Assisted π-stacking: a strong synergy between weak interactions

Article abstract of DOI:10.1039/c8cc07207h

Synergy between a pair of weak non-covalent interactions can predispose a molecular self-assembly towards a specific pathway. We report assisted π-stacking, a synergy between aromatic π-stacking and n → π* interactions that exhibits an unprecedented strength and thermal stability. Natural bond orbital analysis reveals the non-additive nature of the interaction.

Full text of DOI:10.1039/c8cc07207h

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Assisted p-stacking: a strong synergy between
weak interactions†
Cite this: Chem. Commun., 2018,
4, 12186
5
a
a
a
ab
Soumik Sao, Sumit Naskar,
Narottam Mukhopadhyay, Mousumi Das* and
ab
Received 5th September 2018,
Accepted 4th October 2018
Debangshu Chaudhuri
*
DOI: 10.1039/c8cc07207h
rsc.li/chemcomm
Synergy between a pair of weak non-covalent interactions can can even be a precursor to cooperativity. From the point of view of
predispose a molecular self-assembly towards a specific pathway. molecular design, it is easier to incorporate a synergistic pair of
We report assisted p-stacking, a synergy between aromatic p-stacking interactions, as opposed to aiming for cooperativity through a
and n - p* interactions that exhibits an unprecedented strength and fortuitous interplay of multiple interactions.
thermal stability. Natural bond orbital analysis reveals the non-additive
nature of the interaction.
We report an unusually strong synergistic interaction
between regular aromatic p-stacking and a much weaker n - p*
interaction. The participation of the secondary n - p* interaction
The properties of a molecular self-assembly are governed by is so critical that switching it off has the dramatic effect of
how its constituent molecules are organized. Unlike covalent disrupting the otherwise stable pi-stacked state. This synergistic
bonding that involves orbital overlap and is therefore strongly interaction, which we refer to as assisted p-stacking, exhibits
directional in nature, non-covalent interactions impose much an extraordinary strength and thermal stability hitherto
1
relaxed orientational restrictions. While such freedom allows unprecedented among the strongest pi-stacked systems.
one to create a variety of assembled structures from the same
Fig. 1a presents the molecules under investigation. Py-2NDI
building blocks, it also presents an inherent challenge in terms and Py-2PBI are covalently bridged dimers of naphthalene diimide
of guiding a self-assembly through a specific pathway, to a (NDI) and perylene bisimide (PBI), respectively. Free rotations
2
,3
desired structure.
around the methylene carbons of the bridging 2,6-lutidinyl group
Control over the self-assembly pathway is achieved by allow Py-2NDI (and Py-2PBI) to switch between open and folded
exploiting a very unique feature of non-covalent interactions, conformations. Unlike the open conformer, where the two
4
namely the non-additivity. Non-additivity governs the way in NDI (PBI) chromophores are far removed and therefore non-
which non-covalent interactions mutually strengthen (or weaken) interacting, the folded conformer can support two distinct non-
3,5
each other, and can in turn bias the course of a self-assembly.
covalent interactions: p-stacking between the NDI (PBI) units,
Such mutual strengthening is manifested in two related, but and a donor–acceptor type interaction between the pyridyl N
somewhat distinct ways: cooperativity and synergy. The former lone pair and the two electron-deficient NDI (PBI) moieties. The
is usually a many-body effect, operating through multiple short- mX-2NDI and mX-2PBI dimers on the other hand, can only
6
,7
11,12
and long-range interactions, and is commonly encountered in support p-stacking,
and therefore serve as negative controls.
8
large hydrogen-bonded systems. Synergy on the other hand, is a
We carried out an in-depth investigation of the electronic
pair or reciprocal effect, where two non-covalent interactions and structural aspects of dimer folding, in order to understand
complement each other such that the resultant association is the nature of the interaction that drives it. Fig. 1b presents the
9
stronger than the sum of individual interactions. A strong optical absorption spectrum of Py-2NDI vis- `a -vis that of mX-2NDI,
synergistic interaction can locally influence the self-assembled in CHCl
structure by favouring certain arrangements over the others, and of an isolated NDI chromophore: 0–0, 0–1 and 0–2 vibronic bands
3
. The spectrum of mX-2NDI exhibits typical characteristics
10
21
appearing at 380, 360 and 340 nm, respectively. Evidently, in
a ‘‘good’’ solvent such as CHCl , the control dimer assumes a
non-interacting, open conformation. In contrast, the spectrum of
Py-2NDI is marked by a distinctly diminished absorbance at the
a
Department of Chemical Sciences, Indian Institute of Science Education and
Research (IISER) Kolkata, Mohanpur 741246, India.
3
E-mail: dchaudhuri@iiserkol.ac.in
b
Centre for Advanced Functional Materials (CAFM), Indian Institute of Science
Education and Research (IISER) Kolkata, Mohanpur 741246, India
Electronic supplementary information (ESI) available: Experimental methods,
0–0 band (A0–0). The corresponding decrease in the 0–0 to 0–1
absorbance ratio, A0–0/A0–1 from 1.13 to 0.96 is a tell-tale sign of an
†
13
synthesis and characterization, and additional figures. See DOI: 10.1039/c8cc07207h H-type excitonic coupling.
1
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NDI units, and how the folded dimer responds to a stepwise
electrochemical reduction. The cyclic voltammogram (CV) of
mX-2NDI is characterized by two reversible, one-electron
reduction waves, appearing at E1/2(1) = ꢀ626 and E1/2(2) =
ꢀ
959 mV. This redox behavior is consistent with the open
conformation of the mX-2NDI dimer, where each NDI unit is
independently and identically reduced in two distinct steps, to
ꢁ
ꢀ
2ꢀ
16
a [NDI] radical anion and eventually to a [NDI] dianion.
Identical values of the faradaic current peaks in the differential
pulse voltammogram (DPV) confirm the sequence of these
single-electron reduction events. In contrast, Py-2NDI exhibits
a distinctly irreversible CV. A pair of strongly interacting NDIs is
expected to undergo four discrete one-electron reduction
1
6
steps. These individual reduction events are better resolved
in the DPV of Py-2NDI, appearing at ꢀ612, ꢀ684, ꢀ900 and
ꢀ
1024 mV vs. SCE. The potential difference between the first
two cathodic peaks that gives a measure of e–e repulsion within
the dianion, is drastically reduced from 333 mV in open mX-2NDI
to 72 mV in folded Py-2NDI. A strong electronic coupling between
the two NDI units of the folded Py-2NDI helps to delocalize the
Fig. 1 (a) Molecular structures of Py-2NDI and Py-2PBI, and the respec-
tive control dimers mX-2NDI and mX-2PBI. The schematic represents
the dynamic folding equilibrium of the dimers. A comparison based on
(
b) normalized optical absorption spectra (B50 mM, normalized at A0–1), dianion state over the dimer, resulting in a reduced coulombic
1
(
c) the aromatic region of the H-NMR spectra, and (d) CV (B10 mM) at
repulsion. Upon further reduction, the repulsion becomes strong
enough to destabilize the folded state, causing a conformational
change, and resulting in an irreversible CV.
ꢀ1
0
.1 V s scan rate vs. saturated calomel electrode in CHCl confirms the
3
presence of excitonic, structural and electronic interactions between
the two NDI units in folded Py-2NDI, as opposed to mX-2NDI that exists
as the open conformer. DPV of the two NDI dimers is shown at the bottom
of panel (d).
17
The results presented so far clearly establish that the
Py-2NDI (and Py-2PBI) exists in the intramolecularly pi-stacked
folded form in CHCl , while the structurally similar mX-2NDI
3
(and mX-2PBI) dimer assumes the open conformation. In order
Identical spectra at high levels of dilution implies that the to elucidate the nature of this unusual p-stacking interaction in
observed phenomenon is purely intramolecular (Fig. S4, ESI†), 2,6-lutidinyl bridged dimers and the role of the pyridyl N lone
resulting from a folded conformation in which two NDI units pair, we carried out a protonation based experiment. The aim was
are cofacially stacked. This observation is particularly surprising to turn-off any lone pair mediated interaction,‡ and monitor its
3
since NDI p-stacking is strongly disfavored in CHCl (association influence on the dimer conformation, using the same set of
ꢀ
1 14
constant o1 M ). While optical absorption spectroscopy experiments as before. Fig. 2a (and Fig. S6a, ESI†) presents the
provides a reliable way to monitor the nature and strength of effect on the optical absorption spectra, in CHCl . Upon proto-
3
exciton coupling between NDI chromophores in the folded state, nation, the A0–0/A0–1 ratio increases from 0.96 to 1.22, signifying
1
H-NMR spectroscopy provides an independent investigation of a change from the intramolecularly folded H-aggregate to the
the structural interactions in different dimer conformations. non-interacting open form. Interestingly, the effect of protona-
1
Fig. 1c presents the aromatic region of H-NMR spectra of the tion is fully reversible, as adding a base (triethylamine, Et
3
N)
3
two dimers in CDCl . The eight NDI protons of mX-2NDI give rise frees up the pyridyl N lone pair and restores the H-aggregated
to a multiplet signal at 8.75 ppm, their chemical shift consistent dimer spectrum. Clearly the availability of the pyridyl N lone
15,16
with that of an unstacked, isolated NDI.
The NDI protons of pair is critical to the stability of the pi-stacked folded dimer in
Py-2NDI, on the other hand, are strongly upfield shifted, appearing CHCl , which cannot be achieved otherwise through p-stacking
3
1
as two sets of doublets at 8.21 and 8.41 ppm. Such a strong alone. In the H-NMR spectrum, two sets of NDI aromatic
shielding of NDI protons in Py-2NDI is a clear evidence of protons of folded Py-2NDI experience a pronounced downfield
15,16
p-stacking in the folded state.
Furthermore, two sets of shift upon protonation (Fig. 2b and Fig. S5, S6b, ESI† for
doublets indicate a rotational displacement of one NDI with Py-2PBI), giving rise to a complex multiplet signal at 8.76 ppm,
respect to the other in the folded geometry, such that one half similar to that of NDI protons of the open mX-2NDI dimer. Once
1
of the NDI protons are shielded better than the other half. This is again, deprotonation using Et
3
N restores the original H-NMR
in perfect agreement with the geometry-optimized structure of the spectrum. Even cyclic voltammetry of the protonated Py-2NDI
Py-2NDI dimer, discussed subsequently. A similar comparison of (Fig. 2c) reproduces to a great extent, the characteristic features of
Py-2PBI vis- `a -vis its negative control mX-2PBI confirms the general- the non-interacting, open dimer: a reversible CV with a pair of
ity of the observed phenomenon (Fig. S6, ESI†).
Cyclic voltammetry in CHCl (Fig. 1d) elegantly captures the
one-electron reduction peaks, separated by 365 mV.
3
In order to unravel the exact nature of the lone-pair assisted
difference between Py-2NDI and mX-2NDI dimer conformations, p-stacking interaction, natural bond orbital (NBO) analysis was
in terms of the extent of electronic interactions between the two performed on the optimized dimer structure. Quite consistent
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non-covalent interaction§ other than regular p-stacking
(Fig. S8, ESI†), thus highlighting the critical role of the pyridyl
N lone pair in assisted p-stacking.
A comparison of the energy difference between folded and
open conformers of Py-2NDI and mX-2NDI dimers reveals the
non-additive nature of assisted p-stacking interactions (Fig. S7,
ESI†). While the folded conformer is more stable in both cases,
the extent of stabilization is greater in the case of assisted
ꢀ
1
pi-stacked Py-2NDI by 3.86 kcal mol . Interestingly, this difference
is much higher than what can be accounted for by the four
ð2Þ
ꢀ1
n
N
- pCQO* interactions (cumulative E
ꢃ
¼ 0:93 kcal mol ).
n!p
N
Thus n - pCQO* assisted p-stacking interaction is much stronger
N
than what is expected from a simple algebraic sum of n - pCQO*
and p-stacking interactions. Though PBI is known to exhibit a
stronger p-stacking than NDI, the assisted p-stacking interaction in
ꢀ
1
Py-2NDI is comparatively stronger (DDEApS = ꢀ3.86 kcal mol
)
ꢀ
1
than that in Py-2PBI (DDEApS = ꢀ3.24 kcal mol , Fig. S7, ESI†).
This too is consistent with the idea of non-additivity: strong
individual interactions do not necessarily give rise to a stronger
Fig. 2 Lone pair assisted p-stacking. Protonation at the pyridyl N (using
HBr/acetic acid) causes changes in the (a)
b) deshielding of NDI aromatic protons, consistent with dimer unfolding.
In both experiments, a subsequent deprotonation (using Et
N, blue curves) synergy.
A0–0/A0–1 ratio, and
(
3
restores the original spectrum. (c) CV of protonated Py-2NDI closely resembles
that of the open mX-2NDI. (d) Geometry optimized structure of folded Py-2NDI
confirms a B191 rotational displacement between the two NDI units. Green
That the strength of n - p* assisted p-stacking in Py-2NDI
and Py-2PBI far exceeds that of a regular p-stacking is also
realized experimentally, from its extraordinary thermal stability
in solution. The optical absorption spectra of Py-2NDI (and
arrows represent the NDI transition dipole moments; n-C12H25 groups are
truncated for clarity. (e) NBO analysis confirms n - pCQO* interactions
N
between the pyridyl N and NDI carbonyl groups. NꢂꢂꢂC interaction distances Py-2PBI) in non-aggregating solvents, such as chlorobenzene at
ð2Þ
(
d
NꢂꢂꢂC) and the calculated interaction energies (E ꢃ ) are noted.
n!p
125 1C (Fig. S9, ESI†) and DMSO at 180 1C (Fig. 3), are virtually
indistinguishable from the corresponding spectra at room
temperature. This is in sharp contrast to a vast majority of
with the results so far, folded conformers of Py-2NDI (and
Py-2PBI) represent the lowest-energy conformations (Fig. 2d
and Fig. S7, ESI†), with the shortest separation between the NDI
chromophores being under 3.3 Å (Fig. S8, ESI†). In the folded
state, one NDI unit is rotationally displaced (y B 191) with
respect to the other, making one half of the NDI protons
chemically distinct from the other half. This accounts for the
observed split doublet feature (at 8.21 and 8.41 ppm) in the
2
0
7,12,21
pi-stacked aggregates of NDI and PBI,
which experience
a pronounced disassembly between 50 and 100 1C, even those
which are cooperatively bound and/or are supplemented by
stronger side-group interactions, such as H-bonding. It is
N
remarkable that n - pCQO* interaction, despite being more
than an order of magnitude weaker than p-stacking, can
strengthen the latter to such unprecedented levels. This, once
again, highlights the non-additive nature of assisted p-stacking.
In conclusion, we have identified a unique class of assisted
p-stacking interaction that is born out of a strong synergy
between two weak interactions: p-stacking and n - p* interactions.
The resultant association is so strong that it not only forms readily
1
H-NMR spectrum, described earlier. Interestingly, in the
folded conformation, the pyridyl N is in close proximity to
the four carbonyl carbons, such that the corresponding Nꢂ ꢂ ꢂC
distances (dN–C B 3.13–3.3 Å) are either smaller than or
comparable to the sum of van der Waals radii (3.25 Å). NBO
overlaps between the pyridyl N lone pair (n
four pCQO*, shown in Fig. 2e, confirm the presence of signifi-
3
in a non-aggregating medium such as CHCl , but also remains
exceptionally stable at elevated temperatures. The significance of
N
) and each of the
1
0,18,19
cant n - p* interactions in the folded conformation.
Further evidence in favour of n - p* interaction comes from
NBO deletion analysis (Table S1, ESI†), wherein deletion of the
said interaction increases electron occupancy at the donor N
atom. Individual strength of these n - p* interactions, esti-
ð2Þ
mated from the second order perturbation energies (E
ꢃ
), are
n!p
ꢀ
1
ꢀ1
in the order of 0.14–0.38 kcal mol (0.1–0.45 kcal mol for
1
0,19
Py-2PBI), and are comparable with the reported values.
It is
surprising that a single lone pair donor (n ) engaging simulta-
N
neously with 4 different carbonyl acceptors (pCQO*) can result
in reasonably strong n - p* interactions. The confidence in
our findings is further bolstered by the fact that the two control
Fig. 3 n - p* assisted p-stacking in folded (a) Py-2NDI and (b) Py-2PBI
exhibit extraordinary thermal stability, up to 180 1C in non-aggregating
DMSO. Normalized optical absorption spectra remain unchanged at higher
dimers (mX-2NDI and mX-2PBI) do not support any secondary temperatures.
1
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this finding should be seen in the light of the fact that the
strength of such assisted p-stacking interaction surpasses all
known pi-stacked aggregates of NDI and PBI. We believe the key
to a strong synergy lies in a judicious organization of the two
non-covalent interaction motifs, the p-stacking NDI/PBI unit and
the position of the lone-pair bearing N atom. In Py-2NDI and
Py-2PBI folda-dimers, this is achieved by having the pyridyl N
atom flanked between two arylene diimide units, such that the
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The authors gratefully acknowledge Department of Science
and Technology (DST), India (Project: EMR/2014/000223 and
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