Formation of different photodimers of isoquinolinone by irradiation of solid molecular compounds

Article abstract of DOI:10.1039/c0ce00489h

[4 + 4] Photodimerization of isoquinolin-3(2H)-one (isoquinolinone) (1) may yield 12 different isomers depending on the relative geometry of the two monomers prior to the reaction. Irradiation of the solid neat compound shows that of all possible dimers only the dimer between the two hetero-rings having an inversion center is produced. On the other hand, exposure to UV light of solid molecular compounds composed of isoquinolinone as the guest molecule and different host molecules shows to produce few other isomers of the dimer. The use of different host molecules affects the packing of the molecules in the crystal. As a result, the relative geometries between two monomer molecules are varied enabling photochemical dimerization to yield different isomers. When 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol (I) is used as the host molecule, two polymorphic forms of the molecular compound are obtained. Irradiation of single-crystal of the two polymorphs yields a mixture of isomers of the dimer. When the host molecule is cyclohexanol-1,2,4,5-tetracarboxylic acid (II) three isomers are expected, and when the host molecule is 1,3-benzenediol (III) a single isomer is formed which is identical to that obtained from the neat isoquinolinone.

Full text of DOI:10.1039/c0ce00489h

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PAPER
Formation of different photodimers of isoquinolinone by irradiation of solid
molecular compounds†‡
Deng-Ke Cao,ab Thekku V. Sreevidya,a Mark Botoshansky,a Gilad Golden,a Jason B. Benedictc
ac
*
and Menahem Kaftory
Received 3rd August 2010, Accepted 29th October 2010
DOI: 10.1039/c0ce00489h
[4 + 4] Photodimerization of isoquinolin-3(2H)-one (isoquinolinone) (1) may yield 12 different isomers
depending on the relative geometry of the two monomers prior to the reaction. Irradiation of the solid
neat compound shows that of all possible dimers only the dimer between the two hetero-rings having an
inversion center is produced. On the other hand, exposure to UV light of solid molecular compounds
composed of isoquinolinone as the guest molecule and different host molecules shows to produce few
other isomers of the dimer. The use of different host molecules affects the packing of the molecules in
the crystal. As a result, the relative geometries between two monomer molecules are varied enabling
photochemical dimerization to yield different isomers. When 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-
diol (I) is used as the host molecule, two polymorphic forms of the molecular compound are obtained.
Irradiation of single-crystal of the two polymorphs yields a mixture of isomers of the dimer. When the
host molecule is cyclohexanol-1,2,4,5-tetracarboxylic acid (II) three isomers are expected, and when the
host molecule is 1,3-benzenediol (III) a single isomer is formed which is identical to that obtained from
the neat isoquinolinone.
photochemical reaction is by monitoring the crystal structure
Introduction
along its progress. Therefore a reaction that undergoes single-
crystal to single-crystal transformation is preferred. A general
method describes the induction of such a homogeneous photo-
chemical reaction by irradiating the crystal with wavelength
corresponding to the chromophore’s absorption tail was pub-
lished.7 However, in most cases if the photochemical reaction is
executed on the neat light sensitive compound, the reaction does
not proceed to full conversion before the crystal disintegrates.
Therefore, monitoring of the reaction process is very limited.
Although, photochemical reactions that are carried out on solid
neat compounds may yield unique product it is limited to a single
one unless the compound crystallizes in different forms. The
advantage of carrying out photochemical reactions on solid
molecular compounds composed of light stable host molecule and
light sensitive guest molecule is that it may yield different products
depending on the structures enforced by the host molecule. Using
such molecular compound provides a method to alter the reactive
molecular environment expressed by different molecular packing
by using different host molecules. The change of the environment
or the molecular packing allows, in some cases, the synthesis of
different products8 or allows specific reactions. Early work on
such systems shows also an example of photochemical reaction
between the guest and the host molecules.9 In a recently reported
work the effect of the environment on molecular properties was
studied.10 The guest molecules undergo photoreaction which
takes place in cavities formed by the host molecules, therefore, in
the cases where the volume of the cavity is sufficiently big to
Traditionally most synthetic chemistry, especially when photo-
chemical reactions are involved, is carried out in solution;
however, there are enormous examples where synthetic chemistry
in the solid state has advantages that do not exist in solution.
Those examples are described in many publications starting from
the work of Schmidt and his co-workers1 followed by many
review articles and books.2 This is especially due to the fact that
in the crystal, the molecules are fixed and if it provides the
geometrical requirements needed for a photoreaction to take
place, the resulted product will be specific and determined by the
relative geometry of the reacting molecules. This advantage
provides the ability to control asymmetric syntheses as pointed
out already in 1975.3 For example, enantioselective reactions can
be performed by the use of inclusion compounds with a chiral
host molecule,4 or by using ionic chiral auxiliaries5 and zeolites.6
One of the best ways to follow and understand the
aSchulich Faculty of Chemistry, Technion-Israel Institute of Technology,
Haifa, 32000, Israel. E-mail: kaftory@tx.technion.ac.il
bState Key Laboratory of Coordination Chemistry, Coordination
Chemistry Institute, Nanjing University, Nanjing, 210093, People’s
Republic of China
cDepartment of Chemistry, University at Buffalo, State University of New
York, Buffalo, New York, 14260-3000, USA
† The paper is dedicated to the memory of Professor Fumio Toda, who
contributed significantly to the field of solid-state chemistry.
‡ CCDC reference numbers 787295–787303. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0ce00489h
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accommodate the product, single-crystal to single-crystal trans-
formations can take place.2,11 If the volume of the product is
significantly larger than that of the reactant, the crystal is likely
to undergo disintegration. In cases where the photochemical
reaction takes place in a single-crystal to single-crystal trans-
formation, monitoring of the structural changes is possible.
In the following example the effect of replacing the host
molecule on the molecular packing will be revealed. The aim of
this example is to demonstrate a method for synthesizing
different isomers in photochemical reactions by varying the host
molecules. It is not claimed that this is a mature method for
preplanned synthesis that can be used regularly because the
molecular packing cannot envisage when one uses different host
molecules. Nevertheless, it is helpful to know that in those cases
when ‘‘nothing helps’’ one could try this method.
neat isoquinolinone, unless it crystallized in different crystallo-
graphic forms (polymorphs).
The crystal structure of isoquinolinone was not published
before and will be described later in this paper.
There are some geometrical prerequisite for photodimerization
to take place. The distances between the atoms to be bonded
1
ꢀ
should be less than 4.2 A, and the lateral shift of the p-orbitals of
the reacting atoms should be small to allow an effective overlap
between them.12 There are two sites for possible bond formation
to form a dimer of isoquinolinone: the benzene ring and the hetero
ring. The product will be dependent on the relative translation of
the two molecular planes and on the symmetry between the two
monomers. As pointed out above, in principle, the crystal struc-
ture of the neat compound provides a single orientation with
a fixed relative translation of the two monomers and therefore if
the geometrical prerequisite for photodimerization is fulfilled,
a single product will be expected. However, in molecular
compounds the use of different host molecules, two monomers
may adopt different relative geometries leading to different
isomers of the dimers. This will be described by the use of
molecular compounds with isoquinolinone as the guest molecule
and three different host molecules, I, II, and III.
Isoquinolinone can undergo [4 + 4] photodimerization to
many products depending on the relative geometry of the two
monomers. The two monomers may be related to each other
either by a crystallographic symmetry or by a non-crystallo-
graphic symmetry (pseudo symmetry). Scheme 1 shows four
ways to align the two monomers with respect to each other (1, 2,
3, or 4). In 1 the two monomers are related by inversion center
(real or pseudo) and dimerization may lead to one of the three
products 5, 6, or 7. The two monomers in 2 are related by mirror
plane (real or pseudo) and dimerization may lead to one of the
dimers 8, 9, or 10. The monomers are related by two fold
symmetry in cases 3, and 4 and dimerization will yield one of the
three products 11, 12, or 13 from 3, and 14, 15, or 16, from
dimerization of 4. Out of the twelve possible dimers it is expected
that only one will be obtained by illumination of a crystal of the
Scheme 1
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experiments was recrystallized from acetic acid and found to be
Experimental
dimer 6 (Scheme 1).
Molecular compounds
Isoquinolin-3(2H)-one (1). A solution of 1 (0.25 mmol, 0.0365
g) in 4 mL of methanol was slowly evaporated at room
temperature to yield yellow prism crystals.
X-Ray crystal structure
X-Ray diffraction intensities were measured either by Kap-
paCCD Nonius Diffractometer or by a Bruker Smart APEX2
CCD diffractometer installed at a rotating anode source (Mo
1,1,6,6-Tetraphenyl-2,4-hexadiyne-1,6-diol (I) : 2(isoquinolin-
3(2H)-one (1)). A mixture of 1 (0.5 mmol, 0.0726 g) and I (0.25
mmol, 0.1034 g) in methanol (10 mL) was refluxed for 30 min in
ꢀ
Ka, l ¼ 0.71073 A). The crystal structures were solved by direct
methods (SHELX97).13 All non-hydrogen atoms were refined
anisotropically. The positions of the hydrogen atoms were
located in difference Fourier maps and refined by riding on their
parent atoms.x
ꢀ
an oil bath (80 C). The mixture was cooled for one night. The
molecular compound crystallized in two modifications: mostly as
yellow prism and the rest as yellow cubes.
Cyclohexane-1,2,4,5-tetracarboxylic acid (II) : 2(isoquinolin-
3(2H)-one (1)), H2O. A mixture of 1 (0.25 mmol, 0.0365 g) and II
(0.125 mmol, 0.0325 g) in 6 mL of methanol was refluxed for
Results and discussion
Crystal structures before irradiation
ꢀ
30 min in an oil bath (80 C). The filtrate was allowed to evap-
The neat guest compound isoquinolin-3(2H)-one (1) crystallized
in a monoclinic P21/c space group as a disordered molecule
consists of a 50 : 50% mixture of pyridone and hydroxypyridine
orate for a few days and resulted in red-yellow plate crystals.
ꢀ
(Fig. 1). As a result of the disorder the CO distance of 1.309(3) A
1,3-Benzenediol (III) : isoquinolin-3(2H)-one (1). A mixture of
1 (0.25 mmol, 0.0365 g) and III (0.25 mmol, 0.0275 g) in 10 mL of
methanol was refluxed for 30 min. The filtrate stood at room
temperature for about two weeks, forming red-yellow prism
crystals.
is longer than that of a C]O bond length observed in the other
x Crystal data. Compound 1: C9H7NO, M ¼ 145.16, monoclinic, a ¼
ꢀ
ꢀ
9.868(2), b ¼ 6.0890(10), c ¼ 12.466(2) A, b ¼ 108.14(3) , U ¼ 711.8(2)
3
ꢀ
A , T ¼ 293 K, space group P21/c (no. 14), Z ¼ 4, 4352 reflections
measured, 1258 unique (Rint ¼ 0.0671), 950 reflections with I > 2s(I)
which were used in all calculations. R[F2 > 2s(F2)] ¼ 0.0661. Compound
1d: C18H14N2O2$2(C2H4O2), M ¼ 410.42, triclinic, a ¼ 7.102(6), b ¼
Irradiation
ꢀ
ꢀ
ꢀ
ꢀ
7.909(6), c ¼ 9.350(5) A, a ¼ 97.37(3) , b ¼ 95.76(3) , g ¼ 101.14(3) ,
3
Single crystal was irradiated either with UV LED in the range
350–390 nm with the maximum of 365 nm set to 95 mW or with
the third (355 nm) harmonics of a circularly polarized pulsed
Nd:VO4 laser. The laser was set to pulse width of 35 ns and
10 kHz. The average power was less than 1 mW for a spot with
a diameter of ꢁ1 mm. The crystal revolved while it was exposed
to the light. Irradiation of powder sample of I–1 was carried out
by sunlight for 8 h. Recrystallization from methanol yielded pale
yellow prism crystals of the molecular compound composed of I
as the host and a dimer of 1. Irradiation of a single crystal of III–
1 shows that the crystal disintegrated after a few percent of
conversion. Irradiation of powder sample of the same molecular
compound yielded insoluble product in most common organic
solvents. Insoluble product was also obtained when pure iso-
quinolinone was irradiated. The product from the last two
ꢀ
ꢁ
U ¼ 506.8(6) A , T ¼ 293 K, space group P1 (no. 2), Z ¼ 1, 5330
reflections measured, 1905 unique (Rint ¼ 0.053), 1032 reflections with I
> 2s(I) which were used in all calculations. R[F2 > 2s(F2)] ¼ 0.0482.
Compound I–1m: C30H22O2$2(C9H7NO), M ¼ 704.79, monoclinic, a ¼
ꢀ
ꢀ
27.1065(2), b ¼ 9.2347(4), c ¼ 16.4901(8) A, b ¼ 115.402(1) , U ¼
3
ꢀ
3728.7(3) A , T ¼ 286 K, space group C2/c (no. 15), Z ¼ 4, 12 369
reflections measured, 3828 unique (Rint ¼ 0.0313), 2565 reflections with
I > 2s(I) which were used in all calculations. R[F2 > 2s(F2)] ¼
0.0416.Compound I–1o: C30H22O2$2(C9H7NO),
M
¼
704.79,
ꢀ
orthorhombic, a ¼ 25.4736(9), b ¼ 8.5626(3), c ¼ 16.9318(6) A, U ¼
3
ꢀ
3693.2(2) A , T ¼ 260 K, space group Pbcn (no. 60), Z ¼ 4, 27 377
reflections measured, 4601 unique (Rint ¼ 0.0391), 3130 reflections with
I > 2s(I) which were used in all calculations. R[F2 > 2s(F2)] ¼ 0.0420.
Compound II–1: C10H12O8$2(C9H7NO)$H2O, M ¼ 568.52, monoclinic,
ꢀ
ꢀ
a ¼ 13.372(3), b ¼ 7.1670(10), c ¼ 28.248(6) A, b ¼ 108.79(3) , U ¼
3
ꢀ
2562.9(10) A , T ¼ 293 K, space group P21/c (no. 14), Z ¼ 4, 15 706
reflections measured, 4683 unique (Rint ¼ 0.059), 2689 reflections with I
> 2s(I) which were used in all calculations. R[F2 > 2s(F2)] ¼ 0.0501.
Compound III–1: C6H6O2$C9H7NO, M ¼ 255.26, monoclinic, a ¼
ꢀ
ꢀ
5.728(1), b ¼ 22.789(4), c ¼ 11.111(2) A, b ¼ 116.90(3) , U ¼ 1293.4(5)
3
ꢀ
A , T ¼ 293 K, space group P21/c (no. 14), Z ¼ 4, 8147 reflections
measured, 2256 unique (Rint ¼ 0.0630), 1341 reflections with I > 2s(I)
which were used in all calculations. R[F2 > 2s(F2)] ¼ 0.0511. Compound
I–1dp: C30H22O2$2(C18H14N2O2)$C2H4O2, M ¼ 557.60, monoclinic,
ꢀ
ꢀ
a ¼ 14.166(3), b ¼ 9.432(2), c ¼ 20.139(4) A, b ¼ 91.88(2) , U ¼
3
ꢀ
2689.4(10) A , T ¼ 293 K, space group P21/c (no. 14), Z ¼ 2, 17 677
reflections measured, 4896 unique (Rint ¼ 0.089), 3162 reflections with I
> 2s(I) which were used in all calculations. R[F2 > 2s(F2)] ¼ 0.0593.
Compound I–1md: C30H22O2$C18H14N2O2, M ¼ 704.49, monoclinic,
ꢀ
ꢀ
a ¼ 26.761(8), b ¼ 9.111(3), c ¼ 16.998(6) A, b ¼ 114.443(9) , U ¼
3
ꢀ
3773(2) A , T ¼ 286 K, space group C2/c (no. 15), Z ¼ 4, 12 247
reflections measured, 3873 unique (Rint ¼ 0.0971), 1294 reflections with
I > 2s(I) which were used in all calculations. R[F2 > 2s(F2)] ¼ 0.0879.
Compound I–1od: C30H22O2$C18H14N2O2, M ¼ 704.49, orthorhombic,
3
ꢀ
ꢀ
a ¼ 25.479(3), b ¼ 8.1154(8), c ¼ 17.987(2) A, U ¼ 3719.3(7) A , T ¼
286 K, space group Pbcn (no. 60), Z ¼ 4, 11 003 reflections measured,
3524 unique (Rint ¼ 0.045), 1655 reflections with I > 2s(I) which were
used in all calculations. R[F2 > 2s(F2)] ¼ 0.0901.
Fig. 1 Hydrogen bonding and intermolecular geometry in 1.
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ꢀ
compounds presented here (1.274–1.285 A). The OC–N bond
formed hydrogen bonded dimers, typically to molecules con-
ꢀ
length of 1.357(3) A is shorter in 1 than in the other crystal
ꢀ
structures with isoquinolinone (1.375–1.382 A). The molecules
sisting of H–N–C]O groups (Fig. 1). The distance between the
ꢀ
potentially reactive atoms (C2 and C9) is 3.713(3) A, and the
lateral shift of the p-orbitals of the atoms C2 and C9 (of
ꢀ
a molecule that is related by inversion center) is 1.476 A.
The molecular compound I–1 crystallized in two polymorphic
forms: monoclinic C2/c space group (I–1m) (Fig. 2) and ortho-
rhombic Pbcn space group (I–1o) (Fig. 3). In both host molecule
(I) is occupying an inversion center and is hydrogen bonded to
two guest molecules (1) via their carbonyl groups (hydrogen
bonding geometry is given in Table 1). The guest molecules are
hydrogen bonded to each other (see Table 1) and each two are
Table 2 Relevant geometry for possible photodimerization in the solid
state
Fig. 2 Hydrogen bonding in I–1m.
Possible dimer
(Scheme 1)
ꢀ
d/A
ꢀ
p/A
ꢀ
l/A
Compound
Atoms
1
I–1m
C2–C9
3.713
3.699
3.711
4.228
3.685
3.749
4.110
3.725
3.839
3.616
3.703
3.732
3.803
3.813
4.072
4.082
3.407
3.504
3.504
3.504
3.487
3.487
3.487
3.329
3.329
3.329
3.329
3.329
3.329
3.495
3.492
3.488
1.476
1.185
1.222
2.366
1.192
1.377
2.176
1.671
1.912
1.412
1.622
1.687
1.839
1.524
2.095
2.121
6
5
5
7
5
5
6
10
10
9
9
8
8
6
9
9
C17–C22
C19–C24
C19–C22
C17–C22
C19–C24
C17–C24
C4–C4A
C7–C7A
C2–C4A
C9–C7A
C2–C2A
C9–C9A
C2–C9
I–1o
II–1
III–1
C2–C4
C7–C9
Fig. 3 Hydrogen bonds in I–1o.
Table 1 Hydrogen bonding geometry
D–H/A
D–H
H/A
D/A
D–H/A
Symop-for-A
1
O11 H1O1 N1
N1 H1N1 O11
1d
0.82
0.86
1.88
1.83
2.684(3)
2.684(3)
166
172
ꢂx, ꢂy + 2, ꢂz
ꢂx, ꢂy + 2, ꢂz
O3 H3 O1
N1 H1 O1
I–1m
1.03(3)
1.08(3)
1.62(3)
1.90(3)
2.641(3)
2.930(3)
169(3)
159(2)
ꢂx, ꢂy + 1, ꢂz
ꢂx ꢂ 1, ꢂy + 1, ꢂz + 1
O1 H1 O2
N1 H1N O2
I–1o
0.82
0.86
1.853
1.88
2.672(2)
2.731(2)
176
170
x, ꢂy + 1, z + 1/2
ꢂx, ꢂy + 1, ꢂz
O1 H1O1 O2
N1 H1N1 O2
II–1
0.82
0.86
1.87
1.844
2.687(2)
2.703(2)
173
175
x, ꢂy + 1, z + 1/2
ꢂx + 1, ꢂy + 1, ꢂz
N1A H1A O1
N1 H1 O1A
O2 H2O O5
O4 H4O O1
O7 H7O O1A
O9 H9O O8
O30 H30A O5
O30 H30B O3
III–1
0.86
0.86
0.75(3)
0.82
0.82
0.87(3)
0.98(2)
0.95(2)
1.88
1.89
2.03(3)
1.74
1.76
1.81(3)
1.95(2)
1.98(2)
2.743(3)
2.754(3)
2.734(3)
2.492(3)
2.565(3)
2.680(3)
2.858(2)
2.926(2)
177
178
155(3)
151
168
173(3)
152(2)
172(2)
ꢂx, ꢂy + 1, ꢂz
ꢂx, ꢂy + 1, ꢂz
ꢂx, y + 1/2, ꢂz + 1/2
ꢂx, y + 1/2, ꢂz + 1/2
ꢂx, yꢂ1/2, ꢂz + 1/2
ꢂx + 1, ꢂy, ꢂz + 1
ꢂx, y + 1/2, ꢂz + 1/2
x, y, z
N1 H1 O1
O2 H2 O3
O3 H3 O1
0.86
0.82
0.82
1.88
1.90
1.81
2.737(3)
2.705(3)
2.616(3)
178
168
165
ꢂx ꢂ 1, ꢂy, ꢂz + 1
x, ꢂy + 1/2, z ꢂ 1/2
ꢂx, ꢂy, ꢂz + 2
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Fig. 4 Hydrogen bonding in II–1.
related by inversion center. The result is that the carbonyl oxygen
of the guest molecule involved in a bifurcated hydrogen bond.
The relevant geometry with relation to the potential photo-
dimerization is given in Table 2.
conformation is different from the one observed in its neat
crystal.14 In the latter the molecule adopts mirror symmetry while
in the present structure, the carboxyl groups orient themselves in
accordance with the hydrogen bonds (Fig. 4). The presence of
four carboxyl groups of the host molecule provides four acceptor
and four donor groups for hydrogen bonding. The two guest
molecules add additional two hydrogen acceptor and two
The molecular compound II–1 crystallized in monoclinic P21/c
space group with a molecule of water. The host molecule has two
carboxyl groups in equatorial and two in axial positions. The
Fig. 5 Hydrogen bonding in III–1.
Fig. 7 Relevant distances in I–1o.
Fig. 6 Relevant distances in I–1m.
Fig. 8 Relevant distances in II–1.
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hydrogen bonded to each other via a dimer-like arrangement. As
a result of this complex hydrogen bonding scheme (Table 1), two
molecules of isoquinolinone are almost parallel (the angle
between the two mean planes is 2.11ꢀ) and related to each other
by a non-crystallographic translation (see 2 in Scheme 1) with an
ꢀ
average interplanar distance of 3.329(4) A.
The molecular compound III–1 crystallized in a monoclinic
P21/c space group. The guest molecules are related by inversion
centers. The carbonyl group in each isoquinolinone involved in
a bifurcated hydrogen bond: with a second isoquinolinone
molecule and with a hydroxyl group of the host molecule. The
hydroxyl group of the host molecule is also hydrogen bonded to
a second host molecule (Fig. 5). The guest molecules in different
planes are related by inversion centers with an intermolecular
Fig. 9 Relevant distances in III–1.
ꢀ
distance of 3.495(4) A.
Photodimerization will take place if the relative geometry
between the reacting centers fulfills the geometric requirements
set up for similar solid state photochemical reactions.1,12 The
three most relevant geometric parameters are: the intermolecular
distances between the reacting atoms (Fig. 1 and 6–9, and d in
Table 2), the interplanar distance (p in Table 2), and the lateral
shift between the p-orbitals of the reacting atoms (l in Table 2).
According to Table 2, irradiation of the 4 molecular
compounds might yield six different dimers out of the 12 theo-
retical dimers shown in Scheme 1, while irradiation of 1 will yield
a single dimer (6 in Scheme 1). However, a few of the possible
dimers such as 7 (from I–1m) and 9 (from III–1) might be failed
to be produced due to the large lateral shift of the two p-orbitals
and therefore a poor overlap that is needed for the photo-
dimerization. Also one should consider the competition between
the productions of different isomers that might affect the relative
yield of some of them when more than a single isomer is expected.
Fig. 10 Hydrogen bonding in 1d : acetic acid.
hydrogen donor groups. The presence of the water molecule
balances the need for a complete use of hydrogen bonding. One
of the carboxyl groups is used to form a dimer to a second host
molecule. Two carboxyl groups are hydrogen bonded to two
other host molecules through water molecule. The remaining
unused carboxylic acid group is hydrogen bonded to the guest
molecule through its hydroxyl group, while the carbonyl oxygen
of the host molecule is left idle. Each two guest molecules are
Crystal structure after irradiation
ꢁ
Compound 1d crystallizes in a triclinic P1 space group with
a molecule of acetic acid (Fig. 10). As expected from the crystal
Fig. 11 Hydrogen bonding in I–1dp.
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ꢀ
these atoms is smaller (2.176 A) in the orthorhombic polymorph,
ꢀ
compared with 2.366 A in the monoclinic polymorph.
Conclusions
It was demonstrated that molecular compounds composed of
light-stable host molecules and light-sensitive guest molecules are
chemical systems that can be manipulated to achieve relative
geometry of the guest molecules to enable photochemical reac-
tions leading to different isomers. In photodimerization the use
of different host molecules imposes different molecular packing
which naturally determine the relative lateral shift and the
distances between the two potentially reactive guest molecules.
As a result it determines whether the photodimerization will take
place and what would be the product.
Fig. 12 Hydrogen bonding and the disorder in I–1md. The two isomers
are shown in green and yellow (hydrogen atoms are absent).
Acknowledgements
The work was supported by the Israel Science Foundation, (499/
08). We would like to thank Prof. Philip Coppens from the
Department of Chemistry, University at Buffalo for the hospi-
tality, for enabling us to use the equipment to perform this
research and for helpful discussions. Partial support of this work
by the US National Science Foundation (CHE0843922) is
gratefully acknowledged.
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ꢀ
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ꢀ
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