Research advances in the synthesis and applications of ferrocene-based electro and photo responsive materials: Ferrocene-based electro and photo responsive materials.

Abstract

Individually, ferrocene and photochromic materials have several important attributes and applications. Ferrocene is a promising candidate of stimuli‐responsive materials due to its electronic, electrochemical and magnetic properties. On the same lines, photo‐sensitive materials are also an important class of stimuli‐responsive materials which can be modified according to the needs. Recently, a lot of keen attention has been given to the combination of ferrocene with different photo‐sensitive moieties to get novel functional materials. This review illustrates the research advances in the combination of ferrocene‐based small molecules, self‐assembled monolayers and polymers with photo‐responsive compounds and their potential applications.

Full text

REVIEW
Research advances in the synthesis and applications of
ferrocene‐based electro and photo responsive materials
Amin Khan | Li Wang | Haojie Yu | Muhammad Haroon | Raja Summe Ullah |
Ahsan Nazir | Tarig Elshaarani | Muhammad Usman | Shah Fahad | Fazal Haq
State Key Laboratory of Chemical
Engineering, College of Chemical and
Biological Engineering, Zhejiang
University, Hangzhou 310027, China
Correspondence
Prof. Li Wang and Haojie Yu, State Key
Laboratory of Chemical Engineering,
College of Chemical and Biological
Engineering, Zhejiang University,
Hangzhou 310027, China.
Email: opl_wl@dial.zju.edu.cn;
hjyu@zju.edu.cn
Funding information
National Natural Science Foundation of
China, Grant/Award Numbers:
21611530689, 51811530097, 51673170 and
21472168
Individually, ferrocene and photochromic materials have several important
attributes and applications. Ferrocene is a promising candidate of stimuli‐
responsive materials due to its electronic, electrochemical and magnetic prop-
erties. On the same lines, photo‐sensitive materials are also an important class
of stimuli‐responsive materials which can be modified according to the needs.
Recently, a lot of keen attention has been given to the combination of ferrocene
with different photo‐sensitive moieties to get novel functional materials. This
review illustrates the research advances in the combination of ferrocene‐based
small molecules, self‐assembled monolayers and polymers with photo‐respon-
sive compounds and their potential applications.
KEYWORDS
Advanced materials, Ferrocene, Organometallic, Photo responsive, Redox responsive
1|INTRODUCTION
Stimuli‐responsive materials have gained much attention
during the past few decades due to stability and flexibil-
ity in their structures.
[1–4]
They can show alternation and
modification in their properties in response to the exter-
nal stimuli, such as light, redox, temperature, pH and
ionic strength.
[5,6]
Among all the available physical stim-
uli, redox and light have dragged much attention.
[7–9]
Redox materials show a change in their electrochemical
behavior due to the loss/gain of electrons. According to
the IUPAC system, the redox materials carry some
groups which can be reduced/oxidized reversibly in
response to the redox stimulus.
[10]
A redox process
depends on the changes in properties of the materials.
Due to these changes (oxidation or reduction), the mate-
rial usually shows different chemical, electronic, optical
and mechanical properties.
[11,12]
These materials can be
used as electrochemical devices such as batteries,
electrochromic devices, optoelectronic devices, biosen-
sors and biofuel cells.
[10,13,14]
Among different
organometallic materials, ferrocene and its derivatives
have gained much importance in this era due to their
electrochemical activity, liquid crystallinity, thermal
and photochemical stability.
[15–17]
Ferrocene–ferricenium
redox system proved useful for different polymeric and
liquid crystalline materials.
[18,19]
The robustness of
ferrocene is the result of its 18‐e structure as compared
with its neighboring late‐transition‐metal neutral
metallocenes with odd‐number‐electron structures such
as 17‐e for manganocene and 19‐e for cobaltocene.
Cobaltocene (19‐electron metallocene) is also considered
good at stability however due to the high chemical stabil-
ity of cobaltocenium, it is challenging to make
cobaltocenium containing derivatives by direct substitu-
tion on cyclopentadiene (Cp) ring, thus presenting obsta-
cles to synthesize well‐defined cobaltocenium‐containing
materials with good solubility. In comparison with other
organometallic materials, robust isoelectronic 18‐e ferro-
cene‐type structures provide high thermal stability, solu-
bility in all common organic solvents, reactivity as a
strong electrophile, stability in air and mild/reversible
Received: 18 May 2018 Revised: 1 August 2018 Accepted: 2 August 2018
DOI: 10.1002/aoc.4575
Appl Organometal Chem. 2018;e4575.
https://doi.org/10.1002/aoc.4575
© 2018 John Wiley & Sons, Ltd.wileyonlinelibrary.com/journal/aoc 1of28

oxidation. All these characteristics made ferrocene an
icon in organometallic chemistry.
[20–22]
Photo‐responsive materials also got attention due to
change in their physical and chemical characteristics
upon irradiation at specific wavelength.
[23–25]
Light as
one of the most influencing parameter can easily be mod-
ulated according to the system ease.
[26–30]
Photo‐respon-
sive materials are synthesized by the incorporation of
photo‐sensitive moieties, such as azobenzene,
[31,32]
stil-
bene,
[33]
spiropyran,
[34]
spirooxazines
[35]
or fulgides
[36]
into the polymeric networks. It is also reported that the
ferrocene‐based system affects the isomerization behavior
of photoresponsive moieties.
[37]
Based on this phenome-
non, much attention has been given to the ferrocene‐
based photochromic systems to extend their existing
applications such as information storage devices,
[38]
molecular switches,
[39]
photo‐mechanical systems,
[40]
micro patterning
[41]
and nonlinear optical media
[42]
to
get multifunctional systems.
[43,44]
This attractive behavior
led the focus towards the combination of electro and
photochromic moieties. In this respect, the combination
of ferrocene with a photoresponsive material can pro-
vide multi responsive molecules.
[45–47]
According to
the best of our knowledge, there is not a single review
that offers outline specifically for ferrocene‐based
electro and photoresponsive materials with various
applications, so in this review, we have tried to fill this
gap in a broader way.
2|FERROCENE‐BASED ELECTRO
RESPONSIVE MATERIALS
The discovery of ferrocene attracted the world due to
its good reduction/oxidation features whereas it also
provides thermal stability due to the strong interaction
between the cyclopentadienyl ring and iron atom.
[48]
This interaction helps towards the synthesis of different
ferrocene‐based derivatives,
[49,50]
polymers
[51]
and
dendrimers.
[52]
Ferrocene as an individual compound
with applications has already been well covered in
the literature so in this section a very brief touch has
been given for an idea. Several ferrocene‐based small
molecules/derivatives, self‐assembled monolayers and
polymers are available but only a few of them will be
covered in this section.
2.1 |Ferrocene‐based small molecules
Ferrocene‐based small molecules are the important class
of materials due to their renowned redox behavior.
[53]
Ferrocene‐based chalcones can undergo radical
quenching and hydroxyl adduct formation.
[49]
Muller
et al. successfully confirmed this behavior by synthesizing
different ferrocene‐based chalcones, which were sub-
jected to different spectroscopic and electro‐responsive
investigations (Scheme 1).
[54]
These different ferrocene‐
based chalcones showed limited cytotoxic properties
against renal and melanoma cancer cell lines. This newly
created ferrocenium species destroyed the cancer cells by
radical action.
[55]
These limited cytotoxic properties were
mainly due to the unique organometallic nature of lipo-
philic ferrocene group. Shago and co‐workers illustrated
well that the antitumor or anticancer effect of the
ferrocenyl group is related to the oxidation state of the
central iron atom. Ferrocenium cations bring activity in
the ferrocene‐based drugs.
The similar behavior was observed by Osella and co‐
workers. They explained that the inhibition of cell growth
takes place due to the oxidation of ferrocene. Redox‐
active enzymes are responsible for this oxidation. The
ferricenium species correlates with oxygen and water to
generate hydroxyl radical. This hydroxyl radical cleaves
the DNA strands which results in cell death.
[56,57]
Later
on, He et al. developed a bifunctional probe based on fer-
rocene and naphthyridine carbamate dimer (NCD) using
a chain of –CO–NH–CH
2
–CH
2
–.
[58]
The obtained results
showed that ferrocene‐based naphthyridine carbamate
dimmer (fecNCD2) had better performance in compari-
son with the previously reported bifunctional probe hav-
ing chain of –CH
2
–(fecNCD1). This system was used as
a label‐free detection of CGG trinucleotide repeat
(Scheme 2).
[58]
Due to the expensive ions detecting tech-
niques, Manibalan et al. synthesized a ferrocene carba-
mate derivative (FCCD) which proved as simple,
inexpensive and an alternate method for the detection
of F
−
.
[59]
High selectivity for F
−
was obtained as com-
pared to other anions due to the stronger chemical inter-
action between F
−
and Si. Electrochemical ferrocene‐
based receptors are regarded as sustainable redox reporter
due to their favorable electrochemical properties, excel-
lent stability and easy derivatization. In the presence of
F
−
, FCCD undergoes a nucleophilic substitution reaction
at the Si–O bond due to the special affinity of F
−
for Si
and leads to the removal of the silyl protecting group
through 1, 6‐quinone‐methide rearrangement with con-
comitant release of reporter ferrocenyl amine (FA). As
the concentration of F
−
increases, the amount of FCCD
dissipates, whereas that of FA accumulates in the reac-
tion medium. The concentration of reacted F
−
is related
to the dissipated amounts of FCCD and the accumulated
amounts of FA, which are linearly related to their electro-
chemical redox couples. Goggins and co‐workers pro-
vided the evidence that the redox reaction is related to a
one electron‐one proton reversible process between ferro-
cene and the ferrocenium cation which in turn offers the
2of28 KHAN ET AL.

cathodic shifts in their redox‐process when complexed to
anions.
[59]
These complexes are easier to oxidize than the
free redox‐active receptor. This kind of ferrocene contain-
ing redox approach provided the use of cheaper unmodi-
fied electrodes, adoption of simple electrochemical
methods and naked eye detection of ions.
[60]
Density
functional calculations are very important in determining
the properties of complex structures.
[61]
Karaoglu et al.
developed some acetate bridged dinuclear Cu (II) com-
plexes with ferrocene‐based benzimidazol ligands to
check their electro responsive properties and explained
them using density functional theory calculations.
[62]
The study was carried out to develop a correlation
between catechol oxidase mimetic activity and the com-
pounds having an electrochemical response. The results
provided an insight that improved catalytic activity of
Cu (II) complexes was mainly because of electroactive
ferrocene unit (Scheme 3).
[62]
The results provided an
insight that improved catalytic activity of Cu (II) com-
plexes was mainly because of electroactive ferrocene unit.
A novel photopolymerizable surfactant with a ferrocenyl
group (11‐ferrocenylundecyl) (ethyl methacrylate) and
dimethylammonium bromide (I
+
), was synthesized
through the reaction of 11‐bromoundecyl ferrocene and
N,N‐(dimethylamino) ethyl methacrylate.
[63]
The small
potential difference and critical micellar concentration
showed that ferrocene‐based compounds have good redox
reversibility and better aggregation/disaggregation. The
prepared compounds showed lyotropic liquid‐crystalline
properties which can be used further as good environ-
mental responsive materials, potential drug carrier and
scavenger for removing dissolved organic impurities in
water.
[63]
Gong et al. synthesized a cyclopalladated ferro-
cene compound, which showed a good cell cytotoxic-
ity.
[64]
Assisted by a dual‐targeting drug delivery system,
the anticancer activity of this ferrocene‐based compound
remained unchanged, but the toxicity to non‐tumorigenic
cell line was remarkably reduced. This provided a new
pathway for the development of cyclopalladated ferro-
cene as an antitumor drug candidate.
[64]
Ferrocene‐based
small molecules have some inherent defects, mainly
related to their instability which can limit their applica-
tion. Researchers have now changed their focus from
small molecules to the immobilization of functional mol-
ecules into self‐assembled monolayers to overcome the
problems of small molecules.
2.2 |Ferrocene‐based self‐assembled
monolayers
Self‐assembled monolayers (SAMs) based on redox active
center have been extensively studied in several configura-
tions which can be prepared in two steps. The first step
relates to the synthesis of ferrocene‐based molecules
whereas the second step corresponds with the SAM mod-
ification on the substrate.
[65–67]
Rudnev et al. explained
the influence of different anions (NO
3
−
, ClO
4
−
,BF
4
−
and PF
6
−
) on the structure and redox behavior of 11‐
ferrocenyl‐1‐undecanethiol (FcC11) assembled on Au
(111) single crystal.
[68]
They found that hydrophobic anions (ClO
4
−
) formed
ion pair upon oxidation of ferrocene moiety (Figure 1a)
SCHEME 1 Inductive and resonance effects of electron donating and electron withdrawing groups based on ferrocene‐containing
chalcone
[54]
KHAN ET AL.3of28

SCHEME 3 Proposed structures of the complex
[62]
SCHEME 2 Synthesis of bifunctional electrochemical probes: fecNCD1 and fecNCD2
[58]
4of28 KHAN ET AL.

while the ion‐pair formation was hindered for hydro-
philic anions (HSO
42−
) due to stronger interactions with
water molecules and excess of hydration energy (Figure 1b).
This study proved useful to understand the composition,
electrochemical properties and interfacial structure of
electro‐active conducting interfaces.
[68]
Similarly, Aiello
et al. synthesized some novel ferrocene‐based derivatives
via carbon‐based linkers grafted upon SiO
2
surface.
[69]
Silanization followed by click chemistry reaction were
used in this indirect grafting procedure to functionalize
the system. The grafting of ferrocene facilitated to get
the good charge retention properties and surface coverage
while it also could be of importance for hybrid memory
device applications (Figure 2).
[69]
The modification of
redox reaction kinetics using ferrocene helps to distin-
guish the oxidation and reduction states in quasi‐revers-
ible reactions.
[70]
Tian et al. explained an effective
method to confirm the behavior of redox properties based
on SAMs carrying electroactive center.
[71]
In this method,
a thin film having gold as a working electrode, nitroben-
zene (NB) as an immiscible organic solvent and
perchloric acid as an aqueous electrolyte solution was
used. The synthesized monolayer turned out to be more
homogeneous due to a significant decrease in NB phase
relative to the aqueous solution and less polar microenvi-
ronment. These systems proved useful in understanding
the nature of electroactive SAMs in the organic phase
(Figure 3).
[71]
The detection of prostate proved very
FIGURE 1 Ferrocene based SAMs in reduced/oxidized form for: (a) 0.1 M HClO
4
and (b) 0.1 M H
2
SO
4
. Adopted with permission from
ref
[68]
. Copyright 2013, Elsevier
FIGURE 2 (a) Direct grafting protocol
on Si and (b) indirect grafting protocol on
SiO
2
. Adopted with permission from
ref
[69]
. Copyright 2013, Elsevier
KHAN ET AL.5of28

challenging and expensive due to its complex behavior.
To overcome this problem, Cevik et al.designeda
ferrocene‐based immunosensor for the detection of pros-
tate (cancer) specific antigen (PSA) having gold (Au) as
an electrode.
[72]
These ferrocene cored polyamidiamine
(Fc‐PAMAM) dendrimers proved useful for the detection
of PSA due to their branching nature. The density of
terminal group in the dendritic structures increases the
functionality of dendrimers. However, a cascade type gen-
eration in PAMAM dendrimer can introduce a dense
outer ammine shell which can lead to decreased electron
transfer. To overcome this problem, ferrocene units were
linked to the structure as a construction center of the sys-
tem. It was elucidated that the ferrocene group enriched
the electron transfer in the covalently bonded ferrocene‐
cored PAMAM dendrimer which made the detection of
PSA feasible. This approach increased the anti‐PSA load-
ing and fixation capacity of the immunosensors leading
to an enhancement in measurement sensitivity and
selectivity (Figure 4).
[72]
Petrizza et al.developedaself‐
assembled monolayer of ferrocene‐based on two steps
modification of the surface.
[73]
It was carried out with
the help of vapor deposition metathesis reaction. The
resultant product carried a homogeneous robust and
densely packed electro responsive monolayer. This system
can be used to explore in the area of sensing and opto-
electronic devices (Figure 5).
[73]
FIGURE 3 Structural changes taking place in monolayers: (a)
low surface density and (b) high ferrocene surface density.
Adopted with permission from ref
[71]
. Copyright 2015, Elsevier
FIGURE 4 Schematic illustration of the PSA system based on Fc‐PAMAM dendrimers having Au as an electrode. Adopted with
permission from ref
[72]
Copyright 2016, Elsevier
6of28 KHAN ET AL.

2.3 |Ferrocene‐based polymers
Organometallic polymers have dragged much attention
due to their several attributes like thermal and chemical
stability. Based on the advantages of amphiphilic block
copolymers, Xiao et al. synthesized an amphiphilic block
copolymer featuring on polyethylene oxide (PEO) as a
hydrophilic block.
[74]
The block copolymer was synthe-
sized by using monomethoxy‐terminated PEO as macro‐
chain transfer agent and 2‐formal‐4‐vinylphenyl ferrocene
carboxylate (FVFC) as a redox‐responsive moiety. The
synthetic reaction was carried out by using reversible addi-
tion‐fragmentation chain transfer (RAFT) polymerization.
The resultant product showed electro responsive behavior
and proved as a prerequisite for the redox‐controlled
release of encapsulants due to the addition of ferrocene
moiety (Scheme 4).
[74]
The Nano patches have always been
an important side of biological applications.
Staff et al. synthesized nanocapsules based on two
different blocks and prepared polyvinylferrocene‐b‐
polymethyl methacrylate (PVF‐b‐PMMA) block in water,
which resulted in the selectively oxidized nano
patches.
[75]
This kind of hydrophobic to hydrophilic tran-
sitions turned out to be very useful in redox‐responsive
release study (Scheme 5).
[75]
The incorporation of ferro-
cene in the polymeric system imparts redox, mechanical,
optoelectronic and magnetic properties.
[76]
Atom transfer
radical polymerization (ATRP) provides controlled molec-
ular weight with narrow molecular weight distribution
which is very useful for getting the desired polymer struc-
tures and applications. Liu et al. synthesized hydrophobic
to hydrophilic block copolymer using polyethylene glycol
SCHEME 4 Synthesis of ferrocene‐
based block copolymer
[74]
SCHEME 5 Modification of PVFc‐b‐PMMA
[75]
FIGURE 5 Pictorial representation of the surface modification of
both ITO and Si/SiO
2
with tetra (tertbutoxy) tin, and the grafting
process to obtain the ferrocene derivative. Adopted with permission
from ref
[73]
Copyright 2016, Wiley Online Library
SCHEME 6 Synthetic route of PEG‐b‐PMAEFc
[77]
. DCC = N,N‐
dicyclohexylcarbodiimide and DMAP = 4‐dimethylaminopyridine
SCHEME 7 Structures of 4A PCL‐CD and PEG‐Fc
[78]
KHAN ET AL.7of28

(PEG) and 2‐(methacryloyloxy) ethyl ferrocene‐carboxyl-
ate (MAEFc) by following ATRP.
[77]
PEG was used as a
macro ATRP agent. It was found that the solution self‐
assembly of the synthesized compound was greatly depen-
dent on the solvent used, composition and concentration of
the polymer, and the addition of host molecules and oxi-
dants. These systems opened up an approach for controlled
drug release study (Scheme 6).
[77]
Keeping advantages of star‐shaped amphiphilic copol-
ymers in drug delivery systems (DDS), Peng et al. synthe-
sized ferrocene‐based cyclodextrins for DDS.
[78]
It was
seen that star‐shaped amphiphilic copolymers had great
potential for the controlled drug release due to stable self‐
assembly behavior. These amphiphilic copolymers exhib-
ited good efficiency and better biocompatibility as a result
of the stable electroactive moiety (Scheme 7).
[78]
The
reduced state of ferrocene strongly interacts with β‐CD as
compared to the barely interacted oxidized state. There-
fore, ferrocene can provide better host‐guest interac-
tion.
[79]
The Polymer brushes are very demanding due to
the ease of selection in side groups. These side groups can
provide desired properties by using controlled polymeriza-
tion techniques. Gan et al. synthesized some ferrocene‐
based polymer brushes carrying different side chain
lengths.
[80]
Surface‐initiated ATRP was used to prepare
the compounds. The results were compared with the ideal
packing model and it was observed that Fc units can
enhance the overall performance of the compound which
proved useful for sensing or drug delivery systems
(Scheme 8).
[80]
Arsenault et al. prepared a tunable pho-
tonic crystals (PCs) display based on ferrocene‐
metallopolymer.
[81]
They showed that electroactive PCs
provided improved results in contrast to other display tech-
nologies due to the usage of tune voltages. This system
solved the problems of previously reported systems having
internal disorders. These systems provided unique proper-
ties due to the ferrocene moiety. Ferrocene group enabled
the multi‐color display under different voltages through-
out the visible region (Figure 6).
[81]
Polyferrocenylsilanes
can provide good thermotropic liquid‐crystalline proper-
ties which are useful for sensors and display technologies.
Feng et al. prepared some redox‐based robust films
containing polyferrocenylsilane with imidazole‐function-
alized chains. Electro grafting method was adopted to
get functionalized surfaces. With keen observation, it
was found that electroactive ferrocene switch enabled
better grafting on the electrode. These findings can be
used further for fuel cells, energy conversion and modi-
fied sensors (Scheme 9).
[82]
Wang et al. proposed a strat-
egy for orthogonal integration of different properties in
which the thermo‐responsiveness of ethylene glycol‐mod-
ified pillar [6] arene and the redox‐induced reversible
color switching of ferrocene/ferrocenium groups are
orthogonally integrated into one system.
[83]
This gives
rise to a material with cooperative and non‐interfering
dual functions, featuring both thermo‐chromism and
warm/cool tone switchability. The obtained bifunctional
material for fabricating smart windows not only regulated
the input of solar energy but also provided a more com-
fortable color tone to improve the feelings and emotions
of people in indoor environments.
[83]
Electron donor‐
acceptor polyimides (PIs) containing different contents
of ferrocene (Fc) as the pendant group, were synthesized
for electrical resistive memory device applications.
[84]
Semiconductor parameter analysis indicated that the syn-
thesized ferrocene‐based PIs possessed nonvolatile flash
memory characteristic with excellent operational stability
and transient response to applied voltage. The simulation
results revealed that the ferrocene species contributed a
lot to the donating electrons. The present work provided
a new material for future electrical resistive memories.
[84]
SCHEME 8 Synthetic route of PFMMA (polyferrocenyl
methylmethacrylate), PFBMA (polyferrocenyl butylmethacrylate)
and PFNMA (polyferrocenyl nonylmethacrylate) whereas “r”
represents the ideal radius of cylindrical column formed by side
chains coiling along the polymer backbone and L
pb
represents the
ideal thickness of the polymer brush
[80]
8of28 KHAN ET AL.

From the above discussion of ferrocene‐based small
molecules, self‐assembled monolayers and polymers, it
can be comprehensively concluded that within the last
few years tremendous efforts were devoted to the area of
stimuli‐responsive materials, driven by the need for pre-
cisely controllable material properties. Being an inherent
defective and unstable system, ferrocene‐based small mol-
ecules are not considered good in the long run. Ferrocene‐
based SAMs and polymers can induce stronger impacts
due to the flexibility of processing, molecular order, versa-
tility, and variety of functional groups. The switching of
ferrocene can be used for many important systems such
as sensing, optical, magnetic and electronic devices, long‐
range charge transport and drug delivery as explained
above. Individually, it can only provide redox stimulus so
to get the maximum out of ferrocene group, this switching
can be combined with another switch like azobenzene to
get multiple sensitive functions in one system.
3|FERROCENE‐BASED ELECTRO
AND PHOTO RESPONSIVE
MATERIALS
Ferrocene derivatives have the property of absorbing
light, which helps to extend the fatigue life of photochro-
mic compounds. Interlinking of ferrocene with different
photosensitive materials can give a lot of functional mate-
rials. Some of the important ferrocene‐based electro‐
photo responsive materials are as follows.
FIGURE 6 (a) Tunable features of electroactive PCs thin films, (b) SEM images of films and (c) color tuning behavior of films. Adopted
with permission from ref
[81]
Copyright 2007, Nature Publishing Group
SCHEME 9 Synthesis of polyferrocenylsilane‐
methylimidazole
[82]
SCHEME 10 Photoisomerization of
ferrocene‐azobenzene derivative
[85]
KHAN ET AL.9of28

3.1 |Ferrocene‐based azobenzenes
The combination of stimuli‐responsive compounds like
ferrocene and azobenzene can provide multifunctional
materials. Azobenzene derivatives have always been an
exciting area of research due to its reversible conforma-
tional changes. They show different phase transitions
such as gel–sol and crystal liquid transitions. Zhang
et al. synthesized ferrocene‐based azobenzene com-
pounds to investigate their photochemical properties in
bulk and solution states.
[85]
Interestingly, the crystal trans
state of the synthesized compound showed photo induced
solid–liquid transition at elevated temperature under UV
(365 nm) irradiation. Temperature and photo illumina-
tion were responsible for this transition. Ferrocene being
a mesomorphic compound also helped in this transition
by intramolecular interactions. They reported that the
addition of azobenzene moiety resulted in crystal liquid
phase transition of the compound in response to different
wavelengths of light (Scheme 10).
[85]
Zhai et al. synthesized three different ferrocene‐
azobenzene based compounds for anion recognition.
[86]
Their photo and electro responsive properties were stud-
ied regarding the effect of nitro and amine substituents
of benzene ring on anion recognition. They found that
nitro group affected the recognition positively due to the
strong hydrogen bonding between the guest and receptor
whereas the amino group affected the sensitivity nega-
tively (Scheme 11).
[86]
Hydrogen bonding and electro-
static interactions are the main sources of ions
recognition that are obtained due to the signal transmis-
sion.
[87]
Ferrocene molecule imparts this effective signal
transmission throughout the system. Li and co‐workers
further provided the reasoning for this unique behavior
of ferrocene. They had corroborated that the free rotation
of two cyclopentadienyl rings of the ferrocene moiety is
restricted via the addition of anions. This addition brings
the two arms of the ferrocene moiety together in one
direction which resulted in the recognition.
[88]
Researchers devoted their studies to design low molecular
weight gelators which can regulate sol–gel transition.
This kind of materials with multifold sensitivity can be
used to engineer the smart systems with more superior
performance as compared with the pure organic counter-
parts. Afrasiabi et al. explained the effect of incorporating
SCHEME 11 Synthesis of ferrocenyl azobenzene compounds
[86]
SCHEME 12 Metallogelator based on ferrocene and azobenzene.
A = ferrocene moiety with redox properties, B = lysine, acting as a
linker between the subunits, C = azobenzene moiety to induce
photoresponsive change and D = a dipeptide moiety to reinforce
hydrogen bonding
[89]
10 of 28 KHAN ET AL.

the photochromic subunit (azobenzene) into the struc-
ture of ferrocene‐peptide conjugate to obtain the discrete
metallogelator (Scheme 12).
[89]
They found good results
due to multifunctional properties of the moieties which
can be applied further for supramolecular self‐assembly
and achieve organogelation.
[89]
Nature is the source of
inspiration for the development of materials that can
change according to the external circumstances. To
mimic this dynamic behavior, the host‐guest system
proved as an effective tool to introduce switching. A
selective control of host assembly and disassembly
behavior was done by Zhu et al. using redox and
photoirradiation with β‐cyclodextrin (β‐CD) complex.
[90]
Two reference compounds (Ref 1 and Ref 2) were also
synthesized to investigate the individual behavior of
azobenzene and ferrocene moiety with β‐CD. They found
that this host‐guest interaction can serve as a potential
candidate for multi‐mode driven supramolecule‐to‐
supramolecule transformations which can be easily iden-
tifiable (Figure 7).
[90]
This idea was, later on, proved very
useful by Nakahata et al. by preparing a self‐healing sys-
tem based on sol–gel phase transition using ferrocene as
guest polymer and cyclodextrin as a host molecule
(Figure 8).
[91]
It was found that the hydrogel based on fer-
rocene can control the self‐healing properties such as re‐
adhesion. This system provided a new sense to the bio-
medical sciences.
[91]
Ahmed et al. reported an interesting addition to the
research of functional nanomaterials by exploiting the
ionic self‐assemblies of diblock copolymers based on
ferrocenylsilane and azobenzene entities
[92]
to get the
photo‐induced patterns. Different strategies were adopted
to tune the structure and properties of the polymers to
yield organometallic surface relief gratings (SRGs). The
novel part of this work was related to the post‐modifica-
tion of SRGs without changing their periodic structure.
It was found that the use of electro and photoactive moi-
eties enabled this supramolecular system to be post‐mod-
ified for surface relief gratings. These gratings can have
strong potential for holographic data storage and
templating (Scheme 13).
[92]
Redox and photo‐active poly-
mer films were prepared by free radical copolymerization
and their ability to store data was checked.
[38]
Three dif-
ferent copolymers were synthesized in this respect. The
prepared films were tested under trans/cis and reduc-
tion/oxidation pattern. The synthesized polymers showed
FIGURE 7 The structures of FVA, Ref 1, Ref 2 and electro‐photo responsive behavior of FVA. Adopted with permission from ref
[90]
.
Copyright 2010, Royal Society of Chemistry
KHAN ET AL.11 of 28

good thermal stability and potential application for high‐
density information stage. They further explained the
mechanism of multi‐state information storage of the
synthesized polymers. According to Xia Xia and co‐
workers, ferrocene and azobenzene‐based polymer films
showed four different storage stages (Figure 9). After
UV irradiation, the initial trans state of azobenzene (state
1) changed to the cis state which can be allocated as state
2. In state 3, the ferrocene group changed its form to
ferricenium ions due to oxidation. Then polymers at state
3 were irradiated with visible light and transformed to
state 4, where azobenzene recovered to its initial state.
Finally, ferricenium ions recovered to initial state by the
chemical or electrochemical stimulus. This whole process
was stable and reversible during the course of
application.
[38]
3.2 |Ferrocene‐based spiropyrans
Spiropyran is another type of photoactive compound
which has closed and open isomerization forms. The
closed form is colorless whereas open form is colored
due to large electric dipole moment. It is considered that
SCHEME 13 Chemical structure of polyferrocenylsilane‐
azobenzene‐based block copolymer
[92]
FIGURE 8 Ferrocene‐based sol–gel phase transition experiment. Adopted with permission from ref
[91]
. Copyright 2011, Nature Publishing
Group
12 of 28 KHAN ET AL.

by bonding spiropyran with ferrocene can exhibit better
stability and reversibility. A lot of work has been avail-
able for the individual use of spiropyran in different
applications but there are fewer reports available for the
combination of this entity with ferrocene. The application
of spiropyrans is usually become limited due to non‐con-
trollable reversion process which results in the loss of pat-
terns. A keen effort has been executed to solve this
problem by incorporating ferrocene moiety with
spiropyran for information storage application.
[93]
The
prepared films exhibited good reversibility, long retention
time and low threshold voltage as compared to the
spiropyran without ferrocene moiety when exposed to
UV/Vis light (Scheme 14).
[93]
The mechanism was
explained in terms of dual stimuli effect. Ferrocene group
was interlinked with spiropyran by electronic interac-
tions. Before irradiation, the absorption spectrum exhib-
ited a prominent band with λ
max
at 334 nm assigned to
π‐π* transition. Another weaker band at a higher wave-
length (456 nm) was attributed to a metal‐to‐ligand
charge‐transfer band. Irradiation of ferrocene‐based
spiropyrans with UV light (365 nm) gave a distinct blue
color. This process resulted in the formation of an
extended π‐conjugation system and the appearance of
an absorption band (λ
max
= 590 nm) in the visible region.
The spectrum can come back to the initial state by irradi-
ation with 586 nm light.
[10]
The stable behavior and
fatigue resistance were mainly enriched due to the
FIGURE 9 Mechanism of multi‐state information storage. Adopted with permission from ref
[38]
. Copyright 2018, Elsevier
SCHEME 14 Photo‐isomerization of
SPFc
[93]
SCHEME 15 Photo‐isomerization of
ferrocene/spiropyran
[94]
KHAN ET AL.13 of 28

addition of a robust system like ferrocene which provided
better switching behavior. Nagashima et al. developed a
system based on spiropyran/merocyanine by incorporat-
ing ferrocene moiety.
[94]
The compound provided an
insight for the photo‐isomerization and redox cycles of
the compound. This type of compounds helped to prog-
ress in the fields of semiconductor and computer technol-
ogy which based on memory (random access and read‐
only) (Scheme 15).
[94]
This research explored new gates
for the memory devices due to the unique stabilization
of merocyanine (a closed form of spiropyran). The reason
behind this stability was based on the electronic effect of
ferrocene which happened due to the possible π‐conjuga-
tion. Takase et al. designed a compound having novel fer-
rocene‐modified bis spirobenzopyran.
[95]
Photochromic
properties of the compound were measured by inclusion
and exclusion of different metal cations. The presence of
two photochromic moieties in spirobenzopyran tethered
two cyclopentadienyl rings of ferrocene with the help of
ethynediyl spacer. They found that due to the inter‐ring
spacing (0.33 nm) present between two cyclopentadienyl
(Cp) rings in ferrocene the stability of the synthesized
compound was enhanced.
[95]
Spiropyran is most com-
monly used for storage applications but its use is limited
due to some common problems like photodegradation
or photo‐oxidation which results in the loss of
reversibility.
3.3 |Ferrocene‐based diarylethenes and
dithienylethenes
Photochromic compounds have gained much attention
due to their potential applications in switching devices
and optical memory media.
[96–99]
By taking this into
account, Yin et al. synthesized a compound carrying
dithienylethenes as photochromic moiety.
[100]
The isom-
erization of dithienylethenes can be altered from ring
open to ring close form and vice versa upon irradiation
of suitable wavelength. The oxidation of dithienylethenes
usually occurs at high potential so this drawback was
tried to solve in this work by the addition of ferrocene
with dithienylethenes which provided ring open and
close process at a low potential. When dithienylethenes
moiety was combined with ferrocene, the resulting
SCHEME 16 Synthesis of ferrocene‐based dithienylethenes
[100]
14 of 28 KHAN ET AL.

compound showed excellent thermal stability and photo-
chromic properties which can be applied for memory
device application (Scheme 16).
[100]
Zuckerman et al.
developed a systematic route to synthesize some unsym-
metrical benzo [b]thienyl‐thienylethene compounds with
a specific focus on conjugation of ferrocene to the benzo
[b] thiophene subunit.
[101]
The compound showed poten-
tial applications for photosensitive switches and optical
storage materials.
[101]
Cai et al. synthesized some photo-
induced electron transfer (PET) generators having
diarylethene (DAE) as a photoswitchable unit and ferro-
cene‐based naphthalimide.
[102]
It was found that ferro-
cene grafted triads can give electro and photoresponsive
properties by changing the source of light or redox cycle
(Figure 10).
[102]
Upon incorporation of ferrocene unit
(Fc), the photochromic efficiency in the anti‐parallel triad
conformer ap‐Fc was blocked to a great extent in the fer-
rocene state but distinctly enhanced in the ferrocenium
state via chemical or electrochemical stimuli, thereby
constructing redox‐gated photochromism. Meanwhile,
the reversible redox between ferrocene and ferrocenium
states also switched ‘OFF/ON’the fluorescence of
naphthalimide chromophore via photoinduced electron
transfer pathway.
[101]
Cai and co‐workers further explained the mechanism
of this ferrocene‐based triad system.
[102]
They suggested
that the fluorescence quenching of naphthalimide in the
ferrocene‐grafted triad system might be attributed to the
PET pathway from the ferrocene unit to naphthalimide
chromophore. The excellent chemical−/electrochemical‐
redox property of ferrocene unit afforded easy manipula-
tion of conversion between two valence states of Fe
II
and
Fe
III
. After adding the oxidizing agent, ferrocene,
diarylethene and naphthalimide based conformer started
to change in oxidation state. The protons of methylene
and two cyclopentadienyl rings in the ferrocene unit
showed four sets of well‐resolved resonances. Upon
increasing the proportion of the oxidizing agent, the sig-
nals of methylene moiety were shifted downfield and
finally, they totally vanished indicating the complete oxi-
dation. The disappeared resonances gradually recovered
upon addition of reducing agent showing the reversible
redox behavior.
[102]
The use of diarylethene is limited
due to the problem of oxidation which renders the entire
sequence irreversible.
3.4 |Ferrocene‐based fulgides
Fulgides can show good thermal irreversible photochro-
mic properties. McCabe et al. designed a series of com-
pounds based on ferrocenyl fulgides.
[103]
It was found
that photochemistry of the synthesized compound was
greatly dependent on the Z/E isomerization of the
alkylidene double bond (Scheme 17).
[103]
Fulgides are
FIGURE 10 Photochromic behavior and the addition of ferrocene moiety in diarylethenes. Abs: THF solution in daylight and FL:
Fluorescence of THF solution excited at isobestic point of 360 nm. Adopted with permission from ref
[102]
. Copyright 2016, Wiley Online
Library
KHAN ET AL.15 of 28

one of the good thermally irreversible photochromic com-
pounds. However, fulgides were thermally reversible at
the beginning of their history. Lately, to show the stability
of fulgides, Baghaffar et al. synthesized ferrocenyl fulgide
based compounds doped in PMMA polymer film.
[104]
Using different annealing temperatures, it was concluded
that ferrocene‐based fulgides showed good results upon
annealing the film at 82 °C and the addition of ferrocene
moiety in fulgide chain also enhanced the stability which
can be used for holography and coatings.
One of the essential parameters that should be ful-
filled by a photochromic moiety to be used as a data stor-
age device is its high resistance to photochemical
degradation. The photochemical stability and fatigue
resistance were greatly enhanced upon incorporation of
ferrocene due to the intramolecular interactions
(Scheme 18).
[104]
3.5 |Ferrocene‐based spirooxazines and
phenyl methane
A novel photochromic compound based on spirooxazine
and ferrocene was synthesized by Yang et al. by esterifica-
tion of 9′‐hydroxy spirooxazine with ferrocene carboxylic
acid.
[105]
It was reported that the new compound pos-
sessed photo‐electromagnetic property owing to the exis-
tence of metal ion (Scheme 19).
[105]
Ferrocene has the
ability to show distinct absorptions due to strong electro-
magnetic characteristics. Sengupta et al. synthesized a
compound which showed strong absorption in the near‐
infrared region.
[106]
It was found that multiple ferrocene
units showed strong absorptions when conjugatively
linked to a carbocation in a symmetric fashion. The
presence of multiple ferrocene units in triphenylmethane
dyes provided large bathochromic shift as well as
SCHEME 18 (a) Structure of ferrocene
dye used and (b) photochemical reactions
of fulgide
[104]
SCHEME 19 Synthesis of ferrocene‐
based spirooxazine
[105]
SCHEME 17 Synthesis of (E)‐1‐ferrocenylethylidene
(isopropylidene) succinic anhydride
[103]
16 of 28 KHAN ET AL.

hyperchromic effects which can be applied to get new
near‐infrared dyes. Such a large bathochromic shift is,
presumably, caused by a lowering of the energy of the
metal to ligand charge‐transfer transitions in the cationic
ferrocenyl styryl chromophore and the effect is further
amplified as a consequence of its trigonal symmetry. This
system provided an approach toward laser optical imag-
ing and biological stains. (Scheme 20).
[106]
3.6 |Ferrocene‐based stilbenes
Stilbenes have two isomeric forms of 1,2‐
diphenylethylene: (E)‐stilbene (trans‐stilbene), which is
not sterically hindered and (Z)‐stilbene (cis‐stilbene),
which is sterically hindered and therefore less stable.
[107]
By keeping this in the account, Huang et al. synthesized
(E)‐stilbene based ferrocenyl complexes to check the over-
all properties and stability of the compound.
[107]
It was
seen that present behavior was similar to analogous naph-
thalene complexes whereas different computational and
experimental results showed that phenyl ring was not
reduced as compared to the double bond which was
reduced. By comparing this behavior with reported ura-
nium (E)‐stilbene complex it was found that the synthe-
sized compound had a greater stability which allowed
access to rare earth alkene complexes.
[107]
The mechanism
for ferrocene‐based stilbenes was explained well by Dhoun
and co‐workers.
[108]
According to them, ferrocene usually
influences the system as a donor whereas stilbene acts as
an acceptor intercepted by a conjugated bridge. Ferrocene
(Fc) generally possess low oxidation potential and is capa-
ble of a facile charge transfer (CT) to an acceptor to yield
stable α‐ferrocenyl carbocations. Owing to their reversible
redox behavior, these chromophores show different
hyperpolarizability values in each of the two (+2 and + 3)
redox states. After the illumination, ferrocene‐based stil-
benes show intense high energy (HE) and weak intensity
low energy (LE) bands in the region of 352–364 nm and
460–476 nm, respectively, which are characteristics of Fc
based stilbene chromophore. The HE bands are assigned
as intraligand π/π* transitions while the LE bands are
metal to ligand charge transfer (MLCT) bands. At this
stage, ferrocene chromophores exhibited electrochemi-
cally reversible oxidation wave which was attributed to
the Fe
III
/Fe
II
redox couple. The cathodic shift increased
the electron density of the ferrocene donor, which resulted
in the rise of energy levels as well as facile oxidation of Fc
to ferrocenium species.
[108]
The preparation of ferrocene‐based photoresponsive
chromophores requires an interdisciplinary approach.
The well‐known organic photoresponsive compounds
exhibit switching upon irradiation between the cis and
trans isomers (such as azobenzenes and stilbenes) or
interconvert between closed and open forms (such as
spiropyrans, diarylethenes, and fulgides). This switching
can be used to functionalize ferrocene‐based systems.
Individually, all the photo‐responsive moieties show
some drawbacks such as irreversible pathways, photo‐oxi-
dation, photobleaching and fatigue resistance over a large
number of repeating cycles. These problems can be sorted
SCHEME 20 Synthetic route of tris[4‐(ferrocenylvinyl)‐3‐methyl phenyl] methylium ion
[106]
KHAN ET AL.17 of 28

out by incorporating ferrocene with the photoactive
moieties. Ferrocene group directly influences the isomer-
ization behavior of photoactive compounds by intramo-
lecular interactions. Ferrocene groups have sensitive
redox states with remarkable chemical and thermal sta-
bility. They have a low lying triplet excited state and are
known to be an effective triplet quencher. All these
attributes make the ferrocene unit a potent candidate
for ferrocene‐based light responsive systems.
4|APPLICATIONS OF
FERROCENE‐BASED ELECTRO AND
PHOTO RESPONSIVE MATERIALS
Ferrocene‐based electro and photochromic materials
have many potential applications because of their multi‐
functional nature. Some of the important applications
have been discussed below.
4.1 |Information storage
With the increasing interest for faster data processing
rates and higher information storage densities, work on
optical data storage has been increased rapidly.
[109]
In
the high‐density optical memory systems, the data can
be written, read and erased according to the require-
ments.
[109,110]
Zhu et al. fabricated an electro‐photo
responsive compound which can form a host‐guest sys-
tem with β‐CD.
[90]
The peak potential in CV shifted
towards more positive values with the increase in the
amount of β‐CD (Figure 11, inset). It was found that the
oxidation of ferrocene viologen azobenzene (FVA) was
very difficult due to the presence of β‐CD.
[90]
The results
indicated that the aqueous environment was more suit-
able for the process whereas the conditions were unfavor-
able when ferrocene got oxidized or azobenzene turned
into cis‐form. Furthermore, it was found that supramolec-
ular complexes based on CD can serve as molecular scale
storage media with different optical data storage capaci-
ties.
[90]
Ma et al. synthesized a spiropyran based molecule
by incorporating ferrocene (SPFc).
[93]
They prepared
SPFc based film which showed reversible electrical
switching properties along with long retention time and
low threshold voltage. Figure 12 represents some spectral
changes, recording pattern and fatigue resistance of the
synthesized film. It has been reported that these types of
films can be used as high‐density optical storage devices.
It should also be noted that spiropyrans can revert
thermally to the closed ring colorless form very quickly,
so the addition of ferrocene was incorporated to avoid
this problem and to increase the fatigue resistance of
the spiropyran‐based systems. Ferrocene being a robust
system can provide sensitive redox states and better
chemical as well as thermal stability.
[93]
It was found that
ferrocene had increased the thermal stability of the col-
ored ring‐open form of spironaphthoxazine. It was due
to the electronic interactions between ferrocene and
quinoidal form of spironaphthoxazine.
[109]
The
spironaphthoxazine molecule can provide stable
FIGURE 11 Absorption spectra of FVA, T: 298 K (a) starting
state, (b) after irradiation for 30 min, (c) after addition of 20
equiv. β‐CD and (d) after irradiation and addition of 20 equiv. β‐CD
Inset: CV curves of FVA. Adopted with permission from ref
[90]
.
Copyright 2010 Royal Society of Chemistry
FIGURE 12 (a) UV/Vis spectrum of SPFc, (b) Exhibition of optical data storage application on SPFc‐PMMA film and (c) recording‐erasing
cycles of SPFc‐PMMA film. Adopted with permission from ref
[93]
. Copyright 2009 American Institute of Physics
18 of 28 KHAN ET AL.

isomerization when combined with ferrocene moiety
(SOFC) for information storage application. Figure 13a
illustrates the two‐photon excitation technique applied
to the optical memory device. Each layer was having a
distance of 5 μm with recognizable data in each layer.
Figure 13b shows the cross‐sectional view of the Z
direction of four recorded layers. It was also found that
SOFC films proved useful for optical data storage due to
the strong interactions between ferrocene and
spironaphthoxazine.
[109]
4.2 |Ions sensing
Ion recognition is one of the important fields due to its
drastic effect on the environment and human's health.
There are several techniques which can be adopted for
the recognition of ions. Amongst all the available tech-
niques, electrochemical processes can offer advantages
of low cost and high sensitivity.
[46]
Ferrocene‐based mate-
rials are considered one of the promising candidates for
the recognition of specific ions.
[111]
Li et al. successfully
FIGURE 13 (a) Recording and read out
results for the multilayer optical memory
and (b) Z direction of the four recorded
patterns in the cross‐sectional view.
Adopted with permission from ref
[109]
Copyright 2005, Wiley online library
SCHEME 21 Synthesis of the probe
molecules 1 and 2
[88]
KHAN ET AL.19 of 28

synthesized two probe molecules for the recognition of F
−
and H
2
PO
−
4
in polar solvents (Scheme 21).
[88]
It was
found that strong hydrogen bonding between amide
group and ions eased the path for the sensing of anions.
These designed probes interestingly allowed the naked‐
eye detection of F
−
and H
2
PO
4
‐
. Figure 14a corresponded
with the change of color by adding F
−
which showed that
probe molecule 1 can sense F
−
whereas probe molecule 2
showed sensing towards F
−
and H
2
PO
4
−
by changing its
color from yellow to salmon pink without any interfer-
ence of other ions (Figure 14b).
[88]
Gonzalez et al. synthesized functionalized ferrocene‐
triazole‐pyrene triads from 1,1′‐bis (azido) ferrocene
through a copper‐catalyzed click reaction.
[112]
The
resulting disubstituted ferrocene derivatives showed
excellent anion, cation and ion pair multichannel recog-
nition.
[112]
Xanthine oxidase (XO) is a type of enzyme
which generates reactive oxygen species. The recognition
of these enzymes is very important as they proved vital in
the catabolism of purines in humans. A bio‐sensor was
designed by using xanthine oxidase (XO) and redox active
platinum electrode.
[113]
It was found that by measuring
the oxidation current of H
2
O
2
, xanthine can be detected
due to the oxidized oxygen to generate H
2
O
2
and uric
acid (Figure 15).
[113]
Ferrocene was employed as an elec-
tron mediator transfer for the fabrication of this bio‐sen-
sor which helped to detect these enzymes. The
interesting fact about all these ferrocene‐based ions rec-
ognition systems is that upon complexation with metal
cations or hydrogen bond formation with anions, they
undergo significant perturbations of the ferrocene/
ferrocenium redox couple. The values of the
FIGURE 14 (a) Probe molecule 1 showing colorimetric changes
due to the addition of different anions in DMSO and (b) probe
molecule 2 showing colorimetric changes due to the addition of
different anions in DMSO. Adopted with permission from ref
[88]
.
Copyright 2013, Elsevier
FIGURE 15 Synthetic route of the xanthine oxidase based biosensor. Adopted with permission from ref
[113]
. Copyright 2014, Elsevier
20 of 28 KHAN ET AL.

corresponding anodic or cathodic oxidation potential
shifts provide information about the strength of the rec-
ognition event.
[114,115]
4.3 |Biological aspects
Ferrocene‐based photoresponsive materials have gained
much attention as an essential part of the biological
applications such as drug delivery, bio‐sensors and bio‐
technology.
[116,117]
Wajs et al. synthesized stimuli‐
responsive nanocapsules (NCs) using host‐guest chemis-
try.
[116]
Figure 16 represents the encapsulation/release
properties of the compounds using redox stimulus,
whereas Figure 17 represents the switching properties
using light stimulus. It has been reported that the encap-
sulation process became more feasible as compared to
the process without stimulus by the addition of light
and redox stimuli.
[116]
These nanocarriers provided a
pathway in different important applications like cos-
metics, drug delivery, and other biomedical applica-
tions.
[116]
Ma et al. synthesized two novel ferrocene‐
based CD derivatives that could self‐assemble into a
supramolecular polymer and gel in different solvents
environments.
As ferrocene units can form a complex with β‐CD with
high binding constant, so it helped for the pathway of dif-
ferent drug delivery and smart materials applications
(Figure 18).
[117]
The interaction of host‐guest molecule
comprising azobenzene and ferrocene as guest units are
very important for biomedical perspectives.
[118]
It has been
reported that external stimuli (ferrocene/azobenzene) can
control the aggregation and disaggregation of the amphi-
philic CD. This type of reactions proved useful in biologi-
cal and medical applications.
[118]
The redox chemistry of
ferrocene helps to decrease the cytotoxicity of UV irradia-
tion which makes the in vivo applications more feasible.
The inclusion of ferrocene in these systems serves as
mediators to provide an oxidative electron path.
[119,120]
Wan et al. designed a photoswitchable‐bioelectrocatalytic
system using host‐guest chemistry.
[121]
Azobenzene and
ferrocene were used as light and redox stimuli respec-
tively. It has been reported that this kind of systems can
be used as optoelectronics and photoswitchable biofuel
cells (Figure 19).
[121]
FIGURE 16 Encapsulation process of NCs; (d)–(g) release process and (g)–(i) ferrocene‐based nanocarriers using a redox stimulus.
Adopted with permission from ref
[116]
. Copyright 2016, Springer link
KHAN ET AL.21 of 28

4.4 |Liquid crystallinity
Liquid‐crystalline materials have unique properties which
make them attractive for both academic and industrial
research, and also for different high‐tech applications such
as optical data storage, photovoltaic cells, panel display
devices and light emitting diodes.
[122]
Amer et al. synthe-
sized ferrocene‐based polymer with azobenzene as a side
FIGURE 17 Switching process of NCs in the presence/absence of light stimulus. Adopted with permission from ref
[116]
. Copyright 2016,
Springer link
FIGURE 18 Possible mechanism and synthetic route of ferrocene‐based β‐CD derivatives. Adopted with permission from ref
[117]
.
Copyright 2017, Royal Society of Chemistry
22 of 28 KHAN ET AL.

group.
[123]
It was concluded that by the addition of
azobenzene moiety, the compound showed liquid crystal-
line properties in the main chain and in the side chain as
well, which can be used further for electronic devices
(Figure 20).
[123]
Figure 21 represents the polarized optical
microscopy (POM) images of the synthesized compound
(poly (N‐phenyldiethanolamine 1,1
/
−ferrocene dicar-
boxylate (PPFD)). It was shown that nematic phases which
were obtained during heating exhibited textures with
schlieren disclinations. These textures and phases
remained sustained during low and high temperature.
These systems can be used for display technologies.
[123]
A
new class of liquid crystals containing ferrocene‐based
mesogens and different linking groups carrying
azobenzene and imine was reported.
[124]
It was seen that
ferrocene directly influenced the liquid crystalline phase
by stabilizing it and proved as a potential candidate for
optoelectronic devices.
[124]
Thermal stability is a crucial
parameter in liquid crystalline systems as liquid crystalline
ordering occurs in a specific temperature range. Ferrocene
groups are chemically and thermally stable in both the
reduced and oxidized states which are important parame-
ters to get the switchable liquid crystalline materials. Due
FIGURE 19 (a) Schematic assembly of azo‐based self‐assembled monolayer and (b) photoresponsive activation and deactivation of redox
active coated surfaces. Adopted with permission from ref
[121]
. Copyright 2011, Royal Society of Chemistry
FIGURE 20 Schematic illustration of the nematic liquid
crystalline phase of ferrocene‐based polyester with azobenzene in
the side chain (MFPAS). Adopted with permission from ref
[123]
.
Copyright 2012, Wiley Online Library
KHAN ET AL.23 of 28

to the stronger electronic interactions between an iron
atom and cyclopentadiene rings, ferrocene group imparts
thermal stability which contributes to the stable switch-
able liquid crystalline systems.
[125]
4.5 |Molecular machines
Stimuli‐responsive molecular machines are one of the
attractive topics of recent years. Several molecular
machineries such as motors, shuttles, switches have
already been reported.
[126,127]
Muraoka et al. designed a
molecular machine based on ferrocene and azobenzene
moieties. In this system, ferrocene played the role of pivot
whereas azobenzene as handle triggered by light (UV/Vis-
ible). They observed that the twisting of pedal parts can be
affected with the photoisomerization of azobenzene
(Figure 22).
[128]
In another work, Willner et al. fabricated
a molecular train based on ferrocene–carboxamide–β‐CD
which served as the locomotive of the molecular train.
[129]
They reported that it can translocate between a trans‐
FIGURE 21 POM images of the texture displayed by PPFD during different stages of heating and cooling (magnification 20×). Adopted
with permission from ref
[123]
. Copyright 2012, Wiley Online Library
24 of 28 KHAN ET AL.

azobenzene station site and an alkyl‐chain railway.
[129]
The stable redox states of ferrocene helped to induce
mechanical twisting of the rotor molecule. These ferro-
cene‐based robust systems might allow for the remote con-
trol of molecular events in larger interlocked molecular
systems. This work has important significance in biologi-
cal supramolecular machines and molecular devices.
When ferrocene interlinks with light triggered moieties it
can deliver many rousing applications. To perform these
applications in a real mode, it is very important to get
appropriate electronic and/or steric interactions between
the ferrocene and photo‐responsive compounds. No
multi‐functionality appears when the interaction is too
weak, and their original functions are perturbed too much
when the interaction is too strong. Up to date, there have
been many reported applications based on ferrocene and
photo‐responsive systems. All these applications also have
some certain limitations such as in the case of informa-
tion storage, ions recognition, liquid crystallinity and
molecular level machines, there is an intense require-
ment of better reversible switching, stronger electronic
interactions and stable chemical/thermal behavior.
Whereas to achieve biological goals it is an essential need
that redox and light responsive systems do not harm the
living cells. All in all, it is expected that ferrocene can
influence the photo‐responsive systems in many better
ways. It is anticipated that this multifunctional coordina-
tion can bring new avenues in organometallics.
5|CONCLUSIONS AND FUTURE
PERSPECTIVE
With the dramatic increasing demand for multifunc-
tional systems, it has become very important to obtain
accurate control of several stimuli groups combined into
one system. For this reason, it is an essential need to
overcome the problems of partial isomerization or irre-
versible changes upon stimulation. In this respect, this
review gives an overlook on some synthetic routes and
comprehensive applications of ferrocene‐based photo‐
chromic moieties. Ferrocene due to the organometallic
nature can serve as a promising candidate for many
potential applications after combining it with different
photochromic moieties. Combination of ferrocene group
with photo‐active moieties can give rise to multifunc-
tional systems. These systems can be applied further
for opto‐materials, sensors for harmful ions (anionic/cat-
ionic), molecular devices and so on. Ferrocene being
one of the good thermally stable compound, a super
aromatic electrophile and a controllable redox material
can serve well for this purpose. But this requires the
development of new methodologies which can take this
organometallic material to the interdisciplinary
research. Despite the great advances in the ferrocene‐
based systems, there are still numerous challenges that
need to be addressed to master the property‐function
relationship and use of these materials in practical
applications. The information about the kinetics and
magnitude of the ferrocene‐based processes are also of
paramount importance for the use of these materials
in certain applications such as switches, actuators and
storage devices. Another aspect that has not been
addressed extensively so far is related to other physico-
chemical changes that can be induced in parallel by
the applied stimulus, such as pH and temperature
changes, which could be unfavorable for certain applica-
tions. Future developments in the ferrocene‐based field
should focus on the novel designs, synthesis and engi-
neering that will allow one to address above mentioned
FIGURE 22 (a) Schematic model of the
light‐induced molecular pedal and (b)
interlocked motions triggered by light.
Adopted with permission from ref
[128]
.
Copyright 2006, Nature Publishing Group
KHAN ET AL.25 of 28

challenges. The design of smart and stable ferrocene‐
based materials that encompass the individual molecu-
lar response and allow a collective molecular contribu-
tion to generate a concerted macroscopic phenomenon
is highly desirable and an intriguing task. New inven-
tions and more detailed research are crucial in this area
to achieve the self‐defined goals of control, accuracy and
sustainability.
ACKNOWLEDGEMENT
Financial supports from the National Natural Science
Foundation of China (21472168, 51673170, 51811530097
and 21611530689), the Fundamental Research Funds for
the Central Universities (2017FZA4023) are gratefully
acknowledged.
ORCID
Li Wang http://orcid.org/0000-0001-9356-9930
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
How to cite this article: Khan A, Wang L, Yu H,
et al. Research advances in the synthesis and
applications of ferrocene‐based electro and photo
responsive materials. Appl Organometal Chem.
2018;e4575. https://doi.org/10.1002/aoc.4575
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