Reversible mechanofluorochromism of aniline-terminated phenylene ethynylenes

Article abstract of DOI:10.1039/c8sc00980e

Seven three-ring phenylene-ethynylene (PE) structural analogs, differing only in the lengths of alkyl chains on terminal aniline substituents, show 50-62 nm bathochromic shifts in emission maxima in response to mechanical force (mechanofluorochromism, MC)

Full text of DOI:10.1039/c8sc00980e

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Reversible Mechanofluorochromism of Aniline-Terminated Phenylene Ethynylenes
Seth A. Sharber,a Kuo-Chih Shih,b Arielle Mann,a Fanny Frausto,a Terry E. Haas,a Mu-Ping
Niehb, Samuel W. Thomas III*a
Received 00th January 20xx,
Accepted 00th January 20xx
Seven three-ring phenylene-ethynylene (PE) structural analogs, differing only in the lengths of alkyl
chains on terminal aniline substituents, show 40-55 nm bathochromic shifts in emission maxima in
response to mechanical force (mechanofluorochromism, MC). These shifts are fully reversible with
heat or solvent fuming. Shearing of these solids yields a transition from green-emitting crystalline
phases to orange-emitting amorphous phases as established by differential scanning calorimetry and
X-ray diffraction. Molecules with shorter alkyl chain lengths required higher temperatures to recover
the hypsochromically shifted crystalline phases after grinding, while the recovery with chain lengths
longer than butyl occurred at room temperature. In addition to this structure-dependent
thermochromism, these compounds retain their MC properties in polymer hosts to various extents.
The crystalline phases of these materials have PE chromophores that are twisted due to non-
covalent perfluoroarene-arene (ArF-ArH) interactions involving perfluorophenyl pendants and the
terminal rings of the PE chromophore, resulting in interrupted conjugation and an absence of
chromophore aggregation. The MC behavior of an analog without the perfluoroarene rings is
severely attenuated. This work demonstrates the general utility of twisted PEs as stimuli-responsive
moieties and reveals clear structure-property relationships regarding the effects of alkyl chain
lengths of these materials.
DOI: 10.1039/x0xx00000x
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is currently expanding the utility of these materials.
Introduction
While mechanisms of MC transitions are diverse, the
general phenomenon depends upon molecular assemblies
This paper describes a series of dialkylaniline-based PEs
that display reversible mechanofluorochromism with alkyl
chain length dependent properties. Mechanofluorochromic
(MC) materials, which show changes in the intensity and/or
wavelengths of luminescence in response to mechanical force,
have experienced extensive development in the last decade.1
This response to mechanical force is often reversible with
application of light, heat, or exposure to solvent vapor
(fuming). MC materials have shown promise for applications
in pressure sensing,2,3 including in biologically relevant
applications and wearable technology.4–6 Other applications of
MC materials have centered on molecular electronics, optical
data storage, secure encoding of information, and logic
gates.7–11 Development of MC responsive solids beyond small
molecule assemblies into polymers12 and functional materials
converting between sufficiently dissimilar states, each with
differing conformations and/or intermolecular interactions
that yield unique optical properties. Including competitive
non-covalent interactions that stabilize differing states,13 such
as weak aromatic interactions that prefer dissimilar crystal
phases,14 or hydrogen-bonded assemblies that can be
disrupted with mechanical force,15 is a common strategy for
designing MC behavior. Structure-property relationships often
provide mechanistic insight for MC materials; these studies
include how chromophore structure impacts the force-induced
changes in luminescence efficiency, the magnitude and
,
direction of spectral shift,14 16–24 as well as the sensitivity to
mechanical force.25–27 Though alkyl side chains are not
traditionally considered key to the optoelectronics of
conjugated materials, the concept of side chain engineering
has become increasingly popular for the optimization of
optoelectronic devices in recent years.28,29 In addition, their
structures can influence MC behavior30–32 through
perturbations of the differences in energies between
polymorphs.33–35 In some thermally reversible MC materials,
the lengths of alkyl substituents can tune the cold-

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interactions, molecular packing, and optical properties to small
structural changes can also render these solids responsive to
mechanical force. As an example, we reported the MC
behavior of one such compound, an octyloxy-substituted PE
with
absorbance
and
fluorescence
that
shifted
bathochromically (from blue to green emissive) upon
application of mechanical force.64
Herein we describe a series of seven 3-ring PE analogs,
each with different lengths of alkyl chains on the nitrogen
atoms of terminal aniline rings. These compounds show
reversible, force-induced bathochromic shifting from green to
orange emission. The objectives of this work are the following:
i) Develop a series of PE-based MC materials with emission
spectra red-shifted to those we reported previously, ii)
determine how the length of the alkyl substituents impact the
force-responsive spectroscopy and thermal reversibility of the
MC transitions, iii) clarify the nature of the polymorphs
accessed before and after MC transition of these materials,
and iv) determine the extent to which polymer films can be
viable hosts for these materials while retaining MC activity.
These results are important for the development of new MC
compounds that respond to different temperatures, improving
our understanding of MC luminogens, and extending the
viability of MC function into polymeric materials.
Fig. 1. Chemical structure and example mechanofluorochromic response of aniline
terminated three-ring phenylene-ethynylenes.
crystallization temperature (Tcc, or the heat-recovery
temperature) for reversion from the force-induced metastable
state; an example of this trend has been observed in cruciform
divinylanthracenes with alkyl substituents.36,37 The ability to
tune sensitivity to external stimulus through straightforward
structural modification is valuable for practical applications of
MC molecular assemblies, as shown in examples of
thermochromic devices.38–40
As a broad category of chromophores, the optoelectronic
properties of PEs can be highly sensitive to perturbation,
making them attractive candidates for stimuli-responsive
materials. PEs generally have small energy barriers for
rotation, while the extent of coplanarity of rings along PEs has
a large influence on the energies of molecular orbitals and
radiative transitions.41,42 There have been numerous creative
examples of controlling the coplanarity (or lack thereof) of PEs,
and thereby their optical properties, through covalent bonding
or non-covalent interactions.43–54 Moreover, the propensity of
solid PEs to aggregate and show bathochromic shifts and
quenched luminescence presents additional opportunities and
challenges for rational design of chemical structure to optimize
PE-based solid-state fluorophores.44,51,55–63 Our group has
reported a class of three-ring PEs in which fluorinated aromatic
side chains interact cofacially with non-fluorinated terminal
rings of the PE chromophore (ArF-ArH interactions), resulting
in large inter-ring torsional angles of 63 – 88ᵒ (Fig. 1) along the
PE backbone.64,65 This structural motif both interrupts
conjugation along the PE and prevents intermolecular
aggregation, yielding solids that have absorbance and
fluorescence spectra either hypsochromically shifted from or
similar to those in dilute solution. In contrast, solids of
compounds lacking these side chain/main chain ArF-ArH
interactions have planarized and/or aggregated PE
chromophores, with absorbance and fluorescence spectra
bathochromically shifted from solution. Our work has also
enabled rational design of these solid-state properties: tuning
the energy of the ArF-ArH interaction through the electronic
effects of substituents, as well as integrating other competing
non-covalent interactions such as hydrogen bonds, can dictate
the solid-state packing structure and the consequent optical
properties. This sensitivity of the balance of non-covalent
Results and Discussion
Scheme 1 shows the series of seven molecules that we
prepared and studied in this work. These three-ring PEs have
the same general structural features: a central perfluorobenzyl
terephthalate ring flanked by substituted arylacetylenes. We
chose p-dialkylaniline terminal rings for this study because the
strong electron donating effect of the amine substituent is
expected to yield particularly strong cofacial stacking
interactions with the perfluorobenzene side chains, which
dictate the twisted conformation of PEs from the perspective
of electrostatic considerations. The correlation of traditional
Hammett-type substituent effects with ArF-ArH interaction
strength have been established in both experimental and
theoretical studies comprising monosubstitued ArH rings.65–69
Scheme 1. Syntheses of aniline-substituted PEs A-R that vary in alkyl chain
lengths.
The series of bis(dialkylaniline) PEs discussed here vary in the
lengths of the N-alkyl chains, ranging from as short as methyl
to as long as n-octadecyl.
The syntheses of these molecules, A-R, in which “R”
designates the length of the alkyl substituents, was
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straightforward (Scheme 1). Dialkylation of 4-iodoaniline While the emission intensity varies among the compounds, a
under basic conditions yielded 1-R, except for 1-1, which is representative sample of the series (A-2, A-4, A-8, A-18
commercially available. Sonogashira coupling with consistently shows turn-on fluorescence at or above 60%
trimethylsilylacetylene and subsequent deprotection gave the water fraction (fw) with roughly a three-fold increase of
terminal arylacetylenes 2-R A final double Sonogashira intensity relative to pure THF solution and gradual
coupling between each of 2-R and the previously described hypsochromic shift in emission spectra with increasing water
2,5-diiodoterephthalate 364 yielded the target compounds A-R
fraction (38 nm shift in λmax from 0% to 90%, Fig. S3). We
each of which were purified by recrystallization.64 A control attribute the observed AIE effect to
combination of
compound bearing non-fluorinated benzyl ester pendants and reduction in the local polarity of nanoprecipitated
)
.
,
,65
a
therefore lacking the ArF-ArH interaction, A-1F0
analogously synthesized from non-fluorinated
diiodoterephthalate precursor 3-F0 29
,
was fluorophores and the prevention of cofacial interactions
2,5- between PE chromophore units by ArF-ArH interactions (vide
infra).
.
Table 1. Summary of optical spectra of all compounds in dilute chloroform solution.
Photophysical properties in solution and solids
Abs. λmax (nm)
Em. λmax (nm)
575
ΦF
τ
(ns)
1.6
1.7
1.6
1.7
1.7
1.7
1.7
2.7
Photophysical properties of the target compounds in
solution are similar throughout the series of compounds (Fig.
2). Absorbance and emission maxima of yellow to orange
emitting chloroform solutions show lower energy optical
spectra with alkyl chain lengths longer than methyl due to
A-1
A-2
A-3
A-4
436
448
450
450
452
455
453
432
0.21
0.19
0.20
0.23
0.19
0.20
0.23
0.41
585
585
588
increased donor character of the terminal aniline substituents A-6
(Fig. S1). All compounds display positive solvatochromism, A-8
which is consistent with the donor-acceptor nature of these PE A-18
chromophores, with emission maxima varying up to 122 nm A-1F0
from hexane (λmax = 502 nm for A-4) to dichloromethane
590
590
588
562
As powders or drop-cast films, compounds A-R are yellow
(DCM) solutions (λmax = 624 nm for A-4). Fluorescence is with green fluorescence, with absorbance maxima ranging
noticeably quenched in higher polarity solvents: for A-4 we from 388-434 nm and emission maxima from 510-550 nm after
observe a quantum yield of 0.72 and lifetime of 2.7 ns in heating to 100-220 ᵒC to remove solvent and allow relaxation
hexane, which decrease to 0.23 and 1.7 ns in chloroform, and toward a more thermodynamically stable phase. This initial
drop sharply to 0.03 and 0.4 ns in orange emitting DCM annealing has only modest effects on the absorbance and
solution. This quenching increases with ET(30) values of the fluorescence of drop-cast films, which have uniform green
solvents, and indicates that non-radiative decay pathways emission. The rapidly deposited films resulting from spin
become dominant in polar solvents (Fig. S2). While kr drops casting, however, are green emissive, yellow/orange emissive,
four-fold between hexane and ethyl acetate, knr increases over or both. Subsequent thermal annealing hypsochromically
thirty-fold. This coincidence of decreasing is consistent with shifts this emission to uniform green color, similar to those
the “positive solvatokinetic effect,”70 of donor-acceptor type observed in drop-cast films. While emission maxima blue-shift
compounds, and demonstrates intramolecular charge transfer somewhat with alkyl chains longer than methyl and ethyl (up
(ICT) in the excited state, in which an increase in dipole to 40 nm shift between A-1 and A-18), minor changes in the
moment of the excited state relative to the ground state with
solvent polarity enhances non-radiative decay.71–76 Further,
position of thin film absorbance and emission spectra between
compounds do not correlate with increasing alkyl chain length.
The luminescence of these films is most akin to compounds
dissolved in toluene, and the spectra are hypsochromically
shifted relative to chloroform solutions by 15-50 nm in
while quenched in pure tetrahydrofuran (THF) solution (ΦF
0.03 for A-4, 0.04 for A-18), these materials show aggregation-
=
induced enhanced emission (AIEE)77–80 in THF/water mixtures,
13,20,81
which often occurs in force responsive materials (Fig. 2).
absorbance λmax and 25-50 nm in emission λmax
.
As
Fig. 2. Emission of A-R compounds in chloroform solution, λex = 450 nm (left) and emission of A-4 in various solvents, λex = 440 nm (center). Right: Fluorescence intensity at
600 nm of A-4 in THF/water mixtures.
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green emitting crystals of A-2, A-4, and A-8 have twisted
crystal structures (Fig. 5) similar to the previously reported A-
1, with ArF-ArH cofacial interactions as the central features.
These interactions occur both intramolecularly and
intermolecularly, resulting in ArF-ArH stacks that propagate
along infinite columns with centroid—centroid distances of 3.6
- 4.0 Å. These ArF-ArH interactions, combined with the
tendency of the ester functional groups to remain coplanar
with the central phenylene ring, result in heavily twisted PE
backbones. The inter-ring torsional angles along the PE
chromophores are between 63.5ᵒ and 88.2ᵒ. The alkyl chains
in structures of A-2, A-4, and A-8 extend in a roughly
perpendicular trajectory from planes defined by the aniline
rings of the PE backbones, which likely maintains close packing
between neighboring stacks of the twisted PEs, as opposed to
the possibility of alkyl chains extending parallel to the aniline
rings, which may disrupt close packing.
Fig. 3. Emission spectra of annealed thin films and single crystals for A-2, A-4,
Notably, the number of intermolecular aromatic
interactions between neighboring PEs decreases with alkyl
chain length. The structure for A-1 shows C-H---π and C-F---π
interactions between neighboring ArF rings and terminal
anilines to terephthalates (2.9 Å closest contacts). These
interactions are mostly conserved in the structure for A-2,
though the number of close contacts between stacks is
reduced (3.0 Å closest contacts). Packing in A-4 changes
significantly; these C-H---π and C-F---π interactions are lost as
neighboring PEs show a 7 Å pitch displacement relative to A-1
and A-2 (Fig. 5 and Fig. S4). As the alkyl chains occupy a
greater volume in the structure, the ArF rings and anilines are
further away from neighboring terephthalates. Only F---F
interactions are present at 2.9 Å in A-4. A dramatic change in
seen in A-8, where there are no close contacts between
neighboring chromophores, and a greater portion of the
volume of the unit cell is occupied only by the octyl chains.
This inhibition of close packing is accompanied by significant
disorder in the alkyl chains of A-4 and A-8 (whole molecule
A-8 and A-1F0. λex = 430 nm for A-2, A-4, and A-1F0. λex = 420 nm for A-8.
determined in our previous work, this lack of a significant red-
shift for this class of PE solids indicates solid-state packing
environments in which the conjugated chromophores lack two
features common in other PE-based materials: i) highly
coplanar aryl rings along the PE backbones and ii)
intermolecular aggregation of PE chromophores.
The
combination of intramolecular and intermolecular ArF-ArH
interactions between the perfluorobenzyl pendants on the
central terepthalate ring and the terminal rings of the PE
backbone both enforce these twisted conformations and
prevent aggregation between the PE chromophores. For
reference, compounds we reported previously that show the
twisted, non-aggregated arrangement in crystal structures
showed modest solution-to-solid hypsochromic shifts relative
to chloroform solutions either in absorbance and/or emission
(15-25 nm in absorbance λmax and 5-20 nm in emission λmax).
In contrast, analogs that showed coplanar and/or aggregated
PE chromophores in their crystal structures showed significant
bathochromic shifts as solids (14 – 42 nm in absorbance λmax
and 60 - 128 nm in emission λmax).65 Only A-1F0, which lacks
fluorinated pendants, demonstrates this bathochromic shift in
recrystallized and annealed solids, and never shows the
hypsochromically shifted green emission.
Crystallography
Crystal structures obtained for compounds A-2, A-4, A-8,
and A-1F0 and provide key insight for the stimuli-responsive
behavior of this series of molecules. As with the annealed
films, single crystals of these compounds appeared as yellow
needles with green fluorescence. Emission spectra of the
single crystals resemble those of the films, as seen in Fig. 3
with only modest shifts between film and crystal emission λmax
(533 vs 530 nm in A-2, 535 vs 556 nm in A-4, 530 vs 526 nm in
A-8, and 597 vs 625 nm in A-1F0). Similar correlations
between excitation spectra and onset wavelengths exist. The
Fig. 4. Crystal structure of A-1F0 viewed along a) coplanar PE backbone b) b-
axis of unit cell. Hydrogen atoms omitted for clarity.
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Fig. 5. a)-d) Crystal structures of A-1, A-2, A-4, A-8, with highlighted close contacts between molecules in neighboring columns dominated by intra and intermolecular ArF-ArH
stacked arenes. e)-f) View down the a-axis of A-1 and A-2. g) Perspective is replicated in A-4. h) Unit cell of A-8 viewed down the b-axis. Hydrogen atoms and disordered carbon
atoms in alkyl chains (A-4, A-8) omitted for clarity.
disorder in A-4), coinciding with significant difficulty in could be obtained supports the conclusion that molecular
obtaining single crystals of these compounds.
These packing in thin films favors the twisted, non-aggregated motif
differences in crystal packing have important consequences for seen in crystal structures. Powder X-ray diffraction (PXRD)
the variation of MC properties (vide infra). The correlation of studies of drop-cast films also supports this conclusion. In
optical spectra and crystal structures in this series of contrast, an orange emitting crystal of A-1F0 (Fig. 4) shows
compounds support all aniline-terminated 3-ring PEs A1-A18 aggregated PE chromophores with coplanar arenes.
having twisted PE backbone without aggregation. This relative
similarity in fluorescence in all cases where crystal structures Reversible Mechanofluorochromism
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Fig. 6. A-1 Characteristic reversible MC behaviour of films drop-cast from chloroform: absorbance (left), emission (center) spectra after annealing, grinding, second
heating, and fuming with solvent vapor, λex = 430 nm. Plot of emission maxima over five heat-grinding cycles (right).
These seven compounds all show easily noticeable not observable whatsoever for A-18 at room temperature, but
bathochromic shifts in both color (yellow to orange, 37-50 nm only momentarily ca. -80 ᵒC by cooling over a dry ice/acetone
shift in absorbance λmax) and fluorescence (green to orange, bath. Therefore, the recorded emission maxima for sheared
50-62 nm shift in emission λmax) in response to mechanical films of A-6 and A-8 do not reach ~600 nm, as do those of A1
-
force (Fig. 6 and Fig. S5-S6). This bathochromic shift is A4, instead showing smaller bathochromic shifts to 550 nm.
consistent with planarization/aggregation of at least some of An interesting feature of this transient response is that films of
the PE chromophores.56,57 Though this transition can be A-8 recover their green emission immediately after grinding,
achieved to some degree with hydrostatic pressure (15,000 and may be sheared continuously with no loss of MC behavior,
psi), powders and films are far more sensitive to shear force with a single film maintaining its activity over the course of at
such as rubbing with
a
spatula.
The extent of the least a year.
Differential scanning calorimetry (DSC) of these molecules
bathochromic shift in optical spectra is similar across the
compounds with modest differences primarily observed in the also demonstrates the effect of alkyl chain length on recovery
emission of the annealed films. This response is fully from the metastable polymorph created upon grinding. After
reversible upon thermal annealing or with solvent vapor thermal annealing, no transitions are visible for these
annealing and does not fatigue over multiple cycles of heating compounds between ambient temperature and their melting
and grinding for any of the target compounds (Fig. 6 and Fig. points. Ground powders, however, display broad exothermic
S7-S8). The films become increasingly waxy with decreased transitions spanning roughly 15-30 ᵒC that are not visible in
melting points with longer alkyl chains (282 ᵒC for A-1 to 76 ᵒC subsequent heating cycles (Fig. 7). Such transitions of
for A-18), and are more easily deformed by rubbing.
metastable polymorphs are often indicative of cold-
The persistence of metastable orange emission from crystallization from an amorphous to crystalline morphology,
ground films varies greatly across the series. For compounds suggesting crystalline-to-amorphous MC transitions upon
with alkyl chains longer than propyl, the orange emission of grinding.33,36,82–97 The Tcc ranges decrease with alkyl chain
ground films relaxes to yellow at room temperature within length from A-1 to A-4, with peaks of 57 ᵒC for A-1 to 32 ᵒC for
minutes. After grinding, the original fluorescence of A-4 mostly A-4 (Table 2). For those molecules with longer alkyl chains, the
recovers at room temperature within 15 minutes. The ground transient nature of the ground phase at room temperature
phase becomes increasingly transient for A-6 and A-8 where yields featureless thermograms (Fig. S9) with the exception of
the orange color may be seen on the timescale of 1 s, and is A-18, which shows an endotherm at 50-60 ᵒC in the first heat
Table 2. Summary of A-R photophyical and thermal data
Thin films Abs, λmax (nm)
Thin films Em. λmax (nm)
Annealed
400
Ground
450
Shift
Annealed
Ground
605
Shift
Tcc max (ᵒC)
57
Tm (ᵒC)
A-1
50
550
55
282 (dec)
A-2
420
412
420
415
424
400
456
455
456
457
417
424
400
455
35
44
37
2
533
530
540
520
530
510
597
595
589
590
550
550
510
602
62
59
50
30
20
0
54
44
32
–
256 (dec)
204 - 206
149 - 151
150 - 151
107 - 108
75 - 76
A-3
A-4
A-6
A-8
0
–
A-18
A-1F0
0
–
-1
5
–
201 - 203
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Fig. 7. (Left) Images of fluorescent drop-cast films of A1 – A4 after grinding while illuminating with a hand-held UV lamp (left). Films were heated at 5 ᵒC / min
and photographs taken for each progressive initialization of recovery toward green emission. (Center) DSC traces of ground powders (red) and pristine powders
(black) (Exo up). Ground powder curves rotated for clarity. (Right) Graph of Tcc maximum for A-R plotted with emission intensity ratio of 530/600 nm after
heating ground powders at 100 ᵒC for 15 minutes.
for both annealed and ground powders, and then multiple Structural analysis of MC transition
reversible endothermic transitions in the second and third
The results of PXRD experiments are consistent with a
heating cycles (Fig. S10). Such transitions have been observed
reversible crystalline-to-amorphous transition upon grinding,
in our lab in related compounds with long alkyl chains, and will
which explains the nature of MC behavior in this system.
be the subject of future study.
Patterns were collected for films over four sequential steps i)
In addition, heating through these exothermic transitions
pristine ii) ground iii) thermally annealed iv) solvent vapor
of A-1 to A-4 recovers most of the fluorescence of the
annealed (fumed) after thermal annealing. X-ray diffraction of
annealed films, while heating to higher temperatures below
pristine and annealed films agree with simulated powder
the melting points completely recovers the original emission
patterns from single crystal structures for A-1, A-2
, A-4, A-8,
spectrum.
photoluminescence of films of these compounds at increasing
temperatures and the gradual recovery process over
temperature range. Emission spectra show the same
Fig. 7 shows the visual response of the
and A-1F0 with strong diffraction peaks at 2θ = 15.7, 14.9,
13.7, 12.8, and 24.8ᵒ, respectively. However, grinding of the
films results in significantly reduced peak intensity or
featureless PXRD patterns (Fig. 8). The intensities of the
crystalline peaks recover after thermal and solvent annealing,
which in conjunction with the exothermic transitions observed
in DSC during thermal recovery, make evident that mechanical
a
behavior; emission λmax of A-1 recovers from 605 nm to 567
nm when heated to 100 ᵒC for 15 minutes after grinding, and
heating at 220 ᵒC results in complete reversion of the λmax to
550 nm. Similar results are seen in A-2, A-3, and A-4.
force causes
a
crystalline-to-amorphous transition.
Furthermore, Fig. 7 shows a gradient of recovered emission
during thermal recovery where compounds with longer alkyl
chain lengths show hypsochromically-shifted emission relative
Subsequent heating through the Tcc range of the ground films
shows some restoration of the crystalline patterns. The peaks
of annealed and fumed films are significantly less intense than
in the pristine film due to disruption of the crystal structure
and reduction of film thickness during the grinding process,
but comparison with the patterns of mechanically ground films
demonstrate clear amorphous-to-crystalline recovery with
annealing or vapor fuming.
to those with shorter alkyl chain lengths at
a given
temperature. For example, in the 50 ᵒC image in Fig. 7, A-4
emission is the most blue-shifted and A-1 emission is the most
red-shifted.
This gradient in recovered emission is
reproducible when heating at a fixed temperature. Annealing
at 100 ᵒC, A-4 achieves full recovery to emission λmax of 545
nm, A-3 recovers to 552 nm, A-2 557 nm, and A-1 to 567 nm;
this behavior persists over 5 heat-grinding cycles (Fig. S8). In
conjunction with decreasing Tcc, the temperatures required for
post-grinding thermochromic recovery decreases with alkyl
chain length. Other families of mechanochromic compounds
Increasing alkyl chain lengths leads to important
differences between the pristine patterns. Compounds A-1
, A-
2, and A-1F0 show greater crystallinity relative to samples with
longer alkyl chain lengths. We see greater amorphous
character (semi-crystalline material) in the pristine patterns of
A-4, A-6, A-8, and an almost entirely amorphous pattern in A-
have
shown
a
similar
structure-property
18 (Fig. S11). The increase in the amorphous nature of the
films presumably originates from the weakening of
intermolecular interactions across adjacent planes of PEs,
particularly in structures for A-4 and A-8. Control compound
relationship.36,37,85,86,89,90,96,98
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A-1F0 also shows
a reversible crystalline-to-amorphous amorphous pattern of A-18. This loss of diffraction peaks
transition with grinding. However, there is no change in the indicates that increasing alkyl chain length decreases
luminescence of the solids with grinding. The bathochromically crystallinity of powder samples to the extent that, while
shifted emission in both the crystalline and amorphous phases pristine films always display green emission corresponding to
of A-1F0, relative to the fluorinated compounds, is consistent twisted, non-aggregated PEs, the long-range order of parallel
with our model for the MC transition in this system, in which columns of infinitely stacked ArF-ArH units is lost. In
initially twisted and non-aggregated PEs become increasingly agreement with photophysical data, these stacks of twisted
planarized and aggregated upon application of mechanical PEs are disrupted with grinding and restored with
force.33
annealing/fuming. Additionally, this peak shifts to smaller 2θ
Comparing the experimental and calculated patterns, it is (corresponding to greater d-spacing) with increasing alkyl
possible to identify reflection planes and corresponding d- chain length, from 5.6 Å in A-1 to 6.9 Å in A-8 (Fig. 8), which is
spacings from the crystal structures that are present in films. consistent with looser molecular packing with longer chains
Importantly, a plane of reflections lying between columns of that occupy more volume and result in elongated inter-
twisted PEs gives rise to the most intense diffraction peak 2
θ
15.7, 14.9, 13.7, and 12.8ᵒ in A-1, A-2, A-4, and A-8
=
chromophore distances. A peak at 2θ = 14.9ᵒ, corresponding
to plane of reflections lying between parallel columns of
,
respectively. This peak is seen in all pristine patterns except coplanar, aggregated PEs is also present in the A-1F0
for A-18 (Fig. S11) and is the only peak that is recovered with diffraction pattern. It should be noted that in the case of A-
significant intensity among all samples in post-grinding 1F0 the peak at 2θ = 24.8ᵒ, corresponding to coplanar
annealed films. This conserved peak corresponds to the well- backbone, which contributes to the bathochromically-shifted
ordered nature of twisted PE columns formed by infinite ArF- emission in solids, is the most intense peak after recovery,
ArH stacks (Fig. 8). The relative intensity of this peak whereas the peak 2θ = 14.9ᵒ is minimally recovered. The fact
decreases with alkyl chain length in conjunction with the that the peak corresponding to ordered columns of PEs is
decreasing crystallinity of film samples, which coincides with significantly recovered in all cases from A-1 through A-8, but
general trends in material properties for these compounds not in A-1F0, suggests the ArF-ArH interactions are essential
(vide infra). The increasingly amorphous nature of the films is for such structural restoration.
observed in the broad amorphous peak from 2
θ
appearing in pristine samples of A-6 and A-8, and the highly with PXRD data, provide an explanation for the decrease in Tcc
= 11-15ᵒ
The crystal structures of A-1, A-2, A-4, and A-8, confirmed
Fig. 8. PXRD patterns of pristine, ground, annealed, and fumed films of A-1, A-2, A-3, A-4, and A-1F0. (Lower right) Plot of alkyl chain length vs d-spacing for conserved peak
2θ = 12.5 - 15.7ᵒ for A-1, A-2, A-4, and A-8, corresponding in each case to the plane of reflections lying between columns of twisted PEs.
8 | J. Name., 2012, 00, 1-3
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Fig. 9. Emission spectra of A-4 in various polymethacrylate hosts a) annealed (left) and ground (center), λex = 430 nm. Plot of emission maximum over five heat-grinding
cycles of A-4 and A-4 20% (w/w) in PIBMA, PHMA (right).
with increasing alkyl chain length. As alkyl chain length doped with A-18 was persistent at room temperature, which
increases, intermolecular aromatic-π interactions are reduced cannot be observed in pure A-18 films.
(vide supra), and alkyl chains, with propensity for disorder,
While maintaining the same conditions for preparation, MC
take up greater volume fractions in the structures, resulting in response of these films can be maintained in other polymer
greater d-spacing between columns of PEs. These trends hosts (Fig. 9 & Fig. S12), though the optical shift in emission
agree with the increasingly amorphous character of pristine λmax is considerably reduced. Comparison of MC properties for
films (Fig. 8 and Fig. S11), as well as the observation of methacrylate films doped at 20% (w/w) with A-4 across PMMA
decreasing material rigidity among compounds with longer (Tg = 99 ᵒC), poly(butyl methacrylate) (PBMA, Tg = 65 ᵒC),
alkyl chains, the powders of which are more easily deformed poly(isobutyl methacrylate) (PIBMA, Tg = 20 ᵒC), and poly(hexyl
while grinding. Further, these structural parameters coincide methacrylate) (PHMA, Tg = -5 ᵒC) hosts showed a dependence
with decreasing melting points as alkyl chain length increases of the optical shift, sensitivity, and reversibility, on the host.
(282, 256, 149, and 107 ᵒC for A-1, A-2, A-4, and A-8 Annealed PMMA films showed both green and orange emitting
respectively). We propose the decrease in Tcc may originate domains. This polymorphism persisted in PBMA and PIBMA
from a complementary weakening of interactions in the films, though green emission dominated in the PIBMA film,
amorphous phase and inhibition of close packing, which and PHMA host yielded uniform green emission. Sensitivity to
results in easier deformation of the supramolecular structures mechanical force was notably higher in PHMA films than the
and more facile thermal restoration of the crystalline phase other hosts, and the MC transition could be achieved without
with increasing chain length. This is consistent with reported breaking or deforming the film. The annealed films showed a
examples of mechanofluorochromic crystalline-to-amorphous gradual hypsochromic shift in emission with decreasing Tg of
transitions where Tcc decreases with increasing alkyl chain polymer host. At the same time, the MC bathochromic shift in
length due to weaker intermolecular interactions between emission decreased (Fig. 9). Furthermore, green emission in
chromophores.85,89,90,98
the PMMA and PBMA films could not be recovered by
annealing; only the lower Tg hosts PIBMA and PHMA showed
reversible MC response over multiple heating-grinding cycles,
Polymer hosts
In the interest of developing MC functional coatings and though the optical shifts in emission maxima are muted
sensors, crystalline molecular assemblies are typically not relative to A-4 films. This variable response across the hosts
practical for device fabrication due to their poor mechanical indicates that thermal recovery from planarized, aggregated
properties. To address this challenge, MC polymers have been assemblies to the twisted, non-aggregated state depends on
reported,12,99–101 some with quantitation of mechanical the relative mobility of assemblies. In a rigid host such a
stress.102 Aside from preparation of homopolymers and PMMA with high Tg, the original fluorescence may not be
supramolecular polymers, other reports have demonstrated recovered after grinding even though A-4 normally recovers its
the utility of MC luminogens incorporated into polymer green fluorescence within minutes at room temperature. In
matrices.26,103,104 Considering the potential for incorporating contrast, hosts well above the Tg at ambient temperature such
the side-chain controlled PE design into functional polymers, as PHMA allows for facile molecular rearrangement, which in
we have assessed the MC behavior of the target compounds turn diminishes the extent of the bathochromic shift. These
doped into various polymethacrylate hosts that differ in glass results not only demonstrate the MC capability of these PEs in
transition temperature (Tg). We found that poly(methyl polymer scaffolds, but reveal a simple means for tuning MC
methacrylate) (PMMA) films doped with 20% (w/w) with A-R behavior.
and drop-cast from chloroform showed reliable MC responses
in all cases with spectral shifts comparable to the pure PE
Conclusions
films. Intriguingly, orange emission after grinding in films
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Seven structurally analogous three-ring PEs with The authors thank Dr. Daniela Morales, X-Ray Diffraction Lab
dialkylaniline terminal substituents show MC behavior that is at University of Connecticut Institute of Materials Science for
fully reversible upon thermal or solvent vapor annealing. support with PXRD measurements.
Mechanical force causes a crystalline-to-amorphous transition
as shown by DSC and PXRD, during which the fluorescence
bathochromically shifts from green to orange as PE
Notes and references
chromophores rearrange from twisted, non-aggregated
assemblies as seen in single crystal structures to increasingly
planarized and/or aggregated states in the amorphous phase.
The MC behavior is lost in the absence of the ArF-ArH
interaction, which dictates the optical properties and crystal
packing of these molecules. The transition is fully reversible
with heat or solvent vapor annealing, and the temperature
required for thermal recovery of the crystalline phase from the
ground phase decreases as the lengths of alkyl chains on the
aniline substituents increase. This trend coincides with
weakening intermolecular interactions in the crystal
structures, allowing for greater molecular mobility of the
chromophores. The MC properties of these compounds
translate to polymer hosts, and can be tuned simply by
changing the environment of the PE through choice of the
polymer. Varying the length of alkyl substituents can be an
effective method for tuning thermal recovery and material
properties in small molecule MC luminogens that show
crystalline-to-amorphous transitions.
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Chemical Science
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DOI: 10.1039/C8SC00980E
TOC Text:
Alkyl chain length tunes the reversion temperature of mechanofluorochromic
phenylene-ethynylenes that show reversible force-induced change of fluorescence
from green to orange.

Chemical Science
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DOI: 10.1039/C8SC00980E
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