Vice versa donor acceptor fluorene-ferrocene alternate copolymer: A twisted ribbon for electrical switching

Article abstract of DOI:10.1039/c5cc04275e

Two donor-acceptor type copolymers (PFFC-1 and PFFC-2) containing ferrocene and fluorene moieties have been successfully synthesized to evaluate the redox triggered optical and electronic properties. Interestingly, PFFC-1 shows a twisted ribbon-like morphology at the liquid interface and switches to a micellar structure on oxidation.

Full text of DOI:10.1039/c5cc04275e

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Vice-Versa Donor Acceptor Fluorene-Ferrocene
Alternate Copolymer: A Twisted Ribbon for
Electrical Switching
8Cite this: DOI: 10.1039/x0xx00000x
†
*
Chanchal Chakraborty, Manas Kumar Bera, Utpal Rana and Sudip Malik
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two donor-acceptor type copolymers (PFFC-1 and PFFC-2) acceptor conjugated polymers that are scarcely seen in
1
3, 14
containing ferrocene and fluorene moieties have been the literature
. In this aspect, we have synthesized
successfully synthesized to evaluate redox triggered optical fluorene-ferrocene based two donor-acceptor copolymers
and electronic properties. Interestingly, PFFC-1 shows twisted to explore by their photophysical, morphological, cyclic
ribbon like morphology at the liquid interface and turns into voltammetry and electrical behaviours supported by
micellar structure by switching oxidation.
theoretical studies.
Alternate conjugated co-polymers containing ferrocene
(
Fc) units have attracted a great extent of attention due to
the optoelectronic properties of conjugated polymer and
1
-2
the redox activity of the Fc units. As Fc is very electron
rich arene, it is easily oxidized to form ferrocenium cation
+
(
Fc ) electrochemically or by weak oxidants. The most
successful way to incorporate Fc moieties in polymer
1
,3-6
backbone involves covalent binding.
The major
aspiration to insert of the Fc moieties in a conjugated
Scheme 1 Synthesis scheme for preparation of polymer PFFC-1 and PFFC-2.
polymer backbone is the evaluation of new materials to Reagent and condition used: (I) CuBr2 on alumina, CCl , refluxed, 5 h (II) 1-
4
7
8
o
be exploited as push-pull systems, molecular switches,
bromooctane, TBAB, KOH, H
2
O, 75 C, 15 mins. (III) Acetic acid, CrO
3
, 6 h (IV) 1,1ʹ-
o
9
ferrocenediboronicacid, Pd(PPh
3
)
4
, K
2
CO
3
, argon, 85 C, 72 h. (V) Malononitrile,
logic gates, and systems based on intramolecular charge
o
o
1
10 C, 5 h (VI) 1,1ʹ-ferrocenediboronicacid, Pd(PPh
3
)
4
, K
2
CO
3
, 85 C, 72 h.
1
0
transfer. In this context, the choice of suitable bridging
ligands among metal centres is of prime importance.
Poly(fluorene)s (PFs) based copolymers are one of the
most potential blue light emitting materials for electronic
and optoelectronic application because of high quantum
yield and thermal stability. Selective substitutions by
electron donating dialkyl or electron withdrawing dicyano
derivative in 9-position of fluorene (Fl) can make these
either electronically rich fluorenes or electronically poor
fluorenes, respectively.
Starting from the Fl, fluorene-ferrocene alternate co-
polymers were synthesized by proper selection of
monomer via Pd-catalyzed cross coupling polymerization
according to the Scheme 1. All the reaction steps and
characterization details are discussed in experiment
1
1
segment of electronic supplementary information (ESI
All these steps provided satisfactory yields and polymers
produced a sufficiently high molecular weight (M ) in GPC
†).
w
study. PFFC-1 exhibited M
whereas PFFC-2 unveiled M
w
= 11643 and PDI = 1.49,
= 15,706 and PDI = 2.47 in
In this consequence, two new conjugated polymers
w
(
Scheme 1) where functionalized Fl moieties would serve
GPC using THF as eluent. In PFFC-1, we intentionally put
two dioctyl chains in 9-position of each Fl unit so that Fl
segment in PFFC-1 turns out to be electron rich unit. In
PFFC-2, a dicyano unit at 9-position of each Fl unit is put
to enrich the electron withdrawing capacity of Fl unit and
the role of donor (PFFC-1) and acceptor (PFFC-2) vice-
versa have been designed to study the oxidation
behaviour of Fc of two polymers by common oxidizing
1
2
agents as well as the switching behaviour of donor-
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Fc acts as donor to produce an efficient charge transfer to Fe(ClO
cyanofluorene segment. Both of the polymers are soluble 1 and PFFC-2 are shown in Fig. 1c. After addition of
in common organic solvents like CH Cl , chloroform, THF FeCl (50 μM), a distinct colour change from yellow to red
etc. in room temperature. H NMR signals (ESI , Fig.S1) is observed in PFFC-1 and it is possibly due to the
of Fc and aromatic region of copolymer PFFC-1 and oxidation of electronically rich Fc centre to ferrocenium
4 3 3
) is different. Effect of addition of FeCl to PFFC-
2
2
3
1
†
1
+
PFFC-2 are compared. In both of the polymers,
H
cation (Fc ). Whereas, in PFFC-2, the yellow colour of the
signals of Fc rings are seen as broad peaks in the range polymer solution does not alter. Owing to the charge
of 4-5 ppm. In PFFC-2, Fc rings are associated with transfer from Fc to cyanofluorene unit, Fc unit in PFFC-2
electron withdrawing cyanofluorene units and as a result is electronically poor and oxidation is not feasible.
1
of this H signals of Fc ring get a downfield shift to exhibit Addition of Fe(ClO
4
)
3
to the solution of both polymers also
1
higher δ value (5.01 and 4. 91 ppm) than Fc H signals of exhibit the similar results as FeCl
PFFC-1 (4.44 and 4.13 ppm). Aromatic protons of Oxidation of Fc unit in PFFC-1 is also confirmed by H
fluorene segments produce broad signals in the range of NMR titration upon successive addition of FeCl to PFFC-
-8 ppm in both of the copolymers. Incorporation of 1 solution (ESI , Fig. S2). Titration studies suggest that
cyanofluorene unit in PFFC-2 is further evidenced by the Fc is converted to Fc in PFFC-1 upon oxidation and the
3
.
1
3
7
†
+
-1
presence of stretching vibration (
FTIR spectra of PFFC-2.

C≡N) at 2205 cm in protons of Fc unit are shifted to downfield (4.18 to 4.25
ppm and 4.45 to 4.58 ppm) than the same of pristine
PFFC-1. Subsequently, the d-d transition band (462 nm)
in UV-Vis spectra of PFFC-1 gets some red shifting to
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2
.5
PFFC-1
PFFC-2
(a)
(
a)
PFFC-1
PFFC-1+FeCl3
488 nm (Fig. 1b). This red shifting is owing to better
2.0
1.5
0
0
0
.03
.02
.01
+
(b)
PFFC-1
PFFC-2
electron communication after oxidation of Fc centre to Fc
15
in PFFC-1. One broad charge transfer (CT) band
1
.0
.5
appears at NIR region at 980 nm (Fig. 1b) after oxidation
0
0.0 06 00
650
700
750
+
Wavelength (nm)
due to the LMCT transitions from Fl segment to Fc which
0.0
become possible due to the generation of hole in the
metal-based HOMO. It is also visually confirmed by the
4
00
600
Wavelength (nm)
800
400
600
800
1000
Wavelength (nm)
16
(
a) (b)
(c)
(e)
colour change of the solution from yellow to red. Upon
oxidation the photoluminescence intensity is enhanced 5
fold in case of PFFC-1 but not in PFFC-2 (Fig. 1c and
(d)
(f)
FeCl
3
ESI
†
,
Fig. S3) owing to the better electronic
+
communication after oxidation to form Fc .
In cyclic voltammetry (CV) study referenced to the Fc/Fc
+
Fig. 1(a) UV-vis spectra of PFFC-1 and PFFC-2, The enlarged portion between
ox
wavelengths range 600-750 nm is given in inset, (b) UV-vis spectra of PFFC-1 couple (ESI
before and after oxidation by FeCl in CHCl . (c) Optical colour change on
oxidation of CHCl solution of two copolymers by FeCl . (I) PFFC-2, (II) PFFC-1, (IV)
PFFC-2+FeCl and (V) PFFC-1+FeCl . (III) and (VI) are images of PFFC-1 under 365
†
, Fig. S4), a reversible oxidation at E1/2 0.05
3
3
V is assigned to the oxidation of Fc moiety of PFFC-1. A
small but significant peak at 0.9 V is attributed to
oxidation of Fl moiety of the copolymer. In PFFC-2, a very
3
3
3
3
nm UV irradiation before and after oxidation, respectively.
low peak due the reversible oxidation of Fc appears at
ox
Incorporation of Fc unit into PFFC alternate copolymer
E1/2 0.002 V (ESI†, Fig. S4) which indicate the oxidation
will influence the usual spectral properties of both Fl and is not facile. The polyfluorene oxidation peak appears at
Fc unit. In CHCl
, PFFC-1 (Fig. 1a) shows two major 0.34 V. These values get some anodic shift because of
3
*
absorption bands, 328 nm ascribed to the π-π transition the incorporation of electron withdrawing cyanofluorene
in Fl segment and 462 nm for d-d transition of Fc moiety unit in PFFC-2 polymer. In PFFC-2, one strong reduction
*
red
with a small hump at ~300 nm that is π-π transition of Fc at E1/2 -1.52 V is ascribed to the cyanofluorene anion
moiety. PFFC-2 also reveals two characteristic formed owing to intramolecular charge transfer from
1
7
absorbance bands at ~ 360 nm and 495 nm in CHCl
3
donor Fc to accepter cyanofluorene unit. The scan rate
solution. Absorption band for π-π transition in Fc unit is dependent cyclic voltammetry studies (ESI , Fig. S5a-b)
also prominent in PFFC-2 ~ 300 nm. Unlike PFFC-1, a and linearity of the plot (ESI , Fig.S6a-b) of oxidative
*
†
†
small but well-defined charge transfer band is appeared peak current vs square root of the scan rate reveal the
at 663 nm in PFFC-2 (inset of Fig. 1a) that significantly oxidation is diffusion controlled in nature. Diffusion
-
9
supports transfer of charge from donor Fc moiety to constants (D) derived by Cottrell equation are 3.3×10
-9
2
electron withdrawing cyanofluorene acceptor moiety. No and 8.7×10 cm /s for PFFC-1 and PFFC-2, respectively
such charge transfer band is observed in PFFC-1 as both which are consistent to the previously reported data of
6
Fl and Fc are electron rich moieties.
this type main chain ferrocene copolymer.
It is important to note that oxidizing behaviour of both DFT optimized structures of PFFC-1, oxidized form of
polymers by common mild oxidizing agent like FeCl
3
and PFFC-1 and PFFC-2 are given (Fig. S7a-c).
Corresponding images of HOMO and LUMO of each
2
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repeating units of copolymers (Fig. S8a-f) provide that the of the left or right handed twisted polymer chains that are
*
HOMO and LUMO of PFFC-1 reside on π and π orbital, successively self-assembled to manifest as the twisted
*
facilitating the π-π transition. After oxidation of PFFC-1 ribbon. The π-π stacking of fluorene unit and van der
3
by FeCl , HOMO of the oxidized product resides on Fl Walls interaction between alkyl chains are responsible to
+
moiety and LUMO is on Fc (Fig. S8c-d), revealing a the self-assembly of individual twisted polymer chain.
possibility of charge transfer that is supported by the This observed twisted ribbon morphology is similar to the
presence of NIR band in UV-vis spectra (Fig.1b). In case twisting of block co-polymers without having chiral group
of PFFC-2, (Fig. S8e-f) the HOMO resides on Fc moiety and their controlled self-assemblies of helical structures
1
9
and LUMO is on Fl moiety. A charge transfer from Fc by mixed solvent effect. We have proposed a possible
HOMO) to Fl (LUMO) in reverse direction is expected schematic representation (Fig. 2a) based on this
and produces a CT band in UV-vis spectra at 663 nm in phenomenon. Again, SEM studies of PFFC-1 polymer in
(
inset of Fig. 1a.
complete hexane or methanol solution do not produce
any characteristic morphology (ESI†, Fig. S9). So, the
polar-nonpolar mixed solvent plays a crucial role to twist
the dihedral angle in Fc moieties and consequent self-
assembly of individual twisted polymer chains.
Self
Assembly
(a)
Right Handed
To get insight about twisting, the circular dichroism (CD)
spectra were performed (ESI†, Fig. S10). In CD spectra
of PFFC-1 in methanol and hexane (1:1), an induced CD
signals appeared at absorption peak of PFFC-1 supports
the presence of twisted aggregates in the solution. The
temperature dependent CD studies of PFFC-1 (ESI†, Fig.
S11a) reveal the CD intensity (+ cotton effect) of Fc
twisting and the CD pattern are not influenced by
temperature. It suggests the presence of a stable twisting
in our system and it also rules out the possibility of
racemization at higher temperature. CD intensity is not
significantly changed with the decrease of PFFC-1
concentration (ESI†, Fig. S11b). SEM images at various
Left Handed
Self
Assembly
5
µm
(
c)
(
b)
(d)
Left handed
Right handed
5 µm
1
1
0 µm
µm
(
e)
(g)
(
f)
concentration (ESI†, Fig. S12) also reveals that PFFC-1
forms the twisted structure even if at very low
concentration. These observations indicate the cotton
5
µm
2 µm
Fig. 2 (a) Schematic representation of PFFC-1 self-assembly in methanol-hexane
interface to form twisted ribbon and micelles after oxidation. (b)-(e) PFFC-1 in effect in the system is not an interchain phenomena.
methanol-hexane interface and (d),(e) PFFC-1 in methanol-hexane interface after
oxidation by FeCl . The red circled area in (b) indicating the right handed twisted
3
ribbon and the rests are left handed.
Chirality generated in PFFC-1 is form dihedral angle
twisting of Fc units which is not dependent on
concentration. Upon oxidation, PFFC-1 generates an
+
−
ionic Fc group that together with Cl counter ions will
assemble to form the inside of the micelle with the
hydrophobic alkyl chains facing outwards in the solution
in order to maximize ionic interactions (Fig. 2d-e).
Structural rearrangements of this type have been
reported earlier by Manners et al. for Fc containing block
copolymers. The CD spectra (ESI†, Fig. S10) of PFFC-1
after oxidation also demonstrates no signal as polymer
chains get micellar aggregation.
To investigate the morphological features of the PFFC-1
during oxidation, SEM studies have been performed by
making the interface layer of methanol and hexane (1:1)
mixture. Long twisted ribbon morphology of PFFC-1 chain
with ~1 µm width is observed (Fig. 2b-e). From the SEM
image in Fig. 2b-c, it is evident that both left and right
handed twisted ribbons are present in our system. As
PFFC-1 contains Fc and Fl having two long alkyl chains,
some change in configuration of the polymer may occur
2
0
1
4a
Charge transfer complex between a metal centre and
organic chromophore provides a novel system for
memory application because formation of metal filament
through redox reaction as well as intermolecular charge
at the interface of polar and non-polar solvent. Fc part
may prefer the polar solvent and alkyl chains non-polar
solvent. This process may introduce some twisting of
dihedral angle in flexible Fc moieties to create some
twisting in individual polymer chains. The twisting of Fc
dihedral angle is recently investigated in ferrocene–
naphthalenediimide donor-acceptor conjugate with long
1
5,21
transfer.
polymers, current density(J)–voltage (V) characteristics of
the ITO/PEDOT:PSS/PFFC-1/Al and
To explore the electrical property of the
1
8
ITO/PEDOT:PSS/PFFC-2/Al at room temperature are
compared in Fig. 3a-b. The device made of PFFC-1
shows an excellent conductance with switching behaviour
whereas the device made of PFFC-2 does not produce
alkyl chain. In the present system, the interface of polar-
nonpolar solvent plays the key role to twist the ferrocene
dihedral angle and subsequently it leads to the formation
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this type of switching performance. Initially, PFFC-1 is in Polymer Science Unit, Indian Association for the Cultivation of Science,
a low-conducting state (OFF-state). When the bias 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata – 700032.
voltage is increased to ~2 V, a transition from the OFF- †Electronic Supplementary Information (ESI) available: [Details of
1
state to a higher conducting state (ON-state) occurs with synthesis and characterization by instrumental techniques, H-NMR, PL,
a moderate ON/OFF ratio~40, presumably because of the CV, CD studies, DFT calculations and SEM images.]. See
+
15
oxidation of Fc groups to Fc . Formation of Fc+ in DOI: 10.1039/c000000x/
PFFC-1 by electrical oxidation generates holes in the
1
6
metal-based HOMO and it enhances the conductivity.
1
Y. Morisaki and Y. Chujo, Macromolecules, 2003, 36, 9319; G.
Masson, A. J. Lough and I. Manners, Macromolecules, 2008, 41,
539.
+
During a sweep at ±2 V, substantial number of Fc is
formed due to oxidation of Fc and makes continuous
conducting channel between the electrodes resulting in
an increment in current. At reverse voltage Fc is reduced
to Fc which blocks the conducting channel. Ability of
chemical oxidation of PFFC-1 is previously confirmed
2
3
H. Khalid, H. Yu, L. Wang, W. A. Amer, M. Akram, N. M. Abbasi,
Z. ul-Abdin and M. Saleem, Polym. Chem., 2014,5, 6879.
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Langa, J. T. L. Navarrete and J. Casado, J. Am. Chem. Soc., 2012,
134, 5675.
+
1
5
optically and electrochemically by addition of FeCl
3
solution whereas PFFC-2 is reluctant to oxidation which is
previously confirmed, so it does not produce any
conductance switching in increasing forward bias.
4
L. Tan, M. D. Curtis and Y. Francis, Macromolecules, 2002, 35,
4628.
5
6
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(b)
(
a)
I-V
7
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9
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Bartholomew and G. C. Bazan, J. Am. Chem. Soc., 2000, 122, 11956.
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Al
Polymer
PEDOT:PSS
ITO
Glass
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Fig.
3
(a) The J–V characteristics of ITO/PEDOT:PSS/PFFC-1/Al and
10 D. M. Guldi, M. Marcaccio, D. Paolucci, F. Paolucci, N.
Tagmatarchis, D. Tasis, E. Va´zquez and M. Prato, Angew. Chem.,
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ITO/PEDOT:PSS/PFFC-2/Al devices at room temperature. (b) Schematic
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1
1
L-H. Xie, C-R. Yin, W-Y. Lai, Q-L. Fan and W. Huang, Prog.
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In summary, two donor-acceptor type copolymers, PFFC-
1
and PFFC-2, containing Fc and fluorene moieties have
been successfully designed and synthesized.
4
34, 194.
Intramolecular charge transfer from Fc to cyanofluorene
segments in PFFC-2 is confirmed by appearance of
charge transfer band in UV-vis spectra. Oxidation of Fc
1
1
2
3
B. Adhikari and H.-B. Kraatz, Chem. Commun., 2014, 50, 5551.
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Int. Ed. 2013, 52, 1049.
+
centers to Fc either by chemically or electrically
1
4
S. Liu, Q. Chen, Y. Chen, B. Zhang, H. Liu, C. Peng and Y. Hu,
Macromolecules, 2014, 47, 373; M. Goel, K. Narasimha and M.
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and H. Goto, J. Mater. Chem. C, 2015, 3, 2024.
generates holes in metal HOMO to produce better
electronic communication in copolymer that reflects in
enhancement of PL intensity and as well as the switching
behaviour between low-conducting state (OFF-state) to a
high conducting state (ON-state) in J-V measurements of
PFFC-1. Interestingly, the PFFC-1 shows twisted ribbon
morphology with an induced CD signal due to solvent
induced twisting in polymer chain and turns into micellar
structure by triggering oxidation. This study has
tremendous impact on the correlation among preparation
of donor-acceptor ferrocene based copolymers and its
oxidation triggered morphological aspects with electronic
properties.
1
1
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Dr. Malik acknowledges CSIR, INDIA (project no.
0
2(0161)/ 13/EMR-II) for the partial support of finance. M.
2
2
0
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J-C. Eloi, D. A. Rider, G. Cambridge, G. R. Whittell, M. A. Winnik
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K. Bera is thankful to CSIR, New Delhi, India for the
fellowship.
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Notes and references
4
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