Fluorescence-Lifetime Imaging and Super-Resolution Microscopies Shed Light on the Directed- and Self-Assembly of Functional Porphyrins onto Carbon Nanotubes and Flat Surfaces

Article abstract of DOI:10.1002/chem.201605232

Functional porphyrins have attracted intense attention due to their remarkably high extinction coefficients in the visible region and potential for optical and energy-related applications. Two new routes to functionalised SWNTs have been established using

Full text of DOI:10.1002/chem.201605232

A Journal of
Accepted Article
Title: Fluorescence Lifetime Imaging and Super resolution
microscopies shed light on the directed- and self-assembly of
functional porphyrins onto carbon nanotubes and flat surfaces
Authors: Boyang Mao, David González Calatayud, Vincenzo Mirabello,
Haobo Ge, Benjamin J Hodges, Robert M. J. Jacobs, Ashley
M. Shepherd, José Alberto Ribeiro Martins, Navaratnarajah
Kuganathan, Jorge Bernardino De La Serna, Stanley W.
Botchway, and Sofia Ioana Pascu
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To be cited as: Chem. Eur. J. 10.1002/chem.201605232
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Chemistry - A European Journal
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Fluorescence Lifetime Imaging and Super resolution microscopies shed
light on the directed- and self-assembly of functional porphyrins onto
carbon nanotubes and flat surfaces
Boyang Mao,#[a],[b] David G. Calatayud,#[a] Vincenzo Mirabello,#[a] Navaratnarajah Kuganathan, Haobo
[c]
[a]
[d]
[d]
[e]
Ge, Robert M. J. Jacobs, Ashley M. Shepherd, José Alberto Ribeiro Martins, Jorge Bernardino
[f]
[a]
[f]
[a]
De La Serna, Benjamin J. Hodges, Stanley W. Botchway, and Sofia I. Pascu*
Abstract: Functional porphyrins have attracted intense attention due
to their remarkably high extinction coefficients in the visible region
and potential for optical and energy-related applications, ranging
from biosensors or photovoltaic based devices. Two new routes to
functionalized SWNTs have been established using a bulky Zn(II)-
porphyrin featuring thiolate groups at the periphery. We probed the
optical properties of this Zinc(II)-substituted, bulky aryl porphyrin and
those of the corresponding new nano-composites with single walled
carbon nanotube (SWNTs) and coronene, as a model for graphene.
We report hereby on: (i) the supramolecular interactions between the
pristine SWNTs and Zn(II)-porphyrin by virtue of π-π stacking, and
(XPS) studies highlighted the differences in the covalent versus non-
covalent attachments of functional metalloporphyrins to SWNTs.
Gas phase density functional theory (DFT) calculations
(implemented in the Cambridge Serial Total Energy Package
CASTEP) indicated that the Zinc(II) porphyrin interacts non-
covalently with SWNTs to form a donor-acceptor complex. Taken
together, the techniques applied here for the characterization work
link findings in the solution, surface and gas phase. The appendage
of the porphyrin chromophore to the surface of SWNTs (covalently,
via
chemistry) affects the absorption and emission properties of the
hybrid system to greater extent than in the case of the
a directed assembly method based on dynamic covalent
(ii) a novel covalent binding strategy based on the Bingel reaction.
a
We investigated the nature of the porphyrin aggregates forming on
flat surfaces by Steady-state and time-resolved fluorescence
emission spectroscopies and imaging techniques including
fluorescence lifetime imaging microscopy (FLIM) together with
Atomic Force Microscopy (AFM) and Super Resolution Stimulated
Emission Depletion microscopy (STED). The functional porphyrins
used acted as dispersing agent for SWNTs and the resulting
nanohybrids showed improved dispersibility in common organic
solvents with respect to the pristine nanotubes. Characterization
techniques were carried out in the thin film phase on a variety of
surfaces, leading to the prediction that supramolecular
polymerization and host-guest functionalities control the
fluorescence emission intensity as well as the fluorescence lifetime
properties. For the first time, X-ray photoelectron spectroscopy
supramolecular functionalization of the SWNTs surface (non-
covalently, via donor-acceptor self-assembly). This represents a
synthetic challenge as well as an opportunity in the design of
functional nanohybrids for future sensing and optoelectronic
applications.
Introduction
Since the discovery of single-walled carbon nanotubes
(
SWNTs)[1] there has been a sustained interest in this material
from both academic and industrial research perspectives. This is
due to the fact that its rigid rod-like tubular structure, coupled
with a highly delocalized extended π-electron system, confers its
unique mechanical and electronic properties thus offering
#
These authors contributed equally to this work.
Dr. B. Mao, Dr. D. G. Calatayud, Dr. V. Mirabello, Dr.H. Ge, B. J.
Hodges, Prof. S. I. Pascu
[
a]
[2]
promises of technological advances. Ongoing studies show
that, upon functionalization of SWNTs with small molecules, it is
possible to develop prototype optoelectronic and photovoltaic
devices.[3] Thus, the interactions between fluorescent molecules,
as well as semiconductive polymers, and the aromatic surface of
SWNTs have attracted significant interest due to the potential
application of these new nanohybrids, particularly in solar cells.[4]
One of most attractive properties of SWNTs is the possibility
to chemically modify their surfaces by two general approaches:
covalent or non-covalent synthetic methodologies. The intrinsic
sp2 nature of the C-C bond of the SWNT allows reaction with
organic molecules bearing specific functional groups that react
with the outer walls of the tubular material. Porphyrins have
been used as versatile chromophores having the ability to act as
dispersing agents for carbon nanomaterials and to decorate
Department of Chemistry
University of Bath
Claverton Down, BA2 7AY, Bath, UK
E-mail: s.pascu@bath.ac.uk
Dr. B. Mao
National Graphene Institute and School of Physics and Astronomy
The University of Manchester
Booth Street East, Manchester M13 9PL
Dr. N. Kuganathan
Department of Materials
Imperial College London
South Kensington, London SW7 2AZ, UK
Dr R. M. J. Jacobs, Dr, A. M. Shepherd
Department of Chemistry
Chemistry Research Laboratory, University of Oxford
Mansfield Road, Oxford OX1 3TA, UK
Dr. J. A. Ribeiro Martins
Centro de Engenharia Biológica and Departamento de Química
Universidade do Minho
Campus de Gualtar, 4710-057 Braga, Portugal
Prof. S. W. Botchway, Dr. J. Bernardino De La Serna,
Central Laser Facility, Rutherford Appleton Laboratory
Research Complex at Harwell
[
[
[
[
b]
c]
d]
e]
C
60,[5] carbon nanotubes , graphene oxide or nanohornes[8] in
[6]
[7]
artificial photosynthetic devices.[9] SWNTs can interact via
aromatic stacking with electron-rich planar molecules resulting in
supramolecular donor-acceptor complexes. Such molecules
including porphyrins, planar, electron-rich, aromatic species are
characterized by remarkably high extinction coefficients in the
visible region. Tailor-made porphyrins designed to include thiol
units suitable for dynamic covalent chemistries have been
[
f]
STFC Didcot OX11 0QX, UK
Supporting information for this article is given via a link at the end
of the document.
1
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designed and their ability to recognize fullerenes or small
molecular acceptors such as naphtyldiimides in a controlled way
has been demonstrated.[10] Such nanohybrids have the potential
to offer valuable new technologies in the field of nano-
optoelectronic devices for energy conversion, sensing and
biological applications.[4c, 11] The recent work by Strano and
collaborators has demonstrated the potential of non-covalent
photoelectron spectroscopy (XPS), as well as fluorescence
imaging techniques we also demonstrated here that the
formation of the porphyrin-SWNTs supramolecular networks can
be controlled through directed and self-assembly giving rise to
extended nano-structures with interesting optical properties.
We characterized these materials in a thin film environment
using fluorescence lifetime measurements via time correlated
single-photon counting (TCSPC) combined with multi-photon
laser scanning confocal microscopy: this allows us to gain
detailed information on the environment of the fluorophor upon
immobilization within the nanomaterial hybrid and estimate the
influence of the different linking strategies on the emerging
materials‘ optical properties by observing their quenching
characteristics in solution and in the solid state.[7, 12]
[
13]
sensors based on SWNTs in biology.
Thiol-derivatized
functional Zn(II) porphyrin-based systems[7] have not been
investigated thus far in the context of generating new SWNTs
.
adducts with targeted optical properties. The combination of the
unique electron transport properties of the SWNTs together with
the promising photo-chemical properties of tailor-made
functional porphyrins could render their new adducts as potential
candidates for dispersible solid state scaffolds in nano-
optoelectronic devices and bio-sensing. Crucially, using X-ray
Figure 1. Schematic diagram of the formation of covalently and non-covalently linked Zn(II)-porphyrin@SWNTs hybrids (2) and (6). (Element Colour: oxygen –
red, sulphur – yellow – nitrogen – purple, light blue – zinc and grey – carbon. The structure of the disulphide linked side-product was obtained via single crystal X-
ray diffraction, a ball and stick representation is used for clarity)
2
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Characterization approaches rely on several different
efficient and novel physical measurement methods coupled
with multi-scale microscopies to identify unequivocally the
existence of a covalent and non-covalently linkage between
small organic molecules and carbon nanomaterials. The
iodohexane, pentane-2,4-dione and benzyl 3-oxobutanonate as
starting materials, and is described elsewhere.[16] The hexyl-
derived pyrrole acts as the basic building unit from which the
functional porphyrins are assembled.[ This synthetic method
allowed the incorporation of a versatile range of peripheral
groups at the porphyrin core, including extended alkyl chains or
bulky substituents. The nature of the metal center can be further
modified to tune their ability in host-guest recognition of
relevance for photovoltaic applications.
The resulting porphyrins were fully characterized in solution
and in the solid state, and X-ray diffraction studies showed that
indeed, under conditions specific for dynamic covalent chemistry,
the thiol groups lead to the formation of disulfie-bridged dimer.
This new compound could be isolated and fully characterized
17]
approach reported hereby can become
a
general
methodology for achieving control of photochemistry and
sensing capabilities of a wide range of optical and energy
materials by tailoring the nature of the interactions between
components using directed- and self-assembly processes in
solution and thin film.
Results and Discussion
(
ESI). Diffusion Ordered Spectroscopy (DOSY) was employed to
Synthesis and structural investigations: Single crystal X-Ray
Diffraction and Super resolution imaging
explore the possibility that porphyrins of type (1) self-aggregate
in the solutions used to perform experiments on the NMR scale.
This method was deemed a convenient spectroscopic tool as it
has been already applied in supramolecular chemistry to identify
inter- and intra-molecular self-assembled aggregates and aid the
understanding of host-guest interactions in solution.[18] Here, we
estimated the experimental diffusion coefficients (Dexp) to predict
the tendency of porphyrin molecules of type (1), and of its free-
base precursor to self-aggregate.[ DOSY spectra of Zn(II)-
porphyrin (1) and of its free-based porphyrin precursor were
acquired at 298 K and are shown in Figure 2 and ESI (Figure
S7). The Dexp for Zn(II)-porphyrin (1) and metal-free porphyrin
We report on the synthesis of novel SWNT-porphyrin
nanohybrids. The porphyrin of choice is a member of a family of
substituted aryl porphyrins known to be soluble in most common
solvents, e.g. toluene, dichloromethane and ethanol, and easily
derivatized (Compound 1). New functional materials were
synthesized via directed- and self- assembly methods: the
19]
synthetic strategies involve, respectively,
a new dynamic
covalent route (based on the thiols-disulphide exchange
involving a Zn(II) porphyrin dithiolated synthon and a thiolated
SWNTs surface) and a simple and straight forward non-covalent
linking strategy.
precursor were estimated to be 1.62 X 10 m s and 4.57 X 10-
-9
2
-1
1
0
2
-1
m s respectively. These findings indicate the low tendency
The non-covalent, outer surface modification of SWNTs was
carried out via supramolecular π-π interaction of the aryl
thioacetate-functionalized hexyl substituted Zn(II)-porphyrin (1)
of these porphyrins to self-aggregate in solution at the
concentration needed to obtain an NMR spectrum (5 mM). This
is consisted with previous reports for simpler porphyrin systems
such as Tetraphenylporphyrin (TPP).[19] It has already been
shown that Zn-porphyrins can be functionalized and their self-
assembly can generate nano-rings of 5-60 porphyrin units.[17a, 20]
Earlier observations by AFM or STM measurements performed
on drop-casted solutions show that such supramolecular
architectures can be described as nano-crystalline aggregates of
porphyrins having a shape of a ring or tower, with sizes varying
in the range of hundreds of nanometers, depending on the
imaging method used to evaluate the size. [7, 11c, 21]
Confocal fluorescence imaging of such films formed on
borosilicate glass (when a solution of the chromophore was
allowed to dry slowly onto a glass surface, vide infra and ESI)
also seemed to suggest that a self-assembled process is likely
to occur, but the precise shape or size could not be asserted
due to the low resolution. We were intrigued by the objects
formed on insulating surfaces (e.g. mica or borosilicate glass) for
the Zn(II) porphyrins and by their optical emissive properties.
(
Figure 1) in a one-step process, leading to the formation of
complex 2.
For the generation of the covalently linked system, the surface
of the SWNTs was first modified to incorporate a thiol group
capable of reacting with the -SH groups of 3 and generating S-S
disulphide bridges. The Bingel reaction was used to generate
thiol groups on the surface of the nanotobes, which in turn can
[
14]
be zate the covalent appendage
Figure 1) via thiol-disulfide exchanges.
The Zn(II)-porphyrin (1) has been specifically chosen to
to the SWNTs surface
(
incorporate four hexyl chains on the exocyclic positions of the
macrocycle ring to enhance its solubility in common organic
solvents. Moreover, two aryl thioacetate side groups have been
included with the intention to help direct the dynamic covalent
linkage of porphyrin to the thiol-functionalized outer surface of
the SWNTs. For the synthesis of the free porphyrin and zinc(II)
porphyrin on a milligram scale, suitable for further synthetic
manipulations, the general reaction methodology proposed by
Twyman and Sanders was adapted.[15] This method involved 1-
3
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Figure 2. a) 1H DOSY NMR spectrum (500 MHz, 298 K, CDCl
) of Compound 1. Diffusion coefficient = 1.62x 10-9 m2 s-1 which limited ability of this Zn(II)
3
prophyrin to aggregate in solution even at ca 5 mM (needed to record a spectrum); b) 3D TMAFM image of Zn(II)-porphyrin (compound 1) self-assembly on mica.
For the TM AFM measurement, Zn(II)-porphyrin sample was prepared by using the spin coating at 3000 rpm of the Zn(II)-porphyrin solution (18.3 µM in CHCl
onto a 2 cm mica substrate. c) Tapping mode AFM image of Zn(II)-porphyrin self-assembly crystal; d) Profile analysis showing the heights of line.
3
)
2
Super resolution imaging, additionally to single- and multi-
photon confocal fluorescence imaging techniques, was
employed to further characterize the sub-structure morphologies
of the tower-like aggregates observed for (1) on surfaces, and
thus aid our understanding thereof. These optical imaging
experiments in thin film were crucially necessary since solution
experiments by DOSY suggested that at ca. 5 mM concentration
functionalized porphyrins (synthesized as described below) did
not yield such an effect at all, and therefore could not be imaged
by super resolution under these conditions. The presence of
Zn(II) center, together with the thiol-protected aryl groups and
the ability to form aromatically stacked aggregates, play a crucial
role in mediating the nature of the self-assembly of these
porphyrins on insulating surfaces. The application of super
resolution, STED, has allowed the observation of these
structures on the nanoscopy scale (<100 nm). As yet, the effect
of such ‘tower’-like structure formation on the optical properties
of the nanomaterial emerging remained unexplored.
As stated above, the pristine strands of SWNTs were
subsequently used as template in an attempt to control the
formation of such cylindrical supramolecular assemblies of
porphyrins offering an alternative synthetic route to the covalent
functionalization of SWNTs which has already attracted broad
attention in the scientific community.[9a, 22]
3
needed to record NMR spectroscopy in CDCl , little convincing
evidence for aggregation was found. Stimulated emission
depletion (STED) microscopy with 775 nm depletion laser was
used to assess the morphology and the emission properties of
the solid aggregates of the Zn(II)-porphyrin (1) previously
observed by confocal fluorescence as well as AFM (Figure 3).
Interestingly, STED deconvolution and surface 3D rendered
image reveals that such Zn(II)-porphyrin (1) microcrystalline
cylinders are roughly 2 m in height. The larger rings of the
tubular structures have a thickness of 611.3±3.7 nm, whilst the
smaller is 323.5±2.7 nm. The tubular sub-structure is formed of
smaller concatenated rings, which grow in size from the bottom
to the top. The smallest ones observed were of 50nm diameter
and the largest 130nm found both at the top and towards the
periphery of the basal plane, and this method does not allow the
exploration of aggregates smaller than ca 30 nm. Interestingly,
the free base porphyrin precursor only displayed a very weak
STED effect, and the size of the porphyrin stacks could not be
fully interpreted by this method, whereas the SWNTs
Figure
1 shows the reaction scheme of the Zn(II)-
porphyrin@SWNTs non-covalent complex (2). A dispersion of
SWNTs in chloroform and a Zn(II)-porphyrin solution in the same
solvent were mixed and stirred for 24 hours at room room
temperature and the solid nano-composites obtained were
collected by nano-filtration and washing with ethanol.
4
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3
Figure 3. Stimulated emission depletion (STED) microscopy of Zn(II)-porphyrin (1) thin film of a CHCl solution drying out on a borosilicate glass surface, λex =
488 nm, STED = 775 nm, laser power intensity = 179 mW, λem = 580 nm. Scalebar: 10 μm. Top image (a), Deconvolved image of a 3D reconstruction obtained
from stacked individual STED images. The image is shown in perspective for better appreciation of the different height of the tubular structures. The X, Y, and Z
scales are in µm. Below, from right to left, deconvolved image of the 3D reconstruction as projected in the XY plane (b). The line profile crossing 3 different tubes
is shown in the graph below (1). The highlighted squared region in image (b) is magnified to the right; each image from left to right (c-f), is showing the same
tubular structure at different z-planes, with a difference between planes of 300nm, being (c) the lowermost region in contact with the surface, and (f) the
uppermost. Below each image can be observed the squared region zoomed in (g-j). The lines profiles drawn in (h) and (j) are represented in the graphs 2, and 3
respectively.
The self-assembly approach provided a simple one-step
reaction method leading to the supramolecular adduct (2): the
formation of the Zn(II)-porphyrin@SWNTs adduct 2 was likely
due to the aromatic stacking between the pyrrole rings of the
Zn(II)-porphyrin and the SWNT framework. During synthesis it
emerged that the formation of disulphide bridged porphyrin
oligomers is favorable over time in the presence of oxygen and
The directed assembly, leading to the disulphide bridged
covalent linkages, was achieved as described in Figure 1.
Spectroscopic investigations were carried out aiming to probe
whether or not this route can lead to a change of the carbon
hybridization in the outer wall of the carbon-based material. The
subsequent loss of the electronic conjugation, which affects the
electron-acceptor or the electron-transport properties was an
expected feature of this approach.[22i] In order to achieve a
covalently linkage between SWNTs and the complex (3), a four-
step path reliant on dynamic exchange known to occur between
thiol-disulfide was devised (Figure 1).
in aqueous environments.
A disulfide dimer side product
featuring two S-S bridges and aromatically stacked porphyrins
was isolated and fully characterized (Figure 4 and ESI).
Therefore, in the non-covalent linked synthesis strategy, the
thioacetate-protected Zn(II)-porphyrin was prepared and used in
fresh solutions.
5
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Figure 4. Molecular structure determinations by synchrotron single-crystal X-ray diffraction: a) the emerging side-product Zn(II)-porphyrin as a
disulfide-bridge dimer, b) cell packing diagram, of the Zn(II)-porphyrin disulfide dimer, view over axis a. Hydrogen atoms were removed for clarity and
c) DFT optimised structures of model (8,8), (9,9) and (10,10) SWNTs.
Chemical modifications of both compound (3) and SWNTs
were introduced prior to the start of the thiol-disulphide reaction.
These aimed to allow the control of the porphyrins position on
the surface of the SWNTs. First, as shown in Figure 1, (Step i)
SWNT cyclopropanation reaction was achieved in the presence
of diethyl bromomalonate and 1,8-diazabicyclo[5.4.0]undecene
Microscopy investigations of the SWNT hybrids by AFM and
TEM
The morphological properties of the two products, denoted
Complexes (2) and (6), were evaluated and compared by
Tapping mode AFM (TM AFM) and TEM. In order to investigate
the different electron-acceptor or electron-transport properties in
covalent and non-covalently linked Zn(II)-porphyrin@SWNTs
complexes, UV-vis, steady-state time-resolved fluorescence
emission studies, together with Raman spectroscopy, were
performed. TM AFM measurements were carried out to further
study the SWNTs nanohybrids and compare their aggregation
with that observed in Zn(II) porphyrins. Interestingly, for SWNTs,
HOPG worked best as the substrate of choice for AFM images
(
DBU). The resulting mixture led to the (outer surface) modified
SWNTs (4). Next, (Step ii) SWNTs strands were further
derivatized by reacting (4) with 2-mercaptoethanol, which acts
as both liner spacer and thiol-precursor linker. At Step iii, Zn(II)-
porphyrin (1) was deprotected, by treatment with excess
hydrazine to obtain the thioacetyl de- protected Zn(II)-porphyrin
(
3). Finally, (Step iv) compounds (3) and (5) were assembled,
aiming to covalently coupled their thiols appendages in the
presence of DBU thus yielding the novel nanohybrid disulphide
bridged adduct (6).
(
ESI). The AFM images in Figure 5 (a) and (b) show that the
covalent Zn(II)-porphyrin@SWNTs nanohybrids is relatively well
dispersed in solution and its diameter is ca 11 nm. The AFM
images in Figure 5 (d) and (e) indicate that non-covalent Zn(II)-
porphyrin@SWNTs nanohybrids are ca. 9 nm in diameter. The
slight difference in diameter between the non-covalently linked
and covalently linked nanohybrids is likely a direct consequence
6
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Figure 5. Tapping Mode AFM imaging on mica surfaces: (a), (b) image and 3D representation of AFM image of covalent Zn(II)-porphyrin@SWNTs nanohybrids
6); (c) Schematic diagram of compound (6); (d),(e) image and 3D representation of non-covalent Zn(II)-porphyrin@SWNTs nanohybrids (2), (f) Schematic
(
diagram of compound (2)
of the nature of the bond between the two components of the
complex (porphyrin and SWNTs) as well as the presence or
absence of a linker. This generated a substantial morphologic
variation, highlighting the significance of choosing a covalent or
supramolecular approach for SWNTs functionalization. Such
differences observed in the estimated diameters of the
nanohybrids (2) vs (6) could be assigned to the following: i) the
different surface arrangement of the Zn(II)-porphyrin on the
surface of SWNTs in the non-covalently linked complex, ii) the
plane of the porphyrin containing the four pyrroles rings is
parallel to principal axis of the SWNT and iii) the aromatic ring
stacking interactions maintain the porphyrin linked to the SWNT.
Within the covalently linked complex (6), the plane of the
porphyrin is likely held at a longer distance with respect to the
principal axis of the SWNT, compared to the case of (2).
However, the diameter differences observed between the two
specimens may also be due to several SWNTs assembled
together and thus forming bundles.
TEM measurements were employed to further characterize
the morphology of SWNTs. The Figure 6b shows TEM
micrograph of non-covalent Zn(II)-porphyrin@SWNTs complex
(2). Dark areas with stronger contrast arise due to the presence
of zinc, while the brightest sites of interest correspond to areas
where there are only lighter elements such as hydrogen, carbon
and nitrogen, consistent with the EDS analyses of different
areas of interest. Figure 6b shows that the SWNT have a rough
surface likely due to the aggregates formed by porphyrin nano-
crystallization. The significant presence of a surface-roughened
SWNTs indicates coverage of the surface, suggesting the
presence of the Zn(II)-porphyrin on the surface of the SWNTs.[23]
7
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improving their homogenous distribution over the surface of the
SWNTs. In complex (2), the donor-acceptor nature of the non-
covalent bond between Zn(II)-porphyrin and SWNTs allows for a
higher degree of porphyrin mobility onto the sp2 surface.
Therefore,
a non-homogeneous distribution of the donor
molecules onto the acceptor surface can facilitate the
aggregation of porphyrin molecules. AFM and TEM (both
deemed reliable techniques to assess the morphology of
materials produced by chemical modification of the surface of
SWNTs) confirm the formation of isolated individual SWNT from
bundled ropes upon derivatisation.[22i]
Both covalently and non-covalently linked Zn(II)-
porphyrin@SWNTs nanohybrids were found to be relatively well
dispersed in organic solvents upon specimen preparation for
microscopies. Pristine SWNTs were found to be more difficult to
disperse in solution even after prolonged treatment of sonication,
and also the intact SWNTs were observed to be aggregated on
the Highly Ordered Pyrolytic Graphite (HOPG) substrate. A
similar conclusion regarding aggregation behavior can be
derived from absorption spectroscopy in the dispersed phase
(
vide infra).
Surface derivatization studies on SWNT hybrids by Raman
and XRD spectroscopy
Raman spectroscopy was applied to evaluate the bulk
properties and probe the degree of crystallinity of graphitic
carbon structure, including whether or not this structure
remained intact following extensive synthetic chemistry
manipulations.[
24]
Figure 7. Solid state Raman spectroscopy of SWNT (830 nm), Bingel reaction
purified SWNT (4), non-covalently linked SWNTs@Zn(II)-porphyrin complex
(
2) and covalently linked SWNTs@Zn(II)-porphyrin complex (6); Inset: Raman
Figure 6. TEM microscopy: (a) the free pristine SWNTs; (b) non-covalent
spectrum RBM.
Zn(II)-porphyrin@SWNTs
nanohybrids
(2);
(c)
covalent
Zn(II)-
porphyrin@SWNTs nanohybrids (6).
The Raman spectra shown in Figure 7 were recorded by
dispersing samples in CHCl :EtOH (1:1) using the excitation
TEM micrograph of the covalent Zn(II)-porphyrin@SWNTs
complex (6) (Figure (c)) shows a rather homogenous
3
6
wavelenght λex of 830 nm. Spectra showed the G band at ca
1581 cm-1 region (primary graphitic mode) corresponding to a
splitting of the E2g stretching mode of graphite, reflecting the
distribution of the complex over the surface of the SWNTs with
regular aggregation points (areas with stronger contrast) and a
lower agglomeration of the SWNTs was observed for 6 vs 2.
These subtle differences in morphology observed in the
micrographs of (2) and (6) might be explained by the covalent
bond of the Zn(II) porphyrin to the SWNTs which strongly
immobilizes the molecules, reducing their agglomeration and
2
presence of sp -hybridized carbons. The Raman spectra of
-1
SWNTs (4) exhibit a G-band at 1585 cm and a D band at 1289
-1
cm . The I
SWNTs (4) (0.16) increased somewhat compared to the I
band intensity ratio of primitive SWNTs (0.11), indicating that
D
/I
G
band intensity ratio of covalent functionalized
D
/I
G
8
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Bingel reaction introduced surface defects and disordered
graphite structures onto the SWNTs. It can be seen that the
1.02 nm and 1.64 nm. The spectra recorded for the covalently
functionalized SWNTs in complex (4) remained largely
unchanged with respect to free SWNTs, which exhibits two
-1
Raman spectra of complex (2) shows G-band at 1588 cm and
-1
-1
-1
D-band at 1292 cm . A 0.17 I
D
/I
G
band intensity ratio was also
maxima RBM peaks at 234 cm and 145 cm , suggesting that
the diameters of the nanotubes are between 1.02nm and 1.64
nm.[25] The non-covalent Zn(II)-porphyrin@SWNTs complex (2)
shows a maximum RBM peaks at 240 cm-1 and 154 cm . This
indicates that in, dispersions of porphyrin (2) in a mixture of
observed form the Raman spectra of complex (2). Compared to
complex (2), the Raman spectra of the covalent Zn(II)-
porphyrin@SWNTs complex (6) exhibits G and D bands at 1588
-1
-
1
-1
cm and 1293 cm respectively. Moreover, the I
D
/I
G
band
/I
intensity ratio slightly increases to 0.19. Compared to the I
D
G
3
CHCl : EtOH 1:1 contains tubes with diameters of 0.99 nm and
band intensity ratio of free SWNTs, the ratio of both complex (2)
and complex (6) show an increase, which suggests that a
supramolecular interaction between of SWNTs and (1) has
occurred. Raman spectroscopy also showed peaks located
between 140 cm-1 to 300 cm , which correspond to radial
breathing modes (RBM) (Figure 7 inset). The free SWNTs
contains tubes with a broad diameter ranging 0.8-1.8 nm, while
1.55 nm. The RBM frequencies of (6) (240 cm-1 and 152 cm-1)
suggest that carbon nanotubes with a diameter of 0.99 nm and
1.57 nm are mainly present.
To verify the nature of the species adsorbed onto the surface
of the SWNTs and the nature of their possible binding modes,
XPS measurements were carried out. In particular, the C1s, O1s
and N1s core levels were evaluated for both Zn(II)-
porphyrin@SWNTs nanohybrids and pristine SWNTs.
-1
-1
there are two significant and sharp RBM peaks at 234 cm and
-1
1
46 cm , which indicate that there are tubes with diameters of
Figure 8. XPS spectra corresponding to: (a) C1s region, (b) N1s region, (c) O1s region for the SWNT, (d) C1s region, (e) N1s region, (f) O1s region for the
SWNT@Zn(II)-porphyrin (non-covalent), (g) C1s region, (h) N1s region and (i) O1s region for the Zn(II)-porphyrin@SWNT hybrid (covalent).
9
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Figure 8 (a), (d) and (g) show the XPS spectra corresponding to
the C1s level and, as observed, C-C, C-O and C=O convoluted
components can be observed in SWNT and both non-covalent
allowed to bind supramolecularly onto the middle part and tip of
a [10,10] capped SWNT and their binding energies were -0.84
eV and -0.66 eV respectively.[
27]
Zn(II)-porphyrin@SWNT
(2)
and
covalent
Zn(II)-
The magnitude of the binding energy depends on the type of
the small molecule involved and the level of theory adopted. The
amount of charge transferred in the porphyrin-SWNT model
used hereby was calculated to be 0.044 electrons in the tip-
binding configuration and 0.086 electrons in the middle-binding
configuration again showing the non-covalent interaction. In the
model reported hereby, the occurrence of van der Waals (vdW)
interactions was considered. As this interaction is attractive, our
calculated binding energy is slightly higher as expected.
However, calculated binding energies and the amount of charge
transferred in the model reported hereby are consistent with our
previous models. In literature a very few computational studies
are available on single porphyrin units absorbed on SWNTs.[
Basiuk et al. have studied the noncovalent functionalization of
carbon nanotubes with porphyrin theoretically.[29] Figure 9a
shows the DFT relaxed geometries of model compounds used in
this study. It is likely that the aromatic stacking of the porphyrin-
SWNTs alters the optical properties of the emerging nano-
composite and ensures the preservation of a non-covalent
monomeric porphyrin unit ready for a controlled donor-acceptor
interlayer association with SWNTs. From the dimer structure
determinations by synchrotron single-crystal X-ray diffraction, it
can be seen that the size of the Zn(II)-porphyrin measured
between the methyl groups of the phenyl rings is 16.55 Å. This
value is larger than the calculated inner space for [8,8], [9,9]
and ]10,10] SWNTs (Figure 9d). Therefore, all of the porphyrin
molecules will bind to the outside of the SWNTs and interact
with their outer surface.
porphyrin@SWNT (6) (in SWNT the C-O and C=O signals come
from the carbonated surface; in the Zn(II)-porphyrin@SWNT
samples they arise from both the carbonates and the CO
groups). However, as observed, there is one additional signal in
the SWNT and non-covalent Zn(II)-porphyrin@SWNT (2)
systems, a weak shoulder located at a binding energy slightly
above that of the C=O (arrow marked in Figure 8 (a) and (d)).
This signal can be associated with the existence of -
interactions between the SWNTs and/or the Zn(II)-porphyrins,
e.g. non-covalent bonding. The lack of this component in the
Zn(II)-porphyrin@SWNT (covalent) suggests that the presence
of Zn(II)-porphyrin systems covalently linked to the surface of
the SWNTs avoid the - interactions between the SWNTs.
Figure 8 (b), (e) and (h) show the XPS spectra corresponding to
the N1s core level of the samples. As observed only in the case
of the Zn(II)-porphyrin@SWNT (covalent) system can be
observed a component at 399.7 eV which can be ascribed to an
N-C chemical environment. This is indicative of the presence of
Zn(II)-porphyrin@SWNT at the SWNTs surface, as expected.
However, the absence of this signal in the spectrum of Zn(II)-
porphyrin@SWNT (non-covalent) Figure 8 (e) and the similarity
between the C1s spectra of pristine SWNTs and Zn(II)-
porphyrin@SWNT (non-covalent) can be ascribed to the
28]
formation
of
Zn(II)-porphyrin
aggregates
and
the
heterogeneously distribution of the Zn(II)-porphyrin molecules.
This last feature is in strong agreement with the TEM and AFM
results. Finally, Figure 8 (c), (f) and (i) show the O1s spectral
decomposition. In all cases C-O and C-O-H convoluted
components can be observed, showing that the covalent Zn(II)-
porphyrin@SWNT sample have a low presence of C-O-H
species at the SWNT surface with respect to the C-O species.
On the other hand, the non-covalent Zn(II)-porphyrin@SWNT
presents the highest level of this species indicating a more
hydroxylated surface of the SWNTs
DFT calculations
To understand further the geometry adopted by an isolated
Zn(II) porphyrin molecule bound non-covalently onto a single,
model, SWNT strand and the nature of its interaction with this
aromatic ‘guest’, DFT calculations using CASTEP[26] code were
performed. A model fragment of a single strand capped [10,10]
nanotube was selected as this is 1.6 nm wide, consistent with
the expectations from Raman spectroscopy, and its ability to
bind supramolecularly with the porphyrin molecule (1) was
estimated using a gas phase modelling approach which has
been successfully applied in previous studies.[11c, 27] One of our
previous modelling considered a composite formed between a
tripodal porphyrin trimer unit and a capped [10,10] SWNT[11c]
whereby two of the three porphyrin units were found to interact
closely with the surface of the SWNT. In that initial model, the
binding energy was calculated to be -2.046 eV (-1.023 eV per
porphyrin unit) and the charge transfer was estimated to be
Figure 9 a) DFT optimized structures of model compounds: a short [10,10]
capped SWNT; b) Zinc (II) centered porphyrin molecule; c) porphyrin molecule
bound supramolecularly onto the tip of a [10,10] capped SWNT (configuration
A); d) porphyrin molecule bound supramolecularly onto the middle part of a
0
.003 electrons clearly indicating non-covalent nature of the
binding. In a related ‘host-guest’ approach, but involving a
different chromophore, napthalenediimide molecule was
[10,10] capped SWNT (configuration B). (black-carbon, red- oxygen, blue-
a
nitrogen, yellow-sulphur, white-hydrogen, zinc-grey).
1
0
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The calculated binding energies for the porphyrin molecule
interaction with the tip of the SWNT (configuration A) and the
sidewall of the SWNT (configuration B) are ─0.69 eV and ─1.55
eV respectively, indicating a π-orbital interaction between the
porphyrin molecule and SWNT. The negative values of binding
energies indicate that both complexes are thermodynamically
stable. Total energies calculated for each systems and the
methodology we adopted to calculate the binding energy are
given in the supplementary information. Mulliken population
charge analysis was carried out to estimate the charge transfer
between porphyrin and SWNT. Calculations showed that for a
model composite with configuration A (tip-binding geometry),
compounds as well as the small metal dependency. Modelling
results also explain the observation that the substitutions of the
side groups and hexyl chains may play a dominant role in
determining fast electron injection from the excited singlet state
of Zn(II)-porphyrin to other coordinate materials such as carbon
nanotubes, graphene oxide or reduced graphene oxide. Such
interactions may be of relevance in the current quest for new
sustainable chemistries approaches to enhance the
performances of solar cells and bio-sensing devices.
0.04 electrons were transferred between a molecule of porphyrin
and the [10,10] SWNT. For the configuration B (where a
porphyrin molecule is bound to the middle of the SWNT), 0.08
electrons were transferred locally between one porphyrin unit
and the small model of a carbon nanotube considered.
The DFT-relaxed model used for computational studies
considered only a very short length for the SWNT modelled (200
nm): this allows a maximum of four porphyrin molecules to bind
configuration A (coating the tip of the SWNTs) and a further 6
porphyrin molecules to bind in configuration B (to the middle part
of the SWNT). This DFT gas-phase modelling estimates
approximately 2 electrons are transferred between 10 molecules
of porphyrin absorbed onto the outer surface of a 500 nm length
of capped SWNT fragment. This electron transfer can be
attributed to the donor-acceptor interaction between a porphyrin
molecule and SWNT. DFT calculations were employed to obtain
a direct view of the equilibrium geometry and the electronic
structures for the frontier orbitals of the Zn(II)-porphyrins. The
crystal structures of the above complexes were used as an input
geometry for gas phase optimization by B3LYP 6-31G(d,p),
whereby a restricted spin was used for the optimization of the
diamagnetic zinc(II) porphyrin complex.
The calculated electronic structures did not show negative
frequencies, indicating that the optimized geometries are in the
inclusive energy minima.[30] Porphyrins with
D4h symmetry
generally show energetically degenerate two lowest unoccupied
molecular orbitals (LUMO+1, LUMO) and nearly degenerate two
highest occupied molecular orbitals (HOMO, HOMO-1), as
shown in Figure 10. Table S3 (ESI) shows the energy level of
each molecular orbital. The calculated energy band gap for
Zn(II)-porphyrin was 3 eV, which indicates the visible light
absorption behavior, and promising potential photovoltaic
properties can be applied in the design of solar cells. Such
energy gap appears is in accordance with results recently
reported for Zn(II)-porphyrin species.[31] Together with the
molecular orbital structure and the calculated energy level, it can
be found that after modification at both meso-positions and β-
positions with aryl thioacetate-functionalized side groups and
hexyl chains respectively, the rich electron density areas were
moved from the macrocyclic ring (shown in HOMO) center to
meso-positions (as shown in LUMO) and β-positions (as shown
in LUMO+1). These results show that the substitutions onto
meso-positions and β-positions of Zn(II)-porphyrin mainly affect
the energy levels for unoccupied orbitals of the Zn(II)-porphyrin.
Since these molecular orbitals are involved in the ligand
Figure 10. a) Two highest occupied and lowest unoccupied molecular orbital
of de-protected Zn(II)-porphyrin (3) from gas phase DFT calculations
UV-vis spectroscopy was carried out to further investigate
the optical absorption behavior of nanocomposites (2) and (6) in
dispersed phases. The supramolecular assembly of Zn(II)-
porphyrin and SWNTs was studied by using optical
spectroscopy (Figure 50 and corresponding experimental
details, ESI). Figures S51a-b (ESI) exhibit the UV-vis
absorption spectra of Zn(II)-porphyrin dispersed in 1:1 o-DCB
and chloroform (3 mL, 1 μM) and titrated with a dispersed
mixture of Zn(II)-porphyrin (1) (0.5 μM) and SWNTs in 1:1
orthodichlorobenzene (o-DCB) and chloroform suspensions.
Figure 11 shows the optical absorption spectra of Zn(II)-
porphyrin, SWNTs, non-covalent Zn(II)-porphyrin@SWNTs
complex (2) and covalent Zn(II)-porphyrin@SWNTs complex (6).
Zn(II)-porphyrin (1) shows a main absorption peak at 417 nm,
while the intact SWNTs do not show any obvious absorption
(
including the phenyl substituent) and the metal it is highly likely
that both contribute to the UV-vis light absorption properties.
This observation may explain the observed similarity
between the UV-vis absorption bands in this family of
1
1
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band in UV-vis spectral range. After the SWNTs are covalently
linked to Zn(II)-porphyrin, the complex (6) show a peak at 420
nm, slightly red shifted compare with the intact Zn(II)-porphyrin
complex, this type of complex cannot be generated when Zn(II)-
porphyrin molecules are covalently attached to the surface of the
SWNTs. Coronene was chosen deliberatelly as the competitor
molecule as it can act as a simple model for the larger carbon-
based materials whilst providing an insight into the possible π-π
interactions between porphyrin and carbon single-walled
nanotubes. It would also facilitate the evaluation of the degree of
supramolecular aggregation (Scheme 1). Data fitting involved a
(
1). The Zn(II)-porphyrin@SWNTs (non-covalent) complex (2)
shows a red-shifted main absorption peak at 436 nm. These
results suggest that the chromatic shift of complex (2) and
complex (6) is due to the nature of the interaction of Zn(II)-
porphyrin with the SWNTs. The covalent interaction between
SWNTs and Zn(II)-porphyrin seems to minimize the extent of the
red shift observed. The UV–vis–NIR spectra of Zn(II)-
1 : 1 binding isotherm model and the association constant, K1:1
,
4
-1
was estimated to be 4.62 x 10 M (ESI). Therefore, coronene is
indeed capable of acting as a competitive candidate likely to
disrupt the interactions between the SWNTs and Zn(II)-porphyrin
systems (Scheme 1).
porphyrin@SWNT (recorded in pure CHCl
3
) shows the typical
S22 transitions, in the region around 960 nm. Partially
overlapping of S22 and metallic M11 transitions characteristic to
SWNTs can be seen at 500-1000 nm in Zn(II)-porphyrin@SWNT.
Since the transition energies are related to the size of the SWNT
bundles and the presence of isolated tubes in solution have
been reported to give rise to sharp, well-resolved peaks,[27] UV–
vis–NIR spectroscopy of the Zn(II)-porphyrin@SWNT composite
suggests that Zn(II)-porphyrins are capable of efficient
attachment onto the surface of SWTNs. Due to the enhanced
dispersibility caused by the attachment of Zn(II)-porphyrin, this
absorbance band may well constitute a further indication of the
de-bundling of the SWNT strands in solution.
A recent report[12] described the complexity of the non-
covalent interactions between aromatic electron-poor molecules,
such as functional naphthalenediimides (NDI), and planar
carbon-based materials such as thermally reduced graphene
oxide (TRGO), as well as the use of coronene to elucidate the
intimate nature of the donor-acceptor system, but, to the best of
our knowledge such competitive experiment involving functional
zinc porphyrins and SWNTs has not been reported on thus far.
Fluorescence spectroscopy in solution and fluorescence
lifetime imaging microscopy (FLIM) on thin film
To investigate the fluorescence emission behavior of Zn(II)-
porphyrin upon covalent and non-covalent SWNTs linked
complexes, two-dimensional fluorescence spectroscopy was
performed. 2D fluorescence contour plotting of free Zn(II)-
porphyrin, SWNTs, non-covalent Zn(II)-porphyrin@SWNTs
complex (2) and covalent Zn(II)-porphyrin@SWNTs complex (6)
were carried out in a 200- 800 nm excitation-emission range.
Free Zn(II)-porphyrin (1) shows a strong emission peak between
5
70 nm and 670 nm (Figure 12a). A strong Rayleigh scattering
line is shown in the two-dimensional contour plot of SWNTs
Figure 12b), but no detectable fluorescence emissions is
(
observed from the SWNTs suspension. After porphyrin (1) was
non-covalently linked to the SWNTs surface via π-π stacking,
the excitation-emission, observed for a Zn(II)-porphyrin (1)
solution, was remarkably quenched. Moreover, a blue shift of
emission maximum wavelength can be observed from 570-670
nm to 530-610 nm (Figure 12c). Such quenching behavior and
associate blue shifting (from 570-670 nm to 520-590 nm) can
also be found in the two-dimensional fluorescence spectra of the
covalently linked complex (6) (Figure 12d). The intensity of the
emission maximum for the covalently linked was observed to
slightly decrease in comparison to the non-covalently linked
complex. The fluorescence quenching of emission maximum
wavelength indicates that an energy transfer process from the
Zn(II)-porphyrin to the SWNTs has occurred, while the blue shift
is generated due to the non-covalent interaction of porphyrin
with the carbon surface. These results confirm the non-covalent
adsorption interaction between Zn(II)-porphyrin and SWNTs and,
at the same time, the covalent nature of the porphyrin-SWNTs
bond in the nanohybrid (6).
Figure 11. UV-vis and UV-Vis-NIR spectroscopy of Zn(II)-porphyrin, intact
SWNTs, non-covalent Zn(II)-porphyrin@SWNTs nanohybrids (2) and covalent
Zn(II)-porphyrin@SWNTs nanohybrids (6).
The extent of a supramolecular interaction between aromatic
molecules and carbon nanostructures has been previously
studied by using a smaller fraction of a sp carbon system.[32]To
demonstrate how the covalently and non-covalently linking
strategies may differ in their effectiveness to connect the
porphyrins with SWNTs, a small aromatic competitor molecule,
coronene, was introduced in suspensions of covalent and non-
covalent Zn(II)-porphyrin systems (2) and (6) . This was
expected to induce the dissociation of the donor-acceptor
complexes and to allow the monitoring of this process in the
dispersed phase by absorbance spectroscopes. While the high
affinity (binding constant) of coronene for the Zn(II)-porphyrin
molecules induce a competitive equilibrium that facilitates the
dissociation of the non-covalent Zn(II)-porphyrin@SWNTs
system and the formation of a new Zn(II)-porphyrin @coronene
2
1
2
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Scheme 1. Addition of coronene (1M solution in toluene) and washing process of the suspensions of the non-covalent and covalent nanohybrids Zn(II)-
porphyrin@SWNTs (2) and (6) in 1:1 toluene : chloroform.
Time correlated single photon counting measurements were
carried out in order to further investigate the excited state
characteristics of Zn(II)-porphyrin (Table 1 and Figure S46 in
ESI). In solution studies, the TCSPC fluorescence decay data of
compound
presence of major component with short lifetime decay (a1 =
3.2 %, t1 = 39 picoseconds) and minor longer lived component
a = 6.8 %, t2 = 1483 picoseconds, Table 1).
1 (2-photon excitation at 810 nm) shows the
9
(
Table 1. Time-Correlated Single Photon Counting (TCSPC) (2
photon excitation, λex= 810 nm) of Zn(II)-porphyrin 1 μM in
DMF:Toluene. Laser power was 5.4 mW.
a1
a2
(%)
2
1(ps)
39±5.0
2 (ps)
m (ps)
(%)
Figure 12. Two-dimensional fluorescence contour plotting of (a) Zn(II)-
porphyrin, μM (1:1 ethanol chloroform); (b) SWNT, 0.5 mg/mL (1:1
ethanol : chloroform); (c) non-covalent Zn(II)-porphyrin@SWNTs complex (2),
mg/mL (1:1 ethanol chloroform) and (d) Zn(II)-porphyrin@SWNTs
covalent) complex (6), 1 mg/mL (1:1 ethanol : chloroform).
1
1.50
93.2
1483.5±2.8
6.8
136.67
1
:
1
(
:
1
3
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Figure 14. Two-photon fluorescence lifetime map (λex
microcrystalline free base porphyrin (a1; ) and associate lifetime profile
distribution across the image (a ); microcrystalline Zn(II)-porphyrin complex (b
and b ), and associate lifetime profile distribution (b ); non-covalent solid
Zn(II)-porphyrin@SWNTs complex (2) (c and c ), and corresponding lifetime
distribution curve c ; solid covalent Zn(II)-porphyrin@SWNTs complex (6) (d
and d ) and corresponding lifetime distribution curve (d3). Images: a1, b1, c1
= 810 nm) of
a
2
3
1
2
3
1
2
3
1
2
and d1 are two photon intensity images. A color coded bar is provided for
direct correlation between the lifetime colour maps a2, b2, c2 and d2 and
lifetime histograms (a3, b3, c3, d3), respectively.
The interactions and consequent energy transfer between
SWNTs and Zn(II)-porphyrin were further investigated using
laser induced single-photon excitation and fluorescence
emission excited state lifetime measurements When in solid
state, the fluorescence decay of Zn(II)-porphyrin@SWNTs could
be fitted to a single component with a single lifetime of
approximately 2000±100 ps following excitation at 473 nm
(Figure 13a and 13 b). An additional relatively broad maximum
in the lifetime distribution curve (Figure 13c) was also observed.
Such broad full width at half of the maximum (FWHM) suggests
the presence of a heterogeneous fluorescence decay population
and also confirmed the formation of various crystalline
supramolecular assemblies and hybrid structures. Figures 13d
and 13e show lifetime emission maps of solid-state covalent
Zn(II)-porphyrin@SWNTs complex (6) using similar conditions to
the measurement carried out for non-covalently linked complex
Figure 13. Single-photon fluorescence lifetime map (λex
= 473 nm) and
corresponding intensity images of: (a and b) solid non-covalent Zn(II)-
porphyrin@SWNTs complex (2) and corresponding lifetime distribution curve
(c); solid covalent Zn(II)-porphyrin@SWNTs (6) complex (d and e) and
corresponding lifetime distribution curve (f). (b and e). Images a and d
represent intensity images on thin films. A color coded bar is provided for
direct correlation between the lifetime colour map (b and e) and lifetime
histograms (c and f).
Table 2. Solid state FLIM data for free base porphyrin precursor (denoted
Prec. In table), Zn(II)-porphyrin (1), (non-covalent) Zn(II)-porphyrin@SWNTs
complex (2) and (covalent) Zn(II)-porphyrin@SWNTs complex (6).
(
2). Shown in Figure 13f, the corresponding lifetime distribution
curve proved a shorter lifetime measured in complex (6),
400±100 ps. A broad FWHM of the corresponding lifetime
χ2
a1(%)
84.1
87.3
98.8
98.7
τ1 (ps)
a2(%)
15.9
12.7
1.2
Τ2 (ps)
τm (ps)
1
distribution curve was also observed. The difference in major
lifetime between the covalent complex (6) and non-covalent
complex (2) also indicates the two types of synthesis strategies
Prec
r
1.3
244±26.0
554±55.0
35±4.5
3369.7±460.1
1715.6±231.6
870.8±75.1
6880.1±228.0
742.0±155.9
765.3±96.6
44.9±6.1
1
2
6
1.17
1.49
1.15
28±11.8
1.3
120.5±67.1
1
4
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result in different electronic interactions between the Zn(II)-
porphyrin and the SWNTs and different degrees of quenching.
To further investigate the role of SWNT as a quencher of zinc
and free base porphyrins, two-photon FLIM for free-base
porphyrin and Zn(II)-porphyrin, as well as solid complexes (2)
and (6) were carried out (Figure 14a-d). For such
measurements, two component systems (1 and 2) were more
accurate models of the decay profiles (Table 2 and Figure S38-
S41, see in ESI). Micro-crystallites of free base porphyrin and
Zn(II)-porphyrin (1) imaged on borosilicate glass showed similar
occurs in the excited state. The donor-acceptor system designed
hereby is predicted to show possible applications in the
construction of novel optoelectronic devices using SWNTs as an
electron acceptor. Interestingly, the non-covalent linking
strategies could help mediate the absorption and emission
properties of the original porphyrin. New kinetically stable
complex systems emerged and this property is crucial for
biosensing applications. The directed, covalent linking strategy
provided a strong and stable method to link small molecule and
SWNTs whereas in the case of the non-covalently linked
strategy, albeit very convenient and rapid, the porphyrin coating
is detachable in the presence of other flat aromatic organic
molecules capable of acting as competitors. This represents a
challenge, as well as an opportunity in designing functional
nanohybrids for future sensing applications.
fluorescence decay profiles with

m
(average lifetime) of
742±155.9 and 765±96.6 picoseconds respectively. Interestingly,
the major component for the non-covalent Zn(II)-
porphyrin@SWNTs complex (2) was significantly reduced (from
554±55.0 to 35±4.5 picoseconds). The same shortening in
lifetime for the first component of the covalent complex (6) can
be seen. In this instance, however, the extent of shortening is
even more pronounced. In agreement with what observed for
the single-photon excitation mechanism, the covalent
functionalization of SWNTs results in a more significant lifetime
Experimental Section
Synthesis of Zn(II)-porphyrin@SWNTs (non-covalent) complex (2): 5 mg
purified single-walled carbon nanotubes were added to 30 mL of ethanol.
The mixture was sonication for 10 min, three times with 5 min intervals,
to make sure majority of the purified SWNTs were dispersed in the
solution. The suspension was added to a 50 mL centrifuge tubes and
centrifugation at 3500 rpm for 30 min. The top layer supernatant was
carefully separated and collected to afford a SWNTs ethanol solution.
Afterwards, 1 mg Zn(II)-porphyrin was added to 30 mL of chloroform and
the solution added to the SWNTs solution. The mixture was left stirring
for 24 hours at room temperature. The mixture was filtrated using a
membrane and the solids were rinsed with excess chloroform. The solid
nanohybrids were collected and fully dried over 60 °C overnight.
shortening. The appearance of
a second longer minor
component suggests instead the presence of unquenched
species free in solid state (free based or zinc porphyrins) or their
supramolecular self-assembled structures.[12]
We have recently reported that these changes in lifetime
upon complexation, particularly the presence of a lower lifetime
component may account for a photo-induced excited state
energy transfer occurring between the closely bound donor-
acceptor systems aromatic chromophores and planar carbon
based materials.[12] Therefore, it is reasonable to conclude that a
Föster Resonance Energy Transfer (FRET) occurs in these
systems, where the porphyrins act as FRET donors and the
SWNTs as the corresponding FRET acceptor.[12, 33]
Bingel reaction for functionalized SWNTs: For a typical reaction, 30 mg of
purified single-walled carbon nanotubes were weighted out and
transferred to a quartz tube. The single-wall carbon nanotubes were
annealed using a furnace under vacuum at 10-3 mbar at 1000 °C for 3
hours under nitrogen protection. The annealing repaired single-wall
carbon nanotubes were added to 15 mL dry ortho-dichlorobenzene (o-
DCB) to obtain a suspension. A 15 min ultrasonic treatment in 80W
sonication bath was introduced to disperse the SWNTs. Afterwards, 1.8
mmol of diethyl bromomalonate and 3.3 mmol of 1,8-
diazabicyclo[5.4.0]undecene (DBU) were added to carbon nanotube
suspension and the mixture was left stirring for 15 hours under nitrogen
protection at room temperature, followed by adding 3.9 mmol
trifluoroacetic acid to quench the reaction. 35 mL ethanol was added to
the mixture and then the mixture was transferred to a 50 mL centrifuge
tube. The mixture was centrifuged at 3500 rpm for 30 min to separate the
solvent and single-walled carbon nanotubes. The pellet of SWNTs was
washed several times with ethanol by repeating this centrifugation
progress. The SWNTs was then collected by a nano-filtrate system and
rinsed with more excess ethanol to fully remove the organic phase. The
sample was dried at 60 °C overnight.
Conclusions
A multi-pronged characterization protocol involving imaging and
spectroscopy methodologies probed the supramolecular
properties of a Zinc(II) substituted bulky aryl porphyrin. This
tailored Zn(II)-porphyrin synthon and pristine SWNTs were used
as building blocks leading to new Zn(II)-porphyrin@SWNTs
adducts which were either covalently or non-covalently linked.
The new nanocomposites materials have been synthesized by
following either supramolecular self-assembly via - stacking or
via a disulfide linker bridging covalently the two components.
The morphology and photochemical behavior were thoroughly
analyzed by AFM, STED, TEM, Raman, XPS, fluorescence
spectroscopies including two-photon excitation spectroscopies.
Experiments showed that the Zn(II)-porphyrin functionalization is
uniform and the two linking strategies also led to different
morphologic characteristics for the functionalized SWNTs, as
observed by AFM. The DFT calculations confirmed that the
Zn(II)-porphyrin of choice and its carbon monohybrids show
potential to be applied in photovoltaics or and bio-sensing
devices. For the first time, XPS was used to evaluate the
presence of π-π interactions as an evidence of non-covalent
interaction between the porphyrin and the SWNT. UV-vis,
Thiol -SH deprotection of Zn(II)-porphyrin: 3 mg Zn(II)-porphyrin was
dissolved in degassed 5 mL CH Cl , then 200 μL hydrazine monohydrate
2 2
was added to this mixture. This solution was stirred under nitrogen
overnight and the solvent removed under reduced pressure. The product
2 2
was re-dissolved in degassed 10 mL CH Cl and washed twice with 20
mL degassed water under nitrogen. The organic phase was dried over
sodium sulphate and the solvent was removed by reduced pressure to
afford SH- de-protected Zn(II)-porphyrin.
fluorescence
spectroscopy
and
multi-photon
confocal
Synthesis of Zn(II)-porphyrin@SWNTs (covalent) complex (6): Bingel
reaction functionalized single-walled carbon nanotubes were trans-
esterified by prolonged stirring in an extensive of 2-mercaptoethanol (5
mL) for 72 hours under nitrogen protection. Afterwards, the products
fluorescence imaging were carried out to shed light into the
nature of the interactions between the porphyrins and SWNTs
scaffolds both in solution and in the solid state. The UV-visible
studies confirmed the different synthesis strategies produced
different light absorption properties. The fluorescence quenching
and florescence lifetime suggests that a FRET mechanism
between the Zn(II)-porphyrin self-assembled tubular aggregates
[
2 2
(COOCH SH) C<SWNTs] were washed with extensive diethyl ether (80
ml). The mixture was separated by filtration and the solid parts on the
filtrate membrane and collected into a glass vial. 1 mg trans-esterified
single-walled carbon nanotubes were added to a 5mM chloroform
(
viewed as the FRET donor) and the SWNTs (FRET acceptor)
1
5
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605232
FULL PAPER
solution of SH- de-protected porphyrins and then 0.2 % conc. solution
containing one equivalent of DBU was added into the mixture drop wise.
The solution was stirred for 72 hours at room temperature under nitrogen
protection. Afterwards, the mixture was filtrated and the solids on the
membrane were rinsed with excess ethanol. The complex was collected
and transferred to a 30 mL glass vial. The resulting nanohybrid (black
powder) was dried at 60 °C overnight.
an XY galvanometer (GSI Lumonics). Fluorescence emission was
collected without de-scanning, bypassing the scanning system and
passed through a colored glass (BG39) filter. The scan was operated in
normal mode and line, frame and pixel clock signals were generated and
synchronized with an external fast microchannel plate photomultiplier
tube used as the detector (R3809-U, Hamamatsu, Japan). These were
linked via
a Time-Correlated Single Photon Counting (TCSPC)
PCmodule SPC830 operating either under single- or two-photon(s)
excitation conditions. Lifetime calculations were obtained using
SPCImage analysis software (Becker and Hickl, Germany) or Edinburgh
Instruments F900 TCSPC analysis software.
Density Functional Theory (DFT) Calculations: The calculations to study
the equilibrium geometry and electronic structures for the frontier orbitals
of the Zn(II)-porphyrins were first performed using density functional
theory (DFT) as employed in Gaussian09, using B3LYP 6-31G(d,p).[34]
This was carried out via the High Performance Computing Facility,Aquila,
University of Bath. In order to determine the structure of the complex
formed and the nature of the interaction between the Zn(II)-porphyrin and
Acknowledgements
a carbon nanotube, DFT calculations were performed using CASTEP
code,[26] which solves the standard Kohn-Sham (KS) equations using
SIP and SWB thank The Royal Society and STFC for funding.
BYM thanks the University of Bath for a studentship (ORS).
JBdlS acknowledges support for a European Union Marie Curie
Career Integration Grant. Rory L. Arrowsmith is thanked for the
initial stages of the DFT calculations of the Zn(II)-porphyrin. Dr
Amy Kieran is thanked for initial experiments in porphyrin
synthesis. The authors thank EPSRC National Service for
Computational Chemistry Software and High Performance
Computational facilities at Imperial College London for support
and computational facilities. Professors Jeremy K. M. Sanders
and Paul Raithy are acknowledged for training, helpful
discussions in supramolecular chemistry and porphyrin
supramolecular chemistry as well as mentorship to SIP and her
group. Dr. Gabriele Kociok-Köhn is thanked for assistance with
X-ray diffraction. B.J.H. thanks the University of Bath for a DTC
studentship. The SIP group thanks to the EPSRC for funding, as
a member of the Centre of Graphene Science. The authors also
acknowledge the ERC for the Consolidator Grant O2SENSE.
plane wave basis sets. For the exchange correlation term the
Generalized Gradient Approximation (GGA) in the Perdew-Burke-
Ernzerhof (PBE) parameterization was employed.[35] Ultrasoft
pseudopotentials were generated using the “on the fly” formalism in
CASTEP. A plane wave basis set with the energy cut-off of 500 eV was
used to expand the wave function. As the inclusion of van der Waals
(
vdW) interactions can improve the binding energy, in this work we have
applied vdW corrections as implemented by Grimme[36] in the CASTEP
package. Structure optimizations were performed using BFGS algorithm
and the forces on the atoms were obtained from the Hellman-Feynman
theorem including Pulay corrections. In all optimised structures, forces on
the atoms were smaller than 0.05 eV/Å and the stress tensor was less
than 0.01 GPa. In all calculations, we used super cells with 40 Å, 40 Å
and 40 Å vacuum spaces along x, y and z directions respectively. This
modelling method makes sure that the nanotubes plus functional groups
do not interact with their periodic images. A single k point (Γ) was used in
all calculations.
Crystal structure determination: Crystals of Zn(II)-porphyrin dimer were
grown from CHCl
data are summarized in Table S1 (SI). Data were collected at 180 K on a
Nonius Kappa CCD with graphite-monochromated Mo-K radiation (λ =
.71073 Å). Also, measurements on the same compound were
performed at the synchrotron radiation source at Station 9.8, Daresbury
SRS, UK, on Bruker SMART CCD diffractometer
Wavelength=0.69040) which provided a higher quality data set. In both
3
and MeOH mixtures. Crystal and structure refinement
α
Keywords: STED Super-resolution imaging • Carbon
Nanotubes • Optically Active Composite Materials • Self-
Assembly • Dynamic Covalent Chemistry of Nanomaterials
0
a
(
cases, the same cellular parameters were obtained and the the
structures were solved by direct methods using the program SIR92.[37]
The refinement and graphical calculations were performed on the
synchrotron data set using the CRYSTALS and CAMERON software
packages. The structures were refined by full-matrix least-squares
procedure on F. All non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were located in Fourier maps
and their positions adjusted geometrically (after each cycle of refinement)
with isotropic thermal parameters. Chebychev weighting schemes and
empirical absorption corrections were applied.[39]
[
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Chemistry - A European Journal
10.1002/chem.201605232
FULL PAPER
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Supramolecular
and
covalent
Boyang Mao, David G. Calatayud,
assemblies can influence the
morphology and optical properties
of porphyrin arrays onto flat,
conductive as well as insulating
surfaces, and lead to the control of
SWNTs functionalization at the
nanoscale. The synthesis of novel
functionalized luminescent single
walled carbon nanotubes is reported.
STED super resolution microscopy,
Fluorescence Lifetime Imaging as well
as XPS, TCSPC, UV-Vis, XRD, NMR,
TEM and AFM probe the complexity
of the emerging covalent and
supramolecular aggregates.
Vincenzo Mirabello, Haobo Ge,
Benjamin J. Hodges, Robert M. J.
Jacobs, Ashley M. Shepherd, José
Alberto Ribeiro Martins, Navaratnarajah
Kuganathan, Jorge Bernardino De La
Serna, Stanley W. Botchway and Sofia I.
Pascu*
(
(Insert TOC Graphic here: max.
width: 5.5 cm; max. height: 5.0 cm))
Page No. – Page No.
Fluorescence Lifetime Imaging and
Super resolution microscopies shed
light on the directed- and self-assembly
of functional porphyrins onto carbon
nanotubes and flat surfaces
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