Thiourea as a template for photodimerization of azastilbenes

Article abstract of DOI:10.1021/ja105166d

In this study we have explored the potential of thiourea (TU) as a template to preorient stilbazoles and bispyridylethylenes (azastilbenes) in the crystalline state. TU is able to preorient eleven azastilbenes toward dimerization in the crystalline state. While cocrystals of these eleven olefins photodimerized to a single dimer expected based on crystal packing, pure crystals of these olefins either were nonreactive or gave a mixture of dimers. The differential photobehavior of the pure crystals and cocrystals highlights the importance of TU in templating the olefins in a photoreactive orientation in the crystalline state. X-ray crystallographic and photochemical studies have identified a few azastilbenes that photodimerize in spite of not being arranged in an ideal orientation in the crystalline state. These as well as a few examples already reported in the literature suggest that it is important to recognize that molecules could experience large amplitude motions in the crystalline state, especially when energized by light. Short-term lattice instability caused by photoexcitation can be effective in driving a photochemical reaction. Thus one should view the crystalline arrangement of molecules upon light exposure as dynamic rather than static as determined from X-ray structure analysis.

Full text of DOI:10.1021/ja105166d

Published on Web 09/03/2010
Thiourea as a Template for Photodimerization of Azastilbenes
Balakrishna R. Bhogala, Burjor Captain, Anand Parthasarathy, and V. Ramamurthy*
Department of Chemistry, UniVersity of Miami, Coral Gables, Florida 33146
Received June 13, 2010; E-mail: murthy1@miami.edu
Abstract: In this study we have explored the potential of thiourea (TU) as a template to preorient stilbazoles
and bispyridylethylenes (azastilbenes) in the crystalline state. TU is able to preorient eleven azastilbenes
toward dimerization in the crystalline state. While cocrystals of these eleven olefins photodimerized to a
single dimer expected based on crystal packing, pure crystals of these olefins either were nonreactive or
gave a mixture of dimers. The differential photobehavior of the pure crystals and cocrystals highlights the
importance of TU in templating the olefins in a photoreactive orientation in the crystalline state. X-ray
crystallographic and photochemical studies have identified a few azastilbenes that photodimerize in spite
of not being arranged in an ideal orientation in the crystalline state. These as well as a few examples
already reported in the literature suggest that it is important to recognize that molecules could experience
large amplitude motions in the crystalline state, especially when energized by light. Short-term lattice
instability caused by photoexcitation can be effective in driving a photochemical reaction. Thus one should
view the crystalline arrangement of molecules upon light exposure as “dynamic” rather than “static” as
determined from X-ray structure analysis.
Introduction
In contrast to solution chemistry, following the original
discovery in 1871 by Libermann,28 photodimerization in the
solid state has been extensively explored with several elegant
approaches. The topochemical postulation of Schmidt suggests
“reaction in the solid state occurs with a minimum amount of
atomic and molecular movement”.29,30 Accordingly, several
strategies have been adopted to bring CdC bonds of the reactive
olefin pair within the required 4.2 Å and parallel conditions in
the solid state so that they would dimerize upon excitation. This
Photocycloaddition of olefins has proven to be a valuable
synthetic tool to construct strained cyclobutane frameworks.1,2
However, this approach works mostly with molecules where the
CdC bond is part of a relatively rigid cyclic system. In solution,
when the CdC bond is part of either an acyclic or a larger cyclic
system (>C8), [2 + 2] cycloaddition is unable to compete with
facile geometric isomerization process. Furthermore, in solution
[2 + 2] cycloaddition yields a stereoisomeric mixture of cyclobu-
tanes, whose ratio depends on the Woodward-Hoffmann selection
rules, electronic and steric factors and stability of diradical
intermediates. The strategy of use of template molecules to align
two olefinic molecules closer in a specific geometry favors a select
dimer.3-11 The pioneering study in this context due to that of de
Mayo and co-workers templated enones at the aqueous-micellar
interface to photodimerize them to a single dimer.12-15 Recently,
this approach has been extended to the use of well-defined water-
soluble cavitands such as cyclodextrins, cucurbiturils, calixarenes,
octa acid and Fujita Pd host.16-27
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13434 J. AM. CHEM. SOC. 2010, 132, 13434–13442
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Role of TU in Photodimerization of Azastilbenes
A R T I C L E S
presentation is concerned with one such strategy, namely,
“templation” in the solid state.
tell example.55,56 In addition to weak interactions indicated
above, confinement provided by well-defined host systems has
also been exploited to template photodimerization in the solid
state. In this context, studies in inorganic hosts such as zeolites
and clays and organic hosts such as bis-urea and cucurbituril
crystals are noteworthy.57-63
The “crystal engineering” approach initiated by Schmidt that
has resulted in identification of chloro and fluoro groups, among
others, as steering groups to close pack olefins within 4.2 Å
requires structural modification of the olefin of interest.31-34
A
The above-reported hydrogen bonded templated strategy
unfortunately suffers from the limitation that a minor variation
in the structure of the olefin requires a corresponding one on
the template as well. Finding a new template for each olefin
(termed “template switching”) is cumbersome. Although a
universal template that would orient all olefins in the solid state
may not be possible, identification of one that would steer more
than a few olefins is worthwhile. Numerous templates have been
reported for 4,4′-BPE. Thiourea (TU) fulfilled our interest in
identifying a template that would orient olefins other than 4,4′-
BPE. We begin the presentation with its use with 4,4′-BPE and
continue with ten different 1-(4-pyridyl)-2-arylethylenes (stil-
bazoles; Scheme 1). A remarkable nine of the eleven investi-
gated olefins were templated by TU in the solid state. As
predicted by X-ray crystal structure they photodimerized to a
single dimer in significant yields (80 to 100%). The CdC bonds
of olefins 10 and 11 though oriented similarly to 3-9 were
separated by more than 4.5 Å and expectedly were not reactive
in the solid state. We close the presentation by disclosing our
results on three more bispyridylethylenes (2,4′-BPE, 2,3′-BPE
and 2,2′-BPE; 12, 13 and 14 in Scheme 1), all of which
photodimerized as TU cocrystals 12c, 13c and 14c, respectively.
better approach using an independent molecule to steer olefins
through noncovalent interactions, termed “templation”, has been
explored by a number of groups during the past two decades.
Early reports on this topic came from the group of Toda, who
used various diols to orient olefins through weak intermolecular
hydrogen bonds.35-40 They achieved photodimerization of
chalcone, dibenzylideneactone, pyridones, coumarin, thiocou-
marin and cyclohexenone in the solid state with the help of
different diol templates for different olefins. A stronger Cou-
lombic interaction between the template and the olefin was
exploited to orient olefins by Ito, Scheffer and their co-workers
for the same goal.41,42 In this approach an olefin’s acid group
(e.g., cinnamic acid) was complexed with a diamine template
(e.g., ethylene diamine) with the resulting salt orienting the
olefins in the solid state toward photodimerization. This method
is thus restricted to olefins containing either a COOH or NH2
group.
The invaluable templating strategy has gained the attention
of a number of groups recently. MacGillivray’s group has
identified 1,3-dihydroxybenzene as template to preorient 4,4′-
bispyridylethylene (4,4′-BPE) toward dimerization in the crys-
talline state.43,44 Vittal and co-workers reported a number of
metal-organic coordination polymers based on the concept of
templated phototodimerization of 4,4′-BPE.45,46 In this context,
one should also note that a few studies have expanded the
strategy to include unsaturated dicarboxylic acids.47-49 Litera-
ture reveals 4,4′-BPE to be a benchmark system to test the value
of a molecule as a solid-state template.50-54 The synthesis of
ladderanes from 1,4-bis(4-pyridyl)-1,3-butadiene and 1,6-bis(4-
pyridyl)-1,5-hexatriene in the solid state using substituted 1,3-
dihydroxy-5-methoxybenzene as templates is a fine show and
Results and Discussion
The present study consisted of the following aspects: crystal-
lization and crystal structure analysis of the cocrystals of thiourea
and olefins followed by irradiation and finally characterization
of photoproducts. Analyses of Cambridge Structural Database
(CSD) suggested that TU forms two types of hydrogen bonds
with a guest molecule (type I and type II, Scheme 2b). We
recognized that these structural motifs could be utilized to
prealign olefins for photodimerization in the crystalline state.
This has been realized in this study with fourteen different
olefins. The methods of cocrystallization, procedures for ir-
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Scheme 1. Olefins and Their Thiourea Cocrystals Investigated in This Studya
a Olefins are numbered as 1, 2, etc. and TU cocrystals as 1c, 2c etc. See crystallographic analysis in the Supporting Information for ratio of individual
components in cocrystals.
Scheme 2. Representation of Thiourea Tapes and Types of Hydrogen Bonds Thiourea Forms with Hydrogen Bond Acceptors
X-ray crystal structure analyses of cocrystals along with CIF
files are provided in the Supporting Information.
the molecules of 1 stack one on top of the other and the
intermolecular packing distance for the 4,4′-BPE molecules
affords their CdC bonds to be 4.15 Å apart, a value fulfilling
Schmidt’s topochemical criterion for photodimerization. Based
on the packing derived from X-ray crystal structure analysis
one would expect a syn-dimer to be formed.
Crystallographic Studies. As 4,4′-BPE is a benchmark
example with several templates, we first tested if TU would
preorient it toward dimerization in the solid state. Analysis by
single crystal X-ray diffraction of 1:1 TU cocrystals of 1c
(Scheme 1) revealed that the structure contains a single molecule
each of TU and 4,4′-BPE in the asymmetric crystal unit. As
can be seen in Figure 1, TU through its two anti hydrogens
interacts with the nitrogen atoms of 1. One TU molecule
interacts with two different molecules of 1, and this helps to
bring them closer. In addition, as expected, the syn hydrogens
of TU interact with the sulfur atoms of the other TU molecules
in the crystal lattice in a zigzag manner to form a one-
dimensional tape. Interaction of TU tapes with 4,4′-BPE on both
sides results in a two dimensionally ordered array as shown in
Figure 1a. One could visualize the packing as a ladder where
the BPE are the rungs and the TU tapes its arms. In this structure
The above packing arrangement prompted us to investigate
4-cyanostilbazole (2), a molecule which like 1 contains two
nitrogen atoms (one pyridyl and one cyano) that can potentially
hydrogen bond with TU. We anticipated 2 to orient similarly
to 4,4′-BPE. The packing arrangement derived from X-ray
crystal structure analyses of 2c is shown in Figure 1b. Just as
in 1c, the anti hydrogen atom of two TU molecules on either
side interacted with the pyridyl and cyano nitrogen atoms of
two molecules of 2. The molecules of 2 do stack one on top of
the other in a parallel fashion leading to a head-head arrange-
ment. In this assembly, the intermolecular packing distances
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Role of TU in Photodimerization of Azastilbenes
A R T I C L E S
Figure 1. Packing arrangements of (a) TU ·4,4′-BPE cocrystal (1c) and (b) TU ·4-cyano stilbazole cocrystal (2c).
Table 1. N-H· · ·N Distances and Angles for 1c and 2ca
d (Å)
∠N-H· · ·N
cocrystal
N-H· · ·N
NH· · ·N HN· · ·N
(deg)
1c (TU · 1)
N(3)-H(3A) · · · N(16A)
N(4)-H(4A) · · · N(5A)
2.30
2.21
2.43
2.10
2.51
2.26
3.050(4)
2.989(4)
3.045(3)
2.944(3)
3.095(4)
3.104(2)
146
150
130
169
126
165
2c (TU)2 · (2)2 N(3)-H(3A) · · · N(9A)
N(4)-H(4A) · · · N(25A)
N(7)-H(7A) · · · N(40A)/CN
N(8)-H(8A) · · · N(24A)/CN
a N-H refers to thiourea and N to pyridyl ring.
Figure 2. Packing arrangements of TU cocrystal with stilbazole 3.
between the CdC carbon atoms are 3.97 Å and 4.12 Å, within
the topochemical limit for photodimerization.
atom. In this context we investigated nine trans-1-(4-pyridyl)-
2-arylethylenes (Scheme 1). To our knowledge no attempts to
steer stilbazoles in the crystalline state have been reported except
for those by MacGillivray’s group on 4-chlorostilbazole and
stilbazole where syn head-head dimers resulted through tem-
plation with resorcinol and Ag ion respectively.66,67 We were
pleased to find TU capable of templating seven of the nine
stilbazoles we investigated toward anti head-tail dimers. The
packing arrangements for all nine stilbazoles are identical (anti
head-tail) although in two the distance between the reactive
olefins exceeded the Schmidt’s topochemically allowed distance
for photodimerization.
Packing arrangement obtained from single crystal X-ray analysis
of 3c containing only one hydrogen-bonding atom (on the reactive
olefin 3) is shown in Figure 2 (note the different packing
arrangements for 2c and 3c in Figures 1b and 2). Notably it is
important to point out that stilbazole forms type II hydrogen
bonding (Scheme 2b) with TU tape (Figure 2). In this two
dimensionally ordered system, the stilbazoles do stack one on top
of another but with a head to tail orientation. This can also be
viewed as the molecules of 3 are interdigitated between two TU
one-dimensional tapes (Figure 3). A similar packing pattern was
noticed with 4c-11c (Figures 4 and 5). The maintenance of the
head-tail alignment with even 9c suggests that the dichloro
substitution often used to align olefins in a head-head fashion was
offset by hydrogen bonding by the TU template.34 In cocrystals
3c-9c the reactive carbon atoms of CdC bond are within 4.2 Å
and in the remaining two cases (10c and 11c) more than 4.4 Å
apart. Based on packing arrangements one would expect anti
head-tail dimers from 3c to 9c.
Examination of the packing arrangements of 1c and 2c (Figure
1) suggests that, when there are two hydrogen bonding centers
in the guest olefin, the preferred packing is likely to be the ladder
type with TU tapes serving as arms. In this context, TU behaves
similarly to 1,3-dihydroxybenzene, 1,8-naphthalene dicarboxylic
acid, Rebek’s imide, and 1,2,4,5-tetracraboxylic acid where a
single template molecule through hydrogen bonding to two
molecules of 4,4′-BPE brings them within Schmidt’s to-
pochemical distance.43 The difference between TU and the
templates mentioned above is that TU forms a tape holding
several guest molecules like the rungs of the ladder while the
others act as an independent 2:2 host to guest complex. Thiourea
forms an infinite tape structure in which the azastilbene
molecules are held within reactive distance. In this context
thiourea as a template differs from the well-examined resorcinol
where two molecules of template hold two molecules of the
olefin. The tape formed by thiourea is comparable to urea and
3-aminophenol reported in the literature.64,65
In 1c and 2c, TU templates the reactants by relatively weak
N-H· · ·N hydrogen bonding interactions rather than strong
O-H· · ·N interactions or strong metal-ligand interactions. The
distance between two N-H donors in a TU molecule is about
2.3 Å, which is shorter than the π-π stacking distance between
two reactant molecules. Therefore, the hydrogen bond interac-
tions (type I) linking the pyridine and cyano groups to the TU
template in 1c and 2c are bent (see the last column in Table 1).
This most likely results from forced short distance between π
clouds of the two olefins. A list of these N-H· · ·N interactions
is provided in Table 1.
We were then interested in probing TU’s ability to template
bispyridylethylenes other than the benchmark 4,4′-BPE. In this
context we examined 2,4′-, 2,3′- and 2,2′-bispyridylethylenes (2,4′-
Having established the ability of TU to template 4,4′-BPE in
the solid state, we proceeded to investigate, more importantly,
if it can template olefins containing only one hydrogen bonding
(66) MacGillivray, L. R.; Reid, J. L.; Ripmeester, J. A.; Papaefstathiou,
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A R T I C L E S
Bhogala et al.
Figure 3. Interdigitation of thiourea-stilbazole tapes results in head-tail orientation of reactive stilbazole units. Note the reactive olefins are held by
different thiourea tapes.
Figure 4. Packing arrangement of stilbazole (a) 4, (b) 5, (c) 6, (d) 7, (e) 8 and (f) 9 in TU cocrystals.
BPE, 2,3′-BPE and 2,2′-BPE respectively). All three BPEs gave
2:1 (TU to BPE) cocrystals from appropriate solvents (see Table
S4 in SI). Examination of the packing arrangements for TU
cocrystals with these BPEs shown in Figure 6 reveals a similarity
between packings of the former two (2,4′-BPE, 2,3′-BPE) with
those of TU-stilbazoles shown in Figures 2, 4 and 5 and would be
expected to yield anti head-tail dimer. With a unique packing
arrangement where the parallel CdC bonds are separated by 8.16
Å, TU-2,2′-BPE cocrystal would not be expected to dimerize.
Photochemistry of TU-Bispyridylethylene and TU-Stil-
bazole Cocrystals. Procedures for irradiation, isolation and
identification of products are provided in Supporting Informa-
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Role of TU in Photodimerization of Azastilbenes
A R T I C L E S
Figure 5. Packing arrangement of stilbazole (a) 10 and (b) 11 in TU cocrystals.
Figure 6. Packing arrangement of (a) 2,4′-BPE (12), (b) 2,3′-BPE (13) and (c) 2,2′-BPE (14) in TU cocrystals.
tion. Briefly, samples of about 8-10 mg of powdered single
crystals of TU-olefin cocrystals 1c-14c spread uniformly
between two Pyrex glass plates over 10-15 cm2 area and sealed
with parafilm were irradiated (>290 nm). For uniform irradiation
the samples were turned around every 4 h and respread between
the plates every 8 h and generally irradiated for 24 h. Irradiation
was discontinued after 12 h in the few cases where the samples
became powdery or gummy. The progress of the reaction was
powdered cocrystals of 2c gave a single product identified to
1
be the syn head-head dimer (syn HH; 16, Scheme 3) by H
1
NMR. As per H NMR no other products were present; the
reaction was clean and reached 100% conversion in 18 h of
irradiation. Cocrystals 3c-8c gave anti head-tail (anti HT, 17,
Scheme 3) as the single product in each case (see Supporting
Information for the 1H NMR spectra). In all examples the yield
of the dimer was >90%. While cocrystals of olefins 1c-8c
photodimerized to a single dimer, pure crystals of the olefins
1-8 either were nonreactive or gave a mixture of dimers.
Perusal of Table 2 reveals the photobehavior of 9c to be
different from what would be predicted based on the packing
pattern shown in Figure 4f. Unlike the expected single dimer,
namely anti head-tail, a mixture of anti head-tail and syn
head-head dimers (18 and 19, Scheme 3) was obtained. We
believe that syn head-head dimer most likely results from pure
crystals of 9 that are present along with 9c. To ascertain this,
single crystals of 9 were irradiated and its structure was obtained
by X-ray crystal structure analysis. Consistent with the packing
arrangement for 9 provided in Supporting Information, irradia-
tion of its crystals gave syn head-head dimer in quantitative
yield. Our attempts to obtain pure crystals of 9c were not
successful. This was probably due to the ability of compound
9 to readily crystallize independently rather than as cocrystals.
1
monitored by recording H NMR, in DMSO-d6, of a small
amount of the sample taken from the above irradiations. Upon
reaction completion the samples were taken in D2O and
extracted with CDCl3 to obtain 1H NMR spectra of the products.
At this stage all TU was removed from the product. Crystal-
lographic details, stereochemistry and yields of dimers are
provided in Table 2. Reactivity and the structure of the dimer
are consistent with the packing arrangement shown in Figures
1, 2, 4, 5 and 6. As expected, the cocrystals 10c and 11c were
unreactive (see Figure 5).
Irradiation of powdered 1c gave a single product that was
identified to be the syn-dimer (15, Scheme 3) by 1H NMR and
1
comparison with authentic sample. According to H NMR no
other products were formed and the reaction was clean. The
conversion could not be taken beyond 90% as the crystallinity
was lost with the progress of photodimerization. Irradiation of
9
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A R T I C L E S
Bhogala et al.
Table 2. Packing Details in Cocrystals 1c-14c. Dimerization Yield and the Nature of Dimer are also Included
no. of symmetry independent
azastilbenes in AU
overall parallel
parallelisma (%)
cocrystal number
space group
disorder ratio
dimer yieldb (%)
TU·1
P21/n
1
2
4
78:22
100
∼90%
1c
(TU)2 ·(2)2
2c
(TU)4 ·(3)4
3c
P21/c
(1) 91:9;
∼9-20%
32
∼100 syn HH
∼100 anti HT
(2) ∼90:10 (not modeled)
Pn
(1) 56:44;
(2) 80:20;
(3) no disorder;
(4) no disorder
56:44
TU·4
4c
TU·5
5c
TU·6
6c
(TU)2 ·(7)2
7c
(TU)2 ·(8)2
8c
TU·9
9c
(TU)2 ·(10)2
10c
(TU)2 ·(11)2
11c
P21/c
C2/c
C2/c
P21/c
1
1
1
2
2
1
2
2
1
1
1
100
80
∼100 anti HT
∼100 anti HT
∼100 anti HT
∼98 anti HT
∼90 anti HT
60:40
59:41
82
(1) 51:49;
94
(2) 57:43
(1) 86:14;
(2) no disorder
j
P1
100
0
Pna21
P21/c
no disorder
∼80c anti HT + syn HH
(1) 50:50;
(2) 77:23
(1) 76:24;
(2) no disorder
84:16
73
no reaction
j
P1
100
100
100
0
no reaction
j
P1
(TU)2 ·(12)2
12c
(TU)2 ·(13)2
13c
(TU)2 ·(14)2
14c
100
j
P1
82:18
100
P21/c
no disorder
∼30% from bulk sample
a Overall parallel alignment of olefinic groups of reacting pairs in a cocrystal was calculated as outlined below with 3c as an example. Of the four
symmetry independent olefin molecules present in an asymmetric unit two are disordered. These four olefin molecules form two reacting pairs (two
disordered molecules forming one pair and the two non-disordered molecules forming the second pair). In the disordered set 56% (major part) of first
molecule (see values in the table) is parallel with 20% (minor part) of second molecule and 44% (minor part) of first molecule is parallel with 80%
(major part) of the second molecule. In this pair 36% [(36 + 36)/200] of the molecules remain crossed. In the non-disordered pair the olefinic bonds are
crossed (100%). Overall (combination of two sets) 68% [(36 + 100)/(100 + 100)] of molecules are crossed and 32% of molecules are parallel. b The
photodimerizations are completed approximately between 15 and 30 h. c In 9c the dimerization went up to 80% in about 36 h (the sample was mixed
and spread between the Pyrex plates and monitored by 1H NMR after every 5 h). Mixture of anti HT and syn HH dimers was obtained in 55:45 ratio.
The reason for the formation of a mixture of isomers could be attributed to the presence of uncomplexed 9 along with 9c where the former one yields
syn HH dimer upon photoexcitation (see Figures S10 and S19).
Scheme 3. Structures of Photoproducts
Excepting 9c, no other cocrystals, 1c-8c, were contaminated
with pure olefin crystals, 1-8.
However, irradiation of ground powder of TU and 2,2′-BPE in
the ratio of 2:1 gave quantitatively syn-dimer (22, Scheme 3). X-ray
quality single crystals of these complexes could not be obtained.
We believe that the reactive form must be a different polymorph
of the cocrystal whose structure is presented in Figure 6.
Irradiation of crushed TU cocrystals with 2,4′- and 2,3′-BPE
gave anti head-tail dimers (20 and 21, Scheme 3) as the single
1
product (see Supporting Information for H NMR). Quantitative
conversion to product was achieved in about 20 h. As expected
based on packing arrangement crystals of 14c were photoinert.
In conclusion, we have been able to preorient twelve bispy-
ridylethylenes and stilbazoles toward dimerization in the solid state
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13440 J. AM. CHEM. SOC. VOL. 132, NO. 38, 2010

Role of TU in Photodimerization of Azastilbenes
A R T I C L E S
Scheme 4. Various Motions That Could Bring the Criss-Crossed
CdC Bonds to Parallel Arrangementa
with a single template, TU. While cocrystals of these olefins
photodimerized to a single dimer expected based on crystal packing,
pure crystals of these olefins either were nonreactive or gave a
mixture of dimers. The differential photobehavior of the pure
crystals and cocrystals highlights the importance of TU in tem-
plating the olefins in a photoreactive orientation in the crystalline
state. Examination of packing arrangements shown in Figures 1,
2, 4, 5 and 6 and the crystallographic data provided in Table 2
suggests that not all olefins are packed in an ideal geometry as
required by topochemical postulate: for example, the CdC bonds
in reactive pairs are criss-crossed in some (2c, 3c and 9c) and the
two ends of the reactive CdC not equidistant in others (see 5c,
6c, 7c); the photodimerization nevertheless occurs up to at least
90% conversion.
Photodimerization under Less than Ideal Topochemical
Conditions. It is important to note that the olefin pairs in most
cocrystals investigated in this study were disordered over two
orientations. The disorder was modeled during the crystal-
lographic analysis. Details of refinement and disorder treatment
are provided in Supporting Information. In addition to the
disorder problem, the CdC bonds in reactive pairs of olefins
in TU cocrystals 2c, 3c and 9c although within 4.2 Å are criss-
crossed (see Table 2 for details). In Table 2 this is given as
“extent of parallel arrangement”. While, in 2c and 3c, the
percentage of molecules that are not parallel is in the range of
68 to 80%, in the case of 9c all molecules are criss-crossed.
Yet these olefins are reactive. Several such examples have come
to light since the original formulation of topochemical postulate
by Schmidt.68-71 A few molecules where the CdC bonds are
not parallel and the distances are more than 4.2 Å have been
reported to react. Similarly, there are examples where the
reactive pairs being within 4.2 Å were photoinert.72,73 Since
the formulation of Schmidt’s topochemical postulate it has
become clear that molecules in a crystal undergo various types
of motions at room temperature and some of these could aid
photoreactivity in the solid state.74,75 However, it is important
to note that some of these motions occur only upon activation
of molecules to the excited state and do not occur at room
temperature in the absence of light.
a Simultaneous rotation marked as 1 is the pedal-like rotation of one of
the two CdC bonds. Markings 2 represent clockwise in-plane movement
of one of the two CdC bonds. Markings 3 represent anti-clockwise in-
plane movement of one of the two CdC bonds.
suggest that, as long as the surrounding molecules allow, even
the criss-crossed pair could photodimerize by adopting a parallel
arrangement upon excitation. This expectation is along the lines
of a suggestion originally put forward by Craig and co-workers
that a reaction not expected based on ground state structure may
indeed occur if localized excitation of a molecular crystal
produces instability of the lattice configuration.79,80 This
instability that could result in large displacements from the
original lattice structure could lead to a photoreaction unexpected
based on ground-state topochemical postulate. Thus even in
systems where the molecules are not fully prealigned, dimer-
ization could occur as long as two molecules are close to each
other and the neighboring molecules allow modest molecular
motion of the excited molecule in the crystal.
In the case of 2c and 3c, more than 65% reactive pairs are
criss-crossed (Table 2). In spite of such an unfavorable
arrangement, 2c gave syn head-head dimer and 3c gave anti
head-tail dimer in quantitative yield. However, 9c where all
reactive pairs are criss-crossed gave a mixture of anti head-tail
1
dimer (see Supporting Information for H NMR spectra). As
illustrated in Scheme 4, one could visualize two possibilities
of bringing the criss-crossed pair to parallel arrangement. In
the first possibility, one of the two molecules displaces along
the molecular plane as visualized in the case of 7-methoxycou-
marin, a motion that could occur in either a clockwise or an
anticlockwise manner.81 The other possibility is a pedal-like
motion of one of the two molecules. Occurrence of pedal-like
motion even in the ground state of stilbene-like molecules has
been well established.82,83 Such motion has been invoked during
templated photodimerization of trans-cinnamide and trans-2,4-
dichlorocinnamic acid.70,71 Thus, occurrence of both types of
motion has precedence. Depending on the above two motions
different isomers would result as is illustrated in Scheme 3 for
2c. Quantitative yield of only the syn head-head dimer from
2c suggests the dimerization to be facilitated by pedal-like
motion and not lateral displacement. Similar arguments could
be made for 3c and 9c.
Doubt on the generality of the postulate that large motions
would not be tolerated in a single crystal has been raised by
the discovery of several photochemically initiated cis-trans
isomerizations of olefins in crystals.76-78 Cis-trans isomer-
ization requires much larger motion than the one required to
bring non-topochemically aligned olefin pairs toward dimer-
ization. Occurrence of cis-trans isomerization in crystals
(68) Murthy, G. S.; Arjunan, P.; Venkatesan, K.; Ramamurthy, V.
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One question that remains to be resolved is whether the pedal
motion occurs before or after excitation. While in 9c it must
(74) Dunitz, J. D.; Scomkaher, V.; Trueblood, K. N. J. Phys. Chem. 1988,
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A R T I C L E S
Bhogala et al.
occur upon excitation, in 2c and 3c a small fraction of molecules
that are oriented properly could absorb the light to yield the
dimer. As the dimerization proceeds more could reorient
themselves toward the product dimer. One should also note that
only one of the two molecules in a reactive pair has to undergo
the pedal motion to become parallel. Further in depth under-
standing of the mechanism of photodimerization of non-
topochemically oriented olefins would require time-resolved
X-ray crystallographic investigations that we hope would be
performed in the near future.84-88
rather than a rule. If large amplitude molecular motions are
possible in the crystalline state, especially when energized
by light, photodimerization from criss-cross olefins should
not be a surprise. In this study we have identified thiourea
(TU) as a useful template to orient bispyridylethylenes and
stilbazoles toward photodimerization. We are pleased that
we were able to preorient 11 of the 14 olefins investigated
toward photodimerization in the crystalline state. Even the
two that did not dimerize were packed similarly to other
stilbazoles but with more than 4.5 Å distance between the
reactive CdC bonds. Most mechanically ground TU-olefin
cosolids showed photochemical behavior similar to that of
single crystals of TU-olefin cocrystals. We are currently
pursuing this aspect of solid-state photochemistry.
Summary
We close the presentation by pointing out that the
topochemical postulate should be considered a guideline
(84) Busse, G.; Tschentscher, T.; Plech, A.; Wulff, M.; Frederichs, B.;
Techert, S. Faraday Discuss. 2002, 122, 105.
Acknowledgment. V.R. is grateful to the National Science
Foundation, USA, for generous financial support (CHE-0848017)
and to Professor K. Venkatesan for valuable discussions.
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Hollins, P.; Mortimore, J. L.; Tobin, M. J. J. Mol. Struct. 2006, 786,
220.
Supporting Information Available: Experimental procedures,
X-ray crystallographic data, CIF files. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Q.; Wulff, M.; Techert, S. J. Am. Chem. Soc. 2009, 131, 15018.
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