Synthesis, photophysical characterization, and surface photovoltage spectra of windmill-shaped phthalocyanine-porphyrin heterodimers and heteropentamers

Article abstract of DOI:10.1002/ejic.200700724

The synthesis and characterization of three nonperipherally substituted, covalently-linked phthalocyanine-porphyrin hybrid molecules is described. The optical and photophysical properties of these compounds are studied in detail. The intramolecular energy

Full text of DOI:10.1002/ejic.200700724

FULL PAPER
DOI: 10.1002/ejic.200700724
Synthesis, Photophysical Characterization, and Surface Photovoltage Spectra
of Windmill-Shaped Phthalocyanine–Porphyrin Heterodimers and
Heteropentamers
Zhixin Zhao,[a] Chun-Ting Poon,[a] Wai-Kwok Wong,*[a] Wai-Yeung Wong,[a]
Hoi-Lam Tam,[b] Kok-Wai Cheah,[b] Tengfeng Xie,[c] and Dejun Wang[c]
Keywords: Energy transfer / Photoluminescence / Photovoltage spectra / Phthalocyanines / Porphyrins
The synthesis and characterization of three nonperipherally
substituted, covalently-linked phthalocyanine–porphyrin hy-
brid molecules is described. The optical and photophysical
properties of these compounds are studied in detail. The in-
tramolecular energy transfer process is investigated and
quantified in terms of the quantum yield of the energy trans-
fer and is found to be highly effective in these molecules.
The photophysical properties of the phthalocyanine core of
these three compounds are significantly modified by the
presence of the porphyrin moieties. These molecules display
interesting optical-power-limiting behavior and a surface
photovoltaic effect.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
are ideal candidates for application in light-harvesting and
molecular photonics. Porphyrins and phthalocyanines rep-
resent two related classes of very versatile macrocyclic or-
ganic molecules and typically display complementary op-
tical properties. Phthalocyanines have two absorption
bands, a strong Q band in the red region (600–700 nm) and
a medium-strength Soret band in the UV range (350 nm),
whereas porphyrins have a strong B band in the region 410–
430 nm and medium-strength Q bands between 510 and
650 nm. Their ground-state absorptions cover the entire vis-
ible region of the spectrum. The first synthesis of a porphy-
rin–phthalocyanine conjugate, a dimer from a zinc porphy-
rin and a zinc phthalocyanine, was reported about two dec-
ades ago by Gaspard et al.,[7] with the ultimate aim of in-
vestigating their photochemical properties. A few papers
have also appeared in recent years in which the authors re-
ported on investigations to exploit the electronic- and pho-
tonic-based cooperation between individual subunits of
porphyrins and phthalocyanines.[8,9]
The efficiency of intramolecular electron- and energy
transfer processes in oxygen-linked chromophores is of
interest as they are suitable as model systems for photosyn-
thesis. Recently, the synthesis and photophysical properties
of a trimer, pentamer, and nanomer, in which a phthalocy-
anine molecule was connected by an ether linkage to two,
four, and eight porphyrin molecules, respectively, and their
transition-metal [ZnII and CoII] complexes were re-
ported.[10] In this paper, we report the synthesis, photophys-
ical, and surface photovoltage properties of nonperipher-
ally-substituted windmill phthalocyanine–porphyrin hetero-
dimers and heteropentamers.
Introduction
The chemistry of light-harvesting and energy-transfer-
ring molecular arrays has attracted a lot of attention over
the years because of the ability of these systems to mimic
the function of chlorophyll in plants during photosynthe-
sis.[1] As a light-harvesting system, they should be able to
capture solar energy over a broad spectral range and chan-
nel the energy swiftly for use in chemical reactions. The abil-
ity of undergoing excited-state energy transfer is an impor-
tant prerequisite for a good molecular photonic device.
Phthalocyanines are an important class of organic com-
pounds, which have many applications as xerographic
photoreceptors,[2] as optical recording,[3] infrared sensors,[4]
and organic photoelectronic devices,[5] and as a photosensi-
tizer in the photodynamic therapy (PDT) of cancer.[6]
Phthalocyanines are 18-π-electron aromatic compounds
that comprise four isoindoles. Two-dimensional aromatic
delocalization over these macrocycles gives rise to their ex-
ceptional optical and electrical properties. Phthalocyanines
are generally stable, rigid compounds with varying proper-
ties that depend upon their central metals, and peripheral
and axial substituents. Porphyrin–phthalocyanine arrays
[a] Department of Chemistry and Centre for Advanced Lumines-
cence Materials, Hong Kong Baptist University,
Waterloo Road, Hong Kong, P. R. China
Fax: +852-3411-7348
E-mail: wkwong@hkbu.edu.hk
[b] Department of Physics and Centre for Advanced Luminescence
Materials, Hong Kong Baptist University,
Waterloo Road, Hong Kong, P. R. China
[c] College of Chemistry, Jilin University,
2699 Qianjin Street, Changchun, 130012, P.R. China
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W.-K. Wong et al.
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tion of phthalonitrile–benzaldehyde, p-tolualdehyde, and
pyrrole in CH2Cl2 in the presence of BF3·OEt2 at room
temperature followed by oxidation with 2,3-dichloro-5,6-di-
Results and Discussion
Synthesis and Characterization
The synthetic routes for the new compounds are shown cyano-1,4-benzoquinone (DDQ) afforded a mixture of por-
in Schemes 1, 2, and 3. The compounds are abbreviated as phyrins. Because the porphyrins bear different numbers of
indicated in the Experimental Section. All the compounds polar groups, the desired ethynyl-linked porphyrin–
were fully characterized by UV/Vis, IR, and 1H NMR spec- phthalonitrile (H2CNTTP) was easily isolated in 15% yield
troscopy, and mass spectrometry. In the preparation of the after two flash silica gel columns in which CHCl3/hexane
heterodimer and heteropentamer, we examined the cyclo- (3:1, v/v) was the eluent (Scheme 1). The UV/Vis absorption
tetramerization of a phthalonitrile-derivatized porphyrin. 3- spectrum of H2CNTTP, which displays a Soret band at
Iodophthalonitrile was synthesized according to literature 422 nm and Q bands at 517, 552, 593, and 649 nm, is typi-
methods.[11] Treatment of 3-iodophthalonitrile with 4-eth- cal of normal porphyrin-free bases. H2CNTTP exhibits the
ynylbenzaldehyde under the Pd-mediated coupling condi- [M + 1]+ peak at m/z = 807.3262, which is in good agree-
tions [triethylamine/thf (3:2, v/v) at room temperature in the ment with the theoretical value of 807.3231, in its MALDI-
presence of Pd(PPh3)2Cl2 and CuI] gave phthalonitrile– TOF mass spectrum, a singlet at δ = –2.78 ppm for the in-
benzaldehyde in 78% yield (Scheme 1). A mixed condensa- ner NH protons of the porphyrin in its 1H NMR spectrum,
Scheme 1. Synthesis of H2CNTTP and ZnCNTTP.
Scheme 2. Synthesis of H2Pc-(H2TTP)4.
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Phthalocyanine–Porphyrin Heterodimers and Heteropentamers
and an N–H and CϵN stretch at 3316 and 2217 cm–1, preparation of other metallophthalocyanines. Cyclotetra-
respectively, in its IR spectrum. The ZnII metalloporphyrin merization of ZnCNTTP with the addition of hydrated zinc
could be prepared quantitatively by refluxing the porphy- acetate and after a heating period of 4 h afforded the all-
rin-free base with an equivalent amount of hydrated zinc zinc chelate ZnPc-(ZnTTP)4 (Scheme 3). The free base and
acetate in a CHCl3/methanol mixture. The ZnII complex, the metalated products are very stable in air and can easily
ZnCNTTP, displays an electronic absorption spectrum be purified by silica gel column chromatography. When
characteristic of normal metalloporphyrin complexes with eluted with a CHCl3/methanol (50:1, v/v) mixture, a green-
a Soret band at 426 nm and two Q bands at 551 and brown band was obtained from which the desired product
591 nm. The formation of the metalloporphyrin ZnCNTTP was isolated. The formation of the phthalocyanine ring is
was further confirmed by the disappearance of the inner characterized by the appearance of phthalocyanine Q bands
NH resonance in the 1H NMR spectrum and the N–H in the electronic absorption spectrum. Thus, the presence of
stretch in the IR spectrum. The IR spectrum of the complex two Q band absorptions at 687 and 718 nm for H2Pc-
exhibits a CϵN stretch at 2217 cm–1. The MALDI-TOF (H2TTP)4 and one Q band absorption at 695 nm for ZnPc-
mass spectrum of ZnCNTTP also exhibits the [M + 1]+ (ZnTTP)4 confirms the formation of the phthalocyanine
peak at m/z = 868.2247, which agrees well with the theoreti- ring for the two compounds. The disappearance of the
cal value of 868.2292. The metal-free phthalocyanine–por- CϵN stretch in the IR spectrum of H2CNTTP and
phyrin conjugate H2Pc-(H2TTP)4 was synthesized by cyclo- ZnCNTTP at 2217 and 2212 cm–1, respectively, further sup-
tetramerization of H2CNTTP by using the lithium pentox- ports the cyclotetramerization of phthalonitrile to phthalo-
ide method (Scheme 2). The initial product of the lithium cyanine. The MALDI-TOF mass spectra of the two com-
pentoxide method was dilithium phthalocyanine (Li2Pc), in pounds reveal the [M + 1]+ peak at m/z = 3230.2765 and
which the lithium ions can readily be displaced by other 3546.8359, which are in good agreement with the calculated
transition-metal ions. Thus, the subsequent addition of values of 3230.2924 and 3546.8530 for H2Pc-(H2TTP)4 and
metal salts to Li2Pc provides a convenient method for the ZnPc-(ZnTTP)4, respectively.
Scheme 3. Synthesis of ZnPc-ZnTTP and ZnPc-(ZnTTP)4.
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The unsymmetrical heterodimer ZnPc-ZnTTP was syn-
thesized in satisfactory yield (28%) by condensation of
ZnCNTTP (1 equiv.) with excess chloro[7,12:14,19-diimino-
21,5-nitrilo-5H-tribenzo[c,h,m][1,6,11]triazacyclopentadeci-
nato(2-)-κN22,κN23,κN24]boron(III) (SubPc, 5 equiv.) in the
presence of hydrated zinc acetate (5 equiv.).[10b] The only
by-product of the reaction was ZnPc, which could readily
be separated from the desired product by column
chromatography. The heterodimer ZnPc-ZnTTP could eas-
ily be eluted from a silica gel column, whereas the by-prod-
uct ZnPc remained stationary on the top of the column
when eluted with a mixture of dichloromethane/methanol
(19:1, v/v). The MALDI-TOF mass spectrum of ZnPc-
ZnTTP displays the [M]+ peak at m/z = 1320.2721, which
is in agreement with the theoretical value of 1320.2683. The
appearance of the Q band at 676 nm in the UV/Vis absorp-
tion spectrum and the disappearance of the CϵN stretch at
2212 cm–1 in the IR spectrum further supports the claim
that the product is a phthalocyanine complex.
Figure 1. UV/Vis absorption spectra of H2TTP, H2Pc, and H2Pc-
(H2TTP)4 (approximately 1.0ϫ10–6 ) in CHCl3.
Absorption Properties
The electronic absorption spectrum of the phthalocyan-
ine–porphyrin conjugates is basically the sum of the absorp-
tion spectra of phthalocyanine and porphyrin moieties, with
a slight bathochromic shift of the Q bands of the phthalocy-
anine moiety. Figure 1 shows the absorption spectra of
H2TTP, H2Pc, and H2Pc-(H2TTP)4 in CHCl3. The two Q
bands assigned to the phthalocyanine moiety of H2Pc-
(H2TTP)4 at 687 and 718 nm are redshifted by 26–32 nm
relative to those of H2Pc, which are at 655 and 692 nm,
respectively. Such a bathochromic shift is probably attrib-
uted to the extension of the conjugated π-system involving
Figure 2. UV/Vis absorption spectra of ZnTTP, ZnPc, ZnPc-
ZnTTP, and ZnPc-(ZnTTP)4 (approximately 1.0ϫ10–6 ) in
CHCl3.
Fluorescence Properties
The absorption, excitation, and fluorescence spectra of
the four phenylethynyl linkers from the attached porphyrins the three conjugated dyads H2Pc-(H2TTP)4, ZnPc-ZnTTP,
and the distortion of the planarity of the phthalocyanine and ZnPc-(ZnTTP)4 are shown in Figures 3, 4, and 5,
ring. The zinc(II) complexes of the phthalocyanine–porphy- respectively. In each array, illumination of the porphyrin
rin conjugates display similar absorption behavior. Figure 2 component results in fluorescence exclusively from the
shows the absorption spectra of ZnTTP, ZnPc, ZnPc- phthalocyanine moiety. For example, the fluorescence spec-
ZnTTP, and ZnPc-(ZnTTP)4 in CHCl3. The phthalocyan- trum of H2Pc-(H2TTP)4 (Figure 3) obtained by excitation
ine Q band for ZnPc at 671 nm is redshifted to 676 and at the Q band of the porphyrin moieties (λexc = 515 nm) is
695 nm for ZnPc-ZnTTP and ZnPc-(ZnTTP)4, respectively. essentially the same as that obtained when excitation was
The bathochromic shift of the phthalocyanine Q band in- carried out at the Q band of the phthalocyanine moiety
creases from 5 to 24 nm as the number of peripheral ZnTTP (λexc = 627 nm). In the fluorescence spectrum, other than
moieties attached to ZnPc increases from 1 to 4. It has been the phthalocyanine emission at λem = 720 nm, which is red-
shown that the electron-donating substituents on metallo- shifted by 23 nm relative to that of H2Pc (λem = 697 nm),
phthalocyanine complexes give rise to red shifts in the Q there is no evidence of any emission from the porphyrin
band absorptions.[12] The shift in the Q band to longer moieties (expected fluorescence at 650 nm). These results
wavelengths for the ZnPc moiety in the spectra of ZnPc- indicate a very efficient photoinduced intramolecular en-
ZnTTP and ZnPc-(ZnTTP)4 suggests that other than the ergy transfer from the porphyrin to the phthalocyanine
extension of π-conjugation, peripheral ZnTTP substituents components in the pentamer. A similar photoluminescence
may also exert electron-donating effects on ZnPc. At high behavior is also observed for the other two oligomers. The
concentration, the absorbance of the phthalocyanine Q fluorescence band of ZnPc-ZnTTP and ZnPc-(ZnTTP)4 rel-
band of ZnPc-ZnTTP and ZnPc-(ZnTTP)4 deviates from ative to that of the ZnPc complex (675 nm) is redshifted by
linearity, which indicates that aggregation might occur 11 and 29 nm to 687 and 704 nm, respectively. The exci-
through the phthalocyanine moiety.
tation spectra (monitored at 800 nm) of the three dyads are
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Phthalocyanine–Porphyrin Heterodimers and Heteropentamers
coincident with their ground-state absorption spectra (Fig-
ures 3–5) and reveal contribution of both phthalocyanine
and porphyrin moieties.
Figure 5. Normalized (B band) electronic absorption, excitation
(monitored at 800 nm) and fluorescence spectra (λexc = 555 nm:
porphyrin moiety, and λexc = 627 nm: phthalocyanine moiety) of
ZnPc-(ZnTTP)4 in toluene (approximately 1.0ϫ10–6 ). The two
emission spectra coincide completely.
Figure 3. Normalized (B band) electronic absorption, excitation
(monitored at 800 nm), and fluorescence spectra (λexc = 515 nm: Q
band of porphyrin moiety, and λexc = 627 nm: Q band of phthalo-
cyanine moiety) of H2Pc-(H2TTP)4 in toluene (approximately
1.0ϫ10–6 ). The two emission spectra coincide completely.
the reference compounds. The fluorescence quantum yields
for all three compounds are exactly the same at both exci-
tation wavelengths corresponding to the porphyrin and
phthalocyanine moieties. The fluorescence quantum yield
of H2Pc-H2(TTP)4 is lower (Φ = 0.30) than that of H2Pc
(Φ = 0.60); for ZnPc-ZnTTP and ZnPc-(ZnTTP)4, the fluo-
rescence quantum yields are 0.25 and 0.18, respectively.
Table 1. Photophysical data of the monomers and heteromers in
toluene.
Compound
λexc [nm]
τ [ns]
Φ
ΦET
H2TTP
H2Pc
ZnPc
515
627
605
0.11
0.77
0.23
0.30Ϯ0.02
0.31Ϯ0.02
0.25Ϯ0.02
0.25Ϯ0.02
0.18Ϯ0.02
0.19Ϯ0.02
H2Pc-H2(TTP)4 515[a]
627[b]
6.1
4.5
6.3
Ͼ0.98
Ͼ0.99
Ͼ0.99
ZnPc-ZnTTP
548[a]
605[b]
ZnPc-(ZnTTP)4 555[a]
627[b]
[a] Upon excitation of porphyrin moiety. [b] Upon excitation of
phthalocyanine moiety.
Figure 4. Normalized (B band) electronic absorption, excitation
(monitored at 760 nm) and fluorescence spectra (λexc = 548 nm:
porphyrin moiety, and λexc = 605 nm: phthalocyanine moiety) of
ZnPc-ZnTTP in toluene (approximately 1.0ϫ10–6 ). Inset is an
expansion of the Q-band region.
Surface Photovoltaic Properties
The energy-transfer efficiency can be quantified by ana-
Surface photovoltage spectroscopy (SPS) is a useful tool
lyzing the fluorescence yield of the complex by excitation to investigate the photophysics of excited states generated
through the porphyrin moiety and the phthalocyanine moi- by absorption in the aggregate state,[16] since the sensitivity
ety separately. Several methods were used to evaluate the of the method is about 108 charge/cm2, or about one ele-
energy-transfer efficiency.[8] The data in Table 1 were esti- mentary charge per 107 surface atoms, which exceeds that
mated according to a literature method.[8] Fluorescence of conventional spectroscopic methods, such as XPS and
quantum yields of the three compounds were measured to Auger spectroscopy, by many orders of magnitude.[17] On
quantify the singlet emission from the phthalocyanine moi- the basis of the principle of SPS, a field-induced surface
ety at both wavelengths (Table 1). ZnPc (Φ = 0.23),[13] H2Pc photovoltage technique (FISPS)[18,19] has been developed to
(Φ = 0.60),[14] and H2TTP (Φ = 0.11)[15] were employed as demonstrate the optoelectronic properties of organic semi-
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W.-K. Wong et al.
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conductors under the effect of an external electric field.[20]
By combining SPS with FISPS, one could determine the
conduction type of the organic semiconductors. The SPS of
ZnTTP, ZnPc-(ZnTTP)4, and H2Pc-(H2TTP)4 are given in
Figure 6. It can be seen that the photovoltaic action spectra
match the absorption spectral response well, which indi-
cates that the bands correspond to a similar electronic tran-
sition. The electrical and photovoltaic properties of solid
porphyrins and phthalocyanines have been attributed to the
semiconducting behavior of the porphyrins and phthalocy-
anines themselves. The π orbital is analogous to the valence
band of the semiconductor, and the π* orbital to the con-
duction band. Photogenerated charge carriers in the π sys-
tem are nonlocalized, and their motion is free within the
energy band of the π system. Photogenerated holes move in
the valance band, while photogenerated electrons move in
the conduction band. For this kind of system, the band to
band transition is characterized as a πǞπ* transition,
which is exhibited mainly as Soret B and Q bands. By com-
paring the SPS data of the three samples, we observe two
characteristics. One is that the surface photovoltage (SPV)
response intensities of H2Pc-(H2TTP)4 are much larger than
those of ZnTTP and ZnPc-(ZnTTP)4, which indicates that
H2Pc-(H2TTP)4 exhibits a higher photoelectric conversion
efficiency. On the other hand, the number of SPV response
peaks of the Q band for ZnTTP and ZnPc-(ZnTTP)4 de-
creases upon formation of the complexes because the en-
ergy band structure and optical constant are changed after
metalation. Figure 7 shows the field-induced surface photo-
voltage spectra (FISPS) of ZnTTP, ZnPc-(ZnTTP)4, and
H2Pc-(H2TTP)4. When a positive field is applied to the irra-
diated ITO electrode, the SPV response intensities of the
three samples are all strikingly enhanced, which demon-
strates that the external electric field is of the same sign as
that of the built-in field and indicates p-type characteristics
of the materials. On the contrary, when a negative electric
field is applied, the SPV response intensities are weak and
even reversed. Moreover, we note that the B and Q bands
of the two samples exhibit a “simultaneous response” with
a change in positive or negative electric field intensity. This
indicates that they correspond to an analogous transition
characteristic, both belonging to the πǞπ* transition. It
can also be shown that the variation rate of B and Q bands
differs with a change in positive or negative electric field
because of the differing separation of the lower energy
levels from the valence band for these transitions.
Figure 6. The SPS spectra of (a) H2Pc-(H2TTP)4, (b) ZnTTP, and
(c) ZnPc-(ZnTTP)4.
Electrochemistry
The redox properties of H2Pc-(H2TTP)4, ZnPc-ZnTTP,
and ZnPc-(ZnTTP)4 were studied by cyclic voltammetry in
CH2Cl2 containing NBu4PF6. Table 2 presents the potential
values in each case at room temperature. The metal-free
compound H2Pc-(H2TTP)4 displays an irreversible anodic
wave and two reversible reduction waves typical of ring-
based processes. The separation between the first and sec-
ond ring reductions in H2Pc-(H2TTP)4 was found to be
Figure 7. The FISPS of (a) ZnPc-(ZnTTP)4, (b) ZnTTP, and (c)
H2Pc-(H2TTP)4.
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Phthalocyanine–Porphyrin Heterodimers and Heteropentamers
within the range reported for ring-based reductions in some and δ0 are the effective excited-state and ground-state ab-
nontransition-metal phthalocyanines.[21] ZnPc-ZnTTP and sorption cross sections, respectively.[24] RSA occurs when
ZnPc-(ZnTTP)4 show a reversible oxidation wave at 0.29 the cross section of the excited state is larger than that of
and 0.04 V, respectively, and both these processes are ring- the ground state. It was observed that our compounds show
based since the central Zn metal is electroinactive. Met- OPL behavior with good δeff./δ0 values (9.94, 11.09, and
alation of H2Pc-(H2TTP)4 by Zn2+ induces a cathodic shift 10.73 for H2Pc-(H2TTP)4, ZnPc-ZnTTP, and ZnTTP-
in the reduction potential from –1.91 to –2.15 V. Displace- (ZnTTP)4, respectively), which slightly exceed or are at least
ment of the two protons in the inner porphyrin or phthalo- comparable to that of C60 (9.06).
cyanine core with a metal ion leads to a shift in the first
ring-based reduction potential to a more negative value as
a result of the π back donation of the filled dπ orbital of
the metal ion into the empty porphyrin or phthalocyanine
π* orbitals.[10b]
Table 2. Electrochemical data for H2Pc-(H2TTP)4, ZnPc-ZnTTP,
and ZnPc-(ZnTTP)4.[a]
Eox [V]
Ered [V]
H2Pc-(H2TTP)4
ZnPc-ZnTTP
ZnPc-(ZnTTP)4
0.33 (i)
0.29 (r)
0.04 (r)
–1.91 (r), –2.25 (r)
–1.30 (r), –1.93 (r)
–2.15 (r)
[a] i = irreversible wave; r = reversible wave.
Optical-Power-Limiting Properties
Optical limiters are devices that can protect sensitive op-
tical sensors against high optical power. In other words,
these devices limit the transmitted intensity of a bright op-
tical beam to some specified maximum (ideally to a safe
level without causing damage to the eye or optical sensor)
but exhibits high transmittance for low-intensity ambient
light levels. The optical-power-limiting (OPL) effect can be
explained by a mechanism known as reverse saturable ab-
sorption (RSA).[22] So far, metallophthalocyanine and
metalloporphyrins continue to attract considerable atten-
tion because the structure–property relationships can be in-
vestigated easily through structural modification by chang- Figure 8. Open aperture Z-scans for optical-limiting measurements
ing the central metal center and ligand substituents.[23] We
at the linear transmittance of 93% at (a) 400 nm and (b) 800 nm.
therefore studied the OPL effect of our phthalocyanine–
porphyrin heterodimer and pentamer. The intensity-de-
pendent transmittance and Z-scan measurements were per-
Conclusions
formed with Ti:Sapphire laser with an output wavelength
of 400 and 800 nm. From the Z-scan curves for the three
complexes measured in CHCl3 (Figure 8), all of the com-
plexes show excellent OPL behavior at both 400 and 800 nm
excitation. It is clear that the complex ZnPc-ZnTTP shows
the strongest OPL capability at 400 nm excitation, whereas
H2Pc-(H2TTP)4 gives the strongest response at 800 nm exci-
tation. Both of them have an OPL performance that is com-
parable to that of the benchmark material C60. At 400 nm,
the transmittance for H2Pc-(H2TTP)4, ZnPc-ZnTTP, and
ZnTTP-(ZnTTP)4 decreases to 52%, 48%, and 49% of their
original linear transmission, respectively (cf. 58% for C60,
see Figure 8a). The corresponding transmittance values
drop to 13%, 23%, and 37%, respectively, at 800 nm (cf.
17% for C60, see Figure 8b). The performance of these re-
verse saturable absorbers can also be compared quantita-
Nonperipherally substituted porphyrin–phthalocyanine
light-harvesting arrays have been prepared by cyclotetra-
merization of substituted phthalonitriles, which are effective
light harvesters and optical power limiters. The porphyrins
are especially efficient at transferring energy to phthalocy-
anines upon excitation at their B or Q bands. The phthalo-
cyanine moieties are also good acceptors of energy and
their photophysical properties can be modified greatly by
the presence of a porphyrin on its nonperiphery.
Experimental Section
Materials: All reactions were carried out under an atmosphere of
dry nitrogen. The chemicals were purchased from Sigma–Aldrich
tively by using the figure of merit factor δeff/δ0, where δeff and used without further purification. 3-Iodophthalonitrile,[11] 4-
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ethynylbenzaldehyde and SubPc[25] were synthesized as published.
All solvents were dried by the standard methods prior to use. Silica
gel 60 (0.04–0.063 mm) for column chromatography was purchased
from Merck. The IR spectra (KBr pellets) were recorded with a
Nicolet Nagba-IR 550 spectrometer and NMR spectra with a Var-
ian INOVA 400 spectrometer. Electrospray ionization high-resolu-
tion mass spectra (ESI-HRMS) were recorded with a QSTAR mass
spectrometer. Electronic absorption spectra in the UV/Vis region
were recorded with a Hewlett–Packard 8453 UV/Vis spectropho-
tometer, steady-state visible fluorescence and PL excitation spectra
with a Photon Technology International (PTI) Alphascan spectro-
fluorimeter, and visible decay spectra with a pico-N2 laser system
(PTI Time Master) with λexc = 337 nm. Electrochemical measure-
ments were made in CH2Cl2 by using a Princeton Applied Research
(PAR) model 273A potentiostat. A conventional three-electrode
configuration consisting of a glassy-carbon working electrode, and
Pt wires as both the counter and reference electrodes were used.
The supporting electrolyte was 0.1  [Bu4N]PF6. Ferrocene was
added as a calibrant after each set of measurements, and all poten-
tials reported were quoted with reference to the ferrocene/ferrocen-
ium (Fc/Fc+) couple at a scan rate of 100 mVs–1. Surface photo-
voltage spectra (SPS) were measured with a solid junction photo-
voltaic cell (ITO/porphyrin films/ITO) by using light-source-mono-
chromator-lock-in detection technique. The principle and setup
diagram of SPS and field-induced surface photovoltage spectro-
scopic (FISPS) measurements have already been described in detail
elsewhere.[26] The measurement was performed under atmospheric
pressure and at ambient temperature (about 20Ϯ2 °C). The investi-
gation of the optical-limiting properties was performed with a
mode-locked Ti:Sapphire laser (Spectra Physics – Tsunami) and Ti-
:Sapphire amplifier (Spectra Physics – Spitfire) at the repetition of
1 kHz. The laser was frequency doubled with an output wavelength
of 400 or 800 nm with a 100-fs pulse width for Gaussian mode by
a frequency double crystal (FDC). The laser beam was then split
into two beams by a beam splitter (BS). One was used as the refer-
ence beam, which was received by a detector (D1), the other was
for the sample measurement and it was focused with a lens (L1, f
= 20 cm). After transmittance of the light beam through the sample
(S), it entered another detector (D2). The sample to be measured
was moved along a rail to change the incidence irradiance on it.
The incident and transmitted energies were detected simultaneously
by the two detectors D1and D2 (LPE-1A) individually. The solution
samples were measured in a 1-mm quartz cell.
0.50 mmol), 4-[2-(2,3-dicyanophenyl)ethynyl]benzaldehyde
(128 mg, 0.50 mmol), and pyrrole (0.14 mL, 20 mmol) were con-
densed in CH2Cl2 (200 mL) with BF3·OEt2 (0.266 mL of a 2.5 
stock solution in CH2Cl2, 0.665 mmol) at room temperature for
1 h. Then DDQ (342 mg, 1.50 mmol) was added and stirring was
continued at room temperature for a further 1 h. Flash chromatog-
raphy (Al2O3) was applied to remove non-porphyrinic components
from the crude reaction mixture. Column chromatography on silica
gel with CH2Cl2/hexane (3:1, v/v) as eluent gave the desired product
as the second red band. Evaporation of the solvent afforded a pur-
ple solid (61 mg, 15%). The purple crude product was recrystallized
from CH2Cl2/methanol. UV/Vis (toluene): λmax (log ε) = 422 (5.71),
517 (4.26), 552 (4.03), 593 (3.74), 649 (3.71) nm. 1H NMR
(400 MHz, CDCl3): δ = –2.78 (s, 2 H, N–H), 2.71 (s, 9 H, CH3),
7.54 (d, J = 8.0 Hz, 6 H, phenyl H), 7.74–8.05 (m, 3 H, phthalo-
nitrile H), 8.03 (d, J = 8.0 Hz, 6 H, phenyl H), 8.10 (d, J = 8.0 Hz,
2 H, phenyl H), 8.80–8.90 (m, 8 H, pyrrole H) ppm. IR (KBr, in):
ν = 3316 (N–H), 2918 (CH), 2217 (CN), 1581, 1473, 1399, 1349,
˜
1219, 1181, 982, 966, 799, 734 cm–1. MALDI-TOF-MS for
C57H38N6 (807.0): calcd. for [M + H]+ 807.3231; found 807.3262.
5,10,15-Tritolyl-20-{4-[2-(2,3-dicyanophenyl)ethynyl]phenyl}-
porphyrin Zinc(II) (ZnCNTTP): A mixture of H2CNTTP (40 mg,
0.05 mmol) and Zn(OAc)2·2H2O (10.8 mg, 0.05 mmol) in CHCl3
(10 mL) and methanol (10 mL) was stirred at room temperature
for 12 h. After removal of the solvent, the solid was dissolved in a
minimum volume of CHCl3 and loaded onto a silica gel column.
Elution with CHCl3/hexane (3:1, v/v) afforded one vivid red band
only. Removal of chloroform by evaporation afforded the product
in an almost quantitative yield (42 mg, 98%). UV/Vis (toluene):
λmax (log ε) = 426 (5.67), 551 (4.35), 591 (3.81) nm. 1H NMR
(400 MHz, CDCl3): δ = 2.71 (s, 9 H, CH3), 7.54 (d, J = 7.6 Hz, 6
H, phenyl H), 7.75–8.03 (m, 3 H, phthalonitrile H), 8.08 (d, J =
7.6 Hz, 6 H, phenyl H), 8.25 (d, J = 7.6 Hz, 2 H, phenyl H), 8.90–
8.99 (m, 8 H, pyrrole H) ppm. IR (KBr, in): ν = 2212 (CN), 2921
˜
(CH), 1583, 494, 1339, 1205, 1180, 1000, 797, 721 cm–1. MALDI-
TOF-MS for C57H38N6Zn (870.0): calcd. for [M + H]+ 868.2292;
found 868.2247.
All-Free-Base Heteropentamer [H2Pc-(H2TTP)4]: To 1-pentanol
(10 mL) was added lithium ribbon (147.1 mg, 21 mmol). The mix-
ture was stirred under nitrogen at 90 °C for 3 h until all of the
lithium was consumed. After cooling of the mixture to room tem-
perature, H2CNTTP (170 mg, 0.21 mmol) was added. The reaction
mixture was then heated to 145 °C and stirred under nitrogen for
4 h. The solvent was removed by vacuum. Chloroform (100 mL)
was added and washed three times with water (100 mL). The chlo-
roform layer was collected, solvents evaporated, and the solid ap-
plied to a silica gel column. Elution with CHCl3/methanol (50:1,
v/v) afforded a green-brown band. Removal of the solvent by evap-
oration afforded a purple solid (61.2 mg, 36%). The purple product
was recrystallized from a mixture of chloroform and hexane. UV/
Vis (CHCl3): λmax (log ε) = 422 (6.09), 517 (4.79), 554 (4.59), 594
Synthesis
4-[2-(2,3-Dicyanophenyl)ethynyl]benzaldehyde: 3-Iodophthalonitrile
(1.78 g, 7.01 mmol), 4-ethynylbenzaldehyde (1.18 g, 7.01 mmol),
Pd(PPh3)2Cl2 (98.1 mg, 0.14 mmol), and CuI (53.4 mg, 0.28 mmol)
were added to a mixture of deaerated triethylamine (15 mL) and
THF (10 mL) under nitrogen. The mixture was stirred overnight,
and then the solvent was removed by distillation under reduced
pressure. Chloroform (100 mL) was added to the yellow product,
which was washed three times with 5% NaHCO3 solution. The
chloroform layer was collected, solvents evaporated, and the yellow
solid applied to a silica gel column. The desired compound was
eluted as the third band by using chloroform/hexane (3:1, v/v),
which after removal of the solvent, afforded a pale yellow solid in
78% yield (1.40 g). 1H NMR (400 MHz, CDCl3): δ = 7.74–7.80 (m,
1
(4.40), 650 (4.57), 687 (4.85), 718 (4.91) nm. H NMR (400 MHz,
CDCl3): δ = 9.15 (br. s, 4 H, phthalocyanine H), 8.52–8.90 (m, 32
H, pyrrole H), 8.33–8.50 (br. s, 8 H, phenyl H), 7.90–8.20 (br. s, 24
H, phenyl H), 7.46–7.76 (br. d, 32 H, phenyl H), 6.81–7.07 (br. s,
8 H, phthalocyanine H), 2.71 (s, 36 H, CH3), –2.79 (br. s, 8 H, N–
H) ppm. IR (KBr, in): ν = 3312 (NH), 2910, 1502, 1469, 1347,
˜
4 H, phenyl H), 7.87–7.94 (m, 3 H, phenyl H), 10.06 (s, 1 H, CHO) 1220, 1182, 1102, 1023, 965, 798, 731 cm–1. MALDI-TOF-MS for
ppm. IR (KBr, in): ν = 2212 (CN), 1696 (CO) cm–1. ESI for C228H154N24 (3229.8): calcd. for [M + H]+ 3230.2924; found
˜
C17H8N2O (256.3): calcd. for [M + Na]+ 279.0529; found 279.0548.
3230.2765.
5,10,15-Tritolyl-20-{4-[2-(2,3-dicyanophenyl)ethynyl]phenyl}-
All-Zinc Heteropentamer [ZnPc-(ZnTTP)4]: To 1-pentanol (10 mL)
porphyrin (H2CNTTP): Samples of p-tolualdehyde (0.18 mL,
was added lithium ribbon (83 mg, 12 mmol). The mixture was
126
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Eur. J. Inorg. Chem. 2008, 119–128

Phthalocyanine–Porphyrin Heterodimers and Heteropentamers
[3] D. Gu, O. Chen, X. Tang, F. Tang, F. Gan, S. Shen, K. Liu,
stirred at 90 °C until all of the lithium was consumed. At this point,
the alkoxide solution was cooled to room temperature, and
ZnCNTTP (130 mg, 0.15 mmol) was added. The reaction mixture
was then heated to 145 °C and stirred under nitrogen for 4 h. The
temperature was then lowered to 80 °C, and Zn(OAc)2·2H2O
(329.3 mg, 1.5 mmol) was added. The reaction mixture was allowed
to stir for an additional 4 h at 80 °C. The reaction mixture was
then cooled to room temperature. Chloroform (100 mL) was added,
and the chloroform layer was washed with water (3ϫ100 mL). The
chloroform layer was collected, concentrated under vacuum, and
loaded onto a silica gel column. Elution with chloroform afforded
the desired product from the second green-brown band. Removal
of the solvent by evaporation afforded a purple solid (46.5 mg,
35%). The purple product was recrystallized from CHCl3/hexane.
UV/Vis (CHCl3): λmax (log ε) = 426 (6.11), 558 (4.86), 603 (4.70),
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1
628 (4.76), 695 (5.19) nm. H NMR (400 MHz, CDCl3): δ = 9.18
(br. s, 4 H, phthalocyanine H), 8.87–9.05 (br. m, 32 H, pyrrole H),
7.92–8.23 (br. s, 32 H, phenyl H), 7.39–7.66 (br. m, 32 H, phenyl
H), 7.03–7.18 (br. s, 4 H, phthalocyanine H), 7.85–7.00 (br. s, 4 H,
phthalocyanine H), 2.68 (s, 36 H, CH ) ppm. IR (KBr, in): ν =
˜
3
2923 (CH), 1731, 1633, 1457, 1337, 1205, 1110, 1067, 997, 797,
720 cm–1. MALDI-TOF-MS for C228H154N24Zn5 (3546.7): calcd.
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Zinc Heterodimer [ZnPc-ZnTTP]: SubPc (216 mg, 0.5 mmol) in
DMSO (10 mL) was heated to 60 °C for 2 h while stirring under
nitrogen. ZnCNTTP (86.7 mg, 0.10 mmol), DBU (2 drops), and
hydrated zinc acetate (108 mg, 0.50 mmol) were added to the solu-
tion. The mixture was heated to 120 °C for 24 h while stirring under
nitrogen. After cooling to room temperature, the green reaction
mixture was poured into dichloromethane (100 mL) and washed
with water (3ϫ100 mL) to remove excess zinc acetate, acetic acid,
and DMSO. The dichloromethane layer was collected and concen-
trated which was then applied to a silica gel column. The blue band
consisting of the desired compound was eluted by using a mixture
of dichloromethane and methanol (95:5, v/v). The by-product ZnPc
remained on the top of the column. Removal of the solvents by
evaporation afforded a purple solid (36.9 mg, 28%). The purple
product ZnPc-ZnTTP was recrystallized from CH2Cl2/methanol.
UV/Vis (CHCl3) : λmax (log ε) = 426 (5.95), 555 (4.67), 610 (4.80),
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1
652 (4.93), 676 (5.47) nm. H NMR (400 MHz, [D6]DMSO): δ =
9.28 (br. s, 4 H, phthalocyanine H), 9.10 (br. s, 1 H, phthalocyanine
H), 9.00 (br. s, 1 H, phthalocyanine H), 8.68–8.87 (br. m, 8 H,
pyrrole H), 8.47–8.65 (br. m, 2 H, phenyl H), 7.98–8.28 (br. m, 12
H, phenyl and phthalocyanine H), 7.50–7.70 (br. m, 8 H, phenyl
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˜
3
1464, 1332, 1093, 997, 797, 721 cm–1. FAB-MS: m/z = 1320.3
[M]+. MALDI-TOF-MS for C81H48N12Zn2 (1320.1): calcd. for
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Acknowledgments
The work described in this paper was partially supported by a grant
from the Research Grants Council of the Hong Kong Special Ad-
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Received: July 10, 2007
Published Online: November 8, 2007
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