Shaping crystals with light: Crystal-to-crystal isomerization and photomechanical effect in fluorinated azobenzenes

Article abstract of DOI:10.1021/ja4063019

Unusually long thermal half-lives of perhalogenated cis-azobenzenes enabled their structural characterization and the first evidence of a crystal-to-crystal cis → trans azobenzene isomerization. Irradiation with visible light transforms a perhalogenated cis-azobenzene single crystal into a polycrystalline aggregate of its trans-isomer in a photomechanical transformation that involves a significant, controllable, and thermally irreversible change of crystal shape. This is the first demonstration of permanent photomechanical modification of crystal shape in an azobenzene.

Full text of DOI:10.1021/ja4063019

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Communication
Shaping Crystals with Light: Crystal-to-Crystal Isomerization
and Photomechanical Effect in Fluorinated Azobenzenes
Oleksandr S. Bushuyev, Anna Tomberg, Tomislav Friscic, and Christopher J Barrett
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4063019 • Publication Date (Web): 07 Aug 2013
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Shaping Crystals with Light: Crystal-to-Crystal Isomerization
and Photomechanical Effect in Fluorinated Azobenzenes
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Oleksandr S. Bushuyev, Anna Tomberg, Tomislav Friščić* and Christopher J. Barrett*
Department of Chemistry and FRQNT Centre for Self-Assembled Chemical Structures, McGill University, 801 Sher-
brooke St. W., H3A 0B8, Montreal, Canada
Supporting Information Placeholder
ization was reported nearly 200 years ago and addressed in
tens of thousands of papers since.
ABSTRACT: Unusually long thermal half-lives of perhalo-
genated cis-azobenzenes enabled their structural characteri-
zation and the first evidence of a crystal-to-crystal cis→trans
We now report that photochemical cis→trans azobenzene
azobenzene isomerization. Irradiation with visible light
isomerization can take place in a crystal-to-crystal fashion.
transforms a perhalogenated cis-azobenzene single crystal
The very long half-lives (ca. 2 months) of cis-forms of azo-
into a polycrystalline aggregate of its trans-isomer in a pho-
benzenes 1 and 2 (Figure 1b,c) allowed their isolation as crys-
tomechanical transformation which involves a significant,
tals capable of cis→trans isomerization12 upon visible light
irradiation. This photomechanical effect is irreversible, which
controllable and thermally irreversible change of crystal
shape. This is the first demonstration of permanent photo-
is a new development in the area of photomechanical azo-
mechanical modification of crystal shape in an azobenzene.
benzene solids, which has so far focused on reversible
switching. Thin crystals of cis-1 and -2 can be shaped by light
into thermally stable cyclic or zigzag forms (Figures 1d, S4-5).
The azobenzene (azo) chromophore is one of the best
studied photochromic groups,1 and its clean, reversible isom-
erization has been investigated in a variety of applications
(Figure 1a).2 While the isomerization efficiency and chemical
stability of the chromophore under multiple isomerization
cycles are high, a recent fascinating aspect of azobenzene-
containing solids is the photomechanical effect,3 i.e. the di-
rect conversion of visible light into mechanical motion based
on the ability of molecular building blocks to change shape
upon irradiation. Such solid-state molecular motion alters
bulk material properties, including shape, thickness, surface
energy etc.4 Photomechanical effect in azobenzene solids was
studied in polymers,5 thin films6 or liquid crystals7 and is
manifested in macroscopic bending of the material or surface
relief gratings formation8 on its surface upon trans
→cis con-
version. Previous work has focused on reversible systems in
which photochemical bending is accompanied by thermal
cis→trans relaxation which restored the object to its initial
shape.3b,c,5-7 However, azobenzene isomerizations in crystals
remain almost unexplored, possibly due to a general opinion
that underlying stereochemical change (120-250 Å3)9 would
be hindered in a crystal and, consequently, that crystalline
azobenzenes are generally not capable of photomechanical
effect. It was recently demonstrated, however, that sufficient-
ly thin azobenzenes crystals allow photochemical trans→cis
isomerization and thermal relaxation10 leading to a rapidly
reversible photomechanical effect. A peculiarity of azoben-
zene crystal chemistry is the paucity of cis-isomer structures,
associated with their short half-lives hindering the isolation
of single crystals.11 The lack of crystallographic data on cis-
azobenzenes is notable considering that azobenzene isomer-
Figure 1. (a) Azobenzene isomerization; (b) cis- and trans-
forms of 1 and 2; (c) cis→trans isomerization of 1 depicted
with molecular structures based on single crystal X-ray dif-
fraction; (d) irreversible bending of a thin crystal of cis-1 by
457 nm light, with the arrow at the top of the figures indicat-
ing the direction of irradiation.
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Both 1 and 2 were synthesized by conventional oxidative
is unprecedented in crystalline azobenzene systems, and has
only very recently15 been reported in alternative16,17 crystalline
photomechanical systems. Earlier trans-azobenzene or pseu-
dostilbene photomechanical systems exhibited motion of
considerably lower extent and only if crystal was less than 5
µm thick, with optimum effect at 1 µm.10 Bending of cis-1 was
accompanied with change of color of the irradiated part of
the crystal from yellow to orange-red, consistent with
cis→trans isomerization. Irradiation of the opposite face of
the crystal allows bending in the opposite direction. This
results first in straightening of the crystal, and then bending
in the opposite direction (Figure 1d, Video S2). After irradiat-
ing both prominent faces, the crystal became inactive to laser
light, which was explained by isomerization of all cis-1 mole-
cules on the surface to trans-1. With the provision of suffi-
cient spatial control, such irreversible bending allows shap-
ing of the crystalline material. Video S3 shows a crystal being
molded into a S-shape by irradiation from two different di-
rections at two different regions. Similar behavior was also
observed for cis-2, although this material was not obtained as
individual needles or plates and polycrystalline aggregates
were used instead (Figure S4).
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coupling, and obtained predominantly as cis-forms (synthet-
ic procedure in SI). The long half-lives of cis-azobenzenes 1
and 2 are in agreement with the recent report by Hecht et
al.13 showed that o-fluorinated azobenzenes can exhibit sig-
nificantly separated trans- and cis-form n-π* absorption
maxima leading to long cis-isomer half-lives. Indeed, the
half-life of cis-1 and -2 determined by kinetic UV-Vis experi-
ments in solution is 60 days in CH2Cl2. The n-π* transitions
for both 1 and 2 are separated by 40 nm (420 nm cis, 460 nm
trans, Figure S1) which in solution allows for switching be-
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tween photostationary states using visible green (trans
→cis)
and blue (cis trans) light. The trans cis isomerization is
→
→
also achievable with ultraviolet light of 320 nm for 1 and 330
nm for 2 via π-π* excitation. Absorption maxima do not
change appreciably with the solvent.
Computational studies of 1 and 2 were done using a densi-
ty functional theory (DFT) method used for similar systems.14
Compound 1 and the parent azobenzene were treated with
B3LYP level of theory with Pople 6-31G(d) basis set. For 2,
calculations required an increased basis set due to the pres-
ence of iodine and so DGDZVP was used. The calculations
(see SI) predict that the energy difference between cis- and
trans-isomers of 1 and 2 are approximately half of the value
calculated for azobenzene: 8.84 kcal mol-1, 8.88 kcal mol-1
and 15.26 kcal mol-1 respectively (Figure 2, also see SI).
That photomechanical behavior of cis-1 is related to isom-
erization is evidenced by powder X-ray diffraction, which
demonstrated the concomitant disappearance of X-ray re-
flections of cis-1 and the appearance of those corresponding
to trans-1 (see SI) upon irradiating a powder sample of cis-1.
We next attempted to follow the transformation of cis-1 into
trans-1 via single crystal X-ray diffraction. Importantly, pho-
tomechanical isomerization of crystalline azobenzenes has
never been ascertained in this manner. It was, therefore, sur-
prising and gratifying to discover that a single crystal of cis-1
upon irradiation with 457 nm laser light transformed into a
polycrystalline sample of trans-1. The transformation is clear-
ly visible from composite diffraction images (Figures 3a-c
show composite images for diffraction of h0l planes, further
ones are provided in the SI Figure S8). The X-ray reflections
of irradiated cis-1 single crystal could be completely indexed
to several differently oriented domains with unit cell parame-
ters identical to those of trans-1. In particular, 903 out of
total 1125 X-ray reflections could be indexed by considering
five such domains (see SI). Indexing of all reflections was
accomplished by considering a total of 16 domains with crys-
tallographic parameters corresponding to those of trans-1.
Although X-ray diffraction data collected from these do-
mains was poor and highly overlapped, using reflections
from one such domain allowed the structure of trans-1 to be
solved and even refined, using a constrained isotropic model,
to a reasonable degree with respect to data quality. Within
the limitations of the available data, the resulting structure
was identical to that of trans-1 obtained from a separately
grown single crystal, unambiguously confirming the crys-
tal→crystal nature18 of the cis→trans transformation in 1.
Figure 2. Energy profiles for trans-cis isomerization of 1 (red)
and the parent azobenzene (blue); the cis-form of 1 is rela-
tively more stable with respect to the corresponding trans-
isomer compared to azobenzene.
Long half-lives of cis-1 and cis-2 allowed the isolation of
both trans- and cis-forms of 1 and 2 and their structural
characterization by single crystal X-ray diffraction. Two con-
comitant polymorphs of cis-1 (SI Table S1) were found: cis-1
and cis-1a Only cis-1 could be obtained as thin needles suita-
ble for photomechanical studies, and is the relevant form for
this work. Crystal structures of trans-1 and -2 reveal planar
molecules, while cis-1 and -2 are V-shaped with intramolecu-
lar angles between phenyl moieties between 74o and 82o (see
SI for further details, Figures S3, S4).
The amenability of cis-1 and -2 crystals to photomechani-
cal shaping is unique in azobenzene chemistry, compared to
previously studied trans-azobenzenes and pseudostilbenes.
Permanent bending in previously studied trans-azobenzene
systems was not achievable due to the rapid equilibrium
formed upon irradiation, in which molecules photochemical-
ly isomerize to the cis-form but also thermally relax back to
the trans-form. As a result, a
To study photoresponsive behavior we irradiated thin nee-
dle crystals of cis-1 with blue 457 nm laser light. Remarkably,
even relatively thick needles of ca. 10-20 µm bent readily and
irreversibly away from the light source, reaching deflection
angles of up to 180° for the tip of the needle (Figure 1d, Video
S1). Controlled photomechanical bending of such magnitude
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are parallel to the crystallographic b-direction which coin-
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cides with the needle crystal long axis. Molecules in neigh-
boring stacks interact by networks of Br···Br interactions
parallel to and perpendicular to the needle axis (Figure 3d,e).
Progressive photomechanical expansion of molecular stacks
which are closer to the source of irradiation might allow the
crystal to bend away from the light source with minimum
fracturing by sacrificing only the Br···Br interactions along
the needle axis (Figure 3e). Although no clear topochemical
relationship was established between the photochemically
generated domains of trans-1 and the parent cis-1 crystal,18
we note a correspondence between arrangements of mole-
cules populating the crystallographic (010) planes of cis-1 and
the (30-2) planes of trans-1. Both planes exhibit a zigzag net
of Br···Br contacts (3.95 Å in cis-1 and 4.02 Å in trans-1), sug-
gesting that this supramolecular motif, normal to the needle
axis of the starting crystal, might be conserved upon photo-
mechanical bending (Figures 3e,f).
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In summary, the exploration of two new perhalogenated
azobenzenes with long-lived cis-forms provided the first
demonstration of an irreversible photomechanical effect in a
crystalline azobenzene solid. Single crystal and powder X-ray
diffraction revealed that the photomechanical effect involved
a single crystal
→ polycrystal transformation, so far unprece-
dented in solid-state chemistry of azobenzenes. The meta-
stable nature of the starting cis-azobenzene and its large
photomechanical response highlight photomechanical shap-
ing as a new exciting opportunity in the solid-state photo-
chemistry of crystalline azobenzenes. We therefore believe
that the irreversible photochemical effect of 1 and 2 opens
new possibilities in the design of photoresponsive azoben-
zene solids.
Figure 3. Composite diffraction images for the h0l planes of
single crystals of: (a) cis-1; (b) trans-1 and (c) cis-1 after 4
hours irradiation at 457 nm, demonstrating the conversion of
a single crystal of cis-1 into polycrystalline trans-1; (d) tenta-
tive model of the photomechanical bending of cis-1 crystal as
cis-molecules progressively isomerize into trans-1 upon light
penetration. Molecular stacks are parallel to the long axis of
the crystal needle. Crystal structure of cis-1: (e) molecular
stacks viewed in the crystallographic ab-plane perpendicular
to irradiation and (f) down the crystallographic b-direction
parallel to the crystal needle axis, displaying the retained
zigzag motif of Br···Br contacts. (g) The (30-2) plane in trans-
1 with molecular arrangement resembling that of cis-1 in (f).
ASSOCIATED CONTENT
Supporting Information
Details of synthesis, videos, images of crystal bending, select-
ed UV-Vis spectra, powder X-ray diffraction data, DFT calcu-
lations and crystallographic data in cif format. This material
is available free of charge via the Internet at
http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
sufficiently high concentration of the cis-form required for
permanent photomechanical bending of the trans-crystal is
difficult to achieve. In the case of cis-1, however, we have not
observed any tendency for reversal of the cis→trans isomeri-
zation in the crystal using eithr blue (457 nm), green (514
nm) or red (633 nm) light. While the irreversible nature of
the isomerization in cis-1 is probably aided by the metastable
nature of the cis-isomer, it is likely that thickness of the crys-
tal or more efficient molecular packing of trans-azobenzene
in the crystal is the key for permanent bending. The latter
possibility is supported by the lower density of cis-1 com-
pared to trans-1. For cis-1 the volume of the unitcell per mol-
ecule if 339.5 Å3 while the corresponding value for trans-1 is
325.5 Å3. We note a similarity between the herein reported
photomechanical shaping of perhalogenated azobenzene
crystals and previously studied bending of haloarene crystals
by mechanical force.19 In crystals of cis-1 and cis-2 molecules
are arranged into stacks which, we propose, could expand
upon photochemical cis→trans isomerization without gener-
ating extensive stress on the nearby stacks. In cis-1 the stacks
Assoc. Prof. Christopher J. Barrett, Assist. Prof. Tomislav
Friščić, Department of Chemistry and FRQNT Centre for
Self-Assembled Chemical Structures, McGill University, 801
Sherbrooke St. W., H3A 0B8 Montreal, Canada. E-mail:
chris.barrett@mcgill.ca; tomislav.friscic@mcgill.ca
ACKNOWLEDGMENT
We acknowledge Prof. D. S. Bohle for aid in obtaining single
crystal X-ray diffraction data. The financial support of the
Vanier Canada Graduate Scholarship is acknowledged
(O.S.B), as well as the support of the NSERC Discovery
Grant, Canada Foundation for Innovation Leader's Oppor-
tunity Fund and FRQNT Nouveaux Chercheurs program.
REFERENCES
1. (a) Bandara, H. M. D.; Burdette, S. C. Chem. Soc. Rev.2012, 41,
1809. (b) Beharry, A. A.; Woolley, G. A. Chem. Soc. Rev. 2011, 40,
4422.(c) Irie, M. Chem. Rev. 2000, 100, 1683.
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Page 4 of 4
2. (a) Russew, M-M.; Hecht, S. Adv. Mater. 2010, 22, 3348.; b)
azobenzene derivatives (CSD codes AZBENC, AZBENC01 and
Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (c) Holme, N. C. R.;
Ramanujam, P. S.; Hvilsted, S. Opt. Lett. 1996, 21, 902. (d) Gorostiza,
P.; Isacoff, E. Y. Science 2008, 322, 395.
3. (a) Mahimwalla, Z.; Yager, K. G.; Mamiya, J.; Shishido, A.;
Priimagi, A.; Barrett, C. J. Polym. Bull. 2012, 69, 967. (b) Yu, Y. L.;
Nakano, M.; Ikeda, T. Nature 2003, 425, 145.(c) Ikeda, T.; Nakano, M.;
Yu, Y. L.; Tsutsumi, O.; Kanazawa, A. Adv. Mater. 2003, 15, 201; (d) P.
Naumov, S. C. Sahoo, B. A. Zakharov, E. V. Boldyreva, Angew. Chem.
Int. Ed. 2013, DOI: 10.1002/anie.201303757
4. (a) Tanchak, O. M.; Barrett, C. J. Macromolecules 2005, 38,
10566. (b) Stiller, B.; Knochenhauer, G.; Markava, E.; Gustina, D.;
Muzikante, I.; Karageorgiev, P.; Brehmer, L. Mater. Sci. Eng. C-
Biomimetic Supramol. Syst. 1999, 8-9, 385.
FECXIU). 46 further structures were found of generally photoinac-
tive azo-groups bound to metals and N-oxide derivatives.
12. Thermo- or photochemical cis→trans azobenzene isomeriza-
tion in the solid state was first studied by Tsuda, M.; Kuratani, K.
Bull. Chem. Soc. Jap. 1964, 37, 1284.
13. (a) Bléger, D.; Schwarz, J.; Brouwer, A. M.; Hecht, S. J. Am.
Chem. Soc. 2012, 134, 20597; (b) Beharry, A. A.; Sadovski, O.; Wool-
ley, G. A. J. Am. Chem. Soc. 2011, 133, 19684.
14. (a) C.R. Crecca, A. E. Roitberg, J. Phys. Chem. A 2006, 110, 8188;
(b) L. Gagliardi, G. Orlandi, F. Bernardi, A. Cembran, M. Garavelli,
Theor. Chem. Acc. 2004, 111, 363.
15. Sun, J.-K.; Li, W.; Chen, C.; Ren, C.-X.; Pan, D.-M.; Zhang, J.
Angew. Chem. Int. Ed. 2013, 52, 6653.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5. (a) White, T. J.; Tabiryan, N. V.; Serak, S. V.; Hrozhyk, U. A.;
Tondiglia, V. P.; Koerner, H.; Vaia, R. A.; Bunning, T. J. Soft Matter
2008, 4, 1796; (b) Serak, S.; Tabiryan, N.; Vergara, R.; White, T. J.;
Vaia, R. A.; Bunning, T. J. Soft Matter 2010, 6, 779; (c) Lee, K. M.;
Smith, M. L.; Koerner, H.; Tabiryan, N.; Vaia, R. A.; Bunning, T. J.;
White, T. J. Adv. Funct. Mater. 2011, 21, 2913; (d) Yamada, M.; Kondo,
M.; Mamiya, J. I.; Yu, Y. L.; Kinoshita, M.; Barrett, C. J.; Ikeda, T.
Angew. Chem. Int. Ed. 2008, 47, 4986.
6. Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288, 1624.
7. (a) Yu, Y. L.; Nakano, M.; Shishido, A.; Shiono, T.; Ikeda, T.
Chem. Mat. 2004, 16, 1637. (b) Yu, H. F.; Ikeda, T. Adv. Mater. 2011,
23, 2149.
8. (a) Priimagi, A.; Cavallo, G.; Forni, A.; Gorynsztejn-Leben, M.;
Kaivola, M.; Metrangolo, P.; Milani, R.; Shishido, A.; Pilati, T.; Res-
nati, G.; Terraneo, G. Adv. Funct. Mater. 2012, 22, 2572. (b) Priimagi,
A.; Saccone, M.; Cavallo, G.; Shishido, A.; Pilati, T.; Metrangolo, P.;
Resnati, G. Adv. Mater. 2012, 24, OP345; (c) Nakano, H.; Tanino, T.;
Shirota, Y. Appl. Phys. Lett. 2005, 87; (d) Nakano, H. J. Phys. Chem. C
2008, 112, 16042.
9. T. Naito, K. Horie, I. Mita, Macromolecules 1991, 24, 2907.
10. (a) Bushuyev, O.; Singleton, T. A.; Barrett, C. J. Adv. Mater.
2013, 25, 1796; (b) Koshima, H.; Ojima, N.; Uchimoto, H. J. Am.
Chem. Soc. 2009, 131, 6890.
16. (a) Naumov, P.; Kowalik, J.; Sointsev, K. M.; Balridge, A.; Moon,
J.-S.; Kranz, C.; Tolbert, L. M. J. Am. Chem. Soc. 2010, 132, 5845; (b)
Morimoto, M.; Irie, M. J. Am. Chem. Soc. 2010, 132, 14172; (c) Zhu, L.;
Al-Kaysi, R. O.; Bardeen, C. J. J. Am. Chem. Soc. 2011, 133, 12569.
17. (a) Kim, T.; Al-Muhanna, M. K.; Al-Suwaidan, S. D.; Al-Kaysi,
R. O.; Bardeen, C. J. Angew. Chem. Int. Ed. 2013, 52, 6889; (b) Ko-
shima, H.; Takechi, K.; Uchimoto, H.; Shiro, M.; Hashizume, D.
Chem. Commun. 2011, 47, 11423; (c) Kobatake, S.; Takami, S.; Muto,
H.; Ishikawa, T.; Irie, M. Nature 2007, 446, 778; (d) Al-Kaysi, R. O.;
Müller, A. M.; Bardeen, C. J. J. Am. Chem. Soc. 2006, 128, 15938.
18. The reaction is of a single-crystal→polycrystal transformation,
see also: (a) Salzillo, T.; Bilotti, I.; Della Valle, R. G.; Venuti, E.; Bril-
lante, A. J. Am. Chem. Soc. 2012, 134, 17671; (b) Kaftory, M.; Botoshan-
sky, M.; Kapon, M.; Shteiman, V. Acta Crystallogr. B. 2001, 57, 791;
(c) Halasz, I. Cryst. Growth Des. 2010, 10, 2817; (d) Friščić, T.; Mac-
Gillivray, L. R. Z. Krist. 2005, 220, 351; (e) Keating, A. E.; Garcia-
Garibay, M. A. Photochemical solid-to-solid reactions in: Molecular
and Supramolecular Photochemistry, Vol. 2 Eds: Ramamurthy, V.;
Schanze, 1998; (f) Gougoutas, J. Z. Pure Appl. Chem. 1971, 27, 305.
19. (a) Reddy, C. M.; Gundakaram, R. C.; Basavoju, S.; Kirchner, M.
T.; Padmanabhan, K. A.; Desiraju, G. R. Chem. Commun. 2005, 3945;
(b) Reddy, C. M.; Kirchner, M. T.; Gundakaram, R. C.; Padmanabhan,
K. A.; Desiraju, G. R. Chem. Eur. J. 2006, 12, 2222; (c) Reddy, M. C.;
Padmanabhan, K. A.; Desiraju, G. R. Cryst. Growth Des. 2006, 6,
2720.
11. The search of the recent edition of the CSD (2013, ConQuest
Version 1.15, build RC5) yielded 18 structures of molecules containing
a free cis-azobenzene unit, of which only three correspond to simple
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