Aromatic donor-acceptor interactions in non-polar environments

Article abstract of DOI:10.1039/c5cc00507h

We have evaluated the strength of aromatic donor-acceptor interactions between dialkyl naphthalenediimide and dialkoxynaphthalene in non-polar environments. ¹H NMR, UV-vis spectroscopy and isothermal titration calorimetry were used to characterise this interaction. We concluded that the strength of donor-acceptor interactions in heptane is sufficient to drive supramolecular assemblies in this and other aliphatic solvents.

Full text of DOI:10.1039/c5cc00507h

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Aromatic donor–acceptor interactions in
non-polar environments†
Cite this:DOI: 10.1039/c5cc00507h
Giles M. Prentice,a Sofia I. Pascu,a Sorin V. Filip,b Kevin R. Westb and G. Dan Pantos-*a
Received 19th January 2015,
Accepted 8th April 2015
DOI: 10.1039/c5cc00507h
www.rsc.org/chemcomm
We have evaluated the strength of aromatic donor–acceptor inter- the interaction of a pyrene guest with a cyclophane host in a
actions between dialkyl naphthalenediimide and dialkoxynaphtha- large range of solvents from water to carbon disulphide, showed
1
lene in non-polar environments. H NMR, UV-vis spectroscopy and a linear free energy relationship between the free energy of
isothermal titration calorimetry were used to characterise this binding and the polarity of the solvent. Cubberley and Iverson
interaction. We concluded that the strength of donor–acceptor in 2001 studied20 the solvent effect on the interaction between
interactions in heptane is sufficient to drive supramolecular assemblies napthalenediimide and dialkoxynaphthalenes in a variety of
in this and other aliphatic solvents.
solvents ranging from water (most polar) to chloroform (least
polar). This also showed a quasi linear relationship between the
Donor–acceptor (D–A) aromatic interactions1–4 have been shown to polarity and free energy of formation (Fig. 1).
be a very useful tool in the field of supramolecular chemistry from
Both articles report a lower binding affinity between the D–A
self-healable polymers5,6 to catenanes,7 rotaxanes,8–10 molecular partners in non-polar solvents. However in both studies no aliphatic
assembly and binding,11 charge separation and transport12,13 solvents were used.
dye sensitised solar cells and organic photovoltaic devices14 or
More recently, Wu¨rthner and Chen investigated21 the self-
scavengers.15 While they have been employed in many different aggregation of perylenediimides (PDI) in a range of solvents from
research avenues, in the majority of cases polar solvent systems water to n-hexane (PDI had different side chains from one class of
are used throughout. The use of such interaction in non-polar solvents to another in order to achieve desired solubility). In
solvent systems in particular that of various donors with 1,3,5- comparison to previous studies a linear relationship between the
trinitrobenzene has been previously reported by optical absorption polarity and free energy of formation was not observed. It was
or dispersed phase NMR methods.16 We report herein our findings shown that there was a decrease in the free energy of association
from a comprehensive study of the aromatic D–A interactions in with decreasing solvent polarity up until a minimum was reached
heptane between two archetypal D–A partners: dialkoxynaphthalene in THF and toluene, followed by an increase in the free energy
(DN, donor) and dialkyl naphthalenediimide (NDI, acceptor). We use
1H NMR, UV-vis and isothermal titration calorimetry to evaluate the
strength of interaction between these molecules.
Although there is debate on the exact mechanism of the D–A
aromatic interaction it is generally considered an electrostati-
cally favourable face-centered stacking interaction between an
electron rich aromatic molecule and an electron deficient
aromatic counterpart.1,2,17 Early studies of aromatic interactions
have shown that there is a significant solvent dependence in all
the systems studied.18 Diederich and Smithrud’s 1990 work19 on
a Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
E-mail: g.d.pantos@bath.ac.uk
b BP Oil UK Ltd, Technology Centre, Whitchurch Hill, Pangbourne, Reading,
Berkshire, RG8 7QR, UK
† Electronic supplementary information (ESI) available: 1H 13C NMR HRMS data,
Fig. 1 Plot of ÀDG1 vs. the solvent polarity (ET 30 scale) for NDI–DN
interaction. The data for n-heptane is from this study while the rest was
taken from ref. 20.
NMR titrations and dilutions, UV-vis titrations, VT NMR, ITC. See DOI: 10.1039/
c5cc00507h
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of binding as the polarity decreased to hexane. It was also found
that polarisable solvents such as CHCl3, CH2Cl2 and DMSO were
very efficient at solvating the PDIs, thus leading to low association
constants, due to high dispersion forces. This trend was also
observed in a study of reverse micelles by Silber et al.16h in which
the binding constant between tetracyanoethane and naphthalene
was four times higher in hexane than in chloroform (we observed
qualitatively a similar behaviour, see ESI†).
We chose to study the interaction between the NDI and DN
cores in order to compare their interaction in heptane with
Iverson’s seminal work20 in more polar media, and because of
their use in the assembling of catenanes and knots in polar
solvents.7,8 The NDI and DN cores used for this work were
functionalised with 2-octyldodecyl alkyl chains to provide solubility
in heptane. (NDI derivatives bearing ethylbutyl, ethylhexyl, 2-nonyl,
Fig. 3 Titration spectra carried out in heptane at 25 1C, initial concen-
tration of 1 25 mM with increasing amounts of 3, 250 mM solution. (Blue =
0, green = 1 and red = 3 equiv. of 3).
dodecyl were also synthesised but were not soluble in heptane at above. In the case of the synthesis of 4, the excess bromide could
41 mM concentration range). We used the same solubilising side be removed under high vacuum.
chain on both moieties in order to eliminate the influence of side
The first indication of interaction between the two cores
chain interactions when comparing homo- with hetero-aggregates came from mixing in a 1 : 1 ratio NDI : DN in heptane (25 mM
(DN 4 was synthesised in order to test if the side chains influence concentration): a red colour is observed while the starting
the assembly). The synthesis of DN derivatives 2 and 3 starts from solutions are tan and light brown (Fig. 3). This behaviour is
commercially available 2-octyldodecan-1-ol which is converted in consistent with the formation of a charge transfer complex
the corresponding bromide in 89% yield (Fig. 2).22 The bromide is between the two cores.
then converted into to the amine in a two step process using
In order to determine quantitatively how effective the association
phthalimide and hydrazine, in a 81% overall yield.23 The 1-amino- between the NDI and DN is in heptane, UV-vis, 1H-NMR and ITC
2-octyldodecane is reacted with 1,4,8,9-naphthalenetetracarboxylic titrations have been performed on the individual components
anhydride (NDA) under standard24 microwave assisted conditions and on mixtures. When studying the association of any set of
to give the desired NDI in 94% yield. The synthesis of the DN molecules via aromatic interactions, the self-association has to
counterparts used the 1-bromo-2-octyldodecane as starting material be taken in account. Dimerisation is the most obvious process
and 1,5- or 2,6-dihydroxynaphthalene. The reaction is conducted that can occur, however supramolecular polymerisation25,26
with excess bromide to prevent monoalkylation. While the reaction must also be considered.
proceeds with good conversion, the dialkoxynaphthalene cannot be
In the case of NDI 1, DN 2 and the 1Á2 complex, variable
separated from the excess alkylating agent. To overcome this temperature 1H NMR experiments were carried out in a heptane :
problem, phthalimide was added to the reaction in order to react octane-d18 : 1,1,2,2-tetrachloroethane 95.1 : 4.8 : 0.1 solvent mixture.
with the excess bromide. The alkylated phthalimide was separated The data was then fitted with an isodesmic polymerisation
from the DN using column chromatography to give the latter in model,25,27 which indicated that at room temperature the majority
47–50% yield over two steps. This procedure not only that of material exists as a monomer in the case of 1 (NDI 5.5 mM)
allowed us to produce pure DN but also yielded the alkylated B85% with 13% dimer, 2% trimer or larger oligomers. In the case of
phthalimide that was used for the synthesis of NDI as described 2, isodesmic polymerisation model could not be fitted to the data,
indicating that no self-aggregation was present. In the case of a 1Á2
mixture (26.9 mM) 72% existed as the 1 : 1 complex, 22% as 2 : 2
species, 5% as 3 : 3 and 1% as higher oligomers. For simplicity,28
we used the 1 : 1 complex as a ‘monomer’ in the isodesmic
polymerisation model. These results confirmed that the dimer-
ization process is dominant in this solvent and that the amount
of larger oligomers is negligible, therefore all the titration data
(1H-NMR, UV-vis, ITC) was fit with either the dimerization or the
1 : 1 mathematical models were used (see ESI†).29,30
1
Dilution experiments were carried out via H-NMR and ITC
but not by UV-vis spectroscopy because of the high extinction
coefficients of the aromatic cores which prevents accurate
analysis of solutions with concentrations higher than 1 mM
Fig. 2 The synthesis of NDI 1 and DN 2, 3 and 4; reaction conditions: (a) Br2,
PPh3, THF, 3 h, 89% (b) potassium phthalimide, 90 1C, 18 h, 84% (c) H4N2, EtOH,
reflux, overnight, 96% (d) NDA, DMF, 140 1C, 30 min, 94% (e) 1,5- or
for NDI and 0.5 mM for 1,5- and 2,6-DN. The 1H-NMR dilution
experiments (25 to 3.6 mM) indicated that there is no signifi-
cant association between the DN cores regardless of the sub-
2,6-dihydroxynaphthalene, K2CO3, MeCN, reflux, 24 h followed by (b),
yield over two steps 47 or 50% for the 1,5- and 2,6-derivative, respectively. stitution pattern, while for the NDI 1 a dimerisation constant of
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Table 1 Self-association constants between the individual donor and
acceptor molecular and the association constants between the donor
and acceptor pairsa
working close to the detection limits of the instrument the
errors associated with these values are large (Table 1 and ESI†).
There are about two orders of magnitude difference between
all Ka determined in water vs. heptane for the 1 : 1 NDI : DN
mixtures (by 1H NMR: 8.3 MÀ1 in heptane; 2000 MÀ1 in
water20). This strengthens the argument (vide supra) that the
solvophobic effects have a lower contribution in heptane when
compared to water.32 It is however noteworthy that the molecules
show no association in chloroform regardless of the presence or
absence of a complementary D–A partner.
We have shown that not only aromatic donor–acceptor
interactions are possible in aliphatic solvents, but also that
they lead to significant association between complementary
aromatic cores. There is little or no geometrical preference for
the interaction between NDI and 1,5- or 2,6-DN isomers, or side
chain dependence. We believe that this works demonstrates that
supramolecular architectures, similar to the numerous examples in
aqueous media, could be constructed in non-polar aliphatic media.
We thank the EPSRC, BP, the EPSRC NMSF, the BMSS and
the University of Bath for Financial Support. SIP thanks the
Royal Society for a University Research Fellowship. We also
thank Dr Nick Buurma for helpful discussions regarding the
ITC experiments and IC2ITC software, and Mr Liam Emmett for
helpful discussions regarding theoretical and experimental
aspects of the project.
Compound
Ka NMRb (MÀ1
)
Ka UVc (MÀ1
)
Ka ITCc (MÀ1
)
1
2
3
3.4 Æ 1
o1
n/a
n/a
n/a
4.2 Æ 0.3
4.9 Æ 0.3
2.4 Æ 0.3
1.7
o1
o1
o1
1 + 2
1 + 3
1 + 4
8.3 Æ 0.2
6.0 Æ 0.3
6.9 Æ 0.9
11.5
11.3
6.1
a
b
Single compound dilutions carried out at 25 1C. The most downfield
peak was monitored over a 25–3.6 mM conc. All the titrations used
heptane : octane-d18 : 1,1,2,2-tetrachloroethane 95.2 : 4.8 : 0.1 as solvent.
c
Minimum value calculated by IC2ITC used see ESI.31
3.4 Æ 1 MÀ1 was determined. These results were confirmed by
ITC experiments (Table 1). The very weak dimerisation con-
stants, which are two orders of magnitude lower than in water,
indicate that the contribution of the solvophobic effect in
heptane is very small when compared to water. (NDI–NDI
dimerisation in water 200 MÀ1 vs. 3.4 MÀ1 in heptane; the
dimerization constant in heptane is similar to those reported in
CH3CN or acetone).20 Titration experiments were carried out via
1H-NMR, ITC and UV-vis in order to determine the association
1
constants between the NDI and DN. The H-NMR experiments
(25 mM initial NDI concentration) indicated the presence of a
significant interaction between the donor (DN) and the acceptor
(NDI) cores with association constants determined for 1 with 2, 1
Notes and references
with 3 and 1 with 4 of 8.3 Æ 0.2, 6.0 Æ 0.3 MÀ1 and 6.9 Æ 0.9 MÀ1
,
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