Undulation induced tuning of electron acceptance by edge-oxidized graphene oxide

Article abstract of DOI:10.1016/j.saa.2018.06.052

Edge-oxidized graphene oxide (EOGO) nanosheets are good acceptors of electrons. We have employed a suitably designed pyrene-tailed fluorescent probe to establish that the electron acceptability of EOGO can be tuned by undulation of the GO sheet. Comparison between EOGO and single-walled carbon nanotubes (SWCNT) on electron acceptance from the probe molecule shows that the efficiency of π-π stacking between pyrene and the graphene sheet plays the key role.

Full text of DOI:10.1016/j.saa.2018.06.052

Accepted Manuscript
Undulation induced tuning of electron acceptance by edge-
oxidized graphene oxide
Ashutosh Ghosh, Sourav Kanti Seth, Pradipta Purkayastha
PII:
DOI:
Reference:
S1386-1425(18)30592-4
doi:10.1016/j.saa.2018.06.052
SAA 16211
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Received date: 10 May 2018
Revised date:
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8 June 2018
14 June 2018
Please cite this article as: Ashutosh Ghosh, Sourav Kanti Seth, Pradipta Purkayastha ,
Undulation induced tuning of electron acceptance by edge-oxidized graphene oxide. Saa
(2018), doi:10.1016/j.saa.2018.06.052
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ACCEPTED MANUSCRIPT
Undulation induced tuning of electron acceptance by edge-oxidized graphene oxide
Ashutosh Ghosh,a Sourav Kanti Setha and Pradipta Purkayastha* a,b
aDepartment of Chemical Sciences, Indian Institute of Science Education and Research
(IISER) Kolkata, Mohanpur 741246, WB, India.
bCentre for Advanced Functional Materials (CAFM), Indian Institute of Science Education
and Research (IISER) Kolkata, Mohanpur 741246, WB, India.
E-mail: ppurkayastha@iiserkol.ac.in (P. Purkayastha)
ABSTRACT
Edge-oxidized graphene oxide (EOGO) nanosheets are good acceptors of electrons.
We have employed a suitably designed pyrene-tailed fluorescent probe to establish
that the electron acceptability of EOGO can be tuned by undulation of the GO sheet.
Comparison between EOGO and single-walled carbon nanotubes (SWCNT) on
electron acceptance from the probe molecule shows that the efficiency of
between pyrene and the graphene sheet plays the key role.
- stacking
Keywords: edge-oxidized graphene oxide; single-walled carbon nanotube;
stacking; fluorescence quenching; electron acceptability

-

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1. Introduction
The two-dimensional carbon monolayers in graphene composed of all-sp2-
hybridized carbons are lightweight and exhibit high electrical and thermal conductivity
having highly tunable surface area with strong mechanical strength and chemical
stability [1-3]. These outstanding physical, mechanical and chemical properties have
made graphene-based materials suitable for electrochemical energy storage,
sustainable energy generation, fuel cell applications, and in supercapacitors and solar
cells [2-4]. Of special interest is graphene oxide (GO) that is obtained by scraping
graphite oxide into layered sheets through sonicating or mechanical stirring [5].
Presence of the epoxy and hydroxyl groups, in combination of the graphene-based
lattice, enable GO to act as effective anchoring sites to immobilize various active
species [6]. However, by controlling coverage, arrangement, and relative ratio of the
epoxy and hydroxyl groups the band gap of GO can be tailored [7-10]. The oxygen
functional groups enhance the dispersibility of GO in solvents in expense of the
performance as reduced GO (rGO) based applications [11]. Under such circumstance,
edge-selective oxidation of graphite is an effective pre-treatment method to enhance
dispersion stability after exfoliation [12-15]. GO mainly contains carboxyl groups on
the edge, and hydroxyl and epoxide groups on the basal plane of graphene oxide [16].
Hence, the edge-oxidized GO (EOGO) sheets will contain carboxyl groups on the
edge of graphite and thus bear higher edge reactivity than that of the basal plane. This
improves the dispersion stability with less destruction of the conjugation state [11]. In
addition, lack of epoxy and hydroxyl groups on the basal plane of the EOGO sheets
shall ensure efficient - interaction with a suitably substituted guest molecule.
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Adoption of GO instead of graphene sheet helps to overcome the major problem of
insolubility in solvents. GO can be reduced to reduced graphene oxide (rGO) that can be
modified to acquire functionalization [16]. Reduction of GO may lead to the formation of
amorphous carbon, that disrupts the sp2-network of graphene [17]. Hence, reduction of GO is
not recommended in studies on electronic properties of graphene. In this piece of work, we
have studied the electron acceptability of graphene by adopting EOGO as the substrate. A
fluorescent probe has been synthesized that has a cationic electron deficient benzothiazole
unit at one end and a pyrene unit on the other (Scheme 1). While the emission signal from
this compound (represented as 1) is principally due to intramolecular charge transfer (ICT),
the pyrene end can participate efficiently in - interaction with the EOGO basal plane. Thus,
the guest-host interaction could develop a combination of electron accepting centres where
one is the cationic benzothiazole unit and the other is the EOGO sp2 carbon framework.
Scheme 1. Molecular structure of 1.
The planar sheets of EOGO can act as electron acceptor thus getting reduced by
gain of electrons [18,19]. In recent past there have been a few works on studying
electron transfer to SWCNTs [20-22]. Guldi et al. found photoinduced electron
transfer within a SWCNT–ferrocene nanohybrid [20]. Alvaro et al. produced a highly
soluble SWCNT derivative functionalized with pentyl esters and pyridyl isoxazoline
groups [21]. The pyridyl-functionalized SWCNTs suffered quenching due to energy
transfer when complexed with a zinc-porphyrin composite. In this context, a
promising work has been reported by Ehli et al. while exploring interactions of
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SWCNTs with a pyrene derivative and two water-soluble porphyrins [22]. They
established formation of stable hybrid between SWCNT and the pyrene-labelled
ammonium compound through - stacking in aqueous medium. The pyrene probe
used by Ehli et al. has distant similarity with the probe that we have used in the present
report [22].
However, there is hardly any quantitative report on the effect of minute undulation
on the sheet to the phenomenon of electron acceptance. In this work, we have used
single-walled carbon nanotubes (SWCNT) as a substrate for the same electron donor
to mimic undulation on the EOGO surface. The SWCNTs were derivatised with
hydroxyl groups to solubilise them in aqueous medium. The principal aim of this study
is to observe the change in the event of electron donation to the EOGO basal plane due
to alteration in the extent of - stacking of the pyrene group in 1 on the carbon
framework of the EOGO and the SWCNT.
2. Experimental
2.1. Materials
All the high purity chemicals and non-aqueous solvents used in the spectroscopic
experiments were bought from Sigma-Aldrich and used as received without further
purification. HPLC grade or Millipore water has been used in all the experiments whenever
required.
2.2. Synthesis of 1
1-Pyrenecarboxaldehyde (1e) was purchased from Sigma-Aldrich and used as
received. 1d (pyren-1-ylmethanol), 1c (1-(bromomethyl)pyrene) and 1b (diethyl (pyren-1-
ylmethyl)phosphonate) were synthesized following previous reports with some modifications
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[14, 16-22]. Briefly, the procedure to synthesize 1b is as follows: 1e (230 mg, 1.0 mmol) was
poured into anhydrous methanol (40 ml) resulting into a suspension and to this excess of
NaBH4 (380 mg, 10.0 mmol) was added in portions at 0˚C. The mixture was stirred for 1 h at
room temperature and then refluxed for 4 h. The mixture was cooled and saturated aq. NH4Cl
was added to get a white precipitate. It was extracted with dichloromethane (5 x 10ml) and
dried with anhydrous Na2SO4. The solvent was evaporated to obtain a pale yellow coloured
solid 1d. Thus obtained 1d (186 mg, 0.8 mmol) was taken into dry THF (4 ml) forming a
suspension and PBr3 (0.114 ml, 1.2 mmol) and was added to the mixture. The mixture was
stirred for 1 h at room temperature. The residue was filtered and washed with Et2O to give the
desired product 1c. The mixture of 1c (147 mg, 0.5 mmol) and an excess amount of triethyl
phosphite (2 ml) was taken in CHCl3 (10 ml) and refluxed overnight in inert atmosphere. The
mixture was settled down to room temperature and the excess triethyl phosphite was
evaporated under reduced pressure. The crude product was purified by column
chromatography using 1-10% methanol/dichloromethane to give white coloured 1b. An
excess amount of NaH (43 mg, 1.8 mmol, 6 equiv.) was added gradually to a solution of
phosphonate derivative 1b (106 mg, 0.3 mmol) in dry THF (30 ml) and the mixture was
stirred for 1 h at room temperature. To this, terephthalaldehyde (41 mg, 0.3 mmol) was added
at once. The mixture was stirred overnight in dark at room temperature. 10 ml of distilled
water was added upon completion of the reaction to destroy excess NaH present in the
solution. The solvents were evaporated using a rotary evaporator (procured from Citizen) and
the crude product was purified by column chromatography using 10-60% of
hexane/dichloromethane mixture as eluent. The solvents were evaporated to obtain the
orange-yellow coloured 1a ((E)-4-(2-(pyren-1-yl)vinyl)benzaldehyde). The mixture of 1a
(33.2 mg, 0.1 mmol) and 3-ethyl-2-methylbenzothiazolium iodide (43 mg, 0.14 mmol) was
dissolved in methanol (20 ml). An excess amount of NaOMe (~10 equiv.) was added to the
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mixture and refluxed overnight. The initial yellow solution turned into wine-red coloured
solution at the end of reflux. The mixture was cooled to room temperature and methanol was
evaporated. The whole process has been represented by Scheme 2. The crude product was
washed with distilled water, dried and purified by column chromatography using 1-10%
methanol/dichloromethane mixture as eluent to obtain the red-brown coloured 1 (3-ethyl-2-
1
((Z)-4-((E)-2-(pyren-1-yl)vinyl)styryl)benzo[d]thiazol-3-ium) (Scheme 1). H-NMR (400
MHz, DMSO-d6): δ (ppm) 8.857 (d, 1 H, J = 9.92 Hz), 8.655 (d, 1 H, J = 16.04 Hz), 8.592 (d,
1 H, J = 8.40 Hz), 8.460 (d, 1 H, J = 8.4 Hz), 8.368-8.286 (m, 6 H), 8.221 (s, 2 H), 8.183 (d,
2 H, J = 8.4 Hz), 8.123-8.089 (m, 4 H), 7.900 (t, 1 H, J = 7.26 Hz), 7.814 (t, 1 H, J = 8.02
Hz), 7.704 (d, 1 H, J = 16.04 Hz), 5.013 (q, 2 H, J = 7.12 Hz), 1.514 (t, 3 H, J = 7.26 Hz),
ESI-MS (m/z): calculated - 492.18 , found - 493.19 [M+H]+.
Scheme 2. (i) NaBH4 / methanol, 1 h at room temperature to refluxed for 4 h; (ii) PBr3 /
THF; (iii) triethyl phosphite, CHCl3, refluxed overnight under N2 atmosphere; (iv) NaH
/THF, terephthalaldehyde; v) 3-ethyl-2-methylbenzothiazolium iodide, NaOMe/methanol,
refluxed overnight.
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2.3. Instruments and methods
The absorption spectra were measured in a Hitachi U-2900 spectrophotometer. The
steady state fluorescence measurements were done in a QM-40 from PTI, Inc. The time
resolved fluorescence anisotropy measurements were done by the method of time-correlated
single photon counting using a picoseconds spectrofluorimeter from Horiba Jobin Yvon IBH
equipped with a FluoroHub single photon counting controller, Fluoro3PS precision
photomultiplier power supply, and FC-MCP-50SC MCP-PMT detection unit. 375 nm
(resolution < 100 ps) laser was used as the excitation pulse.
The anisotropy, r can be represented as:
IVV GIVH
r 
IVV 2GIVH
,
where, IVH and IVV are the intensities obtained from the excitation polarizer oriented
vertically and the emission polarizer oriented in horizontal and vertical positions,
respectively. The factor G is defined as:
IHV
IHH
G 
,
where, the I terms refer to parameters same as mentioned above for the horizontal position of
the excitation polarizer.
2.4. Quantum chemical calculations
For the present semi-empirical calculations, we have used the commercial package,
HYPERCHEM 6.0 (Hypercube Inc., FL). The geometries of the molecules were optimised in
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the ground state using the AM1 method. The calculations yielded the energies and dipole
moments in the ground state of the compound. The transition energies (Ei ) to different
j
excited electronic states correspond to the excitation of an electron from the orbital i
(occupied in the ground state) to the orbital j (unoccupied in the ground state). To find the
relative stability of the different rotational conformers (rotamers), we have preset one of the
torsional angles in 1 (between the pyrene and the phenyl ring; and the phenyl ring and the
benzothiazole moiety) as the case required and rotated the other at steps of 10 degrees. The
structures were then optimized.
3. Results and discussion
3.1. Theoretical findings
Geometry optimization of 1 reveals that the compound exists as a cis-trans hybrid.
The potential energy surface diagram obtained by changing the various dihedral angles in the
compound showed that combination of trans configuration of the pyrene end and cis
configuration of the benzothiazole end with respect to the central phenyl ring provides the
most stable geometry in aqueous medium (Fig. 1). Although the activation energy for the
trans-pyrene-trans-benzothiazole looks lesser (27.3 kcal mol-1) compared to the trans-pyrene-
cis-benzothiazole (30.0 kcal mol-1) (Fig. 1a) but the dipole moment of the former is much
higher than the latter which imposes instability to the all-trans conformer (Fig. 1b) [23].
Moreover, the NMR spectrum for 1 shows signal for one trans proton (see ESI†) confirming
the existence of the configuration (i).
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(b)
(a)
24
-7140
-7160
-7180
-7200
(ii)
(iii)
20
(i)
(ii)
(iii)
16
12
(i)
0
30
60
90
120 150 180
0
30
60
90
120 150 180
Torsion angle (degrees)
Torsion angles (degrees)
Fig. 1. (a) Solvated potential energy surface diagrams and (b) change in dipole moments at
different torsion angles to achieve the three stable configurations as numbered from (i) to
(iii). The dihedral angles of (i) the benzothiazole moiety and (ii) the pyrene moiety with
respect to the central phenyl ring have been changed from 0 to 180o. In (iii) the dihedral angle
of the pyrene moiety has been changed from 0 to 180o keeping the benzothiazole moiety
fixed at trans configuration. The Roman numerals in the potential surface plots indicate the
stable configurations as shown in the bottom panel.
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3.2. Steady state spectroscopic results
0.06
0.04
0.02
0.00
0.20
0.15
0.10
0.05
0.00
(b)
(a)
0 g/ml
6 g/ml
0 g/ml
2.85 g/ml
250 300 350 400 450 500 550 600
250 300 350 400 450 500 550 600
Wavelength (nm)
Wavelength (nm)
(d)
(c)
0 g/ml
0 g/ml
2.85 g/ml
6 g/ml
400
450
500
550
600
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
(e)
8
6
4
2
SWCNT
EOGO
0
1
2
3
4
5
6
[Q] (g/ml)
Fig. 2. (a, b) Absorption and (c, d) emission spectra of 1 with various concentrations of
EOGO and SWCNT, respectively in each case in aqueous medium; (e) shows comparative
quenching of fluorescence from 1 by EOGO and SWCNT. The samples were excited at 380
nm and the emission intensities in (e) have been monitored at 467 nm.
Fig. 2 shows the steady state spectroscopic results on interaction of 1 with EOGO and
SWCNT in aqueous dispersion. From the absorption spectral changes it is quite clear that 1
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interacts very differently on the two substrates. The absorbance of the * band at 385 nm
decreases profusely on addition of up to 2.85 g/ml of EOGO (Fig. 2a); whereas, it changes
very little even on addition of up to 6 g/ml of SWCNT (Fig. 3b). Due to interaction of the
pyrene moiety of 1 with the carbonaceous -electron rich substrates (EOGO and SWCNT),
there may be two types of operative phenomena: (i) - stacking with the substrate and (ii)
electron transfer from the donor to the acceptor. Both these events would affect the
absorption spectrum of the donor molecule by enhancement and reduction of absorbance,
respectively. The process which dominates will reflect in the spectral change as observed in
the absorption spectral changes in Fig. 2a and 2b. This is also reflected in the steady state
fluorescence spectrum of 1 where the intensity decreases 4 times more for EOGO (Fig. 2c)
compared to that with SWCNT (Fig. 2d). No shift in the emission maximum of 1 in SWCNT
and corresponding fluorescence quenching confirms the existence of the probe outside the
nanotube. The -electron rich pyrene moiety could either form excimer or stack to the
SWCNT surface through - stacking interaction. Dominance of the latter interaction is
perceived from the extent of quenching in fluorescence in compliance to Ehli et al. [22]. In
both the cases the F0/F vs. quencher concentration plots deviate from linearity (Stern-Volmer
plots) with an upward curvature indicating static and dynamic quenching of the fluorophore
(Fig. 2e) [24]. Enhancement in F0/F is more than 3 times for EOGO compared to SWCNT.
The modified Stern Volmer equation used to get the quenching profile is
F
F
0


1KD

Q

1KS

Q


(Eq. 1), where F0 and F are the fluorescence intensities in
absence and presence of quencher Q, KD and KS are the quenching constants for the dynamic
and static quenching processes. The static quenching in the ground state due to probe-
substrate interaction is perceived from the absorbance changes and the dynamic quenching
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may be depicted to electron transfer from 1 to the substrates. To calculate KD and KS, we
F
F
0
have modified the Stern Volmer equation further as
1Kapp Q
  (Eq. 2), where
F
F
1
[Q]




0
K 
1


KD  KS

 KDKS

Q
(Eq. 3) [24]. Plotting Kapp against [Q] yields a


app

straight line with intercept KD+KS and slope KDKS (Fig. 3a and 3b). The individual values for
KD and KS are deduced from the solutions of the quadratic equations generated in each case.
The KD values for EOGO and SWCNT were found to be 211.5 mg/l and 102.5 mg/l,
respectively and the corresponding KS values were 3.5 mg/l and 0.47 mg/l, respectively.
Greater values of KS and KD for EOGO compared to those for SWCNT indicate better static
interaction of 1 with EOGO and hence higher electron transfer.
350
(b)
(a)
2500
2000
1500
1000
500
300
250
200
150
100
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
1
2
3
4
5
6
[SWCNT] (g/ml)
[Q] (g/ml)
Fig. 3. Comparison of the apparent quenching constants (Kapp) for 1 in (a) EOGO and (b)
SWCNT to determine KD and KS separately.
3.3. Confirmation of electron transfer
We have confirmed electron transfer from 1 to the graphene layer by introducing
dimethylamine (DMA), a sacrificial electron donor, and prior to the addition of 1 into the
EOGO system. DMA is insoluble in water, however, the graphene sheets in EOGO allowed
solubilisation of about 4 mM of DMA in the aqueous medium. Due to donation of electrons
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to the EOGO sheets and thereby quenching their thirst of electrons, it is expected that
quenching of fluorescence of 1 by EOGO will be restricted to a large extent as the sheets will
not be in need of electron from the donor 1. Fig. 4 shows that indeed quenching of emission
from 1 by EOGO could be reduced to less than half by this technique that confirmed the
electron accepting property of EOGO.
400
450
500
550
600
650
Wavelength (nm)
Fig. 4. Fluorescence emission from 1 in presence of 2.85 g/ml EOGO (black) and 3.94 mM
DMA added to 2.85 g/ml EOGO (red). Excitation of the sample is done at 380 nm in
aqueous medium.
3.4. Time resolved studies
Time resolved emission and anisotropy decay provide consolidation to the above
findings as shown in Fig. 5 and Table 1. The faster component (1) is attributed to the excited
state electron transfer process and the relatively slower one (2) denotes the local emission of
1. It is clearly observed that the electron transfer got quenched much quicker with EOGO
compared to that with SWCNT. The local emission lifetime was affected concomitantly in
the two systems.
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(a)
(b)
1000
100
10
1000
0 g/ml
0 g/ml
100
10
2.28 g/ml
3 g/ml
0
10
20
Time (ns)
30
40
0
10
20
Time (ns)
30
40
Fig. 5. Time resolved decay of fluorescence emission of 1 with gradual addition of (a) EOGO
and (b) SWCNT in aqueous medium. The samples were excited at 375 nm.
Table 1. Time resolved decay data for 1 on interaction with EOGO and SWCNT. Values in
parentheses represent the percentage contribution to the fit and the 2 value show the
goodness of the fits. The lifetime values are within 5% error limit. The emission at 470 nm
has been monitored to acquire the data.
Substrate added to 1
τ 1 (ns)
τ2 (ns)
χ2
Concentration (g/ml)
EOGO
0
3.12 (65)
2.81 (53)
2.36 (35)
2.04 (32)
1.61 (22)
3.12 (65)
2.92 (59)
2.76 (56)
2.31 (40)
5.55 (35)
5.11 (47)
4.43 (65)
4.24 (68)
3.86 (78)
5.55 (35)
5.39 (41)
5.08 (44)
4.46 (60)
1.09
1.14
1.10
1.17
1.15
1.09
1.11
1.06
1.06
0.57
1.14
1.71
2.28
0
SWCNT
1.00
2.00
3.00
To establish the rigidity of interaction of 1 with the EOGO and the SWCNT
substrates, we performed the anisotropy decay studies (Table 2). The data show that with
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increase in concentration of either EOGO or SWCNT the decay time () increases indicating
mutual absorptive interaction. However, at similar concentrations of the respective substrates
the magnitude of  is much higher for EOGO compared to SWCNT. The degree of stability
of 1 on EOGO is achieved at double the concentration by SWCNT. This is a direct evidence
of sturdiness of 1 on EOGO compared to SWCNT. Since the adsorption on the substrates
takes place through - stacking interaction between the pyrene moiety of 1 and the sp2
carbon skeleton, it is understandable that the degree of electron transfer from 1 to EOGO or
SWCNT is controlled by the extent of the strength of the - stacking interaction. Fig. 6
explains the situation of 1 with EOGO and SWCNT pictorially.
Table 2. Time resolved anisotropy decay data for 1 on interaction with EOGO and SWCNT.
The 2 value show the goodness of the fits. The lifetime values are within 5% error limit.
The emission at 470 nm has been monitored to acquire the data.
Substrate added to 1
τ (ns)
χ2
Concentration (g/ml)
EOGO
0
0.285
0.57
0.85
1.42
0
0.151
0.172
0.174
0.184
0.221
0.15
1.08
1.03
1.01
1.09
1.18
1.02
1.09
1.13
SWCNT
1.5
0.17
3.0
0.20
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Fig. 6. Pictorial representation of alignment of 1 with EOGO basal plane and SWCNT
surface.
4. Conclusions
In summary, by applying a tactically synthesized ICT probe (1) to EOGO and
SWCNT, we could establish that - stacking on the carbonaceous acceptor surface of
EOGO determines the extent of electron transfer from an electron donor to the graphene
based acceptor. Undulations on the graphene surface affect the efficiency of electron transfer
to a great extent. The undulation on graphene was mimicked by using SWCNT that has a
curved surface and similar carbon skeleton as graphene. It was clearly observed that
adsorption of the probe on EOGO and SWCNT was different, in consequence of which we
found difference in the extent of electron transfer from 1 that we could correlate with the
strength of - stacking between the probe and the carbon skeletons of graphene.
Acknowledgements
This work was supported by the Science and Engineering Research Board of the
Department of Science and Technology, Government of India thorugh project number
EMR/2015/000950. AG and SKS acknowledge the Council of Scientific and Industrial
Research, New Delhi for their fellowships.
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Graphical abstract
(e)
8
6
4
2
SWCNT
EOGO
0
1
2
3
4
5
6
[Q] (g/ml)
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Highlights
 A tactically synthesized ICT probe form different - stacking on EOGO and
SWCNT.
 The acceptor surface of EOGO determines the extent of electron transfer from an
electron donor.
 Undulations on the graphene surface affect the efficiency of electron transfer to a
great extent.
 The undulation on graphene was mimicked by using SWCNT that has a curved
surface.
 Adsorption of the probe on EOGO and SWCNT was different and hence different
extent of electron transfer.
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