Substituent Effects That Control Conjugated Oligomer Conformation through Non-covalent Interactions

Article abstract of DOI:10.1021/jacs.7b00878

Although understanding the conformations and arrangements of conjugated materials as solids is key to their prospective applications, predictive power over these structural factors remains elusive. In this work, substituent effects tune non-covalent interactions between side-chain fluorinated benzyl esters and main-chain terminal arenes, in turn controlling the conformations and interchromophore aggregation of three-ring phenylene-ethynylenes (PEs). Cofacial fluoroarene-arene (ArF-ArH) interactions cause twisting in the PE backbone, interrupting intramolecular conjugation as well as blocking chromophore aggregation, both of which prevent the typically observed bathochromic shift observed upon transitioning PEs from solution to solid. This work highlights two structural factors that determine whether the ArF-ArH interactions, and the resulting twisted, unaggregated chromophores, occur in these solids: (i) the electron-releasing characteristic of substituents on ArH, with more electron-releasing character favoring ArF-ArH interactions, and (ii) the fluorination pattern of the ArF ring, with 2,3,4,5,6-pentafluorophenyl favoring ArF-ArH interactions over 2,4,6-trifluorophenyl. These trends indicate that considerations of electrostatic complementarity, whether through a polar-π or substituent-substituent mechanism, can serve as an effective design principle in controlling the interaction strengths, and therefore the optoelectronic properties, of these molecules as solids.

Full text of DOI:10.1021/jacs.7b00878

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Substituent Effects That Control Conjugated Oligomer Conformation
through Non-covalent Interactions
Seth A. Sharber,† Rom Nath Baral,† Fanny Frausto,† Terry E. Haas,† Peter Muller,‡
̈
and Samuel W. Thomas III*,†
†Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States
‡Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: Although understanding the conformations and
arrangements of conjugated materials as solids is key to their
prospective applications, predictive power over these structural
factors remains elusive. In this work, substituent effects tune non-
covalent interactions between side-chain fluorinated benzyl esters
and main-chain terminal arenes, in turn controlling the
conformations and interchromophore aggregation of three-ring
phenylene-ethynylenes (PEs). Cofacial fluoroarene−arene (ArF−
ArH) interactions cause twisting in the PE backbone, interrupting
intramolecular conjugation as well as blocking chromophore aggregation, both of which prevent the typically observed
bathochromic shift observed upon transitioning PEs from solution to solid. This work highlights two structural factors that
determine whether the ArF−ArH interactions, and the resulting twisted, unaggregated chromophores, occur in these solids: (i)
the electron-releasing characteristic of substituents on ArH, with more electron-releasing character favoring ArF−ArH
interactions, and (ii) the fluorination pattern of the ArF ring, with 2,3,4,5,6-pentafluorophenyl favoring ArF−ArH interactions
over 2,4,6-trifluorophenyl. These trends indicate that considerations of electrostatic complementarity, whether through a polar−π
or substituent−substituent mechanism, can serve as an effective design principle in controlling the interaction strengths, and
therefore the optoelectronic properties, of these molecules as solids.
offers versatility in interaction directionality and structurally
tuned packing motifs,7 which has proven useful in altering
molecular packing for efficient electron transport.8 Chalcogen
interactions have been similarly integrated between chromo-
phoric units to increase backbone coplanarity and inter-
molecular coupling in conjugated polymers.9,10
INTRODUCTION
■
The accurate prediction of the properties of conjugated
materials in dilute solution has become relatively straightfor-
ward. The rational design of key properties of these materials as
solid-state ensembles, however, remains an elusive goal.1
Control over the optoelectronic behavior of these solids is
paramount for efficient function of these materials in their
many potential applications. Optical properties of solids depend
not only on chemical structures of chromophoric units, but also
on conformation and intermolecular coupling, which non-
covalent forces can direct. The weak nature of these individual
interactions presents a long-standing challenge for under-
standing the packing of molecules as solids, as well as predicting
bulk material properties.
Incorporating directional non-covalent interactions into
conjugated molecules offers the potential for dictating
conformations and intermolecular packing in crystals and can
therefore yield control over optoelectronic properties. For
example, hydrogen bonding is a powerful tool in crystal
engineering, useful in constructing low-band-gap materials by
driving self-assembly across the breadth of the nanoscale:
intramolecular hydrogen bonds can restrict conformation and
increase conjugation length,2,3 whereas intermolecular hydro-
gen bonding can guide formation of molecular assemblies with
charge-transfer character and dipolar interactions as well as
photoactive nanostructures.4−6 In addition, halogen bonding
Fluorine substitution in conjugated systems has been
especially useful for controlling molecular assembly through
strong interactions between aromatic rings, most notably in the
near-sandwich dimer of the perfluoroarene−arene system
widely adopted as a supramolecular synthon across materials
science and biology.11−16 Fluoroarene−arene interactions offer
reliability17 and tunability18,19 in the preferred orientations of
conjugated units while making only small steric alterations in
fluorine substitution.15,20 In-depth theoretical studies of the
cofacial interactions of substituted arenes and fluorinated arenes
have yielded important insights that further our understanding
of aromatic interactions.21,22 Recent efforts have enhanced our
understanding of how substituents influence the interactions
between aromatic rings beyond the polar−π model by
illuminating the importance of dispersion interactions and the
local nature of interactions between substituents.21−33 These
considerations are important not only for understanding
Received: January 25, 2017
© XXXX American Chemical Society
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aromatic interactions but also for manipulating the fluoro-
arene−arene (ArF−ArH) synthon as a supramolecular control
unit.
From the perspective of applications of conjugated materials,
oligomers (OPEs) and polymers (PPEs) of phenylene-
ethynylene (PE)-linked backbones have proven useful in
chemical sensors,34,35 photovoltaics,36 electronics,37,38 and
solid-state luminescent devices.39,40 In addition, the low barriers
to rotation (∼1 kcal/mol) of PE linkages allow for a wide range
of accessible conformers both in solution and in the solid state,
presenting an opportunity for functional materials as well as a
challenge for exerting control over conformation.41,42 There-
fore, the use of chemical structure to bias PE conformation, and
the resultant magnitude of electronic coupling, has been
important in several breakthroughs in PE chemistry: (i) Rigid,
sterically hindered groups such as pentiptycenes prevent
interchain aggregation and enable reproducibly bright emission
from PPEs for applications in chemoresponsive films and light-
emitting devices;34,43−49 the electronic effects of pentiptycenes
can also play key roles in determining conformational
preferences of PEs.50 (ii) Long-lived phosphorescence in
tethered, twisted tolanes demonstrates the dramatic depend-
ence of PE photophysics on intramolecular conformation and
the utility of biasing geometry.51−54 (iii) Enforcing coplanarity
of the PE backbone through intramolecular hydrogen bonding
deepens the potential energy well for PE bond rotation in
solution, shifting fluorescence bathochromically.2,55
Our group recently reported several examples of three-ring
PEs in which discrete interactions between aromatic rings in
side chainsthe function of which, until recently,56 has
typically been restricted to imparting solubility to otherwise
insoluble conjugated materialsand aromatic rings in main
chains determined the conformations along the PE backbones
in the solid state. Edge−face interactions between non-
fluorinated rings reinforced coplanar PE conformations, while
cofacial interactions between perfluoroarene pendants57 and
aromatic rings in the conjugated backbones yielded PE
chromophores twisted from coplanarity by 55−60° (Figure
1).58 These changes in PE conformations yielded different
optical properties in the solid state, with perfluoroarene
pendants yielding hypsochromically shifted optical spectra.
Such side-chain-based approaches to control of optical spectra
contrast with conventional approaches based on altering the
structures of conjugated backbones. Herein we report that
substituent effects can tune the strengths of fluoroarene−arene
interactions involving the side chains and main chains of this
class of PE compounds, strongly influencing PE conformations,
interchromophore aggregation, and the resulting properties of
solid-state PEs.
Figure 1. Top: OPE structural perturbations described in this work
designed to modify the electrostatic complementarity of cofacially
interacting rings. Bottom: X-ray crystal structure of previously
reported three-ring OPE H-F5, showing how cofacially interacting
ArF and ArH rings both yield a twisted (57−58° torsional angles
between arenes) conformation and prevent aggregation of PE
backbones. Hydrogen atoms have been removed for clarity.
synthesis of these compounds follows a simple sequence of
esterification of previously reported60,61 2,5-diiodoterephthaloyl
chloride with either pentafluorobenzyl alcohol or 2,4,6-
trifluorobenzyl alcohol, followed by Sonogashira coupling
with the appropriate substituted arylethynylene to yield each
target compound (Scheme 1).
Spectroscopy of R-F5 Series. Table 1 summarizes the
absorbance and fluorescence spectra of the compounds in the
R-F5 series, with R = NMe2, OMe, H, CO2Me, CF3, CN, and
NO2 as both solutions in chloroform and as drop-cast films.
Figure 2 shows spectra for NMe2-F5, CO2Me-F5, and NO2-
F5 (see the Supporting Information (SI) for spectra of all
compounds); Figure 3 shows pictures of the photolumines-
cence from these solutions and solids. Generally, the trends in
spectra of these compounds in dilute organic solution are
expected on the basis of simple models of substituent effects.
The central terephthalate ring in each of these compounds is a
relatively electron-poor unit, enabling donor−acceptor charac-
teristics with relatively electron-rich terminal arenes. Con-
sequently, the absorbance and emission spectra of NMe2-F5
(436 nm/575 nm) and OMe-F5 (385 nm/465 nm) were
shifted bathochromically relative to the other R-F5 derivatives
(360−373 nm/410−425 nm).
RESULTS AND DISCUSSION
■
PE Design and Synthesis. The core designs of molecules
reported herein are symmetrical three-ring PEs comprising a
central benzyl terephthalate and identical terminal rings. We
targeted two series of molecules in this work, the difference
between them being the level of fluorination of the benzyl
substituents (the “ArF” ring in ArF−ArH interactions): (i)
2,3,4,5,6-pentafluorobenzyl and (ii) 2,4,6-trifluorobenzyl. In
each series, the electronic effect of substituents on the main-
chain terminal rings (the “ArH” ring) is varied using different
para substituents R, ranging from dimethylaniline (σp = −0.83),
which is reported to participate in a charge-transfer complex
with hexafluorobenzene,21 to nitrobenzene (σp = 0.78).59 The
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spectral shifts observed: (i) a hypsochromic or slight batho-
chromic shift in absorbance and fluorescence and (ii) a strong
bathochromic shift in absorbance and fluorescence.
Scheme 1. Synthesis of Two Series of Three-Ring PEs, R-F5
and R-F3, with R = NMe2, OMe, H, CO2Me, CF3, CN, or
NO2
Molecules in the first category are those in which R is an
electron-donating group, hydrogen atom, or a modest electron-
withdrawing group (CO2Me, CF3). The absence of strong
bathochromic shifting upon transitioning to the solid suggests
the absence of one or both of the factors that can lead to the
well-known bathochromic shifting of PE-based materials
increased coplanarity of conjugated rings or increased
interchromophore coupling through aggregation.63
Conversely, molecules in the second category showing strong
bathochromic shifts in both solid state absorbance (15−40 nm)
and fluorescence maxima (>100 nm) are those with either of
the two most potent electron-withdrawing groups examined
(CN, NO2). These results suggest that the PE backbones of
CN-F5 and NO2-F5 are coplanar, aggregated, or both. An
interesting effect of this trend is that chromophores with more
donor−acceptor character (such as OMe-F5) can have larger
band gaps than those with less donor−acceptor character (CN-
F5 or NO2-F5), contradicting the common paradigm for
chromophore design.64
Crystallography of R-F5 Series. With the exception of
CN-F5, we have determined X-ray crystal structures for all
compounds in the F5 series, including for two different
polymorphs of CO2Me-F5a violet- and a blue/green-
emitting polymorph. Although the structure of CF3-F5 is a
partial structure, it enables us to determine the conformation of
the conjugated backbone. As expected on the basis of the
spectra described above, all R-F5 structures except for NO2-F5
and the blue/green-emitting polymorph of CO2Me-F5 had
packing motifs similar to those described for OMe-F5 and H-
F5 in our previous report. The most obvious common attribute
among these structures is the presence of infinite stacks of
arenes comprising intramolecular and intermolecular cofacial
ArF−ArH interactions (Figure 4). The three new compounds
in the R-F5 series that show this packing motifNMe2-F5,
CO2Me-F5 (violet), and CF3-F5have geometric parameters
consistent with ArF−ArH cofacial interactions, such as closest
CArF···CArH contacts between 3.4 and 3.5 Å, and centroid-
centroid distances of 3.7−3.8 Å. The ArF and ArH rings do not
form perfect sandwich dimers, but instead the ArH rings extend
out farther from the core terephthalate than the ArF rings by
∼1 Å; such displacements have been calculated to yield larger
interaction energies than perfect sandwich dimers due the
possibility for closer distances between rings.21
To assess substituent effects on the spectroscopy of these
molecules as solids, we prepared thin films by drop casting from
chloroform onto planar quartz substrates. As many of these
compounds showed polymorphic behavior, we annealed films
at 100−115 °C for 15 min, followed by slow cooling to ambient
temperature, to ameliorate the issue of preparation-dependent
morphologies of the solid samples.62 Although all data shown
here were obtained from drop cast films, thermally annealed
spun cast films showed similar behavior. The fluorescence
spectra of all R-F5 compounds were reproducible upon thermal
annealing. For example, CO2Me-F5 forms both violet-emissive
and blue-emissive phases in samples of crystals, powders, and
films; these samples, however, converge on violet emission after
slowly cooling from the melt. In addition, because the
crystalline nature of these materials made the acquisition of
absorbance spectra difficult due to scattering, we display
fluorescence excitation spectra as substitutes. The SI contains
absorbance spectra for those samples that yielded reasonably
low levels of scattering.
Unlike those of solutions, the absorbance and fluorescence
spectra of these molecules as drop-cast thin films did not show
a consistent correlation of spectral position with electron-
donating ability of the substituents on the terminal rings.
Instead, the longest wavelength maxima of absorbance and
fluorescence originated from solids with powerful electron
donors (R = NMe2) or powerful electron acceptors (R = CN,
NO2). Especially instructive in this analysis is to correlate the
spectral shifts upon transition from solution to solid state,
which are shown as individual columns in Table 1. We classify
the molecules into two distinct categories on the basis of the
Table 1. Absorbance and Emission Parameters for the R-F5 Series of Compounds in CHCl3 and as Thin Films Drop-Cast from
a
CHCl3
absorbance
emission
λmax (nm)
solution
λmax (nm)
solution
film
shift (nm)
film
shift (nm)
ΦF
τ (ns)
NMe2-F5
OMe-F5
H-F5
436
385
362
363
360
365
373
400
375
342
338
335
380
415
−36
−10
−20
−25
−25
15
575
465
425
415
410
410
413
555
460
440
400
390
520
525
−20
−5
15
0.26
0.47
0.51
0.46
0.38
0.48
<0.01
2.0
2.5
1.7
0.7
0.3
0.6
−
CO2Me-F5
CF3-F5
CN-F5
−15
−20
110
112
NO2-F5
42
a
All thin films were heated to 100 °C for 15 min after solvent evaporated. Reported quantum yields ΦF and lifetimes τ are for CHCl3 solutions.
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Figure 2. Optical spectra of NMe2-F5, CO2Me-F5, and NO2-F5 in chloroform solution and as drop-cast thin films. All thin films were heated to
100 °C for 15 min after solvent evaporated at ambient temperature. Dashed lines are solution excitation spectra.
Figure 4. Single-crystal X-ray structures of two compounds in the R-
F5 series that show conserved packing motifs with intramolecular and
intermolecular ArF−ArH interactions resulting in twisted PE back-
bones. Hydrogen atoms have been omitted for clarity. Thermal
ellipsoids are shown at 50% probability.
Figure 3. Photographs of fluorescence from solutions (CHCl3) and
solids of R-F5 compounds.
These ArF−ArH interactions play an important role in
minimizing intramolecular orbital overlap by enforcing twisted
PE backbones, with inter-ring torsional angles of 60−63° (CF3-
F5), 67−70° (CO2Me-F5), and 81−83° (NMe2-F5). These
favored conformations appear to be due to geometric
restrictions imposed by the short benzylic ester tether between
the PE backbone and the pentafluorobenzene ring. In all
examples of this packing motif, the ester functional groups
adopt similar conformations, with the alkoxy groups of the
esters oriented syn to the alkyne substituentsthe opposite
rotamers with the alkoxy groups oriented anti to the alkyne
substituents would not leave sufficient length of the tether for
ArF and ArH rings to interact intramolecularly. This geometric
restriction is apparent in the crystal structures of two
polymorphs of CO2Me-F5 (vide infra), in which the distance
between the alkoxy oxygen and the centroid of the nearest
terminal PE ring is longer for the coplanar polymorph with anti
rotamers (7.6 Å) than for the twisted polymorph with syn
rotamers (5.2 Å). A crystal structure of the coplanar polymorph
can be found in the Supporting Information.
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In addition to enforcing highly twisted conformations,
cofacial interactions between pendant ArF rings and conjugated
ArH rings also preclude interchromophore aggregation. Except
for NO2-F5 and the blue/green-emitting polymorph of
CO2Me-F5, in no cases did we find any significant inter-
molecular cofacial interactions of the conjugated PE chains that
would be necessary to form J- or H-aggregates.65 We attribute
this to a combination of two factors: (i) the blocking of all faces
of the terminal conjugated phenylene rings of these molecules
with perfluorinated arenes from non-conjugated pendants, and
(ii) shielding of the PE cores by the flanking perfluorinated
ringsfor example, interactions between fluorine atoms of C−
F bonds and the faces of electron poor terephthalates are key
short intermolecular contacts between adjacent stacks (Figure
5).
Figure 5. Single-crystal X-ray structure of CO2Me-F5, visualized along
the crystallographic b-axis, highlighting C−F/terephthalate edge-face
contacts between stacks of chromophores. Hydrogen atoms have been
omitted for clarity.
Figure 6. (a) Single-crystal X-ray structure of NO2-F5. Hydrogen
atoms have been omitted for clarity, and all thermal ellipsoids are
displayed at 50% probability. (b) Highlight of nitro−terephthalate−
terephthalate−nitro cofacial aggregate motifs; dashed lines indicate
interatomic distances <3.56 Å. For clarity, all perfluorobenzene rings
have been omitted, as have all atoms except the nitroarene ring from
the top and bottom contributors. Atoms that participate in these
cofacial stacks are shown as 50% probability thermal ellipsoids; others
are shown as small spheres.
The crystal structure of NO2-F5, however, contrasts strongly
with structures in Figures 1 and 4. Although its conjugated PE
backbone is twisted, with torsional angles between 62 and 67°,
there are no cofacial intramolecular ArF−ArH interactions. The
closest intramolecular contact between the two rings is 6.6 Å
(Figure 6a). Furthermore, the conformation of the benzoate
esters, with the carbonyl groups syn to the alkyne substituents
on the terephthalate rings, precludes the cofacial interactions
observed in the other R-F5 structures. Because the ArF rings do
not effectively enshroud the conjugated backbone, interchain
aggregation of PE chromophores take place in NO2-F5. Each
benzoate ester group interacts cofacially, through complemen-
tary dipole−dipole interactions, with both a nitro group on one
face and a terephthalate ring on the other face, creating nitro−
terephthalate−terephthalate−nitro cofacial stacks throughout
the structure (Figure 6b), with C···C, C···O, and N···O
intermolecular contacts with distances less than 3.56 Å. Each
PE molecule participates in four such stacks, resulting in a
network of aggregated chromophores. Lastly, the blue/green-
emitting polymorph of CO2Me-F5 shows both highly coplanar
PE chromophores (torsional angles ∼2°) and an arrangement
of the chromophores in slipped-stacks, enabling electronic
coupling between chromophores. A 0.6 eV larger calculated
HOMO−LUMO gap from single point energy DFT calcu-
lations for the twisted polymorph than for the planar
polymorph highlights the importance of conformation of
these molecules (SI). Emission from the blue/green-emitting,
coplanar polymorph is not observed in annealed thin films,
suggesting that the twisted, violet-emitting polymorph is
favored.
We therefore conclude that the behavior of molecules that do
not show strong bathochromic shifting is due to inhibition of
both (i) interchromophore coupling (aggregation) and (ii)
coplanarity of the PE backbones, due to the supramolecular
synthon of ArF−ArH interactions between non-conjugated
pendants and terminal rings of the PEs. This conclusion is
similar to our previous report on a smaller breadth of
substituents. In contrast, the strong electron-withdrawing
character of the nitro group in NO2-F5 weakens analogous
ArF−ArH cofacial interactions, resulting in other packing
arrangements, yielding inter-PE aggregation causing batho-
chromic shifting of absorbance and emission of films. DFT
calculations on an individual molecule of NO2-F5, using atomic
coordinates identical to those determined from the crystal
structure, also support this conclusion. Consistent with the
differences of the onset of absorbance spectra in solution, the
calculated HOMO−LUMO gap of NO2-F5 was only ∼0.1 eV
less than that calculated for individual molecules from the
crystal structures of H-F5 and CO2Me-F5 (see SI), suggesting
the importance of intermolecular aggregation in the observed
solution-to-solid bathochromic shift of NO2-F5. We therefore
further infer that the cyano group of CN-F5 also weakens ArF−
ArH cofacial interactions sufficiently to disrupt the packing
E
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a
Table 2. Absorbance and Emission Parameters for the R-F3 Compounds in CHCl3 and as Drop-Cast Thin Films
absorbance
λmax (nm)
emission
λmax (nm)
solution
film
shift (nm)
solution
film
545
shift (nm)
ΦF
τ (ns)
NMe2-F3
OMe-F3
H-F3
430
382
363
360
355
360
375
410
350
377
375
340
375
400
−20
−32
14
565
460
420
413
405
405
412
−20
0
0.39
0.47
0.50
0.45
0.38
0.52
<0.01
2.5
2.2
1.0
1.0
0.3
0.4
−
460
490
70
99
CO2Me-F3
CF3-F3
CN-F3
15
512
b
b
−15
15
395/465
505
−10/60
100
NO2-F3
25
540
128
a
All thin films were heated to 100 °C for 15 min after solvent evaporated. Reported quantum yields ΦF and lifetimes τ are for CHCl3 solutions.
Two differently emitting polymorphs were observed consistently in thin films.
b
Figure 7. Fluorescence excitation and emission spectra of NMe2-F3, OMe-F3, and H-F3 in CHCl3 solution and as thin films drop cast from CHCl3.
All thin films were heated to 100 °C for 15 min after solvent evaporated at ambient temperature. Dashed lines are solution excitation spectra.
motif shown in Figures 1 and 3, resulting in PE coplanarity,
aggregation, or both.
cofacial interactions through replacement of the fluorine atoms
in the 3- and 5-positions with hydrogen atoms. We expected
this to reduce the ArF−ArH interaction energy for substituted
benzenes; in demonstrating the local nature of substituent
effects in ArF−ArH interactions, Wheeler and co-workers have
shown that the regiochemistry of substituents is key to the
strengths of cofacial interactions.66
Table 2 and Figure 7 summarize the absorbance and
fluorescence spectra of compounds in the R-F3 series as both
dilute chloroform solutions and drop cast thin films (see the SI
for the spectra of all molecules); Figure 8 shows photographs of
the fluorescence of these samples. Any differences between the
solution-phase spectra of these compounds and those of R-F5
Spectroscopy of R-F3 Series. Given that substituent
effects on ArH rings yield a predictable trend as to whether the
ArF−ArH supramolecular synthon dictates the packing of these
conjugated molecules, in turn determining their solid-state
optical properties, we reasoned that decreasing the level of
fluorination of the ArF rings could have a similar effect. We
therefore prepared the R-F3 series of PEs, which substitute
2,4,6-trifluorobenzyl esters for the pentafluorobenzyl esters
used in the R-F5 series. These pendants retain the fluorine in
the 4-position, which is usually closest to the para substituents
on the ArH rings, while potentially changing the strengths of
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contrasts with the analogue CF3-F5, which although showing a
blue phase bathochromically shifted from solution in freshly
prepared films, converges exclusively on the violet-emitting
phase upon annealing at 100 °C.
Crystallography of R-F3 Series. Crystallography experi-
ments described below justify extending our model correlating
substituent effects, optical spectroscopy, and crystallography to
the R-F3 series. Single-crystal X-ray structures of those
compounds that did not show significant bathochromic shifts
in optical spectra and had the strongest electron-donating
substituents on the ArH rings, in this case NMe2-F3 and OMe-
F3 have packing motifs similar to those of most of the members
of the R-F5 series, with intramolecular and intermolecular
ArF−ArH interactions that extend throughout the structure
(Figure 9). In these examples, the ArF rings in each molecule
Figure 8. Photographs of the fluorescence of solutions in chloroform
and films drop-cast from chloroform demonstrating substituent effect
on solid behavior in compounds with 2,4,6-trifluorinated pendants.
compounds with identical conjugated backbones are less than
10 nm. We therefore conclude that effects of the trifluorinated
and pentafluorinated arene substituents on the distribution of
conformations and the electronic structure of the conjugated
chromophore backbones are similar. Although our previous
work has demonstrated that inductive effects of analogous non-
conjugated pendants on the frontier molecular orbital (FMO)
energies of three-ring PEs can occur in solution, such shifts
require more significant differences in structures than
investigated here.67
Figure 9. X-ray crystal structures of NMe2-F3 and OMe-F3, with
hydrogen atoms omitted. The thermal ellipsoids are shown at 50%
probability.
The absorbance and fluorescence spectra of these com-
pounds as thermally annealed drop-cast thin films show a trend
similar to that described above for the R-F5 series of
compoundssubstituents that are electron-donating yield
hypsochromic shifts in solids relative to spectra in solution,
while those that are electron-withdrawing generally yield
bathochromic shifts. The key difference between the two
series, however, is the point at which this change in behavior
occurs. As described above, for the R-F5 series, it occurs
between R = CF3 (σp = 0.54) and R = CN (σp = 0.66), with
polymorphism that converges on the hypsochromically shifted
phase in CO2Me-F5 and CF3-F5. In contrast, for the R-F3
compounds, this change occurs between R = OCH3 (σp =
−0.27) and R = H (σp = 0).59 For example, while OMe-F3
shows a 30 nm hypsochromic shift of absorbance maxima and
no significant shift of emission maxima comparing solid to
solution, H-F3 shows 14 and 70 nm bathochromic shifts,
respectively (Figure 7). R-F3 compounds with σp > 0 show
similarly large bathochromic shifts.
An exception to this trend is CF3-F3, which has been the
only compound investigated in this work that shows persistent
polymorphism. Thin films of this compound have emission
spectra either bathochromically or hypsochromically shifted
from solution, depending on the method of fabrication. For
example, slow cooling of CF3-F3 from the melt results in a
solid with violet fluorescence (hypsochromically shifted from
solution), blue fluorescence (bathochromically shifted from
solution), or both. Films prepared by slow evaporation from
dichloromethane emit blue light, while films prepared by slow
evaporation from chloroform gave a mixture of both violet- and
blue-emitting phases. Complete switching of one phase to the
other was not observed upon heating these films to any
temperatures below melting (196−198 °C). This behavior
are tilted decidedly toward the ArH rings in the same molecule,
resulting in closest C···C distances between the interacting
rings of 3.47 Å (NMe2-F3) and 3.53 Å (OMe-F3), while
centroid−centroid distances are 3.91−3.95 Å.
Consistent with our previous observations of this packing
motif, the conjugated PE chains of these two molecules are
strongly twistedin these cases they are nearly perpendicular,
with torsional angles of 80−82°. Also, hydrogen bonds
comprising carbonyl oxygen acceptors and C−H bonds of
the 2,4,6-trifluorobenzene rings (C···O distances of 3.25−3.26
Å) exist between adjacent stacks. These interactions appear to
be important for preventing aggregation of the PE
chromophores. Therefore, the absence of both coplanarity
and PE aggregation in these crystal structures is consistent with
the lack of strong bathochromic shifts observed in the
fluorescence spectra of films.
We have also determined a complete structure for H-F3
(Figure 10) and a partial structure for CN-F3. These crystal
structures do not show ArF−ArH cofacial interactions, instead
showing PE conformations for H-F3 (13°−16° torsions) that
are generally coplanar. In addition, a partial structure of CN-F3
suggests torsional angles of 7−8° (see SI). These structures also
show substantial overlap between the π-systems of PE
backbones in slipped stacks consistent with J-aggregates, with
inter-PE C···C distances as short as 3.27 Å and slip angles
between stacked PEs of 44−46°. The observations in these
crystal structures of geometrical factors commonly implicated
in bathochromic shifting of PEsplanarization and aggrega-
tionare therefore consistent with the bathochromic shifting
we observe in the R-F3 compounds for which σp ≥ 0.
In addition, we have determined crystal structures for two
differentially emissive polymorphs of CF3-F3, each of which
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Figure 10. Two views of the X-ray crystal structure of H-F3, with
hydrogen atoms omitted for clarity. Thermal ellipsoids in top image
are shown at 50% probability.
shows packing motifs consistent with those described above.
The emission spectra of each of the two crystals show excellent
spectral match with one of the two emission bands observed
from thin films (Figure 11). Diffusion of cyclohexane into a
concentrated solution of CF3-F3 in chloroform yielded a
violet-emitting prism, the crystal structure of which shows the
twisted PE motif enforced by ArF−ArH interactions, together
with C−H/π and hydrogen-bonding interactions, similar to
NMe2-F3 and OMe-F3. Conversely, slow evaporation of
dichloromethane with layered hexanes yielded a blue-emitting
crystal, the crystal structure of which has highly planarized PE
backbones and intermolecular stacking contacts consistent with
J-aggregation, similar to H-F3 and CN-F3. Consistent in both
crystal structures are apparent F···F interactions between
terminal trifluoromethyl groups (F···F distances as close as
2.84 Å), suggesting that the inclusion of other competitive non-
covalent interactions influences crystal packing of these
molecules, especially with reduced interaction energy for the
ArF−ArH cofacial interactions relative to more electron-
donating substituents.
Summary of Structure−Property Relationships in R-
F5 and R-F3 Compounds. The similarities of these two series
of molecules are extensive, as they differ only in the degree of
fluorination of the benzylic ester groups, which are not π-
conjugated to the PE backbone. The lack of significant change
in solution-phase spectra between R-F5 and analogous R-F3
derivatives highlights the subtle nature of this chemical change,
relative to the manner by which the electronic structures of
conjugated materials are typically tuned. In contrast, the
behavior of these materials as solids, the required state for most
applications of conjugated materials, depends strongly on
specific non-covalent interactions of the side chains and main
chains. We note several trends connecting chemical structure,
Figure 11. Top: Fluorescence emission spectra of CF3-F3 in dilute
chloroform solution, as a thermally annealed drop-cast thin film, and of
two polymorphic single crystals. Middle and bottom: X-ray crystal
structures of the violet- (middle) and blue-emitting single-crystal
polymorphs of CF3-F3. Hydrogen atoms have been omitted for
clarity. Thermal ellipsoids are shown at 50% probability.
crystal structure, and spectroscopy of these two series of
compounds:
1. Optical spectra depend critically on whether cofacial
ArF−ArH interactions occur between the fluorinated
aromatic pendants and terminal rings of the PE
backbones. In all cases described above for which these
discrete non-covalent interactions occur, the optical
spectra of the solids are not significantly batho-
chromically shifted from solution. The crystallographic
evidence suggests that this feature is due to a
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combination of two factors: (i) twisting of the PE
backbones induced by the ArF−ArH interactions, with
torsional angles between 55 and 83°, and (ii) insulation
of the PE backbones from interchromophore aggrega-
tion. Conversely, in all cases in which the ArF−ArH side-
chain/main-chain interactions do not take place,
significant bathochromic shifting of the PE optical
spectra occurs in the solid state. On the basis of the
crystal structures of such compounds, we attribute these
observations to interchromophore aggregation, PE
planarization, or both.
2. The presence or absence of side-chain/main-chain ArF−
ArH interactions correlates with the electronic effect of
the substituent in monosubstituted ArH rings. In the R-
F5 series of compounds, the para substituent of the ArH
rings is key in the relative strengths of the ArF−ArH
interaction, with the traditional Hammett coefficient
serving as an empirically suitable parameter to correlate
with our observations. For each series, substituents with
smaller σp values (larger interaction energies) showed
ArF−ArH interactions, while those with larger σp values
(smaller interaction energies) lacked ArF−ArH inter-
actions; shifts in optical spectra followed these trends, as
described in point 1 above (Figure 12). Although
dispersion interactions also play a key role in ArF−
ArH interactions generally, including in observed
substituent effects, our observed trend concerning
traditional Hammett-style substituent effects is in line
with previous experimental and theoretical studies of
ArF−ArH interactions of monosubstituted ArH rings.
We have used the σp parameter throughout this paper, as
it captures more accurately the order of substituent
effects in the R-F3 series (particularly for R = OCH3),
and predicts more closely the trend of calculated ArF−
ArH interaction energies, as reported by Wheeler and
Houk,68 for the particular group of substituents chosen
for this study. Although the interacting rings in this paper
are displaced in at least one dimension, instead of a
perfect sandwich dimer as modeled by Wheeler and
Houk,68 Gung and Amicangelo have shown that such
correlations persist in parallel displaced interaction
geometries using singly substituted ArH rings.21
3. The threshold of electronic substituent effect of ArH
rings necessary to induce ArF−ArH interactions depends
on the fluorination pattern of the ArF rings. Consistent
with the idea that there is a significant electrostatic
component to the substituent effects observed here,
reducing the degree of fluorination of the ArF rings of
the pendants generally requires increased electron-
donating character of the ArH substituent to observe
ArF−ArH interactions and prevent chromophore
aggregation. The complex environment of the crystal
lattice makes it difficult to ascribe this observation to one
particular cause, as numerous factors are changing with
this simple chemical modification, including: (i) reduced
complementarity of local substituent−substituent and
substituent−ring interactions of the ArF−ArH rings, (ii)
reduced complementarity of potential polar−π inter-
actions in the cofacial geometry, (iii) the potential for
additional non-covalent interactions, such as C−H
hydrogen bond donation from the ArF rings, and (iv)
changes in the magnitude of dispersion interactions due
to the C−H for C−F substitutions.
Figure 12. Correlation of (i) observed solution-to-solid shift of
absorbance and fluorescence emission spectra, (ii) appearance of side-
chain/main-chain ArF−ArH interactions in single-crystal X-ray
structures, and (iii) calculated interaction energy, relative to R = H,
for the ArF−ArH sandwich dimer, as published by Wheeler and
Houk.68 These data are summarized for both the R-F5 series (top)
and R-F3 series (bottom) of compounds.
CONCLUSION
■
This work presents practical outcomes in solid-state functional
materials from the extensive theoretical and experimental work
regarding substituent effects in the non-covalent interactions of
aromatic rings. The low barriers to rotation and known
sensitivity to both conformational changes and intermolecular
aggregation make phenylene-ethynylenes an ideal class of
materials for exploring these concepts. In these examples,
substituent effects play a key role in determining whether ArF−
ArH interactions are present, which has important consequen-
ces for the conformations and electronic structures of these
solids. Although the factors governing the strengths of these
cofacial interactions are complex, we find here that substituent
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electronic effects of the ArH ring provide a useful structure−
property trend regarding whether ArF−ArH interactions occur
in structurally analogous PEs. In addition, this concept of
complementarity of the ArF and ArH rings can extend to ArF
rings with a smaller extent of fluorination, with larger apparent
electrostatic substituent effects from the ArH ring required to
observe ArF−ArH interactions consistently.
ACKNOWLEDGMENTS
■
The authors thank the U.S. Department of Energy, Basic
Energy Sciences (DE-SC0016423), for generous support of this
research. Data for X-ray crystal structures were obtained on
instrumentation supported by the National Science Foundation
(CHE-1229426).
Many of the efforts and successes in the field of conjugated
materials have originated from designing, tuning, and
optimizing the connectivity of only those atoms and groups
that are part of the π-conjugated portions of small molecules
and polymers, while side chains are often included only as a
necessity to enable solubility. This work highlights that discrete,
rationally designed non-covalent interactions of non-conjugated
substituents can have dramatic effects on solid-state properties
by dictating conformations and interchain packing of
conjugated materials. It is currently not clear, however, to
what extent these design principles will extend to other classes
of conjugated systems. Ongoing topics of research under
consideration in our laboratory involve the potential to harness
the local nature of substituent effects through regiochemistry of
substitution, the extent to which such interactions between side
chains and main chains persist as the number of repeating units
along the conjugated backbone increases, and the role, if any,
that the strength of these interactions has in the sensitivity of
optical properties to applied mechanical force.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b00878.
Full experimental section including detailed descriptions
of synthesis, NMR spectra, optical spectra, and
descriptions of crystallography (PDF)
X-ray crystallographic data for NMe2-F5 (CIF)
X-ray crystallographic data for CO2Me-F5 (violet
fluorescence) (CIF)
X-ray crystallographic data for CO2Me-F5 (blue
fluorescence) (CIF)
X-ray crystallographic data for CF3-F5 (CIF)
X-ray crystallographic data for NO2-F5 (CIF)
X-ray crystallographic data for NMe2-F3 (CIF)
X-ray crystallographic data for OMe-F3 (CIF)
X-ray crystallographic data for H-F3 (CIF)
X-ray crystallographic data for CO2Me-F3 (CIF)
X-ray crystallographic data for CF3-F3 (violet fluores-
cence) (CIF)
X-ray crystallographic data for CF3-F3 (blue fluores-
cence) (CIF)
X-ray crystallographic data for CN-F3 (CIF)
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AUTHOR INFORMATION
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Corresponding Author
*sam.thomas@tufts.edu
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Samuel W. Thomas III: 0000-0002-0811-9781
Notes
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