A p-quinodimethane-bridged porphyrin dimer

Article abstract of DOI:10.1002/chem.201302023

A p-quinodimethane (p-QDM)-bridged porphyrin dimer 1 has been prepared for the first time. An unexpected Michael addition reaction took place when we attempted to synthesize compound 1 by reaction of the cross-conjugated keto-linked porphyrin dimers 8 a and 8 b with alkynyl/aryl Grignard reagents. Alternatively, compound 1 could be successfully prepared by intramolecular Friedel-Crafts alkylation of the diol-linked porphyrin dimer 14 with concomitant oxidation in air. Compound 1 shows intense one-photon absorption (OPA, λ_max=955 nm, ε=45400 M^-1 cm^-1) and a large two-photon absorption (TPA) cross-section (σ⁽²⁾ _max=2080 GM at 1800 nm) in the near-infrared (NIR) region due to its extended π-conjugation and quinoidal character. It also exhibits a short singlet excited-state lifetime of 25 ps. The cyclic voltammogram of 1 displays multiple redox waves with a small electrochemical energy gap of 0.86 eV. The ground-state geometry, electronic structure, and optical properties of 1 have been further studied by density functional theory (DFT) calculations and compared with those of the keto-linked dimer 8 b. This research has revealed that incorporation of a p-QDM unit into the porphyrin framework had a significant impact on its optical and electronic properties, leading to a novel NIR OPA and TPA chromophore.

Full text of DOI:10.1002/chem.201302023

DOI: 10.1002/chem.201302023
A p-Quinodimethane-Bridged Porphyrin Dimer
Wangdong Zeng,[a] Masatoshi Ishida,[b] Sangsu Lee,[b] Young Mo Sung,[b] Zebing Zeng,[a]
Yong Ni,[a] Chunyan Chi,*[a] Dongho Kim,*[b] and Jishan Wu*[a, c]
Abstract:
A
p-quinodimethane (p-
photon absorption (OPA, lmax
=
1 displays multiple redox waves with
a small electrochemical energy gap of
0.86 eV. The ground-state geometry,
electronic structure, and optical proper-
ties of 1 have been further studied by
density functional theory (DFT) calcu-
lations and compared with those of the
keto-linked dimer 8b. This research
has revealed that incorporation of a p-
QDM unit into the porphyrin frame-
work had a significant impact on its op-
tical and electronic properties, leading
to a novel NIR OPA and TPA chromo-
phore.
QDM)-bridged porphyrin dimer 1 has
been prepared for the first time. An
unexpected Michael addition reaction
took place when we attempted to syn-
thesize compound 1 by reaction of the
cross-conjugated keto-linked porphyrin
dimers 8a and 8b with alkynyl/aryl
Grignard reagents. Alternatively, com-
pound 1 could be successfully prepared
by intramolecular Friedel–Crafts alkyl-
955 nm, e=45400mꢀ1 cmꢀ1) and a large
two-photon absorption (TPA) cross-
section (s(2)max =2080 GM at 1800 nm)
in the near-infrared (NIR) region due
to its extended p-conjugation and qui-
noidal character. It also exhibits
a short singlet excited-state lifetime of
25 ps. The cyclic voltammogram of
Keywords: chromophores
infrared · polycyclic hydrocarbons ·
porphyrinoids · quinodimethane
· near-
ACHTUNGTRENNUNGation of the diol-linked porphyrin
dimer 14 with concomitant oxidation in
air. Compound 1 shows intense one-
AHCTUNGTRENNUNG
Introduction
structures play a pivotal role in determining the electronic
and optical properties of p-conjugated systems (including
both polycyclic hydrocarbons and porphyrinoids), with qui-
noidal p-conjugated systems usually displaying a smaller
HOMO–LUMO energy gap, unique reactivity, and distinct
NIR absorption/emission compared to their aromatic ana-
logues.[8] One of the most fundamental representatives of
quinoidal p-conjugated systems is the p-quinodimethane (p-
QDM) unit, which can be depicted as resonance of a quinoi-
dal form and a biradical form with recovery of an aromatic
sextet ring.[9] p-QDM and its tetraphenyl-substituted deriva-
tives such as Thieleꢀs hydrocarbon and the more extended
Tschitschibabinꢀs hydrocarbon have been investigated previ-
ously, with emphasis on their ground-state geometries and
electronic structures.[10] However, the intrinsic instability of
these quinoidal compounds arising from their pronounced
biradical character made it difficult to obtain clear-cut con-
clusions and also limited their practical applications. Tetra-
cyano-substituted p-QDM (i.e., tetracyanoquinodimethane,
TCNQ),[11] extended oligo-para-phenylene quinodimeth-
Conjugated porphyrin oligomers and tapes are of interest
due to their unique electronic and optical properties, such as
NIR absorption/emission and large nonlinear optical (NLO)
responses.[1] They show great promise for applications in or-
ganic opto-electronics and photonics; for example, NIR-
light-excited photodynamic therapy,[2] NIR photodetectors,[3]
and NIR electroluminescence devices.[4] So far, three major
types of conjugated porphyrin oligomers have been report-
ed: (1) doubly- or triply-edge-fused porphyrin tapes,[5]
(2) alkyne-bridged porphyrin strands,[6] and (3) para-phenyl-
ene-bridged twisted or planarized porphyrin dimers.[7] In all
cases, the geometry and the extent of p-conjugation between
the porphyrin units have significant impact on their optical
and electronic properties. On the other hand, quinoidal
[a] W. Zeng, Dr. Z. Zeng, Y. Ni, Prof. C. Chi, Prof. J. Wu
Department of Chemistry, National University of Singapore
3 Science Drive 3, 117543 (Singapore)
Fax : (+65)6779-1691
AHCTUNGTRENNUNG
anes,[10c,12] and their thiophene analogues[13] represent anoth-
E-mail: chmcc@nus.edu.sg
er attractive class of low-band-gap systems with improved
stability and tunable ground states. For example, tetracyano-
oligothiophenequinodimethanes show progressive changes
from a closed-shell to an open-shell singlet biradical ground
state with increasing chain length, and they can also be used
as stable n-type semiconductors.[13] Incorporation of a p-
QDM moiety into a p-conjugated hydrocarbon framework
leads to quinoidal polycyclic hydrocarbons with interesting
properties. For example, bis(phenalenyl)s, in which two
chmwuj@nus.edu.sg
[b] Dr. M. Ishida, S. Lee, Y. M. Sung, Prof. D. Kim
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
Fax : (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
[c] Prof. J. Wu
Institute of Materials Research and Engineering, A*STAR
3 Research Link, 117602 (Singapore)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302023.
phenACHUTNGRENNUaG AHCTNUTGREGlNNNU enyl moieties are bridged by a p-QDM unit (or
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Chem. Eur. J. 2013, 19, 16814 – 16824

FULL PAPER
a naphthoquinodimethane or anthraquinodimethane unit)
show significant singlet biradical character in the ground
state, as reported by Kubo et al.[14] Our group synthesized
the first stable open-shell heptazethrene and octazethrene
derivatives, in which a p-QDM and naphthoquinodimethane
unit are incorporated into dibenzopentacene and dibenzo-
hexacene frameworks, respectively.[15] Other fused p-systems
incorporating a p-QDM subunit include indenofluorene de-
rivatives as reported by Haley and Tobe et al.[16]
subsequent reductive dehydroxylation would afford the de-
sired compound 1 (Scheme 1). Thus, the diketo-porphyrin
dimers 8a and 8b were first prepared according to a modi-
fied literature procedure.[7] The porphyrin monoboronic
ester 5 was first prepared starting from monobromo-porphy-
rin 2[17] by sequential Suzuki coupling, bromination, metala-
tion, and Miyaura borylation in an overall yield of 72%.
Suzuki coupling reaction between 5 and dimethyl 2,5-
dibromoACHTUNGTRNEUNG
terephthalate 6[18] gave the terephthalate-bridged
In this context, we anticipated that a p-QDM-bridged por-
phyrin dimer 1 would show interesting optical and electronic
properties (Figure 1). In addition, a closed-shell resonance
Zn-porphyrin dimer 7a in 48% yield. Demetalation of 7a
with concentrated HCl (6m) followed by metalation with
Ni(acac)2 afforded the terephthalate-bridged Ni-porphyrin
dimer 7b in 91% yield. Hydrol-
ysis of the methyl ester moiety
in 7b with aqueous LiOH gave
the corresponding carboxylic
acid, which was then converted
into the carboxylic chloride
with oxalyl chloride. Subse-
quent intramolecular Friedel–
Crafts acylation with SnCl4
gave the diketo-Ni-porphyrin
dimer 8a in an overall yield of
92%. A similar approach to the
diketo-Zn-porphyrin dimer 8b
was hampered by demetalation
and decomposition. Alterna-
tively, compound 8b could be
obtained in 48% yield by de-
metalation of 8a with trifluoro-
acetic acid (TFA) and concen-
trated sulfuric acid followed by
treatment with Zn
ACHTUNGTRENNUNG
methanol/chloroform.
pounds 8a and 8b were then
subjected to nucleophilic addi-
tion reactions with alkynyl- and
Figure 1. Two possible resonance forms of p-quinodimethane-bridged porphyrin dimer 1.
aryllithium
reagents
or
form 1a and an open-shell biradical form 1b may both con-
tribute to the ground-state structure of 1 (Figure 1), and it
was of interest to ascertain whether it has a ground-state
singlet biradical character like other p-QDM-embedded p-
systems (e.g., heptazethrene).[15] Herein, we report the chal-
lenging synthesis of 1 and detailed investigations of its opto-
electronic properties and ground-state structure by various
methods, including OPA, TPA, transient absorption (TA),
cyclic voltammetry (CV), and differential pulse voltammetry
(DPV), as well as by theoretical DFT calculations.
Grignard reagents. Interestingly, 1,4-addition (Michael addi-
tion) took place, with simultaneous oxidative dehydrogena-
tion in air, giving the b-substituted diketo-porphyrin dimers
in all cases instead of the desired 1,2-addition products
(diols). For example, reaction of 8a with triisopropylsilyl-
AHCTUNGTREGeNNNU thynylmagnesium bromide gave the b-substituted porphy-
rin dimer 9 in 38% yield, and reaction of 8b with 4-tert-bu-
tylphenylmagnesium bromide afforded the b-substituted Mg
porphyrin dimer 10 in 41% yield. The replacement of Zn2+
ion by Mg2+ ion during the reaction of 8b is interesting and
may be ascribed to the labile character of the Zn porphyrin.
Although compounds 9 and 10 showed low solubility, their
structures were supported by clean 1H NMR spectra and
high-resolution mass spectra (see the Supporting Informa-
tion (SI)). Similar Michael addition reactions have been ob-
served for some keto-fused porphyrin monomers.[19]
Results and Discussion
Synthesis: Our initial strategy towards the target compound
1 was based on the diketo-porphyrin dimers 8a (M=Ni) or
8b (M=Zn). It was envisaged that these might undergo nu-
cleophilic addition with alkynyl or aryl Grignard reagents at
the ketone sites to give the corresponding diols, and that
In light of the above findings, an alternative synthetic
strategy was adopted to synthesize the target compound 1.
The new approach was based on intramolecular Friedel–
Chem. Eur. J. 2013, 19, 16814 – 16824
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C. Chi, D. Kim, J. Wu et al.
Scheme 1. Reagents and conditions: (a) p-tolylboronic acid, [Pd
MeOH, RT, 10 h, 96%; (c) pinacolborane, [Pd(PPh3)2Cl2], C2H4Cl2, Et3N, 85 8C, 4 h, 80%; (d) [Pd
(6m), 5 min, 95%; ii) Ni(acac)2, toluene, reflux, overnight, 96%; (f) i) LiOH, H2O, dioxane, H2O, reflux, 24 h; ii) oxalyl chloride, benzene, RT, 8 h;
iii) SnCl4, RT, 8 h, overall 92%; (g) i) TFA/H2SO4, RT, 1 h, 50%; ii) Zn(OAc)2, CHCl3/MeOH (2:1), RT, overnight, 91%; (h) triisopropylsilylethynylmag-
nesium bromide, THF, RT, 48 h, 41%; (i) 4-tert-butylphenylmagnesium bromide, THF, RT, 48 h, 38%; (j) [Pd(PPh3)4], Cs2CO3, toluene, reflux, 48 h,
55%; (k) i) HCl (conc.), CH2Cl2, RT, 2 h, 95%; ii) Zn(OAc)2, CHCl3/MeOH, RT, 12 h, 95%; (l) mesitylmagnesium bromide, THF, RT, 24 h, 85%;
(m) i) excess BF3·OEt2, CH2Cl2, 10 min; ii) air oxidation, two steps, 15–20%.
A
ACHTUNGTRNEN(UNG OAc)2, CHCl3/
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
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Bridged Porphyrin Dimer
FULL PAPER
Crafts alkylation of the diol 14 followed by oxidative dehy-
drogenation (Scheme 1). To this end, the dialdehyde-linked
porphyrin dimer 13 was first prepared as a key intermediate.
To synthesize dimer 13, we first tried to reduce the tere-
ACHTUNGTRENNUNGphthalate-bridged porphyrin dimer 7a or 7b with lithium
aluminum hydride and then oxidize the resulting diols with
pyridinium chlorochromate.[20] However, the overall yields
from these two steps were lower than 5% because of signifi-
cant decomposition of the porphyrin. We next tried Suzuki
coupling reaction between 5 and 2,5-dibromoterephthalalde-
hyde,[21] but again the yield of 13 was lower than 5% due to
significant deboronation of 5. Finally, we found that Suzuki
coupling between 5 and a protected dialdehyde, namely 2,2’-
(2,5-dibromo-1,4-phenylene)bis(1,3-dioxolane) 11,[22] pro-
ceeded smoothly and gave the intermediate 12 in 55%
yield. To remove the protective group of the aldehyde in 12,
we tried to use either pyridinium p-toluenesulfonate (PPTS)
in acetone/water[23] or TFA/H2O in CH2Cl2,[24] but in both
cases only partial deprotection was observed. However, con-
centrated HCl in CH2Cl2 worked well and subsequent metal-
Figure 2. OPA spectra of 1 (d), 8a (c), 8b (a), and 10 (g) in
CH2Cl2.
and in solution in air. The structures of all new compounds
were identified by NMR and mass spectra (see the SI).
lization with ZnACHTUNGTRENNUNG(OAc)2 gave the desired compound 13 in an
overall yield of 95%. 13 was then subjected to nucleophilic
addition reactions with aryl Grignard reagents such as 4-tert-
butylphenylmagnesium bromide and mesitylmagnesium bro-
mide followed by intramolecular Friedel–Crafts alkylation
(cyclization) by using boron trifluoride diethyl etherate as
a Lewis acid. With 4-tert-butylphenyl as the substituent on
the p-QDM moiety, the cyclization reaction took place, as
confirmed by mass spectrometry. However, the cyclized
product was not stable in air and was sensitive to protic sol-
vents (e.g., methanol and etha-
Optical properties: The steady-state OPA spectrum of 1 to-
gether with the spectra of the diketo-porphyrin dimers 8a,
8b, and 10 in CH2Cl2 are shown in Figure 2 and the data are
collected in Table 1. Compound 1 shows an intense Q-band
with a maximum at 955 nm (e=45400 mꢀ1 cmꢀ1), together
with several distinct peaks at 405, 457, 579, and 851 nm, in-
dicating extensive p-conjugation in this p-QDM-bridged
system. In comparison, the p-phenylene-bridged diol 14 ex-
nol) and silica gel. High-polari-
ty products were observed and
the mass spectrum showed the
Table 1. Photophysical and electrochemical properties of porphyrin dimers 8a, 8b, 10, and 1.
1
1
2
=
=
EC
opt
lmax
emax
[mꢀ1 cmꢀ1
lem
Eox
Ered
HOMO LUMO Eg
Eg
t [ps]
s(2)
max
2
presence
of
over-oxidized
[nm]
G
]
[nm] [V]
[V]
[eV]
[eV]
[eV] [eV]
[GM]
products in air. When the
more bulky mesityl group was
used, the addition product diol
14 could be separated in pure
form and subsequent intramo-
lecular Friedel–Crafts alkyla-
tion with concomitant dehy-
drogenation in air gave the de-
8a 412
49700
45000
43900
64600
50100
43400
42500
45300
38000
47100
39300
39700
42100
47400
48300
45000
43900
45400
508
671
800
0.68
0.87
1.18
ꢀ0.96
ꢀ1.12
ꢀ1.71
5 (t1),
40 (t2)
–
ꢀ5.36
ꢀ3.94
1.42 1.35
650 (1500 nm)
8b 521
688
782
865
10 429
534
0.48
0.60
1.03
ꢀ0.98
ꢀ1.13
ꢀ1.69
900
ꢀ5.19
ꢀ3.9
1.29 1.19 660
1.19 1.15 334
1600 (1800 nm)
1490 (1800 nm)
0.25
0.46
0.86
ꢀ1.23
sired product
1 in 15–20%
706
829
922
940
–
ꢀ1.50 ꢀ4.97
ꢀ3.78
yield. The low isolated yield
was mainly due to demetala-
tion and decomposition of the
porphyrin chromophore during
the reaction, and the sensitivi-
ty of 1 to silica gel. A rapid
flash column had to be used to
purify compound 1 as a longer
operation time on a silica gel
column led to significant de-
composition of the material.
Compound 1, however, shows
good stability in the solid state
1
405
457
579
851
955
0.01
0.14
0.67
0.93
ꢀ1.14
ꢀ1.36 ꢀ4.70
ꢀ3.84
0.86 1.08 25
2080 (1800 nm)
lmax: absorption maximum. emax: molar extinction coefficient at the absorption maximum. lem: emission maxi-
1
1
2
=
=
2
mum. Eox and Ered are half-wave potentials of the oxidative and reductive waves, respectively, with poten-
tials relative to the Fc/Fc+ couple. HOMO and LUMO energy levels were calculated according to equations
HOMO=ꢀ(4.8 + Eoxonset) and LUMO=ꢀ(4.8 + Eredonset), in which Eox
and Ered
are the onset potentials
onset
onset
of the first oxidative and reductive redox waves, respectively. EgEC: electrochemical energy gap derived from
LUMO–HOMO. Egopt: optical energy gap derived from lowest-energy absorption onset in the absorption spec-
tra. t is the singlet excited lifetime obtained from fs-TA spectroscopy. s(2)
is the two-photon absorption
max
cross-section at the longest wavelength maximum.
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C. Chi, D. Kim, J. Wu et al.
hibits a weak Q-band extending to 600 nm. The planarized
diketo-porphyrin dimers also show intense NIR absorption
due to effective p-conjugation between the two porphyrin
units. The metal ions in the porphyrin have an obvious
impact on the absorption spectrum, with a gradual red-shift
on going from Ni porphyrin 8a to Zn porphyrin 8b to Mg
porphyrin 10, with longest-wavelength absorption maxima at
800, 865, and 922 nm, respectively. Such a trend is also ob-
served for porphyrin monomers with different metal ions.[25]
The optical energy gaps (Egopt) were estimated to be 1.08,
1.35, 1.19, and 1.15 eV for 1, 8a, 8b, and 10, respectively,
from the onsets of the lowest-energy absorption bands.
porphyrin complexes.[1b,1d,26] In contrast, the TA spectrum of
1 displayed different spectral features; relatively intense ex-
cited-state absorption bands centered around 520 and
660 nm and two ground-state bleaching bands around 460
and 580 nm, respectively, were observed. The kinetic profile
of 1 showed very fast decay (t=25 ps), which is supported
by its non-emissive feature beyond 1000 nm. Taking into ac-
count the structure of 8b, the p-QDM conjugated bridge in
1 may significantly contribute to the electronic perturbation
of the zinc porphyrin.
TPA measurements were carried out for 1, 8a, 8b, and 10
by using
a wavelength-scanning open-aperture Z-scan
Femtosecond transient absorption measurements were
then conducted to explore the excited-state dynamics of
1 and the diketo-porphyrin dimers 8a, 8b, and 10 (Figure S1
in the SI and Table 1). Zn complex 8b and Mg complex 10
exhibited relatively longer singlet excited-state lifetimes of
660 and 334 ps, respectively, which are consistent with their
NIR emissive character (Table 1 and Figure S2 in the SI). Ni
porphyrin 8a displayed a TA spectrum with very fast bi-ex-
ponential decay (t1 =5 ps and t2 =40 ps) as well as non-
emissive features, suggesting the participation of a metal-
centered excited state (e.g., d-d state). These rapid excited-
state deactivations and the excited-state absorption band
around 550 nm underwent blue-shifts, which is characteristic
behavior of the excited-state deactivations of ordinary NiII
method in the wavelength range from 1200 to 2000 nm, in
which one-photon absorption contribution is negligible
(Figure 3 and Figure S3 in the SI; the TPA spectra are
plotted at lex/2). All of the compounds showed a large TPA
response enhanced by resonance in the OPA spectral contri-
bution of a Q-like band. The TPA cross-section maxima
(s(2)max) of 2080 GM (1800 nm) for 1, 650 GM (1500 nm) for
8a, 1600 GM (1800 nm) for 8b, and 1490 GM (1800 nm) for
10 are given in Table 1. Compared with the simple aromatic
porphyrin monomer, which usually shows a small TPA
cross-section (<100 GM),[5c,27] the keto-bridged p-phenylene
dimers 8a, 8b, and 10 demonstrate that effective electronic
coupling between the two porphyrin units through the pla-
narized phenylene p-bridge enhances the TPA activity. This
*
Figure 3. OPA (c and left vertical axis) and TPA spectra ( and right vertical axis) of 8a (a), 8b (b), 10 (c), and 1 (d) in THF. TPA spectra are plotted
at lex/2.
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Bridged Porphyrin Dimer
FULL PAPER
is in good agreement with the properties of the reported
planar porphyrin dimers.[28] Moreover, compared to the non-
conjugated p-phenylene-bridged porphyrin dimer (com-
pound 6 in ref. [7]), the p-QDM-bridged dimer 1 exhibits
a much longer OPA wavelength and a remarkably enhanced
TPA response in the NIR range. It is known that polarizable
quinoidal electronic structures (e.g., tetraphenyl-fused qui-
noidal porphyrin monomer[29]) may be relevant to TPA en-
hancement. Therefore, the p-QDM bridge in 1 plays an im-
portant role in the nonlinear optical response as well as ef-
fective p-overlaps.
8a, ꢀ5.19 and ꢀ3.90 eV for 8b, and ꢀ4.97 and ꢀ3.18 eV for
10, respectively, from the onset of the first oxidation and re-
duction waves. Accordingly, the electrochemical energy gaps
(EgEC) of 1, 8a, 8b, and 10 were determined to be 0.86, 1.42,
1.29, and 1.19 eV, respectively. Therefore, the change of
metal ion from Ni2+ in 8a to Zn2+ in 8b to Mg2+ in 10 led
to negative shifts of the first oxidation and reduction waves,
leading to a higher-lying HOMO and LUMO energy levels
and a lower energy gap. Compound 1 showed an even
higher-lying HOMO energy level and a smaller energy gap,
but it still exhibited good stability against oxidation in air.
Electrochemical properties: Cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) measurements were
conducted on 1, 8a, 8b, and 10 in CH2Cl2 solution contain-
DFT calculations: Compound 1 shows sharp peaks in its
1H NMR spectrum in solution even at elevated tempera-
tures (e.g., up to 1008C in C2D2Cl4), indicating a closed-shell
electronic structure in the ground state. This implies that the
singlet biradical resonance form 1b makes only a minor con-
tribution to the ground-state electronic configuration of
1 (Figure 1). To gain deeper insight into the ground-state ge-
ometry and electronic structure and optical properties, DFT
calculations were performed for zinc complexes 1 and 8b
with the B3LYP/6-31G (d) level basis set. As shown in
Figure 5, the optimized structures of both 1 and 8b indicate
ing
0.1m
tetrabutylammonium
hexafluorophosphate
(Bu4NPF6) (Figure 4, Table 1, and Figure S4 in the SI). Com-
Figure 4. Cyclic voltammograms of 8a (c), 8b (a), 10 (g), and
1 (d) in CH2Cl2 with 0.1m Bu4NPF6 as supporting electrolyte, AgCl/Ag
as reference electrode, an Au disk as working electrode, and a Pt wire as
counter electrode, obtained at a scan rate of 50 mVsꢀ1
.
Figure 5. DFT-optimized structures of 8b (top) and 1 (bottom) with se-
lected bond lengths [ꢂ] and dihedral angles [o] between the mean planes
of the porphyrin core and the bridging phenyl ring. The NICS(1) values
at the centers of the bridging six-membered rings for compound 8b and
1 are also shown.
pound 1 showed four quasi-reversible oxidation waves with
half-wave potentials (Eox1/2) at 0.01, 0.14, 0.67, and 0.93 V
and two quasi-reversible reduction waves with half-wave po-
tentials (Ered1/2) at ꢀ1.14 and ꢀ1.36 V (vs Fc+/Fc). Com-
pound 8a showed three quasi-reversible oxidation waves
1/2
Eox at 0.68, 0.87, and 1.18 V and three quasi-reversible re-
that the central phenylene bridge is structurally constrained
between two porphyrin units by methane carbon and car-
bonyl groups, respectively. The overall molecular geometries
1/2
duction waves Ered at ꢀ0.96, ꢀ1.12, and ꢀ1.71 V. Similarly,
compound 8b exhibited three quasi-reversible oxidation
1/2
ꢀ
waves with Eox at 0.48, 0.60, and 1.03 V and three quasi-re-
are twisted due to steric repulsion between the C H of the
1/2
versible reduction waves with Ered at ꢀ0.98, ꢀ1.13, and
central six-membered (i.e., benzenoid or quinonoid) ring
and the adjacent pyrrole unit, with torsion angles of 25.68
and 25.18, respectively. Bond length analysis around the
bridging ring in 8b indicates effective bond resonances, sug-
gesting a typical “benzenoid-like” ring structure. A negative
nuclear-independent chemical shift (NICS(1)) value of
ꢀ1.69 V. Compound 10 displayed three quasi-reversible oxi-
1/2
dation waves with Eox at 0.25, 0.46, and 0.86 V and two
1/2
quasi-reversible reduction waves with Ered at ꢀ1.23 and
ꢀ1.50 V. The HOMO and LUMO energy levels were evalu-
ated as ꢀ4.70 and ꢀ3.84 eV for 1, ꢀ5.36 and ꢀ3.94 eV for
Chem. Eur. J. 2013, 19, 16814 – 16824
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www.chemeurj.org
16819

C. Chi, D. Kim, J. Wu et al.
ꢀ3.13 ppm confirms the aromatic character of the central
phenylene ring. In contrast, the clear bond length alterna-
tion of the corresponding ring and positive NICS(1) value of
+1.41 ppm found for 1 demonstrate a characteristic quinoi-
dal electronic structure.
The frontier HOMO and LUMO density maps of 8b can
be interpreted in terms of hybrid orbitals of the benzene
MO and the a2u orbital and eg orbital of zinc porphyrins, re-
spectively (Figure S5 in the SI). In contrast, the electron
densities of 1 are homogeneously delocalized over two por-
phyrin units across a p-QDM unit in both the HOMO and
LUMO (Figure 6). The corresponding a2u and eg orbitals of
strategy involving the diketo-porphyrin dimers 8a and 8b
gave unexpected Michael addition products. The optical and
electrochemical properties and ground-state geometries and
electronic structures of the products have been systematical-
ly investigated by various experimental methods and theo-
retical calculations. The p-QDM porphyrin dimer 1 has
a closed-shell ground-state quinoidal structure, which leads
to an extended p-conjugated system with a small energy
gap. This distinct electronic character of 1 gives rise to non-
linear optical enhancement (i.e., larger TPA cross-section
values) in the NIR region. Our synthetic approach for ob-
taining 1 may, in principle, be extended to the synthesis of
further p-extended naphthoquinodimethane- or anthraqui-
nodimethane-bridged porphyrin dimers or oligomers. Inves-
tigation of the p-conjugation effects on these electronic sys-
tems would be highly attractive for the development of
open-shell singlet biradical materials in their ground state.
Research in this area is currently underway in our laborato-
ry.
Experimental Section
General: All reagents and starting materials were obtained from commer-
cial suppliers and used without further purification. Anhydrous tetrahy-
drofuran (THF) and dichloromethane were distilled under a nitrogen at-
mosphere over sodium and calcium hydride, respectively. 2,2’-(2,5-
DibromoACTHUNRTGNEUNG-1,4-phenylene)bis(1,3-dioxolane) (11) was prepared according
to the literature.[22] Column chromatography was performed on silica gel
60 (Merck 40–60 nm, 230–400 mesh). All NMR spectra were recorded on
a Bruker AMX500 spectrometer. All chemical shifts are quoted in ppm,
relative to tetramethylsilane, using the residual solvent peak as a refer-
ence standard. Atmospheric pressure chemical ionization mass spectrom-
etry (APCI MS) was performed on a Finnigan TSQ 7000 triple-stage
quadrupole mass spectrometer. MALDI-TOF mass spectra were mea-
sured on a Bruker Autoflex MALDI-TOF instrument using tetracyano-
quinodimethane (TCNQ) as the matrix. High-resolution mass spectra
(HRMS) were recorded on a Finnigan MAT95XL-T with FAB ionization
source or on a Finnigan MAT 95ꢃP spectrometer. UV/Vis absorption
Figure 6. Frontier molecular orbital profiles and energy diagram of 1 ob-
tained by B3LYP/6-31G* level calculation.
the zinc porphyrins in the HOMO and LUMO of 1 are se-
verely perturbed by electronic coupling through the p-QDM
bridging unit. This resulted in the narrower HOMO–LUMO
energy gap of 1.414 eV for 1. On this basis, the vertical tran-
sitions of quinoidal 1 are crucial for the quinoidal porphy-
rin-like transition behavior. A simulated stick spectrum of
spectra were recorded on
a Shimadzu UV-3600 spectrophotometer.
Cyclic voltammetry and differential pulse voltammetry measurements
were performed in dry CH2Cl2 on a CHI 620C electrochemical analyzer
with a three-electrode cell, using 0.1m Bu4NPF6 as supporting electrolyte,
AgCl/Ag as reference electrode, a gold disk as working electrode, and
a Pt wire as counter electrode. The potential was externally calibrated
against the ferrocene/ferrocenium couple.
1
obtained by time-dependent (TD)-DFT calculations
(B3LYP/6-31G* method) was in good agreement with the
experimental spectrum (Figure S7 and Table S3 in the SI).
The lowest-energy band in the NIR region is mostly contrib-
uted by HOMO–LUMO excitation (900.6 nm, f=1.2145)
(Figure S6 and Table S1 in the SI). In the case of diketo 8b,
the predicted transitions can be understood in terms of the
typical Goutermannꢀs four-orbital model (Figure S6 and
Table S2 in the SI).[30] Thus, the distinct photophysical and
electrochemical properties of quinoidal porphyrin dimer
1 are supported by theoretical calculations.
The femtosecond time-resolved transient absorption spectrometer used
for this study consisted of a femtosecond optical parametric amplifier
(Quantronix, Palitra-FS) pumped by a Ti:sapphire regenerative amplifier
system (Quantronix, Integra-C) operating at 1 kHz repetition rate and an
accompanying optical detection system. The generated OPA pulses had
a width of ~100 fs and an average power of 1 mW in the range 450–
800 nm, and were used as pump pulses. White light continuum (WLC)
probe pulses were generated using a sapphire window (2 mm thick) by
focusing of a small portion of the fundamental 800 nm pulses, which were
picked off by a quartz plate before entering into the OPA. The time
delay between pump and probe beams was carefully controlled by
making the pump beam travel along a variable optical delay (Newport,
ILS250). The intensities of the spectrally dispersed WLC probe pulses
were monitored by means of a miniature spectrograph (OceanOptics,
USB2000+). To obtain the time-resolved transient absorption difference
signal (DA) at a specific time, the pump pulses were chopped at 25 Hz
and absorption spectral intensities were saved alternately with or without
a pump pulse. Typically, 6000 pulses were used to excite samples and to
Conclusion
In summary, the p-QDM-bridged porphyrin dimer 1 has
been prepared for the first time by an intramolecular
Friedel
ACHTUNGTRENNUNG–Crafts alkylation approach, whereas a synthetic
16820
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16814 – 16824

Bridged Porphyrin Dimer
FULL PAPER
obtain the TA spectra at a particular delay time. The polarization angle
between pump and probe beam was set at the magic angle (54.78) using
a Glan laser polarizer with a half-wave retarder to prevent polarization-
dependent signals. The cross-correlation fwhm in the pump–probe experi-
ments was less than 200 fs, and the chirp of the WLC probe pulses was
measured as 800 fs in the 400–800 nm region. To minimize chirp, all-re-
flection optics was used in the probe beam path, and a quartz cell of
2 mm path length was employed. After completing each set of fluores-
cence and TA experiments, the absorption spectra of all compounds were
carefully checked to rule out the presence of artifacts or spurious signals
arising from, for example, degradation or photo-oxidation of the samples
in question.
acetone (10 mL) was added, whereupon the solution was poured into
water and the resulting mixture was extracted with CH2Cl2. The organic
layer was washed with water and dried over anhydrous MgSO4. The sol-
vent was removed under vacuum and the residue was purified by column
chromatography (silica gel; hexane/CH2Cl2, 1:1) to afford the corre-
1
sponding monobromo-porphyrin (2.15 g) in 95% yield. H NMR (CDCl3,
500 MHz): d=9.62 (d, J=4.5 Hz, 2H), 8.81 (d, J=4.5 Hz, 2H), 8.76 (d,
J=4.5 Hz, 2H), 8.65 (d, J=4.5 Hz, 2H), 8.09 (d, J=7.5 Hz, 2H), 7.56 (d,
J=7.5 Hz, 2H), 7.32 (s, 4H), 2.72 (s, 3H), 2.67 (s, 6H), 1.86 (s, 12H),
2.16 ppm (s, 2NH); 13C NMR (CDCl3, 125 MHz): d=139.38, 138.75,
138.16, 137.92, 137.52, 134.36, 127.83, 127.52, 120.46, 118.98, 101.98, 21.60,
21.52, 21.47 ppm; HRMS (MALDI-TOF): m/z calcd for C45H39BrN4:
714.2358; found: 714.2284; error=ꢀ1.4 ppm. Zn
ACHTUNGRTENUN(NG OAc)2 (3.47 g,
The two-photon absorption spectrum was measured in the NIR region
using the open-aperture Z-scan method with 130 fs pulses from an optical
parametric amplifier (Light Conversion, TOPAS) operating at a repetition
rate of 2 kHz generated from a Ti:sapphire regenerative amplifier system
(Spectra-Physics, Hurricane). After passing through a 10 cm focal length
lens, the laser beam was focused and passed through a 1 mm quartz cell.
Since the position of the sample cell could be controlled along the laser
beam direction (z axis) by using a motor-controlled delay stage, the local
power density within the sample cell could be simply controlled under
constant laser intensity. The transmitted laser beam from the sample cell
was then detected by the same photodiode as used for reference monitor-
ing. The on-axis peak intensity of the incident pulses at the focal point,
I0, ranged from 40 to 60 GWcmꢀ2. For a Gaussian beam profile, the non-
linear absorption coefficient could be obtained by curve-fitting of the ob-
served open-aperture traces T(z) with the following equation:
15.85 mmol) was added to a solution of the monobromo-porphryin inter-
mediate (2.15 g, 3.01 mmol) in CHCl3/MeOH (2:1; 400 mL) and the mix-
ture was stirred overnight. The solvent was removed under vacuum and
the residue was purified by column chromatography (silica gel; hexane/
CH2Cl2, 1:1) to afford the desired product 4 (2.24 g) in 96% yield. m.p. >
3008C; 1H NMR (CDCl3, 500 MHz): d=9.71 (d, J=4.5 Hz, 2H), 8.87 (d,
J=4.5 Hz, 2H), 8.83 (d, J=4.5 Hz, 2H), 8.73 (d, J=4.5 Hz, 2H), 8.08 (d,
J=8.0 Hz, 2H), 7.54 (d, J=8.0 Hz, 2H), 7.29 (s, 4H), 2.70 (s, 3H), 2.64
(s, 6H), 1.83 ppm (s, 12H); 13C NMR (CDCl3, 125 MHz): d=150.86,
150.41, 150.32, 149.65, 139.58, 139.27, 138.77, 137.69, 137.25, 134.31,
133.28, 132.95, 131.97, 131.08, 127.78, 127.40, 121.35, 119.98, 21.66, 21.56,
21.52 ppm; HRMS (MALDI-TOF): m/z calcd for C45H37BrN4Zn:
776.1493; found: 776.1457; error=ꢀ3.94 ppm; elemental analysis calcd
(%) for C45H37BrN4Zn: C 75.52, H 5.49, N 7.83; found: C 75.43, H 5.54,
N 7.76.
bI0ð1 ꢀ eꢀa
Þ
2
0
Compound 5: A mixture of compound 4 (2 g, 2.58 mmol), 4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane (0.45 mL, 3.1 mmol), triethylamine (4.43 mL,
5.60 mmol), and [PdACHTUNGTRNEG(UN PPh3)2Cl2] (84.2 mg, 0.12 mmol, 3 mol%) in 1,2-di-
TðzÞ ¼ 1
2a0½1 þ ðz=z0Þ ꢁ
chloroethane (200 mL) was degassed and stirred at 858C for 4 h under an
argon atmosphere. The reaction mixture was then cooled to room tem-
perature, washed with water, and dried over Na2SO4. The solvent was re-
moved under vacuum and the residue was purified by column chromatog-
raphy (silica gel; hexane/CH2Cl2, 1:2) to afford the desired product 5
(1.7 g) in 80% yield; m.p. > 3008C; 1H NMR (CDCl3, 500 MHz): d=9.84
(d, J=4.5 Hz, 2H), 8.92 (d, J=4.5 Hz, 4H), 8.76 (d, J=4.5 Hz, 2H), 8.12
(d, J=7.5 Hz, 2H), 7.54 (d, J=7.5 Hz, 2H), 7.29 (s, 4H), 2.71 (s, 3H),
2.65 (s, 6H), 1.86 (s, 12H), 1.82 ppm (s, 12H); 13C NMR (CDCl3,
125 MHz): d=154.32, 150.71, 149.67, 149.50, 139.84, 139.35, 139.21,
137.42, 137.11, 134.34, 133.22, 132.63, 131.71, 130.22, 127.65, 127.28,
121.95, 119.05, 85.23, 25.39, 21.62, 21.56, 21.52 ppm; HRMS (MALDI-
TOF): m/z calcd for C51H49BN4O2Zn: 824.3240; found: 824.3215; error=
ꢀ2.40 ppm; elemental analysis calcd (%) for C51H49BN4O2Zn: C 74.14, H
5.98, N 6.78; found: C 74.06, H 6.08, N 6.67.
where a0 is the linear absorption coefficient, l is the sample length, and
z0 is the diffraction length of the incident beam. Having obtained the
nonlinear absorption coefficient, the TPA cross-section s(2) of one solute
molecule (in units of GM, where 1 GM=10ꢀ50 cm4 sphotonꢀ1 moleculeꢀ1
)
could be determined by using the following relationship:
10ꢀ1
s
ð2ÞNAd
hv
b ¼
where NA is the Avogadro constant, d is the concentration of the com-
pound in solution, h is Planckꢀs constant, and n is the frequency of the in-
cident laser beam.
Synthetic procedures and characterization data
Compound 3: A mixture of compound 2 (2 g, 3.20 mmol), 4,4,5,5-tetra-
methyl-2-(p-tolyl)-1,3,2-dioxaborolane
(250 mL), and aqueous Na2CO3 solution (2m, 60 mL) was degassed by
bubbling a stream of nitrogen through it for 30 min. [Pd(PPh3)4] (370 mg,
(1.05 g, 4.8 mmol), toluene
Compound 7a: A mixture of compound 5 (1.7 g, 2.06 mmol), dimethyl
2,5-dibromoterephthalate (288 mg, 0.82 mmol), Cs2CO3 (2.68 g,
8.24 mmol), and [PdACTHUNTRGNE(UNG PPh3)4] (98 mg, 0.082 mmol) in toluene (75 mL) was
AHCTUNGTRENNUNG
0.32 mol) was added and the mixture was further degassed by three
freeze-pump-thaw cycles. The reaction mixture was heated at 808C for
24 h under a nitrogen atmosphere. After cooling to room temperature, it
was poured into water and the resulting mixture was extracted with
CH2Cl2. The organic layer was washed with water and dried over anhy-
drous MgSO4. The solvent was removed under vacuum and the residue
was purified by column chromatography (silica gel; hexane/CH2Cl2, 1:1)
to afford the desired product 3 (2.03 g) in 98% yield; m.p. > 3008C;
1H NMR (CDCl3, 500 MHz): d=10.11 (s, 1H), 9.26 (d, J=4.5 Hz, 2H),
8.84 (d, J=4.5 Hz, 2H), 8.82 (d, J=4.5 Hz, 2H), 8.73 (d, J=5.0 Hz, 2H),
8.10 (d, J=7.5 Hz, 2H), 7.54 (d, J=7.5 Hz, 2H), 7.30 (s, 4H), 2.70 (s,
3H), 2.65 (s, 6H), 1.85 ppm (s, 12H); 13C NMR (CDCl3, 125 MHz): d=
139.47, 139.41, 138.11, 137.75, 137.32, 134.41, 127.79, 127.30, 119.99,
117.72, 103.99, 21.65, 21.53, 21.48 ppm; HRMS (APCI-HR): m/z calcd
for C45H41N4 [M+1]: 637.3326; found: 637.3340; error=2.2 ppm; elemen-
tal analysis calcd (%) for C45H40N4: C 84.87, H 6.33, N 8.80; found: C
84.73, H 6.42, N 8.70.
degassed by two freeze-pump-thaw cycles and stirred at 1208C for 48 h.
After cooling to room temperature, the solvent was removed under
vacuum and the residue was purified by column chromatography (silica
gel; hexane/CH2Cl2, 1:1) to afford the desired product 7a (754 mg) in
1
58% yield; m.p. > 4008C; H NMR (CDCl3, 500 MHz): d=9.22 (s, 2H),
9.14 (d, J=4.5 Hz, 4H), 8.94 (d, J=4.5 Hz, 8H), 8.81 (d, J=4.5 Hz, 4H),
8.16 (m, 4H), 7.57 (d, J=8.0 Hz, 4H), 7.31 (s, 8H), 2.90 (s, 6H), 2.73 (s,
6H), 2.67 (s, 12H), 1.91 ppm (s, 24H); 13C NMR (CDCl3, 125 MHz): d=
167.52, 150.37, 150.12, 149.86, 149.63, 139.36, 139.10, 137.51, 137.08,
137.08, 136.07, 134.73, 134.49, 134.44, 134.36, 132.56, 131.40, 131.16,
130.73, 128.82, 127.72, 127.31, 120.84, 119.49, 117.21, 51.91, 21.76, 21.55,
21.52 ppm; HRMS (MALDI-TOF): m/z calcd for
C100H82N8O4Zn2:
1586.5042; found: 1586.5096; error=3.40 ppm; elemental analysis calcd
(%) for C100H82N8O4Zn2: C 75.51, H 5.20, N 7.04; found: C 75.48, H 5.31,
N 6.89.
Compound 7b: HCl (6m, 100 mL) was added to a solution of compound
7a (750 mg, 0.473 mmol) in CH2Cl2 (200 mL). The mixture was stirred at
room temperature for 5 min, then washed with water, and the organic
phase was dried over Na2SO4. The solvent was removed under vacuum to
Compound 4: Compound 3 (2.02 g, 3.17 mmol) was dissolved in CHCl3
(500 mL) and N-bromosuccinimide (NBS; 621 mg, 3.5 mmol) was added
in small portions. The mixture was stirred for 30 min at 08C and then
Chem. Eur. J. 2013, 19, 16814 – 16824
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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16821

C. Chi, D. Kim, J. Wu et al.
afford the intermediate free porphyrin, which was used for the next step.
A mixture of the free porphyrin (656.8 mg, 0.45 mmol) and nickel(II)
acetylacetonate (1.16 g, 4.5 mmol) in toluene (250 mL) was heated at
reflux for 24 h. The solution was then cooled to room temperature,
washed with water, and dried over anhydrous sodium sulfate. The solvent
was removed under vacuum and the residue was purified by column
chromatography (silica gel; CH2Cl2) to afford compound 7b (679 mg) in
Compounds 8a and 8b: A mixture of 7b (600 mg, 0.381 mmol) and
LiOH·H2O (10.5 g, 28.2 mmol) in dioxane (450 mL) and water (45 mL)
was heated under reflux for 36 h under a nitrogen atmosphere. The sol-
vents were evaporated and the solid was purified by column chromatog-
raphy (silica gel; CH2Cl2 + 1% AcOH) to give the corresponding acid
(550 mg) as
a crude product. The acid was taken up in benzene
(200 mL), oxalyl chloride (12 mL) was added, and the mixture was stirred
at 508C overnight. The solvent and excess oxalyl chloride were then re-
moved under vacuum. The residue was redissolved in dry CH2Cl2
(160 mL) and SnCl4 (8 mL) was added. After stirring at room tempera-
ture for 8 h, CH2Cl2 (200 mL) was added and the solution was neutralized
with aqueous NaOH. The organic phase was washed with water and
dried over Na2SO4. The solvent was removed under vacuum and the resi-
due was purified by column chromatography (silica gel; hexane/CH2Cl2,
2:1) to afford the desired product 8a (530 mg) in 92% yield; m.p. >
4008C; 1H NMR (CDCl3, 500 MHz): d=9.48 (d, J=5.0 Hz, 4H), 9.10 (s,
2H), 9.01 (s, 2H), 8.53 (d, J=5.0 Hz, 2H), 8.47 (d, J=5.0 Hz, 2H), 8.36
(d, J=5.0 Hz, 2H), 8.27 (d, J=5.0 Hz, 2H), 8.26 (d, J=5.0 Hz, 2H), 7.80
(d, J=8.0 Hz, 4H), 7.45 (d, J=8.0 Hz, 4H), 7.22 (s, 4H), 7.17 (s, 4H),
2.60 (s, 6H), 2.57 (s, 6H), 2.55 (s, 6H), 1.96 (s, 12H), 1.88 ppm (s, 12H);
13C NMR (CDCl3, 125 MHz): d=182.17, 145.62, 144.42, 144.06, 142.93,
142.04, 140.28, 139.30, 138.92, 138.67, 138.19, 138.06, 137.81, 136.71,
135.87, 135.61, 135.38, 135.30, 134.43, 134.18, 133.13, 133.00, 132.31,
132.27, 131.84, 127.98, 127.92, 124.86, 121.54, 117.68, 107.86, 21.49, 21.45,
21.39, 21.35, 21.30 ppm; HRMS (MALDI-TOF): m/z calcd for
C98H74N8Ni2O2: 1511.4714; found: 1511.4626; error=ꢀ0.52 ppm; elemen-
tal analysis calcd (%) for C98H74N8Ni2O2: C 77.79, H 4.93, N 7.41; found:
C 77.68, H 5.01, N 7.36.
1
96% yield; m.p. > 4008C; H NMR (CDCl3, 500 MHz): d=9.03 (s, 2H),
8.88 (d, J=5.0 Hz, 4H), 8.76 (d, J=5.0 Hz, 4H), 8.72 (d, J=5 Hz, 4H),
8.62 (d, J=5.0 Hz, 4H), 7.60 (br, 4H), 7.50 (d, J=7.5 Hz, 4H), 7.28 (s,
4H), 7.23 (s, 4H), 2.91 (s, 6H), 2.66 (s, 6H), 2.61 (s, 12H), 1.98 (s, 24H),
1.77 ppm (s, 12H); 13C NMR (CDCl3, 125 MHz): d=166.91, 143.14,
142.83, 142.57, 142.44, 141.42, 139.10, 138.17, 137.72, 137.34, 135.78,
134.43, 133.64, 132.58, 131.61, 131.32, 131.04, 127.78, 127.56, 119.04,
117.67, 115.76, 51.97, 21.50, 21.44, 21.37 ppm; HRMS (MALDI-TOF):
m/z calcd for
C100H82N8Ni2O4: 1574.5166; found: 1574.5137; error=
ꢀ1.8 ppm; elemental analysis calcd (%) for C100H82N8O4Ni2: C 76.15, H
5.24, N 7.10; found: C 76.03, H 5.34, N 7.01.
Compound 12: A mixture of 5 (1.7 g, 2.06 mmol), 2,2’-(2,5-dibromo-1,4-
phenylene)bis(1,3-dioxolane) (310 mg, 0.82 mmol), Cs2CO3 (2.68 g,
8.24 mmol), and [PdACHTUNGTRENNUNG(PPh3)4] (98 mg, 0.082 mmol) in dry toluene
(100 mL) was degassed by two freeze-pump-thaw cycles and stirred at
1208C for 48 h. After cooling to room temperature, the solvent was re-
moved under vacuum and the residue was purified by column chromatog-
raphy (silica gel; hexane/CH2Cl2, 1:1) to afford the desired product 12
(728.2 mg) in 55% yield; m.p. > 4008C; 1H NMR (CDCl3, 500 MHz):
d=9.22 (d, J=4.5 Hz, 4H), 8.95 (d, J=4.5 Hz, 8H), 8.83 (d, J=4.5 Hz,
4H), 8.72 (s, 2H), 8.18 (d, J=7.5 Hz, 2H), 8.12 (d, J=7.5 Hz, 2H), 7.57
(m, 4H), 7.34 (s, 8H), 5.71 (s, 2H), 3.61 (m, 4H), 3.21 (m, 4H), 2.71 (s,
6H), 2.69 (s, 12H), 1.95 (s, 12H), 1.90 ppm (s, 12H); 13C NMR (CDCl3,
125 MHz): d=150.43, 150.20, 150.09, 142.10, 139.87, 139.36, 139.30,
139.09, 137.66, 137.50, 137.06, 134.45, 134.35, 132.61, 132.48, 131.68,
131.29, 130.64, 127.74, 127.32, 120.80, 119.46, 116.37, 101.74, 65.00, 21.81,
21.74, 21.54 ppm; HRMS (MALDI-TOF): m/z calcd for C102H86N8O4Zn2:
1614.5355; found: 1614.5320; error=ꢀ2.16 ppm; elemental analysis calcd
(%) for C102H86N8O4Zn2: C 75.69, H 5.36, N 6.92; found: C 75.59, H 5.43,
N 6.84.
Compound 8a: (254 mg, 0.168 mmol) was dissolved in a mixture of TFA
(18 mL) and concentrated H2SO4 (40 mL). The mixture was stirred at
room temperature for 1 h and then poured into iced water. The solution
was neutralized with aqueous NaHCO3 and extracted with CH2Cl2. The
organic layer was washed with water and dried over Na2SO4. The solvent
was removed under vacuum and the residue was purified by column
chromatography (silica gel; hexane/CH2Cl2, 1:1) to afford the free por-
1
phyrin intermediate (119 mg) in 50% yield. H NMR (CDCl3, 500 MHz):
d=9.59 (d, J=5.0 Hz, 2H), 9.49 (s, 2H), 9.11 (s, 2H), 8.59 (d, J=5.0 Hz,
2H), 8.48 (d, J=5.0 Hz, 2H), 8.46 (d, J=5.0 Hz, 2H), 8.40 (d, J=5.0 Hz,
2H), 8.30 (d, J=5.0 Hz, 2H), 7.95 (d, J=8.0 Hz, 4H), 7.48 (d, J=8.0 Hz,
4H), 7.21 (s, 8H), 2.63 (s, 6H), 2.59 (s, 6H), 2.57 (s, 6H), 1.94 (s, 12H),
1.86 (s, 12H), 0.14 ppm (s, 4NH); 13C NMR (CDCl3, 125 MHz): d=
183.53, 141.35, 139.22, 139.07, 138.28, 138.12, 137.86, 137.59, 137.16,
136.93, 136.39, 134.00, 128.04, 127.97, 127.82, 125.40, 122.87, 119.67,
109.67, 21.71, 21.62, 21.52, 21.48, 21.45 ppm. The free porphyrin (119 mg)
Compound 13: Concentrated HCl (100 mL) was slowly added to a solu-
tion of compound 12 (720 mg, 0.446 mmol) in CH2Cl2 (50 mL). The mix-
ture was stirred under argon at room temperature overnight, then
washed with water. The organic layer was washed with water and dried
over Na2SO4. The solvent was removed under vacuum and the residue
was purified by column chromatography (silica gel; hexane/CH2Cl2, 1:3)
to afford the free porphyrin dialdehyde intermediate in 95% yield.
1H NMR (CDCl3, 500 MHz): d=9.86 (s, 2H; CHO), 9.28 (s, 2H), 8.99 (d,
J=4.0 Hz, 4H), 8.92 (d, J=4.5 Hz, 4H), 8.90 (d, J=4.5 Hz, 4H), 8.77 (d,
J=4.5 Hz, 4H), 8.18 (d, J=7.5 Hz, 2H), 8.12 (d, J=7.5 Hz, 2H), 7.60 (d,
J=7.5 Hz, 4H), 7.36 (s, 8H), 2.69 (s, 6H), 2.63 (s, 12H), 1.95 (s, 12H),
1.92 ppm (s, 12H); 13C NMR (CDCl3, 125 MHz): d=190.65, 145.33,
139.55, 139.49, 138.85, 138.16, 138.03, 138.00, 137.64, 134.56, 134.46,
132.47, 127.94, 127.57, 120.94, 119.39, 111.49, 21.81, 21.76, 21.56 ppm. The
free porphyrin (594 mg, 0.423 mmol) was dissolved in CHCl3/MeOH (2:1;
was dissolved in CHCl3/MeOH (2:1; 100 mL) and ZnACHTUNTRGNE(UNG OAc)2 (347 mg,
1.6 mmol) was added. The mixture was stirred at room temperature over-
night. The solvent was then removed under vacuum and the residue was
purified by column chromatography (silica gel; hexane/CH2Cl2, 1:1) to
afford the desired product 8b (117 mg) in 91% yield; m.p. > 4008C;
1H NMR (CDCl3, 500 MHz): d=9.67 (d, J=5.0 Hz, 2H), 9.52 (s, 2H),
9.19 (s, 2H), 8.63 (d, J=5.0 Hz, 2H), 8.60 (d, J=5.0 Hz, 2H), 8.49 (d, J=
5.0 Hz, 2H), 8.43 (d, J=5.0 Hz, 2H), 8.42 (d, J=5.0 Hz, 2H), 7.96 (d, J=
8.0 Hz, 4H), 7.48 (d, J=8.0 Hz, 4H), 7.21 (s, 8H), 2.63 (s, 6H), 2.59 (s,
6H), 2.57 (s, 6H), 1.93 (s, 12H), 1.86 ppm (s, 12H); 13C NMR (CDCl3,
125 MHz): d=183.86, 153.19, 153.08, 152.56, 151.61, 150.66, 150.46,
148.38, 146.46, 141.52, 139.09, 138.88, 138.71, 138.10, 137.95, 137.83,
137.58, 137.52, 137.08, 136.98, 134.19, 134.11, 133.90, 132.69, 132.43,
131.98, 131.87, 131.72, 131.28, 127.94, 127.84, 127.63, 126.76, 123.57,
120.28, 110.71, 29.71, 21.74, 21.64, 21.52, 21.48, 21.44 ppm; HRMS
(MALDI-TOF): m/z calcd for C98H74N8O2Zn2: 1523.4590; found:
1523.4502; error=5.77 ppm; elemental analysis calcd (%) for
C98H74N8O2Zn2: C 77.11, H 4.89, N 7.34; found: C 76.98, H 4.99, N 7.23.
200 mL), ZnACHTUNGTRENNUNG(OAc)2 (464 mg, 2.12 mmol) was added, and the mixture was
stirred overnight under argon. The solvent was then removed under
vacuum and the residue was purified by column chromatography (silica
gel; hexane/CH2Cl2, 1:5) to afford the desired product 13 (614 mg) in
95% yield; m.p. > 4008C; 1H NMR ([D8]THF, 500 MHz): d=9.77 (s,
2H), 9.16 (s, 2H), 9.10 (d, J=4.5 Hz, 4H), 8.87 (d, J=4.5 Hz, 4H), 8.84
(d, J=4.5 Hz, 4H), 8.70 (d, J=4.5 Hz, 4H), 8.10 (m, 4H), 7.57 (d, J=
7.5 Hz, 4H), 7.34 (s, 8H), 2.70 (s, 6H), 2.64 (s, 12H), 1.93 (s, 12H),
1.92 ppm (s, 12H); 13C NMR (CDCl3
+ 1% pyridine, 125 MHz): d=
Compound 9: Triisopropylsilylethynyllithium (2m in THF, 10 mL) was
added to a solution of 8a (50 mg, 0.0328 mmol) in anhydrous THF
(5 mL) and the mixture was stirred at room temperature for 48 h. The
solvent was then removed under vacuum and the residue was purified by
column chromatography (silica gel; hexane/CH2Cl2, 1:1) to afford com-
pound 9 (27 mg) in 41% yield; m.p. > 4008C; 1H NMR (C2D2Cl4,
500 MHz, 1008C): d=9.40 (d, J=4.0 Hz, 2H), 9.00 (s, 2H), 8.52 (d, J=
190.98, 150.53, 150.37, 149.89, 146.31, 140.30, 139.54, 139.36, 139.27,
137.36, 137.30, 136.80, 134.48, 134.42, 132.62, 131.79, 131.54, 131.26,
130.64, 127.64, 127.05, 121.04, 119.43, 111.92, 31.24, 29.69, 29.66 ppm;
HRMS (MALDI-TOF): m/z calcd for C98H78N8O4Zn2: 1526.4831; found:
1526.4812; error=ꢀ1.23 ppm; elemental analysis calcd (%) for
C98H78N8O2Zn2: C 76.90, H 5.14, N 7.32; found: C 76.81, H 5.23, N 7.21.
16822
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16814 – 16824

Bridged Porphyrin Dimer
FULL PAPER
3.5 Hz, 2H), 8.45 (d, J=4.5 Hz, 2H), 8.31 (d, J=3.5 Hz, 2H), 8.23 (d, J=
4.0 Hz, 2H), 8.02 (d, J=3.5 Hz, 2H), 7.80 (d, J=6.5 Hz, 4H), 7.44 (d, J=
6.5 Hz, 4H), 7.22 (s, 4H), 7.07 (s, 4H), 2.59 (s, 6H), 2.57 (s, 6H), 2.49 (s,
6H), 2.02 (m, 6H), 1.95 (s, 12H), 1.86 ppm (s, 36H); due to poor solubili-
ty, a 13C NMR spectrum could not be acquired even after scanning for
24 h; HRMS (MALDI-TOF): m/z calcd for C120H115N8Ni2O2Si2 [M+1]:
1873.7383; found: 1873.7320; error=ꢀ3.4 ppm; elemental analysis calcd
(%) for C120H114N8Ni2O2Si2: C 76.92, H 6.13, N 5.98; found: C 76.86, H
6.22, N 5.89.
131.74, 131.62, 131.18, 130.47, 130.10, 129.39, 128.58, 127.66, 127.51,
127.15, 123.36, 122.96, 121.42, 29.71, 21.44, 21.41, 21.37, 21.04, 20.62 ppm;
HRMS (MALDI-TOF): m/z calcd for C116H96N8Zn2: 1728.6341; found:
1728.6321; error=ꢀ1.16 ppm; elemental analysis calcd (%) for
C
116H96N8Zn2: C 80.40, H 5.58, N 6.47; found: C 80.31, H 5.65, N 6.39.
Compound 10: Compound 8b (50 mg, 0.0328 mmol) was dissolved in an-
hydrous THF (5 mL) under nitrogen and 4-tert-butylphenylmagnesium
bromide (2m in THF, 10 mL) was added. The mixture was stirred at
room temperature for 48 h. The solvent was then removed under vacuum
and the residue was purified by column chromatography (silica gel;
hexane/CH2Cl2, 2:1) to afford compound 10 (21 mg) in 38% yield; m.p.
Acknowledgements
The work at Singapore was supported by a BMRC grant (10/1/21/19/642),
an MOE Tier 2 grant (MOE2011-T2–2–130), MOE AcRF FRC grant R-
143–000–510–112, the MINDEF-NUS JPP Program (MINDEF-NUS-
JPP-12–02–05), and IMRE core funding (IMRE/12–1P0902). The work at
Yonsei University was supported by World Class University (WCU) pro-
grams (R32–2010–10217–0) and an AFSOR/APARD grant (no. FA2386–
09–1–4092). We thank Professor Harry L. Anderson of Oxford University
for valuable discussion.
1
> 4008C; H NMR (CDCl3, 500 MHz): d=9.41 (s, 2H), 9.08 (s, 2H), 8.51
(d, J=3.5 Hz, 4H), 8.34 (d, J=3.5 Hz, 4H), 8.00 (d, J=3.5 Hz, 2H), 7.97
(d, J=3.5 Hz, 2H), 7.48 (d, J=7.0 Hz, 4H), 7.24 (m, 12H), 6.68 (s, 4H),
2.64 (s, 6H), 2.59 (s, 6H), 1.94 (s, 12H), 1.75 (s, 12H), 1.42 ppm (s, 18H);
due to poor solubility, a 13C NMR spectrum could not be acquired even
after scanning for 24 h; HRMS (MALDI-TOF): m/z calcd for
C
118H98Mg2N8O2: 1706.7514; found: 1706.7501; error=ꢀ0.76 ppm; ele-
mental analysis calcd (%) for C118H98Mg2N8O2: C 82.94, H 5.78, N 6.56;
found: C 82.98, H 5.80, N 6.52.
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Compound 14: Compound 13 (305 mg, 0.2 mmol) was dissolved in anhy-
drous THF (20 mL) under argon and 2-mesitylmagnesium bromide solu-
tion (1.0m in diethyl ether, 10 mL, 10 mmol) was added. The mixture was
stirred at room temperature for 36 h and then poured into water. The re-
sulting mixture was extracted with CH2Cl2 (100 mL) and the organic
layer was washed with water and dried over Na2SO4. The solvent was re-
moved under vacuum and the residue was purified by column chromatog-
raphy (silica gel; hexane/ethyl acetate, 3:1) to afford the desired product.
Recrystallization from CH2Cl2/MeOH afforded pure 14 as a red solid
(300 mg) in 85% yield; m.p. > 4008C; 1H NMR (CDCl3, 500 MHz): d=
9.26 (d, J=4.5 Hz, 2H), 8.95 (m, 8H), 8.88 (d, J=4.5 Hz, 2H), 8.84 (d,
J=4.5 Hz, 2H), 8.78 (d, J=4.5 Hz, 2H), 8.59 (d, J=4.5 Hz, 2H), 8.20 (d,
J=7.5 Hz, 2H), 8.17 (d, J=7.5 Hz, 2H), 7.59 (m, 4H), 7.36 (s, 4H), 7.34
(s, 4H), 7.28 (s, 2H), 6.72 (d, 2H, J=3 Hz), 5.37 (s, 4H), 2.74 (s, 6H),
2.69 (s, 6H), 2.67 (s, 6H), 2.04 (s, 6H), 1.97 (s, 6H), 1.86 (s, 6H), 1.75 (s,
6H), 1.27 (s, 12H), 1.20 ppm (s, 6H); 13C NMR (CDCl3, 125 MHz): d=
150.53, 150.20, 150.17, 149.99, 149.85, 149.75, 149.34, 141.72, 139.92,
139.59, 139.41, 139.34, 139.31, 139.23, 139.13, 139.08, 137.52, 137.45,
137.08, 136.24, 135.86, 134.57, 134.38, 132.76, 132.46, 132.42, 131.94,
131.73, 131.59, 130.66, 130.54, 130.38, 128.66, 127.77, 127.58, 127.41,
127.37, 120.60, 119.44, 119.08, 118.66, 22.12, 21.70, 21.66, 21.55, 19.88,
19.73 ppm; HRMS (MALDI-TOF): m/z calcd for
C116H102N8O2Zn2:
1766.6709; found: 1766.6781; error=4.08 ppm; elemental analysis calcd
(%) for C116H102N8O2Zn2: C 78.67, H 5.81, N 6.33; found: C 78.58, H
5.90, N 6.26.
Compound 1: Boron trifluoride diethyl etherate (3 mL) was added to
a solution of compound 14 (100 mg, 0.0566 mol) in dry CH2Cl2 (10 mL)
at room temperature under argon, whereupon a brown color immediately
developed. After 10 min, methanol (10 mL) and water (20 mL) were
added to quench the reaction. The organic layer was separated and dried
over Na2SO4. The solution was stirred with silica gel (0.5 g) in CH2Cl2
(10 mL) for 2 h in air to ensure complete oxidation. After filtration, the
solvent was removed from the filtrate under vacuum and the residue was
purified by flash column chromatography (silica gel; CH2Cl2) to afford
the target compound 1 as a light-green solid (15 mg, 15%); m.p. >
4008C; 1H NMR (CDCl3, 500 MHz): d=8.88 (d, J=4.0 Hz, 2H), 8.53 (d,
J=4.5 Hz, 2H), 8.48 (d, J=4.5 Hz, 2H), 8.46 (d, J=4.5 Hz, 2H), 8.41 (d,
J=4.5 Hz, 2H), 8.24 (d, J=4.5 Hz, 2H), 8.09 (s, 2H), 8.00 (d, J=4.5 Hz,
4H), 7.77 (s, 2H), 7.50 (d, J=8.0 Hz 4H), 7.31 (s, 4H), 7.19 (s, 4H), 7.16
(s, 4H), 2.68 (s, 6H), 2.66 (s, 6H), 2.55 (s, 6H), 2.48 (s, 6H), 2.43 (s,
12H), 1.93 (s, 12H), 1.86 ppm (s, 12H); 13C NMR (CDCl3 + 1% pyri-
dine, 125 MHz): d=150.65, 150.56, 149.60, 149.38, 149.16, 148.38, 147.16,
142.86, 140.05, 139.54, 138.89, 138.82, 138.80, 138.54, 138.49, 137.20,
137.04, 136.77, 136.51, 135.64, 135.44, 135.24, 133.86, 133.54, 132.61,
Chem. Eur. J. 2013, 19, 16814 – 16824
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: May 27, 2013
Revised: August 2, 2013
Published online: October 22, 2013
16824
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