Acetylene bridged porphyrin-monophthalocyaninato ytterbium(iii) hybrids with strong two-photon absorption and high singlet oxygen quantum yield

Article abstract of DOI:10.1039/c2dt12034h

Several acetylene bridged porphyrin-monophthalocyaninato ytterbium(iii) hybrids, PZn-PcYb, PH₂-PcYb and PPd-PcYb, have been prepared and characterized by ¹H and ³¹P NMR, mass spectrometry, and UV-vis spectroscopy. Their photophysical and photochemical properties, especially the relative singlet oxygen (¹O₂) quantum yields and the two-photon absorption cross-section (σ₂), were investigated. These three newly synthesized compounds exhibited very large σ₂ values and substantial ¹O₂ quantum yields upon photo-excitation, making them potential candidates as one- and two-photon photodynamic therapeutic agents.

Full text of DOI:10.1039/c2dt12034h

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PAPER
Acetylene bridged porphyrin–monophthalocyaninato ytterbium(III) hybrids
with strong two-photon absorption and high singlet oxygen quantum yield
Hanzhong Ke,*a Wenbin Li,a Tao Zhang,b Xunjin Zhu,*b Hoi-Lam Tam,c Anxin Hou,d Daniel W. J. Kwong*b
and Wai-Kwok Wong*b
Received 26th October 2011, Accepted 16th January 2012
DOI: 10.1039/c2dt12034h
Several acetylene bridged porphyrin–monophthalocyaninato ytterbium(III) hybrids, PZn–PcYb,
PH2–PcYb and PPd–PcYb, have been prepared and characterized by 1H and 31P NMR, mass
spectrometry, and UV-vis spectroscopy. Their photophysical and photochemical properties, especially
the relative singlet oxygen (1O2) quantum yields and the two-photon absorption cross-section (σ2), were
investigated. These three newly synthesized compounds exhibited very large σ2 values and substantial
1O2 quantum yields upon photo-excitation, making them potential candidates as one- and two-photon
photodynamic therapeutic agents.
2
energy of the emissive F5/2 state of Yb(III) is about the same as
Introduction
that of the phthalocyanine T1 state and back-energy transfer
becomes significant, only 1O2 phosphorescence, not Yb(III) emis-
Photodynamic therapy (PDT) is a light-activated treatment used
clinically to destroy diseased tissues and tumors.1–3 It is also cur-
sion, was observed in the NIR region. The strong absorption of
rently under clinical trials for the eradication of localized bac-
terial infections.4 The selectivity of this therapy is based on a
preferential accumulation of a photosensitizer in the target
tissues. After irradiation with light of an appropriate wavelength
and in the presence of oxygen, this photosensitizer will induce
cellular and tissue damage by generating reactive oxygen species
(ROS) such as singlet oxygen (1O2).5 As the first generation
photosensitizers, porphyrin derivatives have been studied exten-
sively and used in clinical work.6 However, the efficiency of this
treatment is limited by their small extinction coefficient at the
body therapeutic window (650–900 nm).7 One way to overcome
this problem is to synthesize a new generation of photosensiti-
zers that absorb strongly at >650 nm. We have previously
reported some examples of monophthalocyaninato lanthanide(III)
complexes (Ln = Er, Yb).8 In the case of the Er(III) complex, the
these monophthalocyaninato Yb(III) complexes in the red (Q-
band) falls into the tissue-transparent spectral region, thus
making these Yb(III) complexes promising candidates for PDT.
Currently, the design and synthesis of photosensitizers with
high two-photon absorption cross-sections is an area of intense
research.9–17 Lower energy (longer wavelength) photons readily
penetrate the skin, allowing them to be absorbed by the photo-
sensitizer localized in the target tissue and promoting these mol-
ecules to the excited state, which sensitizes the molecular
1
oxygen nearby to produce O2. When designing a two-photon
absorbing (TPA) sensitizer for TPA-PDT applications, one
should not only consider the two-photon absorption cross-
1
section, but also its efficiency to generate O2, as well as other
crucial biological properties, such as low dark cytotoxicity and
selective uptake by cancer cells.
4
energy of the emissive I13/2 state of Er(III) is much lower than
In this study, we aim to develop new porphyrin–monophthalo-
cyaninato Yb(III) hybrids with high TPA cross-sections (σ2) and
high efficiency to generate 1O2 for potential application in
TPA-PDT. Through the direct acetylene linkage, π-conjugation
between porphyrin and monophthalocyaninato Yb(III) is length-
ened and enlarged σ2 is expected to be achieved by increasing
the intramolecular charge-transfer contribution or coherent
domain through conjugation. Furthermore, as both porphyrin
that of the T1 state of phthalocyanine and efficient energy trans-
fer takes place, which results in the sensitization of the emissive
Er(III) ion without the observation of 1O2 phosphorescence in the
NIR region. However, in the case of the Yb(III) complexes, the
aFaculty of Material Science & Chemistry Engineering, China
University of Geosciences, Wuhan, Hubei 430074, P. R. China.
E-mail: kehanz@yahoo.cn
1
and monophthalocyaninato Yb(III) can generate O2, it is poss-
bDepartment of Chemistry and Centre for Advanced Luminescence
Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong,
P. R. China. E-mail: xjzhu@hkbu.edu.hk, dkwong@hkbu.edu.hk,
wkwong@hkbu.edu.hk; Fax: +852-3411-5862; Tel: +(852)3411-7011
cDepartment of Physics, Hong Kong Baptist University, Waterloo Road,
Hong Kong, P. R. China
ible that a hybrid containing both moieties might exhibit an
enhanced 1O2 quantum yield.
Herein, we describe the synthesis of porphyrin–monophthalo-
cyaninato Yb(III) hybrids with a direct acetylene linkage and the
investigation of their photophysical properties, particularly their
TPA properties and 1O2 quantum yields.
dCollege of Chemistry and Molecular Sciences, Wuhan University,
430072 Wuhan, P. R. China
4536 | Dalton Trans., 2012, 41, 4536–4543
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Scheme 1 Synthesis of PZn–PcYb, PH2–PcYb, and PPd–PcYb: (a) i. NBS, CH2Cl2, 0 °C; ii. Zn(OAc)2, CHCl3, reflux; (b) (trimethylsilyl)acetylene,
Pd(PPh3)4, CuI, THF–TEA, 45 °C; (c) TBAF, CH2Cl2, r.t.; (d) DBU, 1-pentanol, 145 °C; (e) i. Ln[N(SiMe3)2]3[LiCl(THF)3]x, toluene, reflux; ii.
NaLOMe, r.t.; (f) Pd(PPh3)4, CuI, THF–TEA, 40 °C; (g) conc. HCl, r.t.; (h) Pd(OAc)2, CHCl3, 40 °C.
Another precursor I(t-Bu)3Pc was synthesized in 25% yield by
condensation of 4-(tert-butyl)-phthalonitrile with 4-iodophthalo-
nitrile. Then the reaction of I(t-Bu)3Pc with Yb[N(SiMe3)2]3·
[LiCl(THF)3]x followed by addition of Na(LOMe) gave I(t-
Bu)3PcYb in 66% yield.8 Finally, PZn–PcYb was obtained from
EPZn and I(t-Bu)3PcYb through Sonogashira cross-coupling
reaction in 58% yield. Selective demetallation of PZn–PcYb
using conc. HCl gave PH2–PcYb in 93% yield. PH2–PcYb was
further metallated with palladium(II) acetate to give PPd–PcYb
in 71% yield (Scheme 1). For comparison, the porphyrin mono-
mers PH2, PZn and PPd were also synthesized and characterized
(Scheme 2).
Results and discussion
Synthetic procedures for PZn–PcYb, PH2–PcYb, and PPd–PcYb
are shown in Scheme 1. The 5,15-diaryl porphyrin (PH2) is
readily prepared in one step by acid-catalysed condensation of
dipyrromethane (prepared in 51% yield from pyrrole and formal-
dehyde) with ethyl acetate substituted p-hydroxyl-benzaldehyde,
followed by in situ oxidation with DDQ.18 Then meso-ethynyl
zinc porphyrin (EPZn) was prepared in three steps from 5,15-
diaryl-porphyrin PH2. Firstly, metallation with zinc acetate furn-
ished the desired zinc porphyrin (PZn) in good yield. Porphyrin
PZn was brominated with one equivalent of NBS. The mono-
brominated porphyrin (BrPZn) was subjected to Sonogashira
ethynylation, giving porphyrin EPZn after removing of the tri-
methylsilyl protecting group (total in 30% yield).19
The newly synthesized compounds were fully characterized
1
by high resolution mass spectrometry, H and 31P NMR, UV-vis
absorption and fluorescence spectroscopy. The MALDI-TOF
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Table 1 Photophysical data of PZn–PcYb, PH2–PcYb, PPd–PcYb,
(t-Bu)3PcYb, PZn, PH2, and PPda
Absorption, λmax/nm
[log(ε/dm3 mol−1
cm−1)]
Emission at
Excitation
λex/nm
298 K, λem/nm
Compound
(τ, Φem × 102)b,c
PZn–P Yb
353 (4.93) 432 (5.32), 432
563 (4.27), 614 (4.49),
647 (4.63), 678 (5.08),
712 (5.21)
621 (5.22 ns,
1.0), 718
(4.96 ns)
PH2–PcYb
PPd–PcYb
349 (4.74), 429 (5.15), 429
532 (3.88), 574 (4.11),
616 (4.26), 648 .45),
678 (4.89), 18 (4.99)
353 (4.79), 426 (5.12), 426
537 (4.19), 576 (4.11),
616 (4.31), 645 (4.48),
676 (4.98), 709 (5.09)
670 (10.30 ns,
1.3), 719
714 (5.12 ns,
0.26)
Scheme 2 Synthesis of PZn and PPd: (a) Zn(OAc)2, CHCl3, reflux;
(b) Pd(OAc)2, CHCl3, 40 °C.
(t-Bu)3PcYb 349 (4.78), 609 (4.45), 349
684 (6.23 ns,
1.9)c
634 (6.05 ns,
4.3)
645 (22.08 ns),
702 nm
(18.96 ns, 6.6)
552, 598
PZn
PH2
PPd
646 (4.42), 676(5.27)
413 (5.61), 540 (4.28) 413
high-resolution mass spectra (HRMS) of PZn–PcYb, PH2–PcYb,
and PPd–PcYb showed the [M]+ or [M + 1]+ peaks at
2057.4245, 1994.5208, and 2098.4160, giving mass accuracies
Δm of ≤4 ppm relative to their corresponding calculated m/z
values of 2057.4112, 1994.5064, and 2098.3951, respectively.
Their isotopic distribution patterns also matched the expected
profiles. PZn–PcYb, PH2–PcYb, and PPd–PcYb displayed a
singlet at δ 64.05, 64.31, and 64.49 ppm (vs. 85% H3PO4) in
their 31P{1H}-NMR spectra for the phosphito groups of the
410 (5.44), 505 (4.10), 410
540 (3.71), 579 (3.56),
635 (3.04)
407 (5.31), 514 (4.28), 407
545 (3.76)
(5.38 ns, 0.15)
a Measurements were performed on 1 × 10−6 M solution in toluene.
b Quantum yields were measured relative to H2TPP in benzene (Φem
=
0.11). c Quantum yield was measured relative to H2(t-Bu)4Pc in toluene
(Φem = 0.67).
−
anionic LOMe ligand, and peaks at δ −6.18, −6.04, and
1
−6.05 ppm (vs. TMS) in their H NMR spectra (in CDCl3) for
the five magnetically equivalent cyclopentadienyl protons,
respectively.
The photophysical properties of the porphyrin–monophthalo-
cyaninato Yb(III) hybrids, as well as those of the reference mono-
mers (t-Bu)3PcYb, PZn, PH2, and PPd are summarized in
Table 1. Fig. 1a shows the absorption spectra of PZn–PcYb,
PH2–PcYb, and PPd–PcYb. The UV-vis spectra of PZn–PcYb,
PH2–PcYb, and PPd–PcYb are very similar, with all of them
exhibit very strong absorption bands in the UV-vis and near-
infrared regions, which arise typically from the porphyrin and
the monophthalocyaninato Yb(III) moieties.20 It should be noted
that the UV-vis spectra of these hybrids are not simply a super-
position of the individual spectra of the porphyrin and the mono-
phthalocyaninato Yb(III) moieties. For instance, compare the UV-
vis spectrum of PZn–PcYb with an equimolar solution of (t-
Bu)3PcYb and PZn, the Soret band arising from the porphyrin
moiety in PZn–PcYb appears to be broadened and red-shifted
(∼19 nm), concomitant with a significant reduction in intensity
(Fig. 1b–1d). In addition, the Q-band at 678 nm for I(t-Bu)3Pc is
markedly split into two peaks at 678 and 712 nm in PZn–PcYb
due to the excitonic and symmetry effects caused by the
porphyrin–ethynyl moiety. The results herein before indicate the
existence of strong interaction between the porphyrin and the
phthalocyanine moieties in PZn–PcYb.
Fig. 1 (a) Absorption spectra of PZn–PcYb (solid), PH2–PcYb (dash),
and PPd–PcYb (dot) solutions in toluene. (b–d) Absorption spectra of
PZn–PcYb (solid), PH2–PcYb (solid), PPd–PcYb (solid) and those of an
equimolar mixture of (t-Bu)3PcYb and the corresponding porphyrin
monomer (PZn, PH2, PPd) (dash), respectively, in toluene.
Steady-state fluorescence measurements at room temperature
reveal a large Qx(0,0) emission band for PH2–PcYb (in toluene)
at 670 nm (τ = 10.30 ns, Φem = 0.013), and a small low energy
Qx(0,0) shoulder at 719 nm (Fig. 2), which coincides with the
emission spectrum of free base porphyrins.21 For the PZn–PcYb,
the spectrum shows a similar behavior to that of zinc-cored
porphyrins in the region Qx(0,0) and Qx(0,1) bands
(600–700 nm).21 The spectrum shows a strong peak at 621 nm
(τ = 5.22 ns, Φem = 0.010), which corresponds to the Qx(0,0)
band and a smaller one around 670 nm, the Qx(0,1) band. The
results are also in accord with spectroscopic data of a reported
porphyrin–phthalocyanine system.17 However, excitation at the
Soret band of the porphyrin moieties of PPd–PcYb results in dra-
matically different behavior with only one weak broad peak at
714 nm (τ = 5.12 ns, Φem = 0.0026).22 It is known that the
heavy-atom effect of the Pd2+ ion which, upon coordination with
4538 | Dalton Trans., 2012, 41, 4536–4543
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Table 2 TPA cross-sections (σ2) and singlet-oxygen quantum yields
(ΦΔ) of PZn–PcYb, PH2–PcYb, PPd–PcYb, (t-Bu)3PcYb, PZn, PH2,
and PPd
ΦΔ at 298 Ka
(λex = 416 nm)
ΦΔ at 298 Kb
(λex = 675 nm)
Compound
σ2 [GM]c
PZn–PcYb
PH2–PYb
PPd–PcYb
(t-Bu)3PcYb
PZn
PH2
PPd
1.38
1.30
1.07
0.79
0.73
0.77
0.63
—
3301
2347
2365
—
—
—
d
—
0.71
0.84
1.08
Fig. 2 (a) Fluorescence spectra of PZn–PcYb (solid), PH2–PcYb
(dash), and PPd–PcYb (dot) in toluene. (b) Fluorescence spectra of
PZn–PcYb (solid) and an equimolar mixture of (t-Bu)3PcYb and PZn
(dash) in toluene by selective excitation of the porphyrin moiety at 432
and 413 nm, respectively.
—
—
—
a Quantum yield of 1O2 was measured relative to H2TPP in toluene
(ΦΔ = 0.66). b Quantum yield of 1O2 was measured relative to ZnPc
in toluene (ΦΔ = 0.58). c TPA cross-sections are expressed in units of
GM, where 1 GM = 10−50 cm4 s photon−1 d Not detected.
.
Fig. 3 Phosphorescence spectra of 1O2 generated from PZn–PcYb
(solid), PH2–PcYb (dash), PPd–PcYb (dot), H2TPP (dash-dot), and
ZnPc (dash-dot) solutions in toluene [Abs (λexc) = 0.05] excited at: (a)
416 nm, (b) 675 nm.
Fig. 4 Photophysical and photochemical pathways of porphyrin–
monophthalocyaninato ytterbium hybrids. (i) Photo-excitation at por-
phyrin Soret band, (ii) fluorescence, (iii) intersystem crossing, (iv) non-
radiative decay, (v) generation of 1O2, (vi) energy transfer, (vii) photo-
excitation at phthalocyanine Q-band, (viii) fluorescence, (ix) inter-
system crossing, (x) non-radiative decay, and (xi) generation of 1O2.
the porphyrin ligand, enhances the intersystem crossing of the
porphyrin moiety from the singlet to the triplet state, and finally
interacts with molecular oxygen (3O2) to generate 1O2. In
addition, the fluorescence arising from phthalocyaninato Yb(III)
complex could not be clearly assigned, as it might overlap with
the fluorescence bands from the porphyrin moiety.8 In any case,
the room temperature visible emission of all of the complexes in
the range 600–727 nm shows quite fast decay time (i.e., <1 μs).
At the same time, the intensity of fluorescence from the por-
phyrin moiety in porphyrin–monophthalocyaninato Yb(III)
hybrids is weaker than that of the corresponding porphyrin
monomer, when monitored at the same absorbance values of the
Soret band. This is indicative of energy transfer from the por-
phyrin to the phthalocyanine units, which was further corrobo-
in Table 2. Fig. 3 shows that PZn–PcYb, PH2–PcYb, and PPd–
PcYb exhibited high efficiency of 1O2 generation whether
excited at the porphyrin Soret band or the Q-band of the phthalo-
cyanine moiety. As shown in Table 2, with excitation at 416 nm,
1
PZn–PcYb, PH2–PcYb, and PPd–PcYb all show very high O2
quantum yields of 1.38, 1.30, and 1.07, respectively, which are
much higher than the corresponding porphyrin monomers (0.71
for PZn, 0.84 for PH2, and 1.08 for PPd).25,26 When excited at
1
1
rated by significant enhancement of O2 generation (vide infra).
675 nm, PZn–PcYb, PH2–PcYb, and PPd–PcYb exhibited O2
As aforementioned, no metal-centered NIR luminescence is
detected. This is because the triplet state energy of the phthalo-
quantum yields of 0.79, 0.73, and 0.77, respectively, which are
higher than that for the corresponding phthalocyanine monomer
(t-Bu)3PcYb (ΦΔ = 0.63) as well. It is now clear that porphyrin–
monophthalocyaninato Yb(III) hybrids linked with an acetylene
group can generate 1O2 with high efficiency, especially when
excited at the λmax of the porphyrin Soret band. This result is
interpreted in terms of the schematic diagram shown in Fig. 4,
which depicts the various photophysical and photochemical pro-
cesses available in these Por–PcYb hybrids.
Upon photo-irradiation, the porphyrin moiety is excited to its
excited singlet state (1Por*–PcYb), which is then deactivated via
fluorescence, or intersystem crossing to its excited triplet state
(3Por*–PcYb), or energy transfer to the phthalocyanine moiety
(Por–1PcYb*), which then undergoes either fluorescence or
cyaninate ligand is about the same as that of the Yb(2F5/2
)
excited state, which allows significant back-energy transfer to
occur.8
To evaluate the photosensitizing efficiency of these com-
pounds, their 1O2 yields (ΦΔ) were determined in toluene by
1
measuring the near-IR phosphorescence intensity of the O2 (at
1270 nm) upon one-photon irradiation at 416 nm, using
5,10,15,20-tetraphenylporphyrin (H2TPP) as the reference (ΦΔ =
0.66),23 and irradiation at 675 nm, using unsubstituted zinc(II)
phthalocyanine (ZnPc) as the reference (ΦΔ = 0.58) (Fig. 3).24
1
The relative O2 quantum yields of these compounds and the
reference monomers PZn, PH2, PPd, and (t-Bu)3PcYb are given
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Conclusions
In conclusion, a series of conjugated porphyrin–monophthalo-
cyaninato Yb(III) hybrids, PZn–PcYb, PH2–PcYb, and PPd–
PcYb, have been synthesized and characterized. The photophysi-
cal and 1O2-generating properties of these hybrids have been
1
examined. They all exhibit large σ2 values and very high O2
quantum yields upon photo-excitation, which make them poten-
tial candidates as one- and two-photon PDT agents. Their solubi-
lity in water can be increased by the introduction of hydrophilic
groups at the porphyrin moiety. Modifications of the porphyrin
chromophore to achieve aqueous solubility for these hybrids,
together with a study of their PDT activities, are currently
underway.
Fig. 5 Open-aperture Z-scan traces of PZn–PcYb (squares), PH2–
PcYb (circles), and PPd–PcYb (triangles) (0.25 mM in toluene) excited
at 800 nm. The average power of the laser beam used was 0.20 mW.
Experimental
General
intersystem crossing to its triplet state (Por–3PcYb*). Since both
excited triplet states (i.e., 3Por*–PcYb and Por–3PcYb*) can
interact with molecular oxygen (3O2) to generate 1O2, these Por–
PcYb hybrids furnish an additional pathway to generate 1O2,
All reactions were carried out under dry nitrogen atmosphere
unless specified otherwise. Solvents were dried by standard pro-
cedures, distilled, and deaerated prior to use. All chemicals were
obtained from Sigma-Aldrich Chemical Company and used
without further purification. (4-formyl-phenoxy)acetic acid ethyl
ester,29 2,2-dipyrrylmethane,30 Yb[N(SiMe3)2]3·[LiCl(THF)3]x,31
PH2 and BrPZn32 were synthesized according to procedures in
the literature. Electronic absorption spectra in the UV-vis region
were recorded with a Varian Cary 100 UV-vis spectrophoto-
meter, steady-state visible fluorescence and photoluminescence
excitation spectra were recorded with a Photon Technology Inter-
national (PTI) Alphascan spectrofluorimeter, and quantum yields
of the visible emissions were determined according to the litera-
ture method33 using tetraphenylporphyrin (H2TPP) as reference
standard (Φem = 0.11 in benzene),34 except for (t-Bu)3PcYb,
1
resulting in an increased channeling of excitation energy for O2
production. This interpretation is consistent with the significantly
lower fluorescence quantum yields (Φem) observed in these
hybrids (0.010 for PZn–PcYb and 0.013 for PH2–PcYb) relative
to those of their corresponding porphyrin monomers (0.043 for
PZn and 0.066 for PH2). At present, we have no data to support
our hypothesis/speculation that the high singlet oxygen quantum
yield (>1) seen in PZn–PcYb and PH2–PcYb is the result of O2
quenching by its two triplet states, 3Por*–PcYb and
Por–3PcYb*. But the phenomenon of photosensitized singlet
oxygen generation from two excited states (S1 and T1) in the
same molecule simultaneously has been amply demonstrated in
many polycyclic aromatic systems, where the energy gap
between the two excited states ΔES1–T1 > 7880 cm−1, which
leads to the singlet oxygen quantum yield approaching the value
of two (e.g., pyrene, ΦΔ = 1.85 0.05).27 We believe that our
hypothesis is a reasonable one as the component porphyrins,
namely, PZn and PH2, showed much lower singlet oxygen
quantum yields of 0.71 and 0.84, respectively. We will address
this important issue by carrying out transient spectroscopic and
lifetime measurement experiments to verify the existence and to
study the photophysical dynamics of the proposed excited states
of these molecules. As for the PPd–PcYb, due to the heavy-atom
effect of the Pd2+ ion, intersystem crossing from the excited
singlet state to the excited triplet state is the major process, so it
exhibits the same 1O2 quantum yield as PPd.
The σ2 values of hybrids PZn–PcYb, PH2–PcYb, and PPd–
PcYb were measured at 800 nm using 100 fs laser pulses by the
open-aperture Z-scan method.28 Fig. 5 shows the Z-scan traces
of these hybrids, from which the absolute σ2 values are deter-
mined and given in Table 2. PZn–PcYb exhibited the largest σ2
value (3301 GM) among these hybrids, i.e., PH2–PcYb (2347
GM) and PPd–PcYb (2365 GM). The large enhancement is
mainly due to the expansion of π-conjugation and the molecular
polarization induced by the asymmetrical (D–π–A) structure
between the porphyrin and phthalocyanine moieties linked
through an ethynyl bridge.
which was measured relative to H2(t-Bu)4Pc in toluene (Φem
=
0.67).35 NMR spectra were recorded with a Varian Unity Inova
400 MHz spectrometer. 1H NMR chemical shifts were refer-
enced to internal CDCl3 and then re-referenced to TMS (δ =
0.00 ppm). High-resolution mass spectra, reported as m/z, were
obtained with
spectrometer.
a
Bruker Autoflex MALDI-TOF mass
Singlet oxygen quantum yield
Singlet oxygen was detected directly by its phosphorescence
emission at 1270 nm using an InGaAs detector on the PTI QM4
luminescence spectrometer. The singlet oxygen quantum yields
(ΦΔ) of all compounds were determined in toluene by comparing
the singlet oxygen emission intensity of the sample solution to
that of a reference material according to eqn (1).36
ꢀ
ꢁ
2
nS
GΔS
AREF
AS
ΦSΔ ¼ ΦRΔEF
ꢀ
ꢀ
ð1Þ
nREF GRΔEF
where ΦΔ is the singlet oxygen quantum yield, GΔ is the inte-
grated emission intensity, A is the absorbance at the excitation
wavelength, n is the refractive index of the solvent. Superscripts
and subscripts REF and S correspond to the reference and
1
sample, respectively. In all cases, the O2 emission spectra were
4540 | Dalton Trans., 2012, 41, 4536–4543
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I(t-Bu)3PcYb. A solution of Yb[N(SiMe3)2]3·[LiCl(THF)3]x
(20 mL, 3.2 mmol) was transferred to a Schlenk flask and the
solvent was removed under vacuum. The residue was re-dis-
solved in CH2Cl2 (10 mL) to give a suspension, which was cen-
trifuged and the clear layer was transferred to another Schlenk
flask with dry I(t-Bu)3Pc (161.6 mg, 0.20 mmol) dissolved in
toluene (40 mL). The resulting solution was refluxed for 3 h.
Upon cooling the reaction mixture to 70 °C, anhydrous NaLOMe
(160 mg, 0.35 mmol) was added and magnetically stirred for
1 h. After the reaction was complete, the solvent was removed
under vacuum and the residue re-dissolved in CHCl3, filtered
and chromatographed on silica gel column using CHCl3−hexane
(v/v, 3 : 2) as the eluent. The second band gave I(t-Bu)3PcYb in
66% yield (190 mg). MALDI-TOF HRMS: calcd for [M]+
1431.1622, found 1431.1637. 1H NMR (CDCl3): δ = −6.07 (s, 5
H), 3.25–3.40 (m, 27H), 7.59 (s, 18 H), 10.60–11.25 (m, 4 H),
14.20–15.50 (m, 8 H) ppm. 31P{1H} NMR (CDCl3): δ =
64.41 ppm. UV-vis (toluene): λmax/nm [log(ε/dm3 mol−1 cm−1)]
= 349 (4.87), 611 (4.48), 682 (5.22).
measured after excitation with the absorbance set at 0.05 in order
to minimize re-absorption of the emitted light.
Two-photon absorption measurements
The two-photon absorption spectra were measured at 800 nm by
the open-aperture Z-scan method using 100 fs laser pulses from
an optical parametric amplifier operating at a 1 kHz repetition
rate generated from a Ti:sapphire regenerative amplifier system.
The laser beam was split into two parts by a beam splitter. One
part was monitored by a photodiode (D1) as the incident inten-
sity reference, I0, and the other was detected as the transmitted
intensity by another photodiode (D2). After passing through a
lens with f = 10 cm, the laser beam was focused and passed
through a quartz cell. The position of the sample cell, z, was
moved along the laser-beam direction (z axis) by a computer-
controlled translatable table so that the local power density
within the sample cell could be changed under the constant inci-
dent intensity laser power level. Finally, the transmitted intensity
from the sample cell was detected by the photodiode (D2). The
photodiode (D2) was interfaced to a computer for signal acqui-
sition and averaging. Each transmitted intensity data point rep-
resents the average of over 100 measurements. Assuming a
Gaussian beam profile, the nonlinear absorption coefficient, β,
can be obtained by curve fitting to the observed open-aperture
traces, T(z), with eqn (2),37 where a0 is the linear absorption
coefficient, l is the sample length (1 mm quartz cell) and z0 is
the diffraction length of the incident beam.
TMSPZn. Under a nitrogen atmosphere, a 250 mL round-bot-
tomed flask was charged with BrPZn (730 mg, 0.90 mmol), (tri-
methylsilyl)acetylene (0.63 mL, 4.5 mmol), Pd(PPh3)4 (100 mg,
0.09 mmol), CuI (27 mg, 0.14 mmol), triethylamine (40 mL),
and THF (80 mL). After stirring for 24 h at 45 °C, the solvents
were evaporated. The residue was purified via chromatography
on silica gel (CH2Cl2) to give the product in 79% yield
(520 mg). MALDI-TOF HRMS: calcd for [M]+ 824.2003; found
824.1992. 1H NMR (400 MHz, CDCl3): δ = 0.62 (s, 9H), 1.41(t,
J = 7.2 Hz, 6H), 4.39 (q, J = 7.2 Hz, 4H), 4.87 (s, 4 H), 7.28 (d,
J = 8.4 Hz, 4H), 8.09(d, J = 8.4 Hz, 4H), 8.95 (d, J = 4.4 Hz,
2H), 9.00 (d, J = 4.4 Hz, 2H), 9.21 (d, J = 4.4 Hz, 2H), 9.76 (d,
J = 4.8 Hz, 2H), 10.01 (s, 1H) ppm. UV-vis (toluene): λmax/nm
[log(ε/dm3 mol−1 cm−1)] = 431 (5.54), 558 (4.20), 599 (3.88).
βI0ð1 ꢁ eꢁα l
Þ
2
0
TðzÞ ¼ 1 ꢁ
ð2Þ
2a0ð1 þ ðz=z0ÞÞ
After obtaining the nonlinear absorption coefficient β, the
TPA cross-section σ2 of the sample molecule (in units of GM,
where 1 GM = 10−50 cm4 s photon−1) can be calculated using
eqn (3),36 where NA is Avogadro’s constant, d is the concen-
tration of the sample compound in solution, h is Planck’s con-
stant, and ν is the frequency of the incident laser beam.
EPZn. Tetrabutylammonium fluoride solution (3 mL, 0.2 M
in THF) was added to a solution of TMSPZn (500 mg,
0.60 mmol) in CH2Cl2 (150 mL) under N2. The reaction mixture
was stirred for 10 min at room temperature, quenched with
water, extracted with CH2Cl2, and dried over anhydrous CaCl2.
After the solvent was evaporated, the residue was chromato-
graphed on silica gel using 10 : 1 hexane–THF as the eluent. The
yield was 420 mg (93%). MALDI-TOF HRMS: calcd for [M +
1000βhν
σ2 ¼
ð3Þ
NAd
1
1]+ 753.1686; found 753.1737. H NMR (400 MHz, CD2Cl2): δ
= 1.40 (t, J = 7.2 Hz, 6H), 4.19 (s, 1H), 4.37 (q, J = 7.2 Hz,
4H), 4.89 (s, 4 H), 7.28 (d, J = 8.4 Hz, 4H), 8.11 (d, J = 8.4 Hz,
4H), 8.98 (d, J = 4.4 Hz, 2H), 9.02 (d, J = 4.8 Hz, 2 H), 9.30 (d,
J = 4.4 Hz, 2H), 9.74 (d, J = 4.8 Hz, 2H), 10.13 (s, 1H) ppm.
UV-vis (toluene): λmax/nm [log(ε/dm3 mol−1 cm−1)] = 428
(5.51), 555 (4.21).
Synthesis
I(t-Bu)3Pc. Under a nitrogen atmosphere, 4-iodophthalonitrile
(254 mg, 1.0 mmol), 4-tert-butylphthalonitrile (924 mg,
5.0 mmol), and DBU (0.9 mL) in dry amyl alcohol (100 mL)
were heated refluxing for 48 h while stirring. After removing
most of the solvent, the mixture was poured into methanol
(100 mL). The precipitate was collected by filtration and further
chromatographed on silica gel column using hexane–THF (v/v,
7 : 1) as the eluent. The yield was 25% (200 mg). MALDI-TOF
HRMS: calcd for [M + 1]+ 809.2572, found 809.2555. 1H NMR
(CDCl3): δ = (−4.00)–(−3.50) (m, 2H), 1.60–2.10 (m, 27H),
7.26–9.00 (m, 12H) ppm. UV-vis (toluene): λmax/nm [log(ε/dm3
mol−1 cm−1)] = 350 (4.77), 601 (4.39), 643 (4.63), 663 (5.07),
696 (5.13).
PZn–PcYb. Under a nitrogen atmosphere, a 100 mL Schlenk
flask was charged with EPZn (190 mg, 0.25 mmol), I(t-
Bu)3PcYb (180 mg, 0.13 mmol), Pd(PPh3)4 (14 mg,
0.013 mmol), CuI (3.6 mg, 0.020 mmol), triethylamine (16 mL),
and THF (80 mL). After stirring for 24 h at 40 °C, the solvents
were evaporated. The residue was purified via chromatography
on silica gel (CHCl3) to give the product in 58% yield (150 mg).
MALDI-TOF HRMS: calcd for [M]+ 2057.4112; found
1
2057.4245. H NMR (CDCl3): δ = (−6.25)–(−6.10) (m, 5H),
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 4536–4543 | 4541

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1.55–1.65 (m, 6H), 3.35–3.70 (m, 27H), 4.50–4.70 (m, 4H),
5.15–5.25 (m, 4H), 7.70 (s, 22H), 8.00–9.00 (m, 8H), 9.20–9.65
(m, 2H), 10.70–11.70 (m, 7H), 14.30–15.90 (m, 8H) ppm. 31P
{1H} NMR (CDCl3): δ = 64.05 ppm.
(q, J = 7.2 Hz, 4H), 4.92 (s, 4H), 7.32 (d, J = 8.4 Hz, 4H), 8.15
(d, J = 8.4 Hz, 4H), 9.12 (d, J = 4.8 Hz, 4H), 9.40 (d, J = 4.8
Hz, 4H), 10.27 (s, 2H) ppm.
PPd. Under a nitrogen atmosphere, PH2 (67 mg, 0.10 mmol)
and palladium(II) acetate (227 mg, 1.0 mmol) were stirred in
CHCl3 (100 mL) for 48 h at 40 °C. After removing the solvent,
the residue was purified via chromatography on silica gel
(CHCl3) to give the product in 65% yield (50 mg). MALDI-TOF
HRMS: calcd for [M]+ 770.1371; found 770.1344. 1H NMR
(CDCl3): δ = 1.42 (t, J = 7.2 Hz, 6H), 4.42 (q, J = 7.2 Hz, 4H),
4.93 (s, 4H), 7.31 (d, J = 8.8 Hz, 4H), 8.12 (d, J = 8.8 Hz, 4H),
9.00 (d, J = 4.8 Hz, 4H), 9.27 (d, J = 4.8 Hz, 4H), 10.26 (s, 2H)
ppm.
PH2–PcYb. 0.62 mL of HCl (36%) was added to a solution of
PZn–PcYb (111 mg, 0.054 mmol) in CH2Cl2 (50 mL) under N2.
The reaction mixture was vigorously stirred for 10 min at room
temperature. The solution was washed sequentially with water,
5% NaHCO3 and water, then dried over Na2SO4. After the
solvent was evaporated, the residue was chromatographed on
silica gel using CHCl3 as the eluent. The yield was 100 mg
(93%). MALDI-TOF HRMS: calcd for [M + 1]+ 1994.5064;
1
found 1994.5208. H NMR (400 MHz, CD2Cl2): δ = −6.04 (s,
5H), −2.25 (br, 2H), 1.50–1.65 (m, 6H), 3.20–3.45 (m, 27H),
4.50–4.65 (m, 4H), 5.15–5.20 (m, 4H), 7.67 (s, 22H), 8.40–8.60
(m, 4H), 9.10–9.20 (m, 2H), 9.30–9.50 (m, 2H), 9.70–9.80 (m,
2H), 10.10–10.30 (m, 1H), 10.60–11.00 (m, 3H), 11.40–11.60
(m, 3H), 14.20–15.80 (m, 8H) ppm. 31P{1H} NMR (CDCl3):
δ = 64.31 ppm.
Acknowledgements
The work described in this paper was partially supported by a
grant from the Research Grants Council of the Hong Kong
Special Administrative Region, P.R. China (HKBU 202509) and
a grant from the Hong Kong Baptist University (FRG1/10-11/
011).
PPd–PcYb. Palladium(II) acetate (68 mg, 0.30 mmol) was
added to a solution of PH2–PcYb (60 mg, 0.030 mmol) in
CHCl3 (35 mL) under N2. After stirring for 48 h at 40 °C, the
solvent was evaporated. The residue was purified via chromato-
graphy on silica gel (CHCl3) to give the product in 71% yield
(45 mg). MALDI-TOF HRMS: calcd for [M + 1]+ 2098.3951;
found 2098.4160. 1H NMR (CDCl3): δ = −6.05 (s, 5H),
1.50–1.70 (m, 6H), 3.25–3.45 (m, 27H), 4.55–4.65 (m, 4H),
5.15–5.20 (m, 4H), 7.64 (s, 22H), 8.40–8.60 (m, 4H), 9.10–9.20
(m, 2H), 9.20–9.40 (m, 2H), 9.60–9.80 (m, 2H), 10.00–10.30
(m, 1H), 10.60–11.00 (m, 3H), 11.30–11.60 (m, 3H),
14.30–15.50 (m, 8H) ppm. 31P{1H} NMR (CDCl3): δ =
64.49 ppm.
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1
1305.2661, found 1305.2673. H NMR (CDCl3): δ = −6.06 (s,
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