Synthesis and photochemical properties of cationic cyclopentadienyliron containing arylazo chromophores

Article abstract of DOI:10.1016/j.inoche.2011.06.011

Two kinds of cationic cyclopentadienyliron complexes containing arylazo chromophores (Azo-Fc) were synthesized through the nucleophilic aromatic substitution reactions of cyclopentadienyliron chlorobenzene complexes. The synthesized Azo-Fc was characterized by LC-MS, IR, and ¹H NMR. The influence of substituents and the aryl ring on spectral data of the synthesized compounds is described. Unlike other cationic cyclopentadienyliron complexes, Azo-Fc hardly underwent photolysis and showed no photo-initiating abilities in the cationic polymerization under photo-irradiation; in addition, trans/cis isomerization by irradiation is possible at different wavelengths.

Full text of DOI:10.1016/j.inoche.2011.06.011

Inorganic Chemistry Communications 14 (2011) 1516–1519
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and photochemical properties of cationic cyclopentadienyliron containing
arylazo chromophores
Guang-Lei Li , Hui Ye , Yu Chen , Bao-Dong Zhao , Tao Wang a,b,⁎
a
a
a
a
a
Department of Organic Chemistry, College of Science, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
State Key Lab of Chemical Resource Engineering, College of Science, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Two kinds of cationic cyclopentadienyliron complexes containing arylazo chromophores (Azo-Fc) were
Received 15 March 2011
Accepted 7 June 2011
Available online 14 June 2011
synthesized through the nucleophilic aromatic substitution reactions of cyclopentadienyliron chlorobenzene
complexes. The synthesized Azo-Fc was characterized by LC–MS, IR, and 1H NMR. The influence of
substituents and the aryl ring on spectral data of the synthesized compounds is described. Unlike other
cationic cyclopentadienyliron complexes, Azo-Fc hardly underwent photolysis and showed no photo-
initiating abilities in the cationic polymerization under photo-irradiation; in addition, trans/cis isomerization
by irradiation is possible at different wavelengths.
Keywords:
Cationic cyclopentadienyliron complex
Synthesis
Photochemistry
© 2011 Elsevier B.V. All rights reserved.
Trans/cis isomerization
Azo compounds, containing two phenyl rings separated by an
azo (\N_N\) bond, are versatile molecules that have received
much attention both in fundamental studies and in applications
which include textile fiber dyeing, biomedical studies, photo-
electronic applications, printing systems, optical storage technology,
and photochemistry [1–5]. Azo compounds which contain a metallic
atom in structure posses the combined properties of the metal and the
azo group, producing materials that can be processed with interesting
magnetic, electronic, optical, and anisotropic properties. Transition
metals present improve the physical properties of these compounds
were synthesized. Unlike other cationic cyclopentadienyliron com-
plexes, Azo-Fc does not easily undergo photolysis and shows no
photo-initiating abilities in cationic photopolymerization under
photoirradiation. However, it can undergo reversible trans/cis
isomerization by photo-irradiation with different wavelengths.
To investigate the effects of the conjugated structure of azobenzene
on spectral character and isomerization behavior, two kinds of cationic
cyclopentadienyliron complexes containing arylazo chromophores
were selected: (η6-4-phenylazodiphenylether) (η5-cyclopentadienyl)
iron hexafluorophosphate (Azo-Fc-1-3) and (η6-1-(4-phenylazo)
naphthoxyl benzene) (η5-cyclopentadienyl) iron hexafluorophosphate
(Azo-Fc-4-6). Because substituents can cause some degree of electronic
effect on the azobenzene, the chloro and methyl groups were
introduced to the phenyl rings respectively. Reversible trans/cis
isomerization makes Azo-Fcs attractive because of its numerous
possible applications to molecular memories and switches. For this
reason, the synthesis and photochemical properties of Azo-Fcs were
studied.
Here, cationic cyclopentadienyliron complexes containing the azo
group were synthesized from chlorobenzene cyclopentadienyliron
complexes and arylazo (Scheme 1). The chlorine atoms of the
chlorobenzene cyclopentadienyliron complex are attacked by nucleo-
philes due to high electrophilicity, whereas the inversion of polarity is
due to electron withdrawal initiated by the metallic moiety. Azo-Fcs
were characterized using 1HNMR, UV–visible and IR spectroscopy, as
well as mass spectrometry.
[6,7].
Some complexes can reversibly isomerize by light irradiation [8–10].
Reversible trans-cis photo-isomerization of azo chromophores may
be used to design and exploit novel photoactive materials [11–13].
Abd-El-Aziz reported the synthesis of a number of neutral and cationic
iron-containing polymers with azo dyes either directly placed in the
backbone or as pendants to the polymer [14–18]. However, reports
on the photochemical and thermal properties of monomolecular
cationic cyclopentadienyliron complexes containing the azo group
remain scarce.
Recently, a great number of cationic cyclopentadienyliron com-
plexes were prepared and studied in the laboratory [19,20]. The
studies were mainly focused on the photo-initiating ability of
these complexes in cationic polymerization systems under visible
light. To improve absorption in the visible light region, several new
cationic cyclopentadienyliron complexes containing the azo group
The strong electronic absorption maximum of the azo compound
can be changed by the conjugated ring. The UV-visible absorption
⁎
Corresponding author at: Beijing University of Chemical Technology 144#, Beijing
−
4
spectra of Azo-Fcs in the 10
in Fig. 1 and Table 1.
2 2
M CH Cl solution at 298 K are shown
1
1
00029, People's Republic of China. Tel.: +86 13691179175; fax: +86 10 64445350.
E-mail address: wangtwj2000@163.com (T. Wang).
387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.06.011

G.-L. Li et al. / Inorganic Chemistry Communications 14 (2011) 1516–1519
1517
Scheme 1. Synthesis route of the cationic cyclopentadienyliron complexes containing the azo group. Reagents and conditions of diazotization: aniline or substituted aniline, –5 to 0 °C;
reagents and conditions of coupling reaction: phenol or 1-naphenol, –5 to 0 °C; reagents and conditions of nucleophilic substitution reaction: cyclopentadienyliron complexes chlorobenzene,
DMF, K CO , 120 °C, 3–5 h.
2 3
The electronic spectra of Azo-Fcs show three absorption bands;
absorption in regions 220–250 and 300–400 nm was strong, owing to
the π–π⁎ electronic transitions involving the whole conjugated
system. The whole conjugated system includes both phenyl rings
and azo groups. In the region 400–550 nm, absorption is weak, due
to the d–π⁎ transition from a Fe(II) d-orbital to a π -orbital of the
⁎
⁎
bridging ligand and azo n–π transition. In contrast with the spectra of
Azo-Fc-1-3, the strong absorption due to π–π⁎ electronic transitions
involving the whole conjugated system, is bathochromically shifted
by almost 50–60 nm because in Azo-Fc-4, Azo-Fc-5, and Azo-Fc-6, the
whole conjugated system includes the phenyl rings, the naphthalene
ring, and the azo groups.
Ferrocenium salts have been used as cationic photoinitiators in the
photo-polymerization of epoxy systems. They undergo photolysis
to generate an iron-based Lewis acid which can coordinate with
an epoxy monomer and cause epoxy ring-opening polymerization.
Photolysis of the photoinitiator is very important; upon evaluation
of the experiments on the synthesized Azo-Fc used as photoinitiators
in the polymerization of epoxy systems, Azo-Fc showed no photo-
initiating ability for epoxy compounds. Thus, the photochemistry of
the synthesized Azo-Fcs needs to be studied.
LC–MS and UV–vis spectroscopy were used to analyze the changes
in Azo-Fcs after irradiation. The products of photolysis stated in
literature, i.e., ferrocene, inorganic ferric salts, or the azobenzene
ligand, were not found.
At the start of irradiation, Azo-Fcs exhibited similar spectral
⁎
changes upon photoirradiation. Absorbance of the azo π–π electronic
Table 1
UV–vis absorption spectral data of Azo-Fcs.
λ
max,1
ε
(M
max,1
λ
(nm)
max,2
ε
(M
max,2
λ
(nm)
max,3
ε
(M
max,3
−
1
cm−1
−1
cm−1
−1
cm−1
)
(nm)
)
)
Azo-Fc-1 239.5 29640
Azo-Fc-2 238.7 20400
Azo-Fc-3 240.4 26520
Azo-Fc-4 235.9 32640
Azo-Fc-5 232.2 34170
Azo-Fc-6 239.5 31370
334.8 32580
325.6 22530
336.6 25930
379.1 17090
371.9 17680
375.5 19070
438.2 790
439.1 580
442.8 584
457.3 191
447.3 157
447.9 167
2 2
Fig. 1. UV–visible absorption spectra of Azo-Fc in CH Cl solution (concentration=
−
4
1
×10 M).

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G.-L. Li et al. / Inorganic Chemistry Communications 14 (2011) 1516–1519
transition band decreases, whereas that of the azo n–π⁎ transition
band increases. As shown by the changes in the UV–vis spectra
of Azo-Fcs in CH Cl upon irradiation, Azo-Fcs did not undergo
2 2
photolysis. This is different from the UV–vis spectral changes of
other previously studied ferrocenium salts. However, this change
in absorption spectrum is similar to that reported by Hiroshi
Nishihara et al. about the photo-isomerization behavior of some
ferrocenylazobenzenes [21,22].
Fig. 2(A), gives two Azo-Fc-2 spectra. The left part shows the
spectral change of Azo-Fc-2 by irradiation with the unfiltered halogen
lamp. The right part shows the spectral change of Azo-Fc-2 by
irradiation with a wide pass-band filter which can cut off light
⁎
N460 nm. Absorbance of the azo π–π electronic transition bands
decreased. However, subsequent irradiation with light N460 nm
⁎
recovered part of the intensity of the azo π–π band. From literature
[23], this spectroscopic behavior is characteristic of the reversible
trans-to-cis isomerization of azobenzene. In Fig. 2(B), two Azo-Fc-4
spectra are given. Different from that of Azo-Fc-2, the filter functioned
Fig. 3. 1H-NMR spectra of Azo-Fc before and after irradiation. Irradiation condition:
2
halogen lamp, emitting wavelength 380–550 nm, 50 W, I=5 mW/cm .
as an interference filter which only allows 540 nm light to go through.
Reversible trans-to-cis isomerization also occurred.
1
Trans-to-cis photo-isomerization behavior was supported by HNMR.
Fig. 3 shows the 1HNMR spectra of Azo-Fc-1 and Azo-Fc-6 before and
after irradiation under halogen irradiation without filtering for 3 min.
1
The H NMR signals of Azo-Fc-1 and Azo-Fc-6 shifted to up-field positions
after irradiation with visible light, which is consistent with the report in
literature [24].
In conclusion, two series of cationic arylazo cyclopentadienyliron
complexes with different substitutes were conveniently synthesized.
UV–vis results show that Azo-Fc has a strong absorption in the
3
00–450 nm region. The introduction of naphthalene in Azo-Fc-4-6
leads to a substantial red-shift of the absorption and the change in
absorption spectra of Azo-Fc caused by photoirradiation at different
wavelengths shows that Azo-Fcs undergo reversible trans/cis isom-
erization, making these compounds attractive owing to their possible
applications in molecular memories and switches.
Fig. 2. (A) The UV–vis absorption spectral change of Azo-Fc-2 (1×10− M) in acetonitrile
4
upon photoirradiation with an unfiltered halogen lamp at room temperature (left) and
Acknowledgements
subsequent irradiation with a monochromatic light at 460 nm (right). (B) The UV–vis
absorption spectral change of Azo-Fc-4 (1×10− M) in CH
4
Cl
upon photoirradiation with
2
2
The authors wish to thank for financial support of national natural
an unfiltered halogen lamp at room temperature (left) and subsequent irradiation with a
monochromatic light at 540 nm (right).
science foundation of China (project grant no. 20676012).

G.-L. Li et al. / Inorganic Chemistry Communications 14 (2011) 1516–1519
1519
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