Syntheses, crystal structure, and spectroscopic properties of meso-meso-linked porphyrin corrole hybrids

Article abstract of DOI:10.1246/cl.130324

Meso-meso directly linked porphyrin (Por)corrole (Cor) hybrids have been successfully achieved for the first time. The meso-meso linkage has been confirmed by X-ray diffraction analysis. The characteristic split Soret absorption band revealed the existenc

Full text of DOI:10.1246/cl.130324

doi:10.1246/cl.130324
936
Published on the web May 31, 2013
Syntheses, Crystal Structure, and Spectroscopic Properties
of Meso-Meso-Linked Porphyrin-Corrole Hybrids
Chen Chen, Yi-Zhou Zhu, Qiao-Jun Fan, Hai-Bin Song, and Jian-Yu Zheng*
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, P. R. China
(Received April 9, 2013; CL-130324; E-mail: jyzheng@nankai.edu.cn)
Meso-meso directly linked porphyrin (Por)-corrole (Cor)
hybrids have been successfully achieved for the first time. The
meso-meso linkage has been confirmed by X-ray diffraction
analysis. The characteristic split Soret absorption band revealed
the existence of strong exciton coupling between porphyrin and
corrole subunits. Time-resolved and steady-state fluorescence
has also been investigated.
R
b
c
d
a
R
NH HN
2
N
HN
N
N
N
N
N
N
N
N
i
ii
Ni
Ni
CHO
+
NH HN
R
h
e
g
f
3
1
Me
Me
F
F
F
F
Me
F
NO2 b:
c:
R=
a:
iii
In the past two decades, covalently linked multiporphyrin
arrays have attracted considerable interest for their great
potential applications, such as artificial photosynthetic systems,
molecular wires, sensors, and nonlinear optical devices.1 Among
the reported structures, meso-meso directly linked porphyrin
(Por) arrays possess an important position because they are
favorable for achieving rapid energy- and electron-transfer
owing to the direct linking and a short center-to-center distance
between Por subunits.2 The strong exciton interaction between
two adjacent Por rings imparts predominant spectroscopic and
photovoltaic properties to the molecules.3
R
R
N
HN
N
N
N
N
N
N
N
N
N
N
Co
R
PPh3
Ni
M
NH HN
R
N
N
6: (M=Ni) 7: (M=2H)
5
4
Corroles (Cors), one carbon short analogs of porphyrins
bearing a direct pyrrole-pyrrole linkage, have recently emerged
as an independent research area.4 In comparison with porphy-
rins, corroles exhibit some interesting properties, including
trianionic character, smaller cavity, lower oxidation potential,
higher fluorescence quantum yield, larger Stokes shift, and
relatively more intense absorption of red light.5 Many efforts
have been devoted to the syntheses of directly linked corrole
dyads.6 With the aim of realizing the combination of specific
properties of Por and Cor, several Por-Cor conjugates have
also been studied.7 To the best of our knowledge, the fascinating
meso-meso-linked porphyrin-corrole hybrids have not been
reported so far. As a continuous focus on the meso-meso-linked
porphyrinoids,8 we present herein the systematic investigation of
synthesis, structure, and photophysical properties of meso-meso
directly linked Por-Cor dyads.
Scheme 1. Synthesis procedures of Por-Cor dyads and
structures of reference compounds studied herein. Reagents
and conditions: (i) CH2Cl2, BF3¢Et2O, EtOH, 0 °C; (ii) CH2Cl2,
THF, DDQ; (iii) CH2Cl2, CH3OH, Co(OAc)2¢4H2O, PPh3.
the mutual ring current effect of the adjacent Por and Cor
chromophores.
The selected low-field chemical shift data of 3b, NiIIPor 6,
and free-base Cor 5b are shown in Figure S10.11 By carefully
analyzing the 1H-1H COSY spectrum (Figure S9)11 and the
previously reported data of corroles and meso-meso-linked
porphyrinoid arrays,2,10 the ¢-H of 3b has been assigned and
labeled by lower-case letter a-f in Scheme 1. Comparing to the
references 6 and 5b, both the inner ¢-protons of porphyrin (Ha
and Hb) and corrole (He and Hf) in 3b show significant upfield
shift due to the large shielding effect of macrocycles.
An acid-catalyzed 2 + 1 condensation method with rela-
tively accessible dipyrromethane and meso-formylporphyrin was
adopted to avoid the difficulty in derivatization of corroles.9
After screening the reaction conditions, BF3¢Et2O/EtOH was
found to be the most efficient catalyst, and the general procedure
was carried out in CH2Cl2 at 0 °C with a subsequent oxidation
by DDQ (Scheme 1). Compounds 3 have been obtained with a
moderate yield of 8% for 3a, 17% for 3b, and 20% for 3c,
respectively. HR-MALDI-TOF mass spectra of all three target
molecules showed obvious [M + H]+ peaks. The 1H NMR
spectrum of 3a displays broad signals, which is consistent with
reported free-base corroles possessing unhindered substituents.10
The spectra of 3b and 3c show much clearer splitting due to the
steric effect of introducing 2,6-substituents. That will facilitate
the assignment of proton signals and the determination of
More intuitive evidence for the formation of meso-meso
linkage was provided by crystallographic analysis of 4a, a cobalt
coordinated complex of 3a (Figure 1). X-ray quality crystals of
4a were obtained by slow diffusion of CH3OH to a CH2Cl2
solution of 4a.12 Porphyrin and corrole subunits in the dyad 4a
are attached to each other at the meso position through a 1.50 ¡
C-C bond, and the dihedral angle between the two planes is 86°.
Meanwhile, the nickel(II)-porphyrin moiety of 4a displays
slight distortion, while the plane of corrole moiety retains planar
conformation with a 2.21 ¡ P-Co bond.
The absorption spectra of 3c, the reference monomer 5c and
6 are shown in Figure 2. Compound 3c displays an obvious split
Soret band. The high-energy Soret band of 3c occurs at the same
wavelength with respect to that of monomer Cor 5c and Por 6
(414 nm), while a new low-energy Soret band appears at 445 nm.
Chem. Lett. 2013, 42, 936-938
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

937
F
F
F
F
F
F
F
F
F
F
N
NH
NH
N
N
N
N
N
NH
NH
NH
N
N
CH2Cl2
Ni
conc. HCl
TFA
NH
NH
HN
F
F
F
F
F
F
F
F
F
F
3c
8
Scheme 2. Synthesis procedure of free-base Por-Cor dyad.
Figure 1. X-ray crystal structure of 4a. Hydrogen atoms are
omitted for clarity. The thermal ellipsoids are scaled to 30%
probability.
Figure 3. Absorption and emission spectra (-ex = 420 nm) of
dyad 8, reference compound 5c and 7 in toluene.
Table 1. Fluorescence quantum yield and fluorescence lifetime
of compound 5c, 7, and 8 in toluene
a
Compound
ΦF
¸/nsa
5c
7
8
0.16
0.11
0.14
3.9
9.6
6.7
Figure 2. Electronic absorption spectra of dyad 3c, reference
compound 5c and 6 in CHCl3.
aMeasured at 298 K.
Such results reflect a strong exciton coupling between the
neighboring porphyrin and corrole.2 The Q-bands of 3c represent
the sum of Q-band of 5c and 6 with a slight bathochromic-shift.
This should be attributed to the weak electronic coupling
between the directly linked chromophores with a nearly
orthogonal conformation supported by the DFT calculations
(Figure S43).2e,11
Since the fluorescent emission of porphyrin was seriously
quenched by the introducing of Ni atom, a demetalization
process is needed before exploring the luminescence properties
of meso-meso directly linked Por-Cor hybrid. We initially
treated 3c with the mixture of trifluoroacetic acid (TFA) and
concentrated H2SO4, a commonly used method, but severe
decomposition was observed. In view of the lability of corroles
toward oxidant, we modified the reaction by using concd HCl
instead of concd H2SO4 and free-base 8 was obtained in 50%
yield (Scheme 2).13 A typical signal of inner 2H of porphyrin
appeared at ¹2.18 ppm (Figure S36).11
also exhibits obvious split Soret bands in toluene because of the
same reason mentioned above. As shown in the inset of
Figure 3, there is a large overlap between the emission of corrole
5c and porphyrin 7. The emission of corrole 5c onsets at higher
energy with respect to porphyrin 7 (600 nm compared to
625 nm), but its maximum emission (659 nm) is slightly longer
than 652 nm for 7. Comparing to the second distinct band at
720 nm of 7, 5c only shows a shoulder in the same region.7b
The fluorescence quantum yield ΦF and lifetime ¸ has further
been determined (Table 1 and Figure S4211), ΦF = 0.11 with
¸ = 9.6 ns for 7 and ΦF = 0.16 with ¸ = 3.9 ns for 5c are in
agreement with the literature data.5,7b A sum of the emission
features of 5c and 7 have been observed for dyad 8, and ΦF and
¸ have been evaluated to be 0.14 and 6.7 ns, respectively. The
values locate between those of 5c and 7. This phenomenon could
be rationalized by the existence of reversible energy transfer
tendency between porphyrin and corrole subunit in dyad 8
because of their nearly iso-energetic singlet excited states (both
extending up at 770 nm). That has been found and demonstrated
by Gryko in amide-linked porphyrin-corrole dyads.7b
Figure 3 showed absorption and emission spectra of free-
base 8, Cor 5c, and Por 7. Similar to compound 3c, the dyad 8
Chem. Lett. 2013, 42, 936-938
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

938
In summary, meso-meso directly linked porphyrin-corrole
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hybrids have been successfully prepared and fully characterized
by NMR, HR-MS, and X-ray diffraction analysis. Crystal
structure and calculation results show that Por and Cor moiety
in the dyads adopt orthogonal conformations, which results in
minimizing the conjugate of neighboring chromophores. Ob-
vious split Soret bands observed in absorption spectra reflect
the existence of strong exciton coupling between the adjacent
porphyrin and corrole. Luminescence properties of the free-base
dyad indicate that reversible energy transfer should exist in the
dyads.
The authors wish to thank the 973 Program (No.
2011CB932902), NSFC (Nos. 21172126 and 21272123) for
their generous financial support.
7
8
9
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12 Crystal data for 4a: CCDC No. 928176, triclinic, space
ꢀ
group P1, a = 16.509(3) ¡, b = 16.672(2) ¡, c =
21.029(3) ¡, ¡ = 67.256(10)°, ¢ = 76.733(12)°, £ =
¹1
68.391(9)°, V = 4937.1(12) ¡3, Z = 2, ® = 0.455 mm
R = 0.0770, wR = 0.1854.
,
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Chem. Lett. 2013, 42, 936-938
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/