 1), so that in each consecutive step of PET only one-donor-one
acceptor is involved. Although papers that deal with multi-donor-
one-acceptor systems (DneA) are rare, such DneA (n > 1) models
may reveal novel features that do not occur in the more common
DeA system, such as the formation of multi-charge, separated
states (MCSS) (Dꢁþ)keAkꢂ (k > 1) [10e12]. MCSSꢂcan provide
þ
stronger reducing or oxidizing species than Dꢁ eAꢁ systems for
artificial photosynthesis to reduce CO2 and split water into dioxy-
gen. Such reduction or water splitting requires multi reducing or
oxidizing equivalents in natural photosynthesis [1,13], which is
cannot be achieved using Dꢁ eAꢁ in artificial photosynthesis.
Other than MCSS, our understanding of PET within DneA remains
limited, especially as there is no specific understanding of how both
þ
ꢂ
* Corresponding author. Tel./fax: þ86 0335 2039067.
E-mail address: zhangxianfu@tsinghua.org.cn (X.-F. Zhang).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.03.010

140
X.-F. Zhang et al. / Dyes and Pigments 87 (2010) 139e143
Fig. 1. Structures of DeA and D4eA. DeA is produced together with DieA (i ¼ 0, 2e4) that needs to be separated out.
ZnPc can function as a good electron acceptor and has been
2.3. Preparation of
b-phenothiazinyl-tri(b-phenoxy)
employed in many DeA systems [14e20], its electrochemistry and
absorption spectrum of ZnPcꢁꢂ having been well documented. PTZ
is a nonplanar molecule in which the two benzene rings form an
angle of 140ꢃ [21]. PTZ tends to form stable PTZꢁþ, which is an
almost coplanar system (the dihedral angle is 176.5ꢃ) so that the
positive charge is delocalized [22]. The PTZ radical cation has also
been well-characterized using UVeVis methods [23e25].
zinc phthalocyanine
A
mixture of 4-phenothiazinyl phthalonitrile (0.152 g,
0.45 mmol), 4-phenoxy phthalonitrile (0.30 g, 1.36 mmol), zinc
acetate dihydrate (0.087 g, 0.40 mmol), two drops of DBU as cata-
lyst and dry quinoline (15 ml) was heated under an argon atmo-
sphere, with stirring, at 190 ꢃC for 5 h. The majority of quinoline
was removed by distillation under vacuum and the resulting
precipitate was filtered and washed with n-hexane and then
refluxed with ethanol (20 mL) and filtered to remove all traces of
quinoline. The green dry product was purified by column chro-
matography using CH2Cl2eTHF (v/v 19:1) as eluent. Yield: (14%). IR
nmax/cmꢂ1: 3063, 1616, 1589, 1485, 1369, 1312, 1277, 1234, 1092,
1041, 930, 868, 744, 694. MS (Maldi-TOF): 1050.2 [M þ 1]þ. 1H NMR
2. Experimental
2.1. Materials and instruments for synthesis
All organic solvents were dried by appropriate methods and
distilled before use. ZnPc was purchased from Tokyo Kaise. 4-
(CDCl3, 22 ꢃC):
d 6.80e6.90 (m, 1H), 7.05e7.15 (d, 6H), 7.20e7.30(t,
phenoxy phthalonitrile and Tetra(
were prepared previously by the current authors [26]. The
synthesis and characterization of Tetra( -Phenothiazinyl) zinc
b-phenoxy) zinc phthalocyanine
7H), 7.30e7.35(s, 4H), 7.35e7.38(m, 2H), 7.39e7.45(t, 6H), 7.45e7.50
(d, 2H), 7.60e7.65(d, 1H), 7.80e7.85(d, 3H), 7.90e8.20(s, 3H). Anal.
calcd. for C62H35N9O3SZn (MW, 1049.2); C, 70.82; H, 3.36; N, 11.99.
Found: C, 70.36; H, 2.98; N, 11.74.
b
phthalocyanine was undertaken according to [27]. All other
reagents were analytical grade and were used as received. 1H NMR
spectra were recorded at room temperature on a Bruker dmx
300 MHz NMR spectrometer. MS spectra were recorded either on
a Bruker APEX II or Autoflex III Maldi-TOF spectrometer. IR spectra
were recorded at room temperature on a Shimadzu FTIR-8900
spectrometer. Samples for C, H, N elemental analysis were dried
under vacuum, and analyzed with a Carlo Erba-1106 instrument.
2.4. Photophysical measurements
DMF and other solvents were dried and freshly distilled before
use. Measurements were carried out at room temperature of 22 ꢃC.
UVevis absorption measurements were made with an HP 8451A or
Shimadzu 4500 spectrophotometer in 10 mm quartz cuvettes.
Fluorescence spectra up to 900 nm were monitored using a Perki-
nElmer LS 55, with 2.5 nm slits. All spectra were corrected for the
sensitivity of the photo-multiplier tube. The fluorescence quantum
yield (Ff) was calculated by Ff ¼ FsA0F0f/(F0As), in which F is the
integrated fluorescence intensity, A is the absorbance at excitation
wavelength, the subscript 0 stands for a reference compound and s
represents samples. Zinc phthalocyanine in DMF was used as the
2.2. Preparation of 4-phenothiazinyl phthalonitrile
The procedure is based on that reported in [27]. A mixture of
4-nitrophthalonitrile (1.04 g, 6 mmol) and phenothiazine (1.20 g,
6 mmol) was stirred in dry DMSO (20 mL) under argon for 64 h at
room temperature, during which time, dry potassium carbonate
(2.5 g, 18.1 mmol) was added in four equal portions over 2 h. The
reaction process was monitored using flash chromatography
employing dichloromethane as mobile phase. After reaction was
complete, the resulting solution was poured into 150 mL ice-water
and filtered. The ensuing solid was washed with cold deionized
water until pH was neutral, washed with cold ethanol, diethyl ether
and then dried. The crude product was recrystallized twice with
ethanol as solvent, which yielded yellþow crystals. Yield: 54%. m.p.:
198e200 ꢃC. MS: m/z ¼ 326.2 [M þ 1] . 1H NMR (300 MHZ, CD2Cl2,
reference (F0 ¼ 0.30) [28]. Excitation wavelengths of 610 nm cor-
f
responding to S0 to S1 transitions were employed. The sample and
reference solutions were prepared with the same absorbance (Ai) at
the excitation wavelength (near 0.09 per cm). All solutions were air
saturated. Fluorescence lifetime of S1 was measured by time-
correlated single photon counting method (Edinburgh FL-900
spectrophotometer) with excitation at 660 nm by a CdS portable
diode laser (50 ps) and emission was monitored at 700 nm. Tran-
sient spectra were recorded in degassed DMF (prepared by
bubbling with Argon for 20 min) with an Edinburgh LP-920 laser
flash photolysis system. A Nd:YAG laser (Continuum surelite II,
22 ꢃC):
d
7.61e7.15 (m, 11 H, Ar-H). IR [(KBr) nmax/cmꢂ1]: 3062
(Ar-H), 2222 (C^N), 1600, 1578, 1546, 1458, 1331, 1258, 1095, 1030,
876, 825.

X.-F. Zhang et al. / Dyes and Pigments 87 (2010) 139e143
141
355 nm and 7 ns FWHM) was used as excitation source. The
analyzing light was from a xenon lamp. The laser and analyzing
light beams perpendicularly passed through a quartz cell with an
optical path length of 1 cm. The signal was displayed and recorded
on a Tektronix TDS 3012B oscilloscope and an Edinburgh LP900
detector. The laser energy incident at the sample was attenuated to
a few mJ per pulse. Time profiles at a series of wavelengths from
which point by-point spectra were assembled were recorded with
the aid of a PC controlled kinetic absorption spectrometer. The
The electronic absorption spectra are given in Fig. 3. Comparing
the spectra with that of ZnPc( -OPh)4 i.e., tetra( -phenoxy) ZnPc, it
is clear that the long wavelength Q band of DeA or D4eA corre-
sponds essentially to that of the ZnPC moiety, i.e., A itself. This is
b
b
owing to the very small
p-electronic interaction between the two
moieties D and A at ground state caused by the sterically favored
orthogonal orientation. The “extra” PTZs do not influence the
electronic properties of PC moiety since the absorption spectra of
DeA and D4eA show little difference.
concentrations of the target compounds were typically 10
providing A355 ¼ 0.3 in a 10 mm cuvette.
mM
Also noted is the absence of the band at 630 nm attributed to the
intermolecular dimer or higher aggregates formed by two or more
phthalocyanines. The Beer’s law is also obeyed according to the
obtained linear relation between absorbance and the concentration
of DeA or D4eA up to 10 mM, which further removes the possibility
3. Results and discussion
of aggregation effect upon photophysics.
The fluorescence emission spectra in DMF measured by exciting
PC moieties at 610 nm are compared in the inset of Fig. 4. With
photoexcitation under the same conditions, the emission intensi-
ties are remarkably different between the four compounds. D4eA
showed the most significant fluorescence quenching with a fluo-
rescence quantum yield (Ff) of 0.040. The Ff of DeA is 0.080, which
The attachment of a single PTZ entity to ZnPc is of the asym-
metrical synthesis and was accomplished by the common cross
condensation method [29], as shown in Fig. 2. The formation of
additional DieA (i ¼ 0 to 4 but not 3) lowers the yield and increases
isolation difficulty. A D4eA, on the other hand, can be prepared by
the symmetrical method without worrying about the formation of
other DieA (i ¼ 0e3) [30].
is 2-fold of that of D4eA. ZnPc(b-OPh)4, the model compound of A,
Fig. 2. The comparison of symmetrical and asymmetrical synthesis for D4eA and DeA, respectively.

142
X.-F. Zhang et al. / Dyes and Pigments 87 (2010) 139e143
only a slight decreased Ff and sf compared with that of ZnPc. The
energy transfer from ZnPc moiety to PTZ is not possible because
PTZ does not absorb in the red region. This quenching must be
owing to the PET from PTZ to S1 of ZnPc within the DeA and D4eA,
which is also indicated from the thermodynamic and transient
absorption studies below.
The rate constant of PET (ket) value can be calculated from the
ꢂ1
fluorescence lifetime using ket ¼ (sf
)
ꢂ (s0
)
ꢂ1. PET within D4eA
f
occurred with ket ¼ 2.28 ꢄ 109 sꢂ1, which is about five times faster
than that in DeA (ket ¼ 0.43 ꢄ 109 sꢂ1). The efficiency of PET within
D4eA is 0.90, calculated by (F0
than that of the DeA (0.60).
/
f
Ff ꢂ 1)/s0f, which is 1.5 times larger
Both phthalocyanies and PTZ’s redox properties have been
extensively studied. The oxidation potential of PTZ appeared at
ꢁ
E(PTZ/PTZ þ) ¼ 0.73 V (vs SCE) and the reduction potential occurred at
ꢁ
ꢂ2.20 V [31], while ZnPc exhibited reduction peak at E(ZnPc ꢂ/ZnPc)
¼
ꢂ0.83 V and oxidation peak at 0.86 V in DMF [32]. PTZ is obviously
easier to be oxidized than ZnPc according to their redox potentials.
PET from PTZ to S1 of ZnPc moiety is thermodynamically favored,
Fig. 3. UVeVis absorption spectra of PTZ, DeA, D4eA and ZnPc(b-OPh)4 in DMF
solutions. The latter three spectra were raised for clear viewing.
ꢁ
since its
D
G is a negative value obtained by:
D
GPET ¼ E(PTZ/PTZ þ)
ꢂ
ꢁ
E(ZnPc ꢂ/ZnPc) ꢂ E00 ꢂ 0.06 ¼ 0.73 e (ꢂ0.83) e 1.84 e 0.06 ¼
ꢂ0.34 eV. PET by the photo excitation of PTZ is even more favored,
exhibited only a slight decrease of emission intensity (Ff ¼ 0.26)
from that of ZnPc (Ff ¼ 0.30).
since
D
GPET ¼ 0.73 e (ꢂ0.83) e 3.22 e 0.06 ¼ ꢂ1.72 eV, which is
much more negative. In practice, however, the light illumination in
UV region mainly populates the S2 state of ZnPc moiety rather than
The fluorescence decays (Fig. 4) were monitored at 700 nm with
laser excitation at 660 nm. The emission of both ZnPc and ZnPc(
OPh)4 decays mono exponentially with slightly different lifetime
f) of 3.54 ns and 3.47 ns, respectively. The decay kinetics of DeA
b-
S1 of PTZ, because the former possesses a much larger 3, especially
in the region above 350 nm. Unlike the previous system in which
the double electron transfer from two donors to the same S2 of ZnPc
moiety is feasible [12], the same process is not possible within
(s
and D4eA, however, is biexponential. sf of D4eA is 0.39 ns (74.6%)
and 3.69 ns (25.4%), while that for DeA is 1.45 ns (95.4%) and
3.20 ns (4.6%), respectively. The long-lived but minor components
are usually attributed to the conformers unfavoring PET. The sf of
the major but short-lived component of D4eA was significantly
reduced to 0.39 ns from 1.45 ns that in the DeA dyad, while the
the current D4eA, because the associated
of 0.26 eV. Also noted is the electron transfer between D and A
without photoexcitation is not allowed owing to
DGPET is a positive value
D
GET ¼ Eox ꢂ Ered ꢂ
C ¼ 0.73 ꢂ (ꢂ0.83) ꢂ 0.06 ¼ 1.50 eV. PET from PTZ to T1 of ZnPc
moiety is also forbidden, since the energy of the T1 state is 0.99 eV
latter was also decreased from 3.47 ns of ZnPc(b-OPh)4.
[33,34], suggesting a
DGPET of þ0.51 eV. In conclusion, based on the
It is clear that the singlet excited state (S1) of ZnPc moiety within
both DeA and D4eA is efficiently quenched by the occurrence of
PTZ, and the number of PTZ moieties showed remarkable effect on
the quenching efficiency. The enhancement of internal conversion
(IC) by PTZs can be excluded, since ZnPc(b-OPh)4 that possess faster
rotational and vibrational OPh groups than phenothiazinyl showed
thermodynamic analysis, only single electron can be transferred
from PTZ to S1 of ZnPc moiety within both DeA and D4eA, hence
the competition occurs in the D4eA between donors for PET.
The occurrence of radical ions generated by PET was confirmed
by the transient absorption spectra measured under the same
conditions. In accordance with the well-known tripletetriplet
excited-state absorption (T1eTn) of ZnPc [33,34], we found a broad
Fig. 4. Steady state (inset) and dynamic fluorescence quenching of ZnPc moiety by the
substitution of PTZ in DMF. The excitation wavelength for steady state fluorescence is
610 nm (Xenon lamp), the fluorescence decay was measured by single photon counting
method with excitation at 660 nm by diode laser (50 ps) and emission at 700 nm. The
values of c2 for the fitting are between 1.10 and 1.25.
Fig. 5. Transient absorption spectra of ZnPc, DꢂA and D4ꢂA (5
ms delay) in argon
saturated DMF with excitation at 355 nm (absorbance w0.30, laser energy w3 mJ) by
Nd:YAG laser.

X.-F. Zhang et al. / Dyes and Pigments 87 (2010) 139e143
143
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around 400e630 nm for ZnPc reference compound, Fig. 5. The
monoexponential decay at 480 nm gave the T1 lifetime of 200 ms.
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ms, which is expected to reflect the back
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(kBET ¼ 1/
s
d) of 2.04 ꢄ 105
s
ꢂ1. The PTZ cation has long been
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is further weakened in comparison with that of DeA, which affords
another support that the more the donors, the faster the PET.
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4. Conclusion
In conclusion, we explored the difference of PET between D4eA
and DeA models. The effect of the number of donors on the rate
constant and efficiency of intramolecular PET is an aspect rarely
discussed before. We showed that PET within D4eA is more than
five times faster than that in DeA. More over, D4eA is significantly
easier to be synthesized and purified than its DeA counterpart.
Together with the previous report that a double-charge separated
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A, we believe that a multi-donor one acceptor model D4eA can
provide opportunity for more efficient photosynthesis mimicking.
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