Solvent-dependent moulding of porphyrin-based nanostructures: solid state, solution and on surface self-assembly

Article abstract of DOI:10.1080/10610278.2016.1158407

A novel porphyrin derivative 1?Zn was synthesised in order to mimic the self-assembly properties of natural light-harvesting antennas and its self-assembly behaviour in solution and in solid state were studied by NMR and X-Ray spectroscopies. The self-assembly of this molecule was triggered in apolar solvents and studied in solution by UV-Vis spectroscopy, suggesting it is able to form slipped face-to-face aggregates, or J-aggregates. The nanoscopic and microscopic morphology of the aggregates was elucidated by atomic force microscopy, revealing the formation of extended two-dimensional structures.

Full text of DOI:10.1080/10610278.2016.1158407

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Solvent-dependent moulding of porphyrin-based
nanostructures: solid state, solution and on
surface self-assembly
Luka Ðorđević, Nicola Demitri & Davide Bonifazi
To cite this article: Luka Ðorđević, Nicola Demitri & Davide Bonifazi (2016): Solvent-dependent
moulding of porphyrin-based nanostructures: solid state, solution and on surface self-
assembly, Supramolecular Chemistry, DOI: 10.1080/10610278.2016.1158407
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Date: 22 March 2016, At: 17:58

Supramolecular chemiStry, 2016
http://dx.doi.org/10.1080/10610278.2016.1158407
Solvent-dependent moulding of porphyrin-based nanostructures: solid state,
solution and on surface self-assembly
Luka Ðorđevića , Nicola Demitrib and Davide Bonifazia,c
b
aDepartment of chemical and pharmaceutical Sciences, iNStm udr trieste, university of trieste, trieste, italy; elettra − Sincrotrone trieste,
trieste, italy; cSchool of chemistry, cardiff university, cardiff, uK
ABSTRACT
ARTICLE HISTORY
A novel porphyrin derivative 1∙Zn was synthesised in order to mimic the self-assembly properties
of natural light-harvesting antennas and its self-assembly behaviour in solution and in solid state
were studied by NMR and X-Ray spectroscopies. The self-assembly of this molecule was triggered
in apolar solvents and studied in solution by UV-Vis spectroscopy, suggesting it is able to form
slipped face-to-face aggregates, or J-aggregates. The nanoscopic and microscopic morphology of
the aggregates was elucidated by atomic force microscopy, revealing the formation of extended
two-dimensional structures.
received 12 January 2016
accepted 16 February 2016
KEYWORDS
porphyrin; nanostructures;
h-bonding; soft-matter
arrays employed by plants and bacteria for photosynthesis
(17, 18). Especially the chlorosome antennae, in the green
photoprotic bacteria (e.g. Chloroflexus aurantiacus), is able
to aggregate and exert its function without the presence
of any protein. For this reason, bacteriochlorophylls (BChl)
have become the main source of inspiration for semi-
synthetic and artificial chlorins and porphyrins in mim-
icking bacterial light-harvesting systems (11, 14, 19–25).
For example, semi-synthetic derivatives from chlorophyll
Introduction
Skilful organisation of organic molecules is an area of
research of special interest as a bottom-up approach for
self-assembled functional dyes that hold great promise
in materials science applications (1–6). In this context, it is
not surprising that, in the area of supramolecular chemis-
try (7–9), extensive efforts have been placed into mimick-
ing the light-harvesting antennae (10–16), chromophoric
CONTACT Davide Bonifazi
BonifaziD@cardiff.ac.uk
© 2016 informa uK limited, trading as taylor & Francis Group

2
L. ÐoRđEVIć ET AL.
stacks and, owing to the second hydroxymethyl group,
should give rise to novel nanostructures, other than the
usual nanorods (29, 35–37). The self-assembly properties
of 1∙Zn was evaluated by a variety of variable-tempera-
ture and concentration-dependent UV-Vis spectroscopic
measurements. Moreover, the formation of different nano-
structures was evaluated under different solvent condi-
tions, given the explicit role that the solvent plays in the
formation of self-assembled nanostructures (38–42).
Results and discussion
Synthesis
Initially, we wanted to develop a high-yielding route to
synthesise the porphyrin derivative 1∙Zn (Scheme 1). We
reasoned that the best way to obtain the final benzyl alco-
hol would be the reduction of the porphyrin di-ester. In
order to obtain this key intermediate, it was desirable to
avoid tedious chromatographic separations of porphyrin
mixtures obtained by statistical Alder-Longo or Lindsey
reactions (16, 43, 44). The best route to the 5,15 substi-
tuted porphyrin is certainly through the MacDonald [2 + 2]
condensation approach (45, 46). At first, the meso-unsub-
stituted dipyrromethane 2 was obtained by reaction of
H2Co with a large excess of pyrrole (as solvent) containing
TFA (trifluoroacetic acid) as catalyst. The dipyrromethane
was reacted with benzaldehyde (under TFA catalysis) in
order to obtain 5,15-diphenylporphyrin derivative (47).
The 10,20 meso-free positions were easily brominated
with NBS (N-bromosuccinimide) in the presence of pyri-
dine as acid scavenger (48). The 5,15-dibromo-10,20-di-
phenylporphyrin 5 was then subjected to a Pd-catalysed
Suzuki-Miyaura cross coupling (49) reaction with the
methyl pinacolboranebenzoate to obtain exclusively the
trans-porphyrin di-ester 6. Reduction from ester to benzyl
alcohol in the presence of LiAlH4 and subsequent metalla-
tion with Zn(oAc)2∙2H2o yielded the final porphyrin 1∙Zn
in a good overall yield. Notably, this sequence of reactions
gave products fairly different in polarity, making it easy to
separate them by single column chromatography.
Figure 1. (colour online) chemical structure of porphyrin 1∙Zn
along with its cartoon representation highlighting the relevant
functional units.
a have been self-assembled in chlorosomal-type aggre-
gates (26–31). Additionally, synthetic porphyrin analogues
revealed to be good models for mimicking chlorosomes by
employing the same self-assembly recognition groups that
are found in BChl’s (32–34). The self-assembly algorithm
for this family of (semi)synthetic derivatives is made of
three key points: a central metal ion, a hydroxy group and
a keto function. Meticulous tailoring of these three groups
enabled a given self-organisation with coordination to the
central metal atom with the hydroxy of another molecule
as main contact that, through a lateral hydrogen-bond
with a keto group of a third molecule, organises the stacks
together with π–π interactions (26–34).
Herein we report a fully synthetic porphyrin 1∙Zn,
a tetrapyrrolic macrocycle carrying a central zinc core
(capable of ligation) and two hydroxymethyl groups that
are trans to each other (Figure 1). The design of this mac-
rocycle should enable self-assembly of the porphyrins in
Scheme 1. (colouronline) Syntheticschemeforporphyrin 1∙Zn. reagents and conditions: (a) (i)tFa, ch2cl2, r.t. (ii) DDQ, reflux (iii) et3 N; (b)
NBS, chcl3/py, 0 °c; (c) methyl 4-pinacolboranebenzoate5, pd(pph3)4 cat., K3po4, thF, reflux; (d) lialh4, thF, 0 °c ➝ r.t.; (e) Zn(oac)2, chcl3,
ch3oh, r.t. abbreviations: tFa – trifluoroacetic acid; DDQ – 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone; NBS – N-bromosuccinimide;
py – pyridine; thF – tetrahydrofuran.

SUPRAMoLECULAR CHEMISTRy
3
Figure 2. (colour online) Nmr spectroscopy experiments for porphyrin 1∙Zn (500 mhz, thF-d8, 298 K, c = 5.0 mm) – (a) 1h-Nmr spectra;
(b) 1h-Nmr; 1h–1h correlation spectroscopy; (c) 1h-13c single quantum correlation and (d) 1h-13c multiple bond correlations.
Figure 3. (colour online) crystallographic structure of porphyrin 1 (a) – thermal ellipsoids are at 50% level; h atoms were omitted for
clarity. molecular packing (b–d) of 1 and 1∙Zn is shown: (b,c) top and side view of 1 with blue dashes showing solvent (thF) filled tubes
and red dashes showing theH-bonding pores; (d) helicoidalarrangementof theo atoms (50% thermalellipoids). Seethe‘crystallographic
data’section in the Supporting information for more details.

4
L. ÐoRđEVIć ET AL.
Figure 4. (colour online) crystallographic structures of porphyrin 1∙Zn (a) – thermal ellipsoids are at 50% level; h atoms were omitted for
clarity. molecular packing (b) of 1∙Zn is shown: two crystallographically independent porphyrin molecules with differently coordinated
ligands: one bound to a thF molecule, while the other is bound to a h2o molecule. See the ‘crystallographic data’ section in the
Supporting information for more details.
(a)
(b)
7,5
6,0
4,5
3,0
1,5
0,0
S
C
e
H
r
Cie3
Cie3
ies3(0.3% CHCl3)
MCHX
Series2(0.3% CHCl3)
l
s10
THF
Series10
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
CH l + MeOH
Ser s1
MCHX (0.07% THF)
Series2
CHCl
Series3
MCHX
Series1(0.3% CHCl3)
Ser
MCHX
3
+ MeOH
0,3
0,2
0,0
500
600
380
410
440
470
500
380
410
440
Wavelength / nm
Wavelength / nm
Figure 5. (colouronline) absorbanceprofiles of 1∙Zn in differentsolvents thatincludethF (purpletrace), chcl3 (pink), mch (withthF, blue
trace) and mch (with chcl3, light blue); inset shows enlarged region where the Q bands absorb; on the right are shown the absorption
profiles after addition of a polar protic solvent such as meoh. Fluorescence profiles are presented in supporting information Figure S11.
(both unsubstituted and para-substituted ones) and are
further away from the porphyrin macrocycle lie together
in the 7.8–7.7 ppm region. Resonances from the phenyl
rings close to the porphyrin macrocycle can be found at
8.2 ppm as a doublet of doublets (with coupling constants
being 7 and 2 Hz for ortho- and meta- couplings, respec-
tively) for the unsubstituted phenyl and at 8.15 ppm as a
doublet (8 Hz for the only possible ortho- coupling). These
findings were confirmed both by homonuclear (CoSy) and
heterenuclear correlations (on single quantum and multi-
ple bond correlation, HSQC and HMBC, respectively).
Characterisation
The porphyrin 1∙Zn was characterised by mono- (1H and
13C) and bi-dimensional (1H-1H and 1H-13C) NMR spectros-
copies (Figure 2). Notably, in a polar aprotic solvent (such
as THF-d8) the o–H resonance appears as a triplet, due to
the coupling to the benzylic –CH2– that, in turn, is a dou-
blet (coupling constant 6 Hz). Additionally, the β-pyrrolic
resonances manifest as two doublets, probably due to the
diminished symmetry in THF-d8 induced by the Zn⋯o
complexation. Protons that are part of the four phenyl rings

SUPRAMoLECULAR CHEMISTRy
5
(a)
(b)
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
380
410
440
380
410
440
Wavelength / nm
Wavelength / nm
(c)
(d)
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
1,2
1,0
0,8
0,6
0,4
0,2
0,0
380
410
440
380
410
440
Wavelength / nm
Wavelength / nm
Figure 6. (colour online) uV-Vis titration experiments for 1∙Zn with meoh (a,c) for chcl3 and mch respectively and pyridine (b,d) for
chcl3 and mch respectively.
(a) 2,0
(b) 150,0
100,0
(c)
1,0
0,8
0,6
0,4
0,2
0,0
Dil.
Conc.
1,6
1,2
0,8
0,4
0,0
70 °C
10 °C
10 °C
70 °C
70 °C
10 °C
10 °C
70 °C
50,0
0,0
500
600
Wavelength / nm
700
380
410
440
380
410
440
Wavelength / nm
Wavelength / nm
Figure 7. (colour online) absorption and fluorescence spectra of 1∙Zn. Variable-temperature uV-Vis experiment (left); variable-
temperature fluorescence experiment (middle; λex = 419 nm) and concentration-depended experiment (right).
Crystals suitable for X-ray spectroscopy were obtained
for 1 (from THF/Cyclohexane vapour exchange) and 1∙Zn
(slow evaporation of a THF solution). Solid structure and
molecular packing of porphyrins 1 and 1∙Zn are shown
in Figures 3 and 4. In the asymmetric unit of crystals for
1, one full and two half porphyrin molecules were found.
Remarkably, a chiral helicoidal arrangement of hydrogen
bonds connects 32 neighbour porphyrins (dO−O = 2.749(8)
Å), forming channels along the a axis occupied by the THF
solvent molecules (50–52). Finally, a slipped face-to-face
stacking with an offset of 4.2 Å was observed, suggesting
that this synthetic variant of chlorin forms J-aggregates.
Although a similar ladder stacks are formed in an already
reported bis[4-(hydroxymethyl)phenyl]porphyrin (16),
porphyrin 1 forms a different hydrogen-bonded arrays
(no hydrogen bonding is present between hydroxyl
and pyrrolic NH) and includes also solvents in the solid
state structure, contrarily to already reported porphyrin.
Molecule 1·Zn model (Figure 4) shows two crystallograph-
ically independent porphyrin molecules with differently
coordinated ligands: one bound to a THF molecule,
while the other is bound to a H2o molecule (Supporting
Information Figure S15). As also observed in the case of
molecule 1, neighbour porphyrins show intermolecular
hydrogen bonds connections (average dO−O = 3.126(7) Å).
Interstitial voids are filled by three additionalTHF molecules

6
L. ÐoRđEVIć ET AL.
(a)
2,8
2,1
1,4
0,7
0,0
0,0
0,5
d / µm
1,0
1,5
(b)
(c)
2,0
1,5
1,0
0,5
0,0
0,0
0,3
d / µm
0,6
2,5
2,0
1,5
1,0
0,5
0,0
0,0
0,3
0,6
d / µm
Figure 8. (colour online) aFm height images in tapping mode with the profile of the white dashed lines. Samples were prepared by spin-
coating a solution of 1∙Zn in chcl3 (a) scale bars 5 and 2 μm, in mch (0.3% chcl3, scale bars 5 and 0.5 μm (b) and mch (0.07% thF, scale
bars 5 and 0.5 μm) (c).
tightly connected to available hydroxyl groups (from por-
phyrins and water).
To further investigate the formation of 1∙Zn assemblies
in CHCl3, MeoH or pyridine were added to disrupt both
hydrogen-bonding or axial zinc coordination interactions,
respectively (Figure 6). By adding MeoH, the Soret band
is steadily bathochromically shifted from λmax = 420 to
425 nm, without the presence of a clear isosbestic point,
most likely indicating the presence of sequential equilibri-
ums, from disaggregation to the Meo⋯Zn complexation.
The pyridine UV-Vis titration in contrast showed marked
bathochromic shift from λmax = 420 to 429 nm with a
clear isosbestic point, recalling an equilibrium between
the formation aggregates and the porphyrin–pyridine
adduct (42). In order to further favour the formation of
aggregates, a relatively concentrated solution of 1∙Zn in
CHCl3 was diluted in the apolar and non-coordinating
MCH solvent (0.3% CHCl3 content). The resulting solution
presents a broader Soret band with λmax = 417 nm, clearly
indicating the formation of aggregates. Successive addi-
tion of different aliquots of MeoH or pyridine showed anal-
ogous behaviour to that observed in CHCl3, namely the
restoring of the monomeric porphyrin species. However,
temperature-dependent UV-Vis studies displayed a strong
Self-assembly in solution
The optical properties of 1∙Zn were investigated in THF,
CHCl3 and the combination of the two with methylcy-
clohexane (MCH). The latter solvent was used to trigger
the self-assembly of the organic nanostructures (Figure 5).
At room temperature, 1∙Zn is dissolved inTHF.The UV-Vis
spectrum displays a sharp Soret band at λmax = 424 nm
(S2 ← S0 transition) accompanied by Q-bands at λmax = 556
and 596 nm, typical of metal porphyrins where the
increased symmetry causes the S1 ← S0 transition to reduce
from four to two bands. In CHCl3, the absorption spectrum
presents changes with respect to that observed from the
THF solution, with the Soret band showing hypso- and
hypo-chromic shift. only slight changes were discerned
with variable temperature (VT) and concentration-
dependent (Supporting Information Figure S12), such as
narrowing of the Soret band. This suggests the formation
of weakly bonded supramolecular assemblies (53).

SUPRAMoLECULAR CHEMISTRy
7
by increasing temperature, the S-band of the aggregates
at 422 nm decreases and the apparent monomer band
at 417 nm was revealed. Concentration-depended stud-
ies showed the disappearance of the 422 nm band and
the appearance of the absorption maximum at 417 nm,
confirming the temperature-depended behaviour and
the reversible nature of the aggregation phenomena
(Figure 7).
(a)
Taken all together, these data suggest the formation
of a bathochromically shifted band that, when com-
pared to that of the monomer, indicates the presence of
slipped face-to-face stacking arrangements, known also
as J-aggregates. This is in line with previous studies per-
formed with zinc chlorins, as well as with their semisyn-
thetic and synthetic variants (12).
1,4
0,7
0,0
(b)
Self-assembly on surface
Prompted by these results, we carried out atomic force
microscopy (AFM) studies to elucidate the morphology of
the aggregates of 1∙Zn as deposited on a surface. In Figure
8 are reported AFM height images of the 1∙Zn nanostruc-
tures as obtained from different solvents spin-coated onto
a freshly cleaved mica surface. As it can be seen in the
images, porphyrin 1∙Zn is able to form two-dimensional
islands on the surface, with a typical height of 2 nm, in
accordance with edge-on stacked height of the macro-
cycle onto the surface (39, 42, 54). Notably, the size of the
islands is depended on the solvent used for the solution.
For example, samples obtained from CHCl3 (Figure 8(a))
show the formation of scattered islands, in agreement with
the formation of weak non-covalent architectures that
were observed by absorption experiments. From a MCH
(0.3% CHCl3) solution (Figure 8(b)) extended aggregates
were also observed. Finally, structures obtained from MCH
(0.07% THF content) shown in Figure 8(c) present them-
selves as heterogeneous structures, including islands dis-
playing different sizes. As observed from spectroscopic
studies, the presence of THF in the sample is responsible
both for H-bonding disruption and the Zn complexation,
the latter disrupting the formation of extended stacked
nanostructures. However, since the amount of THF is very
low (0.07%) the formation of two-dimensional structures
is still observed.
0,0
0,5
1,0
d / µm
1,5
2,0
Figure 9. (colour online) aFm height images in tapping mode (a)
scale bar 2 μm with the profile of the white dashed line (b). Sample
were prepared by spin-coating a solution of 1∙Zn in chcl3 + meoh.
narrowing of the Soret band accompanied by a clear dis-
appearing of the blue-shifted band centred at around
435 nm (Supporting Information Figure S13). This result
indicated the formation of weak aggregates, although
stronger than in the case of CHCl3. Finally, in an attempt
to further increase the content of apolar MCH, a more apo-
lar solution of 1∙Zn could be prepared in a MCH-diluted
THF solution (0.07% THF content). Notably, the absorption
spectrum compared to that obtained in a THF solution,
where 1∙Zn is molecularly dissolved, appears to be broad-
ened exhibiting a loss in oscillator strength. Apparently,
the energy of the absorption maximum does not shift and
temperature-dependent and concentration-dependent
studies were thus performed. VT-UV-Vis revealed that,
Scheme 2. (colour online) Schematic representation of the aggregation of 1∙Zn in solution and when it is transferred onto the surface.

8
L. ÐoRđEVIć ET AL.
In order to appreciate the effect of H-bonding and
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MeoH disrupted the formation of the aggregates favour-
ing the presence of individual molecules on the surface,
confirming the results observed from the spectroscopic
measurements.
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Disclosure statement
No potential conflict of interest was reported by the authors.
Supplemental material
CCDC-1442721 and 1442722 contain the supplementary
crystallographic data for compounds 1 and 1∙Zn. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.
org/10.1080/10610278.2016.1158407
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Funding
DB gratefully acknowledges the EU through the ERC
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ORCID
Luka Ðorđević
Nicola Demitri
Davide Bonifazi
http://orcid.org/0000-0002-8346-7110
http://orcid.org/0000-0003-0288-3233
http://orcid.org/0000-0001-5717-0121

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