Directed polymorphism and mechanofluorochromism of conjugated materials through weak non-covalent control

Article abstract of DOI:10.1039/c9tc01301f

Understanding and manipulating crystal polymorphism can provide novel strategies for materials discovery in organic optoelectronics. In this paper, a series of seven ester-terminated three-ring phenylene ethynylenes (PEs) exhibit structure-dependent polymorphism wherein alkyl chain length modulates the propensity to form violet or green fluorescent solid phases, as well as tunable thermal and mechanofluorochromic (MFC) transitions. These compounds harness "soft" non-covalent control to achieve polymorphism: the electronic substituent effect of the ester groups weakens the fluoroarene-arene (ArF-ArH) interactions that typically direct crystal packing of this class of compounds, increasing competitiveness of other interactions. Small structural modifications tip this balance and shift the prevalence of violet- or green-emitting polymorphs. Compounds with short alkyl chain lengths show both violet and various green fluorescent polymorphs, while the violet fluorescent form dominates with alkyl lengths longer than butyl. Further, thermally induced green-to-violet fluorescent crystal-to-crystal transitions occur for single crystals of two derivatives. Finally, the PEs show reversible violet-to-green mechanofluorochromism (MFC), with temperature required for reversion of this MFC decreasing with alkyl chain length. We therefore present this design of directional but weak interactions as a strategy to access polymorphs and tunable stimuli-responsive behavior in solids.

Full text of DOI:10.1039/c9tc01301f

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Directed Polymorphism and Mechanofluorochromism of Conjugated Materials through
Weak Non-Covalent Control
Seth A. Sharbera, Arielle Manna, Kuo-Chih Shihb, William J. Mullin, Mu-Ping Niehb, Samuel W.
Thomas IIIa*
Received 00th January 20xx,
Accepted 00th January 20xx
Understanding and manipulating crystal polymorphism can provide novel strategies for materials discovery in organic
optoelectronics. In this paper, a series of seven ester-terminated three-ring phenylene ethynylenes (PEs) exhibit structure-
dependent polymorphism wherein alkyl chain length modulates the propensity to form violet or green fluorescent solid
phases, as well as tunable thermal and mechanofluorochromic (MFC) transitions. These compounds harness “soft” non-
covalent control to achieve polymorphism: the electronic substituent effect of the ester groups weakens the fluoroarene-
arene (ArF-ArH) interactions that typically direct crystal packing of this class of compounds, increasing competitiveness of
other interactions. Small structural modifications tip this balance and shift the prevalence of violet- or green-emitting
polymorphs. Compounds with short alkyl chain lengths show both violet and various green fluorescent polymorphs, while
the violet fluorescent form dominates with alkyl lengths longer than butyl. Further, thermally induced green-to-violet
fluorescent crystal-to-crystal transitions occur for single crystals of CO2-1 and CO2-3. Finally, the PEs show reversible
violet-to-green mechanofluorochromism (MFC), with temperature required for reversion of this MFC decreasing with alkyl
chain length. We therefore present this design of directional but weak interactions as a strategy to access polymorphs and
tunable stimuli-responsive behavior in solids.
DOI: 10.1039/x0xx00000x
www.rsc.org/
control over phase behavior can inform design and discovery of
new responsive materials.17–22
Introduction
The challenge of predicting conformation and molecular
assembly in the solid state makes the bottom-up design of
functional organic materials difficult.1–3 This goal is particularly
elusive in the design of responsive materials, which hold promise
for numerous applications by exhibiting phenomena such as
mechanofluorochromic (MFC), photochromic, thermochromic, and
vapochromic materials.4–6 Since access to solid phases with
different optoelectronic properties is important in many such
designs, crystal polymorphs can provide important structural
insights in the distinct properties of interconvertible phases.
Molecules that present numerous directional interactions of similar
energies can form responsive materials that switch between
packing motifs with different photophysical properties.5,7–11
Although important advances in the theoretical prediction of
polymorphs12 have emerged, their complex energetic landscapes
make rational design of polymorphs and reversibly-responsive
solids uniquely challenging, requiring extensive experimental effort
to identify, isolate, and characterize polymoprhs.10,13–16 Therefore,
strategies that enable predictable access to polymorphism and
Some common structural themes exist among compounds that
switch between polymorphs in response to stimuli, such as
conformational flexibility or torsionally distorted structures that
pack inefficiently, for example in the aggregation-induced emission
(AIE) luminogens.4,5,23–28 In addition, supramolecular strategies
using directional interactions—synthons such as hydrogen bonding,
halogen bonding, and fluoroarene-arene (ArF-ArH) interactions—
can direct packing and yield predictable assembly motifs through
development
of
structure-property
relationships.7,20,29–32
Furthermore, polymorphs can emerge when the energetics of
competing directional interactions are judiciously balanced, as in
cases of synthon polymorphism.27,29,33,34 As an example, Zou and
You reported diaryltriazolo-pyrimidines in which different
regioisomeric aryl linkages alter stacking interactions predictably
and determine whether bathochromic or hypsochromic MFC
behavior occurs.35 Other design strategies for responsive materials,
often based on AIEgens, include setting up competitions between
hydrogen bonding and aromatic stacking interactions to yield
interconvertible phases with different optical properties.36–39
Therefore, purposeful integration of competition between
directional interactions, especially those with readily tunable
strengths, can form the basis for rational design of stimuli-
responsive transitions between polymorphs.
a. Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, MA
02155, USA. E-mail: sam.thomas@tufts.edu
b. Department of Chemical & Biomolecular Engineering, University of Connecticut,
97 North Eagleville Road, Storrs, CT 06269, USA
†Electronic supplementary information (ESI) available: Experimental methods,
synthetic methods, NMR spectra, HRMS, photophysical data, DSC data, PXRD data,
crystallographic tables. CCDC 1897095 - 1897099

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Figure 1. Weak non-covalent control in ester-terminated PEs. a) Substituents in PE main chain affect the ArF-ArH stacking interaction that twists the backbone. When X = CO2Me,
aromatic stacking interactions between PEs become competitive with ArF-ArH programmed twisting, resulting in polymorphism. b) Lengths of alkyl chains alter polymorphism in
ester-terminated PEs by tipping the balance of competing interactions; short chain lengths result in multiple polymorphs, while long chain lengths favor the violet fluorescent phase.
Although discovery of MFC materials is often empirical, explicit benzoate alkyl chains. In contrast with the strong ArF-ArH
consideration of the properties of polymorphs can yield structural interactions of the aniline-terminated PEs, these compounds with
and mechanistic insight to their behavior.9,40–44 Among known MFC ArF-ArH interactions weakened by the electron-withdrawing nature
materials, electronic substituent effects and positional isomerism of the ester substituents display prevalent polymorphism. The
affect both electronics of conjugated units and crystal packing, lengths of the alkyl chains influence both thermally induced crystal-
enabling dramatic tuning of optical contrast, mechanical sensitivity, to-crystal transitions and MFC properties of these compounds.
and thermal response.41,45–49 In contrast, although alkyl pendants Evidence from fluorescence, crystallography, X-ray scattering, and
typically only modulate crystal packing, their lengths can tune MFC thermal properties of polymorphs reveals that balancing the
shifts, sensitivity to mechanical force, and thermal recovery competitions between non-covalent interactions can enable
temperature due to alterations in packing and variation of relative polymorphism as well as new responsive behavior.
stability of phases.40,50–56
Our group has reported tunable MFC properties of three-ring
PEs that harness directional, discrete interactions of pendant arenes
Results and Discussion
1. Design and Synthesis of Target Compounds
to program PE conformation and assembly, and thereby control
Our previous work shows that ArF-ArH cofacial stacking
luminescence of these solids.57-59 Fluoroarene-arene (ArF-ArH)
interactions between ArF side chains and ArH rings in PE main
interactions between non-conjugated side chains and PE main
chains twist the conjugated arenes out of coplanarity, reducing
both intramolecular and intermolecular electronic coupling
chains yield twisted backbones that stack in electronically isolated
assemblies, resulting in hypsochromically shifted optical spectra
and hypsochromically shifting luminescence relative to other
and reversible MFC behavior.57,58 We further found that electron-
structures.57–59 In agreement with previous studies that
donating substituents on the non-fluorinated ring favor these
showed a correlation between Hammett parameters and
cofacial ArF-ArH interactions and the resulting twisted geometries,
aromatic interaction strength, we found electronic substituent
while electron-withdrawing groups can weaken the ArF-ArH
interactions such that other packing motifs with bathochromically
shifted fluorescence emerge.58 In addition, we also demonstrated
alkyl chain length-dependent MFC properties of a series of aniline-
terminated derivatives of this common PE scaffold—increasing the
lengths of N-terminal alkyl chains lowered the temperatures
required for thermal recovery after grinding.59
effects on the ArH ring can dictate whether these solids exhibit
these ArF-ArH interactions.60,58 In this particular series of PEs,
methyl ester substituents represented a boundary of whether
We present here a series of electron-withdrawing ester-
terminated three-ring PE analogs that share identical perfluorinated
aromatic side chains and differ only in the length of terminal
Scheme 1. Synthesis of ester-terminated PEs with varying alkyl chain lengths.
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ArF-ArH interactions occurred or not. We previously
determined crystal structures of two polymorphs, one of
which had twisted PEs due to ArF-ArH interactions, while the
other had coplanar PEs that aggregated, and no ArF-ArH
interactions (Figure 1).
We suspected that small changes in alkyl chain length
could influence the apparently delicate balance between these
two polymorphs, and therefore prepared a series of analogs of
this compound, with varying ester alkoxy chain lengths.
Scheme 1 shows the synthesis of these compounds, which
followed reported methods:61 ethynylation of methyl 4-
iodobenzoate with trimethylsilyl acetylene followed by
simultaneous deprotection of ester and alkyne gave 4-
ethynylbenzoic acid (1). Esterification of this common
intermediate with n-alkyl alcohols yielded 2-R, Sonogashira
coupling of which with 2,5-diiodoterephthalate 3 afforded
target compounds CO2-R with alkyl groups ranging from ethyl
to n-octadecyl.59
2. Properties of PEs in Solution and Thin Films
Figure 2. (Top) Absorbance and emission spectra of CO2-R compounds in
chloroform solution ( ~ 35000 M-1cm-1 (315 nm) in all solvents). (Bottom)
Excitation and emission spectra in thermally annealed thin films, λex = 330 nm for
solution and film spectra.
All molecules in the CO2-R series show nearly identical
optical properties in solution because differences in alkyl chain
lengths do not alter the conjugated -system directly (Figure
2). Dilute samples of all CO2-R samples in chloroform appear
colorless with violet emission (Figure 2). Small bathochromic
shifts of up to 4 nm occur with increasing chain length
(absorbance λmax = 363 – 367 nm, emission λmax = 415 – 419
nm), while fluorescence quantum yields (~0.45) and lifetimes
(~1.0 ns) do not depend on alkyl chain length. In addition,
optical properties of these compounds depend only slightly on
solvent polarity: the emission λmax for CO2-1 ranges from 417
nm in DMF to 409 nm in hexane with no discernable trend,
while quantum yields decrease slightly in higher polarity
solvents (0.53 in hexane to 0.36 in DMF, Figure S1 and Table
S1).
In contrast, drop-cast films of this series of compounds
show some differences in photophysical spectra but no clear
trend based on alkyl chain length, all of which exhibit violet
fluorescence after slow evaporation under a watch glass and
heating to 100-130 °C to drive packing towards the most stable
phases.58 In all cases, colorless and uniform violet emission
after annealing showed relatively similar or hypsochromically
shifted absorbance and emission spectra relative to
chloroform solutions. These films have absorbance λmax of
342 – 355 nm and emission λmax from 385 – 414 nm,
representing hypsochromic shifts of 10-20 nm when compared
to those in chloroform solution. In contrast, the thermal and
mechanical properties of these solids vary regularly with chain
length (Table 1): melting points decrease with increasing chain
length (214 ᵒC for CO2-1 to 110 ᵒC for CO2-18). In addition,
solids with longer alkyl pendants (up until octyl) are
increasingly soft, and qualitatively more mechanically
compliant, suggesting an overall decrease in crystallinity.
Compound CO2-18 breaks this trend, and noticeably more
crystalline and brittle, qualitatively, than CO2-6 and CO2-8. We
speculate that dispersion interactions between the long
octadecyl chains of CO2-18 become increasingly important in
this solid, increasing its rigidity.
3. Polymorphism
To better understand how alkyl chain length influences
phase behavior, we first describe here polymorphs we have
isolated of the methyl ester derivative, CO2-1. As described
above, we previously reported single crystal X-ray structures of
two polymorphs of CO2-1:58 single crystals with violet emission
(Em. λmax = 410 nm), referred to here as CO2-1A, have severely
twisted backbones due to cofacial stacking between ArF side
chains and terminal ArH units (Figure 1).58 Fluorescence
spectra of CO2-1 violet emitting annealed films (Em. λmax = 405
nm) correspond well with those of these CO2-1A single
crystals, suggesting the presence of this packing motif in violet
emitting solids. Blue-green emitting prisms of CO2-1 (CO2-1B,
Favg = 2.2 ns) grown by slow evaporation of dichloromethane
layered with hexanes (10:1 vol/vol) pack with coplanar PE
backbones aggregated in slipped-stacks. These crystals (Em.
λmax = 465 nm) emit a spectrum similar to the blue-green
fluorescent domains that often appear as minority domains in
drop-cast thin films (λmax = 480 nm).58 These two polymorphs
show distinct thermal properties: CO2-1A exhibits
endothermic transitions at 139 and 173 ᵒC, while CO2-1B
shows a broad endotherm from 145 – 165 ᵒC.
In addition to these forms we published previously, we
have discovered two new forms of CO2-1: i) bluish-green
emitting needles CO2-1C (a pseudopolymorph, vide infra)
grown from dichloromethane/hexanes as described above for
which we have solved a crystal structure bearing resemblance
to CO2-1B, and ii) a bright green emitting powder, CO2-1D,
grown from chloroform, for which X-ray quality single crystals
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similar richness in solid-state phase behavioDr.OTI:h1r0e.e10p3o9/lyCm9ToCr0p1h3s0o1Ff
the propyl derivative, violet emitting needles CO2-3A (λmax = 422
nm), green emitting needles CO2-3B (λmax = 483 nm), and bright
green emitting powder CO2-3C (λmax = 530 nm) were obtained by
slow evaporation from chloroform or dichloromethane solutions.
DSC thermograms of CO2-3A show no transitions below the melt,
whereas CO2-3B displays two overlapping endotherms from 80 –
120 ᵒC. CO2-3C shows a transition near room temperature at 20 ᵒC,
as well as an endotherm at 110 °C, similar to CO2-3B (Figure 3). As
with CO2-1, crystal polymorphs CO2-3A and CO2-3B frequently
occur together from slow evaporation methods, whereas powder
CO2-3C, though reproducibly obtained by rapid evaporation of
CH2Cl2, emerges only infrequently among the others. Lastly, we
observed two polymorphs for both CO2-2 and CO2-4—violet
emitting crystals CO2-2A and CO2-4A (λmax = 400 and 415 nm,
respectively), for which we have solved crystal structures, and
green emitting powders CO2-2B and CO2-4B (λmax = 520 nm for
both), which appear with far less frequency than the violet-emitting
polymorphs, and have not yielded single crystals.
have not been obtained. The emission spectrum of CO2-1C
(Favg = 1.1 ns) is nearly identical to CO2-1B, and exhibits a
similar DSC thermogram to CO2-1A, with the addition of a
unique endotherm at 62 ᵒC, which we attribute to the
liberation of solvent molecules (vide infra). Polymorph CO2-
1D, which emerges only occasionally, emits to the red of the
While violet and green emitting domains are frequently
obtained in drop-cast films, the relative ratios of these polymorphs
obtained by fast evaporation appear to depend on alkyl chain
length. Spin-coated films show a mixture of bluish-green and violet
emitting domains in CO2-1, and blue emission (mixed phases) in
CO2-2, CO2-3, and CO2-4 that gradually blue-shifts with alkyl chain
length. Emission λmax shifts from 474 nm in CO2-1 to λmax = 431 nm
in CO2-4 (Figure S4), which signals the increasing dominance of
violet polymorphs over blue-green emitting polymorphs in mixed-
phase films under these preparation conditions. Melting and slow-
cooling thin films with mixed domains results in convergence to
uniform violet fluorescence for all compounds regardless of the
preparation method, indicating that the more hypsochromically
emitting polymorphs are more stable.
Figure 3. Emission Spectra (left) and DSC thermograms (right) for polymorphs of CO2-1
and CO2-3 obtained by recrystallization. Solid lines used for polymorphs whose crystal
structures have been solved, and dashed lines used to indicate absence of
a
corresponding solved crystal structure. λex = 330 nm.
other polymorphs (λmax = 516 nm) and displays a weak
endotherm at 140 ᵒC in the absence of other transitions.
Violet emitting crystals and powders of CO2-1 are nearly always
obtained whether solvent evaporates slowly or quickly. Blue-green
emitting needles frequently appear together with the violet-
emitting crystals, while bright green-emitting powder appears
occasionally. Other compounds with short alkyl pendants display
With the exception of some green-emitting domains of CO2-18
drop-cast films (Figure S3), we were unable to observe any
polymorphism of molecules with longer alkyl pendants, which is
similar to observations in other reports of polymorphic and
responsive materials with variable lengths of alkyl pendants.62 CO2-
Table 1. Photophysical data of CO2-R compounds in solution, drop-cast films from CHCl3, and melting temperatures.
Solution (CHCl3)
Annealed Films
PE
Abs λmax (nm)
363
Em λmax (nm)
415
φf
τ (ns)
1.1
1.0
1.1
0.9
1.0
1.0
1.0
Abs λmax (nm)
Em λmax (nm)
405
Tm (°C)
214-215
185-186
158-159
148-149
128-130
123-124
112
CO2-1
CO2-2
CO2-3
CO2-4
CO2-6
CO2-8
CO2-18
0.45
0.47
0.45
0.46
0.43
0.45
0.45
350
349
346
355
354
355
355
364
418
414
363
417
414
366
419
412
367
419
414
363
418
408
365
418
410
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Single crystal X-ray structures of fluorescent prisms and
needles of CO2-2A, CO2-3A, and CO2-4A, grown by slow
evaporation from ethyl acetate, acetone, and chloroform,
support the assignment of violet-emitting polymorphs to
twisted PEs that resist aggregation. Similar to the methyl ester
derivative (vide supra), structures of CO2-2A and CO2-3A
feature twisted backbones (69-76 and 67-77ᵒ torsional angles,
respectively), as well as coplanar interactions between side
chain perfluorinated arenes and the terminal benzoate rings of
the conjugated PE backbone (3.9-4.1 Å ArF-ArH centroid-
centroid distances). In these structures, intermolecular ArF-
ArH stacking extends above and below the PEs, yielding infinite
columns of twisted units, which pack with each other in
herringbone-like structures. Adjacent columns of stacked PEs
are diagonally offset from one another across the molecular
long axis (Figure S5), accommodating edge-face interactions
between terminal benzoates and terephthalates (3.3 – 3.5 Å
closest contacts), as well as between pendant perfluoroarenes
and terephthalates (3.0 – 3.1 Å closest contacts).58 The ArF-
ArH interactions in CO2-2A and CO2-3A are slightly elongated
relative to CO2-1A (3.8 Å ArF-ArH centroid-centroid distance).
Ethyl and propyl side chains are further displaced laterally with
respect to the PE backbones (Figure 4b) as the ArF pendants
extend at ~18ᵒ angles away from the plane of the central
terephthalate, rather than the 8ᵒ offset in CO2-1A. These
minor differences aside, the ethyl or propyl chains of these
derivatives yield only small changes in crystal packing relative
to CO2-1A, and otherwise lead to increased unit cell volume
per PE and decreased calculated density with respect to CO2-
1A (Table S2).
Figure 4. a) Structures from violet emitting crystals of CO2-1 - CO2-4 showing twisted
PEs. b) View of PEs along face of ArF ring, showing lateral displacements with respect
to PE main chain. c) Intramolecular and intermolecular ArF-ArH stacking in CO2-3. d)
Only intermolecular ArF-ArH stacking observed in CO2-4. Second PE highlighted
magenta for clarity, with short contacts in ArF-ArH displayed. Hydrogens omitted for
clarity.
6, CO2-8, and CO2-18 yielded only colorless powders with violet
fluorescence regardless of preparation conditions. We attribute the
prevalence of polymorphism throughout the compounds with
shorter alkyl chains to “soft” non-covalent control: spectroscopic
results indicate that although ArF-ArH interactions with moderately
electron-withdrawing alkyl esters (p = 0.45 for –CO2Me)63 are
strong enough to cause the twisted, non-aggregated packing motifs
to be thermodynamically favorable, such interactions are
sufficiently weak to accommodate alternative arrangements, such
as coplanar, aggregated PE backbones yielding blue-green
fluorescence. The relative absence of polymorphism in CO2-6, CO2-
8, and CO2-18 suggests that interactions of alkyl chains shifts this
delicate balance to favor programmed twisting more thoroughly.
The butyl chains of CO2-4A, however, change crystal
packing more notably compared to those with shorter alkyl
chains. The PE backbone is less twisted (43–61ᵒ), and
intramolecular ArF-ArH stacking is absent (6.1-6.2Å centroid-
centroid distances). The butyl chains of adjacent molecules
occupy the regions of space that contain ArF rings in the other
twisted structures, which blocks intramolecular ArF-ArH
stacking (Figure S6). Rather than the vertical, columnar
stacking of twisted PEs when intramolecular ArF-ArH stacking
4. Structures of Violet Emitting Crystals
Figure 5. Structures of blue-green emitting crystals showing coplanar PE backbones
and aggregated packing of PEs in a) CO2-3B and b) CO2-1C, which includes a CHCl3
molecule. Hydrogens omitted for clarity, and aggregated PEs highlighted magenta.
occurs, the strictly intermolecular ArF-ArH interactions of CO2-
4A yield networks of diagonally stacked PEs (Figures S5 and
S6). Therefore, although they do not strongly impact the
optical properties of the violet emitting phases, the lengths of
alkyl chains do influence the mode of crystal packing. Based on
their optical properties, we therefore suspect that solids CO2-
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6, CO2-8, and CO2-18 lack intramolecular stacking between spectra from TD-DFT calculations (B3LYP/6-31G(d,p)) exhibit a
PEs and side chains, while still having twisted structures and 53 nm red-shift in the absorbance λmax between these twisted
intermolecular ArF-ArH interactions throughout.
and planar structures (339 and 392 nm, respectively). Similar
results are observed for the CO2-3 polymorphs. Excitation
5. Structures and Crystal-to-Crystal Transitions of Blue-Green spectra show a 45 nm red-shift between single crystals of CO2-
3A and CO2-3B (λ = 380 and 425 nm, respectively), and their
Figure 6. Thermal transition of CO2-3B. (Left) PXRD ( = 1.54184 Å) shows patterns of green and violet emitting polymorphs at room temperature and after
heating to 120 C, with accompanying photos of green emissive CO2-3B before and after heating. Corresponding excitation and emission spectra (top and
bottom right, respectively) of the polymorphs before and after heating indicate transformation to violet fluorescence after heating. ex = 330 nm.
Emitting Polymorphs
corresponding simulated UV-Vis spectra exhibit a 66 nm red-
shift in absorbance λmax (334 and 400 nm, respectively) as a
result of increased conjugation in the planarized structure.
In addition, blue-green emitting crystals CO2-1B (Figure S8)
and CO2-3B (Figure 6) undergo transitions to crystalline violet-
emitting polymorphs upon heating, while preserving crystal
habit. Corresponding with the broad transition seen in DSC
(Figure 3), violet emitting domains appear within prisms of
CO2-1B (roughly 2 x 0.2 x 0.2 mm) at 150 ᵒC and propagate
along the length of the crystals, suggesting a nucleation-and-
growth type mechanism of the phase transition.10 Emission
spectra of these crystals after heating (λmax = 413 nm) are
nearly identical to those when polymorph CO2-1A is also
heated to 200 ᵒC, including shoulders in excitation spectra at
375 nm. The green-emitting polymorph CO2-3B of the propyl
ester behaves similarly: green emitting needles bend and twist
between 80 – 120 ᵒC while their fluorescence becomes violet
(max = 413 nm).
Although we were unable to solve the unit cells of heated
crystals, powder X-ray diffraction (PXRD) confirms the
structural transformations. Diffractograms of CO2-1A and CO2-
1B at room temperature show different patterns that become
more similar after heating (Figure S8). In agreement with the
DSC data, PXRD analysis of the CO2-3B also confirms a thermal
transition between polymorphs upon heating to 120 °C (Figure
S6). Based on the spectroscopic changes, we conclude that the
samples after heating have twisted, non-aggregated packing
To better understand the impact of packing on the
different optical properties of polymorphs, we determined
single crystal X-ray structures of additional blue-green emitting
polymorphs of CO2-1 and CO2-3. Each of these crystal
structures exhibit generally coplanar PE rings and slipped-
stacked pendants that do not interact with PE main chains. A
needle of CO2-3B grown by slow evaporation of
dichloromethane/hexanes (10:1 vol/vol) shows PEs with 21ᵒ
torsional angles aggregated in infinite slipped stacks (3.4 Å
closest contacts), similar to the coplanar structure CO2-1B
(Figure 1, F (avg) = 2.2 ns). Alternating columns of aggregated
PEs in CO2-3B are nearly perpendicular to one another with
close contacts between carbonyls and and neighboring carbon
atoms in perfluoroarenes (2.9 Å C=O---C-F distances). In
addition, pseudopolymorph CO2-1C grown by the same
method shows a nearly identical structure to CO2-1B,58 but
includes one chloroform molecule per unit cell (Figure 5),
which likely accounts for the endothermic transition at 62 ᵒC
(Figure 3).
TD-DFT calculations on individual molecules from these
crystal structures support the importance of conformation in
their optical properties (Figure S7).
experimental excitation spectra of CO2-1A and CO2-1B single
crystals reveals a 50 nm red-shift in the spectrum shoulder (λ =
375 and 425 nm, respectively). Likewise, simulated UV-Vis
Comparing the
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motifs based on the fluorescence spectra being similar to
solved twisted structures.
While there is little difference in the green fluorescence
obtained by grinding any of the CO2-R compounds, temperatures
Given the similarity in crystal packing between the CO2-1B required for thermal recovery depend strongly on alkyl chain
and CO2-3B structures, we conclude that the longer alkyl length, ranging from room temperature for CO2-4, CO2-6, and CO2-
chains of the propyl derivative facilitating these thermal 8 to 100 ᵒC for CO2-1. For example, recovery of emission to λmax
transitions at lower temperatures, such as by stabilizing the
twisted structures of the A phases relative to the planar,
aggregated structures of the B phases. Alkyl pendants can
modulate thermally induced phase transitions via changes in
the mode of crystal packing,64 changes in lattice energies of
stable vs metastable phases,56,65,66 or changes in the balance
of phase enthalpy and entropy.67 Furthermore, given the
similarities in crystal density between the twisted and planar
forms (CO2-1B: 1.513 and CO2-1A: 1.583 g/cm3, CO2-3B: 1.556
and CO2-3A: 1.522 g/cm3) the changes in crystal stability are
likely due to alkyl chains tipping the balance of non-covalent
interactions to favor the twisted phases over the coplanar,
aggregated phase, rather than close packing considerations.
Figure 7. Mechanofluorochromism in CO2-R films. (Left) Representative MFC
response in emission spectra of CO2-1, grinding with a spatula and thermally
annealing at 180 ᵒC. (Right) Plot of emission maxima of ground films while heating
at 10 ᵒC increments to 100 ᵒC demonstrating how compounds with longer alkyl
Similar hypsochromically-shifting polymorphic transitions occur
for polymorphs CO2-1C and CO2-1D from 150 – 200 ᵒC, as well as in
CO2-2B powder from 60 – 130 ᵒC, which agrees with the endotherm
observed at 56 ᵒC (Figure S2). Only CO2-4B powder, which shows
no thermal transitions below its melting point by DSC, does not
change from green fluorescence upon heating, even though melting
followed by slow-cooling gives uniform violet emission. Despite
these thermally induced polymorphic transitions among CO2-1 to
CO2-4, blue and green emitting phases are generally unresponsive
to solvent vapor fuming, showing unchanged fluorescence after
exposure to CH2Cl2 vapor. In all cases, however, melting and slow-
cooling of any of these polymorphs for CO2-1 to CO2-4 results in
convergence on violet emitting phases, suggesting that twisted
arrangements of PEs with some form of ArF-ArH directed packing
resulting in violet emission give stable phases, whereas the blue and
green emitting phases with coplanar/aggregated PEs are
metastable.
chain recover violet fluorescence across lower temperatures. See corresponding
images and spectra, Fig S10. λec = 330 nm.
~415 nm occurs at 30 ᵒC for CO2-4 and remains unchanged for
increasing temperatures thereafter, whereas CO2-3, CO2-2, and
CO2-1 show a gradient in their recovered emission with λmax = 455,
468, and 470 nm, respectively, at 30 °C (Figure 8 and corresponding
Figure S12). At 40 ᵒC, CO2-3 shows near complete recovery to λmax
= 418 nm, while CO2-2 and CO2-1 require higher temperatures and
recover gradually up until 90 ᵒC, changing only slightly at even
higher temperatures.
With alkyl chains longer than propyl, recovery of violet emission
occurs at room temperature. Upon grinding CO2-4, bright green
fluorescence dissipates within seconds, after which a less intense
emission spectrum with λmax = 440 nm hypsochromically shifts to
418 nm within 2 minutes (Figure S13). CO2-6 and CO2-8 recover
their violet emission on the order of one second at room
temperature, allowing for mechanical “writing” into the waxy films
that disappears rapidly (Video S2). Films of CO2-18 notably break
from this trend, exhibiting persistent green emission at room
temperature after shearing (Em. λmax = 472 nm, Δλmax = 62 nm) and
requiring thermal annealing to 80 ᵒC to recover (Figure S14). Given
the greater rigidity and crystallinity of CO2-18 powders relative to
CO2-8 and CO2-6, we conclude that the extensive dispersion
interactions of alkyl chains alters solid packing and phase stability to
change the nature of solid phase transitions in CO2-18. Among
these compounds, increasing alkyl chain length may decrease
temperatures required to recover violet emission after grinding by
lowering the kinetic barrier to transition from metastable
coplanar/aggregated assemblies to crystalline, twisted assemblies.
This general trend of decreasing MFC thermal recovery
temperatures with alkyl chain length is consistent with our
observations on the overall occurrence of polymorphism in these
compounds and decreasing temperatures in thermal polymorphic
transitions that converge on violet emitting phases with alkyl chain
length.
6. Reversible Mechanofluorochromism of Powders and Films
In conjunction with these accessible polymorphs, this family of
CO2-R compounds exhibits reversible MFC upon mechanical
grinding of powders and films, resulting in consistent bathochromic
shifts from violet to green emission with max of emission of 36 –
80 nm (Table S3). The responses of these sheared films reverse
fully upon heating below their melting temperatures or fuming with
CH2Cl2 solvent vapor, and repeating this reversible process for CO2-
1 over multiple cycles does not fatigue the MFC response (Figure
S10). In conjunction with the crystal structures of blue-green
emitting polymorphs, we attribute this MFC to increased backbone
planarization and/or aggregation of initially twisted structures.59,68
Thermograms obtained from DSC of CO2-1 after grinding exhibits
an irreversible exothermic peak from 40 – 50 ᵒC in the first heat,
suggesting a crystalline-to-amorphous transition with grinding and
cold-crystallization during thermal recovery (Figure S11). However,
thermograms of other CO2-R ground powders did not show any
differences from recrystallized powders, which suggests small
differences in energy between the ground and annealed phases.
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of Science User Facility operated for the DDOOIE: 1O0.1ff0i3c9e/Co9fTSCc0i1e3n0c1eF
by Brookhaven National Laboratory under Contract No. DE-
SC0012704. The authors would like to thank Dr. Lin Yang, who
manages the Life Science X-ray Scattering (LiX) at Brookhaven
National Laboratory for support with WAXS measurements
and Dr. Daniela Morales, X-Ray Diffraction Lab at University of
Connecticut Institute of Materials Science for support with
PXRD measurements.
Conclusions
This series of seven ester-terminated PEs with relatively weak
ArF-ArH interactions show alkyl chain length dependent
polymorphism related both to irreversible thermochromic green-to-
violet fluorescent crystal-to-crystal transitions, as well as reversible
violet-to-green mechanofluorochromism. Shorter chain lengths lead
to greater co-incidence of green emitting polymorphs
(planar/aggregated) with the violet emitting phase (programmed
twisting), while longer chain lengths preferentially stabilize the
violet emitting phase, leading both to lower temperatures of
crystal-to-crystal transitions and lower thermal recovery
temperatures in the MFC process. Crystallography reveals how
these trends relate to the changes in PE packing and interactions
with chain length; as alkyl chains modulate the intramolecular vs
intermolecular nature of the ArF-ArH programmed twisting,
conformational distortions of the twisted motif determine its
relative stability and ease of transition from planar/aggregated to
the twisted structures.
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