Complexes of aryl-substituted porphyrins and naphthalenediimide (NDI): Investigations by synchrotron X-ray diffraction and NMR spectroscopy

Article abstract of DOI:10.1039/c1dt10880h

New donor-acceptor hybrids of Zn(ii)-metallated 5,15-diaryl porphyrins have been designed and synthesised via the porphyrin interactions with an electron acceptor molecule, di-n-hexyl N-substituted 1,2,4,8-naphthalenetetracarboxylic diimide (NDI). Binding interactions within these supramolecular complexes were investigated in the solid state by synchrotron X-ray diffraction and probed in solution by ¹H NMR spectroscopy. The systematic modulation of the porphyrin π-density was achieved, for the first time as multiple methoxy and fluorine groups were introduced as substituents to the 5,15-diaryls of the porphyrin. For these, the variation of the porphyrin-NDI binding strengths determined by ¹H NMR titrations was shown, using the Swain's type dual parameter approach, to be closely linked with the peripheral substitution pattern of the diaryl porphyrins validated by crystallography. The new 1:1 donor-acceptor complexes formed display characteristic features of the aromatic-stacked systems, i.e. the parallel arrangement and short interplanar separation between the substituted porphyrin and NDI. Synthetic modification of electron-density on the porphyrin surface by introducing substituents at peripheral sites of functionalised porphyrins represent a general solution towards electronically tunable aromatic surfaces: an understanding of their solution and solid state behaviour will significantly improve the rational design of new functional donor-acceptor supramolecular materials with potential applications ranging from new energy materials to dye-sensitised solar cells, photovoltaics and future drug delivery devices.

Full text of DOI:10.1039/c1dt10880h

Dalton
Transactions
An international journal of inorganic chemistry
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Volume 40 | Number 41 | 7 November 2011 | Pages 10777–11052
ISSN 1477-9226
COVER ARTICLE
Pascu et al.
Complexes of aryl-substituted
porphyrins and naphthalenediimide:
investigations by synchrotron XRD
and NMR
1477-9226(2011)40:41;1-R

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PAPER
Complexes of aryl-substituted porphyrins and naphthalenediimide (NDI):
investigations by synchrotron X-ray diffraction and NMR spectroscopy†
Lok H. Tong,a,b Paolo Pengo,a William Clegg,c John P. Lowe,d Paul R. Raithby,d Jeremy K. M. Sandersa and
Sofia I. Pascu*d
Received 10th May 2011, Accepted 13th June 2011
DOI: 10.1039/c1dt10880h
New donor–acceptor hybrids of Zn(II)-metallated 5,15-diaryl porphyrins have been designed and
synthesised via the porphyrin interactions with an electron acceptor molecule, di-n-hexyl N-substituted
1,2,4,8-naphthalenetetracarboxylic diimide (NDI). Binding interactions within these supramolecular
complexes were investigated in the solid state by synchrotron X-ray diffraction and probed in solution
by 1H NMR spectroscopy. The systematic modulation of the porphyrin p-density was achieved, for the
first time as multiple methoxy and fluorine groups were introduced as substituents to the 5,15-diaryls of
the porphyrin. For these, the variation of the porphyrin–NDI binding strengths determined by 1H
NMR titrations was shown, using the Swain’s type dual parameter approach, to be closely linked with
the peripheral substitution pattern of the diaryl porphyrins validated by crystallography. The new 1 : 1
donor–acceptor complexes formed display characteristic features of the aromatic-stacked systems, i.e.
the parallel arrangement and short interplanar separation between the substituted porphyrin and NDI.
Synthetic modification of electron-density on the porphyrin surface by introducing substituents at
peripheral sites of functionalised porphyrins represent a general solution towards electronically tunable
aromatic surfaces: an understanding of their solution and solid state behaviour will significantly
improve the rational design of new functional donor–acceptor supramolecular materials with potential
applications ranging from new energy materials to dye-sensitised solar cells, photovoltaics and future
drug delivery devices.
Introduction
Attractive interactions between aromatic molecules play signifi-
cant roles in biological and molecular recognition1 and in material
science.2 Recently, composites incorporating aromatic systems
such as porphyrins and naphthalenediimides have become of
importance due to their potential role in organic electronics,
dye-sensitised solar cells and photovoltaic applications.3 Naph-
thalenediimides and related materials are emerging as electron
acceptors and electron-transporting materials that have been
explored for applications as components for new materials with
interesting optical properties in the solid state.4 An understanding
of their ability to form tunable donor–acceptor complexes with
Scheme 1 Synthesis of 5,15-diaryl porphyrins. Reagents and conditions:
(i) H2, Pd/C, THF, 2% Et3N, 2 h, r.t.; (ii) TFA; (iii) MeOH, r.t.; (iv) DDQ;
(v) Zn(OAc)2, MeOH–CHCl3. (R1 = various substituents).
aDepartment of Chemistry, University of Cambridge, Lensfield Road, CB2
1EW, Cambridge, UK
bDepartment of Inorganic, Analytical and Applied Chemistry, University of
Geneva, 30 Quai Ernest-Ansermet, CH-1211, Geneva 4, Switzerland
porphyrins should be of significant importance towards the
rational design of the new generation of hybrid materials for
optoelectronics.
The electronic nature and geometrical requirements for inter-
molecular interactions of simple aromatics can be rationalised
by simple quadrupolar models,5–7 but this is still a lively area of
exploration.8 Strong interactions are observed between porphyrin
cDepartment of Chemistry, University of Newcastle upon Tyne, NE1 7RU,
Newcastle upon Tyne, UK
dDepartment of Chemistry, University of Bath, BA2 7AY, Bath, UK.
E-mail: s.pascu@bath.ac.uk; Fax: 44 1225 386231; Tel: 44 1225 386627
† Electronic supplementary information (ESI) available. CCDC refer-
ence numbers 715661–715666, 766219–766220, 767031–767033. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10880h
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10833–10842 | 10833
©

Fig. 1 (a) Schematic representations and substitution pattern of the 5,15-diaryl substituted porphyrins (1–16); (b) Structures of the N-disubstituted
diimides (I–III) employed in the complexation studies. (iPr = iso-propyl).
planes due to their large and planar conjugated structure, giving
rise to self aggregates in solution and offset packing arrangements
in the solid state. Although the use of porphyrins to construct
molecular tweezers for p-complexation of aromatic electron poor
species has been demonstrated,9 the first solid state evidence for
p-complexes of porphyrins with aromatic acceptors was only
recently reported.10 Earlier work has been concerned with the
determination of solution structures of p-complexes, including
those of several metallated porphyrins with aromatic substrates.
Porphyrins have been under intense investigation because of their
unique properties leading to catalytic roles in photochemical
to organic transformations. It has been demonstrated that the
variation of the electronic properties on the peripheral sites alters
the properties of the macrocycles, but only a couple of reports
on the factors affecting the strength of aromatic interactions
have been published and only brief discussions on the factors
affecting the strength of these interactions have been reported.11,12
Importantly, only the electronic effects of peripheral substituents
on the porphyrin metal centre have received attention thus far.
The electronic effect of peripheral substituents on the porphyrin
metal centre has received intensive attention: Walker and co-
workers showed that the coordination of the metal centre by
free-base porphyrins is sensitive to the substitution pattern at the
periphery.13
report was published to date.14 Here we describe for the first time
combined solution and solid state investigations of the effect of the
systematic modulation of the p-density in a series of symmetrically
substituted 5,15-diaryl porphyrins in their interactions with
electron deficient molecules such as di-n-hexyl N-substituted
1,2,4,8-naphthalenetetracarboxylic diimide (NDI). Our work
aims to validate a new, entirely accessible route towards tuning
the p -surface of porphyrins and was undertaken to demonstrate
that the electronic structures of the zinc porphyrin, and therefore
the association properties in solution and in the solid state,
can be tuned as required by applications with significance in
biological, materials science and future photovoltaics. This
could provide a general solution for constructing electronically
controllable donor–acceptor hybrids, and aims to enhance our
understanding of the attractive interactions between aromatic
molecules.
An understanding of such effects is crucial in the quest
for new energy-harvesting composites incorporating porphyrins
and organic electronics and the rational design of new donor–
acceptor synthetic precursors for energy materials. Substituent
effects in other p-complexations, such as in recognition of
2,4,7-trinitrofluorenone within the tweezer-like cavity of two
co-facially substituted acridine units, have been applied in
the effective separation of nitro-substituted polycyclic aromatic
compounds.15 Establishing a reliable way to vary the p-electronic
density of porphyrins can be beneficial to the rational design
of porphyrin-based systems for separation of aromatic com-
pounds via aromatic interactions such as selective extraction
of higher fullerenes16 and in the design and synthesis of new
Despite the fact that the peripheral substituents have been used
to probe the properties of porphyrin macrocycles, the, electronic
effects on the thermodynamic stabilities of the donor–acceptor
complexes involving porphyrins have received only limited
attention, and to the best of our knowledge only one such
10834 | Dalton Trans., 2011, 40, 10833–10842
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©

materials with optoelectronic properties for dye-sensitised solar
cell applications.17
The subsequent oxidation with DDQ followed by zinc insertion
using Zn(OAc)2 in a CHCl3–MeOH mixture yielded the corre-
sponding zinc metallated porphyrin. Free-base porphyrin 6 was
obtained in high yield by reacting the zinc counterpart 5 with
dilute hydrochloric acid. Treatment of 6 with Ni(OAc)2 afforded
the corresponding nickel-metallated porphyrin 7.
Full characterisation of the porphyrins and acceptor molecules
was obtained by 1H, 13C NMR and UV-Vis spectroscopies,
high resolution mass spectrometry (HRMS) or MALDI-TOF
and elemental analysis. Additionally, characterisation of the
fluorine-containing porphyrins 8–13 was obtained by 19F NMR
spectroscopy. In all cases, the symmetric structures gave rise to
simple 1H NMR spectra with a well resolved meso proton singlet
resonance in the 10 ppm region of the spectrum. The position of
this resonance allowed its use as a probe to monitor the titration
experiment (vide infra) and as a diagnostic tool for the electron
density in the porphyrin p-system.
Due to the structural similarities within each of the fluorine-
or methoxy-substituted series investigated, and within the experi-
mental timescale of the 1H NMR experiments, we propose that, all
compounds are similarly solvated and that any changes in solva-
tion upon complex formation are likely to be similar throughout
the series. The electron-poor character of di-n-hexyl substituted
NDI I should render it an effective acceptor, as demonstrated
in earlier studies.23 Its complementary geometrical dimensions
with the substituted porphyrins under investigations were expected
to provide suitable p-surfaces for aromatic interactions with the
porphyrins.
We now report the formation of new porphyrin–NDI compos-
ites in solution and their solid state structures. Explorations into
the substituent effects of the 5,15-diaryl symmetrically substituted
porphyrins on the strength of p-complexation with a dialkyl N-
substituted naphthalenediimide (NDI) are also discussed. Due
to the highly conjugated nature of the porphyrin ring systems,
the presence of electron-donating or withdrawing substituents on
the peripheral meso-phenyls can affect the physical and chemical
properties of the macrocycle, hence the strength of the interactions
within the donor–acceptor complex in solution and the nature of
the aromatic stacking within the resulting complex in the solid
state. An understanding of these effects will significantly improve
the rational design of new donor–acceptor hybrid materials.
Results and discussions
Experimental design and syntheses
To investigate the peripheral substituent effects on aromatic
interactions between electron-rich porphyrins and electron-poor
acceptors, di-N-substituted naphthalenediimide I and related
electron deficient molecules II and III were selected and their
complexations in CHCl3 solutions with porphyrins 1–16 were
examined (Fig. 1).
Free base, zinc- and nickel-metallated 5,15-diaryl symmetrically
substituted porphyrins 1–16 were synthesised in high yield via
modification of established procedures.18 With the exception of
6,19 720 and 1621 all porphyrin donors used are new and have
been synthesised as described in Scheme 1. The optimum reaction
time for the acid catalysed condensation between deprotected
5,5¢-dibenzyl-3-3¢-di-(n-hexyl)-4,4¢-dimethyl-dipyrromethane and
substituted benzaldehyde varied from 4 to 16 h depending on
the reactivity of the individual aromatic aldehyde. The di-n-hexyl
substituted NDI I and related acceptor molecule II (di-2,6-iPrC6H3
N-substituted NDI) and III (di-n-hexyl N-substituted pyromellitic
diimide) were prepared following published procedures22 or via
adapted synthetic methods.
For 1–16, multiple substituents were introduced onto the periph-
eral aryl rings to alter the electronic properties of the porphyrin p-
density. An increasingly depleted electron density in the porphyrin
p-system was gained by increasing the number of electron-
withdrawing groups. The opposite electronic effect was achieved
using electron-donating methoxy substituents. Methoxylated (1–
4) and fluorinated (8–13) porphyrins bear various numbers of
methoxy groups and fluorines respectively. Free-base porphyrin 6
and its zinc- and nickel-metallated analogues (5 and 7 respectively)
possess no peripheral substitutions but phenyl rings at the meso
positions. For porphyrins 14, 15 and 16 each of the peripheral
sites are substituted with one 3-allyloxy or 4-nitro groups or two
3,5-di-tert-butyl groups respectively, but synthetic challenges did
not allow any further functionalisation for tuning the electron
density. For all porphyrins 1–16, four flexible hexyl side chains
were incorporated into the b-pyrrolic positions of the porphyrin
framework to enhance the solubility and minimise aggregation in
common solvents.
Investigations in the solid state by X-ray diffraction
The molecular structures of six of the zinc porphyrins (2, 8, 10,
12, 13 and 16) were determined in the solid state by single-crystal
X-ray diffraction after using high-intensity synchrotron radiation
(ESI†). Typically, extremely small and weakly diffracting single
crystals suitable for X-ray analysis were grown by slow diffusion
of methanol into a chloroform or dichloromethane solution of
porphyrins. The porphyrin frameworks in these compounds are
all essentially flat, with slight deviations from the molecular plane
through core atoms of the porphyrins in the range of up to 0.1
˚
A. The internal dimensions of the zinc metallated porphyrins fall
within the normal ranges.24 The substituted diaryl units lie close
to perpendicular to the molecular plane as observed previously in
the free-base 5,15-diaryl porphyrins.20
Four new donor–acceptor complexes (or co-crystallites) A–D,
incorporating naphthalenediimide I and porphyrins 5, 6, 14 and 15
respectively, were isolated and characterised by X-ray diffraction.
X-ray quality single crystals were grown by the slow diffusion of
MeOH into a mixture of equimolar ratio of porphyrin and NDI
in CH2Cl2. In each case the 1 : 1 porphyrin : acceptor molecule
stacking was observed in the solid state with the NDI surface
appears to stack over the porphyrin plane. The main feature in
these co-crystallites is the offset arrangement of the components,
which seems most prominent in the case of complex A (Fig.
2). The mean plane separation between the porphyrin and the
˚
diimide molecules is 3.23 A, suggesting the aromatic stacking
interactions between the molecules. The closest approach of a
Aromatic aldehydes bearing electron-donating methoxy groups
required longer reaction times than their fluorinated analogues.
neighbouring atom in the NDI molecule to the zinc atom (i.e.
˚
the Zn(1)–N(5) distance) is 3.31 A. The tilt between the planes of
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Dalton Trans., 2011, 40, 10833–10842 | 10835
©

Fig. 3 (a) The molecular structure of complex B; (b) Projection of NDI
onto the free-base porphyrin 6 (hexyl chains of both components are
not shown); (NDI is symmetric with respect to an inversion centre) (c)
Packing arrangement along b axis (hexyl chains of both components
are not shown). Hydrogen atoms and solvated molecules are omitted for
clarity. (Key: grey, carbon; blue, nitrogen; red, oxygen).
Fig. 2 (a) The molecular structure of complex A. The asymmetric unit
contains only half of the acceptor and the porphyrin molecules, the other
half was generated by symmetry (NDI is symmetric with respect to an
inversion centre); (b) Projection of NDI onto the zinc porphyrin 5 (hexyl
chains of both components are not shown); (c) Packing arrangement along
b axis (hexyl chains of both components are not shown). Hydrogen atoms
and solvated molecules are omitted for clarity. (Key: grey, carbon; blue,
nitrogen; purple, zinc; red, oxygen).
state structure of co-crystallite C shows polymer-like columns
containing alternating molecules of diimide and porphyrin stacked
along the crystallographic b axis (Fig. 4). Within these stacks
the porphyrin and acceptor molecular make close face-to-face
contacts.
these two molecules (i.e. porphyrin 5 and acceptor molecule I) is
1.28◦. The overall packing arrangement in complex A is shown
in Fig. 2c. This structure showed no major distortions of the
geometry of the individual molecular components indicating that
this is a complex held together by weak noncovalent interactions.
Internal dimensions of the porphyrin such as zinc-nitrogen bond
lengths fall within expected ranges.24 In the diimide molecules
˚
the mean deviation from the plane for all atoms is only 0.03 A
(excluding hexyl chains). However, the porphyrin plane appears
to be slightly concave towards the NDI molecule. The metal
centre Zn(1) is protruding away from the porphyrin core, being
˚
displaced from the porphyrin plane by 0.15 A. As a consequence,
the mean deviation of any of the core porphyrin atoms, excluding
˚
the aliphatic side groups, from the plane of this core is 0.11 A. Out-
of-plane deformations of the phenyl rings are observed. The phenyl
rings lie approximately perpendicular to the porphyrin plane and
adopt a syn orientation, with relatively large displacements of
˚
0.48 A of the ipso carbons above the molecular plane. This
deformation results in a substantial angle between the porphyrin
plane and the para hydrogen.
The structure of complex B displays a strong resemblance
to that of A (Fig. 3). These complexes are isostructural and
¯
crystallise in the same space group (P1). The structures have almost
identical unit cells and they differ only in the presence/absence of
zinc and the replacement of CHCl3 by CH2Cl2 as the solvate.
The n-hexyl substituents in both cases are found to adopt an
‘up–up’ orientation on one side and a ‘down–down’ orientation
on the opposite side. The complex formed between NDI with
porphyrin 14 also showed the equimolar ratio of porphyrins and
acceptor molecule. In contrast to co-crystallites A and B, where
discrete pairs of porphyrin-acceptor units were observed, the solid
Fig. 4 (a) The molecular structure of complex C; (b) Projection of
NDI onto the zinc porphyrin 14 (hexyl chains of both components are
not shown); (Both components, porphyrin and NDI, are symmetric with
respect to independent inversion centres) (c) Packing arrangement along
b axis (hexyl chains of both components are not shown). Hydrogen atoms
are omitted for clarity. (Key: grey, carbon; blue, nitrogen; purple, zinc; red,
oxygen).
10836 | Dalton Trans., 2011, 40, 10833–10842
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©

˚
The mean plane separation between the porphyrin and the
spacing between the components is 3.19 A while in TCNQ–
II
˚
˚
diimide molecules in co-crystallite C is 3.30 A and the tilt between
Zn (OEP) it is 3.16 A.
these planes is 2.4◦. The position of the NDI molecule in C is
laterally displaced away from the centre such that the closest
contact between the metal centre labelled Zn(1) and the nearest
Solution studies by 1H NMR spectroscopy
Aromatic interactions between porphyrins and acceptor molecules
I–III were probed in solution by 1H NMR titrations in CDCl3 at
298 K. 1H NMR dilution studies tested the aggregation behaviour
of the porphyrins alone: dilution curves from 15 to 1 mM were
constructed for the two most highly substituted porphyrins, 1
and 13, which showed the greatest differences between these
structures (ESI†). Throughout the series, dilution experiments
showed only very small variations of meso shifts (<0.01 ppm),
suggesting that the level of aggregation of porphyrins is rather
low in this concentration range. Under the dilute experimental
conditions used we approximate that the degree of aggregation
for all porphyrins 1–16 would be similar. We expect that the
absolute magnitude of the binding strengths might be affected
by approximately the same amount but this factor is not large
enough to alter the overall outcomes and trends resulting from
these binding studies. A dilute concentration of porphyrin (5 mM)
˚
atom in the adjacent diimide is 3.31 A, to the carbonyl carbon
C(38). Instead of the discrete pair of porphyrin–acceptor observed
in A and B, and similar to the supramolecular structure of C, the
solid state structure of D shows columns that contain alternating
molecules of diimide and porphyrin that are stacked along the
crystallographic b axis (Fig. 5). Within these stacks the porphyrin
and acceptor molecules show close face-to-face contacts and the
position of the diimide molecule is laterally displaced away from
the porphyrin centre. The zinc porphyrin and NDI molecules are
˚
held apart by a Zn–O bond of 2.326(18) A. As before, the structural
parameters of individual molecules and the interplanar distances
between the donor and acceptor component fall within the normal
range and are very close to those found for the complex C.
1
proved to be optimum for H NMR studies on the entire series,
and this was used throughout to minimise the extent of porphyrin
aggregation during the experiment.
For the titration experiment, aliquots of the acceptor in
chloroform were sequentially added to the chloroform solution of
the substituted porphyrins. The addition of the acceptor molecule
I caused increasing upfield shifts to the meso resonance of the zinc
porphyrins. All other porphyrin resonances remained essentially
unaffected by the complexation process. Chemical exchange of
the aromatic substrate between the complexed and un-complexed
form is rapid on the chemical shift timescale and only the
exchange-averaged observed shift was observed.
For metalloporphyrins studied, the chemical shift of the meso
protons at each acceptor concentration composed a data point and
a data set of 20 points was collected from each titration study (Fig.
6). The data sets were analysed by EQNMR25 and the fitting to
a 1 : 1 binding isotherm afforded values of association constant
Kmeso using initial meso chemical shifts (d0) and the limiting
complexation-induced meso shifts of the porphyrins (dcomplex).
Evidence for the formation of 1 : 1 adducts in solution comes from
Job plot analysis. A typical plot between porphyrin 1 and NDI
molecule (acceptor I) is shown in Fig. 7. The plot of the mole
fraction (c) multiplied by the complexation-induced shift of the
meso proton (Dd) versus the mole fraction c shows a maximum
at c = 0.5. The use of EQNMR gave a good fit for the 1 : 1
donor–acceptor stoichiometry. Porphyrin and dihexyl substituted
naphthalenediimide I are therefore in equilibrium with the 1 : 1
donor–acceptor complex (eqn (1)), with the association constant
Kmeso (eqn (2)).
Fig. 5 (a) The molecular structure of complex D; (b) Projection of NDI
onto the zinc porphyrin 15 (hexyl chains of both components are not
shown); Connectivity and structure have been established: full details are
in the CIF.† (c) Packing arrangement along a axis (hexyl chains of both
components are not shown). Hydrogen atoms are omitted for clarity. (Key:
grey, carbon; blue, nitrogen; purple, zinc; red, oxygen).
Interestingly, for all complexes A–D, a “staircase-like” arrange-
ment of the hexyl groups present either side of the porphyrin
molecules was ‘followed’ by the hexyl groups in the diimide. This
may allow a more efficient packing in solid state. Additional
Van der Waals interactions between the chains may help stabilise
these structures. The formation of porphyrin–NDI complexes
was therefore established via X-ray structure determinations. The
structures show characteristic features of the p–p stacked systems,
such as the parallel arrangement of the acceptor and the short
interplanar spacing between the two components. The porphyrin–
acceptor contacts seen in these complexes are comparable to pre-
viously reported cases where 7,7¢,8,8¢-tetracyanoquinodimethane
(TCNQ) molecules interact with metallated octaethylporphyrins
(OEP).10 For example, in TCNQ–CuII(OEP), the interplanar
Porphyrin + NDI ꢀ Complex
(1)
[Complex]
Kmeso
=
(2)
[Porphyrin][NDI]
The association constants of Zn(II) porphyrins with NDI (I)
are given in Table 1. A typical plot of complexation-induced
chemical shift of meso protons of porphyrin 1 versus the addition
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Dalton Trans., 2011, 40, 10833–10842 | 10837
©

Table 1 Association constants (Kmeso, inclusive of standard deviation
errors from the data fitting calculated by EQNMR), meso chemical shifts
(d0) and maximum limiting complexation-induced 1H NMR shifts of the
meso protons (Dd = d0 - dcomplex) for the complexation reactions between
zinc porphyrins and the acceptor molecule I (5 mM, CDCl3, 298 K,
500 MHz)
Number/type of non-
Porphyrin Kmeso (M-1) d0 (ppm) Dd (ppm) hydrogen substituents
1
110 4.2
79 1.7
67 1.2
61 1.6
58 1.1
54 1.0
42 1.3
24 1.0
13 1.9
19 0.5
18 0.3
67 1.3
47 1.4
70 4.9
10.256
10.236
10.218
10.176
10.187
10.172
10.176
10.151
10.128
10.263
10.268
10.194
10.215
10.200
0.351
0.346
0.352
0.350
0.340
0.318
0.320
0.302
0.324
0.427
0.412
0.312
0.355
0.285
6/OMe
4/OMe
2/OMe
2/OMe
0
2/F
2/F
4/F
6/F
8/F
10/F
2
3
4
5
8
9
Fig. 6 The complexation of a 5,15-diaryl substituted porphyrin (R = H,
F and OMe groups) and a dihexyl substituted NDI. Protons coloured in
pink indicate the 1H NMR probe used to follow the complex formation in
solution.
10
11
12
13
14
15
16
2/OCH2CH CH2
2/NO2
4/C(CH3)3
number of nitro, allyloxy and tert-butyl substituents could not
be achieved synthetically thus far, only the binding strengths
within the methoxylated (1–4) and fluorinated (8–13) porphyrin
series with the acceptor I were analysed further. The chemical
shifts of meso protons of the 5,15-diaryl substituted porphyrins
appear to correlate with the extent of substitution with fluorines
and methoxy groups. The substitution-free porphyrin 5 may be
considered as a “reference” in the peripheral substitution study
as it binds stronger to I than the fluorinated porphyrins but
weaker than the methoxylated porphyrins. As evident from Table
1
1, the meso H NMR chemical shifts showed a general upfield
shift with increasing number of electron-donating groups (except
for the case of 4) while the opposite trend is observed with
increasing number of fluorines (except 12 and 13). The observed
variations of the meso chemical shifts seem to point towards
the long distance and systematic influences through substitutions
on the meso aryl rings. Overall, a noticeably weaker binding
of NDI with the fluorinated porphyrins (8–13) was observed
compared with the methoxy-substituted ones (1–4). It appears
that electronic effects of both substituents operate generally in
a cumulative manner, with some exceptions (vide infra). The
binding strength increases slightly with increasing number of
methoxy groups. The opposite trend with a general decrease in
association constant was observed upon progressive incorporation
of fluorine. The cumulative argument may apply to account for
the gradual decrease in affinity, from substitution-free porphyrin
Fig. 7 Typical plot (Job) of mole fraction (c) against mole fraction
multiplied by complexation-induced shift of the meso proton (c ¥ Dd)
of the complexation of porphyrin 1 and acceptor molecule I. Similar plots
were obtained for all zinc porphyrins studied here.
of acceptor I is given in the ESI.† The complexation-induced shift
(Dd) for porphyrins 1–5 and 8–13 is in the range 0.30 to 0.43
ppm indicating essentially identical geometries for the resulting
donor–acceptor complexes. These similarities are maintained
in the solid state for complexes A–C as confirmed by X-ray
diffraction.
The binding strengths between acceptor I and Zn(II)-porphyrins
5, 14–16 were first examined. The association constant between I
and the dinitro-substituted porphyrin 15 (Kmeso = 47 1.4 M-1)
appears to be smaller than that between I and “unsubstituted”
porphyrin 5 (Kmeso = 58
1.1 M-1 respectively), but those
5 to bis-(3,4,5-trifluorophenyl) substituted porphyrin 11 (Kmeso
13
=
between I and allyloxy- and tert-butyl substituted porphyrins 14
and 16 (Kmeso = 67 1.3 and 70 4.9 M-1 respectively) were
found to be somewhat larger. This prompted an investigation
of the substituent effects operating on the binding strength
due to differences in electronic demands of the substituents at
peripheral aryl sites. The association constants listed in Table 1
appear to be strongly dependent on the electron withdrawing or
releasing characteristics of the peripheral substituents: porphyrins
with donating groups show enhanced binding, whereas those
bearing electronegative fluorines display reduced affinities. Since
the systematic modulation of electron density by altering the
1.9 M-1). Incorporation of further fluorine substituents
showed insignificant effect on the binding strength. However, the
possibility that the fluorine ortho-substituted porphyrins 12 and
13 can exhibit an electron donating character in these systems
cannot be ignored. Such an unusual property has been previously
observed and proposed to be arising from the overlap between
the electron clouds of the fluorines and the p system of the
porphyrin.13i
The use of Hammett type linear free energy analysis treatment
was initially attempted to account for the trend of the binding
strengths observed in the complexation studies for all systems
10838 | Dalton Trans., 2011, 40, 10833–10842
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©

in the selected set, with the exception of porphyrins 12 and 13,
since Hammett constant for ortho fluorine was unavailable. The
fact that a trend line could not be deduced from the Hammett
plot suggested that this model is not suitable for analysing the
peripheral substituent effects on the binding strengths for these
donor–acceptor complexes (ESI†). This may be due to the fact
that substituents are not directly attached to the porphyrin core,
but placed at the aryl rings where the electronic ‘information’ can
only be transmitted indirectly through the meso-aryl bonds.
The dual parameter approach developed by Swain,26 which
analyses the resonance and inductive contributions separately,
was applied to the data to probe whether this approximation
constitutes a more suitable model for the interpretation of the
binding results. Here, the sigma (s) values for the substituents used
in the Hammett approach may be separated into two components,
sI (inductive) and sR (resonance), as given by eqn (3) (also see
Table S3, ESI†). To take into account the possible positional
dependence of the resonance contribution, two parameters sigma
meta-(sm) and sigma-para (sp) are introduced. For a given
porphyrin donor j, the overall electronic contribution from the
peripheral substituents depends also on the total number of meta
substituents (Njm) and para substituents (Njp). The Swain eqn (4)
was derived as described in the ESI,
fit gives r the value of -0.43 0.04 with a correlation factor (R2) of
0.94. The different sigma-R values for meta and para substituents
may reflect possible differences in efficiency for the resonance effect
to be transmitted to the porphyrin core. These values are only
apparently different, because the error associated with beta-para
(bp) renders this value very close to that of beta-meta (bm), i.e. 0.65
0.04 versus 0.73 0.11. It appears that the substituents introduced
into the peripheral phenyls exert their influence somewhat more
effectively through the inductive than the resonance route. The
resonance communication between the porphyrin core and the
periphery may be hindered by the perpendicular geometry of
the aryl rings. The correlation obtained in the fitting process
seems close to linear both for the methoxylated (1–4) and
fluorinated series (8–11), and therefore consistent with a simple
electrostatic explanation: the stacking interactions seem to reflect
the electrostatic potential on the surface of the porphyrins and are
sensitive to the nature and peripheral substitution patterns.
Metallation effects on solution binding constants remain unpre-
dictable within the experimental error. For example, the binding
strength of the free-base and substitution-free porphyrin 6 to
I (e.g. Kmeso = 55 1.5 M-1) is very close to that of its zinc-
metallated analogue 5 (Kmeso = 58 1.1 M-1). Here, comparable
binding strengths found for porphyrins 5 and 6 may be due to
the structurally similar, flat, frameworks yielding isostructural
complexes A and B, as confirmed by X-ray diffraction (Fig.
2 and 3). Steric effects appear to play a role in the binding
interactions. The crystal structure of 7 shows a non-planar ruffled
conformation of the nickel porphyrin (ESI†),20 in strong contrast
to the largely undistorted flat porphyrin surface observed in the
structure of its zinc counterpart 5 (Fig. 2) or free base analogue
6 (Fig. 3). In solution, only a very small induced chemical
shift difference (Dd) was detected by 1H NMR spectroscopy
at the complexation between acceptor I and substitution-free
nickel porphyrin 7, suggesting that the binding interactions within
this donor–acceptor system are negligible (ESI†). The electronic
contributions by Zn(II) and Ni(II) to the p-electron clouds are
not fully understood but their structural effects on the porphyrin
framework are evident from the X-ray structure determination.
Such differences could explain the ineffective overlapping of the
p-systems in the intermolecular binding process for the Ni(II)
porphyrins.
(3)
s = asI + bsR
K j
⎡ ⎤
= r N jm (sI + bmsR ) + N jp (sI + bpsR )
⎢ ⎥
log
(4)
⎣
⎦
KH
and used in the fitting of the experimental data (see Table S4,
ESI and Fig. 8). The values for sigma-I (sI) and sigma-R (sR)
of fluorine and methoxy groups have been reported.27 The Zn(II)
porphyrins considered for the fitting of Swain eqn (4) are listed
in Table S4 (ESI†) together with the relevant parameters used.
From the data, it is noted the resonance contribution to the
substituent parameter sigma-meta (sm) is 0.65, while the inductive
contribution is 1 by definition (i.e. sm = sI + 0.65sR). The same
applies to the sigma-para (sp) (i.e. sp = sI + 0.73sR). With the beta
values in hand, the overall substituent parameter, i.e. the sum over
all the electronic contributions for all the substituents in the meso
phenyl rings can be computed (Fig. 8) (the actual plot). The linear
The binding experiment between porphyrin
5 and the
peripherally crowded bis-N-(2¢,6¢-di-iso-propylphenyl)-
naphthalenediimide II showed no complexation induced
shifts by 1H NMR. In fact, the perpendicular orientation adopted
by the 2,6-di-iso-propylphenyl groups in the crystal structure (i.e.
giving an average distance between the plane of the aromatic core
˚
and the further methyl hydrogens of ca. 4.10 A yields a sterically
isolated NDI acceptor unit (ESI, Fig. S8†). This strengthens the
argument that the donor–acceptor interactions are mediated by
steric factors. A similar geometrical arrangement for a substituted
aryl groups was proposed to originate from the unfavourable
clashes between the sterically demanding isopropyl groups and
the carbonyl groups.28
The binding of the bis-(3,4,5-trimethoxyphenyl) substituted
porphyrin 1 to di-n-hexyl N-substituted pyromellitic diimide III
(Kmeso = 10 0.7 M-1) was found to be significantly weaker than
that of NDI I (Kmeso = 110 4.2 M-1). On the other hand, the
binding strength between the bis-(2,3,4,5,6- pentafluorophenyl)
Fig. 8 Linear trend relevant to the subset of selected Zn(II)-porphyin
donors (methoxy- and fluoride-substituted). The trend line shown was
fitted for the Swain eqn (4) (MicroMathꢀR ScientistꢀR 2.01) implementing a
least squares algorithm (Data is collated in Table S4, ESI†). The linear fit
gives r the value of -0.43 0.04 with a correlation factor (R2) of 0.94.
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Dalton Trans., 2011, 40, 10833–10842 | 10839
©

substituted porphyrin 13 and the acceptor III and (Kmeso = 9
0.8 M-1) is comparable to that of NDI I (Kmeso = 18 0.3 M-1).
It was expected that the more effective electron-accepting nature
of III with respect to I would result in a stronger binding to
the porphyrin donors.29 The binding strengths of 1 and 13 with
pyromellitic diimide III showed negligible differences. This is in
sharp contrast to that observed with naphthalenediimide (NDI) I
(ª 100 M-1) and suggests that acceptor molecule III is less sensitive
to the electronic changes of the porphyrin core than acceptor I.‡
Such a decrease in the binding strength can be attributed to the
smaller size of the pyromellitic diimide acceptor III.30 Thus, an
acceptor molecule bearing a large and unshielded surface area
such as I constitutes the ideal design for sensing the variations of
p-density in Zn(II) metallated porphyrins and for probing the effect
of the substituent on the strength of donor–acceptor interactions
in solution.
Experimental
Air sensitive reactions were performed using standard vacuum
line and Schlenk techniques and were carried out under an inert
environment (argon or nitrogen). All solvents were distilled prior
to use and obtained from solvent stills (Et3N ex. CaH2, THF ex.
Na). 3-Allyloxybenzaldehyde,32 5,5¢-dibenzyl-3-3¢-di-(n-hexyl)-
4,4¢-dimethyldipyrromethane,18 5,15-bis(3,5-di-tert-butylphenyl)-
2,8,12,18-tetra-n-hexyl-3,7,13,17-tetramethylzincporphyrin 16,21
N,N¢-di-n-hexylnaphthalene-1,4,5,8-tetracarboxylic diimide22
I
and N,N¢-di-n-hexylpyromellitic diimide22 III were prepared
according to the published procedures. All other chemicals
were purchased from Aldrich, Avocado or Strem in reagent
grade quality or better and used without further purification.
Screw-cap NMR tubes with disposable non-absorbing septa for
NMR titrations were purchased from Fluorochem. Micro-volume
gastight syringes for fluid measurements were obtained from
Hamilton.
Conclusions
1
All H, 13C and 19F NMR measurements were carried out in
chloroform-d at room temperature (298 K) and recorded on a
Brucker DPX-400 or DRX-500 instrument at 400.13 and 500.20
MHz respectively. Chemical shifts (d) are quoted in parts per
million (ppm) and are expressed relative to TMS (1H, 13C). The
downfield direction is taken as positive. Fluorine chemical shifts
were referenced to an external CFCl3 reference. High Resolution
Mass Spectra (HRMS) were performed on ABI/Sciex Q-Star pul-
sar. The spectra were run in positive ion and the parts per million
(ppm) values were worked out using the analyst software. HRMS
MALDI-TOF analyses were carried out at the EPSRC Mass
Spectrometry service at Swansea. UV/Vis spectra were obtained
on a Hewlett Packard 8452A diode array spectrometer using a
10 mm path quartz cell versus a pure-solvent reference. Elemental
analyses were performed in the University of Cambridge, UK.
Column chromatography was performed on Merck 60 silica gel
(230–400 mesh).
Systematic modulation of the p-density of the aromatic surface of
5,15-diaryl substituted Zn(II) porphyrins was achieved by varia-
tion of the number of substituents at peripheral positions. Up to
six substituents (i.e. F or OMe groups) were introduced gradually
at the periphery of the porphyrin. Comparison of the binding
strength in solutions of porphyrin donors with dihexyl substituted
naphthalenediimide allowed the investigation of substituent effect
on electronic properties of the porphyrins. The binding in solutions
led to 1 : 1 donor–acceptor complexes and their structures were
confirmed by X-ray diffraction studies. In solution, the binding
strengths of NDI are primarily governed by the intrinsic electronic
properties of the peripheral substituent and the total number of
substituents present in a given porphyrin, although steric factors
seem to influence the binding strengths too. Incorporation of
increasing numbers of electron-donating groups produces more
electron-rich aromatic surfaces, as indicated by the progressively
enhanced binding affinities, while the opposite cumulative elec-
tronic effect is observed with the electron-withdrawing fluorine
atoms. An almost linear correlation is obtained for the binding
strength and the peripheral substitution pattern in the Swain’s
type dual parameter model: this suggests an overestimation of
the resonance contribution in the Hammett constants in this
alternative approach. This might indicate that the presence of a
largely undistorted zinc porphyrin core is crucial for measurable
interactions in solutions to be achieved. Synthetic modification
of electron-density on the porphyrin surface by introducing sub-
stituents at peripheral sites of large, flat and sterically unhindered
aromatics may represent a general solution towards electronically
tuneable aromatic surfaces. Such an approach may be extended to
other related planar systems, for example, expanded porphyrins
with larger aromatic surfaces31 and lead to the rational design of
new supramolecular hybrids, with applications ranging from new
energy materials for dye-sensitised solar cells and photovoltaic
applications to future sensors and drug delivery devices.
General procedure for co-crystallisations between porphyrins and
naphthalenediimides
Equimolar ratio of porphyrin 5 (2.10 mg, 2.00 mmol) and di-
n-hexyl substituted naphthalenediimide I (0.90 mg, 2.00 mmol)
was mixed and dissolved in a minimum amount of CH2Cl2.
Slow diffusion of MeOH into the solution mixture afforded
extremely small, but well-formed crystals of complex A. The
other porphyrin–NDI complexes, B (between 6 and I), C (be-
tween 14 and I) and D (between 15 and I) were prepared
by a similar co-crystallisation procedure from their respective
components mixed in equimolar ratio in CHCl3 or CH2Cl2 at room
temperature.
1H NMR dilution experiments
A concentrated porphyrin solution (1 and 13) of known con-
centration (2 ml, 30 mM) was prepared in CDCl3. Aliquots of
this solution were sequentially added to 0.5 mL of CDCl3 in
an NMR tube and the H spectra were recorded at 500 MHz,
298 K. A total of 9 data points were obtained for each dilution
experiment.
‡ When 1,3,5-trifluorobenzene was used as the acceptor molecule, no
detectable changes in the meso chemical shift were observed with either por-
phyrins 1 and 13. Binding studies with di-n-hexyl N-substituted perylene
diimide and cyano-containing aromatics (e.g. 1,3,5-tricyanobenzene and
TCNQ) were hampered by their limited solubility in chlorinated solvents.
1
10840 | Dalton Trans., 2011, 40, 10833–10842
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The Royal Society of Chemistry 2011
©

1H NMR titrations and curve fitting procedure
the ESI) in a layered fashion yielded the desired materials of high
purity and with moderate yields.
All binding studies were performed using solutions derived
from crystalline materials. Any solvated methanol present in the
materials was removed under reduced pressure prior to use in order
to eliminate a possible MeOH–Zinc(II) coordination. Titrations
to estimate the strengths of binding interactions in solution
were performed maintaining constant porphyrin concentrations
(similar to the method described in Reference 2f): each titration
was carried out using 5 mM solutions of the porphyrin ‘host’
(1 mL) in CDCl3. To this porphyrin ‘host’ solution, aliquots of
the ‘guest’ solution (containing a mixture of the ‘guest’ acceptor,
200 mM and the porphyrin ‘host’, 5 mM) in CDCl3 were added
directly into the NMR tube. The sample was then shaken well
and the 1H NMR spectrum was recorded immediately (500 MHz,
298 K). Aliquots were added in a 2.5 mL interval (i.e. 0.1 eq.)
until reaching the 1 : 1 ratio. Further additions were made at a
25 mL interval and therefore a total of 20 data points for each
titration were obtained. The association constant for each titration
was evaluated based on the observed change in chemical shift of
the meso protons, except free-base porphyrin 7 where variation
of the NH protons was also used. Shifts from corresponding
peaks were measured and fitted using a 1 : 1 binding model.
Titration experiments were carried out at least 2 times and the
errors have been estimated by the fitting programme as standard
deviations from the mean. Titration data were converted to
association constants by using EQNMR.25 The fitting of eqn
X-ray diffraction experiment
Crystallographic data of extremely small crystals of co-crystallites
were collected using the synchrotron radiation source at Station
9.8, Daresbury SRS, UK, on a Bruker SMART CCD diffrac-
tometer. The structures were solved by direct methods using
the program SIR92.33 The refinement and graphical calculations
were performed using the CRYSTALS34 program suite. Crystallo-
graphic and refinement data of porphyrins 2, 8, 10, 12, 13 and 16,
as well as the diimide III are given in the ESI.†
Crystal data A. C147H184N10O4Zn2Cl2, M = 2356.72, Z = 1,
¯
˚
˚
◦
triclinic space group P1, a◦= 13.629(2) A, b = 16.504(3) A, c =
◦
˚
16.554(3) A, a = 112.529(2) , b = 96.328(2) , g = 104.184(2) , V =
3
-1
˚
3246.8(10) A , T = 120(2) K, m = 0.468 mm . Of 24 110 reflections
measured, 11 363 were independent (Rint = 0.033). Final R = 0.0615
(6579 reflections with I > 3s(I)) and wR = 0.0621.
Crystal data B. C147H187N10O4Cl3, M = 2264.42, Z = 1, triclinic
¯
˚
˚
˚
space group P1, a = 13.465(2) A, b = 16.552(3) A, c = 16.697(3) A,
a = 112.645(2)◦, b = 95.228(2)◦, g = 103.286(2)◦, V = 3275.0(10)
3
-1
˚
A , T = 120(2) K, m = 0.127 mm . Of 24 342 reflections measured,
11 417 were independent (Rint = 0.028). Final R = 0.1088 (5490
reflections with I > 3s(I)) and wR = 0.1160.
Crystal data C. C92H114N6O6Zn, M = 1465.28, Z = 1, triclinic
R
R
(4) was performed using the software MicroMathꢀ Scientistꢀ for
WindowsTM, Version 2.01 (MicroMath, Inc.) implementing a least
squares algorithm.
¯
˚
˚
˚
space group ◦P1, a = 9.207(1◦) A, b = 12.543(1) A, c = 17.287(1) A,
◦
3
˚
a = 88.83(2) , b = 86.38(3) , g = 87.52(3) , V = 1990.2(3) A , T
= 150(2) K, m = 0.367 mm-1. Of 11 416 reflections measured, 4720
were independent (Rint = 0.07). Final R = 0.0348 (3440 reflections
with I > 3s(I)) and wR = 0.0396.
General procedure for porphyrin synthesis18
Crystal data D. C87H106Cl2N8O9Zn, M = 1544.73, Z = 2,
Palladium on carbon (10%, typically 0.1 g for every 1 g of
dipyrromethane used) was added to a solution of 5,5¢-dibenzyl-
3,3¢-di(n-hexyl)-4,4¢-dimethyldipyrromethane (2.00 g, 3.27 mmol)
(1 eq.) in THF (100 mL, containing 1% Et3N), the resulting black
suspension was placed under hydrogen for 2 h. The catalyst was
filtered off through a plug of Celite and the filtrate concentrated
to give an off-white solid which was dried under vacuum. To this
cold degassed trifluoroacetic acid (20 mL) was added by cannula
under argon at 0 ◦C. After 1 h a degassed methanolic solution
of substituted aromatic aldehyde was cannulated into the mixture
at -25 ◦C. The reaction was allowed to warm up with stirring
over a period of time depending on the reactivity of individual
the aldehyde, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
(0.89 g, 3.93 mmol) was then added and stirring continued
overnight. Et3N (40 mL) was added and the solvent removed
by evaporation. The crude free-base porphyrin product was
redissolved in CH2Cl2 (300 mL) and washed with H2O (4 ¥
400 mL), dried over Na2SO4 and filtered. Excess of
Zn(OAc)2·2H2O (ca. 5 eq.) was added to a stirred solution of the
appropriate free-base porphyrin (1 eq.) in CHCl3–MeOH solution
(10% MeOH) and the mixture was refluxed for 20 min. The
solution was washed with H2O, dried over Na2SO4, filtered and
passed through a plug of silica gel. The solvent was removed under
reduced pressure. The desired product was typically purified by
column chromatography over silica gel (as described in the ESI†).
Recrystallisation using a combination of solvents (as described in
˚
˚
monoclinic space group P21, a = 15.952(6) A, b = 9.578(4) A,
◦
3
˚
˚
c = 27.437(10) A, b = 93.065(6) , V = 4186(3) A , T = 150(2)
K, m = 0.352 mm-1. Of 13 163 reflections measured, 4843 were
independent (Rint = 0.044). Final R = 0.1264 (3771 reflections with
I > 3s(I)) and wR = 0.1390.
Acknowledgements
We thank EPSRC (LHT, SIP) for financial support and the Royal
Society for a University Research Fellowship (SIP). EPSRC is
also thanked for a Senior Research Fellowship (PRR). We thank
Fatima Merzoug and the Erasmus Scheme, University of Poitiers
1
and University of Bath, for some of the preliminary H NMR
titration studies. We thank the EPSRC Mass Spectroscopy Service
at Swansea for assistance and STFC for funding and access to SRS
Daresbury. We thank Drs John Warren, J. E. Davis and Simon Teat
for crystallographic support and helpful discussions.
Notes and references
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