Synthesis and photochemical characterization of a zinc phthalocyanine-zinc porphyrin heterotrimer and heterononamer

Article abstract of DOI:10.1039/b508478d

The synthesis of two phthalocyanine-porphyrin covalently linked heteromolecules are described. Intramolecular energy transfer is investigated and quantified in terms of the quantum yield of energy transfer and found to be highly effective in both molecules. The photophysical properties of both molecules are modified greatly by the presence of the porphyrin moieties on the phthalocyanine core. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b508478d

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F U L L P A P E R
Synthesis and photochemical characterization of a zinc
phthalocyanine–zinc porphyrin heterotrimer and heterononamer
Zhixin Zhao, Tebello Nyokong and M. David Maree*
Rhodes University, Grahamstown, 6140, South Africa. E-mail: d.m.maree@ru.ac.za;
Fax: +27 466225109; Tel: +27 466038683
Received 6th June 2005, Accepted 2nd September 2005
First published as an Advance Article on the web 9th September 2005
The synthesis of two phthalocyanine–porphyrin covalently linked heteromolecules are described. Intramolecular
energy transfer is investigated and quantified in terms of the quantum yield of energy transfer and found to be highly
effective in both molecules. The photophysical properties of both molecules are modified greatly by the presence of
the porphyrin moieties on the phthalocyanine core.
hydrous zinc acetate, lithium and propan-1-ol, were purchased
Introduction
from Sigma-Aldrich and used without further purification.
Phthalocyanine complexes have received much attention in
Dimethyl sulfoxide (DMSO) was dried over alumina before
the literature for various photochemical applications such as
use. All other solvents were dried by standard methods prior
infrared sensors,1 nonlinear optics,2 photodynamic therapy,3
to use. Silica gel 60 (0.04–0.063 mm) for chromatography was
optical recording4 and as xerographic photoreceptors.5 The
purchased from Merck.
phthalocyanines are generally stable, rigid compounds with
varying properties dependent upon their central metals, pe-
ripheral substituents and axial substituents. Optical applications
Equipment
abound due to the uniquely intense red absorbing band referred
to as the Q-band in these compounds. Porphyrins are naturally
occurring entities and are structurally similar to phthalocyanines
but are generally less rigid and less stable (chemically as well as
photochemically) than phthalocyanines. Porphyrins generally
have an intense B-band (∼420 nm for zinc tetraphenylporphyrin)
and less intense Q-band. Many porphyrins have triplet lifetimes
above 1 ms and high quantum yields of triplet formation
which make them interesting molecules for study in photo-
catalytic applications. The covalent linking of phthalocyanines
to porphyrins was first achieved by Gaspard et al.6 A dimer
of these compounds where one porphyrin unit was linked
to a phthalocyanine unit were shown to exhibit interesting
charge transfer properties by Tran-Thi et al.7 Metal-free8 and
zinc8 containing covalently linked phthalocyanine–porphyrin
dimers have also been reported. Ethynyl linked phthalocyanine–
porphyrin dimers have also been synthesised by two groups.9,10
Metal-free, zinc and magnesium containing phthalocyanine–
porphyrin nonamers as a mixture of isomers due to tetra-
substitution of porphyrin dimers have also been prepared.11
We have recently reported on a pentamer consisting of one
phthalocyanine molecule and four porphyrin molecules con-
taining zinc12 or cobalt13 central metals. A mutual quenching of
excited singlet states of both the zinc phthalocyanine (ZnPc) and
zinc tetraphenylporphyrin (ZnTPP) components were observed
for the zinc pentamer.12 Intramolecular electron and energy
transfer process efficiencies in covalently linked chromophores
are of interest as model systems for the reaction centre of
photosynthesis. In this paper we report on the synthesis and
photophysical properties of a covalently linked trimer (one
phthalocyanine and two porphyrins) as well as a covalently
linked nonamer (one phthalocyanine and eight porphyrins).
UV-Vis spectra were recorded on a Varian 500 UV-Vis/NIR
spectrophotometer. IR (KBr pellets) were recorded on a Perkin-
Elmer Spectrum 200 FTIR spectrometer. 1H NMR spectra
were recorded using a Bruker EMX 400 NMR spectrometer.
Fluorescence excitation and emission spectra were recorded
with a Varian Eclipse spectrofluorimeter. Flash photolysis
experiments were performed with light pulses produced by a
Quanta-Ray Nd:YAG laser providing 400 mJ, 90 ns pulses of
laser light at 10 Hz, pumping a Lambda-Physik FL3002 dye
(pyridine 1 dye in methanol). Single pulse energy was 2 mJ. The
analyzing beam source was from a Thermo Oriel xenon arc
lamp, and a photomultiplier tube was used as detector. Signals
were recorded with a digital real-time oscilloscope (Tektronix
TDS 360). MALDI–TOF mass spectra of the nonamer ZnPc–
(ZnTPP)8 was obtained at Tohoku University using a Perseptive
Biosystem MALDI-TOF Mass Voyager DE-SI2 spectrometer
with dithranol as matrix. The MALDI-TOF mass spectra of
the rest of the complexes were determined using Perseptive
Biosystems Voyager DE-PRO Biospectrometry Workstation and
Processing Delayed Extraction Technology at the University of
Cape Town in South Africa.
Methods
Fluorescence quantum yields were determined in DMSO by
the comparative method16,17 using either zinc phthalocyanine
(ZnPc, UF = 0.237) or zinc tetraphenylporphyrin (ZnTPP, UF =
0.0397) as references. Fluorescence lifetimes were determined
from the absorbance and emission spectra using the Strickler–
Berg equation as described previously.18 Triplet lifetimes (sT)
were determined by exponential fitting of the laser flash
photolysis experimental data using OriginPro 7.5 software.
Triplet quantum yields (UT) were determined by a comparative
method19,20 (singlet depletion) using zinc phthalocyanine (0.657)
as standard with a laser flash photolysis setup described above.
Deaerated solutions were introduced into a 10 mm × 10 mm
spectrophotometric cell and irradiated at the relevant Q-band
maxima. The relevant equations will be given at the sections
where they are required.
Experimental
Materials
4,5-Dichlorophthalonitrile14 and 5-hydroxy-10,15,20-triphenyl-
porphyrin15 were synthesized as in the literature. 4-Hydroxy-
benzaldehyde, benzaldehyde, pyrrole, 1,8-diazabicyclo[5,4,0]-
unde-7-ene (DBU), sodium methoxide, potassium carbonate,
3 7 3 2
D a l t o n T r a n s . , 2 0 0 5 , 3 7 3 2 – 3 7 3 7
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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Synthesis
Preparation of 4,5-bis[5-(4-phenoxy)-10,15,20-triphenyl por-
H). IR(KBr) COC 1202 cm−1; MALDI-TOF-MS m/z: Calc. =
1966.98; Found = 1962.93 [M − 4]+.
phyrin]-1,2-dicarbonitrile ligand P(CNOTPP)2. A mixture of
5-hydroxy-10,15,20-triphenylporphyrin (P-OH)PP, 126.2 mg,
0.2 mmol), 4,5-dichlorophthalonitrile (13.2 mg, 0.066 mmol)
and K2CO3 (138 mg, 1 mmol) in dry DMSO (20 ml) was heated
at 90 ◦C for 6 h with stirring under a nitrogen atmosphere. The
reaction progress was monitored by thin layer chromatography
(TLC). After cooling to room temperature, the purple reaction
mixture was poured into 100 ml dichloromethane and washed
three times with 100 ml water. The dichloromethane was
removed by evaporation. Column chromatography on silica
gel with dichloromethane as eluent gave two bands; the first
band was found to be the product. The second band was the
starting material. Evaporation of the dichloromethane afforded
83.4 mg (91%) of purple solid. The purple crude product was
recrystallized from dichloromethane with absolute methanol.
UV-Vis [CHCl3, kmax/nm (log e)]: 420 (5.72), 515 (4.45), 551
Preparation of 2,3,9,10,16,17,23,24-octakis[5-(4-phenoxy)-10,
15,20-triphenylporphyrin zinc(II)] phthalocyanine zinc(II) (ZnPc–
(ZnTPP)8).
A mixture of P(ZnCNOTPP)2 (151.6 mg,
0.1 mmol) and sodium methoxide (5 mg) was added to distilled
methanol (10 ml). Anhydrous ammonia gas was bubbled
through the strirred suspension for 1 hour. The suspension
was then refluxed for 6 h with continued addition of ammonia
gas, forming the diiminoisoindole which was not isolated. The
methanol was removed, then hydrous zinc acetate (43.2 mg,
0.2 mmol), DBU (2 drops) and propan-1-ol (10 ml) were
added. The mixture was heated at 95 ◦C for 2 h with stirring
under a nitrogen atmosphere. The reaction procedure was
monitored by UV-Vis spectroscopy. After cooling to room
temperature, the purple solution was poured into 20 ml
dichloromethane, and washed three times with 100 ml water.
The dichloromethane layer was collected, evaporated and the
solid applied to a silica gel column. A series of purple bands
was eluted by dichloromethane. The desired compound was
eluted using dichloromethane with 2% methanol. Removal
of dichloromethane and methanol by evaporation afforded
21.7 mg (14%) of a dark-purple solid. The dark-purple product
was recrystallized from dichloromethane with hexane. UV-Vis
[DMSO, kmax/nm (log e)]: 356 (4.97), 429 (6.31), 561 (4.91),
1
(4.14), 591 (3.99), 646 (3.91); H NMR (400 MHz, CDCl3),
d 8.84–8.89 (m, 16H, pyrrole H), 8.15–8.39 (m, 16H, phenyl
H), 7.84 (s, 2H, phthalonitrile H), 7.59–7.80 (m, 22H, phenyl
H), −2.73 (s, 4H, N–H). IR(KBr) CN 2230, COC 1210 cm−1;
MALDI-TOF-MS m/z: Calc. = 1385.61; Found = 1387.61
[M + 2]+.
Preparation of 4,5-bis[5-(4-phenoxy)-10,15,20-triphenyl por-
phyrin]-1,2-dicarbonitrile zinc(II) P(ZnCNOTPP)2. A mixture
of P(CNOTPP)2 (138.6 mg, 0.1 mmol) and hydrous zinc acetate
(108 mg, 0.5 mmol) in dimethylformamide (DMF) (30 ml) was
heated at 70 ◦C for 1 h with stirring under nitrogen. After cooling
to room temperature, the purple reaction mixture was poured
into 100 ml dichloromethane and washed three times with 100 ml
water to remove excess zinc acetate, acetic acid and DMF. Col-
umn chromatography on silica gel with dichloromethane as
eluent gave one band. Removal of dichloromethane by evap-
oration afforded 144.5 mg (97%) of a vivid red solid. The crude
product was recrystallized from dichloromethane with absolute
methanol. UV-Vis [CHCl3, kmax/nm (log e)]: 428 (5.97), 522
1
602 (4.69), 681 (5.09); H NMR (400 MHz, CDCl3), d 8.87
(broad s, 72H, phthalocyanine H and pyrrole H), 8.21(broad s,
40H, phenyl H), 7.75 (broad s, 80H, phenyl H), 7.15 (broad s,
32H, phenoxy H). IR(KBr) COC 1210 cm−1; MALDI-TOF-MS
m/z: Calc. = 6114.79; Found = 6114.93 [M]+.
Results and discussion
Synthesis
The synthetic route to the compounds presented is shown in
Schemes 1 and 2. The compounds are abbreviated as indicated
in the Experimental section. The synthesis of P(CNOTPP)2
using a standard chloride group displacement reaction was
completed sooner than expected with very high yield (91%).
Most of the porphyrin reagent was converted to product, the
lost yield is attributed to the purification procedure. The high
yield may be due to the increased nucleophilic nature of the
attacking oxygen on the porphyrin species. This compound was
characterised by IR (CN stretch at 2230 cm−1 and aromatic
ether at 1210 cm−1), UV-Vis spectroscopy (porphyrin B-band
at 420 nm, and a split Q-band at 591 nm and 646 nm) as
1
(3.81), 561 (4.62), 600 (4.30); H NMR (400 MHz, CDCl3),
d 8.94–9.00 (m, 16H, pyrrole H), 8.16–8.40 (m, 16H, phenyl H),
7.84 (s, 2H, phthalonitrile H), 7.59–7.81 (m, 22H, phenyl H);
IR(KBr) CN 2232, COC 1206 cm−1; MALDI-TOF-MS m/z:
Calc. = 1513.18; Found = 1512.17 [M]+.
Preparation of 2,3-bis[5-(4-phenoxy)-10,15,20-triphenyl por-
phyrin zinc(II)] phthalocyanine zinc(II) (ZnPc–(ZnTPP)2).
Chloro[7,12,14,19-diimino-21,5-nitrilo-5H -tribenzo[c,h,m]-
[1,16,1]triazacycloopentadecinato-(2−)-N22,N23,N24]
boron
well as H NMR. The synthesis of P(ZnCNOTPP)2 using a
1
(SubPc) was synthesized according to literature methods.21
SubPc (86.4 mg, 0.2 mmol) in DMSO (10 ml) was heated at
60 ◦C for 2 h with stirring under nitrogen. P(ZnCNOTPP)2
(151.6 mg, 0.1 mmol), DBU (2 drops) and hydrous zinc acetate
(43.2 mg, 0.2 mmol) were added to the solution. The mixture
was heated to 120 ◦C for 24 h with stirring under nitrogen. After
cooling to room temperature, the green reaction mixture was
poured into 100 ml dichloromethane and washed three times
with 100 ml water to remove excess zinc acetate, acetic acid and
DMF. The dichloromethane layer was collected, evaporated and
the solid applied to a silica gel column. A series of purple bands
was eluted by dichloromethane. The desired compound (blue
bands) was eluted using dichloromethane with 5% methanol.
The biproduct ZnPc remained on the top of column. Removal
of dichloromethane and methanol by evaporation afforded
12.7 mg (6%) of a blue solid. The blue product ZnPc–(ZnTPP)2
was recrystallized from dichloromethane with methanol.
UV-Vis [DMSO, kmax/nm (log e)]: 350 (4.72), 428 (5.73), 561
(4.32), 607 (4.45), 675 (5.13); 1H NMR (400 MHz, DMSO-d6),
d 9.42 (s, 2H, phthalocyanine H), 9.05 (d, 6H, phthalocyanine
H), 8.77–8.90 (m, 16H, pyrrole H), 8.07–8.25 (m, 16H, phenyl
H), 7.49–7.75 (m, 22H, phenyl H), 6.74 (d, 6H, phthalocyanine
general porphyrin metallation reaction with hydrous zinc acetate
in DMF gave a satisfactory yield (96%). This compound was
characterized by IR (CN stretch at 2232 cm−1 and aromatic
ether at 1206 cm−1), UV-Vis spectroscopy (porphyrin B-band
at 428 nm, and a split Q-band at 561 nm and 600 nm) as
1
well as H NMR sectroscopy. The mass spectrum revealed a
molecular ion peak [M]+ at 1512.18 m/z which is in good
agreement with the calculated molecular weight of 1513.18 g
mol−1. An attempt was made to synthesise the cobalt product
P(CoCNOTPP)2 using the P(CNOTPP)2 ligand and hydrous
cobalt acetate in DMF. This compound was found to be
unstable upon purification as it slowly decomposed to the
starting materials. This is probably due to the cobalt porphyrin
being acid-sensitive as well as oxygen sensitive. The synthesis of
the porphyrin–phthalocyanine trimer ZnPc–(ZnTPP)2 was done
from P(ZnCNOTPP)2 with unsubstituted subphthalocyanine
(SubPc) to give the desired product in low yield (6%). The
low yield is partly due to the extensive purification procedure
required. An attempt was also made to synthesise the trimer
ZnPc–(ZnTPP)2 using the statistical condensation method with
P(ZnCNOTPP)2 and phthalonitrile, but the yield was found
1
to be far lower than with the SubPc method. The H NMR
D a l t o n T r a n s . , 2 0 0 5 , 3 7 3 2 – 3 7 3 7
3 7 3 3

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Scheme 1 Synthesis of the trimer ZnPc–(ZnTPP)2.
spectrum of this compound displayed two sets of signals of the
non-peripheral protons on the phthalocyanine at 9.42 ppm (2H
as a singlet) and 9.05 ppm (6H as a doublet). This is typical of
a disubstituted phthalocyanine as the two protons at 9.42 ppm
are present on the ring that contains the two porphyrin moeities
and the other 6H are present on the unsubstituted benzene
rings where they have one neighbouring proton that splits
them into a doublet. Mass spectroscopy revealed a molecular
ion peak at 1962.93 m/z [M − 4]+ which is 4 less than the
calculated molar mass of 1966.98 g mol−1. The synthesis of
the porphyrin–phthalocyanine nonamer ZnPc–(ZnTPP)8 was
done from P(ZnCNOTPP)2 which was firstly converted into
the diiminoisoindoline precursor by bubbling the phthalonitrile
with ammonia gas and after chromatography gave a 14% yield
120 protons. Finally, the broad peak at 7.15 ppm integrated
for 32 protons and this was due to the phenyl protons on
the phenoxy groups that connect the porphyrin moeities to
the phthalocyanine core. The mass spectrum of this compound
displayed a molecular ion peak at 6114.93 m/z, Fig. 2. which was
in agreement with the calculated value of 6114.93 g mol−1. It has
been shown by Marcuccio22 et al. that large phthalocyanine type
structures do not always give satisfactory elemental analysis.
Absorption properties
The UV-Vis spectrum confirms the formation of a phthalocya-
nine ring by the appearance of a phthalocyanine Q-band. Fig. 3
shows the electronic absorption spectra of ZnPc–(ZnTPP)2,
ZnPc–(ZnTPP)8, ZnTPP and ZnPc in DMSO. The UV-Vis
spectrum of a metallophthalocyanine–metalloporphyrin conju-
gate should represent the spectrum of each of the components
ZnPc and ZnTPP. The UV-Vis spectrum of ZnPc–(ZnTPP)2
and ZnPc–(ZnTPP)8 (Fig. 3) does indeed show a combination
of the respective spectra of ZnTPP and ZnPc with a slight
bathochromic (red) shift for the Q-bands of the ZnPc moieties
(from 670 nm to 675 nm for ZnPc in ZnPc–(ZnTPP)2 and
670 nm to 681 nm for ZnPc in ZnPc–(ZnTPP)8). The shifting
1
of the target phthalocyanine product. The H NMR spectrum
of this compound (Fig. 1) displayed four broad peaks due to the
extensive overlapping of the numerous protons in this molecule.
The integration of these peaks, however, revealed that all protons
could be accounted for. The broad peak at 8.87 ppm integrated
for 72 protons and this was due to the 64 pyrrole protons on the
porphyrin ring as well as the 8 non-peripheral phthalocyanine
protons. The broad peaks at 8.21 ppm and 7.75 ppm were due
to the phenyl protons on the porphyrin rings and integrated for
3 7 3 4
D a l t o n T r a n s . , 2 0 0 5 , 3 7 3 2 – 3 7 3 7

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Scheme 2 Synthesis of the nonamer ZnPc–(ZnTPP)8.
Fig. 2 Mass spectrum of ZnPc–(ZnTPP)8.
Fig. 1 1H NMR (aromatic region on top and complete spectrum below)
there are no conformation changes. The molecules synthesised
obey Beers law up to 1 × 10−5 M for ZnPc–(ZnTPP)2 and
2 × 10−5 M for ZnPc–(ZnTPP)8 in DMSO. The deviation from
linearity was observed at the phthalocyanine Q-band, at high
concentrations, thus indicating that aggregation occurs through
the phthalocyanine moeities.
of ZnPc–(ZnTPP)8.
of the ZnPc Q-bands in ZnPc–(ZnTPP)2 and ZnPc–(ZnTPP)8
to longer wavelengths compared to ZnPc alone suggests that
ZnTPP peripheral substituents exert electron-donating effects
on the ZnPc, since electron-donating peripheral substituents on
the MPc complexes gives rise to red shifts in the Q absorption
wavelengths.23 The intensity of the B-band relative to the Q-
band is lower for ZnTPP compared to the trimer and nonamer
(Fig. 3). The position of the porphyrin B-bands in terms of
wavelength did not shift at all. The fact that the B-band of
the ZnTPP components did not shift excludes the possibility
of a charge transfer interaction in the ground states of these
two molecules. A decrease in the B-band intensity is due to
conformational changes of the porphyrin moeity upon bonding
to the phthalocyanine,7 hence the observation of an intense B-
band relative to Q-band for the porphyrin is an indication that
Fluorescence properties
The fluorescence spectra of both the ZnPc–(ZnTPP)2 (Fig. 4)
and ZnPc–(ZnTPP)8 (Fig. 5) showed extensive energy transfer
between the porphyrin and phthalocyanine moeities. For both
these compounds, irradiation of the porphyrin B-band vibration
at 403 nm led to emission at the porphyrin B- and Q-bands as
well as the Q-band of the phthalocyanine (678 nm) (Fig. 4).
This excitation leads to the formation of a p–p* singlet state
of the porphyrin which results in the fluorescence from the
porphyrin molecule. From the fact that emission is observed
D a l t o n T r a n s . , 2 0 0 5 , 3 7 3 2 – 3 7 3 7
3 7 3 5

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Fig. 5 Normalized (Q-band) electronic absorption, excitation and
fluorescence spectra of 2.0 × 10−6 M ZnPc–(ZnTPP)8 in DMSO. Insert:
expansion of the Q-band region.
Fig. 3 The UV-Vis spectrum of ZnTPP (2.7 × 10−6 M), ZnPc (4.9 ×
10−6 M), ZnPc–ZnTPP2 (3.9 × 10−6 M) and ZnPc–ZnTPP8 (1.7 ×
10−6 M) in DMSO. Insert: expansion of the Q-band region of the ZnTPP.
molecules follow upon excitation. Fluorescence quantum yields
were determined using the following eqn (1):
Fx · Astd · g2(x)
UxF = UsFtd
(1)
Fstd · Ax · g2(std)
Where UF and UsFtd are the fluorescence quantum yields of the
sample and standard respectively; Fx and Fstd the areas under
the emission curves of the sample and standard respectively;
Ax and Astd, the absorbances of the sample and standard at the
excitation wavelength respectively (absorbance ranged between
0.04 and 0.05) and g2(x) and g2(std) are the refractive indices of
the solvents used for the sample and standard respectively. Both
the sample and the standard were excited at the same relevant
wavelength. The UF values obtained were the average of three
independent experiments in DMSO. The fluorescence quantum
yields (using the phthalocyanine emission for calculations) upon
excitation at 603 nm (porphyrin Q-band) (Table 1) were found
to decrease in the following order: single molecule ZnTPP
(0.039)7 > trimer ZnPc–(ZnTPP)2 (0.009 0.001) > nonamer
ZnPc–(ZnTPP)8 (0.005 0.001). This decrease was expected as
the steady-state emission spectra clearly showed energy transfer
taking place from the porphyrin units to the phthalocyanine
moeity. The fluorescence quantum yield (UF) calculated for the
zinc phthalocyanine moeity (excitation at 640 nm) in the het-
eromers had lower values compared to the ZnPc single molecule
(0.14 for ZnPc–(ZnTPP)2 and 0.06 for ZnPc–(ZnTPP)8). This
may result from fluorescence quenching due to the porphyrin
rings on the periphery of the phthalocyanine by charge transfer.7
Fluorescence lifetimes were determined using the Strickler–Berg
equation utilising the absorbance and fluorescence spectra of
dilute solutions of ZnPc–(ZnTPP)2 and ZnPc–(ZnTPP)8. The
Strickler–Berg equation has been shown to be accurate for non-
aggregated phthalocyanines.14 The same trend observed for the
fluorescence quantum yields was obtained for the fluorescence
x
Fig. 4 Normalized (Q-band) electronic absorption, excitation and
fluorescence spectra of 1.8 × 10−6 M ZnPc–(ZnTPP)2 in DMSO. Insert:
expansion of the Q-band region.
also from the phthalocyanine molecule suggests that the excited
porphyrin molecule transfers energy to the phthalocyanine
molecule. These two processes are in competition with the
internal conversion processes. In both cases the porphyrin B-
band had a far lower fluorescence intensity than expected, due
to charge transfer to the Pc molecule. This effect was amplified
in ZnPc–(ZnTPP)8 (3× more than in ZnPc–(ZnTPP)2) as seen
in Fig. 4 and 5 for the former compound, thus indicating a more
efficient intramolecular energy transfer process taking place
in this molecule. The excitation spectra (emission monitored
at 720 nm) (Fig. 4 and 5) for both complexes displayed two
interesting features. The first was the relatively low ratio of the
intensity of the porphyrin B-band to the phthalocyanine Q-band
in the excitation spectra of both complexes. This phenomenon
is indicative of the extensive energy transfer taking place.
Secondly, the excitation spectra was slightly different from the
absorption spectra in terms of Q-band absorbtion wavelength
for both compounds, suggesting that changes in the nature of the
lifetimes as follows: single molecule (2.7
(ZnTPP)2 (1.6
0.3 ns) > ZnPc–
0.3 ns) > ZnPc–(ZnTPP)8 (1.2
0.2 ns).
To quantify the energy transfer process one may determine
the energy transfer constant (kEt) and concurrently the energy
transfer quantum yield (UEt) from the fluorescence quantum
Table 1 Photophysical data of the single molecules and heteromers used in this study. Solvent = DMSO
Molecule
UF (Exciting kmax
)
sF/ns
UT
sT/ls
kEt/(×109 s−1
)
UEt
ZnPc
ZnTPP
0.23 (640)
0.039 (603)
4.75a
2.7
0.65
0.83
330
1400
—
—
—
—
ZnPc–(ZnTPP)2
ZnPc–(ZnTPP)2
ZnPc–(ZnTPP)8
ZnPc–(ZnTPP)8
0.009 0.001 (603)
0.14 0.05 (640)
0.005 0.001 (603)
0.06 0.02 (640)
1.6 0.3
—
1.2 0.2
—
0.10 0.02
—
0.04 0.01
—
260 30
—
170 30
—
1.2 0.1
—
2.5 0.2
—
0.77 0.08
—
0.95 0.1
—
a From ref. 20.
3 7 3 6
D a l t o n T r a n s . , 2 0 0 5 , 3 7 3 2 – 3 7 3 7

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yields of the respective single molecules and heteromers as well
as the fluorescence lifetime of the single molecule (2.7 ns)24 from
eqn (2) and (3), which have been derived before:7
Conclusion
Phthalocyanine–porphyrin conjugates that are linked through
covalent linkers are effective light harvesters. The porphyrins are
especially efficient at transferring energy to phthalocyaninines
upon excitation at their B- or Q-bands. The zinc phthalocyanine
used in this study was also a good acceptor of this energy
and its photophysical properties were modified greatly by the
presence of the porphyrins on its periphery. The nonameric
molecule, ZnPc–(ZnTPP)8, which had eight porphyrins linked
to the phthalocyanine had an energy transfer quantum yield of
unity which is quite remarkable.
ꢀ
ꢁ
1
UF(monomer)
kEt
=
− 1
(2)
sS UF(heteromer)
kEt
+
UEt
=
kEt
(3)
1
s
S
Where UF (single molecule) is the fluorescence quantum yield
of the single molecule (ZnPc of ZnTPP), sS is the lifetime of
the excited singlet state. Using the fluorescence data above,
efficiency of the energy transfer process was calculated using eqn
(2) and (3) and kEt of 1.2 ( 0.1) × 109 s−1 for ZnPc–(ZnTPP)2
was observed. This value increases for ZnPc–(ZnTPP)8 to 2.5
Acknowledgements
This work has been supported by the National Research
Foundation of South Africa as well as Rhodes University.
We wish to thank Dr Takamitsu Fukuda for assistance with
MALDI-TOF spectral analysis.
(
0.2) × 109 s−1 indicating an increase by a factor of two, on
going from the trimer to the nonamer. These values were then
used to calculate the quantum yield of energy transfer (UEt)
using eqn (3) and found to be 0.77 ( 0.08) for ZnPc–(ZnTPP)2
and near unity (0.95 0.1) for ZnPc–(ZnTPP)8, showing that
the nonamer experiences more quenching than the trimer.
References
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Triplet properties
The influence of the porphyrin rings upon the triplet properties
of the zinc phthalocyanine moeity was studied by laser flash
photolysis. Excitation for these studies was performed at 640 nm
(the Pc absorption) only due to intrumental limitations. Un-
substituted zinc phthalocyanine has a triplet quantum yield of
0.657 in DMSO and a triplet lifetime of 330 ls7 in DMSO. The
triplet lifetime of the studied compounds was determined by
laser flash photolysis and the analysis of the decays was done by
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somewhat, compared to the respective single molecules, and
this decrease was dependant on the number of porphyrin rings
present on the molecules. The ZnPc–(ZnTPP)2 had a triplet
lifetime of 260 ( 30) ls and ZnPc–(ZnTPP)8 170 ( 30) ls.
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DAsSampleesStd
DAsStdesSample
UsTample = UsTtd
(4)
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Where DASsample and DASstd are the changes in the singlet state
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ZnPc, respectively; UsTtd is the triplet quantum yield for ZnPc.
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ZnPc component) decreased with an increase in porphyrin rings
attached peripherally to the zinc phthalocyanine unit. These
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(
0.01) for the ZnPc–(ZnTPP)8. All these data confirm that
effective energy transfer takes place and that the porphyrin units
in turn act as quenchers of the photophysical processes taking
place in these molecules.
D a l t o n T r a n s . , 2 0 0 5 , 3 7 3 2 – 3 7 3 7
3 7 3 7