Inserting Porphyrin Quantum Dots in Bottom-Up Synthesized Graphene Nanoribbons

Article abstract of DOI:10.1002/chem.201705252

Diels–Alder copolymerization of tetraphenylcyclopentadienone, a precursor for cove graphene nanoribbons (cGNRs), with bifunctional porphyrins yields defined nanostructures comprised of a single cGNR-porphyrin-cGNR heterojunction within each ribbon. ¹

Full text of DOI:10.1002/chem.201705252

A Journal of
Accepted Article
Title: Inserting Porphyrin Quantum Dots in Bottom-Up Synthesized
Graphene Nanoribbons
Authors: Wade Perkins and Felix Raoul Fischer
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To be cited as: Chem. Eur. J. 10.1002/chem.201705252
Link to VoR: http://dx.doi.org/10.1002/chem.201705252
Supported by

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COMMUNICATION
Inserting Porphyrin Quantum Dots in Bottom-Up Synthesized
Graphene Nanoribbons
[
a]
[a,b,c]
Wade Perkins, Felix R. Fischer
*
Abstract: Diels-Alder copolymerization of tetrapheneylcycopenta-
dienone, a precursor for cove graphene nanoribbons (cGNRs), with
bifunctional porphyrins yields defined nanostructures comprised of a
strategy can be extended to a variety of bifunctional linkers (see
Supporting Information), we herein focus on the integration of a
disubstituted tetraphenylporphyrin (H
2
(TPP)) and its metal
1
3
single cGNR-porphyrin-cGNR heterojunction within each ribbon. C-
NMR labeling and high-resolution mass spectrometry of solubilized
polymer intermediates indicates that every porphyrin is covalently
linked to two extended segments of cGNRs. UV-Vis absorption and
complexes into cGNR-H (TPP)-cGNR heterostructure.
a
2
1
3
13
Mass spectrometry (MS) and C-NMR spectroscopy of C-
labeled poly-phenylene intermediates underscores the exquisite
structural control over monomer sequence in the cGNR-
fluorescence emission spectroscopy reveal
a
strong electronic
2
H (TPP)-cGNR heterojunction. Electronic characterization of the
correlation between the porphyrin and the adjacent cGNR segments
that can be attenuated through reversible metalation of the porphyrin
core. Our versatile bottom-up synthetic strategy provides access to
structurally well defined, functional GNR-quantum dot-GNR
heterostructures within a single graphene nanoribbon.
resulting metalloporphyrin-cGNR hybrid materials by UV-Vis
absorption and fluorescence emission spectroscopy shows
strong electronic communication between the porphyrin and
cGNR segments. We further demonstrate that reversible binding
of primary amine ligands to the axial coordination site of the
metalloporphyrin core can serve as a tool to direct the assembly
of
cGNR-Zn(TPP)-cGNR
heterostructures
on
T
he design and implementation of carbon-based functional
photolithographically patterned substrates.
nanoelectronic materials into device architectures relies on the
development of synthetic tools capable of providing a precise
and reproducible control over the structure of materials at the
nanometer scale. Recent advances in the bottom-up synthesis
of semiconducting graphene nanoribbons (GNRs), quasi-one
dimensional strips of single-layer graphene, have enabled the
preparation of carbon-based nanomaterials with exquisite
[
1-5]
control over the width,
the crystallographic symmetry (e.g.
[
1-13]
[14]
[15-16]
armchair,
zig-zag ), and the edge structure (cove,
[
1, 17-20]
chevron
) both in solution and on metal surfaces. While
bottom-up synthesized GNRs have been touted for their intrinsic
[
21-31]
[25, 29-32]
[16,
exotic electronic,
magnetic,
and optical properties,
2
7, 28, 33-34]
examples for the deterministic assembly of functional
bottom-up synthesized GNRs heterostructures have thus far
been limited to uncontrolled copolymerization of molecular
[
6, 13, 18, 20]
precursors on metal surfaces
molecule model systems in solution.
or the study of small-
[
35-37]
We herein report the solution-based bottom-up synthesis
and characterization of a GNR heterostructure comprised of two
segments of solubilized cove GNRs (cGNRs) linked by a
substituted tetraphenylporphyrin core (1, Scheme 1) acting as a
highly tunable molecular quantum dot (QD). While our synthetic
[a]
W. Perkins, Prof. Dr. F. R. Fischer
Department of Chemistry,
University of California Berkeley
699 Tan Hall, Berkeley, CA-94720, U.S.A.
E-mail: ffischer@berkeley.edu
Scheme 1. Synthesis of cove-type GNRs featuring a single porphyrin at the
center of the ribbon. Reaction conditions: a) Ph O, 230 °C, 24 h, 40% b) FeCl
, 24 °C, 2 h, 55%. * 99.5% C isotopically labelled.
[
b]
c]
Materials Science Division,
Lawrence Berkeley National Laboratory,
Berkeley, CA-94720, U.S.A.
2
3
,
13
2 2 3 2
CH Cl , CH NO
[
Kavli Energy NanoSciences Institute at the University of California
Berkeley and the Lawrence Berkeley National Laboratory,
Berkeley, CA-94720, U.S.A.
The deterministic bottom-up synthesis of cGNR-porphyrin-
cGNR heterojunctions is depicted in Scheme 1. 5,15-bis(4-
ethynylphenyl)-10,20-diphenylporphyrin (2) serves as the
precursor for the porphyrin core in 1. The solubilized cGNR
Supporting information for this article is available on the WWW
under http://dx.doi.org/XX.XXXX/anie.2016.
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Chemistry - A European Journal
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COMMUNICATION
segments are derived from tetraphenylcyclopentadienone
monomer 3. Diels-Alder polymerization of 3 in the presence of 2
(
2
[3]/[2] = 24) in Ph O at 230 °C yields the GNR polymer
precursor 4 featuring a central porphyrin core extended on either
side by chains of poly-3 along with the expected homopolymer
poly-3. Size exclusion chromatography (SEC) analysis of the
crude polymer mixtures (calibrated to polystyrene standards)
show a monomodal size-distribution centered around M
9000, a polydispersity index Đ = 1.8, and an average degree
of polymerization X = 35 (determined by H-NMR end-group
n
=
2
M
1
n
analysis of the characteristic porphyrin resonances at –2.8 ppm).
Matrix-assisted laser desorption/ionization (MALDI) mass
spectrometry (Figure 1a) reveals the presence of two distinctive
–
1
families of polymers separated by Dm/z = 662 g mol , the mass
of the tetraarylporphyrin 2. The repeat unit within both polymer
–
1
families (Dm/z = 829 g mol ) corresponds to the mass of the
monomer unit resulting from the Diels-Alder reaction and
cheletropic extrusion of CO from 3. To gain insight into the
efficiency of the functionalization of both alkynes in the
tetraarylporphyrin 2 we followed the copolymerization of 3 with
1
3
13
9
9.5% C labeled 2. Characteristic C NMR resonances for the
isotopically enriched terminal alkyne C-atom shift from d = 79
ppm in 2 to d = 131 ppm upon lateral extension of both ends of
1
3
the porphyrin core with segments of poly-3. The absence of
C
labeled alkyne resonances (d 79 ppm) in the crude
=
polymerization mixture indicates that both functional ends of 2
have reacted and were efficiently incorporated into the extended
polymer backbone. The crude polymerization mixture is thus
comprised of only two distinctive polymeric species, the
homopolymer poly-3 and the copolymer 4 featuring a single
porphyrin core extended on both sides by chains of poly-3.
Fractionation of the crude polymer mixture through column
2
chromatography over SiO yields poly-3 and 4 in 45% and 40%
Figure 1. a) MALDI of crude polymer mixture containing poly-3 and copolymer
isolated yield, respectively. While the MALDI mass spectrometry
4
. MALDI of b) purified poly-3 and c) copolymer 4 fractionated through column
13
of poly-3 (Figure 2b) shows only one characteristic family of
chromatography. d) C NMR of purified poly-3 (black) and copolymer 4 (red).
13
–
1
The arrow indicates the characteristic resonance signal for the 99.5%
labelled aromatic carbon atoms resulting from the reaction of both terminal
C
peaks separated by the monomer mass (Dm/z = 829 g mol ),
the corresponding MALDI of (Figure 1c) contains
4
alkynes in 2 with 3.
predominantly (> 95%) the desired copolymer alongside trace
amounts of poly-3. Figure 1d shows the aromatic region of the
1
3
C NMR spectra of fractionated poly-3 and copolymer 4. The
2
Both cGNRs and cGNR-H (TPP)-cGNR heterostructure 1
1
3
diagnostic resonance associated with the 99.5% C labeled
terminal alkyne in 2 appears at d = 131 ppm. Oxidative
cyclodehydrogenation of both poly-3 and 4 with an excess of
feature solubilizing hexadecyl side chains and form stable
dispersions after sonication and centrifugation in THF. The
respective UV-Vis absorption spectra are depicted in Figure 2b.
A broad absorption at lmax = 556 nm characteristic for cGNRs
FeCl
heterostructure 1 in 96% and 97% isolated yield, respectively.
Raman spectroscopy (l 532 nm) of fully
cyclodehydrogenated 1 features characteristic radial breathing
3
gave solubilized cGNR and cGNR-porphyrin-cGNR
2
dominates the spectrum of 1. The lmax of cGNR-H (TPP)-cGNR
E
=
heterostructure 1 is only slightly shifted (~6 nm) to shorter
wavelength when compared to the absorption of pristine cGNRs.
A second prominent absorption at l = 433 nm in the spectrum of
–
1
–1
–1
like mode (RBLM) (194 cm FWHM = 78 cm ), D (1322 cm
–
1
–1
–1
FWHM = 62 cm ), and G (1600 cm FWHM = 38 cm ) peaks
with a ratio I /I = 1.2 reminiscent of pristine cGNRs along with
1
0 2
can be attributed to the corresponding S ®S transition (Soret
D
G
[38]
band) in the porphyrin core, while the characteristic Q-bands
are obscured by the dominant absorption of the cGNR segments.
The bathochromic shift (~10 nm) of the Soret band in 1, when
compared to the precursor 2 (lmax = 423 nm), can be attributed
to an efficient electronic coupling with the extended p-system of
the adjacent cGNRs. An inherent strength of porphyrins as the
central component in functional GNR heterostructures is the
ability to reversibly tune the electronic structure of the porphyrin
[
15]
higher order 2D, D+G and 2G peaks (Figure 2a). We conclude
that the incorporation of a single porphyrin core at the center of
a cove-type GNR does not perturb the structure of the parent
cGNR segments in 1.
2
+
through late-stage metalation. Coordination of Zn ions to the
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COMMUNICATION
free-base porphyrin in 1 is mirrored in a diagnostic shift (~10 nm)
of the Soret band to longer wavelengths (1•Zn in Figure 2b)
quantitative, reflected in a rigid shift of the Soret band, and is
fully reversible. Treatment of 1•Zn with trifluoroacetic acid in
2
+
3+
(
coordination of Ru or Al ions induces a negligible shift,
2 2
CH Cl regenerates the free-base porphyrin 1.
2
+
Supporting Information, Figure 2SI). The complexation of Zn is
E 2
Figure 2. a) Raman spectra (l = 532 nm) of pristine cGNRs and cGNR-H (TPP)-cGNR heterostructure 1. Inset shows a magnification of a region of the
–1
spectrum associated with the radial breathing like mode (RBLM) at 194 cm . b) UV-Vis spectra of dilute dispersions of cGNRs (black) (normalized to the
absorption maximum of the GNR), 1 (red), and metalated 1•Zn (blue) (normalized to the Soret band). EEM fluorescence spectra for c) cGNRs and d) cGNR-
H
2
(TPP)-cGNR heterostructures 1.
Figure 2c,d shows the excitation emission matrix (EEM)
deposited Pt traces (Figure 3a). Self-assembled monolayer
1
fluorescence spectra for cGNRs and cGNR-H
heterostructure 1, respectively. Excitation of pristine cGNRs at
Ex = 560 nm leads to a broad fluorescence emission centered
around lEm = 705 nm (Figure 2c). The emission of the cGNR-
(TPP)-cGNR heterostructure 1 upon excitation at the same
2
(TPP)-cGNR
(SAM) of either N-(3-(triethoxysilyl)propyl)hexane-1,6-diamine
(NH
selectively grown on the exposed Al
CH -SAM do not adhere to Pt traces). While the primary amine
in NH -SAM can reversibly bind to metalated 1, the CH -SAM
2
-SAM), or dodecyltrimethoxysilane (CH
3
-SAM) were
l
2
O
3
substrate (NH
2
-SAM and
3
H
2
2
3
wavelength (lEx = 560 nm) decreases significantly (Figure 2d)
serves as a reference to account for dispersion interactions
between solubilizing alkyl chains in 1 and an aliphatic SAM.
Figure 3b shows a representative Raman map of the G peak
and no longer represent the emission maximum. If, however the
2
cGNR-H (TPP)-cGNR heterostructure 1 is excited at lEx = 425
nm, close to the Soret band of the porphyrin core, a very broad
emission lEm = 650–720 nm featuring emission characteristics of
both the porphyrin and cGNR is observed. The energy transfer
from the excited state of the porphyrin core (lEx = 425) to the
cGNR (lEm = 705 nm) further support an efficient electronic
communication between the central porphyrin and the extended
cGNR segments.
intensity attributed to 1•Zn on
functionalized with NH -SAM. The spatial distribution of the
Raman signature of 1•Zn indicates that the interaction between
a
patterned substrate
2
2
+
the Zn and the primary amine directs the assembly of 1•Zn
exclusively on the NH -SAM functionalized substrate. The
2
significance of this selective metal ligand coordination is further
supported by two control experiments. The spatial distribution of
G peak intensity in Raman maps of 1•Zn on CH
3
-SAM (Figure
Axial coordination of a ligand to the metal in cGNR-Zn(TPP)-
cGNR heterostructures can be used as a tool to direct the self-
assembly of functional GNRs on patterned substrates. Figure 3a
3
c) or cGNRs on NH -SAM functionalized substrates (Figure 3d)
2
do not indicate a preference for adhesion to the SAM over the
photo lithographically patterned Pt traces.
2 3
shows an Al O substrate decorated with photolithographically
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COMMUNICATION
The bottom-up solution-based
Wade Perkins, Felix R. Fischer*
synthesis
and
of
selective
graphene
fractionation
Page No. – Page No.
nanoribbon-porphyrin-graphene
nanoribbon heterostructures is
Inserting Porphyrin Quantum Dots
in
Bottom-Up
Synthesized
reported.
fluorescence spectroscopy reveal
strong coupling between the
Absorption
and
Graphene Nanoribbons
a
graphene nanoribbon and the
central porphyrin quantum dot.
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