The interplay of soft-hard substituents in photochromic diarylethenes

Article abstract of DOI:10.1016/j.jphotochem.2016.04.001

A series of diarylethenes with substituents of different size and chemical nature was synthesised showing that beside some intermolecular interactions involving the central diarylethene core, lateral groups clearly play a key role in the crystal packing a

Full text of DOI:10.1016/j.jphotochem.2016.04.001

Accepted Manuscript
Title: The interplay of soft-hard substituents in photochromic
diarylethenes
Author: Rossella Castagna Valentina Nardone Giorgio Pariani
Emilio Parisini Andrea Bianco
PII:
S1010-6030(16)30032-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.04.001
JPC 10190
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
15-1-2016
1-4-2016
1-4-2016
Please cite this article as: Rossella Castagna, Valentina Nardone, Giorgio Pariani,
Emilio Parisini, Andrea Bianco, The interplay of soft-hard substituents in
photochromicdiarylethenes, JournalofPhotochemistryandPhotobiologyA:Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2016.04.001
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The interplay of soft-hard substituents in
photochromic diarylethenes
Rossella Castagnaa*, Valentina Nardonea,b, Giorgio Parianic, Emilio Parisinib, Andrea Biancoc
a Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da
Vinci 32, 20133, Milano (Italy).
b
Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133
Milano (Italy).
c Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (Italy).
1

Highlights
 Diarylethene crystals with substituents of different size and chemical nature were
obtained
 X-ray, time-resolved FT-IR, DSC were performed to rationalize the thermal
transitions processes
 The role of van der Waals interactions of lateral groups drives the crystal packing
formation
 Different substituents favour either an amorphous or crystalline state after thermal
treatment
2

Abstract
A series of diarylethene molecules with substituents of different size and chemical nature
was synthesised showing that beside some intermolecular interactions involving the central
diarylethene core, lateral groups clearly play a key role in the crystal packing arrangements.
These structural features were further analyzed in relation to the thermal data obtained by
differential scanning calorimetry (DSC) and monitored using FT-IR spectroscopy, thus
providing a rationalization of the observed thermal transitions processes. The role of van
der Waals interactions is crucial in driving crystal packing formation towards loosely
packed arrangements characterized by large hydrophobic contact areas. Interestingly, some
functional substituents favour an amorphous state after thermal treatment, a peculiar feature
that can be exploited to design uniform photochromic layers.
Introduction
Progress in the development of photochromic switches requires a detailed characterization
of the behaviour of photoactive compounds in the solid state. Indeed, for some applications,
completely amorphous thin films are needed, whereas for others, fully crystalline materials
are required, possibly in the form of single crystals. For example, photochromic materials
showing remarkable optical contrast[1,2] in specific spectral regions are attractive as active
layers in photolithography, both in the form of molecules dispersed within a polymer
matrix[3,4] and pure as a thin amorphous evaporated layer[5]. In the latter, the quality of the
optical layer is the key to the success of the lithography process, as aggregation and
crystalline domains can cause light scattering. For application of photochromic switches
where crystalline materials are preferred, the long-range molecular packing order in the
crystalline phase and the limited free volume for the photoconversion process may inhibit
photochromic reactions. Interestingly, Irie and co-workers successfully developed thermally
3

irreversible and fatigue-resistant photochromic crystals of furylfulgide[6] and
diarylethenes[7–9].
As a result of the relative orientation of the molecules in the lattice, photochromic crystals
may show specific anisotropic physical properties such as, for instance, dichroism[10,11].
Moreover, the cooperative structural changes of diarylethenes molecules that are regularly
packed in a crystal may determine macroscopic mechanical modifications of the single
crystal upon irradiation. In selected photochromic crystals, a change in surface
morphology[12] or a dramatic shape change occurs upon light irradiation[7,13–15]. Based
on this morphological behaviour, a photodriven nano-scale actuator, consisting in a rod-like
crystal that bends towards the direction of the incident light, has been developed[16].
Beside twisted and bent crystals that are known to be present in nature[17], crystals that can
be morphologically modified using light have been obtained[18] by exploiting the chiral
properties[19–22] of photoactive molecules. These light-induced changes are caused by the
shrinkage of the irradiated portion of the crystal[23] and show crystal thickness
dependence[24]. This behaviour is determined by a photoisomerization gradient inside the
crystal; in fact, the light-induced conversion of the photochromic layer in the solid state is
not uniform throughout the sample, and depends on both the thickness of the material and
the illumination source[25]. Photochromic co-crystals[26–28] have also been studied for the
development of hybridized structures of novel photonic devices. However, not only are
multi-component crystals difficult to obtain, but whenever they form, energy transfer from
one component to the other may prevent cyclization.
Despite the wealth of solid state diarylethene optical and photomechanical switches
described to date, a large scale rational approach to the design of diarylethene molecules that
takes into account the intermolecular forces governing the molecular packing in the lattice is
still missing. Indeed, the effect of intermolecular interactions on the crystal packing has been
4

stimulating an interesting and still on-going debate in the field of photochromic
switches[29–35].
Here we show how the subtle interplay of substituents of different size and chemical nature
affects the packing, the morphology and the aggregation of active diarylethenes in the solid
state. Specifically, we have designed a series of molecules that share the same 1,2-bis-(2-
methyl-5-phenyl-3-thienyl)perfluorocyclopentene central moiety, but with their phenyls
differently substituted in the para-position (Fig. 1). We show that, beside some
intermolecular interactions involving the central moiety of the diarylethenes, lateral groups
play a key role in guiding and stabilizing the crystal packing arrangements. We relate the
nature and the extent of the observed intermolecular interactions within each crystal to the
Differential Scanning Calorimetry (DSC) profile of the corresponding molecule, thus
providing a rationalization of the progression of the thermal transitions in the studied
materials. Overall, extended Van der Waals interactions guide nucleation and crystal growth
both from solution and from the melt, as shown by the evolution of IR spectra with time.
Figure 1 - Photochromic reaction in diarylethenes and chemical structure of the molecules under investigation.
5

Materials and Methods
All the photochromic compounds DTE-1 to DTE-7 were synthesized according to the
procedures reported in the Supplementary Material
Crystals obtained from solution: crystals of DTE-1 to DTE-7 were obtained by slow solvent
evaporation or by vapour diffusion, as described in detail in the Supplementary Material.
Melt derived crystals: DTE-7 powder was heated up to 130°C at a 10°C/min rate, kept for
10 minutes at this temperature to ensure the complete melting of the powder, fast cooled to
65°C and then maintained for three hours at this temperature to perform isothermal
crystallization. Then, the sample was slowly cooled down to room temperature.
1H NMR spectra were collected using a Bruker ARX 400. Mass spectroscopy was carried
out using a Bruker Esquire 3000 plus.
UV−vis absorption spectra UV-vis spectra were recorded with a Varian Cary 5000
spectrophotometer from dilute hexane solution (<10-5 M) and from thin film of the pure
compound deposited onto quartz. The spectrophotometer was equipped with the integrating
sphere (DRA2500) to measure the diffuse transmittance in the case of thin films. Starting
from the open form, the photochromic reaction was triggered by illumination of the solution
or the thin film using quasi-monochromatic UV light with wavelength corresponding to the
absorption maximum of the open form, until the photostationary state (PSS) was reached.
FTIR spectra were recorded using a Nicolet NEXUS FTIR interferometer (DTGS detector).
In order to monitor the melting and isothermal melt crystallization process of DTE-7, the
compound was dissolved in diethyl ether and drop-casted as a thin film on a ZnSe substrate.
The substrate was then set into a variable temperature cell Linkam (THMS 600) and
positioned under the objective of the IR-microscope stage. Two different thermal programs
were set:
6

i) the specimen was heated from room temperature up to 150°C in an argon atmosphere at a
10°C/min heating rate. IR spectra were collected (256 scans, resolution 4 cm-1) at different
temperatures.
ii) the specimen was heated from room temperature up to 150°C in an argon atmosphere
with a 20°C/min heating rate. In order to monitor the isothermal crystallization process the
specimen was cooled down to the desired crystallization temperature. The temperature was
kept constant following the evolution of the crystallization process. IR spectra were
collected every 100 seconds with 32 scans and 4 cm-1 resolution.
A baseline correction was applied to all the IR spectra. In general, the area of the bands was
calculated by integrating each spectrum in the desired region (keeping the spectral
boundaries constant). In order to compare the different areas of the spectra during the
crystallization process, normalization to the peak area at 1600 cm-1 was applied. All these
operations were performed using OMNIC 8.0 (Thermo Electron Corporation). The Avrami
model[36–38] was used to fit the FTIR data from the isothermal crystallization process. A
guess of the induction time was made from the change in the position of the CH stretching
peak. The initial values of A0 and Amax were determined as the mean of 10 points in the
minimum and maximum of the normalized FTIR area at the beginning and at the end of the
crystallization process, respectively. A0, Amax, kapp, and n were then determined by the best
fit with a least-squared method as a function of induction time, which was varied in the
proximity of the guess value and used as a fixed parameter in the fitting procedure. The best
induction time was then obtained as the one minimizing the fitting residuals.
DFT calculations have been performed using Gaussian 09 (G09RevD.01) with a basis set 6-
31G(p,d) and B3LYP as functional. The results of the calculations are reported in the
Supplementary Material.
For the Differential Scanning Calorimetry (DSC) analysis, the aluminium DSC pan was
filled with approximately 3.5 mg of compound powder. DSC thermograms for all samples
7

were measured with a DSC SEIKO 6300, under nitrogen. The general procedure for DSC
scans consisted of a heating step from 0 to 250°C with a heating rate of 20°C/min and a
cooling scan from 250°C to 0°C at a 5°C/min rate. For compound DTE-7, further DSC
analyses were conducted by heating from 0 to 250°C and cooling back to 0°C at a scan rate
of 1°C/min.
The X-ray† data for DTE-2, DTE-4, DTE-5. DTE-6 and DTE-7 were collected at 100K on
a Bruker X8 Prospector APEX-II/CCD diffractometer, using CuKα radiation (λ=1.54050).
All structures were solved by direct methods and refined using the SHELX-97 package[39].
All the structures were analyzed by Mercury[40], and the images were generated using
PyMol[41] and Chimera[42]. The cif files of DTE-1 and DTE-3 were downloaded from the
Cambridge Structural Database[43] (CCDC entries 652100 and 185944 respectively)
Results and discussion
1,2-Bis-(5-chloro-2-methyl-3-thienyl)perfluorocy-clopentene
(
9)
was synthetized as
previously reported[44]. This photochromic dichloride precursor is a versatile substrate that
provides different symmetrically or asymmetrically substituted diarylethenes by Suzuki
cross-coupling with either boronic acid or pinacol ester (Scheme 1).
The asymmetrically substituted DTE-2 was synthesized by a one-pot coupling reaction,
where the two boronic derivatives were sequentially introduced. Since in the symmetrically
substituted dithienylethenes the coupling involving boronic derivatives bearing the cyano
group led to the product in low yield, we introduced 4-cyanophenylborinic acid as a first
coupling reactant in excess with respect to the 1,2-bis-(5-chloro-2-methyl-3-
thienyl)perfluorocyclopentene
9. This allowed maximization of the yield of the
asymmetrical dithienylethene DTE-2 (38%). DTE-3 was also obtained in good yield (27%).
On the contrary, despite the long reaction time, DTE-1 was produced in a very low yield
8

(6%). The same synthetic procedure was followed for DTE-4, but with a lower overall yield.
The symmetrical dodecyloxy-substituted derivative DTE-7 was also collected, while the
symmetrical carboxy-substituted dithienylethene was retained in the silica column during
the purification of the crude.
As an alternative method, the Suzuki-Miyaura reaction was also carried out under super-
heated conditions using 1,1'-bis(di-tert-butylphosphino)ferrocene palladium dichloride
(Pd(dbpf)Cl2), which is a commercially available, air and water-stable catalyst. According to
Moseley et al.[45], aryl chlorides react with sterically and electronically demanding boronic
acids to give in most cases complete conversion. Here, both 4-cyanophenylboronic acid
pinacol ester and 4-dodecyloxybenzene boronic acid (synthesized from 4-bromophenol, as
reported
in
Scheme
1)
the
catalyst,
and
1,2-bis-(5-chloro-2-methyl-3-
thienyl)perfluorocyclopentene
9
were simultaneously introduced in a capped vial and heated
under microwave to obtain DTE-5. The overall yield strongly increased (86%), with a
remarkable improvement also in the formation of DTE-1 (32%). DTE-7 was also collected
from the same reaction in good yield (28%).
DTE-6 was synthesised in 90% yield from DTE-3 by cleavage of the methoxy group and
subsequent reaction with 1-bromohexane (Scheme 1).
9

Scheme 1 - Synthetic route for DTEs under investigation
The UV-vis spectra of all the synthesized DTEs are reported in Fig. 2. In particular, the
spectra at the photostationary state for the open and closed forms in hexane solution (solid
lines) and for the closed form in thin film (dotted line). They show the typical electronic
features of diarylethenes: upon irradiation with UV light, the transparent open form converts
into a coloured closed form that is thermally stable. The closed form has a broad absorption
band in the visible region whose position shifts in wavelength depending on the chemical
structure of the molecule[2]. For all DTEs, photochromism occurs both in solution and in
the solid state (all the UV-vis spectra in the solid state are reported in Supplementary
Material Fig. S5). A remarkable broadening and a red-shift of the visible band is observed
for all the compounds at the solid state in comparison to the solution. This is consistent with
10

the data reported in literature for films of amorphous diarylethenes, where bathochromic
shifts as large as 20nm in comparison with that of the hexane solution were observed [46–
48]. A similar behaviour is usually observed in high-concentration solutions, where
intermolecular interactions are stronger and transitions at lower energies arise [46]. A lower
conversion to the coloured form is also reported in bulk amorphous films, which may
explain the differences in the absorption spectra at the PSS state below 400nm. It is also
worth noting that, for non-amorphous materials, a scattering contribution is present in the
spectra at the solid state and, since the scattering effect is more pronounced at shorter
wavelengths, the differences between the spectra are enhanced in the UV region.
11

Figure 2 – UV-vis spectra of different DTEs compounds. Solid line: spectra in hexane solution in the open form (blue line) and in
the PSS state (red line), scale bar on the left. Dotted line: spectra in the solid state measured on a thin film of pure compound
deposited onto quartz substrate, scale bar on the right. For each compound both in solution and in the solid state the spectra of
the closed form are reported at the photostationary state conversion (PSS), obtained by illumination of the sample at the
absorption peak of the open form.
Absorption maxima of the DTE in hexane solution with the corresponding absorption
coefficients are summarized in Table S3. All the DTEs studied herein that bear withdrawing
groups (i.e. DTE-1, DTE-2, DTE-4 and DTE-5) show red-shifted absorption bands, both in
the open and in the closed form. This is due to the more extended conjugation provided by
12

the CN triple bond or the C=O double bond. Moreover, a decrease in the molar extinction
coefficient (ε) is registered for the diarylethenes substituted with electron-withdrawing
groups. In the closed form, the asymmetrically substituted dithienylethenes (i.e. DTE-2
,
DTE-4 and DTE-5) have the actinic bands in the visible range located at lower energies (i.e.
597, 597 and 603 nm, respectively) as the push-pull effect adds to the extension of the
conjugation system. The symmetrically donor substituted DTE (i.e. DTE-3, DTE-6 and
DTE-7) show very small differences in the absorption maxima, thus indicating that the
length of the alkyl chain has a negligible effect in solution.
Crystal structures
In solution, open form-dithienylethenes exist as an equilibrium of two conformational
isomers, one with anti-parallel C2 and the other with parallel Cs symmetry, only the former
being photoactive. In the solid state, all the compounds described herein feature two
thiophene rings that are out-of-plane with respect to the ethene bridge and an anti-parallel
conformation of the substituents. Moreover, in all the reported structures the distance
between the reactive carbons is shorter than 4.2Å (details in the Supplementary Material
Table S4), which is a fundamental requirement for the maintenance of photochromism in
crystals[49]. These two elements are consistent with the evidence that all the DTE crystals
described herein are photochromic.
The crystal packing arrangement of the molecules is strongly influenced by the size and the
nature of the substituents introduced. The cyano-cyano substituted DTE-1[50], the
asymmetrical substituted DTE-2 and two of the four available polymorphs of the methoxy-
methoxy substituted DTE-3[51], which all bear small substituents, show a very similar
packing arrangement. The packing arrangement of DTE-2 is shown in Fig. 3. At the two
ends of the molecule, the relative orientation of the thiophene and the benzene rings is
essentially identical (the torsion angle of the benzonitrile and the methoxybenzene moieties
13

with respect to their corresponding thiophenes is -18.08° and 20.22°, respectively). In the
lattice, next-neighbouring molecules form an anti-parallel packing arrangement stabilized by
an S-π interaction between the sulfur at the cyanophenyl end of the molecule and the phenyl
ring of the benzonitrile moiety of the partner molecule (the angle is 84.69° and the distance
3.64Å). Conversely, the other thiophene ring of the reference molecule and the methoxy-
substituted ring of the interacting molecule are almost parallel (18.48°), featuring a centroid-
centroid distance of 3.91Å. and a S-centroid distance of 3.24Å. Moreover, a stabilizing π-π
interaction is established between two benzonitrile rings from two molecules belonging to
two different dimers (the distance between the two centroids is 4.15Å).
Figure 3 - Crystal packing of DTE-2 (on the left), and highlight of the stabilizing interactions described in the text (on the right). A
very similar packing arrangement is also found for DTE-1 and DTE-3, not shown here.
In DTE-5, the methoxy group of DTE-2 is replaced by a long (C12) alkoxy chain. The
alkoxy chains of two next neighbouring molecules are essentially parallel and form Van der
Waals contacts (4.1-4.5Å) along their entire extended conformation. Driven by this
interaction, a stacking arrangement of the molecules is formed along the b axis of the DTE-
14

5
unit cell, resulting in an antiparallel interdigitation of the long aliphatic chains, a feature
that is also shared by the DTE-7 structure presented herein (see Fig. S2 in the
Supplementary Material).
In the DTE-5 crystal, like in DTE-2, within the pair of more closely interacting molecules
the thiophene ring of the first molecule is almost perpendicular to the benzonitrile ring of the
next-neighbouring molecules (84.47°, with a sulphur-benzene distance of 3.64Å) and, at the
alkoxy-substituted end of the dithienylethene, the thiophene and the benzene of the two next
neighbouring molecules are almost parallel (17.68°, with a 4.50Å distance between the
centroids).
In DTE-4 intermolecular hydrogen bonding involving the carboxylic acid moiety at one end
of the molecule drives the packing arrangement, where head-to-head dimers are formed in
the crystal (2.64Å). An electrostatic interaction between the thiophene S atom and the
carboxylic group of the next nighboring molecule (3.31Å) further stabilizes the observed
conformation (Fig. 4).
Figure 4 - Crystal packing of DTE-4 in a head-to-head dimer conformation with a bidentate hydrogen bonding between the
carboxylic acid, electrostatic interaction between the thiophene S atom and the carboxylic acid and hydrophobic contacts
between the alkyl chains.
Compounds DTE-3, DTE-6 and DTE-7 are all symmetrically substituted, and carry an
alkoxy-benzene moiety of different length (C1, C6, and C12, respectively). DTE-6
crystallizes with half a molecule in the asymmetric unit and the torsion angle between the
15

thiophene ring and the benzene ring is -14.88°. Within the pair formed by the two more
closely interacting molecules in the DTE-6 crystal, S-π interactions are present, where the
thiophene ring of one molecule is almost perpendicular to the centroid of the benzene ring of
the partner molecule, with a distance of 3.87Å. In the DTE-6 crystal, the alkoxy chains
show structural disorder. Although two well-defined alternative conformations of the alkoxy
chains could be clearly seen and accounted for in the final model, the relatively large
thermal parameters of the atoms suggest a certain degree of residual dynamic disorder in the
molecule even if the data were collected at low temperature. This is in agreement with the
disorder observed in two polymorphs of the same molecule that were previously obtained
with a different crystallization solvent (acetone) at room temperature[52].
Interestingly, the crystal structure of DTE-7 shows very different torsional angles between
the thiophene rings and the benzene rings at the two ends of the molecule, namely 32.47° on
one side and completely planar on the other side (2.31°). The long aliphatic chains of DTE-7
account for a large portion of the molecule, and the packing arrangement is guided by the
optimization of the number and extent of van der Waals contacts between these long
hydrophobic substituents. Each DTE-7 molecule makes two distinct sets of short C-C
contacts (in the range of 4.2-4.5Å) with the next neighbouring molecules in the lattice (Fig.
5), forming a packing arrangement that extends indefinitely and accounts for all the
stabilizing interactions in the crystal. These hydrophobic interactions lead to the formation
of an interdigitated arrangement of the chains similar to that observed for the alkyl moieties
of DTE-5. Interestingly, alkyl chain interdigitation is not observed in DTE-4, where a hard
substituent such as the carboxy group strongly dominates the crystal packing arrangement,
even in the presence of a long alkoxy chain. In DTE-7, short intermolecular S-π interactions
involving the aromatic rings connect next-neighbouring molecules to form an extended
network spanning throughout the crystal. A further stabilizing intermolecular interaction
16

occurs between a sulphur atom of one molecule and the thiophene ring of the next-
neighbouring molecule, with a S-centroid distance of 4.11Å.
Figure 5 - (a) Crystal packing of DTE-7 highlighting the main intermolecular interactions described in the text. (b) Chain
interdigitation.
Although stabilizing S-π contacts are seen in all the structures described herein, the longest
S-π distance is observed in DTE-7, where the major stabilization energy to the packing is
provided by the contacts between the long hydrophobic chains.
A general decrease in the calculated crystal density in going from DTE-1 to DTE-7 (Table
1) highlights the increasing contribution of the relatively weak van der Waals contacts
17

between the long aliphatic chains with respect to the more directional interactions that hold
the shorter molecules together in the lattice. The only notable exception is DTE-6, where the
observed conformational disorder highlights the lack of well-defined structural constraints
on the short (C6) alkoxy chains. Here, the intermolecular packing arrangement is mostly
driven by the interactions between the central moieties of the molecules, resulting in the
relatively high calculated crystal density for this compound (Table 1).
Thermal behavior
Differential Scanning Calorimetry (DSC) analysis was performed in order to study the
possible occurrence of thermal transitions and to provide information on the solid phase.
DSC scans are reported in the Fig. S6 of the Supplementary Material, while the melting
temperatures of the DTEs are summarized in Table 1. The melting temperatures of all the
DTEs studied herein are remarkably different, going from 90°C to 216°C; moreover, they do
not relate to the differences in molecular weight among the members of the series. Rather,
the melting temperature values are consistent with the diverse sets of stabilization
interactions that these different molecules form in the solid state. Indeed, interactions
between neighbouring molecules in the crystal explain why molecules bearing the smallest
side groups (DTE-1
,
DTE-2 and DTE-3) show melting temperatures higher than those of
DTE-6 and DTE-7). Interestingly, in
species with one or two alkyl chains (as for DTE-5
,
the DTE-1 packing arrangement, the nitrile groups form a π-π interaction with the benzene
rings of adjacent molecules; these contacts provide further stabilization energy, which likely
accounts for the sharp increase in density and melting temperature of this compound with
respect to DTE-2 and DTE-3, whose packing arrangements are otherwise very similar. In
the case of DTE-3, DTE-6 and DTE-7, the longer the alkyl chains the lower the melting
temperature. Moreover, crystal stability appears to be higher (e.g. higher melting
temperature) when only one long alkyl chain is present instead of two (see DTE-5 with
18

respect to DTE-7). In the case of DTE-4, the presence of one long alkyl chain is
counterbalanced by the strong, stabilizing bidentate hydrogen bonding interaction between
carboxyl groups.
Table 1 - Melting temperature and crystal density of different diarylethene crystals obtained from solution measured with DSC.
Melting
Diarylethene
temperature
(°C)
216
Density g/cm3
DTE-1
DTE-2
DTE-3
DTE-4
DTE-5
DTE-6
DTE-7
1.526
1.514
1.451
1.354
1.356
1.380
1.220
170
147
147
130
120
90
It is worth noting that the presence and the length of the alkyl chains as well as the melting
temperature of the different crystals show a good correlation with the calculated crystal
densities. The highest density, and therefore the most efficient packing, is found for DTE-1
,
followed by DTE-2 and DTE-3. Notably, DTE-2 and DTE-3 are characterized by a
virtually identical packing arrangement. In the alkyl-substituted compounds, crystal
densities are smaller, likely due to the increased size and more elongated shape of the
molecules. Within the molecules carrying the long (C12) alkoxy chain, the packing
arrangement of DTE-4 and DTE-5, which are both asymmetric as they also carry a short
substituent (COOH and CN, respectively), have similar density, while the symmetrically-
substituted DTE-7 is more loosely packed..
As for DTE-7, this is the only photochromic compound showing a crystallization transition
peak during the DSC cooling scan from the melt phase. In fact, an endothermic peak is
recorded at 50°C during the DSC scan (from 250°C to 0°C at 1°C/min cooling rate). At an
initial visual inspection, melt-derived DTE-7 single crystals obtained from isothermal
crystallization show exactly the same crystal habit as those obtained from solution. The X-
ray diffraction analysis of several crystals from the same batch always provided the same
19

unit cell parameters as those of the crystal obtained by evaporation, indicating that
recrystallization from the melt affords the same DTE-7 crystal form. The peculiar behaviour
of DTE-7 suggests that the presence of two long alkoxy chains (C12) facilitates crystal
formation from the melt phase. To investigate such behaviour, both the melting and the
crystallization process were followed stepwise by IR spectroscopy.
Role of soft substituents
The FT-IR spectra of DTE-7 at room temperature (25°C) and after the melting process (at
150°C) are reported in Fig.6. The spectrum interpretation was obtained by the normal mode
analysis performed with the DFT calculations.
Figure 6 – FT-IR of a DTE-7 in the solid state at 25°C (red) and in the melt state at 150°C (blue).
A shift of both the asymmetric and symmetric stretching bands during the melting process is
clearly visible in the C-H stretching region (3000-2700 cm-1), which is consistent with the
data reported in the literature for alkyl chains in liquid and crystalline phases (see for
example[53]). At the same time, a broadening of the same bands occurs, causing the loss of
the band structure. A backward shift to shorter wavenumbers is reported for the cooling
process.
In the fingerprint region, the spectra are dominated by bands related to the normal modes of
the perfluorocyclopentene and to the bending vibrations of the aliphatic chains (1350-1200
cm-1). The presence of –CH2- rocking at 720 cm-1 (which is active only for long methylene
20

chains) and bands in the CC stretching region (1020 cm-1) supports the occurrence of an
intramolecular order of the lateral chains.
During the melting process, large changes occur in the 1350-1200 cm-1 region, consisting in
i) the destructuration of the bands at 1280 and 1290 cm-1 and ii) the increase of the intensity
ratio between the bands at 1280/1290 cm-1 and the band at 1240 cm-1. These bands refer to
normal modes that involve the C-F stretching of the perfluorocyclopentene and the
methylene wagging of the lateral chains. This finding is consistent with the decrease of long-
range order in the sample as melting occurs. In the process, the coupling of the two modes
changes, thus resulting in wider bands. Another evidence of the increase in chain disorder
comes from the rocking band at about 720 cm-1. This band, which is very weak in the melted
form, becomes stronger in the crystalline form thanks to the increase of long-range order.
A clear fingerprint of the crystalline state is the band at 1023 cm-1 that lies in the CC and CO
region. The band is clearly evident in the crystalline phase (obtained both from solution and
by cooling of the melt) while it disappears in the melt. According to the DFT calculations,
this normal mode originates from a combination of CC stretching, partially involving the
CO stretching (see figure S7 in the Supplementary material).
Isothermal crystallization mechanism
To elucidate the contributions of the different moieties to the crystal packing, the melting
and the isothermal crystallization of DTE-7 were monitored by FT-IR spectroscopy during
isothermal crystallization at 60°C and 65°C. The intensity of the CH stretching bands shows
no phase dependence, as previously reported.[54] Although the total intensity does not vary,
both the asymmetric and symmetric CH stretching wavenumbers shift while the melt
converts to the crystal phase. The evolution of the peaks position during the crystallization
process is reported in Fig. 7.
21

Figure 7 - Peak position of the asymmetrical CH stretching of DTE-7 (left) and the symmetrical CH stretching (right) as a function of
time during isothermal crystallization at 60 °C and 65 °C.
As expected, bands shift towards shorter wavenumbers as the transition from melt to crystal
occurs. The starting time of the isothermal crystallization depends on the temperature, that is
the lower the temperature the shorter the starting time. Given a certain temperature of
isothermal crystallization, both symmetrical and asymmetrical CH stretching modes show the
same transition time.
Figure 8 - A detail of the FT-IR of DTE-7 in the C-C stretching partially involving the C-O stretching region during iso-thermal
crystallization at 60°C.
22

The same transition appears when monitoring the evolution of the infrared band at 1023 cm-1
(Fig. 8), which, according to DFT calculations (see figure S7 in the Supplementary material),
is sensitive to the regularity of the alkoxy lateral chains, i.e. the more transplanar the lateral
chain the higher the band intensity. Hence, it is possible to follow the intensity evolution of
the band at 1023 cm-1 as a marker of the degree of crystallinity of the sample. Accordingly,
the crystallinity fraction (Xc) can be calculated from the area of the marker band, as follows:
where At is the band intensity during isothermal crystallization at the time t, A0 is the
intensity at the beginning of the isothermal crystallization and Amax is the intensity at the end
of the process.
The crystallization fraction obtained from the normalized area of the band at 1023 cm-1 is
reported in Fig. 9 as a function of the crystallization time for isothermal cooling at 60°C and
65°C. In passing from the melt to the solid state, an increase of the infrared intensity is
evident when following the isothermal crystallization process at both temperatures.
Moreover, there is a correlation between the shift in the position of the CH stretching band
and the evolution of the intensity of the 1023 cm-1 band. Referring to the same temperature,
the two transitions start at the same time and have a similar duration, as can be evidenced by
comparing Fig. 7 and Fig. 9. We conclude that the two phenomena can be ascribed to the
same physical transition and are both markers of the crystallization process.
The kinetics of the isothermal crystallization of polymers[55–57], small molecules[58,59]
and alloys[60] is described by means of the Avrami or modified Avrami model. The former is
used for the evolution of the crystallization process under isothermal conditions, while the
latter applies to non-isothermal processes[54]. Since we have performed an isothermal
23

cooling and monitored the evolution of the target band at 1023 cm-1, we apply the standard
Avrami model, as follows:
where k is the crystallization rate constant, which depends on the nucleation and growth rates,
t is the time of the crystallization, t0 is the induction time, and n is the Avrami exponent. The
exponent value is related to the nature of the nucleation and to the geometry of the growing
crystals, namely: n=4 volume nucleation and three dimensional growth, n=3 volume
nucleation and two dimensional growth, n=2 volume nucleation and one dimensional growth,
n=1 surface nucleation and one dimensional growth[60].
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
0
1000
2000
3000
4000
5000
6000
7000
t (s)
Figure 9 - Evolution of the crystallization fraction of DTE-7 obtained from the FT-IR normalized area of the 1023 cm-1 band for
isothermal crystallization at 60°C (black dots) and 65°C (red dots). The solid line represents the Avrami fitting at each
temperature.
The Xc experimental data are fitted according to the model and the best fitting parameters are
reported in Table 2. The Avrami fittings are reported in Fig. 9 for both crystallization
temperatures. As expected, the induction time is shorter for the crystallization at 60°C.
24

On the other hand, the difference in the intensity ΔA increases with the temperature, since a
slower crystallization process leads to a higher degree of crystallinity. Moreover, according to
the model, the best fit occurs for a value of n close to 2, which means that the crystallization
starts from a heterogeneously nucleated sample and follows a one-dimensional growth of the
crystals confined in a thin layer. Therefore, crystals grow from different nucleation seeds and
enlarge as lamellae of constant thickness until all the material has transitioned to the solid
state.
Table 2 - Best Avrami fitting parameters for isothermal crystallization at 60°C and 65°C.
60°C
(1.9± 0.3) x 10-5
65°C
(1.9± 0.2) x 10-5
kapp (s-n)
t0 (s)
N
1780 ± 20
5345 ± 20
2.0 ± 0.3
2.0 ± 0.2
Amax
A0
0.290 ± 0.001
0.095 ± 0.006
0.195
0.400 ± 0.001
0.178 ± 0.007
0.222
∆퐀 퐀
퐀
Conclusions
Although in 1,2-dithienylethenes electroactive substituents are usually introduced to tune the
electronic transitions (i.e. color) of the molecule, their role in affecting the molecular
aggregation or regular packing mode in the solid state cannot be neglected. In all the crystal
structures reported herein, the stabilization energy in the lattice is provided by intermolecular
interactions involving both the diarylethene core and the lateral substituents. This is
particularly evident when side groups that induce hard interactions, such as for instance
carboxylic acid, are present. However, soft lateral groups providing weak Van der Waals
interactions between molecules can also have a strong influence on the crystal packing when
they are extended over long alkyl chains. Here, we showed that different substituents
dramatically affect the intermolecular packing arrangement in the crystals, likely driving the
nucleation and crystallization process. We have demonstrated that the extent and the nature of
the interactions they form in the crystals correlate well with the melting temperatures of the
25

compounds. Moreover, we have shown the unique role of long alkyl chains (DTE-7) in
inducing re-crystallization from the melt state in a 3D-zipper fashion. Relatively to all the
other compounds of the series, the unique ability of DTE-7 to re-crystallize from the melt
suggests the existence of a threshold, determined by the number and the length of the alkyl
chains, above which they are sufficiently strong to drive the molecular recognition process
and assembly at the crystalline state both from solution and from the melt. Moreover, by
correlating FT-IR spectroscopy data with DFT calculations, the crystallization process of the
diarylethene substituted with two long alkyl chains (DTE-7) has been followed over time and
rationalized. Finally, the application of the Avrami model suggests that the nucleation process
results in the one dimensional linear growth of a thin lamella.
Acknowledgements
The European Union (“Marie Curie” FP7 IRG grant DETACH to E.P. and OPTICON project, FP7 to A.B. and
C.B.) is gratefully acknowledged for financial support.
The authors thank L. Brambilla for help with FT-IR spectra measurements and A. Cristina for graphical support.
Notes and References
† Ccdc: 1016933-1016937 contain the supplementary crystallo-graphic data for this paper. These data can be
obtained free of charge from the Cambridge crystallographic data center via www.ccdc.cam.ac.uk/data_request/cif.
Electronic Supplementary Material available: [Synthetic procedure, DFT calculation, crystal data, UV-vis spectra
and DSC traces]. See DOI: 10.1039/b000000x/
Corresponding Author
* Rossella Castagna – rossella.castagna@polimi.it
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Table of contents
Substituents are crucial in driving the intermolecular packing arrangement in 1,2-dithienylethenes and affecting the physical
properties of the solid-state materials.
29