Covalently Attached Porphycene-Ferrocene Dyads: Synthesis, Redox-Switched Emission, and Observation of the Charge-Separated State

Article abstract of DOI:10.1021/acs.inorgchem.5b02078

Two new porphycenes functionalized with ferrocenyl pendants have been synthesized and characterized spectroscopically and structurally. The porphycene-based emission in porphycene-ferrocene dyads was switched on and off by the reversible control of the ferrocenyl pendant redox states. Transient absorption spectroscopy with a femtosecond laser-pulsed technique has successfully detected the picosecond charge-separated excited state of the dyad upon Q-band excitation of the porphycene ring.

Full text of DOI:10.1021/acs.inorgchem.5b02078

Communication
pubs.acs.org/IC
Covalently Attached Porphycene−Ferrocene Dyads: Synthesis,
Redox-Switched Emission, and Observation of the Charge-Separated
State
Masaaki Abe,*,†,‡ Hiroaki Yamada,† Toru Okawara,†,§ Mamoru Fujitsuka,⊥ Tetsuro Majima,*,⊥
and Yoshio Hisaeda*,†,∥
†Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka
819-0395, Japan
⊥Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan
∥Center for Molecular Systems, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
S
* Supporting Information
charge-separated (CS) excited state of the dyad has been
ABSTRACT: Two new porphycenes functionalized with
ferrocenyl pendants have been synthesized and charac-
terized spectroscopically and structurally. The porphycene-
based emission in porphycene−ferrocene dyads was
switched on and off by the reversible control of the
ferrocenyl pendant redox states. Transient absorption
spectroscopy with a femtosecond laser-pulsed technique
has successfully detected the picosecond charge-separated
excited state of the dyad upon Q-band excitation of the
porphycene ring.
successfully characterized by femtosecond-pulsed transient
absorption spectroscopy. Despite numerous studies on ferro-
cene-appended porphyrins,13 porphycene has been decorated
only through axial binding to the metal center.14
Compounds 2 and 3 were synthesized by reacting 1-OH with
ferrocenyl acid chlorides FcC(O)Cl and FcCH2C(O)Cl,
respectively, in dry N,N′-dimethylformamide−CH2Cl2 in the
presence of 4-(dimethylamino)phenol. After column chroma-
tography and recrystallization, compounds 2 and 3 were
obtained as deep-purple solids in 23 and 34% yield, respectively,
and were fully characterized by spectroscopic means and
electrospray ionization time-of-flight mass spectrometry.15
The molecular structure of 3 was determined by single-crystal
X-ray diffraction analysis and is shown in Figure 1. The
orphycene,1,2 a constitutional isomer of porphyrin, has a
P
rectangular 18π-electron system of the tetrapyrrolic macro-
cycle and shows intriguing aspects from basic science to
applications, including catalysis,3 hydrogen-atom-transfer dy-
namics,4 coordination chemistry,5 photodynamic therapy,6
small-molecule binding,7 and materials chemistry.8 Relative to
the porphyrins, the porphycenes show red-shifted Q bands with
larger intensity, which is ascribed to the symmetry reduction of
the tetrapyrrolic framework. In order to tune the physicochem-
ical properties of the porphycene and to develop porphycene-
based advanced functions, the choice of substituents on the
periphery of the porphycene ring is crucial. The modification of
pyrrole β positions has been extensively studied.2,9 In contrast,
meso-position modification, in which ethylenic carbon atoms in
porphycene are relatively inert for activation, is limited.10
In this work, we have successfully introduced a redox-active
ferrocenyl (Fc) group at the 9 position of 2,7,12,17-tetra-n-
propylporphycene (H2TPrPc, 1-OH) as the first example of
porphycene bearing a functional coordination motif at the
porphycene periphery through covalent attachment. Among the
meso-modified porphycenes, a 9-hydroxy-substituted derivative
of 1-OH,11 which is obtained by reacting 9-acetoxy-2,7,12,17-
tetra-n-propylporphycene (1-OAc)12 with sodium methoxide in
dry tetrahydrofuran (THF),11a is useful as a synthetic precursor
for tagging functional pendants selectively at the 9 position of the
porphycene ring. In this Communication, two new porphycene−
ferrocene dyads, 2 and 3, are reported with their redox and
photoluminescent properties. Electrochemical control of the
porphycene-based red emission has been achieved. Also, the
Figure 1. Molecular structure of 3·CH2Cl2 at 100 K (50% probability
level): (a) top view; (b) side view.
introduction of the Fc pendant to the 9 position of the
porphycene ring was unambiguously identified (Figure 1a), and
the iron atoms are separated from the center of the porphycene
ring by 10.5 Å. The porphycene ring is almost planar (Figure 1b).
Cyclic voltammetry (CV) profiles of 2 and 3, along with those
of reference compounds 1-OAc and PhOC(O)Fc, in 0.1 M n-
Received: September 10, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02078
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Bu4NPF6−CH2Cl2 are depicted in Figure 2. Compound 1-OAc
underwent two successive reductions (E1/2 = −1.22 and −0.86 V
Figure 3. Emission spectra (CH2Cl2, 298 K) of (a) 1-OAc, (b) 1-OAc +
ferrocene (1:1 mixture), (c) 2, and (d) 3. λex = 563 nm (absorbance =
0.1). Inset: Pictures showing CH2Cl2 solutions of parts a−d upon UV
excitation.
Figure 2. CV profiles (0.1 M n-Bu4NPF6−CH2Cl2; v = 100 mV s−1) of
(a) 1-OAc, (b) PhOC(O)Fc, (c) 2, and (d) 3. Concentration of the
compounds = 1.0 mM. Working electrode = glassy carbon. Counter
electrode = platinum coil. Reference electrode = Ag/AgCl.
vs Ag/AgCl) and one oxidation (E1/2 = +1.03 V), all of which are
ascribed to porphycene-based processes11 (Figure 2a). The
PhOC(O)Fc compound showed (Figure 2b) a one-electron
oxidation wave at E1/2 = +0.79 V. Compound 2 showed a CV
profile that is ascribed to two distinct redox centers, Fc and
porphycene, with redox potentials almost identical with those of
the corresponding monomers (Figure 2c), indicating negligible
electronic communication between the two centers in the
ground state. For 3 (Figure 2d), the Fc-based wave was
significantly shifted to the negative side (E1/2 = +0.55 V vs +0.81
V for 2) as a result of a decreasing inductive effect of CO upon
insertion of a methylene spacer.
UV−vis spectroelectrochemistry (0.1 M n-Bu4NPF6−
CH2Cl2) was studied for 3 to confirm the assignment of the
two oxidation processes in CV.15 In the first oxidation process,
only a minimal spectral change was observed because of the
occurrence of an oxidation at the Fc site. In the second oxidation
process, a substantial spectral change was then observed with
clean isosbestic points at 660, 540, 400, and 390 nm to give a
spectrum similar to that of a monocationic radical of H2TPrPc,16
confirming that the second oxidation takes place at the
porphycene ring.
Free-base porphycenes are emissive in red upon UV- and
visible-light excitation. As shown in Figure 3, compound 1-OAc
is emissive at 645 nm with a shoulder at 700 nm at 298 K.
However, the emissions from 2 and 3 are considerably weak,
suggesting that the attachment of the Fc pendant is responsible
for quenching of the porphycene-based emission in an
intramolecular fashion. Observation of the porphycene-based
emission of 1-OAc in the presence of 1 equiv of ferrocene ruled
out an intermolecular quenching mechanism. Emission quantum
yields (toluene, 298 K, λex = 370 nm) were determined to be 2.2
and 1.1% for 2 and 3, respectively, relative to 28% for 1-OAc.
The porphycene-based emission from the dyads was switched
on and off by changing the oxidation level of the Fc pendant.17
Figure 4 shows emission spectral changes of 2 and 3 upon
electrochemical oxidation of the Fc pendant to Fc+. When the
potential was applied at +0.90 V to convert 2 to the cationic
radical 2+, the initial tiny emission (650 nm) was continuously
increased (Figure 4a). The identical behavior was also obtained
for 3 (Figure 4b). These observations indicate that the quenching
Figure 4. Luminescence recovery upon one-electron oxidation of the
ferrocenyl pendant for (a) 2 and (b) 3 in CH2Cl2 containing n-Bu4NPF6
(0.1 M). Top: Luminescence spectra. λex = 563 nm. Bottom: CV profiles
and electrode potentials applied. Optical path length = 0.5 mm. Working
electrode = platinum mesh. Counter electrode = platinum rod.
Reference electrode = Ag/AgCl. T = 298 K.
ability of the neutral Fc pendant is reduced when it is oxidized to
Fc+. The on/off behavior in emission was reversible. For neutral
dyads, Fc behaves as an efficient electron donor to the Fc-
localized excited state, allowing quenching of the porphycene-
localized emission to be observed. In contrast, the cationic Fc+
pendant prepared electrochemically is no more efficient as an
electron donor, thereby allowing the intensity of the porphycene-
based emission to be increased.
Femtosecond-pulsed transient absorption spectroscopy was
successfully used to study the excited-state property of 3.14
Transient absorption spectra of 3 in THF with excitation at 620
nm (4 μJ pulse−1) are presented in Figure 5a. After excitation of
0.4 ps, a broad absorption band was observed in the near-IR
region (850−950 nm), which was assigned to the formation of a
singlet excited state of the porphycene ring.14 This absorption
band subsequently decayed continuously until 20.4 ps to give a
distinct spectrum with new peaks at 850 and 750 nm due to the
porphycene radical anion,18 which suggested the formation of a
CS state as a result of intramolecular electron transfer from Fc to
porphycene in the excited state. An absorption band of Fc+
(∼800 nm), however, was not detected because of its
comparatively smaller intensity (ε ∼ 103 M−1 cm−1).19 Curve
B
DOI: 10.1021/acs.inorgchem.5b02078
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
15H00952) and “Coordination Programming” (Grant
24108730), Grants-in-Aid for Scientific Research (A) (Grant
21245016) and (B) (Grant 25288031), the Global COE
Program “Science for Future Molecular Systems”, and the
Nanotechnology Platform (Grant NPS13103) from MEXT.
M.A. also acknowledges financial support from the Tokuyama
Science Foundation.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02078.
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Experimental details (PDF)
X-ray crystallographic data of 3 in CIF format (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: mabe@sci.u-hyogo.ac.jp (M.A.).
*E-mail: majima@sanken.osaka-u.ac.jp (T.M.).
*E-mail: yhisatcm@mail.cstm.kyushu-u.ac.jp (Y.H.).
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Present Addresses
‡M.A.: Graduate School of Material Science, University of
Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297,
Japan.
(14) Maeda, D.; Shimakoshi, H.; Abe, M.; Fujitsuka, M.; Majima, T.;
Hisaeda, Y. Inorg. Chem. 2010, 49, 2872−2880.
(15) Details are given in the Supporting Information.
§T.O.: Department of Creative Engineering, National Institute of
Technology, Kitakyushu College, 5-20-1 Shi-i, Kokuraminami-
ku, Kitakyushu 802-0985, Japan.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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(19) Kubo, M.; Mori, Y.; Otani, M.; Murakami, M.; Ishibashi, Y.;
Yasuda, M.; Hosomizu, K.; Miyasaka, H.; Imahori, H.; Nakashima, S. J.
Phys. Chem. A 2007, 111, 5136−5143.
We appreciate financial support from Grants-in-Aid for Scientific
Research on Innovative Areas “Stimuli-responsive Chemical
Species for the Creation of Functional Molecules” (Grant
C
DOI: 10.1021/acs.inorgchem.5b02078
Inorg. Chem. XXXX, XXX, XXX−XXX