Competition between π-π And C-H/π Interactions: A Comparison of the Structural and Electronic Properties of Alkoxy-Substituted 1,8-Bis((propyloxyphenyl)ethynyl)naphthalenes

Article abstract of DOI:10.1002/chem.201502363

The structural and electronic consequences of π-π and C-H/π interactions in two alkoxy-substituted 1,8-bis- ((propyloxyphenyl)ethynyl)naphthalenes are explored by using X-ray crystallography and electronic structure computations. The crystal structure of analogue 4, bearing an alkoxy side chain in the 4-position of each of the phenyl rings, adopts a π-stacked geometry, whereas analogue 8, bearing alkoxy groups at both the 2- and the 5-positions of each ring, has a geometry in which the rings are splayed away from a π-stacked arrangement. Symmetry-adapted perturbation theory analysis was performed on the two analogues to evaluate the interactions between the phenylethynyl arms in each molecule in terms of electrostatic, steric, polarization, and London dispersion components. The computations support the expectation that the π-stacked geometry of the alkoxyphenyl units in 4 is simply a consequence of maximizing π-π interactions. However, the splayed geometry of 8 results from a more subtle competition between different noncovalent interactions: this geometry provides a favorable anti-alignment of C-O bond dipoles, and two C-H/π interactions in which hydrogen atoms of the alkyl side chains interact favorably with the π electrons of the other phenyl ring. These favorable interactions overcome competing π-π interactions to give rise to a geometry in which the phenylethynyl substituents are in an offset, unstacked arrangement.

Full text of DOI:10.1002/chem.201502363

DOI: 10.1002/chem.201502363
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&
Stacking Interactions
Competition Between p–p and CÀH/p Interactions: A Comparison
of the Structural and Electronic Properties of Alkoxy-Substituted
1,8-Bis((propyloxyphenyl)ethynyl)naphthalenes
Bradley E. Carson,[a] Trent M. Parker,[a, b] Edward G. Hohenstein,[a, b] Glen L. Brizius,[a]
Whitney Komorner,[a] Rollin A. King,[c] David M. Collard,*[a] and C. David Sherrill*[a, b, d]
In memory of Professor Herbert O. House
Abstract: The structural and electronic consequences of p–p
and CÀH/p interactions in two alkoxy-substituted 1,8-bis-
((propyloxyphenyl)ethynyl)naphthalenes are explored by
using X-ray crystallography and electronic structure compu-
tations. The crystal structure of analogue 4, bearing an
alkoxy side chain in the 4-position of each of the phenyl
rings, adopts a p-stacked geometry, whereas analogue 8,
bearing alkoxy groups at both the 2- and the 5-positions of
each ring, has a geometry in which the rings are splayed
away from a p-stacked arrangement. Symmetry-adapted per-
turbation theory analysis was performed on the two ana-
logues to evaluate the interactions between the phenyle-
thynyl arms in each molecule in terms of electrostatic, steric,
polarization, and London dispersion components. The com-
putations support the expectation that the p-stacked geom-
etry of the alkoxyphenyl units in 4 is simply a consequence
of maximizing p–p interactions. However, the splayed geom-
etry of 8 results from a more subtle competition between
different noncovalent interactions: this geometry provides
a favorable anti-alignment of CÀO bond dipoles, and two
CÀH/p interactions in which hydrogen atoms of the alkyl
side chains interact favorably with the p electrons of the
other phenyl ring. These favorable interactions overcome
competing p–p interactions to give rise to a geometry in
which the phenylethynyl substituents are in an offset, un-
stacked arrangement.
light-emitting diodes.[7] The charge transport properties of con-
jugated oligomers and polymers depend strongly on their mo-
lecular packing, with contributions from both the orientation
and distance between the p-systems of neighboring mole-
cules.[8] Side chains also have a strong influence on the packing
of conjugated units. The latter rises from the segregation of
semi-rigid conjugated segments and flexible side chains, and
from side chain crystallization of regularly-placed linear alkyl
substituents.[9]
Introduction
Interactions between p-conjugated units (p–p interactions) are
ubiquitous in chemistry. They contribute to a variety of phe-
nomena including the structure of biomolecules (proteins,[1]
DNA/RNA[2]), drug binding,[3] and catalytic function.[4] p-Interac-
tions are also important to charge transport in semiconducting
p-conjugated materials in the development of devices such as
organic thin-film transistors,[5] photovoltaic solar cells,[6] and
Gaining an understanding of the propensity for p-systems to
stack with one another and the influence of intermolecular in-
teractions on the properties of such assemblies is crucial to the
further development of diverse fields. A good understanding
of p–p interactions includes facets such as strength, geometry
dependence, and tunability through changes in the nature of
substituents or aromatic units. Such questions have been ad-
dressed both experimentally and theoretically (for reviews see
refs.[10]).
[a] Dr. B. E. Carson, T. M. Parker, Dr. E. G. Hohenstein, Dr. G. L. Brizius,
W. Komorner, Prof. D. M. Collard, Prof. C. D. Sherrill
School of Chemistry and Biochemistry, Georgia Institute of Technology
901 Atlantic Dr., Atlanta, GA 30332-0400 (USA)
Fax: (+1) 404-894-7452
E-mail: david.collard@chemistry.gatech.edu
sherrill@gatech.edu
[b] T. M. Parker, Dr. E. G. Hohenstein, Prof. C. D. Sherrill
Center for Computational Molecular Science and Technology
Georgia Institute of Technology
Frustratingly, theoretical and experimental results have been
at odds regarding substituent effects in p-stacked systems. Nu-
merous experiments are consistent with the Hunter–Sanders
rules,[11] which posit that substituent effects in p-stacking inter-
actions are governed by electrostatic factors: electron-with-
drawing substituents reduce the extent of unfavorable Cou-
lombic interactions between the electron-rich p-clouds, thus
strengthening the interaction, whereas electron-donating sub-
901 Atlantic Dr., Atlanta, GA 30332-0400 (USA)
[c] Prof. R. A. King
Department of Chemistry, Bethel University, St. Paul, MN 55112 (USA)
[d] Prof. C. D. Sherrill
School of Computational Science and Engineering
Georgia Institute of Technology
Atlanta, GA 30332 (USA)
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/chem.201502363.
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stituents have the opposite effect. By contrast, high-level
wavefunction-based theoretical studies show definitively that
in the gas phase, both electron-donating and electron-with-
drawing substituents lead to stronger p–p interactions in cofa-
cial geometries.[12] To date, only limited experimental work
seems to support the latter conclusion.[13] Other theoretical
work by Wheeler and co-workers indicates that substituents do
not actually tune the p-electron density, but instead exert their
influence by direct, through-space interactions with the other
p-system[14] or in fact with just the nearest atoms of the other
p-system.[15] The recently developed functional-group-based
partitioning of symmetry-adapted perturbation theory[16] (F-
SAPT) supports the Wheeler–Houk view for substituted cofacial
benzene dimers.[17]
ometry.[19] Cozzi and Siegel used 2-D NMR spectroscopic tech-
niques to determine the barrier to rotation of the phenylene
units,[20] which they found to correlate with Hammett spara
values, supporting the Hunter–Sanders rules if one assumes
that the barrier is increased or decreased according to the sub-
stituent effect on the p-stacking interaction. This interpretation
of the data is, however, somewhat problematic in that sub-
stituents will affect both the potential energy minimum as well
as the transition state for rotation, so that variations in mea-
sured barriers (DG°) do not necessarily correspond directly to
variations in cofacial p–p interactions.
Results and Discussion
To better understand how substituents affect p-stacking, we
report the synthesis, structural characterization, and theoretical
analysis of two alkoxy-substituted 1,8-bis(phenylethynyl)naph-
thalenes, 4 and 8. The 1,8-naphthlene core holds two conju-
gated phenylethynyl units in close proximity such that these
molecules can be considered as models for two interacting
chains of poly(phenylene ethynylene).[18] The availability of
such molecular models for interacting repeat units of conjugat-
ed polymers affords the opportunity to gain detailed structural
information from X-ray crystallography and to perform compu-
tations to gain further insights into the factors that influence
p-stacking. Several prior reports have described p-stacking in-
teractions of arenes that are attached to a 1,8-naphthalenediyl
scaffold. House reported the X-ray crystal structures and bond
rotation energy barriers of various substituted 1,8-diphenylnap-
thalenes and noted significant deformation from a cofacial ge-
Synthesis and structural characterization of 1,8-bis(phenyl-
ethynyl)naphthalenes 4 and 8
For this study, we explored two alkoxy-substituted 1,8-bis((pro-
pyloxyphenyl)ethynyl)naphthalenes: an analogue bearing
alkoxy groups in the 4-position of the phenyl rings (4), and
a second that bears alkoxy groups at both the 2- and the 5-po-
sitions of each ring (8). The synthesis of the bis((4-propyloxy-
phenyl)ethynyl) derivative 4 is shown in Scheme 1A. Coupling
of 4-iodo-1-propyloxybenzene (1) with propargyl alcohol under
Sonogashira conditions[21] gave alcohol 2, which was converted
into terminal acetylene 3 under oxidizing conditions. Installa-
tion of ethynyl groups in this fashion proved more effective
than making use of trimethylsilylacetylene followed by desilyla-
tion because separation of the monopropargylated synthetic
intermediate from byproducts was simplified because of the
Scheme 1. Synthesis of substituted 1,8-bis(phenylethynyl)naphthalenes 4 and 8. a) 1,8-bis((4-propyloxyphenyl)ethynyl)naphthalene (4). b) 1,8-Bis(4-propyloxy-
phenyl)ethynylnaphthalene (8).
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polarity introduced in this step by incorporation of an alcohol.
Coupling of 3 and 1,8-diiodonaphthalene under Sonogashira
conditions afforded 4.[22]
The synthesis of the bis((2,5-dipropyloxyphenyl)ethynyl) de-
rivative 8 followed a similar route, as shown in Scheme 1B.
Treatment of 1,4-dipropoxybenzene with one equivalent of
iodine resulted in monoiodo arene 5, although formation of
some diiodinated by-product could not be avoided. In optimiz-
ing the overall process, treatment of the crude product of this
reaction with an excess of propargyl alcohol under Sonoga-
shira coupling conditions provided the monopropargylated
benzene, 6, which was easily separated from the bis(propargyl)
compound by column chromatography. The alcohol was then
oxidized to acetylene 7 to afford 2-ethynyl-1,4-dipropyloxyben-
zene and subsequently coupled to 1,8-diiodonaphthalene
under the same Sonogashira coupling conditions to afford 8.
The structures of 1,8-bis(phenylethynyl)naphthalenes 4 and 8
1
were verified by H NMR spectroscopy, mass spectrometry, and
elemental analysis (see the Supporting Information for spectra).
Signals corresponding to the naphthalene hydrogen atoms
show a small but distinct dependence on the substitution of
the phenylethynyl unit. Whereas signals for H2 and H4 of 4
appear as doublets (2J(2,3)=2J(3,4)=8 Hz), those of 8 appear
as distinctive doublet of doublets (2J(2,3)=2J(3,4)=7 Hz; 3J=
1 Hz).
Single crystals of 4 and 8 that were suitable for X-ray crystal-
lography were prepared by slow evaporation of solvent from
a 1:10 solution of ethyl acetate in hexanes over 12 h at room
temperature. The crystal structure of the (4-propyloxyphenyl)
analogue 4 is shown along three axes in Figure 1 (left). As ex-
pected, the naphthalene unit is predominantly planar. Howev-
er, there is some in-plane distortion of the substituents at the
1- and 8-positions, with an angle defined by carbon atoms C8-
C1-C13 of 978 (and aC1-C8-C13’=978). Crowding of the phe-
nylethynyl substituents is further accommodated by a bending
around the ethynyl carbon atoms (aC1-C11-C12 + aC11-C12-
C13=3498; aC8-C11’-C12’ + aC11’-C12’-C13’=3498). There is
very little out-of-plane distortion, with the ethynyl substituents
lying in the plane of the naphthalene (Figure 1, left middle).
The phenyl rings of the phenylethynyl arms of 4 are arranged
in a tilted stack at an angle 1158 relative to the naphthalene
scaffold (Figure 1, bottom left), which is similar to the tilting of
the phenyl rings of 1,8-diphenylnaphthalene (i.e., an analogue
of the skeleton of 4 without the ethynylene linkages).[19b] Such
tilted stacks are common to the crystal structure of fused and
linear conjugated arenes,[23] in organic semiconductors,[24] and
columnar discotic phases.[25] The phenyl rings of 4 are almost
parallel, with an intercentroid distance of 3.7 Š. Relative to the
normal to the planes represented by the phenyl rings, there is
a stacked vertical separation, RV of 3.1 Š and a horizontal dis-
placement RH of 1.8 Š. Accordingly, this arrangement of arenes
mimics that commonly found in crystalline regions of conju-
gated polymers, in discotic liquid crystals, and in stacks of DNA
base pairs. It is similar to, but somewhat closer than, the calcu-
lated geometry of the energetic minimum of the parallel-dis-
placed configuration of the benzene dimer (RV =3.5 Š, RH =
1.7 Š).[26]
Figure 1. Molecular structures and key parameters determined by X-ray crys-
tallography. Left: 1,8-bis((4-propyloxyphenyl)ethynyl)naphthalene (4).
Right: 1,8-Bis((2,5-dipropyloxyphenyl)ethynyl)naphthalene (8). Note that
angles reflect distortions in three dimensions and propyl groups are omitted
for clarity
This stacked architecture is in sharp contrast to the geome-
try of the bis(2,5-dipropyloxyphenyl) analogue 8 (Figure 1,
right). In this case, in addition to a greater in-plane distortion,
there is a significant amount of out-of-plane deformation. The
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in-plane distortion of the substituents at the 1- and 8-positions
as characterized by the angle C8-C1-C13 is 1038 (and aC1-C8-
C13’=1028), compared with 978 for the respective positions of
4 (Figure 1, top right). In addition, the ethynylene linkages de-
viate from linearity for 8 (aC1-C11-C12+aC11-C12-C13=
3478; aC8-C11’-C12’+aC11’-C12’-C13’=3478). There is signif-
icant out-of-plane bending that is characterized by a torsional
angle t (C16-C1-C8-C16’)=268 (Figure 1, middle right). The
combination of in-plane and out-of-plane distortions results in
a structure in which the phenyl rings are not arranged in
a stacked fashion. In this case, and in contrast to the situation
for 4, the phenyl rings are almost parallel with the naphthalene
scaffold, tilted by ca. 158 and 88 (Figure 1, bottom right), com-
pared with 1158 in 4, and they are significantly offset from one
another with a vertical separation of RV =3.1 Š and horizontal
displacement of RH =4.7 Š. Although there is precedence for
dramatic in-plane and out-of-plane bending of arene units,[27]
including 1,8-disubstituted anthracenes,[28] we were surprised
by the dissimilarity of the geometries of 4 and 8 that arise
from differences in the substitution pattern of the phenyle-
thynyl arms. Alkoxy and alkyl side chains are common features
on conjugated polymers and oligomers, including poly(pheny-
lene acetylenes), and discotic liquid crystals. These side chains
are accommodated by, and support the formation of, the
stacked arrangement of the p-systems.
viously been used by our group under the name aug-cc-
pVDZ’). This level of theory not only gives reasonably accurate
energies[33] for noncovalent interactions, but also decomposes
the interaction into the physically meaningful components of
electrostatics, exchange-repulsion (sterics), induction (polariza-
tion), and London dispersion forces to better understand the
physical nature of the interaction. Functional group SAPT can
further break down these interaction energy components into
contributions from each molecular sub-fragment (a group of
atoms chosen by the user).
In the course of analyzing nonbonding contacts in 4 and 8
and their potential influence on the preferred geometries in
these compounds, it is important to bear in mind that crystal-
packing forces may also exhibit a significant influence. Howev-
er, gas-phase optimizations of 4 and 8 at the B3LYP-D/aug-cc-
pVDZ level of theory result in conformations similar to those in
the crystal structures (see Figure S25 and S26 of the Support-
ing Information).[34] Intramolecular interactions are enhanced
slightly (see Figure S29), but our conclusions remain the same
using these optimized geometries. The unusually strained geo-
metries of 4 and 8 led to difficulties in performing the geome-
try optimization using standard quantum program packages.
Enhancements to the optimizer in the Psi4 program[35] allowed
the optimizations to be completed.
In the computations, the naphthalene and ethynyl linkers
were removed from each compound for simplicity of analysis
and to provide the distinct molecular fragments necessary for
SAPT analysis. Severed covalent bonds were capped with hy-
drogen atoms subject to constrained optimizations (keeping
all other nuclei fixed) at the B3LYP-D/aug-cc-pVDZ level of
theory. F-SAPT fragments were chosen to be the propyl groups
(one in each monomer for 4, and two in each monomer for 8)
and the aryl ring plus attached oxygen atoms, as indicated in
Figure 2.
The minor distortions from a cofacial geometry in 4, and the
major deviations away from a p-stacking geometry in 8 might
be ascribed to a combination of steric and Coulombic repul-
sions between electron-rich phenyl rings, with the dialkoxy-
substituted phenyl rings in 8 being more electron-rich and
thus, perhaps, resulting in greater distortions. Such an interpre-
tation would be consistent with the analysis of Cozzi and
Siegel[20] for the rotation barriers in their biarylnaphthalenes,
indicating that p–p interactions are stronger with electron-
withdrawing substituents and weaker for electron-donating
substituents (one of the Hunter–Sanders rules). On the other
hand, recent high-accuracy quantum chemical computations in
the gas phase indicate that all substituents, regardless of elec-
tron-donating or electron-withdrawing character, stabilize p–p
interactions in cofacial phenyl rings.[12] Based on the prior ex-
perimental results reported by Carey et al. for 3,5-substituted
arenes,[13b,c] it is surprising that 8 does not adopt a geometry
that is even more aligned for p-stacking than that of 4; appa-
rently, the proximity of the arenes causes C-ÀH/p interactions
to draw the arenes out of plane and dominate the conforma-
tion of the molecule. Hence, we performed electronic structure
computations to understand this difference.
All SAPT computations were performed within Psi4. Con-
strained optimizations to place capping hydrogen atoms were
performed using the Q-Chem package.[36] Figure S27 and S28
contain the F-SAPT0/jun-cc-pVDZ energy components for the
interaction between the pairs of mono- or di-propyloxyben-
zenes (models of the arms in 4 and 8, respectively). The overall
interaction energies between each pair of model fragments
are provided in the color-coded diagrams in Figure 2. The di-
propyloxybenzenes of the model for 8 interact nearly twice as
strongly as the mono-propyloxybenzenes of 4 (interaction en-
ergies of À6.9 versus À3.6 kcalmolÀ1; more negative values in-
dicate stronger intermolecular attractions). The phenyl rings of
4 are at a vertical separation of RV =3.1 Š and a horizontal dis-
placement of RH =1.8 Š. The interaction of the oxyphenyl por-
tion of the molecule alone accounts for 70% (À2.5 kcalmolÀ1
)
Computational analysis
of the model propyloxybenzene–propyloxybenzene interaction
for 4. This interaction is dominated by dispersion, but contains
significant attractive electrostatics (see Figure S27). The interac-
tion of each oxyphenyl unit with each propyl fragment
(À0.5 kcalmolÀ1) and the interaction of the propyl fragments
(À0.1 kcalmolÀ1) both arise primarily from dispersion forces.
The offset-stacked nature of the mono-propyloxybenzene units
of 4 keeps the electronegative oxygen atoms from being
The interactions between the substituted phenyl rings of 4
and 8 were examined by using density-fitted functional group
symmetry-adapted perturbation theory[16,17,29] truncated at
zeroth-order intramonomer correlation[30] (F-SAPT0) combined
with the jun-cc-pVDZ basis set,[31] which is Dunning’s aug-cc-
pVDZ set[32] without diffuse functions on hydrogen and with-
out diffuse d functions on heavy atoms (this basis set has pre-
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Figure 2. Interaction energies (kcalmolÀ1) and geometries: a) propyloxybenzenes as models of interactions in 4; b) para-dipropyloxybenzenes as models of in-
teractions in 8; c) propyloxybenzenes as models for units of 4 in the phenyl ring geometry of 8 (model system 8S); and d) para-dipropyloxybenzenes as
models for units of 8 in the phenyl ring geometry of 4 (model system 4S) are presented on the left of each section (light brown to brown). F-SAPT0/jun-cc-
pVDZ[16] interaction energies between propyl and/or aryl molecular fragments are presented at the top of each section by color. The diamonds indicate the
phenyl carbon atoms that are connected through the ethynyl linkages to the 1,8-naphthalene scaffold. Negative values for interaction energies indicate an at-
tractive interaction.
stacked directly on top of each other, which would otherwise
make an unfavorable contribution to the electrostatic term.
The phenyl rings of 8 are angled 358 from being parallel and
are displaced more horizontally (RV =3.1 Š, RH =4.7 Š) than in
4. For cofacial benzene dimers, all substituents are known to
increase the strength of p–p interactions in gas-phase compu-
tations, regardless of electron-donating or electron-withdraw-
ing character.[12a] Thus, it may be surprising that the disubsti-
tuted phenylenes in 8 adopt a less cofacial geometry than
does 4. The interaction of the two model oxygen-substituted
aryl fragments in 8 is À5.2 kcalmolÀ1, which is due to electro-
statics and dispersion. The favorable anti-alignment of two
polar OÀC bonds of 8 contributes significantly to this favorable
electrostatic stabilization (see Figure 2b for geometry). In addi-
tion, the crystal structure of 8 contains two close CÀH/p con-
tacts, in which a hydrogen atom of a propyl chain of one
phenyl ring is in a position to interact favorably with the p
cloud of the second phenyl ring (with distances of 3.1 and
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interaction. Moreover, the p-stacked geometry in 4S also re-
moves the two favorable CÀH/p interactions and CÀO dipole
anti-alignment that were found in 8. Overall, then, total inter-
actions between two dipropyloxybenzenes are significantly
more favorable (À6.9 kcalmolÀ1) in the experimentally ob-
served splayed geometry of 8 than they are in the p-stacked
geometry 4S (À3.7 kcalmolÀ1), even though the p–p interac-
tion strength is increased by additional substituents in 4S rela-
tive to 4.
We have performed a similar comparison between the
mono-propyloxy substituted system 4 in its native, offset-
stacked geometry and a hypothetical geometry in which we
take the geometry of 8 and switch its dipropyloxy substitution
pattern on each phenyl for a single mono-propyloxy substitu-
ent (model 8S). The experimentally observed p-stacked geom-
etry 4 is significantly more favorable for interactions between
two mono-propyloxybenzenes (À3.6 kcalmolÀ1) than is the
more open geometry of 8S (À1.6 kcalmolÀ1). This is simply
a consequence of the stacked geometry being more favorable
for p–p interactions (À2.5 vs. À1.4 kcalmolÀ1 in 4 vs. 8S) and
Figure 3. Side view of 8 illustrating one of the two CÀH/p interactions.
3.2 Š between the hydrogen atoms and the centers of the aro-
matic rings; see Figure 3 for an illustration of one of these CÀ the propyl groups of 8S being too far apart to interact.
H/p contacts). Computations indicate that these interactions
are dispersion dominated, with each CÀH/p interaction contri-
buting À0.6 kcalmolÀ1 to the interaction energy. A large spatial
separation of the phenyl rings is necessary to achieve this ar-
rangement of mutual CÀH/p interactions and CÀO bond anti-
alignment, thereby explaining the deviation of 8 from any tra-
ditional p-stacking geometry.
In summary, computations based on models for subunits of
4 and 8 indicate that the propyloxyphenyl subunits of 4 inter-
act with À3.6 kcalmolÀ1 of energy resulting from an optimal
parallel-displaced p-stacking geometry. The dipropyloxyphenyl
subunits of 8 interact more strongly (À6.9 kcalmolÀ1 overall) in
a conformation that has a larger separation of the p systems,
but also two favorable CÀH/p interactions of À0.6 kcalmolÀ1
each and a favorable CÀO dipole anti-alignment in the sub-
stituents. Thus, each system adopts a geometry that maximizes
the overall sum of the available noncovalent interactions. Per-
forming the computational experiment of switching the sub-
stituents between models 4 and 8 at fixed phenyl ring geome-
try results in interactions that are less than ideal, including
steric clashes for 4S, and weakly interacting propyloxyben-
zenes for 8S without the stabilization of the CÀH/p interac-
tions.
To further investigate the surprising differences between the
expected and observed geometry of the p systems in 8, com-
putational experiments were performed on hypothetical
“switched” model systems in which the propyloxy substituents
are exchanged between compounds 4 and 8. Compound 4
had its single propyloxy substituent on each phenyl switched
for two propyloxy substituents to yield model 4S. This hypo-
thetical structure illustrates what the dipropyloxy compound 8
would look like if it adopted an offset-stacked arrangement of
the phenyl rings (like that in 4) instead of the splayed arrange-
ment it actually has [see Figure 2(c),(d)]. Comparing the F-
SAPT energetics for 8 versus 4S provides further insight into
why the dipropyloxy compound 8 adopts its “native” splayed
structure instead of an offset-stacked one like that of com-
pound 4. For this comparison we compute only the interac-
tions between the alkoxy-substituted phenyl arms (replacing
the linkage to the backbone with a capping hydrogen atom),
as in the previous discussion of the F-SAPT results for 4 and 8.
The substituent geometry was refined by a constrained optimi-
zation, keeping the coordinates of the phenyl rings fixed.
The attraction between the two dioxyphenyl moieties in the
stacked phenyl ring geometry of 4S (À4.0 kcalmolÀ1) is much
greater than that between the two oxyphenyl components in
4 (À2.5 kcalmolÀ1). This is consistent with previous observa-
tions that increasing substitution of phenyl rings leads to in-
creased attraction for p–p stacking.[12] However, a steric clash
between two of the propyl groups in the stacked dipropyloxy-
benzene model 4S (+2.7 kcalmolÀ1, 4S·PrB-4S·PrA’ in Fig-
ure 2(d)) more than cancels out this enhancement of the p–p
Conclusion
We have presented one of the first detailed case studies of
a competition between CÀH/p and p–p interactions, and we
have explored how this competition influences molecular
structure in the two alkoxy-substituted 1,8-bis((propyloxyphe-
nyl)ethynyl) naphthalenes examined. Additional studies on re-
lated systems (e.g., with different alkoxy substituents, or with
electron-withdrawing substituents) would certainly be warrant-
ed; nevertheless, the present comparison yields several inter-
esting conclusions. The rather different geometries adopted by
4 and 8 cannot be understood by considering p–p interactions
alone (indeed, with respect to p–p interactions only, the 2,5-di-
propyloxy phenyl groups in 8 actually prefer a more stacked
geometry like that in 4). Instead, other interactions (such as
the favorable anti-alignment of CÀO bond dipoles and the fa-
vorable CÀH/p interactions that can result from the less p-
stacked geometry of 8) must also be taken into account. By
using the newly-developed fragment-based symmetry-adapted
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(100 mL), and H2O (100 mL). The solvent was removed under re-
duced pressure and the residue subjected to column chromatogra-
phy (ethyl acetate/hexanes, 1:20) followed by recrystallization from
MeOH to give 8 (482 mg, 65%) as a light-beige solid. M.p.=79–
808C; 1H NMR (CDCl3): d=0.91 (t, J=7 Hz, 6H; CH3), 1.08 (t, J=
7 Hz, 6H; CH3), 1.61 (sextet, 3J=7 Hz, 2H; CH2), 1.86 (sextet, 3J=
7 Hz, 2H; CH2), 3.37 (t, J=7 Hz, 4H; OCH2) 3.89 (t, J=7 Hz, 4H;
OCH2), 6.72–6.75 (m, 4H; Ar-H), 6.78–6.82 (m, 2H; Ar-H), 7.47 (t, J=
7 Hz, 2H; naphthyl H3 and H6), 7.81 (d, J=7 Hz, 2H; naphthyl-H),
7.88 ppm (d, J=7 Hz, 2H; naphthyl-H); 13C NMR (CDCl3): d=10.7
(CH3), 10.9 (CH3), 22.8 (CH2), 23.0 (CH2), 69.7(OCH2), 71.3 (OCH2),
93.9 (CꢀC), 94.1 (CꢀC), 113.8, 114.3, 117.1, 117.8, 121.7, 125.8, 129.5,
131.4, 134.3, 135.0, 152.8 (CO-Ar), 154.3 ppm (CO-Ar); IR (AT-IR,
neat): n˜ =2957, 2930, 2870, 2359, 2196, 1595, 1495, 1216,
978 cmÀ1; MS (EI): m/z (%): 560.3 (100) [M+], 517.3 (60) [M+ÀC3H7];
HRMS: m/z calcd for C38H40O4: 560.29029; found: 560.29266 (D=
4.2 ppm); elemental analysis calcd (%) for C38H40O4: C 81.45, H 7.20,
O 11.42; found C 81.36, H 7.26, O 11.52.
perturbation theory (F-SAPT), we show that electrostatic effects
alone are also insufficient to explain the geometric preferences
in these systems. The delicate interplay between different non-
covalent interactions must therefore be kept in mind in future
efforts to understand and design the structures of p-conjugat-
ed materials.
Experimental Section
General procedures
All reactions were conducted in freshly-distilled solvents in oven-
dried and argon-charged glassware. All reagents were used as re-
ceived from commercial suppliers without additional purification.
Analytical thin-layer chromatography (TLC) was performed on pre-
coated aluminum-backed plates purchased from Sorbent Technolo-
gies (silica gel 60 F254; 0.25 mm thickness). Flash column chroma-
tography was performed on silica gel 60 (230–400 mesh ASTM)
1
from Sorbent Technologies. H and 13C NMR spectra were recorded
1
with a Varian Gemini spectrometer (300 MHz for H and 75 MHz for
Acknowledgements
13C) in deuterated chloroform (CDCl3) and referenced to peaks for
chloroform (CHCl3) at d=7.26 ppm (1H) and d=77.0 ppm (13C).
Chemical shifts are reported in parts per million (ppm). Abbrevia-
tions for signal multiplets are as follows: s, singlet; d, doublet; t,
triplet; m, multiplet (br=broad).
We thank Dr. Les Gelbaum for assistance with NMR, Mr. David
Bostwick for mass spectra, and Dr. Kenneth Hardcastle of
Emory University for collection and assignment of the crystal
structure for compounds 4 and 8. This work was supported in
part by the U.S. National Science Foundation (Grants No. ECS-
437925 and CHE-1300497).
IR spectra of neat films were collected with a Nicolet 4700 FTIR
fitted with an ATR attachment from SmartOrbit Thermoelectronic
Corp. Mass spectra were recorded with a MALDI Micromass TOF
Spec2E instrument or by using EI with a Waters 70SE instrument.
Elemental analyses were obtained from Atlantic Microlabs (Nor-
cross, Georgia). X-ray crystal structures were collected with
a Bruker Apex-II CCD by Kenneth Hardcastle at the Emory Universi-
ty X-ray facility.
Keywords: density functional calculations
interactions pi interactions stacking interactions
substituent effects
·
noncovalent
·
·
·
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444.20716 (D=4.0 ppm).
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Received: June 18, 2015
Published online on November 16, 2015
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19175
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